Study On Characteristics Of Surface Acidity And Denitration Mechanism Of VPO/TiO₂ Catalyst

Nitrogen oxides（NOx）are one of the air pollutants,and selective catalytic reduction（NH3-SCR）technology is commonly used to remove them.Coal-fired flue gas denitrification has been fully implemented,and commercial V₂O₅-WO3/TiO₂ catalysts are usually used for control,while the temperature of industrial flue gas such as coke ovens and sintering is lower（100-300℃）,and the existing V₂O₅-WO₃/TiO₂ active window At 300-400℃,it cannot be directly used for medium and low temperature flue gas denitration.The development of low-temperature SCR catalysts with high activity has broad application prospects for the control of NOx emissions from industrial flue gas.The influence of the preparation conditions on the denitration activity was studied experimentally,and a series of V₂O₅/TiO₂ catalysts were prepared by the sol-gel method.The results show that when V/P=5（molar ratio）,HCl/V=3（molar ratio）,V/W=30（mass ratio）,and the calcination temperature is 450℃,the VPO/TiO₂ catalyst has the best activity.The NOx conversion rate can reach 98%at 200℃.The V₂O₅/TiO₂catalyst was characterized by XRD,BET,SEM,XPS,H₂-TPR and UV-vis methods.The research results show that V₂O₅ and（VO）2P2O7 are the main crystal phases of the catalyst,and the catalyst has a larger specific surface area and better redox performance.The pyridine infrared（PY-IR）test results show that the Lewis acid on the catalyst surface is caused by the coupling of the coordinated unsaturated vanadium（IV）ions exposed on the surface and the V=O double bond.Br（?）nsted acid is mainly produced by P-OH and V-OH groups.When V/P=5（molar ratio）,HCl/V=3（molar ratio）,V/W=30（mass ratio）,and the calcination temperature is 450℃,the maximum total acid content of the catalyst is 7300μmol·g^-1.The linear fitting results of the acid site characteristics of the catalyst and the denitrification activity show that the low-temperature denitrification activity of all V₂O₅/TiO₂ catalysts is positively correlated with the weak Lewis acid content,so they have good low-temperature denitrification activity.The denitration mechanism of the V₂O₅/TiO₂ catalyst was studied,and the results showed that the NH₃ on the catalyst surface existed in the form of NH4⁺or coordinated NH₃.NO is adsorbed on the surface of the catalyst,forming active intermediates such as N-O absorption peak,NO₂ asymmetric absorption peak,bidentate nitrate and monodentate nitrate.The V₂O₅/TiO₂ catalyst follows the L-H and E-R mechanisms at the same time,but the L-H mechanism has a faster reaction speed than the E-R mechanism,and a"fast SCR"reaction occurs at the same time.The crystal models of V₂O₅ and（VO）2P2O7 were constructed using DFT software,and the adsorption process of reactants（NO and NH3）on the catalyst surface was studied.From the analysis of adsorption energy,bond length change and Mulliken charge population analysis,it is found that NH3 is more stable than NO adsorption.The NH3 molecules on the surface of the catalyst are adsorbed on the Br（?）nsted acid sites in the form of NH4⁺,and the coordinated NH3 is adsorbed on the Lewis acid sites,forming a stable chemical adsorption.The study found that the adsorption stability at the Br（?）nsted acid sites is stronger than that of the Lewis acid.