Quaternary metal oxides

Metal clusters in metal oxide systems have not been well-characterized or abundantly investigated up to the present time. Only isolated examples of metal-metal bonded units in oxide lattices have appeared from time to time. It will be the thesis of this presentation to show that highly unusual structures determined by strong metal-metal bonding will be found in ternary and quaternary metal oxide systems, and that opportunities abound for creative work on the synthesis, theory and structure-property relationships of such compounds. Because of the well-known correlation of d-electron population and d-orbital radial extension with metal-metal bond formation,... 

Figure 13 Reaction pathway for the electrochemical incineration of p-benzoqui-none at a Pt anode covered with a quaternary metal oxide film. (From Ref. 54.)...

Houk LL, Johnson SK, Feng J, Houk RS, Johnson DC. Electrochemical incineration of benzoquinone in aqueous media using a quaternary metal oxide electrode in the absence of a soluble supporting electrolyte. J Appl Electrochem 1998 28 1167-1177. 

A number of ternary and quaternary metal oxides of perovskite and related structures can be prepared by employing hydroxide, nitrate and cyanide solid solution precursors as well [20]. For example, hydroxide solid solutions of the general formula Lnj-jMj/OHJj where Ln = La or Nd and M = Al, Cr, Fe, Co or Ni) and La where (where M Ni and... 

Johnson et al. (Houk et al. 1998 Johnson et al. 1999) studied the incineration of 4-chlorophenol and benzoquinone using quaternary metal-oxide anodes (Ti, Ru, Sn, and Sb). They demonstrated that this type of electrode is stable and electrochemi-cally active for the oxidation of organic compounds when it is used in the absence of a soluble supporting electrolyte, with a Nation membrane as solid-state electrolyte however, the electrolysis time for complete COD and TOC removal was excessively long and current efficiency was low. 

Binary Mixed Metal Oxides to Quaternary Metal Oxides... 

Numerous examples of applications of DSC to the study of the stabiUty of binary, ternary and quaternary metal oxides can be found in the Uterature, and we will not focus on this type of systematic characterization. Fewer studies have been performed to determine the acid-base character of mixed oxides using adsorption calorimetry of probe molecules. 

In catalysis, oxides with well defined acidic and basic properties are used in different forms that have found application in numerous catalytic applications in the gas-solid and liquid-solid heterogeneous catalysis [3, 46, 47], Among the most used oxide materials in catalysis, we And (i) bulk oxides (one component metal oxides) (ii) doped and moditied oxides (iii) supported metal oxides (dispersed active oxide component onto a support oxide component) (iv) bulk and supported binary metal oxides to quaternary metal oxides (mixed oxide compositions) (v) complex oxides (e.g., spinels, perovskites, hexa-aluminates, bulk and supported hydrotalcites, pillared clays, bulk and supported heteropolyacids, layered silicas, etc.). 

Bulk and supported mixed oxide compositions, from binary metal oxides to quaternary metal oxides, consist in general of large crystalline phases possessing low surface area values (typically from 1 to 10m g ). Examples of oxides of this type of catalytic relevance are V-Nb-0, Mo-Nb-O, Co-Ti-0, Ni-Ti-0, etc. The acid-base properties of mixed metal oxides have been found to change with the nature of the constituents and their relative concentrations, preparation (co-precipitation and sol-gel synthesis among are the most popular methods used), and pre-treatments procedures. Appropriately choosing the mentioned variables, mixed oxides can be used to prepare catalysts with the desired-acid-base characteristics. 

Table II. Characteristics of metal-metal bonded ternary and quaternary molybdenum oxides.

In 1988, Cava and co-workers also prepared (88a) a quaternary oxide, Ba/K/Bi/O, and observed superconductivity at -28 K. This compound was the first "non-transition metal" oxide with a Tc above the legendary "alloy record" of 23 K. Further studies indicated (88a) that the optimum composition for "high temperature" superconductivity in this system was Ba0 6K0 4BiO3 x, having a Tc of 30.5 K (Figure 17). The samples were multiphase, and the superconducting fraction varied from 3 to 25%. Superconductivity for the rubidium-substituted compound was observed at -28.6 K. 

SILICATES (Soluble). The most common and commercially used soluble silicates are those of sodium and potassium. Soluble silicates are systems containing varying proportions of an alkali metal or quaternary ammonium ion and silica. The soluble silicates can be produced over a wide range of stoichiometric and nonstoicluometric composition and are distinguished by the ratio of silica to alkali. This ratio is generally expressed as the weight percent ratio of silica to alkali-metal oxide (SiOj/MjO). Particularly with lithium and quaternary ammonium silicates, the molar ratio is used. 

POMs can he immobilized onto anion-exchange resins and surface-modified metal oxides with quaternary ammonium cation- or amino-functional groups via anion-exchange. Jacobs and coworkers tethered Venturello s catalyst [P04(W0(02)2)4]3-on a commercially available nitrate-form resin with alkylammonium cations and have carried out the epoxidation of allylic alcohols and terpenes with this supported catalyst [166, 167]. The regio- and diastereoselectivity of the parent homogeneous catalysts were preserved in the supported catalyst. For bulky alkenes, the reactivity of the POM catalyst was superior to that of Ti- 3 zeolite with a large pore size. The catalytic activity of the recycled catalyst was maintained completely after several cycles. 

The synthesis of a MeAPO molecular sieve typically uses an aqueous reaction mixture formed by combining a dissolved form of the divalent metal, orthophosphor1c acid, a reactive alumina, and an amine or quaternary ammonium templatlng agent (R>. The metal Is typically Introduced as the acetate or sulfate salt, or as the metal oxide dissolved In dilute phosphoric acid. A synthesis mixture Is prepared In one of two ways ... 

Use of the combustion method in an atmosphere of air or oxygen to prepare complex metal oxides seems obvious. In the last three to four years, a variety of oxides have been prepared using nitrate mixtures with a fuel such as glycine or urea. It seems that almost any ternary or quaternary oxide can be prepared by this method. All the superconducting cuprates have been prepared by this method, although the resulting products in fine particulate form have to be heated to an appropriate high temperature in a desired atmosphere to obtain the final cuprate [18], Table 2 lists typical materials prepared by the combustion method. 

The astute reader will recognize that this compound is a metal-metal bonded adaptation of the well-known hollandite structure (19). Further, the metal cluster configuration is like that in CsNb Cl.. (20), though the latter compound possesses a double layer structure rather than the tunnel structure observed here. Finally these new structures provide only a glimpse of what is possible, and an excitement about new compounds of unusual structure and properties yet to come from studies of highly reduced ternary and quaternary refractory metal oxide systems. 

Transition metal oxidants such as permanganate, ruthenium tetroxide and diromium(VI) oxide are convenient and efficient reagents for routine cleavage reactions. The use of phase transfer catalysts (quaternary ammonium and phosphonium ions, primarily) has made it possible to solubilize transition metal oxides such as permanganate and chromatt in nonaqueous solvents, and to therdry increase the scope of these reactions substantially. ... 

Figure 20 Crystal structures of various ternary and quaternary phosphide oxides of the alkaline earth, rare earth, and actinoid metals. Alkaline earth (rare earth, actinoid), transition metal, phosphorus, and oxygen atoms are drawn as large light grey, medium grey, filled, and open circles, respectively. Some relevant coordination polyhedra around the oxygen atoms and the transition metal-phosphorus bonds are emphasized...

It was regarded as a suboxide and also as metal modification. Today it is described as a metastable metal phase, which is stabilized by small amounts of oxygen. A further stabilization of P-W occurs by the presence of different foreign elements, such as K, Be, P, B, As, Ce, Th, and Al. All these elements have in common that they form rather stable ternary and quaternary tungsten oxides. 

The project goals are to significantly improve both the kinetic performance of the electrocatalyst powder at low noble metal loading and its utilization in the cathode layers through layer structure development. Limitations in the catalyst performance will be addressed through combinatorial discovery of supported catalyst compositions and microstructures. The discovery of these new catalyst formulations will be carried out under conditions that have been scaled for commercial powder production. A large variation of binary, ternary and quaternary noble metal -transition metal alloys and mixed metal-metal oxide catalyst compositions will be screened. To improve the utilization/performance of the catalyst in MEAs,... 

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