Role of WO₃ in NO Reduction with NH₃ over V₂O₅-WO₃/TiO₂: A New Insight from the Kinetic Study

Abstract

WO₃ role in NO reduction over V₂O₅-WO₃/TiO₂ was studied by comparing the kinetic parameters of the selective catalytic reduction (SCR) reaction, the non selective catalytic reduction (NSCR) reaction and the C–O reaction. The results show that the SCR reaction over V₂O₅/TiO₂ was promoted after WO₃ incorporation, while the NSCR reaction and the C–O reaction were both restrained.

Graphical Abstract

1 Introduction

Now, selective catalytic reduction (SCR) of NO by NH₃ is the major technology to control the emission of nitrogen oxides from coal fired plants [1–3] and V₂O₅-WO₃/TiO₂ is widely used as the commercial catalyst [4, 5]. WO₃ in V₂O₅-WO₃/TiO₂ can increase the SCR activity, widen the temperature range and improve N₂ selectivity [6, 7]. So far, the mechanism of WO₃ role in the SCR reaction over V₂O₅-WO₃/TiO₂ has been widely studied [8, 9] and the promotion was generally attributed to the increase of the acidity on V₂O₅/TiO₂, the delay of the loss of the BET surface, the inhibition of the transformation of monomeric vanadyl to crystalline V₂O₅, and the improvement of the reducibility of V component [10].

The non selective catalytic reduction (NSCR) reaction and the catalytic oxidation of NH₃ to NO (C–O reaction) can simultaneously happen during the SCR reaction over V₂O₅-WO₃/TiO₂ especially at higher temperatures, resulting in the formation of N₂O and the decrease of NO conversion at higher temperatures [11]. Therefore, the SCR performance (including NO _x conversion and N₂ selectivity) depends on the competition of the SCR reaction, the NSCR reaction and the C–O reaction. Previous studies mainly aimed at WO₃ role in the SCR reaction. However, WO₃ role in the side reactions (i.e. the NSCR reaction and the C–O reaction) was seldom investigated. In our previous work, a global kinetics of NO reduction over V₂O₅-WO₃/TiO₂ (including the SCR reaction, the NSCR reaction and the C–O reaction) was built [12] and their kinetic parameters can be obtained from the steady-state kinetic study. In this work, WO₃ role in NO reduction over V₂O₅-WO₃/TiO₂ was studied by comparing the kinetic parameters of NO reduction over V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂. The results show that the SCR reaction over V₂O₅/TiO₂ was promoted after WO₃ incorporation due to the promotion of NH₃ adsorption and the increase of oxidation ability. Meanwhile, the C–O reaction was restrained due to the decrease of V⁵⁺ concentration. As the SCR reaction contributed to NO reduction while the C–O reaction contributed to NO formation, NO _x conversion over V₂O₅/TiO₂ obviously increased after WO₃ incorporation. Furthermore, the NSCR reaction was restrained due to the decrease of V⁵⁺ concentration, resulting in an obvious increase of N₂ selectivity.

2 Experimental

2.1 Catalyst Preparation

V₂O₅-WO₃/TiO₂ (with 5 wt% V₂O₅ and 10 wt% WO₃) and V₂O₅/TiO₂ (with 5 wt% V₂O₅) were prepared by the conventional impregnation method using NH₄VO₃, (NH₄)₁₀W₁₂O₄₁ (when used) as precursors, H₂C₂O₄·2H₂O as the dissolving promoter and P25 as support [7, 13].

2.2 Characterization

BET surface areas, H₂-temperature programmed reduction (H₂-TPR), X-ray diffraction patterns (XRD), X-ray photoelectron spectra (XPS) and in situ DRIFT spectra were performed on a nitrogen adsorption apparatus (Quantachrome, Autosorb-1), a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx), an X-ray diffractionmeter (Bruker-AXS D8 Advance), an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250) and a Fourier transform infrared spectrometer (Nicolet IS 50), respectively.

2.3 Catalytic Test

The SCR reaction and temperature programmed desorption of NH₃ (NH₃-TPD) were performed on a fixed-bed quartz tube micro-reactor [14]. 100 mg of catalyst with 40–60 mesh and 200 mL min⁻¹ of the simulated flue gas containing 500 ppm of NO, 500 ppm of NH₃, 2 % of O₂, and balance of N₂ were used and the corresponding gas hourly space velocity (GHSV) was 1.2 × 10⁵ cm³ g⁻¹ h⁻¹. The concentrations of NO, NO₂, NH₃ and N₂O in the inlet or outlet were determined online by an infrared spectrometer (Thermo SCIENTIFIC, ANTARIS, IGS Analyzer).

2.4 Steady State Kinetic Study

Gaseous NH₃ concentration in the inlet was kept at 500 ppm, while gaseous NO concentration varied from 300 to 700 ppm [15, 16]. To minimize the inner diffusion and external diffusion, a very high GHSV of 4.8 × 10⁶ − 1.2 × 10⁷ cm³ g⁻¹ h⁻¹ was adopted to keep NO conversion <15 % [17].

3 Results

3.1 SCR Performance

Figure 1 shows that the SCR activity of V₂O₅-WO₃/TiO₂ was higher than that of V₂O₅/TiO₂ at 150–500 °C. Meanwhile, N₂O selectivity of NO reduction over V₂O₅/TiO₂ was much higher than that over V₂O₅-WO₃/TiO₂. They suggest that the SCR performance of V₂O₅/TiO₂ obviously improved after WO₃ incorporation, which was consistent with the result of previous study [6]. Moreover, NH₃ conversions of both V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ were close to NO conversions at 150–250 °C, and higher than NO conversions at 300–500 °C. It suggests that the C–O reaction happened at 300–500 °C over V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂.

Fig. 1

SCR performance of: a V₂O₅/TiO₂; b V₂O₅-WO₃/TiO₂

3.2 Characterization

The characteristic peaks appeared in the XRD patterns of V₂O₅-WO₃/TiO₂ and V₂O₅/TiO₂ mainly corresponded to anatase (JCPDS: 21-1272) and rutile (JCPDS: 21-1276) (shown in Fig. 2) [7]. Meanwhile, other characteristic peaks corresponding to tungsten oxides and vanadium oxides cannot be clearly observed. It suggests that the loaded tungsten oxides and vanadium oxides were well dispersed on P25. The BET surface areas of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ were 41.6 and 49.3 m² g⁻¹, respectively.

Fig. 2

XRD patterns of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂

XPS spectra of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ over the spectral regions of V2p and W 4f are shown in Fig. 3. The binding energies of V 2p 3/2 on V₂O₅/TiO₂ mainly appeared at 515.6 and 517.1 eV, which were attributed to V⁴⁺ and V⁵⁺ on V₂O₅/TiO₂, respectively [18, 19]. After WO₃ was loaded, no significant changes can be observed in the spectral region of V 2p. Meanwhile, the binding energies of W 4f species mainly appeared at 35.6 and 37.5 eV, which were assigned to W⁶⁺ on the surface [20]. The percentages of V, Ti, O and W species on V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ were calculated from the XPS spectra. As shown in Table 1, the percentages of V⁵⁺ on V₂O₅/TiO₂ decreased from 5.9 to 3.0 % after WO₃ incorporation.

Fig. 3

XPS spectra of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ over the spectral regions of V 2p and W 4f

Table 1 Percentages of V, Ti, O and W species on V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂

H₂-TPR profiles (Fig. 4) show a clear reduction peak at approximately 504 °C on V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂, which was attributed to the reduction of V⁵⁺ on the surface [10, 21]. Some H₂ consumption can be clearly observed on V₂O₅-WO₃/TiO₂ below 300 °C, while it cannot be observed on V₂O₅/TiO₂. It suggests the oxidation ability of V⁵⁺ on V₂O₅-WO₃/TiO₂ was much higher than that over V₂O₅/TiO₂ [10]. Figure 4 also shows that the area of the first reduction peak of V₂O₅-WO₃/TiO₂ was much less than that on V₂O₅/TiO₂. It suggests that V⁵⁺ concentration on V₂O₅-WO₃/TiO₂ was much less than that of V₂O₅/TiO₂, which was consistent with the result of XPS analysis.

Fig. 4

H₂-TPR profiles of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂

Figure 5 shows NH₃-TPD profiles of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂. The integration of NH₃-TPD profiles (Fig. 5) shows that the capacities of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ for NH₃ adsorption at 200 °C were approximately 101 and 131 μmol g⁻¹, respectively. It suggests that the adsorption of NH₃ on V₂O₅/TiO₂ was promoted after WO₃ incorporation.

Fig. 5

NH₃-TPD profiles of V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂

3.3 In situ DRIFT Study

After the introduction of NH₃ at 250 °C for 30 min, V₂O₅/TiO₂ was mainly covered by coordinated NH₃ bound to the Lewis acid sites (at 1220, 1246 and 1602 cm⁻¹) and ionic NH₄ ⁺ bound to the Brønsted acid sites (1422 and 1675 cm⁻¹) [13]. After NO+O₂ were then introduced, adsorbed NH₃ species (i.e. coordinated NH₃ and ionic NH₄ ⁺) gradually decreased (shown in Fig. 6a). It suggests that the Eley–Rideal mechanism (i.e. the reaction between gaseous NO and adsorbed NH₃ species) can contribute to NO reduction over V₂O₅/TiO₂.

Fig. 6

a In situ DRIFT spectra taken at 250 °C upon passing NO+O₂ over NH₃ presorbed V₂O₅/TiO₂; b In situ DRIFT spectra taken at 250 °C upon passing NH₃ over NO+O₂ presorbed V₂O₅/TiO₂; c In situ DRIFT spectra taken at 250 °C upon passing NO+O₂ over NH₃ presorbed V₂O₅-WO₃/TiO₂; d In situ DRIFT spectra taken at 250 °C upon passing NH₃ over NO+O₂ presorbed V₂O₅-WO₃/TiO₂

After the introduction of NO+O₂ at 250 °C for 30 min, V₂O₅/TiO₂ was mainly covered by monodentate nitrite (at 1610 cm⁻¹) [22]. After NH₃ was then introduced, adsorbed monodentate nitrite rapidly diminished. At last, V₂O₅/TiO₂ was mainly covered by coordinated NH₃ and ionic NH₄ ⁺ (shown in Fig. 6b). It suggests that the Langmuir–Hinshelwood mechanism (i.e. the reaction between adsorbed NO_x and adsorbed NH₃ species) can contribute to NO reduction over V₂O₅/TiO₂.

In situ DRIFT spectra taken at 250 °C upon passing NO+O₂ over NH₃ presorbed V₂O₅-WO₃/TiO₂ and those upon passing NH₃ over NO+O₂ presorbed V₂O₅-WO₃/TiO₂ were similar to those over V₂O₅/TiO₂ (shown in Fig. 6c, d). Therefore, both the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism can contribute to NO reduction over V₂O₅-WO₃/TiO₂.

3.4 Steady-State Kinetic Study

To investigate WO₃ role in the SCR reaction, the NSCR reaction and the C–O reaction during NO reduction over V₂O₅-WO₃/TiO₂, the steady-state kinetic study was performed. Figures 7a–c and 8a–c show the dependences of the rates of NH₃ conversion, NO _x conversion and N₂O formation on gaseous NO concentration during NO reduction over V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂. The SCR reaction, the NSCR reaction and the C–O reaction all contributed to NH₃ conversion [11]. The SCR reaction and the NSCR reaction contributed to NO reduction, while the C–O reaction contributed to NO formation [23]. Therefore, the rates of NH₃ conversion ($\delta _{{{\text{NH}}_{3} }}$) and NO _x conversion ($\delta _{{{\text{NO}}_{{\text{x}}} }}$) can be described as follows [24]:

Fig. 7

Dependences of the rates of NH₃ conversion (a), NO _x conversion (b), N₂O formation (c), the SCR reaction (d), the C–O reaction (e) during NO reduction over V₂O₅/TiO₂ on gaseous NO concentration

Fig. 8

Dependences of the rates of NH₃ conversion (a), NO _x conversion (b), N₂O formation (c), the SCR reaction (d), the C–O reaction (e) during NO reduction over V₂O₅-WO₃/TiO₂ on gaseous NO concentration

$$\delta _{{{\text{NH}}_{{\text{3}}} }} \; = \;\delta _{{{\text{SCR}}}} \; + \;\delta _{{{\text{NSCR}}}} \; + \;\delta _{{{\text{C}}{-}{\text{O}}}}$$

(1)

$$\delta _{{{\text{NO}}_{{\text{x}}} }} \; = \;\delta _{{{\text{SCR}}}} \; + \;\delta _{{{\text{NSCR}}}} \; - \;\delta _{{{\text{C}}{-}{\text{O}}}}$$

(2)

where, δ _SCR, δ _NSCR and δ _C–O were the rates of the SCR reaction, the NSCR reaction and the C–O reaction, respectively.

Then, δ _C–O can be calculated as [25, 26]:

$$\delta _{{{\text{C}}{-}{\text{O}}}} \; = \;\frac{{\delta _{{{\text{NH}}_{{\text{3}}} }} - \delta _{{{\text{NO}}_{{\text{x}}} }} }}{2}$$

(3)

The rates of the SCR reaction and the NSCR reaction were equal to the rates of N₂ formation and N₂O formation, respectively. Figures 7 and 8 show that both δ _NSCR and δ _C–O of V₂O₅/TiO₂ were much higher than those of V₂O₅-WO₃/TiO₂. However, δ _SCR of V₂O₅/TiO₂ was generally much less than that of V₂O₅-WO₃/TiO₂. It suggests that the SCR reaction over V₂O₅/TiO₂ was promoted after WO₃ incorporation, while both the NSCR reaction and the C–O reaction were restrained remarkably. Therefore, both NO_x conversion and N₂ selectivity of NO reduction over V₂O₅/TiO₂ obviously increased after WO₃ incorporation.

4 Discussion

The SCR reaction through the Eley–Rideal mechanism, the NSCR reaction and the C–O reaction during NO reduction over V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ can be approximately described as [27]:

$$\text{N}{{\text{H}}_{3}}_{(\text{g})}\ \to \ \text{N}{{\text{H}}_{3}}_{(\text{ad})}$$

(4)

$${\text{V}}^{{5 + }}={\text{O}}\; + \;{\text{NH}}_{{3\left( {{\text{ad}}} \right)}} \; \to \;{\text{V}}^{{4 + }} {-}{\text{OH}}\; + \;{\text{NH}}_{2}$$

(5)

$$\text{N}{{\text{O}}_{(\text{g})}}\ +\ \text{N}{{\text{H}}_{2}}\ \to \ {{\text{H}}_{2}}\text{O}\ +\ {{\text{N}}_{2}}$$

(6)

$${\text{V}}^{{5 + }}={\text{O}}\; + \;{\text{NH}}_{2} \; \to \;{\text{V}}^{{{\text{4 + }}}} {-}{\text{OH}}\; + \;{\text{NH}}$$

(7)

$$\text{N}{{\text{O}}_{(\text{g})}}\ +\ \text{NH}\ +\ {{\text{V}}^{5+}}=\text{O}\ \to \ {{\text{N}}_{2}}\text{O}\ +\ {{\text{V}}^{\text{4+}}}{-}\text{OH}$$

(8)

$${{\text{V}}^{5+}}=\text{O}\ +\ \text{NH}\ +\ \frac{1}{2}{{\text{O}}_{2}}\ \to \ {{\text{V}}^{\text{4+}}}{-}{\text{OH}\ +\ \text{N}{\text{O}}_{(\text{g})}}$$

(9)

$${{\text{V}}^{\text{4+}}}{-}\text{OH}\ +\ \frac{1}{4}{{\text{O}}_{2}}\ \to \ {{\text{V}}^{5+}}=\text{O}\ +\ \frac{1}{2}{{\text{H}}_{2}}\text{O}$$

(10)

Meanwhile, NO reduction through the Langmuir–Hinshelwood mechanism can be approximately described as [28]:

$$\text{N}{{\text{O}}_{(\text{g})}}\ \to \ \text{N}{{\text{O}}_{(\text{ad})}}$$

(11)

$${{\text{V}}^{5+}}=\text{O}\ +\ \text{N}{{\text{O}}_{(\text{ad})}}\ \to \ {{\text{V}}^{\text{4+}}}{-}{\text{O}}{-}\text{NO}$$

(12)

$$\text{N}{{\text{H}}_{3}}_{(\text{ad})}\ +\ {{\text{V}}^{\text{4+}}}{-}{\text{O}}{-}\text{NO}\ \to \ {{\text{V}}^{\text{4+}}}{-}{\text{O}}{-}{\text{NO}}{-}\text{N}{{\text{H}}_{\text{3}}}\ \to \ {{\text{V}}^{\text{4+}}}{-}\text{OH}\ +\ {{\text{H}}_{2}}\text{O}\ +\ {{\text{N}}_{2}}$$

(13)

The kinetic equations of Reactions 6 and 8 can be described as [29]:

$$\left.\frac{\text{d}[\text{N}_{2} ]}{\text{dt}}\right\vert_{\text{E-R}} \; = \; - \frac{\text{d}[{\text{NH}}_{2} ]}{\text{dt}}\; = \;k_{1} [{\text{NO}}_{(\text{g})} ][{\text{NH}}_{2} ]$$

(14)

$$\frac{\text{d}[{{\text{N}}_{2}}\text{O}]}{\text{dt}}\ =\ -\frac{\text{d}[\text{NH}]}{\text{dt}}\ =\ {{k}_{2}}[\text{N}{{\text{O}}_{(\text{g})}}][{{\text{V}}^{5+}}=\text{O}][\text{NH}]$$

(15)

where, k ₁, k ₂, [NH₂], [NH], [V⁵⁺=O] and [NO_(g)] were the kinetic constants of Reactions 6 and 8, the concentrations of NH₂, NH and V⁵⁺ on the surface, and gaseous NO concentration, respectively.

The kinetic equations of Reactions 5 and 7 can be described as:

$$\frac{\text{d}[\text{N}{{\text{H}}_{2}}]}{\text{dt}}\ =\ {{k}_{3}}[{{\text{V}}^{5+}}=\text{O}][\text{N}{{\text{H}}_{3}}_{(\text{ad})}]$$

(16)

$$\frac{\text{d}[\text{NH}]}{\text{dt}}\ =\ {{k}_{4}}[{{\text{V}}^{5+}}=\text{O}][\text{N}{{\text{H}}_{2}}]$$

(17)

where, k ₃, k ₄ and [NH_3(ad)] were the kinetic constants of Reactions 5 and 7, and the concentration of NH₃ adsorbed, respectively.

Furthermore, the kinetic of Reaction 9 can be approximately described as:

$$\frac{\text{d}[\text{N}{{\text{O}}_{(\text{g})}}]}{\text{dt}}\ =\ -\frac{\text{d}[\text{NH}]}{\text{dt}}\ =\ {{k}_{5}}[\text{NH}][{{\text{V}}^{5+}}=\text{O}]$$

(18)

where, k ₅ was the kinetic constant of Reaction 9.

NH concentration on the surface would not vary as the reaction reached the steady-state. Thus,

$$-\frac{\text{d}[\text{NH}]}{\text{dt}}\ =\ {{k}_{2}}[{{\text{V}}^{5+}}=\text{O}][\text{NH}][\text{N}{{\text{O}}_{(\text{g})}}]\ +\ {{k}_{5}}[{{\text{V}}^{5+}}=\text{O}][\text{NH}]\ -\ {{k}_{4}}[{{\text{V}}^{5+}}=\text{O}][\text{N}{{\text{H}}_{2}}]\ =\ 0$$

(19)

Therefore,

$$[\text{NH}]\ =\ \frac{{{k}_{4}}[\text{N}{{\text{H}}_{2}}]}{{{k}_{2}}[\text{N}{{\text{O}}_{(\text{g})}}]\ +\ {{k}_{5}}}$$

(20)

Then, Eqs. 15 and 18 can be transformed as follows [12]:

$${{\delta }_{\text{NSCR}}}\ =\ \frac{\text{d}[{{\text{N}}_{2}}\text{O}]}{\text{dt}}\ =\ {{k}_{2}}\frac{{{k}_{4}}[\text{N}{{\text{H}}_{2}}]}{{{k}_{2}}[\text{N}{{\text{O}}_{(\text{g})}}]\ +\ {{k}_{5}}}[\text{N}{{\text{O}}_{(\text{g})}}][{{\text{V}}^{5+}}=\text{O}]=\frac{{{k}_{4}}[\text{N}{{\text{H}}_{2}}][{{\text{V}}^{5+}}=\text{O}]}{\frac{{{k}_{5}}}{{{k}_{2}}[\text{N}{{\text{O}}_{(\text{g})}}]}\ +\ 1}$$

(21)

$${{\delta }_{\text{C-O}}}\ =\ \frac{\text{d}[\text{N}{{\text{O}}_{(\text{g})}}]}{\text{dt}}\ =\ {{k}_{5}}\frac{{{k}_{4}}[\text{N}{{\text{H}}_{2}}]}{{{k}_{2}}[\text{N}{{\text{O}}_{(\text{g})}}]\ +\ {{k}_{5}}}[{{\text{V}}^{5+}}=\text{O}]\ =\ \frac{{{k}_{4}}[\text{N}{{\text{H}}_{2}}][{{\text{V}}^{5+}}=\text{O}]}{\frac{{{k}_{2}}[\text{N}{{\text{O}}_{(\text{g})}}]}{{{k}_{5}}}\ +\ 1}$$

(22)

Furthermore, the kinetic equation of Reaction 13 can be described as:

$$\left.\frac{\text{d}[\text{N}_2 ]}{\text{dt}}\right| _{\text{L-H}} =k_{6} [\text{V}^{\text{4+}}{-}{\text{O}}{-}\text{NO}{-}{\text{NH}}_{\text{3}} ]$$

(23)

where, k ₆ and [V⁴⁺–O–NO–NH₃] were the rate constant of NH₄NO₂ decomposition and NH₄NO₂ concentration on the surface, respectively.

Then, the SCR reaction rate can be described as:

$$\left. \delta _{\text{SCR}} = \frac{\text{d}[\text{N}_2 ]}{\text{dt}}\right| _{\text{L-H}} + \left. \frac{\text{d}[\text{N}_2]}{\text{dt}}\right|_{\text{E-R}} = k_{6} [{\text{V}}^{\text{4+}}{-}{\text{O}}{-}{\text{NO}}{-}{\text{NH}}_{{\text{3}}} ] + k_{1} [{\text{NH}}_{2} ] [{\text{NO}}_{(\text{g})} ]$$

(24)

Our previous study demonstrated that NH₄NO₂ concentration on the surface can be approximately regarded as a constant at the steady-state [12, 30], so the rate of the SCR reaction through the Langmuir–Hinshelwood mechanism was nearly independent of gaseous NO and NH₃ concentrations. Our previous study also demonstrated that NH₂ concentration on the surface at the steady-state was approximately independent of gaseous NO and NH₃ concentrations [12, 31], so the rate of the SCR reaction through the Eley–Rideal mechanism was nearly in direct proportion to gaseous NO concentration. Therefore, the parameters of k ₆[V³⁺–O–NO–NH₃] and k ₁[NH₂] can be obtained after the linear regression of δ _SCR with gaseous NO concentration. Furthermore, the parameters of k ₄[NH₂][V⁵⁺=O] and k ₂/k ₅ can also be obtained after the non-linear regression of δ _C–O with gaseous NO concentration. δ _c-o of V₂O₅-WO₃/TiO₂ and V₂O₅/TiO₂ were both close to 0 at 200–350 °C, so k ₅ was approximately 0. Then, k ₄[NH₂][V⁵⁺=O] at 200–350 °C can be approximately obtained from the average values of $\delta _{{{\text{N}}_{2} {\text{O}}}}$ (hinted by Eq. 21) [12].

The parameters of k ₆[V³⁺–O–NO–NH₃], k ₁[NH₂], k ₄[NH₂][V⁵⁺=O] and k ₂/k ₅, which resulted from the regression of Figs. 7 and 8, were listed in Table 2. k ₁ was the reaction kinetic constant of Reaction 6, so it of V₂O₅/TiO₂ would be equal to that of V₂O₅-WO₃/TiO₂. [NH₂] on V₂O₅/TiO₂ was proportional to the products of k ₃, [NH_3(ad)] and [V⁵⁺=O] (hinted by Eq. 16). k ₃ was related to the oxidation ability of V⁵⁺ on the surface, so it of V₂O₅/TiO₂ was less than that of V₂O₅-WO₃/TiO₂ (hinted by the H₂-TPR analysis). NH₃-TPD analysis shows that the adsorption of NH₃ on V₂O₅/TiO₂ was promoted after WO₃ incorporation. Although [V⁵⁺=O] on V₂O₅/TiO₂ was higher than that on V₂O₅-WO₃/TiO₂, both k ₃ and [NH_3(ad)] of V₂O₅/TiO₂ were less than those of V₂O₅-WO₃/TiO₂. Therefore, [NH₂] on V₂O₅/TiO₂ was much less than that on V₂O₅-WO₃/TiO₂. As a result, k ₁[NH₂] of V₂O₅/TiO₂ obviously increased after WO₃ incorporation (shown in Table 2), resulting in an obvious promotion of the SCR reaction through the Eley–Rideal mechanism (hinted by Eq. 14).

Table 2 Kinetic parameters of NO reduction over V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂

As V⁵⁺ concentration on V₂O₅-WO₃/TiO₂ was much less than that on V₂O₅/TiO₂, the probability of two V⁵⁺ on the adjacent sites for the over-activation of NH₃ to NH on V₂O₅-WO₃/TiO₂ was much less than that of V₂O₅/TiO₂. It suggests that k ₄ of V₂O₅-WO₃/TiO₂ was much less than that of V₂O₅/TiO₂. Although [NH₂] on V₂O₅-WO₃/TiO₂ was much higher than that of V₂O₅/TiO₂, both k ₄ and [V⁵⁺=O] of/on V₂O₅-WO₃/TiO₂ was much less than those of/on V₂O₅/TiO₂. Therefore, k ₄[NH₂][V⁵⁺=O] of V₂O₅/TiO₂ obviously decreased after WO₃ incorporation (shown in Table 2), resulting in a remarkable inhibition of the NSCR reaction (hinted by Eq. 21).

As V⁵⁺ concentration on V₂O₅-WO₃/TiO₂ was much less than that on V₂O₅/TiO₂, the probability of three V⁵⁺ on the adjacent sites for the deep oxidation of NH₃ to gaseous NO on V₂O₅-WO₃/TiO₂ was much less than that of V₂O₅/TiO₂. It suggests that k ₅ of V₂O₅-WO₃/TiO₂ was much less than that of V₂O₅/TiO₂. Meanwhile, k ₂ was the reaction kinetic constant of Reaction 8, so it of V₂O₅/TiO₂ would be approximately equal to that of V₂O₅-WO₃/TiO₂. Therefore, k ₅/k ₂ of V₂O₅/TiO₂ obviously decreased after WO₃ incorporation (shown in Table 2). Meanwhile, k ₄[NH₂][V⁵⁺=O] of V₂O₅/TiO₂ obviously decreased after WO₃ incorporation. Hinted by Eq. 22, the C–O reaction over V₂O₅/TiO₂ was remarkably inhibited after WO₃ incorporation.

5 Conclusion

The steady-state kinetic study demonstrated that the SCR reaction over V₂O₅/TiO₂ was promoted after WO₃ incorporation due to the promotion of NH₃ adsorption and the increase of oxidation ability. Meanwhile, the C–O reaction was restrained due to the decrease of V⁵⁺ concentration. As the SCR reaction contributed to NO reduction while the C–O reaction contributed to NO formation, NO_x conversion over V₂O₅/TiO₂ obviously increased after WO₃ incorporation. Furthermore, the NSCR reaction was restrained due to the decrease of V⁵⁺ concentration, resulting in an obvious increase of N₂ selectivity.