Time-of-flight mass spectrometry experiments demonstrated that laser ablation generated and mass selected Au₂TiO₄⁻ cluster anions can unexpectedly oxidize three CO molecules in an ion trap reactor. This is an improvement on the more commonly observed oxidation of at most two CO molecules by a doped cluster. Quantum chemistry calculations were performed to rationalize the reactions. The lowest energy isomer of Au₂TiO₄⁻ contains a superoxide unit, the participation of which in CO oxidation can be promoted by the Au₂ dimer. The Au₂ dimer functions as a rather flexible electron reservoir in each CO oxidation step in terms of the release and storage of electrons to drive the dissociation of superoxide to peroxide and then to lattice oxygen atoms, which can be removed by reaction with CO molecules. This gas-phase study enriches our understanding toward the nature of reactive oxygen species involved in gold-catalyzed oxidation reactions.

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 O²⁻ 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 O₂²⁻ 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 OO 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.

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 O²⁻ 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 O₂²⁻ 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 OO 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.

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas-phase and condensed-phase studies. Herein, methane activation by the diatomic cation MoC⁺ is presented. MoC⁺ ions have been prepared and mass-selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time-of-flight mass spectrometer. Bare metal Mo⁺ and MoC₂H₂⁺ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C₂H₄ elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC⁺ with CH₄. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the CC coupling (doubletquartet) and hydrogen atom transfer (quartetsextet) steps. The carbon atom in MoC⁺ plays a key role in methane activation because it becomes sp³ hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the CH bond activation and CC coupling involved in methane transformation over molybdenum carbide-based catalysts.

The reactivity of metal oxide clusters toward hydrocarbon molecules can be changed, tuned, or controlled by doping. Cerium-doped vanadium cluster cations CeV₂O₇⁺ are generated by laser ablation, mass-selected by a quadrupole mass filter, and then reacted with C₂H₄ in a linear ion trap reactor. The reaction is characterized by a reflectron time-of-flight mass spectrometer. Three types of reaction channels are observed: 1) single oxygen-atom transfer , 2) double oxygen-atom transfer , and 3) CC bond cleavage. This study provides the first bimetallic oxide cluster ion, CeV₂O₇⁺, which gives rise to CC bond cleavage of ethene. Neither Ce_xO_y^± nor V_xO_y^± alone possess the necessary topological and electronic properties to bring about such a reaction.

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.

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 activation of CH bonds in alkanes is currently a hot research topic in chemistry. The atomic oxygen radical anion (O^−.) is an important species in CH activation. The mechanistic details of CH activation by O^−. radicals can be well understood by studying the reactions between O^−. containing transition metal oxide clusters and alkanes. Here the reactivity of scandium oxide cluster anions toward n-butane was studied by using a high-resolution time-of-flight mass spectrometer coupled with a fast flow reactor. Hydrogen atom abstraction (HAA) from n-butane by (Sc₂O₃)_NO⁻ (N=1–18) clusters was observed. The reactivity of (Sc₂O₃)_NO⁻ (N=1–18) clusters is significantly sizedependent and the highest reactivity was observed for N=4 (Sc₈O₁₃⁻) and 12 (Sc₂₄O₃₇⁻). Larger (Sc₂O₃)_NO⁻ clusters generally have higher reactivity than the smaller ones. Density functional theory calculations were performed to interpret the reactivity of (Sc₂O₃)_NO⁻ (N=1–5) clusters, which were found to contain the O^−. radicals as the active sites. The local charge environment around the O^−. radicals was demonstrated to control the experimentally observed size-dependent reactivity. This work is among the first to report HAA reactivity of cluster anions with dimensions up to nanosize toward alkane molecules. The anionic O^−. containing scandium oxide clusters are found to be more reactive than the corresponding cationic ones in the CH bond activation.

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.

Oxygen-rich scandium cluster anions ScO_3–5⁻ are prepared by laser ablation and allowed to react with n-butane in a fast-flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. The ScO₃⁻ and ScO₄⁻ clusters can react with n-butane to produce ScO₃H⁻, ScO₃H₂⁻, and ScO₄H⁻, while the more oxygen-rich cluster ScO₅⁻ is inert. The experiment suggests that unreactive cluster isomers of ScO₃⁻ and ScO₄⁻ are also present in the cluster source. Density functional theory and ab initio methods are used to calculate the structures and reaction mechanisms of the clusters. The theoretical results indicate that the unreactive and reactive cluster isomers of ScO_3,4⁻ contain peroxides (O₂²⁻) and oxygen-centered radicals (O^.−), respectively. The mechanisms and energetics for conversion of unreactive O₂²⁻ to reactive O^.− species are also theoretically studied.

Vanadium–silver bimetallic oxide cluster ions (V_xAg_yO_z⁺; x=1–4, y=1–4, z=3–11) are produced by laser ablation and reacted with ethane in a fast-flow reactor. A reflectron time of flight (Re-TOF) mass spectrometer is used to detect the cluster distribution before and after the reactions. Hydrogen atom abstraction (HAA) reactions are identified over VAgO₃⁺, V₂Ag₂O₆⁺, V₂Ag₄O₇⁺, V₃AgO₈⁺, V₃Ag₃O₉⁺, and V₄Ag₂O₁₁⁺ ions, in which the oxygen-centered radicals terminally bonded on V atoms are active sites for the facile HAA reactions. DFT calculations are performed to study the structures, bonding, and reactivity. The reaction mechanisms of V₂Ag₂O₆⁺+C₂H₆ are also given. The doped Ag atoms with a valence state of +1 are highly dispersed at the periphery of the V_xAg_yO_z⁺ cluster ions. The reactivity can be well-tuned gradually by controlling the number of Ag atoms. The steric protection due to the peripherally bonded Ag atoms greatly enhances the selectivity of the V–Ag bimetallic oxide clusters with respect to the corresponding pure vanadium oxide systems.

Cerium oxide cluster cations (Ce_mO_n⁺, m=2–16; n=2m, 2m±1 and 2m±2) are prepared by laser ablation and reacted with acetylene (C₂H₂) in a fast-flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Reactions of stoichiometric Ce_mO_2m⁺ (m=2–6) with C₂H₂ produce Ce_mO_2m−2⁺ clusters, which indicates a “double-oxygen-atom transfer” reaction Ce_mO_2m⁺+C₂H₂Ce_mO_2m−2⁺+(CHO)₂ (ethanedial). A single-oxygen-atom transfer reaction channel is also identified as Ce_mO_2m⁺+C₂H₂Ce_mO_2m−1⁺+C₂H₂O (at least for m=2 and 3). Density functional theory calculations are performed to study reaction mechanisms of Ce₂O₄⁺+C₂H₂, and the calculated results confirm that both the single- and double-oxygen-atom transfer channels are thermodynamically and kinetically favourable.

Vanadium–silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O₂ background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V₂O₅)_n(SiO₂)_m]⁺ (n=1, m=1–4; n=2, m=1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V₄O₁₀⁺ with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (O_t^.) were found in all of the stable isomers. These O_t^. radicals are active sites of the clusters in reaction with CH₄. The O_t^. radicals in [V₂O₅(SiO₂)_1–4]⁺ clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH₄ activation on silica-supported vanadium oxide catalysts.

Density functional theory (DFT) calculations are used to investigate the reaction mechanism of V₃O₈+C₂H₄. The reaction of V₃O₈ with C₂H₄ produces V₃O₇CH₂+HCHO or V₃O₇+CH₂OCH₂ overall barrierlessly at room temperature, whereas formation of hydrogen-transfer products V₃O₇+CH₃CHO is subject to a tiny overall free energy barrier (0.03 eV), although the formation of the last-named pair of products is thermodynamically more favorable than that of the first two. These DFT results are in agreement with recent experimental observations. The (O_b)₂V(O_tO_t)^. (b=bridging, t=terminal) moiety containing the oxygen radical in V₃O₈ is the active site in the reaction with C₂H₄. Similarities and differences between the reactivities of (O_b)₂V(O_tO_t)^. in V₃O₈ and the small VO₃ cluster [(O_t)₂VO_t^.] are discussed. Moreover, the effect of the support on the reactivity of the (O_b)₂V(O_tO_t)^. active site is evaluated by investigating the reactivity of the cluster VX₂O₈, which is obtained by replacing the V atoms in the (O_b)₃VO_t support moieties of V₃O₈ with X atoms (X=P, As, Sb, Nb, Ta, Si, and Ti). Support X atoms with different electronegativities influence the oxidative reactivity of the (O_b)₂V(O_tO_t)^. active site through changing the net charge of the active site. These theoretical predictions of the mechanism of V₃O₈+C₂H₄ and the effect of the support on the active site may be helpful for understanding the reactivity and selectivity of reactive O^. species over condensed-phase catalysts.