A Study of Support Effects for the Water-Gas-Shift Reaction over Cu
Round 1
Reviewer 1 Report
The authors have tried to find out the effect of support on Cu/MAO catalysts for WGS. This paper presents an interesting result regarding the systematic changes of catalyst composition. This paper can be published after major revision.
1. Please provide CO conversion as a percent form.
2. The authors conducted WGS experiments under ideal conditions with only CO and H2O. Can you provide the activity of a catalyst developed under real conditions?
3. Note the use of subscripts in the text and references.
Author Response
The authors have tried to find out the effect of support on Cu/MAO catalysts for WGS. This paper presents an interesting result regarding the systematic changes of catalyst composition. This paper can be published after major revision.
- Please provide CO conversion as a percent form.
To ensure that we maintained differential conditions, the conversions were less than 15% in all cases. Indeed, we varied the amount of catalyst used in some experiments in order to maintain differential conditions. Therefore, we do not feel the conversion in any given experiment is meaningful. We have tried to clarify this point in the revised manuscript.
- The authors conducted WGS experiments under ideal conditions with only CO and H2O. Can you provide the activity of a catalyst developed under real conditions?
As stated in the manuscript, the goal of this work was to understand how Cu interacts with different support oxides, rather than develop a catalyst with optimal properties. That is why we chose to work with low Cu loadings under differential reaction conditions. Nonetheless, the trends and findings from this work should still be valid at higher pressures and in the presence of reaction products.
- Note the use of subscripts in the text and references.
We corrected the subscripts in Figure 2, Conclusions and References.
Reviewer 2 Report
Please check the attachment.
Comments for author File: 
Comments.pdf
Author Response
In this manuscript, authors reported some catalysts prepared by ALD methods, presenting good performance of WGS reaction. However, some contents in this manuscript need to be updated and more information need to be provided to make the manuscript to be published. Some questions are listed below:
Question I: In the introduction, author didn’t m mention why they choose to use MAO as support. From previous WGS studies, Cu species or interface of Cu-metal oxide was discussed as catalytic sites and inert oxide materials were applied to be supports dispersing active Cu species. Could author comment the advantages or benefits of using MAO?
Although Al2O3 is the most common support for catalysts, it can react with some of the oxides that we deposited. Because MAO is a compound oxide, it is less reactive with other oxides. We have modified the Introduction in order to clarify this point.
Question II: The control experiment of MAO support for WGS should be included.
This experiment was performed, and we now include a statement in the revised manuscript, stating that the MAO had no activity for WGS.
Question III: Cu dispersion and particle size of Cu on MAO with ZnO, CeO2, Mn2O3, CoO films cannot be measured by N2O chemisorption. Did authors check with STEM and EDS? high dispersion of Cu on these metal oxide films could be an important effect to activities of WGS.
We did attempt to use STEM and EDS to examine these surfaces. However, the low weight loading of Cu and poor contrast of Cu with heavier elements like Zn, Mn and Co made the results difficult to interpret. Since the catalysts were prepared using similar vapor-phase methods, we are confident the dispersions are similar. A statement to this effect was added.
Question IV: Besides of reaction rate, authors could also demonstrate the conversion, selectivity or yields or TOF of different catalysts along with increase of temperature. Such comparison could be clearer to show which catalyst presents better performance.
The selectivity for WGS on Cu is 100%, making yield and conversion identical. As discussed above, conversions are not meaningful in this case because the reactions were carried out under differential conditions.
Question V: Time-on-stream should be considered to test if catalyst deactivated along with the WGS reaction, because deactivation is one of the major issues of current WGS catalysts.
We completely agree with the reviewer. Because aging studies in an academic laboratory are difficult (Results will depend on temperature, reactant concentrations, etc.), we simulated the aging process by treating the catalyst in H2 at 1073 K.
Question VI: author should check their languages and typos. For example, the subscriptions in Figure 2. The sentence in line 105 “This is primarily due to the increased sample mass, since 105 the surface area per gram will change with the additional mass of the films” is a little confused. It could be rephrased or separated to two sentences. Authors should resubmit after compensating more experiments, answering questions and revising their manuscript.
We corrected the subscripts in Figure 2, Conclusions and References. The sentence in line 108 was rephrased to:
“This is primarily due to an increase in the sample mass, leading to a decrease in the surface area per mass. Because the ALD films were so thin, the pore volume and average pore diameter will not change.”
Reviewer 3 Report
The article has shown interesting work concerning Cu catalyst used in WGS process. However there some aspects need to be clarified.
1. TPD-TGA measurements
Authors wrote: ,,For purposes of this study, the most important point is that the sharp propene peak at 500 K that was observed on MAO was completely absent on all of the ALD-modified supports’’- but on the figures are still visible.
2. Why were the copper catalysts calcined and reduced at so high temperature?
3. The authors wrote ,,The Cu dispersions on the MAO supports were estimated using N2O adsorption, using the following procedure’’ and then they wrote opposite sentence ,,Unfortunately, it was not possible to measure Cu dispersions by N2O reaction on catalysts containing reducible oxides due to reaction with the oxide films’’
4. What about Tamman temperature of Cu?
Author Response
1. TPD-TGA measurements
Authors wrote: ,,For purposes of this study, the most important point is that the sharp propene peak at 500 K that was observed on MAO was completely absent on all of the ALD-modified supports’’- but on the figures are still visible.
The sharp propene peak in Figure 3a) at 500 K is indicative of the reaction of 2-propanol on MAO. This peak was absent from oxide thin film samples, although separate peaks were observed at higher temperatures which are indicative of reaction on the oxide films not the MAO. The statement: “If patches of MAO had remained uncovered by the oxide overlayers, one would expect to see reaction of the 2-propanol at that temperature [38]” is specifically referring to the peak at 500 K indicative of reaction on the MAO.
2. Why were the copper catalysts calcined and reduced at so high temperature?
A calcination temperature of 773 K was required to remove ligands from the ALD precursors. Oxidation at this temperature does not appear to cause sintering. The reduction temperature for the fresh catalyst was 623 K, which is a common temperature for reducing Cu. The following statement was added to the manuscript at line 216.
“Oxidation at 773 K was necessary to remove the precursor ligands, while reduction at 623 K was sufficiently low to minimize sintering.”
3. The authors wrote ,,The Cu dispersions on the MAO supports were estimated using N2O adsorption, using the following procedure’’ and then they wrote opposite sentence ,,Unfortunately, it was not possible to measure Cu dispersions by N2O reaction on catalysts containing reducible oxides due to reaction with the oxide films’’
The N2O method was used to measure the samples with only Cu on the inert MAO. In the presence of a reducible oxide film, the film would interfere with the dispersion measurement.
4. What about Tamman temperature of Cu?
Tamman temperature of Cu is 678 K. On the fresh catalyst, reduction was carried out at 623 K to minimize sintering. For the stability study, the catalysts were treated with H2 at 1073 K, a temperature significantly above the Tamman temperature in order to simulate aging. The following statement was added to the manuscript at line 242.
“A brief reduction at 1073 K, a temperature significantly above the Tamman temperature of Cu, 678 K, was chosen in order to simulate what would happen after long-time aging.”
Reviewer 4 Report
The subject of this review paper is of great interest in the area of hydrogen production, with a focus on the Cu catalysts for water-gas shift reaction. Different oxides were added to MgAl2O4 support by atomic layer deposition and their effect on the catalytic performance of Cu catalysts was evaluated.
I have some comments/suggestions for the authors:
1. Table 1: Pore volume and average pore diameter of the supports must be added to the table.
2. Lines 152-153: The sentence “If patches of MAO had remained uncovered by the oxide overlayers, one would expect reaction to see reaction of the 2-propanol at that temperature [38].” must be rewritten.
3. Table 2: The difference between Cu/MAO and Cu/MAO(1073K) should be explained in the table caption (the first was reduced at 623 K and the second at 1073 K).
4. The choice of the reduction temperatures (623 and 1073 K) should be explained in the text.
5. Item 3: The technique used to measure the final metal oxide loading (Table 1) and Cu loading (Table 2) should be cited (XRF, AA, ICP-OES, etc.).
Author Response
The subject of this review paper is of great interest in the area of hydrogen production, with a focus on the Cu catalysts for water-gas shift reaction. Different oxides were added to MgAl2O4 support by atomic layer deposition and their effect on the catalytic performance of Cu catalysts was evaluated.
I have some comments/suggestions for the authors:
1. Table 1: Pore volume and average pore diameter of the supports must be added to the table.
We added pore volume and average pore diameter of MAO in Table 1. Thin ALD films do not change the pore structure of the support.
2. Lines 152-153: The sentence “If patches of MAO had remained uncovered by the oxide overlayers, one would expect reaction to see reaction of the 2-propanol at that temperature [38].” must be rewritten.
We corrected the typo in the manuscript.
“If patches of MAO had remained uncovered by the oxide overlayers, one would expect to see reaction of the 2-propanol at that temperature [38].”
3. Table 2: The difference between Cu/MAO and Cu/MAO(1073K) should be explained in the table caption (the first was reduced at 623 K and the second at 1073 K).
This was added to the caption of Table 2:
“All samples were reduced at 623 K with the exception of Cu/MAO (1073K) which was reduced at 1073 K.”
4. The choice of the reduction temperatures (623 and 1073 K) should be explained in the text.
As discussed in our response to question 2 from Reviewer 3, the reduction temperature of 623 K is typical for reducing Cu. The following statement was added to the manuscript at line 216.
“Oxidation at 773 K was necessary to remove the precursor ligands, while reduction at 623 K was sufficiently low to minimize sintering.”
As discussed in our response to question 4 from the Reviewer 3, the following statement was added to the manuscript at line 242.
“A brief reduction at 1073 K, a temperature significantly above the Tamman temperature of Cu, 678 K, was chosen in order to simulate what would happen after long-time aging.”
5. Item 3: The technique used to measure the final metal oxide loading (Table 1) and Cu loading (Table 2) should be cited (XRF, AA, ICP-OES, etc.).
The following statement was added to the manuscript at line 272.
“The weight loading was determined gravimetrically.”
Round 2
Reviewer 2 Report
Authors provided more information in the revised manuscript. This manuscript can be accepted after polishing the language and correcting typos.
