Precursor-mediated dissociation of n-butane on a PdO(101) thin film

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 sigma-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 similar to 215K 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 sigma-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 sigma-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 eta(1)(2H) configuration with two coordinate H-Pd bonds per molecule. Our DFT calculations also show that sigma-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 sigma-complexes play a general role as precursors in alkane activation on transition metal oxide surfaces. (C) 2010 Elsevier B.V. All rights reserved.