Thermolysis Of Molecular Precursor Research Articles

ポリマーから誘導されたケイ素系セラミックス<br>─合成・性質・応用に関する総説─

This review presents the synthesis, characterization techniques, processing and potential applications of silicon-based ceramic materials derived from organosilicon polymers. The Si-ceramics are prepared by thermolysis of molecular precursors. The influence of the initial molecular structure of the precursor on the properties of the final ceramic material and its applications is discussed. The thermolytic decomposition of suitable Si-based polymers provides materials which are denoted as polymer-derived ceramics (PDCs). In particular, this procedure is a promising method for the preparation of ternary and multinary silicon-based ceramics in the system SiCNO. There is no other synthetic approach known to produce e.g. SiCO or SiCN based ceramics. In the case of PDCs route, common preceramic polymers are poly(organosilazanes), poly(organosilylcarbodiimides) and poly(organosiloxanes). One basic advantage of the PDC route is that the materials can be easily shaped in form of fibers, layers or bulk composite materials by applying processing techniques established in the plastic industry. The PDCs in general exhibit enhanced thermomechanical properties, i.e., temperature stabilities up to approximately 1500°C. Recent investigations have shown that in some cases the high temperature stability in terms of decomposition and/or crystallization can be increased even up to 2000°C if the preceramic polymer contains some amount of boron. The composition and microstructure of the PDC are a result of the molecular structure of the preceramic polymer. Therefore, the observed differences in the macroscopic properties are also closely related to the variation of composition and solid state structure of these materials.

Preparation and characterization of molybdenum and tungsten nitride nanoparticles obtained by thermolysis of molecular precursors

Treatment of Mo(NtBu)2Cl2 with [K(Ph2pz)(THF)]6 (pz = pyrazolyl) in tetrahydrofuran afforded Mo(NtBu)2(Ph2pz)2 (85%), while treatment of W(NtBu)2(NHtBu)2 with 3,5-diphenylpyrazole afforded W(NtBu)2(Ph2pz)2 (97%). The complexes M(NtBu)2(Ph2pz)2 were characterized completely by spectral and analytical data, and by an X-ray crystal structure determination for Mo(NtBu)2(Ph2pz)2. Thermolysis of M(NtBu)2(Ph2pz)2 at 800 °C under nitrogen afforded 2–3 nm metal nitride nanoparticles that were embedded in an amorphous carbon–oxygen matrix, as determined by X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. X-Ray photoelectron spectroscopy also revealed the presence of metal oxide phases, which were amorphous by X-ray powder diffraction. The nanoparticles prepared at 800 °C were insoluble. Thermolysis of M(NtBu)2(Ph2pz)2 at 425 °C afforded amorphous 2–3 nm nanoparticles, as determined by X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. X-Ray photoelectron spectroscopy suggested a molybdenum(IV) nitride and W2N/WN, as well as MoO3 and an oxidized tungsten nitride. Materials prepared at 425 °C were not embedded in a matrix, and were soluble in tetrahydrofuran. Infrared and NMR spectroscopy suggested the presence of surface organic fragments that contain alkyl-substituted phenyl groups. Such surface groups are most likely derived from decomposition of the heterocyclic ligands. Simultaneous differential thermal analysis/thermogravimetric analysis indicated that the amorphous nitride materials convert to crystalline nanoparticles consistent with M2N phases between 600–700 °C, along with a weight loss that may correspond to dinitrogen evolution. The amorphous carbon–oxygen matrix also forms upon heating from 425 to 800 °C. This work provides the first description of tungsten nitride nanoparticles, as well as the first description of soluble group 4–6 nanoparticles.

Thermolytic Molecular Precursor Route to Active and Selective Vanadia–Zirconia Catalysts for the Oxidative Dehydrogenation of Propane

The thermolytic molecular precursor route was employed in attempts to obtain highly dispersed and structurally well-defined vanadia–zirconia catalysts for the oxidative dehydrogenation of propane. The vanadia–zirconia materials were prepared by cothermolysis of OV(OtBu)3 and Zr(OCMe2Et)4 in a nonpolar solvent, at relatively low temperatures. Prior to calcination, these materials have relatively high surface areas, are amorphous, and appear to be highly dispersed. After calcination to 773 K, nanocrystalline zirconia and various VOx species, which appear to be dispersed on the zirconia surface, can be observed by PXRD and Raman, DR-UV-vis, and 51V NMR spectroscopies. Higher vanadia loadings and/or an increased calcination temperature (823 K) resulted in formation of ZrV2O7 (as demonstrated by Raman, DR-UV-vis, and 51V NMR spectroscopies). In general, the VOx/ZrO2 materials obtained from the molecular precursor method possess a greater surface area and exhibit a higher dispersion of VOx species than materials of the same composition prepared by conventional impregnation methods. For catalysts derived from the alkoxides, the maximum rate for propane oxidative dehydrogenation is more than double that observed for VOx/ZrO2 materials prepared by impregnation, under similar conditions. Likewise, higher selectivities to propene (by more than 15%) were observed with materials derived from the alkoxides. However, analysis of the rate data for the new catalysts revealed that the active sites were also more active in catalyzing the secondary oxidation of propene. As a result of this, the ratio of secondary combustion to oxidative dehydrogenation rate constants (k3/k1) was similar for materials prepared by either wet impregnation or thermolysis of molecular precursors.