Sol-gel synthesized V2O5/TiO2 catalysts for NH3-SCR: Effect of calcination temperature on performance

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Although commercial V 2 O 5 /TiO 2 catalysts are economically viable for NH 3 -SCR, their poor catalytic activities limit their use to operating temperatures greater than 300℃, which prevents their use under low-temperature exhaust conditions. In this study, we employed a one-step sol-gel method to produce V 2 O 5 /TiO 2 catalysts and then compared their catalytic performances and physicochemical characteristics with those of a conventional impregnated V 2 O 5 /TiO 2 catalyst. This one-step approach resulted in catalysts that exhibited improved NO conversions, and notably, the activity of sol-gel catalysts produced under optimized conditions was almost twice that of the conventional catalyst. In this study, catalyst calcination temperature was adjusted between 250 and 550℃. X-ray diffraction showed the crystallinity of the anatase TiO 2 phase increased with calcination temperature, but that calcination temperatures (> 500℃) caused sintering and reduced BET surface area as determined using N 2 adsorption-desorption isotherms. X-ray photoelectron spectroscopy and NH 3 temperature-programmed desorption demonstrated that catalysts calcined at temperatures between 350 and 500°C had optimal amounts of V 4+ species, surface oxygen, and acidic sites, which are essential for catalytic activity. This study highlights that the one-step sol-gel technique provides a simple, cost-effective means of synthesizing high-performance V 2 O 5 /TiO 2 catalysts for low-temperature NH 3 -SCR applications. NOx removal NH3-SCR Sol-gel synthesis V2O5/TiO2 Calcination Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Air pollution caused by the energy sector has emerged as a serious environmental issue in parallel with industrial expansion and energy consumption. In particular, nitrogen oxides (NO X ), emitted by power plants and automobiles, are major air pollutants [ 1 , 2 ]. Not only is NO x harmful but it also reacts with other air pollutants, such as volatile organic compounds (VOCs), to form ozone and fine particulate matter [ 3 , 4 ]. In addition, because power plants are considering switching from fossil fuels to ammonia (NH₃) to reduce CO₂ emissions, NO X emission has become an even greater concern [ 5 ]. Selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) are considered candidate technologies for NO X removal using NH 3 . SNCR is a cost-effective and small-scale method that does not require a catalyst but does require high operating temperatures (850–1100°C) and has a relatively poor NO X removal performance [ 6 , 7 ]. Thus, as environmental regulations have become more stringent, SCR is favored for industrial applications. Metal oxide-based catalysts are critical components of NH 3 -based SCR. However, although various advanced catalysts, including rare-earth metal-based oxides [ 8 , 9 ] and non-stoichiometric metal oxides [ 10 ], have been actively researched, V 2 O 5 /TiO 2 -based catalysts are used preferentially due to their low cost, compatibility with mass production, and long-term stability [ 11 , 12 ]. Despite the economic advantages of traditional V 2 O 5 /TiO 2 catalysts, their low catalytic activities and high activation temperatures result in a narrow operating temperature range (300–400°C) [ 13 ], and the temperature of NO X containing exhaust gases from waste-fired power plants and automobiles is generally less than 300°C [ 14 – 16 ]. In addition, SCR units are being increasingly installed downstream of desulfurization to minimize the effects of SO X , which further reduces SCR operating temperatures. Thus, industrial changes are driving the development of V 2 O 5 /TiO 2 catalysts that operate at temperatures below 300℃. Previous studies on V 2 O 5 /TiO 2 catalysts for NO x removal have mainly focused on using dopants to enhance catalytic activities and decrease operating temperatures. Zhao et al. formed oxygen vacancies in a V 2 O 5 /TiO 2 catalyst using F (fluorine) as a dopant. F doping induced a charge imbalance between Ti and V ions, generating free radicals that enhanced catalytic activity [ 17 ]. By doping V 2 O 5 /TiO 2 catalysts with Cr, Yang et al. increased the acid site to active SCR site ratio [ 18 ]. Zhao et al. synthesized a Ti self-doped V 2 O 5 /TiO 2 for SCR [ 19 ], and the resulting charge imbalance enhanced catalytic activity by inhibiting the anatase-to-rutile phase transformation of TiO 2 , promoting the formation of oxygen vacancies and enhancing redox and surface acidity. Li et al. doped V 2 O 5 /TiO 2 catalysts with boron (B) [ 20 ] and found anatase TiO 2 was stabilized by B, which prevented its transformation into rutile TiO 2 . Since the anatase phase has better textural properties than the rutile phase, the B-doped V 2 O 5 /TiO 2 catalyst demonstrated higher catalytic activity. To summarize, heteroatom doping of V 2 O 5 /TiO 2 catalysts enhances catalytic activity by inducing charge imbalances and stabilizing the anatase phase, preventing anatase to rutile transition. Charge imbalances and oxygen vacancies can be produced in V 2 O 5 /TiO 2 catalysts in various ways. Kang et al. synthesized several V 2 O 5 /TiO 2 catalysts using aerosol, xerogel, and sequential aerosol-impregnation methods [ 21 ]. The V 2 O 5 /TiO 2 catalyst prepared using the sequential aerosol-impregnation method showed the highest catalytic activity due to its strong acidity and well-dispersed vanadium sites. Cheng et al. compared the sol-gel and hydrothermal methods for preparing the catalytic V 2 O 5 /TiO 2 fibers for the simultaneous removal of dust and NO X [ 22 ]. They found that sol-gel prepared catalysts had higher activity and attributed this to greater redox activity and acidity. Despite these efforts, doping and other preparation methods have not been commercialized due to their cost and complexity. Here, we synthesized V 2 O 5 /TiO 2 catalysts using a one-step sol-gel method and compared their SCR activities with that of a V 2 O 5 /TiO 2 catalyst prepared by conventional impregnation. In addition, we investigated the effect of the calcination temperature, which primarily determines the chemical and physical properties, and hence the catalytic activities, of sol-gel prepared catalyst. Specifically, we examined charge imbalances, surface areas, and crystallinities of catalysts as a function of calcination temperature. This study aimed to answer the following questions: (1) How does the NH₃ SCR activity of a catalyst prepared using the sol-gel approach compare to that of a conventional impregnated catalyst? (2) How do the physical and chemical properties of a catalyst change with calcination temperature? (3) What is the optimal calcination temperature for a sol-gel prepared V 2 O 5 /TiO 2 catalyst? (4) How effective are catalysts prepared in this manner compared to similar catalysts prepared in other ways? 2. Experimental 2.1. Preparation of the sol-gel catalyst V 2 O 5 /TiO 2 catalysts were synthesized using a one-step sol-gel method [ 23 – 25 ] using ammonium metavanadate (NH 4 VO 3 , Sigma Aldrich, > 99%) and titanium isopropoxide (Ti[OCH(CH 3 ) 2 ] 4 , TTIP, Sigma Aldrich, > 97%), respectively. To achieve a V 2 O 5 loading of 7 wt% in the final product, ammonium metavanadate was first dissolved in deionized water, and then the requisite amount of TTIP was added dropwise under continuous stirring. The resulting slurry was stirred for 1 h to hydrolyze the TTIP and dried at 80°C with stirring. The resulting xerogel was then calcined at 250 to 550°C at 50°C intervals for 12 h. The prepared catalysts were denoted V 2 O 5 /TiO 2 (X), where X represents the calcination temperature. Catalysts became darker when the calcination temperature was increased (Fig. 1 ). To compare the catalytic activity of the sol-gel V 2 O 5 /TiO 2 catalysts with that of a conventional impregnated catalyst, the conventional catalyst was prepared using a sequential process by TiO 2 support synthesis followed by V 2 O 5 impregnation. The TiO 2 support was synthesized using the same procedure used for the sol-gel but without ammonium metavanadate. After hydrolyzing the TTIP, the xerogel obtained was calcined at 450°C to produce the TiO 2 support, which was then impregnated with ammonium metavanadate to achieve a 7 wt% V 2 O 5 loading. The impregnated support was then subjected to calcination for 12 h at 450°C and named I-V 2 O 5 /TiO 2 (450). 2.2. Catalytic activity measurements NH3-SCR catalytic activities were measured using the experimental setup described in our previous study [ 3 ]. A fixed-bed reactor with an inner diameter of 9.6 mm was installed in an electrically heated furnace (Fig. 2 ), and the catalyst was packed into the center of the heating zone. The reactor was heated at 5°C∙min − 1 to 150, 200, or 230°C and then a reaction gas mixture containing 170 ppm of NO, 170 ppm NH 3 , and 5 vol% of O 2 in N 2 was introduced at a total flow rate of 1500 mL∙min-1. Before entering the fixed-bed reactor, the gas mixture was mixed and preheated. A pressure gauge was installed on top of the reactor to monitor pressure changes, and an on-line gas analyzer (Testo 350, Testo SE, Germany) was used to determine effluent gas compositions. NO conversion was estimated using input and output concentrations. The reaction was conducted for at least 5 h to ensure steady-state conditions, after which NO conversion was measured. \(\:{X}_{NO}\left(\%\right)=\frac{{\left[NO\right]}_{inlet}\:-{\left[NO\right]}_{outlet}}{{\left[NO\right]}_{inlet}}\times\:10\) 0% \(\:{\left[NO\right]}_{inlet}\) : Concentration of NO flowing into the reactor \(\:{\left[NO\right]}_{outlet}\) : Concentration of NO effluent from the reactor 2.3. Characterization of the catalysts X-ray fluorescence (XRF) was performed using Epsilon 3-XLE (PANalytical, Netherlands) to determine catalyst compositions. Catalyst powders were pelletized before XRF. X-ray diffraction (XRD) was used to determine the crystal structures of catalysts using X'Pert PRO (PANalytical, Netherlands). XRD patterns were obtained in the 20° to 80° 2θ range at a scanning rate of 2°∙min − 1 . N 2 adsorption-desorptions were measured to estimate textural parameters (surface area, total pore volume, and mean pore diameter) using a 3-Flex instrument (Micromeritics, USA). Prior to measurements, samples were degassed under vacuum at 150°C to remove adsorbed water. Surface acidities were determined by NH3 temperature-programmed desorption (NH3-TPD) on a BELCAT (BEL Japan, Inc., Japan). Catalyst samples were first degassed under Ar flow for 2 h at 150°C, cooled to 100°C, and exposed to a gas mixture containing 5 vol% NH 3 in He to saturate the catalyst surface. After removing physically adsorbed NH 3 by purging with He, the temperature was increased to 500°C at 8°C∙min − 1 , and the desorbed gas was detected using a thermal conductivity detector (TCD). Samples were then subjected to X-ray photoelectron spectroscopy (XPS) (K-Alpha; Thermo Fisher Scientific, USA). 3. Results and discussion To compare the catalytic activities of samples prepared using the one-step sol-gel and impregnation methods, the NO conversions of V 2 O 5 /TiO 2 (450) (one-step sol-gel) and I-V 2 O 5 /TiO 2 (450) (impregnation method) calcined at 450℃ (Fig. 3 ). V 2 O 5 /TiO 2 (450) exhibited better catalytic activity than I-V 2 O 5 /TiO 2 (450) at all temperatures. In particular, at 230°C, which is considered the highest NH 3 -SCR temperature for flue gas produced by waste-fired heaters and automobiles, V 2 O 5 /TiO 2 (450) NO conversion was almost twice that of I-V 2 O 5 /TiO 2 (450). Previous studies on the use of V 2 O 5 /TiO 2 catalysts for NH 3 -SCR have used two-step sol-gel methods to synthesize TiO 2 supports. These methods involved the initial synthesis of TiO 2 via the sol-gel method and subsequent impregnation of the TiO 2 support with V 2 O 5 [ 26 – 28 ]. On the other hand, we propose a one-step sol-gel approach whereby the V 2 O 5 precursor is directly incorporated into the synthesis. This approach simplifies the synthesis and results in catalysts with higher NO conversions than catalysts prepared using the two-step method. To optimize the devised process, the calcination temperature was adjusted from 250 to 550°C in 50°C increments. Since TiO 2 and V 2 O 5 are formed simultaneously during calcination, the temperature used critically determines the physical and chemical characteristics of the catalyst [ 29 ]. Catalytic performance increased with calcination temperature from 250℃ and peaked at 450°C, beyond which it declined (Fig. 3 ). The significantly lower NO conversions observed at temperatures below 300°C supported the notion that insufficient thermal activation hinders the formation of catalytically active species. On the other hand, catalyst sintering and porosity loss probably reduced conversion at temperatures > 500°C. Notably, although V 2 O 5 /TiO 2 (450) had the highest NO conversion, catalysts calcined at temperatures between 350 and 500℃ had comparable catalytic activities. The NO conversion of V 2 O 5 /TiO 2 (450) without doping was equivalent to that of a recently reported B-doped V 2 O 5 /TiO 2 catalyst (0.5 wt% B on TiO 2 ) under similar reaction conditions [ 20 ]. Considering that V 2 O 5 /TiO 2 (450) was synthesized using a one-step procedure without heteroatom doping, it would appear that its performance could be enhanced by doping. XRF was used to confirm that changes in catalytic activity were not due to chemical composition differences (Table 1). The weight ratio of V 2 O 5 to TiO 2 in the catalysts prepared was around 7:93, a V 2 O 5 loading of 7% as was intended. This result demonstrates that the described sol-gel technique provides a reproducible means of synthesizing catalysts. Table 1. Compositions of the sol-gel catalysts determined by XRF. Calcination temperature ( o C) 200 300 350 400 450 500 550 V 2 O 5 93.2 93.3 93.2 93 93.2 93.1 93.2 TiO 2 6.8 6.7 6.8 7.0 6.8 6.9 6.8 XRD was used to analyze the crystalline structure of the prepared catalysts (Fig. 4 a). Samples calcined at > 350°C showed a distinct peak corresponding to the anatase phase of TiO 2 [ 30 ]. Below this temperature, insufficient calcination prevented the anatase formation, which probably contributed to the reduced catalytic activity (Fig. 3 ). The peaks of anatase phase steadily increased with calcination temperature. However, at 550℃, a rapid increase in crystallinity was observed, suggesting sintering of crystalline V 2 O 5 /TiO 2 (550) and reduced NO conversion. The enlarged XRD pattern of prepared catalysts (Fig. 4 b) suggested that the major peak of the crystalline rutile TiO 2 phase, which is located at 27.3°, was hardly visible. Rutile TiO 2 is less catalytically active than anatase TiO 2 [ 22 ], and according to earlier studies, V 2 O 5 /TiO 2 catalysts calcined at < 600℃ contain the rutile phase [ 13 , 20 ]. However, we found the catalyst prepared using the sol-gel method and calcined at 550℃ contained almost no rutile. This finding indicates that the presence of V 2 O 5 precursor during calcination inhibited the anatase to rutile phase transition, and thus, enhanced the activity of the catalyst produced. In addition, the absence of vanadium oxide peaks suggested that these oxides were well dispersed and/or present in an amorphous form on the anatase support. Catalyst textural characteristics were examined using N 2 adsorption-desorption isotherms. All catalysts, with the exception of V 2 O 5 /TiO 2 (550), displayed Type IV isotherms (Fig. 5 ), suggesting that mesopores formed in the TiO 2 support during sol-gel synthesis. On the other hand, V 2 O 5 /TiO 2 (550) showed markedly fewer micro- and mesoporous structures than the catalysts calcined at lower temperatures (Fig. 4 ). Furthermore, this structural collapse caused a sharp drop in the NO conversion of V 2 O 5 /TiO 2 (550) (Fig. 3 ). Interestingly, all samples except V 2 O 5 /TiO 2 (550) exhibited two hysteresis loops in the relative pressure (P/P 0 ) ranges of 0.4–0.6 and 0.8–1.0, suggesting the presence of a bimodal pore size distribution and/or a complex network of interconnected narrow and large pores. Mesopore formation during hydrolysis and calcination suggests that the co-presence of V 2 O 5 and TiO 2 precursors might have led to the development of a multi-porous structure. Table 2 displays the BET surface areas, pore volumes, and mean pore diameters of the catalysts produced. Notably, pore volume and surface area decreased with calcination temperature, and V 2 O 5 /TiO 2 (250) and V 2 O 5 /TiO 2 (300) had large surface areas and pore volumes, but low anate TiO 2 crystallinity, indicating reduced catalytic activity for NH 3 -SCR. While the textural characteristics of V 2 O 5 /TiO 2 (400), V 2 O 5 /TiO 2 (450), and V 2 O 5 /TiO 2 (500) were comparable, V 2 O 5 /TiO 2 (350) had largest surface area and pore volume. However, these features were not reflected by their catalytic activities. Interestingly, V 2 O 5 /TiO 2 (450) had the highest NO conversion, suggesting that textural properties alone are not the primary determinant of catalytic performance. Table 2. Textural properties of the one-step prepared catalysts. Catalysts Surface area (m2∙g-1) Total pore volume (cm3∙g-1) Mean pore diameter (nm) V 2 O 5 /TiO 2 (250) 358.88 0.32 3.58 V 2 O 5 /TiO 2 (300) 382.39 0.34 3.57 V 2 O 5 /TiO 2 (350) 242.63 0.25 4.12 V 2 O 5 /TiO 2 (400) 112.16 0.18 6.31 V 2 O 5 /TiO 2 (450) 90.37 0.18 8.01 V 2 O 5 /TiO 2 (500) 64.14 0.16 9.84 V 2 O 5 /TiO 2 (550) 15.85 0.05 12.66 Surface acidities of catalysts were investigated by NH 3 -TPD (Fig. 6 ). According to earlier studies, desorption bands at higher temperatures are indicative of strong Lewis or Bronsted acid sites, whereas those at lower temperatures are ascribed to weak Bronsted acid sites [ 31 , 32 ]. The TPD spectra of all samples, except V 2 O 5 /TiO 2 (550), showed three major desorption bands located at 150, 250, and 360℃. On the other hand, V 2 O 5 /TiO 2 (550) showed only two bands at ~ 150℃ and ~ 250℃. The desorption bands at 150, 250, and 360℃ were ascribed to weak Bronsted acid, strong Bronsted acid, and medium-strength Bronsted acid sites, respectively. V 2 O 5 /TiO 2 (250) and V 2 O 5 /TiO 2 (350) had larger desorption band areas than V 2 O 5 /TiO 2 (400), V 2 O 5 /TiO 2 (450), and V 2 O 5 /TiO 2 (500), which are considered to be proportional to the number of acid sites. This increase in band areas was probably due to larger surface areas. However, despite these high acid site numbers and surface areas, their lower XRD-determined crystallinities (Fig. 4 ) hindered their catalytic activities. Previous studies have also suggested that NH 4 + adsorbed on weak Bronsted acid sites are active species for NH 3 -SCR [ 20 , 33 ]. Although specific surface area followed the order V 2 O 5 /TiO 2 (350) > V 2 O 5 /TiO 2 (400) > V 2 O 5 /TiO 2 (450) > V 2 O 5 /TiO 2 (500), these samples have similar desorption band areas around 250°C, and thus, presumably numbers of active sites. This implies that these catalysts have a comparable numbers of active sites, and thus, similar catalytic activities (Fig. 3 ). However, the poor NO conversion of V 2 O 5 /TiO 2 (550) was attributed to a markedly smaller desorption band area and BET surface area. XPS was used to examine the surface characteristics of catalysts and their elemental compositions and oxidation states. Figure 7 a displays V 2p spectra. An asymmetric band at 514–519 eV was ascribed to V 2p3/2 for all samples, suggesting that V 4+ and V 5+ oxidation states coexisted. V 4+ is responsible for the lower energy peak at ~ 516 eV [ 34 ], whereas V 5+ is responsible for the higher energy peak at ~ 517 eV [ 35 ]. Gaussian curve fitting was used to deconvolute the asymmetric band and isolate the contributions of these two peaks. Previous studies have shown that V 4+ is more strongly associated with surface-active species than other oxidation states [ 32 ]. Measured V 4+ /(V 4+ + V 5+ ) ratios for each catalyst are shown in Table 3. Interestingly, even without doping, V 2 O 5 /TiO 2 catalysts prepared using the one-step sol-gel method showed ratios similar to those of V 2 O 5 /TiO 2 catalysts doped with heteroatoms [ 20 ]. V 4+ /(V 4+ + V 5+ ) ratio increased as calcination temperature increased, suggesting an increase in catalytic activity. These results are consistent with our catalytic activity measurements (Fig. 3 ), which indicated that the V 2 O 5 /TiO 2 (250 and 300) catalysts provided less NO conversion than the V 2 O 5 /TiO 2 (400–500) catalysts. Despite the relatively low V 4+ /(V 4+ + V 5+ ) ratio of V 2 O 5 /TiO 2 (350), its catalytic activity was enhanced due to abundant surface oxygen, as discussed below. On the other hand, V 2 O 5 /TiO 2 (550) had a higher V 4+ /(V 4+ + V 5+ ) ratio, but its NO conversion was lower than those of other catalysts due to its lower surface area and acidity. The O 1s spectra of all catalysts are shown in Fig. 7 b. Like V 2p spectra, O 1s spectra also exhibited an asymmetric band, indicating the presence of oxygen in different chemical states. Lattice oxygen was represented by a peak at ~ 529 eV, whereas surface oxygen adsorbed on oxygen vacancy or defect sites correspond to the peak at ~ 531 eV [ 36 , 37 ]. According to earlier research, oxygen vacancies and defect sites, which are regarded as catalytically active entities, are associated with surface-adsorbed oxygen [ 19 ]. The asymmetric band was also deconvoluted into two peaks by Gaussian curve fitting. Surface-adsorbed oxygen to total oxygen ratios are provided in Table 3. Higher ratios at lower calcination temperatures suggested the presence of surface oxygen vacancies. V 2 O 5 /TiO 2 (250) and V 2 O 5 /TiO 2 (300) were not considered because of their significantly lower crystallinities. V 2 O 5 /TiO 2 (400–500) catalysts showed comparable ratios, as was expected from their similar NO conversions (Fig. 3 ). Notably, V 2 O 5 /TiO 2 (350) had a higher surface oxygen ratio than V 2 O 5 /TiO 2 (400–500), which potentially compensated for its lower V 4+ /(V 4+ + V 5+ ) ratio, and explained the similar NO conversions observed for V 2 O 5 /TiO 2 (350–500) Catalyst Ti 2p spectra are shown in Fig. 7 c. The two prominent peaks at ~ 459 eV and 465 eV, respectively, were ascribed to Ti 2p3/2 and Ti 2p1/2. XRD showed the anatase phase rather than the rutile phase dominated in TiO 2 samples (Fig. 4 ), resulting in no significant difference in Ti 2p spectra. Slight peak shifts caused by Ti-O-V interactions at the different calcination temperatures were undetectable, presumably due to the relatively low V content. In conclusion, V 2 O 5 /TiO 2 catalysts synthesized using a one-step sol-gel method showed higher catalytic activity for the conversion of NO than the catalyst made by impregnation. The catalyst properties of the synthesized catalysts were significantly impacted by calcination temperature. Notably, the porous structure collapsed at higher temperatures and anatase TiO 2 crystallinity decreased at lower temperatures, reducing catalytic activity. NH3-TPD showed surface acidities were similar for samples calcined between 350–500°C but reduced sharply for catalysts calcined at above 550°C. In addition, at lower calcination temperatures, XPS showed a higher concentration of oxygen vacancies but a lower V 4+ fraction, both of which indicate active species for NH 3 -SCR. In summary, our findings show that the crystalline TiO 2 and V 2 O 5 phase, which is necessary for NH 3 -SCR, is not formed at calcination temperatures lower than 300℃. However, the chemical and textural properties of the catalysts deteriorated at temperatures higher than 500°C. As a result, the 300 to 500°C temperature range is ideal for creating V 2 O 5 /TiO 2 catalysts using the one-step sol-gel approach for NH 3 -SCR. Table 3. Ratios of vanadium to oxygen oxidation states as determined from XPS spectra of V 2p and O 1s. Catalysts V 2p O 1s V 4+ / (V 4+ + V 5+ ) Surface-adsorbed O / total O V 2 O 5 /TiO 2 (250) 0.20 0.20 V 2 O 5 /TiO 2 (300) 0.33 0.23 V 2 O 5 /TiO 2 (350) 0.30 0.21 V 2 O 5 /TiO 2 (400) 0.46 0.11 V 2 O 5 /TiO 2 (450) 0.42 0.13 V 2 O 5 /TiO 2 (500) 0.36 0.13 V 2 O 5 /TiO 2 (550) 0.44 0.14 4. Conclusions This study shows that the one-step sol-gel method greatly increases the catalytic activity of V 2 O 5 /TiO 2 catalysts for NO X reduction compared to the traditional impregnation method. The sol-gel catalysts exhibited higher NO conversion at all temperatures tested and better performance at 230℃, a temperature typical of industrial exhaust gases. Calcination temperature, which crucially determined the crystallinity, porosity, and acidity of the catalysts, was optimized. According to XRD measurements, TiO 2 preserved the anatase phase in all catalysts regardless of calcination temperature. Observations suggested that the presence of V 2 O 5 precursor during TiO 2 formation through hydrolysis of TTIP hinders the anatase to rutile phase transition. Though increasing temperature increased catalyst activity, calcination at temperatures > 550°C significantly reduced porosity and specific surface area as described by BET surface analysis and N 2 adsorption-desorption experiments. Conversely, insufficient calcination at temperatures < 300°C led to poor V 2 O 5 and TiO 2 crystallinities and reduced catalytic efficiency. According to our NH 3 -TPD study, catalysts calcined between 350 and 500°C retained a constant surface acidity and had adequate Bronsted acid sites to facilitate NH 3 adsorption and activation. Furthermore, XPS analysis showed that calcination in the 350–500°C range provided optimum V 4+ content and surface oxygen vacancies, which are known to increase redox activity for NH 3 -SCR. These results demonstrate that the one-step sol-gel method effectively generates highly active V 2 O 5 /TiO 2 catalysts and obviates the need for heteroatom doping, as the V 2 O 5 /TiO 2 catalysts prepared in this study exhibited catalytic activity comparable to that of doped catalysts. Furthermore, the optimal calcination range of 350–500°C was found to guarantee balance between crystallinity, porosity, and surface activity. Declarations Acknowledgments This study was supported by the Research Program funded by the SeoulTech (Seoul National University of Science and Technology). 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Chin J Chem Eng 44:377–383 Kang M, Choi J, Kim YT, Park ED, Shin CB, Suh DJ, Yie JE (2009) Effects of preparation methods for V 2 O 5-TiO 2 aerogel catalysts on the selective catalytic reduction of NO with NH 3. Korean Journal of Chemical Engineering 26:884–889 Cheng J, Li W, Wang X, Liu H, Chen Y (2022) Vanadium-based catalytic fibers for selective reduction of NO by NH3 and their potential use on co-processing of dust and NOx. Chemical Engineering Journal 431:133694 Kang D, Lim HS, Lee M, Lee JW (2018) Syngas production on a Ni-enhanced Fe2O3/Al2O3 oxygen carrier via chemical looping partial oxidation with dry reforming of methane. Appl Energy 211:174–186. https://doi.org/https://doi.org/10.1016/j.apenergy.2017.11.018 Kang D, Lim HS, Lee JW (2020) Mesoporous Fe2O3–CeO2–Al2O3 oxygen carrier for chemical looping dry reforming with subsequent water splitting. Ind Eng Chem Res 59:15912–15920 Kang D, Lee M, Lim HS, Lee JW (2018) Chemical looping partial oxidation of methane with CO2 utilization on the ceria-enhanced mesoporous Fe2O3 oxygen carrier. Fuel 215:787–798 Cha W, Chin S, Park E, Yun S-T, Jurng J (2013) Effect of V2O5 loading of V2O5/TiO2 catalysts prepared via CVC and impregnation methods on NOx removal. Appl Catal B 140:708–715 Kang M, Park ED, Kim JM, Yie JE (2007) Manganese oxide catalysts for NOx reduction with NH3 at low temperatures. Appl Catal A Gen 327:261–269 Cha W, Le HA, Chin S, Kim M, Jung H, Yun S-T, Jurng J (2013) Enhanced low-temperature NH3-SCR activity of a V2O5/TiO2 composite prepared via chemical vapor condensation and impregnation method. Mater Res Bull 48:4415–4418 Soleimanzadeh H, Niaei A, Salari D, Tarjomannejad A, Penner S, Grünbacher M, Hosseini SA, Mousavi SM (2019) Modeling and optimization of V2O5/TiO2 nanocatalysts for NH3-Selective catalytic reduction (SCR) of NOx by RSM and ANN techniques. J Environ Manage 238:360–367. https://doi.org/https://doi.org/10.1016/j.jenvman.2019.03.018 Thamaphat K, Limsuwan P, Ngotawornchai B (2008) Phase characterization of TiO2 powder by XRD and TEM. Agriculture and Natural Resources 42:357–361 Zhou X, Huang X, Xie A, Luo S, Yao C, Li X, Zuo S (2017) V2O5-decorated Mn-Fe/attapulgite catalyst with high SO2 tolerance for SCR of NOx with NH3 at low temperature. Chemical Engineering Journal 326:1074–1085 Zhong Q, Zhang T, Li Y, Ma W, Qu H (2011) NO (or NH3)+ O2 adsorption on fluorine-doped vanadia/titania and its role in the mechanism of a two-step process characterized by EPR. Chemical engineering journal 174:390–395 Zhang Y, Guo W, Wang L, Song M, Yang L, Shen K, Xu H, Zhou C (2015) Characterization and activity of V2O5-CeO2/TiO2-ZrO2 catalysts for NH3-selective catalytic reduction of NOx. Chinese Journal of Catalysis 36:1701–1710 Demeter M, Neumann M, Reichelt W (2000) Mixed-valence vanadium oxides studied by XPS. Surf Sci 454:41–44 Silversmit G, Depla D, Poelman H, Marin GB, De Gryse R (2004) Determination of the V2p XPS binding energies for different vanadium oxidation states (V5+ to V0+). J Electron Spectros Relat Phenomena 135:167–175 Zhao X, Huang L, Li H, Hu H, Han J, Shi L, Zhang D (2015) Highly dispersed V2O5/TiO2 modified with transition metals (Cu, Fe, Mn, Co) as efficient catalysts for the selective reduction of NO with NH3. Chinese Journal of Catalysis 36:1886–1899 Liu Z, Li Y, Zhu T, Su H, Zhu J (2014) Selective catalytic reduction of NO x by NH3 over Mn-promoted V2O5/TiO2 catalyst. Ind Eng Chem Res 53:12964–12970 Cite Share Download PDF Status: Published Journal Publication published 08 Jul, 2025 Read the published version in Korean Journal of Chemical Engineering → Version 1 posted Reviewers agreed at journal 08 Apr, 2025 Reviewers invited by journal 08 Apr, 2025 Editor assigned by journal 08 Apr, 2025 First submitted to journal 03 Apr, 2025 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-6372367","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":439876957,"identity":"1d1e1875-7d16-42a0-9fad-47ab702a1d5a","order_by":0,"name":"Se-Jun Kwon","email":"","orcid":"","institution":"Yeungnam University","correspondingAuthor":false,"prefix":"","firstName":"Se-Jun","middleName":"","lastName":"Kwon","suffix":""},{"id":439876958,"identity":"3f3e52d1-31c1-4d25-a774-fc6c05b2f550","order_by":1,"name":"Junyoung Lee","email":"","orcid":"","institution":"Seoul National University of Science \u0026 Technology","correspondingAuthor":false,"prefix":"","firstName":"Junyoung","middleName":"","lastName":"Lee","suffix":""},{"id":439876959,"identity":"6e92cb76-5661-41f0-a4b0-55593bb0c0ce","order_by":2,"name":"Byung Chan Kwon","email":"","orcid":"","institution":"Yeungnam University","correspondingAuthor":false,"prefix":"","firstName":"Byung","middleName":"Chan","lastName":"Kwon","suffix":""},{"id":439876960,"identity":"be96498c-2f5f-459a-afe8-bec8d1f403e1","order_by":3,"name":"Dohyung Kang","email":"","orcid":"","institution":"Seoul National University of Science \u0026 Technology","correspondingAuthor":false,"prefix":"","firstName":"Dohyung","middleName":"","lastName":"Kang","suffix":""},{"id":439876961,"identity":"98cc24db-30ce-44d2-a5e7-4a752a2e377e","order_by":4,"name":"No-Kuk Park","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2klEQVRIiWNgGAWjYFACNgbGBiAlwd4AFeAhWgvPAZK1SCQQqcWcvS1NcuYOu3zJmW/MHvMw2Mkz8Jx9gFeLZc+xY5IbzyRbzpbOMTfmYUg2bOBtN8CrxeBGepvkwzZmAznpHDNpHgbmBAZ+NvwOg2qpN5CTPAPSUk+MljSgw9oOG0hL8IC0HE5g4G0joOXMsWTLmWeOG0j2pJVJzjE4btjGc4yAluNthjd7d1QbSBw/vE3iTUW1PD9PGn4tYACOGAYOYEAZgOKJGADRwv6AKMWjYBSMglEw8gAAfDg7amjb5pYAAAAASUVORK5CYII=","orcid":"","institution":"Yeungnam University","correspondingAuthor":true,"prefix":"","firstName":"No-Kuk","middleName":"","lastName":"Park","suffix":""}],"badges":[],"createdAt":"2025-04-03 23:31:44","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6372367/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6372367/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11814-025-00509-x","type":"published","date":"2025-07-08T15:57:32+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":80212632,"identity":"4416550f-9613-4c05-b732-8e07580489f4","added_by":"auto","created_at":"2025-04-09 09:00:59","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":611820,"visible":true,"origin":"","legend":"\u003cp\u003ePhotographic images of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts synthesized via the one-step sol-gel method at different calcination temperatures: (a) V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(250), (b) V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(300), (c) V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350), (d) V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400), (e) V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450), (f) V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(500), and (g) V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/456df3617bb76a6c57230fc5.png"},{"id":80212627,"identity":"4fe94d3d-3569-4a8a-b991-97564e7c10b7","added_by":"auto","created_at":"2025-04-09 09:00:59","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":58778,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of the experimental setup used for measuring NH\u003csub\u003e3\u003c/sub\u003e-SCR catalytic activities.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/9990f6ba9606fb9319d99400.png"},{"id":80212628,"identity":"190a0538-09d8-4276-9434-545fc99e7cc8","added_by":"auto","created_at":"2025-04-09 09:00:59","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":41428,"visible":true,"origin":"","legend":"\u003cp\u003eNO conversion as a function of reaction temperature for sol-gel catalysts prepared using different calcination temperatures and preparation methods.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/a37842df9d85b3fcf40cbbed.png"},{"id":80212629,"identity":"36c8e7bb-b361-472f-9e3b-8f677e4e127b","added_by":"auto","created_at":"2025-04-09 09:00:59","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":102180,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of catalysts synthesized via the one-step sol-gel method at various calcination temperatures: (a) full-range scan (20–80°), (b) magnified view of the low-angle region (20–30°).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/de4e83b39754b7456de3a28d.png"},{"id":80212635,"identity":"541dde4a-13dd-4819-86aa-a6d0ca578677","added_by":"auto","created_at":"2025-04-09 09:01:00","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":32456,"visible":true,"origin":"","legend":"\u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms of catalysts synthesized using the one-step method and calcined at various temperatures.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/d6a6a0268e579347bb67a8ab.png"},{"id":80212634,"identity":"8e7f4b5d-ae7a-4617-8ca9-d0801a3ffff3","added_by":"auto","created_at":"2025-04-09 09:00:59","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":27010,"visible":true,"origin":"","legend":"\u003cp\u003eNH\u003csub\u003e3\u003c/sub\u003e-TPD spectra of catalysts synthesized using one-step sol-gel method and calcined at different temperatures.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/61b893cc0f355ec4e7b9ddc9.png"},{"id":80213860,"identity":"c08fd54f-881e-4c22-9777-f13e33da0ad3","added_by":"auto","created_at":"2025-04-09 09:09:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":238335,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of catalysts synthesized by the one-step sol-gel method at various calcination temperatures: (a) V 2p, (b) O 1s, and (c) Ti 2p.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/cf8e7285bfca8550096a31f5.png"},{"id":86699370,"identity":"fcc522b1-643b-427a-8002-829de303629f","added_by":"auto","created_at":"2025-07-14 16:08:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1853468,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6372367/v1/d8c674fc-3a69-4cc9-9d50-8e7203a13da1.pdf"}],"financialInterests":"","formattedTitle":"Sol-gel synthesized V2O5/TiO2 catalysts for NH3-SCR: Effect of calcination temperature on performance","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAir pollution caused by the energy sector has emerged as a serious environmental issue in parallel with industrial expansion and energy consumption. In particular, nitrogen oxides (NO\u003csub\u003eX\u003c/sub\u003e), emitted by power plants and automobiles, are major air pollutants [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Not only is NO\u003csub\u003ex\u003c/sub\u003e harmful but it also reacts with other air pollutants, such as volatile organic compounds (VOCs), to form ozone and fine particulate matter [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. In addition, because power plants are considering switching from fossil fuels to ammonia (NH₃) to reduce CO₂ emissions, NO\u003csub\u003eX\u003c/sub\u003e emission has become an even greater concern [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSelective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) are considered candidate technologies for NO\u003csub\u003eX\u003c/sub\u003e removal using NH\u003csub\u003e3\u003c/sub\u003e. SNCR is a cost-effective and small-scale method that does not require a catalyst but does require high operating temperatures (850\u0026ndash;1100\u0026deg;C) and has a relatively poor NO\u003csub\u003eX\u003c/sub\u003e removal performance [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Thus, as environmental regulations have become more stringent, SCR is favored for industrial applications. Metal oxide-based catalysts are critical components of NH\u003csub\u003e3\u003c/sub\u003e-based SCR. However, although various advanced catalysts, including rare-earth metal-based oxides [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] and non-stoichiometric metal oxides [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], have been actively researched, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e-based catalysts are used preferentially due to their low cost, compatibility with mass production, and long-term stability [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDespite the economic advantages of traditional V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts, their low catalytic activities and high activation temperatures result in a narrow operating temperature range (300\u0026ndash;400\u0026deg;C) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], and the temperature of NO\u003csub\u003eX\u003c/sub\u003e containing exhaust gases from waste-fired power plants and automobiles is generally less than 300\u0026deg;C [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. In addition, SCR units are being increasingly installed downstream of desulfurization to minimize the effects of SO\u003csub\u003eX\u003c/sub\u003e, which further reduces SCR operating temperatures. Thus, industrial changes are driving the development of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts that operate at temperatures below 300℃.\u003c/p\u003e \u003cp\u003ePrevious studies on V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts for NO\u003csub\u003ex\u003c/sub\u003e removal have mainly focused on using dopants to enhance catalytic activities and decrease operating temperatures. Zhao et al. formed oxygen vacancies in a V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalyst using F (fluorine) as a dopant. F doping induced a charge imbalance between Ti and V ions, generating free radicals that enhanced catalytic activity [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. By doping V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts with Cr, Yang et al. increased the acid site to active SCR site ratio [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Zhao et al. synthesized a Ti self-doped V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e for SCR [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], and the resulting charge imbalance enhanced catalytic activity by inhibiting the anatase-to-rutile phase transformation of TiO\u003csub\u003e2\u003c/sub\u003e, promoting the formation of oxygen vacancies and enhancing redox and surface acidity. Li et al. doped V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts with boron (B) [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] and found anatase TiO\u003csub\u003e2\u003c/sub\u003e was stabilized by B, which prevented its transformation into rutile TiO\u003csub\u003e2\u003c/sub\u003e. Since the anatase phase has better textural properties than the rutile phase, the B-doped V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalyst demonstrated higher catalytic activity. To summarize, heteroatom doping of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts enhances catalytic activity by inducing charge imbalances and stabilizing the anatase phase, preventing anatase to rutile transition.\u003c/p\u003e \u003cp\u003eCharge imbalances and oxygen vacancies can be produced in V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts in various ways. Kang et al. synthesized several V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts using aerosol, xerogel, and sequential aerosol-impregnation methods [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalyst prepared using the sequential aerosol-impregnation method showed the highest catalytic activity due to its strong acidity and well-dispersed vanadium sites. Cheng et al. compared the sol-gel and hydrothermal methods for preparing the catalytic V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e fibers for the simultaneous removal of dust and NO\u003csub\u003eX\u003c/sub\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. They found that sol-gel prepared catalysts had higher activity and attributed this to greater redox activity and acidity. Despite these efforts, doping and other preparation methods have not been commercialized due to their cost and complexity.\u003c/p\u003e \u003cp\u003eHere, we synthesized V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts using a one-step sol-gel method and compared their SCR activities with that of a V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalyst prepared by conventional impregnation. In addition, we investigated the effect of the calcination temperature, which primarily determines the chemical and physical properties, and hence the catalytic activities, of sol-gel prepared catalyst. Specifically, we examined charge imbalances, surface areas, and crystallinities of catalysts as a function of calcination temperature. This study aimed to answer the following questions: (1) How does the NH₃ SCR activity of a catalyst prepared using the sol-gel approach compare to that of a conventional impregnated catalyst? (2) How do the physical and chemical properties of a catalyst change with calcination temperature? (3) What is the optimal calcination temperature for a sol-gel prepared V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalyst? (4) How effective are catalysts prepared in this manner compared to similar catalysts prepared in other ways?\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1. Preparation of the sol-gel catalyst\u003c/h2\u003e\n \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts were synthesized using a one-step sol-gel method [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e] using ammonium metavanadate (NH\u003csub\u003e4\u003c/sub\u003eVO\u003csub\u003e3\u003c/sub\u003e, Sigma Aldrich, \u0026gt;\u0026thinsp;99%) and titanium isopropoxide (Ti[OCH(CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e4\u003c/sub\u003e, TTIP, Sigma Aldrich, \u0026gt;\u0026thinsp;97%), respectively. To achieve a V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e loading of 7 wt% in the final product, ammonium metavanadate was first dissolved in deionized water, and then the requisite amount of TTIP was added dropwise under continuous stirring. The resulting slurry was stirred for 1 h to hydrolyze the TTIP and dried at 80\u0026deg;C with stirring. The resulting xerogel was then calcined at 250 to 550\u0026deg;C at 50\u0026deg;C intervals for 12 h. The prepared catalysts were denoted V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(X), where X represents the calcination temperature. Catalysts became darker when the calcination temperature was increased (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eTo compare the catalytic activity of the sol-gel V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts with that of a conventional impregnated catalyst, the conventional catalyst was prepared using a sequential process by TiO\u003csub\u003e2\u003c/sub\u003e support synthesis followed by V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e impregnation. The TiO\u003csub\u003e2\u003c/sub\u003e support was synthesized using the same procedure used for the sol-gel but without ammonium metavanadate. After hydrolyzing the TTIP, the xerogel obtained was calcined at 450\u0026deg;C to produce the TiO\u003csub\u003e2\u003c/sub\u003e support, which was then impregnated with ammonium metavanadate to achieve a 7 wt% V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e loading. The impregnated support was then subjected to calcination for 12 h at 450\u0026deg;C and named I-V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2. Catalytic activity measurements\u003c/h2\u003e\n \u003cp\u003eNH3-SCR catalytic activities were measured using the experimental setup described in our previous study [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e]. A fixed-bed reactor with an inner diameter of 9.6 mm was installed in an electrically heated furnace (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e), and the catalyst was packed into the center of the heating zone. The reactor was heated at 5\u0026deg;C∙min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 150, 200, or 230\u0026deg;C and then a reaction gas mixture containing 170 ppm of NO, 170 ppm NH\u003csub\u003e3\u003c/sub\u003e, and 5 vol% of O\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e was introduced at a total flow rate of 1500 mL∙min-1. Before entering the fixed-bed reactor, the gas mixture was mixed and preheated. A pressure gauge was installed on top of the reactor to monitor pressure changes, and an on-line gas analyzer (Testo 350, Testo SE, Germany) was used to determine effluent gas compositions. NO conversion was estimated using input and output concentrations. The reaction was conducted for at least 5 h to ensure steady-state conditions, after which NO conversion was measured.\u003c/p\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u0026nbsp;\u003cspan class=\"mathinline\"\u003e\\(\\:{X}_{NO}\\left(\\%\\right)=\\frac{{\\left[NO\\right]}_{inlet}\\:-{\\left[NO\\right]}_{outlet}}{{\\left[NO\\right]}_{inlet}}\\times\\:10\\)\u003c/span\u003e\u0026nbsp;\u003c/span\u003e0%\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left[NO\\right]}_{inlet}\\) :\u0026nbsp;\u003c/span\u003e\u003c/span\u003e\u003c/strong\u003eConcentration of NO flowing into the reactor\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left[NO\\right]}_{outlet}\\) :\u0026nbsp;\u003c/span\u003e\u003c/span\u003e\u003c/strong\u003eConcentration of NO effluent from the reactor\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3. Characterization of the catalysts\u003c/h2\u003e\n \u003cp\u003eX-ray fluorescence (XRF) was performed using Epsilon 3-XLE (PANalytical, Netherlands) to determine catalyst compositions. Catalyst powders were pelletized before XRF. X-ray diffraction (XRD) was used to determine the crystal structures of catalysts using X\u0026apos;Pert PRO (PANalytical, Netherlands). XRD patterns were obtained in the 20\u0026deg; to 80\u0026deg; 2\u0026theta; range at a scanning rate of 2\u0026deg;∙min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. N\u003csub\u003e2\u003c/sub\u003e adsorption-desorptions were measured to estimate textural parameters (surface area, total pore volume, and mean pore diameter) using a 3-Flex instrument (Micromeritics, USA). Prior to measurements, samples were degassed under vacuum at 150\u0026deg;C to remove adsorbed water. Surface acidities were determined by NH3 temperature-programmed desorption (NH3-TPD) on a BELCAT (BEL Japan, Inc., Japan). Catalyst samples were first degassed under Ar flow for 2 h at 150\u0026deg;C, cooled to 100\u0026deg;C, and exposed to a gas mixture containing 5 vol% NH\u003csub\u003e3\u003c/sub\u003e in He to saturate the catalyst surface. After removing physically adsorbed NH\u003csub\u003e3\u003c/sub\u003e by purging with He, the temperature was increased to 500\u0026deg;C at 8\u0026deg;C∙min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and the desorbed gas was detected using a thermal conductivity detector (TCD). Samples were then subjected to X-ray photoelectron spectroscopy (XPS) (K-Alpha; Thermo Fisher Scientific, USA).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eTo compare the catalytic activities of samples prepared using the one-step sol-gel and impregnation methods, the NO conversions of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) (one-step sol-gel) and I-V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) (impregnation method) calcined at 450℃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) exhibited better catalytic activity than I-V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) at all temperatures. In particular, at 230\u0026deg;C, which is considered the highest NH\u003csub\u003e3\u003c/sub\u003e-SCR temperature for flue gas produced by waste-fired heaters and automobiles, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) NO conversion was almost twice that of I-V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450). Previous studies on the use of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts for NH\u003csub\u003e3\u003c/sub\u003e-SCR have used two-step sol-gel methods to synthesize TiO\u003csub\u003e2\u003c/sub\u003e supports. These methods involved the initial synthesis of TiO\u003csub\u003e2\u003c/sub\u003e via the sol-gel method and subsequent impregnation of the TiO\u003csub\u003e2\u003c/sub\u003e support with V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e [\u003cspan additionalcitationids=\"CR27\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. On the other hand, we propose a one-step sol-gel approach whereby the V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e precursor is directly incorporated into the synthesis. This approach simplifies the synthesis and results in catalysts with higher NO conversions than catalysts prepared using the two-step method.\u003c/p\u003e \u003cp\u003eTo optimize the devised process, the calcination temperature was adjusted from 250 to 550\u0026deg;C in 50\u0026deg;C increments. Since TiO\u003csub\u003e2\u003c/sub\u003e and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e are formed simultaneously during calcination, the temperature used critically determines the physical and chemical characteristics of the catalyst [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Catalytic performance increased with calcination temperature from 250℃ and peaked at 450\u0026deg;C, beyond which it declined (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The significantly lower NO conversions observed at temperatures below 300\u0026deg;C supported the notion that insufficient thermal activation hinders the formation of catalytically active species. On the other hand, catalyst sintering and porosity loss probably reduced conversion at temperatures\u0026thinsp;\u0026gt;\u0026thinsp;500\u0026deg;C. Notably, although V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) had the highest NO conversion, catalysts calcined at temperatures between 350 and 500℃ had comparable catalytic activities.\u003c/p\u003e \u003cp\u003eThe NO conversion of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) without doping was equivalent to that of a recently reported B-doped V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalyst (0.5 wt% B on TiO\u003csub\u003e2\u003c/sub\u003e) under similar reaction conditions [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Considering that V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) was synthesized using a one-step procedure without heteroatom doping, it would appear that its performance could be enhanced by doping.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eXRF was used to confirm that changes in catalytic activity were not due to chemical composition differences (Table\u0026nbsp;1). The weight ratio of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e to TiO\u003csub\u003e2\u003c/sub\u003e in the catalysts prepared was around 7:93, a V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e loading of 7% as was intended. This result demonstrates that the described sol-gel technique provides a reproducible means of synthesizing catalysts.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"9\" nameend=\"c9\" namest=\"c1\"\u003e \u003cp\u003eTable\u0026nbsp;1. Compositions of the sol-gel catalysts determined by XRF.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c10\" namest=\"c10\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCalcination temperature (\u003csup\u003eo\u003c/sup\u003eC)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e350\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c9\" namest=\"c8\"\u003e \u003cp\u003e550\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c10\" namest=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e93.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e93.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e93.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e93.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e93.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c9\" namest=\"c8\"\u003e \u003cp\u003e93.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c10\" namest=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c9\" namest=\"c8\"\u003e \u003cp\u003e6.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c10\" namest=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eXRD was used to analyze the crystalline structure of the prepared catalysts (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). Samples calcined at \u0026gt;\u0026thinsp;350\u0026deg;C showed a distinct peak corresponding to the anatase phase of TiO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Below this temperature, insufficient calcination prevented the anatase formation, which probably contributed to the reduced catalytic activity (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The peaks of anatase phase steadily increased with calcination temperature. However, at 550℃, a rapid increase in crystallinity was observed, suggesting sintering of crystalline V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550) and reduced NO conversion.\u003c/p\u003e \u003cp\u003eThe enlarged XRD pattern of prepared catalysts (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) suggested that the major peak of the crystalline rutile TiO\u003csub\u003e2\u003c/sub\u003e phase, which is located at 27.3\u0026deg;, was hardly visible. Rutile TiO\u003csub\u003e2\u003c/sub\u003e is less catalytically active than anatase TiO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], and according to earlier studies, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts calcined at \u0026lt;\u0026thinsp;600℃ contain the rutile phase [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, we found the catalyst prepared using the sol-gel method and calcined at 550℃ contained almost no rutile. This finding indicates that the presence of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e precursor during calcination inhibited the anatase to rutile phase transition, and thus, enhanced the activity of the catalyst produced. In addition, the absence of vanadium oxide peaks suggested that these oxides were well dispersed and/or present in an amorphous form on the anatase support.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCatalyst textural characteristics were examined using N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms. All catalysts, with the exception of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550), displayed Type IV isotherms (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), suggesting that mesopores formed in the TiO\u003csub\u003e2\u003c/sub\u003e support during sol-gel synthesis. On the other hand, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550) showed markedly fewer micro- and mesoporous structures than the catalysts calcined at lower temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Furthermore, this structural collapse caused a sharp drop in the NO conversion of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eInterestingly, all samples except V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550) exhibited two hysteresis loops in the relative pressure (P/P\u003csub\u003e0\u003c/sub\u003e) ranges of 0.4\u0026ndash;0.6 and 0.8\u0026ndash;1.0, suggesting the presence of a bimodal pore size distribution and/or a complex network of interconnected narrow and large pores. Mesopore formation during hydrolysis and calcination suggests that the co-presence of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e precursors might have led to the development of a multi-porous structure.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;2 displays the BET surface areas, pore volumes, and mean pore diameters of the catalysts produced. Notably, pore volume and surface area decreased with calcination temperature, and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(250) and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(300) had large surface areas and pore volumes, but low anate TiO\u003csub\u003e2\u003c/sub\u003e crystallinity, indicating reduced catalytic activity for NH\u003csub\u003e3\u003c/sub\u003e-SCR.\u003c/p\u003e \u003cp\u003eWhile the textural characteristics of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400), V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450), and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(500) were comparable, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350) had largest surface area and pore volume. However, these features were not reflected by their catalytic activities. Interestingly, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450) had the highest NO conversion, suggesting that textural properties alone are not the primary determinant of catalytic performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Tabb\" border=\"1\"\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"5\" nameend=\"c5\" namest=\"c1\"\u003e \u003cp\u003eTable\u0026nbsp;2. Textural properties of the one-step prepared catalysts.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCatalysts\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurface area\u003c/p\u003e \u003cp\u003e(m2∙g-1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTotal pore volume\u003c/p\u003e \u003cp\u003e(cm3∙g-1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003eMean pore diameter\u003c/p\u003e \u003cp\u003e(nm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(250)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e358.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e3.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(300)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e382.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e3.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e242.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e4.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e112.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e6.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e90.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e8.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(500)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e64.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e9.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e15.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e12.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eSurface acidities of catalysts were investigated by NH\u003csub\u003e3\u003c/sub\u003e-TPD (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). According to earlier studies, desorption bands at higher temperatures are indicative of strong Lewis or Bronsted acid sites, whereas those at lower temperatures are ascribed to weak Bronsted acid sites [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The TPD spectra of all samples, except V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550), showed three major desorption bands located at 150, 250, and 360℃. On the other hand, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550) showed only two bands at ~\u0026thinsp;150℃ and ~\u0026thinsp;250℃. The desorption bands at 150, 250, and 360℃ were ascribed to weak Bronsted acid, strong Bronsted acid, and medium-strength Bronsted acid sites, respectively.\u003c/p\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(250) and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350) had larger desorption band areas than V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400), V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450), and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(500), which are considered to be proportional to the number of acid sites. This increase in band areas was probably due to larger surface areas. However, despite these high acid site numbers and surface areas, their lower XRD-determined crystallinities (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) hindered their catalytic activities.\u003c/p\u003e \u003cp\u003ePrevious studies have also suggested that NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e adsorbed on weak Bronsted acid sites are active species for NH\u003csub\u003e3\u003c/sub\u003e-SCR [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Although specific surface area followed the order V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350)\u0026thinsp;\u0026gt;\u0026thinsp;V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400)\u0026thinsp;\u0026gt;\u0026thinsp;V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450)\u0026thinsp;\u0026gt;\u0026thinsp;V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(500), these samples have similar desorption band areas around 250\u0026deg;C, and thus, presumably numbers of active sites. This implies that these catalysts have a comparable numbers of active sites, and thus, similar catalytic activities (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, the poor NO conversion of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550) was attributed to a markedly smaller desorption band area and BET surface area.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eXPS was used to examine the surface characteristics of catalysts and their elemental compositions and oxidation states. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea displays V 2p spectra. An asymmetric band at 514\u0026ndash;519 eV was ascribed to V 2p3/2 for all samples, suggesting that V\u003csup\u003e4+\u003c/sup\u003e and V\u003csup\u003e5+\u003c/sup\u003e oxidation states coexisted. V\u003csup\u003e4+\u003c/sup\u003e is responsible for the lower energy peak at ~\u0026thinsp;516 eV [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], whereas V\u003csup\u003e5+\u003c/sup\u003e is responsible for the higher energy peak at ~\u0026thinsp;517 eV [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Gaussian curve fitting was used to deconvolute the asymmetric band and isolate the contributions of these two peaks. Previous studies have shown that V\u003csup\u003e4+\u003c/sup\u003e is more strongly associated with surface-active species than other oxidation states [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMeasured V\u003csup\u003e4+\u003c/sup\u003e/(V\u003csup\u003e4+\u003c/sup\u003e + V\u003csup\u003e5+\u003c/sup\u003e) ratios for each catalyst are shown in Table\u0026nbsp;3. Interestingly, even without doping, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts prepared using the one-step sol-gel method showed ratios similar to those of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts doped with heteroatoms [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. V\u003csup\u003e4+\u003c/sup\u003e/(V\u003csup\u003e4+\u003c/sup\u003e + V\u003csup\u003e5+\u003c/sup\u003e) ratio increased as calcination temperature increased, suggesting an increase in catalytic activity. These results are consistent with our catalytic activity measurements (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), which indicated that the V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(250 and 300) catalysts provided less NO conversion than the V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400\u0026ndash;500) catalysts. Despite the relatively low V\u003csup\u003e4+\u003c/sup\u003e/(V\u003csup\u003e4+\u003c/sup\u003e + V\u003csup\u003e5+\u003c/sup\u003e) ratio of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350), its catalytic activity was enhanced due to abundant surface oxygen, as discussed below. On the other hand, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550) had a higher V\u003csup\u003e4+\u003c/sup\u003e/(V\u003csup\u003e4+\u003c/sup\u003e + V\u003csup\u003e5+\u003c/sup\u003e) ratio, but its NO conversion was lower than those of other catalysts due to its lower surface area and acidity.\u003c/p\u003e \u003cp\u003eThe O 1s spectra of all catalysts are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb. Like V 2p spectra, O 1s spectra also exhibited an asymmetric band, indicating the presence of oxygen in different chemical states. Lattice oxygen was represented by a peak at ~\u0026thinsp;529 eV, whereas surface oxygen adsorbed on oxygen vacancy or defect sites correspond to the peak at ~\u0026thinsp;531 eV [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. According to earlier research, oxygen vacancies and defect sites, which are regarded as catalytically active entities, are associated with surface-adsorbed oxygen [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe asymmetric band was also deconvoluted into two peaks by Gaussian curve fitting. Surface-adsorbed oxygen to total oxygen ratios are provided in Table\u0026nbsp;3. Higher ratios at lower calcination temperatures suggested the presence of surface oxygen vacancies. V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(250) and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(300) were not considered because of their significantly lower crystallinities. V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400\u0026ndash;500) catalysts showed comparable ratios, as was expected from their similar NO conversions (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Notably, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350) had a higher surface oxygen ratio than V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400\u0026ndash;500), which potentially compensated for its lower V\u003csup\u003e4+\u003c/sup\u003e/(V\u003csup\u003e4+\u003c/sup\u003e + V\u003csup\u003e5+\u003c/sup\u003e) ratio, and explained the similar NO conversions observed for V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350\u0026ndash;500)\u003c/p\u003e \u003cp\u003eCatalyst Ti 2p spectra are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec. The two prominent peaks at ~\u0026thinsp;459 eV and 465 eV, respectively, were ascribed to Ti 2p3/2 and Ti 2p1/2. XRD showed the anatase phase rather than the rutile phase dominated in TiO\u003csub\u003e2\u003c/sub\u003e samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), resulting in no significant difference in Ti 2p spectra. Slight peak shifts caused by Ti-O-V interactions at the different calcination temperatures were undetectable, presumably due to the relatively low V content.\u003c/p\u003e \u003cp\u003eIn conclusion, V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts synthesized using a one-step sol-gel method showed higher catalytic activity for the conversion of NO than the catalyst made by impregnation. The catalyst properties of the synthesized catalysts were significantly impacted by calcination temperature. Notably, the porous structure collapsed at higher temperatures and anatase TiO\u003csub\u003e2\u003c/sub\u003e crystallinity decreased at lower temperatures, reducing catalytic activity. NH3-TPD showed surface acidities were similar for samples calcined between 350\u0026ndash;500\u0026deg;C but reduced sharply for catalysts calcined at above 550\u0026deg;C. In addition, at lower calcination temperatures, XPS showed a higher concentration of oxygen vacancies but a lower V\u003csup\u003e4+\u003c/sup\u003e fraction, both of which indicate active species for NH\u003csub\u003e3\u003c/sub\u003e-SCR.\u003c/p\u003e \u003cp\u003eIn summary, our findings show that the crystalline TiO\u003csup\u003e2\u003c/sup\u003e and V\u003csup\u003e2\u003c/sup\u003eO\u003csup\u003e5\u003c/sup\u003e phase, which is necessary for NH\u003csup\u003e3\u003c/sup\u003e-SCR, is not formed at calcination temperatures lower than 300℃. However, the chemical and textural properties of the catalysts deteriorated at temperatures higher than 500\u0026deg;C. As a result, the 300 to 500\u0026deg;C temperature range is ideal for creating V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts using the one-step sol-gel approach for NH\u003csub\u003e3\u003c/sub\u003e-SCR.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Tabc\" border=\"1\"\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c3\" namest=\"c1\"\u003e \u003cp\u003eTable\u0026nbsp;3. Ratios of vanadium to oxygen oxidation states as determined from XPS spectra of V 2p and O 1s.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c4\" namest=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCatalysts\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eV 2p\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003eO 1s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eV\u003csup\u003e4+\u003c/sup\u003e / (V\u003csup\u003e4+\u003c/sup\u003e + V\u003csup\u003e5+\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003eSurface-adsorbed O / total O\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(250)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003e0.20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(300)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003e0.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(350)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(400)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(450)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003e0.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(500)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003e0.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e(550)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis study shows that the one-step sol-gel method greatly increases the catalytic activity of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts for NO\u003csub\u003eX\u003c/sub\u003e reduction compared to the traditional impregnation method. The sol-gel catalysts exhibited higher NO conversion at all temperatures tested and better performance at 230℃, a temperature typical of industrial exhaust gases. Calcination temperature, which crucially determined the crystallinity, porosity, and acidity of the catalysts, was optimized.\u003c/p\u003e \u003cp\u003eAccording to XRD measurements, TiO\u003csup\u003e2\u003c/sup\u003e preserved the anatase phase in all catalysts regardless of calcination temperature. Observations suggested that the presence of V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e precursor during TiO\u003csub\u003e2\u003c/sub\u003e formation through hydrolysis of TTIP hinders the anatase to rutile phase transition. Though increasing temperature increased catalyst activity, calcination at temperatures\u0026thinsp;\u0026gt;\u0026thinsp;550\u0026deg;C significantly reduced porosity and specific surface area as described by BET surface analysis and N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption experiments. Conversely, insufficient calcination at temperatures\u0026thinsp;\u0026lt;\u0026thinsp;300\u0026deg;C led to poor V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e crystallinities and reduced catalytic efficiency. According to our NH\u003csub\u003e3\u003c/sub\u003e-TPD study, catalysts calcined between 350 and 500\u0026deg;C retained a constant surface acidity and had adequate Bronsted acid sites to facilitate NH\u003csub\u003e3\u003c/sub\u003e adsorption and activation. Furthermore, XPS analysis showed that calcination in the 350\u0026ndash;500\u0026deg;C range provided optimum V\u003csup\u003e4+\u003c/sup\u003e content and surface oxygen vacancies, which are known to increase redox activity for NH\u003csub\u003e3\u003c/sub\u003e-SCR. These results demonstrate that the one-step sol-gel method effectively generates highly active V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts and obviates the need for heteroatom doping, as the V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts prepared in this study exhibited catalytic activity comparable to that of doped catalysts. Furthermore, the optimal calcination range of 350\u0026ndash;500\u0026deg;C was found to guarantee balance between crystallinity, porosity, and surface activity.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThis study was supported by the Research Program funded by the SeoulTech (Seoul National University of Science and Technology).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003ePark Y-K, Kim B-S (2023) Catalytic removal of nitrogen oxides (NO, NO2, N2O) from ammonia-fueled combustion exhaust: A review of applicable technologies. Chemical Engineering Journal 461:141958\u003c/li\u003e\n\u003cli\u003eCheng X, Bi XT (2014) A review of recent advances in selective catalytic NOx reduction reactor technologies. 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Ind Eng Chem Res 59:15912\u0026ndash;15920\u003c/li\u003e\n\u003cli\u003eKang D, Lee M, Lim HS, Lee JW (2018) Chemical looping partial oxidation of methane with CO2 utilization on the ceria-enhanced mesoporous Fe2O3 oxygen carrier. Fuel 215:787\u0026ndash;798\u003c/li\u003e\n\u003cli\u003eCha W, Chin S, Park E, Yun S-T, Jurng J (2013) Effect of V2O5 loading of V2O5/TiO2 catalysts prepared via CVC and impregnation methods on NOx removal. Appl Catal B 140:708\u0026ndash;715\u003c/li\u003e\n\u003cli\u003eKang M, Park ED, Kim JM, Yie JE (2007) Manganese oxide catalysts for NOx reduction with NH3 at low temperatures. Appl Catal A Gen 327:261\u0026ndash;269\u003c/li\u003e\n\u003cli\u003eCha W, Le HA, Chin S, Kim M, Jung H, Yun S-T, Jurng J (2013) Enhanced low-temperature NH3-SCR activity of a V2O5/TiO2 composite prepared via chemical vapor condensation and impregnation method. Mater Res Bull 48:4415\u0026ndash;4418\u003c/li\u003e\n\u003cli\u003eSoleimanzadeh H, Niaei A, Salari D, Tarjomannejad A, Penner S, Gr\u0026uuml;nbacher M, Hosseini SA, Mousavi SM (2019) Modeling and optimization of V2O5/TiO2 nanocatalysts for NH3-Selective catalytic reduction (SCR) of NOx by RSM and ANN techniques. J Environ Manage 238:360\u0026ndash;367. https://doi.org/https://doi.org/10.1016/j.jenvman.2019.03.018\u003c/li\u003e\n\u003cli\u003eThamaphat K, Limsuwan P, Ngotawornchai B (2008) Phase characterization of TiO2 powder by XRD and TEM. Agriculture and Natural Resources 42:357\u0026ndash;361\u003c/li\u003e\n\u003cli\u003eZhou X, Huang X, Xie A, Luo S, Yao C, Li X, Zuo S (2017) V2O5-decorated Mn-Fe/attapulgite catalyst with high SO2 tolerance for SCR of NOx with NH3 at low temperature. 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Ind Eng Chem Res 53:12964\u0026ndash;12970\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"NOx removal, NH3-SCR, Sol-gel synthesis, V2O5/TiO2, Calcination","lastPublishedDoi":"10.21203/rs.3.rs-6372367/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6372367/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAmmonia-based selective catalytic reduction (SCR) is essential for removing nitrogen oxides (NO\u003csub\u003eX\u003c/sub\u003e) emitted from industrial furnaces and automobiles. Although commercial V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts are economically viable for NH\u003csub\u003e3\u003c/sub\u003e-SCR, their poor catalytic activities limit their use to operating temperatures greater than 300℃, which prevents their use under low-temperature exhaust conditions. In this study, we employed a one-step sol-gel method to produce V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts and then compared their catalytic performances and physicochemical characteristics with those of a conventional impregnated V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalyst. This one-step approach resulted in catalysts that exhibited improved NO conversions, and notably, the activity of sol-gel catalysts produced under optimized conditions was almost twice that of the conventional catalyst. In this study, catalyst calcination temperature was adjusted between 250 and 550℃. X-ray diffraction showed the crystallinity of the anatase TiO\u003csub\u003e2\u003c/sub\u003e phase increased with calcination temperature, but that calcination temperatures (\u0026gt;\u0026thinsp;500℃) caused sintering and reduced BET surface area as determined using N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms. X-ray photoelectron spectroscopy and NH\u003csub\u003e3\u003c/sub\u003e temperature-programmed desorption demonstrated that catalysts calcined at temperatures between 350 and 500\u0026deg;C had optimal amounts of V\u003csup\u003e4+\u003c/sup\u003e species, surface oxygen, and acidic sites, which are essential for catalytic activity. This study highlights that the one-step sol-gel technique provides a simple, cost-effective means of synthesizing high-performance V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e catalysts for low-temperature NH\u003csub\u003e3\u003c/sub\u003e-SCR applications.\u003c/p\u003e","manuscriptTitle":"Sol-gel synthesized V2O5/TiO2 catalysts for NH3-SCR: Effect of calcination temperature on performance","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-09 09:00:55","doi":"10.21203/rs.3.rs-6372367/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-04-08T09:49:29+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-08T05:38:33+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-08T04:39:45+00:00","index":"","fulltext":""},{"type":"submitted","content":"Korean Journal of Chemical Engineering","date":"2025-04-03T19:31:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"ff513c8c-6ae3-4ec7-86fb-32c4777243a3","owner":[],"postedDate":"April 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-07-14T16:01:39+00:00","versionOfRecord":{"articleIdentity":"rs-6372367","link":"https://doi.org/10.1007/s11814-025-00509-x","journal":{"identity":"korean-journal-of-chemical-engineering","isVorOnly":false,"title":"Korean Journal of Chemical Engineering"},"publishedOn":"2025-07-08 15:57:32","publishedOnDateReadable":"July 8th, 2025"},"versionCreatedAt":"2025-04-09 09:00:55","video":"","vorDoi":"10.1007/s11814-025-00509-x","vorDoiUrl":"https://doi.org/10.1007/s11814-025-00509-x","workflowStages":[]},"version":"v1","identity":"rs-6372367","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6372367","identity":"rs-6372367","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}