Catalysis on evaporated metal films. V Reactions between cyclic hydrocarbons and deuterium

Abstract

Catalytic exchange reactions have been studied by means of a mass spectrometer between deuterium and cyclopentane over palladium and cyclohexane over rhodium, palladium, tungsten, platinum, nickel, oriented nickel and oriented rhodium. Activation energies, frequency factors and distributions of initial products were determined. The reactions involved multiple exchange by repeated second-point adsorption. Half the hydrogen atoms were readily exchanged, and a study of the temperature-dependence of the product distributions showed that it required an activation energy of 4 to 8 kcal/mole in excess of that for adsorption-desorption before the remaining half could be exchanged. Considerations of molecular geometry revealed that in order to exchange the second half of the hydrogen atoms the molecule had to 'turn over' on the surface, and for cyclopentane the process for turning over must require second-point adsorption by the same carbon atom, but with cyclohexane there is the alternative process of exchanging two adjacent equatorial hydrogen atoms. Theories are developed which account satisfactorily for the observed initial product distributions. The pressure-dependence of reaction rate was determined for cyclohexane over palladium. Deuterium was strongly adsorbed and cyclohexane weakly adsorbed. Exchange of cyclohexane over oriented films of nickel and rhodium showed marked differences in distribution compared with unoriented films, suggesting different catalytic activity of different crystal planes. Only exchange was observed with benzene over nickel, but there was simultaneous exchange and deuteration over palladium. The initial product pattern over palladium showed that a redistribution reaction (in the sense of Wagner et al. (1952) did not occur during deuteration and that benzene was adsorbed mainly as phenyl radicals during the exchange reaction. The reaction between cyclopropane and deuterium was examined over rhodium and simultaneous exchange and deuteration were observed. As determined by the temperature range for reaction the order of reactivity in exchange was cyclopropane>cyclopentane>cyclohexane, and the order of catalytic activity of the metals was similar to that observed previously for the exchange of other saturated hydrocarbons.