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Pinning down the water-gas shift mechanism

Study identifies catalytically active sites and key reaction intermediates in classic industrial chemical process

by Mitch Jacoby
September 28, 2019 | A version of this story appeared in Volume 97, Issue 38


Computer model depicting the reverse water-gas shift reaction intermediate.
Credit: Pacific Northwest National Laboratory
This carboxylate intermediate plays a key role in water-gas shift chemistry. Pd = light gray; C = dark gray; Al = blue; O = red; H = white.

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.


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