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
First published on 21st October 2016
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.
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.
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).
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![]() | (1) |
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.
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.
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
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.
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 SO 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.
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
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.
![]() | ||
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.
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. |
This journal is © The Royal Society of Chemistry 2016 |