ON THE ELECTRON DENSITY IN GRAPHENE LAYERS: AN APPROXIMATION TO A PREDICTION OF CARBON REACTIVITY

摘要

Over the last few years, modeling of carbon gasification reactivity has entered a new phase, which has been made possible not only by the advances in computational quantum chemistry [1] but also by progress in experimental studies. Indeed, much has been accomplished in the last few decades to eliminate empiricism from kinetic treatments of gasification reactions. However, our ability to further quantify the important details of carbon reactivity is now at a critical juncture. The experimental determination of active and reactive surface areas -- while now straightforward in principle [2,3] -- is complicated by stringent accuracy requirements because the active sites, and especially the reactive ones, are often a very small subset of the total surface area of the carbon material. The experimental measurements that are available suggest that the turnover frequency at the free carbon sites -- while largely independent of carbon type -- is dependent not only on temperature but also on the extent of carbon burnoff. This induced heterogeneity effect is difficult to quantify experimentally and it is on this key issue that carbon kineticists can (and should!) seek the help of computational quantum chemists. Before attempting to analyze theoretically the kinetics of interactions between a gaseous molecule (O_2, NO, CO_2, H_2O) and the carbon surface, it is necessary to evaluate the theory in terms of its consistency with some well known facts about the electronic structure and surface chemistry of graphene layers. At the same time, it is expected that such a theoretical analysis will clarify the key details of this electronic structure and thus help with the more accurate quantification of gasification reactivity. The electron density within the graphene sheet and at its edges -- particularly its sensitivity to oxygen surface coverage -- is of immediate interest. The affinity of the carbon surface for an oxidizing gas is assumed to be dependent on the electron density at the free carbon sites. The propensity for product formation (e.g., CO and CO_2 desorption) is dependent on the C-C bond strength, which in turn is governed by the electron density in the graphene layers. The objective of this paper is to make an initial contribution in this field. In our previous studies, both semi-empirical and ab initio methods have been used to evaluate the electron densities in graphene layers by considering 1-, 2-, 4- and 7-ring systems. Here we pay special attention to edge termination of such model compounds for the graphene layers as well as to the criteria for selecting the most appropriate spin multiplicities.