Mechanism Study Of Mercury Adsorption On Mn Doped CeO₂Surfaces

Vast consumption of coals is the main reason for the serious mercury pullution prob-lem in China. With the introduction of new Emission standard of air pullutants for thermal power plants, and also for the pressure from the international negotiations, China will con-inue to tighten the emission control of mercury in coal-fired boilers. Taking into account the current level of economic development in China, the combination of catalytic oxidation from Hg0to Hg2+followed by WFGD is a promising and pratical strategy for mercury re-moval. Thus, high-performance catalyst to reach efficient transformation from Hg0to Hg2+must be developed. However, the mercury oxidation efficiency on commercial vanadium SCR catalyst is limited, along with some issues of its own, developing new SCR catalyst to achieve simultaneously efficient oxidation of Hg0will have important practical value.Mn-Ce composite oxide has a promising prospect in mercury removal. This paper takes the Mn-Ce composite oxide as the research object to investigate the adsorption mech-anism of mercury on different active sites of Mn dopping ceria by means of periodic density functional theory and experiments.Experimentally, Mn-Ce composite oxide was prepared by co-precipitation, the mer-cury adsorption experiments at150℃and200℃showed Mn doping greatly improves its absorbing ability for mercury, the surface morphology of oxygen species is extremely im-portant. With mercury being adsorbed, the surface lattice oxygen species transforms into chemisorpted state, which suggests the mercury absorbing ability of Mn-Ce composite oxide might origins from oxygen species in the surface lattice or those bind strongly with the metal ions, the element mercury adsorption will weaken the interaction from the active oxygen species with the substrate and result into some chemisorpted states.Theoretically, using density functional theory with the on-site Coulomb correction (DFT+U). Hg adsorption on clean surfaces and H-compensated surfaces with different oxygen species were investigated over Mn substitutionally doped CeO2(Ⅲ) model. The results shows that Mn doping do promote the mercury Adsorption. In the clean doped case, the intact surface and reoxidized surface (O2/Ov, with O2adsorbed on oxygen vacancy) are both favourable for Hg binding. The adsorbed Hg exists in oxidized form, which results into the reduction of the doped Mn. Besides, the transformation from superoxo (O2-) to peroxo CO22-) also strengthen the interaction between Hg with the substrate surface. In view of thermodynamics stability, the O2/Ov species in clean doped surface will be one primary active sites for mercury adsorption on Mn-Ce composites. For the H-added surfaces, the electron deficit induced by Mn doping were well compensated, which leads to stabilization of its intact surface. H-compensated intact surface is also favourable for Hg adsorption, but it is the two neighbouring Ce4+ions which were reduced after the adsorption instead of the doped Mn. The formation of oxygen vacancy on Mn doped surface is found to be unfavourable for Hg adsorption, which explains the importance of active oxygen species fairy well.