Study of the Kinetics of Reduction of IrO₂ on TiO₂ (Anatase) by Temperature-Programmed Reduction

Round 1

Reviewer 1 Report

The paper is written well generally and is almost fully understandable. However, according to me, the following comments should be addressed before it can be considered for publication:

1.       There are many methods to calculate the activation energy of a solid chemical reaction by thermal analysis, such as the Flynn-Wall-Ozawa method, Friedman-Reich-Levi method, and Kissinger-Akahira-Sunose method. Why the author chooses the Kissinger method？

2.       The description of the current situation at home and abroad is not deep enough.

3.       Lines 120-160, missing units of variables.

4.       The model only validates the case where the heating rate is 10℃/min, and it is recommended to verify the situation under other heating rates. To see whether the model is appropriate and whether the activation energy needs to be adjusted.

5.       The article should give a specific chemical reaction equation.

Author Response

The paper is written well generally and is almost fully understandable. However, according to me, the following comments should be addressed before it can be considered for publication:

Thank you very much for your positive opinion about our work.  Please see below my answer to your questions.

1. There are many methods to calculate the activation energy of a solid chemical reaction by thermal analysis, such as the Flynn-Wall-Ozawa method, Friedman-Reich-Levi method, and Kissinger-Akahira-Sunose method. Why the author chooses the Kissinger method？

We have used a few model-free methods in the past; Redhead and Kissinger equations (Energy Technol. 2021, 2100491), Polanyi Wigner plots (Top. Catal. 2018, 61, 487), Leading edge analysis (Langmuir 2009, 25, 4543), and model-based methods (Chem. Eng. J. 2020, 401, 126015).  In this work we wanted to compare both approaches to analyze the experimental results for the reduction of IrO₂. I have added a few lines further explaining our choice in the introduction section.

2. The description of the current situation at home and abroad is not deep enough.

I am not certain to understand the meaning of “at home and abroad “ in the above sentence.  I have added more on the state of art of the Ir-based catalysts in the introduction section and hope that the added material is what the referee has asked for.

3. Lines 120-160, missing units of variables.

Basic units are now added.

4. The model only validates the case where the heating rate is 10℃/min, and it is recommended to verify the situation under other heating rates. To see whether the model is appropriate and whether the activation energy needs to be adjusted.

Modelling for the 40^oC/min is now added in the supporting information (figure S2).  The results did not change, it seems that the n-order model still best describes the first peak while the second peak is best described by a combination of two models (phase boundary for the front end and n-nucleation for the tail of the peak).

5. The article should give a specific chemical reaction equation.

The equation is given on page 1, it is now numbered.

Reviewer 2 Report

The work by S. Batubara et al entitled “Study of the kinetics of reduction of IrO₂ on TiO₂ (anatase) by temperature-programmed reduction.” describes the reduction of IrO₂ on TiO₂ anatase, when using an optimal Ir loading of 3.5 wt.%.

The work is interesting and data interpretation suitable, but one major aspect I would like to see is at least some XRD data to clearly identify the species (initial, during transformation –or at least after first region, and in the final state, after reduction).

Other smaller observations:

1.      Line 14: Rutile=TiO₂, when referring to IrO₂ it should be made clear that it is the rutile polymorph of IrO₂.

2.      Line 40: The reduction reaction should be written separately and bear a number (1)

3.      Line 131–and many more: I believe “beta” β is missing from the text.

4.      Was the effect of the IrO₂ reduction on the TiO₂ support investigated? That is the rutile/anatase

ratio before, during and after reduction (TiO₂)?

5.      The Ir-source is a Ir(III) salt, whereas the oxide contains Ir(IV). The authors might want to add a reaction to account for this redox process (reaction (2)).

6.      Lines 315-316: Multiple regions can be theoretically seen in Fe₂O₃ reduction, since mixtures Fe(II)/Fe(III) and also Fe(II) oxide can form, before Fe(0) is produced.

7.      Rephrase line 322:” Ir4f of Ir metal is separated by about 1.5 eV from that of Ir4+, only)”

8.      Finally, I suggest at least some XRD data to clearly show the nature of the reactants and products, as well as possibly other intermediate phases formed (during first reduction region, for instance), this is an important piece of data in my opinion. And phase conversion of TiO₂ support could be interesting as well (rutile/anatase/etc.).

Author Response

The work by S. Batubara et al entitled “Study of the kinetics of reduction of IrO₂ on TiO₂ (anatase) by temperature-programmed reduction.” describes the reduction of IrO₂ on TiO₂ anatase, when using an optimal Ir loading of 3.5 wt.%.

The work is interesting and data interpretation suitable, but one major aspect I would like to see is at least some XRD data to clearly identify the species (initial, during transformation –or at least after first region, and in the final state, after reduction).

Thank you very much for your positive opinion and support of our work. Please find below my answer to your queries.

Initial and final XRD results are now added in the supporting material (figure S3).  There is no noticeable difference between the initial phase and the one heated to 120^oC.

Other smaller observations:

1. Line 14: Rutile=TiO₂, when referring to IrO₂it should be made clear that it is the rutile polymorph of IrO₂.

OK

2. Line 40: The reduction reaction should be written separately and bear a number (1)

OK

3. Line 131–and many more: I believe “beta” β is missing from the text.

Yes, this is due to pdf conversion (checked in the revised version).

4. Was the effect of the IrO₂reduction on the TiO₂ support investigated? That is the rutile/anatase ratio before, during and after reduction (TiO₂)?

TiO2 used is a pure anatase phase and stayed that way after the complete TPR runs, we have not studied IrO2 on the rutile phase.

4. The Ir-source is a Ir(III) salt, whereas the oxide contains Ir(IV). The authors might want to add a reaction to account for this redox process (reaction (2)).

OK, added and numbered.

5. Lines 315-316: Multiple regions can be theoretically seen in Fe₂O₃reduction, since mixtures Fe(II)/Fe(III) and also Fe(II) oxide can form, before Fe(0) is produced.

Yes, clarified further

6. Rephrase line 322:” Ir4f of Ir metal is separated by about 1.5 eV from that of Ir4+, only)”

Ok, rephrased for further clarity.

8. Finally, I suggest at least some XRD data to clearly show the nature of the reactants and products, as well as possibly other intermediate phases formed (during first reduction region, for instance), this is an important piece of data in my opinion. And phase conversion of TiO₂support could be interesting as well (rutile/anatase/etc.).

We have added XRD in the supporting material.  A weak signal related to IrO2 is seen.   We have previously studied the anatase to rutile transformation of this same TiO2 (Sigma Aldrich), it starts to convert by 720^oC (Catalysis Today, 240, 242 (2015)).

Reviewer 3 Report

The manuscript addresses the study of the kinetics of reduction of IrO2 on TiO2 by temperature-programmed reduction. 

Overall paper is well structured and well written.

The research gap is clearly stated. However the direct application and need for this is not clearly explained at the end of the introduction.

Methodology must include reproducibility for the experiments.

Only one reference (12) is used for the kinetic study. These should be confirmed with more refeences.

Table 4 with kinetic parameters has to include the error for each of them. 

Why is the phase-controlled order much more different to others? This must be explained.

Conclusions are too broad. They must include main findings with obtained values and also future recommendations

Author Response

The manuscript addresses the study of the kinetics of reduction of IrO2 on TiO2 by temperature-programmed reduction. 

Overall paper is well structured and well written.

# Thank you very much for your positive opinion about our work.  Please see below my answer to your queries and suggestions.

The research gap is clearly stated. However the direct application and need for this is not clearly explained at the end of the introduction.

#More information is added in the introduction section now (page 3).

Methodology must include reproducibility for the experiments.

# Added.

Only one reference (12) is used for the kinetic study. These should be confirmed with more references.

# More references are added (13, 14, 28-33).

Table 4 with kinetic parameters has to include the error for each of them. 

# Experimental errors are added.

Why is the phase-controlled order much more different to others? This must be explained.

# The simplified equation for phase controlled order is "3 (1-a)^(2/3)".  In our case n was found to be 2.7 when approximated to 3 it gives (n-1)/n = 2/3; this may indicate that it is a contracting sphere process.  We have added further discussion on this in the manuscript.

Conclusions are too broad. They must include main findings with obtained values and also future recommendations

# The conclusion is changed in line with the referee’s request.

Round 2

Reviewer 2 Report

The authors have addressed most of my observations. One in particular however, remains. If the title of the paper is "Study of the kinetics of reduction of IrO₂ on TiO₂ (anatase) by temperature-programmed reduction.", I must say I’m surprised not seeing Ir in the XRD pattern, and even more so, that IrO₂ peaks are actually bigger. So, I must add, is this a reduction, or an oxidation that the authors have modelled?

Author Response

Thank you for your prompt answer.  I have added in figure S4 the XRD after reduction that shows the presence of metallic iridium.  Apologies for misunderstanding your query, I thought it was about the detection of Ir as an element.  XRD detects crystalline phases (long range order) and is not easily used for low % of metals dispersed on an oxide because the small particle size hinders their detection. The presence of Ir(111) at about 40.6 degree is however clear after reduction.  We have previously studied Iridium content of the same sample by XPS (reference 27) but I do not think it is important to add XPS spectra again here.

I do hope that I have answered correctly the referee's query.