Density Functional Study of H2O adsorption and dissociation on Au/a-Fe2O3

CAROLINA ZUBIETA; PATRICIA BELELLI; LEANDRO F. FORTUNATO; SILVIA A. FUENTE; RICARDO M. FERULLO

Montevideo

Congreso; QUITEL 2016, 42nd International Congress of Theoretical Chemists of Latin Expression; 2016

The chemical interaction between Au and H2O is of great interest because it takes place in several reactions of catalytic importance such as the water-gas shift reaction (WGSR) and CO2 dissociation. On supported metal-based catalysts, two reaction mechanisms can generally be considered for the WGS reaction, the regenerative (or redox) mechanism and the associative (or adsorptive) mechanism. The associative mechanism consists in the dissociative adsorption of H2O on small gold particles followed by spillover of activated hydroxyl groups onto adjacent sites of the support. This is followed by reaction of CO at the Au?support interface giving an intermediate of general formula COyHx, finally producing CO2 and H2. Enhanced catalytic activity of Au/Fe2O3 catalysts compared with Fe2O3 was explained on the basis that Au/Fe2O3 contains more active OH groups. In this work we evaluate the adsorption and dissociation of water on a model catalyst formed by five Au atoms on the Fe-terminated (0001) surface of hematite (α-Fe2O3). The calculations were performed within density functional theory including an on-site Coulomb term (DFT+U; U=4 eV) as implemented in the VASP (Vienna Ab-Initio Simulation Package) code. The results indicate that whereas water adsorbs in the O-down orientation on a surface Fe ion on the clean hematite surface, on Au5/hematite it adsorbs preferentially at the metal?support interface in the H-down orientation (an unstable structure on clean hematite) and doubly bonded with the surface, namely, with a surface O ion and the Au particle (Fig. 1). From this stable configuration the water molecule dissociates by overcoming a low activation barrier (around 0.1 eV), a process as favorable as on clean hematite. Hydroxyl groups resulted to be linked at the metal?oxide interface in the dissociative final state. In this way, supported Au particle is able to provide the necessary surface sites for the adsorption of species for further reactions with active OH groups present at the peripheral Au atoms. According to a recent theoretical work, CO tends to adsorb preferentially on positively Au atoms of tiny hematite-supported Au clusters. Our results show that after H2O dissociation the Au atom involved in this mechanism (number 5; Fig. 1) acquires a positive charge (0.23e). Thus, in relation with the WGSR, the CO molecule should preferentially adsorb on this Au atom to form the COyHx intermediate, whose decomposition would finally yield to CO2 and H2.