The redispersion of iridium on SiO 2 and γ-Al 2O 3 supports with chlorine-containing gases

Abstract

Iridium metal particles (1–100 nm in size) on SiO 2 (Aerosil, Degussa) and γ-Al 2O 3 (Merck, 125 m 2 g −1) supports have been exposed in a flow system at selected temperatures in the range 470–830 K to Cl 2 or HCl mixed with one or two of the following gases: N 2, He, O 2, H 2O, 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-IrCl 3 was formed exclusively on SiO 2 and Al 2O 3 by treatment with Cl 2. At T r< 750 K it built up in layers around Ir particles, but at higher T r enhanced mobility resulted in the formation of thin IrCl 3 sheets. Transport could be induced at lower temperatures, by the addition of O 2, H 2O, CO, or NO to the treatment stream. However, under most conditions crystalline products (IrCl 3, IrO 2, 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-Al 2O 3 after treatment with C1 2 + CO ( 4 < C 2 CO < 2.5 ) or with Cl 2 + NO ( Cl 2 NO2 ) in N 2. The products are believed to be anionic iridium carbonyl- or nitrosyl-chloride complexes anchored on the alumina surface.