Metal oxide, solid solutions

Foster, P.K. Welch, A.J.F. (1956) Metal-oxide solid solutions. Part 1. Lattice constant and phase relationships in ferrous oxide (wiistite) and in solid solutions of ferrous oxide and manganous oxide. Trans. Faraday Soc. 52 1626-1635... 

Metal oxide solid solutions have attracted the interest of many research groups since the 1950s (323-329), for two reasons. First, solid solutions have been used to investigate the effect that a guest cation (M) has on the... 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as tf-type Fe2 Ty03 and type Ni1 LyO. Thick film resistors are also made from transition-metal oxide solid solutions. Glass-bonded By 2 Pb2yRu207 having the pyrochlore [12174-36-6] structure is typical. 

The first two forms of degradation can be suppressed by forming solid solutions of catalysts with certain isostructural compounds which possess high Brunauer-Emmett-Teller (BET) surface areas of superior thermal stability. However, these studies demonstrate that transition metal oxide solid solutions are doubtful candidates for use as automotive catalysts for a variety of reasons. 

PR Kurek. Oxidation and Hydrolysis of Cyanides Using Metal Chelates on Supports of Metal Oxide Solid Solutions. US Patent 5,476,596, December 19, 1995. 

Supported metal catalysts have been prepared usually by the impregnation method. More finely dispersed metal catalysts were reported to be prepared by superficial reduction of dilute mixed metal oxide solid solutions (ref. 1). The dispersion of metal may be improved by direct synthesis of metal-oxide composite, because the chance of the aggregation of metal atoms is diminished in the direct synthesis. Moreover, clusters with a different structure may be obtained by the direct synthesis because the conditions of cluster formation are different. 

We now extend the work to in situ measurements on metal ions adsorbed at the metal oxide/aqueous solution interface. In this report, our previous results are combined with new measurements to yield specific information on the chemical structure of adsorbed species at the solid/aqueous solution interface. Here, we describe the principles of emission Mossbauer spectroscopy, experimental techniques, and some results on divalent Co-57 and pentavalent Sb-119 ions adsorbed at the interface between hematite (a-Fe203) and aqueous solutions. 

Let us now compare the internal oxidation of nonmetallic (oxide) solid solutions with the internal oxidation of metal alloys. The role of the (neutral) point defect... 

The third way to prepare CNT-ceramic composite powders is via the synthesis of CNT by a CCVD process, in situ in the ceramic powder. A ceramic powder which contains catalytic metal particles at a nanometric size, appropriate to the formation of CNTs, is treated at a high temperature (600-1100°C), in an atmosphere containing a hydrocarbon or CO. In the method reported in 1997 by the present authors,27 iron nanoparticles are generated in the reactor itself, at a high temperature (>800°C), by the selective reduction in H2/CH4 (18% CH4) of an a-Al203 based oxide solid solution ... 

The first step taking place at no/ Nb<0.7 lowers the local structure symmetry due to the formation of the C4V local structure of the [NbOFs] " anion and facilitates the deposition of niobium. Good experimental conditions for the deposition process can be expected in such a case. However, at no/ Nb>l, inhomogeneous niobium deposits containing probably potassium-niobium phase and niobium oxide solid solution were obtained. However, a non-metal deposit could be obtained as well. 

There are a few catalytic applications in which other mixed oxide structures are cited. Among them are pyrochlore compounds such as La2Zr207, spinels such as AB2O4, and lanthanum beta aluminates, all of them with well-defined crystalline structures. There are also a few examples of oxide solid solutions made up of a rare-earth oxide and a transition-metal oxide. 

This paper describes a new synthesis strategy of preparing thermally stable mesostructured transition metal oxides, namely, two-step synthesis (TSS). Basically, the synthesis course involves two steps (1) formation of a mesostructured transition metal oxide solid mediated by surfactant in a basic aqueous solution and (2) treatment of the solid product in an acidic organic solvent containing the respective precursor from which the solid product was produced. The final material synthesized according to such a method is thermally stable and structurally mesoporous with high surface area and uniform pores arranged disorderedly. 

Many studies carried out with bimetallic materials show that Pt-Ru is today one of the best options to oxidize methanol. Thus, a simple way to prepare active Pt-Ru catalysts involves the deposition of metallic nanoparticles from a suspention onto the carbon microparticles by the method known as formic acid method [24]. Considering that the crystal structures of Pt and Ru are different, Pt being fee and Ru hep, the final crystal structure of the alloy depends on the composition. For Ru atomic fractions up to 0.6-0.7, the two metals form solid solutions in which Ru atoms replace Pt lattice points in the fee structure. The opposite situation, Pt atoms replacing Ru atoms in the hep structure is found for Ru atomic fractions higher than about 0.7. However, the crystal structure seems to depend also on the physical state of the material. When nanoparticles are prepared by reduction of ionic metal species, there is at least one report [25] claiming that the fee stmeture prevails up to 80 at.%Ru. On the other hand in sputtered films the hep structure is predominant even at low Ru fractions [26]. 

If AO and BO are mutually soluble, then the situation will be different, especially if the stabilities of the two oxides are comparable. For example, a (Co, Ni) O product layer is formed when Co-Ni alloys are oxidized [51]. Since Co is preferentially oxidized (because it is less noble), the Co content of the alloy decreases. Also, the mole fraction of CoO in the product phase increases in the direction from the metal to the gas. This indicates that the mobility of Co ions in the oxide solid solution is greater than that of the Ni ions. The quantitative treatment of the growth of this quasi-binary reaction product has been given only in an approximate form using a number of simplifications because of mathematical difficulties. However, in some recent studies the application of this approximate theory has been shown to describe the tarnishing of (Co, Ni) or (Mn, Fe) alloys satisfactorily [59]. 

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