Reducibility is an essential characteristic of oxide catalysts in oxidation reactions following the Mars−van Krevelen mechanism. A typical descriptor of the reducibility of an oxide is the cost of formation of an oxygen vacancy, which measures the tendency of the oxide to lose oxygen or to donate it to an adsorbed species with consequent change in the surface composition, from M n O m to M n O m−x. The oxide reducibility, however, can be modified in various ways: for instance, by doping and/or nanostructuring. In this review we consider an additional aspect, related to the formation of a metal/oxide interface. This can be realized when small metal nanoparticles are deposited on the surface of an oxide support or when a nanostructured oxide, either a nanoparticle or a thin film, is grown on a metal. In the past decade, both theory and experiment indicate a particularly high reactivity of the oxygen atoms at the boundary region between a metal and an oxide. Oxygen atoms can be removed from interface sites at much lower cost than in other regions of the surface. This can alter completely the reactivity of a solid catalyst. In this respect, reducibility of the bulk material may differ completely from that of the metal/oxide surface. The atomistic study of CO oxidation and water-gas shift reactions are used as examples to provide compelling evidence that the oxidation occurs at specific interface sites, the actual active sites in the complex catalyst. Combining oxide nanostructuring with metal/oxide interfaces opens promising perspectives to turn hardly reducible oxides into reactive materials in oxidation reactions based on the Mars−van Krevelen mechanism.

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The adsorption of Ag and Au atoms and Ag 4 and Au 4 clusters on the stoichiometric TiO 2 (anatase) and ZrO 2 (tetragonal) (101) surfaces has been investigated using DFT+U calculations with and without the inclusion of van der Waals (VdW) forces. We have considered the role of VdW interactions on the physical properties of the adsorbed species using three different approaches: two variants of the pair-wise force field method proposed by Grimme (DFT+D2 and DFT+D2'), and the vdW-DF method where the vdW contribution is expressed directly as a function of the electron density. The results show that already at the level of metal atoms and small clusters the inclusion of vdW interactions can change the order of stability of various isomers and, more important, can result in major corrections to the adsorption energies. These, in turn, can affect other properties as for instance the structure and chemical reactivity of supported metal particles on oxides or their diffusion properties.

The structure, stability, and electronic properties of a series of zirconia nanoparticles between 1.5 and 2 nm in size, (ZrO 2±x ) n within the n = 13 to n = 85 range, have been investigated through density functional theory (DFT) based calculations. On the methodological side we compare results obtained with standard DFT functionals with the DFT+U approach and with hybrid functionals. As representative models, octahedral and truncated octahedral morphologies have been considered for the zirconia nanoparticles. Partly truncated octahedral nanoparticles with ZrO 2 stoichiometry display the highest stability. On the contrary, nanoparticles with octahedral and cuboctahedral (totally truncated octahedral) shapes are less stable due to oxygen deficiency or excess, respectively. We show that the calculated formation energies scale linearly with the average coordination number of the Zr ions and converge to the bulk value as the particle size increases. The formation energy of a neutral oxygen vacancy in the nanoparticles has also been investigated. In comparison to the ZrO 2 (101) surface of tetragonal zirconia, we found that three-and four-coordinated O atoms have similar formation energies. However, the two-coordinated O ions on the surface of the nanoparticles have considerably smaller formation energies. In this respect the effect of nanostructuring can be substantial for the reactivity of the material and its reducibility. The low-coordinated sites create defective states in the electronic structure and reduce the effective band gap, which can result in enhanced interaction with deposited species and modified photocatalytic activity.

Transition Metal (TM) atom adsorption on γ-graphyne is here studied to unravel the electronic and magnetic properties tuning of this 2D carbon allotrope, with possible repercussions on molecular storage, sensing, and catalytic properties. A thorough density functional theory study, including dispersion, of the structural, energetic, diffusivity, magnetic, and doping properties for all 3d, 4d, and 5d TM adatoms adsorbed on γ-graphyne is provided. Overall, TMs strongly chemisorb on γ-graphyne acetylenic rings, except d 10 group XII TMs which physisorb. Diffusion energy barriers span 0.5-3.5 eV and adatom height with respect the γ-graphyne sheet seems to be governed by TM atomic radius. All TMs are found to give n-doped γ-graphyne, where charge transfer decays along d series due to the increasing electronegativity of TMs. Middle TMs infer noticeable magnetism to γ-graphyne, yet magnetism is heavily quenched for early and late TMs. The large adsorption energies close to parent TM bulk cohesive energies, the high diffusion energy barriers, and the coulombic repulsion between positively charged TM adatoms provide a good environment for TMs to disperse over the graphyne.