Advanced PtCo Catalysts Based on Platinum Acetate Blue for the Preferential CO Oxidation in H₂-Rich Mixture

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the manuscript, advanced PtCo/ZSM-5 (Si/Al=15 and 28) catalysts for CO-PROX were prepared using platinum acetate blue (PAB) of the empirical formula Pt(СН₃СОО)_2.5 as a novel precursor for the zeolite impregnation. It was found that PtCo/ZSM-5 catalysts with only 0.1-0.2 wt.% Pt are extremely effective for CO oxidation. The as-prepared materials were well characterized by several techniques. The manuscript can be accepted after addressing the following comments:

(1)   “Therefore, the choice of a suitable support and synthesis conditions along with chemical precursor for introducing the active metal is of great importance.” In my opinion, it is not enough to obtain this statement. I suggest authors to discuss it with appropriate references.

(2)   Why did author choose zeolite with the Si/Al=15 and 18 as raw material? The authors have to add comments with respect to this aspect.

(3)   The catalytic performance data of 0.1Pt/Z-15/28 after 200+300 °C heat treatment and 0.2Pt/Z-15/28 after 300 °C heat treatment should be presented in Table 1.

(4)   Line 138, the formation of “Co/Pt/Z catalysts” should be unified.

(5)   The discussion of Figure 2 is lacking. “2 heating - cooling cycles” should be replaced with “Two heating - cooling cycles”. What is “0.2PtCo/Z-152”? Is there no cooling mode in the 2nd cycle?

(6)   The survey XPS spectra of all synthesized catalysts should be shown in supporting information.

(7)   Table 4 should be optimized. I suggest authors to divide Table 4 into two tables.

(8)   Why did author choose to heat the sample at 200 and 300 °C? The authors have to add comments with respect to this aspect.

 

 

Comments on the Quality of English Language

The English of the manuscript should be further polished.

Author Response

Dear Reviewer, 

First of all, we would like to express our gratitude for the high assessment of our work and very useful comments and questions. Our responses to the comments made and the corresponding corrections are listed below. We hope that the corrections made will improve our material and make it more logical. 

• “Therefore, the choice of a suitable support and synthesis conditions along with chemical precursor for introducing the active metal is of great importance.” In my opinion, it is not enough to obtain this statement. I suggest authors to discuss it with appropriate references.

This phrase, in our opinion, should have concluded the material presented above. But, we are afraid that this is not entirely clear, so we moved it to the beginning of the introduction. In this regard, we have significantly restructured the introduction and replaced two references 11 and 44 with more convincing ones. The revised introduction, along with all corrections, is below.

1. Introduction

Oxidation of carbon monoxide remains one of the most studied reactions due to its enormous fundamental and applied significance [1-5]. Recently, special attention of researchers has been attracted to the preferential oxidation of CO in H₂-rich gas mixture (CO-PROX). It is an effective way of the hydrogen purification from CO impurities for the application in fuel cells [6-8]. Catalysts for this process should be active, selective and stable in the temperature range of 65-85 °C corresponding to the operating window of fuel cells [9]. Supported nanoparticles of noble metals are widely used in the oxidation of CO, among them Pt base catalysts are the most suitable for the CO-PROX process [10-13]. Rational design of catalysts with reduced noble metal loading for the CO oxidation remains an urgent task, the solution of which requires knowledge of the process mechanism and the nature of active centers. With decreasing of metal loading a metal dispersity, electronic state and catalytic behavior varies greatly [14-16]. The choice of a suitable support and synthesis conditions along with chemical precursor for introducing the active metal becomes of great importance [17-20]. The properties of Pt catalysts can be improved by using reducible oxides as supports or by an addition of transition metal oxide as promoters [14-17].

Rational design of catalysts with reduced noble metal loading for the CO oxidation remains an urgent task, the solution of which requires knowledge of the process mechanism and the nature of active centers. This task is complicated by the fact that the metal dispersity, electronic state and catalytic behavior varies greatly with decreasing of metal loading [18-20]. Oxide supports seem to be promising candidates for stabilizing highly dispersed particles of the active phase, even for single atoms and small clusters due to strong metal-oxide interactions [21-24]. To prepare high performance Pt based catalysts for CO-PROX different metals (Ni, Fe, Ce, Co and etc) or oxides are additionally introduced as promoters [25-28]. Among them, cobalt oxides should be noted due to their own high activity [4, 29-31] and a strong synergistic effect with platinum [32-34]. The properties of bimetallic catalysts are determined by the formation of metal-metal or metal-oxide interfaces, and they strongly depend on the synthesis method and conditions, the ratio of components and the choice of support.

Zeolites are considered as the perspective supports for the formation and stabilization of highly dispersed active species with the controllable electronic states of metals for different redox reactions [35-40]. The type of framework and the Si/Al ratio influence the catalyst structure and catalytic properties [41-43], in CHA zeoite the activity in the CO oxidation increased with decreasing pore size and optimal Si/Al molar ratio of 17 [44]. The use of BEA to stabilize Pd nanoparticles led to their almost complete oxidation and low activity in the CO oxidation compared to Pd/ZSM-5 [45].  High performance of ZSM-5 zeolites modified by platinum metal groups in the oxidation reactions is due to synergistic effect of metals and acid sites of zeolites resulting in [46-48]. Acid sites on the zeolite surface favor the formation and stabilization of highly dispersed MeOx particles that play an important role in the oxidation both of CO and hydrocarbons [45, 49].

Different types of Pt modified zeolites were used for CO-PROX [6, 50-52], transition metals or oxides are used as promotors to improve their catalytic performance [53, 54]. To achieve high activity of bimetallic catalysts, it is necessary to ensure their maximum proximity on the surface of the support [55-57]. A variety of chemical and physical methods can be used for the preparation of metal modified zeolites, but chemical methods based on impregnation, ion-exchange and deposition–participation remain the main ones [11, 17, 23]. In the case of low loaded catalysts the procedure for introducing active components is of great importance, original method of laser electrodispersion (LED) was applied for the selective deposition of highly dispersed Pt particles on the zeolite surface and Co pre-modified ZSM-5 with Si/Al of 15, 28 and 40 [51, 53]. Pt- and PtCo-ZSM-5 with low Si/Al of 15 and 28 were more efficient in CO-PROX. The use of such zeolites provides the best conditions for Pt and Co interaction and improved catalytic properties. This fact along with known data that both noble and transition metal-modified ZSM-5 were more active in the CO oxidation compared to alumina and BEA supported catalysts [45, 58] is the background of this work. But a question of the influence of the Si/Al ratio in zeolite ZSM-5 on the catalytic performance of PtCo/ZSM-5 prepared by traditional method of impregnation in CO-PROX reaction remains not entirely clear.

A variety of chemical and physical methods can be used for the preparation of metal modified zeolites, but chemical methods based on impregnation, ion-exchange and deposition –participation remain the main ones [11,17,23]. Therefore, the choice of a suitable support and synthesis conditions along with chemical precursor for introducing the active metal is of great importance. The use of BEA to stabilize Pd nanoparticles led to their almost complete oxidation and low activity in the CO oxidation compared to Pd/ZSM-5 [45]. Both Pt and transition metal-modified ZSM-5 surpassed in their activity and selectivity in CO-PROX catalysts based on alumina and zeolite BEA [53, 58]. The question of the influence of the Si/Al ratio in zeolite ZSM-5 on the catalytic performance remains not entirely clear.

In this work the polynuclear Pt₉(CH₃COO)₂₃ complex known as platinum acetate blue (PAB) was used as a novel precursor for Pt loading in zeolite. This X-ray amorphous substance of the empirical formula Pt(СН₃СОО)_2.5 was synthesized and characterized as has been described previously [59].  Based on this data ZSM-5 with Si/Al=15 and 28 was taken for the preparation of Pt/ and PtCo/zeolites. Polynuclear Pt₉(CH₃COO)₂₃ complex known as platinum acetate blue (PAB) was used as a novel precursor for Pt loading. This X-ray amorphous substance of the empirical formula Pt(СН₃СОО)_2.5 was synthesized and characterized as has been described previously [59]. The impregnation of ZSM-5 with PAB and its two- step decomposition at 200 and then at 300°C results in the stabilization of highly dispersed PtOx species on the zeolite surface. The properties of Pt-modified zeolite were improved by the addition of Co(СН₃СОО)₂ followed by calcination at 450°C. Prepared materials with reduced 0.1-0.2 wt% Pt were studied by SEM, TEM, EDX, XPS and DRIFTS methods and tested in the CO-PROX reaction. The relationships between synthesis conditions, structure and catalytic behavior of composites were found. The best synergistic effect of Pt and Co was observed when they both were located tog ether on the surface of a zeolite with Si/Al=15, which has the highest acidity.

• Why did author choose zeolite with the Si/Al=15 and 18 28 as raw material? The authors have to add comments with respect to this aspect.

We also took this remark into account in the revised introduction where we presented available data that ZSM-5 zeolites with these values of the Si/Al ratio are the most promising candidate for the formation of highly active bimetallic catalysts. We hope that now our choice of raw material has become more convincing.

• The catalytic performance data of 0.1Pt/Z-15/28 after 200+300 °C heat treatment and 0.2Pt/Z-15/28 after 300 °C heat treatment should be presented in Table 1.

The influence of different heat treatment modes on the catalytic characteristics of the samples was studied using two series of catalysts with different Pt loading. These data are summarized in Table 1. For catalysts with 0.1%Pt, it was shown that treatment at 300˚C has a negative effect on the activity of the catalysts for both types of zeolites with different Si/Al ratios. Using another series of catalysts with 0.2%Pt as an example, it was found that a two-step increase in temperature (c, 200+300, ˚C), on the contrary, has a positive effect on activity. As can be seen from Table 1 both series of catalysts are characterized by similar activity under the same heat treatment (a, 200 ˚C). It seems that the introduction of additional information will only overload Table1. This will not lead to the identification of new regularities in the influence of the heat treatment mode on catalytic properties, but will only complicate the perception of the material in the article. To prepare bimetallic PtCo-catalysts we used 0.1Pt/Z and 0.2Pt/Z heated only at 200°C (a).

(4)   Line 138, the formation of “Co/Pt/Z catalysts” should be unified.

Thank you for noticing this inaccuracy, we corrected to PtCo/Z-15.

• The discussion of Figure 2 is lacking. “2 heating - cooling cycles” should be replaced with “Two heating - cooling cycles”. What is “0.2PtCo/Z-152”? Is there no cooling mode in the 2nd cycle?

Of course you are absolutely right. Two heating - cooling cycles are presented in Figure 2. And “0.2PtCo/Z-152” is a typo, we have corrected this negligence.

• The survey XPS spectra of all synthesized catalysts should be shown in supporting information.

Thank you, we’ve done it. XPS spectra of all synthesized catalysts are shown in the supporting information.

(7)   Table 4 should be optimized. I suggest authors to divide Table 4 into two tables.

In accordance with this recommendation we divided Table 4 into two Tables 4 and 5. Corresponding changes have been made to table titles and to the text.

Table 4. Pt4f_7/2 binding energies, the fractions of Pt atoms in different states and the Pt/Si+Al ratio on the surface of Pt/Z catalysts

 

Eb, eV		71.2-71.3	72.2-72.4	73.5-74.0	Element ratio
Sample	Conditions	Electronic state, at.,%
		Pt0	Pt2+	Pt4+	Pt/(Si+Al)
0.2Pt/Z-15(a)	initial	44	29	27	0.004
0.2Pt/Z-15(c)	initial	74	12	14	0.007
0.6Pt/Z-28(a)	initial	24	50	26	0.02
	spent	74	20	6	0.004

 

Table 5. Pt4f_7/2 and Co2p_3/2 binding energies, the fractions of Pt and Co atoms in different states, and atomic ratios of elements on the surface of PtСo/Z and Сo/Z catalysts

  

Eb, eV		71.2 71.3	72.2 72.4	73.5 74.0	781.6	779.8	Element ratio
Sample	Conditions	Electronic state, at.,%
		Pt0	Pt2+	Pt4+	Co2+	Co3+	Pt/(Si+Al)	Co/(Si+Al)	Co/Pt
0.2PtCo/Z-15	initial	42	33	25	74	26	0.001	0.045	45
	spent	65	20	15	74	26	0.001	0.044	44
0.2PtCo/Z-28	initial	40	28	32	75	25	0.001	0.093	93
	spent	48	28	24	65	35	0.001	0.089	89
2.5Co/Z-15	initial	-	-	-	72	28	-	0.045	-
1.7Со/Z-281	initial	-	-	-	83	17	-	0.025	-

¹ data from our previous work [71]

 

 (8)   Why did author choose to heat the sample at 200 and 300 °C? The authors have to add comments with respect to this aspect.

This comment is added to the text. “The temperature of 200°C (a) was chosen as the initial because the decomposition of the platinum acetate complex occurs at this temperature [59]. To select heat treatment conditions that provide the best characteristics of the samples, two more high temperature modes (b, 300°C) and (c, 200+300,°C) were also tested.”

 

The explanation of this choice was in the text (lines 360-367) “As can be seen from Table 4 for the 0.2Pt/Z-15 catalyst, two-stage heat treatment (c) at 200˚C and then at 300˚C increases the fraction of metallic platinum on the surface in comparison with heating only at 200˚C (a). At the same time, part of the platinum remains in an oxidized state; the simultaneous presence of metallic and oxidized Pt states favors the catalytic oxidation [53]. The oxidized platinum weakens unwanted CO adsorption and promotes oxidation. Also, two-stage processing slightly increases the total platinum content on the surface.”

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

To Authors,

Concerning the manuscript “Advanced PtCo Catalysts based on Platinum Acetate Blue for the Preferential CO Oxidation in H2-Rich Mixture” (ID: catalysts-3105675) by Marina Shilina, Irina Krotova, Sergey Nikolaev, Natalia Cherkashina, Igor Stolarov, Olga Udalova, Sergey Maksimov, Tatiana Rostovshchikova:

1. Please provide the company names and grades for the chemicals used in this work.

2. Not many scholars are familiar with DRIFTS. Spend some time to give an impression on the method and why is suitable for the present study (e.g., Kubelka-Munk units)?

3. In TEM, the d-spacing is determined from picture analysis or are they just indicated? For instance, in Fig. 6 b) and d), it is very hard to make a difference between 2.2, 2.6 and 2.8 A.

4. In lines 529-531, the sentence is somewhat overstated since it is valid only for one (hydrogen-rich) electrode side.

Comments on the Quality of English Language

Minor language changes 

Author Response

Dear Reviewer, 

First of all, we would like to express our gratitude for the high assessment of our work and very useful comments and questions.Our responses to the comments made and the corresponding corrections are listed below.

 

1. Please provide the company names and grades for the chemicals used in this work.

Thanks for the comment. We’ve done it.

 

2. Not many scholars are familiar with DRIFTS. Spend some time to give an impression on the method and why is suitable for the present study (e.g., Kubelka-Munk units)?

 

In accordance with this remark we have added the following explanations to the Results (2.2. DRIFT spectroscopy of the adsorbed CO)  and  to the Materials and Methods.

“This method is widely used to study supported catalyst because of the high sensitivity of CO stretching frequency to the structure of the binding sites [60-67]. There are well-established values for the vibrational frequency on supported Pt catalysts [14, 60, 62-64]. When CO is adsorbed on oxidized cationic Pt sites, its values lie in the range above 2100 cm^-1, and can be blueshifted to 2170 cm^-1 depending on the cation charge and support [60, 62, 63]. When CO is adsorbed on Pt-metal particles, the vibrational frequency is redshifted and decreases from 2100 to 2020 cm^-1 when passing from isolated platinum atoms to small clusters. In addition, the cluster formation is accompanied by the appearance of additional bands (below 2000 cm^-1) associated with the bridging adsorption of CO on several neighboring Pt atoms at once [14, 65].”

 “The IR spectra were transformed into the Kubelka–Munk function [75]:

F(R) = a/s,  where a is the absorption and s is the scattering.”

 

3. In TEM, the d-spacing is determined from picture analysis or are they just indicated? For instance, in Fig. 6 b) and d), it is very hard to make a difference between 2.2, 2.6 and 2.8 A.

 

Thanks for the comment. This is a rather typical situation, when our eyes see a little (or no) difference between d-spacing on figures of the articles. Actually, the lattice d-spacing values were calculated from the fast Fourier transformation (FFT) patterns for planes visible in high-resolution TEM images  (See Materials and Methods) and inserted on figures of the manuscript. This approach is used widely and described elsewhere (https://doi.org/10.1016/j.apcatb.2017.02.038; https://doi.org/10.1016/j.cattod.2020.06.061; https://doi.org/10.1016/j.micromeso.2020.110089).

 

3. In lines 529-531, the sentence is somewhat overstated since it is valid only for one (hydrogen-rich) electrode side.

Thank you, we have made this clarification.

 

“In the H₂ excess they provide the 100% CO conversion in the wide temperature range from 50 to 130°C that corresponds to the tolerable CO values (below10 ppm) for hydrogen-rich electrode side of proton exchange membrane fuel cells (PEMFC).”

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript can be accepted in its present form.