Bulk carbide

After reduction and surface characterization, the iron sample was moved to the reactor and brought to the reaction conditions (7 atm, 3 1 H2 C0, 540 K). Once the reactor temperature, gas flow and pressure were stabilized ( 10 min.) the catalytic activity and selectivity were monitored by on-line gas chromatography. As previously reported, the iron powder exhibited an induction period in which the catalytic activity increased with time. The catalyst reached steady state activity after approximately 4 hours on line. This induction period is believed to be the result of a competition for surface carbon between bulk carbide formation and hydrocarbon synthesis.(6,9) Steady state synthesis is reached only after the surface region of the catalyst is fully carbided. 

Figure 4.1 summarizes the different routes that can potentially lead to carbon deposition during FTS (a) CO dissociation occurs on cobalt to form an adsorbed atomic carbon, which is also referred to as surface carbide, which can further react to produce the FT intermediates and products. The adsorbed atomic carbon may also form bulk carbide or a polymeric type of carbon. Carbon deposition may also result (b) from the Boudouard reaction and (c) due to further reaction and dehydrogenation of the FTS product (what is commonly called coke), a reaction that should be limited at typical FT reaction conditions. Carbon formed on the surface of cobalt can also spill over or migrate to the support. This is reported to readily occur on Co/A1203 catalysts.43 The chemical nature of the carbonaceous deposits during FTS will depend on the conditions of temperature and pressure, the age of the catalyst, the chemical nature of the feed, and the products formed. 

CHX and hydrocarbon wax are, respectively, the active intermediates formed by the hydrogenation of surface carbide and products of FTS formed by chain growth and hydrogenation of CHX intermediates. The hydrocarbon wax can contain molecules with the number of carbon atoms in excess of 100. Bulk carbide refers to a crystalline CoxC structure formed by the diffusion of carbon into bulk metal. Subsurface carbon may be a precursor to these bulk species and is formed when surface carbon diffuses into an octahedral position under the first surface layer of cobalt atoms. 

Johnson et al.67 studied CO hydrogenation on bimetallic catalysts consisting of cobalt overlayers on W (100) and (110) single crystals at 200°C, 1 bar at a H2/ CO ratio of 2. AES spectra showed the postreaction Co/W surfaces to have high coverages of both carbon and oxygen, with carbon line shapes characteristic of bulk carbidic carbon.67 The catalytic activity apparently could not be correlated with surface carbon level.67... 

The formation of bulk cobalt carbide is quite a slow process since it requires the diffusion of carbon into the cobalt bulk. It was reported that the full conversion of unsupported and reduced Co to Co2C only occurred after 500 h of exposure to pure CO at 230°C. Increasing the reaction temperature resulted in a faster rate of carburization.81 Bulk cobalt carbides are considered to be thermodynamically metastable species, and therefore Co2C will decompose to hep cobalt and graphite, while Co3C will decompose to fee cobalt and methane. Thermal decomposition of bulk carbides under an inert atmosphere is believed to occur at 400°C.81 Hydrogenation of the bulk carbides is believed to be a fast process and occurs around 200°C.82 83... 

Early work at the Bureau of Mines on Co/Th02/kieselguhr catalysts showed that bulk carbide was not an intermediate in the FTS, nor was it catalytically active.82 Excessive amounts of carbides, produced by CO exposure prior to the... 

Recent work done by Xiong et al.84 on Co/AC (activated carbon) catalysts showed that a Co2C species formed during the catalyst reduction in hydrogen at 500°C. Evidence for the carbide in the Co/AC catalysts was obtained by x-ray diffraction and XPS measurements, and the formation of this Co2C species reduced the FTS activity over the Co-based catalysts. The presence of bulk carbide also seems to enhance alcohol selectivity.85... 

Several workers have reported that bulk carbide does not form readily during normal FTS conditions.76 82 Bureau of Mines work, using laboratory XRD measurements, showed that detectable amounts of bulk carbide were not formed under synthesis conditions.82... 

To summarize, from literature there does not seem to be much consensus on whether bulk cobalt carbide forms during realistic FTS conditions. Bulk carbide is generally considered a metastable species. However, it is clear that it may form under upset conditions. Furthermore, there is strong evidence to show that if bulk cobalt carbide is present, it is deleterious in terms of both catalyst activity and selectivity. With this in mind, it would be prudent to operate the catalyst in a regime (sufficiently high H2/CO ratio) where bulk carbide formation is avoided. 

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... 

A point which has not been examined is the nature of the surface during exchange reactions carried out at high temperatures such as those required for the exchange of methane. Surface carbides may be formed under these conditions. The inactivity of iron films and the comparatively small activity of cobalt films at 300° for the exchange of ethane 19) may possibly be due to the tendency of these metals to form not only surface but also bulk carbides. 

In analogy to the organosilicon method for the production of bulk carbides, the formation of ceramic coatings by pyrolysis of organosilane layers on silica gel is studied. As a ceramic precursor, the aminosilane APTS is used. 

Table 2 shows the initial results for the catalytic dry reforming of methane using bulk carbides of niobium, tantalum, molybdenum and tungsten, prepared by CH4 TPR. The conversions and yields obtained over P-M02C and a-WC are very similar to those predicted by thermodynamic considerations, and thus these materials are efficient catalysts for methane dry reforming. At atmospheric pressure, deactivation was observed over both catalysts after about 8 hours on stream. Examination of the post-catalytic samples by powder XRD (Figure 2) revealed that as the reaction proceeded the active P-M02C was oxidised and converted to... 

Guczi and coworkers [4.61] studied CO chemisorption on glassy and crystalline FeNiB alloys in the presence and in the absence of hydrogen using XPS and UPS. They found that CO chemisorption at 300 K is characteristic of the surface structure. At S70 K, no difference could be observed in the mode of chemisorption because only dissociative carbon was present. However, the reactivity differences observed in the CO + H2 reaction could be ascribed to the difference in the surface transformation of the carbidic species. The authors suggested that this species can be stabilized by the small ensemble size characteristic for glassy and partially crystallized samples, whereas the main route of the dissociative carbon on crystallized samples is the inactive bulk carbide formation. This phenomenon was found to be influenced by the alloy composition and by the presence of hydrogen. 

XPS spectra of and Mo. for alumina supported carbides are very different from those of bulk carbides (ref.4). Besides W and Mo carbides, W or Mo oxidized are present in large amounts, probably as W(Mo)+4, Mo+5 and W(Mo)+ ... 

Such oxides are probably partly formed during the passivation treatment. However, while with bulk carbides these oxides are easily reduced by a treatment under hydrogen at 300°C or 500°C (ref.4,5), with supported carbides similar reducing treatments do not seem to markedly influence the proportion of oxide phases. Such a stability of these oxides could be related to the formation of combinations with the alumina support (Ref.6). The presence of carbidic surface phase was also checked by the C1 peak at a binding energy of 282.5 eV assigned to carbidic carbon (Fig. 3a). It can be mentioned that carburization can be increased by a treatment in CO and H2 at 300°C as can be seen in figure 3b. 

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