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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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.
The authors declare no competing interests.
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Nelson, N.C., Nguyen, MT., Glezakou, VA. 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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