II International Symposium on Carbon for Catalysis ABSTRACTS

OP-I-17
NOVEL NANOPOROUS CARBON MATERIALS FOR STYRENE CATALYSIS
Carlsson J.M., Scheffler M.
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14 195 Berlin, Germany
e-mail: johanc@fhi-berlin.mpg.de
Production of styrene by dehydrogenation of ethylbenzene (EB) is an important process in
chemical industry [1]. The technical catalyst for this process is iron oxide, but experiments
indicate that the surface gets covered by a layer of carbon materials under reaction conditions [2].
This has led to the suggestion that carbon materials could be used as catalysts’ for this reaction
[3-7]. Subsequent experiments have also shown that different carbon materials such as carbon
nanofilaments [4], onion-like carbon [5], carbon nanotubes [6] or nanoporous carbon molecular
sieves [7] are indeed efficient catalysts for oxidative dehydrogenation (ODH) of ethylbenzene, but
the origin of the catalytic activity remains unclear. This is in particular the case for the nanoporous
carbon materials (NPC) since their structure is not well characterized. Such nanoporous carbon
(NPC) constitutes an intermediate class of materials which are less ordered than graphite due to
defects and voids, but still not completely amorphous. However, the defect concentration appears
to be crucial, since Raman measurements of active carbon materials show that these materials
contain a significant amount of defects [5]. In addition, XPS-experiments indicate that the
presence of functional oxygen groups is vital to obtain a chemically active material [4,5].
We have therefore carried out an extensive study of NPC and how oxygen can assist ODH of
ethylbenzene by density-functional calculations (DFT). The building blocks of NPC were first
identified by a systematic study of defects in graphene and nanotubes: the ``motifs'' [8]. These
motifs were embedded in a graphene sheet in a supercell containing up to 112 atoms. Our DFT
calculations revealed that the atomic relaxation transforms defects into combinations of nonhexagonal
rings. These motifs lead to strain and local buckling of the structure. They also induce
defect states close to the Fermi level, leading to that some of them being charged, which may
facilitate molecule dissociation. Curvature modifies the properties of the motifs by lowering their
formation energy in nanotubes and mixing the defect states with the π-band, such that the defect
states lose their localized character. These motifs can then be combined to build a model of a NPC
material with a certain concentration of non-hexagonal rings in the structure. This model indicates
that NPC materials with up to 1 % motifs appear to have a comparable heat of formation to single
wall nanotubes.
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