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 < 350 °C (i), 350 °C < T < 700 °C (ii), and 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.
Research on Chemical Intermediates – Springer Journals
Published: Oct 31, 2016
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