Preparation And Characterization Of Supported Nano-Structured Au/TiO₂ Catalyst With High Thermal Stability

1,The support TiO₂ was synthesized by sol-gel method, and a series of Au/TiO₂ catalysts were prepared by the deposition-precipitation method. The effect of several variable parameters on the activity of the catalysts,such as the calcination temperature, the gold loading, the pH value of reaction solution and the base used to neutralise the HAuCl4 solution, were investigated carefully. The optimal preparation condition was confirmed by using the ambient CO oxidation as probe reaction. It has been found that the best calcination temperature is between 200³00℃; the best pH value and precipitator were 9 and NaOH, respectively; the more gold contents the better catalytic activity and thermal stability when the gold loading is in the range of 0.5%⁵wt%. The presence of Cl- ions leads to a partial decrease of the catalytic activity of Au/TiO₂. The following Au/TiO₂ catalysts were prepared under the optimal condition.2,The Au/TiO₂ catalysts prepared by deposition-precipitation method show high thermal stability and a definite ability to prevent poisoning by SO2. The circular test was carried out on Au/TiO₂ catalyst with different concentration of CO, the CO conversion over this catalyst was always 100% at ambient temperature after undergoing three circulations and a continuous reaction of 2160 min. However,the oxidation activity of the catalyst pretreated by SO2 remarkably decreases. The XPS results show that the metallic state of Au particles is well maintained after SO2 pretreatment. According to the results of in-situ IR, it is deduced that the formation of sulfate and its occupying the vacancies of TiO₂ may have prevented the adsorption of O2, resulting in the decrease of the catalytic activity.3,The mesoporous TiO₂ was synthesized using poly(alkylene oxide)triblock copolymer (designated EO20PO70EO20; P123) as the organic template, and the Au/TiO₂ catalyst was prepared by the deposition-precipitation method. For comparison, an industrial TiO₂ and another kind of TiO₂ synthesized by sol-gel method were also used as supports to prepare the Au/TiO₂ catalysts. The N2 adsorption-desorption, X-ray diffraction, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy techniques were employed to characterize the structure and the morphology of the catalysts. The results show that the TiO₂ synthesized using P123 as the organic template possesses uniform mesoporous structure with a narrow pore size distribution centered at ⁶.1nm. After supporting Au, the mesoporous structure of TiO2 is well maintained only with the pore size decreasing to ⁵.4 nm. The gold in the mesoporous TiO2-supported catalyst calcined at 400℃exists as Au0. The dispersion and the crystal size of Au on the three kinds of TiO2 show much difference. On the TiO2 prepared using organic template P123, the crystal size of Au is 1-5nm after calcination at 400℃, while on the other two supports, the Au crystal size increases up to ¹0 nm. For ambient CO oxidation, the former Au/TiO2 catalyst exhibits very high activity and thermal stability, even after calcination at 420℃, the CO conversion over this catalyst is still more than 90%. But for the industrial TiO2 and that synthesized by sol-gel method, the corresponding catalysts show low thermal stability and poor catalytic activity for CO oxidation.