Promoter and poisoning effects on NO-catalyzed dissociation on bimetallic RhCu(111) surfaces

A systematic density functional study of NO dissociation on Rh(111), Cu(111), and two different RhCu(111) bimetallic surfaces was conducted using a periodic supercell approach. Properties of the transition states, energy barriers, and transition state theory reaction rates were obtained for relevant situations involving NO adsorbed on different compositions of fcc and hcp sites. The results demonstrate that the superior activity of Rh versus Cu toward NO dissociation has its origin in the stronger adsorption on the former surface, which permits NO dissociation without the need for extra energy to the system. Moreover, the presence of Cu atoms on a Rh surface exerts a promoter effect by decreasing the energy barrier with respect to the value for the single crystal surface. In contrast, the presence of Rh atoms on a Cu surface has a poisoning effect. Analysis of the adsorption energies provides some important clues as to the modification of the corresponding energy barriers induced by the presence of the second metal. The results indicate that it is possible to improve the catalytic properties of Rh surfaces by adding small quantities of Cu, in agreement with previous experimental findings.