Preadsorbed water-promoted mechanism of the water-gas shift reaction

Abstract

In this work, a detailed first principle study of the mechanism of the heterogeneous catalytic water-gas shift (WGS) reaction on a rhodium cluster is presented. A large number of possible reaction mechanisms relevant to the WGS reaction are explored, and as many as 34 possible pathways are located due to the unsaturated nature of the metal cluster and the multitude of intermediate species binding configurations. Brønsted-Evans-Polanyi relationships and the Sabatier principle are used to locate the kinetic and thermodynamic paths occurring on the rhodium cluster. A detailed potential energy diagram of the kinetically favored mechanism is presented that shows that the RDS of the reaction are the water dissociation, formate association and formate decomposition elementary reactions, with free energy barriers (ΔG‡) of 24.2, 25.9, and 27.0 kcal/mol, respectively. The poisoning effect of coadsorbed CO and the beneficial effect of preadsorbed water on the kinetic rate of this reaction is demonstrated and a water-mediated mechanism is proposed. In the watermediated mechanism favorable H-bonding interactions stabilize Zundel-cations adsorbed to the metal cluster, which manifest a lower energy path for the dissociation of water. Participation of coadsorbed water in this mechanism explains the promoting effect water vapor pressure has on the reaction kinetics (positive reaction order with respect to water) by lowering the free energy of the rate-determining step barrier by 4.0 kcal/mol and causing a 10-fold increase of the reaction rate for the water dissociation elementary reaction step. In addition a presumable preadsorbed water-mediated mechanism is shown to have an even lower free energy barrier (16,9 kcal/mol) causing a 2000-fold increase of the elementary reaction rate for water dissociation. It is suggested that operation of heterogeneous WGS catalysts in a cyclic fashion where water is first preadsorbed might enhance current catalyst performance and CO conversion turnover frequencies. © 2008 American Chemical Society.