Cerium-Copper Oxides Synthesized in a Multi-Inlet Vortex Reactor as Effective Nanocatalysts for CO and Ethene Oxidation Reactions

Round 1

Reviewer 1 Report

1. Please explain why oxygen was not analyzed in the EDX and ICP studies. The authors sum the percentage of copper and cerium, in each case the sum is 100%. Where is the oxygen?
2. line 217-218: "Raman spectroscopy is used to study surface groups, not structural studies as the authors indicated.
3. Catalytic studies: please determine the thermodynamic equilibrium and again discuss the results based on this  thermodynamic equilibrium. 

Author Response

Replies to Reviewer’s comments (Please, read the attached document)

The relative modifications for each Reviewer are evidenced in different colors in the article text

Replies to Reviewer #1 (the modifications are evidenced in green color)

Comments and Suggestions for Authors:

1. Please explain why oxygen was not analyzed in the EDX and ICP studies. The authors sum the percentage of copper and cerium, in each case the sum is 100%. Where is the oxygen?

Reply: Thank you for your question. In the article, we used both EDX and ICP techniques in order to check the correspondence between the nominal composition and the actual one. ICP is one of the most reliable techniques to assess the metal content in a mixed oxide catalyst. However, the analytical procedure involves an acid treatment to solubilize the metallic cations, which was carried out using a solution of ascorbic acid, hydrochloric acid and nitric acid in water. In this case, the quantity of oxygen initially present in the metal oxide cannot be quantified, since this element is also present in the solvent (water) and in the acids. Hence, we have expressed the metal content in terms of percentage of Ce and Cu ions with respect to the total quantity of cations (Ce + Cu) in the materials, without considering oxygen. In order to allow a direct comparison, we reported the EDX results using the same notation. An analogous way of expressing the chemical composition (without considering oxygen) has also been frequently used in the literature [1–8]. Both the techniques evidenced a good agreement between the nominal composition and the actual one. Nevertheless, for the sake of clarity, we modified the caption of Table 2, explicitly describing this type of percentage. Moreover, we added a new table in the supporting information section (Table S1) that includes the atomic percentages of oxygen and the other elements, as measured by EDX. As well, we added a more complete description of the experimental method used for ICP-MS in the Materials and Methods section.

Literature references

[1] P. Venkataswamy, K.N. Rao, D. Jampaiah, B.M. Reddy, Nanostructured manganese doped ceria solid solutions for CO oxidation at lower temperatures, Appl. Catal. B Environ. 162 (2015) 122–132. https://doi.org/10.1016/j.apcatb.2014.06.038.

[2] J. Liu, Y. Li, H. Liu, D. He, Transformation of CO₂ and glycerol to glycerol carbonate over CeO2–ZrO2 solid solution —— effect of Zr doping, Biomass and Bioenergy. 118 (2018) 74–83. https://doi.org/10.1016/j.biombioe.2018.08.004.

[3] E. Sartoretti, C. Novara, A. Chiodoni, F. Giorgis, M. Piumetti, S. Bensaid, N. Russo, D. Fino, Nanostructured ceria-based catalysts doped with La and Nd: How acid-base sites and redox properties determine the oxidation mechanisms, Catal. Today. (2022). https://doi.org/10.1016/J.CATTOD.2021.11.040.

[4] H. Wang, B. Jin, H. Wang, N. Ma, W. Liu, D. Weng, X. Wu, S. Liu, Study of Ag promoted Fe₂O₃@CeO₂ as superior soot oxidation catalysts: The role of Fe₂O₃ crystal plane and tandem oxygen delivery, Appl. Catal. B Environ. 237 (2018) 251–262. https://doi.org/10.1016/j.apcatb.2018.05.093.

[5] K. Kim, J. Do Yoo, S. Lee, M. Bae, J. Bae, W.C. Jung, J.W. Han, A Simple Descriptor to Rapidly Screen CO Oxidation Activity on Rare-Earth Metal-Doped CeO₂: From Experiment to First-Principles, ACS Appl. Mater. Interfaces. 9 (2017) 15449–15458. https://doi.org/10.1021/acsami.7b01844.

[6] Y. Wang, D. Yang, S. Li, L. Zhang, G. Zheng, L. Guo, Layered copper manganese oxide for the efficient catalytic CO and VOCs oxidation, Chem. Eng. J. 357 (2019) 258–268. https://doi.org/10.1016/j.cej.2018.09.156.

[7] S. Fernandez-garcia, L. Jiang, M. Tinoco, A.B. Hungria, J. Han, G. Blanco, J.J. Calvino, X. Chen, Enhanced Hydroxyl Radical Scavenging Activity by Doping Lanthanum in Ceria Nanocubes, J. Phys. Chem. C. 120 (2016) 1891–1901. https://doi.org/10.1021/acs.jpcc.5b09495.

[8] C. Herzig, J. Frank, A. Nenning, M. Gerstl, A. Bumberger, J. Fleig, A.K. Opitz, A. Limbeck, Combining electrochemical and quantitative elemental analysis to investigate the sulfur poisoning process of ceria thin film fuel electrodes †, (2022). https://doi.org/10.1039/d1ta06873c.

Changes:

2.2 Characterization techniques section

[…]The elemental composition of the samples was also investigated using an Inductively Coupled Plasma Mass Spectrometer (iCAP Q ICP-MS). For this analysis, 100 mg of each sample were dissolved in an aqueous solution of hydrochloric acid (1 M), nitric acid (1M) and ascorbic acid (0.5 M), which was then stirred for 8 h to completely solubilize the powder. […]

3.1. Structural and textural properties

Catalyst	Elemental composition (wt.%)
	EDX*			ICP
	Ce	Cu	Tot.	Ce	Cu	Tot.
5%CuCeOx	97	3	100	97	3	100
10%CuCeOx	92	8	100	90	10	100
20%CuCeOx	82	18	100	79	21	100
30%CuCeOx	67	33	100	67	33	100
60%CuCeOx	32	68	100	37	63	100

 […]Table 2. EDX and ICP analysis of the CuCeO_x catalysts: the elemental content is expressed in percentage of Ce and Cu with respect to the total amount of cations (Ce + Cu).

 

 

 

 

 

 

 

 

 

^*The values are estimated over three different areas.

[…]

Supporting information section

Table S1. EDX* of the CuCeO_x catalysts: the elemental content is expressed in percentage (wt.%) of each element in the catalysts.

Catalysts	Ce	O	Cu	Tot.
5%CuCeOx	78	19	2	100
10%CuCeOx	72	22	6	100
20%CuCeOx	63	24	14	100
30%CuCeOx	55	18	27	100
60%CuCeOx	26	20	54	100

 

 

 

 

 

*The values are estimated over three different areas.

 

 

1. line 217-218: "Raman spectroscopy is used to study surface groups, not structural studies as the authors indicated.

Reply: We agree with you that Raman spectroscopy can be fruitfully employed for the study of surface groups. However, it can also provide interesting information about the structural properties of crystalline solids, since many solid oxides are characterized by Raman-active vibrational modes [9–15]. This technique can be therefore used to investigate which phases are present in some samples, as well as their degree of order or disorder [13,16]. As also reported in the literature, the Raman spectra of ceria-based oxides can give insights into the presence of different types of defects in the crystal lattice [17–20]. Their quantity can even be estimated and compared through the calculation of the D/F_2g ratio [21,22]. Moreover, the formation of a segregated CuO phase can be observed [11]. For all these reasons, we decided to employ Raman spectroscopy for the study of our catalytic materials, gaining new information about the structural properties of these mixed oxides.

Literature references

[9] J. Wang, J. Chen, L. Peng, H. Zhang, Z. Jiang, K. Xiong, Q. Yang, J. Chen, N. Yang, On the CuO-Mn₂O₃ oxide-pair in CuMnO_x multi-oxide complexes: Structural and catalytic studies, Appl. Surf. Sci. 575 (2022). https://doi.org/10.1016/j.apsusc.2021.151733.

[10]K.H. Cho, S. Park, H. Seo, S. Choi, M.Y. Lee, C. Ko, K.T. Nam, Capturing Manganese Oxide Intermediates in Electrochemical Water Oxidation at Neutral pH by In Situ Raman Spectroscopy, Angew. Chemie - Int. Ed. 60 (2021) 4673–4681. https://doi.org/10.1002/anie.202014551.

[11]K.R. Reddy, Green synthesis, morphological and optical studies of CuO nanoparticles, J. Mol. Struct. 1150 (2017) 553–557. https://doi.org/10.1016/j.molstruc.2017.09.005.

[12]M. Verma, V. Kumar, A. Katoch, Sputtering based synthesis of CuO nanoparticles and their structural, thermal and optical studies, Mater. Sci. Semicond. Process. 76 (2018) 55–60. https://doi.org/10.1016/j.mssp.2017.12.018.

[13]S. Ballauri, E. Sartoretti, C. Novara, F. Giorgis, M. Piumetti, D. Fino, N. Russo, S. Bensaid, Wide range temperature stability of palladium on ceria-praseodymia catalysts for complete methane oxidation, Catal. Today. (2021). https://doi.org/10.1016/J.CATTOD.2021.11.035.

[14]A. Baylet, P. Marécot, D. Duprez, P. Castellazzi, G. Groppi, P. Forzatti, In situ Raman and in situ XRD analysis of PdO reduction and Pd° oxidation supported on γ-Al₂O₃ catalyst under different atmospheres, Phys. Chem. Chem. Phys. 13 (2011) 4607–4613. https://doi.org/10.1039/c0cp01331e.

[15]W.H. Weber, R.J. Baird, G.W. Graham, Raman investigation of palladium oxide, rhodium sesquioxide and palladium rhodium dioxide, J. Raman Spectrosc. 19 (1988) 239–244. https://doi.org/10.1002/jrs.1250190404.

[16]N. Guillén-Hurtado, J. Giménez-Mañogil, J.C. Martínez-Munuera, A. Bueno-López, A. García-García, Study of Ce/Pr ratio in ceria-praseodymia catalysts for soot combustion under different atmospheres, Appl. Catal. A Gen. 590 (2020) 117339. https://doi.org/10.1016/j.apcata.2019.117339.

[17]Z. Wu, M. Li, J. Howe, H.M. Meyer, S.H. Overbury, Probing defect sites on CeO₂ nanocrystals with well-defined surface planes by raman spectroscopy and O₂ adsorption, Langmuir. 26 (2010) 16595–16606. https://doi.org/10.1021/la101723w.

[18]S. Agarwal, X. Zhu, E.J.M. Hensen, L. Lefferts, B.L. Mojet, Defect chemistry of ceria nanorods, J. Phys. Chem. C. 118 (2014) 4131–4142. https://doi.org/10.1021/jp409989y.

[19]C. Andriopoulou, A. Trimpalis, K.C. Petallidou, A. Sgoura, A.M. Efstathiou, S. Boghosian, Structural and Redox Properties of Ce_{1– x}Zr_xO_2−δ and Ce_0.8Zr_0.15RE_0.05O_2−δ (RE: La, Nd, Pr, Y) Solids Studied by High Temperature in Situ Raman Spectroscopy, J. Phys. Chem. C. 121 (2017) 7931–7943. https://doi.org/10.1021/acs.jpcc.7b00515.

[20]E. Sartoretti, C. Novara, M. Fontana, F. Giorgis, M. Piumetti, S. Bensaid, N. Russo, D. Fino, New insights on the defect sites evolution during CO oxidation over doped ceria nanocatalysts probed by in situ Raman spectroscopy, Appl. Catal. A Gen. 596 (2020) 117517. https://doi.org/10.1016/j.apcata.2020.117517.

[21]S. Agarwal, X. Zhu, E.J.M. Hensen, B.L. Mojet, L. Lefferts, Surface-Dependence of Defect Chemistry of Nanostructured Ceria, J. Phys. Chem. C. 119 (2015) 12423–12433. https://doi.org/10.1021/acs.jpcc.5b02389.

[22]E. Sartoretti, C. Novara, F. Giorgis, M. Piumetti, S. Bensaid, N. Russo, D. Fino, In situ Raman analyses of the soot oxidation reaction over nanostructured ceria-based catalysts, Sci. Rep. 9 (2019) 3875. https://doi.org/10.1038/s41598-019-39105-5.

 

1. Catalytic studies: please determine the thermodynamic equilibrium and again discuss the results based on this thermodynamic equilibrium. 

Reply: Thank you for the interesting suggestion. We studied the thermodynamic equilibrium for the CO oxidation reaction (CO + ½ O₂ ↔ CO₂) through a process simulation (which was carried out using the software ASPEN Plus), in order to study the equilibrium composition of the gas mixture used during the catalytic tests (1000 ppm CO + 10% O₂ in N₂) as a function of the temperature. As reported in the following graph (Figure A), a real thermodynamic equilibrium between CO and CO₂ in these conditions only exists at very high temperature, namely above 2000 °C. The oxidation temperatures of our tests are instead much lower (see the light blue area in the picture). In other words, the catalytic phenomena studied occur in a temperature range far from the thermodynamic equilibrium, in which the complete CO oxidation to CO₂ is thermodynamically favored.

Similar considerations can be outlined for the ethene oxidation reaction: at the studied temperatures (25 – 500 °C) and reaction conditions (ambient pressure, gas containing 500 ppm of ethene + 10% of O₂ in N₂), the thermodynamic equilibrium is totally shifted towards the formation of the complete oxidation products (namely CO₂ and H₂O), while the formation of ethene is not thermodynamically favored.

 

Figure A. Thermodynamic equilibrium composition of the gaseous mixture fed to the reactor during the CO oxidation catalytic tests (1000 ppm CO + 10% O₂ in N₂) at ambient pressure as a function of the temperature, in terms of ratio between the CO or CO₂ moles and the total moles of CO_X = CO + CO₂. The light blue area highlights the temperature range in which the catalytic activity of our samples was investigated.

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper reports the study of performed a systematic study In this study, a set of CuCeOx catalysts was prepared via the coprecipitation method using a Multi-Inlet Vortex Reactor: the Cu wt.% content is 5, 10, 20, 30 and 60. Moreover, pure CeO2 and CuO were synthesized for comparison purposes. The physico-chemical properties of this set of samples were investigated by complementary techniques, e.g., XRD, N2 physisorption at -196 °C, Scanning Electron Microscopy, XPS, FT-IR, Raman spectroscopy and H2-TPR. Then, the CuCeOx catalysts were tested for the CO and ethene oxidation reactions. As a whole, all the prepared samples presented good catalytic performances towards the CO oxidation reaction (1000 ppm CO, 10 vol.% 18 O2/N2): the most promising catalyst was the 20%CuCeOx (complete CO conversion at 125 °C), which exhibited a long-term thermal stability. Similarly, the oxidative activity of the catalysts were evaluated using a gaseous mixture containing 500 ppm C2H4, 10 vol.% O2/N2. Accordingly, for the ethene oxidation reaction, the 20%CuCeOx catalyst evidenced the best catalytic properties. The elevated catalytic activity towards CO and ethene oxidation was mainly ascribed to synergistic interactions between CeO2 and CuO phases, as well as to the high amount of surface-chemisorbed oxygen species and structural defects. The experimental results are interesting and informative, but some data were not well presented. Details are listed below.

 

• Authors synthesized cerium-copper oxides catalysts.The author presented the compositional analysis of metals with EDS. However, since EDS is not accurate, it is necessary to add data by analyzing it with ICP-AES..
• The author need to add data confirmed though EDS mapping to determine whether it is alloy of CuCeOx.

Author Response

Replies to Reviewer #2 (the modifications are evidenced in blue color). Please, read the attached document.

Comments and Suggestions for Authors

This paper reports the study of performed a systematic study In this study, a set of CuCeOx catalysts was prepared via the coprecipitation method using a Multi-Inlet Vortex Reactor: the Cu wt.% content is 5, 10, 20, 30 and 60. Moreover, pure CeO2 and CuO were synthesized for comparison purposes. The physico-chemical properties of this set of samples were investigated by complementary techniques, e.g., XRD, N2 physisorption at -196 °C, Scanning Electron Microscopy, XPS, FT-IR, Raman spectroscopy and H2-TPR. Then, the CuCeOx catalysts were tested for the CO and ethene oxidation reactions. As a whole, all the prepared samples presented good catalytic performances towards the CO oxidation reaction (1000 ppm CO, 10 vol.% 18 O2/N2): the most promising catalyst was the 20%CuCeOx (complete CO conversion at 125 °C), which exhibited a long-term thermal stability. Similarly, the oxidative activity of the catalysts were evaluated using a gaseous mixture containing 500 ppm C2H4, 10 vol.% O2/N2. Accordingly, for the ethene oxidation reaction, the 20%CuCeOx catalyst evidenced the best catalytic properties. The elevated catalytic activity towards CO and ethene oxidation was mainly ascribed to synergistic interactions between CeO2 and CuO phases, as well as to the high amount of surface-chemisorbed oxygen species and structural defects.

The experimental results are interesting and informative, but some data were not well presented. Details are listed below.

 

1. Authors synthesized cerium-copper oxides catalysts. The author presented the compositional analysis of metals with EDS. However, since EDS is not accurate, it is necessary to add data by analyzing it with ICP-AES.

Reply: We agree with you on the fact that the ICP is a more accurate technique than EDS to evaluate the chemical composition of a solid. Indeed, we have performed this type of analysis, using ICP-MS. The obtained results are presented in Table 2, together with the EDX results for the sake of comparison (this Table is also reported in this document, few lines below). As a whole, the two techniques are in good agreement and the actual proportion between Ce and Cu atoms is in line with the nominal one. Moreover, we clarified the caption of Table 2 and we added some details about the procedure used for ICP-MS (which can be found in the Materials and Methods section).

Changes:

2.2 Characterization techniques section

[…]The elemental composition of the samples was also investigated using an Inductively Coupled Plasma Mass Spectrometer (iCAP Q ICP-MS). For this analysis, 100 mg of each sample were dissolved in an aqueous solution of hydrochloric acid (1 M), nitric acid (1M) and ascorbic acid (0.5 M), which was then stirred for 8 h to completely solubilize the powder. […]

3.1. Structural and textural properties

Table 2. EDX and ICP analysis of the CuCeO_x catalysts: the elemental content is expressed in percentage of Ce and Cu with respect to the total amount of cations (Ce + Cu).

Catalyst	Elemental composition (wt.%)
	EDX*			ICP
	Ce	Cu	Tot.	Ce	Cu	Tot.
5%CuCeOx	97	3	100	97	3	100
10%CuCeOx	92	8	100	90	10	100
20%CuCeOx	82	18	100	79	21	100
30%CuCeOx	67	33	100	67	33	100
60%CuCeOx	32	68	100	37	63	100

 

 

 

 

1. The author need to add data confirmed though EDS mapping to determine whether it is alloy of CuCeOx.

Reply: Thank you for your observation. The EDS mapping could be an interesting technique for the estimation of the elemental dispersion on the catalysts. However, as you mentioned in your comments, the EDS is not highly precise as a quantification technique for quantification. Moreover, the mapping resolution in a FESEM microscope unfortunately does not allow to reliably investigate small nanoparticles such as those in our samples (<20 nm, see Table 1 and Fig. 2). Nevertheless, thanks to the information about the chemical composition and textural properties of the catalysts obtained with other types of analyses, we observed the presence of metal oxides and not of metallic alloys in our samples.

If there is a metallic alloy, it should be detected by XRD analysis, since alloys most likely have a different textural organization comparing with cerium oxide or copper oxide. In our analysis, as evidenced by Figure 1, no additional peaks can be assigned to specific alloys, while the typical peaks of cerium oxide or copper oxide were detected. Furthermore, the XPS analysis does not evidence metallic alloys (Figure 5). The high-resolution XPS spectrum of Cu, for example, pointed out the presence of Cu²⁺, while metallic Cu is absent. Analogous results were obtained by the deconvolution of Ce spectrum. The O 1s XPS spectrum deconvolution shows a shift to higher Binding Energy when the copper content increases, suggesting that two difference oxide phases are present (i.e., cerium oxide and copper oxide), at least in the Cu-rich samples. Raman spectroscopy also evidenced the presence of cerium oxide; furthermore, it highlighted a progressive distortion of ceria structure with increasing the copper loading, suggesting that a part of copper ions are directly inserted inside the ceria crystal lattice. At high Cu loading, some Raman peaks indicated the presence of CuO. As a whole, all these results seem to exclude the presence of metallic cerium or metallic copper alloys inside our samples.

Changes:

In 3.2.1. Surface oxidation states

[…] The Cu LMM Auger spectra are reported in Figure S3. The Auger peaks are centered in the range 917.0-917.7 eV, confirming the presence of  Cu²⁺ species in the CuO phase and the absence of Cu metallic phase which could be ascribed to the presence of possible alloy [74,77,78]. […]

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you to the author for correction. Accept