Antonio Ruiz Puigdollers

CO Oxidation on Au Nanoparticles Supported on ZrO2: Role of Metal/Oxide Interface and Oxide Reducibility

In this paper, we report the results of density functional theory calculations on CO oxidation on a model Au/ZrO2 catalyst, in which the support is a non‐reducible oxide (zirconia). Low‐temperature CO oxidation on supported gold nanoparticles occurs through direct interaction of an activated O2 molecule with adsorbed CO to form CO2. However, at room temperature or above the reaction follows another path as O2 desorbs from Au at 170 K. On a reducible oxide, such as TiO2, CO is directly oxidized by a lattice oxygen in a gold‐assisted Mars van Krevelen mechanism. The role of the oxide reducibility is thus essential, as the first step of the reaction is the formation of an oxygen vacancy. Contrary to what one would expect, the reaction can occur also on zirconia. We show that this is a result of the reduced oxygen vacancy formation energy at the gold/zirconia interface. The role of ambient oxygen is to reoxidize the support with a slightly activated process. The results point towards the importance of distinguishing between bulk reducibility and surface reducibility, as the latter can be strongly affected by phenomena such as deposition of metal nanoparticles or oxide nanostructuring.

Tuning the charge state of Ag and Au atoms and clusters deposited on oxide surfaces by doping: a DFT study of the adsorption properties of nitrogen- and niobium-doped TiO2 and ZrO2

The charge state of Ag and Au atoms and clusters (Ag4 and Au4, Ag5 and Au5) adsorbed on defective TiO2 anatase(101) and tetragonal ZrO2(101) has been systematically investigated as a function of oxide doping and defectivity using a DFT+U approach. As intrinsic defects, we have considered the presence of oxygen vacancies. As extrinsic defects, substitutional nitrogen- and niobium-doping have been investigated, respectively. Both surface and sub-surface defects and dopants have been considered. Whereas on surfaces with oxygen vacancies or Nb-doping, atoms and clusters may become negatively charged, N-doping always leads to the formation of positively charged adsorbates, independently of the supporting material (TiO2 or ZrO2). This suggests the possibility to tune the electronic properties of supported metal clusters by selective doping of the oxide support, an effect that may result in complete changes in chemical reactivity.

TiO2 and ZrO2 in biomass conversion: why catalyst reduction helps

Biomass refers to plant-based materials that are not used for food or feed. As an energy source, lignocellulosic biomass (lignin, cellulose and hemicellulose) can be converted into various forms of biofuel using thermal, chemical and biochemical methods. Chemical conversion implies the use of solid catalysts, usually oxide materials. In this context, reducible oxides are considered to be more active than non-reducible oxides. But why? Using density functional theory DFT + U calculations with the inclusion of dispersion forces, we describe the properties of anatase TiO2, a reducible oxide, and tetragonal ZrO2, a non-reducible oxide, the (101) surfaces in this context. In particular, we focus on the role of surface reduction, either by direct creation of oxygen vacancies via O2 desorption, or by treatment in hydrogen. We show that the presence of reduced centres on the surface of titania or zirconia (either Ti3+ or Zr3+ ions, or oxygen vacancies) results in lower barriers and more stable intermediates in two key reactions in biomass catalytic conversion: ketonization of acetic acid (studied on ZrO2) and deoxygenation of phenol (studied on TiO2). We discuss the role of Ru nanoparticles in these processes, and in particular in favouring H2 dissociation and hydrogen spillover, which results in hydroxylated surfaces. We suggest that H2O desorption from the hydroxylated surfaces may be a relevant mechanism for the regeneration of oxygen vacancies, in particular on low-coordinated sites of oxide nanoparticles. Finally, we discuss the role of nanostructuring in favouring oxide reduction, by discussing the properties of ZrO2 nanoparticles of diameter of about 2 nm. This article is part of a discussion meeting issue ‘Providing sustainable catalytic solutions for a rapidly changing world’.

Theory of Ferromagnetism in Reduced ZrO2–x Nanoparticles

Bulk ZrO2 is both nonreducible and nonmagnetic. Recent experimental results show that dopant-free, oxygen-deficient ZrO2–x 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.

Role of Metal/Oxide Interfaces in Enhancing the Local Oxide Reducibility

Oxide reducibility is an important property in catalysis by metal-oxides. The reducibility of an oxide can be substantially modified when an interface is created between the oxide and a metal. Here we discuss two types of interfaces. One consists of gold nanoparticles deposited on anatase TiO2 or tetragonal ZrO2 (101) surfaces; these are traditional direct catalysts (metal deposited on an oxide). The second example consists of a metal support, Pt or a Pt3Zr alloy, where a ZrO2 nanofilm is deposited; this is representative of an inverse catalyst (oxide on metal). We designed models of these systems and analyzed by means of first principle calculations a key descriptor of the oxide reducibility, the cost of formation of an oxygen vacancy. We show that this cost is dramatically reduced when the oxide is interfaced with the metal. The effect on catalytic reactions is analyzed by computing the energy profiles for the CO oxidation reaction on Au/TiO2 and Au/ZrO2 model catalysts. Despite the very different nature of the two oxide supports, reducible for TiO2 and non-reducible for ZrO2, the same Au-assisted Mars–van Krevelen mechanism is found, with similar barriers.

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 O2 desorption. The formation of an OsH2 surface precursor state to water desorption is the rate-determining 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.