In Situ X-ray Diffraction as a Basic Tool to Study Oxide and Metal Oxide Catalysts
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsI read this paper with interest.
It is a very good review paper on in situ methods using X-ray diffraction.
The historical background of the development of XRD, as well as a detailed description of the principles and instrumentation, are carefully described with references to published papers.
The sample setup and characterization and analysis procedures are also clearly explained with specific examples. In particular, studies on in-situ XRD on the reducibility of metal oxides are described in detail.
In particular, in-situ XRD studies on the reducibility of metal oxides, dry reforming of methane, steam reforming of methanol/ethanol, and water-gas shifts are detailed as examples of catalyst activation processes and catalyst studies under reaction conditions. Details on the catalyst deactivation mechanism are also described, along with experimental results. In addition, recent developments in in situ XRD instrumentation and areas of wide-ranging research are introduced, and future prospects for materials science research using this technique are discussed.
As mentioned above, this review article has a very good textbook-like structure for understanding the usefulness of in situ XRD methods in materials science research.
Therefore, I recommend that this paper be published in Catalysts. There is nothing in particular that needs to be revised in terms of content.
Author Response
We are greatly thankful for comments.
Reviewer 2 Report
Comments and Suggestions for Authors
In principle, I found the paper to be well-written and clearly structured. It also significantly contributes to the understanding of the application of XRD in catalysis.
However, I believe that a few additional points and improvements could make your paper more comprehensive and useful for a broader readership. In particular, in the context of a general review paper. I would like to provide some suggestions:
1. Discuss the unique position of XRD among other characterization methods used in catalysis.
2. As in heterogeneous catalysis, surfaces are of crucial importance, discuss how XRD can provide information about surface structures and crystallographic changes during catalytic reactions. Have you got any specific examples where XRD has been used in studying surface phenomena.
3. Comment on the common, in catalytic papers, use of XRD for crystallite size evaluation via Sherrer or Williamson-Hall methods. Provide practical examples from the literature to illustrate the application of XRD in nanoparticle characterization.
4. I suggest revising the caption of Fig. 4 to match the order of presentation in the graph.
5. I propose redesigning Fig. 6 to make it more cohesive and user-friendly. (the presented graphs are of various characters from molecular to microscale representation – this can be unified, or you can divide the mechanisms into classes instead of mixing them, Oswald ripening is not well represented, it just represents the NPs migration, in the case of the fouling plural form should be changed into a singular form, also where the carbon deposit is formed nanoparticles of active phase can be separated from the support … etc. ).
6. I Suggest incorporating noteworthy examples like the K-Fe-O catalyst for styrene production and the dramatic structural changes resulting from potassium loss. \
7. Discuss the role of XRD in studying mixed oxide catalysts, like e.g. formation of layered and tunneled nanostructures through alkali promotion of transition metal oxides. Highlight other relevant representative systems to make the review more comprehensive and reflective of the field.
I understand that it may not be possible to cover all aspects comprehensively in a short review. However, from my perspective as a reviewer, the addition of the above-mentioned points and systems would greatly enhance the value of your paper.
Author Response
We are greatly thankful for the suggested corrections to the manuscript and have tried our best to cover them all.
- Discuss the unique position of XRD among other characterization methods used in catalysis.
“In order to study the structure of a catalyst in detail, a wide range of methods is commonly used, including X-ray and neutron diffraction, spectroscopy techniques, thermal analysis, adsorption techniques, transmission and scanning electron microscopy. Heterogeneous catalysis is a surface phenomenon, so the main interest for the researcher is the state of the surface. The catalyst can be in the form of massive solids, but in the most cases nanoparticles of the catalytically active phase are located on the surface of a support. XRD is best known for its ability to define a bulk structure of crystalline materials. Therefore, the question may arise as to how the results obtained using XRD can shed light on the surface related catalysis phenomenon. Indeed, XRD struggles with a cases when a number of bulk atoms is comparable with ones of the surface (typically for highly dispersed systems, with dimensions <1 nm), but can provide useful information about the support. In the many cases, XRD truly shines because the knowledge of the structure of the active phase – composition, lattice constants, strain state, atomic arrangement and defect structure all can be invaluable for any reasoning about the active state of the catalyst. Moreover, the changes in the bulk nanostructure will affect the surface structure through the gradient of bulk lattice energy to the surface free energy. The specific changes in the defect structure also may affect the surface reordering. That is, before studying the state of the surface, it is necessary to know the structure of the foundation on which the catalysis phenomenon occurs.”
- As in heterogeneous catalysis, surfaces are of crucial importance, discuss how XRD can provide information about surface structures and crystallographic changes during catalytic reactions. Have you got any specific examples where XRD has been used in studying surface phenomena.
The surface sensitivity is actually the greatest weakness of XRD. Typically, additional techniques such as XPS, FTIR, and others are required to overcome this problem. We mentioned some examples of how XRD could be used directly, but these examples are of little practical use.
“or directly using grazing incidence diffraction optionally with microfocused beams (generally requires a synchrotron and mostly limited by a model atomically flat systems) [10,11].”
- Comment on the common, in catalytic papers, use of XRD for crystallite size evaluation via Sherrer or Williamson-Hall methods. Provide practical examples from the literature to illustrate the application of XRD in nanoparticle characterization.
“Along with the phase analysis, the size of particles is a key characteristic of catalyst [67–69]. Scherrer equation is commonly used to estimate crystallite size (effective length of diffraction coherence, obtained in the direction of the diffraction vector) using integral breadth of XRD peak [70]. Large particles may contain several crystallites, or domains with different orientations and it is important to compare crystallite sizes with those obtained with other methods. Since the reflection width is affected by both microstrains and crystallite size, it is important to distinguish between their contributions. In this case a Williamson-Hall method is often applied to estimate “mean values” of crystallite size and strain [71].”
- I suggest revising the caption of Fig. 4 to match the order of presentation in the graph.
Caption was corrected accordingly.
- I propose redesigning Fig. 6 to make it more cohesive and user-friendly. (the presented graphs are of various characters from molecular to microscale representation – this can be unified, or you can divide the mechanisms into classes instead of mixing them, Oswald ripening is not well represented, it just represents the NPs migration, in the case of the fouling plural form should be changed into a singular form, also where the carbon deposit is formed nanoparticles of active phase can be separated from the support … etc. ).
The figure 6 was divided into two parts (Fig. 6,7). We hope that the modified images of sintering and fouling have become more representative.
Figure 6. The common types of deactivation mechanisms: a) – sintering via atomic Ostwald ripening and crystallite migration; c) – catalyst surface and pores carbonization.
Figure 7. The common types of deactivation mechanisms: a) – catalyst sulfur poisoning and carbonyle leaching; b) – thermal driven growth of the BaWO4 inner layer at the BCFZ membrane blocking the oxygen transport (figure adapted from data obtained in [225]).
- I Suggest incorporating noteworthy examples like the K-Fe-O catalyst for styrene production and the dramatic structural changes resulting from potassium loss.
According to reviewer recommendation, we have added examples of potassium loss for K-Fe-O catalyst for styrene production
“The industrial catalytic dehydrogenation of ethylbenzene accounts for ~90% of the styrene produced worldwide. Muhler et al. [220,238,239] used in situ XRD to elucidate the origin of activation and deactivation of a potassium promoted iron oxide catalyst. Authors indicated that reduction process during the start-up period causes the release of mobile K+ ions, which in turn react with trivalent iron oxides to form of the active phase KFeO2. Under reaction conditions, KFeO2 exists as is a thin layer supported on a solid solution of K2Fe22O34 in Fe3O4, but it is metastable at room temperature and in the presence of moisture. Deactivation of the catalyst may take place by complete reduction of KFeO2 to Fe3O4 and KOH. The formation of a liquid phase (KOH) causes the loss of potassium and irreversible deactivation of a catalyst..”
- Discuss the role of XRD in studying mixed oxide catalysts, like e.g. formation of layered and tunneled nanostructures through alkali promotion of transition metal oxides. Highlight other relevant representative systems to make the review more comprehensive and reflective of the field. I understand that it may not be possible to cover all aspects comprehensively in a short review. However, from my perspective as a reviewer, the addition of the above-mentioned points and systems would greatly enhance the value of your paper.
We are greatly thankful for the suggested corrections to the manuscript and recommendation to expand number of systems to make the review more comprehensive. According to reviewer recommendation, we have added examples of alkali promoted transition metal oxides. In the part of «Study of the process of deactivation of catalysts» the example of K loss for K-Fe-O as the deactivation mechanism was added, in the part of «Study of the preparation procedure» the references illustrating the preparation porous mixed-valent manganese oxides by hydrothermal treatment of layered precursor and by thermal decomposition of KMnO4 were inserted.
Author Response File: 
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