Structures and CO-adsorption reactivities of nickel oxide cluster cations studied by ion mobility mass spectrometry

Structures and CO-adsorption reactivities of nickel oxide cluster cations were investigated by ion mobility mass spectrometry. The series of Ni_nO_n-2⁺, Ni_nO_n-1⁺ and Ni_nO_n⁺ cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into an ion-drift cell. From the arrival time distributions of Ni_nO_n⁺ and Ni_nO_n-1⁺ in the ion mobility spectrometry, structural transition from two-dimensional (2D) ring to three-dimensional (3D) compact structures was found at n = 5. In addition, 2D and 3D structural isomers were found to coexist for Ni₅O₅⁺, Ni₆O₅⁺ and Ni₇O₆⁺. By adding CO gas to buffer gas in the ion-drift cell, Ni₄O₃⁺ and Ni₅O₄⁺ cluster cations were found to be more reactive for the CO adsorption reactions than Ni₄O₄⁺ and Ni₅O₅⁺. Under the pseudo-first-order approximation, rate constants for CO-adsorption were determined to be (8.4 ± 0.7) × 10^-11 cm³ molecule^-1 s^-1 for Ni₄O₃⁺ and (9.6 ± 0.8) × 10^-11 cm³ molecule^-1 s^-1 for Ni₅O₄⁺. These rate constants are 2 orders of magnitude faster than those for Ni₄O₄⁺ and Ni₅O₅⁺, which have reported previously. These differences of rate constants can be originated in the structures of the nickel oxide cluster ions.