Microwave-Assisted CO Oxidation over Perovskites as a Model Reaction for Exhaust Aftertreatment—A Critical Assessment of Opportunities and Challenges

Round 1

Reviewer 1 Report

The manuscript entitled “Microwave-Assisted CO Oxidation over Perovskites as a Model Reaction for Exhaust Aftertreatment – a Critical Assessment of Opportunities and Challenges” is a very logical, scientifically sound and well-written paper and I believe should be accepted for publication. It contains the results of studies on the development of the proper procedure for obtaining meaningful results by strictly controlling the most important parameters. The choice of parameters shows a deep understanding of catalysis, the studied catalytic system, as well as the effect of microwaves on the state of the catalyst under operating conditions. The research clearly shows that the assumptions often made by scientists are an oversimplification of reality and therefore may lead to substantial errors in temperature determination and hence erroneous conclusions. This is a very valuable paper and I will recommend it to all my colleagues the minute it is published.

As mentioned above, the paper is very well-written with a lot of care put into the linguistic side. Below are some minute linguistic errors, which I have found, to help improve this already excellent manuscript (in this case: to make it perfect/flawless):

1. Line 118 “in steady-state conditions” should be changed to “under steady-state conditions”

Line 269 “occurs at varying conditions” should be changed to “occurs under varying conditions”

Line 532 “at relatively modest conditions” should be changed to “under relatively modest conditions”

(The rest of the manuscript, including the captions to figures has the correct word, i.e., “under”).

2. Line 329 „a large enough” is not correct without a continuation (large enough that/to…)  Please change to e.g., one of the three options: “implies a substantial/pronounced/very large penetration depth”

3. Line 334 “equation4” should be “equation 4”

4. Line 371 “This in turn lead to” should be “This in turn led to”

5. Approximated errors of the determination in the form of error bars on Figure 8 would be beneficial for the reader.

6. Lines 470-471 “N2” should be “N₂”

Author Response

Dear reviewer,

thank you very much for assessing our manuscript. We appreciate the valuable input and the positive comments very much.

We addressed all the linguistic comments 1-4 and 6 and searched the manuscript for further comparable errors, which we did not find.

As for comment 5, we very much agree, error bars on the catalytical conversion data could be beneficial for the reader. However, we have a very difficult situation in quantifiying those, as the contribution to the measurement error from the FTIR-analyzer will be neglegible vs. for example contributions from FOT positioning, temperature fluctuations brought about by the systems cooler (esp. at higher MW-power values) and similar factors. For this reason we originally decided to not include error bars as they might instill a false sense of precision with regard to the measurement error. We have however added a short explanation in the experimental part (lines 477-480) to make our reasoning more clear to the reader.

We hope that this answer addresses your concern sufficiently. Thank you again and best regards on behalf of all the authors.

Daniel Röhrens

Author Response File: Author Response.pdf

Reviewer 2 Report

This is an excellent submission and using monomode microwave heating authors have identified different reaction parameters under which a model CO oxidation catalytic system has been studied. Authors have indeed identified a very valid concern and somewhat of a mystery created owing to interaction of hot catalytic surfaces with crystalline facets where variations in selectivity can be truly immense with cool or relatively-less-temperature-equilibrated reactant gases of varying polarities.

This is a very valuable submission, and this reviewer recommends publication without any reservation. Authors may want to implement items listed in their future work as this field is largely unexplored. 

1.     The choice of LSC as the catalytic powder is interesting and since it is commercially supplied authors adding XRD and SEM results of these materials is useful although one could argue that the SEM data is unable to resolve difference in the size of these materials and hence surface areas upon catalysis. A higher resolution electron imaging can be useful either in this study or their future works than the images provided in Figure 6.

2.     Kindly correct the data label from ‘IR’ to ‘IR Pyrometer’ in the Figure 3a to be in agreement with the figure 3b. IR pyrometers are indeed a poor choice for dense powders to be able to attain reliable data. 

3.     Authors should also mention glow rate values in l/g/h as it is relatable to powders than catalytic monoliths for example where sccm is used.

4.     In Figure 8a, authors have applied up to 200 sccm and have found CO oxidation activity being inversely related to the flow rate. Is there any clarity on what happens if the flow rate is increased further, by an order of magnitude larger than the middle data point? One would assume it to decline further but to what extent remains a question and this can be interesting to explore in a follow up work using even noble metal catalytic systems. Additionally, large flows are industrially more amenable. 

It is not entirely unreasonable to assume that microwave heating is largely unexplored for catalysis since a lot remains to be implemented from the reaction design point of view that researchers feel less certain about, after all reproducibility is the key and authors by submitting this study here have taken on a noble role of making MW heating method more relevant to the field of emission control or otherwise. 

This work is entirely suitable for publication in this journal. 

Author Response

Dear reviewer,

thank you very much for assessing our manuscript and the kind words. We have considered all your comments and taken the valuable input to improve this updated submission.

In detail we changed figure 6 to also show SEM images in 50 kx magnification before and after catalysis so that it is clearer that no apparent change in morphology or size is discernible. Additionally, we added "particle size" in Line 309 and information about the magnification in the caption to figure 6.

We changed the label in figure 3a as you proposed to make it uniform with the other figure.

In lines 392-394 we added rate values in l/g/h.

Regarding comment 4, this is a valid aspect. In general, we would have preferred to cover a larger parameter window. However, we found that for flowrates higher than 300 sccm there is a considerable pressure drop and reliable flow conditions could no longer be ensured. This is because of the small diameter of the reactor and the feed tubes. At even higher applied flowrate values the flow will drop off to near zero, so to make sure we have a reliable measurement, we decided to set 200 sccm as the upper boundary. We included a short explanation of this situation in lines 394-396 and hope it will be sufficient.

We agree with the reviewer that more research should be done in the future in more industry-relevant dimensions and process conditions and we are currently working into that direction.

Thank you again for considering our manuscript and best regards on behalf of all the authors.

Daniel Röhrens

Author Response File: Author Response.pdf