The effect of carbon monoxide on the oxidation of four C₆ to C₈ hydrocarbons over platinum, palladium and rhodium

The oxidations of benzene, toluene and 1-hexene, alone and in mixtures with isooctane and carbon monoxide, have been investigated over alumina-supported platinum, palladium and rhodium under three different conditions. In the absence of CO, oxidation of all hydrocarbons is complete at the lowest temperature with platinum. However, inclusion of CO greatly inhibits hydrocarbon oxidation on platinum. There is also moderate inhibition of palladium, but with rhodium oxidation of the aromatics is somewhat enhanced by the presence of carbon monoxide. The activity order then becomes Rh>Pd>Pt. Conversely, unsaturates inhibit CO oxidation to a lesser extent, this effect being greatest for hexene and toluene with palladium and least with benzene on rhodium. In mixtures of the four hydrocarbons, with or without CO present, the order of removal is always hexene followed by toluene, benzene and isooctane with rhodium exhibiting the largest separation between toluene and benzene. This order appears to be largely governed by competitive adsorption between hydrocarbons of different adsorption strength since the pattern of reactivity is very different when the unsaturates are oxidised alone. Under very lean conditions platinum is the most active for the oxidation of benzene alone, palladium for toluene and rhodium for hexene alone. However, relative activities are strongly dependent on conditions since the reactions exhibit very different kinetic orders in oxygen with each metal. The oxidations of hexene and toluene are positive order in oxygen over Pt and Pd but negative order over Rh. On the other hand the oxidation of benzene seems to be positive order in oxygen for Pt, negative order with Pd and near zero order with Rh. Some of these trends can also be rationalised in terms of relative strengths of adsorption. (C) 2000 Elsevier Science B.V.