In light of impending stricter emission standards, attention has been focused on the reduction of NO emissions, which are caused by power plants and diesel vehicles. Selective catalytic reduction (SCR) with NH₃ is one of the most widely adopted technologies for the removal of NO from stationary sources and diesel vehicle exhausts. Commercial V₂O₅-WO₃/TiO₂ catalysts display superior efficiency in NO_x reduction at 300-400 ℃, but present some issues, such as the toxicity of vanadium oxides and the formation of N₂O and high concentrations of SO₃ at high temperatures ^[1-3]. Therefore, the development of new catalysts with low or no vanadia loading is critical for solving the problems mentioned above.

Ceria and ceria-containing materials are an important class of catalyst support and have been widely used as catalysts or structural promoters in heterogeneous catalytic reactions because of their favorable reducibility and remarkable oxygen storage capability ^{[4, 5]}. CeO₂-WO₃ catalysts have demonstrated favorable SCR activity above 200 ℃, even under a space velocity of 500000 h⁻¹, and broadened the activity window relative to that of the commercial catalysts. These catalysts also exhibited higher poisoning resistance towards SO₂ and alkali metals ^[6-9]. Wang et al. ^[10] studied a series of CeO₂-ZrO₂ catalysts modified with transition metal oxides and found that their excellent redox properties could improve the performance of the SCR reaction. Liu et al. ^[11] investigated the effect of Ce on the activity and alkali resistance of a V₂O₅-TiO₂ catalyst, and found that the addition of Ce not only reduced the vanadium loading in V₂O₅-TiO₂ but also enhanced the activity and alkali resistance of this catalyst. At the same time, a series of MoO₃-doped CeO₂-ZrO₂ (CeO₂-TiO₂) catalysts were studied for use in SCR with NH₃, and the added MoO₃ was found to significantly enhance the activity of the CeO₂-ZrO₂ catalyst over a wide temperature range ^[12].

SO₂ is an inevitable poisoning component in flue gas, and sulfation of SCR catalysts usually occurs when they are used in industry ^[13-15]. Manganese- and iron-based catalysts commonly suffer from severe deactivation induced by SO₂ ^[16-18]. However, CeO₂ pretreated with SO₂ was found to exhibit excellent SCR activity; this might be attributed to the sulfation effect ^[19]. There are three methods for catalyst sulfation: gas-phase, liquid-phase and precursor-sulfate methods. Results of X-ray photoelectron spectroscopy experiments described in the literature ^[20] and our previous work ^[16] suggest that only one sulfur species (SO₄²⁻) is formed on the catalyst surface. However, the sulfation mechanism for ceria-based catalysts and the influence of sulfation on the surface acidity, reducibility and NO adsorption-desorption behavior of the catalysts have not yet been elucidated. In the present paper, the sulfation of CeO₂-ZrO₂ SCR catalysts was systematically studied, taking into account the adsorbed NH₃ and NO_x species and the redox properties of the catalysts.

The CeO₂-ZrO₂ catalyst was prepared by a homogeneous precipitation method ^[21]. Ammonium cerium nitrate, zirconium nitrate and excess urea solution were mixed in the desired molar ratio (1:1:20). The resultant solution was heated to boiling with stirring until precipitation was observed. The above mixture was then aged overnight, filtered and washed with deionized water. The precipitate was calcined at 500 ℃ for 5 h. The resultant powder was denoted as CeZr. To prepare the sulfated CeO₂-ZrO₂ catalyst, CeZr was immersed in concentrated sulfuric acid (6 mol/L) for 72 h. The catalyst was filtered, washed with deionized water and then calcined at 500 ℃ for 5 h. The resultant powder was denoted S-CeZr.

Activity measurements were carried out in a fixed-bed quartz reactor (inner diameter 5 mm) using 100 mg catalyst (40-60 mesh). The feed gas mixture contained 0.05% NO, 0.05% NH₃ and 3% O₂, and was balanced with N₂. The total flow rate of the feed gas was 200 mL/min, and the space velocity was about 120000 mL/(g_cat·h). The concentrations of NO, NO₂ and NH₃ were continually monitored by Fourier transform infrared (FTIR) spectroscopy (Gasmet FTIR DX-4000). Activity data were collected until the reaction reached a steady state after being maintained for 30 min at each temperature point.

Characterization of the Brunauer-Emmett-Teller (BET) surface areas of the samples was carried out using a Micromeritics ASAP 2020 apparatus. The crystal structure was determined from X-ray diffraction (XRD) measurements (Rigaku, D/MAX 2500 V/PC) from 20° to 80° at a step rate of 10°/min and operated at 50 kV and 200 mA with Cu K_α radiation.

Temperature-programmed reduction of H₂ (H₂-TPR) experiments were performed up to 1000 ℃ on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx) under 10% H₂/Ar gas flow (50 mL/min) with a heating rate of 10 ℃/min. Temperature-programmed desorption of NO (NO-TPD) experiments were performed in a fixed-bed quartz reactor. Before the test, each sample was pretreated in He at 350 ℃ for 1 h and the sample was purged under 1000 ppm NO at room temperature. After isothermal desorption under N₂ at 30 ℃, the temperature was raised to 630 ℃ at a rate of 10 ℃/min.

In situ IR spectroscopy was recorded using an FTIR spectrometer (Nicolet 6700) equipped with a SMART collector and an MCT detector. The catalyst was loaded into a Harrick IR cell and heated to 350 ℃ under N₂ at a total flow rate of 100 mL/min. The background spectrum was collected in an atmosphere of flowing N₂ and was subtracted from the spectrum of each sample. The spectra were recorded by accumulating 32 scans. To minimize the influence of absorbance from different samples, the absorbance intensity was set to 3.00 for every sample at 300 ℃.

Fig. 1 illustrates the SCR performance of CeZr and S-CeZr catalysts in terms of NO_x conversion and N₂O formation in the temperature range 150-400 ℃. Below 200 ℃, CeZr presented higher activity than S-CeZr. However, when the temperature was above 200 ℃, S-CeZr exhibited considerably greater efficiency in NO_x reduction than did CeZr. Almost 100% NO reduction was achieved between 300 and 350 ℃ using S-CeZr and a relatively high space velocity of 120000 mL/(g_cat·h). When the temperature was higher than 350 ℃, both catalysts showed lower activity in the SCR process than they did at lower temperatures. This can be attributed to the oxidation of NH₃ to N₂, N₂O, or NO. Moreover, N₂O was also formed during the reaction. When using the CeZr catalyst, the concentration of N₂O gradually increased from about 4 ppm (this could be a result of error in the equipment) to nearly 10 ppm with increasing temperature. In contrast, for the S-CeZr catalyst, the N₂O concentration remained essentially the same from 150 to 400 ℃. These results indicate that the N₂ selectivity of the CeZr catalyst could be improved by sulfation.

To investigate the oxidation of NH₃ and NO using the two catalysts, the concentrations of NH₃, N₂ (calculated from the difference in the NH₃, N₂O and NO concentrations), N₂O and NO during NH₃ oxidation at 400 ℃ were determined and are provided in Fig. 2(a). The concentration of NO₂ was below 2 ppm. The major product when using the CeZr catalyst was N₂, with minor amounts of N₂O and NO also produced. In contrast, for the S-CeZr catalyst, although a considerable amount of N₂ was obtained, the concentrations of N₂O and NO were less than 10 ppm. Fig. 2(b) presents the NO to NO₂ ratios, indicating the oxidation of NO to NO₂, at 250-350 ℃. The S-CeZr catalyst exhibited a lower ability for NO oxidation than did CeZr, and almost 30% of the NO was oxidized to NO₂ at 400 ℃. Based on these results, it is proposed that the oxidation ability of CeZr was reduced by sulfation.

The BET surface areas for CeZr and S-CeZr were 93.5 and 66.0 m²/g, respectively. This indicates that sulfation decreased the catalyst surface area. However, considering the higher activity of S-CeZr at higher temperatures, it is evident that the SCR performance was not only determined by surface area, but also by the chemical properties of the catalyst, including reducibility, acidity and adsorption/desorption behavior. XRD patterns of the CeZr and S-CeZr samples are shown in Fig. 3. All of the peaks can be attributed to the cubic fluorite phase of CeO₂; however, the crystallinity was not ideal, possibly because of the formation of cerium-zirconium solid. According to these results, sulfation with concentrated sulfuric acid did not damage the CeZr crystal.

Fig. 4 shows the H₂-TPR profiles of the catalysts. The peak centered at 544 ℃ was assigned to the reduction of surface Ce⁴⁺ to Ce³⁺ in the CeZr catalyst ^{[22, 23]}. For the S-CeZr sample, a more intense peak was observed at 586 ℃. Based on previous studies of the TPR profiles of SO₂-treated catalysts, this peak could be attributed to the overlap of peaks corresponding to the reduction of surface cerium and the decomposition of sulfur ^{[16, 24]}. Moreover, the initial H₂ consumption (400-450 ℃) for CeZr was greater than that for S-CeZr, and the onset temperature was lower for CeZr than for S-CeZr. These results indicate that the reducibility of the CeZr catalyst was reduced after sulfation. This may originate from the tight binding of sulfur to the surface cerium atoms.

Fig. 5 shows the NH₃-TPD profiles of CeZr and S-CeZr catalysts in the temperature range 50-600 ℃. Only a single peak at 150 ℃ was detected for the CeZr catalyst, and the concentration of NH₃ gradually decreased with increasing temperature. For the S-CeZr catalyst, a broader band was observed at 130-300 ℃, and thereafter the NH₃ concentration decreased. These results show that the acidity of the sulfated catalyst was stronger than that of CeZr. Furthermore, the amount of NH₃ desorption was higher for S-CeZr than that for CeZr, especially in the higher temperature region, which indicates that a strong enhancement of the surface acidity occurred on sulfation.

Fig. 6 displays the NO-TPD profiles of CeZr and S-CeZr. The two intense peaks at 233 and 451 ℃ were ascribed respectively to the weak and strong adsorption of nitrite or nitrate species on the catalyst surface, especially at basic cerium sites. The two peaks in the NO₂ profile at 215 and 435 ℃ were assigned to the nitrate species. For the S-CeZr samples, the area under all the peaks was reduced and the spectrum was shifted to the lower temperature region. These results correlated with those from the NO oxidation experiments shown in Fig. 2(b); there was minimal and weak NO adsorption on the surface of the S-CeZr catalyst.

To further identify the nature and configuration of the surface nitrite and nitrate species, in situ IR spectra were recorded for both catalysts during the TPD process and are shown in Fig. 7. Five peaks, at 1213, 1455, 1534, 1560 and 1606 cm⁻¹ were observed in the FT-IR spectrum of the CeZr sample at 100 ℃. These peaks were attributed to bridged nitrate (1606 cm⁻¹), bidentate nitrate (1560 cm⁻¹) and monodentate nitrate (1534 cm⁻¹) species. The peaks at 1455 and 1213 cm⁻¹ were ascribed to bridged and monodentate nitrite species, respectively ^[25-27]. On increasing the temperature to 300 ℃, the intensity of all the peaks was reduced and the peak at 1455 cm⁻¹ almost disappeared. At 400 ℃, evidence of some adsorbed nitrate species could still be obtained from the spectrum. For the S-CeZr samples, shown in Fig. 7(b), bidentate nitrate species were not observed and only weakly adsorbed bridged and monodentate nitrate species formed on the catalyst. These results suggest that the suppression of NO adsorption by sulfation is mainly caused by a decrease in the adsorption of bidentate nitrate species. Sulfation also lowers the number of other NO adsorption sites, and thus reduces the amount of adsorbed bridged nitrate and monodentate nitrite species. With an increase in temperature, the monodentate nitrite species desorbed first from the surface below 300 ℃, and only a small proportion of the monodentate nitrate species could be seen in the spectra at higher temperatures.

Fig. 8(a) shows the in situ FT-IR spectra of the CeZr catalyst at 150 ℃. At first, peaks corresponding to Lewis and weak Brnsted acid sites could be observed in the spectra. When the sample was purged with NO and O₂ for 3 min, new peaks attributed to monodentate nitrite species appeared in considerable quantities over 20 min. The number of surface acid sites gradually decreased, and at the same time the peak between 1000 and 1250 cm⁻¹ disappeared or overlapped with the large band from the monodentate nitrite. The peaks at 1443 cm⁻¹ (Brnsted acid sites) and 1602 cm⁻¹ (Lewis acid sites) also became weaker during the first 10 min. This indicates that NH₃, whether adsorbed on Lewis or Brnsted acid sites, is still active at 150 ℃. When NH₃ and NO were applied in reverse order (Fig. 8(b)), newly adsorbed acid sites were observed in the spectrum after 1 min and increased in number with increasing exposure time. However, the pre-adsorbed nitrite and nitrate species were inactive under the NH₃/O₂ gas flow. As discussed above, these species were mainly bound to the cerium atoms, and covered the ceria to some extent. This could have limited the number of surface redox sites supported by the ceria, and led to the suppression of NH₃ activation by the catalyst. This could in turn result in the relatively low SCR activity of the catalysts in the low temperature region.

Fig. 8(c) and (d) show the in situ FT-IR spectra for the reaction with the CeZr catalyst at 300 ℃. Because of the weak adsorption of the NH₃ molecules on the surface at this temperature, only weak Lewis and Brnsted acid sites could be observed after full pre-adsorption of NH₃. As soon as NO and O₂ were introduced into the gas flow, considerable amounts of bidentate and monodentate nitrate and nitrite species were bound to the surface and their amount increased as the exposure time was increased up to 10 min. When the gas order was reversed (Fig. 8(d)), a considerable amount of nitrate and nitrite species were observed in the spectrum on application of NO. After introducing NH₃ and O₂ into the gas flow, the peaks attributed to bridged nitrate and bidentate nitrate species decreased in intensity with increasing exposure time, whereas the intensities of the monodentate nitrate and nitrite species were almost unchanged after 30 min. These results indicate that some of the adsorbed nitrate species could participate in the SCR reaction with gaseous or weakly adsorbed NH₃ in the relatively high temperature region.

Fig. 9 displays the in situ FT-IR spectra for the reaction using the S-CeZr catalyst. Large amounts of Brnsted acid sites and newly formed Lewis acid sites were obtained after NH₃ treatment. However, most of the Brnsted acid sites and the band near 1300 cm⁻¹ (Lewis acid sites) were inactive on addition of NO and O₂ at 150 ℃, except for the peak centered at 1150 cm⁻¹, which originated from the CeZr catalyst. These results indicate that the newly generated surface acid sites, including both Lewis and Brnsted acid sites, bond so strongly with the surface that they cannot react with gaseous or adsorbed NO_x. Fig. 9(b) shows the in situ IR spectra with reversed gas order. At first, some weakly adsorbed NO_x species were observed at 150 ℃. When NH₃ and O₂ were introduced into the gas flow, these adsorbed NO_x species disappeared. Thereafter, peaks corresponding to NH₃ bound to the Lewis and Brnsted acid sites appeared and increased in intensity after 3 min.

The in situ FT-IR spectra of the S-CeZr catalyst at 300 ℃ are shown in Fig. 9(c) and (d). After 5 min of NO and O₂ flow, both Brnsted and Lewis acid sites at the surface were consumed. These results suggest that although the NH₃ adsorbed on the Brnsted acid sites exhibited low activity at low temperatures, it was reactive at 300 ℃. When the order of gas flow was reversed, only weak nitrite species were observed in the IR spectrum after the introduction of NO gas. Once NH₃ and O₂ were introduced, the signals corresponding to these nitrite species disappeared and considerable amounts of NH₃ molecules were detected.

Based on the results and discussion above, the SCR reaction mechanism for CeZr and S-CeZr should be the same (Fig. 10). The different SCR performance of these two catalysts results from their surface acidity and redox properties. The CeZr samples have excellent reducibility, which leads to higher oxidation ability towards NO or NH₃, whereas the S-CeZr samples provide large amounts of Brnsted acid sites that are strongly bound to the surface. This means that the amount of NO bound to the S-CeZr surface is lower and the bonding is weaker compared with that in CeZr. Combined with the results on the SCR performance, we can conclude that at low temperatures the redox properties of the catalysts contribute more to the activity than does the surface acidity. At high temperatures, however, the SCR activity is primarily determined by the surface acidity, especially the strong Brnsted acid sites. At low temperatures, the activity may be limited by higher surface acidities, which would result in tightly bonded NH₃ molecules that cannot be easily activated by the lattice oxygen atoms. In contrast, at high temperatures, a higher reducibility may improve the oxidation of NH₃ with O₂ and result in low N₂ selectivity. Therefore, although the S-CeZr samples exhibited low activity below 200 ℃, their high-temperature activity was enhanced and formation of N₂O was reduced at temperatures above 200 ℃.

CeZr catalysts showed higher SCR performance than S-CeZr samples at low temperatures (<200 ℃) but lower activity at high temperatures (>200 ℃). S-CeZr catalysts contained greater quantities of surface acid (mainly Brnsted acid) sites and had higher surface acidities than CeZr. In addition, the reaction mechanisms of the CeZr and S-CeZr catalysts were studied at 150 and 300 ℃, and we concluded that a temperature- dependent pathway is followed. At low temperatures the redox properties of the catalyst contribute more to the activity than does the surface acidity, and at high temperatures the SCR performance is mainly determined by the surface acidity, especially by the strong Brnsted acid sites.