Investigation On Performance And Mechanism Of Selective Catalytic Oxidation Of NH Over Pt-V/TiO₂ Catalysts

The adoption of"Green ammonia（NH₃）"as a fuel for marine engines has been widely recognized as one of the efficient approaches for reducing carbon emissions from shipping industry.However,there was NH₃ in the exhaust gas of NH₃-fuel engines due to the incomplete combustion of NH₃ fuel,which will cause serious harm to human health and ecological environment.The global shipping industry is in urgent need of an efficient and economical technique for removing slipped NH₃ in the exhaust gas of NH₃-fuel engines,so as to promote the application of"Green ammonia"fuel in shipping field.Compared with other NH₃ removal techniques,selective catalytic oxidation（SCO）technique is more suitable for removing NH₃ in high-temperature exhaust gas.As a typical noble-transition-metal composite catalyst,Pt-V/TiO₂ catalyst exhibits good NH₃ removal performance and has become a research hotspot in NH₃-SCO field.But there is little research on the performance of Pt-V/TiO₂ catalysts for removing high-concentrations NH₃ in the exhaust gas of NH₃-fuel engines,and the relevant reaction mechanism is unclear.In this work,a series of Pt-V/TiO₂catalysts were synthesized via impregnation method,and the synergistic effect between Pt and V,as well as the influence of transition metal and rare earth element modification on NH₃-SCO performance of Pt-V/TiO₂ catalyst were investigated systematically.The NH₃-SCO mechanism modified Pt-V/TiO₂ catalysts was revealed by characterization tests and density functional theory（DFT）calculation.The main research content and conclusions of this work are as follows:（1）The effects of key factors such as the loading amounts of Pt and V,as well as the Pt-V impregnation sequence,on the performance of Pt-V/TiO₂ catalysts were investigated,and the synergistic catalytic mechanism of Pt-V on the surface of TiO₂ was explored.The results showed that with the Pt doping amount increasing from 0.02 wt.%to 0.32 wt.%,the minimum temperature corresponding to 100%NH₃ conversion efficiency obtained by Pt_xV_0.5/TiO₂ catalyst（0.5 wt.%V）decreased from 330oC to 230oC.With V loading amount increasing from 0.25 wt.%to 3 wt.%,the N₂ selectivity of Pt_0.04Vy/TiO₂ catalyst（0.04 wt.%Pt）increased at first and then decreased in the range of 230 to 450oC.The NH₃-SCO activities of Pt-V/TiO₂ catalysts with different Pt-V impregnation sequences are ranked from high to low as V_0.5/Pt_0.04/TiO₂>Pt_0.04/V_0.5/TiO₂>Pt_0.04V_0.5/TiO₂.When the inlet NH₃concentration was 5000 ppm and the space velocity was 60000 h^-1,the NH₃ conversion efficiency obtained by V_0.5/Pt_0.04/TiO₂ catalyst in the range of 225-450oC was 100%,and the corresponding N₂ selectivity was higher than 49%.The Pt and V species on the surface of Pt-V/TiO₂ catalysts prepared through two-step impregnation method exist in the form of nanoclusters.The main reactions on the surfaces of Pt and V nanoclusters are NH₃ catalytic oxidation and NH₃ selective catalytic reduction（NH₃-SCR）reactions,respectively.There is a synergistic effect between above two reactions.The adsorbed NH₃ on catalyst surface will be first oxidized to NO_x and then reduced to N₂,which was the internal selective catalytic reduction（i-SCR）mechanism.This synergistic effect is closely related to the valence states of Pt and V species.There are abundant Pt⁰ and V³⁺species on the surface of V_0.5/Pt_0.04/TiO₂catalyst.Pt⁰ and V³⁺species possess high NH₃-SCO activity and NH₃-SCR activity,respectively,which is conducive to remove NH₃ via synergistic catalytic reactions.（2）A comparative study was conducted on the influence of several common transition metals（Cu,Nb,W,Mn,Fe,Ni,Cr）on NH₃-SCO performance of V_0.5/Pt_0.04/TiO₂ catalyst.The results showed that compared with other transition-metal modified Pt-V/TiO₂ catalysts,Cu modified Pt-V/TiO₂ catalyst possessed better NH₃-SCO performance.The activity test results of catalysts with different Cu loading amounts indicated that,when the inlet NH₃concentration was 5000 ppm and the space velocity was 60000 h^-1,the N₂ selectivity of Cu_0.5V_0.5/Pt_0.04/TiO₂ catalyst in the range of 225-375oC was higher than 80%.The enhancement mechanism of Cu modification on NH₃-SCO performance of V_0.5/Pt_0.04/TiO₂catalyst has been studied.The introduction of Cu results in a hybridization between Cu,V,and O atoms on catalyst surface,thereby decreasing the bonding strength between O atoms and surrounding V and Cu atoms.This not only facilitates O atoms to break away from the lattice and result in the generation of oxygen vacancies,but also conducive to the formation of V⁵⁺/V⁴⁺and Cu²⁺/Cu⁺redox pairs,thereby improving the redox performance of catalyst.There is an efficient synergistic catalytic effect between Cu₂V₂O₇ and Pt species on the surface of catalyst,which can convert NH₃ into N₂ and H₂O through i-SCR mechanism.（3）The influence of several common rare-earth elements（Er,Ce,Pr,La,Ho,Gd,Nd,Sm,Dy,Tb）on NH₃-SCO performance of V_0.5/Pt_0.04/TiO₂ catalyst was investigated.The results indicated that compared with other rare-earth elements modified V_0.5/Pt_0.04/TiO₂catalysts,Er modified V_0.5/Pt_0.04/TiO₂ catalyst exhibited better NH₃-SCO performance.The activity test results of catalysts with different Er loading amounts indicated that,when the inlet NH₃ concentration was 5000 ppm and the space velocity was 60000 h^-1,Er_0.5V_0.5/Pt_0.04/TiO₂ catalyst achieved>80%N₂ selectivity in a wide temperature range（225-450oC）.The NH₃-SCO performance and reaction mechanism of Er modified V_0.5/Pt_0.04/TiO₂ catalyst have been studied with emphasis.It was found that Er modification leads to a significantly change in the atomic arrangement and electron distribution in crystals on catalyst surface.This is conducive to the formation of more lattice defects and more V sites with unsaturated coordination,thereby increasing the amounts of Lewis acid sites and Br?nsted acid sites.Besides,the introduction of Er also significantly improved the adsorption energy of NH₃ on V sites,which is beneficial to enhance the adsorption and activation of NH₃,thereby improving the NH₃-SCO performance.（4）The effects of Cu and Er co-modification on NH₃-SCO performance of V_0.5/Pt_0.04/TiO₂ catalyst were evaluated,and the relevant reaction mechanism was investigated.The results indicated that Cu-Er co-modification could significantly enhance the NH₃-SCO performance of V_0.5/Pt_0.04/TiO₂ catalyst.The activity test results of catalysts with different loading amounts of Cu-Er showed that the N₂ selectivity of Cu_0.35Er_0.15V_0.5/Pt_0.04/TiO₂ catalyst was higher than 90%in a wide temperature range of 225-450oC,which is significantly superior to the majority of existing NH₃-SCO catalysts.The introduction of Cu-Er significantly increases the amounts of Lewis acid sites on catalyst surface and enhances the redox performance,thereby improving NH₃-SCO performance of V_0.5/Pt_0.04/TiO₂ catalysts from the aspects of NH₃ adsorption and NH₃ catalytic conversion.The introduction of Cu-Er significantly decreases the energy barrier of the reaction between-NH₂ species and bidentate nitrate species from 84.0 k J/mol to 55.3 k J/mol,which is conducive to the generation of NH₂NO₃ species.This was beneficial to efficiently convert NH₃ into N₂ and H₂O.