ADSORPTION SITE OF ALKANES ON METALS AND ASSOCIATED HYDROGEN PRESSURE EFFECTS

A series of arguments, based on experimental results concerning mainly methane and ethane, in favour of an associative mechanism of the hydrocarbon with an adsorbed H atom for the adsorption step of alkanes on metal surfaces are reviewed. The reaction scheme for the adsorption step is written: A-H + H(a) + zS = [A-H2]a = A(a) + H-2 + wS. As w has a smaller value than z, a coverage function in hydrogen, of the type (1-theta(H))z if the adsorption step is rate determining, or (1-theta(H))z-w if the adsorption step is near to equilibrium, contributes to the kinetic equation of the catalytic reaction. Due to the large value of z (8 or more), this function contains a hydrogen pressure and a temperature dependence of the same type as measured on the rate of the catalytic reaction. It also contains a demanding character.