In Situ Regeneration and Deactivation of Co-Zn/H-Beta Catalysts in Catalytic Reduction of NO x with Propane

: Regeneration and deactivation behaviors of Co-Zn/H-Beta catalysts were investigated in NO x reduction with C 3 H 8 . Co-Zn/H-Beta exhibited a good water resistance in the presence of 10 vol.% H 2 O. However, there was a signiﬁcant drop off in N 2 yield in the presence of SO 2 . The formation of surface sulfate and coke decreased the surface area, blocked the pore structure, and reduced the availability of active sites of Co-Zn/H-Beta during the reaction of NO reduction by C 3 H 8 . The activity of catalyst regenerated by air oxidation followed by H 2 reduction was higher than that of catalyst regenerated by H 2 reduction followed by air oxidation. Among the catalysts regenerated by air oxidation followed by H 2 reduction with different regeneration temperatures, the optimal regeneration temperature was 550 ◦ C. The textural properties of poisoned catalysts could be restored to the levels of fresh catalysts by the optimized regeneration process. The regeneration process of air oxidation followed by H 2 reduction could recover the active sites of cobalt and zinc species from sulfate species, as well as eliminate coke deposition on poisoned catalysts. The regeneration pathway of air oxidation followed by H 2 reduction is summarized as initial removal of coke by air oxidation and ﬁnal reduction of the sulfate species by H 2 .

Sulfur tolerance is a great challenge for deNO x catalysts. Unfortunately, Co-based zeolites exhibited unsatisfactory activity in the presence of SO 2 [11,12]. On the other hand, coke formation, originating from hydrocarbons, also resulted in the deactivation of deNO x catalysts for HC-SCR [13]. H 2 reduction is widely used for the regeneration treatment of catalysts deactivated by SO 2 , such as NO x storage-reduction (NSR) catalysts [14][15][16][17][18], and catalytic reduction of NO by NH 3 (NH 3 -SCR) catalysts [19,20]. In the case of catalysts deactivated by coke deposition, air calcination was considered as an efficient regeneration method [21][22][23][24][25]. Hence, a regeneration process that combines H 2 reduction with air oxidation may be a potential technology for in situ regeneration of deNO x catalysts in HC-SCR. To our knowledge, no reports focused on the regeneration of HC-SCR catalysts deactivated by dual impacts of SO 2 and coke deposition.
In the present paper, Co-Zn/H-Beta was chosen as a deNO x catalyst for C 3 H 8 -SCR, because it showed good catalytic activity [26]. The regeneration of Co-Zn/H-Beta catalysts deactivated by SO 2 and coke deposition was performed in a combined in situ process of air oxidation and H 2 reduction. The effects of the regeneration sequence and regeneration temperature on the regeneration efficiency were investigated. Figure 1 illustrates the influences of SO 2 and H 2 O on the activity of Co-Zn/H-Beta at 450 • C for 80 h. Co-Zn/H-Beta had excellent catalytic activity, with 95% N 2 yield obtained for C 3 H 8 -SCR at 450 • C without addition of SO 2 and H 2 O. The catalytic activity decreased slightly in the presence of 10 vol.% H 2 O. Upon removing H 2 O from the feeding gas, N 2 yield almost returned to its original level. This demonstrates that Co-Zn/H-Beta displays a good water resistance. However, N 2 yield declined significantly during 50-200 ppm SO 2 co-feeding for 15 h. Only 64% N 2 yield was achieved when 200 ppm SO 2 was added into the feeding gas. Upon switching off the SO 2 , N 2 yield increased from 64% to 71.5%, which was far away from the initial activity of 95%. When both 10 vol.% H 2 O and 200 ppm SO 2 were added simultaneously for 7 h, N 2 yield further dropped from 71.5% to 62%. Upon switching off the SO 2 and H 2 O, a partial recovery of catalytic activity was observed. However, N 2 yield decreased gradually after aging for 40 h without adding SO 2 and H 2 O. After the stability experiment, the color of the catalyst turned from light gray to black. This demonstrates that carbon deposition occurs on Co-Zn/H-Beta catalysts during the reaction of NO reduction by C 3 H 8 . as an efficient regeneration method [21][22][23][24][25]. Hence, a regeneration process that combines H2 reduction with air oxidation may be a potential technology for in situ regeneration of deNOx catalysts in HC-SCR. To our knowledge, no reports focused on the regeneration of HC-SCR catalysts deactivated by dual impacts of SO2 and coke deposition. In the present paper, Co-Zn/H-Beta was chosen as a deNOx catalyst for C3H8-SCR, because it showed good catalytic activity [26]. The regeneration of Co-Zn/H-Beta catalysts deactivated by SO2 and coke deposition was performed in a combined in situ process of air oxidation and H2 reduction. The effects of the regeneration sequence and regeneration temperature on the regeneration efficiency were investigated. Figure 1 illustrates the influences of SO2 and H2O on the activity of Co-Zn/H-Beta at 450 °C for 80 h. Co-Zn/H-Beta had excellent catalytic activity, with 95% N2 yield obtained for C3H8-SCR at 450 °C without addition of SO2 and H2O. The catalytic activity decreased slightly in the presence of 10 vol.% H2O. Upon removing H2O from the feeding gas, N2 yield almost returned to its original level. This demonstrates that Co-Zn/H-Beta displays a good water resistance. However, N2 yield declined significantly during 50-200 ppm SO2 co-feeding for 15 h. Only 64% N2 yield was achieved when 200 ppm SO2 was added into the feeding gas. Upon switching off the SO2, N2 yield increased from 64% to 71.5%, which was far away from the initial activity of 95%. When both 10 vol.% H2O and 200 ppm SO2 were added simultaneously for 7 h, N2 yield further dropped from 71.5% to 62%. Upon switching off the SO2 and H2O, a partial recovery of catalytic activity was observed. However, N2 yield decreased gradually after aging for 40 h without adding SO2 and H2O. After the stability experiment, the color of the catalyst turned from light gray to black. This demonstrates that carbon deposition occurs on Co-Zn/H-Beta catalysts during the reaction of NO reduction by C3H8.  Figure 2 presents the influence of regeneration sequence on the activity of the poisoned catalyst at a fixed regeneration temperature of 450 °C. Compared with the poisoned Co-Zn/H-Beta-D catalysts, the regenerated catalysts exhibited higher activity. The activity of regenerated catalysts decreased in the order of: Co-Zn/H-Beta-R (O2 + H2, 450 °C) > Co-Zn/H-Beta-R (H2 + O2, 450 °C) > Co-Zn/H-Beta-R (H2, 450 °C) > Co-Zn/H-Beta-R (O2, 450 °C). Interestingly, Co-Zn/H-Beta-R (O2 + H2, 450 °C) displayed higher activity than Co-Zn/H-Beta-R (H2 + O2, 450 °C). This suggests that combined regeneration is better than single regeneration, and air oxidation followed by H2 reduction is an optimal regeneration sequence for the deactivated Co-Zn/H-Beta catalyst in C3H8-SCR.  . This suggests that combined regeneration is better than single regeneration, and air oxidation followed by H 2 reduction is an optimal regeneration sequence for the deactivated Co-Zn/H-Beta catalyst in C 3 H 8 -SCR.  This indicates that the optimal regeneration temperature was 550 °C. Compared to off-site treatment of solution washing [27] and in situ regeneration by H2 reduction for deactivated deNOx catalysts [28], the in situ regeneration process of air oxidation followed by H2 reduction showed more convenient operation and higher regeneration efficiency, respectively. Thus, although this comparison may be taken with caution because different reaction conditions were employed, the in situ regeneration process of air oxidation followed by H2 reduction is a promising technology for the regeneration of deactivated Co-Zn/H-Beta catalyst.  Table 1 illustrates the textural properties of the samples. Compared with the Co-Zn/H-Beta-F sample, a significant decrease in surface area, microporous area, and pore volume was detected for the Co-Zn/H-Beta-D catalysts. This implies that sulfate species and coke were deposited both on the surface and in the micropores of the deactivated catalysts. For the regenerated catalysts, surface area, microporous area, and pore volume greatly increased after both the combined regeneration process and single regeneration process. Co-Zn/H-Beta-R (O2 + H2, 550 °C) even showed similar values of textural properties to the fresh sample. This implies that air oxidation followed by H2 reduction at 550 °C could eliminate the sulfates and coke deposited over the deactivated Co-Zn/H-Beta.  This indicates that the optimal regeneration temperature was 550 • C. Compared to off-site treatment of solution washing [27] and in situ regeneration by H 2 reduction for deactivated deNO x catalysts [28], the in situ regeneration process of air oxidation followed by H 2 reduction showed more convenient operation and higher regeneration efficiency, respectively. Thus, although this comparison may be taken with caution because different reaction conditions were employed, the in situ regeneration process of air oxidation followed by H 2 reduction is a promising technology for the regeneration of deactivated Co-Zn/H-Beta catalyst.  This indicates that the optimal regeneration temperature was 550 °C. Compared to off-site treatment of solution washing [27] and in situ regeneration by H2 reduction for deactivated deNOx catalysts [28], the in situ regeneration process of air oxidation followed by H2 reduction showed more convenient operation and higher regeneration efficiency, respectively. Thus, although this comparison may be taken with caution because different reaction conditions were employed, the in situ regeneration process of air oxidation followed by H2 reduction is a promising technology for the regeneration of deactivated Co-Zn/H-Beta catalyst.  Table 1 illustrates the textural properties of the samples. Compared with the Co-Zn/H-Beta-F sample, a significant decrease in surface area, microporous area, and pore volume was detected for the Co-Zn/H-Beta-D catalysts. This implies that sulfate species and coke were deposited both on the surface and in the micropores of the deactivated catalysts. For the regenerated catalysts, surface area, microporous area, and pore volume greatly increased after both the combined regeneration process and single regeneration process. Co-Zn/H-Beta-R (O2 + H2, 550 °C) even showed similar values of textural properties to the fresh sample. This implies that air oxidation followed by H2 reduction at 550 °C could eliminate the sulfates and coke deposited over the deactivated Co-Zn/H-Beta.  Table 1 illustrates the textural properties of the samples. Compared with the Co-Zn/H-Beta-F sample, a significant decrease in surface area, microporous area, and pore volume was detected for the Co-Zn/H-Beta-D catalysts. This implies that sulfate species and coke were deposited both on the surface and in the micropores of the deactivated catalysts. For the regenerated catalysts, surface area, microporous area, and pore volume greatly increased after both the combined regeneration process and single regeneration process. Co-Zn/H-Beta-R (O 2 + H 2 , 550 • C) even showed similar values of textural properties to the fresh sample. This implies that air oxidation followed by H 2 reduction at 550 • C could eliminate the sulfates and coke deposited over the deactivated Co-Zn/H-Beta. The X-ray diffraction patterns (XRD) of Co-Zn/H-Beta catalysts are shown in Figure 4. The position of the main diffraction peak around 2θ = 22.4 • is generally taken as evidence of lattice contraction/expansion of the Beta structure [29,30]. The peaks at 21.8 • , 25.1 • , 28.4 • , 29.3 • , and 43.6 • were assigned to Beta-type zeolite. The deactivated and regenerated catalysts preserved the typical Beta crystal structure. The diffraction peaks of Co 3 O 4 (PDF#73-1701), CoO (PDF#71-1178), and Zn(OH) 2 (PDF#74-0094 and PDF#71-2115) were detected for all samples. However, the diffraction peak intensity of Zn(OH) 2 in the deactivated catalyst was weaker than in the fresh and regenerated samples. No new peak related to sulfate species was observed on any sample. This implies that sulfate species might exist as amorphous bulk species, which could not be detected by XRD. The X-ray diffraction patterns (XRD) of Co-Zn/H-Beta catalysts are shown in Figure 4. The position of the main diffraction peak around 2θ = 22.4° is generally taken as evidence of lattice contraction/expansion of the Beta structure [29,30]. The peaks at 21.8°, 25.1°, 28.4°, 29.3°, and 43.6° were assigned to Beta-type zeolite. The deactivated and regenerated catalysts preserved the typical Beta crystal structure. The diffraction peaks of Co3O4 (PDF#73-1701), CoO (PDF#71-1178), and Zn(OH)2 (PDF#74-0094 and PDF#71-2115) were detected for all samples. However, the diffraction peak intensity of Zn(OH)2 in the deactivated catalyst was weaker than in the fresh and regenerated samples. No new peak related to sulfate species was observed on any sample. This implies that sulfate species might exist as amorphous bulk species, which could not be detected by XRD. To identify the state of surface species on various catalysts, the samples were measured by XPS. In Figure 5, the significant movement of the binding energy value of Co 2p3/2 was observed for both deactivated and regenerated samples, compared to the fresh sample. The binding energy of Co 2p3/2 shifted toward a lower value (778.6 eV) in the deactivated catalyst. However, the Co 2p3/2 peaks of regenerated catalysts were shifted to a higher binding energy (781.4-782.7 eV). In Co 2p spectra, shake-up peaks were observed for all samples. This means that metallic cobalt was absent in all samples, because the spectrum of metallic cobalt does not contain shake-up satellite structure at all [31]. Table 1 illustrates the results of Co 2p in the fresh, deactivated, and regenerated catalysts. To identify the state of surface species on various catalysts, the samples were measured by XPS. In Figure 5, the significant movement of the binding energy value of Co 2p 3/2 was observed for both deactivated and regenerated samples, compared to the fresh sample. The binding energy of Co 2p 3/2 shifted toward a lower value (778.6 eV) in the deactivated catalyst. However, the Co 2p 3/2 peaks of regenerated catalysts were shifted to a higher binding energy (781.4-782.7 eV). In Co 2p spectra, shake-up peaks were observed for all samples. This means that metallic cobalt was absent in all samples, because the spectrum of metallic cobalt does not contain shake-up satellite structure at all [31]. Table 1 illustrates the results of Co 2p in the fresh, deactivated, and regenerated catalysts. According to the position of the Co 2p 3/2 peak and interval between Co 2p 3/2 and Co 2p 1/2 , CoO, Co 3 O 4 , Co(OH) 2 , and ZnCo 2 O 4 were present in the fresh sample [32,33]. For the deactivated catalyst, the lowest binding energy of Co 2p 3/2 (778.6 eV) and highest interval between Co 2p 3/2 and Co 2p 1/2 (17.2 eV) were observed, which was quite different from other catalysts in Table 1. This implies that sulfate species were generated on the surface of the deactivated catalyst. For Co-Zn/H-Beta-R  [33]. CoAl 2 O 4 was recognized to be inactive for NO-SCR [34,35]. Thus, the catalytic activity of Co-Zn/H-Beta-R (H 2 + O 2 , 450 • C) was lower than that of Co-Zn/H-Beta-R (O 2 + H 2 ) catalysts ( Figure 2). In the case of Co-Zn/H-Beta-R (O 2 + H 2 ) catalysts, the binding energy value of Co 2p 3/2 was from 781.4 to 782.5 eV, and the interval between Co 2p 3/2 and Co 2p 1/2 was about 15.1 eV. This indicates that CoO, Co 3 O 4 , Co(OH) 2 , and ZnCo 2 O 4 were present in Co-Zn/H-Beta-R (O 2 + H 2 ) catalysts [32,33]. Therefore, the regeneration process of air oxidation followed by H 2 reduction could promote the recovery of the deactivated cobalt species.

Structural and Textural Properties of Catalysts
According to the position of the Co 2p3/2 peak and interval between Co 2p3/2 and Co 2p1/2, CoO, Co3O4, Co(OH)2, and ZnCo2O4 were present in the fresh sample [32,33]. For the deactivated catalyst, the lowest binding energy of Co 2p3/2 (778.6 eV) and highest interval between Co 2p3/2 and Co 2p1/2 (17.2 eV) were observed, which was quite different from other catalysts in Table 1. This implies that sulfate species were generated on the surface of the deactivated catalyst. For Co-Zn/H-Beta-R (H2 + O2, 450 °C), the highest binding energy of Co 2p3/2 (782.7 eV) and lowest interval between Co 2p3/2 and Co 2p1/2 (14.6 eV) were detected among all catalysts. Cobalt species mainly exist as a CoAl2O4 state on Co-Zn/H-Beta-R (H2 + O2, 450 °C) [33]. CoAl2O4 was recognized to be inactive for NO-SCR [34,35]. Thus, the catalytic activity of Co-Zn/H-Beta-R (H2 + O2, 450 °C) was lower than that of Co-Zn/H-Beta-R (O2 + H2) catalysts ( Figure 2). In the case of Co-Zn/H-Beta-R (O2 + H2) catalysts, the binding energy value of Co 2p3/2 was from 781.4 to 782.5 eV, and the interval between Co 2p3/2 and Co 2p1/2 was about 15.1 eV. This indicates that CoO, Co3O4, Co(OH)2, and ZnCo2O4 were present in Co-Zn/H-Beta-R (O2 + H2) catalysts [32,33]. Therefore, the regeneration process of air oxidation followed by H2 reduction could promote the recovery of the deactivated cobalt species.  Figure 6 presents XPS spectra of the Zn 2p regions for the fresh, deactivated, and regenerated catalysts. For all samples, binding energy of Zn 2p3/2 and interval between Zn 2p3/2 and Zn 2p1/2 were 1022-1023 eV and 23 eV, respectively, which could be attributed to Zn(OH) + [36] or ZnO [37]. Compared to the fresh sample, a lower binding energy of Zn 2p3/2 was observed for the deactivated sample, which was similar with the variation trend of Co 2p3/2 lines. The binding energy of Zn 2p3/2 shifting to a lower value may be due to the formation of sulfate species on the surface of the deactivated catalysts. For Co-Zn/H-Beta-R (O2 + H2) catalysts, the position of the binding energy of Zn 2p3/2 and Zn 2p1/2 was the same as the fresh sample. However, a lower binding energy of Zn 2p3/2 and Zn 2p1/2 was also observed for Co-Zn/H-Beta-R (H2 + O2, 450 °C), which was similar with the deactivated sample. Thus, regeneration sequence was important to the regeneration of poisoned catalysts. The peak intensity of both Co 2p3/2 and Zn 2p3/2 was enhanced with the increase of regeneration temperature from 450 to 550 °C, meaning that a regeneration temperature of 550 °C is optimal.  Figure 6 presents XPS spectra of the Zn 2p regions for the fresh, deactivated, and regenerated catalysts. For all samples, binding energy of Zn 2p 3/2 and interval between Zn 2p 3/2 and Zn 2p 1/2 were 1022-1023 eV and 23 eV, respectively, which could be attributed to Zn(OH) + [36] or ZnO [37]. Compared to the fresh sample, a lower binding energy of Zn 2p 3/2 was observed for the deactivated sample, which was similar with the variation trend of Co 2p 3/2 lines. The binding energy of Zn 2p 3/2 shifting to a lower value may be due to the formation of sulfate species on the surface of the deactivated catalysts. For Co-Zn/H-Beta-R (O 2 + H 2 ) catalysts, the position of the binding energy of Zn 2p 3/2 and Zn 2p 1/2 was the same as the fresh sample. However, a lower binding energy of Zn 2p 3/2 and Zn 2p 1/2 was also observed for Co-Zn/H-Beta-R (H 2 + O 2 , 450 • C), which was similar with the deactivated sample. Thus, regeneration sequence was important to the regeneration of poisoned catalysts. The peak intensity of both Co 2p 3/2 and Zn 2p 3/2 was enhanced with the increase of regeneration temperature from 450 to 550 • C, meaning that a regeneration temperature of 550 • C is optimal.  Figure 7 presents the H2-temperature programmed reduction (H2-TPR) of Co-Zn/H-Beta catalysts. The TPR peak centered at around 345 °C is ascribed to the reduction of Co3O4 [38]. The peaks centered at 423 °C and 512 °C could correspond to the reduction of CoOx on the catalyst surface and in the catalyst pore, respectively [38]. The broad peaks centered at 550 °C and 565 °C could be ascribed to the reduction of sulfate and CoAl2O4, respectively. The high temperature reduction peaks of 620 °C and 800 °C may be assigned to the reduction peaks of Zn(OH) + and ZnO, respectively. The reduction peak of sulfate (550 °C) is clearly detected for Co-Zn/H-Beta-D and Co-Zn/H-Beta-R (O2, 450 °C) catalysts. Thus, air oxidation could not remove sulfate on deactivated catalysts. The reduction peak of CoAl2O4 was observed for Co-Zn/H-Beta-R (H2 + O2, 450 °C), but not for Co-Zn/H-Beta-R (O2 + H2, 450 °C), Co-Zn/H-Beta-R (O2 + H2, 550 °C), and Co-Zn/H-Beta-R (H2, 450 °C). This may be due to the diffusion of Co species on the surface into the pore of zeolite, and interaction with extraframework Al 3+ cations at high temperature during the SCR reaction and regeneration process, resulting in the formation of CoAl2O4 [39]. This means that air oxidation can promote the formation of CoAl2O4, while H2 reduction could inhibit CoAl2O4 formation. CoAl2O4 was recognized to be inactive for HC-SCR [34,35]. Therefore, air oxidation followed by H2 reduction is an optimal regeneration sequence for the deactivated Co-Zn/H-Beta catalysts in C3H8-SCR.   The TPR peak centered at around 345 • C is ascribed to the reduction of Co 3 O 4 [38]. The peaks centered at 423 • C and 512 • C could correspond to the reduction of CoO x on the catalyst surface and in the catalyst pore, respectively [38]. The broad peaks centered at 550 • C and 565 • C could be ascribed to the reduction of sulfate and CoAl 2 O 4 , respectively. The high temperature reduction peaks of 620 • C and 800 • C may be assigned to the reduction peaks of Zn(OH) + and ZnO, respectively. The reduction peak of sulfate (550 • C) is clearly detected for Co-Zn/H-Beta-D and Co-Zn/H-Beta-R (O 2 , 450 • C) catalysts. Thus, air oxidation could not remove sulfate on deactivated catalysts. The reduction peak of CoAl 2 O 4 was observed for Co-Zn/H-Beta-R (H 2 + O 2 , 450 • C), but not for Co-Zn/H-Beta-R (O 2 + H 2 , 450 • C), Co-Zn/H-Beta-R (O 2 + H 2 , 550 • C), and Co-Zn/H-Beta-R (H 2 , 450 • C). This may be due to the diffusion of Co species on the surface into the pore of zeolite, and interaction with extra-framework Al 3+ cations at high temperature during the SCR reaction and regeneration process, resulting in the formation of CoAl 2 O 4 [39]. This means that air oxidation can promote the formation of CoAl 2 O 4 , while H 2 reduction could inhibit CoAl 2 O 4 formation. CoAl 2 O 4 was recognized to be inactive for HC-SCR [34,35]. Therefore, air oxidation followed by H 2 reduction is an optimal regeneration sequence for the deactivated Co-Zn/H-Beta catalysts in C 3 H 8 -SCR.  Figure 7 presents the H2-temperature programmed reduction (H2-TPR) of Co-Zn/H-Beta catalysts. The TPR peak centered at around 345 °C is ascribed to the reduction of Co3O4 [38]. The peaks centered at 423 °C and 512 °C could correspond to the reduction of CoOx on the catalyst surface and in the catalyst pore, respectively [38]. The broad peaks centered at 550 °C and 565 °C could be ascribed to the reduction of sulfate and CoAl2O4, respectively. The high temperature reduction peaks of 620 °C and 800 °C may be assigned to the reduction peaks of Zn(OH) + and ZnO, respectively. The reduction peak of sulfate (550 °C) is clearly detected for Co-Zn/H-Beta-D and Co-Zn/H-Beta-R (O2, 450 °C) catalysts. Thus, air oxidation could not remove sulfate on deactivated catalysts. The reduction peak of CoAl2O4 was observed for Co-Zn/H-Beta-R (H2 + O2, 450 °C), but not for Co-Zn/H-Beta-R (O2 + H2, 450 °C), Co-Zn/H-Beta-R (O2 + H2, 550 °C), and Co-Zn/H-Beta-R (H2, 450 °C). This may be due to the diffusion of Co species on the surface into the pore of zeolite, and interaction with extraframework Al 3+ cations at high temperature during the SCR reaction and regeneration process, resulting in the formation of CoAl2O4 [39]. This means that air oxidation can promote the formation of CoAl2O4, while H2 reduction could inhibit CoAl2O4 formation. CoAl2O4 was recognized to be inactive for HC-SCR [34,35]. Therefore, air oxidation followed by H2 reduction is an optimal regeneration sequence for the deactivated Co-Zn/H-Beta catalysts in C3H8-SCR.   However, the deactivated catalyst displayed another stage of weight losses at temperatures above 650 • C, which corresponded to the combustion of coke and sulfate deposited on the catalyst. The regenerated (Co-Zn/H-Beta-R (H 2 + O 2 , 450 • C)) sample showed slighter weight losses than the deactivated sample did at temperatures above 650 • C. Interestingly, no significant weight losses were observed for the regenerated (Co-Zn/H-Beta-R (O 2 + H 2 , 550 • C)) sample. This further indicates that a combined in situ regeneration process of air oxidation followed by H 2 reduction at 550 • C is efficient for regeneration of Co-Zn/H-Beta in C 3 H 8 -SCR. In summary, the regeneration pathway is illustrated in Scheme 1. Coke on the deactivated catalyst was initially removed by air oxidation at 550 • C. Finally, the sulfate species were reduced to the active cobalt and zinc species by H 2 at 550 • C. However, the deactivated catalyst displayed another stage of weight losses at temperatures above 650 °C, which corresponded to the combustion of coke and sulfate deposited on the catalyst. The regenerated (Co-Zn/H-Beta-R (H2 + O2, 450 °C)) sample showed slighter weight losses than the deactivated sample did at temperatures above 650 °C. Interestingly, no significant weight losses were observed for the regenerated (Co-Zn/H-Beta-R (O2 + H2, 550 °C)) sample. This further indicates that a combined in situ regeneration process of air oxidation followed by H2 reduction at 550 °C is efficient for regeneration of Co-Zn/H-Beta in C3H8-SCR. In summary, the regeneration pathway is illustrated in Scheme 1. Coke on the deactivated catalyst was initially removed by air oxidation at 550 °C. Finally, the sulfate species were reduced to the active cobalt and zinc species by H2 at 550 °C.

Catalyst Preparation
Beta zeolite with an atomic ratio Si/Al = 25 was purchased commercially in H-form from Nankai University (Tianjin, China). Co-Zn/H-Beta catalysts were synthesized according to the method described elsewhere [40], using Co(NO3)2 and Zn(NO3)2 as precursors. All the chemicals were purchased from Macklin Inc. (Shanghai, China). The cobalt and zinc content of Co-Zn/H-Beta was 2 wt.%. The fresh samples were noted as Co-Zn/H-Beta-F.

Deactivation and Regeneration of Catalysts
In the deactivation process, the samples were exposed to a mixture of 50-200 ppm SO2, 600 ppm NO, 25 ppm NO2, 600 ppm C3H8, 6 vol.% O2, 10 vol.% H2O, and balance of N2 at 450 °C for 45 h. The  However, the deactivated catalyst displayed another stage of weight losses at temperatures above 650 °C, which corresponded to the combustion of coke and sulfate deposited on the catalyst. The regenerated (Co-Zn/H-Beta-R (H2 + O2, 450 °C)) sample showed slighter weight losses than the deactivated sample did at temperatures above 650 °C. Interestingly, no significant weight losses were observed for the regenerated (Co-Zn/H-Beta-R (O2 + H2, 550 °C)) sample. This further indicates that a combined in situ regeneration process of air oxidation followed by H2 reduction at 550 °C is efficient for regeneration of Co-Zn/H-Beta in C3H8-SCR. In summary, the regeneration pathway is illustrated in Scheme 1. Coke on the deactivated catalyst was initially removed by air oxidation at 550 °C. Finally, the sulfate species were reduced to the active cobalt and zinc species by H2 at 550 °C.

Catalyst Preparation
Beta zeolite with an atomic ratio Si/Al = 25 was purchased commercially in H-form from Nankai University (Tianjin, China). Co-Zn/H-Beta catalysts were synthesized according to the method described elsewhere [40], using Co(NO3)2 and Zn(NO3)2 as precursors. All the chemicals were purchased from Macklin Inc. (Shanghai, China). The cobalt and zinc content of Co-Zn/H-Beta was 2 wt.%. The fresh samples were noted as Co-Zn/H-Beta-F.

Catalyst Preparation
Beta zeolite with an atomic ratio Si/Al = 25 was purchased commercially in H-form from Nankai University (Tianjin, China). Co-Zn/H-Beta catalysts were synthesized according to the method described elsewhere [40], using Co(NO 3 ) 2 and Zn(NO 3 ) 2 as precursors. All the chemicals were purchased from Macklin Inc. (Shanghai, China). The cobalt and zinc content of Co-Zn/H-Beta was 2 wt.%. The fresh samples were noted as Co-Zn/H-Beta-F.

Deactivation and Regeneration of Catalysts
In the deactivation process, the samples were exposed to a mixture of 50-200 ppm SO 2 , 600 ppm NO, 25 ppm NO 2 , 600 ppm C 3 H 8 , 6 vol.% O 2 , 10 vol.% H 2 O, and balance of N 2 at 450 • C for 45 h.