Supported Au catalysts prepared from Au phosphine complexes and As-precipitated metal hydroxides: Characterization and low-temperature CO oxidation

Supported Au catalysts were prepared by attaching Au phosphine complexes, Au(PPh3)(NO3) (1) and [Au-9(PPh3)(8)](NO3)(3) (2), on as-precipitated metal hydroxides M(OH)(x)* (*, as-precipitated; M = Mn2+, Co2+, Fe3+, Ni2+, Zn2+, Mg2+, Cu2+, Ti4+, Ce4+, and La3+), followed by temperature-programmed calcination in a flow of dry air. The obtained Au catalysts showed high catalytic activities in low-temperature CO oxidation. Among the obtained Au catalysts 1/Mn(OH)(2)* and 1/Co(OH)(2)* were most highly active even at 203 K. 1/Fe(OH)(3)* and 1/Ti(OH)(4)* also catalyzed CO oxidation at low temperatures 203-273 K, whereas 1/Fe2O3 and 1/TiO2 prepared by supporting 1 on conventional Fe2O3 and TiO2 showed negligible activity under the similar reaction conditions. It was estimated by TEM and XRD that the mean diameter of Au particles in 1/Fe(OH)(3)* was about 2.9 nm, which was about 10 times smaller than that for 1/Fe2O3. EXAFS for 1/Ti(OH)(4)* revealed that the coordination number of Au-Au bond was 8-10, while that for 1/TiO2 was 11.0, which also indicates that Au particle size for 1/Ti(OH)(4)* is smaller than that for 1/TiO2. The catalysts obtained by attaching the Au complexes on commercially available metal hydroxides also showed negligible activity for the low-temperature CO oxidation under identical conditions. These results demonstrate that supported Au catalysts with small Au particles, tremendously active for the low-temperature CO oxidation, can be prepared by attaching the Au phosphine complexes on the as-precipitated metal hydroxides. Sodium cations exhibited positive effect on the Au catalysis, whereas chloride anions drastically decreased the CO oxidation activity. (C) 1997 Academic Press.