Oxides Oxycarbides

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

It has been demonstrated that highly selective and active catalysts can be formed by activating either oxidized molybdenum carbide Mo2C or molybdenum oxide Mo03 itself with a hydrocarbon/hydrogen mixture. The active catalysts obtained from the two sources are similar in their catalytic behaviour and are probably both molybdenum oxycarbide (MoO Cy). They are selective for the isomerization of a number of n-hydrocarbons with the main products always consisting of monomethyl isomers but with an important contribution from the dibranched products,... 

In summary, Mo2N Ov and Mo2CAOy are found to be more active per unit area than the sulfated hematite for all the reactions studied, despite the fact that the particle surface is heavily oxidized. In terms of turnover rate, the oxycarbide is found to be 2.4 to 3 times more active than the oxynitride. The hydrogen present in the reaction medium may be activating the oxycarbide and oxynitride catalysts, by partially removing surface oxygen. This is consistent with results from TG-MS experiments.22 The order of activity for the oxynitride and oxycarbide catalysts is the following HDS > HYD > HDO HDN. 

The catalytic activity of Fe carbides, molybdenum oxynitride and oxycarbide has been evaluated for coal liquefaction and heteroatom removal of model compounds related to coal. Preliminary results show that the LP nanoparticles are active catalysts for coal liquefaction. In fact, they are more active for heteroatom removal than a molybdenum promoted sulfated hematite, even though surface characterization indicates that as introduced into the reactor they exhibit surface oxidation. 

Although a number of Si02 precursors are used industrially, most are used for CVD of silica insulating layers in electronic device manufacture1. As such, they will not be discussed here. Many of the silicon oxycarbide precursors discussed above can be oxidized to Si02 and thus can be considered precursors. However, this is simply a processing approach. The chemistry remains the same as described above. Thus, further discussion is not warranted. 

Oxycarbides have also been described (176). They are of interest for catalysis because oxophilic metals may not form carbides easily in preparations from metal oxides by treatment with methane, oxycarbides rather than carbides may form. Furthermore, carbides and nitrides are usually passivated after preparation they are slowly exposed to oxygen or air to form a thin protective oxide layer on the metal carbide surface. In this passivated form, these materials are not pyrophoric and can be transported through the air. After insertion into a catalytic reactor, they must be reactivated either by methane or ammonia treatment, as used in their preparation, or by reduction with hydrogen. In these cases, pure carbide or nitride is not always reformed at the surface an oxycarbide or oxynitride is often formed instead. 

The carbides can be prepared from the oxide and carbon or by using CO as a source of carbon. While economical, the process may produce an oxycarbide. Although oxycarbides may be useful, their properties often are confused with those of the pure system. In group IVB, this combined oxygen is difficult to detect by extraction techniques. 

The reaction between the oxide, AI2O3, and C proceeds through the formation of the oxycarbide, AI4O4C. Below 1800°C only the oxycarbide is formed, whereas above this T a further reduction leads to the carbide. Consequently, this reaction should be carried out in vacuum followed by purification at high T. However, when held in vacuum at ca. 2100°C, the carbide will vaporize A1 preferentially to give a C-rich product. 

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