| Catalytic abatement of gas-phase pollutants is one of promising technologies in the area of air pollution control, such as selective catalytic reduction of NOx with NH3(NH3-SCR) and catalytic combustion of volatile organic compounds (VOCs). Recently, the development of low-temperature active and highly sulfur/chlorine tolerance catalyst has been spotlighted in environment catalysis field. The first principles method, as it could effectively give the details at atomic level for materials, has been widely applied in computational physics and chemistry, materials and theoretical chemistry. Moreover, the application of this method can save lots of experimental resources. Hence, if this method was employed in environment catalysis field, it will be beneficial to the theoretical understanding of environment catalytic reaction processes and make a considerable promotion for the designing of catalyst with high activity, selectivity and stability. Therefore, in this dissertation, we use CeO2based materials as model catalysts to investigate their physical and chemical properties from first principles under environmental catalytic conditions, providing theoretical guidelines of catalytic activity and poising mechanism.Firstly, we investigated the geometric and electronic structures, Oxygen formation energy, oxygen adsorption and activation over Mn substitutionally doped CeO2(111) surface. The results indicated that the Mn doping to ceria could reduce the formation energies for both first and second oxygen vacancies, resulting in the rising of activity. The adsorbed O2could be effectively activated over Mn doped catalyst to form superoxo (O2-) and peroxo species (O22-), while only peroxo species could form over pure ceria. In redox cycle, the doping of Mn induces a Mn3d-O2p gap states between the O2p valance band and Ce f band, which is much easier for electrons donating and accepting. This is the physical origin of the promotion effects for Mn doped CeO2system.Secondly, a theoretic method combining firstly principles theory and ab initio thermodynamics was applied to investigate the SO2poisoning in realistic temperature and pressure. Surface Ce(SO4)2was identified to be the most stable poisoning structure on Mn doped and undoped CeO2surfaces and it was found that the introduction of Mn would enhance the thermal stability of the surface sulfate. From the P-T phase diagrams, it represented that the decomposing temperature of the sulfate on Mn doped CeO2surface was about150℃higher than that on the undoped CeO2surface. The results also indicated that the Lewis acidity of the catalysts could be enhanced by slightly sulfating, which might make some positive effect on catalytic performances for the abatement of basic pollutants.Finally, based on Cl-related environmental catalysis system, the adsorption and transformation of Cl species over catalysts has been taken into consideration. The thermodynamics results indicated that Cl atom will strongly adsorbed on oxygen vacancy, lead to its deactivation. And in typical operating conditions of Cl-related environmental catalysis (quite low Cl2/O2ratio), the chlorinated ratio of CeO2vacancies is very limited. It was identified that there was strong H-bond interaction between surface OH groups and separated Cl atoms, which helped to the formation of HCl under oxygen-rich condition but Cl2under slightly oxidizing or stoichiometric conditions. By introduction of extra H2O or other H resources, the coverage of surface OH radicals could be increased, which in turn benefits the selectivity to HCl over Cl2. |
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