PNNL

Abstract

Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles using 355 nm laser pulses that were synchronized with pulses of nitrogen gas admitted to the irradiation chamber. The gas pulse stabilized the nascent clusters that are largely anhydrous [CexOy] ions and neutrals. These initially-formed species react with water, principally forming closed-shell (c-s) oxohydroxy species that are described by the general formula [CexOy(OH)z]-. In general, the extent of hydroxylation varies from a value of 3 OH per Ce atom when x = 1 to a value slightly greater than 1 for x > 8. The Ce3 and Ce6 species deviate significantly from this trend: the x = 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x = 2 and x = 4. Conversely, the x = 6 cluster is significantly less hydroxylated. Density functional theory (DFT) modeling of the cluster structures show that the hydrated clusters are hydrolyzed, and contain one-to-multiple hydroxide moieties, but not datively bound water. DFT also predicts an energetic preference for formation of highly symmetric structures as the size of the clusters increases. The calculated structures indicate that the ability of the Ce3 oxyhydroxide to accommodate more extensive hydroxylation is due to a more open, hexagonal structure in which the Ce atoms can participate in multiple hydrolysis reactions. Conversely the Ce6 oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the c-s clusters, open-shell (o-s) oxyhydroxides and superoxides are also formed, and they become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to stabilize a non-bonding electron. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases, however this trend is interrupted at Ce13, which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure.