Preparation and characterization of small silica-supported iridium particles from iridium trisacetylacetonate precursor

This study treats the synthesis of silica-supported iridium metal particles by several different methods. Iridium trisacetylacetonate was first deposi ted on silica either by sublimation or by impregnation from a toluene solut ion. Infrared study showed no difference between the methods, each of which produced Ir(acac)(3) physisorbed at the surface, The physisorbed precursor was transformed by two methods and the reactions were followed by in situ infrared spectroscopy. In the first method, the solid was first heated unde r a flow of oxygen to produce surface iridium oxide, which was then reduced under hydrogen at different temperatures to provide iridium metal support particles. In the second method a physisorbed precursor was directly reduce d under a hydrogen flow. Electron microscopy showed that both methods produ ced narrow distributions of metallic particle sizes between 1 and 5 mm, but for the first method very large metallic aggregates were also observed. Th e chemisorption of hydrogen, oxygen, and CO on the resultant supported meta l materials at 25 degrees C was investigated as a means of determining the dispersion of the samples. A discrepancy between the dispersions deduced fr om chemisorption of H-2, O-2, and CO and from electron microscopy on the re sultant supported metal materials was tentatively interpreted as an indicat ion that some very small Ir particles, present on the silica surface, were not detected. (C) 2000 Academic Press.