Cerium Oxyhydroxide Clusters: Formation, Structure, and Reactivity

Abstract

Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles by 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 [Ce(x)O(y)] ions and neutrals. These initially formed species react with water, principally forming oxohydroxy species that are described by the general formula [Ce(x)O(y)(OH)(z)](-) for which all the Ce atoms are in the IV oxidation state. In general, the extent of hydroxylation varies from a value of three OH per Ce atom when x = 1 to a value slightly greater than 1 for x >or= 8. The Ce(3) and Ce(6) species deviate significantly from this trend: the x = 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x = 2 and 4. Conversely, the x = 6 cluster is significantly less hydroxylated than its x = 5 and 7 neighbors. Density functional theory (DFT) modeling of the cluster structures shows 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 Ce(3) 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 Ce(6) oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the fully oxidized (Ce(IV)) oxyhydroxides, reduced oxyhydroxides (containing a Ce(III) center) are also formed. These become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to accommodate a reduced Ce(III) moiety. In addition, the spectra offer evidence for the formation of superoxide derivatives that may arise from reaction of the reduced oxyhydroxides with dioxygen. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases; however, this trend is interrupted at Ce(13), which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure.

Study of Cluster Anions Generated by Laser Ablation of Titanium Oxides: A High Resolution Approach Based on Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

“…Regardless of the CC and TiO 2 BB numbers, the cluster anions never accommodated more than 3 H 2 O moieties. These results are not consistent when comparing them with a previous study concerning cerium oxy-hydroxide anions [25]. Aubriet et al showed that the number of H 2 O moieties increases with the cluster anion size, i.e., the number of CeO 2 BB.…”

“…This was more specifically supported by the results reported in the literature discussing the investigation of alumina [26][27], silica [29], chromia [35,36], or ceria [25] oxyanions and oxy-hydroxide anions. A portion of the oxy-hydroxide cluster anions was obtained by the ion-molecule reaction in the gas phase between metal oxide ions and water molecules, which involves the (1) incorporation of "water" in the cluster ion, and (2) formation of hydroxyl groups.…”

“…In this respect, an alternative formalism may be used to account for the different anions observed in the mass spectrum with a "structural" connotation. This was previously proposed by Aubriet et al for oxohydroxide cerium cluster anions [25] To support the hypothesis of a two-step mechanism for cluster ion formation, an additional MS/MS experiment was performed. After transfer in the FTICR MS cell, the ions were excited in an orbit, which is close to the detection plates.…”

“…In that case, no gas was added to the ICR cell to be sure that the reactions were conducted at a static pressure, allowing the kinetic constant to be extracted from experimental results. Residual pressure was thought, as previously reported by Aubriet et al [25], to be mainly associated with water vapor. In the latter case, the anion distribution was studied depending on the reaction time.…”

“…Indeed, the +IV oxidation number is the more stable oxidation state of titanium. This explanation was recently proposed to account for the differences observed in the comparative study of CeO 2 by LA coupled to FTICR MS with an external ion source and TOF-MS [25]. In addition, the different time frame of both analyzers (FTICR and TOF) must also be considered to explain the observed differences.…”