Tim Kox

In this work, we have used ab initio molecular dynamics at room temperature to study the adsorption and dissociation of a thin water film on the Co3O4 (111) surfaces, considering the Co-rich and O-rich terminations known as the A-type and B-type surface terminations, respectively. We investigate the occupation of active sites, the hydrogen bond network at the interface and the structural response of the surfaces to water adsorption. On both terminations, water adsorbs via a partial dissociative mode. The contact layer is populated by molecular water as well as OH groups and surface OH resulting from proton transfer to the surface. The B-termination is more reactive, with a higher degree of dissociation in the contact layer with water (46%). On the B-terminated surface water barely adsorbs on the Co2+ sites and almost exclusively binds and dissociates on the Co3+ sites. The interaction with the surface consists mostly, of Co3+-Ow bonds and proton transfer exclusively to the 3-fold unsaturated surface Os1. Hydrogen bonds between water molecules in the aqueous film dominate the hydrogen bond network and no hydrogen bonds between water and the surface is observed. The A-terminated surface is less reactive. Water binds covalenlty on Co2+ sites, with a dissociation degree of 13% . Proton transfer occurs mostly on the 3-fold unsaturated surface oxygens Os1. Besides, short-lived surface OH arising from proton transfer to 3-fold unsaturated surface oxygens Os2 are observed. H-bonding to surface Os1 and Os2 constitute 12.7% and 19.8% of the H-bond network, respectively, and the largest contribution is found among the water molecules (67.4%). On both surfaces, the coordination number of the active sites drives the relaxations of the outermost atoms positions to the their bulk counterparts. The occupation of active sites on the B-termination could reach up to 3 adsorbates per Co3+ leading to a binding motif in which the Co is octahedrally coordinated and which was observed experimentally.

CoFe2O4 is a promissing catalytic material for many chemical reactions. We have used ab initio molecular dynamic simulations to study the structure and reactivity of the A- and B-terminations of the low index CoFe2O4 (001) surfaces to water adsorption at room temperature. Upon adsorption, water partly dissociates on both termination with a higher dissociation degree on the A- termination (30% versus 19%). The 2-fold coordinated Fe3+(tet) in the tetrahedral voids and the 5-fold coordinated Fe3+(oct) in the octahedral voids are the main active sites for water dissociation on the A- and B-termination, respectively. Molecular water, hydroxydes and surface OH resulting from proton transfer to surface oxygens are present on the surfaces. Both water free sur- face terminations undergo reconstruction. The outermost Fe3+(tet) on the A-termination and B-termination move towards the nearby unoccupied octahedral voids. In the presence of a thin film of 32 water molecules, the reconstructions are partially and completely lifted on the A- and B-termination, respectively.

Co₃O₄, MgCo₂O₄ and MgO materials have been synthesized using a simple co-precipitation approach and systematically characterized. The total conversion of toluene to CO₂ and H₂O over spinel MgCo₂O₄ with wormlike morphology has been investigated. Compared with single metal oxides (Co₃O₄ and MgO), MgCo₂O₄ with the highest activity has exhibited almost 100% oxidation of toluene at 255 °C. The obtained results are analogous to typical precious metal supported catalysts. The activation energy of toluene over MgCo₂O₄ (38.5 kJ/mol) is found to be much lower than that of Co₃O₄ (68.9 kJ/mol) and MgO ((87.8 kJ/mol)). Compared with the single Co and Mg metal oxide, the as-prepared spinel MgCo₂O₄ exhibits a larger surface area, highest absorbed oxygen and more oxygen vacancies, thus highest mobility of oxygen species due to its good redox capability. Furthermore, the samples specific surface area, low-temperature reducibility and surface adsorbed oxygenated species ratio decreased as follows: MgCo₂O₄ > Co₃O₄ > MgO; which is completely in line with the catalytic performance trends and constitute the reasons for MgCo₂O₄ high excellent activity towards toluene total oxidation. The overall finding supported by ab initio molecular dynamics simulations of toluene oxidation on the Co₃O₄ and MgCo₂O₄ suggest that the catalytic process follows a Mars–van Krevelen mechanism.

The spinel Co₃O₄ has many beneficial properties for potential use in catalysis. In operando, water is always present and alters the properties of the catalyst. We have used ab initio molecular dynamics to understand the effect of water and solvation on the structure and reactivity of the Co₃O₄ (001) A-type and B-type surface terminations. Water adsorbs on both terminations via a partial dissociative mode, and the A-termination is seen to be more reactive. On this surface, a higher degree of dissociation is observed in the topmost layers of the crystal in contact with water. Water dissociates more frequently on the Co²⁺ sites (about 75%) than on the adjacent Co³⁺ sites, where the degree of dissociation is about 50%. Increasing water coverage does not change the degree of water dissociation significantly. OH⁻ adsorption on the Co²⁺ sites leads to a reduction of the amount of reconstruction and relaxation of the surface relative to the clean surface at room temperature. Proton transfer within the water films and between water molecules and surface has localized character. The B-terminated interface is less dynamic, and water forms epitactic layers on top of the Co³⁺ sites, with a dissociation degree of about 25% in the contact layer.