The adsorption and reactions of hydrocarbons on Molybdenum single crystal surfaces; when clean and in the presence of co-adsorbed sulfur or carbon

Abstract

The chemisorption and reactions of thiophene, 1,3-butadiene, butenes, and n-butane on clean Mo(100), and with sulfur or carbon overlayers, have been investigated using thermal desorption spectroscopy (TDS). The predominant reaction at low additive coverage (0–0.2 monolayers of S or C), and at low ambient pressure (10 -10 Torr) for unsaturated hydrocarbons is decomposition. Butane is the least reactive of the hydrocarbons, and a large fraction (95%) desorbs molecularly at all additive coverages. As additive (S or C) coverage increases the amount of decomposition decreases, enabling other reaction pathways to become more probable. Hydrogenation, partial dehydrogenation, and isomerization reactions are detected. Molecular binding on the additive overlayers was also found to be very different. On sulfur overlayers the binding of the hydrocarbons was weak (physisorption), usually on the order of the heat of sublimation (9–10 kcal/mol). However, molecular binding on carbon overlayers was stronger: the heat of desorption was 17–23 kcal/mol for thiophene and butadiene, 12–15 kcal/mol for the butenes, and 11 kcal/mol for butane. In addition, isomerization of 1-butene to 2-butene, occurred on the carbon overlayer. It is suggested that the metal sites control the reactions observed (except for isomerization). This accounts for the similarity in surface reaction product distributions, and explains why the difference in molecular hydrocarbon binding between the sulfur covered and carbon covered surface plays a minor role in these reactions.