Jing-Heng Meng

Reactivity of Atomic Oxygen Radical Anions Bound to Titania and Zirconia Nanoparticles in the Gas Phase: Low-Temperature Oxidation of Carbon Monoxide

Titanium and zirconium oxide cluster anions with dimensions up to nanosize are prepared by laser ablation and reacted with carbon monoxide in a fast low reactor. The cluster reactions are characterized by time-of-flight mass spectrometry and density functional theory calculations. The oxygen atom transfers from (TiO(2))(n)O(-) (n = 3-25) to CO and formations of (TiO(2))(n)(-) are observed, whereas the reactions of (ZrO(2))(n)O(-) (n = 3-25) with CO generate the CO addition products (ZrO(2))(n)OCO(-), which lose CO(2) upon the collisions (studied for n = 3-9) with a crossed helium beam. The computational study indicates that the (MO(2))(n)O(-) (M = Ti, Zr; n = 3-8) clusters are atomic radical anion (O(-)) bonded systems, and the energetics for CO oxidation by the O(-) radicals to form CO(2) is strongly dependent on the metals as well as the cluster size for the titanium system. Atomic oxygen radical anions are important reactive intermediates, while it is difficult to capture and characterize them for condensed phase systems. The reactivity pattern of the O(-)-bonded (TiO(2))(n)O(-) and (ZrO(2))(n)O(-) correlates very well with different behaviors of titania and zirconia supports in the low-temperature catalytic CO oxidation.

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 ).

Reactivity of Oxygen Radical Anions Bound to Scandia Nanoparticles in the Gas Phase: CH Bond Activation

The activation of C-H bonds in alkanes is currently a hot research topic in chemistry. The atomic oxygen radical anion (O(-·)) is an important species in C-H activation. The mechanistic details of C-H 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)(N)O(-) (N=1-18) clusters was observed. The reactivity of (Sc2O3)(N)O(-) (N=1-18) clusters is significantly sizedependent and the highest reactivity was observed for N=4 (Sc8O13(-)) and 12 (Sc24O37(-)). Larger (Sc2O3)(N)O(-) clusters generally have higher reactivity than the smaller ones. Density functional theory calculations were performed to interpret the reactivity of (Sc2O3)(N)O(-) (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 C-H bond activation.

Structural stability and electronic state of transition metal trimers.

Ground state geometries were searched for transition metal trimers Sc3, Y3, La3, Lu3, Ti3, Zr3, and Hf3 by density functional methods. For all the studied trimers, our calculation indicates that the ground state geometries are either equilateral triangle (Zr3 and Hf3) or near equilateral triangle (Ti3, Sc3, Y3, La3, and Lu3). For rare earth trimers Sc3, Y3, La3, and Lu3, isosceles triangle (near equilateral triangle) at quartet state is the ground state. Isosceles triangle at doublet state is the competitive candidate for the ground state. For Zr3 and Hf3, equilateral triangle at singlet state is the most stable. For Ti3, isosceles triangle (near equilateral triangle) at quintet state gives the ground state. For Sc3, Zr3, and Hf3, where experimental results are available, the predicted geometries are in agreement with experiment in which the ground state is equilateral triangle (Zr3) or fluxional (Sc3 and Hf3). For Y3, the calculated geometry is in agreement with experimental observation and previous theoretical study that Y3 is a bent molecule for the ground state. For La3, our calculation is in excellent agreement with previous theoretical study based on density functional methods.

Activation and Transformation of Ethane by Au2VO3+ Clusters with Closed‐Shell Electronic Structures

The study of chemical reactions between gold-containing heteronuclear oxide clusters and small molecules can provide molecular level mechanisms to understand the excellent activity of gold supported by metal oxides. While the promotion role of gold in alkane transformation was identified in the clusters with atomic oxygen radicals (O(-.)), the role of gold in the systems without O(-.) is not clear. By employing mass spectrometry and quantum chemistry calculations, the reactivity of Au2 VO3(+) clusters with closed-shell electronic structures toward ethane was explored. Both the dehydrogenation and ethene elimination channels were identified. It is gold rather than oxygen species initiating the C-H activation. The Au-Au dimer formed during the reactions plays important roles in ethane transformation. The reactivity comparison between Au2 VO3(+) and bare Au2(+) demonstrates that Au2 VO3(+) not only retains the property of bare Au2(+) that transforming ethane to dihydrogen, but also exhibits new functions in converting ethane to ethene, which reveals the importance of the composite system. This study provides a further understanding of the reactivity of metal oxide supported gold in alkane activation and transformation.