Carbides formation mechanism

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

Catalyst preparation is crucial in successful Fischer-Tropsch synthesis. Appropriate catalyst composition and delicate pretreatment and operating conditions are all necessary preconditions to achieve the desired results. Catalyst disintegration brought about by oxidation and carbide formation is a serious problem that can be prevented only by using catalysts with adequate chemical and mechanical stability under appropriate operating conditions. 

Infrared mcasurcnienls of CO adsorption on nickel surfaces by Martin er qL gave some further insight on the formation mechanism of surface carbide. 

The kinetics of formation of this zirconium diboride platelet reinforced zirconium carbide have been discussed, as have possible formation mechanisms [36] and detailed microstructural and orientation relationships between the phases [37]. These materials, in addition to being very refractory, are quite hard. Potential applications typically involve wear resistance, either at low to moderate temperatures or for short times at very high temperatures, such as in biomedical and rocket nozzle or rocket motor application [35]. 

Here the use of pyrolytic graphite coated graphite tubes is helpful, as the diffusion of the analyte solution into the graphite and thus the risk of the carbide formation are decreased. Alternatively, flushing the furnace with nitrogen can be helpful. Indeed, in the case of Ti a nitride is then formed which in contrast to the carbide can be dissociated easier. Other thermochemical means to decrease interferences, as discussed earlier, are known as matrix modification. The addition of a number of substances, such as Pd-compounds or Mg(N03)2, has been shown to be successful for the realization of a matrix-free vapor doud formation (see e.g. Ref. [278]). The mechanisms involved also relate to surface effects in the furnace (see e.g. Ref. [279]) and are in themselves a spedfic field of research. 

In the steam cracking of hydrocarbons, a small portion of the hydrocarbon feed gases decomposes to produce coke that accumulates on the interior walls of the coils in the radiant zone and on the inner surfaces of the transferline exchanger (TLX). Albright et identified three mechanisms for coke formation. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts. The resulting filamentous coke often contains iron or nickel positioned primarily at the tips of the filaments. This filamenteous coke acts as excellent collection sites for coke formed by mechanisms 2 and 3. Mechanism 2 results in the formation of tar droplets in the gas phase, often from aromatics. These aromatics are often produced by trimerization and other reactions involving acetylene. Some, but not all, of these droplets collect... 

A.O. Konstantinov, C.I. Harris and E. Janzen, Electrical properties and formation mechanism of porous silicon carbide, Appl. Phys. Lett., 65, 2699-2701 (1994). 

Figure 9.4 Carbide cycle mechanism of carbon formation.

Another interesting reaetion is the thermolysis of [Ru3(CO)i2] with [2.2]para-cyclophane this has been shown to yield several cluster species, two of which have proved valuable intermediates in elucidating the mechanism of carbide formation in the octahedral cluster [Ru6C(CO)i4(/i3-Ci6Hi6)] (Scheme 7). 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

The authors have extensively studied the reaction conditions and have suggested a reaction mechanism for boron or boron carbide formation from BCI3. 

Thus, whether the reactions involve mechanisms via T2 and/or T3 depends on the metal and the oxidation conditions. Carbide formation is favored by high oxidation states as well as by high positive charge and low s-orbital population on the metal center, whereas the oxidative addition would be preferable on the metal center with an intermediate or low oxidation state. Accordingly, the authors suggested that MO2 (M = W, Mo) is the best oxidation state for this class of metal oxides toward methane activation and predicted that the active phase in the DHAM reaction may be W and Mo oxycarbides (MO2C2). 

Ni/Co > Ni/Pt > Ni/Rh > Ni/Fe > Ni > Fe/Mo > Fe/Cr > Fe/Co > Fe/Pt > Fe/Rh > Fe > Ni/Mo > Fe/Mo > Co/Mo > Co > Pt > Cu. Depending on whether the mechanism proceeds via VLS or surface-addition mechanism, the above order may be related to the ease of carbide formation and carbon diffusion through the nanocluster interior, or the morphology/composition of the catalyst surface and rate of carbon diffusion on the catalyst surface, respectivelyFigure 6.83 shows some recent mechanistic proposals, based on experimental and theoretical data. The VLS-based proposals (Figure 6.83, top and middle) illustrate the following basic steps ... 

The materials currently used in the production of medical devices include stainless steels, cobalt-base alloys, titanium-base alloys, platinum-base alloys, and nickel-titanium alloys. Steels were the first modern metallic alloys to be used in orthopedics and initial problems with corrosion were overcome by modifying the composition of the steel with the addition of carbon, chromium, and molybdenum. Carbon was added at low concentrations (ca. 0.03-0.08%) to initiate carbide formation, while the addition of chromium (17-19%) facilitated the formation of a stable surface oxide layer and the presence of molybdenum (2.0-3.0%) was found to control corrosion. The compositions of stainless steels used can vary widely. Table V shows the limits for the chemical compositions of three different alloys containing eleven different elements together with the mechanical properties for the samples after annealing and cold working. 

BK Yen. X-ray diffraction study of mechanochemical synthesis and formation mechanisms of zirconium carbide and zirconium silicides. J AUoys Compos 268 266, 1998. 

The carbide mechanism, however, does not explain the formation of oxygenates in FTS products. 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

---