Investigation of chemisorbed molecular states for oxygen on rhodium (111)

Using density-functional theory, we provide the first conclusive evidence o f the existence of a molecularly chemisorbed state for oxygen on the Rh (11 1) surface. Four species are identified: a paramagnetic state above the bri dge site with a binding energy of 1.95 eV, a more weakly bound paramagnetic state above the top site with a binding energy of 0.95 eV, and two nonmagn etic states above the face-centered-cubic (fcc) and hexagonal-close-packed (hcp) hollow sites each with a binding energy of 1.98 eV. We compare these results with our calculations of the binding energy for atomic oxygen on th e fcc and hcp hollow sites and an upper bound on the dissociation barrier t o understand major portions of the dissociation reaction coordinate. Combin ing our data with the experimental and theoretical results for oxygen disso ciation on many other fcc (111) metal surfaces, we conclude that all these metal surfaces possess similar minima corresponding to physisorption, molec ular chemisorption, and dissociative chemisorption. Despite these similarit ies, the differing binding energies and barrier heights account for the var ying mechanisms of oxygen dissociation on different fcc (111) surfaces. (C) 2000 American Institute of Physics. [S0021-9606(00)70633-7].