The effect of carbon monoxide on the oxidation of four C-6 to C-8 hydrocarbons over platinum, palladium and rhodium

The oxidations of benzene, toluene and I-hexene, alone and in mixtures with isooctane and carbon monoxide, have been investigated over alumina-support ed platinum, palladium and rhodium under three different conditions. In the absence of CO, oxidation of all hydrocarbons is complete at the lowest tem perature with platinum. However, inclusion of CO greatly inhibits hydrocarb on oxidation on platinum. There is also moderate inhibition of palladium, b ut with rhodium oxidation of the aromatics is somewhat enhanced by the pres ence of carbon monoxide. The activity order then becomes Rh>Pd>Pt. Converse ly, unsaturates inhibit CO oxidation to a lesser extent, this effect being greatest for hexene and toluene with palladium and least with benzene on rh odium. In mixtures of the four hydrocarbons, with or without CO present, th e order of removal is always hexene followed by toluene, benzene and isooct ane with rhodium exhibiting the largest separation between toluene and benz ene. This order appears to be largely governed by competitive adsorption be tween hydrocarbons of different adsorption strength since the pattern of re activity is very different when the unsaturates are oxidised alone. Under v ery lean conditions platinum is the most active for the oxidation of benzen e alone, palladium for toluene and rhodium for hexene alone. However, relat ive activities are strongly dependent on conditions since the reactions exh ibit Very different kinetic orders in oxygen with each metal. The oxidation s of hexene and toluene are positive order in oxygen over Pt and Pd but neg ative 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 orde r with Ph. Some of these trends can also be rationalised in terms of relati ve strengths of adsorption. (C) 2000 Elsevier Science B.V. All rights reser ved.