Multifold enhancement in water dissociation with Ag/Ni bimetallic alloy surfaces

Abstract

Water dissociation is an indispensable step in industrial as well as in natural processes. It is the rate-determining step in the industrially important water gas shift reaction. The barrier for the dissociation of H2O on metal surfaces is very high and therefore, the probabilities of H2O dissociation on these metal surfaces in the energy range of industrial relevance are very low. Thereby, there is a clear need of a better catalyst for this reaction. Bimetallic alloys have unique electronic and catalytic properties as compared to the respective monometals. In this present study, we have investigated the catalytic role played by the Ag/Ni bimetallic alloy surfaces towards H2O dissociation. This is a thermodynamically stable alloy and has already been synthesized experimentally as nanoparticles [J. Phys. Chem. C. 113 (2009) 1155–1159], [J. Phys. Chem. C. 114 (2010) 14,309–14,318] and this can aid the surface phenomenon. Systematic density functional theory calculations were performed on a series of overlayer starting from pure Ag(111) and we report the Ag-based catalyst, Ni9_Ag(111) (full monolayer coverage of Ni on Ag top layer) to act as a better catalyst with a barrier for H2O dissociation of 0.13 eV compared to 1.78 eV for pure Ag(111). A detailed analysis behind this lowering of the barrier is found to be governed by the large shift in the d-band center value of the metal due to alloying. Molecular orbital pictures provided further insight into the interactions of H2O with the surfaces. Transition state calculations also showed that the subsequent addition of Ni atom to Ag(111) surfaces decreases the dissociation barriers gradually. Finally, the semi-classical tunneling probability is computed using minimum energy path including the effect of surface temperature with the lattice atom motion. We observed an increase in surface temperature increases the dissociation probability, where, the extent of increase is strongly dependent on the change in the barrier heights.