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

Ruiyang Qu , Dong Ye, Chenghang Zheng, Xiang Gao*, Zhongyang Luo, Mingjiang Ni and Kefa Cen
State Key Laboratory of Clean Energy Utilization, College of Energy Engineering, Zhejiang University, Hangzhou 310027, P. R. China. E-mail: xgao1@zju.edu.cn

Received 9th September 2016 , Accepted 20th October 2016

First published on 21st October 2016


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 NOx 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[thin space (1/6-em)]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[thin space (1/6-em)]ln(x) with x = CNOx(out)/CNOx(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.
image file: c6ra22571c-f1.tif
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.
image file: c6ra22571c-f2.tif
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 3Eg, 1A1g and 2B1g 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


image file: c6ra22571c-f3.tif
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.


image file: c6ra22571c-f4.tif
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[double bond, length as m-dash]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 (Oβ); the second peak (531.0–532.0 eV) is attributed to chemical adsorbed oxygen (Oα), including surface defect-oxide and hydroxyl groups.21 Given the deposition of NH4HSO4 on the catalyst surface, the relative concentration ratio of Oα/(Oα + Oβ) 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 Oα/(Oα + Oβ).

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 (SBET), total pore volumes (Vp) 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.
image file: c6ra22571c-f5.tif
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


image file: c6ra22571c-f6.tif
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 V5+/(V5+ + V4+)%
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 2p1/2 and S 2p3/2 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[double bond, length as m-dash]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[double bond, length as m-dash]S[double bond, length as m-dash]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
image file: c6ra22571c-f7.tif
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.


image file: c6ra22571c-f8.tif
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).

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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.

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