THERMAL SURFACE CHEMISTRY OF 1-, 3-, AND 4-METHYL-CYCLOHEXENE ON SI(111)7X7 - DEHYDROGENATION MECHANISM AND THE EFFECTS OF METHYL SUBSTITUTION

摘要

Recent studies of 6-membered cyclic olefins on Si(111)7 X 7 have revealed a surprisingly high level of reactivity as demonstrated by the catalytic dehydrogenation reaction of cyclohexene to benzene. The present work examines the room-temperature adsorption of 1-, 3-, and 4-methyl-cyclohexene on clean, sputtered and oxidized Si(111)7 X 7 surfaces by thermal desorption spectrometry. AU three methyl-cyclohexene isomers are found to undergo dehydrogenation reactions to methyl-cyclohexadiene and toluene predominantly at 500-600 K. The dehydrogenated products appear to depend on the relative location of the methyl group with respect to the C=C bond in the cyclic olefin. The reactivities of the three isomers are compared to those found for cyclohexene and 1-methyl-1,4-cyclohexadiene. Furthermore, the mass 78 desorption peak observed at similar to 650 K is attributed to a methyl-elimination reaction that produces benzene and/or cyclohexadiene. A large post-exposure of H atoms prior to the thermal desorption of methyl-cyclohexene (cyclohexene) adsorbed on Si(111)7 X 7 at room temperature is found to inhibit toluene (benzene) evolution and concomitantly enhance molecular desorption. However, no significant reduction in methyl-cyclohexadiene (cyclohexadiene) evolution is found with the H post-exposure. Based on our present and previously reported results, we propose a primary dehydrogenation pathway that involves repositioning of the adsorbed olefin to sequentially interact with new Si (dangling bond and/or back bond) active sites by diffusion to other dangling-bond site and/or rotation about the adsorption site. The apparent ineffectiveness of the H post-exposure to inhibit methyl-cyclohexadiene (cyclohexadiene) evolution suggests a second dehydrogenation pathway that involves a stationary olefin interacting with two neighbouring Si back bonds simultaneously. Finally, the adsorption and dehydrogenation efficiency of methyl-cyclohexene are both found to be less than that of cyclohexene and appear to depend on a steric hindrance effect due to the methyl group and a methyl-to-surface interaction that reduces the mobility of the olefin on the Si surface, respectively. References: 37