Structural Evolution of Iridium Oxide Cluster Anions Ir_nO_m^– (n = 5–8) with Sequential Oxidation: Binding Mode of O Atoms and Ir Framework

Abstract

Geometric structures of small cluster anions of iridium oxide Ir n O m – (n = 5–8, m = 0–14) were investigated by ion mobility mass spectrometry (IMMS) and theoretical calculation to clarify the evolutional behavior of the binding modes of the O atoms and the motif of the Ir frameworks as a function of m. The collision cross sections (CCSs) for n = 5–7 determined by IMMS showed a monotonic increase with m, whereas those for n = 8 dropped abruptly by ∼10% at m = 11, which otherwise gradually increased with m. For n = 5, detailed comparison between the experimental CCS values and those calculated for various structural isomers revealed that the O atoms in Ir5O m – are preferentially bonded to terminal sites of the Ir5 – cores throughout m = 1–8: the Ir5 – cores retained compact motifs up to m = 5 but distorted significantly for m = 6–8. The experimental CCS values for Ir6O6 – and Ir7O7 – were also explained by on-top binding of O atoms while retaining the motifs of the Ir frameworks. The CCS values for n = 8 suggest that the O atoms are sequentially bonded to terminal sites of the cubic Ir8 frameworks for m = 1–6 and of the deformed Ir8 frameworks for m = 7–11. These on-top binding modes of the O atoms on small Ir clusters are in sharp contrast to the μ2 mode of the O atoms at bridge sites on the clusters of other transition metals and are ascribed to non-face-centered cubic (fcc) Ir frameworks. The sudden drop in the CCS values of Ir8O m – at m = 11 was associated with bond breakage in the Ir framework induced by transition of the binding mode of O atoms from an on-top motif to a μ2 bridging motif. These results suggest that the small Ir clusters exhibit novel oxidation catalysis by taking advantage of the unique chemical properties of O atoms adsorbed on the on-top sites on non-fcc Ir clusters.

NO Bond Cleavage on Gas-Phase Irn+ Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

“…For dissociative adsorption of NO, one common trend is that the O atom prefers to adsorb as terminal-O (μ 1 ). This was also found for dissociative adsorption of O 2 onto anionic Ir clusters, as revealed by ion mobility measurements . In addition, Zhou and coworkers reported that up to three O atoms adsorb on-top on pyramidal Ir 5 + .…”

Structural assignments of yttrium oxide cluster cations studied by ion mobility mass spectrometry

“…A similar feature was repeatedly observed in our previous study of the structure assignment of transition metal oxide cluster ions. 31,34–45 For example, similar metal–oxygen–metal bonding structures were observed for the metal oxide cluster ions of group 4 (Ti 35 ), group 5 (V 37 and Nb 31 ), group 6 (Cr 39 ), group 8 (Fe 40 ), group 9 (Co 41 ), group 10 (Ni 42 ), group 11 (Cu 43 ), and group 12 (Zn 44 ).…”

Electrochemical evaluation of the de-/re-activation of oxygen evolving Ir oxide

“…Ion mobility mass spectrometry (IMMS) on small Ir-O clusters indicated that the bridging m2-O motif is favored by increasing the cluster size and the number of O atoms bound on Ir atoms. 53 On this ground, we tentatively assign the excessive cathodic charge appearing after the OER to the protonation of the surface bridging O species, oxidized during the preceding OER, occurring simultaneously with the injection of a higher number of species with low mass, e.g. H + /H 2 O, than in the subsequent sweeps.…”