Thin layers of pure TiO
2 and TiO
2 doped by different amounts of Fe
2
O
3 have been prepared by the sol–gel method with tetraisopropyl orthotitanate and Fe(NO
3)3. Physico-chemical properties of catalysts were characterized by BET Adsorption, x-ray Diffraction (XRD), FE-SEM, as well as Raman and UV-Vis spectroscopy. The photocatalytic activity of the obtained materials was investigated in the reaction of complete oxidation of p-xylene in gas phase under the radiation of UV (λ=365 nm) and LED (λ=470 nm) lamps. It has been found that the particle size of all samples was distributed in the range 20–30 nm. The content of the rutile phase in Fe-doped TiO
2 samples varied in the range 6.8 to 41.8% depending on the Fe content. Iron oxide doped into TiO
2 enables the photon absorbing zone of TiO
2 to extend from UV towards visible waves as well as to reduce its band gap energy from 3.2 to 2.67 eV. Photocatalytic activities of the TiO
2 samples modified by Fe
3+ have been found to be higher than those of pure TiO
2 by about 2.5 times.
In this investigation, the thin film of degussa P25 was obtained by dip coating method and calcined at 450 °C for 2 h (P25-450-2) and used as photocatalyst for gas-phase photooxidation of xylene. The physico-chemical properties of calcined P25-450-2 powder was studied by the methods of BET adsorption, XRD, FTIR, UV–vis, Raman spectroscopies, SEM, TEM, carbon dioxide temperature-programmed desorption . The thickness of the film was determined on the Alpha Step IQ KLA—Ctencor equipment and the point of zero charge (PZC) of the sample was determined by salt addition method. P25-450-2, having a band gap of 3.155 eV, is advisable to use UV lamps in photocatalytic reactions. The kinetics of gas-phase photooxidation of xylene reaction on the thin films of P25-450-2 under UV illumination was studied using a gradientless flow circulating system at atmospheric pressure and 40 °C. The obtained results showed that the kinetics of the given reaction should be written by fractional equations describing the dependence of the reaction rate on the concentration of adsorbed molecules of xylene and oxygen, dissociative adsorbed water vapor, and also on the total intensity of light. The reaction was proposed to follow the Langmuir-Hinshelwood mechanism.
The catalysts TiO2 and TiO2 doped with Fe and V were prepared using the sol–gel method. TiO2-modified samples were obtained in the form of a thick film on pyrex glass sticks and tubes and were used as catalysts in the gas phase photo-oxidation of p-xylene. The physico-chemical characteristics of the catalysts were determined using the methods of Brunauer–Emmett–Teller adsorption, x-ray diffraction, and infrared, ultraviolet and visible and Raman spectroscopies. The experimental results show that the introduction of V did not expand the region of light absorption, but slightly reduced the size of the TiO2 particles, and reduced the number of OH-groups, which should decrease the photocatalytic activity and efficiency of the obtained catalysts compared to those of pure TiO2. The Fe-doped TiO2 samples, in contrast, are characterized by an extension of the spectrum of photon absorption to the visible region with wavenumbers λ up to 464 nm and the values of their band gap energy decreased to lower quantities (up to 2.67 eV), therefore they should have higher catalytic activity and conversion efficiency of p-xylene in the visible region than the original sample. For these catalysts, a combined utilization of radiation by ultraviolet (λ = 365 nm) and visible (λ = 470 nm) light increased the activity and the yield in p-xylene conversion by a factor of around 2–3, as well as making these quantities more stable in comparison with those of TiO2–P25 Degussa.
In this work zeolites HY, HZSM-5 and mixes of zeolites with γ-Al2O3 in different ratios were taken as carriers for 0.8 wt% Pd catalysts. Physico-chemical characteristics of the catalysts were determined by methods of Brunauer–Emmett–Teller (BET)–N2 adsorption, x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), hydrogen pulse chemisorption (HPC) and NH3 adsorption–desorption. The activity of catalysts was studied at 225–450 °C, at 0.1 and 0.7 MPa with molar ratio of H2:n-C6H14 = 5.92 and n-hexane concentration 9.2 mol%. Mixing of γ-Al2O3 with zeolite made acidity of catalyst weaken and led to a decrease of Pd cluster size, to an increase of Pd dispersity and a reduction of the extent of Pd in the case of catalyst Pd/HY; but for the catalyst Pd/HZSM-5 such mixing led to the reverse effect. That is why the increase of activity in the first case and the decrease of activity in the second case have been observed. It has been found that the optimal ratio of mixed carrier is γ-Al2O3:HY = 2.5:1 and the optimal calcined temperature of NH4ZSM-5 to obtain HZSM-5 is 500–550 °C. An increase of reaction pressure from 0.1 to 0.7 MPa remarkably increased the activity, selectivity and stability of Pd-based catalysts.
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