Preparation and Catalytic Performance of Vanadium-Molybdenum Catalysts Supported on TiO₂ Supports from Different Sources

Abstract

Selective catalytic reduction (NH₃-SCR) technology is the predominant technique for industrial flue gas denitrification, with its improvement dependent on the development of highly efficient catalysts. As the most widely employed support for vanadium-based DeNO_x catalysts, TiO₂ exerts significant influences on the dispersion state of active species and the catalytic performance through its physicochemical properties. In this work, three VMoO_x@TiO₂ catalysts were prepared using three different Ti precursors. Multiple characterization techniques were employed to systematically investigate the effects of different support origins on the surface morphology, acid-base properties, and redox performance of the catalysts. The results revealed that the tetrabutyl titanate-derived support possesses the largest specific surface area and most abundant mesoporous structure, facilitating highly dispersed V and Mo species. XPS analysis demonstrated that V⁴⁺ proportion increased from 38.9% to 56.6% and Mo⁶⁺ proportion increased from 50.2% to 78.9% from Catalyst A to C, attributed to enhanced V–O–Mo bridge bond electron transfer. DFT calculations confirmed that the electron transfer driving force on Catalyst C was the largest, and the NH₃ adsorption energy at Lewis acid sites reached −102.4 kJ/mol, significantly exceeding those of Catalysts A and B. The potential energy surface analysis revealed that the rate-determining step exhibited the lowest energy barrier on Catalyst C, consistent with its superior DeNO_x activity: it reaches complete NO conversion at 350 °C and displays optimal sulfur resistance. This study integrates experimental characterization with theoretical computation to establish the structure–activity relationship between TiO₂ support properties and the DeNO_x performance of vanadium-molybdenum catalysts, providing both theoretical guidance and a scientific basis for the rational design of high-performance SCR DeNO_x catalysts.

1. Introduction

Nitrogen oxides (NO_x) are primary atmospheric pollutants discharged from the coal power, steel and cement industries [1,2]. They trigger acid rain, photochemical smog and PM_2.5, seriously endangering ecological and human health [3,4,5]. Rising environmental standards and widespread ultra-low emission limits raise stricter demands for NO_x abatement. Its high efficiency, stable performance and mature processes make NH₃-SCR the dominant denitrification method for industrial stationary flue gas [6,7,8]. However, the catalytic system of this technology lacks sufficient low-temperature denitrification activity, making it incompatible with low-temperature flue gas from non-power industries including cement, glass and waste incineration [9,10]. Therefore, scholars worldwide have extensively researched and developed low-temperature SCR catalysts.

To overcome these constraints, numerous low-temperature and non-vanadium catalysts have been explored. Manganese-based oxides (MnO_x/TiO₂, Mn/Cu-cordierite) are the most active at low temperature but suffer from poor SO₂/H₂O resistance, which SiO₂ modification can partly relieve [11,12,13]. Cerium-based catalysts, including CeTiO_x solid solutions and transition-metal- (especially Mo-) doped CeO₂(-TiO₂), gain strong redox capacity and acidity through DFT-guided design [10,14,15]. LDH-derived NiMnAlO_x, Co–Mn–Fe–Al–LDO and TiO₂/CoMnFeO_x give near-complete conversion with high N₂ selectivity, while Cu–Mn spinel, CuO/red-mud and Fe-SSZ-13 broaden the active window and clarify NH₃ activation at unsaturated cationic sites [16,17,18,19]. Vanadium-based systems are likewise refined via polymeric vanadyl sites, Ce/Ta co-doping and reuse of spent V₂O₅–WO₃/TiO₂ [20,21]. The typical formulation of commercial NH₃-SCR catalysts is V₂O₅–WO₃(MoO₃)/TiO₂, where V₂O₅ serves as the primary active component, WO₃ or MoO₃ functions as the promoter to broaden the temperature window and suppress SO₂ oxidation, and TiO₂ (anatase phase) acts as the support to provide a high specific surface area and favorable dispersion of active species [22,23,24]. Tungsten-based catalysts possess outstanding acidity, redox capacity and adsorption performance [25]. Tungsten doping is a widely adopted modification strategy for denitrification catalysts, and existing studies have verified that it can enhance the poison resistance and catalytic activity of catalysts [26,27]. Vanadium-based catalysts are widely adopted as commercial denitrification catalysts owing to their favorable sulfur resistance and catalytic activity [28,29,30]. Molybdenum doping increases the total amount of Brønsted and Lewis acid sites on catalysts, strengthens the adsorption and activation of NH₃, results in high NO_x conversion efficiency, and simultaneously restrains ammonia slip [31].

The structural and surface properties of the support play a critical role in determining the DeNO_x performance of SCR catalysts [32,33]. In this catalytic system, the TiO₂ support not only provides physical scaffolding for the active components, but its crystal phase structure, specific surface area, surface hydroxyl density, and other physicochemical properties also directly influence the dispersion state, chemical environment, and interaction strength between the active species V and Mo and the support, thereby exerting decisive effects on the DeNO_x performance of the catalyst [34,35,36]. Studies have demonstrated that the crystal structure and surface vacancy characteristics of supports directly govern the dispersion and catalytic activity of active species [37]. Doping SiO₂ into TiO₂ to construct TiO₂–SiO₂ composite supports can inhibit the crystallization of TiO₂ and increase the specific surface area, thereby enhancing the wide-temperature deNO_x activity and sulfur resistance [13]. Co-doping of Ce and Ta into the TiO₂ support has also been shown to broaden the active temperature window of vanadia-based catalysts through synergistic effects [38]. Furthermore, the preparation of low-temperature SCR catalysts using TiO₂ recovered from spent catalysts has confirmed the regulatory role of TiO₂ source on the distribution of active species [39]. However, most existing work has focused on post-treatment modification or elemental doping of a pre-formed TiO₂ support, whereas the influence of the TiO₂ precursor source itself on the final catalyst has received insufficient systematic attention [40,41]. In industrial practice, the TiO₂ used as catalyst support can be derived from various sources, including commercial titanium dioxide pigment, TiO₂ prepared by hydrolysis of titanyl sulfate (TiOSO₄), and TiO₂ synthesized via sol–gel- methods using organic titanium precursors such as tetrabutyl titanate. These TiO₂ materials from different sources exhibit significant differences in phase composition, particle size distribution, specific surface area, and surface chemical properties [42,43,44].

Based on the above research background, this work selected three industrially representative TiO₂ precursors—titanium dioxide powder (direct use type), titanyl sulfate (hydrolysis-precipitation type), and tetrabutyl titanate (organic alkoxide hydrolysis type)—to prepare TiO₂ supports, followed by loading V₂O₅–MoO₃ via incipient wetness impregnation to prepare catalysts VMoO_x@TiO₂-A, VMoO_x@TiO₂-B, and VMoO_x@TiO₂-C, respectively. Multiple characterization techniques (SEM, TEM, XRD, XPS, NH₃-TPD, H₂-TPR, CO₂-TPD and BET) were combined with DFT calculations to reveal the effects of support origin on catalyst structure and surface chemistry, and the NH₃-SCR activity and sulfur resistance were evaluated in a fixed-bed reactor. This study aims to elucidate the structure–activity relationship between TiO₂ support origin and DeNO_x performance of vanadium-molybdenum catalysts, providing scientific basis for the optimized selection of industrial SCR catalyst supports.

2. Results and Discussion

2.1. Surface Morphology and Composition Analysis

Figure 1 presents the scanning electron microscopy (SEM) images of the three catalysts VMoO_x@TiO₂-A, VMoO_x@TiO₂-B, and VMoO_x@TiO₂-C. As can be observed, the three catalysts exhibit distinctly different surface microstructural features. The VMoO_x@TiO₂-A catalyst (with titanium dioxide powder as support) displays relatively regular particle morphology with a uniform particle size distribution, while obvious agglomeration exists among particles, which is consistent with the characteristics of commercial titanium dioxide powder possessing a relatively large particle size and small specific surface area. The VMoO_x@TiO₂-B catalyst (prepared by titanyl sulfate hydrolysis) exhibits a finer and more dispersed nanoparticle morphology, with inter-particle stacking forming abundant pore structures, indicating that the titanyl sulfate hydrolysis process favors the formation of high-surface-area TiO₂ supports. The VMoO_x@TiO₂-C catalyst (prepared by tetrabutyl titanate hydrolysis) shows amorphous to weakly crystalline morphological features, with fine and uniformly dispersed particles and a porous fluffy surface structure, which is attributed to the unique gel network structure formed during the sol–gel process of organic titanium precursors, generating a hierarchical meso-microporous channel system after calcination.

Figure 1. SEM images of ((a1): Mag = 10KX, (a2): Mag = 20KX, (a3): Mag = 50KX) VMoO_x@TiO₂-A, ((b1): Mag = 10KX, (b2): Mag = 20KX, (b3): Mag = 50KX) VMoO_x@TiO₂-B, and ((c1): Mag = 10KX, (c2): Mag = 20KX, (c3): Mag = 50KX) VMoO_x@TiO₂-C.

Figure 2 presents the transmission electron microscopy (TEM) images of the three catalysts. From the high-resolution TEM images, lattice fringes of TiO₂ can be identified in all three catalysts, but their clarity and distribution patterns differ. Clear anatase-phase TiO₂ lattice fringes with an interplanar spacing of approximately 0.37 nm corresponding to the anatase (201) plane can be observed in VMoO_x@TiO₂-A, indicating that the titanium dioxide powder support possesses good crystallinity. In VMoO_x@TiO₂-B, the TiO₂ grain size is significantly reduced with an increased lattice fringe density, and the high dispersion characteristics of V and Mo oxide species on the support surface can be observed in some regions, with no obvious V₂O₅ or MoO₃ lattice fringes detected, suggesting that the active components achieved favorable monolayer or sub-monolayer dispersion on the high-surface-area support. VMoO_x@TiO₂-C exhibits the finest TiO₂ nanocrystallites with short and disordered lattice fringes, indicating that TiO₂ prepared by the sol–gel method possesses lower crystallinity. Such weakly crystalline features are conducive to forming more surface defect sites and oxygen vacancies, providing additional anchoring sites for active components [45,46].

Figure 2. TEM images of VMoO_x@TiO₂-A (a1–a3), VMoO_x@TiO₂-B (b1–b3), and VMoO_x@TiO₂-C (c1–c3).

Figure 3 shows the XRD patterns of three catalysts. Compared with standard PDF cards, all three catalysts exhibit characteristic diffraction peaks of anatase TiO₂ (PDF#21-1272) at 2θ = 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, 62.7°, and 70.3°, corresponding to the (101), (004), (200), (105), (211), (204), and (220) crystal planes, respectively. The VMoO_x@TiO₂-B catalyst additionally displays distinct TiOSO₄ (PDF#49-0467) diffraction peaks, with major characteristic peaks located at 2θ = 16.2°, 21.4°, 23.6°, 27.6°, 28.1°, and 34.8°, among which the (111) plane diffraction peak at 23.6° exhibits the highest intensity. This indicates that the titanyl sulfate precursor retains partial sulfate structure after calcination without complete transformation into anatase TiO₂, which also provides a chemical microenvironment distinct from anatase TiO₂ for anchoring the active components. The diffraction peaks of VMoO_x@TiO₂-A are sharp with high intensity, indicating that the titanium white support possesses high crystallinity and large crystallite size. The diffraction peak intensity of VMoO_x@TiO₂-C is significantly reduced with increased full width at half maximum (FWHM). According to the Scherrer equation, its crystallite size is markedly decreased, suggesting that tetrabutyl titanate forms low-crystallinity nano-anatase TiO₂ after hydrolysis and calcination. No characteristic diffraction peaks of V₂O₅ or MoO₃ are detected in the patterns of all three catalysts, indicating that V and Mo oxides exist in a highly dispersed amorphous state at low loadings.

Figure 3. XRD patterns of the three catalysts.

2.2. Chemical Properties Analysis of Catalysts

To gain deeper insight into the chemical states and relative contents of surface elements on the three catalysts, X-ray photoelectron spectroscopy (XPS) was employed for surface chemical analysis, with the results shown in Figure 4. In Figure 4a, the V 2p_3/2 spectra of the three catalysts can be fitted with two peak components, located at approximately 516.4 eV and 517.2 eV, which are attributed to V⁴⁺ and V⁵⁺ species, respectively [47]. As shown in Table 1, the relative proportions of V⁵⁺ and V⁴⁺ differ significantly among the three catalysts. From VMoO_x@TiO₂-A to VMoO_x@TiO₂-C, the V⁴⁺ proportion exhibits an increasing trend of 38.9% → 45.9% → 56.6%. This result indicates that different TiO₂ supports exert significant regulatory effects on the chemical valence distribution of vanadium species. The TiO₂ support prepared from the organic titanium precursor (tetrabutyl titanate) possesses stronger electron-donating capability on its surface, promoting the partial reduction of V⁵⁺ to V⁴⁺, and its higher specific surface area and abundant surface defect sites allow vanadium species to exist in a more dispersed low-polymeric state, which is favorable for the stabilization of V⁴⁺. According to the “acid–redox” dual-cycle mechanism proposed by Topsoe et al., the efficient cycling of the V⁵⁺/V⁴⁺ redox couple is crucial for the sustained progress of the NH₃-SCR reaction [48].

Figure 4. XPS spectra of V 2p_3/2 (a), O 1s (b), and Mo 3d (c) for the three catalysts.

Table 1. Relative proportions of elemental species on the three catalysts.

Catalyst	V (At%)		O (At%)			Mo (At%)
	V5+	V4+	Oα	Oβ	Oγ	Mo6+	Mo4+
VMoOx@TiO2-A	61.1	38.9	31.0	45.6	23.4	69.8	31.2
VMoOx@TiO2-B	54.1	45.9	30.5	47.9	21.6	71.5	28.5
VMoOx@TiO2-C	43.4	56.6	32.0	45.1	22.9	78.9	21.1

Figure 4b presents the O 1s XPS peak fitting results of the three catalysts. The O 1s spectra of all three catalysts can be fitted into three peak components: the peak at approximately 529.8 eV is attributed to lattice oxygen O_α, the peak at approximately 531.2 eV corresponds to chemisorbed oxygen/surface oxygen vacancy sites O_β, and the peak at approximately 532.5 eV is assigned to surface hydroxyl oxygen or oxygen species in adsorbed water O_γ [49]. As shown in Table 1, the O_α proportions of the three catalysts show minor differences, indicating that the bulk lattice oxygen content is essentially comparable among the three catalysts. It is should be noted that the chemisorbed oxygen O_β plays a crucial role in the SCR DeNO_x reaction. Studies have demonstrated that chemisorbed oxygen Oβ possesses high oxidation activity and can effectively oxidize NO to NO₂, thereby promoting the fast SCR reaction. The O_β content is relatively abundant in all three catalysts (all above 45%), with VMoO_x@TiO₂-B exhibiting the highest O_β proportion of 47.9%, indicating the strongest NO oxidation capability.

The Mo 3d XPS spectra of the three catalysts in Figure 4c reveal that Mo species coexist in two valence states of Mo⁶⁺ and Mo⁴⁺ after peak deconvolution [50]. As shown in Table 1, the valence distribution trend of Mo species is opposite to that of V species—from catalyst A to C, the Mo⁶⁺ proportion increases (69.8% → 71.5% → 78.9%), while the V⁴⁺ proportion also increases (38.9% → 45.9% → 56.6%). This intriguing complementary variation pattern suggests possible electron transfer interactions between V and Mo species. On high-surface-area supports, V and Mo species are highly dispersed and form stronger V–O–Mo bridge bond interactions. Mo⁶⁺ can transfer electrons to V⁵⁺ species through V–O–Mo bridge bonds, i.e., Mo⁶⁺ + V⁵⁺ → Mo⁶⁺ + V⁴⁺ (or electron transfer from Mo to V promotes the partial reduction of V⁵⁺ to V⁴⁺). The presence of high-valence Mo⁶⁺ is favorable for the formation of Lewis acid sites on the catalyst surface, promoting NH₃ adsorption and activation [51]. Furthermore, the high Mo⁶⁺ proportion indicates that Mo species maintain a high oxidation state on high-surface-area supports, which is beneficial for their function of suppressing SO₂ oxidation at V sites [52].

Figure 5 presents the NH₃ temperature-programmed desorption (NH₃-TPD) profiles of the three catalysts. NH₃-TPD is an important technique for evaluating the surface acid strength and acid amount of catalysts, which is of critical significance for understanding the NH₃ adsorption–activation behavior in the SCR reaction [53]. The NH₃-TPD curves of all three catalysts exhibit two distinct NH₃ desorption peaks: a low-temperature desorption peak (100–300 °C region) and a high-temperature desorption peak (300–550 °C region), corresponding to NH₃ desorption from weak acid sites and medium-strong acid sites, respectively. The low-temperature desorption peak is primarily attributed to NH₃ desorption from Lewis acid sites, while the high-temperature desorption peak corresponds to NH₃ desorption in the form of NH₄⁺ from Brønsted acid sites (such as V–OH and Mo–OH). Comparative analysis of the NH₃-TPD results reveals that VMoO_x@TiO₂-B and VMoO_x@TiO₂-C exhibit larger NH₃ desorption peak areas, particularly in the high-temperature desorption peak region, indicating that these two catalysts possess more medium-strong acid sites, which is favorable for maintaining effective NH₃ adsorption and activation over a wider temperature range. This is consistent with the high dispersion of active components V and Mo, forming more exposed acid sites on these two catalysts.

Figure 5. (a) NH₃-TPD curves and (b) acid amount diagram of the three catalysts.

Figure 6 presents the H₂ temperature-programmed reduction (H₂-TPR) profiles of the three catalysts. The low-temperature reduction peak (approximately 350–500 °C) is primarily attributed to the reduction of highly dispersed surface V⁵⁺ species to V³⁺ and the partial reduction of Mo⁶⁺ to Mo⁴⁺. The high-temperature reduction peak (approximately 500–700 °C) corresponds to the deep reduction of bulk V₂O₅ and the further reduction of MoO₃. It is noteworthy that the VMoO_x@TiO₂-C catalyst exhibits the lowest low-temperature reduction peak temperature, indicating that the V and Mo species on this catalyst possess the strongest low-temperature reducibility, i.e., the highest redox activity. This corroborates the XPS analysis result showing that this catalyst has the highest surface V⁴⁺ proportion. The low-temperature reduction peak temperature of VMoO_x@TiO₂-B is intermediate, while that of VMoO_x@TiO₂-A is the highest, indicating that V species on the titanium dioxide powder support have the weakest reducibility.

Figure 6. (a) H₂-TPR curves and (b) H₂ consumption of the three catalysts.

Figure 7 presents the CO₂ temperature-programmed desorption (CO₂-TPD) profiles of the three catalysts. CO₂-TPD is used to characterize the distribution and strength of basic sites on the catalyst surface. The CO₂-TPD curves of the three catalysts show that CO₂ desorption mainly concentrates in the low-temperature region (100–300 °C), corresponding to CO₂ desorption from weak basic sites, while CO₂ desorption in the high-temperature region (>400 °C) is relatively low, indicating that the catalyst surface is dominated by acid sites with relatively few basic sites. Comparative analysis reveals that the CO₂ desorption amounts of the three catalysts show minor differences (within 10%). The fewer basic sites can inhibit the adsorption of SO₂ (an acidic gas) on the catalyst surface, thereby enhancing the sulfur resistance of the catalysts.

Figure 7. (a) CO₂-TPD curves and (b) base amount diagram of the three catalysts.

Figure 8 presents the N₂ adsorption–desorption isotherms and pore size distribution curves of the three catalysts. The results show that the N₂ adsorption–desorption isotherms of all three catalysts belong to Type IV isotherms accompanied by H₂-type hysteresis loops, indicating that all catalysts possess mesoporous structures. The data in Table 2 show that the VMoO_x@TiO₂-A catalyst has the smallest BET specific surface area with a smaller pore volume and larger average pore diameter, which is consistent with the characteristics of the titanium dioxide powder raw material having larger particles and limited specific surface area. The specific surface area of the VMoO_x@TiO₂-B catalyst is significantly increased, with pore size distribution concentrated in the smaller mesopore range, indicating that TiO₂ nanoparticles generated from titanyl sulfate hydrolysis form abundant mesopores through inter-particle stacking. The VMoO_x@TiO₂-C catalyst exhibits the largest specific surface area, which is attributed to the three-dimensional gel network structure formed during the sol–gel process of the organic titanium precursor. The larger specific surface area provides more anchoring sites for high dispersion of active components, facilitating the formation of more exposed active sites and thereby enhancing catalytic activity. Meanwhile, the abundant mesoporous structure facilitates the diffusion and mass transfer of reactant molecules, reducing internal diffusion resistance and improving the effective utilization of the catalyst.

Figure 8. (a) N₂ adsorption–desorption isotherms and (b) pore size distribution curves of the three catalysts.

Table 2. Specific surface area and pore structure data of the three catalysts.

Catalyst	Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)
VMoOx@TiO2-A	76.5	0.17	5.8
VMoOx@TiO2-B	103.2	0.21	3.9
VMoOx@TiO2-C	131.6	0.25	4.6

2.3. Catalytic DeNO_x Activity

Figure 9a shows the NO_x conversion as a function of temperature for the three catalysts in the temperature range of 150–450 °C. As can be observed, all three catalysts exhibit a characteristic volcano-shaped activity profile with DeNO_x activity first increasing and then decreasing with rising temperature. In the low-temperature region (<250 °C), the catalytic activity is kinetically limited, resulting in low denitrification efficiency. As the temperature increases, the reaction rate gradually rises and catalytic activity improves. The VMoO_x@TiO₂-C catalyst demonstrates the most outstanding low-temperature DeNO_x activity, achieving high NO_x conversion at relatively lower temperatures with the widest activity temperature window. The activity of VMoO_x@TiO₂-B is intermediate, while VMoO_x@TiO₂-A exhibits the lowest low-temperature activity, requiring higher temperatures to achieve desirable denitrification efficiency. This is attributed to the larger specific surface area and high abundance of surface acid sites of the VMoO_x@TiO₂-C catalyst, where the synergistic effects of multiple favorable factors result in the optimal DeNO_x catalytic performance.

Figure 9. (a) DeNO_x activity and (b) sulfur resistance performance of the three catalysts.

Figure 9b shows the variation of DeNO_x activity over time for the three catalysts in the presence of SO₂. Upon introduction of 100 ppm SO₂ into the reaction atmosphere, all three catalysts exhibit different degrees of activity decline, but the decline rates and ultimate stabilized activities differ significantly. The VMoO_x@TiO₂-A catalyst shows the most pronounced activity decrease in the presence of SO₂, indicating the weakest sulfur resistance. This is primarily because the lower specific surface area results in low active site density, and the proportion of effective active sites lost due to SO₂ poisoning is larger. The VMoO_x@TiO₂-B and VMoO_x@TiO₂-C catalysts demonstrate better sulfur resistance stability. In particular, the VMoO_x@TiO₂-C catalyst maintains relatively high DeNO_x activity even after prolonged exposure to SO₂. Its excellent sulfur resistance can be attributed to the following factors: the high specific surface area and abundant pore structure provide a dense distribution of active sites, ensuring that sufficient active sites remain to sustain the DeNO_x reaction even when some sites are poisoned by SO₂; the strong surface acidity and weak basicity suppress the chemisorption and oxidation of SO₂ to SO₃; and the strong redox capability facilitates the regeneration of partially sulfated catalyst active sites at elevated temperatures.

2.4. Theoretical Computational Study of Catalysts

To reveal the intrinsic mechanism behind the different catalytic performances induced by various titanium sources, theoretical calculation analyses were carried out on the three catalysts. The detailed theoretical calculation methods are presented in the Supplementary Materials. Figure 10a presents the comparison of DFT-calculated adsorption energies of NH₃ and NO on the three catalysts. The computational results clearly demonstrate that the absolute values of NH₃ and NO adsorption energies on Catalyst C (prepared from tetrabutyl titanate) are significantly higher than those on Catalyst A (titanium dioxide pigment) and Catalyst B (titanyl sulfate). The adsorption energy of NH₃ at V⁵⁺ Lewis acid sites increased from −68.3 kJ/mol on Catalyst A to −85.7 kJ/mol on Catalyst B and −102.4 kJ/mol on Catalyst C. This trend is highly consistent with the experimental NH₃-TPD results. Catalyst C exhibited the largest NH₃ desorption peak area, indicating the most abundant acidic sites and strongest NH₃ adsorption capacity. The high specific surface area of Catalyst C enables V species to exist in a highly dispersed monomeric form, exposing more accessible V⁵⁺ Lewis acid sites. Meanwhile, its higher V⁴⁺ proportion (56.6%) implies a stronger electron-donating ability of the V centers. It should be clarified that the Lewis acid site here is the coordinatively unsaturated V⁵⁺ cation, which acts as an electron acceptor owing to its empty d orbital; the nitrogen lone pair of NH₃ is donated into this empty d orbital to form an N → V dative (coordinate) bond.

Figure 10. (a) DFT-calculated adsorption energies of NH₃ and NO on three catalysts and (b) V–O–Mo bridge bond electron transfer energies and Mulliken charge distribution.

The adsorption energy of NH₃ forming NH⁴⁺ on V–OH Brønsted acid sites was stronger, measuring −121.5 kJ/mol for Catalyst A, −138.2 kJ/mol for Catalyst B, and −152.8 kJ/mol for Catalyst C. The NH₃ adsorption energy at Brønsted acid sites was universally higher than that at Lewis acid sites by approximately 50–55 kJ/mol. The adsorption energy of NO at V=O sites was relatively weak (−22.6 to −38.9 kJ/mol), falling within the range of physisorption to weak chemisorption.

Figure 10b reveals the regulatory mechanism of V–O–Mo bridge bond electron transfer on catalyst activity. The DFT-calculated V⁵⁺ → V⁴⁺ electron transfer energies (through V–O–Mo bridge bonds) were −0.32 eV for Catalyst A, −0.56 eV for Catalyst B, and −0.78 eV for Catalyst C. This electron transfer process can be described as:

V⁵⁺–O–Mo⁴⁺ → V⁴⁺–O–Mo⁶⁺

(1)

The negative electron transfer energy indicates that the V⁴⁺–O–Mo⁶⁺ configuration is thermodynamically more stable. The driving force for this electron transfer in Catalyst C is the largest (−0.78 eV), which corresponds perfectly with the experimental XPS results. Catalyst C exhibits the highest V⁴⁺ proportion (56.6%) and the highest Mo⁶⁺ proportion (78.9%), forming a complementary distribution pattern of simultaneous V⁴⁺ and Mo⁶⁺ enrichment.

From Catalyst A to C, the positive charge on V atoms progressively decreased (+1.82 → +1.65 → +1.48 |e|), indicating that V acquired a higher electron density (corresponding to V⁵⁺ → V⁴⁺ reduction). Simultaneously, the positive charge on Mo atoms progressively increased (+2.14 → +2.38 → +2.52 |e|), reflecting the oxidation of Mo to higher oxidation states (Mo⁴⁺ → Mo⁶⁺). This synergistic electron transfer between V and Mo through V–O–Mo bridge bonds represents the characteristic interaction formed when V and Mo species are highly dispersed on high-surface-area carriers (Catalyst C).

From the perspective of catalytic performance, the V⁵⁺/V⁴⁺ redox cycle is essential for the sustained progression of NH₃-SCR reactions. DFT calculations demonstrate that the stable existence of V⁴⁺ on Catalyst C lowers the V⁵⁺/V⁴⁺ redox potential, facilitating rapid re-oxidation and regeneration of V sites during the reaction process. Meanwhile, high-oxidation-state Mo⁶⁺ favors the formation of additional Lewis acid sites, promoting NH₃ adsorption and activation. Furthermore, high-valence Mo⁶⁺ effectively suppresses the oxidation reaction of SO₂ → SO₃ on V sites, thereby enhancing the sulfur resistance of the catalyst.

2.5. NH₃-SCR Reaction Pathway and Potential Energy Surface

Figure 11 presents the potential energy surfaces of the NH₃-SCR reaction on the three catalysts. The reaction proceeds through the classical acid-redox dual-cycle pathway, encompassing the following key steps.

Figure 11. DFT-calculated potential energy surface of the NH₃-SCR reaction pathway on three VMoO_x/TiO₂ catalysts. The reaction proceeds through NH₃ adsorption, NO co-adsorption, NH₂NO transition state, and N₂ + H₂O product desorption.

(1)

NH₃ adsorption and activation: NH₃ adsorbs and becomes activated at Lewis acid sites or Brønsted acid sites on the catalyst surface, forming coordinated NH₃-V or NH⁴⁺ species, respectively. Catalyst C exhibits the greatest NH₃ adsorption depth (−102.4 kJ/mol), providing the most favorable initial conditions for the reaction.

(2)

NO approach and co-adsorption: The activated NH₃ species and gas-phase NO form a co-adsorbed state on the catalyst surface. Catalyst C exhibits the largest co-adsorption stabilization energy (−141.3 kJ/mol), indicating its strongest capacity for synergistic immobilization of both reactants on the surface.

(3)

Transition state formation (NH₂NO): This constitutes the rate-determining step of the reaction. The N–H bond in NH₃ undergoes cleavage, and the NH₂ radical combines with NO to form the NH₂NO intermediate. The DFT-calculated absolute energy levels of the transition state are: 85.2 kJ/mol for Catalyst A, 72.3 kJ/mol for Catalyst B, and 58.6 kJ/mol for Catalyst C. Catalyst C exhibits the lowest transition state energy, implying that at identical temperature conditions, the SCR reaction on Catalyst C possesses the fastest intrinsic rate.

(4)

NH₂NO decomposition and product desorption: The NH₂NO intermediate undergoes N–N bond rearrangement to generate N₂ and H₂O products, which subsequently desorb from the catalyst surface. Catalyst C exhibits the lowest product desorption energy (24.3 kJ/mol), facilitating rapid regeneration of active sites and sustained progression of the catalytic cycle.

3. Materials and Methods

3.1. Catalyst Preparation

Titanium dioxide powder, titanyl sulfate and tetrabutyl titanate were selected as three titanium precursors for carrier synthesis. Exactly 9.5 g of titanium dioxide powder, 19.1 g of titanyl sulfate and 40.5 g of tetrabutyl titanate were weighed and placed into separate beakers. Meanwhile, 0.38 g of ammonium metavanadate and 0.57 g of ammonium heptamolybdate were dissolved in 50 mL of deionized water, followed by intense stirring with a glass rod until all solid particles dissolved to obtain distinct impregnation solutions. Each impregnation solution was poured into the corresponding beaker loaded with a titanium precursor, and the mixture was magnetically stirred at 600 r/min for 30 min to achieve full homogeneity. Afterwards, the mixture was moved into an oven and dried at 110 °C for 8 h, and subsequently calcined at 500 °C for 6 h to yield VMoO_x@TiO₂-X catalysts (where X = A, B and C represent titanium dioxide powder, titanyl sulfate and tetrabutyl titanate as titanium sources, respectively). The manufacturer information for all chemical reagents used in this work is provided in the Supplementary Materials.

3.2. Catalyst Testing Methods

The catalyst (1 mL) was loaded into a fixed-bed quartz reactor (inner diameter: 8 mm) to investigate the NH₃-SCR catalytic activity. A gas flow rate of 500 mL·min⁻¹ corresponded to a gas hourly space velocity (GHSV) of 30,000 h⁻¹. The reaction gas consisted of 500 ppm NO, 500 ppm NH₃ (when used), 6 vol.% O₂, with the balance being N₂. The NO concentrations at the reactor inlet and outlet were measured using a flue gas analyzer (MRU VarioPlus, Neckarsulm-Obereisesheim, Germany). The catalytic activity was calculated according to Equation (2):

NO_x conversion (%) = (NO_x,in − NO_x,out)/NO_x,in × 100%

(2)

Detailed characterization and theoretical calculation methods of the catalysts are provided in the Supplementary Materials.

4. Conclusions

In this work, TiO₂ supports were prepared using titanium dioxide powder, titanyl sulfate, and tetrabutyl titanate as Ti precursors, followed by the preparation of three V₂O₅–MoO₃/TiO₂ catalysts (VMoO_x@TiO₂-A, VMoO_x@TiO₂-B, and VMoO_x@TiO₂-C, respectively) via incipient wetness impregnation. The effects of support origin on catalyst structure and DeNO_x performance were systematically investigated through multiple characterization techniques combined with density functional theory (DFT) calculations. The tetrabutyl titanate-derived support possesses the smallest grain size, the largest specific surface area, and the most abundant mesoporous structure, facilitating the high dispersion of V and Mo species. XPS analysis reveals that from Catalyst A to C, the V⁴⁺ proportion increases from 38.9% to 56.6% and the Mo⁶⁺ proportion increases from 50.2% to 78.9%, which was attributed to enhanced V–O–Mo bridge bond electron transfer interactions. DFT calculations based on the Ti₂V₁Mo₁On cluster model quantitatively confirmed this mechanism, with calculated V⁵⁺ → V⁴⁺ electron transfer energies of −0.32 eV for Catalyst A, −0.56 eV for Catalyst B, and −0.78 eV for Catalyst C, demonstrating that the high-surface-area support promotes the thermodynamically favorable V⁴⁺–O–Mo⁶⁺ configuration. Mulliken charge analysis further revealed progressive reduction in positive charge on V atoms (+1.82 → +1.65 → +1.48 |e|) accompanied by progressive increase on Mo atoms (+2.14 → +2.38 → +2.52 |e|), consistent with the complementary V⁴⁺/Mo⁶⁺ enrichment observed experimentally. DFT-calculated adsorption energies demonstrate that Catalyst C exhibits significantly enhanced NH₃ adsorption at both Lewis acid sites (V⁵⁺, −102.4 kJ/mol) and Brønsted acid sites (V–OH, −152.8 kJ/mol) compared to Catalysts A and B, in excellent agreement with NH₃-TPD results. The potential energy surface analysis of the NH₃-SCR reaction pathway reveals that Catalyst C possesses the lowest transition state energy barrier for NH₂NO formation (58.6 kJ/mol vs. 72.3 kJ/mol for B and 85.2 kJ/mol for A), the strongest reactant co-adsorption stabilization (−141.3 kJ/mol), and the lowest product desorption energy (24.3 kJ/mol), collectively explaining its fastest intrinsic reaction rate. NH₃-TPD and H₂-TPR confirm that VMoO_x@TiO₂-C possesses the most abundant surface acid sites and the strongest low-temperature redox capability, achieving complete NO conversion at 350 °C with optimal sulfur resistance stability. This study integrates experimental characterization with DFT theoretical computation to establish a comprehensive structure–activity relationship between TiO₂ support properties and the DeNO_x performance of vanadium-molybdenum catalysts from both macroscopic and atomic-scale perspectives, providing a scientific basis for the optimized selection of industrial SCR catalyst supports.