Other Applications of Metal Oxides

As delineated in the Introduction, numerous research groups have focused on the preparation and application of transition metal nanoparticles in the zerovalent form during the last two decades but, parallel to this development, research in the area of nanosized metal oxides has also increased. The areas of application of metal oxide nanoparticles range from catalysis [4] to semiconductors (e.g., ZnO, ZnS, CdSe) [5]. In the area of heterogeneous catalysis, many different preparative methods have been described [4], Other methods are based on the hydrolysis of transition metal salts in microemulsions [3-8], These and other approaches have been reviewed and will not be specifically treated here. Rather, the main focus is on the author s own research. 

The core of the earth is composed of a mass of metals ( 90% iron, 9% nickel, and VYo other metals), while the earth s crust—the outer shell extending to a depth of about 13 km—is dominated by oxygen (47%) [1], Not surprisingly, metal oxides constitute the most abundant and most accessible of inorganic materials in nature [1,2]. The fact that the periodic table is dominated by metals means, further, that a wide variety of metal oxides can be formed, spanning a broad range of physical and chemical properties. These properties provide the basis for the many scientific and technological applications of metal oxides, some selected examples of which are collected in Table 1 [3]. 

Thermoelectric gas sensors are other field of possible application of metal oxides. The thermoelectric effect is the direct conversion of temperature differences to electric voltage. A thermoelectric device... 

The important properties of metal oxides that are of certain interest in different fields of applied science and technology are connected with magnetic, electrical, dielectric, optical, acid-base, and redox behavior. In particular, transition metal oxides with redox properties are of interest for applications in catalysis [1]. Metal oxides and in particular transition metal oxides can possess more than one stable oxidation state making them possible to catalyze reactions which necessitate electron exchanges between reactant(s) and surface active sites (e.g., oxidation, dehydrogenation, etc.). Conversely, other kinds of metal oxides are known to be almost complete irreducible. Oxides of copper, nickel, cobalt, molybdenum, are only few examples of reducible... 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

The number of publications concerning utilization of the EISA process for fabrication of different structured materials is counted in the hundreds, which is far beyond the possibilities of this chapter to review in depth. Rather, we intend to provide a brief introduction into EISA and its application to the fabrication of functional thin films for electronic applications (e.g., electro-chromic layers and solar cells), with a special focus on fabrication of crystalline mesoporous films of metal oxides. Attention will also be given to techniques used to evaluate the pore structure of the thin films. For the other aspects of the EISA process, for example its mechanism,4 strategies for preparation of crystalline porous metal oxides,5 mesoporous nanohybrid materials,6 periodic organic silica materials,7,8 or postgrafting functionalization of mesoporous framework,9 we kindly recommend the reader to refer to the referenced comprehensive reviews. 

Since this initial work there has been a plethora of literature on mesoporous molecular sieves. In addition to the silica and aluminosilicate frameworks similar mesoporous structures of metal oxides now include the oxides of Fe, Ti, V, Sb, Zr, Mn, W and others. Templates have been expanded to include nonionic, neutral surfactants and block copolymers. Pore sizes have broadened to the macroscopic size, in excess of 40 nm in diameter. A recent detailed review of the mesoporous molecular sieves is given in ref [73]. Vartuli and Degnan have reported a Mobil M41S mesoporous-based catalyst in commercial use, but to date the application has not been publicly identified.[74]. 

Other applications of supported liquid membranes have been related to metal speciation. For example, recently a system for chromium speciation has been developed based on the selective extraction and enrichment of anionic Cr(VI) and cationic Cr(III) species in two SLM units connected in series. Aliquat 336 and DEHPA were used respectively as carriers for the two species and graphite furnace atomic absorption spectrometry used for final metal determination. With this process, it was possible to determine chromium in its different oxidation states [103]. 

In the past few decades, a specific kind of colloidal system based on monodis-perse size has been developed for various industrial applications. A variety of metal oxides and hydroxides and polymer lattices have been produced. Monodisperse systems are obviously preferred since their properties can be easily predicted. On the other hand, polydisperse systems will exhibit varying characteristics, depending on the degree of polydispersity. 

Elemental carbon has many important applications. The diamond is a precious gem, known to mankind for ages graphite is used as an electrode and has numerous other applications carbon-14 isotope is used in carbon dating and the isotope carbon-13 in tracer studies and NMR. Carbon black is used in paints, pigments and inks. Activated carbon is used as an adsorbent for purification of water and separation of gases. Coke is used for electrothermal reduction of metal oxides to their metals. These applications are discussed below in more detail. 

Hydroxylamine is used as a reducing agent in many inorganic and organic synthetic reactions. Other applications of this compound include purification of aldehydes and ketones dehairing of hides as an antioxidant for fatty acids to stabilize lower oxidation states of metal ions for analysis and in photography. 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

Some other synthetic applications of the oxidation of positively charged metals in electrochemical cells have been described previously - ... 

Representative alloys containing antimony arc described in the Tabic L Metallic antimony is an effective pearlitizing agent for producing pearlitic cast iron, The principal use of antimony, however, is in the form of the oxide Its major application is as a flame retardant for plastics and textiles, Other applications of importance are in glass, pigments, and catalysts. 

The brief review emphasizes the useful catalytic activities of metal oxides, i.e., Ru02, towards 02-evolution but points to their limitations as a result of surface recombination with intermediate H-atoms. Possible routes to circumvent these difficulties could involve elimination of surface H-atom through the application of homogeneous H2-evolution catalysts (see Sect. 4.3), and compartmentalization of the oxidation catalyst from the H2-evolution catalyst, i.e., liposomes. Alternatively, reduction of other substrates rather than water i.e. C02, could lead to intermediate carbonous species being insensitive to oxidation by intermediate O-species. 

Since mesoporous materials contain pores from 2 nm upwards, these materials are not restricted to the catalysis of small molecules only, as is the case for zeolites. Therefore, mesoporous materials have great potential in catalytic/separation technology applications in the fine chemical and pharmaceutical industries. The first mesoporous materials were pure silicates and aluminosilicates. More recently, the addition of key metallic or molecular species into or onto the siliceous mesoporous framework, and the synthesis of various other mesoporous transition metal oxide materials, has extended their applications to very diverse areas of technology. Potential uses for mesoporous smart materials in sensors, solar cells, nanoelectrodes, optical devices, batteries, fuel cells and electrochromic devices, amongst other applications, have been suggested in the literature.11 51... 

Both the discovery of new synthesis processes for nanostructured materials and the demonstration of the highly reactive properties of these materials have increased rapidly within recent years. The new synthesis processes have made available nanostructured materials in a wide variety of compositions of metal oxides and metals supported on metal oxides, which have led to recognition of their exceptional chemical, physical, and electronic properties. The objective of this review is to provide recent results on synthesis of nanostructured materials using the novel processes that were developed in these laboratories recently and to contrast them to other important, new methods. Because some of the most important applications of nanostructured materials are as catalysts for chemical processing, several key reports on enhanced catalytic reactivity of nanostructured grains will be discussed along with the pertinent theory responsible for controlling both activity and selectivity of these new catalysts. 

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