SPECTROSCOPIC STUDIES OF METHANOL DECOMPOSITION ON PD(111)

The thermal decomposition of methanol on a clean Pd{111} surface has been investigated using X-ray photoelectron spectrometry (XPS), secondary ion mass spectrometry (SIMS) and thermal desorption spectrometry (TDS). In this work, we observe that the methanolic C-O bond activation on Pd{111} is a strongly coverage dependent process. At lower and higher coverages, XPS data suggest that CH3Oads is the: principle product. At near monolayer coverage the methanolic C-O bond dissociates at 175 K to produce CH3,ads, which is characterized by a C 1s binding energy of 283.8 eV, and is stable up to 400 K. A stepwise dehydrogenation of CH3,ads to form CH2,ads (methylene) and CHads (methylene) is observed above 400 K. Surface methyl and its dehydrogenated forms are characterized both by the chemical shift of the XPS C 1s peak from 283.8 to 284.8 eV and by positive ion SIMS peaks with m/e = 15, 14 and 13. The gaseous products detected by TDS are methanol, CO, hydrogen, methane and water. This observation implies that the methanolic C-O bond cleavage is contingent upon a special molecular arrangement of precursors before reaction and that the CH3,ads product is stabilized by ancillary reaction products, such as CO.