Non-hydrogenous Materials and Carbon

In this chapter we consider the analysis of the spectra from non-hydrogenous materials ( 11.1) with chlorine ( 11.1.1) and some minerals ( 11.1.2) as examples. Carbon ( 11.2) in its allotropic forms of diamond ( 11.2.1), graphite ( 11.2.2) and the fullerenes and their derivatives ( 11.2.3) has been studied extensively by INS spectroscopy. 

In addition to the pure forms of carbon, there are a wide range of carbons of varying crystallinity and hydrogen content that are industrially important. These materials form a continuum from almost pure carbon to those with carbon-hydrogen ratios typical of organic compounds. The materials include amorphous hydrogenated-carbon ( 11.2.4) and a range of industrial carbons ( 11.2.5) such as coal, catalyst supports, catalyst coke and carbon blacks. Finally, metal carbonyl complexes ( 11.2.6) are also considered. 

The major difficulty with studying non-hydrogenous compounds is lack of sensitivity. This largely accounts for why there are few studies of 

Deuterium is a special case. Selective deuteration is probably the most common form of sample manipulation and is often highly informative. Complete deuteration is less common, but with sufficient sample is capable of giving excellent spectra. The smaller cross section and larger mass both conspire to reduce the sensitivity but it is still larger than for virtually all other elements, so the INS spectra of fully deuterated compounds are dominated by the deuterium modes. In Fig. 11.1 we compare the spectra (normalised to one mole) of CeHs and CeDe. The effect of the difference in cross section and amplitude of vibration is clear. (The INS spectra of partially deuterated systems are discussed along with the spectra of the parent compound). 

It should be noted that the vast majority of non-hydrogenous samples are studied by coherent INS spectroscopy of single crystals this is particularly the case for magnetic systems. Coherent INS is outside the scope of this book (see [1,2] for an introduction) and these systems are not considered ftirther. There are a few coherent INS studies of non-hydrogenous and perdeuterated organic compounds these are included in the list of compounds given in Appendix 4. 

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The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

The gasification of hydrocarbons to produce hydrogen is a continuous, non-catalytic process (Figure 10-2) that involves partial oxidation of the hydrocarbon. Air or oxygen (with steam or carbon dioxide) is used as the oxidant at 1095— 1480°C (2000-2700°F). Any carbon produced (2-3 wt% of the feedstock) during the process is removed as a slurry in a carbon separator and pelleted for use either as a fuel or as raw material for carbon-based products. 

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