| Titanium dioxide has been used in many fields due to its chemical stability, optical properties, non-toxicity, and low cost, such as photocatalysis, environmental remediation, selective catalytic reduction, and as a support for heterogeneous catalysis, fine function ceramics and coatings. However, TiO2has some shortcomings, low surface area and poor mechanical strength. Single support has been not satisfied the demand of catalysts. Resently, mixed oxides such as TiO2-ZrO2, TiO2-SiO2, TiO2-Bi2O3, and TiO2-Al2O3, have been used as supports. Hence, the Al2O3-TiO2mixture is becoming more popular because of its unique chemical and physical properties.Although TiO2-supported catalysts have been widely investigated, to the best of our knowledge there has been no report concerning the effect of the addition of Al2O3on the phase transformation of TiO2and the formation of active species. In previous work, Pd/Al2O3(5wt.%)-TiO2catalyst showed a higher catalytic activity than those of either Pd/TiO2or Pd/Al2O3for the total oxidation of ethanol. However, it is still unclear as to how Al2O3affects the crystal structure of TiO2and the catalytic activity. The present study focuses on the influences of adding Al2O3on the structural properties of TiO2and the catalytic performance of Pd catalysts for ethanol oxidation. Meanwhile, the by-products of ethanol oxidation were analyzed by online. The last, the supports and catalysts were characterized by N2adsorption/desorption, X-ray diffraction (XRD), DRIFTS,27Al NMR, NH3-TPD, Pd dispersion determine, UV-vis and XPS techniques, the specific details as follows:(1) The results of N2adsorption/desorption showed that the Al2O3-modified TiO2composite exhibited an increase in specific surface area. Sharp decreases in the surface area and pore volume were mainly due to sintering of the mixed composite at higher calcination temperatures. The specific surface area is not a key factor for the catalytic performance.(2) The results of XRD showed that Al atoms are incorporated into the lattice of TiO2. The addition of A12O3evidently stabilizes the anatase phase and inhibits crystal transformation from the anatase to rutile structure.(3) The DRIFTS and XPS spectra of the supports showed that the Ti-O-Ti bond was obviously shifted. Hence, the vibrational band of the Ti-O-Ti bridge was affected by the introduction of Al3+, indicating the formation of Ti-O-Al bridges in the Al2O3-TiO2support. The bonds were favorable for stabilizing the anatase phase.(4)27Al NMR spectra of the samples showed three27Al resonances corresponding to four-, five-, and six-coordinated aluminum, with six-coordinated aluminum being the major component in the samples. A partial structural transformation involving the conversion of four-and six-coordinated aluminum into five-coordinated aluminum or migration were been seen, which affected its chemical properties.(5) The results of NH3-TPD showed that only the weak acid site was present in TiO2. After the introduction of Al2O3, the increase of acidity, a broad acid sites distribution, and creation of new acidity. (6) The dispersion of Pd on the TiO2carrier was lower. This can be attributed to the low surface area and larger Pd particles. Furthermore, the close contact of the dispersed phase and support could create a strong interaction (Pd-TiO2). The addition of A12O3changes the electronic structure of TiO2and weakens the strong interaction between Pd and TiO2, which is favorable for dispersion of Pd on the surface of Al2O3-TiO2.(7) The XPS results showed that Ti species exist+4(TiO2). The electronegativity of Al in A12O3, decrease the strong interaction between Pd and Ti. The O2-is the main species in catalysts.(8) TiO2support absorbs lights mainly in ultraviolet region, whereas the absorption intensity for ultraviolet light in Al2O3-TiO2mixed support obviously increases, and the absorption in light region increases.(9) The catalytic activity test showed that the addition of A12O3enhanced the catalytic activity (75-175℃) over the Pd/AlTi catalyst. After worked10hours at700℃, the conversion of ethanol was also100%. A better stability of Pd/Al2O3-TiO2catalyst was observed.(10) The catalyst was prepared by acetic acid showed a better catalytic than citric acid. Pd-based catalyst was prepared by complete precipitation and altrosonic assist precipitation revealed a better catalytic activity for ethanol oxidation. The total conversion of ethanol was almost175℃. The addition of0.5%V or1%Ce to Pd-based catalysts showed no obvious difference, whereas decreased the yield of acetaldehyde and ethyl acetate. |
PDF Full Text Request