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

Isolated nickel-doped aluminum oxide cations (NiO _m )(Al ₂ O ₃ ) _n (AlO) ⁺ with m = 1-2 and n = 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 ^-1 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 m = 2 series. The identified structures of the oxides ( m = 1) correspond to inserting a Ni-O moiety into an Al-O bond of the corresponding (Al ₂ O ₃ ) _1-3 (AlO) ⁺ cluster, yielding either a doubly or triply coordinated Ni ²⁺ center. The m = 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 ²⁺ atom unchanged, or forms a biradical comprising a terminal oxygen radical anion Al-O ^•- and a Ni ³⁺ 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.