Supported Au Catalysts Prepared from Au Phosphine Complexes and As-Precipitated Metal Hydroxides: Characterization and Low-Temperature CO Oxidation

Abstract

Supported Au catalysts were prepared by attaching Au phosphinecomplexes, Au(PPh3)(NO3) (1) and [Au9(PPh3)8](NO3)3(2), on as-precipitated metal hydroxidesM(OH)x*(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 catalysts1/Mn(OH)x* and1/Co(OH)x* were most highly active even at 203 K.1/Fe(OH)3* and1/Ti(OH)4* also catalyzed CO oxidation at low temperatures 203–273 K, whereas1/Fe2O3and 1/TiO2prepared by supporting1on conventional Fe2O3and TiO2showed negligible activity under the similar reactionconditions. It was estimated by TEM and XRD that the mean diameter of Au particles in1/Fe(OH)3* was about 2.9 nm, which was about 10 times smaller than that for1/Fe2O3. EXAFS for1/Ti(OH)4* revealed that the coordination number of Au–Au bond was 8–10, while that for1/TiO2was 11.0, which also indicates that Au particle size for1/Ti(OH)4* is smaller thanthat for1/TiO2. The catalysts obtained by attaching the Au complexes on commercially available metal hydroxides also showednegligible activity for the low-temperature CO oxidation underidentical conditions. These results demonstrate that supportedAu catalysts with small Au particles, tremendously active for thelow-temperature CO oxidation, can be prepared by attaching theAu 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.