Surface oxygen dynamics and H2 oxidation on cobalt spinel surface probed by 18O/16O isotopic exchange and accounted for by DFT molecular modeling: facile interfacial oxygen atoms flipping through transient peroxy intermediate

Surface dynamics of reactive oxygen species (ROS) and hydrogen oxidation on cobalt spinel nanocatalyst, faceted predominantly with (100), were investigated by means of pulsed 16O2/18O2 isotopic exchange and thermoprogrammed surface reaction investigations, corroborated by periodic spin-unrestricted DFT-PW91+U modelling of the principal surface reaction molecular events. Three temperature windows [T 700 °C (iii)] where identified and associated with diatomic oxygen species of superoxo (CoO–(O2)−–CoO, peroxo (CoT–(O2)2−–CoO) (i), and monoatomic metaloxo (CoT–O, CoO–O) nature (ii), and with oxygen vacancies VO (iii). A new oxygen isotopic exchange pathway was proposed that involves peroxy transient intermediates produced during ROS oxygen surface diffusion. Flipping of the supra- (RSO) and intra-facial (lattice) oxygen moieties within the [18Osup–16Oint]2− peroxy unit requires 0.39 eV only, opening an easy pathway for rapid isotopic exchange without explicit formation of energetically more costly oxygen vacancies. The latter may occur effectively at T > 700 °C. The catalytic activity of ROS species was probed by H2 oxidation reaction. The diatomic ROS reactivity (below 160 °C) is characterized by E a = 16 kcal/mol, and for monoatomic species (between 160 °C and 300 °C) it falls to E a = 9.2 kcal/mol. It was shown that suprafacial dehydroxylation of ROS generated water is energetically less costly (E a = 1.15 eV) than intrafacial dehydroxylation (E a = 1.71 eV) entailing removal of water associated with the lattice oxygen. Thus, the former may operate even at relatively low temperatures (below 300–350 °C). The appearance of significant amount of H 2 16 O in the reaction products is related to easy isotopic 18O/16O scrambling via transient peroxo intermediates, and is not diagnostic of direct involvement of the Mars van Krevelen mechanism.