Probing Topological Electronic Effects in Catalysis: Thiophene Adsorption on NiMoS and CoMoS Clusters

A general two-step theoretical approach to study electronic redistributions in catalytic processes is presented. In the first step, density functional theory (DFT) is used to fully optimize two geometries: the cluster representing the catalyst and the cluster plus adsorbed molecule system. In the second step, the converged electron density is divided into multipoles centered on atomic sites according to a distributed multipole analysis which provides detailed topological information on the charge redistribution of catalyst and molecule before and after adsorption. This approach is applied to thiophene adsorption on the 10 (1) over bar0 metal edge of Ni(Co)MoS catalysts and compared to the same reaction on bare MoS2. Calculated adsorption energies, geometries and multipole analysis indicate weak thiophene chemisorption on both cases. A Coulombic bond model showed that surface metal-sulfur bond strengths in Ni(Co)MoS promoted catalysts are considerably smaller than in bare MoS2, thus confirming the origin of the enhancement of hydrodesulfurization (HDS) activity in these catalysts.