REDOX AND CARBONYLATION CHEMISTRY OF IRIDIUM SPECIES IN THE CHANNELS OF H-ZSM-5 ZEOLITE

Reduction of Ir/H-ZSM-5 by hydrogen yields Ir-0 clusters exhibiting a large positive XPS shift (1.4 eV) and an upward IR shift (25-30 cm(-1) ) of linearly bonded CO, both suggesting a strong electron deficiency. Ir clusters (< 1 nm) can be readily and rapidly reoxidised by the hig hly acidic hydroxyls (protons) with subsequent formation of Ir+ (CO)(2 ) gem-dicarbonyl even with residual CO. Reductive carbonylation of Ir (CO)(2) in the presence of larger amounts of CO and H2O results in th e formation of an iridium cluster carbonyl species, most likely tetra- Ir carbonyl, Ir-4 (CO)(12) which is unstable and can be reversibly tra nsformed into a precursor compound Ir+ (CO)(2). Displacement of NH3 li gands originally coordinated to unreduced Ir3+ cations yields the form ation of Ir3+ (CO)(2). This species undergoes stepwise reduction to Ir +(CO)(2) and Ir-0-CO via the water-gas shift reaction.