Dissociative adsorption of methane on Pd(679) surface

Abstract

The reaction of methane with a Pd(679), Pd(S)−[7(111) × (310)], single crystal was studied using a high-pressure cell and Auger electron spectroscopy, low energy electron diffraction, and temperature programmed desorption techniques. Methane was dissociatively chemisorbed at 1 Torr and at surface temperatures of ⩾. 400 K with the formation of surface carbon and hydrogen species. The surface hydrogen also diffused into the bulk of the crystal. The initial dissociative sticking probability of methane on Pd(679) was 10 −9 − 10 −7 with an activation energy of ∼ 10.7 kcal/mol. The extent of CH 4 decomposition was strongly temperature dependent in that the surface carbon formed fractional monolayers (ML) at 400–500 K but produced multilayer coverages, e.g. 10 ML, at 600 K. The multilayer carboneous layers were disordered irrespective of the surface coverage. Oxidative titration cycles that included post-saturation of the carbon- and hydrogen-covered surfaces with oxygen followed by temperature programmed oxidation (TPO) led to the desorption of CO, CO 2 and H 2. No H 2O formation was observed in the TPO cycles, and the desorbed H 2 was stoichiometrically equivalent to carbon originating from the methane decomposition. The desorption data, along with CO chemisorption experiments, indicated that carbon species resulting from CH 4 decomposition on Pd(679) formed clusters that were located primarily in the vicinity of stepped and kinked edges and left the majority of the Pd surface bare as further adsorption sites. The overall reaction, albeit stepwise executed, amounted to partial oxidation reactions CH 4 + 1 2 O 2 → CO + 2 H 2 and CH 4 + O 2 → CO 2 + 2H 2. These reaction stoichiometries indicate that Pd metal possesses the basic chemical properties for low temperature catalytic oxidative reforming of CH 4 to CO, CO 2, and H 2.