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

SILVIA A. FUENTE; C E ZUBIETA; LEANDRO FORTUNATO; RICARDO M. FERULLO; PATRICIA G. BELELLI

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

1.50 1.06 2.44 1 2 3 4 5   5   4   2   3   1     2.16 2.89 2.00 0.99 2.34 1   2   4   5   3   1.19 1.41          Initial state                                                    Transition state                                                 Final state                      Figure 1: Initial, transition and final states in H2O dissociation on Au5/α-Fe2O3.               The chemicalinteraction between Au and H2O is of great interest because it takesplace in several reactions of catalytic importance such as the water-gas shiftreaction (WGSR) and CO2 dissociation. On supported metal-basedcatalysts, two reaction mechanisms generally can be considered for the WGSreaction, the regenerative (or redox) mechanism and the associative (oradsorptive) mechanism. The associative mechanism consists in the dissociativeadsorption of H2O on small gold particles followed by spillover of activatedhydroxyl groups onto adjacent sites of the support. This is followed byreaction of CO at the Au?support interface generating a COyHxintermediate, finally yielding CO2 and H2. Enhancedcatalytic activity of Au/Fe2O3 catalysts compared with Fe2O3was explained on the basis that Au/Fe2O3 contains moreactive OH groups. In this work we evaluate the adsorption and dissociation of water ona model catalyst formed by five Au atoms on the Fe-terminated (0001) surface ofhematite (α-Fe2O3). The calculations were performed withindensity functional theory including an on-site Coulomb term (DFT + U; U = 4 eV)as implemented in the VASP (Vienna Ab-Initio SimulationPackage) code. The results indicate that meanwhile wateradsorbs in the O-down orientation on a surface Fe ion on the clean hematitesurface, on the Au5/hematite system it adsorbs preferentially at themetal?support interface in a H-down orientation (an unstable structure on cleanhematite) and doubly bonded with the support, namely, with a surface O ion andthe Au particle (Fig. 1). From this stable configuration the water moleculedissociates by overcoming a low activation barrier (around 0.1 eV), a processas favorable as on clean hematite. Hydroxyl groups resulted to be linked at themetal?oxide interface in the dissociative final state. In this way, supportedAu particle is able to provide the necessary surface sites for the adsorptionof species for further reactions in order to react with active OH groupspresent at the peripheral Au atoms. According to a recent theoretical work [1],CO tends to adsorb preferentially on positively Au atoms of tiny hematite-supportedAu particles. Our results show that after H2O dissociation, the Auatom that participates in this mechanism (number 5; Fig. 1) acquires a positivecharge (0.23e). Thus, concerning particularly with the WGSR, the CO moleculeshould preferentially adsorb on this Au atom to form a COyHxintermediate, whose decomposition would yield finally to CO2 and H2.