Maria Veronica Ganduglia-Pirovano

Role of ceria in oxidative dehydrogenation on supported vanadia catalysts

The effect of the suppport on oxidative dehydrogenation activity for vanadia/ceria systems is examined for the oxidation of methanol to formaldehyde by use of well-defined VO(x)/CeO(2)(111) model catalysts. Temperature-programmed desorption at low vanadia loadings revealed reactivity at much lower temperature (370 K) as compared to pure ceria and vanadia on inert supports such as silica. Density functional theory is applied and the energies of hydrogenation and oxygen vacancy formation also predict an enhanced reactivity of the vanadia/ceria system. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f-states.

Electronic and nuclear chemical reactivity

The local softness and the Fukui function emerge from density functional theory as measures of local electronic reactivity. We obtain here an exact linear integral relation between the Fukui functions of insulators or molecules and the probability density of the frontier orbitals of Kohn–Sham theory. The same linear map holds between the local softness and the local Kohn–Sham density of states at the Fermi level for metals. The kernel in those relations is the inverse of the transpose of the potential response function (PRF) of Kohn–Sham theory. The PRF has the form of the static Hartree dielectric function with an exchange and correlation interaction added to the bare Coulomb interaction. The exact static dielectric function also has the Hartree form, but with a renormalized polarization propagator. The map is norm preserving for systems with energy gaps such as insulators and molecules and norm reducing or screening for systems with a finite density of states above the ground state such as normal metals...

Resolving the atomic structure of vanadia monolayer catalysts: monomers, trimers, and oligomers on ceria

Supported vanadium oxide catalysts have received considerable attention owing to their high activity for selective oxidation reactions. The reactivity has been shown to depend strongly on the oxide support, with reducible oxides (e.g., ceria, titania, and zirconia) exhibiting much higher turnover frequencies for oxidative dehydrogenation (ODH) reactions than irreducible oxides (e.g., silica and alumina). Structural characterization of the catalysts has been performed primarily using Raman and UV/Vis spectroscopy (see Ref. [4, 6,7] and references therein), as well as X-ray absorption spectroscopy. These results have been used to postulate that vanadia catalysts consist of isolated and polymer structures that wet the supporting oxide (so-called “monolayer catalysts”). To elucidate the surface chemistry of vanadia, different model systems, such as vanadia single crystals and thin films as well as vanadia clusters supported on planar metal oxide substrates, have been studied experimentally by surface-science techniques and computational means. To rationalize structure–reactivity relationships, welldefined systems are required. Of the reducible metal oxide supports that are known to be highly active in ODH reactions, ceria is particularly suited, because well-ordered thin films can be grown with a known surface termination. Previously, the structure and reactivity of vanadia supported on CeO2(111) has been studied using photoelectron spectroscopy (PES) and temperature-programmed desorption (TPD). 15] However, the atomic structure of ceria-supported vanadia monolayer catalysts has not been resolved. Herein, using a combination of high-resolution scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), and PES with synchrotron radiation, we unambiguously demonstrate the formation of monomeric O=VO3 species on the CeO2(111) surface at low vanadia loadings. For the first time, we show a direct relationship between the nuclearity of vanadia species (monomeric vs. polymeric) as observed by STM and their vibrational properties. We show that ceria stabilizes the vanadium + 5 oxidation state, leading to partially reduced ceria upon vanadium deposition. These experimental results are fully supported by density functional theory (DFT) calculations. The results indicate that ceria surfaces stabilize small vanadia species, such as monomers and trimers, that sinter into two-dimensional, monolayer islands. Such stabilization probably plays a crucial role in the enhanced activity observed for ceriasupported vanadia in ODH reactions. Indeed, low-nuclearity species revealed reactivities at much lower temperatures than those with higher nuclearity, which in turn show strong similarities to the reactivity of vanadia clusters supported on alumina and silica. 13] Figure 1 presents compelling evidence for the presence of vanadia monomers on ceria at low coverage (ca. 0.3 V atoms nm ). The STM image in Figure 1 a shows that highly dispersed and randomly distributed species are formed upon deposition of vanadium in an ambient oxygen atmosphere onto a CeO2(111) thin film (see the Experimental Section). The absence of preferential nucleation sites indicates a strong interaction between vanadia species and the underlying ceria support. In the atomically resolved image (inset of Figure 1a), the two protruding spots (ca. 3 in diameter and 1.2 in height) appear to be monomers positioned atop protrusions in the ceria substrate. The apparent height of these vanadia species depends on the tunneling bias and monotonically decreases from approximately 1.8 at 2.2 V to approximately 0.9 at 3 V. At certain voltages, a dark “halo” is visible around a few of the monomeric species (see Figure 1a), which may be related to defect structures of the ceria film. The IR spectrum corresponding to the sample shown in Figure 1a is depicted in Figure 1d with an absorption feature at 1006 cm . This peak is assigned to the vanadyl (V=O) stretching vibration on the basis of comparison with other vanadia systems and reference compounds in which the V=O [*] M. Baron, Dr. H. Abbott, Dr. O. Bondarchuk, Dr. D. Stacchiola, Dr. A. Uhl, Dr. S. Shaikhutdinov, Prof. Dr. H.-J. Freund Fritz Haber Institute of the Max Planck Society Chemical Physics Department Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+ 49)30-8413-4105 E-mail: shamil@fhi-berlin.mpg.de

Reactivity kernels, the normal modes of chemical reactivity, and the hardness and softness spectra

Chemical reactivity theory provides a basis for predicting the reactive proclivities of molecular or condensed systems. The frontier‐orbital concepts of Fukui, as generalized by Parr and collaborators within the framework of density‐functional theory, were developed further by us in a previous paper (I) [J. Chem. Phys. 101, 8988 (1994)]. Nevertheless, five aspects of the theory still require further development; the reactivities are defined as local responses to global stimuli instead of nonlocal responses to local stimuli; there are ambiguities associated with the existence of energy bands in condensed systems; the theory is static and does not properly incorporate the internal dynamics of the reacting systems; the theory focuses on responses without a corresponding definition of chemical stimuli; and no connection is made with the potential energy surface and the reaction pathway. In the present paper, we concentrate on gapless systems extended in at least one dimension, metals, semimetals, and insulato...

Surface core level shifts of clean and oxygen covered Ru(0001)

We present the results of high resolution core level photoelectron spectroscopy employed to investigate the electronic structure of clean and oxygen covered Ir(111) surface. Ir 4f7/2 core level spectra are shown to be very sensitive to the local atomic environment. For the clean surface we detected two distinct components shifted by 550meV, originated by surface and bulk atoms. The larger Gaussian width of the bulk component is explained as due to experimentally unresolved subsurface components. In order to determine the relevance of the phonon contribution we examined the thermal behaviour of the core level lineshape using the Hedin-Rosengren theory. From the phonon- induced spectral broadening we found the Debye temperature of bulk and surface atoms to be 298 and 181K, respectively, which confirms the softening of the vibrational modes at the surface. Oxygen adsorption leads to the appearance of new surface core level components at 200meV and +230meV, which are interpreted as due to first-layer Ir atoms differently coordinated with oxygen. The coverage dependence of these components demonstrates that the oxygen saturation corresponds to 0.38ML, in good agreement with recent density functional theory calculations.

Formaldehyde Formation on Vanadium Oxide Surfaces V2O3(0001) and V2O5(001): How does the Stable Methoxy Intermediate Form?†

Hydroxy-mediated methoxy formation or stabilization is probably an important process in many methanol adsorption systems. Hydrogen atoms originating from the scission of the methanol O-H bond react with the substrate and form water. This process may result 1) in the production of additional surface defects as reactive centers for methoxy formation and 2) in the stabilization of methoxy groups by suppression of methanol formation.

Adlayer core-level shifts of admetal monolayers on transition metal substrates and their relation to the surface chemical reactivity

Using density-functional theory, we study the electronic and structural properties of a monolayer of Cu on the fcc(100) and (111) surfaces of the late 4d transition metals, as well as a monolayer of Pd on Mo bcc(110). We calculate the ground states of these systems, as well as the difference of the ionization energies of an adlayer core electron and a core electron of the clean surface of the adlayer metal. The theoretical results are compared to available experimental data and discussed in a simple physical picture; it is shown why and how adlayer core-level binding energy shifts can be used to deduce information on the adlayers chemical reactivity. \textcopyright{} 1996 The American Physical Society.

Partial oxidation of methanol on well-ordered V2O5(001)/Au(111) thin films

The partial oxidation of methanol to formaldehyde on well-ordered thin V(2)O(5)(001) films supported on Au(111) was studied. Temperature-programmed desorption shows that bulk-terminated surfaces are not reactive, whereas reduced surfaces produce formaldehyde. Formaldehyde desorption occurs between 400 K and 550 K, without evidence for reaction products other than formaldehyde and water. Scanning tunnelling microscopy shows that methanol forms methoxy groups on vanadyl oxygen vacancies. If methanol is adsorbed at low temperature, the available adsorption sites are only partly covered with methoxy groups after warming up to room temperature, whereas prolonged methanol dosing at room temperature leads to full coverage. In order to explain these findings we present a model that essentially comprises recombination of methoxy and hydrogen to methanol in competition with the reaction of two surface hydroxyl groups to form water.

Nucleation of gold atoms on vanadyl-terminated V2O3(0001)

The adsorption of Au atoms on a vanadyl-terminated V2O3 film grown on Au(111) is studied by means of low-temperature STM and DFT+U calculations. The adatoms preferentially bind in an O Au O bridge configuration between two adjacent V O groups. Missing vanadyl groups that have been identified as the characteristic surface defect do not offer an attractive binding environment and are only sparsely occupied with Au. On the other hand, varying concentrations of V2O3 bulk defects that modulate the oxide electronic structure are found to affect the spatial distribution of Au atoms on the oxide surface.

Oxygen Defects at Reducible Oxide Surfaces: The Example of Ceria and Vanadia

Cerium and vanadium oxide-based systems play a major role in a variety of technological applications, with the reducibility of the systems being crucial to their functionality in the applications. The in-depth understanding and control of the type, density, and distribution of oxygen vacancies provide a means to influence the electronic structure and to tailor the systems’ functionality. Hence, a great deal of experimental and theoretical work has been devoted to the study of partially reduced ceria and vanadia, both surfaces and bulk. Here, theoretical data for structural and electronic properties and energetic quantities related to the formation and interaction of neutral oxygen vacancies at the CeO2(111) and V2O5(001) surfaces are reviewed, discussed and compared. Experimental findings on oxygen defects in ceria and vanadia are briefly reported. Special attention is given to the fate of the electrons left in the system upon vacancy formation, the vacancy-induced lattice relaxation, whether vacancies agglomerate or repel each other, and the ability of state-of-the-art quantum-mechanical methods to provide an accurate decription of the geometric and electronic structures of the partially reduced oxide systems as well as reliable oxygen defect formation energies.