Improvement of Methane Combustion Activity for Pd/ZrO2 Catalyst by Simple Reduction/Reoxidation Treatment
Round 1
Reviewer 1 Report
In this work, the author explored the synthesis, methane combustion activity of ZrOx-Pd catalysts upon different reduction/re-oxidation treatment. Different TP and TPR studies have been conducted.
The author tried to report an experimental observation of activation in isothermal reaction. However, there is no strong experimental evidences that could support the conclusions the author made. Therefore, a rejection is recommended.
In figure 1, it is better to include the standard XRD patterns.
There is no direct evidence that would support the cartoon in figure 6.
Also, how could the author claim that 'as-formed oxygen species on the palladium were converted into more reactive species through repeated reduction/re-oxidation cycles under the reactant stream'. The author should consider more detailed surface and structure characterization before jumping to any coarse conclusions.
Author Response
I appreciate the valuable comments provided by the reviewer. As for the points you pointed out, we did our best to make the following changes based on reviewer ’s comments.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript by C. Kim et al. reports improved methane combustion activity of Pd/ZrO2 by reduction/re-oxidation treatments. The catalytic activity increased during the initial stages of isothermal reaction, and the light-off temperature was lowered as the number of cycles increased in the cyclic temperature-programmed reaction. From the CH4-TPR results, it was confirmed that the reduction temperature decreased significantly after activation. The observation of the CH4-TPR peak at relatively low temperatures is directly proportional to the catalytic activity of methane combustion. I think this work is well presented and is overall suitable for publication in Catalysts. Before final acceptance, the authors are suggested to address some comments and concern below.
The size of Pd nanoparticles determined by CO chemisorption is around 4-37 nm, Table 1. It is surprising that no any Pd/PdOx peaks are present in XRD patterns. In addition, TEM images of these samples can provide direct evidence of the nanoparticle size. The author ascribed the activity difference to calcination temperatures for non-activated and activated samples respectively. However, they should note that the different Pd size (thus different active/reactive surface area/sites). I would suggest a normalization of the activity by the surface area of Pd. The authors claim that the as-formed oxygen species on the palladium were converted into more reactive species through repeated reduction/re-oxidation cycles. However, the removal and re-introduction of oxygen species are both present with and without pre-activation. Why did it only occur through repeated reduction/re-oxidation cycles? The authors should try to explain in details on the activation mechanism.Author Response
I appreciate the valuable comments provided by the reviewer. As for the points you pointed out, we did our best to make the following changes based on reviewer ’s comments.
Author Response File: Author Response.pdf
Reviewer 3 Report
The paper titled “Improvement of Methane Combustion Activity for Pd/ZrO2 Catalyst by Simple Reduction/Re-oxidation Treatment” is a well-written work. It describes the results of the 1wt% Pd/ZrO2 catalyst activation by simple pretreatment procedures. The obtained activation effects were found to depend on the temperature of the initial calcination. The paper can be recommended for publication in Catalysts after minor revision. The comments are as follows.
Fig 2 shows light-off curves for three TP cycles. Due to activation under the reaction conditions, the activity was improved in second and third cycles, which is quite typical for this system. The presence of hysteresis loop in heating-cooling cycles is also well known. It is not clear how does it correlate with the activation procedures applied. Then, in Fig. 3, TP profiles for “activated” and “non-activated” samples are compared. According to the data presented, for two of five samples, the pretreatment worsens the activity. In this term, I recommend to replace the “activation” by “pretreatment”. The second question here is for which cycles are data in Fig. 3 presented? The authors have used CH4-TPR characterization of the samples. What is about the traditional H2-TPR characterization that can give more information about palladium? The effect of strong metal-support interaction in the Pd/ZrO2 system should be considered in more detail. For instance, the following paper should be taken into account: Molecules. 2016. DOI: 10.3390/molecules21101289. As-prepared and calcined at varied temperatures catalysts only were characterized by phys.-chem. methods. At the same time, all these characteristics after the catalytic tests are also very important. How were textural properties or Pd particle sizes changed under the action of reaction medium? Moreover, catalytic cycling was done in a range 300-600 °C, while the first sample in the row was calcined at just 500 °C. It means that such changes can be quite significant.Technical remarks:
Line 40: Full stop sign should be after the square brackets. Line 50: Zero in Pd0 should be in upper index.Author Response
I appreciate the valuable comments provided by the reviewer. As for the points you pointed out, we did our best to make the following changes based on reviewer ’s comments.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The author has made corrections based on the comments.
Reviewer 2 Report
The authors have addressed my comments, so I recommend the manuscript to be accepted as it is.
