Mixed metal oxides formation

A systematic study of differently supported Ru catalysts showed that carbon catalysts provide very high selectivities to higher hydrocarbons (C10-C20) and the CNT-supported catalyst is among the most active systems of all [138]. In parts this is related to the inertness of carbon preventing the formation of hardly reducible mixed metal oxides with the support, such as CoAl204 [139,140], which is, besides coking, the main reason for catalyst deactivation. The carbon surface functionalized with oxygen... 

In our experiments we screened zeolites, ion-exchange resins, heteropoly compounds and mixed metal oxides. Several alcohols were used to show the range of applicability. The selectivity was assessed by testing the formation of side products in a suspension of catalyst in alcohol (e.g. SZ in 2-ethylhexanol) under reflux for 24 hours. Under the reaction conditions, no by-products were detected by GC analysis. 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

The formation of rutile mixed phases in the system Ti-Ni-Sb-0 is described in detail in [3.92], When nickel ions are to be included in the pigment and antimony trioxide is used to provide the antimony ions, oxygen must be present to ensure that nickel and antimony occupy the titanium sites in the lattice. If oxygen is excluded, only small amounts of nickel and antimony are taken up in solid solution, and no useful products are formed. The presence of oxidizing agents (e.g., nitrates) is necessary for the production of antimony-containing mixed metal oxide pigments. 

The W-Nb mixed metal oxide was prepared by sonicating the solution containing the precursors. Treatment of the mixed metal oxide precursors in H2S produced the Nb doped WS2 nanotubes (Fig. 14). The mechanism of formation of the com-... 

C-C Bond Formation Through Multifunctional Catalysis By Mixed Metal Oxides... 

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

The deposition of mixed metal oxides is based on the deposition techniques and precursors used for the formation of metal oxides discussed earlier in this chapter. The most important publications in the held of high-temperature superconducting materials produced from metal enolates since the date of release of Rees book in 1996 are summarized in Table 8. 

Figure 17 Formation of 2D-hexagonal mesoporous mixed metal oxides by liquid crystal templating mechanism [134,135]...

Macroporous VPO Phases. - The macroscale templating of bulk mixed metal oxide phases in the presence of colloidal sphere arrays typically consists of three steps shown in Figure 18. First, the interstitial voids of the monodisperse sphere arrays are filled with metal oxide precursors. In the second step, the precursors condense and form a solid framework around the spheres. Finally, the spheres are removed by either calcination or solvent extraction leading to the formation of 3D ordered macroporous structures [137]. 

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... 

In line with the XRD results, the DSC/FTIR study of CR also suggests the formation of hydrotalcite-like phase [13]. High Mg-containing CS is more thermally stable than CR. With increase in calcination temperatures, the FTIR spectrum evolution reveals the formation of mixed metal oxides, i.e. spinel-type phase [40]. The decomposition of catalyst precursors is also reflected in the XRD study which shows that the resultant oxides are largely amorphous although weak spinel-type feature can be observed [41]. 

Other changes can occur to render the surface catalytically inactive for high surface area and/or supported metal oxides. Metal oxides can exist in various crystallographic structures, of which only a limited set (or only one) may be active. The transformation of one form to another can represent a deactivation of the catalyst. The formation of mixed metal oxides by reaction between the oxide and the support may represent a system with little or no catalytic activity as compared to the original catalyst. 

This method has been developed largely by Klemm et al. and is discussed in a recent review,7 which gives details on catalysts, procedure, and possible mechanisms Examples include formation of the peri-condensed compounds 35 (29-39%), from bridging at positions 4 and 5 of the phenanthrene molecule 149 at ca. 630°C in the presence of various sulfided mixed metallic oxide catalysts,6,36 and 80 (18%), from similar bridging in the triphenylene molecule 150 at 500°C.37 Since the sulfur bridge forms by substitution at the sterically most hindered positions of these hydrocarbon substrates, method... 

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