Gold particles smaller than 10nm in diameter, immobilized with α-Fe
2O
3, Co
3O
4, or NiO, were prepared by calcining in air at 400°C. The coprecipitates were obtained from an aqueous solution of HAuCl
4 and the nitrate of Fe, Co, or Ni. The ultrafine gold particles were hemispherical in shape and were strongly held by the host oxides. In most cases, hemispherical gold crystallites were deposited directing their flat (111) plane toward α-Fe
2O
3 (110), Co
3O
4 (111), and NiO (111). The gold particles with a mean diameter of 4.1nm immobilized on α-Fe
2O
3 were more electron deficient than evaporated gold particles of the same size, and much more than the bulk metal. The ultrafine gold particles thus immobilized with 3d transition metal oxides were extremely active for the oxidation of CO at temperatures below 0°C and appreciably selective for CO sensing.
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