Nitrogen oxides (NOx) from the combustion of fossil fuels in vehicles or coke in the electrical power plants have resulted in serious environmental problems due to their promotion of acid rain, photochemical smog, ozone depletion, and greenhouse gases. The selective catalytic reduction (SCR) of NOx with NH3 is the most effective method for the removal of NOx from stationary sources and diesel engines [1-3]. V2O5/TiO2-based catalysts have been widely used in industry to eliminate NOx for their high NOx removal efficiency and strong resistance to poisoning by SO2 that is common in flue gases [3-5]. Nevertheless, these catalysts still suffer from the high activity for SO2 oxidation to SO3, which cause corrosion and plugging of the reactor [6], and the high operating temperatures (300-400 ℃) that cause high energy consumption. Low temperature SCR has aroused great interest in the past two decades [7-10]. Transition metal oxides like Fe2O3 [11], MnOx [12-14], CuO [15] and V2O5 [16, 17] have shown good activity for low temperature SCR reaction. However, these catalysts are easily deactivated in the presence of SO2 and H2O by the blocking of the active sites. Therefore, a high resistance to SO2 and H2O poisoning is of concern for low temperature SCR catalysts for NOx removal.
Pillared interlayer clays (PILCs) are unique two dimensional zeolite-like materials prepared by intercalation of inorganic cationic clusters into clay layers followed by heating. Researchers have paid much attention to PILCs because of their large specific surface area, high surface acidity and good thermal stability. A series of PILCs were synthesized and used as catalysts for the SCR reaction of NOx with NH3 by Yang et al. [18, 19]. These showed high activity in the SCR reaction that was better than the traditional V2O5-based catalysts. TiO2-PILC has a large surface area and pore size, high thermal and hydrothermal stability as well as high resistance to SO2 [20]. The activity of V2O5/TiO2-PILC [21] and Fe/TiO2-PILC [22] catalysts can be improved by the presence of H2O and SO2. Although PILCs-based catalysts showed high sulfur resistance in the NH3-SCR reaction, there are no reports on the mechanisms of the resistance to SO2 over the V2O5/TiO2-PILC catalysts. Even the investigations of SO2 interaction with vanadia/titania catalysts are not comprehensive. Orsenigo et al. [23] studied the role of sulfates in NOx reduction and SO2 oxidation, and suggested that the buildup of sulfates at the catalyst surface likely occurred first at or near the vanadyl sites and increased both the Brnsted and Lewis acidity of the catalyst and enhanced the reactivity in the de-NOx reaction. However, their work did not include confirming experimental evidence from surface science methods. Baxter’s group [24] used in situ FTIR and XPS to prove that a stable sulfate species was formed on titania but not on vanadia. In summary, there was no exact determination on the interaction between SO2 and the vanadia/titania catalysts.
Understanding the effects of SO2 on SCR activity over PILCs catalysts is important for the development and application of the appropriate catalysts. In this study, the effects of SO2 on the NH3-SCR reaction over a V2O5/TiO2-PILC catalyst were investigated. X-ray fluorescence (XRF), X-ray diffraction (XRD), N2 adsorption-desorption measurements, temperature- programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) were used to characterize the catalysts and identify the interaction between SO2 and the catalysts.
TiO2-PILCs were synthesized by the established procedures [25, 26]. The starting clay was a purified grade montmorillonite powder from Nanocor Company. The cation exchange capacity (CEC) of the clay was 145 meq/100 g. The pillaring agent, a solution of partially hydrolyzed Ti polycations, was prepared by adding TiCl4 into HCl solution (2 mol/L). The mixture was then diluted by the slow addition of distilled water with stirring to reach a final Ti concentration of 0.82 mol/L. The amount of HCl solution corresponded to the final concentration of 0.11 mol/L. The solution was aged for 8 h at room temperature, which was the pillaring solution. Clay (10 g) was dispersed in 2.0 L of deionized water and the slurry was stirred for 24 h. The pillaring solution was then slowly added into the suspension of clay with vigorous stirring until the amount of pillaring solution reached the required Ti/clay ratio of 10 mmol/g. The product was left in the solution for 24 h. Subsequently, the mixture was separated by centrifugation and washed with deionized water until the liquid was free of chloride ions as indicated by the silver nitrate test. The samples were dried at 120 ℃ for 12 h and then calcined at 400 ℃ for 4 h.
The TiO2-PILCs supported vanadia catalysts were prepared by the impregnation of TiO2-PILCs with aqueous solutions of NH4VO3 in oxalic acid. The samples were dried at 105 ℃ for 4 h and then calcined at 250 ℃ for 1 h and 450 ℃ for 3 h. The obtained V2O5/TiO2-PILC catalysts were labeled as nV/TiO2-PILC, where n referred to the vanadium amount (mass fraction,%) on the support. Besides the pillared clay catalysts, V2O5/TiO2 and V2O5-MoO3/TiO2 catalysts were also prepared using a similar method for comparison. These catalysts contained 4% V2O5 and 6% MoO3 and were denoted as 4V/TiO2 and 4V6Mo/TiO2, respectively.
The SCR activity measurement was carried out in a fixed bed quartz microreactor (i.d. = 8 mm) with 0.2 mL catalyst (40-60 mesh) at atmospheric pressure. The flue gas was simulated by blending different gaseous reactants that contained 0.1% NO,0.1% NH3,8% O2,0.05% SO2 (when used),10% H2O (when used), and balanced with He. The total flow was 100 mL/min with the GHSV of 30000 h-1. The gas mixtures in the reactor outlet that contained NO, NO2, N2O, and N2 was analyzed by a gas chromatograph (GC-2014C, Shimadzu) equipped with a TCD detector and a Fourier transform infrared (FT-IR) spectrometer (Tensor 27, Bruker). The NO conversion (X) was calculated by
where “in” and “out” represented inlet and outlet of the reactor, respectively.
Elemental analysis of the samples was carried out on an X-ray fluorescence spectrometer (Magix PW2403, PAN alytical). The XRD patterns were measured on a Bruker D8 Advance diffractometer operated at 50 kV and 40 mA using Cu Kα radiation (λ = 0.154 nm) for 2θ = 5°-80° with a step size of 7.2°/min. The specific surface areas, pore volumes and micropore volumes of the samples were measured by a physical adsorption instrument (Micromeritics ASAP 2020). Specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method. All the samples were degassed at 250 ℃ under vacuum for 12 h, and N2 was adsorbed at -196 ℃. In situ DRIFTs were carried out using an FT-IR spectrometer (Nicolet 6700, Thermo) equipped with an in situ diffuse reaction chamber and a high sensitivity mercury cadmium telluride (MCT) detector cooled by liquid nitrogen. The samples were first treated at 110 ℃ in N2 flow for 30 min to remove water and impurities on the surface of the catalysts. All spectra were collected at a resolution of 4 cm-1 by an accumulation of 32 scans. The TPD spectra were obtained by a quantitative gas analysis (QGA) system (HIDEN analytical). For each experiment, the catalyst was preconditioned at 110 ℃ in N2 at a flow rate of 30 mL/min and then cooled to 40 ℃. The catalyst samples were then treated with 1% SO2/N2 or (1% SO2+8% O2)/N2 at 40 ℃ for 1 h. The total flow rate was 30 mL/min. Subsequently, the samples were purged with N2 for 0.5 h before the TPD experiments. The TPD run was conducted from 50 to 900 ℃ at a heating rate of 10 ℃/min.
XRD patterns of the montmorillonite and nV/TiO2-PILC catalysts are shown in Fig. 1. The XRD pattern of the parent clay exhibited a peak at 2θ = 7.1°, which was assigned to the basal (001) reflection, indicating the order of the clay layers [20]. The diffraction at 2θ = 19.7° was assigned to the summation of hk indices of (02) and (11), and the diffraction at 2θ = 35.0° was the summation of hk indices of (13) and (20) [19]. The peaks at 2θ = 26.5° and 28.0° were reflections of a quartz impurity [27]. No reflection was observed at 2θ = 7.1° over the TiO2-PILC support and nV/TiO2-PILC catalysts. The disappearance of the regular basal spacing was attributed to the delaminated clay, which generated a “house card” structure as previously reported [19, 27]. The XRD patterns of the nV/TiO2-PILC catalysts also showed the characteristic diffraction peaks of the anatase phase of titania (JCPDS No. 24-0913). The crystalline phase of V2O5 was not observed on the catalysts, suggesting that vanadia existed in amorphous or highly dispersed state on the surface of the support [28].
The N2 adsorption isotherms and the pore size distributions of the clay (1),4V/TiO2-PILC (2),4V/TiO2 (3) and 4V6Mo/TiO2 (4) catalysts are shown in Fig. 2. The BET surface areas and pore volumes are summarized in Table 1. The adsorption isotherm of the clay was type II, which was characteristic of macroporous solids. The adsorption-desorption isotherms formed a hysteresis loop of the H3 type, which was typical of non-uniform slit-like pores according to IUPAC classification [29]. The 4V/TiO2-PILC catalyst showed a type I N2 adsorption isotherm and type H4 hysteresis loops, implying a typical microporous solid that had uniform slit-like pores. The transformation of the adsorption isotherm and hysteresis loops illustrated that TiO2 was successfully pillared in the interlayers of the clay. There was a sharp peak at the pore diameter of approximately 4 nm for the 4V/TiO2-PILC catalyst (Fig. 2(b-2)), suggesting that there were mesopores with a uniform pore size in the pillared clay. From Table 1, one can see that the elemental composition changed after pillaring modification, indicating that TiO2 was exchanged into the clay. The BET surface area (ABET) was increased greatly from 9 m2/g of the clay to approximately 210 m2/g of the 4V/TiO2-PILC catalysts, which was also much larger than that of the traditional V2O5/TiO2 catalysts.
The catalytic performance of the nV/TiO2-PILC catalysts for the SCR reaction of NO by NH3 is shown in Fig. 3. The pure TiO2-PILC support showed a low activity for NO removal (Fig. 3(a)), and only 60% NO was converted at 500 ℃. When vanadia was loaded on the TiO2-PILC, its activity was enhanced significantly under the same reaction conditions, attaining nearly total NO conversion at 300 ℃. The 4V/TiO2-PILC catalyst exhibited a higher catalytic performance and displayed a wider operating temperature window from 260 to 500 ℃ than that of the 3V/TiO2-PILC and 5V/TiO2-PILC catalysts, revealing that 4% vanadia loading was the optimum amount. The NO conversion over the 4V/TiO2-PILC catalyst reached 80% at 160 ℃, and maintained at a high level (＞90%) in the temperature range of 260-500 ℃.
Fig. 3(b) shows the effects of SO2 and H2O on the catalytic performance of the pillared clay catalyst and the traditional vanadia-based catalysts. The 4V/TiO2-PILC,4V/TiO2 and 4V6Mo/TiO2 catalysts exhibited a similar catalytic activity in the absence of SO2 or H2O between 100 and 350 ℃. After the addition of 0.05% SO2 and 10% H2O, the NO conversion over all the samples increased slightly at the low temperature range (＜150 ℃), which was attributed to sulfation of the catalyst surface that increased the Brnsted acid site density, which correlated well with the increase in SCR catalytic activity [24]. For the 4V/TiO2 and 4V6Mo/TiO2 catalysts, obvious decreases of the NO conversion were observed in the presence of SO2 and H2O in the temperature range of 160-400 ℃. However, the inhibition effect of SO2 and H2O on the 4V/TiO2-PILC catalyst was negligible when the temperature was above 160 ℃. The NO conversion maintained a high level (＞96 %) in the range of 250-400 ℃ (Fig. 3(b)). The tolerance to SO2 and H2O of the three catalysts was in order of 4V/TiO2-PILC ＞ 4V6Mo/TiO2 ＞ 4V/TiO2. The concentrations of N2O formed over the 4V/TiO2-PILC catalyst above 300 ℃ were lower compared with the other two catalysts, implying that the 4V/TiO2-PILC catalyst had high N2 selectivity at high temperature.
The effects of SO2 and H2O on the activities of the 4V/TiO2-PILC and 4V6Mo/TiO2 catalysts are shown in Fig. 3(c). In the presence of SO2 and H2O, the NO conversion over 4V/TiO2-PILC and 4V6Mo/TiO2 gradually decreased with time from 97% to 65% and from 84% to 69%, respectively, after 25 h. During the first 10 h on stream, the NO conversion over the 4V/TiO2-PILC catalyst was higher than that of 4V6Mo/TiO2 catalyst. After 11 h, the NO conversion over the 4V/TiO2-PILC catalyst was lower than that of the 4V6Mo/TiO2 catalyst. These results showed that the stability of the 4V/TiO2-PILC catalyst was still not to our satisfaction, although it had good initial activity for the NH3-SCR reaction in the presence of SO2 and H2O.
In order to investigate SO2 adsorption on the catalysts, temperature-programmed desorption of SO2 (SO2-TPD) experiments were conducted. Fig. 4 shows the profiles of SO2 (m/z = 64) signals with temperature. For the 4V/TiO2 and 4V/TiO2-PILC catalysts, a weak peak at 92 and 108 ℃ was detected, respectively (Fig. 4(a)), which was attributed to the physisorption of SO2 on the catalyst [30]. There was also a broad SO2 desorption band at 550-850 ℃ for each catalyst, which was assigned to the decomposition of bulk sulfate species on titania formed by the interaction of SO2 with lattice oxygen. For the 4V6Mo/TiO2 catalyst, no obvious SO2 desorption peak at low temperature was observed. Moreover, the intensity of the SO2 desorption band at high temperature was much weaker than that of the two others. The 4V6Mo/TiO2 catalyst exhibited the least SO2 desorbed amount, which was possibly due to the inhibition by Mo of SO2 adsorption [31]. When the three catalysts were exposed to (1% SO2 + 8% O2)/N2 at 40 ℃ for 1 h, their SO2 desorption behavior changed (Fig. 4(b)). The SO2 desorption peak at low temperature disappeared and a new broad SO2 desorption band at 350-600 ℃ appeared for each catalyst, which were due to the decomposition of chemisorbed sulfate and/or sulfite species on the titania surface [23, 29]. These results indicated that the presence of O2 promoted the oxidation of SO2 and reduced physisorbed SO2. The significant decrease of the SO2 desorption band at high temperature (＞ 700 ℃) suggested that the interaction between SO2 and the lattice oxygen of the catalyst was inhibited by the presence of O2, which reduced the format ion of sulfate species on the catalyst. The SO2 desorption amount from the 4V/TiO2-PILC catalyst was comparative more than that from the 4V6Mo/TiO2 catalyst, possibly due to the adsorption of SO2 molecules on the clay.
To investigate SO2 poisoning of the SCR catalysts, the in situ DRIFT technique was used. The adsorption mechanism of sulfate species on metal oxides has been reported in the literature [32, 33, 34]. The sulfate infrared spectra show the interaction modes of the sulfate species with the surface, from the change of the number of S=O bonds in the sulfate species. Normally, the ν(S=O) stretching mode of ionic sulfate with a bond number of 1.5 is observed at 1100 cm-1. However, with increasing bond number, the stretching frequency shifts from 1300-1200 cm-1 for bond numbers of 1.6-1.7 to 1400 cm-1 for double bonds. Corresponding to the increasing bond number, the binding character of sulfate changes from ionic to covalent [35]. Fig. 5(a) shows the DRIFT spectra of the 4V/TiO2-PILC catalyst as a function of exposure time. After exposing the 4V/TiO2-PILC catalyst to SO2, four peaks at 1373,1359,1344, and 1275 cm-1 appeared. Their intensities increased with exposure time. In other studies [36, 37, 38], the peaks at 1373,1359 and 1344 cm-1 were attributed to the S=O stretching frequencies of chemisorbed sulfate and/or sulfite species, which indicated covalently bonded sulfate species on the surface of TiO2 [39]. The band at 1275 cm-1 was assigned to ionic SO42- species [36]. The band shift to lower frequencies indica ted that the bond number of S=O decreased, implying that the binding mode of the sulfate species with the catalyst changed from covalent to ionic.
A broad band in the range of 1200-1100 cm-1 over the 4V/TiO2 and 4V6Mo/TiO2 catalysts was observed (Fig. 5(b)), which was assigned to bulk sulfate species. For the 4V/TiO2 and 4V6Mo/TiO2 catalysts, the DRIFT results were consistent with the SO2-TPD data. However, no bulk sulfate species was detected on the 4V/TiO2-PILC catalyst. Compared to the other two catalysts, the intensity of the bands at 1359 and 1344 cm-1 over the 4V/TiO2-PILC catalyst was higher (Fig. 5(b)), indicating more chemisorbed sulfate and/or sulfite species on the 4V/TiO2-PILC catalyst. This would explain the larger SO2 desorption band of the 4V/TiO2-PILC catalyst in the SO2-TPD profile.
Fig. 6 shows the interaction of SO2 and NH3 on the catalyst. In the absence of SO2, four bands were observed for the 4V/TiO2-PILC catalyst at 260 ℃ (Fig. 6(a)). The weak bands at 1598 and 1256 cm-1 were attributed to the asymmetric and symmetric bending vibrations of the N-H bonds in NH3 coordinately linked to Lewis acid sites. The bands at 1674 and 1430 cm-1 were due to the asymmetric and symmetric deformation vibrations of the N-H bonds in ammonium ions formed by the chemisorption of NH3 on Brnsted acid sites [22, 40, 41]. After the addition of 0.05% SO2 to the feed, four new peaks at 1377,1359,1344, and 1270 cm-1 appeared. Their intensities increased with time in the SO2 atmosphere. All these peaks were characteristic peaks of surface sulfate and/or sulfite species, indicating the adsorption of SO2 on the catalyst. From Fig. 6(b), one can see that the intensity of the peak at 1272 cm-1 over the 4V/TiO2 and 4V6Mo/TiO2 catalysts was stronger than that on the 4V/TiO2-PILC catalyst, revealing the existence of more ionic SO42- species on both the 4V/TiO2 and 4V6Mo/TiO2 catalysts. In other words, less ionic SO42- species were formed on the surface of the 4V/TiO2-PILC catalyst than on the 4V/TiO2 and 4V6Mo/TiO2 catalysts, impl ying that the conversion of chemisorbed SO2 to SO42- was inhibited on the 4V/TiO2-PILC catalyst in the presence of NH3. The amounts of ionic SO42- species on the three catalyst were in the order of 4V/TiO2-PILC ＜ 4V6Mo/TiO2 ＜ 4V/TiO2, which was reversed to that of NH3-SCR activity over the three catalysts in the presence of SO2 and H2O. This showed that the accumulation of ionic SO42- species on the catalyst was one reason that led to the deactivation of the catalyst in the SCR reaction. In addition, for the 4V/TiO2 catalyst, the bands at 1359 and 1344 cm-1 were very weak, indicating that part of the sulfate species on the 4V/TiO2 catalyst surface was transformed to ionic SO42- species due to the presence of a hydrogen donator (NH3). Weak bands at 1160 and 1140 cm-1 over the 4V/TiO2 and 4V6Mo/TiO2 catalysts were also observed, showing the formation of the bulk sulfate species, but this band was not detected on the 4V/TiO2-PILC catalyst.
The DRIFT experiments of the 4V/TiO2-PILC catalyst were also conducted in a NO+O2+SO2 atmosphere. As shown in Fig. 7, three peaks at 1629,1600 and 1348 cm-1 appeared in the absence of SO2, which were all assigned to the formation of nitrate species on the surface [42, 43, 44]. The addition of SO2 resulted in the appearance of sulfate species bands at 1371,1348 (overlapped with the band of nitrate species) and 1278 cm-1. Moreover, their intensity increased with exposure time. In contrast, the intensity of the bands of the nitrate species decreased gradually with the introducing of SO2. The results showed that the existence of SO2 promoted the reduction of the nitrate species.
We also investigated the interaction of SO2 and the reaction gases. The DRIFT spectra of the catalysts in a flow of NO+NH3+O2 with and without SO2 at 260 ℃ are illustrated in Fig. 8. The experiment was carried out by treating the catalysts in a flow of NO+NH3+O2 for 60 min first, and then 0.05% SO2 was introduced into the feed. From Fig. 8(a), one can see that the characteristic bands of nitrate species at 1630,1600, and 1385 cm-1 were detected in the absence of SO2, but almost no N-H vibration band related to ammonia species was detected, which was possibly due to the consumption by the reaction between NO and NH3. New bands appeared at 1362 and 1277 cm-1 after the introduction of SO2 and their intensity increased with time. Meanwhile, the bands at 1630,1600, and 1385 cm-1 assigned to nitrate species disappeared gradually, further illustrating that the existence of SO2 improved the reduction of nitrate species, which could be correlated with the good resistance of 4V/TiO2-PILC to SO2 poisoning. From Fig. 8(b), one can see that there were obvious differences among the DRIFT spectra of the three catalysts. For the 4V/TiO2 and 4V6Mo/TiO2 catalysts, the bands due to the nitrate species (1629 and 1600 cm-1) still could be detected in the presence of SO2, revealing that the nitrate species could be maintained for some time on the surface under the reaction atmosphere. The intens ity of the band at 1277 cm-1 attributed to ionic SO42- species over the 4V/TiO2-PILC catalyst was much weaker than that over the others, showing that the amount of ionic SO42- species over the 4V/TiO2-PILC catalyst was negligible. The broad bands at 1115 cm-1 for the 4V/TiO2 and 4V6Mo/TiO2 catalysts also illustrated the existence of bulk sulfate species on the surface. The results were consistent with the other results in the above experiments.
From the in situ DRIFTs experiments, it was found that surface sulfate and/or sulfite species and ionic SO42- species were formed on the catalysts, but the amount of ionic SO42- species on the surface of the 4V/TiO2-PILC catalyst was the least among the three catalysts. This was one reason why the 4V/TiO2-PILC catalyst had better resistance to SO2 poisoning than the two others.
In order to further explain the formation of surface ionic SO42- species on the catalysts, XPS experiments were carried out to analyze the surface oxygen species of the catalysts. According to Fig. 9, the O 1s spectra exhibited two peaks due to different oxygen-containing chemical bonds. The first peak at 530.3 eV was attributed to the lattice oxygen O2- (expressed by Oβ) and the peak at 531.8 eV was assigned to surface adsorbed oxygen (Oα), including O2-, O22- and O-. The strong and broad peak at 532.3 eV over the 4V/TiO2-PILC catalyst was attributed to surface hydroxyl, which existed on the interlayer of the clay [45, 46]. The XPS data showed that the molar ratios of Oads/Olatt on the three catalysts surface increased in the order of 4V/TiO2-PILC ＜ 4V6Mo/TiO2 ＜ 4V/TiO2. The surface with more adsorbed oxygen species was more susceptible to sulfur poisoning than the surface without adsorbed oxygen species [47]. The surface oxygen species oxidize adsorbed SO2 to SO42-. When less Oα species existed on the surface, less ionic SO42- species were formed on the catalyst. This is a plausible interpretation of the formation of less ionic SO42- species on the 4V/TiO2-PILC catalyst than the two others. On the other hand, surface ad sorbed oxygen (Oads) is often thought to be more reactive in oxidation reactions due to its higher mobility than lattice oxygen (Olatt), and it is beneficial for NO oxidation to NO2 in the SCR reaction and facilitates the “fast SCR” reaction, which improve the catalytic performance of the catalyst [46, 48].
In order to further identify the amounts of surface adsorbed oxygen over the catalysts, O2-TPD experiments were carried out. It was known that physically adsorbed oxygen O2 and chemically adsorbed oxygen O22-/O2-/O- species are much easier to desorb than lattice O2- species [49]. As shown in Fig. 10, the O2-TPD profiles of the three catalysts displayed several broad oxygen desorption peaks from 100 to 850 ℃. Based on the results reported in the literature [50, 51], we attributed the peaks in the range of 100 to 500 ℃ to the desorption of chemisorbed oxygen (Oads). The oxygen desorption peak at 750 ℃ over the 4V6Mo/TiO2 catalyst was assigned to the decomposition of MoO3 [52]. From Fig. 10, one can see that the intensity of the oxygen desorption peak over the 4V/TiO2-PILC catalyst was much weaker than that over 4V/TiO2 and 4V6Mo/TiO2, indicating that the amount of oxygen species on its surface was much less than that over the others. The results agreed with the XPS analysis, further illustrating that the formation of ionic SO42- species was correlated with the amount of surface adsorbed oxygen on the catalyst.
V2O5/TiO2-PILC,4V/TiO2 and 4V6Mo/TiO2 catalysts were prepared and used in the SCR reaction of NO by NH3. The 4V/TiO2-PILC catalyst showed higher catalytic activity with a broader operating temperature window and higher N2 selectivity, as well as higher tolerance to SO2 and H2O in the SCR of NO by NH3. The accumulation of ionic SO42- species on the catalyst was one reason for the deactivation of the catalyst, and the formation of ionic SO42- species was correlated with the amount of surface adsorbed oxygen species on the catalyst.
2016,
Vol. 37 Issue (6): 888-897 
PDF (21859 619KB)
2016,
Vol. 37 Issue (6): 888-897 PDF (21859 619KB)