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

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

montevideo

Congreso; XLII Congreso Latinoamericano de Químicos Teóricos de Expresión latina (QUITEL 2016); 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 generally can 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 generating a COyHx intermediate, finally yielding 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 meanwhile water adsorbs in the O-down orientation on a surface Fe ion on the clean hematite surface, on the Au5/hematite system it adsorbs preferentially at the metal?support interface in a H-down orientation (an unstable structure on clean hematite) and doubly bonded with the support, 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 in order to react with active OH groups present at the peripheral Au atoms. According to a recent theoretical work [1], CO tends to adsorb preferentially on positively Au atoms of tiny hematite-supported Au particles. Our results show that after H2O dissociation, the Au atom that participates in this mechanism (number 5; Fig. 1) acquires a positive charge (0.23e). Thus, concerning particularly with the WGSR, the CO molecule should preferentially adsorb on this Au atom to form a COyHx intermediate, whose decomposition would yield finally to CO2 and H2.