CO dissociation on potassium-promoted aluminium

Abstract

Several catalytic reactions involving CO bond cleavage on transition metal surfaces are promoted by the presence of alkali atoms, and considerable interest has focused on the nature of strongly perturbed metal–CO potentials. Here I present experimental data on CO bond weakening and dissociation in carbon monoxide co-adsorbed with potassium on the (100) surface of aluminium. Atomic carbon and oxygen were observed, as well as precursor states to dissociation with CO stretch frequencies as low as 1,060 cm−1. Thus, despite its free-electron character, aluminium responds to alkali adsorption in much the same way as typical transition metals. This implies that CO bond weakening on alkali-promoted metal surfaces is caused by perturbations of the joint density of alkali and substrate sp-electrons, and that the role of d-states is not crucial. Vibrational and electronic spectra show that aluminium oxide and aluminium carbide form on annealing to 700 K. Neither CO2 nor high-temperature CO desorption from the alkali-promoted aluminium surface was detected.