HYDROGENATION AND H-D EXCHANGE-REACTIONS OF C3-C6 SUBSTITUTED ALKENES ON FE(100)

The adsorption and reaction of propylene, 1-butene, isobutene, cis- and trans-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene were studied on H-saturated Fe(100) (designated Fe(100)-H). Molecular desorption of both propylene and 1-butene occurs below 200 K, without incorporation of deuterium on Fe(100)-D. However, there are reaction pathways that form alkenes above 200 K and readily incorporate deuterium. These reactions are attributed to the beta-hydride elimination of stable alkyls. Some alkane is formed from propylene and 1-butene, but the yield from beta-hydride elimination to form the alkenes is much greater. The extent of H-D exchange in the alkene indicates that the surface alkyls formed are predominantly terminal. Multiple H-D exchange into the alkenes occurs to a minor extent and is attributed to surface defects; apparently internal alkyls (secondary, tertiary) form only at defect sites. Isobutene forms surface alkyls to a much lesser extent than does 1-butene, presumably due to additional steric hindrance about the carbon-carbon double bond. However, isobutyl groups appear to have a much greater propensity for alkane formation than do propyl and n-butyl. The internal alkenes cis-2-butene, trans-2-butene, and 2-methyl-2-butene yield less alkane than does 1-butene, and the sterically hindered tetrasubstituted 2,3-dimethyl-2-butene does not hydrogenate to a measurable extent.