Of oxide surfaces

It was noted that the content of functional groups on the surface of studied A1,03 was 0,92-10 mol/g of acid character for (I), FOS-IO mol/g of basic character for (II). The total content of the groups of both types was 1,70-lO mol/g for (III). The absence of appreciable point deviations from a flat area of titration curves in all cases proves simultaneously charges neutralization character on the same adsoi ption centers and non-depending on their density. The isoelectric points of oxide surfaces have been detenuined from titration curves and have been confirmed by drift method. 

The bonding agent technique is usually not applicable to the metal particles in the composite. However, the surface of the metal is almost invariably covered by a thin (40-80 A) oxide layer [50]. The free energy of oxide surfaces is normally quite large (10 mJ/m ) to allow quick wetting by most organic polymers (40-60 mJ/m ). Additionally, the metal surface may provide two... 

Though as yet in its infancy, the application of laser Raman spectroscopy to the study of the nature of adsorbed species appears certain to provide unusually detailed information on the structure of oxide surfaces, the adsorptive properties of natural and synthetic zeolites, the nature of adsorbate-adsorbent interaction, and the mechanism of surface reactions. 

Commercial Alkylation of Paraffins and Aromatics Edwin K. Jones The Reactivity of Oxide Surfaces E. R. S. Winter... 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

Recently, it has been demonstrated that coordination vacancies on the surface metal cations are relevant to the unique redox reactivity of oxide surfaces]2]. Oxidation of fonnaldehyde and methyl formate to adsorbed formate intermediates on ZnO(OOOl) and reductive C-C coupling of aliphatic and aromatic aldehydes and cyclic ketones on 1102(001) surfaces reduced by Ar bombardment are observed in temperature-prognunmed desorption(TPD). The thermally reduced 1102(110) surface which is a less heavily damaged surface than that obtained by bombardment and contains Ti cations in the -t-3 and +4 states, still shows activity for the reductive coupling of formaldehyde to form ethene]13]. Interestingly, the catalytic cyclotrimerization of alkynes on TiO2(100) is also traced in UHV conditions, where cation coordination and oxidation states appear to be closely linked to activity and selectivity. The nonpolar Cu20( 111) surface shows a... 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

Despite the success in modeling catalysts with single crystals and well defined surfaces, there is a clear need to develop models with higher levels of complexity to address the catalytically important issues specifically related to mixed oxide surfaces. The characterization and design of oxide surfaces have not proven to be easy tasks, but recent progress in identification of the key issues in catalytic phenomena on oxide surfaces by in-situ characterization techniques on an atomic and molecular scale brings us to look forward to vintage years in the field. 

Over the past decade or so, our understanding of oxide surfaces has markedly improved, especially in the case of the model oxide surface-Ti02(l 10). In particular, the influence of defects such as oxygen vacancies on the surface reactivity has been demonstrated in exquisite detail and it is clear that they play a crucial role in many surface reactions. As we have shown in this chapter, scanning probes have been instrumental in evaluating the surface reactivity as they allow individual defects, reactants, and products to be imaged on relatively short timescales (seconds or minutes). 

Thermodynamic considerations imply that all crystals must contain a certain number of defects at nonzero temperatures (0 K). Defects are important because they are much more abundant at surfaces than in bulk, and in oxides they are usually responsible for many of the catalytic and chemical properties.15 Bulk defects may be classified either as point defects or as extended defects such as line defects and planar defects. Examples of point defects in crystals are Frenkel (vacancy plus interstitial of the same type) and Schottky (balancing pairs of vacancies) types of defects. On oxide surfaces, the point defects can be cation or anion vacancies or adatoms. Measurements of the electronic structure of a variety of oxide surfaces have shown that the predominant type of defect formed when samples are heated are oxygen vacancies.16 Hence, most of the surface models of... 

Another key contribution of the Schwarz group was the recognition of the dramatic influence of oxide surfaces on bulk solution pH. In a landmark 1989 paper, Noh and Schwarz [7] demonstrated the method of mass titration, in which successive additions of oxide cause stepwise shifts in solution pH. This procedure is illustrated in Figure 6.7 [7], As indicated in Figure 6.1, the protonation-deprotonation chemistry of the surface hydroxyl groups is coupled to the liquid-phase pH. In mass titration, as the mass (or more appropriately, the surface area) of oxide in solution increases, the solution pH is brought to the PZC of the oxide, at which point no driving force for proton transfer exists... 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

Roden EE, Zachara JM (1996) Microbial reduction of crystalline iron(III) oxides influence of oxide surface area and potential for cell growth. Environ Sci Technol 30 1618-1628 Roden EE, Urrutia MM (2002) Influence of biogenic Fe(II) on bacterial reduction of crystalline Fe(lll) oxides. Geomicrobio J 19 209-251... 

Although oxides are generally considered to be insulators, it is sometimes possible to monitor the surface of an oxide film by STM. Basically two mechanisms that explain the imaging of oxide surfaces by in situ STM are considered ... 

Hadjiivanov, K.k and Vayssilov, G.N. (2002) Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule. 

The Raman spectrum of VCI4 has been reported and values for the force constants and various thermodynamic parameters of the molecules have been calculated. VCI4 has been used to investigate the structure of oxide surfaces. The number of OH groups per unit area follows from the amount of HCl liberated, and the arrangement of such groups can then be determined by e.s.r. spectroscopy. ... 

The initial slopes obtained are different from the first loading to the third, indicating clearly that there is not one unique zirconium monohydride site on the surface of silica, thus showing the heterogeneity of oxide surface, which leads to different active sites. 

D. Alfe and M. J. Gillan, The Energetics of Oxide Surfaces by Quantum Monte Carlo, J. Phys. Condens. Matt. 18 (2006), F435. 

Mackrodt, W.C. (1988) Atomistic simulation of oxide surfaces. Phys. Chem. Min. 15 228-237... 

Sahai, N. Sverjensky D.A. (1997 a) Evaluation of externally consistent parameters for the triple layer model by the systematic analysis of oxide surface titration data. Geochim. Cosmochim. Acta 61 2801-2826 Sahai, N. Carroll, S.A. Roberts, S. O Day,... 

The following table summarizes techniques useful for the study of oxide surfaces (20), Applications of a number of these are described in the papers by Volta, et al, Busca, Deo and Wachs, Okuhara, et al, A transient technique is reported by Rigas, et al. 

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