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Dynamic coordination of cations and catalytic selectivity on zinc–chromium oxide alloys during syngas conversion

Abstract

Metal oxide alloys (for example AxByOz) exhibit dramatically different catalytic properties in response to small changes in composition (the A:B ratio). Here, we show that for the ternary zinc–chromium oxide (ZnCrO) catalysts the activity and selectivity during syngas (CO/H2) conversion strongly depend on the Zn:Cr ratio. By using a global neural network potential, stochastic surface walking global optimization and first principles validation, we constructed a thermodynamics phase diagram for Zn–Cr–O that reveals the presence of a small stable composition island, that is, Zn:Cr:O = 6:6:16 to 3:8:16, where the oxide alloy crystallizes into a spinel phase. By changing the Zn:Cr ratio from 1:2 to 1:1, the ability to form oxygen vacancies increases appreciably and extends from the surface to the subsurface, in agreement with previous experiments. This leads to the critical presence of a four-coordinated planar Cr2+ cation that markedly affects the syngas conversion activity and selectivity to methanol, as further proved by microkinetics simulations.

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Fig. 1: Thermodynamics and structures of bulk ZnCrO at different compositions.
Fig. 2: Surface structures and phase diagram for ZnCr2O4 and Zn3Cr3O8 crystals under reaction conditions.
Fig. 3: Syngas conversion mechanisms and kinetics.
Fig. 4: Energetics and electronic structure analyses of CH3O adsorption.

Data availability

All the data are available within the article (and Supplementary Information) and from the corresponding authors upon reasonable request.

Code availability

The software code for LASP and the NN potentials used within the article are available from the corresponding author upon request or on the website http://www.lasphub.com.

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Acknowledgements

This work was supported by the National Key Research and Development Program of China (2018YFA0208600) and the National Science Foundation of China (21573149, 21533001 and 91745201).

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Z.-P.L. conceived the project and contributed to the design of the calculations and analyses of the data. S.M. carried out most of the calculations and wrote the draft of the paper. S.-D.H. wrote the neural network code and contributed to the analyses of the data. All the authors discussed the results and commented on the manuscripts.

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Correspondence to Zhi-Pan Liu.

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Supplementary Information

Supplementary Methods, Supplementary Figs. 1–9, Supplementary Tables 1–9, Supplementary References.

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Ma, S., Huang, SD. & Liu, ZP. Dynamic coordination of cations and catalytic selectivity on zinc–chromium oxide alloys during syngas conversion. Nat Catal 2, 671–677 (2019). https://doi.org/10.1038/s41929-019-0293-8

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