Modification of SmMn₂O₅ Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions

Round 1

Reviewer 1 Report

Authors prepared a series of Ag-modified manganese-mullite (SmMn2O5) catalysts with different Ag contents (1, 3, and 6 wt.%) via a citric acid sol-gel method. The prepared catalysts are characterized and tested for the catalytic soot oxidation. The work is interesting and authors presented the results well. I recommend the work for publication after minor revision.

From the TEM observation, it seems the nanoparticles are significantly aggregated. Comment on the significance on this.

Catalytic activity for soot oxidation section should be improved with more discussion. I suggest authors to compare the catalytic activity of their material with previously reported materials. 

Reusability and stability of the catalyst? comment on this.

Author Response

Reviewers #1

Authors prepared a series of Ag-modified manganese-mullite (SmMn₂O₅) catalysts with different Ag contents (1, 3, and 6 wt.%) via a citric acid sol-gel method. The prepared catalysts are characterized and tested for the catalytic soot oxidation. The work is interesting and authors presented the results well. I recommend the work for publication after minor revision.

Response: We sincerely thank the reviewer’s positive comment.

Comment 1. From the TEM observation, it seems the nanoparticles are significantly aggregated. Comment on the significance on this.

Response: Thanks for the comment. We added the corresponding discussion about the particle aggregation of the catalyst in Section 2.1.3 TEM observation in the revised manuscript.

Revision:

As the solid was obtained by calcination at 800 °C for 5 h, aggregation between nanoparticles occurred significantly, which leads to the reduction of the free surface with the secondary elimination of the grain boundary area and low specific surface area of the mixed oxides.

Comment 2. Catalytic activity for soot oxidation section should be improved with more discussion. I suggest authors to compare the catalytic activity of their material with previously reported materials.

Response: We thank the reviewer for raising this important issue. The comparison of the prepared catalysts with related literature was added and the corresponding instructions are added in Section 2.2 Catalytic activity for soot oxidation and Table S1 in Supporting information.

Revision: 

(Section 2.2)

The catalytic performance of this material was similar to those modified SmMn₂O₅ [37-39] and superior to those of many other previously reported compounds, such as Ag [40, 41], Pt [42, 43], Pd [44], Cu [45], and Ce-based catalysts [15, 46, 47], as briefly summarized in Table S1.

(Supporting Information)

Table S1. Comparative studies of catalytic activity for soot oxidation among various materials and our designed catalysts.

References

[S1] Chen, Y.; Shen, G.; Lang, Y.; Chen, R.; Jia, L.; Yue, J.; Shen, M.; Du, C.; Shan, B. Promoting soot combustion efficiency by strengthening the adsorption of NO_x on the 3DOM mullite catalyst. J. Catal. 2020, 384, 96-105, dor.org/10.1016/j.jcat.2020.02.006.

[S2] Feng, Z.; Liu, Q.; Chen, Y.; Zhao, P.; Peng, Q.; Cao, K.; Chen, R.; Shen, M.; Shan, B. Macroporous SmMn₂O₅ mullite for NO_x-assisted soot combustion. Catal. Sci.  Technol. 2017, 7, 838-847, dor.org/10.1039/c6cy02478e.

[S3] Chen, Y.; Du, C.; Lang, Y.; Jia, L.; Chen, R.; Shan, B. Carboxyl-modified colloidal crystal templates for the synthesis of three-dimensionally ordered macroporous SmMMNN₂O₅ mullite and its application in NO_x-assisted soot combustion. Catal. Sci.  Technol. 2018, 8, 5955-5962, dor.org/10.1039/c8cy01663a.

[S4] Nossova, L.; Caravaggio, G.; Couillard, M.; Ntais, S. Effect of preparation method on the performance of silver-zirconia catalysts for soot oxidation in diesel engine exhaust. Appl. Catal. B. 2018, 225, 538-549, dor.org/10.1016/j.apcatb.2017.11.070.

[S5] Huo, Z.; Zhao, P.; Miu, P.; Ren, L.; Tan, B.; Feng, N.; Wan, H.; Guan, G. Enhanced catalytic oxidation of soot over 3DOM LaMnO₃ by adding Ag and CeO₂: Improving the generation and delivery of active oxygen species. Appl. Surf. Sci. 2022, 600, 154204, dor.org/10.1016/j.apsusc.2022.154204.

[S6] Zhang, H.; Zhu, Y.; Wang, S.; Zhao, M.; Gong, M.; Chen, Y.. Activity and thermal stability of Pt/Ce_0.64Mn_0.16R_0.2O_x (R=Al, Zr, La, or Y) for soot and NO oxidation. Fuel Process. Technol. 2015, 137, 38-47, dor.org/10.1016/j.fuproc.2015.03.027.

[S7] Xu, H.; Zeng, L.; Cui, L.; Guo, W.; Gong, C.; Xue, G. In-situ generation of platinum nanoparticles on LaCoO₃ matrix for soot oxidation. J. Rare Earths. 2022, 40, 888-896, dor.org/10.1016/j.jre.2021.06.009.

[S8] Lee, J.; Jo, D.; Choung, J.; Kim, C.; Ham, H.; Lee, K-Y. Roles of noble metals (M = Ag, Au, Pd, Pt and Rh) on CeO₂ in enhancing activity toward soot oxidation: active oxygen species and DFT calculations. J. Hazard. Mater. 2021, 403, 124085, doi.org/10.1016/j.jhazmat.2020.124085.

[S9] López-suárez, F.; Bueno-lópez, A.; Illán-gómez, M.; Adamski, A.; Ura, B.; Trawczynski, J. Copper catalysts for soot oxidation:alumina versus perovskite supports. Environ. Sci. Technol. 2008, 42, 7670-7675, doi.org/10.1021/es8009293.

[S10] Wang, M.; Zhang, Y.; Yu, Y.; Shan, W.; He, H. Insight into the better performance of Co than Pt on Ce-Sn catalyst for soot oxidation. Fuel 2023, 346, 128379, doi.org/10.1016/j.fuel.2023.128379.

[S11] Piumetti, M.; Bensaid, S.; Russo, N.; Fino, D. Nanostructured ceria-based catalysts for soot combustion: Investigations on the surface sensitivity. Appl. Catal. B. 2015, 165, 742-751, doi.org/10.1016/j.apcatb.2014.10.062.

[S12] Guan, B.; Huang, Y.; Lin, H.; Huang, Z. Promoting effects of barium substitution on the catalytic performances of FeCeO_2−δ for soot oxidation. Ind. Eng. Chem. Res. 2018, 57, 8635-8646, doi.org/10.1021/acs.iecr.8b01005.

Comment 3. Reusability and stability of the catalyst? comment on this.

Response: Thank you for the good question. Although the reusability and stability of the catalyst is not the focus of this work, a cycled soot-TPO test was performed taking 3%Ag/SMO as an example, and the results are shown in Fig. S1. A short quotation about this result was also added in Section 2.2 Catalytic activity for soot oxidation in the revised manuscript.

Revision:

(Section 2.2)

The reusability of the catalysts was evaluated via a cycled soot-TPO test, taking 3%Ag/SMO as an example (Fig. S1). Compared with the fresh catalyst, the used counterparts even exhibit somewhat improved activity in O₂, maybe due to the redispersion of the metal on the mullite during the TPO runs.

(Supporting Information)

Compared with the fresh Ag/SMO, the used counterparts even exhibit somewhat improved activity in O₂, with a decrease of T₅₀ by 11-13 °C. It has been reported that SMO itself has superior thermal stability [S13], and silver nanoparticles can be redispersed upon high-temperature treatments in oxidizing conditions due to its low Tammann temperature (ca. 344 °C) [S14, S15]. In this case, Ag nanoparticles in Ag/SMO may be redispersed into smaller Ag nanoparticles during the TPO runs (with the ending temperature at 600 °C). Smaller Ag nanoparticles are more favorable for oxidation reactions [S16], which accounts for the improved activity of the used Ag/SMO catalyst.

Fig. S1. Cycled soot oxidation profiles of 3Ag/SMO. Reaction conditions: 10% O₂/N₂, GHSV = 100,000 h⁻¹, heating rate = 5 °C/min.

References

[S13] Yang, J.; Zhang, J.; Liu, X.; Duan, X.; Wen, Y.; Chen, R.; Shan, B. Origin of the superior activity of surface doped SmMn₂O₅ mullites for NO oxidation: A first-principles based microkinetic study. J. Catal. 2018, 359, 122-129, doi.org/10.1016/j.jcat.2018.01.002.

[S14] Shimizu, K.; Katagiri, M.; Satokawa, S.; Satsuma, A. Sintering-resistant and self-regenerative properties of Ag/SnO₂ catalyst for soot oxidation. Appl. Catal. B. 2011, 108-109, 39-46, doi.org/10.1016/j.apcatb.2011.08.003.

[S15] Qu, Z.; Huang, W.; Cheng, M.; Bao, X. Restructuring and redispersion of silver on SiO₂ under oxidizing/reducing atmospheres and its activity toward CO oxidation. J. Phys. Chem. B 2005, 109, 15842–15848, doi.org/10.1021/jp050152m.

[S16] Bukhtiyarov, A.; Stakheev, A.; Mytareva, A.; Prosvirin, I.; Bukhtiyarov, V. In situ XPS study of the size effect in the interaction of NO with the surface of the model Ag/Al₂O₃/FeCrAl catalysts. Russ. Chem. Bull. 2015, 64, 2780-2785,doi.org/10.1007/s11172-015-1225-7.

Author Response File: Author Response.pdf

Reviewer 2 Report

The presented paper devoted to study a series of manganese-mullite (SmMn₂O₅) modified by silver with different loadings for soot oxidation. The main aim of this work was to find a correlation between amount of introduced silver and catalytic performance in soot oxidation. To characterize the prepared catalysts the combination of complement physico-chemical methods such as XRD, XPS, TEM, Raman spectroscopy and others have been applied. Overall, it is an interesting study and the data obtained are new and actual. The test and analysis procedures are detailed and reliable. The manuscript can be accepted with the following revisions:

1)      Page 6. Lines 169-170. “XPS measurements are influenced by some factors, such as the penetration depth (generally a few nanometers)…”. Usually, term of “penetration depth” uses for X-rays which achieve several micrometers, while escape depth of photoelectron from the matter is actually a few nanometers. The authors are kindly asked to use correct terminology such as depth of probing, depth of analysis or something else like this instead of “penetration depth”.

2)      Figure 4. “Ag3d” and “O1s” are confused in Figure caption. Also, to increase visability of XPS spectra presented in Figure 4, perhaps, it would be helpful to make thinner curves corresponding to raw XPS spectrum (black curve) and fitted one (red curve).

3)      How many particles have been used for building of histograms and the mean particle sizes presented in Figure 3? It should be mentioned in the paper.

4)      The authors are kindly asked to add information to Experimental part concerning XPS analysis: how surface elemental composition has been calculated, what type of background were used, how binding energy scale was preliminary calibrated etc.

5)      Perhaps, it would be also helpful to add information concerning the carbon content on the surface in Table 2 and C1s spectra as well. Taking into account that the studied materials have been prepared from citric acid, some impurities of the latter one could be presented on the surface of Ag/SMO catalysts. If there is only adventitious carbon on the surface of the studied materials, it should be also mentioned. It is essential important since remaining citric acid could give signal to O1s spectra close to O_ads discussed in the paper.

6)      From presented text, it is not clear at which conversion of soot the selectivities given in Table 3 have been calculated.

7)      The highest shift of a peak assigned to Mn-O stretching vibrations in Raman spectra presented in Figure 2 is observed for 3Ag/SMO catalyst. However, the corresponding discussion and further comparison with data obtained by other methods are missing, while they are in good agreement. The authors are kindly asked to extend corresponding discission by stressing it.

8)      The standard X-ray diffraction pattern of SmMn₂O₅ phase should be given for comparison on the Figure 1a.

Author Response

Reviewers #2

The presented paper devoted to study a series of manganese-mullite (SmMn₂O₅) modified by silver with different loadings for soot oxidation. The main aim of this work was to find a correlation between amount of introduced silver and catalytic performance in soot oxidation. To characterize the prepared catalysts, the combination of complement physico-chemical methods such as XRD, XPS, TEM, Raman spectroscopy and others have been applied. Overall, it is an interesting study and the data obtained are new and actual. The test and analysis procedures are detailed and reliable. The manuscript can be accepted with the following revisions.

Response: We sincerely thank the reviewer’s positive comment.

Comment 1. Page 6. Lines 169-170. “XPS measurements are influenced by some factors, such as the penetration depth (generally a few nanometers)…”. Usually, term of “penetration depth” uses for X-rays which achieve several micrometers, while escape depth of photoelectron from the matter is actually a few nanometers. The authors are kindly asked to use correct terminology such as depth of probing, depth of analysis or something else like this instead of “penetration depth”.

Response: Thanks for the kind reminder. We changed the “penetration depth” to “probing depth” in Section 2.1.4 XPS analysis in the revised manuscript.

Revision:

  …such as the probing depth (generally a few nanometers)…

Comment 2. Figure 4. “Ag 3d” and “O 1s” are confused in Figure caption. Also, to increase visibility of XPS spectra presented in Figure 4, perhaps, it would be helpful to make thinner curves corresponding to raw XPS spectrum (black curve) and fitted one (red curve).

Response: Thanks for the good suggestion. we revised the figure caption and redrew the Fig. 4 as suggested by the reviewer.

Revision:

Fig. 4. (a) Mn 2p, (b) O 1s and (c) Ag 3d XPS spectra of the catalysts.

Comment 3. How many particles have been used for building of histograms and the mean particle sizes presented in Figure 3? It should be mentioned in the paper.

Response: Thanks for the comment. The statistical method of Ag size distribution was specified in Section 2.1.3 in the revised manuscript.

Revision:

100-150 silver nanoparticles were measured for size distribution statistics of each sample.

Comment 4. The authors are kindly asked to add information to Experimental part concerning XPS analysis: how surface elemental composition has been calculated, what type of background were used, how binding energy scale was preliminary calibrated etc.

Response: Thanks for the good suggestion. As suggested by the reviewer, the corresponding information about XPS analysis was added in Section 3.3 Characterizations in the revised manuscript.

Revision:

The elemental composition of the catalyst was derived through a simple conversion formula according to the atomic ratios given by XPS without applying any standardization procedure. To obtain more information of Mn and O species, the obtained XPS spectra were deconvoluted employing the XPS peak software after deducting the background signals using the Shirley algorithm. The C 1s line of adventitious hydrocarbon on air exposed samples with a binding energy of 284.6 eV was used as the reference to calibrate the XPS results.

Comment 5. Perhaps, it would be also helpful to add information concerning the carbon content on the surface in Table 2 and C 1s spectra as well. Taking into account that the studied materials have been prepared from citric acid, some impurities of the latter one could be presented on the surface of Ag/SMO catalysts. If there is only adventitious carbon on the surface of the studied materials, it should be also mentioned. It is essential important since remaining citric acid could give signal to O1s spectra close to O_ads discussed in the paper.

Response: It is a good question. Citric acid sol-gel is a conventional preparation method, which has been widely applied in the literature (Appl. Catal. B, 102 (2011) 251-259; Appl. Catal. B, 225 (2018) 538-549; Catalysts, 9 (2019) 483). For example, Nossova et al. (Appl. Catal. B, 225 (2018) 538-549) prepared Ag/ZrO₂ catalysts by citric acid-assisted sol-gel (SG) and incipient wetness impregnation (IM). They systematically analyzed the XPS peaks of C 1s in Ag/ZrO₂. They found three peaks at ca. 285, 286.5 and 289 eV due to C-C, C=O, and free COO- species, respectively (Fig. R1). There was no significant difference about C 1s XPS peaks among the two samples. As the Ag(NO₃)₃ precursor used in the impregnation process did not contain any carbon species, this similarity implied that the carbon in citric acid could be even removed at a calcination temperature of 550 °C, and the carbon in the final samples was mainly related to the adsorption of hydrocarbons and CO₂ from the ambient environment.

In our work, the mullite was synthesized by citric acid-assisted sol-gel method and was calcined at 800 °C, which was much higher than that adopted in Nossova’s study. As shown in Fig. S2, the signals assigned to C=O and free COO- species are much weaker and are related to the adsorption of hydrocarbons and CO₂ from the ambient environment. The C 1s line of adventitious hydrocarbon on air exposed samples with a binding energy of 284.6 eV was used as the reference to calibrate the XPS results. The corresponding figure and analysis were added in Supporting Information.

Fig. R1. C 1s XPS spectra for the Ag (30)/ZrO₂-SG (A) and Ag(30)/ZrO₂-IM (B) catalysts.

Revision:

(Section 3.3)

The C 1s line of adventitious hydrocarbon on air exposed samples with a binding energy of 284.6 eV was used as the reference to calibrate the XPS results.

(Supporting Information)

In this work, the mullite was synthesized by citric acid-assisted sol-gel method and was calcined at 800 °C for 5 h. The C 1s spectra of the samples are shown in Fig. S2, with three peaks at ca. 285, 286.5 and 289 eV assgined to C-C, C=O, and free COO- species, respectively [S17]. The signals of C 1s spectra are related to the adsorption of hydrocarbons and CO₂ from the ambient environment. The citric acid-assisted sol-gel method appeared to have no influence on the presence of carbon on the catalyst surface.

Fig. S2. C 1s spectra for the catalysts.

References

[S17] Nossova, L; Caravaggio, G.; Couillard, M.; Ntais, S. Effect of preparation method on the performance of silver-zirconia catalysts for soot oxidation in diesel engine exhaust. Appl. Catal. B. 2018, 225, 538-549, doi.org/10.1016/j.apcatb.2017.11.070.

Comment 6. From presented text, it is not clear at which conversion of soot the selectivities given in Table 3 have been calculated.

Response: Thanks for figuring out the missing information. The selectivity towards CO₂ was obtained based on the total CO_x emitted during the soot-TPO test instead of CO_x emitted at a certain soot conversion or a certain reaction temperature. The calculation was updated in Section 3.4 in the revised manuscript.

Revision:

C_CO and C_CO2 were defined as the total CO and CO₂ release in the outlet gas during the soot-TPO test, respectively, which were obtained by integrating the outlet CO_x concentrations over time.

Comment 7. The highest shift of a peak assigned to Mn-O stretching vibrations in Raman spectra presented in Figure 2 is observed for 3Ag/SMO catalyst. However, the corresponding discussion and further comparison with data obtained by other methods are missing, while they are in good agreement. The authors are kindly asked to extend corresponding discussion by stressing it.

Response: Thanks for the good suggestion. We did omit this experimental evidence. The corresponding discussion was added in Section 2.1.2 and 2.1.2, respectively.

Revision:

In Section 2.1.1:

Overall, a largest crystal cell shrinkage occurs over 3Ag/SMO, implying the maximum lattice distortion in the mullite resulting from the addition of 3 wt.% silver.

In Section 2.1.2:

The highest shifts appear in the Raman peaks of 3Ag/SMO due to creation of more lattice defects, which agrees with the variations of crystal cell parameters obtained by XRD.

Comment 8. The standard X-ray diffraction pattern of SmMn₂O₅ phase should be given for comparison on the Figure 1a.

Response: Thanks for the good suggestion. The XRD pattern of We updated Fig. 1a in the revised manuscript.

Revision:

Fig. 1. (a) Normal and (b) slow-scanning XRD patterns of the catalysts.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thanks for the authors' reply. The manuscript has been greatly improved. I have no more questions.