What formate electro-oxidation can teach us about CO poisoning on Pt during biomass oxidation

What formate electro-oxidation can teach us about CO poisoning on Pt during biomass oxidation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article What formate electro-oxidation can teach us about CO poisoning on Pt during biomass oxidation Silvia Favero, Zhe Meng, Henrik H. Kristoffersen, Jan Rossmeisl, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7263225/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Catalyst deactivation due to *CO poisoning is a persistent challenge in the electrochemical oxidation of biomass-derived molecules such as glycerol and glucose. On platinum catalysts, *CO forms readily as a reaction intermediate, blocking active sites and requiring high overpotentials for removal—often leading to undesired overoxidation of valuable products. Understanding the fundamental origins of *CO formation is thus critical for designing more selective and stable catalysts. Since biomass oxidation can be extremely complex and involve a multitude of adsorbates and products, here we use a simplified model system, formate oxidation, to investigate *CO formation on Pt in alkaline pH . Starting from operando surface-enhanced infrared spectroscopy, we show that the adsorption configuration of formate determines if the surface will be poisoned by *CO. Oxygen-bound formate (*OOCH) undergoes direct and stable oxidation to CO₂, while carbon-bound formate (*COOH) disproportionates to form *CO–*OH, initiating poisoning. These insights offer a mechanistic foundation for designing Pt-based catalysts that resist *CO formation by selectively stabilizing *OOCH over *COOH intermediates, with broader implications for improving biomass electrooxidation performance. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction The oxidation of small organic molecules is gaining increasing attention for fuel cells and as a less energy-intensive alternative to hydrogen production, while also enabling the generation of value-added chemicals under ambient conditions. Various organic compounds have been explored for this purpose, 1 including glucose (targeting mainly gluconic acid), 2 glycerol (for which 1,3-dihydroxyacetone - DHA, glycolic acid and glyceric acid are common target producs), 3 5-hydroxymethylfurfural (HMF) (oxidized to 2,5-furandicarboxylic acid - FDCA), 4 and lower alcohols (targeting formic and acetic acid from methanol and ethanol oxidation respectively). 5 Although each of these molecules follows distinct oxidation pathways and require specific catalyst optimizations, some general trends can be identified. Notably, noble metals—particularly platinum—exhibit high catalytic activity for the oxidation of many of these substrates. For instance, platinum is the best-performing metal for the oxidation of glycerol, especially in alkaline media, where glycerol oxidation starts at around 0.4V vs RHE. 1 In the case of glucose oxidation, Pt and Au are the most selective catalysts for the partial oxidation to gluconic acid, with glucose electrooxidation starting from potentials as low as 0.2V vs RHE on Pt. 6 Finally, noble metals like Pt and Pd also display high intrinsic activity for the deep oxidation of alcohols to CO 2 . 7 Beyond their high activity, Pt-catalyzed reactions share a common limitation: platinum suffers from rapid deactivation due to the formation of *CO-like intermediates (where * denotes an adsorbed species). In the case of glycerol electrooxidation, the formation of *CO has been confirmed by surface-enhanced IR spectroscopy (SEIRAS), starting from potentials as low as 0.2V vs RHE. 8 In-situ infrared spectroscopy also confirmed the formation *CO during glucose oxidation on polycrystalline Pt in alkaline media 6 , 9 , 10 , and on Pt(100) in neutral pH. 6 Finally, platinum is similarly vulnerable to *CO poisoning during the oxidation of C1 and C2 alcohols. 7 The formation of *CO is particularly critical in these reactions, not only because it decreases the activity of the catalyst, but also because it requires positive potentials (> 0.6V vs RHE) to oxidize *CO to CO 2 . Such positive potentials can also catalyze the further oxidation of the reactants to something less economically valuable than the target product. 11 Therefore, understanding and ultimately avoiding the formation of *CO on platinum would benefit a wide range of reactions. However, due to the complex network of possible products, studying the reaction mechanisms of the oxidation of molecules like glycerol can be extremely challenging. Operando and surface-sensitive spectroscopy measurements are valuable tools in this pursuit, as they can monitor the real-time formation and evolution of adsorbates, as a function of the applied potential. Still, assigning specific peaks to given intermediate remains challenging. Infrared spectroscopy detects the frequency of vibrational modes that cause a change in the dipole moment of the adsorbate. However, for the case of glycerol, the problem remains that many common products—carboxylates, aldehydes, ketones, acyl groups, and carbonates—exhibit overlapping peaks in the 1100–1750 cm⁻¹ range. As highlighted in our recent review on the challenges of glycerol oxidation, 11 these overlapping signals can be hard to distinguish, a problem common to many biomass oxidation reactions. Moreover, once an intermediate adsorbs on a catalyst surface, its vibrational frequency may shift, making ex-situ product characterization insufficient for identifying species observed in situ. Density Functional Theory (DFT) simulations can aid in predicting vibrational frequencies, but this predictions alore are insufficient to distinguish between similar species—for example, COO stretching in lactic acid, glycolic acid, and glyceric acid. We encounter this limitation when using SEIRAS to study glycerol electrooxidation on Pt. While the technique is very sensitive to surface species and can provide excellent time and potential resolution in the production and consumption of different species, assigning each peak to an adsorbate can be extremely challenging. One approach that we took to assign these intermediates and to gain insight into the reaction was to simplify the problem by looking at a reduced part of the reaction pathway, by starting from a known intermediate. Being especially interested in *CO production we started from a likely precursor of *CO: formic acid. After a thorough literature review, we realized that while formic acid oxidation had been previously studied, 12 – 22 the nature of the active intermediates and the pathway for the formation of *CO are still debated. Moreover, almost all the works had focused on acidic conditions while biomass oxidation, including glycerol, generally display higher selectivity to value-added products in alkaline conditions. Therefore, we decided to study the electrooxidation of formic acid on Pt, as a model system to study *CO formation and poisoning dynamics in Pt-catalysed biomass oxidation. Formic acid oxidation was initially studied for fuel cells and electrolyzers because of its low thermodynamic potential (-0.12 V vs RHE) and the assumption that, having two oxygen atoms, it might avoid *CO formation typical of alcohol oxidation. However, it quickly became clear that *CO poisoning still occurs on Pt during formic acid oxidation, leading to rapid catalyst deactivation. 23 , 24 From the studies of formic acid oxidation on Pt in acidic media 12 – 22 , there is a general agreement on the existence of two parallel pathways: one involving direct formate oxidation, and the other proceeding through the formation of a *CO intermediate. The direct pathway dominates in the cathodic scan, while *CO formation is primarily observed in the anodic scan—yet both routes coexist. Figure 1 summarizes the pathways for the direct (blue) and indirect (orange) oxidation of formic acid, that have been previously proposed in literature. Initially, the direct pathway was attributed to bidentate formate (adsorbate 1 in Fig. 1 , with the two oxygen atoms bounded to the platinum surface) by Samjeske et al. 25 , 26 and Cuesta et al. 27 , 28 This assignment followed the observation through surface enhanced infrared spectroscopy in acidic conditions, that bidentate formate appears in the region of direct formic acid oxidation. 27 , 28 However, later contrasting experimental evidence has shown that the direct oxidation mechanism is more complex. For example, it was shown that the measured current is not proportional to the coverage of adsorbed bidentate formate, 29 and that adsorbed bisulfates, which reduces the coverage of bidentate formate, has a positive effect on the reaction. 30 DFT simulations also showed that the cleavage of the C-H bond from bidentate formate requires high activation energy, 31 while the same step would be much easier for monodentate formate adsorbed via the oxygen atom (species 3 in Fig. 1 ). 32 This evidence suggests that monodentate *OOCH might be the reactive species, as proposed by other investigations. 32 , 33 However, this hypothesis is in contrast with spectroscopic observations, as well as other DFT predictions, which show that the bidentate configuration is 0.7eV more favourable than the monodentate one. 32 To explain these discrepancies, Herrero and co-workers showed by theoretical calculations that the adsorption of bidentate formate creates pockets where monodentate *OOCH can be stably formed, and where C-H cleavage can happen with virtually no energy required. 32 In this scenario, monodentate *OOCH would be the active intermediate, but the surface would be dominated by bidentate *OOCH, explaining the spectroscopic results. Nevertheless, this mechanism has not been experimentally confirmed, and DFT simulations have also shown that other mechanisms are equally possible. For example, DFT simulations have also shown that *COOH (species 2 in Fig. 1 ) could be the key intermediate in both direct and indirect oxidation, and that neither monodentate or bidentate *OOCH are heavily involved in the reaction. 21 Other reports have proposed that monodentate formic acid is the active species, while bidentate *OOCH may poison the surface. 33 Kwon et al. showed that formic acid oxidation on Pt kinetics have a strong pH dependence, with the maximum in current density located at the pKa of formic acid. They proposed that the reaction proceeds through the oxidation of formate, rather than formic acid, explaining the positive effect of increasing the pH from 0 to 4, which extends the fraction of formate. On the other side, higher pH would cause an activity decline, due to the increasing competition with *OH adsorption. 18 Despite the crucial effect of pH, almost all the fundamental studies of this reaction are still performed in acidic pH, leaving room for valuable information to be extracted by studying the reaction in alkaline conditions. The pathway for indirect oxidation of formic acid is also still debated. Some studies point to the possibility that direct and indirect oxidation might share common steps and reaction intermediates, 21 , 34 and that *CO formation appears site demanding. 32 , 35 – 37 However, various mechanisms have been proposed for the formation of *CO. Bagger et al. have proposed that *CO formation happens through the disproportionation reaction of *COOH and requires the presence of adsorbed protons (pathway following species 2, 5 and 6 in Fig. 1 ). The requirement of *H was shown experimentally by the increase in the anodic activity, when the lower limit of the cyclic voltammogram is increased, with cycles completed between 0.4 and 1V vs RHE showing no hysteresis. 21 However, Feliu and co-workers reported *CO formation in a potential window located around the potential of zero total charge, where both protons and anions adsorptions are favoured but neither dominate. 38 , 39 They proposed that indirect oxidation starts from monodentate adsorbed formate (*OOCH) and proceed through a formate intermediate where both the carbon and the oxygen are adsorbed (species 8 in Fig. 1 ). They also suggest that adsorbed *H might not be a prerequisite for the reaction, but that *CO formation can only happen at potentials where both protons and anions adsorption is favourable. 32 In summary, despite decades of study, the mechanisms of formic acid oxidation—especially CO formation—remain unresolved. Most work has been done in acidic media, while alkaline conditions remain understudied, despite their relevance to biomass conversion. This disconnect has left critical knowledge gaps that limit our ability to design selective and stable catalysts for practical applications. In this work, we address this gap by using operando surface-enhanced infrared absorption spectroscopy (SEIRAS) to unravel the mechanism of formate oxidation on Pt in alkaline media. By directly probing the adsorbed species under working conditions, we identify the key intermediates responsible for the direct and indirect pathways and reveal how adsorption geometry dictates the reaction pathway. Results General observations from the cyclic voltammogram Figure 2 a shows the cyclic voltammogram (CV) of polycrystalline Pt in alkaline electrolyte in the presence of formate (1M formate, 0.1M NaOH). This scan shows a hysteresis between the anodic and cathodic peaks, typical of the electrooxidation of formate on Pt. 12 – 22 In particular, in the anodic scan, the oxidation current peaks at around 0.35V (peak 1) and then starts to drop, followed by a sharper peak at 0.6V vs RHE (peak 2). On the cathodic scan, the current reaches a higher peak at 0.4V vs RHE (peak 3). Figure 2 b shows the results of operando SEIRAS measurements, as a function of time and applied potential. From the bottom to the top of the graph, data are shown for increasing time. The orange lines at the bottom show the stable background (in 0.1M NaOH, at OCP) before the addition of formate, light orange lines show the change in spectra after the addition of 1M formate, at open circuit potential (0.9V vs RHE). Then, a cyclic voltammogram is performed between 1 and 0V vs RHE, shown in colors ranging from purple to yellow. Spectra are color-coded to match the potential of the cyclic voltammogram in Fig. 2 a. At OCP, 4 peaks can be observed at 1630 cm − 1 , 1590 cm − 1 , 1380 cm − 1 , 1350 cm − 1 . The peak at 1630 cm − 1 is assigned to interfacial water, 20 while the remaining ones are assigned to the C-H and C = O (symmetric and asymmetric) stretching of formate in solution (Fig. 2 b). 40 , 41 Starting from 0.6V vs RHE, in concomitance with the detection of an anodic current, new peaks appear, caused by the formation of new adsorbate. Two peaks are first observed at 1440–1460 cm − 1 and 1270–1290 cm − 1 , which are assigned to the O-C = O stretching mode of *OOCH, and the C = O stretching mode of *COOH respectively, following previous studies 18 , 20 , 25 . To confirm the assignment, we used density functional theory (DFT) to estimate the vibrational frequencies of the stretching and bending modes of the proposed intermediates (Fig. 3). For *OOCH, the frequencies are 1305 cm − 1 and 1555 cm − 1 for O-C-O bond symmetric stretching and asymmetric stretching modes, respectively. Other frequencies for *OOCH structure can be found in Supporting Information Table S2. The reason why experiments cannot detect O-C-O symmetric stretching mode frequency (1305 cm − 1 ) is because this frequency overlaps with experimental C = O asymmetric stretching of formate in solution (1350 cm − 1 ). 40,41 For *COOH, the frequencies are 1189 cm − 1 and 1686 cm − 1 in C-OH bond asymmetric stretching and C = O bond asymmetric stretching modes, respectively. Other frequencies for *COOH structure can be found in Supporting Information Table S1 . The 1686 cm − 1 frequency can’t be detected from experiments because the frequency would overlap with the experimental strong bulk water (1630cm − 1 ). The calculated frequency results, summarized in Fig. 3b, exhibit strong agreement with experimental observed values. While minor deviations are present, they do not exceed 100 cm⁻¹, which is within an acceptable range for theoretical approximations. 42 Finally, at potentials of around 0.2V two very sharp peaks appear at 1700–1790 cm − 1 , and at 1930–1970 cm − 1 . These peaks are located in the CO adsorption region, but are around 100 cm − 1 lower than those observed in CO-saturated NaOH, which are caused by the characteristic C ≡ O stretching mode of linear-bonded (2010–2060 cm − 1 ) and bridge-bonded (1830–1870 cm − 1 ) CO. 20 , 22 This suggests that the *CO species observed here are partially hydrated (i.e. bound to a neighbouring *OH species, and here denoted as *CO-*OH), as shown in Fig. 3b and discussed in more details later. Additionally, we highlight the site preference for CO adsorption. As shown in Table S3, *CO adsorbing on Pt surface via fcc (hollow, ΔE = 0.32 eV, referenced to *COOH = 0 eV) site is energetically favored compared with bridge (ΔE = 0.35 eV) or linear (ΔE = 0.42 eV) configurations. However, experimental observations on Pt(111) surfaces indicate the presence of only bridge and linearly bonded *CO; this discrepancy between theory and experiments is consistent with the literature for *CO binding on Pt(111). 43 , 44 To better track the formation of the reaction intermediates, Fig. 2 c shows the intensity of each adsorbate as a function of time and applied potential. When scanning in the cathodic direction, first no reaction is detected due to *OH poisoning, 18 , 19 , 25 , 26 then a positive current is observed below 0.6V vs RHE (peak 3), which is accompanied by first the formation of a small amount of O-bonded formate (*OOCH), followed by the rapid formation of C-bonded formate (*COOH). At 0.2V vs RHE, the current becomes negative, indicating the formation of *H, accompanied by the formation of *CO bridge and the consumption of *COOH. In the anodic scan, the current positive of 0.2V vs RHE (peak 1) is accompanied by an increase in *CO linear, a decrease in *CO bridge and the disappearance of *COOH. Finally, at 0.6V vs RHE, in conjunction with peak 2, both the bridge and linear *CO are consumed. These results suggest that the O-bonded formate is responsible for the direct oxidation pathway and C-bonded formate is responsible for the indirect oxidation pathway. In the cathodic scan, the presence of only the *OOCH intermediate in the rising side of peak 3 suggests that *OOCH is responsible for the direct oxidation pathway. We note that previous DFT simulations have shown that oxidation of bidentate *OOCH is energy demanding and have proposed that monodentate *OOCH formed in pockets of bidentate *OOCH could be the active species. 32 This explanation would be in line with our findings, as the coverage of monodentate *OOCH would be too low and its residence time too short to be detected by operando SEIRAS. At potentials less positive than 0.4V vs RHE, the appearance of the *COOH intermediate is accompanied by a fast decrease in the current, which suggests that C-bonded formate is more stable and difficult to oxidize compared to O-bonded formate. Finally, below 0.1V vs RHE, the simultaneous decrease in *COOH signals and increase in bridge *CO-*OH suggests that C-bonded formate is the active species for the formation of *CO. As the potential is increased again in the anodic scan (> 0.2V vs RHE), formate oxidation is restored as shown by the current increase. However, this time the surface is covered by a combination of bridge and linear *CO, and formic acid oxidation proceeds through the indirect route, with lower reaction rates. Finally, positive of 0.55V vs RHE, *CO oxidation becomes thermodynamically favourable, and *CO (bridge and linear) is converted to CO 2 . Above 0.7V vs RHE, the surface is rapidly poisoned by *OH adsorption, in combination with a sharp current increase. It can also be observed that in all the measurements *CO bridge is formed first, reaches a plateau and is then slowly substituted by *CO linear. *CO linear only requires one adsorption site, as opposed to the bridged form which requires two. Therefore, we hypothesize that bridged *CO is the favoured species, which is converted to linear *CO at high coverages due to the limited availability of two adjacent sites for the formation of bridge *CO. Elucidating the *CO formation mechanism From Fig. 2 , it appears clear that the decrease in reaction rate and the *CO bridge formation is faster in the hydrogen underpotential deposition (H UPD ) region of platinum, where we expect adsorbed *H is expected to be present on the terraces of Pt. This is in line with previous proposals suggesting that *CO formation can happen through a chemical disproportionation reaction, where the adsorbed formic acid reacts with hydrogen, to form water and *CO (*COOH + *H à *CO + H 2 O). 21 This suggests that the ideal catalysts should destabilize *H, to avoid the formation of *CO, and that operating at low coverages of *H (positive of the the H UPD ) region should avoid the indirect oxidation pathway. However, when performing formate oxidation in alkaline conditions at a lower scan rate of 2mVs − 1 (Figure S1 ), one can notice the formation of *CO starting at potential as high as 0.5V vs RHE in the cathodic scan. Even though *CO formation still accelerates in the H UPD region, this shows that *COOH disproportionation can happen at higher potentials, leading to the gradual poisoning of active sites. To elucidate the mechanism of *CO formation in alkaline pH we monitored the formation of reaction intermediates in CV scans, while gradually limiting the lower potential from the 0V vs RHE usually employed, to higher values. In fact, if disproportionation required the presence of adsorbed *H, one would expect that limiting the lower potential to 0.2V vs RHE would prevent the formation of *CO. The results are shown in Fig. 4 . As expected, and as shown before, if the potential is scanned all the way down to 0V vs RHE, *OOCH, *COOH, *CO bridge and *CO linear are formed. However, the formation of *CO bridge (following that of *COOH) is still observable when the lower potential is limited to 0.2V vs RHE and, to a small extent, even by stopping at 0.3V vs RHE. This shows that *CO can form at potentials above 0.3V vs RHE. Finally, if the potential is scanned between 0.4 and 1V vs RHE, only *OOCH is observed while both *COOH and *CO are absent. On top of confirming that *CO can form outside of the H UPD region, these results suggest that disproportionation occurs only from the *COOH intermediate, in absence of which *CO is not detected. On the other hand, *OOCH leads to the fast and direct oxidation of formate. Therefore, we propose that the binding mode of formate controls the reaction pathway, with O-bonded and C-bonded formate causing respectively the direct and indirect oxidation of formate. Another interesting observation is that the wavenumber at which *CO bridge and linear are detected shows a large redshift compared to “pure” CO, i.e. the case where CO gas is directly bubbled in the electrolyte. To illustrate this, Fig. 5 a and 5 b show the position of the formate-derived *CO intermediates as a function of potential (in blue), compared to the signal obtained from saturated CO gas at the same applied potential (shown in pink). The values differ by up to 100 cm − 1 and so does the potential dependency of the peak position. Linear and bridge-bound *CO from gaseous CO exhibits a typical potential dependence of ∼60 cm – 1 V – 1 , known as the Stark tuning slope 14 , 15 , 45 – 47 and caused by: i) electrostatic interactions between the electric field caused by the charged surface and the outer Helmholtz plane, and the dipole moment of the adsorbed *CO, 47 ii) the compression or dissipation of the CO adsorbed layer, 46 and iii) the change in *CO coverage. 45 On the contrary, the formate-derived linear *CO exhibits a potential dependence of ∼120 cm – 1 V – 1 and bridged *CO shows an even higher value of ∼150 cm – 1 V – 1 . These values cannot be solely explained by the Stark effect associated with the electric field or changes in coverage. We have reported this phenomenon before, in the context of methanol oxidation. 48 Similarly to what we observed here, methanol-derived *CO showed a red-shift of up to 60 cm – 1 compared to pure *CO and displayed a potential dependence of ∼120 cm – 1 V – 1 . To test whether this shift originates from interactions with the electrolyte, or intermediates from the molecule of interest, we performed the same methanol oxidation experiments in deuterated electrolyte (1M NaOD in D 2 O). As expected, pure *CO showed negligible changes upon deuterium substitution. On the contrary, we reported a measurable shift of ∼20 cm – 1 V – 1 on methanol-derived *CO in the deuterated electrolyte, suggesting the presence of a partially hydrogenated *CO adsorbate, interacting with hydrogen from a hydroxy group. For the case of formate (Fig. 5 ), the red-shift compared to pure *CO indicates again the existence of *CO in a partially hydrogenated state but no significant shift is observed in the deuterated electrolyte. Therefore, we propose that *COOH degradation takes the form of *COOH à *CO-*OH, where *CO-*OH indicate the adsorption of *CO and *OH on contiguous Pt atoms, with a weak C-O bonding still existing between the C atom in *CO and the O atom in *OH, as shown in Fig. 5 e. This mechanism would explain why *CO formation can happen above H UPD ; the lower vibrational frequency of formate-derived *CO, compare to pure *CO; and why the first one does not change in deuterated electrolyte. Below 0.2V vs RHE, the *CO formation becomes faster, but all the observations remain valid, indicating that adsorbed *H facilitate *CO formation but is not directly involved in the reaction. Interestingly, the red-shift of *CO vibrational frequencies was not reported in acidic electrolyte, 18 , 25 for which *CO formation above 0.2V vs RHE was also not detected. This suggests that the mechanism of *CO formation is different in acidic (*COOH + *H à *CO + H 2 O) and alkaline (*COOH à *CO-*OH) conditions, with only the first one requiring adsorbed protons. This observation could additionally explain the lower activity reported in alkaline electrolyte, 18 which on top of increasing competition with *OH, suffer from a slow but continuous formation of *CO species above H UPD . Another indication of the proposed *COOH ◊ *CO-*OH mechanism in alkaline pH is shown in Fig. 5 c,d. This shows the electrochemical and spectroscopical results of a step test where we applied a potential of 0.4V vs RHE for 2 minutes, and then left the system equilibrate to OCP for 10 seconds. At the start of the experiment the system is at OCP and no adsorbates are observed. When a potential of 0.4V vs RHE is applied, as expected, *COOH is formed and rapidly converted to *CO bridge. More surprisingly, if the potential control is stopped, the electrode reaches an open circuit potential of around 0.25V, where *CO bridge is converted back to *COOH. This suggests that *COOH disproportionation is reversible and that the *OH co-produced in *CO formation remains weakly bound to *CO. To confirm the existence of the *CO-*OH species and confirm our assignment of the 1700–1790 and 1930–1970 cm − 1 to bridge and linear *CO-*OH, we also simulated their vibrational frequencies using DFT, and the results are summarized in Fig. 6 . We started from *COOH intermediate, which adsorbs monodentately via the C atom, with a C-OH bond length of 1.37 Å. By elongating the C-OH bond, bidentate adsorption via both C and O becomes possible but this significantly increases the energy. Thus, we systematically fixed the C-OH bond length at distance between 1.46 and 2.50 Å, extending from the stable *COOH structure (1.37 Å). After relaxation, *CO-*OH (bridge) structure was meta-stable from 1.80 to 2.50 Å, while *CO-*OH (linear) structure was meta-stable from 1.58 to 2.50 Å. CO-*OH intermediate is always less stable than *COOH. This indicates that the conversion of *CO-*OH to *COOH is spontaneous in absence of an applied potential, as also observed experimentally (Fig. 5 c-e). For comparison, *CO (bridge) and *CO (linear) were modeled by setting the C-OH bond length to a sufficiently large value (indicated as ∞ in Fig. 6 ), effectively removing the OH group and eliminating its interaction with CO. Frequencies of other vibrational modes and corresponding energies are provided in Supporting Information Tables S3-S5. Figure 6 a summarizes the frequencies of C = O stretching of the simulated species, as a function of the C-OH bond length, starting from *COOH (in yellow), to *CO-*OH bridge to *CO bridge (in orange), and to *CO-*OH linear to *CO linear (in red). Shaded areas indicate experimental frequency ranges for *CO-*OH (bridge and linear) and *CO (bridge and linear). The color coding for the *COOH and *CO-*OH intermediates was selected to match that of other figures in the manuscript. The vibrational frequencies of *CO-*OH (bridge) are closer to those of *COOH than those of *CO-*OH (linear), suggesting greater structural similarity. However, the DFT-calculated energy differences (ΔE, relative to *COOH = 0 eV) indicate that the bridge configuration is less stable than the linear one. These ΔE values are plotted in Fig. 6 b. For *CO-*OH (bridge), DFT frequencies align with experimental data when the C-OH bond length is 1.80–2.40 Å, corresponding to ΔE of 0.65–0.90 eV. For *CO-*OH (linear), agreement occurs at C-OH bond lengths of 2.20–2.30 Å at an energy cost of ΔE between 0.59–0.63 eV. At these bond lengths, the vibrational frequencies of bridged and linear *CO-*OH species is ≈ 100 cm − 1 lower than the prediction of *CO, matching the experimental difference between “pure” *CO and formate-derived *CO. However, energetically, neither intermediate appears stable enough to be a detectable species from *COOH to *CO. Their experimental observability may arise from solvation effects not included in our calculations, where water molecules could help stabilize these intermediates. Conclusion Previous studies of formic acid oxidation on Pt have concluded that: i) the reaction can proceed through a direct and indirect pathway, ii) indirect oxidation leads to the formation of *CO, which poisons the surface, and iii) at high potentials (> 0.6 V vs RHE) activity drops as a results of *OH adsorption. However, up until now the active species prior to direct oxidation to CO 2 has been the subject of debate, as summarized in Fig. 1 , with reports proposing monodentate *OOCH, bidentate *OOCH, *COOH or formic acid as the active species. Regarding the indirect activation pathway, most works propose that *CO is formed through a disproportionation reaction, where *COOH reacts with protons to form *CO and water. However, other mechanisms have also been reported, for example involving *OOCH as the active species. The majority of the proposed mechanisms are based on DFT-simulations, and since multiple pathways are theoretically possible, here we have used operando surface enhanced IR-spectroscopy to gain further insight, by monitoring the formation of reaction intermediates experimentally. Our results show that, when cycling the potential above 0.4V vs RHE, a stable oxidation current can be achieved, and no hysteresis is observed between the cathodic and anodic scan. Under these conditions, bidentate *OOCH is the only detectable adsorbate, suggesting that oxygen-bound formate drives direct oxidation. On the contrary, our results show that *COOH (which dominates below 0.4V vs RHE) always leads to the formation of *CO, and while *CO formation is accelerated by the presence of adsorbed protons, *CO is formed at any potential as long as formate is adsorbed through the carbon atom. We also found that formate-derived *CO in alkaline media shows a ≈ 100 cm − 1 red-shift, compared to “pure” *CO (derived from bubbling CO gas). This shift, which was not reported in acidic conditions, is attributed to an alkaline-specific mechanism, where *COOH disproportionation leads to the formation of adsorbed *CO, weakly bound to an adjacent adsorbed *OH (Fig. 7 ). A lack of change in the *CO peak position in deuterated electrolyte confirmed that the red-shift of*CO is not due to interaction with the electrolyte. The proposed mechanism was also confirmed by DFT simulations of the *CO-*OH vibrational frequency and by a chronoamperometry experiment showing that once *CO-*OH is formed on the surface, it is converted back to *COOH if the system is left to relax to OCP. The mechanism summarized in Fig. 7 allows us to deduce design criteria for active formate oxidation catalysts. Since the *OOCH and *COOH intermediates catalyze respectively the direct and indirect oxidation of formate, our results indicate that suppressing CO formation requires stabilizing oxygen-bound formate adsorption. Since the binding energies of *COOH and *OOCH are not related, catalysts with strong *OOCH and weak *COOH binding energies can be easily found. 21 However, the binding energy of *OOCH and *OH do scale linearly, so these catalysts (such as Ag) are oxidized before they can adsorb formate. 21 One potential approach to overcome this limitation is to reduce OH coverage by carefully controlling electrolyte conditions. However, our findings also reveal that in alkaline media *COOH disproportionation can happen in absence of adsorbed protons, constraining the potential window for stable direct oxidation. Beyond formic acid oxidation, these findings have broader implications for biomass oxidation, where CO poisoning limits catalytic efficiency. Firstly, the intermediates identified here build a valuable library of FTIR vibrational frequencies that will aid identification of species in more complex reactions. Secondly, we have shown that CO vibrational frequencies provide direct insight into coordination environments and formation mechanisms — for example, “hydrated” or “hydrogenated” CO exhibits characteristic red-shifts. Aditionally, we have shown how isotopic labeling experiments can futher add the elucidaiton of both the cooridation environment and the formation mechanism of *CO. Our findings are directly relevant for reactions like glycerol oxidation, which can feature formic acid as a reaction intermediate, to supress its further oxidation to *CO. Additionally, some findings are more broadly applicable to *CO formation, even if not from formic acids. For example, our work highlights the crucial role of adsorption geometry in determining reaction pathways: oxygen-bound intermediates (*OOCH) favor direct oxidation, while carbon-bound intermediates (*COOH) lead to *CO formation. The role of the adsorption configuration has been already reported for various reactions, including methanol and glycerol oxidation on Pt and Au, where adsorption configuration governs selectivity and C–C cleavage pathways. 8 , 49 – 51 however, the role of the adsorption configuration on *CO formation for such reactions has not yet been investigated. Finally, we have also highlighted how *CO formation differs in alkaline and acidic pH, which could be crucial for future catalyst design strategies. Overall, this study closes a critical knowledge gap by experimentally elucidating the intermediates that govern *CO formation on Pt in alkaline media, advancing mechanistic understanding beyond formic acid oxidation to the broader field of biomass conversion. Experimental Electrocatalysts preparation Operando SEIRAS measurements were performed using a Si hemisphere (radius 22mm from Pier optics), on which an enhancement layer of Pd was deposited, followed by a thin film of Pt. The preparation method is described elsewhere. 48 , 52 In brief, the Si surface was prepared by dropcasting 1mL of 40% NH 4 F solution for a minute. Subsequently, palladium was depositing by drop-casting a solution of 1% HF–1 mM PdCl 2 for 5 min at room temperature. Finally, platinum was deposited by immersing the Si hemisphere in a Pt plating solution for 5 minutes, at 50˚C. The plating solution was obtained by mixing 30mL of LECTROLESS Pt 100 basic solution (30 mL, Electroplating Engineering of Japan Ltd.), 8mL of 28% NH 3 solution, and ultrapure water. SEIRAS Measurement The Si prism was mounted on an in-house developed electrochemical cell, featuring a Pt rod counter electrode and Hg/HgO reference. All the measurements were performed in either 0.1M NaOH or 1M formate, 0.1M NaOH. The SEIRAS spectra were obtained with an FTIR Nicolet iS50 (Thermo Fisher Scientific) spectrometer equipped with a Mercury-Cadmium-Telluride (MCT) detector. The optical path was fully replaced with N 2 gas. The measurements were performed with a 4 cm – 1 resolution in the 500–4000 cm – 1 spectral range; 32 scans were averaged, giving a spectral collection every 10s. The SEIRAS spectra were recorded using a custom-made single reflection ATR optics system at an incident angle of 67°. For the isotopically labelled experiments, 1M NaOD (99.5% from Merck) in D 2 O (99.9% from Merck) was used. Before each experiment, nitrogen was bubbled in 1M NaOH or 1M NaOD for 15 minutes to remove oxygen. Then the Pt surface was cleaned by performing 10 cyclic voltammograms between 0 and 1V vs RHE, at 20mVs − 1 . The system was then left to equilibrate at open circuit potential and a spectrum was recorded as reference. 1M Formate was subsequently added to the electrolyte and the experiment started. All the resulting spectra are shown as absorbance vs potential (in V vs RHE). Absorbance units are defined as log( I 0 / I ) where I 0 and I represent the reference spectra and the spectra of interest respectively. Computational Details We use density functional theory (DFT) to calculate energies and frequencies of possible formic acid oxidation intermediates. In our modelling, the metal slab is 4 × 4 × 4 with face-centered cubic (FCC) (111) facet with adsorbing intermediates. The 2 bottom layers are fixed to emulate bulk metal. To avoid interlayer interactions, the distance between metal slabs (periodic images) is set to 17 Å. Atomic simulation environment (ASE) program 53 was employed for the atomic structures. GPAW program, 54 , 55 was employed for DFT calculations with revised Perdew − Burke − Ernzerhof exchange-correlation functional, 56 400 eV energy cutoff for plane-wave basis sets and 4 × 4 × 1 Monkhorst-Pack k-point sampling. The maximum force on each atom is set to 0.03 eV/Å for relaxed atomic structures and frequencies. The DFT calculations and python scripts are accessible online at: https://erda.ku.dk/archives/d57d50eea957e20eb94a73a5d770bb51/published-archive.html Declarations Consent to Publish declaration: not applicable Consent to Participate declaration: not applicable Funding S.F. acknowledges the RSC collaboration grant C23-0818957162, and the Engineering and Physical Sciences Research Council (EPSRC) program grant EP/W031019. I.E.L.S. acknowledges the Royce Institute (EP/P02520X/1). Y.K. acknowledges the financial support by the Japan Society for the Promotion of Science (JPSP) KAKENHI Grant Number 25K01880 and by the Japan Science and Technology Agency (JST) under the Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE) program (grant no JPMJAP2422). Z.M., H.H.K., and J.R. acknowledge financial support from the Independent Research Fund Denmark grant no. 1127-00372B and the Danish National Research Foundation, Center for High Entropy Alloy Catalysis (CHEAC) DNRF149. Author Contribution SV did all the electrochemical experiments and the FTIR in Osaka and wrote the paperZM did all the theoretical modelling workHK and JR supervised all the theoretical model workIS supervised the electrochemistry work and corrected and discussed the manuscriptMT supervised all the work and contributed to the writing of the paper and discussionsYK contributed to the discussions and supervision of the FTIR measurements and their interpretation Data Availability The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. References Luo, H. et al. Progress and Perspectives in Photo- and Electrochemical-Oxidation of Biomass for Sustainable Chemicals and Hydrogen Production. Adv Energy Mater 11 , (2021). Brouzgou, A. & Tsiakaras, P. Electrocatalysts for Glucose Electrooxidation Reaction: A Review. Top Catal 58 , 1311–1327 (2015). Kumar, M., Meena, B., Yu, A., Sun, C. & Challapalli, S. Advancements in catalysts for glycerol oxidation via photo-/electrocatalysis: a comprehensive review of recent developments. Green Chem. 25 , 8411–8443 (2023). Yang, Y. & Mu, T. Electrochemical oxidation of biomass derived 5-hydroxymethylfurfural (HMF): pathway, mechanism, catalysts and coupling reactions. Green Chemistry 23 4228–4254 (2021). Siwal, S. S., Thakur, S., Zhang, Q. B. & Thakur, V. K. Electrocatalysts for electrooxidation of direct alcohol fuel cell: chemistry and applications. Materials Today Chemistry 14, 100182 (2019). Mello, G. A. B., Cheuquepán, W., Briega-Martos, V. & Feliu, J. M. Glucose electro-oxidation on Pt(100) in phosphate buffer solution (pH 7): A mechanistic study. Electrochim Acta 354 , (2020). Xue, J., Wu, X. & Feng, L. Pt/Mn3O4cubes with high anti-poisoning ability for C1 and C2 alcohol fuel oxidation. Chemical Communications 58 , 2371–2374 (2022). Garcia, A. C. et al. Strong Impact of Platinum Surface Structure on Primary and Secondary Alcohol Oxidation during Electro-Oxidation of Glycerol. ACS Catal 6 , 4491–4500 (2016). Faverge, T. et al. In Situ Investigation of d-Glucose Oxidation into Value-Added Products on Au, Pt, and Pd under Alkaline Conditions: A Comparative Study. ACS Catal 13 , 2657–2669 (2023). Neha, N. et al. Revisited Mechanisms for Glucose Electrooxidation at Platinum and Gold Nanoparticles. Electrocatalysis 14 , 121–130 (2023). Ye, H. et al. Progress and Challenges in Electrochemical Glycerol Oxidation: The Importance of Benchmark Methods and Protocols. ChemCatChem, 17 , e00152 (2025) ShyamYadav, R., Gebru, M. G., Teller, H., Schechter, A. & Kornweitz, H. The origins of formic acid electrooxidation on selected surfaces of Pt, Pd, and their alloys with Sn. J Mater Chem A Mater 12 , 3311–3322 (2024). Herrero, E. & Feliu, J. M. Understanding formic acid oxidation mechanism on platinum single crystal electrodes. Current Opinion in Electrochemistry, 9, 145–150 (2018). Yan, Y. G. et al. Ubiquitous strategy for probing ATR surface-enhanced infrared absorption At platinum group metal-electrolyte interfaces. Journal of Physical Chemistry B 109 , 7900–7906 (2005). Blizanac, B. B., Arenz, M., Ross, P. N. & Markovic, N. M. Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode. J Am Chem Soc 126 , 10130–10141 (2004). Yang, T. et al. Formic acid electro-oxidation: Mechanism and electrocatalysts design. Nano Res 16 , 3607–3621 (2023). Fang, Z. & Chen, W. Recent advances in formic acid electro-oxidation: From the fundamental mechanism to electrocatalysts. Nanoscale Adv 3 , 94–105 (2021). Joo, J., Uchida, T., Cuesta, A., Koper, M. T. M. & Osawa, M. The effect of pH on the electrocatalytic oxidation of formic acid/formate on platinum: A mechanistic study by surface-enhanced infrared spectroscopy coupled with cyclic voltammetry. Electrochim Acta 129 , 127–136 (2014). John, J., Wang, H., Rus, E. D. & Abrun, D. Mechanistic Studies of Formate Oxidation on Platinum in Alkaline Medium., 116, 5810-5820 (2012). Samjeske, G. Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at Platinum in Acidic Solution by Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy, 110, 16559–16566 (2006). Bagger, A. et al. Correlations between experiments and simulations for formic acid oxidation. Chem Sci 13 , 13409–13417 (2022). McPherson, I. J., Ash, P. A., Jacobs, R. M. J. & Vincent, K. A. Formate adsorption on Pt nanoparticles during formic acid electro-oxidation: Insights from: In situ infrared spectroscopy. Chemical Communications 52 , 12665–12668 (2016). Capon, A. & Parsons, R. The Oxidation of Fomric Acid at Noble Metal Electrodes part III: Intermediates and Mechanism on Platinum Electrodes, J Electroanal Chem Interfacial Electrochem , 45 , 205–231 (1973). Beden, B., Bewick, A. & Lamy, C. A Comparative Study of Formic Acid Adsorption on a Platinum Electrode by both Electrochemical and EMIRS Technique, J. Electroanal. Chem vol., 150 , 505-511 (1983). Samjeské, G., Miki, A., Ye, S. & Osawa, M. Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at Platinum in Acidic Solution by Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy. J Phys Chem B 110 , 16559–16566 (2006). Osawa, M. et al. The role of bridge-bonded adsorbed formate in the electrocatalytic oxidation of formic acid on platinum. Angewandte Chemie - International Edition 50 , 1159–1163 (2011). Cuesta, A., Cabello, G., Gutiérrez, C. & Osawa, M. Adsorbed formate: The key intermediate in the oxidation of formic acid on platinum electrodes. Physical Chemistry Chemical Physics 13 , 20091–20095 (2011). Cuesta, A., Cabello, G., Osawa, M. & Gutiérrez, C. Mechanism of the electrocatalytic oxidation of formic acid on metals. ACS Catal 2 , 728–738 (2012). Xu, J. et al. On the mechanism of the direct pathway for formic acid oxidation at a Pt(111) electrode. Physical Chemistry Chemical Physics 15 , 4367–4376 (2013). Perales-Rondón, J. V., Herrero, E. & Feliu, J. M. Effects of the anion adsorption and pH on the formic acid oxidation reaction on Pt(111) electrodes. Electrochim Acta 140 , 511–517 (2014). Wang, H. F. & Liu, Z. P. Formic acid oxidation at Pt/H2O interface from periodic DFT calculations integrated with a continuum solvation model. Journal of Physical Chemistry C 113 , 17502–17508 (2009). Ferre-Vilaplana, A., Perales-Rondón, J. V., Buso-Rogero, C., Feliu, J. M. & Herrero, E. Formic acid oxidation on platinum electrodes: A detailed mechanism supported by experiments and calculations on well-defined surfaces. J Mater Chem A Mater 5 , 21773–21784 (2017). Gao, W., Keith, J. A., Anton, J. & Jacob, T. Theoretical elucidation of the competitive electro-oxidation mechanisms of formic acid on Pt(111). J Am Chem Soc 132 , 18377–18385 (2010). Wang, H. F. & Liu, Z. P. Formic acid oxidation at Pt/H2O interface from periodic DFT calculations integrated with a continuum solvation model. Journal of Physical Chemistry C 113 , 17502–17508 (2009). Macia, M. D., Herrero, E., Feliu, J. M. & Aldaz, A. Formic Acid Self-Poisoning on Bismuth-Modified Pt(755) and Pt(775), 1, 87-89 (1999). Kita, H. & Lei, H.-W. Oxidation of Formic Acid in Acid Solution on Pt Single-Crystal Electrodes . Journal of Electroanalytical Chemistry, 388 , 167-177 (1995). Maciá, M. D., Herrero, E. & Feliu, J. M. Formic acid self-poisoning on adatom-modified stepped electrodes. Electrochim Acta, 47 , 3653–3661 (2002). Grozovski, V., Climent, V., Herrero, E. & Feliu, J. M. Intrinsic activity and poisoning rate for HCOOH oxidation on platinum stepped surfaces. Physical Chemistry Chemical Physics 12 , 8822–8831 (2010). Grozovski, V., Climent, V., Herrero, E. & Feliu, J. M. Intrinsic activity and poisoning rate for HCOOH oxidation at Pt(100) and vicinal surfaces containing monoatomic (111) steps. ChemPhysChem 10 , 1922–1926 (2009). Beckingham, B. S., Lynd, N. A. & Miller, D. J. Monitoring multicomponent transport using in situ ATR FTIR spectroscopy. J Memb Sci 550 , 348–356 (2018). Lucks, C. et al. Formic acid interaction with the uranyl(vi) ion: Structural and photochemical characterization. Dalton Transactions 42 , 13584–13589 (2013). Scaranto, J. & Mavrikakis, M. HCOOH decomposition on Pt(111): A DFT study. Surf Sci 648 , 201–211 (2016). Feibelman, P. J. et al. The CO/Pt(111) Puzzle a. Journal of Physical Chemistry B 105 , 4018–4025 (2001). Grinberg, I., Yourdshahyan, Y. & Rappe, A. M. CO on Pt(111) puzzle: A possible solution. J Chem Phys 117 , 2264–2270 (2002). Uddin, J. & Anderson, A. B. Trends with coverage and pH in Stark tuning rates for CO on Pt(111) electrodes. Electrochim Acta 108 , 398–403 (2013). Lambert, D. K. Vibrational Stark Effect of Adsorbates at Electrochemical Interfaces. Electrochim Acta 41 , 623–630 (1996). Stamenkovic, V., Chou, K. C., Somorjai, G. A., Ross, P. N. & Markovic, N. M. Vibrational properties of CO at the Pt(111)-solution interface: The anomalous stark-tuning slope. Journal of Physical Chemistry B 109 , 678–680 (2005). Katayama, Y. et al. Direct Observation of Surface-Bound Intermediates during Methanol Oxidation on Platinum under Alkaline Conditions. Journal of Physical Chemistry C 125 , 26321–26331 (2021). Li, Y. et al. PtAu alloying-modulated hydroxyl and substrate adsorption for glycerol electrooxidation to C3 products. Energy Environ Sci 17 , 4205–4215 (2024). Fernández, P. S., Martins, C. A., Martins, M. E. & Camara, G. A. Electrooxidation of glycerol on platinum nanoparticles: Deciphering how the position of each carbon affects the oxidation pathways. Electrochim Acta 112 , 686–691 (2013). Meng, Z., Tran, D., Hjelm, J., Kristoffersen, H. H. & Rossmeisl, J. Insight into Selectivity Differences of Glycerol Electro-Oxidation on Pt(111) and Ag(111). ACS Catal 14 , 2455–2462 (2024). Katayama, Y. et al. Surface (Electro)chemistry of CO2 on Pt Surface: An in Situ Surface-Enhanced Infrared Absorption Spectroscopy Study. Journal of Physical Chemistry C 122 , 12341–12349 (2018). Hjorth Larsen, A. et al. The atomic simulation environment - A Python library for working with atoms. Journal of Physics Condensed Matter 29 , 273002 (2017). Enkovaara, J. et al. Electronic structure calculations with GPAW: A real-space implementation of the projector augmented-wave method. Journal of Physics Condensed Matter 22 , 253202 (2010). Mortensen, J. J., Hansen, L. B. & Jacobsen, K. W. Real-space grid implementation of the projector augmented wave method. Phys Rev B Condens Matter Mater Phys 71 , (2005). Hammer, B., Hansen, L. B. & No, J. K. Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals. Phys Rev B 59 , 7413–7421. Additional Declarations No competing interests reported. Supplementary Files SIZM.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7263225","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":508682399,"identity":"43412767-8a3c-44c5-87f8-05385e564784","order_by":0,"name":"Silvia Favero","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Silvia","middleName":"","lastName":"Favero","suffix":""},{"id":508682400,"identity":"b4f9205a-eb99-446d-bb68-0cf502234034","order_by":1,"name":"Zhe Meng","email":"","orcid":"","institution":"University of Copenhagen","correspondingAuthor":false,"prefix":"","firstName":"Zhe","middleName":"","lastName":"Meng","suffix":""},{"id":508682401,"identity":"0b96d1c4-b325-49ea-ad67-b84cb19d71aa","order_by":2,"name":"Henrik H. Kristoffersen","email":"","orcid":"","institution":"University of Copenhagen","correspondingAuthor":false,"prefix":"","firstName":"Henrik","middleName":"H.","lastName":"Kristoffersen","suffix":""},{"id":508682402,"identity":"0656d0c2-df6e-4d98-95ff-a3741612f884","order_by":3,"name":"Jan Rossmeisl","email":"","orcid":"","institution":"University of Copenhagen","correspondingAuthor":false,"prefix":"","firstName":"Jan","middleName":"","lastName":"Rossmeisl","suffix":""},{"id":508682403,"identity":"335ce1f0-d841-4b99-94e2-8cd309247b73","order_by":4,"name":"Ifan E. L. Stephens","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Ifan","middleName":"E. L.","lastName":"Stephens","suffix":""},{"id":508682404,"identity":"51fb1500-aa76-460b-82cf-335618009066","order_by":5,"name":"Maria-Magdalena Titirici","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Maria-Magdalena","middleName":"","lastName":"Titirici","suffix":""},{"id":508682405,"identity":"db1085bc-2509-46a5-9788-36df3dd2db9f","order_by":6,"name":"Yu Katayama","email":"data:image/png;base64,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","orcid":"","institution":"SANKEN, The University of Osaka","correspondingAuthor":true,"prefix":"","firstName":"Yu","middleName":"","lastName":"Katayama","suffix":""}],"badges":[],"createdAt":"2025-07-31 14:23:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7263225/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7263225/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":90494007,"identity":"71152773-c860-449d-8616-5c6f9a9f7b76","added_by":"auto","created_at":"2025-09-03 10:17:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":189182,"visible":true,"origin":"","legend":"\u003cp\u003eSchematics of the mechanisms of formic acid oxidation proposed in the literature (semi-transparent arrows). In blue are shown the mechanism for direct oxidation, while in orange are shown the mechanisms proposed for indirect oxidation, through the formation of carbon monoxide. The mechanism suggested in this work is highlighted with full arrows and boxes around the proposed intermediates: bidentate *OOCH (in blue), mono-dentate *OOCH (in yellow), *COOH (in green), bridge-bonded *CO-*OH (in orange) and linear bonded *CO-*OH (in red). All these intermediates, except mono-dentate *OOCH, are detected by SEIRAS. Mono-dentate *OOCH is reported to be only stable in pockets of bi-dentate *OOCH\u003csup\u003e 32\u003c/sup\u003e and to be easily oxidized, leading to short residence times and very low coverages, explaining the lack of SEIRAS detection.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/7200ae0dd2b6c16d812868a2.png"},{"id":90496293,"identity":"b72de5ff-4a69-4a42-a386-eca885d46665","added_by":"auto","created_at":"2025-09-03 10:41:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":289759,"visible":true,"origin":"","legend":"\u003cp\u003ea) cyclic voltammogram in 0.1M NaOH and 1M formic acid. The cycle starts at OCP (0.9V vs RHE), then the potential is scanned anodically up to 1V vs RHE and cathodically to 0V vs RHE, at a scan rate of 10mVs\u003csup\u003e-1\u003c/sup\u003e. Data were collected in an in-situ cell, described in the experimental section, at 25˚C. The working electrode is polycrystalline Pt deposited on a Si optical prism b) Background-subtracted in-situ surface-enhanced FTIR spectra, collected during the cyclic voltammogram shown in part a. The time increases from the bottom to the top of the graph. The background was collected at OCP in 0.1M NaOH (orange curves at the bottom of the graph), then formic acid was added and spectra were collected at OCP (light orange curves). \u0026nbsp;Finally, the potential was cycled as described in part a and the spectra are color-coded according to the applied potential (with the same colours reported in the cyclic voltammogram). c) The applied potential and current are shown in black, as a function of time. The magnitude of the adsorbate peaks reported in part b is also reported as a function of time. \u0026nbsp;\u0026nbsp;\u0026nbsp;\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/66573651d791e65b9b8bf2d7.png"},{"id":90494014,"identity":"55e57d09-e5d3-471c-93d4-a536c3e95d2a","added_by":"auto","created_at":"2025-09-03 10:17:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":175180,"visible":true,"origin":"","legend":"\u003cp\u003ea) Summary of the IR peaks assignments and b) the experimental and DFT predicted vibrational frequencies of the proposed intermediates. Details of the DFT simulations can be found in the “methods” section\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/eecaefebcff21612fecc6df3.png"},{"id":90495100,"identity":"e4684f49-170a-43e4-8db4-2ca0870636fb","added_by":"auto","created_at":"2025-09-03 10:25:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":386635,"visible":true,"origin":"","legend":"\u003cp\u003eFrom left to right are shown the electrochemical results of cyclic voltammograms, the corresponding SEIRAS spectra, and the magnitude of each SEIRAS peak as a function of time and applied potential. All the data were collected in 0.1M NaOH with 1M formic acid, at room temperature, with a scan rate of 10mVs\u003csup\u003e-1\u003c/sup\u003e, with an anodic potential limit of 1V vs RHE, and the cathodic limit increasing from the top to the bottom graph, between 0, 0.2, 0.3 and 0.4 V vs RHE. The SEIRAS spectra shown in the second column are colour-coded to match the potential colour of the cyclic voltammogram, and are background-subtracted, with the background collected at OCP in 0.1M NaOH.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/cec7173ec6d2449685c42119.png"},{"id":90494021,"identity":"58fa97a5-f132-4564-b041-961ccb0f4fc0","added_by":"auto","created_at":"2025-09-03 10:17:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":143723,"visible":true,"origin":"","legend":"\u003cp\u003ea) and b) show the peak position of formate-derived *CO linear and *CO bridge respectively, as a function of the applied potential. In pink are the results obtained in CO-saturated 0.1M KOH (adapted from Katayama et al.). c) and d) show the electrochemical and spectroscopic results of a potential step experiment performed in an \u003cem\u003eoperando \u003c/em\u003eSEIRAS cell, with polycrystalline Pt as working electrode, in 0.1M NaOH with 1M formate, at room temperature. c) The potential is shown in a continuous line: the system started at OCP, then 0.45V was applied for 2min, then the system was left to relax to OCP for 10s, followed by 2 min at 0.45V and finally the system relaxes again to OCP. In the same graph is shown the measured current (reported as squared data points). d) SEIRAS results for the step measurements, from the bottom to the top of the graph spectra are reported for increasing time, the spectra in light green are collected at 0.2V vs RHE, and those in blue were collected at OCP. e) schematic representation of the reversible *COOH →*CO-*OH step\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/0ac5612494fe4ba57700cf8a.png"},{"id":90494012,"identity":"76a3429b-088c-4b38-8c2a-2da2b30d6eaa","added_by":"auto","created_at":"2025-09-03 10:17:23","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":66978,"visible":true,"origin":"","legend":"\u003cp\u003eCalculated C=O asymmetric stretching frequencies (cm\u003csup\u003e-1\u003c/sup\u003e) (a) and DFT energy differences (ΔE/eV, referenced to *COOH = 0 eV) (b) as functions of C-OH bond length for intermediates *COOH, *CO-*OH (bridge), *CO-*OH (linear), *CO (bridge), and *CO (linear). Color coding matches Figure 3. A C-OH bond length of ∞ represents fully separated CO and OH groups. Experimental ranges for *CO-*OH (bridge), *CO-*OH (linear), *CO bridge and *CO linear are shown as shaded areas; all other points are from DFT.\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/c6f76e9e0f0c9a02a8873b79.png"},{"id":90495102,"identity":"463113ff-d70c-486f-9938-be898d50f1fc","added_by":"auto","created_at":"2025-09-03 10:25:23","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":114553,"visible":true,"origin":"","legend":"\u003cp\u003eSummary of the reaction pathway proposed in this work. Above 0.4 V vs RHE, bi-dentate formate (*OOCH) is adsorbed via the oxygen atom, leading to the fast and stable direct oxidation to CO\u003csub\u003e2\u003c/sub\u003e. Below 0.4V vs RHE, formate adsorbs via the carbon atom (*COOH). This adsorption configuration leads to the formation of *CO and the gradual poisoning of the catalyst. In acidic media, as previously reported, *COOH oxidation to *CO requires the presence of protons. On the contrary, in this work, we have shown that in alkaline media *COOH oxidation proceeds via a *CO-*OH intermediate, which can form in absence of protons.\u003c/p\u003e","description":"","filename":"Fig7.png","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/649ea0ce3b6dcc988f243028.png"},{"id":92570015,"identity":"d2d4feb2-01cd-49ae-9ac7-e10745d73256","added_by":"auto","created_at":"2025-10-01 07:32:29","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1839815,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/9a2de376-3ff5-4a9c-b8cc-eda6ef85c3aa.pdf"},{"id":90495385,"identity":"afe87d62-bd0f-4366-8d66-e95f2b97d451","added_by":"auto","created_at":"2025-09-03 10:33:23","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":488848,"visible":true,"origin":"","legend":"","description":"","filename":"SIZM.docx","url":"https://assets-eu.researchsquare.com/files/rs-7263225/v1/856ca7d6e906be2ddf628a0f.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"What formate electro-oxidation can teach us about CO poisoning on Pt during biomass oxidation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe oxidation of small organic molecules is gaining increasing attention for fuel cells and as a less energy-intensive alternative to hydrogen production, while also enabling the generation of value-added chemicals under ambient conditions. Various organic compounds have been explored for this purpose,\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e including glucose (targeting mainly gluconic acid),\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e glycerol (for which 1,3-dihydroxyacetone - DHA, glycolic acid and glyceric acid are common target producs), \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e 5-hydroxymethylfurfural (HMF) (oxidized to 2,5-furandicarboxylic acid - FDCA),\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e and lower alcohols (targeting formic and acetic acid from methanol and ethanol oxidation respectively).\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eAlthough each of these molecules follows distinct oxidation pathways and require specific catalyst optimizations, some general trends can be identified. Notably, noble metals\u0026mdash;particularly platinum\u0026mdash;exhibit high catalytic activity for the oxidation of many of these substrates. For instance, platinum is the best-performing metal for the oxidation of glycerol, especially in alkaline media, where glycerol oxidation starts at around 0.4V vs RHE.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e In the case of glucose oxidation, Pt and Au are the most selective catalysts for the partial oxidation to gluconic acid, with glucose electrooxidation starting from potentials as low as 0.2V vs RHE on Pt.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Finally, noble metals like Pt and Pd also display high intrinsic activity for the deep oxidation of alcohols to CO\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e Beyond their high activity, Pt-catalyzed reactions share a common limitation: platinum suffers from rapid deactivation due to the formation of *CO-like intermediates (where * denotes an adsorbed species). In the case of glycerol electrooxidation, the formation of *CO has been confirmed by surface-enhanced IR spectroscopy (SEIRAS), starting from potentials as low as 0.2V vs RHE.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e \u003cem\u003eIn-situ\u003c/em\u003e infrared spectroscopy also confirmed the formation *CO during glucose oxidation on polycrystalline Pt in alkaline media \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, and on Pt(100) in neutral pH.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Finally, platinum is similarly vulnerable to *CO poisoning during the oxidation of C1 and C2 alcohols.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e The formation of *CO is particularly critical in these reactions, not only because it decreases the activity of the catalyst, but also because it requires positive potentials (\u0026gt;\u0026thinsp;0.6V vs RHE) to oxidize *CO to CO\u003csub\u003e2\u003c/sub\u003e. Such positive potentials can also catalyze the further oxidation of the reactants to something less economically valuable than the target product.\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eTherefore, understanding and ultimately avoiding the formation of *CO on platinum would benefit a wide range of reactions. However, due to the complex network of possible products, studying the reaction mechanisms of the oxidation of molecules like glycerol can be extremely challenging. \u003cem\u003eOperando\u003c/em\u003e and surface-sensitive spectroscopy measurements are valuable tools in this pursuit, as they can monitor the real-time formation and evolution of adsorbates, as a function of the applied potential. Still, assigning specific peaks to given intermediate remains challenging. Infrared spectroscopy detects the frequency of vibrational modes that cause a change in the dipole moment of the adsorbate. However, for the case of glycerol, the problem remains that many common products\u0026mdash;carboxylates, aldehydes, ketones, acyl groups, and carbonates\u0026mdash;exhibit overlapping peaks in the 1100\u0026ndash;1750 cm⁻\u0026sup1; range. As highlighted in our recent review on the challenges of glycerol oxidation,\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e these overlapping signals can be hard to distinguish, a problem common to many biomass oxidation reactions. Moreover, once an intermediate adsorbs on a catalyst surface, its vibrational frequency may shift, making ex-situ product characterization insufficient for identifying species observed in situ. Density Functional Theory (DFT) simulations can aid in predicting vibrational frequencies, but this predictions alore are insufficient to distinguish between similar species\u0026mdash;for example, COO stretching in lactic acid, glycolic acid, and glyceric acid.\u003c/p\u003e\u003cp\u003eWe encounter this limitation when using SEIRAS to study glycerol electrooxidation on Pt. While the technique is very sensitive to surface species and can provide excellent time and potential resolution in the production and consumption of different species, assigning each peak to an adsorbate can be extremely challenging. One approach that we took to assign these intermediates and to gain insight into the reaction was to simplify the problem by looking at a reduced part of the reaction pathway, by starting from a known intermediate. Being especially interested in *CO production we started from a likely precursor of *CO: formic acid. After a thorough literature review, we realized that while formic acid oxidation had been previously studied, \u003csup\u003e\u003cspan additionalcitationids=\"CR13 CR14 CR15 CR16 CR17 CR18 CR19 CR20 CR21\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e the nature of the active intermediates and the pathway for the formation of *CO are still debated. Moreover, almost all the works had focused on acidic conditions while biomass oxidation, including glycerol, generally display higher selectivity to value-added products in alkaline conditions. Therefore, we decided to study the electrooxidation of formic acid on Pt, as a model system to study *CO formation and poisoning dynamics in Pt-catalysed biomass oxidation.\u003c/p\u003e\u003cp\u003eFormic acid oxidation was initially studied for fuel cells and electrolyzers because of its low thermodynamic potential (-0.12 V vs RHE) and the assumption that, having two oxygen atoms, it might avoid *CO formation typical of alcohol oxidation. However, it quickly became clear that *CO poisoning still occurs on Pt during formic acid oxidation, leading to rapid catalyst deactivation. \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e From the studies of formic acid oxidation on Pt in acidic media \u003csup\u003e\u003cspan additionalcitationids=\"CR13 CR14 CR15 CR16 CR17 CR18 CR19 CR20 CR21\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, there is a general agreement on the existence of two parallel pathways: one involving direct formate oxidation, and the other proceeding through the formation of a *CO intermediate. The direct pathway dominates in the cathodic scan, while *CO formation is primarily observed in the anodic scan\u0026mdash;yet both routes coexist.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the pathways for the direct (blue) and indirect (orange) oxidation of formic acid, that have been previously proposed in literature. Initially, the direct pathway was attributed to bidentate formate (adsorbate 1 in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, with the two oxygen atoms bounded to the platinum surface) by Samjeske et al.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e and Cuesta et al.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e This assignment followed the observation through surface enhanced infrared spectroscopy in acidic conditions, that bidentate formate appears in the region of direct formic acid oxidation.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e However, later contrasting experimental evidence has shown that the direct oxidation mechanism is more complex. For example, it was shown that the measured current is not proportional to the coverage of adsorbed bidentate formate,\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e and that adsorbed bisulfates, which reduces the coverage of bidentate formate, has a positive effect on the reaction.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e DFT simulations also showed that the cleavage of the C-H bond from bidentate formate requires high activation energy,\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e while the same step would be much easier for monodentate formate adsorbed via the oxygen atom (species 3 in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e This evidence suggests that monodentate *OOCH might be the reactive species, as proposed by other investigations.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e However, this hypothesis is in contrast with spectroscopic observations, as well as other DFT predictions, which show that the bidentate configuration is 0.7eV more favourable than the monodentate one.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e To explain these discrepancies, Herrero and co-workers showed by theoretical calculations that the adsorption of bidentate formate creates pockets where monodentate *OOCH can be stably formed, and where C-H cleavage can happen with virtually no energy required.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e In this scenario, monodentate *OOCH would be the active intermediate, but the surface would be dominated by bidentate *OOCH, explaining the spectroscopic results. Nevertheless, this mechanism has not been experimentally confirmed, and DFT simulations have also shown that other mechanisms are equally possible. For example, DFT simulations have also shown that *COOH (species 2 in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) could be the key intermediate in both direct and indirect oxidation, and that neither monodentate or bidentate *OOCH are heavily involved in the reaction.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Other reports have proposed that monodentate formic acid is the active species, while bidentate *OOCH may poison the surface.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eKwon et al. showed that formic acid oxidation on Pt kinetics have a strong pH dependence, with the maximum in current density located at the pKa of formic acid. They proposed that the reaction proceeds through the oxidation of formate, rather than formic acid, explaining the positive effect of increasing the pH from 0 to 4, which extends the fraction of formate. On the other side, higher pH would cause an activity decline, due to the increasing competition with *OH adsorption.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Despite the crucial effect of pH, almost all the fundamental studies of this reaction are still performed in acidic pH, leaving room for valuable information to be extracted by studying the reaction in alkaline conditions.\u003c/p\u003e\u003cp\u003eThe pathway for indirect oxidation of formic acid is also still debated. Some studies point to the possibility that direct and indirect oxidation might share common steps and reaction intermediates,\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e and that *CO formation appears site demanding.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e However, various mechanisms have been proposed for the formation of *CO. Bagger et al. have proposed that *CO formation happens through the disproportionation reaction of *COOH and requires the presence of adsorbed protons (pathway following species 2, 5 and 6 in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The requirement of *H was shown experimentally by the increase in the anodic activity, when the lower limit of the cyclic voltammogram is increased, with cycles completed between 0.4 and 1V vs RHE showing no hysteresis.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e However, Feliu and co-workers reported *CO formation in a potential window located around the potential of zero total charge, where both protons and anions adsorptions are favoured but neither dominate.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e They proposed that indirect oxidation starts from monodentate adsorbed formate (*OOCH) and proceed through a formate intermediate where both the carbon and the oxygen are adsorbed (species 8 in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). They also suggest that adsorbed *H might not be a prerequisite for the reaction, but that *CO formation can only happen at potentials where both protons and anions adsorption is favourable.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eIn summary, despite decades of study, the mechanisms of formic acid oxidation\u0026mdash;especially CO formation\u0026mdash;remain unresolved. Most work has been done in acidic media, while alkaline conditions remain understudied, despite their relevance to biomass conversion. This disconnect has left critical knowledge gaps that limit our ability to design selective and stable catalysts for practical applications.\u003c/p\u003e\u003cp\u003eIn this work, we address this gap by using operando surface-enhanced infrared absorption spectroscopy (SEIRAS) to unravel the mechanism of formate oxidation on Pt in alkaline media. By directly probing the adsorbed species under working conditions, we identify the key intermediates responsible for the direct and indirect pathways and reveal how adsorption geometry dictates the reaction pathway.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003eGeneral observations from the cyclic voltammogram\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the cyclic voltammogram (CV) of polycrystalline Pt in alkaline electrolyte in the presence of formate (1M formate, 0.1M NaOH). This scan shows a hysteresis between the anodic and cathodic peaks, typical of the electrooxidation of formate on Pt. \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e In particular, in the anodic scan, the oxidation current peaks at around 0.35V (peak 1) and then starts to drop, followed by a sharper peak at 0.6V vs RHE (peak 2). On the cathodic scan, the current reaches a higher peak at 0.4V vs RHE (peak 3). Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb shows the results of \u003cem\u003eoperando\u003c/em\u003e SEIRAS measurements, as a function of time and applied potential. From the bottom to the top of the graph, data are shown for increasing time. The orange lines at the bottom show the stable background (in 0.1M NaOH, at OCP) before the addition of formate, light orange lines show the change in spectra after the addition of 1M formate, at open circuit potential (0.9V vs RHE). Then, a cyclic voltammogram is performed between 1 and 0V vs RHE, shown in colors ranging from purple to yellow. Spectra are color-coded to match the potential of the cyclic voltammogram in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea.\u003c/p\u003e\n \u003cp\u003eAt OCP, 4 peaks can be observed at 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1590 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1380 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The peak at 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is assigned to interfacial water, \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e while the remaining ones are assigned to the C-H and C\u0026thinsp;=\u0026thinsp;O (symmetric and asymmetric) stretching of formate in solution (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb). \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e Starting from 0.6V vs RHE, in concomitance with the detection of an anodic current, new peaks appear, caused by the formation of new adsorbate. Two peaks are first observed at 1440\u0026ndash;1460 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1270\u0026ndash;1290 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which are assigned to the O-C\u0026thinsp;=\u0026thinsp;O stretching mode of *OOCH, and the C\u0026thinsp;=\u0026thinsp;O stretching mode of *COOH respectively, following previous studies \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. To confirm the assignment, we used density functional theory (DFT) to estimate the vibrational frequencies of the stretching and bending modes of the proposed intermediates (Fig.\u0026nbsp;3).\u003c/p\u003e\n \u003cp\u003eFor *OOCH, the frequencies are 1305 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1555 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for O-C-O bond symmetric stretching and asymmetric stretching modes, respectively. Other frequencies for *OOCH structure can be found in Supporting Information Table S2. The reason why experiments cannot detect O-C-O symmetric stretching mode frequency (1305 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is because this frequency overlaps with experimental C\u0026thinsp;=\u0026thinsp;O asymmetric stretching of formate in solution (1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003csup\u003e40,41\u003c/sup\u003e For *COOH, the frequencies are 1189 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1686 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in C-OH bond asymmetric stretching and C\u0026thinsp;=\u0026thinsp;O bond asymmetric stretching modes, respectively. Other frequencies for *COOH structure can be found in Supporting Information Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e. The 1686 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e frequency can\u0026rsquo;t be detected from experiments because the frequency would overlap with the experimental strong bulk water (1630cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The calculated frequency results, summarized in Fig.\u0026nbsp;3b, exhibit strong agreement with experimental observed values. While minor deviations are present, they do not exceed 100 cm⁻\u0026sup1;, which is within an acceptable range for theoretical approximations.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003eFinally, at potentials of around 0.2V two very sharp peaks appear at 1700\u0026ndash;1790 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and at 1930\u0026ndash;1970 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. These peaks are located in the CO adsorption region, but are around 100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e lower than those observed in CO-saturated NaOH, which are caused by the characteristic C\u0026thinsp;\u0026equiv;\u0026thinsp;O stretching mode of linear-bonded (2010\u0026ndash;2060 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and bridge-bonded (1830\u0026ndash;1870 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) CO.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e This suggests that the *CO species observed here are partially hydrated (i.e. bound to a neighbouring *OH species, and here denoted as *CO-*OH), as shown in Fig. 3b and discussed in more details later. Additionally, we highlight the site preference for CO adsorption. As shown in Table S3, *CO adsorbing on Pt surface via fcc (hollow, \u0026Delta;E\u0026thinsp;=\u0026thinsp;0.32 eV, referenced to *COOH\u0026thinsp;=\u0026thinsp;0 eV) site is energetically favored compared with bridge (\u0026Delta;E\u0026thinsp;=\u0026thinsp;0.35 eV) or linear (\u0026Delta;E\u0026thinsp;=\u0026thinsp;0.42 eV) configurations. However, experimental observations on Pt(111) surfaces indicate the presence of only bridge and linearly bonded *CO; this discrepancy between theory and experiments is consistent with the literature for *CO binding on Pt(111).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003eTo better track the formation of the reaction intermediates, Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the intensity of each adsorbate as a function of time and applied potential. When scanning in the cathodic direction, first no reaction is detected due to *OH poisoning, \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e then a positive current is observed below 0.6V vs RHE (peak 3), which is accompanied by first the formation of a small amount of O-bonded formate (*OOCH), followed by the rapid formation of C-bonded formate (*COOH). At 0.2V vs RHE, the current becomes negative, indicating the formation of *H, accompanied by the formation of *CO bridge and the consumption of *COOH. In the anodic scan, the current positive of 0.2V vs RHE (peak 1) is accompanied by an increase in *CO linear, a decrease in *CO bridge and the disappearance of *COOH. Finally, at 0.6V vs RHE, in conjunction with peak 2, both the bridge and linear *CO are consumed.\u003c/p\u003e\n \u003cp\u003eThese results suggest that the O-bonded formate is responsible for the direct oxidation pathway and C-bonded formate is responsible for the indirect oxidation pathway. In the cathodic scan, the presence of only the *OOCH intermediate in the rising side of peak 3 suggests that *OOCH is responsible for the direct oxidation pathway. We note that previous DFT simulations have shown that oxidation of bidentate *OOCH is energy demanding and have proposed that monodentate *OOCH formed in pockets of bidentate *OOCH could be the active species.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e This explanation would be in line with our findings, as the coverage of monodentate *OOCH would be too low and its residence time too short to be detected by \u003cem\u003eoperando\u003c/em\u003e SEIRAS.\u003c/p\u003e\n \u003cp\u003eAt potentials less positive than 0.4V vs RHE, the appearance of the *COOH intermediate is accompanied by a fast decrease in the current, which suggests that C-bonded formate is more stable and difficult to oxidize compared to O-bonded formate. Finally, below 0.1V vs RHE, the simultaneous decrease in *COOH signals and increase in bridge *CO-*OH suggests that C-bonded formate is the active species for the formation of *CO. As the potential is increased again in the anodic scan (\u0026gt;\u0026thinsp;0.2V vs RHE), formate oxidation is restored as shown by the current increase. However, this time the surface is covered by a combination of bridge and linear *CO, and formic acid oxidation proceeds through the indirect route, with lower reaction rates. Finally, positive of 0.55V vs RHE, *CO oxidation becomes thermodynamically favourable, and *CO (bridge and linear) is converted to CO\u003csub\u003e2\u003c/sub\u003e. Above 0.7V vs RHE, the surface is rapidly poisoned by *OH adsorption, in combination with a sharp current increase. It can also be observed that in all the measurements *CO bridge is formed first, reaches a plateau and is then slowly substituted by *CO linear. *CO linear only requires one adsorption site, as opposed to the bridged form which requires two. Therefore, we hypothesize that bridged *CO is the favoured species, which is converted to linear *CO at high coverages due to the limited availability of two adjacent sites for the formation of bridge *CO.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eElucidating the *CO formation mechanism\u003c/h3\u003e\n\u003cp\u003eFrom Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, it appears clear that the decrease in reaction rate and the *CO bridge formation is faster in the hydrogen underpotential deposition (H\u003csub\u003eUPD\u003c/sub\u003e) region of platinum, where we expect adsorbed *H is expected to be present on the terraces of Pt. This is in line with previous proposals suggesting that *CO formation can happen through a chemical disproportionation reaction, where the adsorbed formic acid reacts with hydrogen, to form water and *CO (*COOH + *H \u0026agrave; *CO\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO).\u003csup\u003e21\u003c/sup\u003e This suggests that the ideal catalysts should destabilize *H, to avoid the formation of *CO, and that operating at low coverages of *H (positive of the the H\u003csub\u003eUPD\u003c/sub\u003e ) region should avoid the indirect oxidation pathway.\u003c/p\u003e\n\u003cp\u003eHowever, when performing formate oxidation in alkaline conditions at a lower scan rate of 2mVs\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Figure \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e), one can notice the formation of *CO starting at potential as high as 0.5V vs RHE in the cathodic scan. Even though *CO formation still accelerates in the H\u003csub\u003eUPD\u003c/sub\u003e region, this shows that *COOH disproportionation can happen at higher potentials, leading to the gradual poisoning of active sites. To elucidate the mechanism of *CO formation in alkaline pH we monitored the formation of reaction intermediates in CV scans, while gradually limiting the lower potential from the 0V vs RHE usually employed, to higher values. In fact, if disproportionation required the presence of adsorbed *H, one would expect that limiting the lower potential to 0.2V vs RHE would prevent the formation of *CO. The results are shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eAs expected, and as shown before, if the potential is scanned all the way down to 0V vs RHE, *OOCH, *COOH, *CO bridge and *CO linear are formed. However, the formation of *CO bridge (following that of *COOH) is still observable when the lower potential is limited to 0.2V vs RHE and, to a small extent, even by stopping at 0.3V vs RHE. This shows that *CO can form at potentials above 0.3V vs RHE. Finally, if the potential is scanned between 0.4 and 1V vs RHE, only *OOCH is observed while both *COOH and *CO are absent. On top of confirming that *CO can form outside of the H\u003csub\u003eUPD\u003c/sub\u003e region, these results suggest that disproportionation occurs only from the *COOH intermediate, in absence of which *CO is not detected. On the other hand, *OOCH leads to the fast and direct oxidation of formate. Therefore, we propose that the binding mode of formate controls the reaction pathway, with O-bonded and C-bonded formate causing respectively the direct and indirect oxidation of formate.\u003c/p\u003e\n\u003cp\u003eAnother interesting observation is that the wavenumber at which *CO bridge and linear are detected shows a large redshift compared to \u0026ldquo;pure\u0026rdquo; CO, i.e. the case where CO gas is directly bubbled in the electrolyte. To illustrate this, Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea and \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb show the position of the formate-derived *CO intermediates as a function of potential (in blue), compared to the signal obtained from saturated CO gas at the same applied potential (shown in pink).\u003c/p\u003e\n\u003cp\u003eThe values differ by up to 100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and so does the potential dependency of the peak position. Linear and bridge-bound *CO from gaseous CO exhibits a typical potential dependence of \u0026sim;60 cm\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e V\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, known as the Stark tuning slope\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e and caused by: i) electrostatic interactions between the electric field caused by the charged surface and the outer Helmholtz plane, and the dipole moment of the adsorbed *CO,\u003csup\u003e47\u003c/sup\u003e ii) the compression or dissipation of the CO adsorbed layer,\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e and iii) the change in *CO coverage.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e On the contrary, the formate-derived linear *CO exhibits a potential dependence of \u0026sim;120 cm\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e V\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e and bridged *CO shows an even higher value of \u0026sim;150 cm\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e V\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. These values cannot be solely explained by the Stark effect associated with the electric field or changes in coverage.\u003c/p\u003e\n\u003cp\u003eWe have reported this phenomenon before, in the context of methanol oxidation.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e Similarly to what we observed here, methanol-derived *CO showed a red-shift of up to 60 cm\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e compared to pure *CO and displayed a potential dependence of \u0026sim;120 cm\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e V\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. To test whether this shift originates from interactions with the electrolyte, or intermediates from the molecule of interest, we performed the same methanol oxidation experiments in deuterated electrolyte (1M NaOD in D\u003csub\u003e2\u003c/sub\u003eO). As expected, pure *CO showed negligible changes upon deuterium substitution. On the contrary, we reported a measurable shift of \u0026sim;20 cm\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e V\u003csup\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e on methanol-derived *CO in the deuterated electrolyte, suggesting the presence of a partially hydrogenated *CO adsorbate, interacting with hydrogen from a hydroxy group. For the case of formate (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e), the red-shift compared to pure *CO indicates again the existence of *CO in a partially hydrogenated state but no significant shift is observed in the deuterated electrolyte. Therefore, we propose that *COOH degradation takes the form of *COOH \u0026agrave; *CO-*OH, where *CO-*OH indicate the adsorption of *CO and *OH on contiguous Pt atoms, with a weak C-O bonding still existing between the C atom in *CO and the O atom in *OH, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee. This mechanism would explain why *CO formation can happen above H\u003csub\u003eUPD\u003c/sub\u003e; the lower vibrational frequency of formate-derived *CO, compare to pure *CO; and why the first one does not change in deuterated electrolyte. Below 0.2V vs RHE, the *CO formation becomes faster, but all the observations remain valid, indicating that adsorbed *H facilitate *CO formation but is not directly involved in the reaction. Interestingly, the red-shift of *CO vibrational frequencies was not reported in acidic electrolyte,\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e for which *CO formation above 0.2V vs RHE was also not detected. This suggests that the mechanism of *CO formation is different in acidic (*COOH + *H \u0026agrave; *CO\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO) and alkaline (*COOH \u0026agrave; *CO-*OH) conditions, with only the first one requiring adsorbed protons. This observation could additionally explain the lower activity reported in alkaline electrolyte,\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e which on top of increasing competition with *OH, suffer from a slow but continuous formation of *CO species above H\u003csub\u003eUPD\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eAnother indication of the proposed *COOH \u0026loz; *CO-*OH mechanism in alkaline pH is shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec,d. This shows the electrochemical and spectroscopical results of a step test where we applied a potential of 0.4V vs RHE for 2 minutes, and then left the system equilibrate to OCP for 10 seconds. At the start of the experiment the system is at OCP and no adsorbates are observed. When a potential of 0.4V vs RHE is applied, as expected, *COOH is formed and rapidly converted to *CO bridge. More surprisingly, if the potential control is stopped, the electrode reaches an open circuit potential of around 0.25V, where *CO bridge is converted back to *COOH. This suggests that *COOH disproportionation is reversible and that the *OH co-produced in *CO formation remains weakly bound to *CO.\u003c/p\u003e\n\u003cp\u003eTo confirm the existence of the *CO-*OH species and confirm our assignment of the 1700\u0026ndash;1790 and 1930\u0026ndash;1970 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to bridge and linear *CO-*OH, we also simulated their vibrational frequencies using DFT, and the results are summarized in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eWe started from *COOH intermediate, which adsorbs monodentately via the C atom, with a C-OH bond length of 1.37 \u0026Aring;. By elongating the C-OH bond, bidentate adsorption via both C and O becomes possible but this significantly increases the energy. Thus, we systematically fixed the C-OH bond length at distance between 1.46 and 2.50 \u0026Aring;, extending from the stable *COOH structure (1.37 \u0026Aring;). After relaxation, *CO-*OH (bridge) structure was meta-stable from 1.80 to 2.50 \u0026Aring;, while *CO-*OH (linear) structure was meta-stable from 1.58 to 2.50 \u0026Aring;. CO-*OH intermediate is always less stable than *COOH. This indicates that the conversion of *CO-*OH to *COOH is spontaneous in absence of an applied potential, as also observed experimentally (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec-e). For comparison, *CO (bridge) and *CO (linear) were modeled by setting the C-OH bond length to a sufficiently large value (indicated as \u0026infin;\u0026thinsp;in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e), effectively removing the OH group and eliminating its interaction with CO. Frequencies of other vibrational modes and corresponding energies are provided in Supporting Information Tables S3-S5.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ea summarizes the frequencies of C\u0026thinsp;=\u0026thinsp;O stretching of the simulated species, as a function of the C-OH bond length, starting from *COOH (in yellow), to *CO-*OH bridge to *CO bridge (in orange), and to *CO-*OH linear to *CO linear (in red). Shaded areas indicate experimental frequency ranges for *CO-*OH (bridge and linear) and *CO (bridge and linear). The color coding for the *COOH and *CO-*OH intermediates was selected to match that of other figures in the manuscript. The vibrational frequencies of *CO-*OH (bridge) are closer to those of *COOH than those of *CO-*OH (linear), suggesting greater structural similarity. However, the DFT-calculated energy differences (\u0026Delta;E, relative to *COOH\u0026thinsp;=\u0026thinsp;0 eV) indicate that the bridge configuration is less stable than the linear one. These \u0026Delta;E values are plotted in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eb. For *CO-*OH (bridge), DFT frequencies align with experimental data when the C-OH bond length is 1.80\u0026ndash;2.40 \u0026Aring;, corresponding to \u0026Delta;E of 0.65\u0026ndash;0.90 eV. For *CO-*OH (linear), agreement occurs at C-OH bond lengths of 2.20\u0026ndash;2.30 \u0026Aring; at an energy cost of \u0026Delta;E between 0.59\u0026ndash;0.63 eV. At these bond lengths, the vibrational frequencies of bridged and linear *CO-*OH species is \u0026asymp;\u0026thinsp;100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e lower than the prediction of *CO, matching the experimental difference between \u0026ldquo;pure\u0026rdquo; *CO and formate-derived *CO. However, energetically, neither intermediate appears stable enough to be a detectable species from *COOH to *CO. Their experimental observability may arise from solvation effects not included in our calculations, where water molecules could help stabilize these intermediates.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003ePrevious studies of formic acid oxidation on Pt have concluded that: i) the reaction can proceed through a direct and indirect pathway, ii) indirect oxidation leads to the formation of *CO, which poisons the surface, and iii) at high potentials (\u0026gt;\u0026thinsp;0.6 V vs RHE) activity drops as a results of *OH adsorption. However, up until now the active species prior to direct oxidation to CO\u003csub\u003e2\u003c/sub\u003e has been the subject of debate, as summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, with reports proposing monodentate *OOCH, bidentate *OOCH, *COOH or formic acid as the active species. Regarding the indirect activation pathway, most works propose that *CO is formed through a disproportionation reaction, where *COOH reacts with protons to form *CO and water. However, other mechanisms have also been reported, for example involving *OOCH as the active species.\u003c/p\u003e\u003cp\u003eThe majority of the proposed mechanisms are based on DFT-simulations, and since multiple pathways are theoretically possible, here we have used \u003cem\u003eoperando\u003c/em\u003e surface enhanced IR-spectroscopy to gain further insight, by monitoring the formation of reaction intermediates experimentally. Our results show that, when cycling the potential above 0.4V vs RHE, a stable oxidation current can be achieved, and no hysteresis is observed between the cathodic and anodic scan. Under these conditions, bidentate *OOCH is the only detectable adsorbate, suggesting that oxygen-bound formate drives direct oxidation. On the contrary, our results show that *COOH (which dominates below 0.4V vs RHE) always leads to the formation of *CO, and while *CO formation is accelerated by the presence of adsorbed protons, *CO is formed at any potential as long as formate is adsorbed through the carbon atom. We also found that formate-derived *CO in alkaline media shows a\u0026thinsp;\u0026asymp;\u0026thinsp;100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e red-shift, compared to \u0026ldquo;pure\u0026rdquo; *CO (derived from bubbling CO gas). This shift, which was not reported in acidic conditions, is attributed to an alkaline-specific mechanism, where *COOH disproportionation leads to the formation of adsorbed *CO, weakly bound to an adjacent adsorbed *OH (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e). A lack of change in the *CO peak position in deuterated electrolyte confirmed that the red-shift of*CO is not due to interaction with the electrolyte. The proposed mechanism was also confirmed by DFT simulations of the *CO-*OH vibrational frequency and by a chronoamperometry experiment showing that once *CO-*OH is formed on the surface, it is converted back to *COOH if the system is left to relax to OCP.\u003c/p\u003e\u003cp\u003eThe mechanism summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e allows us to deduce design criteria for active formate oxidation catalysts. Since the *OOCH and *COOH intermediates catalyze respectively the direct and indirect oxidation of formate, our results indicate that suppressing \u003cem\u003eCO formation requires stabilizing oxygen-bound formate adsorption.\u003c/em\u003e Since the binding energies of *COOH and *OOCH are not related, catalysts with strong *OOCH and weak *COOH binding energies can be easily found.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e However, the binding energy of *OOCH and *OH do scale linearly, so these catalysts (such as Ag) are oxidized before they can adsorb formate. \u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e One potential approach to overcome this limitation is to reduce \u003cem\u003eOH\u003c/em\u003e coverage by carefully controlling electrolyte conditions. However, our findings also reveal that in alkaline media *COOH disproportionation can happen in absence of adsorbed protons, constraining the potential window for stable direct oxidation.\u003c/p\u003e\u003cp\u003eBeyond formic acid oxidation, these findings have broader implications for biomass oxidation, where CO poisoning limits catalytic efficiency. Firstly, the intermediates identified here build a valuable library of FTIR vibrational frequencies that will aid identification of species in more complex reactions. Secondly, we have shown that CO vibrational frequencies provide direct insight into coordination environments and formation mechanisms \u0026mdash; for example, \u0026ldquo;hydrated\u0026rdquo; or \u0026ldquo;hydrogenated\u0026rdquo; CO exhibits characteristic red-shifts. Aditionally, we have shown how isotopic labeling experiments can futher add the elucidaiton of both the cooridation environment and the formation mechanism of *CO. Our findings are directly relevant for reactions like glycerol oxidation, which can feature formic acid as a reaction intermediate, to supress its further oxidation to *CO. Additionally, some findings are more broadly applicable to *CO formation, even if not from formic acids. For example, our work highlights the crucial role of adsorption geometry in determining reaction pathways: oxygen-bound intermediates (*OOCH) favor direct oxidation, while carbon-bound intermediates (*COOH) lead to *CO formation. The role of the adsorption configuration has been already reported for various reactions, including methanol and glycerol oxidation on Pt and Au, where adsorption configuration governs selectivity and C\u0026ndash;C cleavage pathways. \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan additionalcitationids=\"CR50\" citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e however, the role of the adsorption configuration on *CO formation for such reactions has not yet been investigated. Finally, we have also highlighted how *CO formation differs in alkaline and acidic pH, which could be crucial for future catalyst design strategies.\u003c/p\u003e\u003cp\u003eOverall, this study closes a critical knowledge gap by experimentally elucidating the intermediates that govern *CO formation on Pt in alkaline media, advancing mechanistic understanding beyond formic acid oxidation to the broader field of biomass conversion.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eExperimental\u003c/h3\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003eElectrocatalysts preparation\u003c/h2\u003e\u003cp\u003e\u003cem\u003eOperando\u003c/em\u003e SEIRAS measurements were performed using a Si hemisphere (radius 22mm from Pier optics), on which an enhancement layer of Pd was deposited, followed by a thin film of Pt. The preparation method is described elsewhere.\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e In brief, the Si surface was prepared by dropcasting 1mL of 40% NH\u003csub\u003e4\u003c/sub\u003eF solution for a minute. Subsequently, palladium was depositing by drop-casting a solution of 1% HF\u0026ndash;1 mM PdCl\u003csub\u003e2\u003c/sub\u003e for 5 min at room temperature. Finally, platinum was deposited by immersing the Si hemisphere in a Pt plating solution for 5 minutes, at 50˚C. The plating solution was obtained by mixing 30mL of LECTROLESS Pt 100 basic solution (30 mL, Electroplating Engineering of Japan Ltd.), 8mL of 28% NH\u003csub\u003e3\u003c/sub\u003e solution, and ultrapure water.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eSEIRAS Measurement\u003c/h2\u003e\u003cp\u003eThe Si prism was mounted on an in-house developed electrochemical cell, featuring a Pt rod counter electrode and Hg/HgO reference. All the measurements were performed in either 0.1M NaOH or 1M formate, 0.1M NaOH. The SEIRAS spectra were obtained with an FTIR Nicolet iS50 (Thermo Fisher Scientific) spectrometer equipped with a Mercury-Cadmium-Telluride (MCT) detector. The optical path was fully replaced with N\u003csub\u003e2\u003c/sub\u003e gas. The measurements were performed with a 4 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e resolution in the 500\u0026ndash;4000 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e spectral range; 32 scans were averaged, giving a spectral collection every 10s. The SEIRAS spectra were recorded using a custom-made single reflection ATR optics system at an incident angle of 67\u0026deg;. For the isotopically labelled experiments, 1M NaOD (99.5% from Merck) in D\u003csub\u003e2\u003c/sub\u003eO (99.9% from Merck) was used.\u003c/p\u003e\u003cp\u003eBefore each experiment, nitrogen was bubbled in 1M NaOH or 1M NaOD for 15 minutes to remove oxygen. Then the Pt surface was cleaned by performing 10 cyclic voltammograms between 0 and 1V vs RHE, at 20mVs\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The system was then left to equilibrate at open circuit potential and a spectrum was recorded as reference. 1M Formate was subsequently added to the electrolyte and the experiment started. All the resulting spectra are shown as absorbance vs potential (in V vs RHE). Absorbance units are defined as log(\u003cem\u003eI\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e) where \u003cem\u003eI\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e and \u003cem\u003eI\u003c/em\u003e represent the reference spectra and the spectra of interest respectively.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eComputational Details\u003c/h3\u003e\n\u003cp\u003eWe use density functional theory (DFT) to calculate energies and frequencies of possible formic acid oxidation intermediates. In our modelling, the metal slab is 4 \u0026times; 4 \u0026times; 4 with face-centered cubic (FCC) (111) facet with adsorbing intermediates. The 2 bottom layers are fixed to emulate bulk metal. To avoid interlayer interactions, the distance between metal slabs (periodic images) is set to 17 \u0026Aring;. Atomic simulation environment (ASE) program\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e was employed for the atomic structures. GPAW program,\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e,\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e was employed for DFT calculations with revised Perdew\u0026thinsp;\u0026minus;\u0026thinsp;Burke\u0026thinsp;\u0026minus;\u0026thinsp;Ernzerhof exchange-correlation functional,\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e 400 eV energy cutoff for plane-wave basis sets and 4 \u0026times; 4 \u0026times; 1 Monkhorst-Pack k-point sampling. The maximum force on each atom is set to 0.03 eV/\u0026Aring; for relaxed atomic structures and frequencies.\u003c/p\u003e\u003cp\u003eThe DFT calculations and python scripts are accessible online at: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://erda.ku.dk/archives/d57d50eea957e20eb94a73a5d770bb51/published-archive.html\u003c/span\u003e\u003cspan address=\"https://erda.ku.dk/archives/d57d50eea957e20eb94a73a5d770bb51/published-archive.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eConsent to Publish\u003c/p\u003e\n\u003cp\u003edeclaration: not applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003edeclaration: not applicable\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eS.F. acknowledges the RSC collaboration grant C23-0818957162, and the Engineering and Physical Sciences Research Council (EPSRC) program grant EP/W031019. I.E.L.S. acknowledges the Royce Institute (EP/P02520X/1). Y.K. acknowledges the financial support by the Japan Society for the Promotion of Science (JPSP) KAKENHI Grant Number 25K01880 and by the Japan Science and Technology Agency (JST) under the Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE) program (grant no JPMJAP2422). Z.M., H.H.K., and J.R. acknowledge financial support from the Independent Research Fund Denmark grant no. 1127-00372B and the Danish National Research Foundation, Center for High Entropy Alloy Catalysis (CHEAC) DNRF149.\u003c/p\u003e\n\u003cp\u003eAuthor Contribution\u003c/p\u003e\n\u003cp\u003eSV did all the electrochemical experiments and the FTIR in Osaka and wrote the paperZM did all the theoretical modelling workHK and JR supervised all the theoretical model workIS supervised the electrochemistry work and corrected and discussed the manuscriptMT supervised all the work and contributed to the writing of the paper and discussionsYK contributed to the discussions and supervision of the FTIR measurements and their interpretation\u003c/p\u003e\n\u003cp\u003eData Availability\u003c/p\u003e\n\u003cp\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLuo, H. \u003cem\u003eet al.\u003c/em\u003e Progress and Perspectives in Photo- and Electrochemical-Oxidation of Biomass for Sustainable Chemicals and Hydrogen Production. \u003cem\u003eAdv Energy Mater\u003c/em\u003e \u003cstrong\u003e11\u003c/strong\u003e, (2021).\u003c/li\u003e\n\u003cli\u003eBrouzgou, A. \u0026amp; Tsiakaras, P. Electrocatalysts for Glucose Electrooxidation Reaction: A Review. \u003cem\u003eTop Catal\u003c/em\u003e \u003cstrong\u003e58\u003c/strong\u003e, 1311\u0026ndash;1327 (2015).\u003c/li\u003e\n\u003cli\u003eKumar, M., Meena, B., Yu, A., Sun, C. \u0026amp; Challapalli, S. Advancements in catalysts for glycerol oxidation via photo-/electrocatalysis: a comprehensive review of recent developments. \u003cem\u003eGreen Chem.\u003c/em\u003e \u003cstrong\u003e25\u003c/strong\u003e, 8411\u0026ndash;8443 (2023).\u003c/li\u003e\n\u003cli\u003eYang, Y. \u0026amp; Mu, T. Electrochemical oxidation of biomass derived 5-hydroxymethylfurfural (HMF): pathway, mechanism, catalysts and coupling reactions. \u003cem\u003eGreen Chemistry\u003c/em\u003e \u003cstrong\u003e23\u003c/strong\u003e 4228\u0026ndash;4254 (2021).\u003c/li\u003e\n\u003cli\u003eSiwal, S. S., Thakur, S., Zhang, Q. B. \u0026amp; Thakur, V. K. Electrocatalysts for electrooxidation of direct alcohol fuel cell: chemistry and applications. \u003cem\u003eMaterials Today Chemistry\u003c/em\u003e \u003cstrong\u003e14, \u003c/strong\u003e100182\u003cstrong\u003e (2019).\u003c/strong\u003e\u003c/li\u003e\n\u003cli\u003eMello, G. A. B., Cheuquep\u0026aacute;n, W., Briega-Martos, V. \u0026amp; Feliu, J. M. Glucose electro-oxidation on Pt(100) in phosphate buffer solution (pH 7): A mechanistic study. \u003cem\u003eElectrochim Acta\u003c/em\u003e \u003cstrong\u003e354\u003c/strong\u003e, (2020).\u003c/li\u003e\n\u003cli\u003eXue, J., Wu, X. \u0026amp; Feng, L. Pt/Mn3O4cubes with high anti-poisoning ability for C1 and C2 alcohol fuel oxidation. \u003cem\u003eChemical Communications\u003c/em\u003e \u003cstrong\u003e58\u003c/strong\u003e, 2371\u0026ndash;2374 (2022).\u003c/li\u003e\n\u003cli\u003eGarcia, A. C. \u003cem\u003eet al.\u003c/em\u003e Strong Impact of Platinum Surface Structure on Primary and Secondary Alcohol Oxidation during Electro-Oxidation of Glycerol. \u003cem\u003eACS Catal\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 4491\u0026ndash;4500 (2016).\u003c/li\u003e\n\u003cli\u003eFaverge, T. \u003cem\u003eet al.\u003c/em\u003e In Situ Investigation of d-Glucose Oxidation into Value-Added Products on Au, Pt, and Pd under Alkaline Conditions: A Comparative Study. \u003cem\u003eACS Catal\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 2657\u0026ndash;2669 (2023).\u003c/li\u003e\n\u003cli\u003eNeha, N. \u003cem\u003eet al.\u003c/em\u003e Revisited Mechanisms for Glucose Electrooxidation at Platinum and Gold Nanoparticles. \u003cem\u003eElectrocatalysis\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 121\u0026ndash;130 (2023).\u003c/li\u003e\n\u003cli\u003eYe, H. \u003cem\u003eet al.\u003c/em\u003e Progress and Challenges in Electrochemical Glycerol Oxidation: The Importance of Benchmark Methods and Protocols. \u003cem\u003eChemCatChem, \u003cstrong\u003e17\u003c/strong\u003e, e00152\u003c/em\u003e (2025) \u003c/li\u003e\n\u003cli\u003eShyamYadav, R., Gebru, M. G., Teller, H., Schechter, A. \u0026amp; Kornweitz, H. The origins of formic acid electrooxidation on selected surfaces of Pt, Pd, and their alloys with Sn. \u003cem\u003eJ Mater Chem A Mater\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 3311\u0026ndash;3322 (2024).\u003c/li\u003e\n\u003cli\u003eHerrero, E. \u0026amp; Feliu, J. M. Understanding formic acid oxidation mechanism on platinum single crystal electrodes. \u003cem\u003eCurrent Opinion in Electrochemistry,\u003c/em\u003e \u003cstrong\u003e9,\u003c/strong\u003e 145\u0026ndash;150 (2018).\u003c/li\u003e\n\u003cli\u003eYan, Y. G. \u003cem\u003eet al.\u003c/em\u003e Ubiquitous strategy for probing ATR surface-enhanced infrared absorption At platinum group metal-electrolyte interfaces. \u003cem\u003eJournal of Physical Chemistry B\u003c/em\u003e \u003cstrong\u003e109\u003c/strong\u003e, 7900\u0026ndash;7906 (2005).\u003c/li\u003e\n\u003cli\u003eBlizanac, B. B., Arenz, M., Ross, P. N. \u0026amp; Markovic, N. M. Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode. \u003cem\u003eJ Am Chem Soc\u003c/em\u003e \u003cstrong\u003e126\u003c/strong\u003e, 10130\u0026ndash;10141 (2004).\u003c/li\u003e\n\u003cli\u003eYang, T. \u003cem\u003eet al.\u003c/em\u003e Formic acid electro-oxidation: Mechanism and electrocatalysts design. \u003cem\u003eNano Res\u003c/em\u003e \u003cstrong\u003e16\u003c/strong\u003e, 3607\u0026ndash;3621 (2023).\u003c/li\u003e\n\u003cli\u003eFang, Z. \u0026amp; Chen, W. Recent advances in formic acid electro-oxidation: From the fundamental mechanism to electrocatalysts. \u003cem\u003eNanoscale Adv\u003c/em\u003e \u003cstrong\u003e3\u003c/strong\u003e, 94\u0026ndash;105 (2021).\u003c/li\u003e\n\u003cli\u003eJoo, J., Uchida, T., Cuesta, A., Koper, M. T. M. \u0026amp; Osawa, M. The effect of pH on the electrocatalytic oxidation of formic acid/formate on platinum: A mechanistic study by surface-enhanced infrared spectroscopy coupled with cyclic voltammetry. \u003cem\u003eElectrochim Acta\u003c/em\u003e \u003cstrong\u003e129\u003c/strong\u003e, 127\u0026ndash;136 (2014).\u003c/li\u003e\n\u003cli\u003eJohn, J., Wang, H., Rus, E. D. \u0026amp; Abrun, D. Mechanistic Studies of Formate Oxidation on Platinum in Alkaline Medium., \u003cstrong\u003e116, \u003c/strong\u003e5810-5820 (2012).\u003c/li\u003e\n\u003cli\u003eSamjeske, G. Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at Platinum in Acidic Solution by Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy,\u003cstrong\u003e110,\u003c/strong\u003e 16559\u0026ndash;16566 (2006).\u003c/li\u003e\n\u003cli\u003eBagger, A. \u003cem\u003eet al.\u003c/em\u003e Correlations between experiments and simulations for formic acid oxidation. \u003cem\u003eChem Sci\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 13409\u0026ndash;13417 (2022).\u003c/li\u003e\n\u003cli\u003eMcPherson, I. J., Ash, P. A., Jacobs, R. M. J. \u0026amp; Vincent, K. A. Formate adsorption on Pt nanoparticles during formic acid electro-oxidation: Insights from: In situ infrared spectroscopy. \u003cem\u003eChemical Communications\u003c/em\u003e \u003cstrong\u003e52\u003c/strong\u003e, 12665\u0026ndash;12668 (2016).\u003c/li\u003e\n\u003cli\u003eCapon, A. \u0026amp; Parsons, R. The Oxidation of Fomric Acid at Noble Metal Electrodes part III: Intermediates and Mechanism on Platinum Electrodes, \u003cem\u003eJ Electroanal Chem Interfacial Electrochem\u003c/em\u003e, \u003cstrong\u003e45\u003c/strong\u003e, 205\u0026ndash;231 (1973).\u003c/li\u003e\n\u003cli\u003eBeden, B., Bewick, A. \u0026amp; Lamy, C. \u003cem\u003eA Comparative Study of Formic Acid Adsorption on a Platinum Electrode by both Electrochemical and EMIRS Technique,\u003c/em\u003e \u003cem\u003eJ. Electroanal. Chem\u003c/em\u003e vol., \u003cstrong\u003e150\u003c/strong\u003e, 505-511 (1983).\u003c/li\u003e\n\u003cli\u003eSamjesk\u0026eacute;, G., Miki, A., Ye, S. \u0026amp; Osawa, M. Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at Platinum in Acidic Solution by Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy. \u003cem\u003eJ Phys Chem B\u003c/em\u003e \u003cstrong\u003e110\u003c/strong\u003e, 16559\u0026ndash;16566 (2006).\u003c/li\u003e\n\u003cli\u003eOsawa, M. \u003cem\u003eet al.\u003c/em\u003e The role of bridge-bonded adsorbed formate in the electrocatalytic oxidation of formic acid on platinum. \u003cem\u003eAngewandte Chemie - International Edition\u003c/em\u003e \u003cstrong\u003e50\u003c/strong\u003e, 1159\u0026ndash;1163 (2011).\u003c/li\u003e\n\u003cli\u003eCuesta, A., Cabello, G., Guti\u0026eacute;rrez, C. \u0026amp; Osawa, M. Adsorbed formate: The key intermediate in the oxidation of formic acid on platinum electrodes. \u003cem\u003ePhysical Chemistry Chemical Physics\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 20091\u0026ndash;20095 (2011).\u003c/li\u003e\n\u003cli\u003eCuesta, A., Cabello, G., Osawa, M. \u0026amp; Guti\u0026eacute;rrez, C. Mechanism of the electrocatalytic oxidation of formic acid on metals. \u003cem\u003eACS Catal\u003c/em\u003e \u003cstrong\u003e2\u003c/strong\u003e, 728\u0026ndash;738 (2012).\u003c/li\u003e\n\u003cli\u003eXu, J. \u003cem\u003eet al.\u003c/em\u003e On the mechanism of the direct pathway for formic acid oxidation at a Pt(111) electrode. \u003cem\u003ePhysical Chemistry Chemical Physics\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 4367\u0026ndash;4376 (2013).\u003c/li\u003e\n\u003cli\u003ePerales-Rond\u0026oacute;n, J. V., Herrero, E. \u0026amp; Feliu, J. M. Effects of the anion adsorption and pH on the formic acid oxidation reaction on Pt(111) electrodes. \u003cem\u003eElectrochim Acta\u003c/em\u003e \u003cstrong\u003e140\u003c/strong\u003e, 511\u0026ndash;517 (2014).\u003c/li\u003e\n\u003cli\u003eWang, H. F. \u0026amp; Liu, Z. P. Formic acid oxidation at Pt/H2O interface from periodic DFT calculations integrated with a continuum solvation model. \u003cem\u003eJournal of Physical Chemistry C\u003c/em\u003e \u003cstrong\u003e113\u003c/strong\u003e, 17502\u0026ndash;17508 (2009).\u003c/li\u003e\n\u003cli\u003eFerre-Vilaplana, A., Perales-Rond\u0026oacute;n, J. V., Buso-Rogero, C., Feliu, J. M. \u0026amp; Herrero, E. Formic acid oxidation on platinum electrodes: A detailed mechanism supported by experiments and calculations on well-defined surfaces. \u003cem\u003eJ Mater Chem A Mater\u003c/em\u003e \u003cstrong\u003e5\u003c/strong\u003e, 21773\u0026ndash;21784 (2017).\u003c/li\u003e\n\u003cli\u003eGao, W., Keith, J. A., Anton, J. \u0026amp; Jacob, T. Theoretical elucidation of the competitive electro-oxidation mechanisms of formic acid on Pt(111). \u003cem\u003eJ Am Chem Soc\u003c/em\u003e \u003cstrong\u003e132\u003c/strong\u003e, 18377\u0026ndash;18385 (2010).\u003c/li\u003e\n\u003cli\u003eWang, H. F. \u0026amp; Liu, Z. P. Formic acid oxidation at Pt/H2O interface from periodic DFT calculations integrated with a continuum solvation model. \u003cem\u003eJournal of Physical Chemistry C\u003c/em\u003e \u003cstrong\u003e113\u003c/strong\u003e, 17502\u0026ndash;17508 (2009).\u003c/li\u003e\n\u003cli\u003eMacia, M. D., Herrero, E., Feliu, J. M. \u0026amp; Aldaz, A. \u003cem\u003eFormic Acid Self-Poisoning on Bismuth-Modified Pt(755) and Pt(775),\u003c/em\u003e\u003cstrong\u003e1, \u003c/strong\u003e87-89 (1999).\u003c/li\u003e\n\u003cli\u003eKita, H. \u0026amp; Lei, H.-W. \u003cem\u003eOxidation of Formic Acid in Acid Solution on Pt Single-Crystal Electrodes\u003c/em\u003e. \u003cem\u003eJournal of Electroanalytical Chemistry,\u003c/em\u003e\u003cstrong\u003e388\u003c/strong\u003e, 167-177 (1995).\u003c/li\u003e\n\u003cli\u003eMaci\u0026aacute;, M. D., Herrero, E. \u0026amp; Feliu, J. M. Formic acid self-poisoning on adatom-modified stepped electrodes. \u003cem\u003eElectrochim Acta, \u003c/em\u003e\u003cstrong\u003e47\u003cem\u003e,\u003c/em\u003e\u003c/strong\u003e 3653\u0026ndash;3661 (2002).\u003c/li\u003e\n\u003cli\u003eGrozovski, V., Climent, V., Herrero, E. \u0026amp; Feliu, J. M. Intrinsic activity and poisoning rate for HCOOH oxidation on platinum stepped surfaces. \u003cem\u003ePhysical Chemistry Chemical Physics\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 8822\u0026ndash;8831 (2010).\u003c/li\u003e\n\u003cli\u003eGrozovski, V., Climent, V., Herrero, E. \u0026amp; Feliu, J. M. Intrinsic activity and poisoning rate for HCOOH oxidation at Pt(100) and vicinal surfaces containing monoatomic (111) steps. \u003cem\u003eChemPhysChem\u003c/em\u003e \u003cstrong\u003e10\u003c/strong\u003e, 1922\u0026ndash;1926 (2009).\u003c/li\u003e\n\u003cli\u003eBeckingham, B. S., Lynd, N. A. \u0026amp; Miller, D. J. Monitoring multicomponent transport using in situ ATR FTIR spectroscopy. \u003cem\u003eJ Memb Sci\u003c/em\u003e \u003cstrong\u003e550\u003c/strong\u003e, 348\u0026ndash;356 (2018).\u003c/li\u003e\n\u003cli\u003eLucks, C. \u003cem\u003eet al.\u003c/em\u003e Formic acid interaction with the uranyl(vi) ion: Structural and photochemical characterization. \u003cem\u003eDalton Transactions\u003c/em\u003e \u003cstrong\u003e42\u003c/strong\u003e, 13584\u0026ndash;13589 (2013).\u003c/li\u003e\n\u003cli\u003eScaranto, J. \u0026amp; Mavrikakis, M. HCOOH decomposition on Pt(111): A DFT study. \u003cem\u003eSurf Sci\u003c/em\u003e \u003cstrong\u003e648\u003c/strong\u003e, 201\u0026ndash;211 (2016).\u003c/li\u003e\n\u003cli\u003eFeibelman, P. J. \u003cem\u003eet al.\u003c/em\u003e The CO/Pt(111) Puzzle a. \u003cem\u003eJournal of Physical Chemistry B\u003c/em\u003e \u003cstrong\u003e105\u003c/strong\u003e, 4018\u0026ndash;4025 (2001).\u003c/li\u003e\n\u003cli\u003eGrinberg, I., Yourdshahyan, Y. \u0026amp; Rappe, A. M. CO on Pt(111) puzzle: A possible solution. \u003cem\u003eJ Chem Phys\u003c/em\u003e \u003cstrong\u003e117\u003c/strong\u003e, 2264\u0026ndash;2270 (2002).\u003c/li\u003e\n\u003cli\u003eUddin, J. \u0026amp; Anderson, A. B. Trends with coverage and pH in Stark tuning rates for CO on Pt(111) electrodes. \u003cem\u003eElectrochim Acta\u003c/em\u003e \u003cstrong\u003e108\u003c/strong\u003e, 398\u0026ndash;403 (2013).\u003c/li\u003e\n\u003cli\u003eLambert, D. K. Vibrational Stark Effect of Adsorbates at Electrochemical Interfaces. \u003cem\u003eElectrochim Acta\u003c/em\u003e \u003cstrong\u003e41\u003c/strong\u003e, 623\u0026ndash;630 (1996).\u003c/li\u003e\n\u003cli\u003eStamenkovic, V., Chou, K. C., Somorjai, G. A., Ross, P. N. \u0026amp; Markovic, N. M. Vibrational properties of CO at the Pt(111)-solution interface: The anomalous stark-tuning slope. \u003cem\u003eJournal of Physical Chemistry B\u003c/em\u003e \u003cstrong\u003e109\u003c/strong\u003e, 678\u0026ndash;680 (2005).\u003c/li\u003e\n\u003cli\u003eKatayama, Y. \u003cem\u003eet al.\u003c/em\u003e Direct Observation of Surface-Bound Intermediates during Methanol Oxidation on Platinum under Alkaline Conditions. \u003cem\u003eJournal of Physical Chemistry C\u003c/em\u003e \u003cstrong\u003e125\u003c/strong\u003e, 26321\u0026ndash;26331 (2021).\u003c/li\u003e\n\u003cli\u003eLi, Y. \u003cem\u003eet al.\u003c/em\u003e PtAu alloying-modulated hydroxyl and substrate adsorption for glycerol electrooxidation to C3 products. \u003cem\u003eEnergy Environ Sci\u003c/em\u003e \u003cstrong\u003e17\u003c/strong\u003e, 4205\u0026ndash;4215 (2024).\u003c/li\u003e\n\u003cli\u003eFern\u0026aacute;ndez, P. S., Martins, C. A., Martins, M. E. \u0026amp; Camara, G. A. Electrooxidation of glycerol on platinum nanoparticles: Deciphering how the position of each carbon affects the oxidation pathways. \u003cem\u003eElectrochim Acta\u003c/em\u003e \u003cstrong\u003e112\u003c/strong\u003e, 686\u0026ndash;691 (2013).\u003c/li\u003e\n\u003cli\u003eMeng, Z., Tran, D., Hjelm, J., Kristoffersen, H. H. \u0026amp; Rossmeisl, J. Insight into Selectivity Differences of Glycerol Electro-Oxidation on Pt(111) and Ag(111). \u003cem\u003eACS Catal\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 2455\u0026ndash;2462 (2024).\u003c/li\u003e\n\u003cli\u003eKatayama, Y. \u003cem\u003eet al.\u003c/em\u003e Surface (Electro)chemistry of CO2 on Pt Surface: An in Situ Surface-Enhanced Infrared Absorption Spectroscopy Study. \u003cem\u003eJournal of Physical Chemistry C\u003c/em\u003e \u003cstrong\u003e122\u003c/strong\u003e, 12341\u0026ndash;12349 (2018).\u003c/li\u003e\n\u003cli\u003eHjorth Larsen, A. \u003cem\u003eet al.\u003c/em\u003e The atomic simulation environment - A Python library for working with atoms. \u003cem\u003eJournal of Physics Condensed Matter\u003c/em\u003e \u003cstrong\u003e29\u003c/strong\u003e, 273002 (2017).\u003c/li\u003e\n\u003cli\u003eEnkovaara, J. \u003cem\u003eet al.\u003c/em\u003e Electronic structure calculations with GPAW: A real-space implementation of the projector augmented-wave method. \u003cem\u003eJournal of Physics Condensed Matter\u003c/em\u003e \u003cstrong\u003e22\u003c/strong\u003e, 253202 (2010).\u003c/li\u003e\n\u003cli\u003eMortensen, J. J., Hansen, L. B. \u0026amp; Jacobsen, K. W. Real-space grid implementation of the projector augmented wave method. \u003cem\u003ePhys Rev B Condens Matter Mater Phys\u003c/em\u003e \u003cstrong\u003e71\u003c/strong\u003e, (2005).\u003c/li\u003e\n\u003cli\u003eHammer, B., Hansen, L. B. \u0026amp; No, J. K. Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals. \u003cem\u003ePhys Rev B\u003c/em\u003e \u003cstrong\u003e59\u003c/strong\u003e, 7413\u0026ndash;7421.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7263225/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7263225/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCatalyst deactivation due to *CO poisoning is a persistent challenge in the electrochemical oxidation of biomass-derived molecules such as glycerol and glucose. On platinum catalysts, *CO forms readily as a reaction intermediate, blocking active sites and requiring high overpotentials for removal—often leading to undesired overoxidation of valuable products. Understanding the fundamental origins of *CO formation is thus critical for designing more selective and stable catalysts.\u003c/p\u003e\n\u003cp\u003eSince biomass oxidation can be extremely complex and involve a multitude of adsorbates and products, here we use a simplified model system, formate oxidation, to investigate *CO formation on Pt in alkaline pH . Starting from \u003cem\u003eoperando\u003c/em\u003e surface-enhanced infrared spectroscopy, we show that the adsorption configuration of formate determines if the surface will be poisoned by *CO. Oxygen-bound formate (*OOCH) undergoes direct and stable oxidation to CO₂, while carbon-bound formate (*COOH) disproportionates to form *CO–*OH, initiating poisoning. These insights offer a mechanistic foundation for designing Pt-based catalysts that resist *CO formation by selectively stabilizing *OOCH over *COOH intermediates, with broader implications for improving biomass electrooxidation performance.\u003c/p\u003e","manuscriptTitle":"What formate electro-oxidation can teach us about CO poisoning on Pt during biomass oxidation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-03 10:17:19","doi":"10.21203/rs.3.rs-7263225/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e96a3c74-5eb7-4f53-bd38-10a5e3e71e2b","owner":[],"postedDate":"September 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-01T07:23:55+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-03 10:17:19","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7263225","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7263225","identity":"rs-7263225","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}