The Formation of Carbides

Since the chromium activity is 0 -18 for the formation of carbide in the steel, each of the standard free-energy lines (A°, B°, C°) derived for the carbides must be corrected (moved upwards) for this lower activity (A, BC ). 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

It is clear that the influence of surface geometry upon catalytic activity is extremely complex and many more studies are required before any definitive relationship between catalytic activity and metal particle size can be established. Such studies will require to take cognisance of such factors as the perturbation of surface structure due to the formation of carbidic residues, as noted by Boudart [289] and by Thomson and Webb [95], and by the modification of catalytic properties on adsorption, as noted by Izumi et al. [296—298] and by Groenewegen and Sachtler [299] in studies of the modification of nickel catalysts for enantioselective hydrogenation. Possible effects of the support, as will be discussed in Sect. 6.3, must also be taken into account. 

Chromium. Phis element increases hardness, improves hardenahilily. and promotes the formation of carbides and for tliese reasons is used in eonstt uelional steels Chrome steels are relatively stable at elevated temperatures and have outstanding wear resistance. Chromium is an important constituent of stainless and heat-resistant steels to be described shortly. 

Synthesis of carbide and nitride films by reactive sputtering. Table 14.3 presents a summary of the experimental conditions for the formation of carbides and nitrides. The phases formed are different from those in Table 14.2. As sputtering is a non-equilibrium technique, it was possible to synthesize the 6-MoC and the fi-WCi x carbides and the... 

Figure 24.1 NEXAFS and AES analysis of VC/V(110). Open circles NEXAFS uptake curve obtained by measuring the fluorescence-yield of carbon K-edge feature at 285.5 eV following the formation of carbide on V(110) at 600 K. Closed circles the carbon/vanadium atomic ratios as a function of carbide formation as derived from the AES C(KLL)/V(LMM) peak-to-...

The TDS results in Figures 24.6 and 24.7 therefore indicate that, on carbide-modified surfaces, the reactivities of vanadium towards saturated and unsaturated C4 molecules are modified in a distinctly different manner. These results support the argument that the formation of carbide reduces the decomposition of 1,3-butadiene and enhances the activation of n-butane. A similar trend is observed for other saturated and unsaturated C4 molecules. A summary of the decomposition probabilties of C4 molecules as a function of carbide formation on the V(110) surface is shown in Figure 24.8. These TDS measurements clearly indicate that the decomposition probabilities of the three unsaturated C4 molecules (1-butene, 1,3-butadiene and iso-butene) are decreased on the carbide-modified surfaces. The HREELS results suggest that the interaction... 

Following all the results presented here, there seems to be a strong difference between the formation of carbides and nitrides by reactive sputtering if the substrate temperature increases, the nitrogen concentration in the film decreases while the carbon concentration increases. 

Figure 11. Change in the C(ls) spectra of CO adsorbed on Ni clusters with temperature. The feature at 286 eV corresponds to molecularly adsorbed CO while that at 284 eV arises due to the formation of carbidic species (reproduced with permission from ref. [27]).

In flameless atomic absorption the analyte often tends to react with the graphite furnace or rod to form carbides. In such cases atomisation is suppressed. Release agents are used to react preferentially with the graphite releasing the analyte on atomisation. An application of this is in the determination of aluminium, barium, beryllium, silicon and tin. A large enhancement of the signal has been observed [47] when calcium (as the nitrate) is added to the analytical solutions. This has been suggested as due to a reduction in the formation of carbide in the presence of calcium. A calcium level of 1000 to 2000 mg l-1 in the solutions has been reported as the optimum in most cases. 

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