posted on 2018-11-27, 00:00authored bySeenivasan Hariharan, Moumita Majumder, Ross Edel, Tim Grabnic, S. J. Sibener, William L. Hase
Direct
chemical dynamics simulations at high temperatures of reaction
between 3O2 and graphene containing varied number
of defects were performed using the VENUS-MOPAC code. Graphene was
modeled using (5a,6z)-periacene,
a poly aromatic hydrocarbon with 5 and 6 benzene rings in the armchair
and zigzag directions, respectively. Up to six defects were introduced
by removing carbon atoms from the basal plane. Usage of the PM7/unrestricted
Hartree–Fock (UHF) method, for the simulations, was validated
by benchmarking singlet-triplet gaps of n-acenes
and (5a,nz) periacenes with high-level
theoretical calculations. PM7/UHF calculations showed that graphene
with different number of vacancies has different ground electronic
states. Dynamics simulations were performed for two 3O2 collision energies Ei of 0.4
and 0.7 eV, with the incident angle normal to the graphene plane at
1375 K. Collisions on graphene with one, two, three, and four vacancies
(1C-, 2C-, 3C-, and 4C-vacant graphene) showed no reactive trajectories,
mainly due to the nonavailability of reactive sites resulting from
nascent site deactivation, a dynamical phenomenon. On the other hand, 3O2 dissociative chemisorption was observed for
collisions on four- (with a different morphology), five- and six-vacant
graphene (4C-2-, 5C- and 6C-vacant graphene). A strong morphology
dependence was observed for the reaction conditions. On all reactive
surfaces, larger reaction probabilities were observed for collisions
at Ei = 0.7 eV. This is in agreement with
the nucleation time measured by supersonic molecular beam experiments
wherein about 2.5 times longer nucleation time for O2 impinging
at 0.4 eV compared with 0.7 eV was observed. Reactivity at both collision
energies, viz., 0.4 and 0.7 eV, showed the following trend: 5C- <
6C- < 4C-vacant graphene. Formation of carboxyl/semiquinone (CO)- and ether (−C–O–C−)-type
dissociation products was observed on all reactive surfaces, whereas
a higher probability of formation of the ether (−C–O–C−)
group was found on 4C-vacant graphene on which dangling carbon atoms
are present in close proximity. However, no gaseous CO/CO2 formation was observed on any of the graphene vacancies even for
simulations that were run up to 10 ps. This is apparently the result
of the absence of excess oxygen atoms that can aid the formation of
larger groups, the precursors for CO/CO2 formation. Although
the results of this study do not provide a conclusive understanding
of the mechanism of graphene/graphite oxidation, this work serves
as an initial study attempting to understand the 3O2 dissociative chemisorption dynamical mechanism on defective-graphene/graphite
surfaces at high temperatures.