Graphite intercalated carbides

This chapter has presented a summary of the important features of research on catalytic materials. It is a changing world and new data will be forthcoming. Nothing has been said about exciting new materials such as metal sulfates, phosphates/ metal-graphite intercalation compounds/ superbasic materials/ and carbides or nitrides/ all of which await industrial exploitation. 

The endohedral metallofullarenes just described (and the alkali metal fullerides described on p. 285) are all formally examples of metal carbides, M cCy, but they have entirely different structure motifs and properties from the classical metal carbides and the more recently discovered metallacarbohedrenes (metcars) on the one hand (both to be considered in Section 8.4) and the graphite intercalation compounds to be discussed in Section 8.3. Before that, however, we must complete this present section on the various forms of the element carbon by describing and comparing the chemical properties of the two most familiar forms of the element, diamond and graphite. 

The alkali metals form only ionic carbides, mostly simple ionic salts of acetylene, M2C2, which liberate acetylene on reaction with moisture. There has been much recent interest in permetalated and hypermetalated hydrocarbon species, or methanides . Most studied in this respect has been lithium, presumably because of its volatility and amenability to calculation. Mass spectrometric and calculational evidence has been presented for CLie, CLis, and C2Li4, but real samples of CLi4, C3Li4, and C5Li4 are preparable. All are pyrophoric powders. The heavier metals form another class of carbide , the graphite intercalation compounds, but as the electron has not been totally freed from the metal, these were considered in the previous section. 

There is Htde evidence of the direct formation of sodium carbide from the elements (29,30), but sodium and graphite form lamellar intercalation compounds (16,31—33). At 500—700°C, sodium and sodium carbonate produce the carbide, Na2C2 above 700°C, free carbon is also formed (34). Sodium reacts with carbon monoxide to give sodium carbide (34), and with acetylene to give sodium acetyHde, NaHC2, and sodium carbide (disodium acetyHde), Na2C2 (see Carbides) (8). 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

Graphite materials produced at 600-1100°C may find applications in lithium batteries and supercapacitors. Currently, similar flakes are produced in a complex process including graphitization at above 2500°C,16 followed by intercalation and exfoliation of graphite15. Here we demonstrate that synthesis of graphite from iron carbide can be done in one step at moderate temperatures. 

Rare-earth elements form MjCj, MjC and MCj carbides. The PujCj-type structure exists from La to Ho, a YjC type is found from Sm to Lu and a CaCj-type compound occurs throughout the series. An intercalation compound, MC, with graphite, has been prepared in the Eu and Yb systems. ... 

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