Aerobic Oxidation of Formaldehyde Catalyzed by Polyvanadotungstates

Three tetra-<i>n</i>-butylammonium (TBA) salts of polyvanadotungstates, [<i>n</i>-Bu₄N]₆[PW₉V₃O₄₀] (<b>PW</b>_<b>9</b><b>V</b>_<b>3</b>), [<i>n</i>-Bu₄N]₅H₂PW₈V₄O₄₀ (<b>PW</b>_<b>8</b><b>V</b>_<b>4</b>), and [<i>n</i>-Bu₄N]₄H₅PW₆V₆O₄₀·20H₂O (<b>PW</b>_<b>6</b><b>V</b>_<b>6</b>), have been synthesized and shown to be effective catalysts for the aerobic oxidation of formaldehyde to formic acid under ambient conditions. These complexes, characterized by elemental analysis, Fourier transform infrared spectroscopy, UV–vis spectroscopy, and thermogravimetric analysis, exhibit a catalytic activity for this reaction comparable to those of other polyoxometalates. Importantly, they are more effective in the presence of water than the metal oxide-supported Pt and/or Au nanoparticles traditionally used as catalysts for formaldehyde oxidation in the gas phase. The polyvanadotungstate-catalyzed oxidation reactions are first-order in formaldehyde, parabolic-order (slow, fast, and slow again) in catalyst, and zero-order in O₂. Under optimized conditions, a turnover number of ∼57 has been obtained. These catalysts can be recycled and reused without a significant loss of catalytic activity

A Hybrid Quantum Mechanical Approach: Intimate Details of Electron Transfer between Type‑I CdSe/ZnS Quantum Dots and an Anthraquinone Molecule

We report a hybrid computational approach to calculate electron transfer between a type-I CdSe/ZnS core/shell quantum dot (QD) with a varying shell thickness and the functionalized anthraquinone (AQ) molecule. This novel approach combines the traditional electron/hole confinement theory in the effective mass approximation for the QD and molecular orbital theory for the AQ molecule. In the present study, the QD’s electron and hole envelope wave functions are solutions of the effective-mass Schrödinger equation, and the AQ wave function is obtained at the density functional level. Electron-transfer rate calculations are based on Marcus’s theory with the coupling strength computed according to an one-electron orbital perturbation model. We show that in a heptane solution, the LUMO of AQ and the 1S_e electron orbital of QD are involved in the charge separation (CS) process. The charge recombination (CR) process, on the other hand, occurs from the singly occupied molecular orbital of the AQ radical (which corresponds to the LUMO in AQ) to a trapped hole state of the QD within the band gap. The calculations support previously reported interpretations of the role of the ZnS shell as a hindrance in the CS and CR process

Synergetic Catalysis of Copper and Iron in Oxidation of Reduced Keggin Heteropolytungstates by Dioxygen

Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O₂-based oxidations is reoxidation of the reduced POM by O₂. We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POM_red (α-PW₁₂O₄₀^4– and α-SiVW₁₁O₄₀^6–) reacting with O₂, is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu­(II) and Fe­(II), followed by a fast oxidation of POM_red by Fe­(III) and Cu­(I) by O₂ to regenerate Fe­(II) and Cu­(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, α-PW₁₂O₄₀^3– and α-SiVW₁₁O₄₀^5–, are far better oxidants of organic substrates than the previously studied POMs, α-SiW₁₂O₄₀^4– and α-AlW₁₂O₄₀^5–, this synergistic Fe/Cu cocatalysis of reduced-POM reoxidation could well facilitate significant new O₂/air-based processes

Reaction Mechanism of Nerve-Agent Hydrolysis with the Cs₈Nb₆O₁₉ Lindqvist Hexaniobate Catalyst

We present a detailed mechanism for the hydrolysis of Sarin catalyzed by Cs₈Nb₆O₁₉ obtained using electronic structure calculations. The initial steps of the reaction involve the adsorption of water and Sarin on the hexaniobate catalyst via nonbonding interactions. Dissociation of the coordinated water molecule generates a hydroxide ion that adds nucleophilically to the coadsorbed Sarin molecule in a concerted manner, following a general base catalysis mechanism. The addition of OH^– to the nerve agent generates a trigonal bipyramidal pentacoordinated phosphorus intermediate that subsequently undergoes facile dissociation forming either HF or isopropanol and a corresponding phosphonic acid. The rate-determining step of the overall reaction is found to be the dissociation of water on the catalyst in concert with the nucleophilic addition of the nascent OH^– to the nerve agent. The calculated barrier for this step is considerably smaller than that measured for bulk base hydrolysis. This work represents a blueprint for future studies aimed to optimize catalysts for base hydrolysis of nerve agents at the gas–surface interface

Structural Modification of TiO₂ Surfaces in Bulk Water and Binding Motifs of a Functionalized C₆₀ on TiO₂ Anatase and Rutile Surfaces in Vacuo and in Water: Molecular Dynamics Studies

The nature of several TiO₂ surfaces in liquid water, as well as the adsorption of a functionalized C₆₀, <b>L*C</b>_<b>60</b> (where L is a carboxylic acid), on TiO₂ anatase and rutile low index surfaces in vacuo and in liquid water have been studied at the self-consistent charge density functional tight-binding (SCC-DFTB) level of theory. It is shown that the SCC-DFTB method provides very good agreement with the high-level DFT data. The typical binding motif of <b>L*C</b>_<b>60</b><b>@TiO</b>_<b>2</b> is found to be the formation of a strong HC¹C­(O²H)­O¹–Ti_5C/O¹–Ti_4C bond with a distance of 2.0–2.1 Å and a weaker HC¹CO¹O²–H···O_2C/O²···H–O_2C hydrogen bond. In some cases, a terminal OH of the linking group coordinates with a Ti–O–Ti bridging oxygen and loses the H to the surface. The adsorption energies in vacuum range between 21 and 82 kcal/mol depending on the surface. The density of states of these species reveals the presence of peaks below the surface conduction band upon ligand adsorption, which is due to the low-lying lowest unoccupied molecular orbitals (LUMOs) of the <b>L*C</b>_<b>60</b>. Electron transfer from the surface to the ligand is thus possible via the initial UV photoexcitation of the surface followed by nonradiative relaxation of the excited electron to the LUMO of the ligand. This pattern was observed for all six surfaces considered in the present work. Solvation of <b>C</b>_<b>60</b><b>@TiO</b>_<b>2</b> in liquid water does not change the qualitative character of surface–ligand binding. In all cases, the ligand remains bound to the surface in the presence of water. The interaction of water molecules with the surface shows various patterns depending on the surface index. Anatase (101) and (100) surfaces favor nondissociative water adsorption, while anatase (001) and rutile (001), (110), and (101) surfaces show dissociative water adsorption which results in OH/OH₂-terminated TiO₂ surfaces. The latter finding is in agreement with several previous DFT studies reported by others

Reaction Mechanism of Nerve-Agent Hydrolysis with the Cs₈Nb₆O₁₉ Lindqvist Hexaniobate Catalyst

We present a detailed mechanism for the hydrolysis of Sarin catalyzed by Cs₈Nb₆O₁₉ obtained using electronic structure calculations. The initial steps of the reaction involve the adsorption of water and Sarin on the hexaniobate catalyst via nonbonding interactions. Dissociation of the coordinated water molecule generates a hydroxide ion that adds nucleophilically to the coadsorbed Sarin molecule in a concerted manner, following a general base catalysis mechanism. The addition of OH^– to the nerve agent generates a trigonal bipyramidal pentacoordinated phosphorus intermediate that subsequently undergoes facile dissociation forming either HF or isopropanol and a corresponding phosphonic acid. The rate-determining step of the overall reaction is found to be the dissociation of water on the catalyst in concert with the nucleophilic addition of the nascent OH^– to the nerve agent. The calculated barrier for this step is considerably smaller than that measured for bulk base hydrolysis. This work represents a blueprint for future studies aimed to optimize catalysts for base hydrolysis of nerve agents at the gas–surface interface

Reaction Mechanism of Nerve-Agent Hydrolysis with the Cs₈Nb₆O₁₉ Lindqvist Hexaniobate Catalyst

We present a detailed mechanism for the hydrolysis of Sarin catalyzed by Cs₈Nb₆O₁₉ obtained using electronic structure calculations. The initial steps of the reaction involve the adsorption of water and Sarin on the hexaniobate catalyst via nonbonding interactions. Dissociation of the coordinated water molecule generates a hydroxide ion that adds nucleophilically to the coadsorbed Sarin molecule in a concerted manner, following a general base catalysis mechanism. The addition of OH^– to the nerve agent generates a trigonal bipyramidal pentacoordinated phosphorus intermediate that subsequently undergoes facile dissociation forming either HF or isopropanol and a corresponding phosphonic acid. The rate-determining step of the overall reaction is found to be the dissociation of water on the catalyst in concert with the nucleophilic addition of the nascent OH^– to the nerve agent. The calculated barrier for this step is considerably smaller than that measured for bulk base hydrolysis. This work represents a blueprint for future studies aimed to optimize catalysts for base hydrolysis of nerve agents at the gas–surface interface

Oxidation of Reduced Keggin Heteropolytungstates by Dioxygen in Water Catalyzed by Cu(II)

The reaction of reduced polyoxometalates (POMs) with dioxygen is centrally important in POM catalysis. We report that this process, as represented by the one-electron-reduced Keggin complexes POM_red (α-AlW₁₂O₄₀^6–, α-SiW₁₂O₄₀^5–, and α-PW₁₂O₄₀^4–), is efficiently catalyzed by copper complexes. The Cu-catalyzed pathway is dominant in the presence of as low as ∼0.1 μM of Cu, a copper concentration that is typically lower than the copper ion contamination in aqueous solutions. The reaction kinetics and mechanism have been comprehensively studied in sodium sulfate buffer at pH 2.0. The catalytic pathway includes a reversible reduction of Cu­(II) by POM_red, followed by a fast reoxidation of Cu­(I) by O₂ to regenerate Cu­(II). The rate constants of the first catalytic steps were determined by three approaches and found to be (1.8 ± 0.3) × 10⁵ and 57 ± 15 M^–1 s^–1 for SiW₁₂O₄₀^5– and PW₁₂O₄₀^4–, respectively. These reactions are thermodynamically more favorable and therefore proceed significantly more quickly than those for the direct outer-sphere electron transfer to O₂. The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions

Parameterization of Reactive Force Field: Dynamics of the [Nb₆O₁₉H_<i>x</i>]^{(8–<i>x</i>)–} Lindqvist Polyoxoanion in Bulk Water

We present results on parameterization of reactive force field [van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A Reactive Force Field for Hydrocarbons. <i>J. Phys. Chem. A</i> <b>2001</b>, <i>105</i>, 9396–9409] for investigating the properties of the [Nb₆O₁₉H_<i>x</i>]^{(8–<i>x</i>)–} Lindqvist polyoxoanion, <i>x</i> = 0–8, in water. Force-field parameters were fitted to an extensive data set consisting of structures and energetics obtained at the Perdew–Burke–Ernzerhof density functional level of theory. These parameters can reasonably describe pure water structure as well as water with an excess of H⁺ and OH^– ions. Molecular dynamics simulations were performed on [Nb₆O₁₉H_<i>x</i>]^{(8–<i>x</i>)–}, <i>x</i> = 0–8, submerged in bulk water at 298 K. Analysis of the MD trajectories showed facile H atom transfer between the protonated polyoxoanion core and bulk water. The number of oxygen sites labeled with an H atom was found to vary depending on the pH of the solution. Detailed analysis shows that the total number of protons at bridging (terminal), η-O (μ₂-O), sites ranges from 3(1) at pH 7, to 2(0) at pH 11, to 1(0) at pH 15. These findings closely reflect available experimental measurements

Near Unity Quantum Yield of Light-Driven Redox Mediator Reduction and Efficient H₂ Generation Using Colloidal Nanorod Heterostructures

The advancement of direct solar-to-fuel conversion technologies requires the development of efficient catalysts as well as efficient materials and novel approaches for light harvesting and charge separation. We report a novel system for unprecedentedly efficient (with near-unity quantum yield) light-driven reduction of methylviologen (MV²⁺), a common redox mediator, using colloidal quasi-type II CdSe/CdS dot-in-rod nanorods as a light absorber and charge separator and mercaptopropionic acid as a sacrificial electron donor. In the presence of Pt nanoparticles, this system can efficiently convert sunlight into H₂, providing a versatile redox mediator-based approach for solar-to-fuel conversion. Compared to related CdSe seed and CdSe/CdS core/shell quantum dots and CdS nanorods, the quantum yields are significantly higher in the CdSe/CdS dot-in-rod structures. Comparison of charge separation, recombination and hole filling rates in these complexes showed that the dot-in-rod structure enables ultrafast electron transfer to methylviologen, fast hole removal by sacrificial electron donor and slow charge recombination, leading to the high quantum yield for MV²⁺ photoreduction. Our finding demonstrates that by controlling the composition, size and shape of quantum-confined nanoheterostructures, the electron and hole wave functions can be tailored to produce efficient light harvesting and charge separation materials