Theoretical Study On The Selective Reduction Of No_x Over Doped Cerium/Titanium Oxide Catalysts

Nitrogen oxide（NOx）is one of the main air pollutants emitted from the main mobile sources and fixed sources（combustion process of coal-fired power plants,industrial flue exhaust gas）.It can cause acid rain,photochemical smog,and ozone destruction.Selective catalytic reduction of NOx with ammonia（NH₃-SCR）is the most effective technology for NOx removal.Therefore,it is very important to design an NH₃-SCR catalyst with good low-temperature activity,high N₂ selectivity,and good resistance to water,sulfur,and alkali metal.A deep understanding of the reaction mechanism and catalytic essence of NH₃-SCR at an atomic scale can provide theoretical guidance and a scientific basis for designing and developing novel NH₃-SCR catalysts.In this dissertation,the density functional theory calculations were used to study ceria and titania-based catalyst models,including Mn-doped ceria,Mn-doped titania,third and fourth-period transition metals,and rare earth metal Ce doped titania.NH₃-SCR reaction mechanisms on these catalyst surfaces were investigated at the atomic scale.Taking Mn-doped ceria/titania as an example,the NH₃-SCR mechanism was revealed.The effect of doping metal and the reason why Mn shows excellent activity at low temperatures were proved.The NH₃-SCR mechanism of Mn-doped TiO₂ was also applicable to different metal-doped TiO₂ catalysts.The descriptors representing the activity and N₂ selectivity were determined.The design and screening of metal-doped TiO₂ catalysts for NH₃-SCR were carried out,and the deep factors affecting the NH₃-SCR activity and N₂ selectivity were found.The poisoning mechanisms of water,sulfur,and alkali metals were also explained.The stabilities of the designed catalysts were evaluated under the conditions of resistance to water,sulfur,and alkali metals.The main studying contents and conclusions are as follows:（1）The mechanism of NH₃-SCR reaction on the Mn-CeO₂（111）surface was studied.According to the lower LUMO energy level of Mn 3d orbital than Ce 4f orbita,Mn is the active site of the reaction.NH₃ is adsorbed on Mn and activated into NH₂,and then spontaneously reacts with gaseous NO to form the intermediate NH₂NO.NH₂NO dissociates two H atoms to form N₂O.Two hydroxyl groups on the surface react with each other to form H₂O and an oxygen vacancy.The O of N₂O heals the oxygen vacancy and forms N₂.According to the reaction energy barrier,the associative desorption of H₂O is the rate-determining step.Furthermore,the key to N₂ formation is the existence of oxygen vacancy.（2）The NH₃-SCR mechanism on the Mn-TiO₂（101）surface was studied.Mn is the active site of the reaction.NH₃ is adsorbed and activated on Mn to form NH₂,and then spontaneously reacts with gaseous NO to form the intermediate NH₂NO.NH₂NO can be easily decomposed to N₂ and surface hydroxyl groups through a synergistic H transfer step with support.The OH on Mn then abstracts the H of the adjacent OH to form H₂O and desorbs.After two reaction cycles,the surface of the reduced catalyst is recovered to its initial state byO₂.The doping of Mn promotes NH₃ adsorption and NH₃ activation.This is because that the 3d orbital of Mn shows a lower LUMO energy level,and it activates the adjacent surface O.The N₂O formation path is also studied.It suggests that the deep oxidation of NH₃ is the main reason for the N₂O formation,which is also why the N₂ selectivity on Mn-TiO₂ decreases with the increasing temperature.（3）Based on the mechanism from（2）,the N₂ and N₂O formation pathways of NH₃-SCR reaction over V,Cr,Mn,Fe,Co,Mo,and Ce doped TiO₂（101）were studied.The apparent activation energies of N₂ and N₂O formation pathways are the descriptors representing the activity and N₂ selectivity for NH₃-SCR on the metal-doped TiO₂ catalyst,respectively,according to the energy barrier analysis of each elementary step.Thus,the two descriptors on all the third and fourth-period transition metal-doped TiO₂（101）were calculated.It is found that Sc-、Mn-、Co-、Ni-、Cu、Zn、Y-、Pd-、Ag-、Cd-TiO₂ show good low-temperature activity,while TiO₂,Cr-,Fe-,Zr-and Ce-TiO₂ show high N₂ selectivity.In addition,it is found that the descriptors have linear relationships with the surface oxygen activity,and the doping metals affect the surface oxygen activity through doping effect,electronic structure,and atomic size,thus affecting the NH₃-SCR activity and N₂ selectivity.（4）The poisoning mechanisms of all the third and fourth-period transition metals and rare earth metal Ce doped TiO₂（101）with H₂O,SO₂,Na,and K were studied.First,the H₂O adsorption on these catalyst surfaces was calculated.It is found that it adsorbs on the same adsorption site with NH₃,and shows a linear relationship with NH₃ adsorption energy.Moreover,the formation of H₂O in the NH₃-SCR reaction also needs much heat for desorption so that H₂O can inhibit the NH₃-SCR reaction.The SO₂ adsorption on these catalyst surfaces was also studied.It is found that there are four different adsorption sites,and the most stable adsorption sites on different catalyst surfaces are different.SO₂ interacts with the catalyst to adsorb and cover the active sites of doping metals in the form of sulfite or sulfate,thus inhibiting the NH₃-SCR reaction.Only the SO₂ on Ag-TiO₂ does not cover the active sites of doping metals,and promotes the adsorption and activation of NH₃,which is beneficial to the NH₃-SCR reaction.The adsorption of alkali metals Na and K was studied.It is found that their adsorption does not cover the active sites of doping metals,but leads to weaker NH₃ adsorption on the catalyst surfaces and more difficult NH₃ activation.This is because the alkali metals reduce the acidity of the active sites of the doping metals and the activity of the surface active oxygen.However,alkali metals can promote the NH₃-SCR reaction over the rare earth metal Ce doped TiO₂ catalyst.It is predicted that Ce can be used to modify the transition metal-doped TiO₂ catalysts to improve the resistance to alkali metal poisoning.