Synergistic Promotion of NO-CO Reaction Cycle by Gold and Nickel Elucidated using a Well-Defined Model Bimetallic Catalyst Surface

A bimetallic catalyst of Au and Ni significantly increased the catalytic activity of the NO-CO reaction in comparison to monometallic Au and Ni catalysts. Unraveling the roles of Au and Ni atoms in the each of the NO-CO reaction steps occurring on the Au-Ni catalyst surface is crucial to reveal the origin of the increased activity. For this purpose, a well-defined Au/Ni(111) model catalyst was prepared, on which CO and NO adsorption, their coadsorption, NO dissociation, CO2 formation, and N-2 formation were investigated using infrared reflection absorption spectroscopy, temperature-programmed desorption/reaction, and density functional theory calculations. In the reaction process, the catalyst surface would be dominantly covered by N and O atoms, which would be removed from the surface by N-2 formation and CO2 formation. O atoms preferentially occupy the Ni hollow sites by segregating N atoms to the adsorption sites made up of Au and Ni atoms. Thus, only the N-2 formation step was affected by the Au atoms. The activation energy for the N-2 formation step, which was assigned as a rate-limiting step, was significantly lowered by the Au atoms, and this effect will contribute to the decrease of the activation energy of the overall NO-CO reaction. These results suggest that, by utilizing the adsorption site preferences among the coadsorbates on the bimetallic surface, the activation energy of a rate-limiting step would be significantly decreased; this could be useful in the development of advanced NOx reduction catalysts.