Surface Science Studies of the Photoactivation of TiO2New Photochemical Processes

The high catalytic activity of gold clusters on oxides has been attributed to structural effects (including particle thickness and shape and metal oxidation state), as well as to support effects. We have created well-ordered gold mono-layers and bilayers that completely wet (cover) the oxide support, thus eliminating particle shape and direct support effects. High-resolution electron energy loss spectroscopy and carbon monoxide adsorption confirm that the gold atoms are bonded to titanium atoms. Kinetic measurements for the catalytic oxidation of carbon monoxide show that the gold bilayer structure is significantly more active (by more than an order of magnitude) than the monolayer.

The Structure of Catalytically Active Gold on Titania

Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges

Efficient and stable algorithms for the calculation of spectral quantities and correlation functions are some of the key tools in computational condensed-matter physics. In this paper basic properties and recent developments of Chebyshev expansion based algorithms and the kernel polynomial method are reviewed. Characterized by a resource consumption that scales linearly with the problem dimension these methods enjoyed growing popularity over the last decade and found broad application not only in physics. Representative examples from the fields of disordered systems, strongly correlated electrons, electron-phonon interaction, and quantum spin systems are discussed in detail. In addition, an illustration on how the kernel polynomial method is successfully embedded into other numerical techniques, such as cluster perturbation theory or Monte Carlo simulation, is provided.

The kernel polynomial method

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Surface chemistry of catalysis by gold

We present a density functional study of reduced and stoichiometric rutile TiO2 (110) surfaces, and of binding of gold monomers and dimers to them. On the stoichiometric TiO2 surface, a Au atom binds to either a five-coordinated Ti atom on the basal plane, or atop a bridging oxygen atom. The two sites have nearly the same binding energy, suggesting diffusion of Au across unreduced regions of TiO2 (110) will be fast. The reduction of the rutile surface, by removal of bridging oxygen atoms, causes a charge redistribution in the system, which extends far from the vacancy site. A Au atom binds strongly to the reduced surfaces: the greater the degree of reduction the stronger the binding. On all reduced surfaces, the preferred binding sites are the vacant bridging oxygen sites. Less stable is the binding to a nearby five-coordinated Ti atom. The binding of Au2 on the reduced surfaces follows a similar pattern. Specifically, if two adjacent vacant sites are available, the optimal structure involves the dimer “d...

Adsorption of gold on stoichiometric and reduced rutile TiO2 (110) surfaces

The most robust treatment of the inverse acoustic scattering problem is based on the reversion of the Born-Neumann series solution of the Lippmann-Schwinger equation. An important issue for this approach to inversion is the radius of convergence of the Born-Neumann series for Fredholm integral kernels, and especially for acoustic scattering for which the interaction depends on the square of the frequency. By contrast, it is well known that the Born-Neumann series for the Volterra integral equations in quantum scattering are absolutely convergent, independent of the strength of the coupling characterizing the interaction. The transformation of the Lippmann-Schwinger equation from a Fredholm to a Volterra structure by renormalization has been considered previously for quantum scattering calculations and electromagnetic scattering. In this paper, we employ the renormalization technique to obtain a Volterra equation framework for the inverse acoustic scattering series, proving that this series also converges absolutely in the entire complex plane of coupling constant and frequency values. The present results are for acoustic scattering in one dimension, but the method is general. The approach is illustrated by applications to two simple one-dimensional models for acoustic scattering.

Inverse scattering theory: renormalization of the Lippmann-Schwinger equation for acoustic scattering in one dimension.

Using density functional theory, we have studied the structure and energetics of the (001) face of γ alumina. Our results address several experimental issues: (1) When the face with tetrahedral aluminum is exposed in the bulk-terminated system, the surface reconstructs extensively, leading to exposure of the higher-density layer. When only a few layers are present, this reconstruction may even lead to the collapse of the system into a different structure. (2) We find that the lowest energy is obtained if the vacant spinel sites lie on octahedral positions. We also find that vacancies are less preferred on the surface than in the bulk. (3) Migration to and from the surface of vacant spinel sites, by hopping of Al atoms between octahedral and tetrahedral cation sites has a rather high barrier. This suggests the vacancy distribution may not reach equilibrium if the material is not annealed carefully during preparation.

Structure of the (001) surface of γ alumina

A theoretical force field for the ground-state vibrations of nonionized glycine $H_2NCH_2COOH$ has been determined from ab initio calculations at the Hartree-Fock level with 4-21G basis set. The experimental infrared spectra of $H_2NCH_2COOH, H_2NCD_2COOH$, $^{15}NH_2CH_2COOH, H_2NCH_2C^{18}O^{18}OH$, and their nitrogen- and oxygen-deuterated isotopomers observed from the matrix-isolated species have been assigned with the aid of the potential energy distributions calculated for each of the normal modes using the theor. force field. The calculated isotopic frequency shifts for $^{15}N, ^{18}O$, and $^2H$ substituted glycines are also found to be in close agreement with the experimental isotopic frequency shifts. The calculated infrared band intensity pattern shows good agreement with the experimental spectrum of glycine. From a complete assignment of the vibrational fundamentals observed from the infrared spectra of glycine and its several isotopic molecules, a stretched framework planar conformer is concluded to be the more stable form, reconfirming the results of the other studies on nonionized glycine.

Amrendra Vijay

Papers

Adsorption of gold on stoichiometric and reduced rutile TiO2 (110) surfaces

Inverse scattering theory: renormalization of the Lippmann-Schwinger equation for acoustic scattering in one dimension.

Structure of the (001) surface of γ alumina

Theoretical study of the ground-state vibrations of nonionized glycine

Ab initio study of the force field, geometry and vibrational assignment of urea