Exploring the role of V₂O₅ in the reactivity of NH₄HSO₄ with NO on V₂O₅/TiO₂ SCR catalysts

Abstract

In this study, attention was focused on the interactions between NH₄HSO₄ and vanadium species in the selective catalytic reduction (SCR) of NO with NH₃, along with the role of vanadium species in the reactivity of NH₄HSO₄ with NO on V₂O₅/TiO₂ catalysts. Both vanadium and sulfate species occupied the TiO₂ surface basic hydroxyl groups; the decreased TiO₂ surface basic sites resulting from the introduction of NH₄HSO₄ in turn promoted the formation of polymeric vanadium species. Given increases in vanadium content, formation of polymeric vanadium species and reactive electrophilic oxygen species on the catalysts occurred, which was an important reason for the enhanced reactivity of NH₄HSO₄ with NO on the high V content catalysts. Besides, a higher electron cloud density around the S atoms in SO₄²⁻ could be detected for the high V content catalysts, on which SO₄²⁻ would be easily reduced to SO₂ during the TPSR process. In situ diffuse reflectance infrared Fourier transform spectroscopy confirmed that NH₄⁺ in NH₄HSO₄ functioned as a reductant during reaction with gaseous NO, while S-containing functional groups were stabilized as tridentate sulfate anions on the catalyst surface.

---

1. Introduction

Selective catalytic reduction of NO with NH₃ is an effective technology to control NO_x emissions from stationary sources.¹ Recently, much attention has been focused on low-temperature SCR technology, since it can be located downstream of desulfurized and particulate control devices, which protects catalysts from being deactivated by high concentrations of dust and SO₂ in the flue gas. Several types of low-temperature SCR catalysts, which exhibit excellent SCR activity between 100 and 300 °C, have been developed.^2–4 However, a certain amount of SO₂ in the flue gas reacts with NH₃ to produce NH₄HSO₄, which adversely affects the SCR reaction. Formation of that product is regarded as the main barrier to the commercialization of low-temperature SCR systems.⁵ Therefore, researching the role of catalyst active components in the reactivity of NH₄HSO₄ with NO constitutes an important step in industrializing low-temperature SCR systems.

The reactivity of NH₄HSO₄ with NO, which is always neglected by many researchers, explains the recovery of SCR catalytic activity after cessation of SO₂ purging at low-temperature regions and protects catalysts from being deactivated by the excess deposition of NH₄HSO₄ during SCR reactions in H₂O- and SO₂-containing flue gas to some extent.^6,7 In the case of the reactivity of NH₄HSO₄ with NO on the TiO₂-based catalysts, Baltin claimed that NH₄HSO₄ formed on the V₂O₅–WO₃/TiO₂ catalysts could be consumed in an NO-containing flue gas at 170 °C.⁸ Our previous study has extensively explored the NH₄HSO₄ reactivity behavior on the V₂O₅/TiO₂ catalysts.⁹ It has been proved that amorphous ammonium sulfate salts react with NO more easily than crystallite salts. Therefore, promoting the NH₄HSO₄ reactivity behavior to avoid the formation of crystallite ammonium sulfate salts on the catalysts would be the best way to protect catalysts from being deactivated by the excess deposition of NH₄HSO₄, on which little research has been conducted, even if V₂O₅/TiO₂ catalyst system is typically used for stationary applications. Conversely, Liu found that NH₄HSO₄ reacts with NO more easily on the V₂O₅/activated carbon (V/AC) catalysts than on the V₂O₅/TiO₂ catalysts.^10–12 The novel NH₄HSO₄ activation process is considered to be the main reason for the enhanced NH₄HSO₄ reactivity behavior on the AC-based catalysts, knowledge of which lays a solid foundation for designing catalysts with superior activity and sulfur tolerance, even though variations in the physicochemical properties between AC and TiO₂ exist.

The active component for V₂O₅/TiO₂ catalysts is the metal oxide, V₂O₅, the effect of which on the reactivity of NH₄HSO₄ with NO still lacks exploration. Liu concluded that the NH₄HSO₄ reactivity behavior on the AC-based catalysts starts to be inhibited as the vanadium content exceeds 5 wt%, the reason for which is the inhibitory effect of high V contents on the NH₄HSO₄ activation process.¹¹ However, some crucial aspects on the role of V₂O₅ in the NH₄HSO₄ reactivity behavior on the TiO₂-based catalysts still lack investigation. (a) It has been proved that vanadium and sulfate species occupy the same sites on the TiO₂;¹³ the decreased TiO₂ surface sites through adding V₂O₅ might cause the appearance of crystallite ammonium sulfate salts on the catalysts, thereby inhibiting the NH₄HSO₄ reactivity behavior; (b) it has been confirmed that polymeric vanadium species with novel reducibility would gradually come out with increasing vanadium content,¹ which might have a promotion effect on the NH₄HSO₄ reactivity behavior. In this study, attention was focused on the role of vanadium species in the reactivity of NH₄HSO₄ with NO, of which the key factor would be discovered.

For the purpose above, NH₄HSO₄ was deposited on the V₂O₅/TiO₂ catalysts with various vanadium contents. X-ray diffraction (XRD) and N₂ adsorption were applied to investigate the catalyst physical properties. Raman, FTIR, and X-ray photoelectron spectroscopy (XPS) were conducted to explore the catalyst surface structure properties and atom environment. Temperature-programmed methods were used to study the effect of vanadium species on the reactivity of NH₄HSO₄ with NO. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was conducted to investigate the detailed NH₄HSO₄ decomposition and reactivity behaviors on the V₂O₅/TiO₂ catalysts.

2. Experimental

2.1 Preparation of the samples

V₂O₅/TiO₂ catalysts (denoted as V/Ti) containing 1, 2, 4, 8 wt% V₂O₅, were prepared using the wet impregnation method. P25, the catalyst support, was immersed into the ammonium metavanadate–oxalic solution. All of the samples were dried at 110 °C overnight, followed by the calcination process in a muffle furnace at 500 °C for 5 h. The obtained catalysts were crushed and sieved into 100 meshes for the deposition of NH₄HSO₄.

NH₄HSO₄ was deposited on the V/Ti catalysts using a previously reported wet impregnation method.¹¹ The as-prepared V/Ti catalyst was immersed into the NH₄HSO₄ solution. Samples were named based on the V₂O₅ content. For examples, ABS-V4/Ti indicates the sample containing 10 wt% NH₄HSO₄ and 4 wt% V₂O₅. The NH₄HSO₄-deposited samples were dried at 110 °C overnight. And then the samples were crushed and sieved into 40–60 meshes for temperature-programmed surface reaction (TPSR).

2.2 Reaction systems

The reactivity of the deposited-NH₄HSO₄ was measured via TPSR with NO. Briefly, 0.3 g NH₄HSO₄-containing catalyst was used for evaluation. The feed gas mixture contained 1000 ppm NO, 5% O₂ and N₂ as the balance gas. The total flow rate was 0.5 L min⁻¹, corresponding to a gas hourly space velocity (GHSV) of 100 000 mL g⁻¹ h⁻¹. The temperature was ramped from 50 °C or 100 °C to 450 °C at a heating rate of 5 °C min⁻¹. The outlet concentrations of NO, NO₂, NH₃ and SO₂ were detected using an FTIR detector (Gasmet gx4000).

The reaction rate constant k was obtained based on previous study of Baltin.⁸ As the concentration of the second partner, NH₃, during a single measuring operation (about 5 seconds) remains almost unchanged, the pseudo-first order reaction rate constant k could be calculated using the following equation:

k = −AV ln(x) with x = C_NOx(out)/C_NOx(out) (1)

where AV is indexed to the area velocity.

2.3 Characterization of the samples

The XRD patterns were obtained with an X-ray diffractometer (RIGAKU D/MAX 2550, Japan) equipped with Cu Kα radiation in the 2θ range of 20–80°.

The specific surface areas and pore volumes of the series V/Ti samples were measured with a Quantachrome Autosorb-1-C through N₂ adsorption and desorption at 77 K. Specific surface areas were obtained according to the Brunauer–Emmett–Teller (BET) equation. Pore volumes were determined with Barrett–Joyner–Halenda (BJH) method from the desorption branches of the isotherms.

Raman spectra were recorded using a Raman spectrometer (LabRamHRUV, JDbin-Yvon, France) under the 514 nm excitation laser light at ambient conditions.

FTIR spectra were obtained using Nicolet 6700 at ambient pressure. The spectra were recorded from 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹, which were averaged from 64 scans.

XPS experiments were conducted on a Thermo ESCALAB 250 electron spectrometer with Al Kα radiation. The binding energies were calibrated using C 1s line at 284.6 eV.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments of the NH₄HSO₄ decomposition and reactivity behaviors on the V/Ti catalysts were performed using an FTIR spectrometer (Nicolet 6700). All the IR spectra were obtained by collecting 64 scans with a resolution of 4 cm⁻¹. Samples were pretreated at 450 °C in N₂ for 1 h. Background spectra were recorded in flowing N₂. Then the samples were heated to certain temperatures in N₂ or 500 ppm NO + 5% O₂ to investigate the dynamic changes in the surface functional groups.

3. Results and discussion

3.1 Reactivity of NH₄HSO₄ with NO on the catalysts

Fig. 1(a) illustrates the NO TPSR profiles with NH₄HSO₄ on the series V/Ti catalysts. Variations in the reaction rate constant reflect the reactivity of NH₄HSO₄ with NO. In the case of ABS-Ti sample, the reaction rate constant begins to increase at ca. 250 °C, revealing a start of this reaction. With increasing temperature, a dramatic increase in the reaction rate constant occurs, the maximum value of which is at ca. 370 °C. The reaction rate constant then begins to decrease and gradually restores to 0, illustrating the depletion of ammonium ions on the catalyst surface. Given increases in vanadium content, an obvious increase in the reaction rate constant could be detected in the temperature region of 100–250 °C; this observation illustrates that the reactivity of NH₄HSO₄ with NO is enhanced on the high V content catalysts. Fig. 1(b) demonstrates the outlet NH₃ concentration during the TPSR process. It seems that the addition of V₂O₅ has a negative effect on the NH₃ release in the higher temperature region during the TPSR process; the enhanced reactivity behavior of NH₄HSO₄ with NO on the high V content catalysts promotes the NH₄⁺ consumption behavior at low-temperature regions, which leads to a reduced number of surface NH₄⁺ during the heat process. Consequently, a decrease in the NH₃ release amount occurs at higher temperatures during the TPSR process for the high V content catalysts.

Fig. 1 (a) TPSR profiles of NO with NH₄HSO₄ deposited on the V/Ti catalysts with various vanadium contents; (b) TPSR profiles of NH₃; (c) TPSR profiles of SO₂; (d) FTIR spectra of the series V/Ti catalysts after the TPSR process.

Fig. 1(c) reveals the outlet concentration of SO₂ during the TPSR process. In the case of NH₄HSO₄ deposited on the TiO₂ catalyst surface, an obvious SO₂ signal begins to be detected at a temperature higher than 300 °C. Then the outlet SO₂ concentration continues to increase with increasing temperature. Given increases in vanadium content, a dramatic increase in the SO₂ release amount could be detected, along with a decrease in the temperature for the maximum outlet SO₂ concentration. This result is supported by the FTIR spectra in Fig. 1(d) to some extent that no obvious characteristic peaks attributed to bidentate SO₄²⁻ could be detected for the samples with V content exceeding 2 wt% after the TPSR process.^14,15

As previously mentioned, V₂O₅ addition has a promotion effect on the reactivity of NH₄HSO₄ with NO at low-temperature regions, the reason for which is still uncovered but is important to design novel catalysts possessing satisfactory activity and sulfur tolerance. Thus, the first step in understanding the role of vanadium species in the reactivity of NH₄HSO₄ with NO is to investigate the interactions between NH₄HSO₄ and vanadium species; knowledge of such effects is beneficial for discovering the key factor in the NH₄HSO₄ reactivity behavior.

3.2 Interactions between NH₄HSO₄ and vanadium species

3.2.1 Catalyst surface structure properties. Vanadium and sulfate species loaded on TiO₂ might interact with certain functional groups on the catalysts. Characteristics of surface hydroxyl groups may reveal the interactions between NH₄HSO₄ and vanadium species. The infrared absorption spectra of the samples are presented in Fig. 2. A negative IR characteristic peak attributed to the TiO₂ surface basic hydroxyl groups appears with increasing V₂O₅ content, indicating the reduced number of basic hydroxyl groups by selective loading of vanadium species on the TiO₂ surface basic sites.¹³ Given the addition of NH₄HSO₄ on the catalyst surface, a strong negative peak centered at 3700 cm⁻¹ can also be detected, suggesting the consumption of TiO₂ surface basic sites through the interaction between sulfate species and TiO₂ support.¹³ It seems that vanadium and sulfate species occupy the same sites on the TiO₂ surface.

Fig. 2 FTIR spectra of the series Ti-based samples.

The Raman spectra of the series samples are presented in Fig. 3. For V4/Ti sample, Raman active peaks attributed to titanium and vanadium species can be clearly detected. According to previous studies, the Raman active peaks centered at 142, 196, 395, 513, 637 cm⁻¹, can be attributed to 3E_g, 1A_1g and 2B_1g modes of anatase TiO₂ phase.¹⁶ And the peak at 1013 cm⁻¹ is assigned to monomeric vanadium oxide species.¹⁷ Given the deposition of NH₄HSO₄ on the V4/Ti catalyst surface, the Raman band attributed to polymeric vanadium species at 974 cm⁻¹ appears with the diminishment of the peak related to monomeric vanadium species. Thus, it could be concluded that the competitive attachment of vanadium and sulfate species to the TiO₂ surface basic hydroxyl groups is an important reason for the transformation of vanadium species from monomeric species to polymeric ones with the deposition of NH₄HSO₄.¹³

Fig. 3 Raman spectra of the V4/Ti and ABS-V4/Ti samples.

3.2.2 Catalyst atom environment. The existence of chemical interactions between NH₄HSO₄ and vanadium species is related to the electron deviation between V atoms and ammonium ions or sulfate anions, which is still unknown from the FTIR and Raman spectra obtained. Thus, the XPS method would be conducted to investigate the variations in the catalyst atom environment with the deposition of NH₄HSO₄.

As is seen in Fig. 4(a), the binding energies of the Ti 2p photoelectron peaks illustrate the existence of Ti atoms in the +4 oxidation state.¹⁸ Given the deposition of NH₄HSO₄ on the catalyst surface, no obvious variations in the binding energies of the Ti 2p photoelectron peaks could be detected, indicating that Ti atom environment remains almost unchanged regardless of the deposition of NH₄HSO₄.

Fig. 4 XPS spectra of the V4/Ti and ABS-V4/Ti samples: (a) Ti 2p; (b) V 2p; (c) O 1s.

For V atoms, the binding energies of the V 2p photoelectron peaks after deconvolution indicate the coexistence of V⁴⁺ and V⁵⁺ in the V4/Ti and ABS-V4/Ti samples.¹⁹ After introducing NH₄HSO₄, the ratio of V⁵⁺/(V⁴⁺ + V⁵⁺) increases to 82.6%, which is higher than that of the sulfate-free sample (V⁵⁺/(V⁴⁺ + V⁵⁺) = 69.3%). This result suggests that adding NH₄HSO₄ increases the valence state of V atoms (Fig. 4(b)). According to previous studies,¹³ S=O structure is responsible for generating acidic sites on the sulfate-containing samples and has a strong ability to attract electrons from basic molecules. It seems that the stronger electron affinity by S⁶⁺ in sulfate anions is the main reason for the increased valence state of V atoms in the NH₄HSO₄-loaded samples.²⁰

According to Fig. 4(c), the deconvolution of the O 1s photoelectron peaks indicates the existence of various oxygen-containing chemical bonds on the catalyst surface. The first peak in the range of 529.5–530.5 eV could be assign to lattice oxygen (O_β); the second peak (531.0–532.0 eV) is attributed to chemical adsorbed oxygen (O_α), including surface defect-oxide and hydroxyl groups.²¹ Given the deposition of NH₄HSO₄ on the catalyst surface, the relative concentration ratio of O_α/(O_α + O_β) increases, which might result from hydration of SO₄²⁻ to form new Bronsted sites on the catalyst surface.²⁰ Consequently, Bronsted sites increase and more hydroxyl groups come out with the introduction of NH₄HSO₄, thereby resulting in a relatively high ratio of O_α/(O_α + O_β).

As is previously mentioned, NH₄HSO₄ and vanadium species sometimes interact on the catalyst surface. The competitive attachment of vanadium and sulfate species to the TiO₂ surface basic hydroxyl groups leads to the transformation of vanadium species from monomeric species to polymeric ones with the deposition of NH₄HSO₄. Considering the stronger electronegativity of sulfate anions, S species would attract electrons from V atoms, thereby resulting in V atoms in an electron-deficit state. Consequently, the valence state of V atoms is increased in the NH₄HSO₄-deposited samples. It seems that interactions between NH₄HSO₄ and vanadium species play an important role in the reactivity of NH₄HSO₄ with NO, for NH₄HSO₄ deposited on the TiO₂ catalyst exhibit a low reactivity. However, the reason for the enhanced reactivity behavior of NH₄HSO₄ with NO on the high V content catalysts is still unknown, the answer of which would be uncovered in the next section.

3.3 The key factor in the reactivity behavior of NH₄HSO₄ with NO

3.3.1 Catalyst physical properties. The XRD patterns of the series catalysts in Fig. S1† illustrate that V and sulfate species are highly dispersed, which exist in an amorphous state on the catalysts. Given increases in V₂O₅ content to 8 wt%, new diffraction peaks indexed to V₂O₅ phase appear, indicating that V loading is beyond the theoretical monolayer coverage on the TiO₂ support. The BET specific surface areas (S_BET), total pore volumes (V_p) of the samples are summarized in Table S1.† With increasing V content, no obvious variations in the catalyst specific surface areas occur, suggesting that catalyst surface area is not an important reason for the enhanced reactivity behavior of NH₄HSO₄ with NO on the high V content catalysts. Therefore, further research, particularly on Raman and XPS, should be conducted to explore the role of vanadium species in the NH₄HSO₄ reactivity behavior.

3.3.2 Catalyst surface molecular information. FTIR spectra in Fig. S2† illustrate that sulfate species are presented as bidentate SO₄²⁻, which are subsequently bonded to the catalyst surface; whereas N species exhibit as NH₄⁺ linked to sulfate sites.²² Raman spectra of the series NH₄HSO₄-deposited V/Ti catalysts are shown in Fig. 5. Given increases in vanadium content, polymeric vanadium species become the dominant V-species, as the absence of the Raman band attributed to monomeric vanadium species illustrates. According to previous studies,¹⁷ formation of polymeric vanadium species possessing novel reducibility has a positive effect on the NH₃ activation process, thereby resulting in an improved SCR activity at lower temperatures for the high V content catalysts. Accordingly, it seems that the as-formed polymeric vanadium species would be closely related to the enhanced reactivity behavior of NH₄HSO₄ with NO on the high V content catalysts, for it has been proved that mechanism of the NH₄HSO₄ reactivity behavior is similar with that of SCR reactions.⁹ As the V₂O₅ content reaches 8 wt%, a strong peak centered at 993 cm⁻¹ attributed to V₂O₅ crystallites comes out,²³ which is consistent with the XRD results.

Fig. 5 Raman spectra of the series V/Ti catalysts.

The XPS spectra of V 2p, O 1s and S 2p are shown in Fig. 6. The deconvolution of the V 2p photoelectron peaks indicates the coexistence of V⁴⁺ and V⁵⁺ in the series NH₄HSO₄-deposited samples (Fig. 6(a)). As the V₂O₅ content increases from 1% to 8%, the relative concentration ratio of V⁵⁺ increases from 49.34% to 82.58% (Table 1), illustrating the ratio of [(vanadium strongly interacting with the support)/(total vanadium)] decreases with increasing vanadium loading,²³ which is consistent with the Raman results that formation of polymeric vanadium species occurs on the high V content catalysts. Based on previous studies, a high content of V⁵⁺ exerts a promotion effect on the dehydrogenation process of ammonium species, which contributes to the enhanced NH₄HSO₄ reactivity behavior on the high V content catalysts to some extent.²⁴ As shown in Fig. 6(b), the increase in vanadium content leads to a declined binding energies of the O 1s photoelectron peaks, implying that the surface oxygen species of the high vanadium content samples own the most electron cloud density; that is the main reason for the formation of more reactive electrophilic oxygen species, which would exert a positive effect on the NH₄HSO₄ activation process, thereby leading to an enhanced reactivity behavior of NH₄HSO₄ with NO.²⁵

Fig. 6 XPS spectra of the series V/Ti samples: (a) V 2p; (b) O 1s; (c) S 2p.

Table 1 XPS data of the series V/Ti catalysts

Sample	V^5+/(V^5+ + V^4+)%
ABS-Ti	—
ABS-V1/Ti	49.34
ABS-V2/Ti	52.50
ABS-V4/Ti	80.04
ABS-V8/Ti	82.58

---

After the curve fitting of the S 2p photoelectron peaks, the obtained two sub-peaks could be attributed to the S 2p_1/2 and S 2p_3/2 transitions of SO₄²⁻.²⁶ Given increases in V content, S 2p peaks shift towards lower binding energies, indicating an increased electron cloud density around the S atoms with +6 formal oxidation number as in SO₄²⁻ (Fig. 6(c)). Based on previous study,²⁷ the NH₄HSO₄ consumption behavior on the catalysts during the heat process is mainly attributed to the reduction of S atoms with +6 formal oxidation number as in SO₄²⁻ to those with +4 formal oxidation number as in SO₂. According to the results mentioned above, the increased vanadium content leads to an increase in the electron cloud density around the S atoms in SO₄²⁻. Therefore, SO₄²⁻ on the high V content catalysts would be easily reduced to SO₂ during the heat process. Consequently, temperature for the maximum outlet SO₂ concentration decreases and the SO₂ release amount increases for the high V content catalysts.

3.3.3 Mechanism of the reactivity behavior of NH₄HSO₄ with NO. To better study the detailed mechanism of the NH₄HSO₄ reactivity behavior with NO on the catalyst surface, special experiments at lower temperatures should be conducted to avoid the NH₄HSO₄ decomposition behavior. Based on the FTIR spectra in Fig. S3,† temperatures at or below 200 °C should be chosen because ammonium sulfate salts remain almost unchanged in this temperature region. As shown in Fig. 7, characteristic peaks at 1247 and 1160 cm⁻¹, which can be assigned to the asymmetric and symmetric stretching frequencies of S=O in bidentate SO₄²⁻, together with a peak at 1432 cm⁻¹ attributed to NH₄⁺, appear.^28,29 Meanwhile, the bands at 2834, 3050, 3262 cm⁻¹, assigned to the stretching vibrations of N–H in NH₄⁺ also come out.³⁰ Upon introduction of NO + O₂, the intensity of the band attributed to NH₄⁺ gradually decreases, indicating the occurrence of the reaction between NH₄⁺ and NO. Meanwhile, the band at 1296 cm⁻¹, the symmetric stretching of the O=S=O vibrations in tridentate SO₄²⁻, comes out during the reaction process, which might be attributed to the gradual consumption of NH₄⁺ bonded to sulfate sites.³¹ During the whole reaction process, no bands attributed to nitrate species or other nitrogenous intermediates are detected, suggesting that the reaction pathway between NH₄⁺ species and gaseous NO dominates in the reactivity of NH₄HSO₄ with NO on the V/Ti catalyst surface. The dehydrogenation of NH₄⁺ by redox sites constitutes the main step in the reactivity of NH₄HSO₄ with NO on the catalysts.¹

Fig. 7 In situ DRIFTS study of the NH₄HSO₄ reactivity behavior on the V4/Ti catalyst surface at 200 °C.

During the reaction process, NH₄⁺ bonded to sulfate sites acts as a reductant and reacts with gaseous NO, as the production of N₂ and H₂O follows.⁹ Meanwhile, a transformation of sulfate species from bidentate sulfate anions to tridentate sulfate anions occurs with the consumption of NH₄⁺ through the reaction with NO. In other words, NH₄⁺ in NH₄HSO₄ functions as a reductant during reaction with gaseous NO, while S-containing functional groups would be stabilized as tridentate sulfate species on the catalyst surface (Fig. 8). According to the FTIR spectra in Fig. 7, it can be concluded that the dehydrogenation process of NH₄⁺ would be the key factor in the reactivity of NH₄HSO₄ with NO on the catalysts, which catalyst redox property determines. Given increases in vanadium content, the as-formed polymeric vanadium species with novel reducibility and reactive electrophilic oxygen species have a positive effect on the NH₄⁺ activation process, thereby resulting in an enhanced reactivity of NH₄HSO₄ with NO at low-temperature regions.

Fig. 8 Reaction routine of NH₄HSO₄ with NO on V₂O₅/TiO₂ catalyst surfaces.

4. Conclusions

This study illustrates the interactions between NH₄HSO₄ and vanadium species, along with the role of vanadium species in the reactivity of NH₄HSO₄ with NO on the V₂O₅/TiO₂ catalysts. Both vanadium and sulfate species occupy the TiO₂ surface basic hydroxyl groups; the declined TiO₂ surface basic sites resulting from the introduction of NH₄HSO₄ leads to the formation of polymeric vanadium species. The reactivity behavior of NH₄HSO₄ with NO is enhanced with increasing vanadium content, the main reason for which is the formation of polymeric vanadium species and reactive electrophilic oxygen species on the catalysts. Besides, a higher electron cloud density around the S atoms in SO₄²⁻ could be detected for the high V content catalysts, on which SO₄²⁻ would be easily reduced to SO₂ during the TPSR process. In situ diffuse reflectance infrared Fourier transform spectroscopy confirms that NH₄⁺ belonging to NH₄HSO₄ functions as a reductant during reaction with gaseous NO, while S-containing functional groups are stabilized as tridentate sulfate anions on the catalyst surface.

Acknowledgements

This work is supported by the Key Research & Development Plan of Shandong Province (No. 2014GJJS0501) and the National High-Tech Research and Development (863) Program of China (No. 2013AA065401).

References

1. G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal., B, 1998, 18, 1–36.
2. T. Boningari, R. Koirala and P. G. Smirniotis, Appl. Catal., B, 2013, 140–141, 289–298.
3. T. Boningari, R. Koirala and P. G. Smirniotis, Appl. Catal., B, 2012, 127, 255–264.
4. W. J. Cha, S. M. Chin, E. S. Park, S. T. Yun and J. S. Jurng, Appl. Catal., B, 2013, 140–141, 708–715.
5. Y. Xi, N. A. Ottinger and Z. G. Liu, Appl. Catal., B, 2014, 160–161, 1–9.
6. S. Zhang, H. Li and Q. Zhong, Appl. Catal., A, 2012, 435–436, 156–162.
7. D. Ye, R. Qu, H. Song, C. Zheng, X. Gao, Z. Luo, M. Ni and K. Cen, RSC Adv., 2016, 6, 55584–55592.
8. G. Baltin, H. Köser and K.-P. Wendlandt, Catal. Today, 2002, 75, 339–345.
9. D. Ye, R. Qu, H. Song, X. Gao, Z. Luo, M. Ni and K. Cen, Chem. Eng. J., 2016, 283, 846–854.
10. Z. Zhu, Z. Liu, H. Niu, S. Liu, T. Hu, T. Liu and Y. Xie, J. Catal., 2001, 197, 6–16.
11. Z. Zhu, H. Niu, Z. Liu and S. Liu, J. Catal., 2000, 195, 268–278.
12. Z. Huang, J. Catal., 2003, 214, 213–219.
13. S. T. Choo, Y. G. Lee, I.-S. Nam, S.-W. Ham and J.-B. Lee, Appl. Catal., A, 2000, 200, 177–188.
14. J. L. Ropero-Vega, A. Aldana-Pérez, R. Gómez and M. E. Niño-Gómez, Appl. Catal., A, 2010, 379, 24–29.
15. J. P. Chen and R. T. Yang, J. Catal., 1993, 139, 277–288.
16. M. J. Scepanovic, M. Grujic-Brojcin, Z. D. Dohcevic-Mitrovic and Z. V. Popovic, Sci. Sintering, 2009, 41, 67–73.
17. C. Wang, S. Yang, H. Chang, Y. Peng and J. Li, Chem. Eng. J., 2013, 225, 520–527.
18. X. Du, X. Gao, Y. Fu, F. Gao, Z. Luo and K. Cen, J. Colloid Interface Sci., 2012, 368, 406–412.
19. J. Haber, Catal. Today, 2009, 142, 100–113.
20. O. Saur, M. Bensitel, A. B. M. Saad, J. C. Lavalley, C. P. Tripp and B. A. Morrow, J. Catal., 1986, 99, 104–110.
21. R. Qu, X. Gao, K. Cen and J. Li, Appl. Catal., B, 2013, 142–143, 290–297.
22. X. Guo, C. Bartholomew, W. Hecker and L. L. Baxter, Appl. Catal., B, 2009, 92, 30–40.
23. P. Ji, X. Gao, X. Du, C. Zheng, Z. Luo and K. Cen, Catal. Sci. Technol., 2016, 6, 1187–1194.
24. D. W. Kwon, S. M. Lee and S. C. Hong, Appl. Catal., A, 2015, 505, 557–565.
25. C. He, Y. Yu, J. Shi, Q. Shen, J. Chen and H. Liu, Mater. Chem. Phys., 2015, 157, 87–100.
26. Y. Wu, L. Qin, G. Zhang, L. Chen, X. Guo and M. Liu, Ind. Eng. Chem. Res., 2013, 52, 16698–16708.
27. J. Li and G.-E. Zhang, Acta Phys.-Chim. Sin., 1992, 8, 123–127.
28. R. Jin, Y. Liu, Y. Wang, W. Cen, Z. Wu, H. Wang and X. Weng, Appl. Catal., B, 2014, 148–149, 582–588.
29. B. Jiang, B. Deng, Z. Zhang, Z. Wu, X. Tang, S. Yao and H. Lu, J. Phys. Chem. C, 2014, 118, 14866–14875.
30. F. Liu, K. Asakura, H. He, W. Shan, X. Shi and C. Zhang, Appl. Catal., B, 2011, 103, 369–377.
31. P. A. Kumar, Y. E. Jeong, S. Gautam, H. P. Ha, K. J. Lee and K. H. Chae, Chem. Eng. J., 2015, 275, 142–151.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22571c

‡ These authors contributed equally to this work and should be considered as co-first authors.

---