A First Principles Study Of Adsorption And Oxidation Of SO₂ And NO By Graphene Oxides

Sulfur dioxide （SO₂） and Nitrogen monoxide （NO） emitted by anthropogenic sources, which lead to photochemical smog and acid rain, are two kinds of harmful air pollutants, and mainly derive from the process of burning of fossil fuels. Extensive efforts have been made to develop various efficient technologies for controling the emissions of SO₂ and NO. Wet flue gas desulfurization technology is still the current mainstream desulfurization technology, but it has lots of defects such as covers a large area, high investment and the by-product gypsum is difficult to use, etc. In recent years, activated carbon flue gas desulfurization technology which has been industrialized application is considered to be a very promising technology. Activated carbon have been widely used and studied for catalytic removal of SO₂ at low temperature （20-150℃）. The catalytic reaction involves adsorption and oxidation of SO2 on carbon materials and hydration, forming sulfuric acid as the end product, in the presence of O2 and H2O.Selective catalytic reduction （SCR） is a successful method for the removal of NO at high temperature （300-450℃） from stationary sources. Very recently, carbonaceous materials, such as activated carbon, activated carbon fibers and carbon nanofibers, are found to be of activity for the catalytic oxidation of NO into NO₂ at room temperature （50-180℃）. The product NO₂ can be removed easily by subsequently water scrubbing. So, it might be possible to develop a new technology based on carbon materials for simultaneously controlling the emissions of SO₂ and NO. However, the practical applications are limited as the conversion efficiency of SO₂ and NO is still object to be improved. Many efforts, including nitrogen doping and metal oxides loading, are tried, but still unsatisfied. A mechanistic understanding at atomic level of the adsorption and oxidation of SO2 and NO on the carbon material is needed for developing more effective catalysts.Activated carbon as adsorbent or catalyst carrier used in flue gas desulfurization and denitration technology has made some achievements. Due to the undefined structure and complex components of activated carbon, it has great difficulty to make clear its mechanism of desulfurization and denitration. Therefore, we choose a carbon material with definite microstructure instead of activated carbon to make some research, which can provide a theoretical basis on revealing the mechanism of desulfurization and denitration of carbon materials.Graphene （GP）, with a well defined atom-thick hexagonal structure, could be functionalized as a model catalyst of carbon materials so as to provide some deep insights into the mechanism of adsorption and oxidation of SO₂ and NO.First principles calculations was used to investigate the effects of epoxy and hydroxyl groups on adsorption and catalytic oxidation of SO₂ and NO on elaborated graphene oxides （GOs）. The adsorption structures, adsorption energies, charge transfer, oxidation pathway and oxidation barrier of SO₂ and NO on elaborated graphene oxides （GOs） are included in the investigation .The main research results are as follows:（1） The hydroxyl groups on GO surface possess bi-functional effects:both enhancing the adsorption of SO₂ through H-bonding interaction and reducing the reaction barrier for its oxidation to SO3, from 0.21 eV down to 0.12 eV. The promotion of oxidation is related to a pre-activation of the surface epoxy group. Based on Bader population, charge difference and electron localization function analysis, a charge transfer channel between hydroxyl, epoxy groups and SO₂ molecule is proposed to explain the pre-activation.（2） The surface hydroxyl groups of GO can enhance the adsorption of NO. The enhancement is derived from a weak covalent interaction between the nitrogen atom of the NO molecule and the GO surface. NO can be oxidized by epoxy groups with rather low barrier （0.01eV）, which helps to explain the low temperature catalytic oxidation. Easy to gain and lose electron is ascribed to the unique electronic structure NO, the highest occupied molecular orbit （HOMO） of which is partially filled. The hydroxyl group possesses no significant effects on the oxidation reaction.