Zirconia thermal decomposition

Inoue, M., Kominami, H. and Inui, T. (1993) Novel synthetic method for the catalytic use of thermally stable zirconia thermal decomposition of zirconium alkoxides in organic media. Applied Catalysis A, 97, L25-30. 

The evidence obtained in compaction experiments is of particular interest in the present context. Figure 3.22 shows the results obtained by Avery and Ramsay for the isotherms of nitrogen on compacts of silica powder. The hysteresis loop moved progressively to the left as the compacting pressure increased, but the lower closure point did not fall below a relative pressure of 0-40. Similar results were obtained in the compaction of zirconia powder both by Avery and Ramsay (cf. Fig. 4.5), and by Gregg and Langford, where the lower closure point moved down to 0-42-0-45p° but not below. With a mesoporous magnesia (prepared by thermal decomposition of the hydrated carbonate) the position of the closure point... 

Zirconia prepared by the thermal decomposition of zirconium salts is often metastable tetragonal, or metastable cubic, and reverts to the stable monoclinic form upon heating to 800°C. These metastable forms apparently occur because of the presence of other ions during the hydrolysis of the zirconium their stabiUty has been ascribed both to crystaUite size and surface energy (152—153) as well as strain energy and the formation of domains (154). 

Figure 9.3 Conversion of thermal decomposition of carbon dioxide in a dense yttria-stabilized zirconia membrane reactor as a function of membrane thickness when a sweep gas is used (top) and when vacuum is applied (bottom) [Itoh et al., 1993]...

Figure 10.6 compares the model and experimental results of direct thermal decomposition of CO2 using a dense yttria-stabilized zirconia membrane shell-and-tube reactor [Itoh et. al., 1993]. The agreement for the reactor conversion is very good. At a CO2 feed rate of less than 20 cm /min and with a membrane thickness of 2,(XX) pm the conversion is significantly enhanced by the use of a permselective membrane for oxygen. Beyond a feed rate of 20 cm /min., however, the difference in conversion between a membrane and a conventional reactor. 

First of all, the space time defined in Eq. (11-5) or (11-6) depends on the volume of the reactor and the total volumetric feed rate. Thus, for a given reactor volume, space time is inversely proportional to the total feed rate. Itoh et al. [1993] studied the use of a dense yttria-stabilized zirconia membrane reactor for thermal decomposition of carbon dioxide. The reactor temperature was not kept constant everywhere in the reactor but varying with the reactor length instead. The resulting temperature profile is parabolic with the maximum temperature at the midpoint of the reactor length. This nonisothermal... 

Vu, T.A. and Heimann, R.B. (1997) Effect of CaO on thermal decomposition during sintering of composite hydroxyapatite-zirconia mixtures for monolithic implants. J. Mater. Sci. Lett., 16, 437-439. 

There are three generic routes to sulfated zirconia (sulfation of the oxide/ hydroxide, sol-gel synthesis with Zr alkoxides and sulfuric acid and, much less studied, the thermal decomposition of zirconium sulfate). Furthermore, many potential structures have been suggested on the basis of various studies (Figure Whether one is correct or there are several... 

A mixture of Ni°/NiO, produced by thermal decomposition of nickel acetate, dispersed on either silica or cordierite supports, was found to be catalytically active for the decomposition of methane without the need for any pre-treatment. Other authors used Ni catalysts supported on zirconia to produce H2 and a high yield of multiwalled carbon nanotubes. Raman spectroscopy suggested that carbon nanotubes formed at temperatures higher that 973 K had more graphite-like structure than those obtained at lower temperatures. They also reported that feed gas containing methane and hydrogen caused slow deactivation of the catalyst, and carbon yield increased with increasing Hg partial pressure in the feed gas. For a commercial Ni catalyst (65% wt Ni supported on a mixture of silica and alumina) it was found that catalyst deactivation depends on the... 

Pure zirconia is obtained via the chlorination and thermal decomposition of zirconia ores, their decomposition with alkali oxides, and lime fusion. The initial stage of the process is based on the chlorination of zircon in the presence of carbon at a temperature of 800-1200 °C in a shaft furnace ... 

Other preparation methods have recently been developed. Sulfated metal oxides have been prepared by a sol-gel method [42,57,58], which involves the formation of a zirconium-sulfate co-gel by adding sulfuric acid to zirconium n-propoxide in isopropyl alcohol. This one step method appears to be simpler than the two step preparation procedures and allows a better control of the variables. It also allows the direct formation of biiunctional catalysts by the addition of chloroplatinic acid to the gel mixture. A new preparation method, named rapid thermal decomposition of precursors in solution (RTDS), which involves the use of hot pressurized water at hydrotheimal conditions to force metal ion precursors to go into phases of oxyhydroxides and oxyhydrosulfates, has been used to produce sulfated zirconia with crystallite sizes below 100 A [59]. 

There are two methods used to make zirconia from zirconyl chloride dihydrate (Zr0Cl2.8H20) thermal decomposition and precipitation. 

The zirconia lumps obtained from the calcination then undergo a size-reduction process, such as ball milling, into the particle size range needed, usually up to -325 mesh. Thermal decomposition is hence an energy-demanding process from which it is not easy to produce zirconia powders with a high purity and fine particle size. 

Zirconia is usually produced fiom zircon, ZrSi04. The first step is to convert zircon to zirconyl chloride by melting the zircon with sodium hydroxide to form NajZrOj. This compound is then treated with HCl to form zirconyl chloride, ZrOCl2 SHjO. Two methods are used to make zirconia fiom zirconyl chloride thermal decomposition and precipitation. The thermal decomposition method is the less costly of the two, but the resulting material is not usually of as high a purity and fine particle size as that produced by precipitation. The precipitation method uses chemical reactions to obtain the zirconia hydroxides as an intermediate material. A final calcination process results in zirconia powder. By controUmg the precipitation and calcination conditions, it is possible to achieve desired particle size and shape, grain size, and specific surface area. 

The oxide of pure zirconium (Zirconia = Zr02) is obtained from sands of zircon (ZrSi04) or baddeleyite (Zr02) by a chemical processes via of chlorination and thermal decomposition, alkali oxide decomposition or lime fusion or by plasma decomposition [30],... 

Although the decomposition of ozone to dioxygen is a thermodynamically favoured process,126 it is thermally stable up to 523 K and catalysts are needed to decompose it at ambient temperature in ventilation systems, in the presence of water vapour and at high space velocity. A limited number of catalysts have been evaluated and active components are mainly metals such as platinum, palladium and rhodium, and metal oxides including those of manganese, cobalt, copper, iron, nickel and silver. Supports that have been used include 7-alumina, silica, zirconia, titania and activated carbon.125,170... 

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