Theoretical study of the water gas shift reaction (WGSR) on a Au/hematite model catalyst

SILVIA A. FUENTE; CAROLINA ZUBIETA; RICARDO M. FERULLO; PATRICIA G. BELELLI

Santa Fe

Congreso; VI San Luis Congress of Surfaces, Interfaces and Catalysis; 2018

Universidad Nacional del Litoral

The chemical interaction between Au and H2O is of great interest due to its occurrence in several reactions of catalytic importance such as the water-gas shift reaction. In this work, we have simulated this reaction using a model catalyst formed by a Au5 particle supported on the Fe-terminated (0001) surface of hematite (alpha-Fe2O3) using the density functional theory (DFT + U). First, the dissociative adsorption of water was studied comparatively on clean hematite and on Au/hematite. While the free Au5 particle has a poor performance to activate one of the O-H bonds, supporting it on hematite becomes highly active, having an activation barrier of only 0.09 eV. This process is even more favorable than pure hematite (Eact = 0.29 eV for the most reactive mechanism). Thus, the presence of Au enables the H2O molecule to adsorb in a configuration in which one of its O-H bonds is strongly activated. In the final dissociated state, the OH group is located at the metal-oxide interface. The second step of the reaction involves the CO adsorption and the formation of a surface intermediate complex. Our calculations show that the formation of the OCOH intermediate at the metal/support interface is not possible with CO bound on Au at bridge position. However, CO can migrate to a top position where it is only 0.17 eV less stable than at bridge. This migration occurs by overcoming an activation barrier of around 0.20 eV. In the next step of the reaction, the OCOH intermediate is formed by the interaction between CO located on top on a Au atom and the OH at the interface. This process takes place by overcoming a similar barrier (0.23 eV). In this complex, CO and OH groups are in cis position. From this configuration, H2 is formed by reacting with an H atom belonging to a OH adsorbed on the hematite surface. At the same time, CO2 is also generated and remains adsorbs at the metal/oxide interface. This process, i.e., the formation of H2 and CO2 from the intermediate, was found to be the rate-limiting step of the reaction.