Ideal, Defective, and Gold--Promoted Rutile TiO2(110) Surfaces: Structures, Energies, Dynamics, and Thermodynamics from PBE+U

Extensive first principles calculations are carried out to investigate gold-promoted TiO2(110) surfaces in terms of structure optimizations, electronic structure analyses, ab initio thermodynamics calculations of surface phase diagrams, and ab initio molecular dynamics simulations. All computations rely on density functional theory in the generalized gradient approximation (PBE) and account for on-site Coulomb interactions via inclusion of a Hubbard correction, PBE+U, where U is computed from linear response theory. This approach is validated by investigating the interaction between TiO2(110) surfaces and typical probe species (H, H2O, CO). Relaxed structures and binding energies are compared to both data from the literature and plain PBE results. The main focus of the study is on the properties of gold-promoted titania surfaces and their interactions with CO. Both PBE+U and PBE optimized structures of Au adatoms adsorbed on stoichiometric and reduced TiO2 surfaces are computed, along with their electronic structure. The charge rearrangement induced by the adsorbates at the metal/oxide contact are also analyzed and discussed. By performing PBE+U ab initio molecular dynamics simulations, it is demonstrated that the diffusion of Au adatoms on the stoichiometric surface is highly anisotropic. The metal atoms migrate either along the top of the bridging oxygen rows, or around the area between these rows, from one bridging position to the next along the [001] direction. Approximate ab initio thermodynamics predicts that under O-rich conditions, structures obtained by substituting a Ti5c atom with an Au atom are thermodynamically stable over a wide range of temperatures and pressures.Comment: 20 pages, 12 figures, accepted for publication in Phys. Rev.

Indirect daylight oxidative degradation of polyethylene microplastics by a bio-waste modified TiO2-based material

Microplastics are recognized as an emerging critical issue for the environment. Here an innovative chemical approach for the treatment of microplastics is proposed, based on an oxidative process that does not require any direct energy source (irradiation or heat). Linear low-density polyethylene (LLDPE) was selected as target commodity polymer, due to its widespread use, chemical inertness and inefficient recycling. This route is based on a hybrid material coupling titanium oxide with a bio-waste, rosin, mainly constituted by abietic acid, through a simple sol-gel synthesis procedure. The ligand-to-metal charge transfer complexes formed between rosin and Ti4+ allow the generation of reactive oxygen species without UV irradiation for its activation. In agreement with theorical calculations, superoxide radical ions are stabilized at ambient conditions on the surface of the hybrid TiO2. Consequently, an impressive degradation of LLDPE is observed after 1 month exposure in a batch configuration under indirect daylight, as evidenced by the products revealed by gas chromatography-mass spectrometry analysis and by chemical and structural modifications of the polymer surface. In a context of waste exploitation, this innovative and sustainable approach represents a promising cost-effective strategy for the oxidative degradation of microplastics, without producing any toxic by-products

Two different mechanisms of stabilization of regular pi-stacks of radicals in switchable dithiazolyl-based materials

Materials based on regular π-stacks of planar organic radicals are intensively pursued by virtue of their technologically relevant properties. Yet, these π-stacks are commonly unstable against π-dimerization. In this computational study, we reveal that regular π-stacks of planar dithiazolyl radicals can be rendered stable, in some range of temperatures, via two different mechanisms. When the radicals of a π-stack are both longitudinally and latitudinally slipped with respect to each other, the corresponding regular π-stacked configuration is associated with a locally stable minimum in the potential energy surface of the system. Conversely, those regular π-stacks in which radicals are latitudinally slipped with respect to each other are stable as a result of a dynamic interconversion between two degenerate dimerized configurations. The existence of two stabilization mechanisms, which can be traced back to the bonding properties of isolated π-dimers, translates into two different ways of exploiting spin-Peierls-like transitions in switchable dithiazolyl-based materials

Nature and role of activated molecular oxygen species at the gold/titania interface in the selective oxidation of alcohols

En este documento se presenta un análisis de los determinantes del perfil de ahorro para hogares e individuos con el objetivo de contrastar la Hipótesis de Ciclo de Vida, utilizando la metodología expuesta en Deaton y Paxson (2000a y 2000b) con datos de la Encuesta Nacional de Ingresos y Gastos (ENIG) para los periodos 1984-1985, 1994-1995 y 2006-2007. Se encontró que para el análisis por hogar no hay evidencia que determine el cumplimiento de la Hipótesis del Ciclo de Vida, mientras que para el análisis por individuo si existe

On the Impact of Solvation on a Au/TiO₂ Nanocatalyst in Contact with Water

Water, the ubiquitous solvent, is also prominent in forming liquid–solid interfaces with catalytically active surfaces, in particular, with promoted oxides. We study the complex interface of a gold nanocatalyst, pinned by an F-center on titania support, and water. The ab initio simulations uncover the microscopic details of solvent-induced charge rearrangements at the metal particle. Water is found to stabilize charge states differently from the gas phase as a result of structure-specific charge transfer from/to the solvent, thus altering surface reactivity. The metal cluster is shown to feature both “cationic” and “anionic” solvation, depending on fluctuation and polarization effects in the liquid, which creates novel active sites. These observations open up an avenue toward “solvent engineering” in liquid-phase heterogeneous catalysis

Fluxionality of Au Clusters at Ceria Surfaces during CO Oxidation: Relationships among Reactivity, Size, Cohesion, and Surface Defects from DFT Simulations

Density functional theory (DFT) calculations are used to identify correlations among reactivity, structural stability, cohesion, size, and morphology of small Au clusters supported on stoichiometric and defective CeO₂(111) surfaces. Molecular adsorption significantly affects the cluster morphology and in some cases induces cluster dissociation into smaller particles and deactivation. We present a thermodynamic rationalization of these effects and identify Au₃ as the smallest stable nanoparticle that can sustain catalytic cycles for CO oxidation without incurring structural/morphological changes that jeopardize its reactivity. The proposed Mars van Krevelen reaction pathway displays a low activation energy, which we explain in terms of the cluster fluxionality and of labile CO₂ intermediates at the Au/ceria interface. These findings shed light on the importance of cluster dynamics during reaction and provide key guidelines for engineering more efficient metal–oxide interfaces in catalysis

Thermodynamic, electronic and structural properties of Cu/CeO(2) surfaces and interfaces from first-principles DFT plus U calculations

The thermodynamic, structural and electronic properties of Cu-CeO(2) (ceria) surfaces and interfaces are investigated by means of density functional theory (DFT+U) calculations. We focus on model systems consisting of Cu atoms (i) supported by stoichiometric and reduced CeO(2) (111) surfaces, (ii) dispersed as substitutional solid solution at the same surface, as well as on (iii) the extended Cu(111)/CeO(2)(111) interface. Extensive charge reorganization at the metal-oxide contact is predicted for ceria-supported Cu adatoms and nanoparticles, leading to Cu oxidation, ceria reduction, and interfacial Ce(3+) ions. The calculated thermodynamics predict that Cu adatoms on stoichiometric surfaces are more stable than on O vacancies of reduced surfaces at all temperatures and pressures relevant for catalytic applications, even in extremely reducing chemical environments. This suggests that supported Cu nanoparticles do not nucleate at surface O vacancies of the oxide, at variance with many other metal/ceria systems. In oxidizing conditions, the solid solutions are shown to be more stable than the supported systems. Substitutional Cu ions form characteristic CuO(4) units. These promote an easy and reversible O release without the reduction of Ce ions. The study of the extended CeO(2)(111)/Cu(111) interface predicts the full reduction of the interfacial ceria trilayer. Cu nanoparticles supported by ceria are proposed to lie above a subsurface layer of Ce(3+) ions that extends up to the perimeter of the metal-oxide interface. (c) 2010 American Institute of Physics. [doi:10.1063/1.3515424