Hydrogen Atoms Research Articles

Dual Acridine/Decatungstate Photocatalysis for the Decarboxylative Radical Addition of Carboxylic Acids to Azomethines.

A concept for the dual use of acridine and tetrabutylammonium decatungstate photocatalysts in the reactions of carboxylic acids is proposed. Imines generated in situ from aldehydes and p-methoxyaniline, as well as other azomethines, were used as radical acceptors. The role of the decatungstate is believed to facilitate the turnover of the acridine photocatalyst by means of hydrogen atom transfer.

Universal inter-molecular radical transfer reactions on metal surfaces.

On-surface synthesis provides tools to prepare low-dimensional supramolecular structures. Traditionally, reactive radicals are a class of single-electron species, serving as exceptional electron-withdrawing groups. On metal surfaces, however, such species are affected by conduction band screening effects that may even quench their unpaired electron characteristics. As a result, radicals are expected to be less active, and reactions catalyzed by surface-stabilized radicals are rarely reported. Herein, we describe a class of inter-molecular radical transfer reactions on metal surfaces. With the assistance of aryl halide precursors, the coupling of terminal alkynes is steered from non-dehydrogenated to dehydrogenated products, resulting in alkynyl-Ag-alkynyl bonds. Dehalogenated molecules are fully passivated by detached hydrogen atoms. The reaction mechanism is unraveled by various surface-sensitive technologies and density functional theory calculations. Moreover, we reveal the universality of this mechanism on metal surfaces. Our studies enrich the on-surface synthesis toolbox and develop a pathway for producing low-dimensional organic materials.

Analytical model for a hydrogen atom in a magnetic field: Implications for the diamagnetic shift

Analytical model for a hydrogen atom in a magnetic field: Implications for the diamagnetic shift

Landau Levels versus Hydrogen Atom

The Landau problem and harmonic oscillator in the plane share a Hilbert space that carries the structure of Dirac’s remarkable so(2,3) representation. We show that the orthosymplectic algebra osp(1|4) is the spectrum generating algebra for the Landau problem and, hence, for the 2D isotropic harmonic oscillator. The 2D harmonic oscillator is in duality with the 2D quantum Coulomb–Kepler systems, with the osp(1|4) symmetry broken down to the conformal symmetry so(2,3). The even so(2,3) submodule (coined Rac) generated from the ground state of zero angular momentum is identified with the Hilbert space of a 2D hydrogen atom. An odd element of the superalgebra osp(1|4) creates a pseudo-vacuum with intrinsic angular momentum 1/2 from the vacuum. The odd so(2,3)-submodule (coined Di) built upon the pseudo-vacuum is the Hilbert space of a magnetized 2D hydrogen atom: a quantum system of a dyon and an electron. Thus, the Hilbert space of the Landau problem is a direct sum of two massless unitary so(2,3) representations, namely, the Di and Rac singletons introduced by Flato and Fronsdal.

Hydrogen diffusion dynamics on [formula omitted] surface: A mechanism of hydrogen-induced failure

Hydrogen-induced failure (embrittlement as the primary source) is a significant concern for retrofitting existing pipelines to transport hydrogen. One of the major factors associated with hydrogen embrittlement of metals is hydrogen diffusion. Therefore, understanding the modes and dynamics of hydrogen diffusion into metals is critical for understanding better and acting to limit hydrogen-induced damage to metals. It is well established that transition metals catalyze the dissociation of molecular hydrogen into atomic hydrogen even at room temperature. The atomic hydrogen diffuses through the metal surface, and this causes degradation. Understanding atomic hydrogen diffusion dynamics is critical to reducing hydrogen embrittlement. This work presents atomic hydrogen diffusion dynamics along and into Fe(100) surfaces. The results indicated that diffusion of atomic hydrogen along the surface is favored over surface-to-subsurface diffusion. Once hydrogen diffuses to the subsurface layer, the barrier for further hydrogen diffusion into the subsequent layers is relatively lower. The methodology presented here of atomic hydrogen diffusion under dynamic relaxation into metal surface can be used to study the effect of the metal's chemical composition and structure on the hydrogen diffusion rate. This allows for screening various strategies to reduce hydrogen diffusion into metals and guide the design of degradation prevention strategies.

Photoenzymatic Asymmetric Hydroamination for Chiral Alkyl Amine Synthesis.

Chiral alkyl amines are common structural motifs in pharmaceuticals, natural products, synthetic intermediates, and bioactive molecules. An attractive method to prepare these molecules is the asymmetric radical hydroamination; however, this approach has not been explored with dialkyl amine-derived nitrogen-centered radicals since designing a catalytic system to generate the aminium radical cation, to suppress deleterious side reactions such as α-deprotonation and H atom abstraction, and to facilitate enantioselective hydrogen atom transfer is a formidable task. Herein, we describe the application of photoenzymatic catalysis to generate and harness the aminium radical cation for asymmetric intermolecular hydroamination. In this reaction, the flavin-dependent ene-reductase photocatalytically generates the aminium radical cation from the corresponding hydroxylamine and catalyzes the asymmetric intermolecular hydroamination to furnish the enantioenriched tertiary amine, whereby enantioinduction occurs through enzyme-mediated hydrogen atom transfer. This work highlights the use of photoenzymatic catalysis to generate and control highly reactive radical intermediates for asymmetric synthesis, addressing a long-standing challenge in chemical synthesis.

Ab initio calculations of the chemisorption of atomic H and O on Pt and Ir metal and on bimetallic Pt x Ir y surfaces

Abstract Understanding the chemisorption of atoms on precious metal surfaces is of substantial interest for the rational design of heterogeneous and electrochemical catalysts. In this study, we report density functional theory (DFT) investigations of the chemisorption of atomic H and O on bimetallic Pt x Ir y (111) surfaces for bifunctional anode catalyst materials in polymer electrolyte membrane (PEM) fuel cells. We found that for both adsorbates, the adsorption on the Pt(111) surface is in general less exothermic than on the Ir(111) surface. Our study has revealed that chemisorption on the bimetallic surfaces becomes more stable with increasing number of Ir surface atoms at the adsorption site. While for hydrogen atoms the ONTOP sites yield the most negative adsorption energies, the chemisorption of oxygen atoms appears to be most stable on the FCC sites for both the mono- and bimetallic surfaces. Using the ab initio thermodynamics approach, we calculated phase diagrams for the chemisorption of H and O atoms on these metal surfaces in order to transfer our findings to finite temperature and pressure conditions. Our theoretical results may provide an improved understanding of the hydrogen oxidation reaction (HOR) and oxygen evolution reaction (OER) on intermetallic Pt x Ir y (111) surfaces and may be helpful for the rational design of new bifunctional PEM fuel cell anode catalyst materials.

Improvement in the Hydrogen-Storage Properties of Mg<sub>2</sub>Ni by Adding LiBH<sub>4</sub>

To improve the hydrogen-storage properties of Mg<sub>2</sub>Ni, LiBH<sub>4</sub> was added by milling in hydrogen atmosphere (reactive mechanical milling, RMM). Mg<sub>2</sub>Ni-10LiBH<sub>4</sub> was prepared with a composition of 90 wt% Mg<sub>2</sub>Ni + 10 wt% LiBH<sub>4</sub>. The quantity of released hydrogen (H<sub>r</sub>) versus temperature T curve for Mg<sub>2</sub>Ni10LiBH<sub>4</sub> was obtained by heating at a rate of 4~5 K in 1.0 bar hydrogen. The hydrogen-storage properties of the sample were investigated. The phases formed were examined from the x-ray diffraction (XRD) patterns of the samples after RMM, and after hydrogen-absorption and release cycling. The XRD pattern of Mg<sub>2</sub>Ni10LiBH<sub>4</sub> showed that this sample contained Mg<sub>2</sub>Ni, Mg<sub>2</sub>NiH<sub>4</sub>, o-LiBH<sub>4</sub>, h-LiBH<sub>4</sub>, Ni, and MgH<sub>2</sub>. The dHr/dT versus T curve exhibited three peaks at 325 K, 563 K, and 600 K, respectively. The peak at 325 K is for the hydrogen release from o-LiBH<sub>4</sub> and h-LiBH<sub>4</sub>. The peak at 563 K is for the hydrogen release from Mg<sub>2</sub>NiH<sub>4</sub> and the peak at 600 K is for the hydrogen release from Mg<sub>2</sub>NiH<sub>4</sub> and MgH<sub>2</sub>. The RMM of Mg<sub>2</sub>Ni with added LiBH<sub>4</sub> creates defects and cracks. RMM facilitates nucleation, increases reactivity, and shortens the diffusion distances of hydrogen atoms. Expansion and contraction of lattices due to cycling has effects similar to, but weaker than, the effects of RMM.

Carbon quaternization of redox active esters and olefins by decarboxylative coupling.

The synthesis of quaternary carbons often requires numerous steps and complex conditions or harsh reagents that act on heavily engineered substrates. This is largely a consequence of conventional polar-bond retrosynthetic disconnections that in turn require multiple functional group interconversions, redox manipulations, and protecting group chemistry. Here, we report a simple catalyst and reductant combination that converts two types of feedstock chemicals, carboxylic acids and olefins, into tetrasubstituted carbons through quaternization of radical intermediates. An iron porphyrin catalyst activates each substrate by electron transfer or hydrogen atom transfer, and then combines the fragments using a bimolecular homolytic substitution (SH2) reaction. This cross-coupling reduces the synthetic burden to procure numerous quaternary carbon---containing products from simple chemical feedstocks.

Per- and polyfluoroalkyls used as cosmetic ingredients - Qualitative study of 765 cosmetic products

Per- and polyfluoroalkyl substances (PFAS) form a vast family comprising more than 4700 synthetic compounds. Their molecules contain a terminal functional group and a hydrophobic carbon tail (alkyl group) at which the hydrogen atoms are totally (in the case of perfluorinated compounds) or partially (in the case of polyfluorinated compounds) replaced by fluorine atoms. Due to the very specific properties of their structure, they have been used in a vast range of applications over the last 70 years. These substances are considered to be of concern for the environment. Their effects on human health are still poorly understood because studies are still too rare, but the cutaneous route could be a significant pathway of penetration.In this context, we made a qualitative study to assess the presence of PFAS in various cosmetics such as hygiene products, skin care products, make-up and perfumes.Among the 765 products studied, we found 11 different PFAS. Polytetrafluoroethylene (PTFE) and perfluorodecalin, present in 25.9% and 22.2% of products containing it, respectively, were the most frequent.Although the presence of this type of ingredient seems to be limited in Europe, make-up appears to be the type of product most likely to contain PFAS.

Magnetite-driven Bio-Fenton degradation of chloroacetanilide herbicides by a newly isolated hydrogen peroxide producing bacterium Desemzia sp. strain C1

The efficiency of the Fenton reaction is markedly contingent upon the operational pH related to iron solubility. Therefore, a heterogeneous Fenton reaction has been developed to function at neutral pH. In the present study, the Bio-Fenton reaction was carried out using magnetite (Fe(II)Fe(III)2O4) and H2O2 generated by a newly isolated H2O2-producing bacterium, Desemzia sp. strain C1 at pH 6.8 to degrade chloroacetanilide herbicides. The optimal conditions for an efficient Bio-Fenton reaction were 10 mM of lactate, 0.5% (w/v) of magnetite, and resting-cells (O.D.600 = 1) of strain C1. During the Bio-Fenton reaction, 1.8–2.0 mM of H2O2 was generated by strain C1 and promptly consumed by the Fenton reaction with magnetite, maintaining stable pH conditions. Approximately, 40–50% of the herbicides underwent oxidation through non-specific reactions of •OH, leading to dealkylation, dechlorination, and hydroxylation via hydrogen atom abstraction. These findings will contribute to advancing the Bio-Fenton system for non-specific oxidative degradation of diverse organic pollutants under in-situ environmental conditions with bacteria producing high amount of H2O2 and magnetite under a neutral pH condition.

XGASS: the scatter of the H i–halo mass relation of central galaxies

ABSTRACT Empirical studies of the relationship between baryonic matter in galaxies and the gravitational potential of their host haloes are important to constrain our theoretical framework for galaxy formation and evolution. One such relation, between the atomic hydrogen (H i) mass of central galaxies (MH i,c) and the total mass of their host haloes (Mhalo), has attracted significant interest in the last few years. In this work, we use the extended GALEX Arecibo SDSS Survey to examine the scatter of the H i–halo mass relation for a representative sample of central galaxies. Our findings reveal a flat median relation at $\mathrm{ log}_{10}(M_{\rm {H\,{\small I},c}}/\rm {M}_{\odot }) \approx 9.40$, across $11.1 \lt \mathrm{ log}_{10}(M_{\rm {halo}}/{{\rm M}_{\odot }}) \lt 14.1$. This flat relation stems from the statistical dominance of star-forming, disc galaxies at low Mhalo in combination with the increasing prevalence of passive, high stellar concentration systems at higher Mhalo. The scatter of this relation and the stellar specific angular momentum of centrals have a strong link (Spearman’s rank correlation coefficient ≥0.5). Comparisons with simulations suggest that the kinematic state of host haloes may be primarily driving this scatter. Our findings highlight that the H i–halo mass parameter space is too complex to be completely represented by simple median or average relations and we show that tensions with previous works are most likely due to selection biases. We recommend that future observational studies, and their comparisons with theoretical models, bin central galaxies also by their secondary properties to enable a statistically robust understanding of the processes regulating the cold gas content within central galaxies of dark matter haloes.

Synthesis of Deuterated Compounds by Flow Chemistry.

Development of the efficient and practical method for the synthesis of deuterated compounds which occupies the broadest area among stable isotopes is one of the most essential issues toward the industrial advance and building a sustainable society. This review describes recent advances in deuteration reactions, where the continuous flow chemistry plays pivotal roles for the successful installation of deuterium atom into diverse organic frameworks, opening new fields of isotope-based synthetic chemistry.

Sprayed Oil-Water Microdroplets as a Hydrogen Source.

Liquid water provides the largest hydrogen reservoir on the earth's surface. Direct utilization of water as a source of hydrogen atoms and molecules is fundamental to the evolution of the ecosystem and industry. However, liquid water is an unfavorable electron donor for forming these hydrogen species owing to its redox inertness. We report oil-mediated electron extraction from water microdroplets, which is easily achieved by ultrasonically spraying an oil-water emulsion. Based on charge measurement and electron paramagnetic resonance spectroscopy, contact electrification between oil and a water microdroplet is demonstrated to be the origin of electron extraction from water molecules. This contact electrification results in enhanced charge separation and subsequent mutual neutralization, which enables a ∼13-fold increase of charge carriers in comparison with an ultrapure water spray, leading to a ∼16-fold increase of spray-sourced hydrogen that can hydrogenate CO2 to selectively produce CO. These findings emphasize the potential of charge separation enabled by spraying an emulsion of liquid water and a hydrophobic liquid in driving hydrogenation reactions.

Cobalt Single-Atom Reverse Hydrogen Spillover for Efficient Electrochemical Water Dissociation and Dechlorination.

Efficient water dissociation to atomic hydrogen (H*) with restrained recombination of H* is crucial for improving the H* utilization for electrochemical dechlorination, but is currently limited by the lack of feasible electrodes. Herein, we developed a monolithic single-atom electrode with Co single atoms anchored on the inherent oxide layer of titanium foam (Co1-TiOx/Ti), which can efficiently dissociate water into H* and simultaneously inhibit the recombination of H*, by taking advantage of the single-atom reverse hydrogen spillover effect. Experimental and theoretical calculations demonstrated that H* could be rapidly generated on the oxide layer of titanium foam, and then overflowed to the adjacent Co single atom for the reductive dechlorination. Using chloramphenicol as a proof-of-concept verification, the resulting Co1-TiOx/Ti monolithic electrode exhibited an unprecedented performance with almost 100 % dechlorination at -1.0 V, far superior to that of traditional indirect reduction-driven commercial Pd/C (52 %) and direct reduction-driven Co1-N-C (44 %). Moreover, its dechlorination rate constant of 1.64 h-1 was 4.3 and 8.6 times more active than those of Pd/C (0.38 h-1) and Co1-N-C (0.19 h-1), respectively. Our research sheds light on the rational design of hydrogen spillover-related electrocatalysts to simultaneously improve the H* generation, transfer, and utilization for environmental and energy applications.

Cobalt Single‐Atom Reverse Hydrogen Spillover for Efficient Electrochemical Water Dissociation and Dechlorination

AbstractEfficient water dissociation to atomic hydrogen (H*) with restrained recombination of H* is crucial for improving the H* utilization for electrochemical dechlorination, but is currently limited by the lack of feasible electrodes. Herein, we developed a monolithic single‐atom electrode with Co single atoms anchored on the inherent oxide layer of titanium foam (Co1−TiOx/Ti), which can efficiently dissociate water into H* and simultaneously inhibit the recombination of H*, by taking advantage of the single‐atom reverse hydrogen spillover effect. Experimental and theoretical calculations demonstrated that H* could be rapidly generated on the oxide layer of titanium foam, and then overflowed to the adjacent Co single atom for the reductive dechlorination. Using chloramphenicol as a proof‐of‐concept verification, the resulting Co1−TiOx/Ti monolithic electrode exhibited an unprecedented performance with almost 100 % dechlorination at −1.0 V, far superior to that of traditional indirect reduction‐driven commercial Pd/C (52 %) and direct reduction‐driven Co1−N−C (44 %). Moreover, its dechlorination rate constant of 1.64 h−1 was 4.3 and 8.6 times more active than those of Pd/C (0.38 h−1) and Co1−N−C (0.19 h−1), respectively. Our research sheds light on the rational design of hydrogen spillover‐related electrocatalysts to simultaneously improve the H* generation, transfer, and utilization for environmental and energy applications.

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C-H Oxidation.

Precise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)-H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site-selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C-H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis-Menten kinetics and increased reaction rates. In addition, C-H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C−H Oxidation

AbstractPrecise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)−H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site‐selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C−H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis–Menten kinetics and increased reaction rates. In addition, C−H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

Hollow carbon sphere featuring highly dispersed Co-Nx sites for efficient and controllable syngas electrosynthesis from CO2

Electrochemical reduction of CO2 and H2O powered by green electricity provides a sustainable way to produce syngas. However, this reaction still suffers from insufficient syngas selectivity, slow charge kinetic process, and poor controllability of the CO/H2 ratio. In this work, we report a hollow carbon sphere electrocatalyst featuring highly dispersed Co-Nx sites (Co-NHCS-x) for efficient and controllable syngas electrosynthesis. The Co-NHCS-400 sample exhibits a high syngas production rate with a steady CO/H2 ratio (≈1:2) over a wide range of operating potential from −0.6 V to −1.1 V vs. RHE. The variation in synthesis temperature can finely tune the coordination environment of Co-Nx in the catalyst. Consequently, the adsorption behaviors of the CO and hydrogen atoms can be manipulated, allowing for the precise adjustment of CO and H2 proportions in the product gas. The findings are potentially applicable to the exploitation of promising carbon-based electrocatalysts for electrochemical CO2 conversion into syngas.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon.

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.