Regioselective Bond-Forming and Hydrolysis Reactions of Doubly Charged Vanadium Oxide Anions in the Gas Phase

The gas-phase reactivity of vanadium-containing dianions, NaV3O92− and its hydrated form H2NaV3O102−, were probed towards sulphur dioxide at room temperature by ion-molecule reaction (IMR) experiments in the collision cell of an ion trap mass spectrometer. The sequential addition of two SO2 molecules to the NaV3O92− dianion leads to the breakage of the stable V3O9 backbone, resulting in a charge separation process with the formation of new V-O and S-O bonds. On the contrary, the H2NaV3O102− hydroxide species reacts with SO2, promoting regioselective hydrolysis and bond-forming processes, the latter similar to that observed for the NaV3O92− reactant anion. Kinetic analysis shows that these reactions are fast and efficient with rate constants of the 10−9 (±30) cm3 s−1 molecule−1 order of magnitude.

Oxidation of isoprene by titanium oxide cluster cations in the gas phase

The heterogeneous oxidation of isoprene (C5H8) by metal-oxide particles, such as the typical mineral aerosols TiO2, plays an important role in the isoprene atmospheric chemistry. However, the underlying mechanism of C5H8 oxidation remains elusive owing to the complexities of aerosol surfaces and reaction channels. Herein, we report the gas-phase reactions of TixOy+ (x = 1-7, y = 1-14) cations with isoprene by using mass spectrometry and density functional theory (DFT) calculations. Five types of reaction channels were observed: association, hydrogen atom transfer (HAT), C-C bond cleavage, combined oxygen atom transfer (OAT) and HAT and combined OAT and C-C bond cleavage. It is noteworthy that formaldehyde is known as the major oxidation product of isoprene/hydroxyl radicals in the atmosphere. In addition, CO has not been observed in the reactions of isoprene with gas-phase ions. Therefore, the reaction mechanisms of CH2O and CO generation observed in Ti2O5+/C5H8 and Ti4O8+/C5H8 systems were further investigated by DFT calculations, and the calculated results are in agreement with the experimental observations. In these two reactions, both Ti and O atoms can be the adsorption sites for C5H8. The reaction channels and mechanistic information gained in these gas-phase model reactions may offer fundamental insights relevant to the corresponding oxidation processes over titanium oxide aerosols in the atmosphere.

Density functional theory (DFT) studies of vanadium-titanium based selective catalytic reduction (SCR) catalysts

Based on density functional theory (DFT) and basic structure models, the chemical reactions on the surface of vanadium-titanium based selective catalytic reduction (SCR) denitrification catalysts were summarized. Reasonable structural models (non-periodic and periodic structural models) are the basis of density functional calculations. A periodic structure model was more appropriate to represent the catalyst surface, and its theoretical calculation results were more comparable with the experimental results than a non-periodic model. It is generally believed that the SCR mechanism where NH₃ and NO react to produce N₂ and H₂O follows an Eley-Rideal type mechanism. NH₂NO was found to be an important intermediate in the SCR reaction, with multiple production routes. Simultaneously, the effects of H₂O, SO₂ and metal on SCR catalysts were also summarized.

Direct hydroxylation of benzene to phenol mediated by nanosized vanadium oxide cluster ions at room temperature

Gas-phase vanadium oxide cluster cations and anions are prepared by laser ablation. The small cluster ions (<1000 amu) are mass-selected using a quadrupole mass filter and reacted with benzene in a linear ion trap reactor; large clusters (>1000 amu) with no mass selection are reacted with C₆H₆ in a fast flow reactor. Rich product variety is encountered in these reactions, and the reaction channels for small cationic and anionic systems are different. For large clusters, the reactivity patterns of (V₂O₅) _n⁺ (n = 6-25) and (V₂O₅) _n O^- (n = 6-24) cluster series are very similar to each other, indicating that the charge state has little influence on the oxidation of benzene. In sharp contrast to the dramatic changes of reactivity of small clusters, a weakly size dependent reaction behavior of large (V₂O₅)_6-25⁺ and (V₂O₅)_6-24O^- clusters is observed. Therefore, the charge state and the size are not the major factors influencing the reactivity of nanosized vanadium oxide cluster ions toward C₆H₆, which is not common in cluster science. In the reactions with benzene, the small and large reactive vanadium oxide cations show similar reactivity of hydroxyl radicals (OH^•) toward C₆H₆ at higher and lower temperatures, respectively; different numbers of vibrational degrees of freedom and the released energy during the formation of adduct complexes can explain this intriguing correlation. The reactions investigated herein might be used as the models of how to realize the partial oxidation of benzene to phenol in a single step, and the observed mechanisms are helpful to understand the corresponding heterogeneous reactions, such as those over vanadium oxide aerosols and vanadium oxide catalysts.

Fe2 O+ Cation Mediated Propane Oxidation by Dioxygen in the Gas Phase

The mass-selected Fe₂ O⁺ cation mediated propane oxidation by O₂ was investigated by mass spectrometry and density functional theory calculations. In the reaction of Fe₂ O⁺ with C₃ H₈ , H₂ was liberated by C-H bond activation to give Fe₂ OC₃ H₆⁺ . Interestingly, when a mixture of C₃ H₈ /O₂ was introduced into the reactor, an intense signal that corresponded to the Fe₂ O₂⁺ cation was present; the experiments indicated that O₂ was activated in its reaction with Fe₂ O(C₃ H₆ )⁺ to give Fe₂ O₂⁺ and C₃ H₆ O (acetone or propanal). A Langmuir-Hinshelwood-like mechanism was adopted in the propane oxidation reaction by O₂ on gas-phase Fe₂ O⁺ cations. In comparison with the absence of Fe₂ O₂⁺ in the reaction of Fe₂ O⁺ with O₂ , the ligand effect of C₃ H₆ on Fe₂ OC₃ H₆⁺ is important in the oxygen activation reaction. The theoretical results are consistent with the experimental observations. The propane oxidation by O₂ in the presence of Fe₂ O⁺ might be applied as a model for alkane and O₂ activations over iron oxide catalysts, and the mechanisms and kinetic data are useful for understanding corresponding heterogeneous reactions.

Sulphur dioxide cooperation in hydrolysis reactions of vanadium oxide and hydroxide cluster dianions

Gas-phase hydrolysis takes place due to the synergistic action of SO₂ and vanadium-containing dianions that succeeds in tightening hydrogen bonds.

Liberation of three dihydrogens from two ethene molecules as mediated by the tantalum nitride anion cluster Ta3N2− at room temperature

The full dehydrogenation of C₂H₄ by gas-phase anions Ta₃N₂⁻ as well as the structure and reactivity of the M–N–C cluster is reported for the first time.

Compositions and Structures of Vanadium Oxide Cluster Ions VmOn(±) (m = 2-20) Investigated by Ion Mobility Mass Spectrometry

Stable compositions and geometrical structures of vanadium oxide cluster ions, VmOn(±), were investigated by ion mobility mass spectrometry (IM-MS). The most stable compositions of vanadium oxide cluster cations were (V2O4)(V2O5)(m-2)/2(+) and (VO2)(V2O5)(m-1)/2(+), depending on the clusters with even and odd numbers of vanadium atoms. Compositions one-oxygen richer than the cations, such as (V2O5)m/2(-) and (VO3)(V2O5)(m-1)/2(-), were predominantly observed for cluster anions. Assignments of these stable cluster ion compositions, which were determined as a result of collision-induced dissociations in IM-MS, can partly be explained with consideration of spin density distribution. By comparing the experimental collision cross sections (CCSs) obtained from ion mobility measurement with CCSs of the theoretically calculated structures, we confirmed the patterned growth of geometrical structures partially discussed in previous theoretical and spectroscopic studies. We showed that even sized (V2O5)m/2(±) where m = 6-12 had right polygonal prism structures except for the anionic V12O30(-), and for the clusters of odd numbers of vanadium m, cations and anions can either have bridged or pyramid structures. Both of the odd sized structures proposed were derivatives from the even sized right polygonal prism structures. The exception, V12O30(-), which had a CCS almost equal to that of the neighboring smaller V11O28(-), should have a structure of higher density than the right hexagonal prism, in which it was proposed to be a captured pyramid structure, derived from V11O28(-).

Reactions of metal cluster anions with inorganic and organic molecules in the gas phase

Progress on the activation and transformation of important inorganic and organic molecules by negatively charged bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others.