Mechanisms for methane and ethane formation in the reaction
of hydrogen with carbonaceous materials
Juan F. Espinal
, Fanor Mondrago
, Thanh N. Truong
Institute of Chemistry, University of Antioquia, Medellı
n, A.A. 1226, Colombia
Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, University of Utah, 315 South 1400 East,
Room 2020, Salt Lake City, Utah 84112, USA
Received 8 December 2004; accepted 6 February 2005
Available online 31 March 2005
A systematic density functional theory (DFT) study of the hydrogen reactions with carbonaceous surfaces was carried out in
order to provide molecular-level understanding on the mechanisms of chemical processes involved in carbon hydrogasiﬁcation.
It was found that hydrogen is dissociatively chemisorbed on the active sites of zigzag and armchair conﬁgurations of carbonaceous
models. In addition, mechanisms for methane and ethane production during the carbon–H
reaction were proposed suggesting that
methane formation is exothermic and ethane formation is also possible but with a much lesser extent. These results agree with avail-
able experimental observations. Rate constants of the rate limiting steps were also calculated using the transition state theory. From
both the thermodynamic and kinetic points of view, methane formation is much easier from zigzag edges rather than from armchair
edges. The large activation energies for both pathways suggest that these reactions are favored at high temperatures.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Char; Molecular simulation; Gasiﬁcation
The reactions between carbonaceous materials and
hydrogen are important in many processes such as lique-
faction, gasiﬁcation, and hydrogen storage in carbon
ﬁbers and nanotubes. During the hydrogasiﬁcation reac-
tion the hydrogen molecule interacts with active sites of
a carbonaceous material leading to hydrogenated struc-
tures, which could be the precursors of hydrocarbons
(HCs) evolution. Methane is the most abundant HC
produced. Although the reactions involved during the
process are relatively simple, their mechanisms are not
Experimental studies on carbon hydrogasiﬁcation
were stimulated by the increasing demand for natural
gas since it is one possible way to produce synthetic nat-
ural gas [1–4]. In addition to methane, there are other
products such as: ethane, ethene, propane and propene,
but their yields are known to be much smaller [2,5].
Karcz and Porada  compared the amounts of these
low molecular weight HCs produced in coal pyrolysis
(under argon atmosphere) and in coal hydrogasiﬁcation
(pyrolysis under hydrogen atmosphere) at a pressure of
2.5 MPa and temperatures up to 1200 K. It was found
that during hydrogasiﬁcation, the evolution of both
methane and ethane was much larger than in pyrolysis.
The same trend was also observed for the other
hydrocarbons but in a smaller extent. Studies on the
mechanism of methane formation during carbon–H
reactions are relatively scarce. An empirical mechanism
proposed by Zielke and Gorin  was obtained from ﬁt-
ting to the observed reaction order. This mechanism
Carbon 43 (2005) 1820–1827
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
Corresponding authors. Fax: +574 210 6565 (F. Mondrago
+1 801 581 4301; fax: +1 801 581 4354 (T.N. Truong).
E-mail addresses: firstname.lastname@example.org (F. Mondra-
n), email@example.com (T.N. Truong).