Surface interfaces in low temperature water-gas shift: The metal oxide synergy, the assistance of co-adsorbed water, and alkali doping

With new developments in polymer electrolyte membrane fuel cells, interest is growing in fuel processor technology for converting feedstocks to hydrogen. One critical step in the process to convert CO and purify hydrogen is low temperature water-gas shift (LTS). Control of the LTS rate can be achieved by designing catalysts in a way that produces and rapidly decomposes the surface formate anion intermediate. In this account, examples are provided to demonstrate various interfacial phenomena important for achieving these goals. The interface between a metal and a partially reducible oxide promotes surface reduction of the oxide to the low temperature range, generating sites for the low temperature activation of H2O on the oxide. Partial reduction of the oxide and surface activation of H2O at low temperature provide a route for formate production at low temperature. Adjacent co-adsorbed water molecules participate in the transition state of formate decomposition, accelerating the formate turnover rate and altering the selectivity to favor dehydrogenation. The rate-limiting-step involves formate C–H bond scission, with the metal abstracting hydrogen at the interface between metal and oxide, serving as a conduit for hydrogen release. Catalysts may be improved by increasing formate mobility on the oxide; furthermore, the optimization of alkali doping levels can electronically promote formate C–H bond scission.