posted on 2016-04-01, 00:00authored byS. T. Akin, S. G. Ard, B. E. Dye, H. F. Schaefer, M. A. Duncan
Cerium oxide cluster
cations, CexOy+, are produced via laser vaporization
in a pulsed nozzle source and detected with time-of-flight mass spectrometry.
The mass spectrum displays a strongly preferred oxide stoichiometry
for each cluster with a specific number of metal atoms x, with x ≤ y. Specifically,
the most prominent clusters correspond to the formula CeO(CeO2)n+. The cluster cations
are mass selected and photodissociated with a Nd:YAG laser at either
532 or 355 nm. The prominent clusters dissociate to produce smaller
species also having a similar CeO(CeO2)n+ formula, always with apparent leaving groups of
(CeO2). The production of CeO(CeO2)n+ from the dissociation of many cluster
sizes establishes the relative stability of these clusters. Furthermore,
the consistent loss of neutral CeO2 shows that the smallest
neutral clusters adopt the same oxidation state (IV) as the most common
form of bulk cerium oxide. Clusters with higher oxygen content than
the CeO(CeO2)n+ masses
are present with much lower abundance. These species dissociate by
the loss of O2, leaving surviving clusters with the CeO(CeO2)n+ formula. Density
functional theory calculations on these clusters suggest structures
composed of stable CeO(CeO2)n+ cores with excess oxygen bound to the surface as a superoxide
unit (O2–).