Gold on titania catalysts, influence of some physicochemical parameters on the activity and stability for the oxidation of carbon monoxide

In the standard deposition-precipitation method for preparing Au/TiO2 (P-25) catalysts, after addition of NaOH to the HAuCl4 solution and introduction of the P-25, the suspension was heated to 343 K. As the pH was increased from 2 to 9, the gold uptake decreased but the specific activity rose as the particles became smaller. Excellent catalysts were however made by conducting the whole preparation at room temperature, showing that heating to 343 K as was done earlier was quite unnecessary. No advantage was found in performing preparations in the dark.In a second preparation method HAuCl4 solution was heated at 343 K before TiO2 addition. This method of preparation, if carried out at pH 6, ensured total deposition of the gold, a relatively high activity for CO oxidation, and represents the best compromise from a practical standpoint.Catalysts made with a high-area anatase (90 m2 g−1) responded to the pH used in the preparation in a similar way as TiO2 (P-25), but the pH for optimum activity covered a wider range (7–9). Preparations made with anatase of high surface area (240 or 350 m2 g−1) gave complete recovery of gold from solution, but had only a low catalytic activity. Maximum specific rate was found with anatase of 37 m2 g−1, the value being only slightly lower than the highest value given with the P-25 support.The safest way for preserving the activity of Au/TiO2 catalysts is by keeping them below ambient temperature. The activity loss observed during long-term CO oxidation experiments could be restored by oxidation at 573 K. Sintered catalysts could be reactivated by treatment with aqua regia and re-deposition of the gold precursor at pH 9.