Simultaneous Enhancement of Photocatalytic CH4 Conversion and H2O2 Production through Water Confinement in Nanoporous Core-Shell Catalysts

Simultaneous Enhancement of Photocatalytic CH4 Conversion and H2O2 Production through Water Confinement in Nanoporous Core-Shell Catalysts | 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 Article Simultaneous Enhancement of Photocatalytic CH 4 Conversion and H 2 O 2 Production through Water Confinement in Nanoporous Core-Shell Catalysts Chenlu Xie, Fanxun Lv, Xiaoyan Wu, Xuan Wang, Yi Yu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6469567/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 17 Feb, 2026 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Aqueous photocatalytic CH 4 oxidation offers a promising route for converting natural gas into liquid oxygenates, a process governed by multi-electron and proton transfer steps at the catalyst-water interface. Here, we demonstrate that spatially confining interfacial water within Au/TiO 2 @pSiO 2 core-shell catalysts – achieved by systematically reducing silica pore size to 1.7 nm – remarkably increases CH 4 conversion threefold and H 2 O 2 production 22-fold compared to Au/TiO 2 . This strategy is generalizable to other semiconductors and co-catalysts, with Pt/TiO 2 @pSiO 2 -1.7 exhibiting high oxygenate yields (32.7 mmol g − 1 h − 1 ) and a 14.1% apparent quantum yield at 365 nm. Kinetic isotope effect (KIE), FTIR and EPR studies reveal that the water confined within pores, with a weakened H-bonding network, alters proton-coupled electron transfer (PCET) pathways. Water oxidation transits to a concerted PCET pathway, favoring •OH production for CH 4 conversion, while oxygen reduction shifts to a 2e − process, directly producing H 2 O 2 . This work highlights the potential of water confinement for designing efficient photocatalyst. Physical sciences/Chemistry/Catalysis/Photocatalysis Physical sciences/Energy science and technology/Fossil fuels/Natural gas Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Aqueous photochemical processes are vital for energy conversion and environmental applications, such as water splitting 1 , O 2 /CO 2 reduction 2 , and the removal of organic pollutants 3 . These processes harness photon energy to carry out chemical reactions under mild conditions and typically involve multi-electron and multi-proton transfer steps that generate reactive oxygen species as key intermediates 3-5 . Among these processes, photocatalytic methane oxidation has emerged as a green route to directly transform abundant and inexpensive natural gas into valuable fuels and chemical feedstocks, such as methanol 6-8 . This approach utilizes photon energy to overcome the high activation barriers of CH 4 , enabling its conversion at room temperature via hydroxyl radicals (•OH) and bypassing the energy-intensive syngas production route 6,9 . Moreover, the aqueous environment offers unique advantages in improving product selectivity by facilitating the desorption of partially oxygenated products (e.g., methanol and formaldehyde) from the catalyst surface 9,10 . Despite extensive research has focused on developing catalysts to enhance charge separation and regulate surface catalytic reactions through band structure engineering 11-13 and co-catalyst modification 9,14-18 , the potential of tuning the aqueous microenvironment in the vicinity of semiconductor to boost CH 4 conversion remains largely unexploited. Aqueous-phase photocatalytic CH 4 oxidation involves a complex network of electron transfer (ET) and proton transfer (PT) reactions at the semiconductor-water interface ( Supplementary Scheme 1 ), where water serves as a reactant, proton source, and solvation environment 19-21 . For hydrophilic oxide semiconductors such as TiO 2 and ZnO, water acts as the primary hole acceptor, generating •OH through proton and electron transfer pathways 9,20 . These •OH radicals activate methane through hydrogen abstraction, forming methyl radicals (•CH 3 ). Simultaneously, oxygen is reduced to superoxide radicals (•OOH), also involving electron and proton transfer, which further contribute to the conversion of •CH 3 into oxygenates 15 . These ET and PT events may occur either stepwise or concertedly via proton-coupled electron transfer (PCET) mechanisms, which are highly sensitive to the solvent environment due to the significant energy required for solvent reorganization induced by charge separation, especially in polar solvents like water 22,23 . Therefore, designing catalyst that modulate the water structure near the semiconductor interface presents a promising avenue for tuning proton/electron transfer pathways and optimizing methane oxidation kinetics. Spatial confinement offers an effective strategy to tune the structure of water 22,24 , and consequently, the dynamics of these interfacial ET and PT processes 25 . Compared to bulk water, confined water exhibits distinct hydrogen-bonding (HB) networks, enthalpic and entropic characteristics, and electronic properties 25,26 . While theoretical and model reaction studies of electrocatalytic half-reactions including H 2 revolution/CO 2 reduction 27-33 , and photochemical water splitting 34-36 have hinted at the potential of tuning water structure for promoting catalytic performance, its application to complex photocatalytic reactions such as methane oxidation remains largely unexplored. Herein, we demonstrate that confining interfacial water near TiO 2 within silica nanopores of core-shell Au/TiO 2 @pSiO 2 catalysts provides a powerful means to modulate its structure and reactivity, and consequently, the PCET pathways. Decreasing the silica pore size from 3.1 nm to 1.7 nm leads to an approximately three-fold enhancement in methane conversion, and surprisingly, a 22-fold increase in H 2 O 2 production compared to Au/TiO 2 . The Au/TiO 2 @pSiO 2 with 1.7 nm pores exhibits a high liquid oxygenates yield of 27.4 mmol g -1 h -1 with 97.2 % selectivity, and 44.6 mmol g -1 h -1 for H 2 O 2 production under 2.5 MPa CH 4 and 1 MPa O 2 at ambient temperature. This confinement strategy is broadly applicable to other metal co-catalysts and semiconductors, with Pt/TiO 2 @pSiO 2 -1.7 catalyst reaching maximum oxygenate yields of 32.7 mmol g -1 h -1 and an apparent quantum yield (AQY) of 14.1% at 365 nm, surpassing other reported TiO 2 -based photocatalysts. Mechanistic investigation, including H/D KIE, in situ electron paramagnetic resonance (EPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and 18 O isotope labeling studies, reveal that the weakened HB network of water confined within the silica pores modulates PCET pathways for both water oxidation reaction (WOR) and oxygen reduction reaction (ORR). Specifically, WOR shifts from a stepwise PCET to a concerted mechanism, enhancing •OH radical production for CH 4 conversion, while ORR shifts from a 1e - to a 2e - process, promoting H 2 O 2 production. These findings underscore the potential of engineering interfacial water structures through spatial confinement to enhance photocatalytic activity and selectivity. Results Synthesis and characterization of Au/TiO 2 @pSiO 2 catalysts TiO 2 is a well-established semiconductor for photocatalytic methane oxidation, with a valence band position sufficient for oxidizing water to •OH radicals 12 , 20 . Noble metal cocatalyst modification is known to enhance CH 4 conversion 9 , 14 , 37 . Therefore, we selected Au/TiO 2 as the model catalyst to study the effect of water confinement, which can be achieved by installing water permeable, nanoporous enclosures to the Au/TiO 2 pore. We chose SiO 2 enclosure due to its facile growth control, controllable pore size, optical transparency in the UV region, and good stability. Furthermore, our previous studies have shown that a thin, non-porous silica shell can prevent oxygenates overoxidation while still allowing •OH radicals transport and convert CH 4 20 , indicating good compatibility of silica with the TiO 2 -based photocatalysts. Gold nanoparticles (NPs) were loaded onto TiO 2 (P25) via NaBH 4 reduction 9 , 38 , and nanoporous silica shell was constructed on Au/TiO 2 using the Stöber method with alkylammonium surfactants as templates (Fig. 1 a) 39 . By varying surfactant carbon chain length or adding auxiliary organics 40 , 41 , we obtained Au/TiO 2 @pSiO 2 -X with different pore sizes (X = pore size). The surface area and porosity of Au/TiO 2 @pSiO 2 -X were characterized by nitrogen physisorption (Fig. 1 e, Supplementary Table 1 ). Using C 16 TAB resulted in mesopores ~ 2.6 nm, while a shorter surfactant, C 12 TAB, yielded smaller pores around 1.7 nm (Fig. 1 f). Adding hexane as pore-expanding agent yielded larger pores around 3.1 nm. For comparison, Au/TiO 2 @SiO 2 without nanopores were synthesized similarly but without CTAB templates ( Supplementary Fig. 1 ). Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) confirmed full encapsulation of Au/TiO 2 by a uniform, amorphous SiO 2 shell across all Au/TiO 2 @pSiO 2 -X catalysts (Fig. 1 b-d, Supplementary Fig. 2–4 ). The silica layer had an average thickness of 8 nm (Fig. 1 b-d), which is tunable by varying tetraethyl orthosilicate (TEOS) ( Supplementary Fig. 5 ). Au NPs ranging 2 6 nm were randomly dispersed on TiO 2 (Fig. 1 b-d, Supplementary Fig. 3 ), with a loading of ~ 0.1 wt% ( Supplementary Table 2 ). UV-visible diffusive reflective spectra (UV-Vis DRS) of Au/TiO 2 and Au/TiO 2 @pSiO 2 -X were nearly identical (Fig. 1 g), suggesting minimal effect of the SiO 2 coating on light absorption. X-ray photoelectron spectra (XPS) showed similar Ti oxidation states among all samples ( Supplementary Fig. 6 ). These results indicate that the nanoporous coating does not significantly modify the intrinsic properties of the semiconductors. Pore-size dependent CH 4 photocatalytic oxidation by Au/TiO 2 @pSiO 2 -X Photocatalytic reactions were conducted in a batch reactor with 100 mL of distilled water under 2.5 MPa CH 4 and 1 MPa O 2 at 25 ºC ( Supplementary Fig. 7, 8 ). Control experiments, including dark and catalyst-free controls ( Supplementary Table 3 ), and a 13 CH 4 isotope-labeling study ( Supplementary Fig. 9 ), validated that oxygenates were generated from photocatalytic methane oxidation over Au/TiO 2 @pSiO 2 . Compared to bare Au/TiO 2 , Au/TiO 2 with a nanoporous silica shell showed significantly enhanced oxygenates yields, which increased with decreasing pore size (Fig. 2 b). Au/TiO 2 @pSiO 2 -3.1 exhibited total oxygenates yields of 18.0 mmol g⁻ 1 h⁻ 1 , with a selectivity of 92.5%. Further reducing the pore size to 1.7 nm led to greater oxygenates yields, achieving 27.4 mmol g⁻ 1 h⁻ 1 — approximately triple that of Au/TiO 2 . The AQY for Au/TiO 2 @pSiO 2 -1.7 was 10.9% at 365 nm ( Supplementary Table 4 ), substantially exceeding Au/TiO 2 (3.3%). Notably, Au/TiO 2 @pSiO 2 also produced an unexpectedly high amount of H 2 O 2 as a valuable byproduct. The H 2 O 2 production also increased with decreasing pore size, reaching 44.6 mmol g⁻ 1 h⁻ 1 for Au/TiO 2 @pSiO 2 -1.7 (Fig. 2 a, c), which is 22 times higher than Au/TiO 2 and comparable to the state-of-the-art catalysts ( Supplementary Table 5 ). A physical mixture of Au/TiO 2 and porous SiO 2 (pSiO 2 -1.7) showed similar catalytic performance to Au/TiO 2 alone, highlighting the importance of the core-shell architecture ( Supplementary Figs. 10 and 11 ). The thickness of silica also plays a pivotal role: an 8 nm silica shell was optimal, while a 30 nm shell reduced yields ( Supplementary Fig. 12 ), likely due to an increased diffusion barrier. It is noteworthy that Au/TiO 2 @SiO 2 without pores achieved a higher yield than Au/TiO 2 but lower than the nanoporous catalysts (Fig. 2 b), likely due to the greater diffusion barrier in non-porous silica. This performance trend signifies the importance of controlling silica pore size for efficient methane conversion. The effect of O 2 pressure on methane conversion rate is also studied in detail. 18 O isotopic labelling confirmed the direct participation of O 2 in forming oxygenated products ( Supplementary Fig. 13 ). Under anerobic conditions, both Au/TiO 2 and Au/TiO 2 @pSiO 2 -X exhibited low yield, which increased with O 2 pressure (Fig. 2 d, Supplementary Fig. 14 ). Au/TiO 2 achieved maximum yield at 0.042 MPa O 2 , whereas Au/TiO 2 @pSiO 2 -2.6 and Au/TiO 2 @pSiO 2 -1.7 required higher O 2 pressures (up to 1 MPa) (Fig. 2 d, Supplementary Fig. 14a, b) , suggesting hindered O 2 transport by the silica shell ( Supplementary Note 1 ). Furthermore, TiO 2 and TiO 2 @pSiO 2 catalysts required higher O 2 pressures to attain optimal yields compared to their Au-incorporated counterparts ( Supplementary Fig. 14c-d, Fig. 15 ), and relocating Au NPs to the SiO 2 surface decreased product yields ( Supplementary Fig. 16 ). These results indicate that the enhanced performance of Au/TiO 2 @pSiO 2 is not due to O 2 enrichment from its high surface area. Instead, O 2 transport within silica shell is hindered in aqueous solution but can be promoted by the supported Au co-catalysts. Further optimization of oxygenate yields was carried out by varying light intensity and increasing water volume (Fig. 2 e, Supplementary Fig. 17 ). By increasing the light flux to 500 mW/cm 2 , we achieved a yield of 62.9 mmol g⁻ 1 h⁻ 1 with 80% selectivity for oxygenates (Fig. 2 e). Au/TiO 2 @pSiO 2 -1.7 also exhibited excellent stability over four cycles with unchanged production and selectivity ( Supplementary Fig. 18 ). XRD and TEM showed no obvious structure change in the spent catalyst ( Supplementary Fig. 19 ), and N 2 adsorption isotherms showed similar pore size distribution and surface area to fresh catalysts ( Supplementary Fig. 20 ). Generalization of the photocatalyst confinement strategy We explored this confinement strategy with different metal cocatalysts and semiconductors. Substituting Au with Pd, Pt or Rh exhibited a similar trend: confined pores around metalTiO 2 surface significantly enhance oxygenates and H 2 O 2 yields compared to bare metalTiO 2 catalysts, with smaller pores yielding higher productions (Fig. 3 a, Supplementary Figs. 21 and 22, Supplementary Note 2 ). Among these, Pt/TiO 2 @pSiO 2 -1.7 achieved the highest oxygenate yields of 32.7 mmol g − 1 h − 1 and an AQY of 1.1 % for CH 4 oxidation at 365 nm, which outperforms the previous benchmark (Fig. 3 b, Supplementary Table 4, 6 ). We further extended our study to other semiconductors, including BiVO 4 and anatase TiO 2 (ATiO 2 ) (Fig. 3 a, Supplementary Fig. 23–25, Supplementary Note 2) . Applying the porous silica coating improved oxygenate yields and H 2 O 2 production for both catalysts. For example, Au/ATiO 2 @pSiO 2 -1.7 achieved oxygenates and H 2 O 2 formation rate of 22.0 and 23.6 mmol g − 1 h − 1 , respectively, significantly surpassing Au/ATiO 2 (Fig. 3 a). These results underscore the versatility of the nanoporous silica core-shell confinement design, demonstrating its broad applicability across different photocatalyst systems. Key reactive oxygen species (ROS) for photocatalytic CH 4 oxidation We first investigated the role of SiO 2 shell in photocarrier separation using steady-state photoluminescence (PL) and photocurrent experiments. PL measurements revealed that the silica shell has a negligible effect on photocarrier separation ( Supplementary Fig. 26a ). Furthermore, Au/TiO 2 showed a higher photocurrent compared to Au/TiO 2 @pSiO 2 ( Supplementary Fig. 26b ), likely due to the low conductivity of silica coating. These results, together with previous findings regarding O 2 transport limitations through the porous silica shell ( Supplementary Note 1 ), demonstrate that improved performance in Au/TiO 2 @pSiO 2 compared to Au/TiO 2 is not due to improved photocarrier separation or enhanced O 2 transport, but rather to effects associated with the structure of the confined water layers in contact with the semiconductors. To unravel the underlying mechanism, we next sought to identify the key ROS responsible for methane activation and H 2 O 2 formation within the confined environment. This can be achieved by introducing sacrificial agents that selectively quench different ROS and monitor the change in methane conversion 15 , 42 . Both holes and •OH radicals are capable of activating methane 9 , 14 , 15 . We used salicylic acid and (NH 4 ) 2 C 2 O 4 to scavenge •OH radicals and holes (h + ), respectively 15 . The addition of salicylic acid nearly completely inhibited CH 4 conversion, showing a stronger suppression than (NH 4 ) 2 C 2 O 4 ( Supplementary Fig. 27 ). This suggests that •OH radicals, rather than h⁺, are the main reactive species for CH 4 activation. The Au/TiO 2 @pSiO 2 catalyst with smaller pores produced more •OH radicals, as monitored by in situ EPR, which is consistent with their superior performance in CH 4 conversion (Fig. 4 a, b). Coumarin was also employed as a fluorescent probe to assess •OH production 15 , which further supports the findings from EPR ( Supplementary Figs. 28 and 29 ). Typically, there are two distinct pathways by which •OH radicals are formed: one involves H 2 O oxidation by holes (1e⁻ WOR), while the other involves O 2 reduction by electrons (ORR) (Supplementary Scheme 1 ). To identify the origin of •OH radicals, we performed 18 O isotopic labeling with benzoic acid as a probe to capture •OH ( Supplementary Fig. 30 ). Under H 2 16 O/ 18 O 2 conditions, over 95% of products corresponded to H 16 OC 6 H 4 CO 2 H (m/z = 137), while with H 2 18 O/ 16 O 2 , approximately 80% of products corresponded to H 18 OC 6 H 4 CO 2 H (m/z = 139). This analysis revealed that •OH radical primarily originate from 1e⁻ WOR rather than from O 2 reduction over Au/TiO 2 @pSiO 2 -1.7 (Fig. 4 c, Supplementary Fig. 31 ). In contrast, for Au/TiO 2 , approximately half of the •OH radicals derive from H 2 O oxidation and the other half from O 2 reduction ( Supplementary Figs. 32 and 33) . Thus, the enhanced 1e⁻ WOR to •OH radicals within the confined SiO 2 pores is responsible for enhanced CH 4 conversion over Au/TiO 2 @pSiO 2 . EPR spectra of DMPO-OOH show that Au/TiO 2 @pSiO 2 with smaller pores produced less •OOH radicals (Fig. 4 d), consistent with their enhanced H 2 O 2 production as the main product in ORR (Fig. 4 e). Electron scavenger experiments using AgNO 3 or NaIO 3 and 18 O 2 /H 2 18 O isotope labelling confirmed that H 2 O 2 is generated exclusively through ORR (Fig. 4 f, Supplementary Fig. 34–36 ). The ORR selectivity to H 2 O 2 is 75% over Au/TiO 2 @pSiO 2 -1.7, compared to 11% over Au/TiO 2 ( Supplementary Fig. 37, Supplementary Note 3 ). The high H 2 O 2 production and negligible •OOH production over Au/TiO 2 @pSiO 2 suggests that the silica nanopores confinement alters the ORR pathway from a 1e⁻ process to a 2e⁻ process. Thus, facilitated 1e⁻ WOR producing •OH and an altered ORR pathway are key features of Au/TiO 2 @pSiO 2 responsible for its superior catalytic performance. Moreover, similar enhancements in •OH production from 1e⁻ WOR and altered ORR pathways were observed with Pt/TiO 2 @pSiO 2 catalysts ( Supplementary Fig. 38–40, Supplementary Note 2 ). These results indicate this alteration is a general phenomenon when semiconductors are spatially confined within nanopores. We hypothesize that both the enhanced 1e⁻ WOR and the modified ORR pathway result from changes in the water structure within these confined spaces compared to bulk water. KIE investigations for CH 4 oxidation process H/D kinetic isotope effect (KIE) can be used to analyze whether PT is involved in the rate-determining step (RDS) 43 – 45 , which would further shed light on the mechanism of altered reactivity and selectivity. For the 1e⁻ WOR that produces •OH radicals, Au/TiO 2 @pSiO 2 –1.7 shows a pronounced KIE value of 2.0–2.3, indicating a strong involvement of PT in the RDS. In contrast, Au/TiO 2 exhibit a low KIE value of 1.1–1.2 (Fig. 5 a, Supplementary Figs. 41 and 42 ), suggesting a distinctly different 1e⁻ WOR pathway. The absence of KIE for Au/TiO 2 is consistent with the widely accepted stepwise proton-electron transfer (SPET) mechanism 35 , 46 , 47 , where water readily deprotonates on TiO 2 before being oxidized by photogenerated hole, making the electron transfer step (OH⁻ + h + → •OH) the RDS. Conversely, the large KIE for Au/TiO 2 @pSiO 2 –1.7 suggests that molecular water confined in the nanopores act directly as the hole acceptor, generating •OH radicals through a concerted proton-electron transfer (CPET) pathway (H 2 O + h + → •OH + H + ). As a result, the slower dissociation and deuteron transfer in D 2 O compared to H 2 O greatly reduce the •OH radical production. At a molecular level, the difference between the SPET pathway for Au/TiO 2 and CPET pathway for Au/TiO 2 @pSiO 2 –1.7 lies in whether the photogenerated holes prefer to react with molecular water or hydroxide anion. The SPET mechanism for Au/TiO 2 indicates h + prefers to react with hydroxide anion despite the greater abundance of water molecules. This can be rationalized by hydroxide anion having greater reactivity towards h + due its negative charge. Consequently, the shift to a CPET mechanism in Au/TiO 2 @pSiO 2 –1.7 upon water confinement within the nanopores indicates that confined water possesses higher reactivity towards h + than bulk water. This enhanced reactivity can be attributed to the weakened HB network and reduced solvation effect in confined water 29 , 35 . On the other hand, the weakened HB network also facilitates proton transfer 48 – 50 , further promoting the CPET pathway. The higher •OH production observed with Au/TiO 2 @pSiO 2 –1.7 compared to Au/TiO 2 suggests that the CPET mechanism, directly utilizing molecular water to react with h + , provides a more effective route for generating the •OH radicals necessary for CH 4 activation compared to the two-step SPET pathway. Thus, confinement-driven changes in water reactivity and PCET pathways are key to enhanced CH 4 conversion for Au/TiO 2 @pSiO 2 –1.7 catalysts. As the WOR producing •OH is key to CH 4 activation, its KIE influences the KIE of overall CH 4 oxidation. Consequently, Au/TiO 2 does not show KIE effect for CH 4 conversion and Au/TiO 2 @pSiO 2 -1.7 show KIE value of 1.3–1.6 (Fig. 5 b, Supplementary Fig. 43–46 ). The KIE decrease over time for Au/TiO 2 @pSiO 2 –1.7, likely due to H/D exchange between CH 4 and D 2 O during reactions ( Supplementary Fig. 46, Supplementary Notes 4 ). On the ORR side, a large KIE of 2.97 is also observed for Au/TiO 2 @pSiO 2 –1.7 (Fig. 5 c), which is expected for the 2e⁻ process that produces H 2 O 2 from oxygen and proton. In stark contrast, a reverse KIE for H 2 O 2 production was observed over Au/TiO 2 , indicating that the confined water significantly modulates the PT and ET dynamics. Notably, the observed shift in ORR pathway from the 1e⁻ process for Au/TiO 2 to the 2e⁻ process for Au/TiO 2 @pSiO 2 can also be attributed to facilitated proton transfer due to water confinement. Compared to the 1e⁻ process, the 2e⁻ process is thermodynamically more viable but requires simultaneous transfer of two protons. Therefore, more efficient proton transfer, along with increased protons generated from the enhanced WOR process, are responsible for the shift to 2e⁻ process and enables efficient production of H 2 O 2 . Confined water characterization The weakened HB network of the confined water was analyzed by in situ DRIFTS measurement at various relative humidity (RH) levels (Fig. 5 d-g, Supplementary Fig. 47–48 ). The broad OH stretching band (2800 − 3600 cm − 1 ) is sensitive to the HB network configuration and can be deconvoluted into five Gaussian sub-bands (Fig. 5 f) 24 , 34 . From Au/TiO 2 to Au/TiO 2 @pSiO 2 -1.7, water in smaller pore shows a significantly larger portion of DA to DDAA ratio along with lower average number of HBs () (Fig. 5 g, Supplementary Table 7 ), indicating a weakened HB network. A similar trend was observed in the HOH bending (1500–1800 cm − 1 ) region (Fig. 5 e), where deconvolution of the spectra can give the ratio between two peaks at 1620 cm⁻¹ and 1680 cm⁻¹ attributed to DA and DDAA water clusters 34 . These results suggest that H 2 O molecules within smaller pores adopt a more linear arrangement with a weaker HB network, leading to properties that are more isolated and prone to oxidation, improved proton conductivity favorable to PT processes, and reduced solvation that better stabilizes the semi-hydrophobic •OH radicals. Therefore, both enhanced methane conversion and H 2 O 2 production of the Au/TiO 2 @pSiO 2 can be attributed to the weakened HB network of the confined water in porous silica (Fig. 6 ): (a) for h + driven WOR, confined water is less stabilized by HB and are thus more reactive, causing a shift from SPET to CPET mechanism that generates more •OH radicals to activate methane; (b) for e − driven ORR, confined water with weakened HB transfers proton more efficiently, enabling a shift from 1e − to 2e − process that directly produces H 2 O 2 . Conclusions In summary, we have demonstrated a core-shell photocatalyst design that leverages spatial confinement to modulate the interfacial water structure, leading to a significant enhancement in both CH 4 conversion and H 2 O 2 production. The confined water with weaker HB bonds significantly modulates the PT and ET dynamics in reaction involving photogenerated electron and holes. Specifically, the confined water exhibits enhanced reactivity towards h + in WOR, causing a shift from SPET to CPET mechanism and generating more •OH radicals to activate methane. Concurrently, the weakened HB network facilitates more efficient proton transfer in ORR, enabling a shift from the 1e⁻ to the 2e⁻ process and directly producing H 2 O 2 . This study provides a fundamental understanding of how interfacial water structure can be engineered to optimize photocatalytic performance, opening new avenues for photocatalyst design. Methods Preparation of M/TiO 2 photocatalyst Metal cocatalyst (Au, Pt, Pd, Rh) were loaded onto TiO 2 (P25) using a simple NaBH 4 reduction method 9 , 38 . Typically, 50 mg of TiO 2 was ultrasonically dispersed in 30 mL of deionized water, after which a specific amount of metal precursor solution (HAuCl 3 •3H 2 O, H 2 PtCl 6 , PdCl 2 , Rh(NO 3 ) 3 ) was added. Following 10 minutes of stirring, 2 mL of freshly prepared 0.1 M NaBH 4 was added. After an additional 30 minutes of stirring, the product was collected by centrifugation, washed with deionized water, and dried in an oven at 60 ℃. Preparation of M/TiO 2 @pSiO 2 -X To synthesize M/TiO 2 @pSiO 2 -1.7 (M denoted to Au, Pd, Pt, Rh), 100 mg of dodecyltrimethylammonium bromide (C 12 TAB) was used as the templating agents, which was dissolved in 10 mL of deionized water and 40 mL of anhydrous ethanol. Subsequently, 50 mg of M/TiO 2 was added, sonicated for 30 mins, followed by the addition of tetraethyl orthosilicate (TEOS). After 15 minutes of stirring, 0.5 mL ammonium hydroxide solution (28–30%) was added, and the mixture was stirred overnight. The product was collected via centrifugation, washed twice with DI H 2 O and ethanol, dried at 60 ℃, and calcined at 550 ℃ for 2 h in static air to remove the C 12 TAB templates. TEOS volume controlled the SiO 2 thickness; 300 µL produced an 8 nm SiO 2 shell. For the M/TiO 2 @pSiO 2 -2.6 sample, the synthesis process was identical, except cetyltrimethylammonium bromide (C 16 TAB) was used as the template. The preparation of M/TiO 2 @pSiO 2 -3.1 followed the same procedure as M/TiO 2 @pSiO 2 -2.6, with the exception that 10 mL of n-hexane was added to the mixture prior to the addition of TEOS. Characterization The crystal structure of the catalysts was characterized using a powder X-ray diffractometer D2 (Bruker, Switzerland) with Cu Kα radiation (λ = 1.542 Å). Morphologies of the photocatalysts were studied using TEM (JEM-1400plus). High-resolution TEM (HRTEM) images, HAADF-STEM images and corresponding EDS maps were collected using a JEOL JEM-F200 field-emission microscope operated at 200 kV. Nitrogen sorption isotherms were collected on a BELSORP-Max 2 apparatus. The metal loading in the photocatalysts was determined by inductively coupled plasma optical emission spectrometer (ICP-OES, Icap7400, Thermo). UV-Vis DRS were recorded on a Cary 5000 spectrometer (Agilent) equipped with an integrating sphere. The absorption spectra were obtained using the Kubelka–Munk transformation. XPS were collected using a Thermo Fisher ESCALAB 250Xi spectrometer with monochromatic Al-Kα X-rays as the excitation source. All spectra were calibrated to the C 1s peak at 284.8 eV. Steady-state PL spectra were recorded on the Horiba Fluorolog-3 Spectrometer at room temperature with an excitation wavelength of 350 nm. Photocurrent tests were conducted with the electrochemical workstation (Gamry Instrument Interface 1010E) in three-electrode system with 0.1 M Na 2 SO 4 as electrolyte (Ag/AgCl as the reference electrode, platinum plate as the counter electrode and catalysts loaded on indium-tin oxide (ITO) as the working electrode). Photocatalytic activity measurements Photocatalytic CH 4 oxidation reaction was conducted in a 250 mL high-pressure reactor with a sapphire window for light irradiation. Typically, 10 mg of catalyst was ultrasonically dispersed in 100 mL of deionized water for 30 min. The reactor was then sealed and purged with ultrapure argon (99.999%) to completely remove the air. Subsequently, the reactor vessel was pressurized with 1 MPa O 2 (99.999%) and 2.5 MPa CH 4 (99.999%). The reactor was irradiated from the top using an LED lamp (365 nm, PLS-LED100C, Perfectlight). The photocatalytic reaction was conducted for 1 hour at 25 o C. A thermocouple was inserted into the solution to directly detect the temperature of the solution. The photoreactor was directly connected to a gas chromatograph (Agilent GC 8890) equipped with a flame ionization detector (FID) and a methanizer for the gas product analysis of CO 2 and ethane. The liquid products (CH 3 OOH, CH 3 OH, and HCOOH) were analyzed by 1 H-NMR (Bruker 400 MHz), with dimethyl sulfoxide (DMSO) added as an internal standard. The amount of HCHO was quantified using the acetylacetone color-development method 21 (details in Supplementary Methods). The concentration of produced H 2 O 2 was determined using the titanium oxalate spectrophotometric method (details in Supplementary Methods). Details of CH 4 conversion rate and oxygenates selectivity calculation, AQY calculation, catalyst recycle tests, and active species trapping experiments are provide in the Supplementary Methods. Isotope experiments for identifying the origin of products For carbon source investigation with isotopic labelled 13 CH 4 : 10 mg Au/TiO 2 @pSiO 2 was dispersed in 5 mL H 2 O, and the reactor was purged with Ar to remove air. The photoreactor was then filled with 0.55 MPa 13 CH 4 (99%), 0.45 MPa 12 CH 4 (99.999%) and 0.2 MPa O 2 (99.999%). After 2 h of light irradiation, gas products were analyzed by gas chromatograph-mass spectroscopy (GC-MS, Shimadzu), and liquid products by 1 H-NMR. For oxygen source investigation with 18 O isotope labeling with 18 O 2 /H 2 16 O and 16 O 2 /H 2 18 O: 10 mg Au/TiO 2 @pSiO 2 was dispersed in 10 mL H 2 16 O or (H 2 18 O), and the reactor was purged with Ar. The photoreactor was charged with 2.5 MPa CH 4 (99.999%) and 0.5 MPa 18 O 2 (98%) (or 16 O 2 ). Products were analyzed by GC-MS after 2 h of light irradiation. EPR measurements In situ electron paramagnetic resonance (EPR) spectra were acquired using a Bruker ESR5000 spectrometer equipped with a 300 W xenon lamp (PLS-SXE300+/UV, Perfectlight) for illumination. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) served as the radical trap. For •OH radical detection, 5 mg of catalyst was dispersed in 5 mL of distilled H 2 O in the dark and purged with ultrapure argon (99.999 vol.%) for 2 min. For •OOH radical detection, 5 mg of catalyst was dispersed into a 5 mL of DMSO with O 2 bubbling for 2 min. Analysis of photogenerated •OH radicals The •OH production was measured using PL with coumarin, which reacts with •OH to form fluorescent 7-hydroxycoumarin (7-HC) (Supplementary Fig. 28). 2 mg catalyst was dispersed in 100 mL of 1.0 mM coumarin solution in dark, irradiated for 10 minutes, then centrifuged and analyzed with a Horiba FluoroLog-3 spectrofluorometer at 332 nm excitation. Investigation of the photochemical pathway of H 2 O 2 production To determine if H 2 O 2 production originates from WOR or ORR, liquid samples from 18 O 2 or H 2 18 O isotope CH 4 oxidation experiments were analyzed. As shown in Supplementary Fig. 36, 2 mL of the liquid samples was injected into a sealed vial (Vial A) and purged with He for 30 min. Another vial (Vial B), containing 200 mg of MnO 2 and 5 mL H 2 O, was also purged with He for 30 min. Then, 2 mL from Vial A was injected into Vial B. After complete decomposition of H 2 O 2 over MnO 2 , 1 mL of gas from the headspace of Vial B was extracted for GC-MS analysis. Investigation of the photochemical pathway for •OH generation To analyze photochemical pathways of •OH production, 18 O 2 isotope experiments and H 2 18 O isotope experiments were conducted. In the 18 O 2 experiments, 2 mg of catalyst was dispersed in 1 mL of 1 mM benzoic acid solution (H 2 16 O as solvent). The reactor was purged with Ar to remove air, and filled with 0.1 MPa 18 O 2 (98%). The reaction was carried out under UV irradiation for 600 s. The products were analyzed by liquid chromatography–mass spectrometry (LC-MS, Thermofisher). Similarly, for the H 2 18 O isotope experiments, the setup was identical except using benzoic acid dissolved in H 2 18 O and 0.1 MPa 16 O 2 . KIE measurements For •OH radical generation, 5 mg of catalyst were dispersed in 2 mL of either D₂O or H₂O, and in situ EPR measurements were performed to track the formation of •OH radicals. For CH 4 conversion, 10 mg of catalysts were dispersed in 10 mL of D 2 O or H 2 O. The reactor vessel was then pressured to 2.5 MPa CH 4 and 1 MPa of O 2 , followed by light irradiation, and liquid products were collected at specific time intervals for analysis. For H 2 O 2 generation, 10 mg of catalysts were dispersed in 10 mL of D 2 O or H 2 O. The reactor vessel was then pressured to 1 MPa of O 2 , followed by light irradiation. The resulting liquid product was collected for analysis. In situ DRIFTS measurements In situ DRIFTS measurements were performed using a Thermo Scientific iS50 FTIR spectrometer equipped with a liquid-nitrogen-cooled MCT/A detector. An infrared cell (Harrick) with ZnSe windows were used for measurements. Each spectrum was recorded by averaging 64 scans at a resolution of 4 cm – 1 . Samples were first pretreated at 673 K (5 K min − 1 ) for 30 min in flowing O 2 (20 mL min − 1 ) to remove impurities adsorbed on the surface. Background spectra were then taken at 303 K in dry Ar. Wet Ar (Ar/H 2 O with controlled humidity) was subsequently introduced (20 mL min − 1 ) and spectra were acquired at the 303K. By using the following equation proposed by previous work 24 , 34 , we determined the average number of hydrogen bonds of each water molecule: = 4 \(\:\times\:\) A DDAA + 3 \(\:\times\:\) A DDA + 3 \(\:\times\:\) A DAA + 2 \(\:\times\:\) A DA Declarations Acknowledgements We thank Prof. P. Liu, Prof. B. Yang, Prof. Y. Huang, and Prof. P. Hu at ShanghaiTech University for the insightful discussion. We thank Prof. F. Yang and Prof. Y. Yang at ShanghaiTech University for help on FTIR experiments, and Dr. S. Chen for help on the EPR measurements. The TEM experiments are supported by the Center for High-resolution Electron Microscopy (CℏEM) at ShanghaiTech University. The NMR, UV-DRS, ICP-OES, N 2 physisorption, XPS, and PL measurements are supported by Analytical Instrumentation Center (#SPST-AIC10112914) at ShanghaiTech University. This work was financially supported by Shanghai Sailing Program (Nos. 23YF1426600) and start-up funding from ShanghaiTech University. Author contributions C.X. conceived the project. F. L. performed the catalyst preparation, characterization, and catalytic tests. X.-Y. W. performed the HR-TEM, HAADF and EDS characterization. Y.Y. and X.-Y. W. helped on TEM image analysis. X.W. performed XPS experiments. C.X., F.L. wrote the manuscript. All authors contributed to the data discussion, and revision of the manuscript. Competing interests : The authors declare no competing interests. Data and materials availability : All data and images are available in the manuscript or the supplementary materials. 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Chem. 9 , 1017-1035 (2023). https://doi.org/https://doi.org/10.1016/j.chempr.2023.02.011 Additional Declarations There is NO Competing Interest. Supplementary Files SIfinal041725.docx Supplementary information Cite Share Download PDF Status: Published Journal Publication published 17 Feb, 2026 Read the published version in Nature Communications → 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-6469567","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":458924379,"identity":"4ee359bd-c691-434e-bf6a-4b10cb3e9de7","order_by":0,"name":"Chenlu Xie","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0001-9215-6878","institution":"ShanghaiTech University","correspondingAuthor":true,"prefix":"","firstName":"Chenlu","middleName":"","lastName":"Xie","suffix":""},{"id":458924380,"identity":"9c31569e-f4c9-4f37-891b-c8ff94dec3e8","order_by":1,"name":"Fanxun Lv","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Fanxun","middleName":"","lastName":"Lv","suffix":""},{"id":458924381,"identity":"bcc91b09-7b9e-4a6a-8869-56d89dd974d1","order_by":2,"name":"Xiaoyan Wu","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Xiaoyan","middleName":"","lastName":"Wu","suffix":""},{"id":458924382,"identity":"5e5eff72-9cfc-43e2-9d84-863d5724fe92","order_by":3,"name":"Xuan Wang","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Xuan","middleName":"","lastName":"Wang","suffix":""},{"id":458924383,"identity":"db47355e-fd00-4742-bc80-36ab590f5b41","order_by":4,"name":"Yi Yu","email":"","orcid":"https://orcid.org/0000-0003-4326-5992","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Yi","middleName":"","lastName":"Yu","suffix":""}],"badges":[],"createdAt":"2025-04-17 08:10:24","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6469567/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6469567/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-026-69719-z","type":"published","date":"2026-02-17T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":85037867,"identity":"e011e300-e662-4288-a1d3-01462b5aecce","added_by":"auto","created_at":"2025-06-20 08:49:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":12129202,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructural characterizations of Au/TiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e@pSiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e with varying silica pore sizes. a\u003c/strong\u003e, Schematic illustration of the preparation of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X catalysts.\u003cstrong\u003e b\u003c/strong\u003e, HR-TEM image of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 catalysts. \u003cstrong\u003ec\u003c/strong\u003e, High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 catalysts. Scale bar: 20 nm. \u003cstrong\u003ed\u003c/strong\u003e, EDS mapping images of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7. Scale bar: 20 nm. \u003cstrong\u003ee\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eN\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms of Au/TiO\u003csub\u003e2 \u003c/sub\u003eand Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X. \u003cstrong\u003ef\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003ePore size distribution of Au/TiO\u003csub\u003e2 \u003c/sub\u003eand Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X. \u003cstrong\u003eg\u003c/strong\u003e, UV-Vis DRS of Au/TiO\u003csub\u003e2 \u003c/sub\u003eand Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/620a073fa7350ce3388327c7.png"},{"id":85038928,"identity":"5d2b9c24-f9d3-45c9-a069-a841b1bb5846","added_by":"auto","created_at":"2025-06-20 08:57:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":813157,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCatalytic performance of photocatalytic methane oxidation. a\u003c/strong\u003e, Schematics of products formation from CH\u003csub\u003e4\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X. \u003cstrong\u003eb\u003c/strong\u003e, Yields of oxygenates from CH\u003csub\u003e4\u003c/sub\u003e oxidation over different photocatalysts.\u003cstrong\u003e c\u003c/strong\u003e, H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production over different photocatalysts. Reaction conditions in (b) and (c): 10 mg of catalysts, 100 mL of H\u003csub\u003e2\u003c/sub\u003eO, 2.5 MPa CH\u003csub\u003e4\u003c/sub\u003e, 1 MPa O\u003csub\u003e2\u003c/sub\u003e, 1 h reaction time, reaction temperature of 25 ± 3 °C, light source of 365 nm UV LED, 80 mW/cm\u003csup\u003e2\u003c/sup\u003e. \u003cstrong\u003ed\u003c/strong\u003e, O\u003csub\u003e2\u003c/sub\u003e partial pressure dependence on Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-2.6. \u003cstrong\u003ee\u003c/strong\u003e, Productivity assessment of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 obtained at different light intensities.\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/cb529450201e9413387d16a7.png"},{"id":85037473,"identity":"f9e9cefe-80ac-4b4d-bdd5-8258d03ebec7","added_by":"auto","created_at":"2025-06-20 08:41:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":770124,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGeneralization of the photocatalyst confinement strategy. a\u003c/strong\u003e, Oxygenated product and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e yields of photocatalytic methane oxidation over core-shell photocatalysts with different metal co-catalysts and semiconductors. Reaction conditions: 10 mg catalysts, 100 mL of H\u003csub\u003e2\u003c/sub\u003eO, 2.5 MPa CH\u003csub\u003e4\u003c/sub\u003e, 1 MPa O\u003csub\u003e2\u003c/sub\u003e, 1 h reaction time, reaction temperature of 25 ± 3 °C, light source: 365 nm UV LED, 80 mW/cm\u003csup\u003e2\u003c/sup\u003e. \u003cstrong\u003eb\u003c/strong\u003e, Summary of oxygenates yields and AQY over reported benchmark photocatalyst for photocatalytic CH\u003csub\u003e4\u003c/sub\u003e oxidation with O\u003csub\u003e2\u003c/sub\u003e at room temperature. The list of catalysts and corresponding reference are as follows: Cu-def-WO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e51\u003c/sup\u003e; Au-CoO\u003csub\u003ex\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e37\u003c/sup\u003e; Au/In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e14\u003c/sup\u003e; N-doped TiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e52\u003c/sup\u003e; Ni-NC/TiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e53\u003c/sup\u003e; TiO\u003csub\u003e2\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e-AuPd\u003csup\u003e20\u003c/sup\u003e; Pd/def-In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e54\u003c/sup\u003e; Pd/WO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e55\u003c/sup\u003e; Au-Pd/ZnO\u003csup\u003e17\u003c/sup\u003e; Defective TiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e56\u003c/sup\u003e; q-BiVO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e21\u003c/sup\u003e; Au-Cu/ZnO\u003csup\u003e15\u003c/sup\u003e; AC-Co\u003csub\u003e1\u003c/sub\u003e/PCN\u003csup\u003e57\u003c/sup\u003e; Ru\u003csub\u003e1\u003c/sub\u003eO\u003csub\u003ex\u003c/sub\u003e/ZnO\u003csup\u003e18\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/7592361af82d421487df1b06.png"},{"id":85037866,"identity":"afd0a9a0-4ab6-4aa1-99c5-2ad7301e8da4","added_by":"auto","created_at":"2025-06-20 08:49:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1908255,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eKey ROS detection for photocatalytic CH\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e oxidation. a, \u003c/strong\u003eIn situ EPR spectra of DMPO‑OH for monitoring the generation of •OH radicals over Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X. \u003cstrong\u003eb\u003c/strong\u003e, Correlation between CH\u003csub\u003e4\u003c/sub\u003e conversion rate with •OH radical production rate over different photocatalyst. \u003cstrong\u003ec\u003c/strong\u003e, \u003csup\u003e18\u003c/sup\u003eO isotopic studies on the origins of •OH radicals over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7, with benzoic acid used as a probe to capture •OH in situ. \u003cstrong\u003ed, \u003c/strong\u003eIn situ EPR spectra of DMPO‑OOH for monitoring the generation of •OOH active species over Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X. \u003cstrong\u003ee, \u003c/strong\u003eCorrelation between H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation and •OOH radical production rate over different photocatalyst. \u003cstrong\u003ef\u003c/strong\u003e, \u003csup\u003e18\u003c/sup\u003eO isotopic studies on the origins of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/e4b9d391019a66e181cbc8f8.png"},{"id":85039036,"identity":"68dee804-8695-4fa2-bd8e-acc546a5772a","added_by":"auto","created_at":"2025-06-20 09:05:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2345313,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eH/D KIE and in situ DRIFTS studies over Au/TiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e@pSiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e catalysts. a, \u003c/strong\u003eComparison in •OH radical production over Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 measured in H\u003csub\u003e2\u003c/sub\u003eO and D\u003csub\u003e2\u003c/sub\u003eO. \u003cstrong\u003eb\u003c/strong\u003e, Comparison in CH\u003csub\u003e4\u003c/sub\u003e conversion rate over Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 measured in H\u003csub\u003e2\u003c/sub\u003eO and D\u003csub\u003e2\u003c/sub\u003eO. \u003cstrong\u003ec\u003c/strong\u003e, Comparison in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation rate over Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 measured in H\u003csub\u003e2\u003c/sub\u003eO and D\u003csub\u003e2\u003c/sub\u003eO. \u003cstrong\u003ed\u003c/strong\u003e, DRIFTS analysis of the OH stretching band of the water in different photocatalyst. \u003cstrong\u003ee\u003c/strong\u003e, DRIFTS analysis of the HOH bending band of the water in different photocatalyst. \u003cstrong\u003ef\u003c/strong\u003e, Spectroscopically observable hydrogen-bonding configurations of H\u003csub\u003e2\u003c/sub\u003eO. \u003cstrong\u003eg\u003c/strong\u003e, Mean H-bonding state of water in Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X samples.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/c1c2d5ec0db0640aca544084.png"},{"id":85037863,"identity":"57c57b33-8c61-4581-81e5-02473d96c730","added_by":"auto","created_at":"2025-06-20 08:49:13","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1438567,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eProposed mechanistic pathways and the role of confined water.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/d77ea0abc7a9ce1769bf9dcb.png"},{"id":103810785,"identity":"d8c084a5-9983-44e9-8546-3d6d98aabf2d","added_by":"auto","created_at":"2026-03-03 08:12:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":27133941,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/12b1fa3f-3428-4ff3-91ea-3d5b9ccff4e1.pdf"},{"id":85037489,"identity":"31cc1a13-1de4-4784-9083-7030e939bead","added_by":"auto","created_at":"2025-06-20 08:41:13","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":9750570,"visible":true,"origin":"","legend":"Supplementary information","description":"","filename":"SIfinal041725.docx","url":"https://assets-eu.researchsquare.com/files/rs-6469567/v1/211ded878a389062712eb9ab.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eSimultaneous Enhancement of Photocatalytic CH\u003csub\u003e4\u003c/sub\u003e Conversion and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e Production through Water Confinement in Nanoporous Core-Shell Catalysts\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAqueous photochemical processes are vital for energy conversion and environmental applications, such as water splitting\u003csup\u003e1\u003c/sup\u003e, O\u003csub\u003e2\u003c/sub\u003e/CO\u003csub\u003e2\u003c/sub\u003e reduction\u003csup\u003e2\u003c/sup\u003e, and the removal of organic pollutants\u003csup\u003e3\u003c/sup\u003e. These processes harness photon energy to carry out chemical reactions under mild conditions and typically involve multi-electron and multi-proton transfer steps that generate reactive oxygen species as key intermediates\u003csup\u003e3-5\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAmong these processes, photocatalytic methane oxidation has emerged as a green route to directly transform abundant and inexpensive natural gas into valuable fuels and chemical feedstocks, such as methanol\u003csup\u003e6-8\u003c/sup\u003e. This approach utilizes photon energy to overcome the high activation barriers of CH\u003csub\u003e4\u003c/sub\u003e, enabling its conversion at room temperature via hydroxyl radicals (\u0026bull;OH) and bypassing the energy-intensive syngas production route\u003csup\u003e6,9\u003c/sup\u003e. Moreover, the aqueous environment offers unique advantages in improving product selectivity by facilitating the desorption of partially oxygenated products (e.g., methanol and formaldehyde) from the catalyst surface\u003csup\u003e9,10\u003c/sup\u003e. Despite extensive research has focused on developing catalysts to enhance charge separation and regulate surface catalytic reactions through band structure engineering\u003csup\u003e11-13\u003c/sup\u003e and co-catalyst modification\u003csup\u003e9,14-18\u003c/sup\u003e, the potential of tuning the aqueous microenvironment in the vicinity of semiconductor to boost CH\u003csub\u003e4\u003c/sub\u003e conversion remains largely unexploited.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAqueous-phase photocatalytic CH\u003csub\u003e4\u003c/sub\u003e oxidation involves a complex network of electron transfer (ET) and proton transfer (PT) reactions at the semiconductor-water interface (\u003cstrong\u003eSupplementary Scheme 1\u003c/strong\u003e), where water serves as a reactant, proton source, and solvation environment\u003csup\u003e19-21\u003c/sup\u003e. For hydrophilic oxide semiconductors such as TiO\u003csub\u003e2\u003c/sub\u003e and ZnO, water acts as the primary hole acceptor, generating \u0026bull;OH through proton and electron transfer pathways\u003csup\u003e9,20\u003c/sup\u003e. These \u0026bull;OH radicals activate methane through hydrogen abstraction, forming methyl radicals (\u0026bull;CH\u003csub\u003e3\u003c/sub\u003e). Simultaneously, oxygen is reduced to superoxide radicals (\u0026bull;OOH),\u0026nbsp;also involving electron and proton transfer, which further contribute to the conversion of \u0026bull;CH\u003csub\u003e3\u003c/sub\u003e into oxygenates\u003csup\u003e15\u003c/sup\u003e. These ET and PT events may occur either stepwise or concertedly via proton-coupled electron transfer (PCET) mechanisms, which are highly sensitive to the solvent environment due to the significant energy required for solvent reorganization induced by charge separation, especially in polar solvents like water\u003csup\u003e22,23\u003c/sup\u003e. Therefore, designing catalyst that modulate the water structure near the semiconductor interface presents a promising avenue for tuning proton/electron transfer pathways and optimizing methane oxidation kinetics.\u003c/p\u003e\n\u003cp\u003eSpatial confinement offers an effective strategy to tune the structure of water\u003csup\u003e22,24\u003c/sup\u003e, and consequently, the dynamics of these interfacial ET and PT processes\u003csup\u003e25\u003c/sup\u003e. Compared to bulk water, confined water exhibits distinct hydrogen-bonding (HB) networks, enthalpic and entropic characteristics, and electronic properties\u003csup\u003e25,26\u003c/sup\u003e. While theoretical and model reaction studies of electrocatalytic half-reactions including H\u003csub\u003e2\u003c/sub\u003e revolution/CO\u003csub\u003e2\u003c/sub\u003e reduction\u003csup\u003e27-33\u003c/sup\u003e, and photochemical water splitting\u003csup\u003e34-36\u003c/sup\u003e have hinted at the potential of tuning water structure for promoting catalytic performance, its application to complex photocatalytic reactions such as methane oxidation remains largely unexplored.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHerein, we demonstrate that confining interfacial water near TiO\u003csub\u003e2\u003c/sub\u003e within silica nanopores of core-shell Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e catalysts provides a powerful means to modulate its structure and reactivity, and consequently, the PCET pathways. Decreasing the silica pore size from 3.1 nm to 1.7 nm leads to an approximately three-fold enhancement in methane conversion, and surprisingly, a 22-fold increase in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production compared to Au/TiO\u003csub\u003e2\u003c/sub\u003e. The Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e with 1.7 nm pores exhibits a high liquid oxygenates yield of 27.4 mmol g\u003csup\u003e-1\u003c/sup\u003e h\u003csup\u003e-1\u003c/sup\u003e with 97.2 % selectivity, and 44.6 mmol g\u003csup\u003e-1\u003c/sup\u003e h\u003csup\u003e-1\u003c/sup\u003e for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production under 2.5 MPa CH\u003csub\u003e4\u003c/sub\u003e and 1 MPa O\u003csub\u003e2\u003c/sub\u003e at ambient temperature. This confinement strategy is broadly applicable to other metal co-catalysts and semiconductors, with Pt/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 catalyst reaching maximum oxygenate yields of 32.7 mmol g\u003csup\u003e-1\u003c/sup\u003e h\u003csup\u003e-1\u003c/sup\u003e and an apparent quantum yield (AQY) of 14.1% at 365 nm, surpassing other reported TiO\u003csub\u003e2\u003c/sub\u003e-based photocatalysts. Mechanistic investigation, including H/D KIE, in situ electron paramagnetic resonance (EPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and \u003csup\u003e18\u003c/sup\u003eO isotope labeling studies, reveal that the weakened HB network of water confined within the silica pores modulates PCET pathways for both water oxidation reaction (WOR) and oxygen reduction reaction (ORR).\u0026nbsp;Specifically, WOR shifts from a stepwise PCET to a concerted mechanism, enhancing \u0026bull;OH radical production for CH\u003csub\u003e4\u003c/sub\u003e conversion, while ORR shifts from a 1e\u003csup\u003e-\u003c/sup\u003e to a 2e\u003csup\u003e-\u003c/sup\u003e process, promoting H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eThese findings underscore the potential of engineering interfacial water structures through spatial confinement to enhance photocatalytic activity and selectivity.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e\n \u003ch2\u003eSynthesis and characterization of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e catalysts\u003c/h2\u003e\n \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e is a well-established semiconductor for photocatalytic methane oxidation, with a valence band position sufficient for oxidizing water to \u0026bull;OH radicals\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Noble metal cocatalyst modification is known to enhance CH\u003csub\u003e4\u003c/sub\u003e conversion\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. Therefore, we selected Au/TiO\u003csub\u003e2\u003c/sub\u003e as the model catalyst to study the effect of water confinement, which can be achieved by installing water permeable, nanoporous enclosures to the Au/TiO\u003csub\u003e2\u003c/sub\u003e pore. We chose SiO\u003csub\u003e2\u003c/sub\u003e enclosure due to its facile growth control, controllable pore size, optical transparency in the UV region, and good stability. Furthermore, our previous studies have shown that a thin, non-porous silica shell can prevent oxygenates overoxidation while still allowing \u0026bull;OH radicals transport and convert CH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e20\u003c/sup\u003e, indicating good compatibility of silica with the TiO\u003csub\u003e2\u003c/sub\u003e-based photocatalysts.\u003c/p\u003e\n \u003cp\u003eGold nanoparticles (NPs) were loaded onto TiO\u003csub\u003e2\u003c/sub\u003e (P25) via NaBH\u003csub\u003e4\u003c/sub\u003e reduction\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e, and nanoporous silica shell was constructed on Au/TiO\u003csub\u003e2\u003c/sub\u003e using the St\u0026ouml;ber method with alkylammonium surfactants as templates (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea) \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. By varying surfactant carbon chain length or adding auxiliary organics\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e, we obtained Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X with different pore sizes (X\u0026thinsp;=\u0026thinsp;pore size). The surface area and porosity of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X were characterized by nitrogen physisorption (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee, \u003cstrong\u003eSupplementary Table\u0026nbsp;1\u003c/strong\u003e). Using C\u003csub\u003e16\u003c/sub\u003eTAB resulted in mesopores\u0026thinsp;~\u0026thinsp;2.6 nm, while a shorter surfactant, C\u003csub\u003e12\u003c/sub\u003eTAB, yielded smaller pores around 1.7 nm (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ef). Adding hexane as pore-expanding agent yielded larger pores around 3.1 nm. For comparison, Au/TiO\u003csub\u003e2\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e without nanopores were synthesized similarly but without CTAB templates (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;1\u003c/strong\u003e).\u003c/p\u003e\n \u003cp\u003eTransmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) confirmed full encapsulation of Au/TiO\u003csub\u003e2\u003c/sub\u003e by a uniform, amorphous SiO\u003csub\u003e2\u003c/sub\u003e shell across all Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X catalysts (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb-d, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;2\u0026ndash;4\u003c/strong\u003e). The silica layer had an average thickness of 8 nm (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb-d), which is tunable by varying tetraethyl orthosilicate (TEOS) (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;5\u003c/strong\u003e). Au NPs ranging 2 6 nm were randomly dispersed on TiO\u003csub\u003e2\u003c/sub\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb-d, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;3\u003c/strong\u003e), with a loading of ~\u0026thinsp;0.1 wt% (\u003cstrong\u003eSupplementary Table\u0026nbsp;2\u003c/strong\u003e). UV-visible diffusive reflective spectra (UV-Vis DRS) of Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X were nearly identical (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eg), suggesting minimal effect of the SiO\u003csub\u003e2\u003c/sub\u003e coating on light absorption. X-ray photoelectron spectra (XPS) showed similar Ti oxidation states among all samples (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;6\u003c/strong\u003e). These results indicate that the nanoporous coating does not significantly modify the intrinsic properties of the semiconductors.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003ePore-size dependent CH\u003csub\u003e4\u003c/sub\u003e photocatalytic oxidation by Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X\u003c/h2\u003e\n \u003cp\u003ePhotocatalytic reactions were conducted in a batch reactor with 100 mL of distilled water under 2.5 MPa CH\u003csub\u003e4\u003c/sub\u003e and 1 MPa O\u003csub\u003e2\u003c/sub\u003e at 25 \u0026ordm;C (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;7, 8\u003c/strong\u003e). Control experiments, including dark and catalyst-free controls (\u003cstrong\u003eSupplementary Table\u0026nbsp;3\u003c/strong\u003e), and a \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eCH\u003csub\u003e4\u003c/sub\u003e isotope-labeling study (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;9\u003c/strong\u003e), validated that oxygenates were generated from photocatalytic methane oxidation over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e. Compared to bare Au/TiO\u003csub\u003e2\u003c/sub\u003e, Au/TiO\u003csub\u003e2\u003c/sub\u003e with a nanoporous silica shell showed significantly enhanced oxygenates yields, which increased with decreasing pore size (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb). Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-3.1 exhibited total oxygenates yields of 18.0 mmol g⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e h⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, with a selectivity of 92.5%. Further reducing the pore size to 1.7 nm led to greater oxygenates yields, achieving 27.4 mmol g⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e h⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e \u0026mdash; approximately triple that of Au/TiO\u003csub\u003e2\u003c/sub\u003e. The AQY for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 was 10.9% at 365 nm (\u003cstrong\u003eSupplementary Table\u0026nbsp;4\u003c/strong\u003e), substantially exceeding Au/TiO\u003csub\u003e2\u003c/sub\u003e (3.3%). Notably, Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e also produced an unexpectedly high amount of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e as a valuable byproduct. The H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production also increased with decreasing pore size, reaching 44.6 mmol g⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e h⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea, c), which is 22 times higher than Au/TiO\u003csub\u003e2\u003c/sub\u003e and comparable to the state-of-the-art catalysts (\u003cstrong\u003eSupplementary Table\u0026nbsp;5\u003c/strong\u003e).\u003c/p\u003e\n \u003cp\u003eA physical mixture of Au/TiO\u003csub\u003e2\u003c/sub\u003e and porous SiO\u003csub\u003e2\u003c/sub\u003e (pSiO\u003csub\u003e2\u003c/sub\u003e-1.7) showed similar catalytic performance to Au/TiO\u003csub\u003e2\u003c/sub\u003e alone, highlighting the importance of the core-shell architecture (\u003cstrong\u003eSupplementary Figs.\u0026nbsp;10 and 11\u003c/strong\u003e). The thickness of silica also plays a pivotal role: an 8 nm silica shell was optimal, while a 30 nm shell reduced yields (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;12\u003c/strong\u003e), likely due to an increased diffusion barrier. It is noteworthy that Au/TiO\u003csub\u003e2\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e without pores achieved a higher yield than Au/TiO\u003csub\u003e2\u003c/sub\u003e but lower than the nanoporous catalysts (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb), likely due to the greater diffusion barrier in non-porous silica. This performance trend signifies the importance of controlling silica pore size for efficient methane conversion.\u003c/p\u003e\n \u003cp\u003eThe effect of O\u003csub\u003e2\u003c/sub\u003e pressure on methane conversion rate is also studied in detail. \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO isotopic labelling confirmed the direct participation of O\u003csub\u003e2\u003c/sub\u003e in forming oxygenated products (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;13\u003c/strong\u003e). Under anerobic conditions, both Au/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X exhibited low yield, which increased with O\u003csub\u003e2\u003c/sub\u003e pressure (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;14\u003c/strong\u003e). Au/TiO\u003csub\u003e2\u003c/sub\u003e achieved maximum yield at 0.042 MPa O\u003csub\u003e2\u003c/sub\u003e, whereas Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-2.6 and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 required higher O\u003csub\u003e2\u003c/sub\u003e pressures (up to 1 MPa) (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;14a, b)\u003c/strong\u003e, suggesting hindered O\u003csub\u003e2\u003c/sub\u003e transport by the silica shell (\u003cstrong\u003eSupplementary Note 1\u003c/strong\u003e). Furthermore, TiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e catalysts required higher O\u003csub\u003e2\u003c/sub\u003e pressures to attain optimal yields compared to their Au-incorporated counterparts (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;14c-d, Fig.\u0026nbsp;15\u003c/strong\u003e), and relocating Au NPs to the SiO\u003csub\u003e2\u003c/sub\u003e surface decreased product yields (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;16\u003c/strong\u003e). These results indicate that the enhanced performance of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e is not due to O\u003csub\u003e2\u003c/sub\u003e enrichment from its high surface area. Instead, O\u003csub\u003e2\u003c/sub\u003e transport within silica shell is hindered in aqueous solution but can be promoted by the supported Au co-catalysts.\u003c/p\u003e\n \u003cp\u003eFurther optimization of oxygenate yields was carried out by varying light intensity and increasing water volume (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;17\u003c/strong\u003e). By increasing the light flux to 500 mW/cm\u003csup\u003e2\u003c/sup\u003e, we achieved a yield of 62.9 mmol g⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e h⁻\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e with 80% selectivity for oxygenates (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee). Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 also exhibited excellent stability over four cycles with unchanged production and selectivity (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;18\u003c/strong\u003e). XRD and TEM showed no obvious structure change in the spent catalyst (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;19\u003c/strong\u003e), and N\u003csub\u003e2\u003c/sub\u003e adsorption isotherms showed similar pore size distribution and surface area to fresh catalysts (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;20\u003c/strong\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eGeneralization of the photocatalyst confinement strategy\u003c/h3\u003e\n\u003cp\u003eWe explored this confinement strategy with different metal cocatalysts and semiconductors. Substituting Au with Pd, Pt or Rh exhibited a similar trend: confined pores around metalTiO\u003csub\u003e2\u003c/sub\u003e surface significantly enhance oxygenates and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e yields compared to bare metalTiO\u003csub\u003e2\u003c/sub\u003e catalysts, with smaller pores yielding higher productions (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, \u003cstrong\u003eSupplementary Figs.\u0026nbsp;21 and 22, Supplementary Note 2\u003c/strong\u003e). Among these, Pt/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 achieved the highest oxygenate yields of 32.7 mmol g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and an AQY of 1.1 % for CH\u003csub\u003e4\u003c/sub\u003e oxidation at 365 nm, which outperforms the previous benchmark (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb, \u003cstrong\u003eSupplementary Table\u0026nbsp;4, 6\u003c/strong\u003e). We further extended our study to other semiconductors, including BiVO\u003csub\u003e4\u003c/sub\u003e and anatase TiO\u003csub\u003e2\u003c/sub\u003e (ATiO\u003csub\u003e2\u003c/sub\u003e) (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;23\u0026ndash;25, Supplementary Note 2)\u003c/strong\u003e. Applying the porous silica coating improved oxygenate yields and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production for both catalysts. For example, Au/ATiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 achieved oxygenates and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e formation rate of 22.0 and 23.6 mmol g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, significantly surpassing Au/ATiO\u003csub\u003e2\u003c/sub\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea). These results underscore the versatility of the nanoporous silica core-shell confinement design, demonstrating its broad applicability across different photocatalyst systems.\u003c/p\u003e\n\u003ch3\u003e\u003cstrong\u003eKey reactive oxygen species (ROS) for photocatalytic CH\u003csub\u003e4\u003c/sub\u003e oxidation\u003c/strong\u003e\u003c/h3\u003e\n\u003cp\u003eWe first investigated the role of SiO\u003csub\u003e2\u003c/sub\u003e shell in photocarrier separation using steady-state photoluminescence (PL) and photocurrent experiments. PL measurements revealed that the silica shell has a negligible effect on photocarrier separation (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;26a\u003c/strong\u003e). Furthermore, Au/TiO\u003csub\u003e2\u003c/sub\u003e showed a higher photocurrent compared to Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;26b\u003c/strong\u003e), likely due to the low conductivity of silica coating. These results, together with previous findings regarding O\u003csub\u003e2\u003c/sub\u003e transport limitations through the porous silica shell (\u003cstrong\u003eSupplementary Note 1\u003c/strong\u003e), demonstrate that improved performance in Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e compared to Au/TiO\u003csub\u003e2\u003c/sub\u003e is not due to improved photocarrier separation or enhanced O\u003csub\u003e2\u003c/sub\u003e transport, but rather to effects associated with the structure of the confined water layers in contact with the semiconductors.\u003c/p\u003e\n\u003cp\u003eTo unravel the underlying mechanism, we next sought to identify the key ROS responsible for methane activation and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e formation within the confined environment. This can be achieved by introducing sacrificial agents that selectively quench different ROS and monitor the change in methane conversion\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. Both holes and \u0026bull;OH radicals are capable of activating methane\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. We used salicylic acid and (NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e to scavenge \u0026bull;OH radicals and holes (h\u003csup\u003e+\u003c/sup\u003e), respectively\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The addition of salicylic acid nearly completely inhibited CH\u003csub\u003e4\u003c/sub\u003e conversion, showing a stronger suppression than (NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;27\u003c/strong\u003e). This suggests that \u0026bull;OH radicals, rather than h⁺, are the main reactive species for CH\u003csub\u003e4\u003c/sub\u003e activation.\u003c/p\u003e\n\u003cp\u003eThe Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e catalyst with smaller pores produced more \u0026bull;OH radicals, as monitored by in situ EPR, which is consistent with their superior performance in CH\u003csub\u003e4\u003c/sub\u003e conversion (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea, b). Coumarin was also employed as a fluorescent probe to assess \u0026bull;OH production\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, which further supports the findings from EPR (\u003cstrong\u003eSupplementary Figs.\u0026nbsp;28 and 29\u003c/strong\u003e). Typically, there are two distinct pathways by which \u0026bull;OH radicals are formed: one involves H\u003csub\u003e2\u003c/sub\u003eO oxidation by holes (1e⁻ WOR), while the other involves O\u003csub\u003e2\u003c/sub\u003e reduction by electrons (ORR) \u003cstrong\u003e(Supplementary Scheme 1\u003c/strong\u003e). To identify the origin of \u0026bull;OH radicals, we performed \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO isotopic labeling with benzoic acid as a probe to capture \u0026bull;OH (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;30\u003c/strong\u003e). Under H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e16\u003c/sup\u003eO/\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e conditions, over 95% of products corresponded to H\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003eOC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eCO\u003csub\u003e2\u003c/sub\u003eH (m/z\u0026thinsp;=\u0026thinsp;137), while with H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO/\u003csup\u003e16\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, approximately 80% of products corresponded to H\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eOC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eCO\u003csub\u003e2\u003c/sub\u003eH (m/z\u0026thinsp;=\u0026thinsp;139). This analysis revealed that \u0026bull;OH radical primarily originate from 1e⁻ WOR rather than from O\u003csub\u003e2\u003c/sub\u003e reduction over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;31\u003c/strong\u003e). In contrast, for Au/TiO\u003csub\u003e2\u003c/sub\u003e, approximately half of the \u0026bull;OH radicals derive from H\u003csub\u003e2\u003c/sub\u003eO oxidation and the other half from O\u003csub\u003e2\u003c/sub\u003e reduction (\u003cstrong\u003eSupplementary Figs.\u0026nbsp;32 and 33)\u003c/strong\u003e. Thus, the enhanced 1e⁻ WOR to \u0026bull;OH radicals within the confined SiO\u003csub\u003e2\u003c/sub\u003e pores is responsible for enhanced CH\u003csub\u003e4\u003c/sub\u003e conversion over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eEPR spectra of DMPO-OOH show that Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e with smaller pores produced less \u0026bull;OOH radicals (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed), consistent with their enhanced H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production as the main product in ORR (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ee). Electron scavenger experiments using AgNO\u003csub\u003e3\u003c/sub\u003e or NaIO\u003csub\u003e3\u003c/sub\u003e and \u003csup\u003e18\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO isotope labelling confirmed that H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e is generated exclusively through ORR (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ef, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;34\u0026ndash;36\u003c/strong\u003e). The ORR selectivity to H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e is 75% over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7, compared to 11% over Au/TiO\u003csub\u003e2\u003c/sub\u003e (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;37, Supplementary Note 3\u003c/strong\u003e). The high H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production and negligible \u0026bull;OOH production over Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e suggests that the silica nanopores confinement alters the ORR pathway from a 1e⁻ process to a 2e⁻ process.\u003c/p\u003e\n\u003cp\u003eThus, facilitated 1e⁻ WOR producing \u0026bull;OH and an altered ORR pathway are key features of Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e responsible for its superior catalytic performance. Moreover, similar enhancements in \u0026bull;OH production from 1e⁻ WOR and altered ORR pathways were observed with Pt/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e catalysts (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;38\u0026ndash;40, Supplementary Note 2\u003c/strong\u003e). These results indicate this alteration is a general phenomenon when semiconductors are spatially confined within nanopores. We hypothesize that both the enhanced 1e⁻ WOR and the modified ORR pathway result from changes in the water structure within these confined spaces compared to bulk water.\u003c/p\u003e\n\u003ch3\u003e\u003cstrong\u003eKIE investigations for CH\u003csub\u003e4\u003c/sub\u003e oxidation process\u0026nbsp;\u003c/strong\u003e\u003c/h3\u003e\n\u003cp\u003eH/D kinetic isotope effect (KIE) can be used to analyze whether PT is involved in the rate-determining step (RDS) \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e, which would further shed light on the mechanism of altered reactivity and selectivity. For the 1e⁻ WOR that produces \u0026bull;OH radicals, Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7 shows a pronounced KIE value of 2.0\u0026ndash;2.3, indicating a strong involvement of PT in the RDS. In contrast, Au/TiO\u003csub\u003e2\u003c/sub\u003e exhibit a low KIE value of 1.1\u0026ndash;1.2 (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea, \u003cstrong\u003eSupplementary Figs.\u0026nbsp;41 and 42\u003c/strong\u003e), suggesting a distinctly different 1e⁻ WOR pathway. The absence of KIE for Au/TiO\u003csub\u003e2\u003c/sub\u003e is consistent with the widely accepted stepwise proton-electron transfer (SPET) mechanism\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e, where water readily deprotonates on TiO\u003csub\u003e2\u003c/sub\u003e before being oxidized by photogenerated hole, making the electron transfer step (OH⁻ + h\u003csup\u003e+\u003c/sup\u003e \u0026rarr; \u0026bull;OH) the RDS. Conversely, the large KIE for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7 suggests that molecular water confined in the nanopores act directly as the hole acceptor, generating \u0026bull;OH radicals through a concerted proton-electron transfer (CPET) pathway (H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;h\u003csup\u003e+\u003c/sup\u003e \u003cstrong\u003e\u0026rarr;\u003c/strong\u003e \u0026bull;OH\u0026thinsp;+\u0026thinsp;H\u003csup\u003e+\u003c/sup\u003e). As a result, the slower dissociation and deuteron transfer in D\u003csub\u003e2\u003c/sub\u003eO compared to H\u003csub\u003e2\u003c/sub\u003eO greatly reduce the \u0026bull;OH radical production.\u003c/p\u003e\n\u003cp\u003eAt a molecular level, the difference between the SPET pathway for Au/TiO\u003csub\u003e2\u003c/sub\u003e and CPET pathway for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7 lies in whether the photogenerated holes prefer to react with molecular water or hydroxide anion. The SPET mechanism for Au/TiO\u003csub\u003e2\u003c/sub\u003e indicates h\u003csup\u003e+\u003c/sup\u003e prefers to react with hydroxide anion despite the greater abundance of water molecules. This can be rationalized by hydroxide anion having greater reactivity towards h\u003csup\u003e+\u003c/sup\u003e due its negative charge. Consequently, the shift to a CPET mechanism in Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7 upon water confinement within the nanopores indicates that confined water possesses higher reactivity towards h\u003csup\u003e+\u003c/sup\u003e than bulk water. This enhanced reactivity can be attributed to the weakened HB network and reduced solvation effect in confined water \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. On the other hand, the weakened HB network also facilitates proton transfer\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e, further promoting the CPET pathway. The higher \u0026bull;OH production observed with Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7 compared to Au/TiO\u003csub\u003e2\u003c/sub\u003e suggests that the CPET mechanism, directly utilizing molecular water to react with h\u003csup\u003e+\u003c/sup\u003e, provides a more effective route for generating the \u0026bull;OH radicals necessary for CH\u003csub\u003e4\u003c/sub\u003e activation compared to the two-step SPET pathway. Thus, confinement-driven changes in water reactivity and PCET pathways are key to enhanced CH\u003csub\u003e4\u003c/sub\u003e conversion for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7 catalysts.\u003c/p\u003e\n\u003cp\u003eAs the WOR producing \u0026bull;OH is key to CH\u003csub\u003e4\u003c/sub\u003e activation, its KIE influences the KIE of overall CH\u003csub\u003e4\u003c/sub\u003e oxidation. Consequently, Au/TiO\u003csub\u003e2\u003c/sub\u003e does not show KIE effect for CH\u003csub\u003e4\u003c/sub\u003e conversion and Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 show KIE value of 1.3\u0026ndash;1.6 (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;43\u0026ndash;46\u003c/strong\u003e). The KIE decrease over time for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7, likely due to H/D exchange between CH\u003csub\u003e4\u003c/sub\u003e and D\u003csub\u003e2\u003c/sub\u003eO during reactions (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;46, Supplementary Notes 4\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eOn the ORR side, a large KIE of 2.97 is also observed for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;1.7 (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec), which is expected for the 2e⁻ process that produces H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e from oxygen and proton. In stark contrast, a reverse KIE for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production was observed over Au/TiO\u003csub\u003e2\u003c/sub\u003e, indicating that the confined water significantly modulates the PT and ET dynamics. Notably, the observed shift in ORR pathway from the 1e⁻ process for Au/TiO\u003csub\u003e2\u003c/sub\u003e to the 2e⁻ process for Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e can also be attributed to facilitated proton transfer due to water confinement. Compared to the 1e⁻ process, the 2e⁻ process is thermodynamically more viable but requires simultaneous transfer of two protons. Therefore, more efficient proton transfer, along with increased protons generated from the enhanced WOR process, are responsible for the shift to 2e⁻ process and enables efficient production of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\n\u003ch3\u003eConfined water characterization\u003c/h3\u003e\n\u003cp\u003eThe weakened HB network of the confined water was analyzed by in situ DRIFTS measurement at various relative humidity (RH) levels (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed-g, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;47\u0026ndash;48\u003c/strong\u003e). The broad OH stretching band (2800\u0026thinsp;\u0026minus;\u0026thinsp;3600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is sensitive to the HB network configuration and can be deconvoluted into five Gaussian sub-bands (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ef) \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. From Au/TiO\u003csub\u003e2\u003c/sub\u003e to Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7, water in smaller pore shows a significantly larger portion of DA to DDAA ratio along with lower average number of HBs (\u0026lt;\u0026thinsp;N\u003csub\u003eHB\u003c/sub\u003e\u0026gt;) (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eg, \u003cstrong\u003eSupplementary Table\u0026nbsp;7\u003c/strong\u003e), indicating a weakened HB network. A similar trend was observed in the HOH bending (1500\u0026ndash;1800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) region (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee), where deconvolution of the spectra can give the ratio between two peaks at 1620 cm⁻\u0026sup1; and 1680 cm⁻\u0026sup1; attributed to DA and DDAA water clusters\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. These results suggest that H\u003csub\u003e2\u003c/sub\u003eO molecules within smaller pores adopt a more linear arrangement with a weaker HB network, leading to properties that are more isolated and prone to oxidation, improved proton conductivity favorable to PT processes, and reduced solvation that better stabilizes the semi-hydrophobic \u0026bull;OH radicals.\u003c/p\u003e\n\u003cp\u003eTherefore, both enhanced methane conversion and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production of the Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e can be attributed to the weakened HB network of the confined water in porous silica (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e): (a) for h\u003csup\u003e+\u003c/sup\u003e driven WOR, confined water is less stabilized by HB and are thus more reactive, causing a shift from SPET to CPET mechanism that generates more \u0026bull;OH radicals to activate methane; (b) for e\u003csup\u003e\u0026minus;\u003c/sup\u003e driven ORR, confined water with weakened HB transfers proton more efficiently, enabling a shift from 1e\u003csup\u003e\u0026minus;\u003c/sup\u003e to 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e process that directly produces H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, we have demonstrated a core-shell photocatalyst design that leverages spatial confinement to modulate the interfacial water structure, leading to a significant enhancement in both CH\u003csub\u003e4\u003c/sub\u003e conversion and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production. The confined water with weaker HB bonds significantly modulates the PT and ET dynamics in reaction involving photogenerated electron and holes. Specifically, the confined water exhibits enhanced reactivity towards h\u003csup\u003e+\u003c/sup\u003e in WOR, causing a shift from SPET to CPET mechanism and generating more \u0026bull;OH radicals to activate methane. Concurrently, the weakened HB network facilitates more efficient proton transfer in ORR, enabling a shift from the 1e⁻ to the 2e⁻ process and directly producing H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. This study provides a fundamental understanding of how interfacial water structure can be engineered to optimize photocatalytic performance, opening new avenues for photocatalyst design.\u003c/p\u003e "},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of M/TiO\u003csub\u003e2\u003c/sub\u003e photocatalyst\u003c/h2\u003e \u003cp\u003eMetal cocatalyst (Au, Pt, Pd, Rh) were loaded onto TiO\u003csub\u003e2\u003c/sub\u003e (P25) using a simple NaBH\u003csub\u003e4\u003c/sub\u003e reduction method\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Typically, 50 mg of TiO\u003csub\u003e2\u003c/sub\u003e was ultrasonically dispersed in 30 mL of deionized water, after which a specific amount of metal precursor solution (HAuCl\u003csub\u003e3\u003c/sub\u003e\u0026bull;3H\u003csub\u003e2\u003c/sub\u003eO, H\u003csub\u003e2\u003c/sub\u003ePtCl\u003csub\u003e6\u003c/sub\u003e, PdCl\u003csub\u003e2\u003c/sub\u003e, Rh(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e) was added. Following 10 minutes of stirring, 2 mL of freshly prepared 0.1 M NaBH\u003csub\u003e4\u003c/sub\u003e was added. After an additional 30 minutes of stirring, the product was collected by centrifugation, washed with deionized water, and dried in an oven at 60 ℃.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of M/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-X\u003c/h2\u003e \u003cp\u003eTo synthesize M/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 (M denoted to Au, Pd, Pt, Rh), 100 mg of dodecyltrimethylammonium bromide (C\u003csub\u003e12\u003c/sub\u003eTAB) was used as the templating agents, which was dissolved in 10 mL of deionized water and 40 mL of anhydrous ethanol. Subsequently, 50 mg of M/TiO\u003csub\u003e2\u003c/sub\u003e was added, sonicated for 30 mins, followed by the addition of tetraethyl orthosilicate (TEOS). After 15 minutes of stirring, 0.5 mL ammonium hydroxide solution (28\u0026ndash;30%) was added, and the mixture was stirred overnight. The product was collected via centrifugation, washed twice with DI H\u003csub\u003e2\u003c/sub\u003eO and ethanol, dried at 60 ℃, and calcined at 550 ℃ for 2 h in static air to remove the C\u003csub\u003e12\u003c/sub\u003eTAB templates. TEOS volume controlled the SiO\u003csub\u003e2\u003c/sub\u003e thickness; 300 \u0026micro;L produced an 8 nm SiO\u003csub\u003e2\u003c/sub\u003e shell. For the M/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-2.6 sample, the synthesis process was identical, except cetyltrimethylammonium bromide (C\u003csub\u003e16\u003c/sub\u003eTAB) was used as the template. The preparation of M/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-3.1 followed the same procedure as M/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-2.6, with the exception that 10 mL of n-hexane was added to the mixture prior to the addition of TEOS.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization\u003c/h2\u003e \u003cp\u003eThe crystal structure of the catalysts was characterized using a powder X-ray diffractometer D2 (Bruker, Switzerland) with Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.542 \u0026Aring;). Morphologies of the photocatalysts were studied using TEM (JEM-1400plus). High-resolution TEM (HRTEM) images, HAADF-STEM images and corresponding EDS maps were collected using a JEOL JEM-F200 field-emission microscope operated at 200 kV. Nitrogen sorption isotherms were collected on a BELSORP-Max 2 apparatus. The metal loading in the photocatalysts was determined by inductively coupled plasma optical emission spectrometer (ICP-OES, Icap7400, Thermo). UV-Vis DRS were recorded on a Cary 5000 spectrometer (Agilent) equipped with an integrating sphere. The absorption spectra were obtained using the Kubelka\u0026ndash;Munk transformation. XPS were collected using a Thermo Fisher ESCALAB 250Xi spectrometer with monochromatic Al-Kα X-rays as the excitation source. All spectra were calibrated to the C 1s peak at 284.8 eV. Steady-state PL spectra were recorded on the Horiba Fluorolog-3 Spectrometer at room temperature with an excitation wavelength of 350 nm. Photocurrent tests were conducted with the electrochemical workstation (Gamry Instrument Interface 1010E) in three-electrode system with 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as electrolyte (Ag/AgCl as the reference electrode, platinum plate as the counter electrode and catalysts loaded on indium-tin oxide (ITO) as the working electrode).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003ePhotocatalytic activity measurements\u003c/h2\u003e \u003cp\u003ePhotocatalytic CH\u003csub\u003e4\u003c/sub\u003e oxidation reaction was conducted in a 250 mL high-pressure reactor with a sapphire window for light irradiation. Typically, 10 mg of catalyst was ultrasonically dispersed in 100 mL of deionized water for 30 min. The reactor was then sealed and purged with ultrapure argon (99.999%) to completely remove the air. Subsequently, the reactor vessel was pressurized with 1 MPa O\u003csub\u003e2\u003c/sub\u003e (99.999%) and 2.5 MPa CH\u003csub\u003e4\u003c/sub\u003e (99.999%). The reactor was irradiated from the top using an LED lamp (365 nm, PLS-LED100C, Perfectlight). The photocatalytic reaction was conducted for 1 hour at 25 \u003csup\u003eo\u003c/sup\u003eC. A thermocouple was inserted into the solution to directly detect the temperature of the solution.\u003c/p\u003e \u003cp\u003eThe photoreactor was directly connected to a gas chromatograph (Agilent GC 8890) equipped with a flame ionization detector (FID) and a methanizer for the gas product analysis of CO\u003csub\u003e2\u003c/sub\u003e and ethane. The liquid products (CH\u003csub\u003e3\u003c/sub\u003eOOH, CH\u003csub\u003e3\u003c/sub\u003eOH, and HCOOH) were analyzed by \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH-NMR (Bruker 400 MHz), with dimethyl sulfoxide (DMSO) added as an internal standard. The amount of HCHO was quantified using the acetylacetone color-development method\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e (details in Supplementary Methods). The concentration of produced H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was determined using the titanium oxalate spectrophotometric method (details in Supplementary Methods). Details of CH\u003csub\u003e4\u003c/sub\u003e conversion rate and oxygenates selectivity calculation, AQY calculation, catalyst recycle tests, and active species trapping experiments are provide in the Supplementary Methods.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eIsotope experiments for identifying the origin of products\u003c/h2\u003e \u003cp\u003eFor carbon source investigation with isotopic labelled \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eCH\u003csub\u003e4\u003c/sub\u003e: 10 mg Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e was dispersed in 5 mL H\u003csub\u003e2\u003c/sub\u003eO, and the reactor was purged with Ar to remove air. The photoreactor was then filled with 0.55 MPa \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eCH\u003csub\u003e4\u003c/sub\u003e (99%), 0.45 MPa \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003eCH\u003csub\u003e4\u003c/sub\u003e (99.999%) and 0.2 MPa O\u003csub\u003e2\u003c/sub\u003e (99.999%). After 2 h of light irradiation, gas products were analyzed by gas chromatograph-mass spectroscopy (GC-MS, Shimadzu), and liquid products by \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH-NMR.\u003c/p\u003e \u003cp\u003eFor oxygen source investigation with \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO isotope labeling with \u003csup\u003e18\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e16\u003c/sup\u003eO and \u003csup\u003e16\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO: 10 mg Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e was dispersed in 10 mL H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e16\u003c/sup\u003eO or (H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO), and the reactor was purged with Ar. The photoreactor was charged with 2.5 MPa CH\u003csub\u003e4\u003c/sub\u003e (99.999%) and 0.5 MPa \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e (98%) (or \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e). Products were analyzed by GC-MS after 2 h of light irradiation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eEPR measurements\u003c/h2\u003e \u003cp\u003eIn situ electron paramagnetic resonance (EPR) spectra were acquired using a Bruker ESR5000 spectrometer equipped with a 300 W xenon lamp (PLS-SXE300+/UV, Perfectlight) for illumination. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) served as the radical trap. For \u0026bull;OH radical detection, 5 mg of catalyst was dispersed in 5 mL of distilled H\u003csub\u003e2\u003c/sub\u003eO in the dark and purged with ultrapure argon (99.999 vol.%) for 2 min. For \u0026bull;OOH radical detection, 5 mg of catalyst was dispersed into a 5 mL of DMSO with O\u003csub\u003e2\u003c/sub\u003e bubbling for 2 min.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eAnalysis of photogenerated \u0026bull;OH radicals\u003c/h2\u003e \u003cp\u003eThe \u0026bull;OH production was measured using PL with coumarin, which reacts with \u0026bull;OH to form fluorescent 7-hydroxycoumarin (7-HC) (Supplementary Fig.\u0026nbsp;28). 2 mg catalyst was dispersed in 100 mL of 1.0 mM coumarin solution in dark, irradiated for 10 minutes, then centrifuged and analyzed with a Horiba FluoroLog-3 spectrofluorometer at 332 nm excitation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eInvestigation of the photochemical pathway of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production\u003c/h2\u003e \u003cp\u003eTo determine if H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production originates from WOR or ORR, liquid samples from \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e or H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO isotope CH\u003csub\u003e4\u003c/sub\u003e oxidation experiments were analyzed. As shown in Supplementary Fig.\u0026nbsp;36, 2 mL of the liquid samples was injected into a sealed vial (Vial A) and purged with He for 30 min. Another vial (Vial B), containing 200 mg of MnO\u003csub\u003e2\u003c/sub\u003e and 5 mL H\u003csub\u003e2\u003c/sub\u003eO, was also purged with He for 30 min. Then, 2 mL from Vial A was injected into Vial B. After complete decomposition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e over MnO\u003csub\u003e2\u003c/sub\u003e, 1 mL of gas from the headspace of Vial B was extracted for GC-MS analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003eInvestigation of the photochemical pathway for \u0026bull;OH generation\u003c/h2\u003e \u003cp\u003eTo analyze photochemical pathways of \u0026bull;OH production, \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e isotope experiments and H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO isotope experiments were conducted. In the \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e experiments, 2 mg of catalyst was dispersed in 1 mL of 1 mM benzoic acid solution (H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e16\u003c/sup\u003eO as solvent). The reactor was purged with Ar to remove air, and filled with 0.1 MPa \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e (98%). The reaction was carried out under UV irradiation for 600 s. The products were analyzed by liquid chromatography\u0026ndash;mass spectrometry (LC-MS, Thermofisher). Similarly, for the H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO isotope experiments, the setup was identical except using benzoic acid dissolved in H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e18\u003c/sup\u003eO and 0.1 MPa \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003eKIE measurements\u003c/h2\u003e \u003cp\u003eFor \u0026bull;OH radical generation, 5 mg of catalyst were dispersed in 2 mL of either D₂O or H₂O, and in situ EPR measurements were performed to track the formation of \u0026bull;OH radicals. For CH\u003csub\u003e4\u003c/sub\u003e conversion, 10 mg of catalysts were dispersed in 10 mL of D\u003csub\u003e2\u003c/sub\u003eO or H\u003csub\u003e2\u003c/sub\u003eO. The reactor vessel was then pressured to 2.5 MPa CH\u003csub\u003e4\u003c/sub\u003e and 1 MPa of O\u003csub\u003e2\u003c/sub\u003e, followed by light irradiation, and liquid products were collected at specific time intervals for analysis. For H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation, 10 mg of catalysts were dispersed in 10 mL of D\u003csub\u003e2\u003c/sub\u003eO or H\u003csub\u003e2\u003c/sub\u003eO. The reactor vessel was then pressured to 1 MPa of O\u003csub\u003e2\u003c/sub\u003e, followed by light irradiation. The resulting liquid product was collected for analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003eIn situ DRIFTS measurements\u003c/h2\u003e \u003cp\u003eIn situ DRIFTS measurements were performed using a Thermo Scientific iS50 FTIR spectrometer equipped with a liquid-nitrogen-cooled MCT/A detector. An infrared cell (Harrick) with ZnSe windows were used for measurements. Each spectrum was recorded by averaging 64 scans at a resolution of 4 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Samples were first pretreated at 673 K (5 K min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) for 30 min in flowing O\u003csub\u003e2\u003c/sub\u003e (20 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) to remove impurities adsorbed on the surface. Background spectra were then taken at 303 K in dry Ar. Wet Ar (Ar/H\u003csub\u003e2\u003c/sub\u003eO with controlled humidity) was subsequently introduced (20 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and spectra were acquired at the 303K. By using the following equation proposed by previous work\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, we determined the average number of hydrogen bonds of each water molecule:\u003c/p\u003e \u003cp\u003e\u0026lt;N\u003csub\u003eHB\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;=\u0026thinsp;4 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e A\u003csub\u003eDDAA\u003c/sub\u003e + 3 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e A\u003csub\u003eDDA\u003c/sub\u003e + 3 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e A\u003csub\u003eDAA\u003c/sub\u003e+ 2 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e A\u003csub\u003eDA\u003c/sub\u003e\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank Prof. P. Liu, Prof. B. Yang, Prof. Y. Huang, and Prof. P. Hu at ShanghaiTech University for the insightful discussion. We thank Prof. F. Yang and Prof. Y. Yang at ShanghaiTech University for help on FTIR experiments, and Dr. S. Chen for help on the EPR measurements. The TEM experiments are supported by the Center for High-resolution Electron Microscopy (CℏEM) at ShanghaiTech University. The NMR, UV-DRS, ICP-OES, N\u003csub\u003e2\u003c/sub\u003e physisorption, XPS, and PL measurements are supported by Analytical Instrumentation Center (#SPST-AIC10112914) at ShanghaiTech University. This work was financially supported by Shanghai Sailing Program (Nos. 23YF1426600) and start-up funding from ShanghaiTech University.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eC.X. conceived the project.\u0026nbsp;F. L. performed the catalyst preparation, characterization, and catalytic tests. X.-Y. W. performed the HR-TEM, HAADF and EDS characterization. Y.Y. and X.-Y. W. helped on TEM image analysis.\u0026nbsp;X.W. performed XPS experiments. C.X., F.L. wrote the manuscript. All authors contributed to the data discussion, and revision of the manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e: The authors declare no competing interests. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData and materials availability\u003c/strong\u003e: All data and images are available in the manuscript or the supplementary materials.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMaeda, K. \u0026amp; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. \u003cem\u003eJ. Phys. Chem. Lett.\u003c/em\u003e \u003cstrong\u003e1\u003c/strong\u003e, 2655-2661 (2010). https://doi.org/10.1021/jz1007966\u003c/li\u003e\n\u003cli\u003eLi, K., Peng, B. \u0026amp; Peng, T. 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Simultaneously Accelerating Carrier Transfer and Enhancing O2/CH4 Activation via Tailoring the Oxygen-Vacancy-Rich Surface Layer for Cocatalyst-Free Selective Photocatalytic CH4 Conversion. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 21069-21078 (2022). https://doi.org/10.1021/acsami.2c03671\u003c/li\u003e\n\u003cli\u003eDing, J.\u003cem\u003e et al.\u003c/em\u003e Asymmetrically coordinated cobalt single atom on carbon nitride for highly selective photocatalytic oxidation of CH\u003csub\u003e4\u003c/sub\u003e to CH\u003csub\u003e3\u003c/sub\u003eOH. \u003cem\u003eChem.\u003c/em\u003e \u003cstrong\u003e9\u003c/strong\u003e, 1017-1035 (2023). https://doi.org/https://doi.org/10.1016/j.chempr.2023.02.011\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6469567/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6469567/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAqueous photocatalytic CH\u003csub\u003e4\u003c/sub\u003e oxidation offers a promising route for converting natural gas into liquid oxygenates, a process governed by multi-electron and proton transfer steps at the catalyst-water interface. Here, we demonstrate that spatially confining interfacial water within Au/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e core-shell catalysts – achieved by systematically reducing silica pore size to 1.7 nm – remarkably increases CH\u003csub\u003e4\u003c/sub\u003e conversion threefold and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production 22-fold compared to Au/TiO\u003csub\u003e2\u003c/sub\u003e. This strategy is generalizable to other semiconductors and co-catalysts, with Pt/TiO\u003csub\u003e2\u003c/sub\u003e@pSiO\u003csub\u003e2\u003c/sub\u003e-1.7 exhibiting high oxygenate yields (32.7 mmol g\u003csup\u003e− 1\u003c/sup\u003e h\u003csup\u003e− 1\u003c/sup\u003e) and a 14.1% apparent quantum yield at 365 nm. Kinetic isotope effect (KIE), FTIR and EPR studies reveal that the water confined within pores, with a weakened H-bonding network, alters proton-coupled electron transfer (PCET) pathways. Water oxidation transits to a concerted PCET pathway, favoring •OH production for CH\u003csub\u003e4\u003c/sub\u003e conversion, while oxygen reduction shifts to a 2e\u003csup\u003e−\u003c/sup\u003e process, directly producing H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. This work highlights the potential of water confinement for designing efficient photocatalyst.\u003c/p\u003e","manuscriptTitle":"Simultaneous Enhancement of Photocatalytic CH4 Conversion and H2O2 Production through Water Confinement in Nanoporous Core-Shell Catalysts","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-20 08:41:08","doi":"10.21203/rs.3.rs-6469567/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b2ff38c1-c1e8-4503-bf98-ab94ab0cd61f","owner":[],"postedDate":"June 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":50336452,"name":"Physical sciences/Chemistry/Catalysis/Photocatalysis"},{"id":50336453,"name":"Physical sciences/Energy science and technology/Fossil fuels/Natural gas"},{"id":50336454,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis"}],"tags":[],"updatedAt":"2026-03-03T08:11:39+00:00","versionOfRecord":{"articleIdentity":"rs-6469567","link":"https://doi.org/10.1038/s41467-026-69719-z","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2026-02-17 05:00:00","publishedOnDateReadable":"February 17th, 2026"},"versionCreatedAt":"2025-06-20 08:41:08","video":"","vorDoi":"10.1038/s41467-026-69719-z","vorDoiUrl":"https://doi.org/10.1038/s41467-026-69719-z","workflowStages":[]},"version":"v1","identity":"rs-6469567","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6469567","identity":"rs-6469567","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}