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

FUENTE SILVIA A; ZUBIETA C.; FERULLO R.; PATRICIA G. BELELLI

Congreso; VI San Luis Conference; 2018

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 (CO + H2O ------ CO2 + H2). 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 (α-Fe2O3) using the density functional theory (DFT + U). First, the dissociative adsorption of water was studied comparatively on clean hematite and on Au/hematite.1 Water adsorbs on pure hematite with its O atom directly attached to a surface Fe ion. Instead, on Au5/hematite it does in its most stable form with one H atom oriented downward (an absent structure on clean hematite) at the metal-support interface and doubly bound to the surface: with a surface O and with the Au particle. Regarding the dissociative process, 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 (Eact) 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. We have modelled the formation of the OCOH intermediate, obtained by the coupling between surface OH (coming from water dissociation) and the incoming CO. According to a recent theoretical work,2CO tends to adsorb preferentially on hematite-supported sub-nano Au cluster over a Au+/Au? pair, mainly bound to Au+ species through its C atom. Thus, the hydroxyl group attached to the Au-support interface activate neighboring Au atoms by capturing CO since a Au pair of this type is formed on the support after water dissociation. 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. Next, it changes easily to the trans isomer, wherein the H atom of OCOH is oriented to the oxide surface. 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.