We have developed a novel and facile approach of hydrothermal redox reaction to prepare cryptomelane-type octahedral molecular sieve (OMS-2) nanorods with tunable concentration of oxygen vacancy defects (OVDs). We demonstrate a giant OVD effect on the catalytic performance of OMS-2. Increasing the OVD concentration considerably enhances the lattice oxygen reactivity, thus tremendously promoting the catalytic activity for the oxidation of benzene.
OMS-2 nanorods with tunable K(+) concentration were prepared by a facile hydrothermal redox reaction of MnSO4, (NH4)2S2O8, and (NH4)2SO4 at 120 °C by adding KNO3 at different KNO3/MnSO4 molar ratios. The OMS-2 nanorod catalysts are characterized by X-ray diffraction, transmission electron microscopy, N2 adsorption and desorption, inductively coupled plasma, and X-ray photoelectron spectrometry. The effect of K(+) concentration on the lattice oxygen activity of OMS-2 is theoretically and experimentally studied by density functional theory calculations and CO temperature-programmed reduction. The results show that increasing the K(+) concentration leads to a considerable enhancement of the lattice oxygen activity in OMS-2 nanorods. An enormous decrease (ΔT50 = 89 °C; ΔT90 > 160 °C) in reaction temperatures T50 and T90 (corresponding to 50 and 90% benzene conversion, respectively) for benzene oxidation has been achieved by increasing the K(+) concentration in the K(+)-doped OMS-2 nanorods due to the considerable enhancement of the lattice oxygen activity.
The octahedral layered birnessite-type manganese oxide (OL-1) with the morphologies of nanoflowers, nanowires, and nanosheets were prepared and characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric/differential scanning calorimetry (TG/DSC), Brunnauer-Emmett-Teller (BET), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS). The OL-1 nanoflowers possess the highest concentration of oxygen vacancies or Mn(3+), followed by the OL-1 nanowires and nanosheets. The result of catalytic tests shows that the OL-1 nanoflowers exhibit a tremendous enhancement in the catalytic activity for benzene oxidation as compared to the OL-1 nanowires and nanosheets. Compared to the OL-1 nanosheets, the OL-1 nanoflowers demonstrate an enormous decrease (ΔT(50) = 274 °C; ΔT(90) > 248 °C) in reaction temperatures T50 and T90 (corresponding to 50 and 90% benzene conversion, respectively) for benzene oxidation. The origin of the tremendous effect of morphology on the catalytic activity for the nanostructured OL-1 catalysts is experimentally and theoretically studied via CO temperature-programmed reduction (CO-TPR) and density functional theory (DFT) calculation. The tremendous catalytic enhancement of the OL-1 nanoflowers compared to the OL-1 nanowires and nanosheets is attributed to their highest surface area as well as their highest lattice oxygen reactivity due to their higher concentration of oxygen vacancies or Mn(3+), thus tremendously improving the catalytic activity for the benzene oxidation.
Anatase TiO2 nanocrystals with internal pores are prepared by a novel facile microwave-assisted hydrolysis of a mixture of TiOCl2 and HF aqueous solutions, followed by calcination at 400 °C. The TiO2 nanocrystals with internal pores are characterized by XRD, TEM, SEM, BET, EDS, and XPS. The formation mechanism of the TiO2 nanocrystals with internal pores is discussed by investigating the role of fluorine and the calcination. The photocatalytic measurement shows that the TiO2 nanocrystals with internal pores exhibit much higher photocatalytic activity for the photodegradation of crystal violet, methyl orange, and 4-chlorophenol than the TiO2 solid nanocrystals. The photocatalytic enhancement is due to the fluorination of TiO2 nanocrystals as well as its unique hollow nanostructure, which results in the higher separation efficiency of photogenerated electrons and holes in the TiO2 nanocrystals with internal pores than in its solid counterpart.
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