Volatile organic compounds (VOCs) are both harmful to human health and the environment; however, catalytic combustion offers a promising method for VOC purification because of its high efficiency without secondary pollution. Although manganese-based catalysts have been well studied for VOC catalytic oxidation, their catalytic activity at low temperature must be improved. Alkali metals as promoters have the potential to modulate the electronic and structural properties of the catalysts, improving their catalytic activity. Herein, a Ce0.65Zr0.35O2 support was prepared by co-precipitation and MnOx/Ce0.65Zr0.35O2 catalysts were obtained through the incipient-wetness impregnation method. The catalytic properties of K-modified MnOx/Ce0.65Zr0.35O2 for toluene oxidation with different molar ratios of K/Mn were investigated. In addition, the catalysts were characterized by XRD, UV/visible Raman, Hydrogen temperature program reduction (H2-TPR), Oxygen temperature programmed desorption (O2-TPD), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance FTIR spectroscopy (DRIFTS) experiments. The results showed that alkali metal doping with K significantly improved the catalytic activity. In particular, when the molar ratio of K/Mn was 0.2, the monolith catalyst Mn/Ce0.65Zr0.35O2-K-0.2 exhibited the best performance with the lowest complete conversion temperature T90 of 242 °C at a GHSV of 12000 h −1 . The XRD results suggested that MnOx was uniformly distributed on the surface of the catalyst and that Mn 4+ partially reduced to Mn 3+ on the addition of K. The Raman spectrum demonstrated that with increasing K content, both the β-and α-MnO2 phases coexisted on the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst, increasing the number of surface defect sites. The H2-TPR experiment results confirmed that Mn/Ce0.65Zr0.35O2-K-0.2 exhibited the lowest reduction temperature and good reducibility. From the O2-TPD experiments, it was clear that Mn/Ce0.65Zr0.35O2-K-0.2 contained the most surface adsorbed oxygen species and excellent lattice oxygen mobility, which benefitted the toluene oxidation activity. In addition, the XPS results suggested that the content of surface adsorbed oxygen species of the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst was the highest among all the tested samples. In addition, toluene-TPSR in N2 as measured by in situ DRIFTs analysis demonstrated that available lattice oxygen was present in the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst. Therefore, the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst exhibited the best redox properties and oxygen mobility of the prepared samples and showed excellent activity toward toluene oxidation. Therefore, it was concluded that the addition of an appropriate amount of K improved the redox performance of the catalyst and increased the number of surface defect sites and mobility of the lattice oxygen of the catalyst as well as the concentration of the surface active oxygen species, thereby significantly improving catalytic ability.
SnO2 nanocrystal and rare-earth Eu3+ ion co-doped SiO2 thin films are prepared by sol-gel and spin coating methods. The formation of tetragonal rutile structure SnO2 nanocrystals with a uniform distribution is confirmed by X-ray diffraction and transmission electron microscopy. Fourier transform infrared spectroscopy is used to investigate the densities of the hydroxyl groups, and it is found that the emission intensity from the 5D0−7F2 transitions of the Eu3+ ions is enhanced by two orders of magnitude due to energy transfer from the oxygen-vacancy-related defects of the SnO2 nanocrystals to nearby Eu3+ ions. The influences of the amounts of Sn and the post-annealing temperatures are systematically evaluated to further understand the mechanism of energy transfer. The luminescence intensity ratio of Eu3+ ions from electric dipole transition and magnetic dipole transition indicate the different probable locations of Eu3+ ions in the sol—gel thin film, which are further discussed based on temperature-dependent photoluminescence measurements.
A series of Cu-ZSM-5 catalysts with different Cu loadings were prepared by an incipient wetness impregnation method. These catalysts were used for studies of selective catalytic reduction (SCR) of NO by NH3. The results showed that Cu-ZSM-5 with 5% Cu loading showed the best catalytic activity, the conversion of NO was more than 80% over the temperature range 198-470°C, and the highest NO conversion was 96.5%. The SCR activity was only influenced slightly by the addition of H2O or SO2. Visible diffraction of the CuO phase was observed when the Cu content was above 5%. The results of steady-state kinetics studies indicated that the SCR reaction over Cu-ZSM-5 with 5% Cu loading was zero-order with respect to NH3, first-order with respect to NO, and nearly half-order with respect to O2. The apparent activation energy for the reaction was found to be 47.7 kJ•mol-1 .
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