Methane diamond synthesis

Partial oxidation of methane to synthesis gas with oxidized diamond loaded Ni catalyst... 

Today there are several other ways of diamond synthesis besides the HPHT method. For example, it is possible to utilize the pressure of a shock-wave generated in an explosion. This process mostly yields powdery products with particle sizes in the range of micrometers (1 mm at max.) that may be employed for industrial purposes as well. Moreover, very small diamonds (5-20 nm) can be made by reacting explosives in confined containers. Diamond films are produced on various substrates by chemical vapor deposition (CVD method using methane as a carbon source. Detonation synthesis and vapor deposition will be described in detail in Chapters 5 and 6. 

The influence of the initial C/H/0 composition of the carbon compounds used for non-equilibrium diamond synthesis on the composition of the deposited films is usually illustratedby the so-called Bachmann triangle, showninFig. 9-45. The Bachmanntriangle summarizes experimental data and indicates the C/H/0 compositions corresponding to deposition of non-diamond films, diamond films, or no deposition at all (which corresponds to domination of etching over deposition). Deposition of diamond films from the gas phase is typically performed at pressures of 10-100 Torr in a mixture of hydrogen with about 1-5% methane. The plasma-chemical conversion of carbon compounds can lead not only to the formation of diamond films but also to so-called diamond-like carbon (DLC). DLC films are... 

The raw material for the synthesis was methane. Powder of Nickel carbonyl (NC) or powder of nano-diamond (ND) was the catalyst. Attempts to synthesize pyro-carbon on copper powder were not successful. Powder with the composition 70%PC, 30%NC, and also the set of powders with various ratios of PC and ND were tested. Anodes made of the powder 70PC30NC showed satisfactory cycle behavior and had specific capacity 180 mAh/(g of powder) (260 mA-h/(g 0f carbon)) (Fig. 3a). The anodes made of powder xPCyND, irrespective of the components ratio, had specific capacity... 

One of such unique coatings is Diamond Like Carbon (DLC). The conventional synthesis of synthetic diamonds requires extremely high temperatures and pressures. By PECVD, Diamond Like Carbon is created under mild conditions by the decomposition of methane in H2/CH4 mixture. The applications of DLC are numerous coatings for cutting tools, optical fibres, electronic devices for reading magnetic tapes, or even protective coatings in chemical reactors. 

Low-pressure synthesis of diamond coatings involves the use of hydrogen and methane gas mixtures. Typical deposition pressures in these syntheses are 10 -10 Pa, and... 

Fig. 5 shows the effect of various supports of nickel-loaded catalysts and reaction temperature on the methane conversion, in the partial oxidation of methane. At methane to oxygen ratio of 5 1, the maximum conversion of methane is 40 %, when reaction (5) proceeded, and 10% when complete oxidation proceeded. Only the oxidized diamond-supported Ni catalyst exceeded 10% conversion above 550 C, indicating that the synthesis gas formation proceeded. Ni-loaded LazOz catalyst afforded considerable methane conversion above 450 °C, but the product is mainly COz. Other supports to nickel showed no or only slight catalytic activity in the partial oxidation of methane. These results clearly show that oxidized diamond has excellent properties in the partial oxidation of methane at a low temperature, giving synthesis gas. Fig. 6 shows the effect of temperature on the product distribution, in the partial oxidation of methane. Above 550 °C, Hz and CO were produced, and below 500 °C, only complete oxidation occurred. The Hz to CO ratio should be 2 according to the stoichiometry. However, 3.2 and 2.8 were obtained at 550 and 600 °C, respectively. 

Oxygen in a small concentration was found to be essential for the synthesis of diamond by hot-filament chemical vapor deposition on silicon in the presence of methane [18] and by magnetoactive microwave discharge [19]. 

---