Preparation,Characterization And Catalytic Property Of MnO₂ With Birnessite Crystalline Nanostructure

How to efficiently apply the sun energy to environmental purification has been a hot spot among scientists in the field of catalytic research.Among the reported papers up to now,most catalysts have only used the ultraviolet and visible light that account for 55%in the whole sun energy to activate,that is to say,catalysts'sun utilization efficiency is far away from enough.Therefore,to prepare a catalyst that can efficiently be driven by the full solar spectrum is of great significance.This paper deeply researched the preparation,characterization,mechanism and novel photoactivation effect of full solar spectrum driven nanostructured birnessite on the one hand.On the other hand,we try to improve the catalytic activity of nanostructured birnessite by doping V⁵⁺.The main research results are as follows:1.The OL-1 nanoflower sample?denoted as OL-NF?was prepared by a facile method of hydrothermal redox reaction between KMnO₄ and Mn?NO₃?₂ in the presence of HNO₃ at 50 ^oC for 72h.The OL-1 nanoflower sample exhibited catalytic activity with very high efficiency for CO oxidation as model reaction under the irradiation of the full solar spectrum,visible-infrared,and infrared light.Based on both the experimental and theoretical evidence,we revealed that the highly efficient catalytic activity with the solar light irradiation not only orignated from solar light driven thermocatalysis,but also was considerably enhanced by a conceptually novel photoactivation effect:Mn ions adsorb photons,leading to d-d transition;MnO₂transfers from ground state to excited state,thus weakening the Mn-O bond and making the lattice oxygen become more active,leading to a siganicant increase in catalytic activity.To prove the existence of photoactivation effect,we found that OL-NF sample exhibited a better thermocatalytic activity with full solar spectrum irradiation than without irradiation under a same reaction temerature.We measured the CO-TPR of OL-NF with and without the full solar spectrum irradiation,indicating irradiation can not only make the lattice oxygen become more active but also produce more number of the lattice oxygen.At the same time,we discoverd the catalytic activity of OL-NF under full solar irradiation increase with light intensity increasing,which was in good agreement to the says that photoactivation rate depends on light intensity.Through Density functional theory?DFT?,we found that the energy of the reduction of the OL-1 super cell(K₈Mn₃₂O₆₄)in the ground state by a CO molecule was 0.11 eV.The energy of the reduction of the OL-1 super cell(K₈Mn₃₂O₆₄)in the excited state by a CO moleculewas-7.39 eV,which was much lower than that of the reduction of the OL-1 super cell in the ground state.This DFT calculation result indicated that the solar light irradiation significantly improved the activity of the lattice oxygen in OL-1.2.With KMnO₄ serving as oxidant,Mn?NO₃?₂ serving as reducing agent,and VOSO₄ serving as the source of V ions,MnO₂ catalysts were prepared at 90 ^oC for24h with different V ions doping amount?atomic mole ratio of V/Mn is 0,1:64,1:32,1:16,1:8,respectively?.The as-prepared precipitates were denoted as OMS-2,V-64MnO₂,V-32MnO₂,V-16MnO₂,V-8MnO₂,respectively.The results of XRD and BET indicated BET surface area of samples depended on its crystalline structure.OMS-2 sample showed a pure cryptomelane,thus leading to a very small surface area,such as 35.5 m² g^-1.V-32MnO₂,V-16MnO₂,V-8MnO₂ samples with many doped V ions showed a pure birnessite structure,thus leading to a large surface area,such as350 m² g^-1.V-64MnO₂ sample with a few doped V ions showed a mixed structure consisting of cryptomelane and birnessite both,thus leading to a surface area between35.5 m² g^-1 and 350 m² g^-1.The result of CO-TPR indicated the existence of V did not increace the activity of lattice oxygen of nanostructured birnessite.The thermal catalytic activity of all samples for CO oxidation at different temperature is measured,which showed that doping V ions in nanostructured birnessite could not increase its catalytic activity.