Carbide phase relations

Metal carbides are generally prepared by the direct reaction of the elements at high temperatures (-2470 K). Reaction of metal oxides with carbon is another important route. Reaction of metal vapour with hydrocarbons also yields metal carbides. Phase relations in carbides of IVA, VA and VIA group elements as well as actinides have been reviewed by Storms [5], SiC has been prepared by the reaction of SiCl and CCl with Na, a similar reaction of CCl and BClj with Na gives B C [2]. SiC is formed by the decomposition of CH3S1H3 or (CHj)jSiClj. Pyrolysis of organosilicon polymer precursors has been employed to prepare SiC [6]. Some of the precursor reactions are discussed in Chapter 4 of this book. Various metal carbides have also been synthesized by sol-gel chemistry [7]. 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

Supported nanoparticles (1-1.5 nm) based on Ru4Pt2 entities have been obtained by using a Ru4Pt2(CO)i8 precursor on carbon black and fullerene soot [63]. XANES analysis showed differences between the interaction of nanoparticles with both carbon black and fullerene supports. In particular, a change in the electronic properties of the nanoparticles on fullerene is proposed this change was related to a strong interaction between the nanoparticle and a surface-atom, probably via the formation of a Ru-carbide phase. 

These results make it clear that relating the catalyst composition to the catalytic activity of an iron Fischer-Tropsch synthesis catalyst will be a demanding task. It appears that the carbide phase is more active than the oxide phase. Furthermore, the data for the promoted and unpromoted show that the initial activity of the x-Fe5C2 phase is as great as the e or e -carbide (Fe2C to Fe2 2C) phases. The present data do not allow us to decide whether it is the presence of potassium or the stability of the iron carbide phases that allows the e or e -carbide (Fe2C to Fe2.2C) phases to remain as the conversion declines to a low level while the x-FesC2 oxidizes as the conversion decreases. 

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