Investigating the Impact of Na₂WO₄ Doping in La₂O₃-Catalyzed OCM Reaction: A Structure–Activity Study via In Situ XRD-MS

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper summarizes the results of an investigation into the suppression of La-carbonate species formation that inhibits the OCM reaction with La2O3. Specifically, the authors use XRD-MS to examine the change in the crystalline state of La2O3 in the presence of CO2 by adding Na2WO4. Na2WO4 is a solid basic oxide, and the strategy of protecting active sites on La2O3 by adsorbing CO2 is common, but the appeal is the change in the crystal structure of La₂O₃. In contrast to the activity shown in Fig. 3, the authors suggest that W1, W3, and W5, including Na2WO4, unlike non-doped M-La2O3, form tetragonal-La2O2CO3 at 800°C in the presence of CO2, which is consistent with the C2- yield and C2-selctivity. However, this is a change in a model test at 800°C, and it is questionable whether the actual reaction gas conditions and temperature (600-650°C) cause changes up to the bulk. In addition, the paper is incomplete in that it does not adequately discuss why the formation of the tetragonal phase leads to improved yields and selectivity.

The reviewer hope that the contents of the paper will be substantially reconsidered, including the other modifications listed below.

 

Pointed out (1) Fig. 1, Lines199-200

The authors claim that the diffraction lines are not visible due to the low Na2WO4 addition. However, the diffraction positions of W1-W5 shift to higher angles with increasing mNa2WO4 addition. This may suggest solid solution of Na2WO4 into La2O3.

 

Pointing out (2) L228

Is the sample Loading amount of 300 mol correct?

 

Pointed out (3) Fig. 2

As is well known, Na2WO4 is a solid base. If so, wouldn't the more basic 5W adsorb CO2 from the lowest temperature?

 

Pointed out (4) L267-268

Where is Fig. 4 showing the relationship between the amount of Na2WO4 doping and the light-off temperature?

 

Pointed out item (5) Caption of Fig4

Is the labeling of each sample correct?

Author Response

The reviewer has clearly outlined the idea of this paper. The structure change from La₂O₃ to La₂O₂CO₃ during carbonation is concluded to be correlative to the OCM activity and selectivity. The reviewer correctly pointed out that the different in situ bulk phases listed in Figure 5 (original Figure 4) are obtained from 800°C, while the activity difference is mostly discussed at ~600°C. This part can be further explained by the data in Figures 3 and 6 (original Figure 2 and 5), where it shows that the phases change and the CO₂ uptake are both at ~600 °C. We should have explained this in more details before submission. As the paper was prepared in a hurry before the deadline of the special edition, this part was not addressed in detail. Thanks for providing this revision as an opportunity so we can improve it. Please find all point-point answer in the attached file with figures and remarks of all revision. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Comments on the manuscript “ Investigating the impact of Na₂WO₄ doping in La₂O₃ catalyzed 2 OCM reaction, a structure-activity study via in situ XRD-MS” by  D. Wang, J. Lang, Z. Qiu, N. Ding, Y. Yang, submitted to MDPI catalysts.

 

The authors describe an attempt to promote the La₂O₃ catalyst for the methane oxidative coupling process (OMC) with varied doses of Na₂WO₄.  Based on the catalytic tests in the OMC reaction as well as on the CO2 sorption measurements and XRD results, the author rationalize the mechanism of Na2WO4 intervention.  Experimentally, however, the impact of the Na2WO4 doping does not seem very significant, and weakly related to the amounts of doping applied in each sample.  Indeed, Figure 3 that presents the main catalytic results would have been much more convincing, if the authors had shown the error bars.  Without the errors having been estimated, the reader cannot conclude about statistical significance of difference between the results exhibiting catalytic performance of each sample.  Still the paper may be publishable, but it needs significant improvement.  Please consider the following points.

 

1) p. 6, line227-229  We read:

“By integrating each online MS peak, the yielded total CO2 uptake molar amounts are 208, 265, 283 and 307 μmol for the M-La2O3, M-La2O3_1W, M-La2O3_3W and M-La2O3_5W samples, respectively.  As the sample loading is around 300 mol (100 mg, with molar mass around 326 g/mol), this CO2 uptake is almost stoichiometry [sic] [...]”.

 

This does not sum up.  The higher the “n” value in M-La2O3_nW, the lower the actual content of La2O3 in the sample.  With the same sample mass of 100 g, the La2O3 content must be higher in M-La2O3_1W than in M-La2O3_5W and yet, according to the above fragment, it corresponds to lower CO2 uptake, respectively, 265 against 307 μmol.  The integrated amounts of CO2 should rather be decreasing with the rising “n”, if the uptake was indeed to be stoichiometric.  In fact, a decreasing tendency in the CO2 uptake as a function of n should be observed, not the other way round.

 

2) p. 8, line 243

“The preceding results have indicated that, in comparison to M-La2O3, the three doped 243 La2O3 M-La2O3_nW catalysts display more resistance to CO2 adsorption.”

 

In fact, again, the actually stated order “208, 265, 283 and 307 μmol for the M-La2O3, M-La2O3_1W, M-La2O3_3W and M-La2O3_5W samples” shows the opposite: the higher n, the more the samples adsorb.

 

3) p. 8, line 248

“Figure 3 a clearly reveals that 248 the OCM reaction light-off temperature of M-La2O3 is higher than the other three samples [...]”

 

I cannot agree with this statement.  Would the authors show more specifically how they have reached such conclusion.  Besides, Figure 3a is totally incomprehensible.

 

4) p. 8, line 253

“For all the four samples, the C2 selectivity always show 253 a singular positive dependence on the Na2WO4 loading level.”

 

But what is an error of those selectivity values?  Are the differences between them of any statistical significance.  And what do the authors mean by “singular dependence”?

 

5) pp. 8-9. line 254-256

“The CH4 conversion rates of M-La2O3, after its later light-off, increased faster than all the other systems to 17.8% at 650 °C, while the M-La2O3_1W and M-La2O3_3W are notably lower at 14.0% and 13.0%, respectively.”

 

What is statistical significance of those differences?  How were they determined and with what errors?

 

6) p. 9, line 262

“The total CH4 conversion rate of M-La2O3_5W also rebounds to 17.3%.”

 

Again, what are the errors of the values presented in Fig. 3 b and c

 

7) p. 9 line 263

“Consequently, tuning the Na2WO4 level doped on La2O3 show a positive effect on the catalyst OCM performance especially in the low temperature region.”

 

This seems not the case; in fact, the COx yield is actually the highest on M-La2O3_5W at the temp. range between 400 and 500 C, cf. the green plot in Figure 3a.

 

8) p. 11  Please, increase lettering in Fig. 5.

Comments on the Quality of English Language

Some linguistic corrections may be needed.  Generally, the English is correct as far as the grammar is concerned.  The usage, however, seems to be wanting from time to time, for instance:

- p. 3, line 108, “doping Na2WO4 over La2O3”, I would suggest “doping Na2WO4 into La2O3”;

- p. 3, line 112 “exhaust”, I think “outlet” might be a better term;

- p. 3, line 124, “samples 123 were found higher conversion” should read “samples were found to have (or to show, or to exhibit) higher conversion”.

Author Response

Thanks for the comments. We have made changes according to the reviewer’s suggestion as below.  We agree that the impact of the Na₂WO₄ doping is not uniformly significant for all the evaluation. As we emphasized in the paper, the modification and evaluation “confirmed a positive dependence between the resistivity of La₂O₃ catalyst to CO₂ adsorption and its low-temperature C₂ selectivity”, not the overall performance. For selectivity and activation temperature, the doped samples show results better than the undoped sample with a trend. But the overall yield is not consistent as we highlighted in Figure 4 b (original Figure 3 b). The 5W sample betters the undoped sample in all evaluation. In the revised version, we further strengthen the discussion between low temperature activity and the C-H activation mechanism behind. Please find all point-point answer in the attached file with figures and remarks of all revision. 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

 

REVIEW COMMENTS

  

Manuscript ID: catalysts-2827986

Title: Investigating the impact of Na₂WO₄ doping in La₂O₃ catalyzed OCM reaction, a structure-activity study via in situ XRD-MS.

Authors: Danyu Wang, Junyu Lang, Zhehao Qiu, Ningxujin Ding, Yong Yang

Journal: Catalysts

 

In this manuscript, Wang and colleagues investigated La₂O₃ as a catalyst for the oxidative coupling of methane (OCM) and the impact of doping Na₂WO₄ on La₂O₃’s catalytic performance. They synthesized one control La₂O₃ catalyst and three La₂O₃ catalysts with concentrations of 1,3, and 5% weight Na₂WO₄, respectively. These catalysts were characterized using X-ray diffraction (XRD), and catalytic performance investigations were carried out in a quartz fixed-bed reactor with online mass spectrometry, which monitored catalytic activity as well as adsorption’s impact on reaction performance. The reviewer notices that great in-situ XRD and MS characterization has been done as well as promising preliminary data has been collected. However, lots of revisions are needed for this manuscript. The reviewer has the following specific comments.

(1) In the introduction of this manuscript the term “high catalytic activity” is used but a criterion to justify this claim (say TOF >1 s^-1) is not provided.

(2) The purity of the Na₂WO₄ nor the La₂O₃ in the catalyst preparation section was not specified. If these chemicals are >99.9% pure, it should be noted.

(3) The authors’ XRD patterns in Figure 1 are presented well, however, this reviewer is concerned that Na₂WO₄ cannot generate diffraction patterns another catalyst characterization method such as X-ray photoelectron spectroscopy (XPS) should be used.

(4) The author calculates the catalyst dopant loading information, it may benefit the quality of this manuscript for the author to measure the dopant loading using AES-ICP.

(5) In the first sentence of this manuscript, OCM was used incorrectly. OCM is the oxidative coupling of methane.

(6) A typo of La in Figure 1’s legend.

(7) All these reactions are highly exothermic. Therefore, rather rather thermodynamics, kinetics is the key to control OCM. This should be included at the end of the 1^st paragraph.

(8) Higher temperatures are required to desorb CH₃ radicals, not C-H activation. It has been well known that C-H bonds are activated at 250 C since the 1990s. 

(9) Overall, authors need to pay more attention to the fundamentals of OCM.

(10) Figure 3 has serious problems. The total selectivity values add up to <90%. What are the other products in addition to CO_x and C₂? Looking at the 1^st and 2^nd catalysts at 600 C, why higher conversions and the same C₂ selectivity lead to lower C₂ yield?? This means the catalytic performance was not measured carefully, which may lead to incorrect conclusions.

(11) What is the active phase for OCM? Na₂WO₄ has been reported since the 1990s as an active catalyst for OCM.  It looks like you used it as a promoter.

(12) The critical point for OCM is to desorb CH₃ radicals from the catalyst surface. CO₂ coverages should benefit this. However, the authors had the reverse ideas, which need to be addressed.

(13) CO₂ update experiments were carried out at a completely different concentration as compared to the reaction conditions, making the conclusion nonconvincing. Please reduce the CO₂ concentration and redo the experiment.

 

 

 

 

 

 

 

 

 

 

 

Author Response

Thanks for the positive evaluation. This paper was finished in a hurry as we need submit before DDL. We specially appreciate the reviewer for the following comments as an opportunity for us to improve this paper. Two experiments have been performed according to your comments and the data and related discussion are added. Please find all point-point answer in the attached file with figures and remarks of all revision. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for your careful revision of the previous manuscript with the addition of relevant discussions.

The reviewer confirmed that this manuscript was revised properly, and that the appeal points and the conclusions of the paper was concise and clear.

This manuscript will be recommended to this journal.

Author Response

Thanks for yor constructive discussion. It certainly improved the quality of this paper.

Reviewer 3 Report

Comments and Suggestions for Authors

My prior comment #1 addresses TOFs. In the revised manuscript, in particular, Fig. S3, you used TOFs incorrectly.  You must use surface metal dispersion for TOFs. The unit of mmolg La2O3-1 hr -1 is not for TOF. Instead, it is for mass-specific reaction rate. Please revise it.

Author Response

Thanks for the valuable comments from the reviewer. The detailed changes are as following and attached. 

Author Response File: Author Response.pdf