Structural Characterization of Nickel-Doped Aluminum Oxide Cations by Cryogenic Ion Trap Vibrational Spectroscopy.

Isolated nickel-doped aluminum oxide cations (NiO)(AlO)(AlO) with = 1-2 and = 1-3 are investigated by infrared photodissociation (IRPD) spectroscopy in combination with density functional theory and the single-component artificial force-induced reaction method. IRPD spectra of the corresponding He-tagged cations are reported in the 400-1200 cm spectral range and assigned based on a comparison to calculated harmonic IR spectra of low-energy isomers. Simulated spectra of the lowest energy structures generally match the experimental spectra, but multiple isomers may contribute to the spectra of the = 2 series. The identified structures of the oxides ( = 1) correspond to inserting a Ni-O moiety into an Al-O bond of the corresponding (AlO)(AlO) cluster, yielding either a doubly or triply coordinated Ni center. The = 2 clusters prefer similar structures in which the additional O atom either is incorporated into a peroxide unit, leaving the oxidation state of the Ni atom unchanged, or forms a biradical comprising a terminal oxygen radical anion Al-O and a Ni species. These clusters represent model systems for under-coordinated Ni sites in alumina-supported Ni catalysts and should prove helpful in disentangling the mechanism of selective oxidative dehydrogenation of alkanes by Ni-doped catalysts.