Study On Low Temperature De-NO_x Performance And Reaction Mechanism Of Supported Vanadium Phosphorus Oxygen Catalyst

At present,the main sources of nitrogen oxides?NO_x?emission in China are coal-fired flue gas and non-electric industry flue gas?coke oven,sintering,etc.?.Selective Catalytic Reduction?SCR?denitrification is the most effective technology to control the emission of NO_x.At present,this technology has been fully implemented in the denitrification of coal-fired flue gas in China.Because of the low temperature of flue gas?140²40??,SO₂and water vapor contained in the flue gas will poison the catalysts,which makes the technology used in non-electric industries such as coke oven and sintering.The application of gas denitrification is hindered.Therefore,in view of the deficiencies of low temperature denitrification catalyst in anti-sulfur/water performance,a kind of oxygen-containing polyacid VPO with specific acidity and oxidation-reduction ability as active component and large specific surface area TiO₂ as carrier was prepared based on enhancing the surface acidity of catalyst and increasing the number of exposure sites of catalyst to inhibit the adsorption of acid gas SO₂ and water vapor.Supported low-temperature denitrification catalyst VPO-TiO₂ was prepared.The low-temperature denitrification activity,physical and chemical properties,denitrification reaction mechanism,anti-sulfur/water and heterogeneous catalytic reaction kinetics of VPO-TiO₂ catalyst were studied and analyzed.VPO-TiO₂ catalyst was prepared by organic phase-impregnation method.The denitrification activity of VPO-TiO₂ catalyst under different preparation conditions was investigated.It was found that the denitrification activity of 10%V?5?PO-TiO₂ catalyst was the best when the molar ratio of V to P was 5,the loading of VPO was 10%and the calcination temperature was 400?.Under the experimental conditions of imported NO concentration of 500 ppm,NH₃/NO of 1,volume concentration of O₂ of 8 vol.%and space velocity of 15 000 h^-1,the highest conversion of NO_x was 98.2%at 180?.In addition,the physical-chemical properties of VPO-TiO₂ catalysts were studied by means of BET,XRD,SEM,XPS,H₂-TPR and NH₃-IR.The results show that the surface of 10%V?5?PO-TiO₂catalyst has high mesoporous aggregation,high crystallinity,good particle dispersion and uniform distribution.In addition,more redox electron pairs(V⁴⁺/V⁵⁺)can be formed on the surface of the catalyst,which promotes the occurrence of"fast SCR"reaction.More chemically adsorbed oxygen species on the surface of the catalyst can promote the conversion of NO_x.The surface of the catalyst has strong Br?nsted acidity.The site is favorable for NH₃ adsorption,and the total reduction peak of the catalyst shifts toward low temperature,showing a good ability of denitrification at low temperature.The adsorption behavior of NO,NH₃ and O₂ on the surface of 10%V?5?PO-TiO₂ catalyst was characterized by in situ diffuse reflectance infrared spectroscopy.There are two kinds of NH₃ adsorbing species on the catalyst surface,one is the coordination NH₃ species on the Lewis acid site,the other is the NH₄⁺species on the Br?nsted acid site,and the adsorption stability of NH₃ on the Br?nsted acid site on the catalyst surface is greater than that of Lewis acid.Bridged nitrate species are the main active species in NH₃-SCR reaction.O₂ promotes the adsorption products of NO on the surface of 10%V?5?PO-TiO₂ catalyst.In the denitrification reaction of catalyst,L-H reaction mechanism and E-R reaction mechanism play a synergistic role.With the aid of density functional simulation,V₂O₉H₈ and V₂P₂O₁₅H₁₂ cluster models were constructed to simulate the surface active crystalline phase of catalysts.NH₃ and NO molecules were adsorbed at different active sites of the surface active crystalline phase of catalysts.It was concluded that NO could not be adsorbed and transformed on the active crystalline phase V₂O₅ of catalysts,and could be adsorbed and transformed on the O2 and O3 sites of the active crystalline phase?VO?₂P₂O₇ of catalysts,but not on the O1 site.The Br?nsted acid sites?V-OH?and Lewis acid sites?V=O?of NH₃molecules adsorbed on the active crystalline phase V₂O₅ and?VO?₂P₂O₇ of the catalyst,and the adsorption energy of the Br?nsted acid sites was the largest and the adsorption stability was good.The denitrification reaction on the active phase V₂O₅ of 10%V?5?PO-TiO₂catalyst only follows the E-R reaction mechanism,while the denitrification reaction mechanism of the active phase?VO?₂P₂O₇ follows both E-R and L-H denitrification reaction mechanism.The anti-poisoning performance of 10%V?5?PO-TiO₂ catalyst was investigated.When the concentration of SO₂ was 200⁶00 ppm and the volume concentration of steam was 4 vol.%,the denitrification rate of the catalyst was 98%,and the denitrification rate did not decrease within 450 minutes.FT-IR characterization showed that there was no sulfate peak formation on the surface of the catalyst,indicating that the catalyst had strong anti-sulfur and anti-steam properties.The effect of process parameters on the denitrification performance of10%V?5?PO-TiO₂ catalyst was investigated.Imported NO concentration?200-800 ppm?and oxygen content?4vol.%-10vol.%?had no significant effect on the denitrification performance of 10%V?5?PO-TiO₂ catalyst;the denitrification rate of the catalyst remained unchanged when GHSV was in the range of 1000-30000 h^-1,and decreased when GHSV was increased to 35000 h^-1;the denitrification activity of the catalyst was stable when NH₃ value was in the range of 0.8-1.4,and the denitrification rate was obvious when NH₃ value was less than 0.8 markedly decreased.The heterogeneous catalytic reaction kinetics of 10%V?5?PO-TiO₂ catalyst catalyst was studied.,the reaction orders of imported NH₃,NO and O₂ concentration were 1.000,1.016and 0.067,respectively.The influence of internal and external diffusion on the catalytic reduction of NO by 10%V?5?PO-TiO₂ catalyst could not be considered.