Chemistry Reference
In-Depth Information
Fig. 11 Schematic representation of the bonding of a (BO
3
), polymeric residue to the
{1011} face of graphite. Reproduced from Ref. 113 page 511 with permission of Elsevier.
representation of the bonding of a (BO
3
)
n
chain to the ''zig-zag'' of the
graphite lattice. On the other hand, bonding of (BO
3
)
n
chains at the ''arm-
chair'' would be less favourable from a steric standpoint. It is in good
agreement with the detailed in-situ controlled atmosphere electron mi-
croscopy study.
116
Controlled atmosphere electron microscopy coupled with in situ elec-
tron diffraction has been used to follow the graphite-oxygen reaction on
B-doped graphite. It was found that that the boron oxide undergoes a
strong interaction with both the graphite edge and the basal plane re-
gions, and this results in a spreading of the oxide to form a uniform thin
adherent film over the entire substrate at about 450 1C. McKee observed
similar phenomena where above the melting point of B
2
O
3
(450 1C), a
glassy residue of the polymeric oxide is formed which blocks the active
sites on the carbon surface.
113
The coated graphite specimens appear to be impervious to attack by
oxygen at temperatures below 815 1C. Above this temperature, however,
there is a weakening of the additive-graphite interaction with the ''arm-
chair'' {1120} faces, and these regions then become vulnerable to attack by
oxygen. It revealed that carbon sites in the zigzag faces are preferentially
poisoned by the strongly bonded adsorbate.
116
In a complementary series of
experiments, it was found that boron carbide is an extremely active catalyst
for the graphite-oxygen reaction even at temperatures as low as 100 1C.
Because boron has one electron less than carbon and an empty p or-
bital, and is thus expected to change the electron acceptor level, a
straightforward explanation in terms of lower Fermi level of the doped
carbons has been invoked to account for the modified electronic and
physicochemical properties. However, such an explanation has been
found insucient
for understanding the modification of carbon
