Reactant-promoted reaction mechanism for catalytic water-gas shift reaction on MgO

Abstract

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 CO 2 in the absence of H 2O. 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 CO 2 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 CO 2 in the presence of water.