Molecular and Electronic Structures, Brönsted Basicities, and Lewis Acidities of Group VIB Transition Metal Oxide Clusters

Density functional study of water-gas shift reaction on M3O(3x)/Cu(111)

Density functional theory (DFT) was employed to study the water dissociation and water-gas shift (WGS) reaction on a series of inverse model catalysts, M(3)O(3x)/Cu(111) (M = Mg, Ti, Zr, Mo, W; x = 1, 2, 3). It has been found that the WGS reaction on Cu can be facilitated by introducing various oxides to lower the barrier of water dissociation. Accordingly, the calculated reaction energy for water dissociation was used as a scaling descriptor to screen the WGS activity of oxide-Cu model catalysts. Our calculations show that the activity towards water dissociation decreases in a sequence: Mg(3)O(3)/Cu(111) > Zr(3)O(6)/Cu(111) > Ti(3)O(6)/Cu(111) > W(3)O(9)/Cu(111), Mo(3)O(9)/Cu(111). It seems that Mg(3)O(3)/Cu(111) is the best WGS catalyst among the systems studied here, being able to dissociate water with no barrier. During the process, both Cu and oxides participate in the reaction directly. The strong M(3)O(3x)-Cu interaction is able to tune the electronic structure of M(3)O(3x) and therefore the activity towards water dissociation. Further studies of the overall WGS reaction on Mg(3)O(3)/Cu(111) show that water dissociation may not be the key step to control the WGS reaction on Mg(3)O(3)/Cu(111) and the removal of H from Mg(3)O(3) can be problematic. The strong interaction between H and O from Mg(3)O(3) blocks the O sites for further water dissociation and therefore the WGS reaction. Our study observes a very different behavior of oxide clusters in such small size from the bigger ones supported on Cu(111) and provides new insight into the rational design of the WGS catalysts.

Alcohol Dehydration on Monooxo W═O and Dioxo O═W═O Species

The dehydration of 1-propanol on nanoporous WO3 films prepared via ballistic deposition at ∼20 K has been investigated using temperature-programmed desorption, infrared reflection absorption spectroscopy, and density functional theory. The as-deposited films are extremely efficient in 1-propanol dehydration to propene. This activity is correlated with the presence of dioxo O═W═O groups, whereas monooxo W═O species are shown to be inactive. Annealing of the films induces densification that results in the loss of catalytic activity due to the annihilation of O═W═O species.

Molybdenum oxides versus molybdenum sulfides: geometric and electronic structures of Mo₃X(y)⁻ (X = O, S and y = 6, 9) clusters

We have conducted a comparative computational investigation of the molecular structure and water adsorption properties of molybdenum oxide and sulfide clusters using density functional theory methods. We have found that while Mo₃O₆⁻ and Mo₃S₆⁻ assume very similar ring-type isomers, Mo₃O₉⁻ and Mo₃S₉⁻ clusters are very different with Mo₃O₉⁻ having a ring-type structure and Mo₃S₉⁻ having a more open, linear-type geometry. The more rigid ∠(Mo-S-Mo) bond angle is the primary geometric property responsible for producing such different lowest energy isomers. By computing molecular complexation energies, it is observed that water is found to adsorb more strongly to Mo₃O₆⁻ than to Mo₃S₆⁻, due to a stronger oxide-water hydrogen bond, although dispersion effects reduce this difference when molybdenum centers contribute to the binding. Investigating the energetics of dissociative water addition to Mo₃X₆⁻ clusters, we find that, while the oxide cluster shows kinetic site-selectivity (bridging position vs terminal position), the sulfide cluster exhibits thermodynamic site-selectivity.

Tetratungsten oxide clusters W4O(n)(-/0) (n = 10-13): structural evolution and chemical bonding

Density functional theory (DFT) calculations are carried out to investigate the electronic and structural properties of a series of tetratungsten oxide clusters, W(4)O(n)(-/0) (n = 10-13). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). A large energy gap (approximately 2.9 eV) is observed for the stoichiometric W(4)O(12) cluster, which reaches the bulk value. The calculations suggest that W(4)O(12)(-/0) have the planar eight-membered ring structures, in which each tungsten atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms. W(4)O(10)(-/0) and W(4)O(11)(-) can be viewed as removing two and one terminal O atoms from W(4)O(12)(-/0), respectively. The W(4)O(11) neutral is an interesting species, which possesses the pentabridged structure. We show that W(4)O(11)(-) contains a localized W(3+) site, which can readily react with O(2) to form the W(4)O(13)(-) cluster, whereas the corresponding neutral W(4)O(13) can be viewed as replacing a terminal oxygen in W(4)O(12) by a peroxo O(2) unit. Molecular orbital analyses are performed to analyze the chemical bonding in the tetratungsten oxide clusters and to elucidate their electronic and structural evolution.

Cluster reactivity experiments: employing mass spectrometry to investigate the molecular level details of catalytic oxidation reactions

Mass spectrometry is the most widely used tool in the study of the properties and reactivity of clusters in the gas phase. In this article, we demonstrate its use in investigating the molecular-level details of oxidation reactions occurring on the surfaces of heterogeneous catalysts via cluster reactivity experiments. Guided ion beam mass spectrometry (GIB-MS) employing a quadrupole-octopole-quadrupole (Q-O-Q) configuration enables mass-selected cluster ions to be reacted with various chemicals, providing insight into the effect of size, stoichiometry, and ionic charge state on the reactivity of catalyst materials. For positively charged tungsten oxide clusters, it is shown that species having the same stoichiometry as the bulk, WO(3)(+), W(2)O(6)(+), and W(3)O(9)(+), exhibit enhanced activity and selectivity for the transfer of a single oxygen atom to propylene (C(3)H(6)), suggesting the formation of propylene oxide (C(3)H(6)O), an important monomer used, for example, in the industrial production of plastics. Furthermore, the same stoichiometric clusters are demonstrated to be active for the oxidation of CO to CO(2), a reaction of significance to environmental pollution abatement. The findings reported herein suggest that the enhanced oxidation reactivity of these stoichiometric clusters may be due to the presence of radical oxygen centers (W-O) with elongated metal-oxygen bonds. The unique insights gained into bulk-phase oxidation catalysis through the application of mass spectrometry to cluster reactivity experiments are discussed.

Matrix Isolation Infrared Spectroscopic and Theoretical Study of Dinuclear Chromium Oxide Clusters: Cr2On (n = 2, 4, 6) in Solid Argon

Dichromium oxide clusters, Cr2O2, Cr2O4, and Cr2O6, have been prepared and characterized by matrix isolation infrared spectroscopy and quantum chemical calculations. Laser-evaporated chromium atoms reacted with O2 in solid argon to form the previously characterized CrO2 molecules, which further reacted with chromium atoms to form Cr2O2 spontaneously on annealing. The Cr2O2 cluster is determined to have a chainlike CrOCrO structure. The rhombic ring isomer, which was predicted to be more stable than the CrOCrO structure, was not formed at the present experimental conditions. The Cr2O4 cluster was formed from the barrierless dimerization of the chromium dioxide molecules, which is characterized to have a planar D2h symmetry. TheCr2O6 cluster was produced under UV light irradiation. It is determined to have a singlet ground state with a nonplanar D2h symmetry.