Bulk metal oxides

2 Bulk Metal Oxides. Extensive Raman chemisorption studies on high surface area alumina and silica supports have been performed because of the industrial importance of these oxides and their weak background Raman vibrations. The most informative studies resulted from the adsorption of pyridine since this probe molecule is very sensitive to the type of acid sites (Brpnsted and Lewis) present on the silica and alumina surfaces. On the alumia support, Lewis pyridine was predominately observed and on the silica support both Lewis and Brpnsted pyridine were observed. The surface concentrations of pyridine on the silica surface were very small in comparison to the pyridine coverages on the alumina surface. The signal intensities were dramatically enhanced by the 

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As an initial example of using DFT calculations to describe phase stability, we will continue our discussion of bulk metal oxides. In this chapter, we are interested in describing the thermodynamic stability of a metal, M, in equilibrium with gas-phase O2 at a specified pressure, Po2, and temperature, T. We will assume that we know a series of candidate crystal structures for the metal... 

Hydrated and Dehydrated Bulk Metal Oxide Catalysts... 

The first Raman spectra of bulk metal oxide catalysts were reported in 1971 by Leroy et al. (1971), who characterized the mixed metal oxide Fe2(MoC>4)3. In subsequent years, the Raman spectra of numerous pure and mixed bulk metal oxides were reported a summary in chronological order can be found in the 2002 review by Wachs (Wachs, 2002). Bulk metal oxide phases are readily observed by Raman spectroscopy, in both the unsupported and supported forms. Investigations of the effects of moisture on the molecular structures of supported transition metal oxides have provided insights into the structural dynamics of these catalysts. It is important to know the molecular states of a catalyst as they depend on the conditions, such as the reactive environment. 

Beyond providing bulk structural information about 3-D metal oxide phases, Raman spectroscopy can also provide information about the terminating (and thus 2-D) surface layers of bulk metal oxides. For example, surface Nb=O, V = O, and Mo=O functionalities were detected by Raman spectroscopy for bulk Nb2Os, and for vanadium-niobium, molybdenum-vanadium, molybdenum-niobium, and vanadium-antimony mixed oxide phases (Guerrero-Perez and Banares, 2004 Jehng and Wachs, 1991 Zhao et al., 2003). 

The inherent complexity of bulk metal oxides also makes the study of oxide surfaces a difficult undertaking. Substantive issues include surface stoichiometry and termination, geometric and electronic structure, and the role of defects on surface properties. Accordingly, activity in oxide surface science has exhibited exponential growth over the past decade as the fascinating scientific issues and varied technological importance of oxide surfaces become more apparent to the international surface science community. Much of the... 

Catalysts based on transition metal oxides are the principal materials investigated in alkane ODH. Ethane is oxidatively dehydrogenated to ethylene with high conversion and selectivity over bulk metal oxide catalysts containing... 

One should finally bear in mind that the presented results are of acceptable quality already at the simplest level of approximation in modern DFT whereas traditional correlated computations lead to satisfactory results only on high expense of computer time, close to making some of them intractable with standard techniques. This may be taken as the optimistic sign of high capability of the method and its potential usefulness in studying bigger systems composed of transition-metal atoms, in particular clusters modeling bulk metal oxides. 

Finely divided Fe is pyrophoric in air, but the bulk metal oxidizes in dry air only when heated. In moist air, Fe rusts, forming a hydrated oxide Fe203-xH20. Rusting is an electrochemical process (equation 21.56) and occurs only in the presence of O2, H2O and an electrolyte. The latter may be water, but is more effective if it contains dissolved SO2 (e.g. from industrial pollution) or NaCl (e.g. from sea-spray or salt-treated roads). Diffusion of the ions formed in reaction 21.56 deposits Fe(OH)2 at places between the points of attack and this is further oxidized to hydrated iron(III) oxide (see Box 7.3). 

Bulk oxides are used extensively in the chemical industry as catalysts for a wide range of chemically important transformations (1,2]. In general, these applications employ one of the two fundamental properties of the oxide surface (1) redox activity, which enables hydrogen abstraction from, or oxygen transfer to, adsorbed substrate molecules and (2) acid-base character (Lewis or Bronsted type), which facilitates catalytic cracking or isomerization. Table I summarizes the most important catalytic bulk metal oxides currently in use and gives an overview of the processes catalyzed by them. Not all of these are currently exploited industrially. 

There are essentially three types of cluster models that are currently used to calculate local properties of bulk metal oxides or to describe adsorption and... 

C [36]. Bulk metal oxide catalysts are often not calcined but only dried at temperatures as low as 300 °C [40]. 

Recent publications show that there is actually no solid absorbent based on bulk metal oxides available that meets the conditions formulated above for application in high-temperature desulfurization processes. Absorbents that have been developed all show one or more undesired characteristics. Disintegration due to a poor mechanic and chemical stability is commonly found. Metal sulfate formation during regeneration with O2 is a considerable problem [5]. Furthermore, absorbents based on bulk metal oxides display a relatively low activity towards the removal of H2S from coal gas. 

Manganese oxide, as supported and as bulk metal oxide, has also been studied for the catalytic combustion of aromatic VOCs. The performance of an - -Al203... 

Burcham, L., Briand, L. and Wachs, I. (2001). Quantification of Active Sites for the Determination of Methanol Oxidation Turnover Frequencies Using Methanol Chemisorption and In Situ Infrared Techniques. 2. Bulk Metal Oxide Catalysts, Langmuir, 17, pp. 6175-6184. 

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