Gold on titania catalysts for the oxidation of carbon monoxide: control of pH during preparation with various gold contents

Au/TiO2 catalysts have been prepared by deposition-precipitation, with the initial pH of a HAuCl4 solution raised to various values between 4 and 11 by the addition of NaOH at room temperature. The optimum pH for high activity proved to be 9; at this pH the main species in solution were anionic Au complexes, from which most of the chlorine had been removed by hydrolysis. At lower pH, the gold complexes contained more chlorine, An particles were larger, and activities were lower. Whereas other workers have used catalysts with more than 2% gold, we have focused on lower loadings: catalysts containing only 0.05-1.9 wt% gold were prepared, and the pH was kept constant at 9 throughout the preparation. When their activities for CO oxidation were determined under conditions of kinetic control, all of them had about the same activation energy and the same high specific activity, suggesting that our method of preparation gave similar distributions of gold particle sizes at all loadings. Their activities were unchanged by calcination up to 573 K; however, a catalyst prepared at pH 6 lost activity progressively as the calcination temperature was raised. By withdrawing samples at critical points during the preparation, we have shown that the adsorbed precursor (the form of which determines the size of the gold particles) is in fairly rapid equilibrium with gold species in solution. This permits easy control of the gold particle size during the preparation and even allows the poor activity of a dried catalyst initially prepared at non-optimum pH to be substantially improved. (c) 2005 Elsevier Inc. All rights reserved.