Applications metal oxide catalysts

As a final example of the application of gas-liquid-particle operation to a process involving a gaseous reactant and a solid catalyst, the possibility of polymerizing ethylene in, for example, a slurry operation employing a metal or metal oxide catalyst can be cited. It has been suggested that the good control of reaction conditions obtained in a slurry-type operation may be of importance in the production of certain types of polyethylene (Rl). 

The oxides often are nonstoichiometric (with an excess or dehcit of oxygen). Many oxides are semiconducting, and their conductivity can be altered by adding various electron donors or acceptors. Relative to metals, the applications of oxide catalysts in electrochemistry are somewhat limited. Cathodic reactions might induce a partial or complete reduction of an oxide. For this reason, oxide catalysts are used predominantly (although not exclusively) for anodic reactions. In acidic solutions, many base-metal oxides are unstable and dissolve. Their main area of use, therefore, is in alkaline or neutral solutions. 

At an industrial scale, the esterification catalyst must fulfill several conditions that may not seem so important at lab-scale. This must be very active and selective as by-products are likely to render the process uneconomical, water-tolerant and stable at relatively high temperatures. In addition, it should be an inexpensive material that is readily available on an industrial scale. In a previous study we investigated metal oxides with strong Bronsted acid sites and high thermal stability. Based on the literature reviews and our previous experimental screening, we focus here on application of metal oxide catalysts based on Zr, Ti, and Sn. 

In general, there are two possibilities to prepare nanocarbon-supported metal(oxide) catalysts. The in situ approach grows the catalyst nanoparticles directly on the carbon surface. The ex situ strategy utilizes pre-formed catalyst particles, which are deposited on the latter by adsorption [94]. Besides such solution-based methods, there is also the possibility of gas phase metal (oxide) loading, e.g., by sputtering [95], which is used for preparation of highly loaded systems required for electrochemical applications not considered here. 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

Lanthanide-containing porous materials have found many applications in various fields [20-22], They are known as active and selective catalysts for synthesis of higher hydrocarbons (mostly ethane and ethylene) from methane [23], which is of considerable importance for utilizing the reserves of natural gas around the World. Cerium oxide has been employed as a catalyst or as a structural promoter for supported metal oxide catalysts... 

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control application. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity per exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

The validity of the method of course also relies on the proposed mechanism for reaction (1) being the real one, meaning that there is some danger of circularity in the reasoning. It is to be hoped that other groups are also going to use this method, in order to widen its field of application. As far as could be ascertained, the method proposed by Parkash77 to determine the number of active sites in metal oxide catalysts by selective gas chemisorption has not... 

Lin MM. Complex metal-oxide catalysts for selective oxidation of propane and derivatives. I. Catalysts preparation and application in propane selective oxidation to acrylic acid. Applied Catalysis, A General. 2003 250(2) 305-318. 

Present knowledge of the electronic, surface, and catalytic properties of non-metals is substantially less advanced than that of metals. This is partly due to the fact that various physicochemical techniques are less rapidly applicable to non-metal catalysts. Relatively few metal oxide catalysts have been investigated by XAES techniques (Table VIII and Ref. 18). 

IR spectroscopy is largely used for the characterization of metal oxide catalysts in relation to their structural features, with additional possible information on their morphology. Several collections of IR, Raman or both IR and Raman spectra of inorganic materials and minerals have been published, and are available electronically. In the following we will briefly review some of the applications of vibrational spectroscopies in the characterization of such materials. 

Vanadium catalysts are among the most significant metal oxide catalysts. For instance, considering only supported catalysts, between 1967 and 2000, 28% of all published papers were concerned with vanadium oxide-based materials [57]. This represents a greater fraction than for any other metal or metal oxide. One reason for this is the wide range of applications in which vanadium oxide catalysts may be employed, examples of which are outlined below. 

The data quality of oxides tend not to be as good as for the metal catalysts, but similar instrumentation could be used for a number of applications for oxidic catalysts. 

Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts... 

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