Refractories, directed metal oxidation

Although the transition metal ehaleogenides usually are quite refractory, direct reaction is feasible in many cases. In some cases (e.g., W and Mo), the oxide is volatile, making the surface at least accessible to reaction. In addition, the metals often have high rates of diffusion in the compounds, thus reducing the surface passivation effect of compound formation. This is probably because diffusion jumps are more probable in the presence of elements that can change their charge states. This property can be helpful in conversion of an oxide to a sulfide via H2S or CS2. 

All metal oxides are reduced when in direct contact with graphite at high temperatures. Even the most refractory oxides will be reduced if the temperature is high enough. 

There are several ways to prepare thin films for use as model catalyst supports.30-31 For the purposes of this review, we will point the reader toward other sources that discuss two of these methods direct oxidation of a parent metal and selective oxidation of one component of a binary alloy. 32 34 The remaining method consists of the deposition and oxidation of a metal on a refractory metal substrate. This method has been used extensively in our group323131 11 and by others33-52-68 and will be the focus of the discussion here. The choice of the metal substrate is important, as lattice mismatch between the film and the substrate will determine the level of crystallinity achieved during film growth. 

When lanthanide metals (M = La - Lu) were heated directly with phosphorus, or M203 with PH3 in air, the binary phosphides MP were produced.55 These decomposed in water and, in air at 700°C, were oxidized to phosphates on heating PrP at 2850 °C, a refractory material, deficient in P but containing free Pr and adsorbed oxygen, was formed. EuP2, obtained by reaction of Eu203 with Pn, contains euro-pium(n).56... 

For opaque materials the reflectance p is the complement of the absorptance. The directional distribution of the reflected radiation depends on the material, its degree of roughness or grain size, and, if a metal, its state of oxidation. Polished surfaces of homogeneous materials are specular reflectors. In contrast, the intensity of the radiation reflected from a perfectly diffuse or Lambert surface is independent of direction. The directional distribution of reflectance of many oxidized metals, refractory materials, and natural products approximates that of a perfectly diffuse reflector. A better model, adequate for many calculation purposes, is achieved by assuming that the total reflectance is the sum of diffuse and specular components pD and ps, as discussed in a subsequent section. 

Figure 1 Compositional evolution of metal condensing at a pressure of 10 atm. The first metal to condense contains 19 3 wt.% Ni and decreases to the cosmic value of 5.7 wt.% Ni as cooling commences. Elements more refractory than Ni are enriched in the early condensates, whereas elements more volatile than Ni are depleted. Oxidation of Fe shifts the composition of the metal in the direction of the heavy arrows (Kelly and Larimer, 1977) (reproduced by permission of Elsevier from Geochim. Cosmochim. Acta, 1977, 41, 93-111).

The most commonly used methods for the preparation of ultrathin oxide films are (1) direct oxidation of the parent metal surface, (2) preferential oxidation of one metal of choice from a suitable binary alloy, and (3) simultaneous deposition and oxidation of a metal on a refractory metal substrate. The detailed procedures for (1) and (2) are discussed elsewhere [7,56,57] procedure (3) is discussed here in detail. Preparation of a model thin-film oxide on a refractory metal substrate (such as Mo, Re, or Ta) is usually carried out by vapor-depositing the parent metal in an oxygen environment. These substrate refractory metals are typically cleaned by repeated cycles of Ar sputtering followed by high-temperature annealing and oxygen treatment. The choice of substrate is critical because film stoichiometry and crystallinity depend on lattice mismatch and other interfacial properties. Thin films of several oxides have been prepared in our laboratories and are discussed below. 

---