The Microstructure Tuning Of MnO₂-based Catalysts And Their Catalytic Performance For VOCs Oxidation

Volatile organic compounds (VOCs) as major air pollutants are not onlyhazardous to human health but also harmful to the environment. It is highly desired tocontrol the emissions of VOCs for which efficient treatment technologies and newmaterials are needed. Manganese dioxide, which is inexpensive, environmentallybenign, and processes unique structural characteristics such as porous structure,mixed-valency (3+, and4+) of Mn, easy release of lattice oxygen, etc., has beenregarded as one of the most promising catalyst for VOCs catalytic combustion.However, the catalytic activity of goods MnO2is often very low, it needs to decoratethe microstructure of MnO2-based material for improving its catalytic performance.In this paper, we facilely utilize its unique chemical and physical properties todevelop several strategies such as tuning the microstructure and doping for improvingits catalytic activity. The main innovation points and results in this study can besummarized as follows：1、Giant Effect of Oxygen Vacancy Defects on the Catalytic Oxidation ofOMS-2Nanorods：OMS-2nanorod samples with different OVD concentration wereprepared by a facile method of hydrothermal redox reaction between Mn(NO3)2andKMnO4at180,90, and70oC, respectively. OMS-180possesses the lowestMn3+/Mn4+atomic ratio (1.0). When the hydrothermal reaction temperature ofKMnO4and Mn(NO3)2decreases from180to90oC, the Mn3+/Mn4+atomic ratio ofthe OMS-2nanorod sample increases from1.0to2.1(OMS-90). Further deceasing thereaction temperature to70oC leads to an increase of Mn3+/Mn4+atomic ratio to3.1(OMS-70). We demonstrate a giant OVD effect on the catalytic performance ofOMS-2. Increasing the OVD concentration considerably enhances its catalytic activityfor the oxidation of benzene. When the Mn3+/Mn4+ratio in the OMS-2nanorodsamples increases from1.0to2.1and3.1, respectively, its T90(corresponding to thebenzene conversion=90%) decreases from339oC to274oC and237oC, respectively.We combine both theoretical and experimental evidence to demonstrate a giant OVDeffect on the catalytic performance of OMS-2. The DFT calculation result suggeststhat the presence of oxygen vacancy makes the lattice oxygen adjacent to the oxygenvacancy more active. The Raman spectrum and CO-TPR analysis results demonstratethat increasing the OVD concentration considerably enhances the lattice oxygenactivity of OMS-2. 2、Tuning the K+concentration in the tunnel of OMS-2nanorods leads to asignificant enhancement of the catalytic activity for benzene Oxidation: OMS-2nanorods with tunable K+concentration were prepared by a facile hydrothermal redoxreaction among MnSO4,(NH4)2S2O8, and (NH4)2SO4at120oC by adding KNO3withdifferent KNO3/MnSO4molar ratio. We demonstrate a giant K+concentration effecton the catalytic performance of OMS-2. Increasing the K+concentration leads to aconsiderable enhancement of the catalytic activity in OMS-2nanorods for benzeneoxidation. An enormous decrease (ΔT50=89oC, ΔT90>160oC) in the reactiontemperature of T50and T90for benzene oxidation has been achieved by increasing theK+concentration in the K+doped OMS-2nanorods. We combine both theoretical andexperimental evidence to demonstrate a giant OVD effect on the catalyticperformance of OMS-2.The DFT calculation result suggests that the presence of K+located at the tunnels of OMS-2can improve its lattice oxygen activity. CO-TPRanalysis result indicates that increasing the K+concentration of the K+doped OMS-2nanorods results in the improvement of the lattice oxygen activity.3、Tremendous Effect of the Morphology of Birnessite-type ManganeseOxide Nanostructures on Catalytic Activity: The octahedral layered birnessite-typemanganese oxide (OL-1) with the morphologies of nanoflowers, nanowires, andnanosheets were prepared. The result of catalytic tests shows that the OL-1nanostructure catalyst exhibits a tremendous enhancement in the catalytic activity forbenzene oxidation. Compared to the OL-1nanosheets, the OL-1nanoflowersdemonstrate an enormous decrease (ΔT50=274°C; ΔT90>248°C) in reactiontemperatures T50and T90for benzene oxidation. We combine both theoretical andexperimental evidence to investigate the origin of the tremendous effect ofmorphology on the catalytic activity for the nanostructured OL-1, which depends onits oxygen vacancy concentration.The DFT calculation result suggests that thepresence of oxygen vacancy makes the lattice oxygen adjacent to the oxygen vacancymore active. The CO-TPR analysis results demonstrate that increasing the OVDconcentration considerably enhances the lattice oxygen activity of OL-1.4、Giant effect of Ce ions on the catalytic oxidation of Ce ions substituteOMS-2ultrathin nanorods: Ce ions substitute OMS-2(CM-120) were prepared by afacile hydrothermal redox reaction between KMnO4and Ce(NO3)3with theKMnO4/Ce(NO3)3molar ratio of3:1at120oC. CM-120with a high BET surface areaof322.1m2g-1is an assembly of ultrathin nanorods,~56nm in length,1~4nm in diameter. The XRD, ICP/OES, XPS, and EXAFS characterization results demonstratethat the structure of the obtained sample is Ce ions substitute K+ions in the tunnel ofOMS-2. We demonstrate that Ce ions substitute K+ions in the tunnel of OMS-2canconsiderable enhance its catalytic activity. Compared to pure OMS-2nanorods(OMS-180), T50and T90of CM-120decrease from269oC and339oC to169oC and210oC, respectively. CM-120also exhibits high catalytic activity for COoxidation. An enormous decrease (ΔT50=104oC, ΔT90=126oC) in the reactiontemperature of T50and T90for CO oxidation has been achieved. We combine boththeoretical and experimental evidence to demonstrate a giant Ce ions effect on thecatalytic performance of OMS-2. The DFT calculation result suggests that Ce ionssubstitute K+ions in the tunnel of OMS-2can improve its lattice oxygen activity.CO-TPR analysis result indicates that Ce ions substitute K+ions in the tunnel ofOMS-2results in the improvement of the lattice oxygen activity.5、Ce ions substitute OMS-2ultrathin nanorods with a conceptually newmechanism exhibit extremely high efficiency full solar spectrum, visible, andinfrared light driven catalytic activity: UV-Vis-IR spectra test suggests Ce ionssubstitute OMS-2ultrathin nanorods(CM-120) show a strong absorption band in theentire solar spectrum region of200~2600nm，which can efficiently transform theabsorbed UV, visible, and infrared light energy to thermal energy. CM-120exhibithigh efficiincey full solar spectrum for VOCs such as benzene, toluene, and acetoneand CO oxidation under the irradiation of a500W Xe lamp. The rCO2of CM-120is30.8and25.7times higher than that of TiO2(P25)(a widely used benchmarkphotocatalyst) and Bi2WO6/TiO2(a near-infrared photocatalyst very recently reported),respectively. The recycled tests reveal that CM-120did not exhibit any reduction ofits catalytic activity after40catalytic cycles, indicating an excellent catalytic stability.When the irradiation with wavelength below420,480,560, and690nm from the Xelamp is filtered out using different cutoff filters, CM-120exhibits efficient visible andinfrared light driven catalytic activity for benezen，toluene, acetone, and CO oxidation.The efficiently absorb the visible and infrared solar energy and the high catalyticactivity of CM-120are the two major reasons that lead to the high full spectrum lightdriven catalytic activity for VOCs and CO oxidation.