Surface structures formed by exposing Ag(111) to atomic oxygen have been studied by X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. From the combination of the experimental and theoretical results, a model is proposed for the 0.5 ML oxygen coverage with a c(4 x 8) periodicity. Moreover, we find that a bulk-like Ag2O phase starts to form at coverages above 0.5 ML.

Metals are commonly oxidized under ambient conditions. Although bulk oxidation has received considerable attention, far less is known about oxidation at the subnanometer scale. This is unfortunate, as metal particles used in heterogeneous catalysis typically range from subnanometer to some nanometers. Here, density functional theory calculations are used to explore oxidation of gas-phase transition metal clusters in the range from the dimer to the dodecamer. Comparisons with the corresponding bulk systems uncover that the decomposition temperature of stoichiometrically oxidized clusters may be lower than for the bulk. Despite pronounced variations in ground state geometries, oxidized clusters closely mimic energetic trends across the periodic table valid for bulk systems.

Hydrogen assisted selective catalytic reduction of NOx over Ag/Al2O3 with either hydrocarbons or ammonia as reducing agents is an emerging technology for lean NOx reduction. Herein, we present a density functional theory study of H-2 dissociation over a representative set of sites present on the Ag/Al2O3 catalyst. Whereas H-2 dissociation over supported Ag ions and oxidized Ag surfaces is found to be facile, dissociation over metallic Ag, defect free Al2O3 and alumina-supported Ag is associated with high barriers. The results are rationalized by analysis of the electronic structure.

X-ray photoelectron spectroscopy and density functional theory calculations are used to investigate NO adsorption at low (100K) and room temperature (RT) over preoxidized Ag(111). At 100K, the data indicates presence of NO and N2O2, with little or no nitrite/nitrate formation. This is consistent with the calculated surface core level shifts and the pronounced barrier for nitrite formation. At RT, the recorded spectra indicate a complex interconversion between adsorbed species with an initial formation of a p(4×4) nitrate overlayer. With increasing NO pressure, the experimental results are best rationalized by partial nitrate decomposition into nitrites and subsequent NO physisorption, which leads to the formation of N2O3-like species.

The importance of environmental awareness is today undisputed and manifests in tighter legislations for emissions of pollutants in stationary and automotive sources. This coincides with introduction of lean burn combustion engines in order to enhance the energy efficiency. These type of engines, however, pose a challenge due to inefficient reduction of nitrogen oxide species (NOX) using the conventional three-way catalyst. Selective catalytic reduction (SCR) with either ammonia (NH3) or hydrocarbons (HC) as reducing agent offers a possible solution to the NOX reduction issue. A promising candidate for NOX reduction under HC-SCR and lean conditions, is silver supported on γ-alumina (Ag/Al2O3). However, despite much effort, many fundamental properties of the catalyst remain elusive. This includes the active phase, the role and charge state of Ag and the reaction mechanisms during reduction. Previous studies have suggested the presence of small Agn clusters, metallic, ionic, and oxidized silver phases. However, the importance and role of each phase is unclear. In order to understand and improve the catalyst, these issues, among others, need clarification. Electronic structure calculations are performed in order to address the active site, the charge state of Ag, and to probe different reaction mechanism for lean NOX reduction over Ag/Al2O3. Structural, energetic, electronic, and thermodynamic properties of Ag phases in different size and oxidation regimes are investigated and their reactivity towards O2, NOX , CO, and H2 is evaluated. Based on NH3 assisted SCR, reduction of NOX is calculated to be facile over partially oxidized Ag and hydroxylated Al2O3 with intermediate formation of NH2NO. Reduction over alumina requires the presence of NO and NO2 wheras nitrites are suggested to limit the reduction over oxidized Ag. The results are used to make general conclusions about lean NOX-SCR over Ag/Al2O3.

Density functional theory calculations and high-resolution core-level spectroscopy are used to explore the remarkable observation of decreased Ag $3d$ binding energy upon silver oxidation. The shift in Ag $3d$ binding energy is investigated at different degrees of oxidation and compared to results for Pd $3d$, which exhibits a normal shift. Analysis of initial-state effects and valence electronic structure shows that the onsite Ag core potential is insensitive to oxidation despite a clear metal-to-oxygen charge transfer. The substantial negative shift for oxidized Ag is instead attributed to final-state effects as screening of the core-hole occurs in metal $s$ states of bonding character.

Composite BaO/MgO nanoparticles have been prepared by chemical vapor synthesis and subsequent annealing in controlled gas atmospheres. High resolution transmission electron microscopy and X-ray diffraction reveal that a part of the obtained nanoparticles can be characterized as support particles with hemispherical BaO phases. The structural and energetic properties of BaO units dissolved inside the MgO host and adsorbed on MgO(100) were investigated by density functional theory (DFT) calculations. Moreover, ab initio thermodynamics was used to explore the shape of BaO and MgO particles in a water environment as a function of temperature. The calculations suggest that the spherical shapes of the segregates result from the growth process and become thermodynamically stabilized by surface hydroxylation.

High-resolution core-level spectroscopy and density-functional theory calculations have been used to investigate CO adsorption on the p(4 x 4) structure of oxidized Ag(111). CO adsorption with subsequent carbonate formation was observed at 100 K. The experimental results are consistent with calculations that reveal low activation barriers to form CO2 and CO32- from adsorbed CO. On the basis of a good match between calculated and experimental shifts in the Ag 3d and O 1s core-level binding energies, a model for a monolayer of carbonates on p(4 x 4)-O/Ag(111) is proposed.

The oxidation of small silver clusters (Ag-n, n <= 9) was investigated through electronic structure calculations based on density functional theory. The adsorption energies of molecular and dissociated adsorption show a pronounced odd/even alternation, with lower energies calculated for even-sized clusters. Molecular adsorption is favored for n <= 5, whereas dissociation is preferred for the larger sizes. Molecular oxygen is adsorbed in atop (Ag, Ag-2, Ag-6, Ag-8) or bridge (Ag-3, Ag-4, Ag-5, Ag-7, Ag-9) configurations, and atomic oxygen is preferably adsorbed in 3-fold hollow positions. Results for stoichiometric (Ag2nOn) clusters were compared to O-2 adsorption on Ag(111), and ab initio thermodynamics was used to estimate the temperature for the oxide-to-metal phase transition. The barrier for O-2 dissociation on Ag-8 was calculated to be higher than the corresponding barrier on Ag(111), which indicates a slower oxidation process. Adsorption of NOx onto the oxidized clusters was found to proceed through a formal reduction of the clusters; that is, NOx is adsorbed as NOx+1 with x = 1, 2.

Density functional theory is used to investigate CO oxidation over an ultrathin MgO film supported on Ag(100). O-2 is found to be activated on MgO/Ag(100) whereas CO is only weakly bonded to the surface. These adsorption properties together with a low activation barrier render the MgO/Ag system an efficient catalyst for CO oxidation at low temperatures. As the predicted mechanism is general in nature, the result is suggested to have implications for a wide range of oxidation reactions.

Using a simple model of a vesicle and a substrate, we have studied the surface diffusion of an adsorbed vesicle. We show that the experimentally observed but unexplained fact, that a neutral (POPC) vesicle adsorbed to a SiO(2) or mica surface does not diffuse but can be moved laterally by an atomic force microscope (AFM) tip, without rupture, can be explained by transient (i.e., temporary) pinning of lipid head groups to surface charges. We studied the surface diffusion for different vesicle adsorption strengths (without any pinning taking place), with the observation that a stronger vesicle-surface attraction leads to slower surface diffusion. However, the surface diffusion was still significant and too high to explain the experimentally observed immobility. When allowing transient lipid pinning between the vesicle and the surface, a 1-2 orders of magnitude decrease in the surface diffusion coefficient was observed. For a lipid adsorption potential of around 20 k(B)T and a lipid pinning potential of about 25 k(B)T, the vesicle is found to be practically immobile on the surface.