Chromia-silica

Fig. 4. Alumina—silica—chromia fiber after 120 h at 1426°C showing crystallization and sintering at contact points. Magnified x5000.

Table 1 lists the wear coatings which have been under evaluation to this point in the program. Wear testing has been completed for almost all the air plasma sprayed (APS) chromium oxides, some of the high velocity oxy-fuel (HVOF) thermal sprayed materials and the slurry-sprayed silica-chromia-alumina (SCA). Microstructural characterization has been completed for all the coatings which have been wear tested. 

Silica/chromia catalysts have also been modified by the incorporation of other oxides such as alumina" or zirconia," by impregnating a silica support with zirconium acetylacetonate or aluminum sec-butoxide or co-gelling the silica gel with appropriate soluble salts. Zirconia-modified catalysts are similar to those with added titanium in their effect but have not been widely reported. Aluminum-modified catalysts have increased activity and provide lower-molecular-weight polymers, but the procedure for their preparation is complicated. Neither type of catalyst is widely described in the literature but they are reported as containing different active sites." ... 

Synthetic adsorbents such as silica, chromia, nnd alumina gels are expensive, but various other metals can be introvluced for improving the catalytic, adsorptive, or regenerative properties of the gels or catalysts. Likewise, various chemicals can be added to the natural adsorbents for enhancing their efficiency. 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

MT-chlor [Mitsui Toatsu Chlorine] A process for recovering chlorine from hydrogen chloride. The hydrogen chloride is mixed with oxygen and passed through a fluidized bed of chromia/silica catalyst. Developed by Mitsui Toatsu and first operated in Japan in 1988. See also Deacon, Kel-Chlor. 

Phillips (1) A process for polymerizing ethylene and other linear olefins and di-olefins to make linear polymers. This is a liquid-phase process, operated in a hydrocarbon solvent at an intermediate pressure, using a heterogeneous oxide catalyst such as chromia on silica/ alumina. Developed in the 1950s by the Phillips Petroleum Company, Bartlesville, OK, and first commercialized at its plant in Pasadena, TX. In 1991, 77 reaction fines were either operating or under construction worldwide, accounting for 34 percent of worldwide capacity for linear polyethylene. 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

C -C, on platinum-silica, 30 352 chain termination bed residence time, 39 255-256 probability, CO pressure effects, 39 258 chemical mechanisms for reactions with deuterium, on chromia, 20 73-84 chemisorption, carbon atom complexes, 32 167-167, 175-176 coupling, 27 235-238 double, 27 238, 239 cracking, 39 283 cyclic, catalysis of, 20 309-311 cyclization, 28 295 degree of strain, 25 135 dehydrogenation of, 19 88, 89 deuteration of, 25 140, 141 dimerization, 20 304... 

The desire to have catalysts that were uniform in composition and catalytic performance led to the development of synthetic catalysts. The first synthetic cracking catalyst, consisting of 87% silica (Si02) and 13% alumina (AI2O3), was used in pellet form and used in fixed-bed units in 1940. Catalysts of this composition were ground and sized for use in fluid catalytic cracking units. In 1944, catalysts in the form of beads about 2.5 to 5.0 mm in diameter were introduced and comprised about 90% silica and 10% alumina and were extremely durable. One version of these catalysts contained a minor amount of chromia (Cr203) to act as an oxidation promoter. 

The reactor equipment used for solution polymerizations is typically glass-lined stainless steel. An example of solution polymerization is the reaction of ethylene in isooctane with a chromia silica alumina catalyst initiator (see Figure 3.23) to form polyethylene. Typical reaction conditions for this polymerization are 150-180°C and 2.1-4.8 MPa (300-700 psi). 

Prom the following thermodynamic data, with the assumptions that the heat capacities of reaction are negligible and that standard conditions (other than temperature) prevail, calculate the temperatures above which (a) carbon monoxide becomes the more stable oxide of carbon, in the presence of excess C (6) carbon is thermodynamically capable of reducing chromia (Cr2Os) to chromium metal (c) carbon might, in principle, be used to reduce rutile to titanium metal and (d) silica (taken to be a-quartz) may be reduced to silicon in a blast furnace. 

Gas-phase epoxidation of propylene with 02/H2 mixtures was accomplished over Ag1267 or Au1268 catalysts dispersed on TS-1 or other Ti-containing supports and Ti-modified high-silica zeolites.1269 Sodium ions were shown to be beneficial on the selectivity of propylene epoxidation with H202 over titanium silicalite.1270 A chromia-silica catalyst is active in the visible light-induced photoepoxidation of propylene by molecular oxygen.1271... 

Surprisingly, the direct evidence of Cr(II) has not ended the controversy, Several groups have since proposed Cr(III) as the active valence. The most direct evidence comes from Kazansky (45) and more recently from Lunsford and co-workers (46,47). Trivalent chromium salts, notably CrCl3, impregnated onto silica and calcined under vacuum, were found to polymerize ethylene. In fact, similar catalysts made from the divalent salts were not active. Unsupported chromia has also been reported to polymerize ethylene weakly (48). However, the attachment of amorphous Cr203 to the silica (30, 46,49) probably does not resemble that of the Cr(VI) and Cr(II) species discussed in Section II, C. It may have little relevance to the commercial catalyst, even if Cr(III) is active. 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

A common feature of the two models is that the metal centers should have at least two coordination vacancies prior to the interaction with ethene—one for the alkyl or the carbene species and one for the coordination of ethylene. On the basis of the results discussed so far (which have demonstrated that a significant fraction of Cr2+ centers is highly coordinatively unsaturated), it can be understood why Cr2+/silica is such a good catalyst, whereas Cr3+ ions on chromia/silica or exposed on extended faces of cy-C CF are not. 

Tretyakov and Filimonov (219) describe a coordinative interaction between benzonitrile and aprotic sites on magnesium oxide, and Zecchina et al. (256) came to the same conclusion for the adsorption of propionitrile, benzonitrile, and acrylonitrile on a chromia-silica catalyst. Chapman and Hair (257) observed an additional chemical transformation of benzonitrile on alumina-containing surfaces, which they describe as an oxidation. Knozinger and Krietenbrink (255) have shown that acetonitrile is hydrolyzed on alumina by basic OH- ions, even at temperatures below 100°C. This reaction may be described as shown in Scheme 2. The surface acetamide (V) is subsequently transformed into a surface acetate at higher temperatures. Additional reactions on alumina are a dissociative adsorption and polymerizations (255) analogous to those observed for hydrogen cyanide by Low and Ramamurthy (258), and a dissociative adsorption. Thus, acetonitrile must certainly be refused as a probe molecule and specific poison. 

The above authors have established that the procedure with the polymeric membrane is not satisfactory. At high current densities, metallic nickel nucleates on the polymeric membrane. Although a reasonable loading of a suspended silica carrier could be achieved, a considerable fraction of the nickel was deposited on the polymeric membrane. The chromia layer method was observed to yield much better results. 

---