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 n O n –2 +, Ni n O n –1 + and Ni n O 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 n O n + and Ni n O 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 Ni 5 O 5 +, Ni 6 O 5 + and Ni 7 O 6 + . By adding CO gas to buffer gas in the ion-drift cell, Ni 4 O 3 + and Ni 5 O 4 + cluster cations were found to be more reactive for the CO adsorption reactions than Ni 4 O 4 + and Ni 5 O 5 + . Under the pseudo-first-order approximation, rate constants for CO-adsorption were determined to be (8.4 ± 0.7) × 10 –11 cm 3 molecule –1 s –1 for Ni 4 O 3 + and (9.6 ± 0.8) × 10 –11 cm 3 molecule –1 s –1 for Ni 5 O 4 + . These rate constants are 2 orders of magnitude faster than those for Ni 4 O 4 + and Ni 5 O 5 +, which have reported previously. These differences of rate constants can be originated in the structures of the nickel oxide cluster ions.