Ni (II) and Mg (II) ions as factors enhancing biohydrogen production by Rhodobacter sphaeroides from mineral springs

Various metal ions play a key role in biohydrogen (H₂) production by phototrophic bacteria through incorporation into or stimulating the responsible enzymes and/or related pathways. The Ni (II) and Mg (II) ions effects on growth and H₂ production by Rhodobacter sphaeroides strain MDC6521 isolated from mineral springs in Armenia were established. The highest growth specific rate was obtained with 4–6 μM Ni²⁺ and 5 mM Mg²⁺. pH of the growth medium changed from 7.0 to 9.2–9.4 during the bacterial growth up to 72 h in spite of Ni²⁺ added but pH increased in different manner with Mg²⁺. In the presence of 2–4 μM Ni²⁺ external oxidation-reduction potential (ORP) decreased to more negative values (−800 ± 15 mV). This decrease of ORP indicated ∼2.7-fold enhanced H₂ yield (9.80 mmol L⁻¹) with Ni²⁺ compared with the control (without Ni²⁺). The H₂ yield determined in the medium with Mg²⁺ was ∼2.2 fold higher than that with 1 mM Mg²⁺. These results reveal new regulatory ways to improve H₂ production by R. sphaeroides those were depending on Ni²⁺ and Mg²⁺ of different concentrations.     

Enhanced hydrogen production from sewage sludge by cofermentation with wine vinasse

The cofermentation of sewage sludge and wine vinasse at different mixing ratios to enhance hydrogen production was investigated. Batch experiments were carried out under thermophilic conditions with thermophilic sludge inoculum obtained from an acidogenic anaerobic reactor. The results showed that the addition of wine vinasse enhances the hydrogen production of sewage sludge fermentation. The highest hydrogen yields, 41.16 ± 3.57 and 43.25 ± 1.52 mL H₂/g VS_added, were obtained at sludge:vinasse ratios of 50:50 and 25:75, respectively. These yields were 13 and 14 times higher than that obtained in the monofermentation of sludge (3.17 ± 1.28 mL H₂/g VS_added). The highest VS removal (37%) was obtained at a mixing ratio of 25:75. Cofermentation had a synergistic effect the hydrogen yield obtained at a sludge:vinasse ratio of 50:50 was 40% higher, comparing to the sum of each waste. Furthermore, kinetic analysis showed that Cone and first-order kinetic models fitted hydrogen production better than the modified Gompertz model.     

Design of Ni/La2O3 catalysts for dry reforming of methane: Understanding the impact of synthesis methods

Fine-tuning of materials properties, particularly the catalytic properties, through innovative synthesis procedures has gained an increased research interest in the last decades. It is well known that synthesis procedures have considerable impact on the physio-chemical properties of the synthesized materials even if the chemical composition is maintained. Herein, we investigated the impact of selected synthesis methods on the catalytic performance of Ni/La₂O₃ for the dry reforming of methane (DRM), a challenging reaction known for severe coking. Although this catalyst has been frequently studied for DRM, however, tuning the structure-activity relationship by varying the synthesis routes has not been reported. Herein, the chosen synthesis techniques; for example the solution combustion synthesis (Ni/La-SC), sol-gel (Ni/La-SG), homogeneous precipitation (Ni/La-HP), solvothermal (Ni/La-ST), and modified oleylamine-assisted synthesis (Ni/La-ME); considerably affected the morphology, metal support interaction (MSI), and surface area of Ni/La₂O₃ catalysts leading to variation in their performance for DRM. The investigated catalysts were thoroughly characterized by using SEM-EDX, TEM, N₂-physisorption, XRD, XPS, and H₂-TPR to understand the structural properties. Their catalytic performance towards the DRM was evaluated by varying the temperature between 550 and 800 °C. DRM experiments demonstrated that among the studied catalysts, Ni/La-SC showed the best performance for DRM with a high catalytic activity and coking resistance. For instance, Ni/La-SC revealed the highest CO₂ and CH₄ conversions i.e. 97.9 ± 1.5% and 96.6 ± 1.8%, respectively at 800 °C. The same sample revealed the highest hydrogen yield i.e. 71.9% and the highest H₂/CO ratio i.e. 1.03 ± 0.013 at the same temperature. The results revealed that Ni/La-SC demonstrated the lowest increment (20.9%) in the Ni crystallite size after DRM reaction, highest durability, and the lowest rate of coke formation (42 ± 5.2 mg C/g_catalyst) over an operating period of 100 h at 800 °C. The outstanding performance of Ni/La-SC catalyst was credited to the small crystallite size of Ni, high Ni⁰/Ni²⁺ ratio, high BET area, and a good dispersion of nickel sites over the La₂O₃ support. The obtained results may open new frontiers for size and shape-controlled synthesis of nanostructured metals/metal oxides catalysts with controllable morphologies and dispersion that can lead to desirable catalytic properties.     

Optimization of photo-hydrogen production based on cheese whey spent medium

Cheese whey wastewater diluted to 10 g lactose/L was initially subjected to dark-fermentation by Enterobacter aerogenes MTCC 2822, and the VFAs-rich spent medium (acetic acid 1900 mg/L, butyric acid 537 mg/L, and traces of propionic acid) was subjected to photo-fermentation through enrichment by Ni²⁺ (0–8 μmol/L), Fe²⁺ (0–100 μmol/L) or Mg²⁺ (0–15 mmol/L) in batch mode by Rhodopseudomonas BHU 01 strain. The maximum cumulative H₂ production (144 ml) and yield (58 mmol) was obtained at 4 μmol Ni²⁺/L. Likewise, Fe²⁺ (60 μmol/L) resulted in maximum cumulative H₂ production (139 ml) and yield (56 mmol). Nevertheless, 6 mmol of Mg²⁺ did not significantly affect H₂ production (110 ml) or yield (44 mmol); the latter value in close proximity with the control (37 mmol). The concomitant reduction in COD was maximum (15.61%) for 4 μmol Ni²⁺/L, followed by 15.33% for 60 μmol Fe²⁺/L, and the least for 6 mmol Mg²⁺/L (14.5%). The observations suggest the role of Fe²⁺ and Ni²⁺ in regulation of nitrogenase and hydrogenase, while that of Mg²⁺ mainly in the biosynthesis of photopigment bacteriochlorophyll (Bchl).     

Structural and catalytic studies of Mg1-xNixO nanomaterials for gasification of biomass in supercritical water for H2-rich syngas production

Nowadays, catalytic supercritical water gasification (SCWG) is undoubtedly used for production of H₂-rich syngas from biomass. The present study reported the synthesis and characterisation of Mg_1-xNi_xO (x = 0.05, 0.10, 0.15, 0.20) nanomaterials that were obtained via self-propagating combustion (SPC) method, and catalysed the SCWG for the first time. It had found that increased the nickel (Ni) content in the catalyst reduced the crystallite size, thus, increased the specific surface area, which influenced the catalytic activity. The specific surface area followed the order of Mg_0.95Ni_0.05O (36.2 m² g⁻¹) < Mg_0.90Ni_0.10O (58.9 m² g⁻¹) < Mg_0.85Ni_0.15O (63.6 m² g⁻¹) < Mg_0.80Ni_0.20O (67.9 m² g⁻¹). From the Rietveld refinement, the Ni that was successfully partial substituted in the cubic crystal structure of MgO resulting in a cell contraction which ascribed the reduction of crystallite size. Increased the amount of Ni also narrowed the pore size distribution ranging between 4.17 nm and 6.23 nm, as well as increased the basicity active site up to 5741.0 μmol g⁻¹ at medium basic strength. All the synthesised nanocatalysts were catalysed the SCWG of OPF (oil palm frond) biomass. Among them, the mesoporous Mg_0.80Ni_0.20O nanocatalyst exhibited the highest total gas volume of 193.5 mL g⁻¹ with 361.7% increment of H₂ yield than that of the non-catalytic reaction.     

In situ growth of NiCoFe-layered double hydroxide through etching Ni foam matrix for highly enhanced oxygen evolution reaction

It is of great significance to develop the nonprecious metal oxide electrocatalysts toward oxygen evolution reaction (OER) for water splitting. Herein we report an in-situ growth of the ternary NiCoFe-layered double hydroxide nanosheets on surface etching nickel foam (NiCoFe LDHs/NF) without adding any nickel precursor. In this method, etching Ni matrix by Fe³⁺ not only provides the slowly released nickel ions, but also intensifies the Fe–Ni interaction between the directly grown active species and Ni foam. Therefore the composition, electronic structure, and morphology of the electrocatalysts can be easily regulated only by adjusting Co²⁺:Fe³⁺ ratio in the precursor solution. The obtained NiCo₁Fe₁ LDH/NF, which is formed in 1:1 Co²⁺:Fe³⁺ solution, has highest content of Ni³⁺ and Co³⁺ active sites and the largest electrochemical active area. It exhibits an outstanding OER performance with a small overpotential of 231 mV at 10 mA cm^?2 and excellent durability at 50 mA cm^?2 in 1.0 M KOH solution.     

Glycerol and mixture of carbon sources conversion to hydrogen by Clostridium beijerinckii DSM791 and effects of various heavy metals on hydrogenase activity

Hydrogen is a carbon-neutral energy feedstock which is produced during fermentation of various carbon sources. The genomes of clostridia encode mainly [Fe-Fe]-hydrogenases. Clostridium beijerinckii DSM791 performed anaerobic fermentation of glycerol in batch culture at pH 7.5 and pH 5.5 and produced H₂. At pH 7.5, the glycerol consumption rate was 3.7 g/g cell mass/h, which was higher than that at pH 5.5. H₂ production reached 5 mmol/h/g cell mass at pH 7.5. The specific hydrogenase activity was ～1.4 fold higher if cells were grown on glycerol compared to cells grown on glucose. Single (Fe²⁺, Fe³⁺, Ni²⁺) or mixed supply of metals (Fe²⁺ and Ni²⁺) increased the specific hydrogenase activity by ～50%. These results suggest that C. beijerinckii DSM791 could be used as a potential H₂ producer. It may help to further enhance H₂ production using different industrial or agricultural wastes where glycerol and other carbon sources are present.     

Single-phase Ni3Sn alloy alkali-leached for hydrogen production from methanol decomposition

Methanol decomposition over alkali-leached Ni₃Sn powder at 513–793 K was investigated. Compared with untreated Ni₃Sn, alkali-leached Ni₃Sn had high catalytic activity and selectivity toward H₂ and CO production above 633 K. A maximum H₂ production rate of 100 × 10⁻³ mol h⁻¹ g-Cat⁻¹ and H₂ selectivity above 95% were attained over alkali-leached Ni₃Sn at 793 K. Alkali-leached Ni₃Sn presented good catalytic activity for 45 h of reaction at 713 K, whereas Ni₃Sn had none. The activation energy was calculated, and its values rapidly decreased from Ni₃Sn to alkali-leached ones. The improvement was attributed to the formation of Ni nanoparticles less than 100 nm in diameter in the alkali-leaching process, which had high activity for methanol decomposition. The improved catalytic activity favored the gradual formation of fine Ni₃Sn particle during the reaction, which served as the active sites for methanol decomposition when the catalytic activity decreased because of carbon deposition on the Ni surface. Results demonstrated that alkali-leached Ni₃Sn was a promising potential catalyst for hydrogen production from methanol.     

Water enhanced methanol decomposition for simultaneous heat recovery and ready-to-use synthesis gas production over CO2 capture enhanced Ni/zeolite 4A catalyst

Methanol decomposition is considered as a “one stone two birds” approach for simultaneously recovering waste heat and affording synthesis gas. However, this approach requires efficient catalysts with high CO selectivity and low selectivity to byproducts. Herein, a rational design of CO₂ capture enhanced Ni/zeolite 4 A catalyst for synthesis gas production by water enhanced methanol decomposition is reported. 5%-Ni/NaA-500 catalyst achieves the Y_H2 of 80.6%, Y_CO of 76.2%, H₂/CO molar ratio of 2.11, high stability, low selectivity to CO₂ and CH₄, and no coke at 325 °C. Ni atoms highly disperse on the surface and microporous of zeolite 4 A, and the strong interaction between Ni atoms and zeolite 4 A inhibits the reduction of Ni atoms. Consequently, Ni³⁺, Ni²⁺ and Ni⁰ coexist in 5%-Ni/NaA-500, and the redox couples of Ni³⁺↔Ni²⁺, Ni²⁺↔Ni⁰, and Ni³⁺↔Ni⁰ will enhance the redox processes during methanol decomposition. CO₂ capture capacity of x%-Ni/NaA-Y below 350 °C promotes the reverse water gas shift reaction by concentrating CO₂ molecules, which hence increases CO selectivity and declines the selectivity to byproducts. The reaction path follows CH₃OH→CH₃O→CH₂O→CHO→CO. This work will pave the way to industrial applications that combine ready-to-use synthesis gas production and heat recovery.     

Thermophilic hydrogen fermentation using Thermotoga neapolitana DSM 4359 by fed-batch culture

Biohydrogen fermentation by the hyperthermophile Thermotoga neapolitana was conducted in a continuously stirred anaerobic bioreactor (CSABR). The production level of H₂ from fermentation in a batch culture with pH control was much higher than without pH control from pentose (xylose) and hexose (glucose and sucrose) substrates. The respective H₂ yield in the batch culture with pH control from xylose and glucose was 2.22 ± 0.11 mol-H₂ mol⁻¹ xylose_consumed and 3.2 ± 0.16 mol-H₂ mol⁻¹ glucose_consumed, which was nearly 1.2-fold greater for xylose and 1.6-fold greater for glucose than without pH control. In the case of sucrose, the H₂ yield from fermentation increased by 40.63%, compared with fermentation in batch cultures without pH control, from 3.52 ± 0.171 to 4.95 ± 0.25 mol-H₂ mol⁻¹ sucrose_consumed. The effects of stirring speed and different pH levels on growth and H₂ production were studied in the CSABR for highly efficient H₂ production. Growth and H₂ production of this bacterial strain in a batch culture with pH control or without pH control using a 3 L bioreactor was limited within 24 h due to substrate exhaustion and a decrease in the culture’s pH. The pH-controlled fed-batch culture with a xylose substrate added in doses was studied for the prevention of substrate-associated growth inhibition by controlling the nutrient supply. The highest H₂ production rates were approximately 4.6, 4.1, 3.9, and 4.3 mmol-H₂ L⁻¹ h⁻¹ at 32, 52, 67, and 86 h, respectively.     

Optimization of the production and quality of biogas in the anaerobic digestion of different types of biomass in a batch laboratory biodigester and pilot plant: Numerical modeling,kinetic study and hydrogen potential

The study aims to evaluate the biogas production and quality from four biomasses (microalgae (MB), sorghum (S), corn stubble (CS), rapeseed oil (RO)) in a digestion process carried out in two batch reactor (6 L) and pilot plant (1.5 m³) agitated mechanically.The substrates were characterized and anaerobic digestion was carried out as batch tests in mesophilic conditions for 30–35 days. Inoculum/substrate ratio was 1:1–2:1. Gas composition and total gas volume produced were monitored. Methane yields of 306, 345, 419, and 740 NL kg VS^?1 were obtained for MB, CS, S, and RO, respectively, in laboratory tests, while in pilot plant tests were 182, 151, 397 and 655 NL kg VS^?1. CH₄ percentage in biogas was 49–60%. The yield of H₂ generated for the four biomasses in the two types of biodigesters has been estimated, obtaining values between 16 and 39 mL g VS^?1.First-order, Modified Gompertz, and Cone models have been applied to evaluate the kinetic parameters on the methane produced in the batch and pilot plant tests, obtaining an excellent fit. ADM1 model with 19 biological processes (disintegration of biomass composite, enzymatic hydrolysis, and digestion of soluble materials mediated by organisms), acid-base equilibria, kinetic study, and liquid-gas transference has been used to fit the cumulative methane volume.     

A novel method for increasing biohydrogen production from food waste using electrodialysis

The impact of continuous removal of volatile fatty acids on fermentative hydrogen production from food waste (FW) in a Continuously Stirred Tank Reactor (CSTR) was evaluated. Two experimental phases were conducted, a control phase and one in which volatile fatty acids were removed continuously from the reactor for the first time by electrodialysis (ED). Hydrogen yields were 64.7 cm³ H₂/g VS and 227.3 cm³ H₂/g VS for control phase, and ED phase respectively. Continuous removal of volatile fatty acids during fermentation not only increased H₂ yields but increased the production of volatile fatty acids (a valuable chemical feedstock) from 0.14 g/g VS to 0.34 g/g VS_.     

One-stage H2 and CH4 and two-stage H2 + CH4 production from grass silage and from solid and liquid fractions of NaOH pre-treated grass silage

In the present study, mesophilic CH₄ production from grass silage in a one-stage process was compared with the combined thermophilic H₂ and mesophilic CH₄ production in a two-stage process. In addition, solid and liquid fractions separated from NaOH pre-treated grass silage were also used as substrates. Results showed that higher CH₄ yield was obtained from grass silage in a two-stage process (467 ml g⁻¹ volatile solids (VS)_original) compared with a one-stage process (431 ml g⁻¹ VS_original). Similarly, CH₄ yield from solid fraction increased from 252 to 413 ml g⁻¹ VS_original whereas CH₄ yield from liquid fraction decreased from 82 to 60 ml g⁻¹ VS_original in a two-stage compared to a one-stage process. NaOH pre-treatment increased combined H₂ yield by 15% (from 5.54 to 6.46 ml g⁻¹ VS_original). In contrast, NaOH pre-treatment decreased the combined CH₄ yield by 23%. Compared to the energy value of CH₄ yield obtained, the energy value of H₂ yield remained low. According to this study, highest CH₄ yield (495 ml g⁻¹ VS_original) could be obtained, if grass silage was first pre-treated with NaOH, and the separated solid fraction was digested in a two-stage (thermophilic H₂ and mesophilic CH₄) process while the liquid fraction could be treated directly in a one-stage CH₄ process.     

Hybrid functional materials forming the metal structure with optimal imperfection for storage of hydrogen in hydride form

The paper investigates the samples of Ni–B and Ni–In composites in the form of films synthesized by an electrochemical method having a 3high degree of defectiveness and shows the electrochemical processes for their preparation, in which the experimental data of the influence doping additions of boron and indium on the hydrogen permeability of synthesized electrochemical complexes based on nickel Ni_x-B_y-H_zand Ni_x-In_y-H_z are given. It is shown that by creating traps (structural, impurity) for hydrogen by introducing additional elements into the metal structure or by changing the structure (intermetallides), also by other methods, the hydrogen solubility of the metal can be changed to a greater or lesser extent depending on the technical requirements. Ni–B samples were obtained in the sulfamate nickel electrolyte using boron compounds of the class of higher polyhedral borates Na₂B₁₀H₁₀. The use of nanoforming boron additives provides measured hydrogen content in Ni_x-B_y-H_{z samples} about of 600 cm³/100 g. Nickel-indium composites were obtained by electrolytic deposition on copper substrates (0,05 mm thick), with electrolyte composition: NiSO₄ × 7H₂O = 140 g/L; Na₂SO₄ × 10H₂O = 20 g/L; and In₂(SO₄)₃ which concentration varied from 1 g/L to 12 g/L. Variations in the amount of In₂(SO₄)₃ ensured the production of Ni–In composites with different component ratios. The obtained data made it possible to select the optimal mode of deposition of Ni–In plating in the In₂(SO₄)₃ electrolyte. In order to determine the effect of each of the electrodepositing parameters on the properties of the samples, only one parameter varies: either the cathode current density or the concentration of indium in the electrolyte. The samples of different concentrations of In are synthesized and studied. Their phase composition was determined. X-Ray phase analysis reveals in the Ni–In composites synthesized from electrolytes with In₂(SO₄)₃ concentration of more than 2 g/L, a phase corresponding to the intermetallide η-In₂₇Ni_10.For the first time in the practice of studying sorption, electrochemical composites Ni_x-In_y, including subsequent thermal desorption of hydrogen, are studied by implanting deuterium into the samples. It is demonstrated spectra of deuterium thermal desorption from Ni₇₀In₃₀D_x composites, which allowed determining the temperature ranges of desorption of ion-implanted deuterium depending on the dose of implantation. It is confirmed that a Ni₇₀In₃₀ composite capable of retaining doped deuterium (hydrogen) has been obtained. It is shown that it is permissible to obtain samples of a Ni_x-In_y-D_z composite with a deuterium content of up to 2 at. D/at. Met., that corresponds to 5,3 wt% (for composites of this composition).     

The effect of Ni,Fe and Mg concentration on photo-hydrogen production by Rhodopseudomonas faecalis RLD-53

In the present study, the effect of Ni²⁺ (0–10 μmol/l), Fe²⁺ (0–200 μmol/l) and Mg²⁺ (0–15 mmol/l) concentration on photo-hydrogen production from acetate was investigated by batch culture. Results showed that under a proper concentration range, Ni²⁺ was able to enhance the hydrogen production rate and the hydrogen yield; Fe²⁺ was able to increase the hydrogen yield, and hydrogen production rate was enhanced only when the culturing time was 24–72 h. Ni²⁺ and Fe²⁺ at a higher concentration inhibited cell growth. When Ni²⁺ and Fe²⁺ concentrations were 4 μmol/l and 80 μmol/l, respectively, maximal hydrogen yield of 2.87 and 2.78 mol H₂/mol acetate was obtained when batch culturing at 35 °C with initial pH 7.0. Mg²⁺ did not significantly affect hydrogen production and hydrogen yield which maintained at about 2.45 mol H₂/mol acetate, but it was favorable to cell growth.     

Carbon dioxide methanation over Ni catalysts prepared by reduction of NixMg3‒xAl hydrotalcite-like compounds: Influence of Ni:Mg molar ratio

A series of supported Ni catalysts have been prepared from Ni_xMg_3?xAl hydrotalcite-like compounds (HTlcs) and the influence of Ni:Mg molar ratio on the structural property and catalytic activity for CO₂ methanation is investigated. The catalysts were characterized by N₂ physical adsorption, X-ray powder diffraction (XRD), temperature-programmed reduction (H₂-TPR), temperature-programmed desorption (CO₂-TPD), H₂ chemisorption, scanning electronic microscopy (SEM), scanning transmission electronic microscopy (STEM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). By reducing HTlcs at 800 °C, well dispersed Ni particles with average size of 5–10 nm are formed. The Ni crystal size decreases with the decrease of Ni:Mg ratio, attributable to the strong interaction between nickel and magnesium oxides. Among the catalysts, Ni₂Mg₁Al-HT shows the highest activity, giving ～93% CO₂ conversion and >99% CH₄ selectivity at 275 °C and SV = 5000 mL g^?1 h^?1. Meanwhile, this catalyst exhibits good stability without obvious sintering and coking. The high activity is related to the large amount of surface Ni⁰ species and medium basic sites. From CO₂-TPD and DRIFTS, it is inferred that CO₂ adsorbs on the medium basic sites, i.e., Ni–Mg(Al)O interface, forming monodentate carbonate. In situ DRIFTS reveals that monodentate carbonate, monodentate formate, and adsorbed CO are the main intermediate species, suggesting that the reaction may proceed via the formate formation route.     

Preparation and characterization of active Ni/MgO in oxidative steam reforming of n-C4H10

Oxidative steam reforming of n-C₄H₁₀ over MgO-supported Ni catalysts is described. The Ni/MgO catalysts were prepared by the impregnation method from aqueous Ni(NO₃)₂ precursor solutions at two pH values. Ni/MgO prepared at pH 7 exhibited considerably higher activity than Ni/MgO prepared from a conventional acidic aqueous precursor solution (pH 3.5). The H₂ formation rate for the modified Ni/MgO was up to 2.3 times that for conventional Ni/MgO under a high space velocity of 1660 L(h g)⁻¹. Furthermore, after reduction at high temperature (1273 K), the modified Ni/MgO showed a higher H₂ formation rate than did Rh/MgO. The superior performance of the modified Ni/MgO was ascribed to stronger resistance to oxidation of Ni⁰ due to the formation of relatively large Ni⁰ particles.     

Nanocrystalline cobalt–nickel–boron (metal boride) catalysts for efficient hydrogen production from the hydrolysis of sodium borohydride

Innovative metal boride nanocatalysts containing crystalline Co–Ni based binary/ternary boride phases were synthesized and used in the hydrolysis of NaBH₄. All the as-prepared catalysts were in high-purity with average particle sizes ranging between ~51 and 94 nm and consisting of different crystalline phases (e.g. CoB, Co₂B, Co₅B₁₆, NiB, Ni₄B₃, Ni₂Co_0·67B_0.33). The synergetic effect of the different binary/ternary boride phases in the composite catalysts had a positive role on the catalytic performances thus, while the binary boride containing phases of unstable cobalt borides or single Ni₄B₃ were not showing any catalytic activity. The Co–Ni–B based catalyst containing crystalline phases of CoB–Ni₄B₃ exhibited the highest H₂ production rate (500.0 mL H₂ min^?1 g_cat^?1), with an apparent activation energy of 32.7 kJ/mol. The recyclability evaluations showed that the catalyst provides stability even after the 5th cycle. The results suggested that the composite structures demonstrate favorable catalytic properties compared to those of their single components and they can be used as alternative and stable catalysts for efficient hydrogen production from sodium borohydride.     

Proton reduction by a Ni(II) catalyst and foot-of-the wave analysis for H2 evolution

A nickel based molecular catalyst [Ni(QCl-tpy)₂]Cl₂·7H₂O (where QCl-tpy = 2-choloro-3-(2,6-di (pyridin-2yl)pyridine-4-yl) quinoline) has been synthesized, characterized by single crystal XRD and other spectroscopic techniques. The complex [Ni^II(QCl-tpy)₂]²⁺ has also been employed for the electrocatalytic proton reduction in DMF/H₂O (95:5 v/v) using trifluoro acetic acid (TFA) as the proton source. It exhibits a reasonably efficient catalytic ability towards proton reduction under organic media. Compared to the parent [Ni^II(tpy)₂]²⁺ during the electro-catalysis, the complex [Ni^II(QCl-tpy)₂]²⁺ behaves as a better catalyst in terms of higher catalytic current and 180 mV of lower overpotential as well. It is expected due to the presence of 2-chloroqinoline moiety in the terpyridine framework. The rate of H₂ evolution was analysed with the use of Foot-of-the Wave Analysis (FOWA) method. The complex shows a TOF of 3.68 s⁻¹ as obtained from Foot-of-the Wave Analysis (FOWA) at the scan rate 100 mVs⁻¹ for 1.0 mM [Ni^II(QCl-tpy)₂]²⁺ complex. The acid base equilibria reveals the dechelation followed by protonation at one of the coordinated pyridine rings of the QCl-tpy ligand. There could be a pendant base effect towards hydrogen evolution due to dechelated pyridine ring of the coordinated QCl-tpy ligand, which acts as a proton relay. Based on the spectroscopic evidence and electrochemical studies a plausible mechanism for the reduction of proton to H₂ has been proposed.