Adsorption and thermal reaction of short-chain alcohols on Ge(100)

The adsorption and thermal decomposition of alcohols (CH₃OH, C₂H₅OH, and C₄H₉OH) on Ge(100) were investigated with temperature-programmed desorption and X-ray photoelectron spectra. At 105 K, CH₃OH adsorbs both molecularly and dissociatively on Ge(100). Chemisorbed CH₃OH molecules dissociate to form surface CH₃O and hydrogen in a temperature range 150-300 K. Surface CH ₃O can dehydrogenate to yield CH₂O as two desorption features, which depend on coverage. At small coverage, surface CH₃O undergoes mainly α-hydrogen elimination to desorb CH₂O at 490 K. At large coverage, another desorption of CH₂O occurs predominantly at 525 K, which is initiated by a recombinative desorption of CH₃OH. A calculation with density functional theory at the B3LYP/6-311+G** level shows that the dissociation of the O-H bond has a much smaller barrier (<40 kJ/mol) than those for C-O bond cleavage (>150 kJ/mol). Desorption of CH₂O results from the moderate barriers (∼110 kJ/mol) for cleavage of the C-H bond of surface CH₃O and weak adsorption energy of CH₂O (-56 kJ/mol). The recombination of surface CH₃O with H occurs at large coverage with an energy barrier 127-140 kJ/mol. Similarly to CH₃OH, C₂H₅OH and C₄H ₉OH undergo the mechanism of thermal reactions through formation of alkoxyl intermediates. The longer-chain alkoxyl decomposes to desorb aldehyde at lower temperature because the interaction of its alkoxyl chain with the surface is stronger. On annealing to ∼570 K, all alkoxyl groups are completely removed from the surface via dehydrogenation and recombination to desorb aldehyde and alcohol, respectively. At a large coverage, the longer-chain alkoxyl undergoes dehydrogenation to a larger extent than recombinative desorption.