In this thesis the reaction mechanisms of carbon reactions with oxygen-containing gases have been studied in detail by using density functional theory calculations. Single layer graphite structures have proved to produce reliable relative comparison although multi layer structures are necessary when more accurate results are needed. As the edge sites are much more active than the basal plane sites, this study focused on two sorts of edge sites: zigzag edge sites and armchair edge sites. The following are main findings and conclusions on the reactions studies in this thesis.
02 -carbon reaction was first studied. It was found that there is a good correlation between the electronic structures and the reactivities of carbon edge sites. Each zigzag edge site has an impaired electron around and very favorable for oxygen adsorption. In contrast, the C-C bonds on the armchair sites have relatively stable triple bond character thus can not adsorb oxygen as readily as zigzag sites. The dissociative chemisorption of O2 on the zigzag and armchair sites results in semiquinone and o-quinone, respectively. On the contrary, o-quinone desorption (in the form of CO) from armchair sites needs a lower energy than semiquinone from zigzag sites. The reason is that, following O2 dissociation, the formation of C=0 double bond makes the triple bond on the armchair edge sites a very weak single bond, while a resonance structure of the zigzag C=0 double bond exists with a C-0 and the aromatic π system intact.
CO2 adsorbs on carbon surface much less favorably than O2. Moreover, only semiquinone oxygen can be formed in C02 -carbon reaction, while both o-quinone and semiquinone are a result of 02 -carbon reaction. The o-quinone desorption with a bond energy ca. 30% lower than that of semiquinone desorption, can drive the 02 -carbon reactions forward at an activation energy 30% lower than that of C02 -carbon reaction. With the further increase in O2 pressure, off-plane epoxy can be formed. The semiquinone oxygen with adjacent epoxy has nearly the same desorption bond energy as o-quinone. This means that the formation of off-plane oxygen can increase the active sites, but not further decrease the activation energy of reactions. While in Yang’s previous unified mechanism, it was thought that the off-plane epoxy is the reason for the 30% lower activation energy of 02 -carbon reaction. This is the key difference between our new mechanism and the previous unified mechanism.
H2O is first physically adsorbed on the virgin graphite surface with negligible change in molecular structure. Chemisorption occurs via O approaching the carbon edge site with one H atom stretching away from the O in the transition state. The stretched H is further disconnected from the O atoms and the remaining OH group is still on the carbon edge site. The disconnected H then moves around the OH group to bond with the H of the OH group and forms H2. Although the adsorption mechanism of H2O is different from CO2, but the final result is quite similar, i.e. producing only semiquinone oxyen. This is the reason why CO2- and H20 -carbon reactions have similar reaction rates and activation energies.
The opposite roles of O2 in NO- and N2O -carbon reactions are caused by the different manners of N2O and NO adsorption on the carbon surface. In the presence of excess O2, most of the active sites are occupied by oxygen groups. In the competition for the remaining active sites, NO is more likely to chemisorb in the form of NO2 and is more thermodynamically favourable than O2. By contrast, the presence of excess O2 makes N2O chemisorption much less thermally stable either on the consecutive edge sites oredge sites isolated by semiquinone oxygen.
Based upon the above studies, a new generalized mechanism is summarized and can account for all the important kinetic findings. The key point is that in C02 /H20 -carbon reaction there is only semiquinone formed, while o-quinone can be formed at relatively low pressure followed by epoxy oxygen at higher pressure in 02 -carbon reaction. This is the reason for the activation energy of 02 -carbon reaction 30% lower compared with that of H20 /C02-carbon reactions. There are two CO peaks at ca. 450 °C and 900 °C during the TPD after O2 adsorption, and the fast and slow decays during TK (transient kinetics) following O2 adsorption or reaction, can also be well explained by our mechanism.
Apparently, it is o-quinone and epoxy oxygen that cause the lower temperature CO peak in TPD and the fast decay in TK. The formation of nitrogen species during NO-carbon reaction makes its activation energy ca. 14%-20% lower than that of 02 -carbon reaction. The higher ratio of atomic oxygen due to the lower dissociative energy of N2O is the reason why N20 -carbon also has ca. 14%-20% lower activation energy compared with 02 -carbon reaction. Moreover, the experimentally observed etched pits produced by O2/O-, CO2-, H2O-, and NOx-carbon reactions can also be well explained by ourgeneralized mechanism.