Structure flexibility of iridium metal aggregates. Comparison with platinum and rhodium

The atomic structure of iridium aggregates occluded in zeolite has been studied by radial electron distribution from X-ray scattering data taken under different atmospheres. The interatomic distances in 1–2 nm naked aggregates are contracted with respect to the bulk distances. The adsorption of hydrogen produces a relaxation but a residual contraction remains unlike in the case of platinum aggregates. The carbon monoxide adsorption produces little effect on the position of surface iridium atoms unlike on platinum and rhodium. The lack of structure flexibility of iridium aggregates is probably due to the higher cohesive energy of iridium.