Benzyl coupling on bismuth-modified Pt(111)

Abstract

It has been found that adsorbed benzyl (O-CH2(a)), produced from methylcyclohexane, can undergo dimerisation to adsorbed bibenzyl (O-CH2-CH2-O) on a bismuth-modified Pt(111) surface. In this case, the bismuth was postdosed to the benzyl adlayer, as in bismuth postdosing thermal desorption mass spectroscopy (BPTDS). The bibenzyl product desorbs to give bibenzyl gas in good yield with respect to the starting benzyl or the reacted methylcyclohexane. This reaction competes poorly with simple hydrogenation of the benzyl species to toluene when adsorbed hydrogen is also present on the bismuth-dosed surface. In the absence of hydrogen, dimerization dominates the chemistry of this intermediate, although some direct desorption of benzyl radical also occurs. The dependence of the activation energy for benzyl coupling on the thickness of the bismuth film suggests that the bismuth resides between the Pt surface and the benzyl adlayer. Benzyl coupling and desorption occurs on Bi/Pt, whereas only its decomposition occurs on clean Pt. This is attributed to two effects: the weakness of C-Bi bond leads to a low barrier for benzyl coupling or desorption; and, (2) bismuth blocks Pt sites needed to stabilize the fragments of decomposition. Both D2-BPTDS and this bismuth-induced coupling reaction in simple BPTDS were used to monitor the coverage of benzyl during its thermal decomposition on bismuth-free Pt(111), and the temperature dependence of the rate of its decomposition. Estimates of the bismuth-carbon bond strengths are also presented.