tagungsband - Deutsche Bunsengesellschaft für Physikalische ...

Sa 19. 5. 2007, 10.00–10.20 A
Kurzvortrag - Hauptthema 14
Simulations of Nanoporous Carbon for Chemical
Applications
Johan M. Carlsson and Matthias Scheffler
Fritz-Haber-Institut der Max-Planck-Gesellschaft,
Faradayweg 4–6, D-14195 Berlin,
Germany
Nanoporous carbon materials (NPC) have the ability
to catalyze dehydrogenation reactions [1], but
the actuating chemical reaction steps are still unclear.
This uncertainty is partly because the atomic
structure of NPC is not well known during actual
reaction conditions. TEM experiments suggest that
NPC derived from hydrocarbons has the form of
crumpled graphene sheets with a significant amount
of non-hexagonal rings in the structure [2]. XPS
indicate that there are also C–O groups present in
chemically active NPC [1], such that determining the
surface structure of NPC in an oxygen atmosphere
is a prerequisite to understand its properties.
We have therefore carried out an extensive study
of NPC using ab-initio thermodynamics. Our densityfunctional
theory (DFT) calculations reveal that
the atomic relaxation transforms vacancies into
combinations of non-hexagonal rings. A large ring
surrounds the void in the lattice and incomplete
hexagons at the edge of the void are transformed
into pentagons. We identify this combination of a
large ring connected to a number of pentagons as
the “motifs of NPC”. 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 [3].
Admitting oxygen to NPC reveals that O 2 dissociation
is exothermal at the motifs. The oxidation
of NPC leads to formation of C–O groups as
the O-atoms attach at the motifs. C–O–C (ether)
groups dominate and C=O (carbonyl) groups may
also appear. The C=O groups are precursors for COdesorption,
but two C–C bonds need to be broken to
desorb the CO-molecule. Our thermodynamic treat-