Preferential site occupation by sulfur ligands regulates hydrogen activation on Pd catalysts for selective hydrogenation

The unique catalytic performance of sulfide-modified Pd is largely attributed to its enhanced selectivity for nitro hydrogenation versus dechlorination. However, the microscopic mechanism for such a high selectivity remains unclear. In this work we investigated the ligand adsorption behavior on the surfaces of a sulfided Pd using both chemical adsorption equilibrium model calculated from the CO chemisorption data and density functional theory calculation. The chemical adsorption data reveal preferential site occupation of sulfide ligands at most active Pd surface sites. There exist three types of Pd sites with varying catalytic activity, that is, corner, edge and terrace sites. For the sulfide-modified surface, the most active Pd corner sites preferentially bind sulfide ligand, followed by the less active Pd edge sites. Then only the least active Pd terrace sites are left exposed. Further kinetic measurements and the binding energy calculations reveal that dechlorination requires a significantly higher activation energy than nitro‑hydrogenation. The exposed less active terrace sites can only generate a weakly activated hydrogen, which is sufficient to reduce the nitro group but insufficient to break the C-Cl bond. The high selectivity due to a high preference in site occupation by sulfide ligands fundamentally suppresses the dechlorinating side reaction. The insights provided in this work not only unravels the quantitative understanding of surface coordination chemistry, but also makes this material design an unambiguous strategy in searching highly selective metal catalysts.