Opposite roles of O₂ in NO- and N₂O-carbon reactions: An ab initio study

Previous experimental studies showed that the presence of O₂ greatly enhances NO-carbon reaction while it depresses N₂O-carbon reaction on carbon surfaces. A popular explanation for the rate increase is that the addition of O₂ results in a large number of reactive carbon-oxygen complexes, and decomposition of these complexes produces many more active sites. The explanation for the latter is that excess O₂ simply blocks the active sites, thus reducing the rate of N₂O-carbon reaction. The contradiction is that O₂ can also occupy active sites in NO-carbon reaction and produce active sites in N₂O-carbon reduction. By using ab initio calculation, we find that the opposite roles of O₂ 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 NO chemisorption is more thermodynamically favorable than O₂ chemisorption. By contrast, the presence of excess O₂ makes N₂O chemisorption much less thermally stable either on the consecutive edge sites or edge sites isolated by semiquinone oxygen. A detailed analysis and discussion of the reaction mechanism of N₂ formation from NO- and N₂O-carbon reaction in the presence of O₂ is presented in this paper.