(208e) An in Situ Formed Metastable Active Site Catalyzes the Formation of the Carboxyl Intermediate during the Water-Gas Shift Reaction

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 clearly established the minor role of the formate (HCOO) intermediate. Moreover, hydrogen titration experiments unambiguously confirmed the intermediacy of carboxyl (COOH) for the first time. The ability to decouple the parallel formate and carboxyl pathways led to the identification of a distinct active site exhibiting regio- and chemoselective H-addition to CO₂ to yield carboxyl. The metastable active site is formed in situ, resulting in hydroxylation of the metal-support interface and electronic restructuring of Pd via reduced coordination. Parallel atomistic simulations of the active site electronic structure and mechanistic landscape provided a framework 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.