Noble Metal Supported On Manganese Dioxied Catalysts And Their Application For Low Temperature CO-SCR

CO selective catalytic reduction?CO-SCR?is considered to be one of the most effective techniques for removing nitrogen oxide?NO_x?from both stationary and mobile sources,of which the synthesis of catalysts with high-efficiency remain a main issue.Currently,commercialized catalysts usually show an optimum performance in the temperature range of 300?⁴00?,with a narrow active temperature area,which is not suitable for the low temperature?<300??SCR process.A high-energy gas preheating device is thus required to achieve the above catalytic condition,raising the emerging for a low-temperature and efficient CO-SCR catalyst.Manganese oxides show a variety of valence(Mn²⁺,Mn³⁺,Mn⁴⁺),excellent redox ability and abundant reactive oxygen species?vacancy oxygen and adsorbed oxygen?,which imply their potential in low-temperature CO-SCR catalysis.The loading of noble metal on the surface of MnO₂leads to the hybridization of the N and O orbitals with the half-filled d-band of the transition metallic,resulting in the antibonding orbital shift above the Fermi level.This reduces the NO dissociation energy barrier and greatly improves the low-temperature catalytic performance of noble metal decorated manganese oxides.In this paper,a series of?-MnO₂ with different morphology were synthesized by the hydrothermal method.The precious metals?Pd,Pt,Au?were loaded on MnO₂ by an impregnation,reduction and precipitation involved process.The catalytic activity at low temperature was evaluated by CO-SCR reaction and the corresponding catalytic regular pattern was researched.The morphology of?-MnO₂ showed an influence on its low-temperature CO-SCR performance.Among which,?-MnO₂ nanotube exhibited the highest activity for NO_x reduction,followed by?-MnO₂ nanorod and nanoflower.The main reason is that the MnO₂ nanotube contains the numerous?200?crystal face,which have the most reactive oxygen species and the most high valence of Mn ions(Mn³⁺and Mn⁴⁺).The specific surface area and pore structure were not the dominant factors affecting SCR reaction,while was determined by the type and content of active crystal face.Then,?-MnO₂ nanotube/noble-metal composites were synthesized with varied methods.Pd-MnO₂ composite with 1 mass%Pd content synthesized by the dipping method exhibited a better low-temperature catalytic activity and thermal stability than that of 1%Pt/MnO₂,1%Au/MnO₂,commercialized PdO and PtO catalysts.Catalytic activity starts at 100?,and the NO conversion rate is close to 100%at 239?.The conversion rate of NO stablized at 95%even after continuous usage for 30 hours at300? and 500? respectively.The cause is that Pd particles have high dispersion and specific surface area.It is exposed more active crystal faces and active sites of high-valence Mn ions.Meanwhile,the synergistic effect between the Pd nanoparticles and?-MnO₂ affects the content of reactive oxygen species in the catalyst.This promotes the conversion of Pd to high actived Pd²⁺resulting in an enhanced redox ability of Pd/MnO₂ composite.Finally,in order to further optimize the low-temperature SCR performance of MnO₂ catalysts,a series of Pd/MnO₂ catalysts with different incorporation of Pd nanoparticles were synthesized.Our results showed that Pd/MnO₂ composite with 1mass%Pd dosage exhibited the highest activity.Pd nanoparticles highly dispersed on the surface of the nanotube,which increases the ratio of high-valence Mn ions and reactive oxygen species.This increases the amount of active sites and enhances the redox ability of the Pd/MnO₂ catalyst.