Two-electron Path Research Articles

From pure water to hydrogen peroxide on a novel 2,5,8-triamino-tri-s-triazine (melem)-derived photocatalyst with a high apparent quantum efficiency

Photocatalytic hydrogen peroxide (H2O2) production is a green process but remains a great challenge. Herein, a novel photocatalyst with high activity for H2O2 production, is developed based on 2,5,8-triamino-tri-s-triazine (melem) by linking it with 2, 3-naphthalene dicarboxylic anhydride (NDA). The obtained melem/NDA hybrid not only exhibited narrowed band gap and obviously enhanced visible light absorption, but also showed reduced charge recombination originated from its spatial distribution in HOMO and LUMO induced by the introduction of NDA as verified by DFT calculations. More significantly, the sufficient LUMO and HOMO positions for the optimal sample, melem/NDA0.5, ensured efficient H2O2 production from pure water via both the oxygen reduction reactions mainly through the two-step one-electron path and the water oxidation reaction through the one-step two-electron path. Consequently, melem/NDA0.5 achieves an apparent quantum efficiency of as high as 6.9 % at 420 nm. This work sheds light on developing high-performance organic photocatalysts for boosting photocatalytic H2O2 production.

Microwave-assisted synthesis of well-defined nitrogen doping configuration with high centrality in carbon to identify the active sites for electrochemical hydrogen peroxide production

While nitrogen (N)-doped carbon can facilitate the four-electron transfer in oxygen reduction reaction (ORR), its effect on the two-electron path is yet to be investigated. To date, the random N-C bond configurations in N-doped carbons make it difficult to identify the active sites for electrochemical reactions. Here, we report a preparation of well-controlled N-C configurations with high centrality as model catalysts for identification of active sites for two-electron transfer ORR. Specifically, the synthesized Pyrrolic-N doped single-wall carbon nanotube and graphene utilizing a microwave-assisted pulse heating method, exhibit excellent activity for two-electron path (H2O2 selectivity of 93.5% and 98.35%). X-ray adsorption near-edge structure spectroscopy measurements indicate that carbon atoms adjacent to Pyrrolic-N are the active sites for two-electron transfer. This work provides a viable way for the fabrication of tailored N configurations in carbon materials and significantly advances our understanding of the N moieties for the electrochemical H2O2 generation.

Histidine versus Cysteine-Bearing Heme-Dependent Halogen Peroxidases: Parallels and Differences for Cl- Oxidation.

The homodimeric myeloperoxidase (MPO) features a histidine as a proximal ligand and a sulfonium linkage covalently attaching the heme porphyrin ring to the protein. MPO is able to catalyze Cl- oxidation with about the same efficiency as chloroperoxidase at pH 7.0. In this study, we seek to explore the parallels and differences between the histidine and cysteine heme-dependent halogen peroxidases. Transition states, reaction barriers, and relevant thermodynamic properties are calculated on protein models. Together with electronic structure calculations, it gives an overview of the reaction mechanisms and of the factors that determine the selectivity between one- and two-electron paths. Conclusions point to the innate oxidizing nature of MPO with the ester and sulfonium linkages hiking up the reactivity to enable chloride oxidation. The installation of a deprotonated imidazolate as a proximal ligand does not shift the equilibrium from one- to two-electron events without influencing the chemistry of the oxidation reaction.

High-Throughput Approach Exploitation: Two-Dimensional Double-Metal Sulfide (M2S2) of Efficient Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

A transition-metal sulfide (M2S2) nanolayer as a catalyst for the oxygen reduction reaction (ORR) has been investigated by the density functional theory (DFT) method to explore the underlying mechanisms of the elementary reaction steps for the ORR process. Both the O2 dissociation and O2 hydrogenation paths are probably possible in the ORR on the M2S2 surface. All of the possible intermediate reaction steps of the ORR are exothermic for O2 hydrogenation. This indicates that the four-electron reaction path (4e– ORR) process is the most favorable path, and it is preferred over the two-electron path (2e– ORR) process. The changes in the reaction free energy diagrams were determined, and these diagrams showed that oxygen hydrogenation (OOH) is the rate-determining step. Meanwhile, different working potentials for our studied catalysts were also considered, and we observed that the double-transition-metal sulfide catalysts are energetically favorable (exothermic) catalysts via a 4e transfer mechanism of the ORR processes. According to the formation energies of the ORR intermediates (*O, *OH, *OOH) and the scaling relations between them on different slabs, the volcano plot for the overpotential of the catalyst is also an important index of the catalytic activities, and we found that a smaller overpotential is appropriate to determine better catalytic activities for the ORR process.

A Photochemical Organocatalytic Strategy for the α-Alkylation of Ketones by using Radicals.

Reported herein is a visible‐light‐mediated radical approach to the α‐alkylation of ketones. This method exploits the ability of a nucleophilic organocatalyst to generate radicals upon SN2‐based activation of alkyl halides and blue light irradiation. The resulting open‐shell intermediates are then intercepted by weakly nucleophilic silyl enol ethers, which would be unable to directly attack the alkyl halides through a traditional two‐electron path. The mild reaction conditions allowed functionalization of the α position of ketones with functional groups that are not compatible with classical anionic strategies. In addition, the redox‐neutral nature of this process makes it compatible with a cinchona‐based primary amine catalyst, which was used to develop a rare example of enantioselective organocatalytic radical α‐alkylation of ketones.

Selenium-Coupled Reduced Graphene Oxide as Single-Atom Site Catalyst for Direct Four-Electron Oxygen Reduction Reaction

A single-atom site catalyst for direct four electron oxygen reduction reaction (ORR) is identified where selenium (ultrasmall amount of Se, ≲1 at. % ) is the active center that couples reduced graphene oxide (rGO) sheets. This metal-free catalyst is found to be undergoing ORR through the direct four-electron pathway in alkaline medium, similar to platinum, whereas rGO prefers a two-electron path, and the experimental observation is correlated with theoretical study. The role of Se-like elements in graphene, which have similar electronegativity as carbon, is unraveled with a detailed density functional theory-based analyses.

Electrochemical Promotion of Oxygen Reduction on Gold with Aluminum Phosphate Overlayer

The activities of Au electrodes with an AlPO4 overlayer were examined for oxygen-reduction reactions in alkaline media. Oxygen molecules on gold catalysts are mainly reduced by a two-electron path, forming hydrogen peroxide with half efficiency. On the AlPO4 overlayer deposited Au, larger current densities corresponding to a nearly four-electron path were recorded within the potential range of approximately 0.7−1.0 V, which were correlated with the decomposition (disproportionation) of hydrogen peroxide. This enhancement was attributed to the electronic interactions and changed activities of the intermediate state, as confirmed by X-ray photoelectron spectroscopy and the voltammetric profiles of hydrogen peroxide, respectively.

Electrochemical reduction of oxygen on platinum nanoparticles in alkaline media

The objective of the study was to investigate the platinum particle size effect on the catalytic activity for oxygen reduction reaction in alkaline solution. Various calibrated Pt particles on Vulcan were used. The catalytic powders were characterised by TEM (transmission electron microscopy) and H-electrochemical adsorption. For oxygen reduction, a small loss of catalytic activity with the particle size decrease is demonstrated: the specific activity is three times lower for very small particles. We have also made a study by RRDE which completes and fully confirms the catalytic activity results. This study has shown that the two-electron path is important on carbon, almost negligible on Pt: a comparison between a pure carbon powder and this powder with deposited Pt particles has shown that the total current density is very small on pure carbon at our working potentials. Although the RRDE study does not permit a fully complete analysis of the minor pathways, that is to say how much HO 2 − produced on carbon is reduced on Pt and how much HO 2 − is directly produced on Pt particles, it appears that the ORR is predominantly governed by the activity of Pt particles dispersed on the carbon support.

Electron transfer. 107. The chromium(VI)-uranium(IV) reaction: parallel one- and two-electron paths

Reduction of HCrO{sub 4}{sup {minus}} with U(IV) in solutions buffered by 2-ethyl-2-hydroxybutanoic acid (HLig) and its salt (Lig{sup {minus}}) is rapid and leads to a mixture of bis chelates of Cr(IV), Cr(V), and Cr(III), each derived from the buffering anion. The ratio of products approaches 3:1:1 and is nearly independent of (Lig{sup {minus}}). Kinetic patterns suggest that the complexes U{sup IV} (Lig{sup {minus}}) and U{sup IV}(Lig{sup {minus}}){sub 2} are active reductants but that U{sup IV}(Lig{sup {minus}}){sub 3} is inactive. With U(IV) in excess, Cr(IV) is slowly reduced to Cr(III), a reaction which must involve intervention of U(V). The U(IV)-Cr(IV) reaction appears to proceed through a precursor complex (K{sub assn} = 5 {times} 10{sup 3} M{sup {minus}1}), the pool of which is depleted both by protonation and extraligation. With Cr(VI) in excess, Cr(IV) is slowly lost by comproportionation (Cr{sup IV} + Cr{sup VI} {yields} 2Cr{sup V}), a transformation which has been observed also in Cr(VI)-As(III) and Cr(VI)-Mo{sub 2}{sup V} systems. Cr(III), formed quickly in the initial stage of conversion in quantity equivalent to Cr(V), cannot arise from the Cr(V)-U(IV) or the Cr(IV)-U(IV) reaction, both of which are too slow, but is instead produced by reduction of Cr(IV) with U(V). ESR studiesmore » show that rapid reductions of excess HCrO{sub 4}{sup {minus}} with Mo{sub 2}O{sub 4}{sup 2+}, HSO{sub 3}{sup {minus}}, or Sn(II) in the authors buffer system likewise yield, along with major quantities of Cr(IV), minor amounts of Cr(V), thus pointing to competitive 1e{sup {minus}} and 2e{sup {minus}} transactions with these reductants. Reduction by As(III) under the same conditions, however, yields only Cr(IV).« less

Mechanisms of homogeneous stoichiometric, catalytic and photocatalytic dihydrogen formation using thiocomplexes

Abstract

Concurrent electrophilic and oxidative pathways for reactions of .alpha.-hydroxyalkyl and .alpha.-alkoxyalkyl complexes of chromium(III) with mercury(II) ions

The reactions of Hg/sup 2 +/ with a number of ..cap alpha..-hydroxyalkyl and ..cap alpha..-alkoxyalkyl complexes of chromium(III) were studied. The lowest members of both series, CrCH/sub 2/OH/sup 2 +/ and CrCH/sub 2/OCH/sub 3//sup 2 +/, react with Hg/sup 2 +/ in an acid-independent electrophilic substitution reaction with the reaction constant rho = 10. The organomercurials formed are unstable and decompose rapidly to yield Hg/sup 0/. All the other organochromium complexes studied react with Hg/sup 2 +/ in acid-dependent electron-transfer reactions. (..cap alpha..-Hydroxyisopropyl(chromium ion, CrC(CH/sub 3/)/sub 2/OH/sup 2 +/, undergoes a one-electron transfer reaction, producing Cr/sup 2 +/ and Hg/sup +/ with k (M/sup -1/ s/sup 1/) = 166 + 467/(H/sup +/), while (..cap alpha..-ethoxyethyl)chromium ion, CrCH(CH/sub 3/)OC/sub 2/H/sub 5//sup 2 +/, reacts in a two-electron redox process, yielding Cr/sup 3 +/ and Hg/sup 0/, k (M/sup -1/s/sup -1/) = 0.535/(H/sup +/). (..cap alpha..-Hydroxyethyl)chromium ion, CrCH(CH)/sub 3/OH/sup 2 +/ reacts by both routes, k(M/sup -1/ s/sup -1/) = 4.11/(H/sup +/), with approx. 15% participation from the one-electron path and approx. 85% from the two-electron path. The effect of Cr/sup 2 +/ and Co(NH/sub 3/)/sub 5/F/sup 2 +/ (a specific scavenger for Cr/sup 2 +/) on the kinetics and stoichiometry ofmore » the reactions is discussed.« less

One- and two-equivalent paths in the reaction of vanadium(II) with mercury(II). A recalculation of kinetic parameters

Kinetic parameters for the one- and two-electron paths in the reaction of vanadium(II) with mercury(II) have been recalculated. The one-electron path (a=ka) is independent of [H+]; at 25°ka= 1·04 ± 0·11 l mol–1 s–1, ΔHa‡= 15·8 ± 0·8 kcal mol–1, and ΔSa‡=–5·6 ± 2·7 e.u. There is now no detectable contribution from an [H+]-independent path for the two-electron reaction (b=kb[H+]–1), and at 25°kb= 8·69 ± 0·02 s–1, ΔHb‡= 14·8 ± 0·2 kcal mol–1, and ΔSb‡=–4·6 ± 0·8 e.u.

Evidence for one- and two-electron paths in the reaction of vanadium(II) with mercury(II)

One- and two-electron transfer steps have been shown to occur concurrently in the reaction of vanadium(II) with mercury(II), and kinetic parameters have been obtained for both paths.