REACTANT-PROMOTED REACTION-MECHANISM FOR CATALYTIC WATER GAS SHIFT REACTION ON MGO

The behavior of reaction intermediates in the catalytic water-gas shift reaction (WGSR) on the MgO surface was studied by means of FT-IR spectroscopy. The hydroxyl groups on top of coordinatively unsaturated Mg atoms reacted with CO to produce three kinds of surface formates of unidentate, bidentate, and bridge types in the order bridge > unidentate > bidentate. Unidentate-type formate was produced at room temperature and decomposed at ca. 450 K. The formation and decomposition of bridge- and bidentate-type formates proceeded at higher temperatures (ca. 400 and 550 K, respectively). In the presence of adsorbed water, unidentate-type formate changed into bridge-type formate and hence most formates are bridge type under reaction conditions. Contrary to previous reports, formate intermediates were never converted to HZ and CO2 in the absence of H2O. While all the formates decompose to CO and surface OH by themselves, adsorbed water promoted the decomposition of formates to the WGSR products HZ and CO2 through electronic interaction between the adsorbed water and the formate. The reaction rate of WGSR at steady state agreed with the decomposition rate of the bridge-type formate intermediate to HZ and CO2 in the presence of water. © 1990.