Calorimetric studies of hydrocarbon adsorption on metal films V. Hydrocarbons on platinum

Heats of adsorption of methane, ethane, propane, ethylene, propylene, acetylene, methylacetylene, allene, and cyclopropane were measured at room temperature in an adsorption calorimeter on platinum films deposited in the 10−9 Torr region. Adsorption of methane produced a relatively low heat at a very low rate, and only a very limited adsorbed amount was achieved. The heat fell strongly as the surface coverage increased. Dehydrogenation of the methane molecule to CH2 and CH3 surface species was proposed to explain this behavior. Initial heats of adsorption of ethane and propane were relatively high, pointing to both cracking and partial dehydrogenation of these alkanes. A strong decrease of the heats with increasing coverage and a rather low extent of adsorption suggested that some specific surface sites were required. Acetylene and methylacetylene gave virtually the same heat which was initially lower than that of ethane and propane, but higher than that of alkenes. The heat did not exhibit any clear-cut dependence on the coverage in the major part of the investigated range. Adsorption of alkynes took place on a higher proportion of the surface compared to alkanes and alkenes. Nondissociative adsorption with the formation of perpendicularly attached surface species CCH2 and CCHCH3, respectively, appears to be compatible with the experimental results. Heats of adsorption of monoalkenes were lower than the heats of alkanes and also of alkynes, and mildly decreased with the increasing surface coverage. Partial dehydrogenation of the hydrocarbons and adsorption of the mobile split-off hydrogen atoms on the energetically most favorable sites were suggested to account for the observed behavior. Adsorption of allene produced an essentially coverage-independent heat which was higher than the heat of monoalkenes. A tentative interpretation involves formation of the same adsorbed structure as in the case of the isomeric methylacetylene. The initial heat of adsorption of cyclopropane was higher than that of alkenes, but slightly lower than that of propane. Partial dehydrogenation and limited, if any, fragmentation of the molecule are likely.