The results of our prior work on the reverse water gas shift reaction (RWGSR) strongly suggested the critical role of formates as intermediates on alumina-supported Pd and Ru catalysts. One key experimental observation, i.e., the production of metal-bound CO even in the absence of H2 in the gas phase and without the formation of formates, howerer, was not satisfactorily explained in our earlier studies, and motivated our current work. Definitive experimental proof for catalytic pathways and active sites during low-temperature RWGSR, thus, remains elusive. Herein, we experimentally address the decades-long mechanistic controversy by studying the RWGSR over a Pd/Al2O3 catalyst. In this new studies isotopic transient kinetic analysis clearly established the minority role of surface formates. Moreover, hydrogen titration experiments unambiguously confirmed the intermediacy of a carboxyl species, for the first time. The ability to decouple the two operative pathways led to the identification of a distinct active site exhibiting regio- and chemoselective H-addition to yield carboxyl. The metastable active site only forms upon H-activation (~250 °C and above). The study demonstrates that interfacial H-atoms behave as electrophilic, one-electron reductants.

New insight into CO2 reduction on Pd/Al2O3

2019 Spring National Meeting

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