Oxide catalysts, surface structure

Regardless of the exact mechanism at work, HCl catalyst pretreatment have been demonstrated to enhance the photocatalytic oxidation of toluene at low concentrations [68,69]. The apparent deactivation of the photocatalyst is noticeably delayed over HCl-pretreated catalyst samples in a manner similar to that seen with cofed toluene and TCE (Fig. 13). However, the pseudo-steady-state level of conversion appears to be nearly identical on both untreated and HCl-pretreated catalysts. Because the batch HCl pretreatment process incorporates a limited quantity of HCl into the catalyst surface structure, this similarity in longterm activity may be the result of surface chlorine depletion. 

Deo, G., Wachs, l.E. and Haber, J. (1994) Supported vanadium oxide catalysts. Molecular structural characterization and reactivity properties. Critical Reviews in Surface Chemistry, 4 (3 4), 141-87. 

Confusion in SMO literature can arise because there is no generally accepted method for determining surface density. As the metric that characterizes the surface oxide of supported metal oxide catalysts, surface density allows one to consider the various structures of the surface oxide on a common scale, independent of total oxide content, preparation method, calcination treatment, and surface area of the support oxide. Surface saturation and monolayer coverage are important threshold surface density values, at which surface oxide crystals form and at which complete consumption of surface hydroxyl groups of the support oxide occurs, respectively. Inconsistencies in these values come about because of (1) differences in their definitions, (2) difficulties in compatibilizing data from different characterization techniques, and (3) the use of support surface area instead of the overall composite SMO. These inconsistencies can make structural comparison of the same SMO composition, such as WO /ZrOj, difficult across different research groups. Calculated properly, however, the surface density metric provides the most simple and useful basis for understanding the relationship between surface nanostructure and catalytic and surface properties. 

Electrocatalytic reactions occur on catalyst surfaces. The catalyst surface structure and chemically bonded or physically absorbed substances on the catalyst surface exert strong influences on catalyst activity and efficiency. X-ray photoelectron spectroscopy (XPS) (also known as electron spectroscopy for chemical analysis (ESCA), auger emission spectroscopy (AES), or auger analysis) is a failure analysis technique used to identify elements present on the surface of the sample. For instance, this can be used to identify Pt and carbon surface chemical species that may present histories of chemical reactions or contamination in the catalyst layer. AES and XPS can also provide depth profiles of element analysis. Wang et al. [41] studied XPS spectra of carbon and Pt before and after fuel cell operation. They observed a significant increase in O Is peak value for each oxidized carbon support, the result of a higher surface oxide content in the support surface due to electrochemical oxidation. However, sample preparation in AES and XPS analysis is critical because these methods are very sensitive to a trace amount of contaminants on sample surfaces, and detect as little as 2-10 atoms on the sample surface. 

A potentiodynamic kinetic model of Pt oxide formation that is consistent with CV data over a wide range of scan rates and for different catalyst surface structures is an important step in understanding Pt oxide formation and reduction. Evaluation of the model against a range of electrochemical and spectroscopic data and comparison with theoretical calculations of reaction pathways and energetics could help furnishing details of reaction mechanisms. 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

At elevated temperatures in the presence of oxygen the aluminium oxide layer catalyzes the formation of blue fluorescent aluminium oxide surface compounds with 4-hydroxy-3-oxo-A -steroid structures [4]. Aluminium oxide acts as an oxidation catalyst for an activated methylene group. 

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

Understanding and controlling oxide surfaces are the key issues for the development of industrial oxide catalysts, but oxide surfaces are in general heterogeneous and complicated, and hence have been little studied so as to put them on a scientific basis by traditional approaches. While studies of the structure of surfaces have focused on metals and semiconductors over the past thirty years, the application of surface science techniques to metal oxides has blossomed only within the last decade[l-3]. 

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. 

V-Sb-oxide based catalysts show interesting catal)dic properties in the direct synthesis of acrylonitrile from propane [1,2], a new alternative option to the commercial process starting from propylene. However, further improvement of the selectivity to acrylonitrile would strengthen interest in the process. Optimization of the behavior of Sb-V-oxide catalysts requires a thorough analysis of the relationship between structural/surface characteristics and catalytic properties. Various studies have been reported on the analysis of this relationship [3-8] and on the reaction kinetics [9,10], but little attention has been given to the study of the surface reactivity of V-Sb-oxide in the transformation of possible intermediates and on the identification of the sxirface mechanism of reaction. 

A nanoporous structure on the surface of the micro channels can be realized via anodic oxidation, thereby considerably enlarging the catalyst surface [17]. Catalysts... 

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