The electron shuffle: Cerium influences samarium 4f orbital occupancy in heteronuclear Ce-Sm oxide clusters

The anion photoelectron (PE) spectra along with supporting results of density functional theory (DFT) calculations on SmO^-, SmCeO_y^-, and Sm₂O_y^- (y = 1, 2) are reported and compared to previous results on CeO^- [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce₂O_y^- (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on Ce_xO_y^- clusters, the PE spectra of SmO^-, SmCeO_y^-, and Sm₂O_y^- (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e^-BE. The Sm centers in SmO and Sm₂O₂ neutrals can be described with the 4f⁵6s superconfiguration, which is analogous to CeO and Ce₂O₂ neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (Z_Ce = Z_Sm - 4). The Sm center in CeSmO₂, in contrast, has a 4f⁶ occupancy, while the Ce center maintains the 4f 6s superconfiguration. The less oxidized Sm centers in both Sm₂O and SmCeO have 4f⁶ 6s occupancies. The 4f⁶ subshell occupancy results in relatively weak Sm-O bond strengths. If this extra 4f occupancy also occurs in bulk Sm-doped ceria, it may play a role in the enhanced O^2- ionic conductivity in Sm-doped ceria. Based on the results of DFT calculations, the heteronuclear Ce-Sm oxides have molecular orbitals that are distinctly localized Sm 4f, Sm 6s, Ce 4f, and Ce 6s orbitals. The relative intensity of two electronic bands in the PE spectrum of Sm₂O^- exhibits an unusual photon energy-dependence, and the PE spectrum of Sm₂O₂^- exhibits a photon energy-dependent continuum signal between two electronic transitions. Several explanations, including the high magnetic moment of these suboxide species and the presence of low-lying quasi-bound anion states, are considered.

Gas-Phase Reactions of Copper Oxide Cluster Cations with Ammonia: Selective Catalytic Oxidation to Nitrogen and Water Molecules

Reactions of copper oxide cluster cations, Cu _nO _m⁺ ( n = 3-7; m ≤ 5), with ammonia, NH₃, are studied at near thermal energies using a guided ion beam tandem mass spectrometer. The single-collision reactions of specific clusters such as Cu₄O₂⁺, Cu₅O₃⁺, Cu₆O₃⁺, Cu₇O₃⁺, and Cu₇O₄⁺ give rise to the release of H₂O after NH₃ adsorption efficiently and result in the formation of Cu _nO _m-1NH⁺. These Cu _nO _m⁺ clusters commonly have Cu average oxidation numbers of 1.0-1.4. On the other hand, the formation of Cu _nO _m-1H₂⁺, i.e., the release of HNO, is dominantly observed for Cu₇O₅⁺ with a higher Cu oxidation number. Density functional theory calculations are performed for the reaction Cu₅O₃⁺ + NH₃ → Cu₅O₂NH⁺ + H₂O as a typical example of H₂O release. The calculations show that this reaction occurs almost thermoneutrally, consistent with the experimental observation. Further, our experimental studies indicate that the multiple-collision reactions of Cu₅O₃⁺ and Cu₇O₄⁺ with NH₃ lead to the production of Cu₅⁺ and Cu₇O⁺, respectively. This suggests that the desirable NH₃ oxidation to N₂ and H₂O proceeds on these clusters.

Ligand-Enhanced CO Activation by the Early Lanthanide-Nickel Heterodimers: Photoelectron Velocity-Map Imaging Spectroscopy of LnNi(CO) n- (Ln = La, Ce)

Heterobimetallic lanthanum-nickel and cerium-nickel carbonyls, LnNi(CO) _n^- (Ln = La, Ce; n = 2-5), were generated using a pulsed laser vaporization/supersonic expansion ion source. These compounds were characterized by photoelectron velocity-map imaging spectroscopy and quantum chemical calculations. The binding motif in the most stable isomers of the n = 2 and 3 clusters consists of one side-on-bonded carbonyl. A new building block of two side-on-bonded carbonyls is favored at n = 4, which is retained at n = 5, evidencing the increase of the number of extremely activated CO molecule in the larger clusters. The experimental and theoretical results demonstrate the ligand-enhanced CO activation by the early lanthanide-nickel heterodimers, which would have important implications for the design of alloy catalysts for activation of a molecular ligand.

Fe2 O+ Cation Mediated Propane Oxidation by Dioxygen in the Gas Phase

The mass-selected Fe₂ O⁺ cation mediated propane oxidation by O₂ was investigated by mass spectrometry and density functional theory calculations. In the reaction of Fe₂ O⁺ with C₃ H₈ , H₂ was liberated by C-H bond activation to give Fe₂ OC₃ H₆⁺ . Interestingly, when a mixture of C₃ H₈ /O₂ was introduced into the reactor, an intense signal that corresponded to the Fe₂ O₂⁺ cation was present; the experiments indicated that O₂ was activated in its reaction with Fe₂ O(C₃ H₆ )⁺ to give Fe₂ O₂⁺ and C₃ H₆ O (acetone or propanal). A Langmuir-Hinshelwood-like mechanism was adopted in the propane oxidation reaction by O₂ on gas-phase Fe₂ O⁺ cations. In comparison with the absence of Fe₂ O₂⁺ in the reaction of Fe₂ O⁺ with O₂ , the ligand effect of C₃ H₆ on Fe₂ OC₃ H₆⁺ is important in the oxygen activation reaction. The theoretical results are consistent with the experimental observations. The propane oxidation by O₂ in the presence of Fe₂ O⁺ might be applied as a model for alkane and O₂ activations over iron oxide catalysts, and the mechanisms and kinetic data are useful for understanding corresponding heterogeneous reactions.

Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

Adsorption of O2 on Anionic Gold Clusters in the 0-1 nm Size Range: An Insight into the Electron Transfer Dynamics from Kinetic Measurements

We systematically studied the adsorption of O₂ on Au _n^- in the size range of 0-1 nm at low temperatures and determined new active sizes with n = 22, 24, 34, and 36. The kinetic measurements more clearly showed the correlation between the reactivity of Au _n^- with O₂ and their electronic properties: the sizes with a closed electron shell are always inert, and the sizes with an unpaired electron can chemically adsorb one O₂ molecule if their adiabatic detachment energies (ADEs) are lower than a threshold around 3.5 eV. This ADE threshold dividing the active and inert Au _n^- is independent of the clusters' sizes, global geometries, and local adsorption sites. According to the widely accepted electron transfer mechanism, this threshold could stand for the case in which the total energy of the Au _n^- and an O₂ roughly equals that of the spin crossover point of the potential surfaces of Au _n-O₂^- and Au _n^-···O₂.

Selective Conversion of Methane by Rh1-Doped Aluminum Oxide Cluster Anions RhAl2O4-: A Comparison with the Reactivity of PtAl2O4. J Phys Chem A 2018;122:3950-3955. [PMID: 29578712 DOI: 10.1021/acs.jpca.8b02483]

Studying the elementary reactions of single-noble-metal-atom-doped species can give theoretical guidance for the design of related single-atom catalysis. Using a combination of mass spectrometry and density functional theory calculations, the reaction of RhAl₂O₄^- with the most stable alkane molecule CH₄ under thermal conditions has been studied. The methane tends to be converted into syngas (free H₂ and adsorbed CO) with activation of four C-H bonds. In sharp contrast, formaldehyde was generated in the previously reported reaction of PtAl₂O₄^- with CH₄. Density functional theory calculations show that the difference in reactivity between RhAl₂O₄^- and PtAl₂O₄^- is found to be due to a higher energy barrier of the third C-H bond activation for the Pt analogue. This work provides the first comparative study on the reactivity of single noble-metal atoms (Rh, Pt) on the same cluster support (Al₂O₄^-) and can be helpful for rational design of single-atom catalysts for selective methane conversion.

Thermal stability of iron-sulfur clusters

The thermal decomposition of free cationic iron-sulfur clusters Fe_xS_y⁺ (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric Fe_xS_y⁺ (y = x) or near stoichiometric Fe_xS_y⁺ (y = x ± 1) clusters is observed. In particular, Fe₄S₄⁺ represents the most abundant product up to 600 K, Fe₃S₃⁺ and Fe₃S₂⁺ are preferably formed between 600 K and 800 K, and Fe₂S₂⁺ clearly dominates the cluster distribution above 800 K. These temperature dependent fragment distributions suggest a sequential fragmentation mechanism, which involves the loss of sulfur and iron atoms as well as FeS units, and indicate the particular stability of Fe₂S₂⁺. The potential fragmentation pathways are discussed based on first principles calculations and a mechanism involving the isomerization of the cluster prior to fragmentation is proposed. The fragmentation behavior of the iron-sulfur clusters is in marked contrast to the previously reported thermal dissociation of analogous iron-oxide clusters, which resulted in the release of O₂ molecules only, without loss of metal atoms and without any tendency to form particular prominent and stable Fe_xO_y⁺ clusters at high temperatures.

Formation of Acetylene in the Reaction of Methane with Iron Carbide Cluster Anions FeC3- under High-Temperature Conditions

The underlying mechanism for non-oxidative methane aromatization remains controversial owing to the lack of experimental evidence for the formation of the first C-C bond. For the first time, the elementary reaction of methane with atomic clusters (FeC₃^- ) under high-temperature conditions to produce C-C coupling products has been characterized by mass spectrometry. With the elevation of temperature from 300 K to 610 K, the production of acetylene, the important intermediate proposed in a monofunctional mechanism of methane aromatization, was significantly enhanced, which can be well-rationalized by quantum chemistry calculations. This study narrows the gap between gas-phase and condensed-phase studies on methane conversion and suggests that the monofunctional mechanism probably operates in non-oxidative methane aromatization.

Liquid-phase catalysis by single-size palladium nanoclusters supported on strontium titanate: size-specific catalysts for Suzuki–Miyaura coupling

Size-specific catalysis by single-size palladium nanoclusters.

Electronic Effects on Room-Temperature, Gas-Phase C-H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O^•). Whenever the radical is delocalized, e.g., in [MgO]_n^•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga₂O₃) to [MgO]₂^•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al₂O₂]^•+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH₃ moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]₂^•+ by Al₂O₃ enables HAT and PCET to compete. Similarly, [ZnO]^•+ activates methane by PCET generating many products. Adding a CH₃CN ligand to form [(CH₃CN)ZnO]^•+ leads to a single HAT product. The CH₃CN dipole acts as an OEF that switches off PCET. [MC]⁺ cations (M = Au, Cu) act by different mechanisms, dictated by the M⁺-C bond covalence. For example, Cu⁺, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu⁺.

Size-Dependent Reactivity of Nano-Sized Neutral Manganese Oxide Clusters toward Ethylene

Neutral manganese oxide clusters with the general composition Mn_2 N O_3 N+x (N=2-22; x=-1, 0, 1) with dimensions up to a nanosize were prepared by laser ablation and reacted with C₂ H₄ in a fast flow reactor. The size-dependent reactivity of C₂ H₄ adsorption on these clusters was experimentally identified and the adsorption reactivity decreases generally with an increase of the cluster size. Density functional theory calculations were performed to study the geometrical and electronic structures of the Mn_2 N O_3 N (N=1-6) clusters. The calculated results indicated that the coordination number and the charge distribution of the metal centers are responsible for the experimentally observed size-dependent reactivity. The highly charged Mn atoms with low coordination are preferential to adsorb C₂ H₄ . In contrast, the neutral manganese oxide clusters are completely inert toward the saturated hydrocarbon molecule C₂ H₆ . This work provides new perspectives to design related materials in the separation of hydrocarbon molecules.

Infrared Spectroscopy of Au+(CH4) n Complexes and Vibrationally-Enhanced C-H Activation Reactions

A combined spectroscopic and computational study of gas-phase Au⁺(CH₄)_n (n = 3–8) complexes reveals a strongly-bound linear Au⁺(CH₄)₂ core structure to which up to four additional ligands bind in a secondary coordination shell. Infrared resonance-enhanced photodissociation spectroscopy in the region of the CH₄a₁ and t₂ fundamental transitions reveals essentially free internal rotation of the core ligands about the H₄C–Au⁺–CH₄ axis, with sharp spectral features assigned by comparison with spectral simulations based on density functional theory. In separate experiments, vibrationally-enhanced dehydrogenation is observed when the t₂ vibrational normal mode in methane is excited prior to complexation. Clear infrared-induced enhancement is observed in the mass spectrum for peaks corresponding 4u below the mass of the Au⁺(CH₄)_n=2,3 complexes corresponding, presumably, to the loss of two H₂ molecules.

Efficient photoelectrochemical water splitting on ultrasmall defect-rich TaOx nanoclusters enhanced by size-selected Pt nanocluster promoters

Formation of nanoclusters has attracted a lot of attention in recent years because of their distinct properties from isolated atoms and bulk solids. Here, we focus on the catalytic properties of supported transition metal oxide nanoclusters, such as TaO₂, with a well-defined size distribution below 10 nm. We show that their catalytic performance can be greatly enhanced by introducing a reaction promoter such as Pt. Different combinations of precisely size-selected, defect-rich TaO_x and Pt nanoclusters are produced by a gas-phase aggregation technique in a special DC magnetron sputtering system. Argon flow rate and aggregation length are carefully optimized to control the sizes of these ultrasmall TaO_x and Pt nanoclusters by using a quadrupole mass filter, and TEM studies reveal the different crystalline nature of TaO_x (amorphous) and Pt (crystalline) nanoclusters. We have further demonstrated the size-dependent photoanode activity of (TaO_x, Pt) nanocluster systems in a photoelectrochemical water splitting reaction, where the Pt nanocluster promoters are found to provide a significant enhancement in the photocurrent density, approximately tripled that was observed from just TaO_x nanocluster catalysts alone. The photocurrent density and photoconversion efficiency tend to reduce when Pt nanoclusters become overpopulated due to blocking of the photosensitive TaO_x surface. Reducing the Pt nanocluster size resolves this problem by incorporating a greater number of smaller nanocluster promoters without blocking TaO_x, which leads to further enhancement in the photocurrent density. The enhanced photocatalytic activity is attributed to synergetic effects introduced by the Pt nanoclusters that act as temporary charge storage sites to facilitate effective separation of a large number of electron-hole pairs, generated from a large number of active sites on the defect-rich amorphous TaO_x nanoclusters upon illumination.

Selective C-H Bond Cleavage in Methane by Small Gold Clusters

Methane represents the major constituent of natural gas. It is primarily used only as a source of energy by means of combustion, but could also serve as an abundant hydrocarbon feedstock for high quality chemicals. One of the major challenges in catalysis research nowadays is therefore the development of materials that selectively cleave one of the four C-H bonds of methane and thus make it amenable for further chemical conversion into valuable compounds. By employing infrared spectroscopy and first-principles calculations it is uncovered herein that the interaction of methane with small gold cluster cations leads to selective C-H bond dissociation and the formation of hydrido methyl complexes, H-Au_x⁺ -CH₃ . The distinctive selectivity offered by these gold clusters originates from a fine interplay between the closed-shell nature of the d states and relativistic effects in gold. Such fine balance in fundamental interactions could prove to be a tunable feature in the rational design of a catalyst.

Catalytic Decomposition of NO by Cationic Platinum Oxide Cluster Pt3O4. J Phys Chem Lett 2017;8:2143-2147. [PMID: 28445054 DOI: 10.1021/acs.jpclett.7b00591]

The catalytic decomposition of NO by cationic platinum oxide cluster Pt₃O₄⁺ was investigated by mass spectrometry and thermal desorption spectrometry. Upon reaction with two NO molecules, molecular oxygen desorbed from the cluster at room temperature to form Pt₃O₄N₂⁺. Then, at temperatures above 400 K, desorption of N₂ from Pt₃O₄N₂⁺ was observed. These processes were confirmed by isotope-labeling experiments, and the energetics of O₂ and N₂ release were determined by density functional calculations. The combination of these elementary steps resulted in the catalytic decomposition of NO by Pt₃O₄⁺.

Tuning the oxidative power of free iron-sulfur clusters

The gas-phase reactions between a series of di-iron sulfur clusters Fe₂S_x⁺ (x = 1-3) and the small alkenes C₂H₄, C₃H₆, and C₄H₈ have been investigated by means of Fourier-transform ion-cyclotron resonance mass spectrometry. For all studied alkenes, the reaction efficiency is found to increase in the order Fe₂S⁺ < Fe₂S₂⁺ < Fe₂S₃⁺. In particular, Fe₂S⁺ and Fe₂S₂⁺ only form simple association products, whereas the sulfur-rich Fe₂S₃⁺ is able to dehydrogenate propene and 2-butene via desulfurization of the cluster and formation of H₂S. This indicates an increased propensity to induce oxidation reactions, i.e. oxidative power, of Fe₂S₃⁺ that is attributed to an increased formal oxidation state of the iron atoms. Furthermore, the ability of Fe₂S₃⁺ to activate and dissociate the C-H bonds of the alkenes is observed to increase with increasing size of the alkene and thus correlates with the alkene ionization energy.

Determining Rate Constants and Mechanisms for Sequential Reactions of Fe+ with Ozone at 500 K

We present rate constants and product branching ratios for the reactions of FeO_x⁺ (x = 0-4) with ozone at 500 K. Fe⁺ is observed to react with ozone at the collision rate to produce FeO⁺ + O₂. The FeO⁺ in turn reacts with ozone at the collision rate to yield both Fe⁺ and FeO₂⁺ product channels. Ions up to FeO₄⁺ display similar reactivity patterns. Three-body clustering reactions with O₂ prevent us from measuring accurate rate constants at 300 K although the data do suggest that the efficiency is also high. Therefore, it is probable that little to no temperature dependence exists over this range. Implications of our measurements to the regulation of atmospheric iron and ozone are discussed. Density functional calculations on the reaction of Fe⁺ with ozone show no substantial kinetic barriers to make the FeO⁺ + O₂ product channel, which is consistent with the reaction's efficiency. While a pathway to make FeO₂⁺ + O is also found to be barrierless, our experiments indicate no primary FeO₂⁺ formation for this reaction.

Thermal Methane Activation by the Metal-Free Cluster Cation [Si2 O4 ]

The thermal reaction of methane with the metal-free cluster cation [Si₂ O₄ ]^.+ has been examined by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. In addition to generating a methyl radical via hydrogen-atom abstraction, [Si₂ O₄ ]^.+ can selectively oxidize methane to formaldehyde. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations.

CO oxidation by the atomic oxygen on silver clusters: structurally dependent mechanisms generating free or chemically bonded CO2

Oxidation of CO by the atomic oxygen on Ag_nO⁻ (n = 1–8) forms free or chemically bonded CO₂.

Liberation of three dihydrogens from two ethene molecules as mediated by the tantalum nitride anion cluster Ta3N2− at room temperature

The full dehydrogenation of C₂H₄ by gas-phase anions Ta₃N₂⁻ as well as the structure and reactivity of the M–N–C cluster is reported for the first time.

Ménage-à-trois: single-atom catalysis, mass spectrometry, and computational chemistry

Genuine, single-atom catalysis can be realized in the gas phase and probed by mass spectrometry combined with computational chemistry.

CO oxidation on Rh-doped hexadecagold clusters

Exploring the unique catalytic properties of gold clusters associated with specific nano-architectures is essential for designing improved catalysts with a high mass-specific activity.

Low-Temperature Disproportionation Reaction of NO on Au6-: A Mechanism Involving Three NO Molecules Promoted by the Negative Charge

Conversion of NO to other nitrogen oxides is an elementary step in its catalytic removal processes. On coinage metal surfaces, two kinds of NO activation mechanisms have been well documented: the unimolecular dissociation of NO generates two adsorbed atoms, and the dissociation of an adsorbed (NO)₂ unit generates an adsorbed O and a free N₂O. In this work, we observed a disproportionation mechanism involving three NO molecules on Au₆^- at a very low temperature (150 K), in which an adsorbed (NO)₂ reacts with a free NO forming an adsorbed NO₂ and a free N₂O. The density functional theory (DFT) calculations indicated that this disproportionation step is significantly exothermic and has a very low activation barrier. The charge distributions on the involved cluster complexes and the correlation between the activity and the electronic properties of Au₆^- indicate the important role of extra negative charge in all reaction steps. The disproportionation mechanism revealed in this work could possibly exist in the NO removal processes on real gold catalysts.

Thermal Methane Activation by [Si2 O5 ].+ and [Si2 O5 H2 ].+ : Reactivity Enhancement by Hydrogenation

The thermal reactions of methane with the oxygen-rich cluster cations [Si₂ O₅ ]^⋅+ and [Si₂ O₅ H₂ ]^⋅+ have been examined using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with state-of-the-art quantum chemical calculations. In contrast to the inertness of [Si₂ O₅ ]^.+ towards methane, the hydrogenated cluster [Si₂ O₅ H₂ ]^.+ brings about hydrogen-atom transfer (HAT) from methane with an efficiency of 28 % relative to the collision rate. The mechanisms of this process have been investigated in detail and the reasons for the striking reactivity difference of the two cluster ions have been revealed.