Reduction of Hydrogenated ZrO2 Nanoparticles by Water Desorption

Reduction of zirconia by water desorption from a hydrogenated surface is the topic of this study. The focus is on the role of nanostructuring the oxide reducibility measured by the cost of formation of oxygen vacancies by water desorption. We have performed density functional theory calculations using the Perdew-Burke-Ernzerhof + U approach and including dispersion forces on the adsorption, dissociation, diffusion of hydrogen on the ZrO2 (101) surface and on Zr16O32, Zr40O80, and Zr80O160 nanoparticles (NPs). The process involves the formation of a precursor state via diffusion of hydrogen on the surface of zirconia. The results show that O vacancy formation via H2O desorption is more convenient than via direct O-2 desorption. The formation of an OsH2 surface precursor state to water desorption is the ratedetermining step. This step is highly unfavorable on the ZrO2 (101) surface both thermodynamically and kinetically. On the contrary, on zirconia NPs, characterized by the presence of low coordinated ions, water desorption becomes accessible such that even at temperatures close to 450 K the reaction becomes exergonic. The study shows the role of nanostructuring on the chemical and electronic properties of an oxide

Functionalization of γ-graphyne by transition metal adatoms

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 d10 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

Structure and Properties of Zirconia Nanoparticles from Density Functional Theory Calculations

The structure, stability, and electronic properties of a series of zirconia nanoparticles between 1.5 and 2 nm in size, (ZrO_2±<i>x</i>)_<i>n</i> within the <i>n</i> = 13 to <i>n</i> = 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+<i>U</i> 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 ZrO₂ 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 ZrO₂(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

Effect of Nanostructuring on the Reactivity of Zirconia: A DFT+<i>U</i> Study of Au Atom Adsorption

The reactivity of zirconia nanoparticles has been investigated by means of DFT+<i>U</i> calculations as a function of the morphology and stoichiometry. For comparative purposes, a single Au atom has been deposited on the stoichiometric and O-deficient regular (101) surface, on the stepped (156) surface, and on nanoparticles in the range of 0.9–1.9 nm in size. We show that, under stoichiometric conditions, nanostructuring leads to enhanced binding energies and redox processes with the supported metal that are not found on the extended surfaces. These new features are due to the structural flexibility and peculiar electronic structure displayed by the nanoparticles. In this respect, nanostructuring of oxide supports can modify and possibly improve the catalytic activity of the deposited metals. In contrast, we show that under reducing conditions nanostructuring stabilizes the O vacancies making zirconia nanoparticles less reactive toward Au adsorption than O-deficient extended surfaces

Theory of Ferromagnetism in Reduced ZrO_2–<i>x</i> Nanoparticles

Bulk ZrO₂ is both nonreducible and nonmagnetic. Recent experimental results show that dopant-free, oxygen-deficient ZrO_2–<i>x</i> nanostructures exhibit a ferromagnetic behavior at room temperature (RT). Here, we provide a comprehensive theoretical foundation for the observed RT ferromagnetism of zirconia nanostructures. ZrO₂ nanoparticles containing up to 700 atoms (3 nm) have been studied with the help of density functional theory. Oxygen vacancies in ZrO₂ nanoparticles form more easily than in bulk zirconia and result in electrons trapped in 4d levels of low-coordinated Zr ions. Provided the number of these sites exceeds that of excess electrons, the resulting ground state is high spin and the ordering is ferromagnetic. The work provides a general basis to explain magnetism in intrinsically nonmagnetic oxides without the help of dopants

Increasing Oxide Reducibility: The Role of Metal/Oxide Interfaces in the Formation of Oxygen Vacancies

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_<i>n</i>O_<i>m</i> to M_<i>n</i>O_{<i>m</i>–<i>x</i>}. 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