Effects of particle size, composition, and support on catalytic activity of AuAg nanoparticles prepared in reverse block copolymer micelles as nanoreactors
Block copolymer (BCP) micelles formed by polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) were used as nanoreactors for preparing stable bimetallic AuAg nanoparticles (NPs) with finely tunable composition and diameters. Micelles loaded with NPs were then deposited onto TiO2 and Al2O3 in order to investigate the catalytic properties of the NPs with respect to CO oxidation. The polymer matrix could be removed by thermal treatment at 400 degrees C under H-2 or O-2. Different parameters, such as support, particle size, and metal composition were varied independently, so that their influence on the catalytic activity for CO oxidation could be separated. The atomic Au/Ag ratio was varied from 1:2 to 2:1, and the highest activity was obtained for an Au/Ag ratio of 1, showing synergy effects of both metals for catalyzing CO oxidation. Using nanosized TiO2 as support and an Au/Ag ratio of 1, the nanoparticle diameter was varied between similar to 3 and similar to 20 nm which led to a variation of activity by a factor of similar to 7 (in a continuous flow reactor at 70 degrees C) with the smallest particles showing the highest turnover frequencies (TOFs). In comparison, regular TiO2 as catalyst carrier showed significantly lower performance than nanostructured TiO2 while no activity was found on non-reducible gamma-Al2O3. Independently, all three parameters (metal particle size, Au/Ag atomic ratio, and the support) showed significant effects on the catalytic activity.