Opposite roles of O-2 in NO- and N2O-carbon reactions: An ab initio study

Previous experimental studies showed that the presence of O-2 greatly enhan ces NO-carbon reaction while it depresses N2O-carbon reaction on carbon sur faces. A popular explanation for the rate increase is that the addition of O-2 results in a large number of reactive carbon-oxygen complexes, and deco mposition of these complexes produces many more active sites. The explanati on for the latter is that excess O-2 simply blocks the active sites, thus r educing the rate of N2O-carbon reaction. The contradiction is that O-2 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 oppo site roles of O-2 are caused by the different manners of N2O and NO adsorpt ion on the carbon surface. In the presence of excess O-2, most Of the activ e 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 che misorption is mon thermodynamically favorable than O-2 chemisorption. By co ntrast, the presence of excess O-2 makes N2O chemisorption much less therma lly stable either on the consecutive edge sites or edge sites isolated by s emiquinone oxygen. A detailed analysis and discussion of the reaction mecha nism of N-2 formation from NO- and N2O-carbon reaction in the presence of O -2 is presented in this paper.