Bimetallic Au-Ag/TiO2 catalyst prepared by deposition-precipitation: High activity and stability in CO oxidation
Résumé
Au-Ag bimetallic catalysts supported on TiO2 were prepared by sequential deposition-precipitation method, silver first and then gold. Au-Ag catalysts with different Au/Ag atomic ratios were synthesized. XANES and H2 TPR results show that gold is more easily reduced in bimetallic catalysts than in Au/TiO2. The difference in gold reducibility might result from interaction between Au and Ag species. The Au-Ag/TiO2 catalysts present better temporal stability than monometallic gold catalysts at 20 °C in the reaction of CO oxidation. Compared to monometallic catalysts, the better catalytic results indicate a synergetic effect between gold and silver in the reaction of CO oxidation. The highest catalytic activity was obtained for the catalyst with an Au/Ag ratio of 1:0.37. Temperature of activation under H2 has also an important consequence on the catalytic activity since conversion increases as temperature increases and reached a maximum for the activation temperature of 550 °C. This optimum results from a compromise between metal particle size, which increases with the activation temperature between 350 and 650 °C, and the bimetallic character of the particles determined by UV-visible and micro-EDS, which improves with the activation temperature. The use of a reducible support such as titania does not lead to much more active catalysts than those supported on nonreducible supports such as silica, probably indicating that the whole reaction takes place mainly on the bimetallic particles.