Oxide surfaces, adsorption geometries

The interactions of diisopropylfluorophosphate (DFP) with model MgO and CaO surfaces have been investigated using density functional (DFT) and Mpller-Plesset second order perturbation techniques [67]. Geometries of considered complexes were fully optimized at the DFT level. The calculated interaction energies and the corresponding thermodynamic properties show that DFP is physisorbed on these two model oxide surfaces and the adsorption on the MgO surface is stronger. 

K.3.1. High-pressure Methanol Oxidation on Pd(l 11). Figures 53a and b show PM-IRAS surface (p—s) and gas-phase (p + s) spectra acquired during methanol exposure and oxidation at mbar pressures. The gas-phase composition, determined by GC and by PM-IRAS, respectively is shown in Figs 53c and d. After exposure of Pd(l 1 1) to 5 mbar of CH3OH at 300 K, PM-IRAS was used to identify adsorbed CO (vco at approximately 1840 cm , typical of approximately 0.3 ML coverage) as well as formaldehyde (pcHj formaldehyde in two different adsorption geometries... 

Lack of structural data for clean metal oxide surfaces (point (iv)), has also hindered quantitative structural determinations of adsorbates on these surfaces. This is particularly true for those techniques providing a rather complete description of the adsorption geometry (e.g. quantitative LEED (LEED-IV)), since at least some knowledge of the clean surface geometry is usually necessary for structure elucidation. The reasons for the lack of structural data for clean metal oxide surfaces are basically identical to the previous three points (i-iii) for adsorbate covered surfaces. 

The general view is that it is the activation of water, step (5.31), coupled with the strong adsorption of COHad (or its product, COad), that is the difficulty here (as in the case of O2 reduction, I o is only of the order of 10 A/cm ). Actually, the status of COad, i.e. whether it is a true reaction intermediate or merely a poison (as it is in the case of H2 oxidation, cf. Section 5.6.3), is quite controversial What is certain is that COad is there on the surface during the reaction — as shown by, e.g., in situ IR spectroscopy — and also that COad obtained via adsorption of CO shows different oxidation kinetics from COad derived form MeOH. What causes this latter phenomenon is not completely imderstood yet, but adsorption geometry (e.g. island formation) is surmised to be the main factor. Also, while CO can easily replace Had, MeOH cannot, and so its adsorption can only start in earnest when the Pt surface is substantially free from Had (he. at > 0.25V, whereas Eq = 0.01 V), thus constituting another reason why MeOH carmot react at low potentials. 

Initially, the application of STM to semiconductor surfaces was the most fmitful. Independently of the studies on surface geometry, research on surface chemistry was of great interest. The STM started to be used to study the early stages of oxidation, " adatoms adsorption, and hydrogen forced surface reconstruction. ... 

Abstract The correct description of the weak interaction between small molecules and oxide surfaces is still a challenge for theory. In the present review, the current status of the cluster approach to the calculation of adsorption geometries and energies by means of quantum-chemical ab initio methods is discussed. In the first part, the physical and chemical contributions to the bonding mechanism are briefly characterized and the different clusters models currently used for treating molecule/surface interactions are presented free clusters, hydrogen saturated clusters and embedded clusters. We continue with a description of the... 

The interaction between small molecules and oxide surfaces is characterized by a delicate balance of attractive and repulsive contributions which have different physical and chemical origin. In most cases, several of these contributions have the same order of magnitude, but different distance and orientation dependence. Reliable results for adsorption geometries and energies can be only obtained if all of these contributions are properly taken into account and are quantitatively correctly described. Otherwise, one will get results of doubtful quality, maybe accidentally even the correct answer. 

Cu and Cu Oxide Surfaces Works performed on Cu surfaces were devoted to Cu-benzotriazole interactions. Indeed, benzotriazole (BTAH or C6N3H5) has been a weU-known corrosion inhibitor for Cu since 1947, as evidenced by a British patent. BTAH adsorption on CU2O was investigated by Blajiev and Hubin with LDA [69]. Several initial geometries were considered. The adsorption of the molecule induces a breakup of the bulk oxide periodicity. It was deduced that interfacial phases between the metal and the oxide and between the oxide and the electrolyte may be created in the conditions in which BTAH is used. Strong chemical interactions exist between the oxide and adsorbed molecules. 

The IR spectrum of NiCO isolated in solid argon gave assignments to Vi, V3 and V5 modes, with isotopic shifts.The IR spectrum of CO adsorbed on Nin clusters shows the presence of 4 (vibrationally-coupled) CO molecules per cluster. FTIR spectra (vCO) were used to probe the effects of co-adsorption of on-top CO on bridge CO on a Ni(lll) surface.The FTIR spectrum of CO on an anodic nickel oxide surface had a band at 2112 cm assigned to CO adsorbed to Ni(II) or Ni(0) sites perturbed by oxidation of neighbouring nickel atoms. The geometry of CO or NO coordination on NiO(lll) thin films was deduced from the vCO and vNO values. 

For ionic systems such as silicates and metal oxides, the interactions between physisorbed molecules and the surface atoms are dominated by electrostatic and repulsive terms. Therefore, empirical potentials are very usefol to gain an overview of possible adsorption geometries. This is particularly appropriate for zeolites, where geometric aspects related to the pore sizes and channel widths provide a first screening. In the case of molecules adsorbed or chemisorbed on metals and for achieving quantitative predictions, one has to resort to quantum mechanical methods. At present, this has been demonstrated only for atoms and rather small molecules such as CO and H2O interacting with surfaces. 

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