Precursor-mediated dissociation of n-butane on a PdO(1 0 1) thin film

Abstract

We investigated the molecular adsorption and dissociation of n-butane on a PdO(1 0 1) thin film using temperature-programmed reaction spectroscopy (TPRS) experiments and density functional theory (DFT) calculations. At low coverage, n-butane adsorbs on PdO(1 0 1) in a molecular state that is more strongly bound than n-butane physisorbed on Pd(1 1 1). This molecularly adsorbed state of n-butane on PdO(1 0 1) corresponds to a σ-complex that forms on the rows of coordinatively unsaturated (cus) Pd atoms of the oxide surface. TPRS results show that a fraction of the n-butane layer undergoes C–H bond cleavage below ∼215 K and that the resulting fragments are completely oxidized by the surface upon continued heating. The evolution of product yields with the n-butane coverage as well as site blocking experiments provide strong evidence that the n-butane σ-complex serves as the precursor to initial C–H bond cleavage of n-butane on PdO(1 0 1). DFT calculations confirm the formation of an n-butane σ-complex on PdO(1 0 1). In the preferred bonding geometry, the n-butane molecule aligns parallel to a cus-Pd row and adopts a so-called η 1(2H) configuration with two coordinate H–Pd bonds per molecule. Our DFT calculations also show that σ-complex formation weakens C–H bonds, causing bond elongation and vibrational mode softening. For methane, we predict that coordination with a cus-Pd atom lowers the barrier for C–H bond cleavage on PdO(1 0 1) by more than 100 kJ/mol. These results demonstrate that dative bonding between alkane molecules and cus-Pd atoms serves to electronically activate C–H bonds on PdO(1 0 1) and suggest that adsorbed σ-complexes play a general role as precursors in alkane activation on transition metal oxide surfaces.