Rare earth oxides, deposition

A large deposit of loparite occurs ia the Kola Peninsula, Russia. The production of REO reaches 6500 t/yr. Loparite contains over 30% of rare-earth oxides from the cerium group. In addition, loparite contains up to 40% titanium oxide and up to 12% niobium and tantalum oxides. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

Table 8.9. NO dissociation over reduced rare-earth oxides and over 1% Pt catalysts deposited on these oxides. Gas Prior to NO dissociation (970 ppm NO), the samples are reduced for 1 h in H2 at 500°C [86]...

The metal surface is covered with cerium oxide and the thickness of the film increases with time of immersion of the sample in the CeCl3 solution. Deposition and growth of islands of rare earth oxide have been observed. The islands may be associated with anodic and cathodic sites located in the microstructure of the metal. 

Decay time measurements of EU2O3, and Tb203 doped and coated on alumina were conducted [82]. The luminescence of the alumina substrate was found to be much shorter than that of the rare-earth oxides. Differences between the decay times of the deposited and doped materials are accounted for by the stronger guest-host interaction and the absence of concentration quenching in the doped material. 

The direct oxidation of propylene by molecular oxygen is a low-selective reaction. The propylene oxide yield can be raised by limiting the conversion rate to a low value, about 10 to 15 per cent, by using more selective catalysts, or by achieving co-oxidation with a more oxidizable compound than propylene (acetaldehyde, isobutyraldehyde etc.). Many patents have been Hied concerning this process, but without any industrial implementation. Among them is the liquid phase oxidation of propylene on a rare earth oxide catalyst deposited on silica gel (USSR), or in the presence of molybdenum complexes in chlorobenzene or benzene (JFP Instiiut Francois du Petrole. Jefferson ChemicalX vapor phase oxidation on modified silver catalysts (BP British Petroleum IFP, or on ... 

Several rare earth element oxides are components of high 7 superconducting materials. Their preparation is discussed elsewhere in this book (see Chapter 2). In general, rare earth metal oxides can be obtained by the chemical vapor deposition of the appropriate metal /S-diketonates or carboxylates [96]. Volatile metal alkoxides also should be potentially useful precursors for the preparation of rare earth oxides by CVD. Although several volatile rare earth element alkoxides have been reported in recent years [97-101], detailed information concerning their decomposition behavior is not available at this time. 

The rare earth oxides are usually deposited onto the support by conventional impregnation techniques from aqueous solutions of the corresponding lanthanoid nitrate [136,137,161]. The impregnated samples are further dried at 373 K, and finally calcined at temperatures ranging from 773 K to 973 K. Though widely variable [162], 5% - 15% are typical weight percent loadings for the rare earth oxide promoters. 

Modifiers, typically deposited onto the surface of higher rare earth oxides by conventional impregnation techniques, may notably change their redox chemistry. The shift occurred in the low-temperature peak of the TPR-H2 diagrams for higher oxide-supported noble metal systems is particularly noticeable [249,267,272,291,357-361]. This enhaneement of the oxide reducibilities is interpreted as due to the low-temperature aetivation of H2 occurred on the surfaee of the metal crystallites, and the subsequent transfer of H atoms onto the oxide supports (spillover) [249,272]. 

Deposition effects of rare earth oxides on the surfaee of iron and stainless steels have also been reported [58]. The eorrosion rate eonstants decreased significantly by the coating in corrosion tests under isothermal eonditions. In thermal cyclic conditions, protective scale spallation completely disappeared for eoated samples. The rare earth effect is more remarkable with elements located on the left part of the lanthanide series (lighter rare earths). For example, eeria coatings strongly modify the microstructure and texture of the wustite (FeO) scale formed during low pressure oxidation of pure iron [59]. Cerium is located in the wustite matrix as a CeFeOs phase which dissolves in FeO in time. 

E. Bormashenko, R. Pogreb, G. Whyman, and M. Erlich, Cassie-Wenzel wetting transition in vibrating drops deposited on rough surfaces Is the dynamic Cassie-Wenzel transition a 2D or ID affair Langmuir 23,6501-6503 (2007). G. Azimi, R. Dhiman, H.-M. Kwon, A. T. Paxson, and K. K. Varanasi, Hydrophobicity of rare-earth oxide ceramics. Nature Mater. 12, 315-320 (2013). 

Choudhary VR, Mulla SAR, Uphade BS (1999) Oxidative coupling of methane over alkaline earth oxides deposited on commercial support precoated with rare earth oxides. Fuel 78 427 37... 

Rare-earth oxide films have also been applied by sputtering (Hussey et al. 1989, Graham 1991) and physical vapour deposition (PVD) routes (Adams et al. 1987, Onay and Saito 1990). For a detailed summary and history of methods used, see Moon and Beimett (1989). 

---