Inhibit H2 Evolution Research Articles

Silver nanoparticles catalyzed electrochemical hydrodechlorination of dichloromethane to methane in N,N-Dimethylformamide using water as hydrogen donor

Silver nanoparticles (Ag NPs) were modified on Ag electrode in situ using the electrochemical redox method. The electrochemical hydrodechlorination (EHDC) performance of Ag NPs in N,N-Dimethylformamide (DMF) for a representative chlorinated volatile organic compound, dichloromethane (DCM), was studied. The results revealed that a large number of Ag NPs (∼7 nm) were successfully deposited on the surface of Ag electrode. Be benefit to Nano effect, Ag NPs exhibited higher EHDC catalytic activity than bulk Ag and inhibited H2 evolution. Under the catalysis of Ag NPs, the dechlorination of two Cl atoms on DCM was changed from synchronous dechlorination to asynchronous dechlorination. The EHDC efficiency of DCM in the three DMFs ranked from high to low as follows: weakly alkaline > Neutral > Weakly acidic. DCM at concentrations of 10–150 mM can be hydrodechlorinated to methane with ≥90% yield and ≥35% current efficiency.

All-Vanadium Redox Flow Battery Electrodes of Carbon Cloth Modified with Tungsten-Bismuth-Based Oxide Nanostructures for an Enhanced V(II)/(VIII) Reaction Kinetics and Inhibited H2 Evolution

All-vanadium redox flow batteries (VRFBs) are the most promising and highly efficient storage systems for renewable energy power plants. VRFBs can store high amounts of energy and are designed to work for long periods and their solutions can be used indefinitely.1 VRFBs are mainly carbon-based electrodes as they have high conductivity, high surface area, and high chemical stability, are relatively cheap and available,2 and can be tailored as desired for specific properties. However, these electrodes' performance is limited in their pristine form due to their resistance, hydrophobicity, and surface structure among other factors, resulting in VRFBs with a low power density. The VRFBs' low power density is usually related to the electrodes’ poor performance. Modifying the carbon electrodes using nanomaterials was proven to increase the power density of the battery. Metal oxides like tungsten oxide3 are known to enhance vanadium reactions when used as composites with carbon nanomaterials, while bismuth is known to inhabit the parasitic hydrogen evolution reaction.4 The material of the electrode and the metal molar ratio and the types of metal oxide modifiers highly affect the final performance of the carbon-based electrodes in VRFBs.In this work, different tungsten-bismuth-based oxide nanostructures were synthesized using the hydrothermal method. It is suggested that using different metal molar ratios, pH, and metal precursors significantly affects the electrode's physical and chemical structure and hence its electrochemical performance. The synthesized BiWOx nanostructures are evaluated using XRD, SEM, FT-IR, EDS, XPS, and water contact angle measurements. The effect of using BiWOx nanostructures as a modifier for carbon cloth electrodes on the kinetics of the VRFBs negative half-cell reaction (V2+/V3+) was evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy. Results showed that using BiWOx as an electrode modifier led to an increase in electrode activity and stability, with BiWOx showing the best stability and BiW2Ox showing the best activity. The enhancement effect of BiWOx nanostructures is related to the interaction between the two metal oxides and the electrode surface, the enhanced hydrophilicity of the modified electrode, and the increase of the catalytic active sites.

Degradation of Chloroform by Zerovalent Iron: Effects of Mechanochemical Sulfidation and Nitridation on the Kinetics and Mechanism.

Chloroform (CF) is a widely used chemical reagent and disinfectant and a probable human carcinogen. The extensive literature on halocarbon reduction with zerovalent iron (ZVI) shows that transformation of CF is slow, even with nano, bimetallic, sulfidated, and other modified forms of ZVI. In this study, an alternative method of ZVI modification─involving simultaneous sulfidation and nitridation through mechanochemical ball milling─was developed and shown to give improved degradation of CF (i.e., higher degradation rate and inhibited H2 evolution reaction). The composite material (denoted as S-N(C)-ZVI) gave synergistic effects of nitridation and sulfidation on CF degradation. A complete chemical reaction network (CRN) analysis of CF degradation suggests that O-nucleophile-mediated transformation pathways may be the main route for the formation of the terminal nonchlorinated products (formate, CO, and glycolic polymers) that have been used to explain the undetected products needed for mass balance. Material characterizations of the ZVI recovered after batch experiments showed that sulfidation and nitridation promoted the formation of Fe3O4 on the S-N(C)-ZVI particles, and the effect of aging on CF degradation rates was minor for S-N(C)-ZVI. The synergistic benefits of sulfidation and nitridation on CF degradation were also observed in experiments performed with groundwater.

Layer-Stacked Zn with Abundant Corners for Selective CO2 Electroreduction to CO

Electric-driven CO2 reduction offers a promising strategy for CO2 conversion into valuable chemicals and fuels. However, developing low cost and efficient catalysts is still a challenge. Although earth-abundant Zn with the capability of converting CO2 to CO is considered to be one of the promising materials, the low selectivity and stability of Zn catalyst limit its practical applications in CO2 reduction. Herein, we report a highly selective and stable layer-stacked Zn catalyst prepared by an efficient and facile electrochemical method for CO2 reduction to CO. The layer-stacked Zn can produce CO with more than 90% Faradaic efficiency at an overpotential of 0.9 V. Notably, the layer-stacked Zn maintained ∼90% CO selectivity in CO2 electrolysis for more than 70 h, which significantly surpasses the durability of the reported Zn-based catalysts to date. In addition, after prolonged CO2 reduction, the robust catalytic performance of layer-stacked Zn can be recovered repeatedly by the simple electrochemical method, which may be linked to the maintained layer-stacked structure even after multiple reactivations. Further analysis suggests that while abundant low-coordinated sites (corners and edges) can be created on layer-stacked Zn, the enhanced catalytic performance in CO2 reduction is mainly correlated with the created corners instead of edges, owing to that corners not only improve the intrinsic CO2 reduction activity but also inhibit H2 evolution simultaneously.

A Honeycomb-Like Porous Crystalline Hetero-Electrocatalyst for Efficient Electrocatalytic CO2 Reduction.

Porous heterostructured electrocatalysts with multifunctionality and synergistic effect have much benefit for efficient electrocatalytic CO2 reduction reaction (CO2 RR), yet it still remains a daunting challenge to explore heterostructures based on covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) in this field. Here, a series of honeycomb-like porous crystalline hetero-electrocatalysts (MCH-X, X= 1-4, X stands for the numbered sample obtained from different MOF doses in the synthesis of the MCH) are synthesized, and these are successfully applied in electrocatalytic CO2 RR. The specially designed heterostructures with integrated porous MOF-template and ultrathin COF-coating enable efficient CO2 adsorption/activation and conversion into CH4 . The best of them, MCH-3, shows greatly inhibited H2 evolution, excellent current density (-398.1mA cm-2 ), and superior (76.7%) to the physical mixture (38.0%), the MOF@COF without the honeycomb-like morphology (47.7%), and the bare COF (37.5%) and MOF (15.9%) at -1.0V. Based on the density functional theory calculations and various characterizations, the vital roles of the MOF in facilitating CO2 adsorption/activation, stabilizing intermediates, and conquering the energy barrier of rate-determining step are intensively studied.

Performance and Mechanisms of Sulfidated Nanoscale Zero-Valent Iron Materials for Toxic TCE Removal from the Groundwater.

Trichloroethylene (TCE) is one of the most widely distributed pollutants in groundwater and poses serious risks to the environment and human health. In this study, sulfidated nanoscale zero-valent iron (S-nZVI) materials with different Fe/S molar ratios were synthesized by one-step methods. These materials degraded TCE in groundwater and followed a pathway that did not involve the production of toxic byproducts such as dichloroethenes (DCEs) and vinyl chloride (VC). The effects of sulfur content on TCE dechlorination by S-nZVI were thoroughly investigated in terms of TCE-removal efficiency, H2 evolution, and reaction rate. X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) characterizations confirmed Fe(0) levels in S-nZVI were larger than for zero-valent iron (nZVI). An Fe/S molar ratio of 10 provided the highest TCE-removal efficiencies. Compared with nZVI, the 24-h TCE removal efficiencies of S-nZVI (Fe/S = 10) increased from 30.2% to 92.6%, and the Fe(0) consumed during a side-reaction of H2 evolution dropped from 77.0% to 12.8%. This indicated the incorporation of sulfur effectively inhibited H2 evolution and allowed more Fe(0) to react with TCE. Moreover, the pseudo-first-order kinetic rate constants of S-nZVI materials increased by up to 485% compared to nZVI. In addition, a TCE degradation was proposed based on the variation of detected degradation products. Noting that acetylene, ethylene, and ethane were detected rather than DCEs and VC confirmed that TCE degradation followed β-elimination with acetylene as the intermediate. These results demonstrated that sulfide modification significantly enhanced nZVI performance for TCE degradation, minimized toxic-byproduct formation, and mitigated health risks. This work provides some insight into the remediation of chlorinated-organic-compound-contaminated groundwater and protection from secondary pollution during remediation by adjusting the degradation pathway.

Degradation of Tetrabromobisphenol A by Sulfidated Nanoscale Zerovalent Iron in a Dynamic Two-Step Anoxic/Oxic Process.

A dynamic two-step anoxic/oxic process using sulfidated nanoscale zerovalent iron (S-nZVI) was employed to degrade tetrabromobisphenol A (TBBPA). In the anoxic stage, TBBPA followed a four-step sequential debromination pathway and was completely transformed to bisphenol A (BPA) with the optimal S/Fe molar ratio of 0.3. S-nZVI inhibited H2 evolution and preserved the reducing capacity of Fe(0). Fe(0), rather than the formed FeS in S-nZVI, was responsible for TBBPA debromination. In the oxic stage, the product BPA was attacked by •OH, transformed to dihydroxybenzenes and benzoquinones, and eventually, achieved mineralization via ring-opening reactions. The sulfidation process facilitated •OH production through a two-electron transfer pathway by surface-bound Fe(II), in which structural Fe(II) in FeS and regenerated Fe(II) from Fe(III) reduction by Fe(0) played significant roles toward total BPA degradation. S-nZVI was transformed to S8 and α-FeOOH after the oxic treatment. After these two steps, complete degradation of TBBPA was achieved. This study demonstrated the feasibility that refractory contaminants could be completely degraded in the dynamic two-step anoxic/oxic process, thus broadening the utility of S-nZVI for environmental applications in water treatment.

Culture conditions affecting H2 production by phototrophic bacterium Rhodobacter sphaeroides KD131

The effect of culture conditions on photo-H2 production was investigated using the photosynthetic bacterium Rhodobacter sphaeroides KD131. When the initial cell concentrations were either below or above a threshold of 0.56g-dcw/L, the H2 production decreased due to an imbalance between the biomass and the substrate. Malate- and succinate-fed cultures exhibited the highest substrate conversion to H2 production, whereas more than 85% of the substrate was utilized for cell growth in acetate- and butyrate-fed cultures. Compared with (NH4)2SO4, glutamate as a nitrogen source was more appropriate for the initial H2 production, but inhibited H2 evolution during extended cultivation due to released NH4+ ion. Even though the KD131 strain grew well under slightly acidic conditions, the pH value should be maintained in a neutral range in order to enhance H2 production. The highest H2 yield of 3.65mol-H2/mol-succinate was achieved when the KD131 strain grew in the succinate–glutamate medium with an initial cell concentration of 0.56g-dcw/L and the pH level controlled to 7.5.

Effects on substrate reduction of substitution of histidine-195 by glutamine in the alpha-subunit of the MoFe protein of Azotobacter vinelandii nitrogenase.

Studies of the substrate-reducing capabilities of an altered nitrogenase MoFe protein (alpha-195(Gln) instead of alpha-195(His)) from a mutant of Azotobacter vinelandii show, contrary to an earlier report [Kim, C.-H., Newton, W. E., and Dean, D. R. (1995) Biochemistry 34, 2798-2808], that the alpha-195(Gln) MoFe protein can reduce N2 to NH3 but at a rate that is <2% of that of the wild type. The extent of effective binding of N2 by this altered MoFe protein, as monitored by the inhibition of H2 evolution, is markedly increased as temperature is lowered but virtually eliminated at 45 degreesC. This inhibition of H2 evolution results in an increase in the ATP:2e- ratio, i.e., the number of molecules of MgATP hydrolyzed for each electron pair transferred to substrate, from ca. 5 (the wild-type level) at 45 degreesC to nearly 25 at 13 degreesC. Like wild-type nitrogenase, the N2 inhibition of H2 evolution reaches a maximum at an Fe protein:MoFe protein molar ratio of ca. 2.5, suggesting that a highly reduced enzyme may not be necessary for N2 binding. N2 binding to the alpha-195(Gln) MoFe protein retains a hallmark of the wild type by producing HD under a mixed N2/D2 atmosphere. The rate of HD production and the fraction of total electron flow allocated to HD are similar to those for wild-type nitrogenase under the same conditions. However, the electrons forming HD do not come from those normally producing NH3 (as occurs in the wild type) but are equivalent to those whose evolution as H2 had been inhibited by N2. N2 also inhibits C2H2 reduction catalyzed by the alpha-195(Gln) nitrogenase. This inhibition is relieved by added H2, resulting in a lowering of the elevated ATP:2e- ratio to that found under Ar. With solutions of NaCN, which contain both the substrate, HCN, and the inhibitor, CN-, reduction of HCN is not impaired with the alpha-195(Gln) nitrogenase, but the inhibition by CN- of total electron flow to substrate, which is observed with the wild-type MoFe protein, is completely absent. Unlike that of the catalyzed reduction of H+, HCN, or C2H2, the extent of azide reduction to either N2 or N2H4 is markedly decreased (to 5-7% of that of the wild type) with the alpha-195(Gln) nitrogenase. Azide, like N2, inhibits H2 evolution and increases the ATP:2e- ratio. Both effects are freely reversible and abolished by CO. Added D2 does not relieve either effect, implying that N2 produced from N3- is not the inhibitory species. The correlation between the extremely low rates of reduction for both N2 and azide by the alpha-195(Gln) nitrogenase and their common ability to inhibit H2 evolution suggests that alpha-histidine-195 may be an important proton conductor to the FeMo cofactor center and specifically required for reduction of these two substrates.

Reduction of cyclic and acyclic diazene derivates by Azotobacter vinelandii nitrogenase: diazirine and trans-dimethyldiazene.

Nitrogenase reduces N2 to NH3, but the mechanistic details are unclear. Diazene (N2H2), a proposed 2e-/2H+ intermediate on the reduction pathway, is labile under typical enzyme assay conditions, and no firm evidence is available on whether or not it can be reduced by or inhibit nitrogenase. In this paper, we compare the interactions of Azotobacter vinelandii (Av) nitrogenase with two diazene analogues: diazirine, a photolabile diazene containing the azo (-N=N-) group in a strained, three-membered ring, and trans-dimethyldiazene, a diazene containing an unstrained trans-disubstituted N=N bond. Diazirine is reduced by nitrogenase under specific conditions to methane, methylamine, and ammonia in a ratio of ca. 1:2:4-5 with a Km value for all three products similar (0.05-0.09 mM) to that of dinitrogen (0.06-0.12 mM). The Km value of diazirine does not depend on the ratio of nitrogenase Fe protein (Av2) to nitrogenase MoFe protein (Av1) at Av2:Av1 ratios of 0.71 and 14.9. Diazirine potently and competitively inhibits acetylene reduction by Av nitrogenase with Ki = 0.03 mM and is predicted to inhibit H2 evolution completely at pressures >> Km. The experimental Henry's Law constant (1.50 M/atm) determined for trans-dimethyldiazene in H2O shows that it has about 20-fold higher solubility than diazirine in water at 30 degrees C. trans-Dimethyldiazene is reduced by nitrogenase under specific conditions to ammonia, methane and methylamine in a ratio of ca. 1:1:1 with Km values for the three products of 0.51-0.58 M. The product ratio does not change significantly when the component ratio (Av2:Av1 ) is varied over 2.06-13.62. trans-Dimethyldiazene reduction is inhibited noncompetitively by CO and C2H2 with Ki values of ca. 0.0008 and 0.006 atm, respectively. The results are discussed with respect to the stereoelectronic differences between the two azo substrates. A "random-edge" reduction is compared with alternative schemes for the diazirine reduction. For trans-dimethyldiazene, initial C-N cleavage is proposed to yield CH4 and a bound CH3N2H species, which is then reduced to CH3NH2 and NH3.

Purification and characterization of nitrogenase from a delta nifW strain of Azotobacter vinelandii.

Deletion of the nifW gene in Azotobacter vinelandii yields a strain (DJ224) that grows poorly under N2 fixing conditions (Jacobson, M. R., Cash, V. L., Weiss, M. C., Laird, N. F., Newton, W. E., and Dean, D. R. (1989) Mol. & Gen. Genet. 219, 49-57). Here we report the purification of nitrogenase from DJ224. The purified Fe protein was indistinguishable from wild-type. The MoFe protein was indistinguishable from the wild-type MoFe protein by the criteria of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, native gel electrophoresis, two-dimensional gel electrophoresis, metal analysis, UV/visible and EPR spectroscopies. It was different by the criteria of CD spectroscopy and specific activities. At a 5:1 molar ratio of Fe protein to MoFe protein, H2 evolution under argon was identical to wild-type, C2H2 reduction was inhibited by 27%, N2 reduction was inhibited by 38%, and CO inhibited H2 evolution by 17%. The above data show that the nifW gene product is not required for: 1) detectable alteration of the polypeptide; 2) the synthesis of the metal portion of FeMo cofactor; or 3) FeMo cofactor insertion. The MoFe protein synthesized in the absence of NifW appears to have an alteration near the FeMo cofactor site, possibly at homocitrate, which causes differential inhibition of different substrates.

Analysis of Azotobacter vinelandii nitrogenase reaction by monoclonal antibodies.

Nitrogenase catalyzes not only the reduction of N2 to NH3 but also the reduction of C2H2 to C2H4 and H+ ion to H2 gas, etc. The detailed mechanism of the nitrogenase reaction is not clear. We have prepared monoclonal antibodies against Component I nitrogenase of A. vinelandii and examined the effects of antibodies on the nitrogenase reactions. A monoclonal antibody designated MA-1 inhibited C2H2 reduction activity strongly but did not inhibit H2 evolution activity. MA-2, on the contrary, inhibited only H2 evolution activity. MA-8 inhibited both C2H2 reduction and H2 evolution activity to the same extent.

Klebsiella pneumoniae nitrogenase. Inhibition of hydrogen evolution by ethylene and the reduction of ethylene to ethane.

Ethylene (C2H4) inhibited H2 evolution by the Mo-containing nitrogenase of Klebsiella pneumoniae. The extent of inhibition depended on the electron flux determined by the ratio of Fe protein (Kp2) to MoFe protein (Kp1) with KiC2H4 = 409 kPa ([Kp2]/[Kp1] = 22:1) and KC2H4i = 88 kPa ([Kp1]/[Kp2] = 21:1) at 23 degrees C at pH 7.4. At [Kp2]/[Kp1] = 1:1, inhibition was minimal with C2H4 (101 kPa). Extrapolation of data obtained when C2H4 was varied from 60 to 290 kPa indicates that at infinite pressure of C2H4 total inhibition of H2 evolution should occur. C2H4 inhibited concomitant S2O4(2-) oxidation to the same extent that it inhibited H2 evolution. Although other inhibitors of total electron flux such as CN- and CH3NC uncouple MgATP hydrolysis from electron transfer, C2H4 did not affect the ATP/2e ratio. Inhibition of H2 evolution by C2H4 was not relieved by CO. C2H4 was reduced to C2H6 at [Kp2]/[Kp1] ratios greater than or equal to 5:1 in a reaction that accounted for no more than 1% of the total electron flux. These data are discussed in terms of the chemistry of alkyne and alkene reduction on transition-metal centres.

Characteristics of N2 fixation in Mo-limited batch and continuous cultures of Azotobacter vinelandii.

Steady-state chemostat cultures of Azotobacter vinelandii were established in a simple defined medium that had been chemically purified to minimize Mo and that contained no utilizable combined N source. Growth was dependent on N2 fixation, the limiting nutrient being the Mo contaminating the system. The Mo content of the organisms was at least 100-fold lower than that of Mo-sufficient cultures, and they lacked the characteristic g = 3.7 e.p.r. feature of the MoFe-protein of nitrogenase. A characteristic of nitrogenase activity in vivo in Mo-limited populations was a disproportionately low activity for acetylene reduction, which was 0.3 to 0.1 of that expected from the rate of N2 reduction. Acetylene was also a poor substrate in comparison with protons as a substrate for nitrogenase, and did not markedly inhibit H2 evolution, in contrast with Mo-sufficient populations. In batch cultures in similar medium or 'spent' chemostat medium inoculated with Mo-limited organisms, the addition of Mo elicited a biphasic increased growth response at concentrations as low as 2.5 nM, provided that sufficient Fe was supplied. In this system V did not substitute for Mo, and Mo-deficient cultures ceased growth at a 25-fold lower population density compared with cultures supplemented with Mo. Nitrogenase component proteins could not be unequivocally detected by visual inspection of fractionated crude extracts of Mo-limited organisms. 35SO42-pulse-labelling studies also showed that the rate of synthesis of the MoFe-protein component of nitrogenase was too low to be quantified. However, the Fe-protein of nitrogenase was apparently synthesized at high rates. The discussion includes an evaluation of the possibility that A. vinelandii possesses an Mo-independent N2-fixation system.

Mechanism of electron transfer in dinitrogen fixation in the system Ti(III)−Mo(III)

Mo(III) and N2 together inhibit H2 evolution in the dinitrogen fixing system Ti(III)−Mo(III) in alcohol-water media. It is concluded that the formation of a Mo2N2 complex changes the collective properties of Ti(III) hydroxide. The catalytic action of Mo(III) in N2 reduction is explained by the possibility of electron transfer to the Mo2N2 complex from a number of Ti(III) atoms situated around the catalyst complex.

Mechanism of electron transfer in dinitrogen fixation in the system Ti(III)−Mo(III)

Mo(III) and N2 together inhibit H2 evolution in the dinitrogen fixing system Ti(III)−Mo(III) in alcohol-water media. It is concluded that the formation of a Mo2N2 complex changes the collective properties of Ti(III) hydroxide. The catalytic action of Mo(III) in N2 reduction is explained by the possibility of electron transfer to the Mo2N2 complex from a number of Ti(III) atoms situated around the catalyst complex.

Light-dependent hydrogen evolution by Scenedesmus

The effect of glucose and the uncoupler Cl-CCP upon hydrogen production was studied in adapted cells of Scenedesmus obliquus D3. Cl-CCP at 10(-5)M concentration completely inhibited the evolution of H2 in the dark and increased the apparent rate of H2 evolution in the light. At 10(-5)M Cl-CCP, photosynthesis and photoreduction by anaerobically adapted algae were only temporarily inhibited; O2 evolution reappeared after approximately 1 hr of illumination if CO2 was present. Increasing the Cl-CCP concentration to 5 x 10(-5)M led to a maximum rate of photohydrogen production and fully inhibited H2 evolution, photoreduction and dark H2 evolution. H2 evolution was accompanied by a release of varying amounts of CO2 in the light, as well as in the dark. Dark CO2 production was stimulated by Cl-CCP. H2 evolution in the light was stimulated by adding glucose to autotrophically grown cells or by growing the cells heterotrophically with glucose; starvation had an opposite effect. Adapted cells released (14)CO2 from the 3 and/or 4 position of specifically labeled glucose, indicating that degradation occurred via the Embden-Meyerhof pathway. The amount of H2 released by autotrophically grown cells was the same either with continuous illumination or with short periods of light, followed by darkness. Scenedesmus mutant No. 11, which is unable to evolve O2 was not inhibited in its capacity to evolve H2 in the light. These data indicate that the evolution of H2 in the light by adapted Scenedesmus depends upon the degradation of organic material and does not require the production of free O2 by photosystem II.