Non-thermal Plasma Synergized With Au Catalyst For The Oxidation Mechanism Of Graphitic Carbon

In China,taking control of the particulate matter contamination is the focus of air pollution governance.With the advancement of urbanization,vehicle contamination has become increasingly significant.As the major source of mobile particulate mattter,diesel vehicles contribute more than 90%of vehicle particulate emissions.Thus,there is an urgent need to develop efficient diesel vehicle exhaust particulate control technologies.The plasma-catalysis hybrid system can achieve higher removal rate at lower temperature and energy consumption.It has a great prospect in the decontamination of diesel exhaust particulate matter.In order to further elevate the removal of particulate matter and promote the synergistic effect between plasma and catalysts,it is necessary to carry out researches on the mechanism of plasma catalysis system to purify particulate matter.In this project,γ-Al₂O₃ and Au/γ-Al₂O₃ are mixed with graphitic carbon in a certain proportion,and the experiments were carried out in a modified in-situ DRIFTS cell and a self-made tubular DBD reactor respectively.The avtivity for catalyzing the oxidation of graphite carbon during the cold start and normal driving phase were studied,and the reaction intermediate product at different discharge time or temperature were also analyzed.We have found that when the graphitic carbon is oxidized by plasma alone,it is initially oxidized to quinone and ether.Quinone and ether will not decompose directly below 300°C,but will undergo ring opening and continue to be oxidized to carboxyl carbonate.CO₂ and CO is produced along with the drop of O-C=O and C=O,and lactone is formed.After the lactone is generated,the ring opens again to form new O-C=O and C=O.In the mutual conversion of lactone and carboxyl carbonate,graphitic carbon is continuously oxidized to produce CO₂ and CO.Whenγ-Al₂O₃ and Au/γ-Al₂O₃ are present,the lattice oxygen M-O_b on the catalyst surface will deprive the O-C=O on the carboxyl carbonate to form monodentate carbonate.M-OH combines with CO to produce hydroxycarbonyl,then hydroxycarbonyl is oxidized to bicarbonate,and finally CO₂ is generated by the decomposition of monodentate carbonate and bicarbonate.The decomposition of monodentate carbonate,in turn,can promote the drop of O-C=O on carboxyl carbonate and accelerate the transfer of oxygen-containing groups on the surface of graphitic carbon.Formation and conversion of bicarbonate can oxidize CO to CO₂and improve the selectivity of CO₂ in the meantime.Compared withγ-Al₂O₃,Au/γ-Al₂O₃ has increased the number of weak and strong Lewis acid sites,which are conducive to the adsorption and decomposition of ozone.It means Au/γ-Al₂O₃ can generate more active oxygen atoms,and accelerate the oxidation of graphite carbon to produce quinones,ethers,lactones and carboxyl carbonate.The increase of M-O_b on Au/γ-Al₂O₃ can promote the transfer of O-C=O groups on carboxyl carbonate.With the increase of the reaction temperature,the monodentate carbonate and bicarbonate which are produced during the oxidation of graphite carbon catalyzed by Au/γ-Al₂O₃ are easier to decompose to CO₂,and at the same time,the selectivity of CO₂ is improved.Then,so as to further elevate the synergistic effect between plasma and catalyst to the oxidation of graphitic carbon,CeO₂ was added to Au/γ-Al₂O₃ for modification.It was found that the loading of CeO₂ brought abundant oxygen vacancies.The oxygen vacancies on Au-CeO₂/γ-Al₂O₃ can directly activate oxygen into superoxide（O₂^-）and peroxide（O^-）.Under plasma discharge,the oxygen vacancies can also activate ozone to produce more more O₂^-and O^-.O₂^-and O^-can effectively assist the plasma oxidation of graphitic carbon at low temperature,accelerate the further oxidation of quinone,ether and lactone,and produce more carboxyl carbonate,monodentate carbonate and bicarbonate.The monodentate carbonate and bicarbonte will not accumulate on the surface of the catalyst,but will decompose in time to produce CO₂.This process will further accelerate the oxidation of graphitic carbon,which also verifies the mechanism of graphite carbon oxidation catalyzed by plasma synergistic withγ-Al₂O₃ or Au/γ-Al₂O₃.