The redispersion of iridium on SiO2 and $gamma;-Al2O3 supports with chlorine-containing gases

Iridium metal particles (1–100 nm in size) on SiO2 (Aerosil, Degussa) and γ-Al2O3 (Merck, 125 m2 g−1) supports have been exposed in a flow system at selected temperatures in the range 470–830 K to Cl2 or HCl mixed with one or two of the following gases: N2, He, O2, H2O, NO, CO or air. The extent of reaction and its effectiveness for the redispersion of agglomerated iridium metal was examined with X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-diffuse reflectance spectroscopy, and temperature-programmed reduction (TPR). α-IrCl3 was formed exclusively on SiO2 and Al2O3 by treatment with Cl2. At Tr< 750 K it built up in layers around Ir particles, but at higher Tr enhanced mobility resulted in the formation of thin IrCl3 sheets. Transport could be induced at lower temperatures, by the addition of O2, H2O, CO, or NO to the treatment stream. However, under most conditions crystalline products (IrCl3, IrO2, Ir(CO)3Cl) were formed which converted on reduction into large Ir agglomerates. A dispersed product phase which yielded small Ir particles on reduction was present on γ-Al2O3 after treatment with C12 + CO (4 < C2CO < 2.5) or with Cl2 + NO (Cl2NO2) in N2. The products are believed to be anionic iridium carbonyl- or nitrosyl-chloride complexes anchored on the alumina surface.