The adsorption and reaction of methanol on Pd(100). I. Chemisorption and condensation

Abstract

The adsorption,desorption, and condensation of methanol on a clean Pd(100) surface has been studied in UHV at 77 K using high‐resolution electron energy loss spectroscopy,thermal desorption spectroscopy, UV photoemission, work function measurements, and low energy electron diffraction. CH3OH adsorbed readily with a high initial sticking probability which remains nearly constant up to monolayer coverages (∼6×1014/cm2) indicating precursor state adsorption kinetics. The first 20% of this methanol spontaneously decomposes, but can largely recombine to desorb as methanol. The nature of this species and other decomposition products are discussed in part II. The remaining methanol chemisorbs nondissociatively having a bonding energy of 10.8 kcal/mol; no long‐range order can be observed. The first physisorbed and later condensed layers are readily distinguishable and have desorption energies of 9.0 and 7.2 kcal/mode, respectively. The adsorption of methanol is accompanied by a continuous work function decrease to a final value of −1.60 eV after the deposition of ∼3 layers. A layer‐by‐layer growth mechanism is observed with physisorbed methanol showing first order kinetics and the absence of hydrogen bonding. This physisorbed layer still alters the surface dipole layer and shows some signs of additional interactions with the surface not present in the subsequent condensed layers. The subsequent condensed layers exhibit fractional order kinetics, which is interpreted to reflect hydrogen bonded ’’chains’’ of methanol molecules similar to that occurring in the melt of crystalline methanol.