Hollow silica nanospheres containing Ir clusters in their cavities (Ir-SiO2(h)) were synthesized using Ir-ammine complex crystals as a template in a reversed micelle system. The Ir-SiO2(h) absorbed more than a three times greater amount of dissociated hydrogen than total Ir metal atoms contained in the sample at 298 K. The hydrogen occlusion mechanism was investigated in this study using in situ FT-IR and in situ XAFS techniques. Stretching bands of Ir-H (2118 cm(-1)) and OH (3253-3390 cm(-1)) were observed using FT-IR when hydrogen was introduced into the Ir-SiO2(h). The Ir-H and H+ formed by the dissociation of hydrogen on iridium metals followed by H+ spillover to the silica support, which might be responsible for excess adsorption of hydrogen. When the sample was heated higher than 473 K, the Ir-H band observed at 2118 cm(-1) disappeared completely. TPD measurements after hydrogen adsorption revealed that the most desorbed components were hydrogen. These results demonstrate that the spillover hydrogen onto the silica support desorbs as hydrogen molecules and the adsorption/desorption cycle is reversible. Changes in the coordination number of iridium species in the Ir-SiO2(h) observed using XAFS measurements suggest that reconstruction of iridium clusters took place during hydrogen adsorption/desorption. (c) 2013 Elsevier B.V. All rights reserved.