The Reduction Of NO By H₂ And The Removal Of Hg⁰ On Pd Catalyst

Coal was the main fossil energy in China,and a large amout of it was used for direct combustion for power generation.Nitrogen oxide and mercury emissions have attracted more and more attention during coal combustion to generate power.Nitrogen oxides were important precursors of acid rain,photochemical smog,the greenhouse effect and other environmental problems,while mercury was the main heavy metal pollutants and could be transformed into the organic mercury,which is harmful to human health through the food chain transformation.H₂ was considered to be the most promising agent for NO reduction because the reduction product is H₂ O and N₂,which did not cause secondary pollution.Precious metals were generally regarded as best catalysts for the reduction of NO by H₂.Compared to Rh and Pt,Pd catalyst was more suitable for H₂ reducing NO since it has relatively low price,good thermal stability and low temperature activity.Meanwhile,Pd was also best catalyst for the mercury adsorption and mercury oxidation.Three catalyst models of Pd?211?,Pd₆/TiO₂ and PdTi_x-1O₂ x were constructed and the density functional theory?DFT?method was adopted to investigate the possible reaction path of removal of NO and Hg on Pd catalysts,and the effect of intermediates during the reduction of NO on the removal of Hg were also been considered in this work.The main conclusions are as follows:1.The reduction of NO by H₂ on three different Pd catalysts was studied,and the main reaction paths were obtained.The main route of N₂ generation change with temperature increasing on the Pd?211?surface.The main route for N₂ formation was two NO react and generate N₂O₂,and then N₂O₂ decompose to produce N₂?dimer path?below about 525 K.However,the main route of N₂ generation was changed into NO hydrogenate and dissociate to produce active N?active N path?,which is favor to N₂ generation above 525 K.The active N gradually also probably hydrogenated to NH₃.N₂,N₂ O and NH₃ were also formed though the active N path on the Pd₆/TiO₂.The formation of N₂ was via the dimmer path,the reducing agent H₂ combine with lattice oxygen to generate H₂ O and by-product NH₃ was not formed on the PdTi_x-1O₂ x.2.H₂ plays different roles in the reduction of NO on different catalysts,which will lead to different reduction products and by-products.The main function of H was the removal the surface O formed from N₂O₂ decomposition on Pd?211?surface.In addition,H also promoted NO hydrogenation dissociation and the generation of NH₃.The main role of H was NO hydrogenation to generate HNO and NHOH,which is contributed to the N-O bond fracture on the Pd₆/TiO₂.HNO and NHOH dissociated to produce N and NH,which finally generate N₂ and NH₃.However,the main function of H₂ was to produce surface oxygen vacancy,which promotes the formation and decomposion of N₂O₂ on PdTi_x-1O₂ x.Because H had no direct effect on the breaking of N-O bonds,the reduction products did not contain NH₃ on Pd Ti_x-1O₂ x.3.The removal of Hg⁰ was studied on three Pd based catalysts,and the mechanism of Hg⁰ removal was clarified.Hg⁰ can be removed by direct adsorption removal and indirect oxidation removal because of the adsorption of Hg⁰ belong to strong chemisorption?117.0 and 103.4 kJ?mol-1?and Hg⁰ is easily oxidized with low reaction barrier on the Pd?211?and PdTi_x-1O₂ x catalyst.However,the adsorption of Hg⁰ was weak?68.5 kJ?mol-1?on Pd₆/TiO₂,so Hg⁰ was removed mainly through indirect oxidation removal.4.The performances of three kinds of catalysts were compared for simultaneous removal of NO and Hg⁰,and a good catalyst with high efficiency was obtained.The energy barrier of NO reduction to N₂ was 75.5 kJ?mol-1 on the PdTi_x-1O₂ x catalyst,and the oxidation energy barrier of Hg⁰ was 6.6 k J?mol-1,which are the lowest in the three catalysts.The results show that NO can be reduced with lower the energy barrier and HgO was easily formed on the PdTi_x-1O₂ x catalyst.In addition,the noble metal content of Pd is least in PdTi_x-1O₂ x catalyst,which not only reduces the amount of Pd,but also maintained the catalytic activity.So Pd Ti_x-1O₂ x is the ideal catalyst for simultaneous removal of NO and Hg⁰.