Photodissociation of Cerium Oxide Nanocluster Cations

Abstract

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(-)).

Investigating Pt Atom Addition to Gas Phase Ce2On Clusters Using Photoionization Efficiency Spectroscopy and Density Functional Theory

“…Ceria anions (Ce n O 2 n +1 – ) with up to 21 cerium atoms were some of the first metal oxide clusters to demonstrate the size-dependent reactivity toward CO oxidation that is normally associated with pure metal clusters. Small ceria cations have been studied by Mafuné and co-workers, with the CO oxidation by Ce n O 2 n + also showing a strong size dependence, consistent with the similar earlier work by Hirabayashi et al Duncan and co-workers studied the relative stabilities of various Ce x O y + clusters via photodissociation, finding consistent dissociation of neutral CeO 2 from the prominent y = 2 x – 1 species . Bimetallic gas-phase clusters, analogous to metal atoms on a small metal oxide “surface”, have been shown to have significantly altered reactivities compared to their undoped metal oxide counterparts.…”

“…Experimental and theoretical studies have been performed on the photodissociation of cerium oxide clusters by Duncan and co-workers, with the loss of CeO 2 , O 3 , O 2 , and O from parent ions observed, depending on the original cluster composition. Low values (∼0.8 eV) for dissociation energies were calculated for loss of O 2 in oxygen-rich species, while moderate values (∼3.8 eV) were determined for loss of CeO 2 for most observed species.…”

Ce in the +4 oxidation state: Anion photoelectron spectroscopy and photodissociation of small CexOyHz− molecules

“…Photodissociation of clusters in other studies has frequently resulted in stoichiometries that reflect units of the bulk material. 16,[48][49][50][51][52][53] Since the only photofragment to which band X could potentially be assigned, based on elimination of all other photofragments, is the non-stoichiomentric CeO 2 H molecule, whose e BE is more than 1 eV lower than the corresponding OH neutral, we lean towards assigning band X in the PE spectrum of CeO 3 H 2 to an electronic hot band, placing the excited state at approximately 0.8 eV higher in energy than the ground state, assuming that the neutral ground state is accessed in both bands X and X .…”

“…Thus, many studies have aimed to understand the unique electronic structure of cerium oxides employing methods such as ligand field theory, 7 fluorescence spectroscopies, [8][9][10][11] infrared vibrational predissociation spectroscopy, 12 matrix infrared spectroscopy, 13,14 x-ray absorption spectroscopy, 15 and photodissociation. 16 In an effort to understand the electronic structure of cerium oxide clusters and their chemical properties, we recently completed studies on the reactivity of cerium suboxide clusters toward water 17 and used density functional theory calculations alongside anion photoelectron (PE) spectroscopy to investigate the electronic structures of Ce, 18 CeO and Ce(OH) 2 , 19 a) Author to whom correspondence should be addressed: cjarrold@indiana.edu Ce x O y (x = 2-5, y = 0-4), 20 and Ce x O y H z (x = 2-3, y = 3-7, z = 1-5) cluster anions and neutrals. 21 In addition to this work, Aubriet et al have studied cerium oxyhydroxide cluster formation with mass spectrometry and density functional theory modeling.…”

Photofragmentation Patterns of Cobalt Oxide Cations ConOm+ (n = 5–9, m = 4–13): From Oxygen-Deficient to Oxygen-Rich Species

“…15 In this regard, mass spectrometry followed by mass-selected photodissociation measurements has been demonstrated as a reliable approach to investigate cluster stability. [16][17][18][19][20] Fa- . 22 Dibble and coworkers were the only ones who dealt so far with the photofragmentation of larger sizes of oxygen-equivalent and oxygen-rich cobalt oxide cations (n = 4−9, m = 4−12).…”