Precursor-mediated adsorption of oxygen on the (111) surfaces of platinum-group metals

The dissociative adsorption of oxygen on the (111) surfaces of platinum, pa lladium, and nickel has been investigated using nb initio local-spin-densit y calculations. For all three surfaces, adsorption is shown to be precursor -mediated and the structural, energetic, vibrational and electronic propert ies of the precursors are in very good agreement with the available experim ental information. The investigation of the transition states shows that on Pt and Pd the barriers for dissociation are comparable to (or at sufficien tly high coverage even higher than) the desorption barriers. In combination with large energies for atomic adsorption, this also leads to a high barri er for associative desorption-in agreement with observation. In contrast, t he dissociation barrier For O-2 on Ni(111) is low and occurs already for a less stretched molecule. The trends in molecular and atomic adsorption and in the dissociation barriers are discussed in relation to the geometric and electronic properties of the substrate and to the sticking probabilities o bserved in molecular-beam experiments.