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.
Abstract
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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