Chemistry Reference
In-Depth Information
precursor, which is in good agreement with previous FTIR study.
113
Only
the latter two groups are likely present when POCl
3
is the precursor.
Ab initio molecular orbital calculations were performed with the
commercial Gaussian 98 software package. Based on experimental ob-
servation that P complexes protect the zigzag sites at point defects in the
carbon lattice, an anthracene model with zigzag sites was selected to
represent the carbon structure (Fig. 10). The results of potential energy
surface scan calculations for these three bonds are summarized in
Table 2. The C(10)-P(15) bond has the highest dissociation energy, while
the O(19)-P(15) bond has the lowest energy. The results suggest that the
O-P bonding in the model structure B is the weakest one.
The oxidation experiments revealed that the site blockage is mainly a
result of the presence of P groups bonded to carbon sites, whereas
metaphosphates (including metal-based) may function as a physical
barrier between the catalyst and carbon surfaces. The presence of O
bonded to both a carbon site and a P group is a critical factor for
maintaining the inhibition effect of P deposits. The loss of such oxygen or
its connecting bond results in loss of oxidation inhibition.
111
It should be pointed that there is a rather controversial concerning the
preferential bonding site of P groups on carbon. Oh and Rodriguez have
reported a complete opposite experimental observation comparing to the
model in Fig. 10. In situ transmission electron microscopy provided a
direct observation of the process showing that at higher temperatures
phosphorus species bonded preferentially to the graphite ''armchair''
{1120} faces leaving the ''zigzag'' {1010} faces vulnerable to attack by
oxygen.
109
In situ electron diffraction analysis indicated also the for-
mation of a chemical bond between the phosphorus and graphite edge
atoms at high temperatures, which lines in the results of XPS
111
and FTIR
study.
113
Fig. 10 Model structures with P groups frommolecular orbital calculations (Bond lengths
in Å ). Reproduced from Ref. 111 page 143 with permission of Elsevier.
Table 2 Bond energies for surface P groups (refer to Fig. 10).
111
Bond
Calculated bond energy (kJ/mol)
Bond dissociation energy (kJ/mol)
C(10)-P(15)
579.8
513.0
C(10)-O(19)
577.4
381.0
O(19)-P(15)
452.7
596.6
