In Situ Regeneration and Deactivation of Co-Zn/H-Beta Catalysts in Catalytic Reduction of NOx with Propane
Round 1
Reviewer 1 Report
The manuscript Catalysts-385005 reports results of the new approach of regeneration of Co-Zn/H-Beta catalyst (deNOx catalyst in HR-SCR) by combined H2 reduction with air. Moreover, the temperature impact on the efficiency of regeneration is discussed. The paper contains interesting experimental results. The subject of the paper is interesting too. Catalyst deactivation is a big problem in the practice of industrial catalytic processes, in chemical technology as well as in car engine. That’s why the problem which is described in the paper is very important from the practical point of view. Loss of catalytic rate with time causes the need of the catalyst regeneration. In my opinion this article is worth being published in Catalysts after a minor revision. Here are some specific remarks that the authors may consider prior to publication of this work:
1. The problem of regeneration of deNOx catalyst has been studied form many years. It is a pity that the authors did not refer to the results of other scientists, thereby proving that their solution is definitely better/comparable with other solutions. In the absence of this comparison, it cannot be determined whether this solution has a chance to be implemented in a longer perspective. This significantly reduces the value of this work and requires it to be supplemented.
2. In Figure 1 Authors presented the stability of Co-Zn/H-Beta in C3H8-SCR under the dual effects of SO2 and H2O. The results were obtained by the changing of the added reagents (H2O or SO2 with various configurations). The presented change in NOx conversion depends not only on the considered reagents. The aging of the catalysts is also the problem in technological practise. Therefore, continuous changes in parameters during catalyst operation cause that their influence on the deactivation level cannot be unambiguously determined.
3. The abbreviations used in the text should be explained. For example, I presume that Co-Zn/H-BEA–F means fresh catalyst, but the readers should be sure and not guess.
4. I don’t agree with the authors that the optimal temperature is 550oC. The temperature 550oC is the best temperature which the authors checked. An optimization problem is the problem of finding the best solution from all feasible solutions. Maybe in temperature 525oC will be the same results.
5. XPS spectra of Co 2p region for fresh, deactivated and regenerated catalysts, presented in Figure 4 showed the difference in binding energy for fresh catalyst and regenerated in 550oC. How to explain almost the same catalyst efficiency for both catalysts?
Author Response
The manuscript Catalysts-385005 reports results of the new approach of regeneration of Co-Zn/H-Beta catalyst (deNOx catalyst in HR-SCR) by combined H2 reduction with air. Moreover, the temperature impact on the efficiency of regeneration is discussed. The paper contains interesting experimental results. The subject of the paper is interesting too. Catalyst deactivation is a big problem in the practice of industrial catalytic processes, in chemical technology as well as in car engine. That’s why the problem which is described in the paper is very important from the practical point of view. Loss of catalytic rate with time causes the need of the catalyst regeneration. In my opinion this article is worth being published in Catalysts after a minor revision. Here are some specific remarks that the authors may consider prior to publication of this work:
Comment 1: The problem of regeneration of deNOx catalyst has been studied form many years. It is a pity that the authors did not refer to the results of other scientists, thereby proving that their solution is definitely better/comparable with other solutions. In the absence of this comparison, it cannot be determined whether this solution has a chance to be implemented in a longer perspective. This significantly reduces the value of this work and requires it to be supplemented.
Response: Thanks for your suggestion. Compared to off-site treatment of solution washing (Applied Catalysis B: Environmental 2001,30, 87-99) and in situ regeneration of H2 reduction for deactivated deNOx catalysts (Applied Surface Science 2017,401,120–126), in situ regeneration process of air oxidation followed by H2 reduction shows the more convenient operation and higher regeneration efficiency, respectively. Thus, although this comparison may be taken with caution because the different reaction conditions was employed, 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 (Line 104-108).
Comment 2: In Figure 1 Authors presented the stability of Co-Zn/H-Beta in C3H8-SCR under the dual effects of SO2 and H2O. The results were obtained by the changing of the added reagents (H2O or SO2 with various configurations). The presented change in NOx conversion depends not only on the considered reagents. The aging of the catalysts is also the problem in technological practice. Therefore, continuous changes in parameters during catalyst operation cause that their influence on the deactivation level cannot be unambiguously determined.
Response: The aging of the catalysts is really the problem in technological practice. Therefore, continuous changes in the presence of SO2 and H2O during catalyst operation cause that their influence on the deactivation level cannot be unambiguously determined in Fig. 1 of the original manuscript. In Fig. 1 of the revised manuscript, we supplemented the longtime stability of catalyst aged for 40 h in the absence of SO2 and H2O. N2 yield decreased gradually after aging for 40 h without adding SO2 and H2O. After the stability experiment, the color of catalyst turned from light gray into black. It demonstrates that not only sulfate formation but also carbon deposition occurs on Co-Zn/H-Beta catalyst during the reaction of NO reduction by C3H8 (Line 81-82).
Comment 3: The abbreviations used in the text should be explained. For example, I presume that Co-Zn/H-BEA–F means fresh catalyst, but the readers should be sure and not guess.
Response: The abbreviations of samples were explained in section 3. Materials and Method (Line216, 220, 222-227).
Comment 4: I don’t agree with the authors that the optimal temperature is 550 oC. The temperature 550 oC is the best temperature which the authors checked. An optimization problem is the problem of finding the best solution from all feasible solutions. Maybe in temperature 525 oC will be the same results.
Response: In fact, we checked the regeneration efficiency of catalyst at 525 oC. However, the regeneration efficiency at 550 oC is still better than that at 525 oC. Therefore, we think that the optimal temperature is 550 oC.
Comment 5: XPS spectra of Co 2p region for fresh, deactivated and regenerated catalysts, presented in Figure 4 showed the difference in binding energy for fresh catalyst and regenerated in 550 oC. How to explain almost the same catalyst efficiency for both catalysts?
Response: Activity of catalysts is determined by Co and Zn species. In XPS spectra, the assignment of Co and Zn species are determined by both binding energy and interval between Co(or Zn) 2p3/2 and Co(or Zn) 2p1/2. Same species (CoO, Co3O4, Co(OH)2, ZnCo2O4 and ZnO ) were present in the fresh sample and Co-Zn/H-Beta-R(O2+H2) catalysts. The peak intensity of both Co 2p3/2 and Zn 2p3/2 enhanced with the increase of regeneration temperature from 450 to 550 °C. Thus, almost the same catalyst efficiency was obtained for both catalysts
Reviewer 2 Report
Comments to the paper “In situ regeneration and deactivation of Co-Zn/H-Beta catalyst in catalytic reduction of NOx with propane” by Hua Pan et al.
In the presented paper deactivation of Co-Zn/H-beta catalyst due to coke formation and SO2 poisoning was studied and a feasible regeneration method was elaborated for catalyst reactivation. First the regeneration sequence (oxidation/reduction) was optimized (Fig. 2), then the temperature of the regeneration (Fig. 3). Authors optimized the regeneration sequence only at 450 C, whereas the optimal regeneration temperature was determined by using oxidation/reduction sequence only. Unfortunately, from these data it cannot be excluded that, e.g., a single heat treatment in reducing atmosphere or reduction followed by oxidation at high temperature (550 C) is also a suitable procedure.
The changes in Fig. 4A and 4B are hard to see and follow. These figures have to be rescaled in order to make peak intensities more visible.
XPS spectra shown in Fig. 5 are ill-defined and therefore it is hard to follow the discussion in the text. Adding fitted component curves would be very useful.
Catalyst preparation method (section 3.1.) strongly suggests that a fraction of Co and/or Zn will occupy ion exchange positions in the zeolite, whereas other fraction will form oxides. Scheme 1 suggests that both metals are in the form of oxide in the zeolite, which is probably not a correct picture.
Author Response
Comment 1: In the presented paper deactivation of Co-Zn/H-beta catalyst due to coke formation and SO2 poisoning was studied and a feasible regeneration method was elaborated for catalyst reactivation. First the regeneration sequence (oxidation/reduction) was optimized (Fig. 2), then the temperature of the regeneration (Fig. 3). Authors optimized the regeneration sequence only at 450 0C, whereas the optimal regeneration temperature was determined by using oxidation/reduction sequence only. Unfortunately, from these data it cannot be excluded that, e.g., a single heat treatment in reducing atmosphere or reduction followed by oxidation at high temperature (550 C) is also a suitable procedure.
Response: We supplemented the relative results. Fig. 2 presents the influence of regeneration sequence on activity of the poisoned catalyst at a fixed regeneration temperature of 450 °C. Compared with the poisoned Co-Zn/H-Beta-D catalyst, the regenerated catalysts exhibited the higher activity. The activity of regenerated catalysts decreased in 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 the higher activity than Co-Zn/H-Beta-R (H2+O2, 450°C). It 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.
Fig. 3 exhibits the effect of regeneration temperatures on activity of the catalysts regenerated by combined regeneration process. The catalytic activity of both Co-Zn/H-Beta-R (O2+H2) and Co-Zn/H-Beta-R (H2+O2) catalysts increased with the regeneration temperature from 450 to 550 °C. The regenerated Co-Zn/H-Beta-R (O2+H2, 550 °C) displays the similar activity to the fresh catalyst (Line 91-104).
Comment 2: The changes in Fig. 4A and 4B are hard to see and follow. These figures have to be rescaled in order to make peak intensities more visible.
Response: In order to make peak intensities more visible, Fig. 4 was rescaled in the revised manuscript.
Comment 3: XPS spectra shown in Fig. 5 are ill-defined and therefore it is hard to follow the discussion in the text. Adding fitted component curves would be very useful.
Response: Thanks for your suggestion. We initially try out best to add fitted curves. However, it is hard to add fitted curves by software of XPS 41, because of low intensity of Co 2p spectra. We had to determine Co and Zn species according to both binding energy and interval between Co 2p3/2 and Co 2p1/2. Some references (J. Catal. 1980, 64, 320-331, J. Catal. 1987, 106, 362-368, J. Phys. Chem. 1992, 96, 9393-9400) could support our results.
Comment 4: Catalyst preparation method (section 3.1.) strongly suggests that a fraction of Co and/or Zn will occupy ion exchange positions in the zeolite, whereas other fraction will form oxides. Scheme 1 suggests that both metals are in the form of oxide in the zeolite, which is probably not a correct picture.
Response: Response: Thanks for your suggestion. Co and/or Zn will occupy ion exchange positions in the zeolite during impregnation, however, they will finally tuned to oxides after calcined in air at 500 0C. We put some oxides into the ion exchange positions in Scheme 1 according to your advice.
Reviewer 3 Report
This paper deals with the regeneration of Co-Zn/H-BEA zeolite catalysts for selective catalytic reduction of NOx with C3H8. However, I do not understand what occurred during the regeneration process, because of insufficient characterization of the samples. For example, for elucidating what was caused by each of air oxidation and H2 reduction for Co-Zn/H-Bea-R(O2+H2), the samples before the H2 reduction should be also characterized. Moreover, the peak assignments in the XRD patterns (Figure 4B) are incorrect. The peak positions are inconsistent with the ICDD card data, so that I do not know what the crystalline phases included in the samples are. Additionally, the S/N ratio of the XPS spectra of Co2p is so low that I do not think that the state of Co species in the samples is adequately characterized. Furthermore, the cokes and adsorbed SO2 molecules formed by the deactivation process should be observed directly, and the change of their amounts by the regeneration process should be shown. Since such results are not shown, I do not understand why the order of air oxidation and H2 reduction influenced the extent of regeneration. So, I do not feel that this paper is acceptable for publication.
3. Materials and Methods
3.3 Catalytic activity measurement
The equation for estimating N2 selectivity is shown; however, the results of the N2 selectivity are missing in this paper.
It is shown that for selective catalytic reduction of NOx, CH4 was used as a reductant, but not C3H8. What is the used reductant?
Author Response
Comment 1: This paper deals with the regeneration of Co-Zn/H-BEA zeolite catalysts for selective catalytic reduction of NOx with C3H8. However, I do not understand what occurred during the regeneration process, because of insufficient characterization of the samples. For example, for elucidating what was caused by each of air oxidation and H2 reduction for Co-Zn/H-Bea-R(O2+H2), the samples before the H2 reduction should be also characterized. Moreover, the peak assignments in the XRD patterns (Figure 4B) are incorrect. The peak positions are inconsistent with the ICDD card data, so that I do not know what the crystalline phases included in the samples are. Additionally, the S/N ratio of the XPS spectra of Co2p is so low that I do not think that the state of Co species in the samples is adequately characterized. Furthermore, the cokes and adsorbed SO2 molecules formed by the deactivation process should be observed directly, and the change of their amounts by the regeneration process should be shown. Since such results are not shown, I do not understand why the order of air oxidation and H2 reduction influenced the extent of regeneration. So, I do not feel that this paper is acceptable for publication.
Response: Thanks for your careful reviewing. The peak assignments in the XRD patterns (Fig. 4) are really incorrect in the original manuscript. The peak positions were reanalyzed in the revised manuscript according to the PDF card data. The diffraction peaks of Co3O4 (PDF#73-1701), CoO (PDF#71-1178), Zn(OH)2 (PDF#74-0094 and PDF#71-2115), Al2O3 (PDF#71-1684 and PDF#74-0323) and Al(OH)3 (PDF#72-0623 and PDF#70-2038) were detected for all samples (line 131-136).
In Fig. 1 of the revised manuscript, we supplemented the longtime stability of catalyst aged for 40 h in the absence of SO2 and H2O. N2 yield decreased gradually after aging for 40 h without adding SO2 and H2O. After the stability experiment, the color of catalyst turned from light gray into black. It demonstrates that not only sulfate formation but also carbon deposition occurs on Co-Zn/H-Beta catalyst during the reaction of NO reduction by C3H8 (Line 81-82).
Fig. 2 presents the influence of regeneration sequence on activity of the poisoned catalyst at a fixed regeneration temperature of 450 °C. Compared with the poisoned Co-Zn/H-Beta-D catalyst, the regenerated catalysts exhibited the higher activity. The activity of regenerated catalysts decreased in 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 the higher activity than Co-Zn/H-Beta-R (H2+O2, 450°C). It 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. Fig. 3 exhibits the effect of regeneration temperatures on activity of the catalysts regenerated by combined regeneration process. The catalytic activity of both Co-Zn/H-Beta-R (O2+H2) and Co-Zn/H-Beta-R (H2+O2) catalysts increased with the regeneration temperature from 450 to 550 °C. The regenerated Co-Zn/H-Beta-R (O2+H2, 550 °C) displays the similar activity to the fresh catalyst (Line 91-104).
Each of air oxidation and H2 reduction for Co-Zn/H-Bea-D was characterized by N2 adsorption isotherms, as shown in Table 1. Additionally, the intensity of the XPS spectra of Co2p is low because low loading of cobalt. We tried out best to add fitted curves. However, it is hard to add fitted curves by software of XPS 41, because of low intensity of Co 2p spectra. We had to determine Co and Zn species according to both binding energy and interval between Co 2p3/2 and Co 2p1/2. Some references (J. Catal. 1980, 64, 320-331, J. Catal. 1987, 106, 362-368, J. Phys. Chem. 1992, 96, 9393-9400) could support our results. Finally, the cokes and adsorbed SO2 molecules formed by the deactivation process was investigated preliminarily by TGA. However, quantitative analysis is difficult for us because of the limitation of experimental condition.
Comment 2: The equation for estimating N2 selectivity is shown; however, the results of the N2 selectivity are missing in this paper.
Response: We have changed NOx conversion to N2 yield in Fig. 1-3, because the results of our work are obtained according the equation (1). Equation (1) is for estimating N2 yield, not N2 selectivity. N2 selectivity is calculated as SN2=(NOin+NO2in- NOout-NO2out-2N2Oout)/( NOin+NO2in-NOout-NO2out). In this work, low concentration of N2O (below 10 ppm) was detected on all samples. Thus, high selectivity of N2 (above 95%) is obtained, and it has been explained in the revised manuscript (Line238-239).
Comment 3: It is shown that for selective catalytic reduction of NOx, CH4 was used as a reductant, but not C3H8. What is the used reductant?
Response: Thanks for your careful reviewing. C3H8 was used in our work. It was revised in the revised manuscript (Line 232).
Round 2
Reviewer 2 Report
Authors mostly made the necessary corrections in their manuscript. Concerning Authors’ response to comment #4, they erroneously state that Co or Zn in ion-exchange position in zeolites could be oxidized to metal oxide by calcination at high temperature. Charge compensating cations in zeolites will not turn to oxide due to the strong stabilization provided by the negatively charged zeolite framework. However, metal cations in zeolites can be reduced in hydrogen to metallic state accompanied by the formation of protonic sites as charge compensating cations. The required reduction temperature strongly depends on the type of cation. For instance, Co2+ ions are especially hard to reduce (>700 degree C).
Author Response
Comment 1: Authors mostly made the necessary corrections in their manuscript. Concerning Authors’ response to comment #4, they erroneously state that Co or Zn in ion-exchange position in zeolites could be oxidized to metal oxide by calcination at high temperature. Charge compensating cations in zeolites will not turn to oxide due to the strong stabilization provided by the negatively charged zeolite framework. However, metal cations in zeolites can be reduced in hydrogen to metallic state accompanied by the formation of protonic sites as charge compensating cations. The required reduction temperature strongly depends on the type of cation. For instance, Co2+ ions are especially hard to reduce (>700 degree C).
Response: Thanks for your suggestion. We have revised Scheme 1 in the revised manuscript. We added Co2+ and Zn2+ into the ion exchange positions in Scheme 1 according to your advice. We agree that Co2+ ions are especially hard to reduce to metallic state by H2 at 550 oC in our work. Based on the results of Co 2p spectra, metallic cobalt is absent in all samples, because the spectrum of metallic cobalt does not contain shake-up satellite structure at all (J. Catal. 1980, 64, 320-331).
Reviewer 3 Report
As I previously mentioned, only the explanation of the experimental results is provided, and the results are discussed insufficiently. Thus, I do not understand why the order of air oxidation and H2 reduction influenced the regeneration of the catalysts. In my opinion, this paper does not reach the level suitable for publication.
1. The peak assignment of the XRD patterns (Figure 4) was corrected. They show the presence of Al2O3 and Al(OH)3. However, I do not understand why Al2O3 and Al(OH)3 were formed.
2. On the basis of XPS spectra of Co2p, it is explained that the order of air oxidation and H2 reduction influenced the formation of CoAl2O4. However, I do not understand why their order influenced CoAl2O4 formation. In addition, the XPS Co2p peaks are so weak that the deconvolution of their peaks was impossible. I do not understand why the states of Co species in the samples were determined from the XPS Co2p spectra with low S/N ratios.
3. For Co-Zn/H-Beta-D and Co-Zn/H-Bea-R(H2+O2, 450 °C) which exhibited low catalytic activities, Zn2p3/2 and 2p1/2 peaks in Figure 6 provided lower binding energy. Despite the fact that their peak positions had some relation to catalytic activities, the state of Zn species with a low binding energy of Zn2p was not described.
Author Response
Comment 1: The peak assignment of the XRD patterns (Figure 4) was corrected. They show the presence of Al2O3 and Al(OH)3. However, I do not understand why Al2O3 and Al(OH)3 were formed.
Response: The diffraction peaks around 2θ=21.8°, 25.1°, 28.4°, 29.3° and 43.6° were assigned to Al2O3 and Al(OH)3 according to PDF#71-1684, PDF#74-0323, PDF#72-0623 and PDF#70-2038. We also doubt the presence of Al2O3 and Al(OH)3. If they exist, it may be due to the fact that part of Al did not form beta structure, but turned into Al2O3 and Al(OH)3 during preparation of the commercial Beta zeolite. We searched other literature about XRD of Beta (Ecotoxicology and Environmental Safety 148 (2018) 585–592; Applied Surface Science 456 (2018) 973–979; J. Phys. Chem. B 2006, 110, 12490-12493), these diffraction peaks were also observed on Beta zeolites, but they were not discussed and assigned in literature. In order to avoid the misunderstanding of readers, we deleted the description of these peaks in the revised manuscript.
Comment 2: On the basis of XPS spectra of Co2p, it is explained that the order of air oxidation and H2 reduction influenced the formation of CoAl2O4. However, I do not understand why their order influenced CoAl2O4 formation. In addition, the XPS Co2p peaks are so weak that the deconvolution of their peaks was impossible. I do not understand why the states of Co species in the samples were determined from the XPS Co2p spectra with low S/N ratios.
Response: We supplemented H2-TPR of Co-Zn/H-Beta catalysts to answer this question, as shown in Fig. 7 of revised manuscript. The TPR peak centered at around 345 °C is ascribed to the reduction of Co3O4. 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. The broad peak centered at 550 °C and 565 °C could be ascribed to the reduction of sulfate and CoAl2O4, respectively. The high temperature reduction peak of 620 °C and 800 °C may be assigned to the reduction peak of Zn(OH)+ and ZnO, respectively. The reduction peak of sulfate (550 °C) is clearly detected on 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 on Co-Zn/H-Beta-R (H2+O2, 450 °C), but not disappeared on 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). It 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. Therefore, air oxidation followed by H2 reduction is an optimal regeneration order for the deactivated Co-Zn/H-Beta catalyst in C3H8-SCR.
Additionally, the low intensity of the XPS spectra of Co2p may be due to the low cobalt loading on catalysts.
Comment 3: For Co-Zn/H-Beta-D and Co-Zn/H-Bea-R(H2+O2, 450 °C) which exhibited low catalytic activities, Zn2p3/2 and 2p1/2 peaks in Figure 6 provided lower binding energy. Despite the fact that their peak positions had some relation to catalytic activities, the state of Zn species with a low binding energy of Zn2p was not described.
Response: For all samples, Zn 2p3/2 BE 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)+ or ZnO. Compare to the fresh sample, the 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 shifts to lower value may be due to the formation of sulfate species on the surface of deactivated catalyst.
Round 3
Reviewer 3 Report
The results of TPD measurements were added to the revised version. Based on these results, it is explained that the air oxidation promoted the formation of CoAl2O4, while the H2 reduction inhibited CoAl2O4 formation. However, I do not understand what Al species in CoAl2O4 derived from. Moreover, it is explained on page 5 that cobalt species mainly exist as CoAl2O4 state on Co-Zn/H-Bea-R(H2+O2, 450°C). However, the XRD pattern exhibited peaks assigned to CoO and Co3O4, but not to CoAl2O4. Thus, I think that minor revisions are required for publication.
In the revised version, the peak assignment of the XRD patterns were corrected. I agree with this correction. The peaks at 21.8°, 25.1°, 28.4°, 29.3°, and 43.6° should be assigned to *BEA-type zeolite. I do not think that these peaks show the formation of Al2O3 or Al(OH)3.
Author Response
Comment 1: The results of TPD measurements were added to the revised version. Based on these results, it is explained that the air oxidation promoted the formation of CoAl2O4, while the H2 reduction inhibited CoAl2O4 formation. However, I do not understand what Al species in CoAl2O4 derived from. Moreover, it is explained on page 5 that cobalt species mainly exist as CoAl2O4 state on Co-Zn/H-Bea-R(H2+O2, 450°C). However, the XRD pattern exhibited peaks assigned to CoO and Co3O4, but not to CoAl2O4. Thus, I think that minor revisions are required for publication.
Response: This may be due to the diffusion of Co species on the surface into the pore of zeolite and interaction with extra-framework Al3+ cations at high temperature during SCR reaction and regeneration process, resulting in the formation of CoAl2O4 (Chem. J. Chin. Univ. 2002, 23, 129-131).
XRD pattern exhibited peaks assigned to CoO and Co3O4, but not to CoAl2O4. It indicates that CoAl2O4 particles are well-dispersed or too small to be detected by XRD. Additionally, Boix et al. (Catalysis Today 133–135 (2008) 428–434) reported that it is very hard to distinguish among these compounds based upon XRD, due to the similar diffractograms of the spinel structure of Co3O4 and CoAl2O4.
Comment 2: In the revised version, the peak assignment of the XRD patterns were corrected. I agree with this correction. The peaks at 21.8°, 25.1°, 28.4°, 29.3°, and 43.6° should be assigned to *BEA-type zeolite. I do not think that these peaks show the formation of Al2O3 or Al(OH)3.
Response: Thanks for your suggestion. It has been revised in the revised manuscript (Line108-109).
