Density functional theory study of sulfur tolerance of CO adsorption and dissociation on Rh-Ni binary metals

The effect of Ni addition to improve the sulfur tolerance of a Rh catalyst for CO dissociation was studied using density functional theory (DFT) methods. Adsorption and dissociation were considered over the (1 1 1) surfaces of binary Rh₁Ni₂ and Rh₂Ni₁ metals with comparison to pure Rh and Ni surfaces. Sulfur adsorption on the Rh ₁Ni₂(1 1 1) surface is 0.21 eV more endothermic than on the Rh(1 1 1) surface, suggesting that a Rh₁Ni₂ bimetallic catalyst has a higher sulfur tolerance than pure Rh catalysts due to a lower surface coverage of the sulfur poison. To compare catalytic activity in the presence of adsorbed sulfur, the CO dissociation rates over the binary and pure metals were calculated with 1/9 sulfur coverage. CO dissociation is fastest on the pure Rh surface under sulfur-free conditions, whereas among sulfur poisoned surfaces, the Rh₁Ni₂ surface shows the fastest CO dissociation rate. The CO dissociation barrier on Rh₁Ni₂ is destabilized less by a S coadsorbate than for the other metals. The addition of Ni atoms to a Rh catalyst improves the sulfur tolerance of the catalyst for CO dissociation by minimizing the repulsion between the adsorbed S atom and the CO dissociation transition state, as evidenced through a projected density of states analysis. The Rh₁Ni₂(2 2 1) stepped surface also shows a lower activation barrier and higher CO dissociation rate in the presence of sulfur than the Rh(2 2 1) stepped surface.