Zi-Yu Li

CO oxidation catalyzed by single gold atoms supported on aluminum oxide clusters.

The single gold atom doped aluminum oxide clusters AuAl3O3(+), AuAl3O4(+), and AuAl3O5(+) have been prepared and mass-selected to react with CO, O2, and mixtures of CO and O2 in an ion trap reactor under thermal collision conditions. The reactions have been characterized by mass spectrometry with isotopic substitution ((16)O2 → (18)O2) and density functional theory calculations. The AuAl3O5(+) cluster can oxidize two CO molecules consecutively to form AuAl3O4(+) and then AuAl3O3(+), the latter of which can react with one O2 molecule to regenerate AuAl3O5(+). The AuAl3(16)O3(+) ions interact with a mixture of C(16)O and (18)O2 to produce the fully substituted (18)O species AuAl3(18)O3-5(+), which firmly identifies a catalytic cycle for CO oxidation by O2. The oxidation catalysis is driven by electron cycling primarily through making and breaking a gold-aluminum chemical bond. To the best of our knowledge, this is the first identification of catalytic CO oxidation by O2 mediated with gas-phase cluster catalysts with single-noble-metal atoms, which serves as an important step to understand single-atom catalysis at strictly a molecular level.

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 PtAl2 O4 (-) 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 Al2 O4 (-) 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.

Methane Activation by Yttrium‐Doped Vanadium Oxide Cluster Cations: Local Charge Effects

Methane activation, involved in the transformation of cheap and abundant natural gas into more valuable organic compounds, is a holy grail in chemistry, and has been studied for decades. Hydrogen atom abstraction (HAA) from CH4 to produce CH3C radicals is considered to be the decisive step in the oxidative dehydrogenation and dimerization of methane. Many metal and nonmetal oxide cluster cations that contain oxygen-centered radicals (O C) as active sites can react with CH4 to generate CH3C under thermal collision conditions. The O C-containing oxide anions usually have lower reactivity than the corresponding cations and can abstract a hydrogen atom only from more reactive alkanes such as ethane or butane. The charged states (cationic versus anionic) can thus significantly influence cluster reactivity. Herein, the experimental and computational studies demonstrate that for a certain charge state, that is the cationic state, the local charge distribution around the O C centers can also change the cluster reactivity toward methane dramatically. Such local charge effects on methane activation can generally exist in other related reaction systems, thus giving an important clue about designing efficient catalysts for methane transformation in practice. To demonstrate the local charge effects on methane activation, heteronuclear oxide clusters, namely, the yttriumdoped vanadium oxide cluster cations, are produced by laser ablation and treated with methane in a fast flow reactor under near-room-temperature-conditions. Methane activation by the V O C centers over pure vanadium oxide cluster cations including V4O10 + [3d] and a few others has been reported. The doping of other metals with a different number of valence electrons and electronegativity from those of vanadium may change the local charge distribution around the V O C centers significantly. Figure S1 (in the Supporting Information) shows that bimetallic V–Y clusters, along with pure V and Y oxide clusters, can be generated. The mass spectra in Figure 1 indicate that V4O10H + , V3YO9H + , and V2Y2O8H + (and the deuterated compounds) are produced upon the cluster reaction with CH4 (and CD4). It suggests that HAA takes place as follows:

Density-functional global optimization of (La2O3)n clusters

Structures of stoichiometric (La(2)O(3))(n) (n = 1-6) clusters have been systematically studied by theoretical calculations. Global minimum structures for these clusters are determined by genetic algorithm based global optimizations at density functional level. The ground state structure for La(6)O(9) was found to be highly symmetric with point group O(h) and the centered oxygen atom has the coordination number as large as six, which is the same as the highest coordination number of oxygen atoms in bulk La(2)O(3). Analysis of the binding energies shows that La(6)O(9) has a high stability among the studied clusters. The energies of the highest occupied∕lowest unoccupied molecular orbitals, vertical ionization energy, and vertical electron affinity of each cluster are provided. Electronic structure of La(6)O(9) is discussed by analysis of the frontier molecular orbitals and unpaired spin density distributions of charged clusters.

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 Au2 VO4 (-) cluster ions which then reacted with CO in an ion-trap reactor to produce Au2 VO3 (-) and then Au2 VO2 (-) . 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.

Thermal Methane Activation by La6O10− Cluster Anions

The first example of a metal oxide cluster anion, La6 O10 (-) 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 La6 O10 (-) has a high vertical electron detachment energy (VDE=4.06 eV) and a high symmetry (C4v ).

Classification of VxOyq Clusters by Δ = 2y+q−5x

Vanadium oxide clusters VxOyq (x ≤ 8, q = 0, ±1) are classified according to the oxidation index (Δ=2y+q−5x) of each cluster. Density functional calculations indicate that clusters with the same oxidation index tend to have similar bonding characters, electronic structures, and reactivities. This general rule leads to the findings of new possible ground state structures for V2O6 and V3O6+ clusters. This successful application of the classification method on vanadium oxide clusters proves that this method is very effective in studying the bonding properties of early transition metal oxide clusters.

Gold(III) Mediated Activation and Transformation of Methane on Au1-Doped Vanadium Oxide Cluster Cations AuV2O6+

Gold in the +III oxidation state (Au(III)) has been proposed as a promising species to mediate challenging chemical reactions. However, it is difficult to characterize the chemistry of individual Au(III) species in condensed-phase systems mainly due to the interference from the Au(I) counterpart. Herein, by doping Au atoms into gas-phase vanadium oxide clusters, we demonstrate that the Au(III) cation in the AuV2O6(+) cluster is active for activation and transformation of methane, the most stable alkane molecule, into formaldehyde under mild conditions. In contrast, the AuV2O6(+) cluster isomers with the Au(I) cation can only absorb CH4. The clusters were generated by laser ablation and mass selected to react with CH4, CD4, or CH2D2 in an ion trap reactor. The reactivity was characterized by mass spectrometry and quantum chemistry calculations. The structures of the reactant and product ions were identified by using collision-induced and 425 nm photo-induced dissociation techniques.

Methane Activation by Iron-Carbide Cluster Anions FeC6(-).

Laser-ablation-generated and mass-selected iron-carbide cluster anions FeC6(-) were reacted with CH4 in a linear ion trap reactor under thermal collision conditions. The reactions were characterized by mass spectrometry and density functional theory calculations. Adsorption product of FeC6CH4(-) was observed in the experiments. The identified large kinetic isotope effect suggests that CH4 can be activated by FeC6(-) anions with a dissociative adsorption manner, which is further supported by the reaction mechanism calculations. The large dipole moment of FeC6(-) (19.21 D) can induce a polarization of CH4 and can facilitate the cleavage of C-H bond. This study reports the CH4 activation by transition-metal carbide anions, which provides insights into mechanistic understanding of iron-carbon centers that are important for condensed-phase catalysis.

Methane Activation by Diatomic Molybdenum Carbide Cations

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 C-C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC(+) plays a key role in methane activation because it becomes sp(3) 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 C-H bond activation and C-C coupling involved in methane transformation over molybdenum carbide-based catalysts.