Pinning down the water-gas shift mechanism

Industry has depended on the water-gas shift (WGS) reaction for more than a century. The WGS process, which combines carbon monoxide and water to form carbon dioxide and hydrogen, is a key component of industrial plants that use hydrogen to process hydrocarbons or to synthesize ammonia or methanol. The intimately related reverse process, RWGS, consumes CO₂, a greenhouse gas, and generates CO, a valuable reagent. This chemistry has a long history, yet scientists continue to debate the nature of the catalytically active site and reaction pathway, key pieces of information needed for improving catalyst performance. Vassiliki-Alexandra “Vanda” Glezakou, János Szanyi, and coworkers at Pacific Northwest National Laboratory hope to end the debate. In a study combining spectroscopy, kinetics, and computations, the team analyzed the RWGS reaction driven by a palladium-alumina catalyst. They found that the key intermediate is a carboxylate species that bridges palladium and aluminum atoms (shown) and that a formate intermediate, proposed by other researchers, is a minor player in this chemistry (Nat. Catal. 2019, DOI: 10.1038/s41929-019-0343-2). The team determined that the catalytically active sites, which form only under reaction conditions, are hydroxides coupled to negatively charged palladium atoms at the Pd-Al₂O₃ interface.