High Surface Area VO_x/TiO₂/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

Abstract

The mode of operation of titania-supported vanadia (VO_x) catalysts for NO_x abatement using ammonia selective catalytic reduction (NH₃-SCR) is still vigorously debated. We introduce a new high surface area VO_x/TiO₂/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VO_x species on amorphous TiO₂ domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NO_x conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH₂ and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NO_x conversion is attributed to the presence of highly dispersed VO_x species and the moderate acidity of VO_x supported on TiO₂/SBA-15.

1. Introduction

The emission of nitrogen oxides (NO_x) into the atmosphere, mainly from the electric-power industry and daily traffic, brings about a tremendous threat to the environment as well as human health. As one of the major atmospheric pollutants, NO_x has attracted increasing attention in recent years. Selective catalytic reduction (SCR) of NO_x with NH₃ is proved to be the most effective technology for the removal of NO_x from stationary and mobile sources [1]. Commercial vanadia-based ammonia SCR catalysts are typically based on vanadia and tungstia (or molybdena) on TiO₂ anatase, but vanadia supported on TiO₂ has been extensively studied as a model catalyst system [2,3,4].

It is known from the literature, that SCR activities depend on the vanadia structure and the support material [5,6,7]. Mesoporous materials such as silica SBA-15 provide higher surface area than conventional powder or planar supports [8,9], and the controlled synthesis of vanadia supported on SBA-15 was reported [10,11]. However, SiO₂ supported VO_x-based catalysts were reported to show only poor SCR performance compared to titania-based systems [5,12]. On the other hand, Segura et al. studied vanadia/titania supported on SBA-15 and attributed the good catalytic NH₃-SCR reactivity to isolated VO_x or TiO₂ species [13]. Reiche et al. prepared high surface area TiO₂, SiO_2, and TiO₂-SiO₂ supported vanadia systems using sol-gel methods and selective vanadia grafting and found TiO₂-based catalysts to show the highest SCR activity. In mixed TiO₂-SiO₂ aerogels, vanadia was reported to be preferentially grafted to Ti sites [5]. Compared to these conventional synthesis techniques, atomic layer deposition (ALD) enables accurate control at the atomic scale without damaging the structure of the original matrix [9,14,15,16]. Although ALD has been widely applied to metal-oxide deposition, such as SiO₂ [17,18,19], TiO₂ [20,21,22,23,24], and VO_x [9,25,26,27,28,29], there are only very few studies on the deposition of vanadia and titania on mesoporous SBA-15 [30], and its application to NH₃-SCR [29].

A limitation of VO_x-based catalysts for NO_x removal by NH₃-SCR is the high and narrow temperature window of acceptable deNO_x efficiencies. While VO_x/SiO₂ catalysts are characterized by a wide temperature window but extremely low NO_x conversion, attributed to the inert support and low acid site adsorption, VO_x/TiO₂ catalysts show higher NO_x conversions within a narrow temperature window (300–400 °C), resulting from the temperature dependence of the adsorption and redox reaction behavior. One may envision that combining both support materials in a controlled manner may improve the overall catalytic performance.

While there is an ongoing debate regarding various aspects of the mechanism of VO_x-based catalysts for NH₃-SCR reaction, it is now widely accepted that the majority route for the catalytic reaction is an Eley-Rideal (E-R) rather than a Langmuir-Hinshelwood (L-H) mechanism [31,32,33], with the overall mechanism including the following four steps: (i) NH₃ adsorption on V⁵⁺=O (Lewis acid sites) or V⁵⁺–OH (Bronsted acid sites), (ii) NO reacting with adsorbed NH₃ to form intermediate NH₂NO at Lewis acid sites (L acid sites) or NH₃NO at Bronsted acid sites (B acid sites), (iii) rearrangement of surface species releasing the intermediate by breaking N-H bonds and coordination bonds between the nitrogenous intermediate and its adsorption site, and by reducing the surface vanadia species from V⁵⁺ to V⁴⁺ with -OH groups or water being adsorbed, (iv) reoxidation of surface V⁴⁺ (V³⁺ at oxygen vacancies) by O₂ accompanied by desorption of surface H₂O [34,35,36], Research interests have been focused on exploring the key step(s) of the whole process on a microscopic scale and explaining the temperature-dependent relation between structure and activity. To this end, literature work may be divided into studies on the nature of the acid sites for reactants adsorption, the redox reaction step, and the desorption of products, respectively.

The first aspect, i.e., the sites for reactants adsorption (L vs B acid sites), has been a matter of debate from the beginning. In brief, L acid sites were related to V=O and B acid sites to V–OH species. Earlier, Topsoe et al. had proposed B acid sites to be the catalytically active sites as they detected a dominant amount of NH₄⁺ adsorbed onto V–OH surface sites [34,37,38,39,40], On the other hand, Ramis et al. found NH₃ adsorbed on V=O sites to be thermally more stable than NH₄⁺, implying that L acid sites were the dominant adsorption sites under reaction conditions [41], More recently, Marberger et al. supported the idea of Lewis acid sites being the active intermediate by observing a faster consumption of NH₃ adsorbed at L acid sites compared to NH₄⁺ adsorbed at B acid sites [42,43], Zhu et al. objected to the standpoint of Lewis acid sites as active sites by concluding that minority L sites (V⁵⁺=O) exhibited higher activity (TOF), while the more abundant B sites (V⁵⁺–OH) dominated the overall reaction [44], Thus, still no consensus has been reached regarding the role of L and B acid sites.

The redox reaction step has been considered to be related to the reactivity and connected to the VO_x surface structures. The use of in situ electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) under reaction conditions has revealed the reducing/reoxidizing processes to be accompanied by changes of the vanadium valence and VO_x surface structure [45,46,47,48]. Feng et al. reported VO_x with polymeric structure to be redox-active and provided an atomic view of the change of the VO_x structure with redox state [49,50,51]. Consistently, density functional theory (DFT) results showed that formation of both oxidized and reduced VO_x, i.e., V⁵⁺ and V⁴⁺, kept the vanadyl bond intact for isolated VO_x on TiO₂, as verified by in situ EPR data for VO_x/TiO₂ exposed to ammonia [52]. All of these results have revealed that the redox reactivity of VO_x species is structure sensitive.

Finally, we turn to the interaction between H₂O and surface VO_x sites. According to DFT calculations by Avdeev et al., adsorbed water changes the VO_x molecular configuration by spontaneous dissociation and formation of surface V–OH groups [53,54,55]. Importantly, lattice oxygen of surface VO_x species was shown to originate from adsorbed water rather than gas-phase oxygen [55]. Oxygen-18 isotope labeling studies have revealed that both terminal V=O and bridging V–O–V bonds readily exchange oxygen with water vapor [56].

To date, a detailed understanding of the role of these factors and the VO_x surface structure is still missing. In our previous work, we have developed a hierarchical NH₃-SCR model catalyst using low surface area silica particles as a platform [12]. Here, we report on a new model catalyst system based on high surface area mesoporous silica SBA-15. In particular, we employ ALD for controlled synthesis of titania and vanadia layers within the pores of the silica matrix and demonstrate its catalytic activity in NH₃-SCR. Using UV-Vis and Raman spectroscopy and, in particular, in situ DRIFTS, we explore the nature of the surface sites upon exposure to different gas atmospheres and temperatures, including NH₃-SCR reaction conditions.

2. Results

2.1. Catalyst Characterization

Table 1. Surface, porosity, and V loading characteristics of the prepared samples.

Samples	Stotal [m2/g]	Dp [nm]	Vtotal [cm3/g]	LV [V/nm2]	V:Ti (wt%) a
SBA-15	952	6.96	0.82	--	--
TiO2/SBA-15+1xVOx	410	6.33	0.49	0.4	1.4:5.8
TiO2/SBA-15+3xVOx	366	6.33	0.43	1.6	5.1:5.9
TiO2/SBA-15+5xVOx	355	6.33	0.43	2.0	6.0:5.6

^a From ICP-OES.

gives a summary of the prepared samples and their surface area and porosity characteristics. Upon ALD deposition of TiO₂ and VO_x on mesoporous silica SBA-15, surface area and pore volume drop by roughly half as reflected more precisely in Table 1, and are accompanied by a reduction in pore diameter. The observed decrease in specific surface area is attributed to a coating of the (rough) mesoporous channels but also to closure of SBA-15 micropores, as discussed previously [57]. Increasing VO_x deposition leads to a further (but much smaller) overall decrease of the specific surface area and pore volume (see Table 1). As shown in Figure S1, after TiO₂ and VO_x deposition, the nitrogen adsorption-desorption isotherms show variations of the hysteresis loop compared to bare SBA-15. In particular, the desorption branches exhibit a bulge towards lower p/p° values, indicating the presence of narrowed mesopores [58]. Thus, despite the use of ALD the pores of the silica matrix are not evenly coated, resulting in pore narrowing and possibly pore blocking, consistent with the observed overall decrease in surface area.

Figure 1a depicts wide-angle XRD patterns of TiO₂/SBA-15+nxVO_x (n = 1, 3, and 5) and TiO₂/SBA-15. None of the diffractograms shows any peaks, thus indicating the absence of crystalline titania and vanadia phases and confirming the amorphous nature of the SBA-15 silica matrix. The broad feature at around 23° has been attributed to the walls of mesoporous silica [59]. To check for microcrystalline domains the sample TiO₂/SBA-15+3xVO_x was examined in more detail using UV Raman spectroscopy (see Figure S2), which was shown to be a sensitive indicator for small (micro)crystalline titania contributions in our previous work [12]. According to Figure S2, there is no indication for the presence of titania microcrystals due to the absence of characteristic anatase (394, 514, and 634 cm⁻¹) and rutile (443 and 610 cm⁻¹) bands [60,61], or V₂O₅ microcrystals. In the context of Raman detection of V₂O₅, it should be mentioned, however, that visible Raman spectroscopy is typically more sensitive to V₂O₅ microcrystals than UV Raman spectroscopy (see discussion below).

Figure 1b shows the corresponding Raman spectra at 532 nm excitation. The spectrum of TiO₂/SBA-15 is characterized by broad features typical for amorphous silica dominating the weaker (surface) titania contributions [62]. Major features include the band at around 450–550 cm⁻¹ associated with four-membered rings (D₁), and the broad band at around 600–900 cm⁻¹ assigned to symmetrical Si–O–Si stretching and three-membered rings (D₂) [63]. The band located at 353 cm⁻¹ is assigned to bending in Si–O–Si bridges [64], whereas the band at 286 cm⁻¹ has been attributed to the A₁ mode of TiO₆ octahedra [65,66]. The small feature at 1064 cm⁻¹ is characteristic of silica TO phonons [67], whereas the weak Raman peak observed at 1122 cm⁻¹ for TiO₂/SBA-15 is attributed to framework Ti–O–Si species [68]. Importantly, the addition of increasing amounts of vanadia leads to the appearance of new Raman signals at around 267, 516, 699, and 1020 cm⁻¹. At first sight, the signals at around 267, 516, and 699 cm⁻¹ show some similarity with those observed for crystalline V₂O₅, however, closer inspection reveals significant differences. The observed signals rather indicate the presence of hydrated vanadia forming xerogels V₂O₅·nH₂O, resembling those discussed previously in the context of silica-supported vanadia, and the peak at 1020 cm⁻¹ is attributed to the V=O stretching vibration mode of tetrahedral VO_x species [69]. Based on the above results from XRD and Raman spectroscopic characterization, we can conclude that the ALD-prepared samples contain amorphous titania and vanadia on silica SBA-15 and that titania at the loading studied here (2 Ti/nm⁻²) does not form a conformal layer on the silica surface allowing vanadia to interact with titania and silica. According to a previous ALD study [29], deposition of 3 Ti nm⁻² and 7 Ti nm⁻² onto SiO₂ leads to the formation of sub-monolayer titania, in agreement with the results obtained here.

In the following, we will focus on the structural and catalytic properties of sample TiO₂/SBA-15+3xVO_x with a vanadium loading density of 1.6 V/nm². Table S1 and Figure 2 summarize the results of the XPS analysis. As shown in Table S1, Si, O, Ti, and V are detected at the catalyst surface, besides C. The Ti/Si and V/Si ratios correspond to 0.10 and 0.12, respectively, suggesting the presence of similar amounts of Ti and V on the surface. The results are consistent with the above picture of a mixed layer and rule out the exclusive formation of large titania or vanadia (3D) aggregates, which would strongly reduce the visibility of Ti and V in XPS. The left of Figure 2 depicts the O 1s, V 2p_1/2, and V 2p_3/2 photoemission located at 532.9 eV, 524.9 eV, and 517.4 eV, respectively. As expected, the former is dominated by the O 1s contribution from SiO₂, by comparison with the literature [69]. Detailed analysis of the V 2p_3/2 region based on literature data for binary vanadia compounds (V₂O₅, V₂O₄, V₂O₃) reveals the presence of V⁵⁺ at 517.6 eV (60%) and V⁴⁺ at 516.3 eV (40%), while a small contribution from V³⁺ at 515.7 eV cannot be ruled out [70,71]. The Ti 2p_3/2 peak is composed of (at least) two contributions located at 458.7 and 459.9 eV, which can be attributed to TiO₂ domains and to Ti–O–Si bonds, respectively [5,72].

Figure 3a depicts diffuse reflectance (DR) UV-Vis spectra of TiO₂/SBA-15+3xVO_x in comparison to TiO₂/SBA-15. To clarify the influence of water from ambient on the catalyst structure, spectra were recorded under ambient (hydrated) and dehydrated conditions. Please note that bare silica SBA-15 does not show any significant UV-Vis absorption. The UV-Vis spectrum of Ti/SBA-15 is characterized by strong absorption at around 280 nm, which has been attributed to charge transfer between an oxygen ligand and the central Ti⁴⁺ ion [73,74], whereas the position of the absorption band is consistent with higher coordinated or oligomeric titania species [75]. The presence of oligomeric species would be in agreement with the literature on SiO₂-supported titania, reporting two-dimensional oligomers of TiO₅ domain at a loading density of about 4 Ti nm⁻², which is higher than the density used in this work (2 Ti nm⁻²) [76].

Regarding the vanadia structure, the observed UV-Vis absorption was analyzed based on the ligand-to-metal charge transfer (LMCT) transitions observed for bulk vanadia reference compounds [77]. To this end, bands at around 240 and 290 nm were reported to originate from isolated tetrahedrally coordinated mono-vanadate ions [78], while the bands at about 270, 340, and 412 nm were assigned to poly-vanadate ions [79]. In the literature, the bands at 308, 371, and 406 nm were assigned to LMCT transitions of monomeric tetrahedral VO₄³⁻, oligomeric tetrahedral VO₃⁻, and polymeric distorted tetrahedral VO₃⁻ ions, respectively [80]. It was also reported that the bands at around 440–510 nm were due to LMCT transitions of V⁵⁺ species with a square pyramidal structure and those at 545–650 nm were due to d-d transitions of V⁴⁺ species with a square pyramidal structure. For crystalline V₂O₅, a broad absorption band with maxima ranging from 440 to 490 nm was observed [77]. The presence of V³⁺ and/or V⁴⁺ species was shown to lead to d-d transitions ranging from 600 to 800 nm [81]. A comparison of the hydrated and dehydrated state of the sample reveals the presence of a broad band at around 380–450 nm, which has been attributed to the coordination of water to V sites leading to major structural changes [74].

After dehydration, the UV-Vis absorption behavior is characterized by contributions at around 280, 320 and 400 nm. Based on the above literature results, these are attributed to monomeric/dimeric, oligomeric, and polymeric VO_x species, respectively. The intensity increase at around 260 nm is assigned to Ti–O–Ti from Ti–OH or possibly V–O–Ti from V–OH. Because of the overlap of charge transfer bands of titania and vanadia [80,82], removal of the titania contribution facilitates the analysis of the surface vanadia structure (see Figure S3) [83]. As a result, Figure S3 shows the presence of an asymmetric broad absorption band located between 250–550 nm for the hydrated and between 250–450 nm for the dehydrated sample, together with the result of a fit analysis based on the above reference data [83,84]. Besides, the ratio of monomeric/dimeric/oligomeric VO_x with tetrahedral coordination to polymeric VO_x with (pseudo-)octahedral coordination was estimated to increase from 0.31 to 12.5 upon dehydration, underlining the structural changes associated with the removal of coordinated water.

Based on the UV-Vis spectra the bandgap energy E_g was determined using the general power-law expression suggested by Davis and Mott based on the absorbance α [85].

The value of the parameter n in Equation (1) was set to 2 following the literature [82,86], showing a linear relationship when plotting (αhν)^1/2 vs hν (Tauc’s method) [87]. Extrapolation of the linear region to (αhν)^1/2 = 0 yields E_g values of 2.99 and 3.30 eV for the hydrated and dehydrated state of the catalyst, respectively. As the E_g value is negatively correlated to the number of covalent V–O–V bonds (CVB) via the relation CVB = 14.03 − 3.95 E_g [82], an increase in E_g implies a decrease in the degree of VO_x polymerization upon dehydration:

$$\alpha h\nu \propto {(h\nu -E_g)}^n$$

(1)

Figure 3b depicts Raman spectra of TiO₂/SBA-15+3xVO_x under hydrated and dehydrated conditions in comparison to the spectrum of TiO₂/SBA-15. As discussed above VO_x-related features are observed at 268, 513, 682, and 1025 cm⁻¹. Upon dehydration, the vanadyl (V=O) stretching mode shows an intensity increase and blueshift from ~1025 to 1038 cm⁻¹, consistent with previous work on VO_x/SiO₂ and VO_x/(TiO₂ + SiO₂) [29,69]. The position of the V=O band at 1038 cm⁻¹ in the visible Raman spectrum is characteristic for tetrahedrally coordinated vanadium ions, and has been associated with monomeric and/or small oligomeric VO_x species, consistent with the results from UV-Vis spectroscopy (see above). The presence of (micro)crystalline V₂O₅ in the dehydrated state of the catalyst can be excluded due to the absence of the characteristic vanadyl feature of V₂O₅ at 994 cm⁻¹ [69]. The shoulder observed at around 1062 cm⁻¹ has been attributed to V–O–Si bridging bonds [88]. The Raman bands observed at 268, 513, and 682 cm⁻¹ for the hydrated state are consistent with the presence of hydrated vanadia forming xerogels V₂O₅·nH₂O, as discussed above. Upon dehydration, these bands disappear due to the structural transformation of surface vanadia, leading to an increased visibility of the support-related bands at 258, 481, and 660 cm⁻¹ discussed below.

2.2. Catalytic Performance

The NH₃-SCR activity was examined within 100–450 °C using a mixture of 500 ppm NH₃, 500 ppm NO, 5% O_2, and N₂ (balance). The temperature was increased from 100 °C to 450 °C in 50 °C steps keeping each temperature for 20 min for equilibration. Analysis of the gas phase at the reactor outlet was performed by quantitative FT-IR spectroscopy. Figure 4 depicts the catalytic performance for NO_x conversion over TiO₂/SBA-15+3xVO_x as a function of temperature.

Table 2. Catalytic performance of TiO₂/SBA-15+3xVO_x in NH₃-SCR as a function of temperature. The feed gas consisted of 500 ppm NH₃, 500 ppm NO, and 5% O₂ balanced with N₂. The flow rate was 50 NmL min⁻¹ (GHSV = 40,000 h⁻¹).

Temperature, °C	100	150	200	250	300	350	400	450
NOx conversion, %	33.4	33.1	51.3	61.1	59.9	60.5	53.2	41.9
N2 selectivity, %	100	99.4	99.3	98.1	97.0	95.1	92.8	90.0

summarizes the NO_x conversion and N₂ selectivity values. The NO_x conversion follows a volcano shape with an increase from 100 to 250 °C, a maximum conversion level at 250–350 °C, and a decrease above 350 °C. The large error bar in the low temperature range is related to influences of the adsorption equilibrium process on the reaction step. At high temperatures a decrease of both NO_x conversion and N₂ selectivity is observed, which may be attributed to increased NH₃ oxidation leading to N₂O generation.

Despite the similar shape of the conversion, the TiO₂/SBA-15+3xVO_x sample shows significantly higher catalytic activity as VO_x/(SiO₂ + TiO₂) samples prepared by sol-gel methods [5]. In this context, it was reported that doping TiO₂ with Si may result in an increase in NO_x conversion and a decrease of the lower temperature limit from 360 °C for pure TiO₂ to 300 °C or lower for Si-doped samples [72]. Interestingly, the catalyst exhibits an excellent N₂ selectivity with values higher than 90% even at 450 °C. This behavior may be attributed to the (micro)porous structure and large surface area of the SBA-15 support, inhibiting the growth of bulk-like V₂O₅ species, which were suggested as main cause for N₂O formation [8]. In contrast to the results shown in Figure 4, VO_x/SiO₂ samples (prepared by ALD) showed only low NO conversions. In fact, the presence of titania was reported to enable V–O–Ti bond formation, which was related to the number and strength of surface acid-base sites [29]. Thus, we can conclude that besides the influence of (micro)structural effects, the improved catalytic performance may be attributed to the controlled preparation of dispersed active sites and a submonolayer of TiO₂ on SiO₂.

2.3. Operando Characterization of TiO₂/SBA-15+3xVO_x

To gain insight into the NH₃-SCR mechanism we applied DRIFTS in the presence of different gas environments and at different temperatures including reaction conditions (operando). Figure 5 shows DRIFT spectra of TiO₂/SBA-15+3xVO_x exposed to a gas mixture of 500 ppm NH₃, 500 ppm NO, and 5% O₂ (balanced with N₂) at different temperatures from 100 °C to 450 °C at a flow rate of 50 NmL/min. In Figure 5a, the black curve (labeled ‘N₂, 300 °C’) corresponds to the catalyst in a nitrogen flow at 300 °C before exposure to the reaction gas mixture. Features located at around 919 and 1036 cm⁻¹ and at around 1849 and 2002 cm⁻¹ have previously been assigned to V–O–V and V=O stretching modes of surface VO_x species and their first overtones, respectively [2,50,89,90]. The feature at 1203 cm⁻¹ has been ascribed to L acid sites on TiO₂ [91]. The shoulder at around 1640 cm⁻¹ and the broad band within the range 3500–3800 cm⁻¹ can be attributed to OH bending and stretching of water, respectively [2,72]. In the OH stretching region, the sharp peak at 3745 cm⁻¹ is characteristic of isolated Si–OH groups, while the small feature at 3656 cm⁻¹ can be associated with V–OH [74]. The Si–OH feature shows an asymmetric broad tail at around 3650 cm⁻¹, which has been assigned to Ti–OH [92].

Upon exposure to the reaction gas mixture, two new features appeared at 1418 and 1455 cm⁻¹, which can be attributed to the asymmetric N–H bending vibration of NH₄⁺ linked to B acid sites, while the shoulder and small feature at 1176 and 1606 cm⁻¹ have been associated with the symmetric and symmetric N–H bending vibrations of NH₃ linked to L acid sites, respectively [93,94]. According to Figure 5a, the observed N–H modes show a different temperature-dependent behavior. Whereas the feature at 1606 cm⁻¹ decreased with temperature and disappeared at about 250 °C, the band at about 1418 cm⁻¹ kept intact.

In Figure 5b, upon exposure to the reaction gas mixture, new IR bands are observed at 2803 and 3050 cm⁻¹, which can be attributed to N–H stretching of NH₄⁺ linked to B acid sites, while the bands located at 3159, 3278, and 3370 cm⁻¹ are assigned to N–H stretching modes of NH₃ coordinated to L acid sites [41,94,95,96]. As the temperature increases, the bands at 3278 and 3370 cm⁻¹ show an intensity decrease and completely disappear at temperatures above 300 °C, while the feature at 3159 cm⁻¹ is detectable until 450 °C. Similarly, the bands at 2803 and 3050 cm⁻¹ related to NH₄⁺ on B acid sites can be observed within the whole temperature range.

Figure 6 depicts DRIFT spectra of TiO₂/SBA-15+3xVO_x exposed to different gas environments at 300 °C at a total flow rate of 50 NmL/min within (a) 2600–3500 cm⁻¹ and (b) 3500–4000 cm⁻¹. Following the assignments discussed in the context of Figure 5, the features at 2808 and 3046 cm⁻¹ are assigned to NH₄⁺ coordinated to B acid sites, and those at 3150, 3275, and 3356 cm⁻¹ to NH₃ bound to L acid sites (see Figure 6a).

Similarly to the effect of temperature, the bands linked to B acid sites (2808 and 3046 cm⁻¹) and the band linked to L acid sites at 3150 cm⁻¹ are observed independent of the composition of the gas atmosphere, while those at 3275 and 3356 cm⁻¹ can only be detected upon exposure to gas mixtures containing NH_3. That is to say, that NH₄⁺ on V–OH sites is stable towards reaction no matter which redox conditions, while NH₃ adsorbed on specific L acid sites was proved to be reactive. In the corresponding high-wavenumber region (see Figure 6b) IR bands are observed at 3655 and 3737 cm⁻¹ in all gas mixtures and are attributed to V–OH and Si–OH stretching vibrations, respectively, as discussed above, while the broad absorption band extending to ~3600 cm⁻¹ originates from Ti–OH (see above) and the background from the fine structure of adsorbed water.

Upon closer inspection, the IR spectra in Figure 6 show gas phase-dependent variations. Upon the first exposure to 20 vol% O₂, no acid site-related bands are observed. However, water-related background peaks are detected indicating the rearrangement of surface hydroxyl groups into water under oxidizing conditions [55]. Switching to the reductive atmosphere, i.e., NH₃/NO, results in the appearance of the five N–H stretching modes related to L and B acid sites (see above), while during the following O₂ treatment the L site related features at 3275 and 3356 cm⁻¹ disappear. On the other hand, the 3150 cm⁻¹ band assigned to NH₃ coordinated to V=O sites, is still detected under oxidizing conditions. In the high-wavenumber region, the Si–OH, Ti–OH, and V–OH signals first show a decrease and then a partial recovery in oxygen. Next, the catalyst was exposed separately to NH₃ and NO, and then to a NO/O₂ mixture oxidation. In the presence of NH₃ reduction, the L site related features completely reappear and the water-related peaks show a weak positive increase indicating that NH₃ itself could be reduced to NH_n thereby transferring hydrogen atoms to adjacent hydroxyl groups to form water. Upon oxidation by NO or NO + O₂ the features at 3275 and 3356 cm⁻¹ disappeared, while the water-related peaks significantly increased, suggesting the promotional effect of NO regarding NH₃ oxidation, leading to intermediate formation (e.g., NH₂–NO or NH–NO), consistent with results from DFT [97], and mass spectroscopy [98].

Finally, the catalyst was exposed to reaction conditions, i.e., an NH₃/NO/O₂ mixture, leading to the re-appearance of all five acid site-related features, while the peaks for Si–OH, Ti–OH, and V–OH decreased in intensity. It is worth mentioning that there is a distinct difference in the IR spectra for exposure to the NH₃/NO mixture and the subsequent exposure to NH₃ and NO. In the former case, the presence of NH₃ was sufficient to competitively adsorb on the surface resulting in H₂O desorption, while in the latter case, NH₃ initially adsorbed on the surface was oxidized by NO resulting in NH₂–NO/N–NO and water formation with reduced competitive adsorption [98]. These above results are consistent with the strength of adsorption following the order NH₃ > H₂O > NH₂–NO intermediate, in agreement with the literature [99,100]. Interestingly, the addition of oxygen (NH₃/NO/O₂) leads to a different, distinct state of the catalyst regarding the presence of surface species, which falls between the behavior observed for NH₃ exposure and subsequent exposure to NH₃ and NO, and is also different to that obtained by subsequent exposure to NH₃/NO and oxygen.

In summary, the DRIFTS results shown in Figure 5 and Figure 6 reveal important differences in the adsorption/reactivity behavior of NH₃ species attached to acid sites. The N–H stretching linked to B acid sites at 2803 and 3050 cm⁻¹, which have been assigned to coordinate and suspended N–H bonds [41], respectively, did not respond to the different atmospheres and different temperatures, suggesting that the B acid sites were active for adsorption but not for reaction. On the other hand, the L acid site related N-H stretching features at 3275 and 3356 cm⁻¹, assigned to -NH₂ and -NH species, showed a more distinct response to the nature of the gas environment and the temperature, while the 3150 cm⁻¹ feature, assigned to adsorbed NH₃ species, remained passive towards reaction. Regarding the reaction mechanism, we draw the following conclusions: (a) NO reacts with NH₃ connected to the inert site (3150 cm⁻¹). (b) NH₃ adsorbs to active L acid site forming NH₂ by H-abstraction (3275 and 3356 cm⁻¹), followed by its interaction with NO leading to NH₂NO intermediate formation and further reaction to N₂ and H₂O. (c) In case of N₂O formation, NH₂NO may be expected to lead to NHNO intermediates and finally N–NO.

Figure 7 depicts the results of the in situ detection of the exhaust gas during different temperature SCR experiments from 100 to 450 °C. Each temperature was kept for 20 min to reach an equilibrium state. When increasing the temperature, desorption peaks were observed indicating changes in the surface concentration as a function of temperature. At lower temperatures, the presence of NH₃ peaks but absence of NO desorption peaks (not shown) suggests a surface reaction following the E-R rather than the L-H route, in agreement with the literature [32,33]. In addition, insight into the reaction mechanism can be obtained. To this end, three steps have been distinguished, i.e., adsorption of reactants (mainly NH₃), reaction, and desorption of products (such as H₂O, NO₂, and N₂O). According to Figure 7, there are low temperature desorption peaks of NH₃ at 25/100 °C and 100/150 °C, which are attributed to NH₃ desorption from the B acid site, while high temperature NH₃ peaks above 150 °C are assigned to desorption from L acid site [5,101,102]. These peaks disappeared at about 300/350 °C, consistent with the disappearance of L acid sites in DRIFTS (see Figure 5). The strong water desorption peak at 25/100 °C originates from condensation of OH groups, e.g., after desorption of NH₃ bound to B acid sites [5]. The first desorption peak of NO₂ at 150/200 °C can be attributed to the distortion and release of NO₂ from nitrite/nitrate species by NO molecules connected to surface oxygen sites [103,104]. The second peak at 200/250 °C shows that NO oxidation is faster than NO₂ desorption at 200 °C. Similarly, the first desorption peak for N₂O detected at 200/250 °C is indicative of its formation on active sites and subsequent chemisorption at temperatures <250 °C [105].

All in all, catalyst characterization of TiO₂/SBA-15+3xVO_x reveals a relatively high surface area (>350 m²/g) and the presence of surface hybrid V–O–M bonds connecting the vanadia aggregates to the substrate. The NH₃-SCR catalytic performance follows a volcano shape with temperature, with an optimal deNO_x value of about 60% at 300 °C. Finally, the operando DRIFTS results reveal N–H stretching features related to B and L acid sites.

3. Discussion

3.1. Surface Vanadia Structure

Previously, highly dispersed vanadia and titania on SBA-15 have been studied in detail under hydrated and dehydrated conditions [74]. Despite the lower loading ranges discussed (V: 0.00001–0.7 V/nm²; Ti: 0.001–0.7 Ti/nm²) valuable structural information for the present sample (1.6 V/nm²; 2 Ti/nm²) was obtained regarding the nature of the surface species, in particular, as all samples are well below monolayer coverage [26,106,107]. This is supported by the absence of Raman features due to crystalline V₂O₅ and TiO₂, implying the presence of amorphous domains, consisting of dispersed titania and vanadia species on the silica surface. On the other hand, the detailed surface composition after successive deposition of titania and vanadia needs closer examination, especially as it was reported that vanadium is preferentially deposited on titanium sites [26,108].

For TiO₂ deposited on SBA-15, the Ti loading was quantified as 2 Ti/nm², which is significantly below monolayer coverage (6–8 Ti/nm²) [106]. DR UV-Vis spectra show an absorbance band at about 260 nm, attributed to titanium in 5-fold coordination (see Figure 3) [76]. The UV-Vis edge energies were determined to be 3.2 and 3.0 for the hydrated and dehydrated state, respectively. Comparison with edge energies of titania structures ranging from isolated tetrahedrally coordinated TiO₄ (4.3 eV) to oligomeric TiO₅ species (3.4 eV), suggests the presence of oligomeric TiO₅ species in the VO_x/TiO₂–SiO₂ sample. Regarding the effect of water on the titania structure, no difference was found between the dehydrated and hydrated state at extremely low Ti loading (<0.05 Ti/nm²), in contrast to higher loadings (> 0.1 Ti/nm²) [74]. In fact, adsorbed water was reported to interact with adjacent titanium atoms (Ti–O–Ti) in oligomeric titania, leading to an edge energy 0.2 eV higher than in the dehydrated state, consistent with the behavior observed here [74]. High temperature calcination is expected to lead to an increase in Ti–O–Ti linkages as part of domain TiO₂ [26] without formation of crystalline TiO₂, as discussed above (see above). While anatase and rutile related Raman bands are absent, the feature at 286 cm⁻¹ is attributed to TiO₆ octahedra as part of amorphous TiO₂ domains on the silica surface.

After vanadia deposition, adsorption-desorption isotherms are characterized by a steep hysteresis, attributed to cavitation-controlled evaporation in a narrow range of the pore neck, while some pore blockage or seepage may not be excluded. The surface area shows only a small decrease with increasing VO_x deposition, while no significant differences in the NLDFT pore-size distribution were observed. This behavior implies that the ALD deposited VO_x did not block the pore openings, but was rather selectively deposited inside the matrix on TiO₂ domain sites, forming ink-bottle pore [109].

Although the ALD method permitted the vanadium precursor to react with all surface adsorption sites, including Ti–OH and Si–OH hydroxyls, preferential deposition seems to be inevitable [29]. According to Table 1, the sample with 1xVO_x deposition shows a lower V:Ti ratio of 0.24, at which vanadium is expected to be mainly loaded on TiO₂ domains [26]. For the 3xVO_x and 5xVO_x samples close to unit V:Ti ratios of 0.85 and 1.08 are observed, which can be interpreted as sub- and over-saturated coverage of VO_x on TiO₂, respectively. It should be mentioned that the VO_x deposition rate showed differences among the three samples. In fact, based on the growth rate for 1xVO_x, leading to a V/Ti ratio of 0.24, the extrapolated V/Ti ratio for 3xVO_x should be 0.72, which is slightly smaller than the actual value of 0.85, while the extrapolated V/Ti ratio for 5xVO_x should be 1.20, a value slightly larger than the observed value of 1.08. The increase in VO_x growth rate is attributed to a change in deposition site from titania to silica when approaching TiO₂ saturation coverage. On the other, the mobility of surface VO_x species was reported to reach its lowest surface free energy (SFE) at elevated temperature under reaction conditions, resulting in a two-dimensional spreading of VO_x species on silica and titania [110,111]. To this end, it is worth mentioning that the IR spectra recorded at elevated temperature under reaction conditions reveal a hydroxyl signal at 3740 cm⁻¹ due to surface Si–OH groups (see Figure 5), indicating that the deposition of TiO₂ and VO_x did not lead to complete coverage of the silica surface.

The UV-Vis DRS and Raman spectra provide structural information on the VO_x surface species (see Figs. 3 and S3). It is well known that the structure of surface VO_x depends sensitively on the gas environment [112]. UV-Vis spectra of the hydrated state are characterized by a 250–550 nm band, consistent with polymeric VO_x with pseudo-octahedral coordination, and by a band at 250–450 nm for the dehydrated catalyst, reflecting the presence of tetrahedrally coordinated oligomeric VO_x species, overlapping with the signal of the TiO₂/SiO₂ substrate in the dehydrated state. The presence of H₂O can hydrate the surface VO_x species leading to the formation of V₂O₅·nH₂O-like gels, as evidenced by Raman features at 267, 516, 699, and ~1020 cm⁻¹ under ambient conditions (see Figure 1 and Figure 3). Regarding the position of the V=O stretching bands (~1020 and 1038 cm⁻¹) also the effect of moisture needs to be taken into account [69]. Upon dehydration, a 13 cm⁻¹ blueshift of the V=O feature is observed (see Figure 3), consistent with the literature [69], leading to the formation of tetrahedrally coordinated vanadium ions. In the dehydrated state, the position of the V=O band at 1038 cm⁻¹ is located between the values 1031 cm⁻¹ and 1042 cm⁻¹ reported for bare TiO₂ and SiO₂, respectively [29,113,114], suggesting the simultaneous existence of vanadia species on both oxides attached via V–O–Ti and V–O–Si bands.

Summarizing, regarding the structure of the VO_x surface species we propose based on our findings that (i) titania is dispersed on the silica surface in domains containing low oligomerized TiO₅, (ii) VO_x is preferentially deposited on titania sites at low VO_x loadings (1xVO_x), inducing the formation of dispersed oligomeric species, (iii) increasing VO_x loading results in the deposition of V on both Ti and Si sites (3xVO_x, 5xVO_x).

3.2. Rate-Determining Step

In agreement with the literature, the observed temperature-dependent efficiency for NO removal follows a volcano shape, with a maximum NO conversion at 250–350 °C. N₂ selectivities are near 100% within 100–300 °C and decrease to 90% as temperatures rise to 450 °C (see Figure 4).

The reason for the reactivity changing with temperature was related to the different processes determining the rate. Generally, the SCR reaction process on the VO_x catalyst could be divided into three steps, i.e., adsorption of reactants, redox reaction, and desorption of products. On the one hand, it was stated that the redox reaction was rate-determining at low temperature, while adsorption of NH₃ on surface acid sites was the rate-determining step (RDS) at high temperature [5,115]. On the other hand, desorption of generated H₂O was considered the low-temperature RDS, and the reductive reaction of NH₃ the high-temperature RDS [116]. To explore possible structural factors related to catalytic reactivity, operando DRIFT was applied.

Based on the DRIFT spectra in Figure 5 and Figure 6 part of the L acid sites are proposed as active sites, due to their gas phase and temperature dependent behavior. These sites can be associated with the N-H stretching of coordinated amide, while the stable L acid sites can be related to the N–H vibration of coordinated NH₃. Thus, the detailed H-abstraction ability is important even for the same type of adsorption sites. According to the literature, dispersed oligomeric VO_x species are believed to be the main active species, providing oxygen (V=O or V–O–V) [7,56,117,118], for H-abstraction of adsorbed NH₃ to form an amide intermediate [98]. To this end, the samples TiO₂/SBA-15-3xVO_x and TiO₂/SBA-15-5xVO_x with a larger fraction of oligomeric VO_x show a better NO_x conversion than TiO₂/SBA-15-1xVO_x (not shown). Also, the changes of the acid site-related signals with increasing temperature demonstrate that at temperatures < 300 °C the H-abstraction step is slower than the adsorption of NH₃ (see Figure 5). The presence of adsorbed NH₃ at low temperatures was evidenced by NH₃ desorption peaks. On the other hand, desorption of water was observed at very low temperatures, indicating that its desorption was slower than its formation by reaction. Thus, we propose the RDS at low temperatures to be related to the desorption of products, especially water. To this end, additional water vapor was reported to suppress the reactivity irreversibly, indicative of the occupation of active sites by water [72]. Our conclusions are also supported by the fact that N₂O formed at temperatures below 200 °C was adsorbed on the surface rather than being released into the gas phase.

Except for the redox properties surrounding the acid site, the acid strength is another important factor concerning the SCR reaction, relevant for both the adsorption of reactants (mainly NH₃ in E-R route) and the desorption of products (such as H₂O, NO_2, and N₂). The acidity of binary oxides and ternary oxides VO_x/TiO₂/SiO₂ has been the subject of previous studies, both on solid solutions and supported systems [5,29,30,108]. For binary systems, the surface acidity was found to increase as VO_x/SiO₂ < TiO₂/SiO₂ < VO_x/TiO₂ [30]. Regarding VO_x/TiO₂/SiO₂ systems prepared by ALD [29], samples with lower VO_x loading (1 V/nm², 3 Ti/nm²) and higher VO_x loading (2 V/nm², 3 Ti/nm² ) were compared towards their acidity behavior, showing a decrease (increase) in weak/medium (strong) strength acid sites with loading. Furthermore, strong acid sites were assigned to VO_x species on titania and weak acid sites to VO_x species on silica [29]. In our study, NH₃ desorption peaks for TiO₂/SBA-15+1xVO_x were detected at higher temperature as compared to 3xVO_x and 5xVO_x (see Figure S4), consistent with the formation of stronger acid sites at lower V loading on titania. The N₂O desorption signal also follows the trend of the VO_x acid sites as 1xVO_x > 3xVO_x > 5xVO_x. For the 1xVO_x sample, even at 300/350 °C, a weak N₂O desorption peak was detected. The 3xVO_x and 5xVO_x samples showed a very similar desorption behavior of reactants and products. At temperatures > 300 °C, no more desorption of NH₃ but an increase in the N₂O concentration was detected, reflecting an increase in the H-abstraction ability. Thus the RDS at high temperatures may be related to the adsorption of NH₃.

3.3. Structure-Activity Relationship

In the sections above, we discussed the structure of VO_x loaded on TiO₂/SBA-15 and several aspects affecting the NH₃-SCR activity. Compared to previous VO_x-based model systems with similar V loading on SiO₂, TiO₂, or SiO₂ + TiO₂ prepared by other techniques, our VO_x/TiO₂/SBA-15 catalyst shows an improved NO_x conversion and operation window [5,119,120,121]. For NH₃-SCR over supported VO_x catalysts, high reactivity has been related to crystalline (anatase) TiO₂ [5,122,123]. Mixing SiO₂ with TiO₂, to be used as support for vanadia, introduces extra acid sites, and is accompanied by the transformation of crystalline into amorphous titania, resulting in a decrease in the catalytic performance compared to bare titania [5,124]. However, by adjusting the Si/Ti ratio, a volcano-shape of the SCR reactivity with Si content was obtained [124], indicating synergetic effects regarding the reactivity, possibly related to the presence of Ti–O–Si surface sites. Kobayashi et al. studied the acidity of TiO₂–SiO₂ mixed systems, also showing a volcano-shaped relationship as a function of composition, with a maximum at a Si/Ti ratio of 1 [124]. On the other hand, according to other studies, using the same sol-gel method, the best SCR activity was observed for lower Si/Ti ratios such as 2/8 [72,124]. The mismatch between the acidity and reactivity may be explained by the RDS discussion above, as within the low and medium temperature range, the desorption step was rate determining for the strong acid sites. In present work, ALD deposition of TiO₂ onto SiO₂ results in the formation of oligomeric TiO₂ domains, with a limited boundary between TiO₂ and SiO₂, in contrast to the more uniform mixture of Ti and Si obtained by the sol-gel method. The improved catalytic performance of the TiO₂/SBA-15+3xVO_x compared to other SiO₂/TiO₂-based VO_x catalysts can therefore be explained by the comparable amounts of V and Ti, allowing V to be deposited on TiO₂ and boundary sites (Ti–O–Si). This leads to the formation of VO_x species supported by hybrid sites (V–O–Ti, V–O–Si), thus increasing the total number of acid sites. Consistent with this scenario, for lower V loading (1xVO_x), significantly lower NO_x conversion is observed (not shown), due to the preferential deposition of V on (amorphous) TiO₂, while for higher V loading (5xVO_x), a similar behavior as for the 3xVO_x catalyst was obtained, due accessibility of both TiO₂ and Ti–O–Si boundary sites.

Meanwhile, it is well-known that the SCR reactivity of VO_x/TiO₂ catalysts shows a narrow temperature window within the high temperature range, strongly decreasing below 300 °C [125], VO_x/SiO₂ catalysts are characterized by low SCR reactivity but a wider temperature adaptability even at low temperatures [126]. Besides, according to the literature, produced H₂O was adsorbed on surface active sites because the activation barriers of the NH₂NO decomposition on the catalyst surface were much lower than those calculated for the gas-phase reaction [127]. Thus, in the low temperature range, the desorption of water and other product molecules was considered rate-determining. Keranen et al. reported that VO_x supported on pure TiO₂ led to a larger number of strong acid sites but less weak/medium ones than on a mixture of TiO₂ and SiO₂ [29], implying strong adsorption of NH₃ and H₂O. In fact, Figure 7 reveals the appearance of desorption peaks of water during heating steps. Thus, a limited mixture of TiO₂ and SiO₂ decreases the surface acid strength, facilitates desorption of H₂O and other products and is therefore expected to improve the SCR activity at low temperatures.

Another aspect that is proposed to influence the low-temperature SCR performance is the preparation of the TiO₂ domains on the SiO₂ support [26]. In contrast to other methods used for the preparation of mixed SiO₂ + TiO₂ supports, such as sol-gel based synthesis [5] and co-precipitation [124], the ALD approach enables controlled deposition of sub-monolayer titania and vanadia, resulting in micro-domain areas of TiO₂ and well dispersed VO_x species. The deposition of VO_x on small domains of an oxide less reducible than vanadia but more reducible than the support is expected to improve the dispersion of surface VO_x species [128]. Furthermore, the high surface area of the mesoporous matrix can be largely preserved by the low-temperature ALD approach and allows to further increase the VO_x loading for higher SCR catalytic performance.

4. Experimental

4.1. Chemicals

The chemicals used for the preparation of SBA-15, including triblock copolymer Pluronic P123 (molecular weight = 5800, EO₂₀PO₇₀EO₂₀) and tetraethylorthosilicate (>99%, TEOS), were purchased from Sigma-Aldrich Chemie GmbH (Munich, Germany). As ALD precursors for TiO₂ and VO_x, titanium tetrachloride (>99%, TiCl₄) and vanadium oxytrichloride (>99%, VOCl₃) were employed, respectively, which were purchased from Sigma-Aldrich Chemie GmbH (Munich, Germany). As oxygen source in the ALD process demineralized water was used.

4.2. Synthesis

For the preparation of SBA-15 [129], 4.0 g P123 was completely dispersed in a mixture of 120 mL HCl solution (2 M) and 30 mL water in a PP bottle at 35 °C with 250 r/min stirring for 2 h. Then 10 mL TEOS was added at 35 °C with 400 r/min stirring for 20 h. After that, the bottle was put in an oven at 85 °C with hydrothermal crystallization treatment for 24 h. Using vacuum filtration, the as-made SBA-15 was collected, followed by calcination to remove the template P123 in a muffle furnace heated to 550 °C for 12 h (heating rate: 1.5 °C/min).

VO_x/TiO₂/SBA-15 samples were prepared by ALD using a deposition system described in detail elsewhere [12]. Briefly, three ALD cycles of titania were deposited onto silica SBA-15. Finally, VO_x was deposited with 1, 3, and 5 cycles leading to different V loadings. For the ALD of TiO₂, the TiCl₄ precursor was evaporated in a feeding bottle at 40 °C and reacted in the reaction chamber at 120 °C for 60 s. Similarly, for VO_x ALD, the VOCl₃ precursor was evaporated at 40 °C and reacted at 60 °C for 60 s. In both processes, water at room temperature acted as oxidant. Between reaction steps the system was purged (N₂, 60 s).

The sample with 3 cycles of TiO₂ on SBA-15 is labeled as ‘TiO₂/SBA-15′, and samples with different cycles of VO_x on TiO₂/SBA-15 as ‘TiO₂/SBA-15+nxVO_x‘ (n = 1, 3, 5). Hydrated samples correspond to samples exposed to ambient conditions for at least one week after synthesis. Samples labeled as ‘dehydrated’ were heated at 500–600 °C for 1 h under a controlled atmosphere (20% O₂/80% N₂) and then cooled to room temperature.

4.3. Catalytic Performance

The SCR performance was measured in a commercial reactor (CCR1000, Linkam Scientific Instruments, Tadworth, UK) in combination with quantitative gas-phase FT-IR spectroscopy, employing a Tensor 27 instrument equipped with a deuterated and L-alanine-doped triglycine sulfate (DLaTGS) detector (Bruker, Mannheim, Germany). Using mass-flow controllers, the inlet gas concentration was adjusted to 500 ppm NO, 500 ppm NH₃, and 5% O₂ (balanced with N₂), by mixing 2000 ppm NO/N₂ (±0.25% abs.), 2000 ppm NH₃/N₂ (±0.25% abs.), O₂ (≥99.999%), and N₂ (≥99.999%). During the NH₃-SCR process, the contents of NO, NH_3, and O₂ in the feed were set to 500 ppm, 500 ppm, and 5%, respectively, balanced with N₂. The weight of the catalyst sample in the reactor bed was 15 mg and the gas mixture passed through the catalyst bed at a flow rate of 50 NmL·min⁻¹, resulting in a gas hourly space velocity (GHSV) of 40,000 h⁻¹. The temperature was increased from 100 °C to 450 °C at 50 °C steps. Each temperature was kept for 20 min to allow for equilibration. The outlet gas mixture was analyzed by an FT-IR spectrometer equipped with a low volume gas cell (25 mL, LFT, Axiom Analytical, Inc, Tustin, CA, USA). The gas cell was heated to 120 °C to avoid the condensation of water produced by the catalytic reaction. IR spectra were continuously recorded with a resolution of 4 cm⁻¹. For quantitative analysis of the IR active gas-phase components (NO, NO₂, NH₃, N₂O) calibration curves were recorded.

By combining the outlet concentrations of NO, NO₂, N₂O, and NH₃ with the inlet concentrations, the N₂ concentration was calculated based on nitrogen mass conservation, assuming that nitrogen was present only as gas-phase molecular species. The NO_x conversion and N₂ selectivity were calculated using the following Equations (2) and (3):

$${\mathrm{NO}}_x\mathrm{conversion}=\frac{{[{\mathrm{NO}}_x]}_{\mathrm{in}}-{[{\mathrm{NO}}_x]}_{\mathrm{out}}}{{[{\mathrm{NO}}_x]}_{\mathrm{in}}}\cdot 100\%$$

(2)

$${\mathrm{N}}_2\mathrm{selectivity}=\frac{{2[\mathrm{N}}_2{]}_{\mathrm{out}}}{{{[\mathrm{NO}}_x]}_{\mathrm{in}}-{{[\mathrm{NO}}_x]}_{\mathrm{out}}+{{[\mathrm{NH}}_3]}_{\mathrm{in}}-{{[\mathrm{NH}}_3]}_{\mathrm{out}}}\cdot 100\%$$

(3)

where the subscripts ‘in’ and ‘out’ represent the inlet and outlet flow, respectively.

4.4. Characterization

N₂ adsorption-desorption measurements were carried out on a NOVA 3000e system (Quantachrome, Boynton Beach, FL, USA) to determine the specific surface area using the Brunauer-Emmett-Teller (BET) method and the porosity characteristics using non-local density functional theory (NLDFT). Quantification of the vanadium content of the samples was done by inductively coupled plasma optical emission spectroscopy (ICP-OES).

XRD experiments were carried out in transmission geometry on an X-ray powder diffractometer (StadiP, Stoe & Cie GmbH, Darmstadt, Germany) with a Mythen 1K (Dectris, Baden-Daettwil, Switzerland) detector. For the measurements, a Cu Kα1 radiation (λ = 1.540598 Å) and a Ge [110] monochromator was employed.

XP spectra were recorded on an SSX 100 ECSA spectrometer (Surface Science Laboratories Inc., Minneapolis, MN, USA), equipped with a monochromatic Al-K_α X-ray source (1486.6 eV), in a constant analyzer energy (CAE) mode at a 36° detection angle with 0.1 eV resolution. Data analysis included subtraction of a Shirley background and a peak-fit analysis using Gaussian−Lorentzian product functions with 45% Lorentzian share. Atomic concentrations were calculated using the following relative sensitivity factors (RSFs): 0.537 (C 1s), 2.930 (O 1s), 0.817 (Si 2p), 7.810 (Ti 2p), and 9.660 (V 2p).

UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) was carried out on a V-770 spectrometer (Jasco, Pfungstadt, Germany) equipped with a praying mantis mirror cell and a high-temperature reaction chamber (Harrick Scientific Products Inc., Pleasantville, NY). Halogen and deuterium lamps were used as light sources in the Vis and UV, respectively. A Peltier-cooled PbS detector was employed for detection. Spectra were recorded from 800 to 200 nm with a spectral resolution of 0.5 nm. MgO was used as a white standard.

Visible Raman spectra (532 nm excitation) were recorded on a Holo Spec f/1.8i spectrometer (Holographic Imaging Spectrograph, Kaiser Optical Systems, Los Altos, CA, USA) equipped with a transmission grating and a charge-coupled device (CCD) detector. 532 nm radiation obtained by frequency doubling from a Nd:YAG laser (Cobolt Inc., Kassel, Germany) was employed for excitation. The spectrometer was calibrated by the emission lines of a standard argon lamp. The laser power was set to 1.6 mW at the position of the sample as measured by a power meter (Ophir, North Logan, UT, USA).

UV Raman spectra (256.7 nm excitation) were performed with a tunable Ti:sapphire solid-state laser (Indigo-S, Coherent, Santa Clara, CA, USA). The Raman scattered light was dispersed in a three-stage spectrometer (TriVista 555, Princeton Instruments, Krailing, Germany) used in subtractive mode and detected by a charged coupled device (CCD) camera (Spec10:2KBUV, Princeton Instruments, Krailing, Germany), cooled to −120 °C with liquid nitrogen. The spectral resolution was 1 cm⁻¹. All spectra were recorded at room temperature by irradiating the samples for 900 s with a laser power of 2.6 mW at a repetition rate of 5 kHz.

DRIFTS was carried on a Vertex 70 spectrometer (Bruker, Mannheim, Germany) equipped with a liquid N₂ cooled mercury-cadmium-telluride (MCT) detector and a commercial Harrick cell (Harrick Scientific Products Inc., Pleasantville, NY, USA). As a background standard, KBr powder was used. The spectral resolution of DRIFTS was set to 1 cm⁻¹. Blank spectra were recorded by passing pure N₂ through the loaded sample, and the actual gas-phase spectra were obtained by subtracting the blank spectra. For in situ/operando experiments, the gas phase at the outlet of the cell was continuously analyzed by quantitative FT-IR spectroscopy as described above.

5. Conclusions

An NH₃-SCR model catalyst was prepared by controlled deposition of titania and vanadia onto a mesoporous high surface area silica support by use of ALD. The final VO_x/TiO₂/SBA-15 catalyst retained a large surface area, characterized by domains of amorphous titania on silica and the formation of dispersed oligomeric VO_x surface species. The presence of similar amounts of deposited Ti and V allows vanadium to be anchored to both TiO₂ and Ti–O–Si boundary sites, thus increasing the total number of acid sites.

Compared to other catalysts with similar composition, the SBA-15-based catalyst used in the present study shows a better NH₃-SCR performance and a wider temperature window for operation. The superior catalytic behavior is attributed to several factors, including the controlled ALD synthesis, which leads to a high dispersion of VO_x oligomeric species due to the high surface area of the mesoporous matrix. Considering the analysis of the RDS at low temperature, desorption of products is of great importance due to their strong interaction with acid sites. The structure of VO_x species with amorphous TiO₂ domains on the SiO₂ substrate, forming hybrid supports of V–O–Ti and V–O–Si, provided more acid sites, especially weak acid sites, thus leading to a better catalytic SCR performance than VO_x/TiO₂ and also VO_x on atomically mixed SiO₂ + TiO₂ supports prepared by sol-gel methods.

Our findings demonstrate the potential of using high surface area VO_x/TiO₂/SBA-15 model catalysts for gaining new insight into the factors determining the mode of operation of supported vanadia catalysts used for NH₃-SCR.