The Redox Chemistry of Gold with High-Valence Doped Calcium Oxide

Understanding the Doping Chemistry of High Oxidation States in Scheelite CaWO4 by Hydrothermal Conditions

Doping chemistry has become one of the most effective means of tuning materials' properties for diverse applications. In particular for scheelite-type CaWO₄, high-oxidation-state doping is extremely important, since one may expand the scheelite family and further create prospective candidates for novel applications and/or useful spectral signatures for nuclear forensics. However, the chemistry associated with high-valence doping in scheelite-type CaWO₄ is far from understanding. In this work, a series of scheelite-based materials (Ca_1-x-y-zEu_xK_y□_z)WO₄ (□ represents the cation vacancy of the Ca²⁺ site) were synthesized by hydrothermal conditions and solid-state methods and comparatively studied. For the bulk prepared by the solid-state method, occupation of high-oxidation-state Eu³⁺ at the Ca²⁺ sites of CaWO₄ is followed by doping of the low-oxidation-state K⁺ at a nearly equivalent molar amount. The Eu³⁺ local symmetry is thus varied from the original S₄ point group symmetry to C_2v point group symmetry. Surprisingly different from the cases in bulk, for the nanoscale counterparts prepared by hydrothermal conditions, the high-oxidation-state Eu³⁺ was incorporated in CaWO₄ at two distinct sites, and its amount is higher than that of the low-oxidation-state K⁺ even though KOH was used as a mineralizer, creating a certain amount of cation vacancies. Consequently, an apparent split emission of ⁵D₀ → ⁷F₀ was first demonstrated for (Ca_1-x-y-zEu_xK_y□_z)WO₄. The doping chemistry of high oxidation states uncovered in this work not only provides an explanation for the commonly observed spectral changes in rare-earth-ion-modified scheelite structures, but also points out an advanced direction that can guide the design and synthesis of novel functional oxides by solution chemistry routes.

Dynamic charge and oxidation state of Pt/CeO2 single-atom catalysts

The catalytic activity of metals supported on oxides depends on their charge and oxidation state. Yet, the determination of the degree of charge transfer at the interface remains elusive. Here, by combining density functional theory and first-principles molecular dynamics on Pt single atoms deposited on the CeO₂ (100) surface, we show that the common representation of a static metal charge is oversimplified. Instead, we identify several well-defined charge states that are dynamically interconnected and thus coexist. The origin of this new class of strong metal-support interactions is the relative position of the Ce(4f) levels with respect to those of the noble metal, allowing electron injection to (or recovery from) the support. This process is phonon-assisted, as the Ce(4f) levels adjust by surface atom displacement, and appears for other metals (Ni) and supports (TiO₂). Our dynamic model explains the unique reactivity found for activated single Pt atoms on ceria able to perform CO oxidation, meeting the Department of Energy 150 °C challenge for emissions.

Inducing wetting morphologies and increased reactivities of small Au clusters on doped oxide supports

Au nanoparticles are promising catalysts for industrially important reactions. Their catalytic activity is known to depend on their charge state and morphology. Using density functional theory calculations, we have studied how the induced charge and dimensionality of small Au clusters can be tuned by doping the oxide support that they are deposited on. We have investigated Au _n clusters of sizes n = 1, 2, 3, and 20 on Al-doped MgO and Mo-doped CaO. We show that substitutionally doping the oxide support with an electron donor changes the cluster morphology from an upright and/or three-dimensional geometry to a flat geometry. This structural wetting transition results in an increase in the negative charge induced on the cluster and a consequent lowering in the dissociation barrier for the O₂ atoms adsorbed on the cluster. We find that the nature of Mo and Al dopants differs: only for the former is it true that the charge state of the dopant atoms depends on the presence or absence of Au nanoparticles and their size.

The Surface Science of Catalysis and More, Using Ultrathin Oxide Films as Templates: A Perspective

Surface science has had a major influence on the understanding of processes at surfaces relevant to catalysis. Real catalysts are complex materials, and in order to approach an understanding at the atomic level, it is necessary in a first step to drastically reduce complexity and then systematically increase it again in order to capture the various structural and electronic factors important for the function of the real catalytic material. The use of thin oxide films as templates to mimic three-dimensional supports as such or for metal particles as well as to model charge barriers turns out to be appropriate to approach an understanding of metal-support interactions. Thin oxide films also exhibit properties in their own right that turn out to be relevant in catalysis. Thin oxide film formation may also be used to create unique two-dimensional materials. The present perspective introduces the subject using case studies and indicates possible routes to further apply this approach successfully.

Gold assisted oxygen dissociation on a molybdenum-doped CaO(001) surface

Using density functional theory (DFT) calculations, we address the adsorption of O₂ and the coadsorption of gold species and oxygen molecules on a Mo-doped CaO(001) surface with 1.25% impurity concentration.

Substrate doping: A strategy for enhancing reactivity on gold nanocatalysts by tuning sp bands

We suggest that the reactivity of Au nanocatalysts can be greatly increased by doping the oxide substrate on which they are placed with an electron donor. To demonstrate this, we perform density functional theory calculations on a model system consisting of a 20-atom gold cluster placed on a MgO substrate doped with Al atoms. We show that not only does such substrate doping switch the morphology of the nanoparticles from the three-dimensional tetrahedral form to the two-dimensional planar form, but it also significantly lowers the barrier for oxygen dissociation by an amount proportional to the dopant concentration. At a doping level of 2.78%, the dissociation barrier is reduced by more than half, which corresponds to a speeding up of the oxygen dissociation rate by five orders of magnitude at room temperature. This arises from a lowering in energy of the s and p states of Au. The d states are also lowered in energy, however, this by itself would have tended to reduce reactivity. We propose that a suitable measure of the reactivity of Au nanoparticles is the difference in energy of sp and d states.

Exploring routes to tailor the physical and chemical properties of oxides via doping: an STM study

Doping opens fascinating possibilities for tailoring the electronic, optical, magnetic, and chemical properties of oxides. The dopants perturb the intrinsic behavior of the material by generating charge centers for electron transfer into adsorbates, by inducing new energy levels for electronic and optical excitations, and by altering the surface morphology and hence the adsorption and reactivity pattern. Despite a vivid scientific interest, knowledge on doped oxides is limited when compared to semiconductors, which reflects the higher complexity and the insulating nature of many oxides. In fact, atomic-scale studies, aiming at a mechanistic understanding of dopant-related processes, are still scarce.In this article, we review our scanning tunneling microscopy (STM) experiments on thin, crystalline oxide films with a defined doping level. We demonstrate how the impurities alter the surface morphology and produce cationic/anionic vacancies in order to keep the system charge neutral. We discuss how individual dopants can be visualized in the lattice, even if they reside in subsurface layers. By means of STM-conductance and x-ray photoelectron spectroscopy, we determine the electronic impact of dopants, including the energies of their eigen states and local band-bending effects in the host oxide. Electronic transitions between dopant-induced gap states give rise to new optical modes, as detected with STM luminescence spectroscopy. From a chemical perspective, dopants are introduced to improve the redox potential of oxide materials. Electron transfer from Mo-donors, for example, alters the growth behavior of gold and activates O2 molecules on a wide-gap CaO surface. Such results demonstrate the enormous potential of doped oxides in heterogeneous catalysis. Our experiments address the issue of doping from a fundamental viewpoint, posing questions on the lattice position, charge state, and electron-transfer potential of the impurity ions. Whether doped oxides are suitable to catalyze surface reactions needs to be explored in more applied studies in the future.

The Active Molybdenum Oxide Phase in the Methanol Oxidation to Formaldehyde (Formox Process): A DFT Study

Methanol is oxidised to formaldehyde by the Formox process, in which molybdenum oxides, usually doped with iron, are the catalyst. The active phase of the catalysts and the reasons for the selectivity observed are still unknown. We present a density functional theory based study that indicates the unique character of Mo(VI)¢Mo(IV) pairs as the most active and selective sites and indicates the active sites on the surface, the controlling factors of selectivity, and the role of the dopant. Iron reduces the energy requirements of the redox Mo(VI)¢Mo(IV) pair by acting as an electron reservoir that sets in if required. Our present study paves the way towards a better understanding of the process.

Nb-doped CaO: an efficient electron donor system

Transition metal atoms incorporated into insulating materials (oxides in particular) can deeply modify their adsorption properties. In particular, charge transfer to adsorbed species can be induced by the presence of substitutional dopants, which introduce new electronic states in the band gap of the host crystal. Here we show, by means of density functional theory calculations, that Nb represents an excellent dopant to turn the rather inactive CaO(100) surface into an electron-rich support. The charge transfer ability of the doped material is shown by comparing the adsorption properties of the electronegative Au atoms on pure and Nb-doped CaO. While in the first case the CaO-Au bonding is relatively weak and the Au atom is essentially neutral, in the Nb-doped system a much stronger adhesion is found due to a net charge transfer from the Nb dopant and to the formation of a gold anion. This mechanism occurs also for Nb in high oxidation states. Nb is thus an excellent modifier of the calcium oxide properties.

Surface defects and their impact on the electronic structure of Mo-doped CaO films: an STM and DFT study

Scanning tunneling microscopy and DFT calculations are used to probe the local electronic structure of a Mo-doped CaO film.