The Performance Of Copper-niobium Doped TiO₂ Composite Oxide For Selective Catalytic Reduction Of NO_x And The Mechanism Of Alkali Resistance

A mass of nitrogen oxides(NOx)in the harmful flue gas are produced by the coal-fired power plants and seriously threaten the environmental and human health.At present,NH3 selective catalytic reduction of NOx(NH3-SCR)is the most widely used and muature denitration technology and the core is developing efficient catalyst.Based on the SCR denitrification of coal-fied power plant in stationary emission and the feature of traditional V2Os-WO3/TiO2 catalyst that a narrow reaction temperature window and a worse alkali resistance,the novel environmental friendly catalyst with excellent activity and alkali resistance was developed.On the basis of self-synthesized TiO2 support,the ternary oxide catalyst was successfully prepared through bimetallic bulk doping and surface loading,which would provide innovative ideas and technical basis for the development of new denitration catalysts.Nb element was selected to modify TiO2 support and then Cu was doped to construct the catalytic system.The optimized catalyst was composed of 0.8%Cu/Ti2NbOx,which showed above 90%NOx conversion and 96%N2 selectivity within the the temperature range of 250-425? under a GHSV of 177,000 h'1.Meanwhile,the catalyst exhibited a good resistance to H2O,SO2 and hydrothermal stability.The insufficient active sites over the support surface and poor reducibility of the catalyst were solved by Nb modification and Cu dopant,respectively.Nb2O5 could promote anatanse TiO2 crystal nucleation and inhibite the surface agglomeration,which would increase the specific surface area and provide abundant binding sites for the active sites.Meanwhile,the acid sites and chemical active oxygen species on the surface of support were efficiently enhanced,there were adequate reaction sites for the reactant adsorbing and activating.Cu mainly existed as isolated Cu2+ and non-isolated Cu+ on the surface Ti=O sites of Ti2NbOx.Cu doping led to a wider low and medium active temperature window.0.8%Cu/Ti2NbOx had a high catalytic efficiency under the synergistic effects between Nb and Cu.In situ DRIFTS technique was employed to study the reaction routes and mechanisms over the surface of catalyst.For the process of adsorption and activation,O2 did not have any effects on NH3 adsorption while it would promote NOx transforming into chelate monodentate and bidentate nitrate species,inducing the occurrence of SCR reaction at 175?.When NH3 and NOx coexisted over the catalyst surface,NOx first formed the bridge nitrate before NH3 adsorption.In the low temperature(200?),SCR reaction mainly followed Langmuir-Hinshelwood(L-H)route.With the temperature increasing,Eley-Rideal(E-R)reaction path was gradually strengthened,then it became the dominating reaction mechanism at high temperature(400?)as the extremely weak adsorption of NOx.Besides,0.8%Cu/Ti2NbOx solved the problem of alkali/alkaline earth poisoning and deactivating the catalyst under the high temperature and high dust distribution of industrial denitration devices at fired-power plant.2%K2O could fully deactivate traditional V2O5-WO3/TiO2 catalyst while 0.8%Cu/Ti2NbOx still maintained 78%activity(no vapour in the simulation reaction mixture gas),showing a well alkali resistance.K2O aggravated the agglomeration phenomenon on the catalyst surface,depositing in the micro pores of V2O5-WO3/TiO2 catalyst,blocking the pore structure,greatly reducing the active sites.However,K2O mainly existed on 0.8%Cu/Ti2NbOx surface and did not destroy the internal pore structure.The experiments demonstrated 2%K2O neutralized nearly all the surface acid sites of V2O5-WO3/TiO2 catalyst but only deceased partial acid sites on 0.8%Cu/Ti2NbOx catalyst.Both the acid sites losing and reducibility decreasing made the traditional V2O5-WO3/TiO2 catalyst deactivation.The amount of isolated Cu2+and chemical active oxygen for 0.8%Cu/Ti2NbOx did not decrease obviously after K2O poisoning.Combined the characterization experiments and molecular simulation results,the mechanism of "directed capture" was proposed that K atoms preferred to coordinate with Nb=O and Nb-OH with a bonding energy of-2.83 eV and-2.33 eV on Ti2NbOx support,forming KNbO3 species,which efficiently avoided the toxic effect of K on Cu.Therefore,the synergistic effect of Nb and Cu still worked in the process of anti-alkali.As the activity of 0.8%Cu/Ti2NbOx catalyst partially reduced after 2%K2O poisoning,the traditional regeneration method was investigated and the new regeneration scheme was proposed.At present,sulfuric acid solution was used in the industrial regeneration process.But it could easily lead to equipment corrosion and secondary pollution ect.This research discovered C2H2O4 and H6O39SiW12 with 0.5%and 1%concentration could realize the same regeneration efficiency with H2SO4.And the regenerated 0.8%Cu/Ti2NbOx catalyst recovered above 90%NOx conversion and 98%N2 selectivity among 300-350? with a well H2O and SO2 resistance.The crystal structure was not changed while the amout of surface chemical active oxygen and the redox cycle of copper species enhanced after regeneration.Also,the structure of KNbO3 was broken and most K species were washed out.Nb species were restored to Nb2O5.The acid washing regeneration process mainly increased the quality and strength of Br(?)nsted acid sites,promoting the formation of active intermediate—NH2 species when in the process of NH3 adsorption and activation,thereby strengthening the SCR activity.