The Activation of Oxygen Species on the Pt/CeO₂ Catalyst by H₂ for NO Oxidation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is devoted to the optimization of the conditions for the preparation of Pt-CeO2 catalysts for the NO oxidation reaction. The authors use different calcination temperatures and additionally reduce the catalysts with hydrogen.

However, the effects of the heat treatment conditions are very insignificant in the opinion of the reviewer. Judging by Fig. 1, an increase in the calcination temperature only worsens the properties of the catalyst, while reduction with hydrogen slightly improves the conversion of NO. In this case, it is necessary to indicate the temperature and time of treatment with hydrogen. The authors do not compare the obtained results with literature data, so it remains unclear what the advantages of the systems used are.

When analyzing the structure of the catalysts, inaccuracies and errors are allowed. For example, when analyzing DRIFTS data on CO adsorption (section 2.3.6.2), almost no references to the literature are given when assigning spectra. In Fig. 8 bands 2166, 2117, which the authors attribute to Ce3+ and Pt2+, are apparently related, according to the literature, to the spectrum of gaseous CO, so their disappearance after purging with nitrogen is quite natural. The bands in the region of 1600-1200, which the authors unreasonably attribute to the adsorption of CO on different forms of platinum, are apparently related to the formation of CO oxidation products, carbonates, oxalates, etc. The characteristics of CO on different forms of platinum and cerium are well known.

(see, for example,(M. Y. Mihaylov, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.05.014

F. C. Meunier Relevance of IR Spectroscopy of Adsorbed CO for the Characterization of Heterogeneous Catalysts Containing Isolated Atoms https://pubs.acs.org/doi/10.1021/acs.jpcc.1c06784)

The authors often draw conclusions about the presence of a Pt-O-Ce bond, in particular, in TEM analysis (lines 190-194), however, a uniform distribution of elements on the surface does not yet indicate their interaction.

Table 1 shows the kinetic parameters for NO oxidation over Pt/CeO2. It is not clear whether the data in the bottom 3 lines are the results of the authors of this article or are literary. And if these are the results obtained in this work, then why is their production and properties not described.

In the opinion of the reviewer, the article cannot be published in this form.

Author Response

Reviewer #1: The article is devoted to the optimization of the conditions for the preparation of Pt-CeO₂ catalysts for the NO oxidation reaction. The authors use different calcination temperatures and additionally reduce the catalysts with hydrogen. However, the effects of the heat treatment conditions are very insignificant in the opinion of the reviewer.

Comments 1: Judging by Fig. 1, an increase in the calcination temperature only worsens the properties of the catalyst, while reduction with hydrogen slightly improves the conversion of NO. In this case, it is necessary to indicate the temperature and time of treatment with hydrogen. The authors do not compare the obtained results with literature data, so it remains unclear what the advantages of the systems used are.

Response: We have indicated the temperature and time of treatment with hydrogen in the H₂ pretreatment procedure of the catalysts in the revised manuscript. In addition, we have also compared catalytic activities of our 1 wt% Pt/CeO₂ catalysts before and after H₂ reduction treatment and the 1 wt% Pt/CeO₂ catalyst reported in the literature [24] in the revised text.

Modification: Lines 127~129/Page 4: During the H₂ pretreatment process, the total flow rate of 10% H₂/N₂ was 200 mL/min. The catalyst was thermally treated in a heating rate of 5 ℃/min at 200 ℃ for 30 min.

            Lines 131~135/Page 4: Compared to the NO reaction rate of 0.13 μmol/(g·h) at 325 ^oC over the 1 wt% Pt/CeO₂ catalyst reported in the literature [24], those over the 1 wt% Pt/CeO₂ catalysts before and after H₂ reduction treatment obtained in the present work were 0.76 and 0.87 μmol/(g·h), respectively. Obviously, our catalyst outperformed the catalyst reported in the literature [24].

 

Comments 2: The bands in the region of 1600-1200 cm^-1, which the authors unreasonably attribute to the adsorption of CO on different forms of platinum, are apparently related to the formation of CO oxidation products, carbonates, oxalates, etc. The characteristics of CO on different forms of platinum and cerium are well known. (see, for example, M. Y. Mihaylov, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.05.014; F. C. Meunier Relevance of IR Spectroscopy of Adsorbed CO for the Characterization of Heterogeneous Catalysts Containing Isolated Atoms https://pubs.acs.org/doi/10.1021/acs.jpcc.1c06784).

Response: Thank the Reviewer for the good comment. We have cited several references to make the appropriate band assignment in the revised text.

Modification: Lines 368~373/Page 12: The CO-DRIFTS spectra of CO adsorption on the catalysts are shown in Figure 9. The band at 2086 cm^-1 was associated with the linear adsorption of CO on Pt nanoparticles. For the catalyst calcined at 500 °C, the band observed at 2117 cm^-1 was attributed to the linear adsorption of CO on the oxidized Pt species, the one at 2166 cm^-1 was linked to the CO adsorbed on the Ce³⁺ ions [33,39], and the ones at 1481 and 1566 cm^-1 were related to the products (e.g., carbonates) of CO oxidation [30].

 

Comments 3: The authors often draw conclusions about the presence of a Pt-O-Ce bond, in particular, in TEM analysis (lines 190-194), however, a uniform distribution of elements on the surface does not yet indicate their interaction.

Response: Thank the Reviewer for the good comment. It can be found from the TEM images (Figure 3a and b) that the lattice fringes of CeO₂ and PtO₂ species are clearly seen in the Pt/CeO₂ catalysts calcined at 500 and 800 ^oC, indicating the possible formation of a Pt-O-Ce-like bond. Such a deduction was confirmed by the subsequent EDS mappings of Pt, O, and Ce elements, in which the Pt species were uniformly dispersed on the CeO₂ support (i.e., there might be the formation of a Pt-O-Ce-like bond).

         We have made up the XPS characterization of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-600 catalysts before and after H₂ pretreatment, and their XPS spectra are shown Figure 7. The positions of the O 1s, Pt 4f, and Ce 3d peaks of the two catalysts after H₂ pretreatment were significantly shifted, revealing the formation of a Pt-O-Ce-like bond in the Pt/CeO₂ catalysts.

Modification: Please see Figures 3a and b and 7.

Figure 3a and b. TEM images of the 1 wt% Pt/CeO₂ catalysts calcined at (a) 500 °C and (b) 800 °C.

Figure 7. (a) O 1s, (b) Pt 4f, and (c) Ce 3d XPS spectra of the as-prepared catalysts.

Modification: Lines 207~212/Page 6: It can be found from the TEM images (Figure 3c₁ and c₂) that the lattice fringes of CeO₂ and PtO₂ species are clearly seen in the Pt/CeO₂ catalysts calcined at 500 and 800 ^oC, indicating the possible formation of a Pt-O-Ce-like bond. Such a deduction was confirmed by the subsequent EDS mappings of Pt, O, and Ce elements, in which the Pt species were uniformly dispersed on the CeO₂ support (i.e., there might be the formation of a Pt-O-Ce-like bond).

            Lines 322~325/Page 10: The XPS spectra of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-600 catalysts before and after H₂ pretreatment are shown Figure 7. The positions of the O 1s, Pt 4f, and Ce 3d peaks of the two catalysts after H₂ pretreatment were significantly shifted, revealing the formation of a Pt-O-Ce-like bond in the Pt/CeO₂ catalysts.

 

Comments 4: Table 1 shows the kinetic parameters for NO oxidation over Pt/CeO₂. It is not clear whether the data in the bottom 3 lines are the results of the authors of this article or are literary. And if these are the results obtained in this work, then why is their production and properties not described?

Response: The data in the bottom 3 lines are from the literature [25,26].

Modification:  Line 169/Page 4: The reference numbers (Refs. 25 and 26) have been indicated in Table 1.

Catalyst	Ea (kJ/mol)		Feed gas composition	Ref.
	Fresh	H2 pretreatment
1 wt% Pt/CeO2-500	23.7	19.5	500 ppm NO, 10% O2, N2 (balance)	This work
1 wt% Pt/CeO2-600	26.7	20.9
1 wt% Pt/CeO2-700	31.1	30.6
1 wt% Pt/CeO2-800	32.2	31.8
0.5 wt% Pt/CeO2-500	31.4	-	500 ppm NO, 10% O2, N2 (balance)	[25]
1.7 wt% Pt/SiO2-500	57.7	-	250 ppm NO, 3.5%O2, N2 (balance)	[26]
0.35 wt% Pt/CeZrO2-500	34.6	-	500 ppm NO, 8% O2, N2 (balance)	[26]

 

Reviewer 2 Report

Comments and Suggestions for Authors

  This manuscript describes the effect of H2 pretreatment of Pt/CeO2 catalyst for NO oxidation, using temperature programmed spectroscopy techniques. While these spectroscopic investigations gave the insights into the electronic states of the catalyst, supports for these findings by other characterization techniques should be necessary. The revision by explanation of following concerns will improve the quality of manuscript suitable for Catalysts.

 

 

1. The particle size is critical to catalytic performance. But the effect of H2 treatment on Pt particle sizes is not clearly unveiled. High-resolution TEM measurement of the catalysts is necessary to confirm the particle sizes.

 

2. X-ray photoelectron spectroscopy is recommended to investigate the oxidation state of the catalysts and effect of H2 pretreatment.

 

3. How is the durability of the catalysts? Time-dependence of NO oxidation activity of the catalyst is important.

 

4. The authors claim that “It is evident that the NO2 signal intensity of the catalyst after H2 reduction treatment is higher than that of the untreated catalyst”. However, it is difficult to understand this conclusion from NO-TPSR results in Fig. 5.

 

5. Fig.10 should be revised to illustrate the catalytic mechanism more clearly. The temperature programmed spectroscopy results identified gaseous molecules with various adsorption structures. Identify which is the most effective for the catalytic performance.

 

Author Response

Reviewer #2: This manuscript describes the effect of H₂ pretreatment of Pt/CeO₂ catalyst for NO oxidation, using temperature programmed spectroscopy techniques. While these spectroscopic investigations gave the insights into the electronic states of the catalyst, supports for these findings by other characterization techniques should be necessary. The revision by explanation of following concerns will improve the quality of manuscript suitable for Catalysts.

Comments 1: The particle size is critical to catalytic performance. But the effect of H₂ treatment on Pt particle sizes is not clearly unveiled. High-resolution TEM measurement of the catalysts is necessary to confirm the particle sizes.

Response: Thank the Reviewer for the good suggestion. We made up the high-resolution TEM images of the 1 wt% Pt/CeO₂-500 catalysts before and after H₂ treatment, as shown in Figure 3c and d. Due to the similar contrast of Pt and Ce, it was difficult to distinguish them in the high-resolution TEM images. Therefore, we did not provide the sizes of Pt particles before and after H₂ treatment.

Modification: Please see Figure 3c and d.

Figure 3c and d. (c, d) HRTEM images of 1 wt% Pt/CeO₂-500 before and after H₂ pretreatment.

            Lines 212~216/Page 7: The high-resolution TEM images of the 1 wt% Pt/CeO₂-500 catalysts before and after H₂ treatment are shown in Figure 3c and d. Due to the similar contrast of Pt and Ce, it was difficult to distinguish them in the high-resolution TEM images. Therefore, the sizes of Pt particles before and after H₂ treatment were not provided.

 

Comments 2: X-ray photoelectron spectroscopy is recommended to investigate the oxidation state of the catalysts and effect of H₂ pretreatment.

Response: Thank the Reviewer for the good suggestion. The related revision has been made, as shown in the modification below.

Modification: Please see Figure 7a-c and Table 3.

Figure 7. (a) O 1s, (b) Pt 4f, and (c) Ce 3d XPS spectra of the as-prepared catalysts.

Table 3. The surface element compositions of the 1 wt% Pt/CeO₂ catalysts obtained by the XPS technique.

Catalyst	Molar ratio
	Pt0/(Pt2++ Pt4+)	Ce3+/Ce4+	Oads/Olatt
1 wt% Pt/CeO2-500	0.86	0.18	0.35
1 wt% Pt/CeO2-600	0.48	0.25	0.32
1 wt% Pt/CeO2-500-H2	0.98	0.49	0.51
1 wt% Pt/CeO2-600-H2	0.86	0.37	0.35

 

Lines 286~326/Page 9~10: The metal chemical valence and adsorbed oxygen species of the catalysts play important roles in catalyzing NO oxidation. The XPS characterization experiments of the Pt/CeO₂ catalysts were performed, and the results are shown in Fig. 7 and Table 3. Each of the asymmetrical O 1s XPS spectra of the catalysts could be divided into three components at binding energies of 529.3, 531.3, and 533.1 eV (Figure 7a), which were attributed to the surface lattice oxygen (O_latt), adsorbed oxygen (O_ads), and adsorbed water or carbonate species, respectively. The O_ads/O_latt molar ratios of Pt/CeO₂-500 and Pt/CeO₂-600 increased from 0.35 and 0.32 to 0.51 and 0.35 after H₂ pretreatment, respectively. The increase in O_ads/O_latt molar ratio enhanced the catalytic activity of the Pt/CeO₂ catalyst for NO oxidation.

The Pt 4f XPS spectra of Pt/CeO₂-500 and Pt/CeO₂-600 are shown in Figure 7b. Pt was present mainly in metallic state (Pt⁰) and platinum oxide (Ptⁿ⁺, i.e., Pt²⁺ and Pt⁴⁺). The deconvoluted components at binding energies of 71.5 and 74.8 eV were related to the 4f_7/2 and 4f_5/2 states of the surface Pt⁰ species, those at binding energies of 72.6 and 75.9 eV were assigned to the 4f_7/2 and 4f_5/2 of the surface Pt²⁺ species, and the those at binding energies of 74.2 and 77.5 eV were ascribed to 4f_7/2 and 4f_5/2 states of the surface Pt⁴⁺ species. The signals of Pt 4f_5/2 were 3.3 eV higher than those of Pt 4f_7/2. After H₂ treatment, the Pt⁴⁺ species nearly disappeared. The Pt⁰/ (Pt²⁺ + Pt⁴⁺) molar ratios on the surface of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-600 catalysts before and after H₂ treatment were from 0.86 and 0.48 to 0.98 and 0.86, respectively. In other words, the rise in Pt⁰/ (Pt²⁺ + Pt⁴⁺) molar ratio increased the activity of the Pt/CeO₂ catalyst after H₂ treatment for NO oxidation.

The deconvolution of Ce 3d XPS spectra (Figure 7c) indicates that both Ce³⁺ and Ce⁴⁺ species exist on the surface of Pt/CeO₂. It is generally believed that oxygen vacancies are generated to maintain the electroneutrality of a catalyst owing to the existence of Ce³⁺. The higher the content of Ce³⁺, the more the oxygen vacancies. The H₂ pretreatment increased the content of Ce³⁺, leading to an increase in oxygen vacancy concentration on the catalyst surface. The Ce³⁺/Ce⁴⁺ molar ratios of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-600 catalysts before and after H₂ treatment were from 0.18 and 0.25 to 0.49 and 0.37, respectively. It is well known that oxygen vacancies on the surface of a catalyst can adsorb O₂ to generate the reactive adsorbed oxygen species. Hence, the more the oxygen vacancies, the better catalytic activity of the Pt/CeO₂ catalyst.

 

Comments 3: How is the durability of the catalysts? Time-dependence of NO oxidation activity of the catalyst is important.

Response: Thank the Reviewer for the good suggestion. We have conducted the catalytic stability test of the 1 wt% Pt/CeO₂-500 catalyst after H₂ pretreatment at 325 ℃ for 10 h. Apparently, no significant decreases in NO conversion were observed in 10 h of NO oxidation under the adopted reaction condition.

Modification: Please see Figure 1c.

Figure 1c. ... and (c) catalytic stability of the 1 wt% Pt/CeO₂-500 catalyst after H₂ pretreatment.

Lines 136~139/Page 4: The catalytic stability test of the 1 wt% Pt/CeO₂-500 catalyst after H₂ pretreatment was carried out at 325 ℃ for 10 h of NO oxidation, and its result is shown in Figure 1c. Apparently, no significant decreases in NO conversion were observed in 10 h of NO oxidation under the adopted reaction condition.

 

Comments 4: The authors claim that “It is evident that the NO₂ signal intensity of the catalyst after H₂ reduction treatment is higher than that of the untreated catalyst”. However, it is difficult to understand this conclusion from NO-TPSR results in Fig. 5.

Response: Thank the Reviewer for the good comment. The mentioned sentence has been deleted in the revised manuscript.

 

Comments 5: Fig.10 should be revised to illustrate the catalytic mechanism more clearly. The temperature programmed spectroscopy results identified gaseous molecules with various adsorption structures. Identify which is the most effective for the catalytic performance？

Response: After H₂ pretreatment, the Ce⁴⁺ species in CeO₂ were partially reduced to the Ce³⁺ species, forming a large number of oxygen vacancies on the catalyst surface. Molecular oxygen could be first adsorbed at the oxygen vacancies and Pt sites, which then reacted with the adsorbed NO. The active adsorbed oxygen species might be the most effective factor for the high activity of the Pt/CeO₂ catalyst.

Modification: Please see the modified figure below.

Figure 11. Catalytic mechanism diagram of NO oxidation over Pt/CeO₂.

Lines 450~454/Page 14: After H₂ pretreatment, the Ce⁴⁺ species in CeO₂ were partially reduced to the Ce³⁺ species, forming a large number of oxygen vacancies on the catalyst surface. Molecular oxygen could be first adsorbed at the oxygen vacancies and Pt sites, which then reacted with the adsorbed NO. The active adsorbed oxygen species might be the most effective factor for the high activity of the Pt/CeO₂ catalyst. 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The article can be published

Author Response

Thank you for your review and support of our manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors revised the manuscript according to the reviewer‘s comment. While a part of the revision satisfied the reviewer, some significant concerns still remain in the manuscript.

 

1. The explanation for the NO-TPSR results shown in Figure 5 is insufficient. The authors should discuss the difference between each sample.

 

2. The authors conducted the temperature-programmed measurements and various spectroscopic techniques to identify the oxidation state of each element in the catalysts and the adsorption mode of the gaseous molecules on the catalysts. However, such information obtained by the authors is not depicted in Figure 11. The current figure can be drawn without the knowledge obtained in the research.

 

Author Response

Comments1: The explanation for the NO-TPSR results shown in Figure 5 is insufficient. The authors should discuss the differences between each sample.

Response: Thank the Reviewer for the good comment. The related revision has been made, as shown in the modification below.

Modification: Lines 251~256/Page 8: As shown in the NO-TPSR profiles of the 1 wt% Pt/CeO₂-500 and 1 wt% Pt/CeO₂-800 samples (Figure 5), the intensity of NO₂ signal was stronger in the sample treated in H₂ at 500 or 800 ℃ than that  in the sample treated in air at 500 or 800 ℃, indicating H₂ treatment was beneficial for the improvement in catalytic activity of the sample. The worse activity of the sample treated at 800 ℃ than the sample treated at 500 ℃ might be due to its sintering.

 

Figure 5. NO-TPSR profiles of the catalysts before and after H₂ pretreatment at 500 and 800 °C.

 

Comments 2: The authors conducted the temperature-programmed measurements and various spectroscopic techniques to identify the oxidation state of each element in the catalysts and the adsorption mode of the gaseous molecules on the catalysts. However, such information obtained by the authors is not depicted in Figure 11. The current figure can be drawn without the knowledge obtained in the research.

Response: Thank the Reviewer for the good comment. We have added the related characterization results of temperature-programmed measurements and various spectroscopic techniques to Figure 11 in the revised manuscript. 

Modification: Please see Figure 11 on Lines 451~453/Page 14: 

 

Figure 11. Catalytic mechanism diagram of NO oxidation over Pt/CeO₂.

Author Response File: Author Response.pdf