Co-reduction process

The possible intermediacy of formaldehyde in CO hydrogenation has been addressed above with regard to the cobalt catalytic system. Fahey has observed a small amount of 1,3-dioxolane (the ethylene glycol acetal of formaldehyde) as a product of the rhodium system (43). Thus, there is evidence that formaldehyde or a complexed form of this molecule could be an intermediate in the CO reduction process by this system. Rhodium catalysts are indeed found to be useful for the hydroformylation of formaldehyde to glycolaldehyde (159-161) methanol is a by-product in these reactions. 

The temperature dependence of the CO reduction process has also been studied. Over the range 250-290 C under 1200 atm of H2/CO, an Arrhenius activation energy of 32 kcal/mol was reported (164). 

The R-Fe-B alloys may be prepared by reducing with calcium, at high temperature, the respective rare-earth oxides, in the presence of a mixture of transition metals in powder form plus some transition metal oxides (Herget 1985a). Two versions of calciothermic processes for making rare-earth alloys were proposed the reduction-diffusion (R-D) process (Cech 1974, McFarland 1973) and the co-reduction process (Herget and Domazer 1975, Domazer and Strnat 1976). The last one consists of a simultaneous reduction of rare-earth and transition metal oxides in the presence of transition metal powder, followed by diffusional alloys formation. The process may be divided into two steps ... 

Since the R-Co alloys have become technically significant, many efforts have been made to lower the cost of the R-Co alloys which are the starting materials for the magnet production. In the last three years new methods have been developed, which avoid the preparation of pure R metals which are later melted with Co and other transition metals, but start with rare earth oxides (R20a) and Co oxide. These processes are known under the names reduction-diffusion process and co-reduction process . 

The co-reduction process has been developed by H.G. Domazer and coworkers at Goldschmidt in Essen, Germany. Similar to the reduction-diffusion process they started with a rare earth oxide and not with a rare earth metal. In contrast to the modified (technological) reduction-diffusion process (see McFarland, 1973) C03O4 is used deliberately in order to achieve a strongly exothermic reaction. Therefore the rare earth oxide and the Co oxide are reduced simultaneously. The summary equation of the process is given below ... 

The scheme of the co-reduction process is given in fig. 14.136. An essential part in the preparation of the alloys is the analysis. This analysis is done with respect to the chemical composition, the phase composition and also with respect to the shape and size of the grains etc. 

Fig. 14.136. Flow chart of the co-reduction process (Herget and Domazer, 1975).

The carbon monoxide chemistry of organoactinide hydrides provides a unique glimpse of a reaction pattern thought to be of key importance in catalytic CO reduction processes migratory insertion of CO into a metal-hydrogen bond [84, 85]. At room temperature, the carbonylation of bis(pentamethylcyclopenta-dienyl)thorium alkoxyhydrides to yield ds-enediolates proceeds at rates that are inversely proportional to the steric bulk of R ... 

A fourth ahoy separation technique is fractional crystallization. If shica is co-reduced with alumina, nearly pure shicon and an aluminum shicon eutectic can be obtained by fractional crystallization. Tin can be removed to low levels in aluminum by fractional crystallization and a carbothermic reduction process using tin to ahoy the aluminum produced, fohowed by fractional crystallization and sodium treatment to obtain pure aluminum, has been developed (25). This method looked very promising in the laboratory, but has not been tested on an industrial scale. 

Since the reforaiing of CH4 produces 1 mole of CO for each 2 moles of H2, the dominant heat effect in the reduction process is the endothermic reduction by hydrogen. However, since the reforming process is canied out with ah as the source of oxygen, the heat content of the nitrogen component is a drermal reservoir for die overall reduction process. 

Stable oxides, such as those of clrromium, vanadium and titanium cannot be reduced to the metal by carbon and tire production of these metals, which have melting points above 2000 K, would lead to a refractoty solid containing carbon. The co-reduction of the oxides widr iron oxide leads to the formation of lower melting products, the feno-alloys, and tlris process is successfully used in industrial production. Since these metals form such stable oxides and carbides, tire process based on carbon reduction in a blast furnace would appear to be unsatisfactory, unless a product samrated with carbon is acceptable. This could not be decarburized by oxygen blowing without significairt re-oxidation of the refractory metal. 

Substituting for UL by co and for the limiting case for the cathodic reduction process when = 0, the activity term Aff is then equal to (the concentration of ions in the bulk solution), and then... 

The ionic current intensity corresponding to the peak at 169 amu was analyzed under isothermal and polythermal conditions [383]. It was found that in a gaseous atmosphere, the intensity changes are in correlation with the CO content and in negative correlation with the C02 content. The presence of CO in vacuum systems equipped with heating elements is usually related to thermo-cycling and desorption of CO by nickel atoms [386]. Based on the above, the presence of NbF4+ ions in mass spectra is most probably related to the niobium reduction process, which can be represented as follows ... 

The present study revealed effects of various rutile/anatase ratios in titania on the reduction behaviors of titania-supported cobalt catalysts. It was found that the presence of rutile phase in titania could facilitate the reduction process of the orbalt catalyst. As a matter of fact, the number of reduced cobalt metal surface atoms, which is related to the overall activity during CO hydrogenation increased. 

The present research showed a dependence of various ratios of rutile anatase in titania as a catalyst support for Co/Ti02 on characteristics, especially the reduction behaviors of this catalyst. The study revealed that the presence of 19% rutile phase in titania for CoATi02 (C0/RI9) exhibited the highest number of reduced Co metal surface atoms which is related the number of active sites present. It appeared that the increase in the number of active sites was due to two reasons i) the presence of ratile phase in titania can fadlitrate the reduction process of cobalt oxide species into reduced cobalt metal, and ii) the presence of rutile phase resulted in a larger number of reduced cobalt metal surface atoms. No phase transformation of the supports further occurred during calcination of catalyst samples. However, if the ratios of rutile anatase were over 19%, the number of active sites dramatically decreased. 

The induced co-deposition concept has been successfully exemplified in the formation of metal selenides and tellurides (sulfur has a different behavior) by a chalcogen ion diffusion-limited process, carried out typically in acidic aqueous solutions of oxochalcogenide species containing quadrivalent selenium or tellurium and metal salts with the metal normally in its highest valence state. This is rather the earliest and most studied method for electrodeposition of compound semiconductors [1]. For MX deposition, a simple (4H-2)e reduction process may be considered to describe the overall reaction at the cathode, as for example in... 

We characterize the reduction process by defining the reduction temperature as the point where the C03O4 concentration has dropped to 50% and the delta reduction temperature as the temperature difference between the points at which 50% C03O4 and 50% Co were reached. These definitions are arbitrary and their values will change with experimental conditions, but they are useful for comparing samples examined at the same conditions. Both of these temperature parameters must be considered when assessing the reduction properties of the samples. 

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

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