Adsorption and Thermal Reaction of Short-Chain Alcohols on Ge(100)

• Published in: Journal of physical chemistry. C 2013-02, Vol.117 (6), p.2760-2768
• Main Authors: Lin, Tsung-Hsiang, Lin, Bo-Yu, Hao, Ting, Chien, Hsiu-Yun, Wang, Jeng-Han, Hung, Wei-Hsiu
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Solid-gas interface; Surface physical chemistry
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp308990x

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 CH3O 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 (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 C4H9OH 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.