Structural rearrangements in chemisorbed hydrocarbon layers: 1,3-butadiene on rhodium/alumina

Abstract

Transmission infrared spectroscopy has been used to study the chemisorption of 1,3-butadiene (C{sub 4}H{sub 6}) on Rh/Al{sub 2}O{sub 3}. Specific interaction between the hydrocarbon and metal surface is first observed at {approx}200 K as a variety of carbonaceous species with varying H content are produced in the adsorbed layer. The resulting structures are rich in CH{sub 2} and CH{sub 3} groups. The spectra between 225 and 300 K are characteristic predominantly of a di-{sigma} (MCH{sub 2}MCHCH=CH{sub 2}) structure that coexists with a C{sub 4} metallocycle species where the C{sub 4} chain is symmetrically coordinated to a metal site. The ratio of unsaturated to saturated species is seen to decrease between 225 and 300 K. Both double bonds in the parent diene are entirely consumed by 300 K. The di-{sigma} species is thermally unstable at T {ge} 300 K. At higher temperatures (300-350 K), C-C bond scission in C{sub 4} species leads to the formation of adsorbed ethylidyne, M{sub 3}CCH{sub 3}, which is thermally stable up to 450 K. Subsequent C-H bond scission, at high temperatures, in the ethylidyne structure as well as in C{sub x}H{sub y}(a) species yields a hydrogen-deficient carbonaceous residue, most likely a polymer.