The Effects Of La³⁺ And Ce³⁺ Doping On The Structure And Performance Of Nano-Structured Au/TiO₂ Catalysts

The pure mesoporous TiO₂ and doped mesoporous TiO₂ by La³⁺ or Ce³⁺ were synthesized by using poly(alkylene oxide)triblock copolymer (designated EO20PO70EO20; P123) as the organic template, which were used as supports to prepare the corresponding supported catalysts by the deposition-precipitation method. The effects of the preparation parameters on the activity of the catalysts,such as the pH value of the reaction solution, the calcination temperature, the doping content of La³⁺ and Ce³⁺, etc., were investigated carefully. The optimal preparation condition was confirmed by using the CO oxidation as probe reaction. It has been found that the best pH value is 8; the best calcination temperature is between 200³00 oC; the optimal doping content of La³⁺ or Ce³⁺ is 0.50% (La/Ti or Ce/Ti molar ratio).The results of CO oxidation indicate that both Au/TiO₂ doped with 0.50% La³⁺ (denoted as Au/0.50%La-TiO₂) and Au/TiO₂ doped with 0.50% Ce³⁺ (denoted as Au/0.50%Ce-TiO₂) show the highest catalytic activity and thermal stability. On Au/0.50%La-TiO₂ and Au/0.50%Ce-TiO₂ catalysts calcined at 450 oC, the CO conversion with CO concentration of 1% in the feed was always 100% at 54 oC during a continuous reaction of 7 hours.The N2 adsorption-desorption, X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy technique and energy dispersive X-ray spectrum were employed to characterize the structure of the catalysts. The results show that the pure TiO₂ possesses high surface area (102.5 m2/g) and uniform mesoporous structure with the pore size mainly centered at ⁴.1 nm. With the doping of 0.50% La³⁺, the surface area and the pore size of the La³⁺ doped mesoporous TiO₂ increase to 121.8 m2/g and ⁴.9 nm, respectively. With the doping of 0.50% Ce³⁺, the surface area and the pore size of the Ce³⁺ doped mesoporous TiO₂ increase to 120.6 m2/g and ⁹.7 nm, respectively. After calcination at 450 oC, the mesoporous structure of the Au/TiO₂ has been destroyed partly, while the mesoporous structures of Au/0.50%La-TiO₂ and Au/0.50%Ce-TiO₂ are well maintained. The gold in the Au/TiO₂ calcined at 450 oC exists as Au0, but the gold in the Au/0.50%La-TiO₂ and Au/0.50%Ce-TiO₂ exist as Au0 and a small portion of Au2O3. The TEM results show that the Au crystallite size is 6⁸ nm in Au/TiO₂ after calcination at 400 oC, while decreased to 5 nm in Au/0.50%La-TiO₂. After calcination at 450 oC, a remarkable growth of Au crystallites can be observed on Au/TiO₂ catalyst with the crystal size exceeding 20nm; while the Au crystallites only slightly increase to 8⁹nm on Au/0.50%La-TiO₂ and Au/0.50%Ce-TiO₂. The results of CO oxidation over the catalysts reveal that all the three catalysts calcined at the temperature less than 400 oC exhibit excellent oxidation activities, the CO can be totally oxidized even at ambient temperature. But after calcination at 450 oC, the temperature corresponding to the full conversion of CO on Au/TiO₂ increases to 86 oC, while on Au/0.50%La-TiO₂ and Au/0.50%Ce-TiO₂, the temperatures are only 53 oC and 54 oC, respectively. Thus it can be seen that a small amount of La³⁺ or Ce³⁺ doping to TiO₂ support has effectively inhibited the sintering of Au crystallites at higher temperature, resulting in the improvement of the oxidation activity and thermal stability of the Au/TiO₂ catalyst.