New insights in the electrocatalytic proton reduction and hydrogen oxidation by bioinspired catalysts: a DFT investigation

In this paper, we present a DFT study of the proton reduction mechanism catalyzed by the complex [Ni(P?(H)N?(H))?](2+), bioinspired from the hydrogenases. A detailed analysis of the reactive isomers is discussed together with the localizations of the transitions states and energy minima. The reactive catalytic species is a biprotonated Ni(0) complex that can show different conformations and that can be protonated on different sites. The energies of the different conformations and biprotonated species have been calculated and discussed. Energy barriers for two different reaction mechanisms have been identified in solvent and in gas phase. Frequency calculations have been performed to check the nature of the energy minima and for the calculations of entropic energetic terms and zero point energies. We show that only one conformation is mostly reactive. All the others species are nonreactive in their original form, and they have to pass through conformational barriers in order to transform in the reactive species.     

Mechanism investigation of ketone hydrogenation catalyzed by ruthenium bifunctional catalysts: insights from a DFT study

In this paper, the mechanism of ketone hydrogenation catalyzed by five Ru bifunctional catalysts with different structural frameworks was studied in detail using density functional theory (DFT). This mechanism contains hydrogen transfer, dehydrogenation of alcohol, and dihydrogen activation fundamental reactions. The involvement of alcohol is also discussed and found with different activities in hydrogen transfer, dehydrogenation and dihydrogen activation steps in five systems. Our calculated results indicate that the weak Ru-H bond, stronger basicity of hydride and stronger X-H acidity will decrease the barrier of the HT step, and that the polar micro-environment of dihydrogen coordinating with Ru catalysts and short hydrogen transfer distance would be able to facilitate the heterolytic splitting of dihydrogen in the dihydrogen activation step.     

Direct comparison of the electrocatalytic oxidation of hydrogen by an enzyme and a platinum catalyst

It is shown that for molecules of Allochromatium vinosum [NiFe]-hydrogenase adsorbed on a pyrolytic graphite electrode the nickel-iron active site catalyzes hydrogen oxidation at a diffusion-controlled rate matching that achieved by platinum.     

DFT investigation on the double hydrogen-bonded system: The oxidation and hydration effect

The trustworthy B3LYP/6-311+G* method is employed to investigate the double hydrogen-bonded system with a special emphasis on the oxidation and hydration effect. Proton transfer occurs spontaneously upon oxidation from the amido group to the adjacent imidazole fragment. The larger the pH value of the environment, the significant the effect on the geometry structure is. The electron population on the HOMO determines the IR vibrational frequency of the H bond, being blue-shift or red-shift. The complex prefers to be oxidized under the basic condition. The weak acidic environment is recommended to prevent the DNA mutation.     

Photocatalytic hydrogen generation from water with iron carbonyl phosphine complexes: improved water reduction catalysts and mechanistic insights

An extended study of a novel visible-light-driven water reduction system containing an iridium photosensitizer, an in situ iron(0) phosphine water reduction catalyst (WRC), and triethylamine as sacrificial reductant is described. The influences of solvent composition, ligand, ligand-to-metal ratio, and pH were studied. The use of monodentate phosphine ligands led to improved activity of the WRC. By applying a WRC generated in situ from Fe(3) (CO)(12) and tris[3,5-bis(trifluoromethyl)phenyl]phosphine (P[C(6)H(3)(CF(3))(2)](3), Fe(3)(CO)(12)/PR(3)=1:1.5), a catalyst turnover number of more than 1500 was obtained, which constitutes the highest activity reported for any Fe WRC. The maximum incident photon to hydrogen efficiency obtained was 13.4% (440 nm). It is demonstrated that the evolved H(2) flow (0.23 mmol H(2) h(-1) mg(-1) Fe(3)(CO)(12)) is sufficient to be used in polymer electrolyte membrane fuel cells, which generate electricity directly from water with visible light. Mechanistic studies by NMR spectroscopy, in situ IR spectroscopy, and DFT calculations allow for an improved understanding of the mechanism. With respect to the Fe WRC, the complex [HNEt(3)](+)[HFe(3)(CO)(11)](-) was identified as the key intermediate during the catalytic cycle, which led to light-driven hydrogen generation from water.     

Regioselective addition of Ti enolates to conjugated enones: New insights into the mechanism based on experimental and DFT investigation

The regioselective addition mechanism of the Ti(IV) enolates derived from α-diazo-β-keto carbonyl compounds and α-diazo-β-keto phosphonates to conjugated enones has been studied on the basis of a hypothetical bridging chloride-controlled theory, by density functional theory (DFT), and experimentally. The DFT results indicate that, for the Ti(IV) enolate 3 derived from α-diazo-β-keto carbonyl compounds, the free energy of the bridging chloride-controlled 1,2-addition transition state is 2.4 kcal/mol higher than that of 1,4-addition, and the calculated enthalpies of 1,2-addition is 4.36 kcal/mol more than that of 1,4-addition. For the Ti(IV) enolate 4 derived from α-diazo-β-keto phosphonates, in contrary, the free energy of the bridging chloride-controlled 1,2-addition transition state is 1.1 kcal/mol lower than that of 1,4-addition, and the calculated enthalpy of 1,2-addition is 3.46 kcal/mol less than that of 1,4-addition. Our findings demonstrate that the nucleophilic addition of these Ti(IV) enolates to conjugated enones was carried out not only kinetically but also irreversibly for the first time.     

The effect of extended conjugation on electrocatalytic CO2 reduction by molecular catalysts and macromolecular structures

Homogenous molecular catalysts have shown significant promise for the selective reduction of CO₂ to single products. However, their practical application in emerging CO₂ reduction technologies is hindered by their limited solubility and stability in aqueous solutions, their diffusion-dependent kinetics, and their poor recyclability. Incorporating discrete molecular catalysts into macromolecular architectures such as covalent organic frameworks is one solution to these limitations that allows for the synthesis of heterogeneous materials with increased activity and stability but that still maintain the selectivity and active-site tunability of discrete molecular catalysts. Forming such macromolecular materials necessarily extends the ligand π-conjugated network, which can have important effects on the electrocatalytic activity. In this review, we discuss recent studies on the effect of extended π-conjugation on the catalytic activity of molecular catalyst and extended macromolecular architectures, with an emphasis on how activity is influenced by charge delocalization, electrostatic effects, and electronic coupling between active sites.     

Linkers and catalysts immobilized on oxide supports: New insights by solid-state NMR spectroscopy

This review article describes classical and modern solid-state NMR methods that allow to gain insight into catalyst systems where one or two metal complexes are bound to oxide supports via bifunctional phosphine linkers, such as (EtO)₃Si(CH₂)₃PPh₂. Many aspects of the immobilized molecular catalysts can be elucidated with the corresponding NMR technique. The bulk of the support can be studied, as well as the interface of the support with the ethoxysilane. With respect to the linkers, their structural integrity and mobility are as easy to investigate by classical CP/MAS and high-resolution magic angle spinning (HRMAS) NMR techniques, as their adsorption behavior. Even electrostatic bonding to the support via phosphonium groups can be proven by solid-state NMR. For the immobilized catalysts, leaching, and even “horizontal” translational mobility effects, as probed by HRMAS NMR under “realistic conditions” in the presence of solvents, are described.     

Copper and copper-palladium catalysts of aliphatic thiols oxidation in biological objects: Quantum-chemical DFT simulation

DFT calculations (M06, PBE0/Def2-TZVP) of coordination compounds used in reactions of selective oxidation of thiols to disulfides were performed. Primary active centers of the catalysts are polynuclear scaffolds {L₂M(μ-OH)₂ML₂}²⁺ and {L₂M(μ-OH)₂M′(μ-OH)₂ML₂}²⁺ (M = Cu^I, Cu^II, Pd^II; M' = Cu^II; L = NH₃). Cu^II ions in combination with PdII ions are capable of formation of polynuclear active center {PdII(μ-OH)₂Cu^II(μ-OH)₂Pd^II}²⁺ bringing together a large number of mutually oriented RS^– groups and thus affecting the rate of formation of disulfide R₂S₂.     

New insights on the mechanism of oxidation of D-galacturonic acid by hypervalent chromium

The pollutant Cr(VI) is known to be very carcinogenic. In conditions of excess of Cr(VI), oxidation of D-galacturonic acid (Galur), the major metabolite of pectin, yields d-galactaric acid (Galar) and Cr(III). The redox reaction takes place through a multistep mechanism involving formation of intermediate Cr(II/IV) and Cr(V) species. The mechanism combines one- and two-electron pathways for the reduction of Cr(IV) by the organic substrate: Cr(VI)→ Cr(IV)→ Cr(II) and Cr(VI)→ Cr(IV)→ Cr(III). This is supported by the observation of the optical absorption spectra of Cr(VI) esters, free radicals, CrO(2)(2+) (superoxoCr(III) ion) and oxo-Cr(V) complexes. Cr(IV) cannot be directly detected; however, formation of CrO(2)(2+) provides indirect evidence for the intermediacy of Cr(II/IV). Cr(IV) reacts with Galur much faster than Cr(V) and Cr(VI) do. The analysis of the reaction kinetics via optical absorption spectroscopy shows that the Cr(IV)-Galur reaction rate inversely depends on [H(+)]. Nevertheless, high [H(+)] still does not facilitate accumulation of Cr(IV) in the Cr(VI)-Galur mixture. Cr(VI) and the intermediate Cr(V) react with Galur at comparable rates; therefore the build-up and decay of Cr(V) accompany the decay of Cr(VI). The complete rate laws for the Cr(VI), Cr(V) and Cr(IV)-Galur redox reaction are here derived in detail. Furthermore, the nature of the five-co-ordinated oxo-Cr(V) bischelate complexes formed in Cr(VI)-Galur mixtures at pH 1-5 is investigated using continuous-wave and pulsed electron paramagnetic resonance (EPR) and density functional theory (DFT).     

Moving protons with pendant amines: proton mobility in a nickel catalyst for oxidation of hydrogen

Proton transport is ubiquitous in chemical and biological processes, including the reduction of dioxygen to water, the reduction of CO(2) to formate, and the production/oxidation of hydrogen. In this work we describe intramolecular proton transfer between Ni and positioned pendant amines for the hydrogen oxidation electrocatalyst [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+) (P(Cy)(2)N(Bn)(2) = 1,5-dibenzyl-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane). Rate constants are determined by variable-temperature one-dimensional NMR techniques and two-dimensional EXSY experiments. Computational studies provide insight into the details of the proton movement and energetics of these complexes. Intramolecular proton exchange processes are observed for two of the three experimentally observable isomers of the doubly protonated Ni(0) complex, [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+), which have N-H bonds but no Ni-H bonds. For these two isomers, with pendant amines positioned endo to the Ni, the rate constants for proton exchange range from 10(4) to 10(5) s(-1) at 25 °C, depending on isomer and solvent. No exchange is observed for protons on pendant amines positioned exo to the Ni. Analysis of the exchange as a function of temperature provides a barrier for proton exchange of ΔG(?) = 11-12 kcal/mol for both isomers, with little dependence on solvent. Density functional theory calculations and molecular dynamics simulations support the experimental observations, suggesting metal-mediated intramolecular proton transfers between nitrogen atoms, with chair-to-boat isomerizations as the rate-limiting steps. Because of the fast rate of proton movement, this catalyst may be considered a metal center surrounded by a cloud of exchanging protons. The high intramolecular proton mobility provides information directly pertinent to the ability of pendant amines to accelerate proton transfers during catalysis of hydrogen oxidation. These results may also have broader implications for proton movement in homogeneous catalysts and enzymes in general, with specific implications for the proton channel in the Ni-Fe hydrogenase enzyme.     

A DNA biosensor based on the electrocatalytic oxidation of amine by a threading intercalator

An electrochemical biosensor for the detection of DNA based a peptide nucleic acid (PNA) capture probe (CP) modified indium tin oxide electrode (ITO) is described in this report. After hybridization, a threading intercalator, N,N′-bis[(3-propyl)-imidazole]-1,4,5,8-naphthalene diimide (PIND) imidazole complexed with Ru(bpy)₂Cl (PIND-Ru, bpy = 2,2′-bipyridine), was introduced to the biosensor. PIND-Ru selectively intercalated to double-stranded DNA (ds-DNA) and became immobilized on the biosensor surface. Voltammetric tests showed highly stable and reversible electrochemical oxidation/reduction processes and the peak currents can directly be utilized for DNA quantification. When the tests were conducted in an amine-containing medium, Tris-HCl buffer for example, a remarkable improvement in the voltammetric response and noticeable enhancements of voltammetric and amperometric sensitivities were observed due to the electrocatalytic activity of the [Ru(bpy)₂Cl] redox moieties. Electrocatalytic current was observed when as little as 3.0 attomoles of DNA was present in the sample solution.     

Influence of reduction energy match among CuO species in CuO-CeO2 catalysts on the catalytic performance for CO preferential oxidation in excess hydrogen

In the present study, we have investigated the reducibility of CuO species on CuO-CeO2 catalysts and the influence of CuO species on the catalytic performance for CO preferential oxidation (CO PROX) in excess hydrogen. It is revealed that the smaller the difference of reduction temperature (denoted as ?T) for two adjacent CuO species is, the higher the catalytic activity of CuO-CeO2 for the PROX in excess hydrogen may be obtained. It means that if the reduction energy of Cu0-Cu2+ pairs matched better, the reduction-oxidation recycle of Cu0-Cu2+ pairs would go on more easily, then the transferring energy of Cu0-Cu2+ pairs would be lesser. Therefore, the CuO-CeO2 catalysts will be largely improved in their catalytic performance if the different CuO species on the catalysts have matched the reduction energy, which would allows them to cooperate effectively.     

Determination of carbon-bonded sulfur in soils by hydriodic acid reduction and hydrogen peroxide oxidation

A sequential extraction method has been developed for the determination of carbon-bonded sulfur in soils. The soil sample has been sequentially reduced with HI and oxidized with hydrogen peroxide, and finally the residue has been digested with a mixture of nitric acid and perchloric acid. All inorganic sulfur components and ester sulfur has been reduced to H₂S by HI except the unreducible sulfur including pyritic sulfur, carbon-bonded sulfur and a previously unidentified sulfur fraction. Whereas a part of the carbon-bonded sulfur has been dissolved in the HI reducing solution another part of carbon-bonded sulfur was removed by hydrogen peroxide oxidation. The total carbon-bonded sulfur compose for oxic soils of the HI-dissolved sulfur and the H₂O₂-oxidized sulfur. However, because the pyritic sulfur can be completely decomposed by H₂O₂, this form of sulfur should be subtracted from the sum of the two sulfur fractions in case of anoxic soils. Unidentified sulfur components were also detected in the residue after the sequential extraction.     

Oxoanion binding by guanidiniocarbonylpyrrole cations in water: a combined DFT and MD investigation

Structures and properties of nonbonding interactions involving guanidinium-functionalized hosts and carboxylate substrates were investigated by a combination of ab initio and molecular dynamics approaches. The systems under study are on one hand intended to be a model of the arginine-anion bond, so often observed in proteins and nucleic acids, and on the other to provide an opportunity to investigate the influence of molecular structure on the formation of supramolecular complexes in detail. Use of DFT calculations, including extended basis sets and implicit water treatment, allowed us to determine minimum-energy structures and binding enthalpies that compared well with experimental data. Intermolecular forces were found to be mostly due to electrostatic interactions through three hydrogen bonds, one of which is bifurcate, and are sufficiently strong to induce a conformational change in the ligand consisting of a rotation of about 180 degrees around the guanidiniocarbonylpyrrole axis. Free binding energies of the complexes were evaluated through MD simulations performed in the presence of explicit water molecules by use of the molecular mechanics Poisson-Boltzmann solvent accessible surface area (MM-PBSA) and linear interaction energy (LIE) approaches. LIE energies were in quantitative agreement with experimental data. A detailed analysis of the MD simulations revealed that the complexes cannot be described in terms of a single binding structure, but that they are characterized by a significant internal mobility responsible for several low-energy metastable structures.     

Stabilization role of a phenothiazine derivative on the electrocatalytic oxidation of hydrogen via Aquifex aeolicus hydrogenase at graphite membrane electrodes

The [NiFe] membrane-bound hydrogenase from the microaerophilic, hyperthermophilic Aquifex aeolicus bacterium (Aa Hase) presents oxygen, carbon monoxide, and temperature resistances. Since it oxidizes hydrogen with high turnover, this enzyme is thus of particular interest for biotechnological applications, such as biofuel cells. Efficient immobilization of the enzyme onto electrodes is however a mandatory step. To gain further insight into the parameters governing the interfacial electron process, cyclic voltammetry was performed combining the use of a phenothiazine dye with a membrane electrode design where the enzyme is entrapped in a thin layer. In the absence of the phenothiazine dye, direct electron transfer (DET) for H(2) oxidation is observed due to Aa Hase adsorbed onto the PG electrode. An unexpected loss of the catalytic current with time is however observed. The effect of toluidine blue O (TBO) on the catalytic process is first studied with TBO in solution. In addition to the expected mediated electron transfer process (MET), TBO is demonstrated to reconnect directly some Aa Hase molecules possibly released from the electrode but still entrapped in the thin layer. On adsorbed TBO the two same processes occur demonstrating the ability of the TBO film to connect Aa Hase via a DET process. Loss of activity is however observed due to the poor stability of adsorbed TBO at high temperatures. Aa Hase immobilization is then studied on electropolymerized TBO (pTBO). The effect of film thickness, temperature, presence of inhibitors and pH is evaluated. Given a film thickness less than 20 nm, H(2) oxidation proceeds via a mixed DET/MET process through the pTBO film. A high and very stable H(2) oxidation activity is reached, showing the potential applicability of the bioelectrode for biotechnologies. Finally, the multifunctional roles of TBO-based matrix are underlined, including redox mediator, Aa Hase anchor, but also buffering and ROS scavenger capabilities to drive pH local changes and avoid oxidative damage.     

Thermal stability and activity in hydrogen oxidation of palladium catalysts supported on fibrous sulfonic cation exchanger in the hydrogen and magnesium forms

The effect of reduced palladium on the thermal stability of the hydrogen and magnesium forms of FIBAN K-1 fibrous sulfonic cation exchanger was studied. The activity of palladium catalyst supported by the H and Mg forms of the cation exchanger in hydrogen oxidation was determined, as influenced by the temperature of treatment of the catalyst with the reaction mixture.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 9, 2004, pp. 1510–1515.Original Russian Text Copyright © 2004 by Egiazarov, Radkevich, Kravchuk, Ivko.     

Effect of cerium oxide on the activity of platinum catalysts of hydrogen and CO oxidation

Effective catalytic systems based on Pt/CeO₂-C were prepared. The use of a cerium oxide addition in the substrate stabilized the platinum catalysts against their poisoning with carbon monoxide at room temperature.     

Quantum-chemical investigation of the mechanisms of oxidation of dimethyl sulfide by hydrogen peroxide and peroxoborates

It was established by the DFT method in the B3LYP/6-311G-d,p approximation that the oxidation of dimethyl sulfide (Me₂S) by peroxides (XOOH) can take place by two mechanisms depending on the nature of X. In the reaction of Me₂S with hydrogen peroxide (X = H) the direct reagent is the HOOH molecule while in the reactions with monoperoxoborate [X = B(OH)₃] and diperoxoborate [X = B(OH)₂OOH] it is a reagent containing the “water oxide” fragment X—(⁺OH)—O^–.