Partial Oxidation of Methanol on the Fe3O4(111) Surface Studied by Density Functional Theory

To understand recent temperature-programmed desorption (TPD) experiments carried out for methanol adsorbed on a Fe3O4(111) single crystal surface by Batista and co-workers, we accomplished a systematic density functional theory study on the various dehydrogenation pathways of methoxy species on that surface. For a mass/charge ratio of 30, these experiments detected two desorption peaks, one centered at about 330 and the second one at about 630 K, indicating that methoxide is partially oxidized to formaldehyde. Yet the origin of these two peaks has not been fully understood. Based on computed activation barriers for the H-transfer from methoxy species to the symmetrically distinct surface oxygen ions using the PBE+U approach and the HSE hybrid functional corrected for dispersion effects, we simulated the corresponding TPD peaks by numerical solution of a Polanyi-Wigner rate equation of first order. The simulated spectra using HSE suggest that the observed peaks are caused by the two inequivalent oxygen ions in the Fe3O4(111) surface. They have comparable activities in the protonation step upon adsorption of methanol but feature distinct reactivities toward the redox or H-transfer step causing significantly different activation barriers.