Hydrogen Elimination Research Articles

Study on the role of Pd and ZrVFe hydrogen storage alloy in Pd/ ZrVFe catalyst for hydrogen elimination performance

Hydrogen energy has been considered to be one of the most potential clean energy, but its widespread availability is limited by hydrogen safety. Herein, a hydrogen-oxygen combination catalyst is designed based on a ZrVFe hydrogen storage alloy loaded with Pd, which exhibits excellent hydrogen elimination capability at low temperature, broad hydrogen concentration range and other complex working environments. The results show that the activity of the Pd/ZrVFe catalyst is dependent on the Pd loadings, and the 2 wt% Pd/ZrVFe catalyst can initiate the hydrogen-oxygen elimination reaction rapidly at ∼30 °C under above 2 vol% hydrogen concentration and 2 L min−1 inlet gas flow with a 96% hydrogen elimination efficiency. The Pd/ZrVFe catalyst has excellent hydrogen elimination performance at 2–20 vol% hydrogen concentration, especially at hydrogen concentration above 10%, the hydrogen elimination efficiency can reach more than 99%, and the highest hydrogen elimination rate is as high as 182.6 mL min−1· g−1. In addition, the Pd/ZrVFe catalyst is almost unaffected by irradiation and aerosol poisoning, and this catalyst provided better resistance to iodine vapor poisoning compared to other catalysts. This work opens a new window for addressing hydrogen safety issues.

Competing Si2CH4-H2 and SiCH2-SiH4 Channels in the Bimolecular Reaction of Ground-State Atomic Carbon (C(3Pj)) with Disilane (Si2H6, X1A1g) under Single Collision Conditions.

The bimolecular gas-phase reaction of ground-state atomic carbon (C(3Pj)) with disilane (Si2H6, X1A1g) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 36.6 ± 4.5 kJ mol-1. Two channels were observed: a molecular hydrogen elimination plus Si2CH4 (reaction 1) pathway and a silane loss channel along with the formation of SiCH2 (reaction 2), with branching ratios of 20 ± 3 and 80 ± 4%, respectively. Both channels involved indirect scattering dynamics via long-lived Si2CH6 reaction intermediate(s); the latter eject molecular hydrogen and silane in "molecular" elimination channels within the rotational plane of the fragmenting intermediate nearly perpendicularly to the total angular momentum vector. These molecular elimination channels are associated with tight exit transition states as reflected in a significant electron rearrangement as visible from the chemical bonding in the light reaction products molecular hydrogen and silane. Once these hydrogenated silicon-carbide clusters are formed within the inner envelope of carbon stars such as of IRC + 10216, the stellar wind can drive both Si2CH4 and SiCH2 to the outside sections of the envelope, where they can be photolyzed. This is of particular importance to unravel potential formation pathways to disilicon monocarbide (Si2C) observed recently in the circumstellar shell of IRC + 10216.

Synthesis of a diruthenium μ-η4-α-diimine complex via dehydrogenative coupling of cyclic amines and its role in dehydrogenative oxidation of pyrrolidine.

The reaction of [Cp‡Ru(μ-H)4RuCp‡] (1: Cp‡ = 1,2,4-tri-tert-butylcyclopentadienyl) with cyclic amines at 180 °C afforded a μ-η4-α-diimine complex, [(Cp‡Ru)2(μ-η4-C2nH4n-4N2)] (5a-c: n = 4, 5, 6), via dehydrogenative coupling of two cyclic amine molecules. An intermediate μ-η2-1-pyrroline complex, [{Cp‡Ru(μ-H)}2(μ-η2-C4H7N)] (2a), was synthesized by the photoreaction of 1 with pyrrolidine and 5a was shown to be formed via the disproportionation of 2a upon thermolysis yielding 1 and a μ-imidoyl complex, [(Cp‡Ru)2(μ-η2:η2-C4H6N)(μ-H)] (3a). Complex 3a was transformed into 5avia the incorporation of 1-pyrroline, which was formed by the reaction of 2a with H2. DFT calculations on the model complexes supported by C5H5 groups at the B3LYP level suggested that the μ-η4-α-diimine ligand is formed via the insertion of a terminal cyclic aminocarbene ligand into the Ru-C bond of the μ-imidoyl group followed by the elimination of hydrogen. Although 5a was inert under an Ar atmosphere, it catalyzed the dehydrogenative oxidation of pyrrolidine under an atmosphere of hydrogen to yield γ-butyrolactam. An active species possessing a terminal cyclic aminocarbene ligand was generated via the heterolytic activation of hydrogen at the Ru-N bond followed by C-C bond cleavage.

Chemodivergence in Pd-catalyzed desymmetrization of allenes: enantioselective [4+3] cycloaddition, desymmetric allenylic substitution and enynylation.

A class of prochiral allenylic di-electrophiles have been introduced for the first time as three-atom synthons in cycloadditions, and a new type of [4+3] cycloaddition involving transition metal-catalyzed enantioselective sequential allenylic substitution has been successfully developed, enabling challenging seven-membered exocyclic axially chiral allenes to be accessed in good yields with good enantioselectivity. Through the addition of a catalytic amount of ortho-aminoanilines or ortho-aminophenols, the racemization of the [4+3] cycloaddition products is effectively suppressed. Mechanistic studies reveal that elusive Pd-catalyzed enantioselective intramolecular allenylic substitution rather than intermolecular allenylic substitution is the enantio-determining step in this cycloaddition. By tuning the ligands, a Pd-catalyzed enantioselective desymmetric allenylic substitution leading to linear axially chiral tri-substituted allenes or a Pd-catalyzed tandem desymmetric allenylic substitution/β-vinylic hydrogen elimination (formal enynylation) leading to multi-functionalized 1,3-enynes is achieved chemodivergently.

Theoretical study of the reactivity of urea, thiourea and some of their hydroxylated derivatives towards free radicals

Urea is an organic compound with the chemical formula CO(NH2)2. It is similar to thiourea of formula CS(NH2)2, except that the oxygen atom is replaced by a sulfur atom. It has been shown that urea and thiourea, have derivatives such as: Enolurea, hydroxyurea, hydroxyhiourea, enolthiourea. Indeed, since the discovery of these different compounds, the results of in vitro tests have shown the capacity of each of them to participate in the antioxidant defence of the body by trapping free radicals. In the present work, a comparative study of the antioxidant properties of urea, enolurea, thiourea, 1-hydroxyurea, hydroxythiourea and enolthiourea was carried out by DFT method M06-2X/6-311++G (d, p). The results of the various calculations allowed to: • Identify the oxygen atoms of the O-H groups, O-H9, O-H7 and O-H8, as the most important sites for the manifestation of the antioxidant activity of hydroxythiourea, hydroxyurea, enolthiourea and Enolurea respectively. • It was found that thiourea and hydroxythiourea are the most antioxidant of the six molecules. • to note that the replacement of the oxygen atom by that of sulfur considerably modifies the antioxidant properties of the urea molecule to note that the mechanism passing by the elimination of atomic hydrogen by homolytic rupture of bond (HAT), as the most probable for the trapping of a radical by each of the molecules.

Mechanistic insights into amide formation from aryl epoxides and amines catalyzed by ruthenium pincer complexes: a DFT study.

The synthesis of amides is of great significance in academia and industrial fields. Herein, density functional theory (DFT) studies were employed to investigate the mechanism of the formation of amides via aryl epoxides and amines catalyzed by ruthenium pincer complexes. The entire reaction mainly comprises three processes: isomerization of epoxides to aldehydes, aldimine condensation, and amide formation. Calculated results showed that bipyridine-based Ru-PNN A1 (PNN = 2-(di-tert-butylphosphinomethyl)bipyridine) pincer complexes could be potential highly catalytic species for the synthesis of amides and that the rate-determining step is the amine-assisted hydrogen elimination with a free energy barrier of 28.0 kcal mol-1. This study might not only provide new insights into the future of the formation of amides by transition-metal complexes but also facilitate the theoretical guidance needed to design novel transition-metal catalysts.

Electro- and photochemical H2 generation by Co(ii) polypyridyl-based catalysts bearing ortho-substituted pyridines.

Cobalt(ii) complexes featuring hexadentate amino-pyridyl ligands have been recently discovered as highly active catalysts for the Hydrogen Evolution Reaction (HER), whose high performance arises from the possibility of assisting proton transfer processes via intramolecular routes involving detached pyridine units. With the aim of gaining insights into such catalytic routes, three new proton reduction catalysts based on amino-polypyridyl ligands are reported, focusing on substitution of the pyridine ortho-position. Specifically, a carboxylate (C2) and two hydroxyl substituted pyridyl moieties (C3, C4) are introduced with the aim of promoting intramolecular proton transfer which possibly enhances the efficiency of the catalysts. Foot-of-the-wave and catalytic Tafel plot analyses have been utilized to benchmark the catalytic performances under electrochemical conditions in acetonitrile using trifluoroacetic acid as the proton source. In this respect, the cobalt complex C3 turns out to be the fastest catalyst in the series, with a maximum turnover frequency (TOF) of 1.6 (±0.5) × 105 s-1, but at the expense of large overpotentials. Mechanistic investigations by means of Density Functional Theory (DFT) suggest a typical ECEC mechanism (i.e. a sequence of reduction - E - and protonation - C - events) for all the catalysts, as previously envisioned for the parent unsubstituted complex C1. Interestingly, in the case of complex C2, the catalytic route is triggered by initial protonation of the carboxylate group resulting in a less common (C)ECEC mechanism. The pivotal role of the hexadentate chelating ligand in providing internal proton relays to assist hydrogen elimination is further confirmed within this novel class of molecular catalysts, thus highlighting the relevance of a flexible polypyridine ligand in the design of efficient cobalt complexes for the HER. Photochemical studies in aqueous solution using [Ru(bpy)3]2+ (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor support the superior performance of C3.

New prebiotic molecules in the interstellar medium from the reaction between vinyl alcohol and CN radicals: unsupervised reaction mechanism discovery, accurate electronic structure calculations and kinetic simulations.

Vinyl alcohol (VyA) and cyanide (CN) radicals are relatively abundant in the interstellar medium (ISM). VyA is the enolic tautomer of acetaldehyde and has two low-lying conformers, characterized by the syn or anti placement of hydroxyl hydrogen with respect to the double bond. In this paper, we present a gas-phase model of the barrierless reactions of both VyA's conformers with CN employing accurate quantum chemical computations in the framework of a master equation approach based on the transition state theory. Our results indicate that both VyA conformers feature a similar reactivity with CN, starting with a barrierless addition to the double bond and followed by different isomerization, dissociation, and/or hydrogen elimination steps. The rate constants computed for temperatures up to 600 K show that several reaction channels are open even under the harsh conditions of the ISM, with the favoured one providing the first feasible formation route of a prebiotic molecule not yet detected in the ISM, namely cyanoacetaldehyde. This finding suggests looking for cyanoacetaldehyde in regions where both VyA and CN have already been detected, like, e.g., Sagittarius B2N or G+0.693-0.027.

Fast Heavy-Ion-Induced Anion-Molecule Reactions on the Methanol Droplet Surface.

To gain insight into complex ion-molecule reactions induced by MeV-energy heavy ion irradiation of condensed matter, we performed a mass spectrometric study of secondary ions emitted from methanol microdroplet surfaces by 2.0 MeV C2+. We observed positive and negative secondary ions, including fragments, clusters, and reaction products. We found that a wider variety of negative ions than positive ions (such as C2H-, C2HO-, C2H5O-, and C2H3O2-) were formed. We performed measurements for deuterated methanol (CH3OD) droplets to identify the hydrogen elimination site of the intermediates involved in the reactions and to reveal the mechanism that generates various negative reaction product ions. Comparing the results of CH3OD with CH3OH droplets, we propose that the primary formation mechanism is association reactions of anion and neutral fragments, such as CH3O- + CO → C2H3O2-. Quantum chemical calculations confirmed that the reactions can proceed with no barrier. This study provides new insights into the importance of rapid anion-molecule reactions among fragments as the mechanism that generates complex molecular species in fast heavy-ion-induced reactions.

Oxidative Addition of Methane and Reductive Elimination of Ethane and Hydrogen on Surfaces: From Pure Metals to Single Atom Alloys.

Oxidative addition of CH4 to the catalyst surface produces CH3 and H. If the CH3 species generated on the surface couple with each other, reductive elimination of C2H6 may be achieved. Similarly, H's could couple to form H2. This is the outline of nonoxidative coupling of methane (NOCM). It is difficult to achieve this reaction on a typical Pt catalyst surface. This is because methane is overoxidized and coking occurs. In this study, the authors approach this problem from a molecular aspect, relying on organometallic or complex chemistry concepts. Diagrams obtained by extending the concepts of the Walsh diagram to surface reactions are used extensively. C-H bond activation, i.e., oxidative addition, and C-C and H-H bond formation, i.e., reductive elimination, on metal catalyst surfaces are thoroughly discussed from the point of view of orbital theory. The density functional theory method for structural optimization and accurate energy calculations and the extended Hückel method for detailed analysis of crystal orbital changes and interactions play complementary roles. Limitations of monometallic catalysts are noted. Therefore, a rational design of single atom alloy (SAA) catalysts is attempted. As a result, the effectiveness of the Pt1/Au(111) SAA catalyst for NOCM is theoretically proposed. On such an SAA surface, one would expect to find a single Pt monatomic site in a sea of inert Au atoms. This is desirable for both inhibiting overoxidation and promoting reductive elimination.

Experimental investigation on the temperature characteristics and hydrogen elimination effect of non-premixed hydrogen-oxygen recombination reaction at ambient temperature

Hydrogen-oxygen recombination is a safe and effective method of eliminating hydrogen before the explosion occurs. Most existing studies have focused on material modification, but the dynamic response of this reaction is rarely reported. In this paper, a series of experiments catalyzed by the Platinum-Carbon catalyst was carried out in a cylindrical container of 250 cm3 to study the effects of hydrogen volume fraction, catalyst position, and catalyst usage status on the temperature characteristics and hydrogen conversion. The results show that the reaction temperature on the catalyst surface not only increases and then decreases with the increase of hydrogen volume fraction but also decreases as the distance between the catalyst and the bottom increases, except for the 40 vol% volume fraction of hydrogen, and the maximum value occurs at the hydrogen volume fraction of 70 vol%. Moreover, there is an uncertainty in the temperature relationship between the catalyst surface and below as the volume fraction of hydrogen increases. The temperature peak structure on the catalyst surface is only influenced by the catalyst position and hydrogen volume fraction, and two temperature peaks can be observed in most cases. However, the hydrogen conversion is not affected by the catalyst position and its usage status.

In-containment hydrogen behavior simulation and hydrogen concentration control under LBLOCA accidents

During a design basis accident in a pressurized water reactor (PWR), the released hydrogen through reactor breaks would induce the hydrogen detonation. The hydrogen passive autocatalytic recombiner (PAR) is widely used as a hydrogen elimination measure in nuclear power plants due to its passive capability, low starting threshold and easy installation. The present work aims to study the hydrogen risk after the occurrence of cold section double-end shear fracture large break loss coolant accident (LBLOCA) by using the 3D computational fluid dynamics program GASFLOW. A full containment model of CPR1000 is built. The hydrogen production rate inside the containment after LBLOCA is calculated from the related physicochemical reactions. The hydrogen transport, hydrogen concentration distribution and temperature distribution inside the containment are simulated. The effects of different roughness of the structure surface on the simulation are investigated, and it is proved that the operation of PAR can control the hydrogen concentration under the safety threshold of 4 vol%. The effects of hydrogen flow rate and PAR’s position on the hydrogen elimination efficiency are studied. Based on these studies, this paper makes some suggestions and theoretical references for the spatial arrangement scheme of PAR in the containment to optimize the hydrogen elimination efficiency.

Chemistry of Hydrogen Peroxide Formation and Elimination in Mammalian Cells, and Its Role in Various Pathologies

Hydrogen peroxide (H2O2) is a compound involved in some mammalian reactions and processes. It modulates and signals the redox metabolism of cells by acting as a messenger together with hydrogen sulfide (H2S) and the nitric oxide radical (•NO), activating specific oxidations that determine the metabolic response. The reaction triggered determines cell survival or apoptosis, depending on which downstream metabolic pathways are activated. There are several ways to produce H2O2 in cells, and cellular systems tightly control its concentration. At the cellular level, the accumulation of hydrogen peroxide can trigger inflammation and even apoptosis, and when its concentration in the blood reaches toxic levels, it can lead to bioenergetic failure. This review summarizes existing research from a chemical perspective on the role of H2O2 in various enzymatic pathways and how this biochemistry leads to physiological or pathological responses.

Silanized Graphene Oxide-Supported Pd Nanoparticles and Silicone Rubber for Enhanced Hydrogen Elimination.

Hydrogen is a dangerous gas because it reacts very easily with oxygen to explode, and the accumulation of hydrogen in confined spaces is a safety hazard. Composites consisting of polymers and catalysts are a common getter, where the commonly used catalyst is usually commercial Pd/C. However, it often shows poor compatibility with polymers, making it difficult to form a homogeneous and stable composite. In this work, palladium chloride (PdCl2) was converted to palladium (Pd) nanoparticles by reduction reaction and supported on graphene oxide (GO) modified by silanization. Spherical Pd nanoparticles with a size of 2–36 nm were uniformly distributed over the Silanized graphene oxide (SGO) matrix. When mixed with Pd/SGO, polymethylvinylsiloxane can be cured to silicone rubber (SR) by B2O3. Afterwards, the vinyl in the polymer can interact with hydrogen under the catalysis of Pd through the addition reaction, thus achieving the purpose of hydrogen elimination. The polymer elastomers with excellent self-healing properties and improved hydrogen elimination performance were prepared and were superior to the commercial Pd/C. In addition, excellent environmental adaptability was also demonstrated. The new getter SR-Pd/SGO provides a new avenue for developing polymer getters with superior properties.

Combined Experimental and Computational Study of the Mechanism of Acceptorless Alcohol Dehydrogenation by POCOP Iridium Pincer Complexes

Iridium pincer complexes of the type (POCOP)Ir (POCOP = 2,6-(tBu2PO)2C6H3) are very productive catalysts for dehydrogenation of secondary alcohols. To our surprise, we found that turnover frequencies demonstrated by (POCOP)IrH2 (IrH2) are higher in more dilute solutions of the catalyst, which triggered a mechanistic study of alcohol dehydrogenation by IrH2. Here, we provide strong evidence that acceleration by dilution is related to the rate-limiting mass transfer of hydrogen, which, so far, has not received much attention in the literature. Using experimental and computational methods, we show that dehydrogenation has two high-barrier steps, namely the reaction of IrH2 with alcohol to give (POCOP)IrH(OR) (IrH(OR)) and subsequent β-elimination in the latter. Depending on the alcohol and reaction conditions, IrH(OR) can be formed via an associative pathway that includes proton transfer to the hydride or a dissociative mechanism that involves hydrogen elimination from IrH2 to give a 14e (POCOP)Ir species. Rapid re-hydrogenation of IrH(OR) or the 14e (POCOP)Ir by dissolved hydrogen is responsible for the rate retardation in more concentrated solutions of the catalyst. The suggested mechanism gives a satisfactory quantitative description of the catalytic cycle, such that kinetic curves and reaction orders in the catalyst can be reproduced.

Unidirectional Elimination of Hydrogen by a Giant Local Field Saves First- and Last-Mile Performances of Semiconductor Devices.

Hydrogen, the smallest element, easily forms bonds to host/dopant atoms in semiconductors, which strongly passivates the original electronic characteristics and deteriorates the final reliability. Here, we demonstrate a concept of unidirectional elimination of hydrogen from semiconductor wafers as well as electronic chips through a giant local electric field induced by compact chloridions. We reveal an interactive behavior of chloridions, which can rapidly approach and take hydrogen atoms away from the device surface. A universal and simple technique based on a solution-mediated three-electrode system achieves efficient hydrogen elimination from various semiconductor wafers (p-GaN, p-AlGaN, SiC, and AlInP) and also complete light emitting diodes (LEDs). The p-type conductivity and light output efficiency of H-eliminated UVC LEDs have been significantly enhanced, and the lifetime is almost doubled. Moreover, we confirm that under a one-second irradiation of UVC LEDs, bacteria and COVID-19 coronavirus can be completely killed (>99.93%). This technology will accelerate the further development of the semiconductor-based electronic industry.

Electrochemical-Oxidation-Promoted Direct N-ortho-Selective Difluoromethylation of Heterocyclic N-Oxides.

An efficient and green electrochemical N-ortho-selective difluoromethylation method of various quinoline and isoquinoline N-oxides has been developed. In this method, sodium difluoromethanesulfinate (HCF2SO2Na) was used as the source of the difluoromethyl moiety, and various N-ortho-selective difluoromethylation quinoline and isoquinoline N-oxides were obtained in good to excellent yields under a constant current. In addition, the reaction was easy to scale up and maintained a good yield. Preliminary mechanism studies suggested that the reaction undergoes a free-radical addition and hydrogen elimination pathway.

The Mechanism of the Davy Test for Strychnine

The chemistry of a spot test for identification of a bioactive compound is important the more if the substance is strychnine. This converts the test in a toxicological assay with forensic application. The Davy test is based on the oxidation of strychnine by means of potassium ferricyanide in the presence of sulphuric acid. This Theoretical Organic Chemistry study reveals the reaction series that occurs during the test, the electron flow is provided step by step. The process is an electron-transfer oxidation, from the alkaloid to the multi-functional final product having two lactams, a ketone and a carboxylic acid. The involved reactions are electrophilic attack to double bond, hydrogen elimination, free radical subtraction, isomerization, alcohol oxidation, epoxide formation, hydrolysis, concerted reaction mechanism to ketone and aldehyde via carbon-carbon fission, and oxidation of aldehyde to carboxylic acid. All is in accordance with the chemical deportment of the involved compounds and is supported by the references.

Reactivity of organogermanium and organotin trihydrides.

The organogermanium and organotin trihydrides (TbbEH3) [E = Ge (3), Sn (7)] with the Tbb substituent were synthesized by hydride substitution (Tbb = 2,6-[CH(SiMe3)2]2-4-(t-Bu)C6H2). Deprotonation of the organoelement trihydrides 3 and 7 was studied in reaction with bases MeLi, BnK and LDA (Bn = benzyl, LDA = lithium diisopropylamide) to yield the deprotonation products (8-11) as lithium or potassium salts. Hydride abstraction from TbbSnH3 using the trityl salt [Ph3C][Al(OC{CF3}3)4] gives the salt [TbbSnH2][Al(OC{CF3}3)4] (12) which was stabilized by thf donor ligands [TbbSnH2(thf)2][Al(OC{CF3}3)4] (13). Tintrihydride 7 reacts with trialkylamine Et2MeN to give as the product of a reductive elimination of hydrogen the distannane (TbbSnH2)2 (14). Transfer of hydrogen was observed in reaction of trihydrides TbbEH3 (E = Ge, Sn) and Ar*GeH3 with N-heterocyclic carbene (NHC). The NHC adduct TbbSnH(iPrNHC) (15) was synthesized at rt and the germanium hydrides exhibit hydrogen transfer at higher temperatures to give Ar*GeH(MeNHC) (16) and TbbGeH(MeNHC) (17).

Tunnelling assisted hydrogen elimination mechanisms of FeCl3/TEMPO.

Metal-TEMPO hybrids are a family of novel and promising catalysts for aerobic oxidation of alcohols, yet the underlying mechanisms have not been understood theoretically. Using density functional theory, we probe the hydrogen abstraction mechanisms of FeCl3/TEMPO on two characteristic substrates, 9,10-dihydroanthracene and benzyl alcohol. We found that the low spin state of FeCl3/TEMPO is more favourable, and that the N atom is the preferred hydrogen acceptor. Moreover, dispersion interactions assist the reaction, as well as nuclear tunnelling, which even at room temperature can speed up the process by almost two orders of magnitude. We also predict that pronounced kinetic isotope effects (KIE) could be observed due to tunnelling. Our findings provide insights into improving the substrate scope and the development of new transformations for the FeCl3/TEMPO system.