Reduction-carburization

Li, S., Li, A., Krishnamoorthy, S., and Iglesia, E. 2001. Effects of Zn, Cu, and K promoters on the structure and on the reduction, carburization, and catalytic behavior of iron-based Fischer-Tropsch synthesis catalysts. Catal. Lett. 77 197-205. 

In order to understand better these interesting systems without complications that might arise due to different preparation procedures, we compared oxygen-treated WC and Mo2C prepared by similar reduction/ carburization procedures from their respective oxides. The effects of pretreatment conditions were also studied. An attempt was made to correlate the kinetic behavior of these catalysts in n-hexane-H2 reactions with their physical properties obtained from X-ray diffraction (XRD), CO chemisorption, temperature-programed reaction (TPR) with flowing H2 or He, temperature programed desorption (TPD) of adsorbed NH3, and X-ray photoelectron spectroscopy (XPS). 

In addition to the catalytic solvothermal synthesis of carbon nanotubes (Fig. 2) [7], SiC nanowires and tubular structures (Fig. 8) were prepared through a solvothermal reduction-carburization route using silicon powders and CCI4 as Si and carbon sources [20], Si3N4 nanorods were also prepared with a similar process [12]. 

The XRD pattern (Figure 7.18(a)) indicates that the obtained sample is cubic phase TiC, with cell constant a= 4.34A. TEM imaging (Figure 7.18(b)) reveals that it consists of nanoparticles of size 10-20 nm [67]. The co-reduction carburization method may be used to prepare other transition metal carbides. For example, as shown in Reaction (20), nanocrystalline ZrC, size 10-25 nm was synthesized on heating at 550 °C for 12 h [68]. 

FIGURE 9.22. Ultiafine WC powders produced by conventional (reduction/carburization) manufacture 40,000 X. The size of the spheres emphasizes the degree of fineness of the development grade. By courtesy of Wolfram Bergbau und Huttenges.m.b.H., Austria. 

Thus, Mo,H-ZSM-5 samples prepared via SSIE proved to be catalyst precursors which, after activation (reduction, carburization), activated CH4 molecules and produced hydrocarbons such as C2H4, C2H6, CgHg, CgHg, benzene, toluene, and naphthalene. At higher Mo contents, however, (inactive) aluminum molybdates, Al2(Mo04)3, formed by extracting framework Al that led to a loss of crystallinity. 

The iron carbide process is alow temperature, gas-based, fluidized-bed process. Sized iron oxide fines (0.1—1.0 mm) are preheated in cyclones or a rotary kiln to 500°C and reduced to iron carbide in a single-stage, fluidized-bed reactor system at about 590°C in a process gas consisting primarily of methane, hydrogen, and some carbon monoxide. Reduction time is up to 18 hours owing to the low reduction temperature and slow rate of carburization. The product has the consistency of sand, is very britde, and contains approximately 6% carbon, mostly in the form of Ee C. 

The characteristics of WC, especially grain size, are determined by purity, particle shape and grain size of the starting material, and the conditions employed for reduction and carburization. The course of the reaction WO3 — W — WC is dependent on temperature, gas flow rates, water-vapor concentration in the gas, and the depth of the powder bed. All these factors affect the coarsening of the grain. 

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

The number of chemical reactions used in CVD is considerable and include thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, carburization, and nitrida-tion. They can be used either singly or in combination (see Ch. 3 and 4). These reactions can be activated by several methods which are reviewed in Ch. 5. The most important are as follows ... 

The quadrupole doublet has an isomer shift corresponding to iron in the ferric or Fe " state. After reduction in H2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet, along with some unreduced iron, which gives rise to two doublet contributions of Fe " and Fe " in the centre. The overall degree of iron reduction, as reflected by the relative area under the bcc ion sextet, is high. Fischer-Tropsch synthesis at 575 K in CO and FI2 converts the metallic iron into the Flagg carbide, Fe5C2. The unreduced iron is mainly present as Fe ". Exposure of the carburized catalyst to the air at room temperature leaves most of the carbide phase unaltered but oxidizes the ferrous to ferric iron. 

Also, manganese added to cobalt on activated carbon catalysts resulted in a decrease in bulk carbide formation during reduction and a decrease in the subsequent deactivation rate.84 Magnesium and yttrium added to the support in alumina-supported cobalt catalysts showed a lower extent of carburization. This was explained by a decrease in Lewis acidity of the alumina surface in the presence of these ions.87... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The TPR-XAFS technique confirmed that doping Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) remarkably promotes the carburization rate relative to the undoped catalyst. The EXAFS results suggest that either the Hagg or e-carbides were formed during the reduction process over the cementite form. A correlation is observed between the a-value of the product distribution and the carburization rate. 

The examples illustrate the strong points of XRD for catalyst studies XRD identifies crystallographic phases, if desired under in situ conditions, and can be used to monitor the kinetics of solid state reactions such as reduction, oxidation, sulfidation, carburization or nitridation that are used in the activation of catalysts. In addition, careful analysis of diffraction line shapes or - more common but less accurate-simple determination of the line broadening gives information on particle size. 

Carbon nitride, 27 214—215 conductivity of, 2 7 204-206 field emission properties of, 27 221 theoretical band structure calculations for, 27 203-204 Carbon nitride films, 2 7 205 Carbon nitride solids, 2 7 203 Carbonochloridic esters, 6 290 Carbon paper, 4 738-739 Carbon profile, in gas carburizing, 26 204 Carbon Raschig-ring tower packing, 22 745 Carbon reduction, of ferrovanadium, 25 518... 

Selected ion monitoring mode, mass spectrometer, 6 431 Selection, in chemical product design, 5 759, 772-776 Selective carburizing, 76 205 Selective catalytic reduction (SCR), 77 719-720, 79 626 See also Nonselective catalytic reduction SCR entries... 

The reduction of WO3 by carbon is used for the production of tungsten carbide (direct carburization). 

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