Carboxyl intermediate formation via an in situ-generated metastable active site during water-gas shift catalysis

Abstract

Definitive experimental proof for catalytic pathways and active sites during the low-temperature water-gas shift reaction remains elusive. Herein, we combine spectroscopic, kinetic and computational analyses to address the decades-long mechanistic controversy by studying the reverse water-gas shift over Pd/Al2O3. Isotopic transient kinetic analysis established the minor role of the formate intermediate, whereas hydrogen titration experiments confirmed the intermediacy of carboxyl. The ability to decouple the parallel formate and carboxyl pathways led to the identification of a distinct active site that exhibits regio- and chemoselective hydrogen addition to CO2 to yield the carboxyl intermediate. The metastable active site is formed in situ, resulting in hydroxylation of the metal–support interface and electronic restructuring. Atomistic simulations of the active site electronic structure and mechanistic landscape provided a framework that is consistent with experimental observations. Our study highlights the dynamic creation of a coordinatively unsaturated metal site caused by substrate adsorption on an adjacent support site.

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Fig. 1: Gas-phase products and observable surface species.
Fig. 2: Formate conversion via a serial pathway.
Fig. 3: Hydrogen titration to reveal the carboxyl pathway.
Fig. 4: Characteristics of the stoichiometric reaction.
Fig. 5: Mechanistic and energetic landscape.
Fig. 6: Hydroxylation causes charge localization on interfacial Pd atoms.

Data availability

The data that support the plots in this paper and other findings of this study are available from the corresponding author on reasonable request.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences and performed at the Environmental Molecular Sciences Laboratory, which is a US Department of Energy Office of Science User Facility located at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is a multi-program national laboratory operated for the US Department of Energy by Battelle. Computational resources were provided by a user proposal at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility. N.N. would like to thank O. Y. Gutierrez for critical feedback during the final stage of the manuscript’s preparation.

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N.C.N. and J.S. conceived the research ideas and designed the experiments. N.C.N. synthesized the catalyst, performed all of the catalyst characterizations, transient kinetics and Fourier-transform infrared spectroscopy experiments. M.-T.N. and V.-A.G. carried out the theoretical studies. R.R. supervised the theoretical calculations. N.C.N., M.-T.N. and J.S. co-wrote the paper. J.S. supervised the project. All of the authors discussed the results and commented on the different versions of the manuscript.

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Correspondence to Vassiliki-Alexandra Glezakou or János Szanyi.

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Nelson, N.C., Nguyen, M., Glezakou, V. et al. Carboxyl intermediate formation via an in situ-generated metastable active site during water-gas shift catalysis. Nat Catal 2, 916–924 (2019). https://doi.org/10.1038/s41929-019-0343-2

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