EXAFS study of the influence of hydrogen desorption and oxygen adsorption on the structural properties of small iridium particles supported on Al2O3

Desorption of hydrogen at high temperature leads to a contraction of the Ir–Ir coordination distance of highly dispersed iridium metal particles. The long metal–support oxygen distance (2.55 A) observed if the metal crystallites are covered with chemisorbed hydrogen disappears after treatment in vacuum at 623 K. Instead, a metal–oxygen coordination in the metal–support interface is now detected with a distance of 2.19 A. It is concluded that the long metal-oxygen distances found for oxide–supported metal catalysts after reduction in H2 originate from an M0–(OH)– interaction or from the presence of chemisorbed hydrogen in the metal–support interface. Admission of O2 at 77 K does not lead to a corrosive oxidation of the metal particles. The adsorption of O2 resulted in the same type of Ir—O bonds as present in the metal–support after evacuation.