Surface decomposition intermetallic compound

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). 

For metals promoting other metals, an interesting case was studied by Hurst and Rideal.2 In the combustion of mixtures of hydrogen and carbon monoxide, using copper as the basic catalyst the ratio of the gases burnt depends on the temperature, and also on the amount of small additions of palladium made to the copper. The proportion of carbon monoxide burnt is increased by addition of palladium, a maximum proportion of carbon monoxide being burnt when 0-2 per cent, of palladium is. present. With further amounts of palladium, the ratio CO H2 burnt falls off slowly until, with 5 per cent, palladium, it is nearly the same as with pure copper. This effect of palladium is ascribed to the introduction of a new type of surface, the line of contact between palladium and copper, though the proof that this is the cause of promotion is perhaps not complete. Mit-tasch and others,3 in elaborate studies of the promotion of various metal catalysts, particularly molybdenum, for the synthesis or decomposition of ammonia, concluded that the formation of intermetallic compounds... 

Figure 3 also shows conversion and selectivity to C2H4 with Sn added to the Pt catalyst[15]. Both the alkane conversion and the selectivity to olefins increase significantly with added Sn. X-ray diffraction and XPS of the Pt-Sn catalyst indicate intermetallic compound formation rather than fee metal, and this surface evidently increases the alkane conversion and reduces the decomposition of olefins. 

It is possible to discharge sodium electrolytically from an aqueous solution if a mercury cathode is used. On the basis of the standard electrode potentials [E (Na, Na) =-2.71V], the decomposition of water to form hydrogen at the cathode would be a far easier process even when allowance is made for the high hydrogen overpotential (q.v.) at mercury. However, the fact that sodium forms intermetallic compounds with mercury which are soluble in mercury and diffuse away so reduces the activity of the sodium at the cathode surface, and its tendency to re-ionise that the discharge of sodium becomes the preferred process. This method is not used commercially because of the high cost of extraction of sodium from its amalgam, and recourse is had to fused electrolytes. 

In section 3 the phenomenon of hydrogen sorption in intermetallic compounds is described. Topics dealt with in this section are activation treatment, pressure-composition isotherms, miscibility gap, sorption hysteresis, lattice expansion, diffusion, sorption kinetics, surface effects, poisoning, impurity effects and decomposition of ternary hydrides. The sorption characteristics of all ternary rare-earth hydrides investigated are listed in several tables given in the appendix. For comparison the sorption characteristics of some selected ternary hydrides based on non-rare-earth metals are given in a separate table. 

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