Transformations of ethylene on the Pd(111) surface: A density functional study

The chemistry of ethylene on group VIII metals is important for the petrochemical and polymer industries. Thus far, a complete first-principles- based conversion network of ethylene as a prototypical olefin system on metal surfaces is lacking. In this paper we present a comprehensive view on transformations of ethylene on Pd(111), based on density functional slab model calculations. Specifically, we characterized the thermodynamics and kinetics of C-H and C-C bond scission as well as H isomerization of a series of species C₂H_x (x = 0-4). Accordingly, dehydrogenation or hydrogenation processes are favored for most species, whereas reactions involving the scission or formation of the C-C bond feature high barriers. Ethynyl (CHC) likely is the precursor of C-C bond scission. Ethylidyne is found to be located in a basin of the potential energy surface, which allows one to rationalize relevant experimental observations. In addition, we note that one should be cautious when using information on the potential energy surface obtained with the unity-bond-index quadratic-exponential-potential (UBI-QEP) method.