Cobalt-Copper Oxide Catalysts for VOC Abatement: Effect of Co:Cu Ratio on Performance in Ethanol Oxidation
, Jana Balabánová 2, Martin Koštejn 2, Květuše Jirátová 2, Jean-Marc Giraudon 3, Jean-Francois Lamonier 3, Jaroslav Maixner 4 and František Kovanda 1,*Round 1
Reviewer 1 Report
The manuscript presents new results concerning the properties of CuO-Co3O4 catalysts in ethanol oxidation. It is clearly written and its technical quality is high. However before acceptance the following points should be fully addressed.
My main comment concerns the deactivation of the used catalysts. Given the easy reducibility of CuO, one can question if the used catalysts are reusable. This point is very important for any application and should be presented in the manuscript. Did the authors try several consecutive runs ? Another related question: did the authors characterize the spent catalysts ?
Another remark concerns the use of the term “mixed oxide”. In fact this expression is normally used when a true solid solution is formed. This is not the case in the present work. Therefore another designation should be used e.g. “mixture of oxides”, “CuO – Co3O4 composite oxides” or other.
Figure 3: the desorption branches of the N2 isotherms should be added to the figure.
Figure 8: units should be added to the Y axis. Arbitrary units cannot be used for the concentration axis.
Author Response
Response to Reviewer #1
The manuscript presents new results concerning the properties of CuO-Co3O4 catalysts in ethanol oxidation. It is clearly written and its technical quality is high. However before acceptance the following points should be fully addressed.
My main comment concerns the deactivation of the used catalysts. Given the easy reducibility of CuO, one can question if the used catalysts are reusable. This point is very important for any application and should be presented in the manuscript. Did the authors try several consecutive runs ? Another related question: did the authors characterize the spent catalysts?
Authors thank the Reviewer for this remark. No deactivation of the catalysts was observed in the four consecutive light-off experiments. This new remark was added into the manuscript (page 11). To the moment, the characterizations of the spent catalysts are not available.
Another remark concerns the use of the term “mixed oxide”. In fact this expression is normally used when a true solid solution is formed. This is not the case in the present work. Therefore another designation should be used e.g. “mixture of oxides”, “CuO – Co3O4 composite oxides” or other.
Authors thank the Reviewer for this remark. Many other authors cited in the References concluded formation of Co-Cu mixed oxides. Our results (mainly powder XRD and Raman spectroscopy) indicated that mixtures of Co3O4 and CuO were formed in the prepared Co-Cu samples and, therefore, we did not use the term « mixed oxides » for their designation. The manuscript was carefully checked to avoid this inconsistency.
Figure 3: the desorption branches of the N2 isotherms should be added to the figure.
The desorption branches were added to the N2 isotherms in the revised Figure 3.
Figure 8: units should be added to the Y axis. Arbitrary units cannot be used for the concentration axis.
Description of Y axes in Figure 8 was corrected ; the concentrations of ethanol, acetaldehyde, carbon monoxide (left) and carbon dioxide (right) are in ppm.
Reviewer 2 Report
The topic of this paper is relevant to the scope of the journal «Catalysis». It deals with preparing and studying of Co-Cu oxide catalysts. In order to investigate the activity of other types of mixed oxides, it was prepared the Co-Cu catalysts by heating of coprecipitated precursors and examined the effect of Co:Cu molar ratio on their physicochemical properties and catalytic performance in the ethanol total oxidation. I think this manuscript may be published after a minor revision as outlined below:
1. It was shown high activity and selectivity of the CoCu41 catalyst. The authors are aware of the fact formation of dispersed CuO particles on the surface of the Co3O4 (data of XRD, H2-TPR and XPS study) lead to high level of catalytic activity and selectivity. But CoCu11 allowed high level of catalytic activity and good selectivity also. How can it be explained that samples with different copper content have similar values of catalytic activity.
2. The role of the sodium is not enough clears from manuscript data.
3. What about catalysts stability?
Author Response
Response to Reviewer #2
The topic of this paper is relevant to the scope of the journal «Catalysis». It deals with preparing and studying of Co-Cu oxide catalysts. In order to investigate the activity of other types of mixed oxides, it was prepared the Co-Cu catalysts by heating of coprecipitated precursors and examined the effect of Co:Cu molar ratio on their physicochemical properties and catalytic performance in the ethanol total oxidation. I think this manuscript may be published after a minor revision as outlined below:
- It was shown high activity and selectivity of the CoCu41 catalyst. The authors are aware of the fact formation of dispersed CuO particles on the surface of the Co3O4 (data of XRD, H2-TPR and XPS study) lead to high level of catalytic activity and selectivity. But CoCu11 allowed high level of catalytic activity and good selectivity also. How can it be explained that samples with different copper content have similar values of catalytic activity.
Authors thank the Reviewer for this remark. The methods mentioned above demonstrated the superiority of the CoCu41 catalyst in terms of CuO dispersion. As stated in the Conclusions, the high activity and selectivity of the CoCu41 catalyst was attributed to the presence of finely dispersed CuO particles on the surface of Co3O4. In contrast to the CoCu11 and CoCu14 samples, evaluation of powder XRD data indicated formation of amorphous copper species (not detectable by XRD) in the CoCu41 catalyst. Presence of such (highly dispersed) copper species can explain the easy reduction of the CoCu41 catalyst due to promotion the hydrogen spillover from Cu to cobalt oxide species. Moreover, only the CoCu41 catalyst showed a strong surface enrichment with Cu and additional XPS peaks, indicating presence of Cu in lower oxidation state, which could be caused by X-ray induced reduction of highly dispersed CuO in the presence of adventitious carbon. All these findings are mentioned in the manuscript. On the other hand, the results of the characterization techniques for the CoCu11 catalyst showed a lower level of CuO dispersion (and consequently reducibility and catalytic activity) compared to the CoCu41 catalyst. Thus, we assume that the high dispersion of CuO on the Co3O4 surface is responsible for superior catalytic activity of the CoCu41 catalyst compared to the CoCu11 one, whose properties in terms of CuO dispersion, etc., are less pronounced.
- The role of the sodium is not enough clears from manuscript data.
Authors thank again the Reviewer for this remark. In general, the basicity of VOC oxidation catalysts can affect their selectivity by modifying the reaction mechanism. However, no such dependence was observed in our case. For example, the maximum acetaldehyde concentrations achieved during the catalytic experiment were practically the same for the Co and Cu catalysts (544 and 541 ppm, respectively), although the catalysts differed in Na concentrations (1.75 and 0.77 wt.%, respectively). This suggests that in this case the presence of sodium did not affect the performance of the catalysts.
- What about catalysts stability?
No deactivation of the catalysts was observed in the four consecutive light-off experiments. This new remark was added into the manuscript (page 11).
Reviewer 3 Report
The work is devoted to study of Cu-Co mixed oxide catalysts for ethanol oxidation.Paper is clear, well-written, and well-reasoned. The material is presented consistently and quite logically. The motivation and aims of work as well as the validity of the methods applied, does not raise any questions. I think that this work can be recomended for publication in Catalysts journal taking into an account next comments:
1) Catalytic activity is presented as T50 and T90 values. In case of Figure 8 aurthors discussed the correlation between T90 values and TPR-CO peak position followed by the conclusion about Mars-Van-Krevelen mechanism. Indeed, kinetic parameters (catalytic reaction rate) can be reliably estimated using the region with low conversion (usually <25%), while in the case of high conversions the absence of diffusion limitations must be confirmed by corresponding calcultions additionally. I strongly recommend submitting information about catalytic activity in form of specific catalytic rates (per gram or per m2) using adequate interval of ethanol conversions.
2) I propose to present all missing but discussed in the text data (for example, O1s, N1s, Na1s, Auger spectra, selectivity to products for all samples, etc.) as a part of Supporting information file.
3) The interpretation of peak near 1080.8 eV as Na1s signal is seems to be incorrect. Usually, Na1s is characterized by peak near 1071-1072 eV. Check it.
4) Aurthors did not discussed the negative values of ethanol conversion (cutted in Figure 5). It is evidently due to adsorption of Ethanol on the catalyst surface followed by its desorption. This moment should be considered.
6) No any information about product analysis during catalytic measurements can be found in Experimental data. I saw the link to work 54, but it will be useful to have direct description within the text.
7) Conclusion: "The XPS confirmed the presence of Co3O4 and CuO in all......" is seemed as incorrect. XPS is not structural data. Correctly discuss "the electronic surface states of Co and Cu similar to those in Co3O4 and CuO".
Author Response
Response to Reviewer #3
The work is devoted to study of Cu-Co mixed oxide catalysts for ethanol oxidation.Paper is clear, well-written, and well-reasoned. The material is presented consistently and quite logically. The motivation and aims of work as well as the validity of the methods applied, does not raise any questions. I think that this work can be recomended for publication in Catalysts journal taking into an account next comments:
1) Catalytic activity is presented as T50 and T90 values. In case of Figure 8 aurthors discussed the correlation between T90 values and TPR-CO peak position followed by the conclusion about Mars-Van-Krevelen mechanism. Indeed, kinetic parameters (catalytic reaction rate) can be reliably estimated using the region with low conversion (usually <25%), while in the case of high conversions the absence of diffusion limitations must be confirmed by corresponding calcultions additionally. I strongly recommend submitting information about catalytic activity in form of specific catalytic rates (per gram or per m2) using adequate interval of ethanol conversions.
Authors thank the Reviewer for this remark. As noted in the Experimental, the inlet ethanol concentration and the mass of catalyst used were the same in all catalytic experiments. It is generally accepted that the T50 temperature is used as a measure of catalytic performance in VOC oxidation, and it is assumed that at 50% conversion, the mass and heat transfer limitations are still sufficiently low and thus the performance of the catalysts can be reliably compared. Considering that the experimental conditions were the same in all catalytic tests, the specific reaction rate R calculated per gram of the catalyst using the T50 temperature would provide the same information (i.e., the R/T50 ratio would be the same for all the catalysts) as the T50 temperature alone. Therefore, the specific reaction rate was not included in the manuscript.
2) I propose to present all missing but discussed in the text data (for example, O1s, N1s, Na1s, Auger spectra, selectivity to products for all samples, etc.) as a part of Supporting information file.
Authors added Supporting information file containing additional figures, including the Cu LMM, N 1s, Na 1s, and O 1s XPS spectra, as well as the temperature dependences of the ethanol, acetaldehyde, carbon monoxide, and carbon dioxide concentrations measured over the examined catalysts, to the manuscript.
3) The interpretation of peak near 1080.8 eV as Na1s signal is seems to be incorrect. Usually, Na1s is characterized by peak near 1071-1072 eV. Check it.
Authors thank the Reviewer for the remark. Really, the peak Na 1s signal was mislabeled as 1080.8 eV. The correct reading should have been 1070.8 eV. The error has been corrected in the manuscript (page 10).
4) Aurthors did not discussed the negative values of ethanol conversion (cutted in Figure 5). It is evidently due to adsorption of Ethanol on the catalyst surface followed by its desorption. This moment should be considered.
Authors thank the Reviewer for this remark. All experiment were started at 50 °C after the equilibrium ethanol adsorption was reached at this temperature. The information was added to the description of the catalytic experiments (page 15).
6) No any information about product analysis during catalytic measurements can be found in Experimental data. I saw the link to work 54, but it will be useful to have direct description within the text.
Composition of the reaction mixture was analyzed by the Agilent 8890 gas chromatograph (Agilent Technologies, USA) coupled with a mass spectrometer. The information was added to the description of the catalytic experiments (page 15).
7) Conclusion: "The XPS confirmed the presence of Co3O4 and CuO in all......" is seemed as incorrect. XPS is not structural data. Correctly discuss "the electronic surface states of Co and Cu similar to those in Co3O4 and CuO".
The sentence in Conclusions was corrected to "The XPS results confirmed the presence of the electronic surface states of Co and Cu similar to those in Co3O4 and CuO in all Co-Cu oxide catalysts”.
Round 2
Reviewer 1 Report
The revised manuscript can be accepted.
