CO Oxidation Promoted by the Gold Dimer in Au2VO3− and Au2VO4− Clusters

Investigations on the reactivity of atomic clusters have led to the identification of the elementary steps involved in catalytic CO oxidation, a prototypical reaction in heterogeneous catalysis. The atomic oxygen species O^.? and O^2? bonded to early‐transition‐metal oxide clusters have been shown to oxidize CO. This study reports that when an Au₂ dimer is incorporated within the cluster, the molecular oxygen species O₂^2? bonded to vanadium can be activated to oxidize CO under thermal collision conditions. The gold dimer was doped into Au₂VO₄^? cluster ions which then reacted with CO in an ion‐trap reactor to produce Au₂VO₃^? and then Au₂VO₂^?. The dynamic nature of gold in terms of electron storage and release promotes CO oxidation and O? O bond reduction. The oxidation of CO by atomic clusters in this study parallels similar behavior reported for the oxidation of CO by supported gold catalysts.     

Activation of Methane Promoted by Adsorption of CO on Mo2C2− Cluster Anions

Atomic clusters are being actively studied for activation of methane, the most stable alkane molecule. While many cluster cations are very reactive with methane, the cluster anions are usually not very reactive, particularly for noble metal free anions. This study reports that the reactivity of molybdenum carbide cluster anions with methane can be much enhanced by adsorption of CO. The Mo₂C₂^? is inert with CH₄ while the CO addition product Mo₂C₃O^? brings about dehydrogenation of CH₄ under thermal collision conditions. The cluster structures and reactions are characterized by mass spectrometry, photoelectron spectroscopy, and quantum chemistry calculations, which demonstrate that the Mo₂C₃O^? isomer with dissociated CO is reactive but the one with non‐dissociated CO is unreactive. The enhancement of cluster reactivity promoted by CO adsorption in this study is compared with those of reported systems of a few carbonyl complexes.     

Thermal Methane Conversion to Formaldehyde Promoted by Single Platinum Atoms in PtAl2O4− Cluster Anions

Identification and mechanistic study of thermal methane conversion mediated by gas‐phase species is important for finding potentially useful routes for direct methane transformation under mild conditions. Negatively charged oxide species are usually inert with methane. This work reports an unexpected result that the bi‐metallic oxide cluster anions PtAl₂O₄^? can transform methane into a stable organic compound, formaldehyde, with high selectivity. The clusters are prepared by laser ablation and reacted with CH₄ in an ion trap reactor. The reaction is characterized by mass spectrometry and density functional theory calculations. It is found that platinum rather than oxygen activates CH₄ at the beginning of the reaction. The Al₂O₄^? moiety serves as the support of Pt atom and plays important roles in the late stage of the reaction. A new mechanism for selective methane conversion is provided and new insights into the surface chemistry of single Pt atoms may be obtained from this study.     

Co‐Adsorption of O2 and CO on Au2−: Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2O2(CO)n]− (n=2–6)

The co‐adsorption of O₂ and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au₂O₂(CO)_n]^? (n=2–6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au₂O₂(CO)_n]^? (n=2–6) complexes were characterized to have a core structure involving one CO and one O₂ molecule co‐adsorbed on Au₂^? with the other CO molecules physically tagged around. The CO stretching frequency of the [Au₂O₂(CO)]^? core ion is observed around =2032–2042 cm^?1, which is about 200 cm^?1 higher than that in [Au₂(CO)₂]^?. This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O₂ and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co‐adsorbed O₂ and CO on Au₂^? may have relevance to the elementary reaction steps on real gold catalysts.     

Thermal Methane Activation by La6O10− Cluster Anions

The first example of a metal oxide cluster anion, La₆O₁₀^? that can activate methane under ambient conditions is reported. This reaction is facilitated by the oxygen‐centered radical (O^??) and follows the hydrogen atom transfer mechanism. The La₆O₁₀^? has a high vertical electron detachment energy (VDE=4.06 eV) and a high symmetry (C_4v).     

The Mixed Gold–Palladium Polyoxo‐Noble‐Metalate [NaAuIII4PdII8O8(AsO4)8]11−

The first fully inorganic, discrete gold–palladium–oxo complex [NaAu^III₄Pd^II₈O₈(AsO₄)₈]^11? has been synthesized in aqueous medium. The combination of single‐crystal XRD, elemental analysis, mass spectrometry, and DFT calculations allowed establishing the structure and composition of the novel polyanion, and UV/Vis studies suggest that it is stable in neutral aqueous media.     

Catalytic CO Oxidation by O2 Mediated by Noble‐Metal‐Free Cluster Anions Cu2VO3–5−

Catalytic CO oxidation by molecular O₂ is an important model reaction in both the condensed phase and gas‐phase studies. Available gas‐phase studies indicate that noble metal is indispensable in catalytic CO oxidation by O₂ under thermal collision conditions. Herein, we identified the first example of noble‐metal‐free heteronuclear oxide cluster catalysts, the copper–vanadium bimetallic oxide clusters Cu₂VO_3–5^? for CO oxidation by O₂. The reactions were characterized by mass spectrometry, photoelectron spectroscopy, and density functional calculations. The dynamic nature of the Cu?Cu unit in terms of the electron storage and release is the driving force to promote CO oxidation and O₂ activation during the catalysis.     

CO Oxidation by Neutral Gold-Vanadium Oxide Clusters

Oxidation of CO by gas-phase atomic clusters is being actively studied to understand the molecular-level mechanisms of heterogeneous CO oxidation over related catalytic surfaces. However, it is experimentally challenging to study CO oxidation by neutral heteronuclear metal oxide clusters because of the difficulty of cluster ionization and detection without fragmentation. Herein, the neutral AuVO_2-4 clusters were experimentally generated and their reactions with CO and O₂ were studied. The experimental results showed that CO adsorption is the dominant channel on the interactions of AuVO₄ and AuVO₃ with CO, and AuVO₂ can pick up an O₂ molecule to generate AuVO₄. Theoretical studies indicated that the oxidation of the trapped CO in AuVO_3,4CO into CO₂ is exothermic while the reaction barriers have to be overcome at the elevated temperatures. A catalytic cycle for CO oxidation by AuVO_2-4 is proposed.     

Thermal Conversion of Methane to Formaldehyde Promoted by Gold in AuNbO3+ Cluster Cations

In addition to generation of a methyl radical, formation of a formaldehyde molecule was observed in the thermal reaction of methane with AuNbO₃⁺ heteronuclear oxide cluster cations. The clusters were prepared by laser ablation and mass‐selected to react with CH₄ in an ion‐trap reactor under thermal collision conditions. The reaction was studied by mass spectrometry and DFT calculations. The latter indicated that the gold atom promotes formaldehyde formation through transformation of an Au?O bond into an Au?Nb bond during the reaction.     

Orientational Preference of Long,Multicenter Bonds in Radical Anion Dimers: A Case Study of π‐[TCNB]22− and π‐[TCNP]22−

The similar shape and electronic structure of the radical anions of 1,2,4,5‐tetracyanopyrazine (TCNP) and 1,2,4,5‐tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon?carbon bond in π‐[TCNP]₂^2? and in π‐[TCNB]₂^2?, in good accord with the Maximin Principle predictions. Instead, the two known structures of π‐[TCNP]₂^2? have a D_2h(θ=0°) and a C₂(θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six‐membered rings). The only known π‐[TCNB]₂^2? structure has a C₂(θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (E_int) and the variation of the E_int components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the E_int(θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of E_int(θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the E_int(θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)^.? ??? cation⁺ interactions are also included in the model aggregate are the experimental data reproduced computationally.     

二元铜族团簇负离子催化CO氧化反应机理

用密度泛函理论B3LYP方法研究了二元铜族团簇负离子AuAg^-, AuCu^-和AgCu^-催化CO氧化反应的详细机理. 计算结果表明: CO在混合团簇中的吸附位顺序为Cu>Au>Ag; O₂也优先吸附到Cu上, 其次为Ag, 最难的为Au; 另外, O₂分子较CO分子易于吸附到混合团簇上. CO氧化反应有三条反应通道, 在热力学和动力学上均容易进行. AuAg-团簇催化CO氧化反应的最优反应通道为CO插入AuAgO₂^-中的Ag―O键形成中间体[Au―AgC(O―O)O]^-, 然后直接分解形成CO₂和AuAgO^-, 或另一个CO分子进攻中间体[Au―AgC(O―O)O]^-形成两分子的CO₂和AuAg^-. 而AuCu^-和AgCu^-催化CO氧化反应的最优反应通道为CO和O₂共吸附到团簇上,然后形成四元环中间体,最后四元环中间体分解形成产物或另一个CO分子进攻四元环中间体从而形成产物. 第二个CO分子的协同效应不明显. AuAg^-和AuCu^-对CO氧化反应催化活性强于Au₂^-团簇, 因此, Ag和Cu掺杂可以提高金团簇的催化活性, 与之前实验研究结果一致.     

CO2 Activation and Hydrogenation by PtHn− Cluster Anions

Gas phase reactions between PtH_n^? cluster anions and CO₂ were investigated by mass spectrometry, anion photoelectron spectroscopy, and computations. Two major products, PtCO₂H^? and PtCO₂H₃^?, were observed. The atomic connectivity in PtCO₂H^? can be depicted as HPtCO₂^?, where the platinum atom is bonded to a bent CO₂ moiety on one side and a hydrogen atom on the other. The atomic connectivity of PtCO₂H₃^? can be described as H₂Pt(HCO₂)^?, where the platinum atom is bound to a formate moiety on one side and two hydrogen atoms on the other. Computational studies of the reaction pathway revealed that the hydrogenation of CO₂ by PtH₃^? is highly energetically favorable.     

Bare Clusters Derived from Protein Templates: Au25+, Au38+ and Au102+

A discrete sequence of bare gold clusters of well‐defined nuclearity, namely Au₂₅⁺, Au₃₈⁺ and Au₁₀₂⁺, formed in a process that starts from gold‐bound adducts of the protein lysozyme, were detected in the gas phase. It is proposed that subsequent to laser desorption ionization, gold clusters form in the gas phase, with the protein serving as a confining growth environment that provides an effective reservoir for dissipation of the cluster aggregation and stabilization energy. First‐principles calculations reveal that the growing gold clusters can be electronically stabilized in the protein environment, achieving electronic closed‐shell structures as a result of bonding interactions with the protein. Calculations for a cluster with 38 gold atoms reveal that gold interaction with the protein results in breaking of the disulfide bonds of the cystine units, and that the binding of the cysteine residues to the cluster depletes the number of delocalized electrons in the cluster, resulting in opening of a super‐atom electronic gap. This shell‐closure stabilization mechanism confers enhanced stability to the gold clusters. Once formed as stable magic number aggregates in the protein growth medium, the gold clusters become detached from the protein template and are observed as bare Au_n⁺ (n=25, 38, and 102) clusters.     

Energies and Spin States of FeS0/−, FeS20/−, Fe2S20/−, Fe3S40/−, and Fe4S40/− Clusters

The structures and energies of the electronic ground states of the FeS^0/?, FeS₂^0/?, Fe₂S₂^0/?, Fe₃S₄^0/?, and Fe₄S₄^0/? neutral and anionic clusters have been computed systematically with nine computational methods in combination with seven basis sets. The computed adiabatic electronic affinities (AEA) have been compared with available experimental data. Most reasonable agreements between theory and experiment have been found for both hybrid B3LYP and B3PW91 functionals in conjugation with 6‐311+G* and QZVP basis sets. Detailed comparisons between the available experimental and computed AEA data at the B3LYP/6‐311+G* level identified the electronic ground state of ⁵Δ for FeS, ⁴Δ for FeS^?, ⁵B₂ for FeS₂, ⁶A₁ for FeS₂^?, ¹A₁ for Fe₂S₂, ⁸A′ for Fe₂S₂^?, ⁵A′′ for Fe₃S₄, ⁶A′′ for Fe₃S₄^?, ¹A₁ for Fe₄S₄, and ¹A₂ for Fe₄S₄^?. In addition, Fe₂S₂, Fe₃S₄, Fe₃S₄^?, Fe₄S₄, and Fe₄S₄^? are antiferromagnetic at the B3LYP/6‐311+G* level. The magnetic properties are discussed on the basis of natural bond orbital analysis.     

Gas‐Phase Synthesis and Reactivity of the Lithium Acetate Enolate Anion, −CH2CO2Li

Aerial pingpong : The lithium acetate enolate anion, the prototypical lithium salt of an α‐deprotonated carboxylate, was prepared in the gas phase by electrospray ionization (ESI) and collision‐induced ionization (CID). Its structure, reactivity, and energetics are presented along with the results of high‐level computations.