SEMI-EMPIRICAL ESTIMATION OF THE THERMODYNAMICS OF GRAPHANE; ASSESSMENT OF THE GRAPHITE-GRAPHANE SYSTEM FOR HYDROGEN STORAGE AND RELEASE; SOME POTENTIAL FORMATION REACTIONS FOR GRAPHANE
BY MICHAEL JEWESS
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This work was completed in September 2009 and put onto the internet on 11 May 2011 as http://myweb.tiscali.co.uk/symmetry/graphanesynthesis.pdf.
Graphane consists of macromolecular layers, each comprising CH units at the vertices of an indefinitely large 2-dimensional array of fused hexagons. Between the layers there are weak attractive van der Waals forces. Graphane has yet to be synthesized in pure bulk form. In order to facilitate the devising of improved syntheses of graphane, we here estimate the thermodynamic properties of graphane by semi-empirical methods. DHf (CH, graphane) and S (CH, graphane) are estimated, respectively, as -8.8 ± 5 kJ mol-1 and 18 ± 4 J K-1 mol-1 at 298.15 K and 1 bar. This makes graphane thermodynamically unstable with respect to graphite and hydrogen gas at 298.15 K unless the pressure is raised above 1 bar, probably very considerably. (It is, however, believed that graphane, once formed, will survive indefinitely at 298.15 K and 1 bar – ie will be kinetically stable.)
Elevated temperature and use of a catalyst will probably be needed if high-pressure hydrogenation of graphite is to produce graphane at an appreciable rate, any increase in temperature needing to be offset by a further increase in pressure. The implications of this for the use of the graphite-graphane system for hydrogen storage and release are discussed.
The following reactions producing graphane are predicted to be thermodynamically favored at 298.15 K and 1 bar: addition polymerisation of acetylene; addition polymerisation of benzene; dehydrogenation of cyclohexene; the reaction of cyclohexene with graphite; and the reduction of graphite with formic acid.
Keywords: thermodynamics, crystallography, graphene, graphene.
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