The Catalytic Performance of CO Oxidation over MnOx-ZrO₂ Catalysts: The Role of Synthetic Routes

Round 1

Reviewer 1 Report

This manuscript mainly explores the effect of synthesis method for MnOx-ZrO2 catalysts on the catalytic performance of CO oxidation. It shows that the catalyst prepared by co-precipitation has higher catalytic activity than that prepared by vacuum impregnation due to different Mn-Zr-O species formation with increasing reaction temperature. Very detailed characterizations were performed on these two types of samples. This work is suitable for Catalysts journal. It can be accepted after considering the following suggestions:

1. As for the sample prepared by vacuum impregnation, where does ZrO2 come from, commercial one or synthesis from ZrO(NO3)2 hydrolysis?

2.Why only IM-400 was analyzed by TEM? Is it special? How about other samples, e.g., above 500 ^oC, an additional Mn2O3 phase was clearly recognized by XRD.

3. Is there any pre-treatment (except calcination) on your catalysts before catalytic reaction?

4. As for XPS data in Figure 5, Does the identified Mn2+ have any reduction peaks in TPR analyzation?

5. On page 8, line 251, it is interesting to see the segregation of Mn cations on ZrO2 surface. Since the sample is prepared by impregnation, using ZrO2 as support. Could the author give a detailed discussion?

6. Even though the author did plenty of characterization on the samples prepared by COP and IM, the evolution of metal precursor/species during sample preparation by COP and IM is different and missing. More discussion on the role of synthetic method for obtaining samples with different structure could be added.

Author Response

Response to Reviews

We are thankful to the referees for useful comments. We made revision of the text according to the recommendations. Since we have changes the structure of manuscript, revisions to the manuscript were marked by color instead of the “Track Changes”

Review 1

This manuscript mainly explores the effect of synthesis method for MnOx-ZrO2 catalysts on the catalytic performance of CO oxidation. It shows that the catalyst prepared by co-precipitation has higher catalytic activity than that prepared by vacuum impregnation due to different Mn-Zr-O species formation with increasing reaction temperature. Very detailed characterizations were performed on these two types of samples. This work is suitable for Catalysts journal. It can be accepted after considering the following suggestions:

1. As for the sample prepared by vacuum impregnation, where does ZrO2 come from, commercial one or synthesis from ZrO(NO3)2 hydrolysis?

ZrO₂ support was prepare by hydrolysis of ZrO(NO₃)₂ solution by addition of a NH₄OH and further calcination at 600°C for 4 hours.

This information has been added into the experimental part.

2. Why only IM-400 was analyzed by TEM? Is it special? How about other samples, e.g., above 500 ^oC, an additional Mn2O3 phase was clearly recognized by XRD.

According to reviewer recommendation, we have added the TEM data for IMP-650 catalyst. TEM results shows the formation of Mn2O3 phase.

2.3 TEM study (morphology, composition and distribution)

For IM-400, TEM images demonstrate large aggregates with sizes of 100-200 nm (Figure 3a,b) and smaller particles of a rounded shape with sizes of 10-20 nm (Figure 3c). The latter particles are associated with the monoclinic modification of ZrO₂, since the Fourier image contains distances of 0.498, 0.364, 0.262 and 0.232 nm corresponding to the (100), (110), (002) and (02-1) planes of zirconia (PDF No. 37-1484), respectively. In addition, disordered particles can be distinguished with an interplanar distance of 0.313 nm corresponding to the (110) MnO₂ plane. The EDX analysis shows that in this region the Mn:Zr ratio is 98:2 at. %. For IM-650, TEM images demonstrate smaller particles of the monoclinic modification of ZrO₂ with sizes of 10-20 nm (Figure 3d, f). Also, there are particles with an interplanar distance of 0.39 and 0.27 nm corresponding to the (211) and (222) Mn₂O₃ planes Figure 3e).

3. Is there any pre-treatment (except calcination) on your catalysts before catalytic reaction?

Before the catalytic test, the catalyst was not pre-treated.

4. As for XPS data in Figure 5, Does the identified Mn2+ have any reduction peaks in TPR analyzation?

Reduction of Mn2+ under TPR conditions does not go to metallic manganese. From the literature data, reduction of Mn2O3, Mn3O4, MnO2 goes to MnO under TPR experiment (H2, 900C). [Stobbe, E.R.; De Boer, B.A.; Geus, J.W. The reduction and oxidation behaviour of manganese oxides. Catalysis Today 1999, 47, 161-167.]  

5. On page 8, line 251, it is interesting to see the segregation of Mn cations on ZrO2 surface. Since the sample is prepared by impregnation, using ZrO2 as support. Could the author give a detailed discussion?

Indeed, this is an interesting and unexpected effect. Two explanations can be considered for the observed increase in the surface Mn/Zr ratio from 0.49 to 2.41-1.25. Firstly, this phenomenon may be due to partial sintering of zirconia particles. As a result, the surface area of zirconia decreases, therefore, the relative XPS signal of Mn grows and a relative XPS signal of Zr decreases. The second explanation is a decrease in the size or shape of particles/agglomerates of manganese oxides due to the MnO2->Mn2O3 transition.

The sentence «Probably, further calcination leads to the “spreads” of manganese over the support surface due to MnO₂→Mn₂O₃ transformation. Another explanation is partial sintering of zirconia particles. As a result, the surface area of zirconia decreases, therefore, the relative XPS signal of Mn grows» has been added into 3.6 part.

6. Even though the author did plenty of characterization on the samples prepared by COP and IM, the evolution of metal precursor/species during sample preparation by COP and IM is different and missing. More discussion on the role of synthetic method for obtaining samples with different structure could be added.

According to reviewer recommendation, we have added the discussion (2.7 Factors determining the catalytic activity part)

2.7 Factors determining the catalytic activity

Two series of MnOx-ZrO₂ catalysts with Mn:Zr=1:4 were prepared by co-precipitation and vacuum impregnation with subsequent calcination at temperatures from 400 to 800°C and then tested in CO oxidation. The activity of the COP catalysts exceed the IM catalyst in the range of calcination temperature from 400 to 700°C (Figure 1). Catalysts prepared by co-precipitation and impregnation have the same chemical composition, but their physicochemical properties differ. In the case of IM series, catalyst contains support ZrO₂ (mon), ZrO₂ (tetr) and Mn oxides as active component. The most active catalyst was obtained at 400°C, its activity in CO oxidation is due to the formation of MnO₂ oxide containing Mn⁴⁺, and a low-temperature reducibility. According to XPS results (Table 2), with an increase in the calcination temperature from 400 to 700°C, segregation of the surface Mn cations occurs. However, it does not lead to an increase in catalytic activity. An increase in the calcination temperature leads to the formation of less active Mn₂O₃, a decrease in surface area, while the TPR profiles shift to the high-temperature region, which indicates a decrease in the oxygen mobility of the catalyst.

For the COP series of catalysts, the dependence of catalytic activity on calcination temperature of the catalyst has a volcano-like shape with a maximum at 650-700°C. For COP-400 and COP-500, an X-ray amorphous state is formed. An increase in the synthesis temperature leads to the formation of a Mn_yZr_1-yO_2-x solid solution with the fluorite structure. An increase in the calcination temperature is accompanied by an increase in the lattice parameter of the Mn_yZr_1-yO_2-x oxide (Table 1). The lattice parameter approaches the value for "pure" zirconium oxide, which is associated with the release of manganese cations from the solid solution structure. Simultaneously, XPS detected the enrichment of the surface with manganese cations (Table 2). According to TPR, an increase in the low-temperature peaks of H₂ consumption occurs, which is attributed to the highly disperse MnOx oxide. The decomposition of the Mn_yZr_1-yO_2-x solid solution into simple manganese and zirconium oxide begins. For COP-800, the amount of the disperse MnOx species decreases (peak 1, Figure 6b); the disperse MnOx species were transformed into Mn₃O₄ species in accordance with the XRD findings.

Noteworthy are the catalysts prepared at 600-700°C. Indeed, XRD, XPS, H₂-TPR, and BET demonstrate the similarity between catalysts prepared at 600-700°C: all of them contain Mn_yZr_1-yO_2-x oxide with a close crystallite size, the charge states of Mn cations on the surface and their specific surface areas are nearly equal, and the profiles of H₂ consumption look quite close. However, the catalytic properties change drastically: T₅₀ increases from 185 to 248°C, and R decreases from 0.099 to 0.044 cm³/(g∙s) for COP-650 and COP-600, respectively. Probably, a tendency to decomposition of the solid solution Mn_yZr_1-yO₂ contributes to the formation of the active state of the catalyst. Several signs indicate the decomposition of the solid solution: (1) a change in parameters of the mixed oxide towards "pure" zirconium oxide, (2) a tendency to form a phase of manganese oxide at 800°C, and (3) an increase in the content of manganese on the oxide surface. An optimum is needed here between an “ideal” solid solution and a completely decomposed catalyst. It can be assumed that during the solid solution decomposition a unique microstructure is formed, in which, on the one hand, oxygen of the Mn_yZr_1-yO_2-x solid solution structure is active in oxidation reactions and, on the other hand, manganese cations segregate to the surface in the form of MnOx grain boundaries or the content of Mn in the near-surface layer of the solid solution, due to which the increase in catalytic activity is ensured.

Interestingly, that at relatively low Mn content (8wt%) catalysts prepared by wet impregnation exhibit better catalytic performance than catalyst synthesized by co-precipitation [12]. In this case, MnOx species acts as an active component. At relatively high Mn content, co-precipitated catalysts predominantly show improved catalytic performance [10,14]. According to most studies with the widely varying manganese content, catalysts with the composition Mn_0.4Zr_0.6O₂ – Mn_0.6Zr_0.4O₂ exhibited the maximum catalytic activity in oxidation reactions [13,14,52,53]. Catalysts with a high Mn loading contain, along with a solid solution, crystalline manganese oxides such as Mn₂O₃ and Mn₃O₄. With an increase in the total amount of Mn in the catalyst, the number of manganese ions introduced into zirconia grows, which means that the number of oxygen vacancies that are active in oxidation reactions also increases [11]. Simultaneously, the introduction of Mn into zirconia leads to a decrease in the size of crystallites and an increase in the specific surface area [33,48]. Due to the multi-phase nature of Mn_0.4Zr_0.6O₂ – Mn_0.6Zr_0.4O₂ catalysts, the possible effects associated with the formation of manganese nanoparticles during decomposition of the solid solution are not so noticeable and it is difficult to detect them. In this work, the influence of the solid solution decomposition processes was studied in the single-phase mixed oxides containing a smaller amount of manganese Mn_0.2Zr_0.8O₂ and their role in increasing catalytic activity was shown.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

My suggestions to revise the article are as follows.

 

1.     It is recommended that the authors put the characterization results of the catalysts prepared by the two methods together, and not in separate paragraphs. The discussion is needed to associate with the activity of the catalyst

 

2.     Why the two catalysts showed different CO oxidation catalytic activities？ The author should give an in-depth discussion from the bulk and surface structure, as well as the physicochemical properties, of the catalyst.

 

3.     Mn K-edge XANES spectra of the MnOx-ZrO2 catalysts prepared by co-precipitation were given. However, I did not find the same characterization results for another catalyst. 

 

4.     The impregnation method is only used for the preparation of loading catalysts, and the active components should generally be less than 10 wt% to ensure the uniform distribution of the active components on the carrier surface.

 

5.     In MnOx-ZrO2 catalysts prepared by impregnation, the ratio of Mn / Zr is 1 / 4. So how many proportions of MnOx is distributed on the ZrO2 surface and how many exists as an agglomeration?

Author Response

Response to Reviews

We are thankful to the referees for useful comments. We made revision of the text according to the recommendations. Since we have changes the structure of manuscript, revisions to the manuscript were marked by color instead of the “Track Changes”

My suggestions to revise the article are as follows.

1. It is recommended that the authors put the characterization results of the catalysts prepared by the two methods together, and not in separate paragraphs. The discussion is needed to associate with the activity of the catalyst

According to reviewer recommendation, we have changed the structure of the article: we put the characterization results of the catalysts prepared by the two methods together. Indeed, this approach improved the clarity of the article.

2. Why the two catalysts showed different CO oxidation catalytic activities？The author should give an in-depth discussion from the bulk and surface structure, as well as the physicochemical properties, of the catalyst.

According to reviewer recommendation, we have added the discussion part in the article (2.7 Factors determining the catalytic activity).

3. Mn K-edge XANES spectra of the MnOx-ZrO2 catalysts prepared by co-precipitation were given. However, I did not find the same characterization results for another catalyst. 

According to reviewer recommendation, Mn K-edge XANES spectra of the MnOx-ZrO2 catalysts prepared by impregnation method have been added in the text.

2.4 XANES study

The Mn K-edge XANES spectra of the IMP and COP catalysts and those of standard manganese compounds are shown in Figure 5 and can provide information on local atomic and local electronic structures of the compounds. The Mn K-edge XANES spectra of the samples obtained represent two regions: the main feature at the Mn K-edge originates from the 1s → 4p dipole transitions and the pre-edge region, which is due to 1s → 3d transitions [20].

The main peak at the Mn absorption edge for COP-400 is observed at 6560.2 eV, and there is also small peak at 6540.7 eV in the pre-edge region (Figure 5b). The main peak of COP-600-700 has a slight shift toward low energy and a doublet structure. Probably, such a spectra shape arises due to the superposition of absorption spectra from manganese atoms located in different local environments.

It is known that the absorption edge shifts to higher energies with an increase in the average oxidation state of Mn. There is a nearly linear relationship between energy positions of electronic transitions and formal oxidation state in the series of Mn-oxides. Spectra of standard manganese compounds are also shown in Figure 5c and were used to determine the correlation between the K-edge excitation energy and the oxidation state of the metal cation using the positions of the K absorption edge of standard manganese oxide compounds: MnO (Mn²⁺), Mn₃O₄ (Mn^2.67+), Mn₂O₃ (Mn³⁺) and MnO₂ (Mn⁴⁺), Mn [21-25]. The experimental value of the absorption edge is about 6549.6 eV, the shifts of the absorption edges for all samples are in the range of ± 0.2 eV, which indicates the average oxidation state of Mn from 3.17+ to 3.24+.

In the case of the IM-catalysts, the XANES spectrum of IM-400 sample correspond to Mn in MnO₂, the spectra of samples IM-600 – IM-650 correspond to Mn in Mn₂O₃ (Figure 5a). The XANES spectrum of IM-800 has a similar shape to that of Mn₃O₄. The charge state of Mn atoms are in accordance with them in corresponding compounds.

 

Figure 5. XANES spectra of the IMP (a) and COP (b) catalysts and those of standard manganese compounds (c).

 

4. The impregnation method is only used for the preparation of loading catalysts, and the active components should generally be less than 10 wt% to ensure the uniform distribution of the active components on the carrier surface.

Agree with the remark. In this work we used the molar ratio of Mn / Zr= 1/4. It corresponds to a mass ratio of 15% MnO2 / 85% ZrO2 or 11.2% Mn/ZrO2. The chemical composition of samples was chosen to compare impregnated and co-precipitated catalysts in order to reveal differences in structural and catalytic properties. On the one hand, it was necessary to increase Mn content in COP series to have possibility to investigate catalyst by methods and fix changes in properties depending on the introduced manganese. On the other hand, it is possible to introduce this amount of Mn into catalyst by wet impregnation. .

Indeed, depending on the concentration of manganese, the nature of the active component may differ. We have added a relevant discussion to the text of the article (2.7 Factors determining the catalytic activity).

2.7 Factors determining the catalytic activity

Two series of MnOx-ZrO₂ catalysts with Mn:Zr=1:4 were prepared by co-precipitation and vacuum impregnation with subsequent calcination at temperatures from 400 to 800°C and then tested in CO oxidation. The activity of the COP catalysts exceed the IM catalyst in the range of calcination temperature from 400 to 700°C (Figure 1). Catalysts prepared by co-precipitation and impregnation have the same chemical composition, but their physicochemical properties differ. In the case of IM series, catalyst contains support ZrO₂ (mon), ZrO₂ (tetr) and Mn oxides as active component. The most active catalyst was obtained at 400°C, its activity in CO oxidation is due to the formation of MnO₂ oxide containing Mn⁴⁺, and a low-temperature reducibility. According to XPS results (Table 2), with an increase in the calcination temperature from 400 to 700°C, segregation of the surface Mn cations occurs. However, it does not lead to an increase in catalytic activity. An increase in the calcination temperature leads to the formation of less active Mn₂O₃, a decrease in surface area, while the TPR profiles shift to the high-temperature region, which indicates a decrease in the oxygen mobility of the catalyst.

For the COP series of catalysts, the dependence of catalytic activity on calcination temperature of the catalyst has a volcano-like shape with a maximum at 650-700°C. For COP-400 and COP-500, an X-ray amorphous state is formed. An increase in the synthesis temperature leads to the formation of a Mn_yZr_1-yO_2-x solid solution with the fluorite structure. An increase in the calcination temperature is accompanied by an increase in the lattice parameter of the Mn_yZr_1-yO_2-x oxide (Table 1). The lattice parameter approaches the value for "pure" zirconium oxide, which is associated with the release of manganese cations from the solid solution structure. Simultaneously, XPS detected the enrichment of the surface with manganese cations (Table 2). According to TPR, an increase in the low-temperature peaks of H₂ consumption occurs, which is attributed to the highly disperse MnOx oxide. The decomposition of the Mn_yZr_1-yO_2-x solid solution into simple manganese and zirconium oxide begins. For COP-800, the amount of the disperse MnOx species decreases (peak 1, Figure 6b); the disperse MnOx species were transformed into Mn₃O₄ species in accordance with the XRD findings.

Noteworthy are the catalysts prepared at 600-700°C. Indeed, XRD, XPS, H₂-TPR, and BET demonstrate the similarity between catalysts prepared at 600-700°C: all of them contain Mn_yZr_1-yO_2-x oxide with a close crystallite size, the charge states of Mn cations on the surface and their specific surface areas are nearly equal, and the profiles of H₂ consumption look quite close. However, the catalytic properties change drastically: T₅₀ increases from 185 to 248°C, and R decreases from 0.099 to 0.044 cm³/(g∙s) for COP-650 and COP-600, respectively. Probably, a tendency to decomposition of the solid solution Mn_yZr_1-yO₂ contributes to the formation of the active state of the catalyst. Several signs indicate the decomposition of the solid solution: (1) a change in parameters of the mixed oxide towards "pure" zirconium oxide, (2) a tendency to form a phase of manganese oxide at 800°C, and (3) an increase in the content of manganese on the oxide surface. An optimum is needed here between an “ideal” solid solution and a completely decomposed catalyst. It can be assumed that during the solid solution decomposition a unique microstructure is formed, in which, on the one hand, oxygen of the Mn_yZr_1-yO_2-x solid solution structure is active in oxidation reactions and, on the other hand, manganese cations segregate to the surface in the form of MnOx grain boundaries or the content of Mn in the near-surface layer of the solid solution, due to which the increase in catalytic activity is ensured.

Interestingly, that at relatively low Mn content (8wt%) catalysts prepared by wet impregnation exhibit better catalytic performance than catalyst synthesized by co-precipitation [12]. In this case, MnOx species acts as an active component. At relatively high Mn content, co-precipitated catalysts predominantly show improved catalytic performance [10,14]. According to most studies with the widely varying manganese content, catalysts with the composition Mn_0.4Zr_0.6O₂ – Mn_0.6Zr_0.4O₂ exhibited the maximum catalytic activity in oxidation reactions [13,14,52,53]. Catalysts with a high Mn loading contain, along with a solid solution, crystalline manganese oxides such as Mn₂O₃ and Mn₃O₄. With an increase in the total amount of Mn in the catalyst, the number of manganese ions introduced into zirconia grows, which means that the number of oxygen vacancies that are active in oxidation reactions also increases [11]. Simultaneously, the introduction of Mn into zirconia leads to a decrease in the size of crystallites and an increase in the specific surface area [33,48]. Due to the multi-phase nature of Mn_0.4Zr_0.6O₂ – Mn_0.6Zr_0.4O₂ catalysts, the possible effects associated with the formation of manganese nanoparticles during decomposition of the solid solution are not so noticeable and it is difficult to detect them. In this work, the influence of the solid solution decomposition processes was studied in the single-phase mixed oxides containing a smaller amount of manganese Mn_0.2Zr_0.8O₂ and their role in increasing catalytic activity was shown.

5. In MnOx-ZrO2 catalysts prepared by impregnation, the ratio of Mn / Zr is 1 / 4. So how many proportions of MnOx is distributed on the ZrO2 surface and how many exists as an agglomeration?

Calculations showed that approximately 1.6 MnOx monolayers should be observed. We agree with the reviewer that in the case of IM catalysts, agglomeration of manganese cations into phases occurs.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper presents the preparation of two sets of MnO_x/ZrO₂ catalysts through two methods (impregnation and coprecipitation), analyzing the physicochemistry of these materials and their performance in the CO oxidation.

The work is presented in an orderly manner and the characterization results obtained through the different techniques are well analysed. The sequence of calcination temperatures of the materials and their preparation methods allowed to correlate the activity of some samples (coprecipitation) with the nature of the MnOx species present.

The following comments are made:

- In experimental, indicate how the activity at 200 °C (cm³/g s) was calculated.

- It is required to place a description of the figures in the legend of Figure 3.

- The Zr3d spectrum does not contribute much and it is recommended to delete Fig. 5 or move it to a supplementary information.

- It is required to place a description of the figures in the legend of Figure 8.

- TPR: Why in the COP-500 and COP-400 samples that have amorphous species, the peak 2 assigned to MnyZr1-yO2-x solid solution?

- Paragraph page 18 lines 365 -367...It is worth mentioning that the H2 consumption...: Discuss the correlation between this higher consumption with the properties of both solids.

- Page 13 line 392: it should say Table 3.

- Table 3 shows that the Mn+3/Mn+4 ratio goes through a maximum for the COP-600 and COP-700 samples, in line to their activity. Please, discuss this aspect.

- Page 13 lines 402-403... T50 increases 402 from 185 to 248°C, and R200 decreases from 0.099 to 0.044 cm3/(g∙s): This difference is not observed between the COP-600 and COP-700 samples. Indeed both are very similar in activity (it is observed in the light off curves of Figure 1a). Please, clarify this point.

Author Response

Response to Reviews

We are thankful to the referees for useful comments. We made revision of the text according to the recommendations. Since we have changes the structure of manuscript, revisions to the manuscript was marked by color instead of the “Track Changes”

Review 3

The paper presents the preparation of two sets of MnO_x/ZrO₂ catalysts through two methods (impregnation and coprecipitation), analyzing the physicochemistry of these materials and their performance in the CO oxidation.

The work is presented in an orderly manner and the characterization results obtained through the different techniques are well analysed. The sequence of calcination temperatures of the materials and their preparation methods allowed to correlate the activity of some samples (coprecipitation) with the nature of the MnOx species present.

The following comments are made:

- In experimental, indicate how the activity at 200 °C (cm³/g s) was calculated.

Catalytic activity was calculated from the formula:

R(CO) = С_о ∙X∙V/m_cat, [cm³(СО)/g∙s]

X = (Р₀ – Р_cur)/ Р₀,

where P₀ is the peak area corresponding to the initial concentration of CO in the reactant mixture;

P_cur is the peak area corresponding to the current concentration of CO at the reactor outlet;

X is the degree of CO conversion;

C₀ is the initial concentration of CO in the mixture (C₀ = 1 vol.%);

V is the feed rate of the reactant mixture, ml/min; and m_cat is the mass of the catalyst, g.

- It is required to place a description of the figures in the legend of Figure 3.

According to reviewer recommendation, the legend of Figure 3 has been changed.

«Figure 3. TEM images (a,d); with FTT image (b,e,f); with EDX spectra (c) of the IM-400 (a,b,c) and IM-650 (d, e, f) catalysts. »

- The Zr3d spectrum does not contribute much and it is recommended to delete Fig. 5 or move it to a supplementary information.

According to reviewer recommendation, the Zr3d spectra were deleted from the manuscript

- It is required to place a description of the figures in the legend of Figure 8.

According to reviewer recommendation, the legend of Figure 8 (4) has been changed.

«Figure 4. TEM images (a,b); with EDX spectra (c) the COP-650 catalyst.»

- TPR: Why in the COP-500 and COP-400 samples that have amorphous species, the peak 2 assigned to MnyZr1-yO2-x solid solution?

Indeed, we agreed with reviewer, that it is not correct to assume that amorphous species contain MnyZr1-yO2-x solid solution.

We have change the description of TPR data « Peak 1 located at 150–300°C is attributed to the reduction of easily reducible and highly dispersed surface manganese species [5,12,13,47-49], while a broad peak 2 at 250-500°C is associated with the reduction of manganese cations in the Mn_xZr_1-xO₂ solid solution [50] or Mn cations in the Mn-Zr-O amorphous species. » (3.5 Results of the TPR-H₂ analysis)

- Paragraph page 18 lines 365 -367...It is worth mentioning that the H2 consumption...: Discuss the correlation between this higher consumption with the properties of both solids.

The origin of the weak high-temperature peak, which is observed at 540–560°C (peak 4), is still debatable. It is usually attributed to the reduction of cations Mn³⁺ → Mn²⁺ in the lattice of zirconia. However, sometimes this peak is associated with the reduction of zirconia itself. According our results, the H₂ consumption of high-temperature TPR peaks for IM series is larger than in the case of COP series (0.5-2.8*10^-4 mol H₂/g versus 0.2*10^-4 mol H₂/g). Previously, we investigates the reduction of Mn_yZr_1-yO_2-x solid solution [Bulavchenko, O.A.; Vinokurov, Z.S.; Afonasenko, T.N.; Tsyrul'Nikov, P.G.; Tsybulya, S.V.; Saraev, A.A.; Kaichev, V.V. Reduction of mixed Mn-Zr oxides: In situ XPS and XRD studies. Dalton Transactions 2015, 44, 15499-15507, doi:10.1039/c5dt01440a]. The reduction of mixed oxides in hydrogen was studied by in situ XRD, TPR, and in situ XPS. It has been shown that the reduction of the solid solutions Mn_yZr_1-yO_2-x proceeds in a wide temperature range of 100–700 °C via two steps. In the first step, at 100–500 °C, Mn cations, which constitute the solid solution, undergo partial reduction. In the second step, at 500–700 °C, Mn cations irreversibly exit to the particle surface. From this point of view, the high value of the H₂ consumption of high-temperature TPR peaks for IM series correlates with formation of surface solid solution and easy exit of Mn ions from the near-surface layer of zirconia particles. In the case of COP catalysts, probably exit of Mn ions from the volume of mixed oxide is complicated.

- Page 13 line 392: it should say Table 3.

Thank you for your comment; in new version of manuscript, we joined Tables 2 and 3.

- Table 3 shows that the Mn+3/Mn+4 ratio goes through a maximum for the COP-600 and COP-700 samples, in line to their activity. Please, discuss this aspect.

According to the literature data, the authors often indicate the presence of a direct correlation between the amount of highly oxidized manganese cations on the catalyst surface and the catalytic activity. In our case, such a correspondence is not observed for COP samples. Indeed, for the most active samples COP-650 and COP-700, the Mn⁺³/Mn⁺⁴ ratio is the highest, 1.70 and 1.86, respectively, while for COP-600 and COP-800 it is 1.44, for COP-500 it is 1.56, for COP-400 - 0.85. Based on the correlation between the activity and the electronic state of manganese, the most active sample should be COP-400. It, like the impregnating sample IM-400, has the maximum amount of Mn4+ and, in addition, a high specific surface area relative to other samples in the series. However, the COP activity depends not only on the electronic state of manganese, but is determined by a combination of various structural factors. Probably, the activity of COP samples is determined by the increase in the manganese content on the surface of the sample in the form of finely dispersed MnOx particles, which are formed at the initial stage of solid solution decomposition.

- Page 13 lines 402-403... T50 increases 402 from 185 to 248°C, and R200 decreases from 0.099 to 0.044 cm3/(g∙s): This difference is not observed between the COP-600 and COP-700 samples. Indeed both are very similar in activity (it is observed in the light off curves of Figure 1a). Please, clarify this point.

Sentence «... T50 increases 402 from 185 to 248°C, and R200 decreases from 0.099 to 0.044 cm3/(g∙s)..» corresponded to the COP-650 and COP-600. To improve clarity of the article, we have changed the sentence to the following: «However, the catalytic properties change drastically: T50 increases from 185 to 248°C, and R200 decreases from 0.099 to 0.044 cm3/(g∙s) for COP-650 and COP-600, respectively.»

Figure 1a shows that the CO conversion curves for samples COP-600 and COP-700 are far from each other. The CO conversion curve for COP-700 is close to the corresponding curve for COP-650. With respect to them, the CO conversion curve for COP-600 is shifted towards higher temperatures and is more close to the curves for COP-500 and COP-400. The R200 value for COP-650 and COP-700 is 0.077 and 0.099 cm3/(g∙s), while for COP-600 it is 0.044 cm3/(g∙s). Thus, there is no contradiction between the R200 value and the position of the CO conversion curves for the COP samples.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The paper has been well modified, and its quality meets the criteria for publication.

Reviewer 3 Report

I am satisfied with the changes incorporated by the authors. I recommend its publication without new revisions.