Mol H2 Research Articles

Mg and Cu incorporated CoFe2O4 catalyst: characterization and methane cracking performance for hydrogen and nano-carbon production

The Cobalt ferrite (CoFe2O4) and cobalt ferrite incorporated with Cu (Cu–CoFe2O4) and Mg (Mg–CoFe2O4) have been synthesized by wet chemical route. In fixed-bed reactor, the CoFe2O4, Cu–CoFe2O4 and Mg–CoFe2O4 were used as catalysts for direct cracking of methane at temperature of 800 °C and 20 mL/min of feed gas flow rate for hydrogen and nano-carbon production. Different characterizations techniques namely XRD, FESEM, XPS, Raman spectroscopy, TGA, BET for fresh and spent catalysts have been executed. The X-ray powder diffraction and Raman spectra confirmed that the fresh catalysts possess a cubic spinel crystal structure. The FESEM images for spent catalysts displayed that filamentous carbons were not formed over catalysts surfaces except a few amount was observed over the spent CoFe2O4 catalyst. XPS results confirmed the purity of the synthesized catalysts and qualitatively evaluate the cation distribution between the tetrahedral and octahedral sites from Co 2p3/2 and Fe 2p3/2 spectra. BET surface area revealed no significant effects in surface area and pore size of CoFe2O4 catalyst by incorporation of Mg and Cu metals. Activity studies showed that incorporation Mg metal on CoFe2O4 improved methane conversion up to 40.03% and hydrogen formation rate of 79.90 mol H2 g−1 min−1 as compared to 31.61% and 66 mol H2 g−1 min−1 for CoFe2O4 catalyst. Whilst, Cu metal incorporation in CoFe2O4 catalyst led to decline the catalyst performance. Structure activity relationship was described in details.

Effects of methane processing strategy on fuel composition, electrical and thermal efficiency of solid oxide fuel cell

Natural gas is a cheap and abundant fuel for solid oxide fuel cell (SOFC), generally integrating the SOFC system with methane pre-treating system for improving the stability of SOFC. In this paper, the accurate effects of methane processing strategy on fuel composition, electrical efficiency and thermal efficiency of SOFC are investigated based on the thermodynamic equilibrium. Steam reforming of methane is an endothermic process and can produce 3 mol of H2 and 1 mol of CO from 1 mol of methane, and thus the electrical efficiency of SOFC is high at the same O/C ratio and equivalent fuel utilization, whereas the thermal efficiency is low. On the contrary, partial oxidation of methane is an exothermal process and only produces 2 mol of H2 and 1 mol of CO from 1 mol of methane, and thus the electrical efficiency of SOFC is low at the same O/C ratio and equivalent fuel utilization, whereas the thermal efficiency is high. When the O/C ratio is 1.5, the electrical efficiency of SOFC is 55.3% for steam reforming of methane, while 32.7% for partial oxidation of methane. High electrical efficiency of SOFC can be achieved and carbon deposition can be depressed by selecting suitable O/C ratio from methane pretreatment according to the accurate calculation and analysis of effects of different methane processing strategies on the electrical efficiency and thermal efficiency of SOFC.

A novel Nb-based hydrogen purification membrane without catalytic palladium overlayer

Recently, group 5 metals and related alloys are considered as promising hydrogen permeation membranes to replace commercial palladium and its alloys. However, almost all these alloy membranes need to be coated with a noble Pd catalytic layer, which is complicated and costly. Therefore, one of the important R&D aspects is to explore "non-Pd" membranes. To address this challenge, we innovatively used vanadium and its oxide layer to replace Pd in the "core" materials of these membranes, and then developed a new type of Nb-based hydrogen purification membrane without a catalytic Pd overlayer. The base-metal Nb15Ti55Fe30 layer, produced by the directional solidification method, not only exhibits excellent hydrogen permeation performance, but also has good hydrogen embrittlement resistance. After testing, the novel composite membrane showed a high hydrogen permeability of 6.07 × 10−8 mol H2 m−1 s−1 Pa−1/2 at 773 K, which is ca. 3.8 times that of pure Pd. More importantly, the price of this novel membrane is at least two orders of magnitude cheaper than that of a Pd membrane. This work opens up a new way for large-scale practical application of Nb-based membranes, and the developed "non-Pd" composite membrane has wide application prospects.

Semidry acid hydrolysis of cellulose sustained by autoclaving for production of reducing sugars for bacterial biohydrogen generation from various cellulose feedstock.

Cellulosic biowastes are one of the cheapest and most abundant renewable organic materials on earth that can be, subsequent to hydrolysis, utilized as an organic carbon source for several fermentation biotechnologies. This study was devoted to explore a semidry acid hydrolysis of cellulose for decreasing the cost and ionic strength of the hydrolysate. For semidry acid hydrolysis, cellulose was just wetted with HCl (0 to 7 M) and subjected to autoclaving. The optimum molar concentration of HCl and period of autoclaving for semidry acid hydrolysis of cellulose were 6 M and 50 min respectively. Subsequent to the semidry acid hydrolysis with a minimum volume of 6 M HCl sustained by autoclaving, the hydrolysate was diluted with distilled water and neutralized with NaOH (0.5 M). The reducing sugars produced from the semidry acid hydrolysis of cellulose was further used for dark fermentation biohydrogen production by Escherichia coli as a representative of most hydrogen producing eubacteria which cannot utilize non-hydrolyzed cellulose. An isolated E. coli TFYM was used where this bacterium was morphologically and biochemically characterized and further identified by phylogenetic 16S rRNA encoding gene sequence analysis. The reducing sugars produced by semidry acid hydrolysis could be efficiently utilized by E. coli producing 0.4 mol H2 mol−1 hexose with a maximum rate of hydrogen gas production of 23.3 ml H2 h−1 L−1 and an estimated hydrogen yield of 20.5 (L H2 kg−1 dry biomass). The cheap cellulosic biowastes of wheat bran, sawdust and sugarcane bagasse could be hydrolyzed by semidry acid hydrolysis where the estimated hydrogen yield per kg of its dry biomass were 36, 18 and 32 (L H2 kg−1 dry biomass) respectively indicating a good feasibility of hydrogen production from reducing sugars prepared by semidry acid hydrolysis of these cellulosic biowastes. Semidry acid hydrolysis could also be effectively used for hydrolyzing non-cellulosic polysaccharides of dry cyanobacterial biomass. The described semidry acid hydrolysis of cellulosic biowastes in this study might be applicable not only for bacterial biohydrogen production but also for various hydrolyzed cellulose-based fermentation biotechnologies.

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH2-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO2, and Ti4Fe2Ox suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH2 and hydrogenation of nano-Ti and Ti4Fe2Ox. The possible reduction of TiO2 during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO2 compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH2 by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH2 down to 74 and 67 kJ/mol for MgH2 modified with nano-TiO2 and Ti4Fe2O0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH2. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti4Fe2O0.3 nanocomposite, ΔHdes = 76 kJ/mol H2 and ΔSdes = 138 J/K·mol H2, correspond to the reported literature values for pure polycrystalline MgH2. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO2 and Ti4Fe2Ox additives. Cycling stability of Mg/Ti4Fe2Ox was substantially improved by introduction of 3 wt% graphite into the composite.

Carboxylates and alcohols production in an autotrophic hydrogen-based membrane biofilm reactor.

Microbiological conversion of CO2 into biofuels and/or organic industrial feedstock is an excellent carbon-cycling strategy. Here, autotrophic anaerobic bacteria in the membrane biofilm reactor (MBfR) transferred electrons from hydrogen gas (H2 ) to inorganic carbon (IC) and produced organic acids and alcohols. We systematically varied the H2 -delivery, the IC concentration, and the hydraulic retention time in the MBfR. The relative availability of H2 versus IC was the determining factor for enabling microbial chain elongation (MCE). When the H2 :IC mole ratio was high (>2.0 mol H2 /mol C), MCE was an important process, generating medium-chain carboxylates up to octanoate (C8, 9.1 ± 1.3 mM C and 28.1 ± 4.1 mmol C m-2 d-1 ). Conversely, products with two carbons were the only ones present when the H2 :IC ratio was low (<2.0 mol H2 /mol C), so that H2 was the limiting factor. The biofilm microbial community was enriched in phylotypes most similar to the well-known acetogen Acetobacterium for all conditions tested, but phylotypes closely related with families capable of MCE (e.g., Bacteroidales, Rhodocyclaceae, Alcaligenaceae, Thermoanaerobacteriales, and Erysipelotrichaceae) became important when the H2 :IC ratio was high. Thus, proper management of IC availability and H2 supply allowed control over community structure and function, reflected by the chain length of the carboxylates and alcohols produced in the MBfR.

Metal hydride mediated water splitting: Electrical energy saving and decoupled H2/O2 generation

Reversible hydrogen absorption/desorption, the very fundamental property of metal hydrides (MHs), can be utilized to innovate the water splitting process. Spontaneous hydrogen absorption by a MH electrode shifts the water reduction potential to 0.3 V above the reversible hydrogen electrode. Then, the absorbed hydrogen can be released to give H2 by thermal dehydrogenation at mild temperature below 70 °C by utilizing low grade heat. The MH mediated water splitting combines the electrochemical hydrogen absorption in Ni-MH batteries and the thermal dehydrogenation in gas phase hydrogen storage, which significantly reduces the electrical energy consumption for water splitting up to 25 kJ (mol H2)−1 compared to an ideal hydrogen evolution catalyst and also automatically leads to decoupled H2/O2 generation. The advantages and mechanisms of the MH mediated water splitting are demonstrated by MH electrodes with surface Pd coating.

Combinations of V2C and Ti3C2 MXenes for Boosting the Hydrogen Storage Performances of MgH2.

Two-dimensional vanadium carbide (V2C) and titanium carbide (Ti3C2) MXenes were first synthesized by exfoliating V2AlC or Ti3AlC2 and then introduced jointly into magnesium hydride (MgH2) to tailor the hydrogen desorption/absorption performances of MgH2. The as-prepared MgH2-V2C-Ti3C2 composites show much better hydrogen storage performances than pure MgH2. MgH2 with addition of 10 wt % of 2V2C/Ti3C2 initiates hydrogen desorption at around 180 °C; 5.1 wt % of hydrogen was desorbed within 60 min at 225 °C, while 5.8 wt % was desorbed within 2 min at 300 °C. Under 6 MPa H2, the dehydrided MgH2-2V2C/Ti3C2 can start to recover hydrogen at room temperature, and 5.1 wt % of H2 is obtained within 20 s at a constant temperature of 40 °C. The reversible capacity (6.3 wt %) does not decline for up to 10 cycles, which shows excellent cycling stability. The addition of 2V2C/Ti3C2 can remarkably lower the activation energy for the hydrogen desorption reaction of MgH2 by 37% and slightly reduce the hydrogen desorption reaction enthalpy by 2 kJ mol-1 H2. It was demonstrated that the combination of V2C and Ti3C2 promotes the hydrogen-releasing process of MgH2 compared with addition of only V2C or Ti3C2, while Ti3C2 impacts MgH2 more significantly than V2C in the hydrogen absorption process of MgH2 at ambient temperatures. A possible mechanism in the hydrogen release and uptake of the MgH2-V2C-Ti3C2 system was proposed as follows: hydrogen atoms or molecules may preferentially transfer through the MgH2/V2C/Ti3C2 triple-grain boundaries during the desorption process and through the Mg/Ti3C2 interfaces during the absorption process. Microstructure studies indicated that V2C and Ti3C2 mainly act as efficient catalysts for MgH2. This work provides an insight into the hydrogen storage behaviors and mechanisms of MgH2 boosted by a combination of two MXenes.

A nickel complex of 2,2-dicyanoethylene-1,1-dithiolate, a catalyst for electrochemical and photochemical driven hydrogen evolution

The reaction of NiCl2, K2(i-mnt) (i-mnt2−= 2,2-dicyanoethylene-1,1-dithiolate) and 1-benzyl-4-amino-pyridinium bromide [BzPyNH2]Br affords a nickel complex, [BzPyNH2]2[Ni(i-mnt)2], a molecular catalyst for electrochemical and photochemical driven hydrogen evolution. As an electrocatalyst, [BzPyNH2]2[Ni(i-mnt)2] can electrocatalyze hydrogen generation from a neutral buffer with a turnover frequency (TOF) of 556 (±3) mol of hydrogen per mole of catalyst per hour (mol H2/mol catalyst/h) at an overpotential (OP) of 837.6 mV. As a photocatalyst, combining with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor, [BzPyNH2]2[Ni(i-mnt)2] photocatalyzes hydrogen evolution in heterogeneous environments with a turnover number (TON) of 18420 (±5) mol H2 per mol of catalyst during 60 h irradiation. The highest apparent quantum yield (AQY) is ∼10% at 469 nm. Several chemical and physical methods are employed to understand catalytic procedures for H2 production.

Substitution of nickel in Mg2Ni and its hydride with elements from groups XIII and XIV: An ab initio study

We have performed ab initio calculations with equilibrium supercells of the Mg2Ni compound and its hydride Mg2NiH4 doped with elements X = Al, Ga, In, Si, Ge and Sn. Two concentrations of X in both structures have been set: (1) every 16th, and (2) every fourth Ni atom has been substituted by X. Total energy calculations yielded the Mg2NiH4 hydrogen absorption enthalpy ΔHabs according to the chemical reaction Mg2Ni + 2H2 → Mg2NiH4. Reduction of the hydrogen absorption enthalpy was reported for both concentrations of X. When doping the Mg2NiH4 hydride with X = In in a low concentration (1), the value of hydrogen desorption enthalpy decreases from 68.22 to 55.96 kJ(mol H2)−1. Doping with X = In in a high concentration (2) further decreases the hydrogen desorption enthalpy to 5.50 kJ(mol H2)−1. Further, the electronic structure of Mg2(Ni–In)H4 hydride with a low In concentration indicates weaker Ni–H bonds in comparison with the pristine Mg2NiH4. Attraction between H and In atoms induced enhanced bonding between Mg and H atoms compared to the pristine Mg2NiH4.

Impact of Grid Gas Requirements on Hydrogen Blending Levels

Abstract The aim of the article is to determine what amount of hydrogen in %mol can be transferred/stored in the Estonian, Latvian and Lithuanian grid gas networks, based on the limitations of chemical and physical requirements, technical requirements of the gas network, and quality requirements. The main characteristics for the analysis of mixtures of hydrogen and natural gas are the Wobbe Index, relative density, methane number, and calorific value. The calculation of the effects of hydrogen blending on the above main characteristics of a real grid gas is based on the principles described in ISO 6976:2016 and the distribution of the grid gas mole fraction components from the grid gas quality reports. The Wärtsila methane number calculator was used to illustrate the effects of hydrogen blending on the methane number of the grid gas. The calculation results show that the maximum hydrogen content in the grid gas (hydrogen and natural gas mix), depending on the grid gas quality parameters (methane number, gross heat of combustion, specific gravity, and the Wobbe Index), is in the range of 5–23 %mol H2. The minimum hydrogen content (5 %mol H2) is limited by specific gravity (&gt;0.55). The next limitation is at 12 %mol H2 and is related to the gross heat of combustion (&gt;9.69 kWh/m3). It is advisable to explore the readiness of gas grids and consumers in Estonia, Latvia and Lithuania before switching to higher hydrogen blend levels. If the applicability and safety of hydrogen blends above 5 %mol is approved, then it is necessary to analyse the possible reduction of the minimum requirements for the quality of the grid gas and evaluate the associated risks (primarily related to specific gravity).

Light-driven hydrogen production with CdSe quantum dots and a cobalt glutathione catalyst.

A photocatalytic hydrogen (H2) production system is reported using glutathione (GSH)-capped CdSe QDs with a cobalt precatalyst, yielding 130 000 mol H2 per mol cobalt over 48 hours. Analysis of the reaction mixtures after catalysis indicates that the active catalyst is a labile complex of cobalt and GSH formed in situ.

Continuous biohydrogen production by a degenerated strain of Clostridium acetobutylicum ATCC 824

Degenerated strains of Clostridium acetobutylicum lack the ability to produce solvents and to sporulate, allowing the continuous production of hydrogen and organic acids. A degenerated strain of Clostridium acetobutylicum was obtained through successive batch cultures. Its kinetic characterization showed a similar specific growth rate than the wild type (0.25 h−1), a higher butyric acid production of 6.8 g·L−1 and no solvents production. A steady state was reached in a continuous culture at a dilution rate of 0.1 h−1, with a constant hydrogen production of 507 mL·h−1, corresponding to a volumetric rate of 6.10 L·L−1 d−1, and a yield of 2.39 mol of H2 per mole of glucose which represents 60% of the theoretical maximum yield. These results suggest that the degeneration is an interesting alternative for hydrogen production with this strain, obtaining a high hydrogen production in a continuous culture with cells in a permanent acidogenic state.

Biomass-formic acid-hydrogen conversion process with improved sustainability and formic acid yield: Combination of citric acid and mechanocatalytic depolymerization

Sustainable and efficient hydrogen production from biomass resources have been highly desired. Here, we report a low-temperature hydrogen production process from lignocellulosic biomass passing through formic acid (the biomass-formic acid-hydrogen conversion process) with improved sustainability and formic acid yield. In the biomass oxidation/hydrolysis step, toxic sulfuric acid has been replaced by sustainable citric acid. In addition, mechanocatalytic depolymerization of biomass has been applied to the biomass pretreatment step, resulting in the remarkable improvement in formic acid yield. In the first step, the citric-acid-impregnated biomass was subjected to a mechanocatalytic depolymerization process for facilitating contact between the substrate and catalysts (MD-CIB). In the second step, the crude formic acid (CF) solution was obtained via combined hydrolysis-oxidation process of the MD-CIB in the presence of dimethyl sulfoxide (DMSO), and hydrogen peroxide (H2O2). The yield of formic acid were measured up to be 50.03%. In the third step, the lignin-free crude formic acid (LCF) solution was recovered by filtration, or distilled formic acid (DF) solution was obtained by distillation of the CF solution. In the final step, hydrogen was generated from LCF or DF solutions through dehydrogenation of formic acid over Pd catalysts supported on amine-functionalized mesoporous silica (KIE-6) with a sodium formate. The turnover frequency (TOF) at 10 min were measured up to be 279.24 mol H2 mol Pd-1h−1. The clean hydrogen, which is produced from biomass passing through formic acid, could be directly applied in fuel cell as an energy source. Our hydrogen production process is expected to be much more efficient, sustainable and economical process for hydrogen production from biomass. In addition, organic acids as a byproduct could be utilized as a platform chemical.

Microscopic structure, hydrogen permeability and hydrogen embrittlement resistance of Nb-Hf-Ni eutectic alloy

The microscopic structure, hydrogen embrittlement resistance and hydrogen permeability Φ of the eutectic alloy Nb-Hf-Ni system were studied in brief. The eutectic structure of {(Nb, Hf)+HfNi} is indexed to be the main phase of the Nb15.6Hf42.2Ni42.2 alloy, which exists as the fine lamellar microstructures with the chemical composition of Nb26Hf49Ni25. The fully eutectic Nb15.6Hf42.2Ni42.2 alloy is brittle at 573 K and 523 K because of the adverse impact of the HfmNin (1 ≤ m ≤ 8; 3 ≤ n ≤ 21) phases on hydrogen embrittlement resistance. The hydrogen permeability Φ of the eutectic Nb15.6Hf42.2Ni42.2 sample is 1.6 × 10−8 [mol H2 m−1 s−1 Pa−0.5] at 673 K, a little bigger than that of pure palladium metal.

Design of hydrogen separatinwg Nb-Ti-Fe membranes with high permeability and low cost

Nowadays, group 5B (Nb-, V-, and Ta-based) alloys are considered prospective candidates to replace commercial Pd-based hydrogen permeation membranes. In recent years, the focus of research efforts towards these metal membranes has been the development of robust/cheaper alloys and the exploration of related phase diagrams. To address this challenge, the phase diagram and solidification path of the Nb-Ti-Fe alloy system have been investigated within the CALPHAD (CALculation of PHAse Diagrams) approach by combining available experimental data. On this basis, Nb-Ti-Fe multi-phase hydrogen-permeable alloys have been designed and fabricated, and their microstructure, hydrogen permeability, and hydrogen embrittlement (HE) resistance were evaluated. The results obtained show four ternary invariant reactions and five phase regions in the liquidus projection, where the compositional region suitable for hydrogen permeation has been established successfully. Small variations in alloy composition (e.g. Nb or Ti/Fe ratios) can lead to large changes in hydrogen permeation properties. This is attributed to the TiFe phase in the eutectic structure transition from a continuous thin layer to discontinuous granules. In particular, Nb5Ti60Fe35 shows the highest permeability at 673 K, 3.82 × 10−8 mol H2 m−1 s−1 Pa−1/2, which, heretofore, is the highest value reported in this alloy system.

A cobalt complex of bis(methylthioether)pyridine, a new catalyst for hydrogen evolution

A new cobalt complex, [(btep)CoBr2], was produced by the reaction of CoBr2 and bis(methylthioether)pyridine (btep), and its structure has been determined by X-ray crystallography. [(btep)CoBr2] shows good activity for the electro- and photocatalytic reduction of water to H2. As an electrocatalyst, [(btep)CoBr2] can provide 591.9 mol of hydrogen per mole of catalyst per hour (mol H2/mol catalyst/h) from neutral water under an overpotential (OP) of 837.6 mV. As a co-catalyst in a photocatalytic system, together with CdS nanorods (CdS NRs) (0.14 mg mL−1) as a photosensitizer, and ascorbic acid (H2A) (0.12 M) as a sacrificial electron donor in an aqueous solution (pH 4.5), [(btep)CoBr2] can afford 9326.4 mol H2 per mole of catalyst over a 40 h irradiation with blue light (λmax = 469 nm). The highest apparent quantum yield (AQY) is ~ 25.5%. The catalytic mechanism for H2 production was investigated by several measurements and analysis.

Hydrogen Generation upon Nanocatalyzed Hydrolysis of Hydrogen-Rich Boron Derivatives: Recent Developments.

ConspectusProduction of hydrogen from nonfossil sources is essential toward the generation of sustainable energy. Hydrogen generation upon hydrolysis of stable hydrogen-rich materials has long been proposed as a possibility of hydrogen disposal on site, because transport of explosive hydrogen gas is dangerous. Hydrolysis of some boron derivatives could rapidly produce large amounts of hydrogen, but this requires the presence of very active catalysts. Indeed, late transition-metal nanocatalysts have recently been developed for the hydrolysis of a few hydrogen-rich precursors.Our research group has focused on the improvement and optimization of highly performing Earth-abundant transition-metal-based nanocatalysts, optimization of remarkable synergies between different metals in nanoalloys, supports including positive synergy with nanoparticles (NPs) for rapid hydrogen generation, comparison between various endo- or exoreceptors working as homogeneous and heterogeneous supports, mechanistic research, and comparison of the nanocatalyzed hydrolysis of several boron hydrides.First, hydrogen production upon hydrolysis of ammonia borane, AB (3 mol H2 per mol AB) was examined with heterogeneous endoreceptors. Thus, a highly performing Ni@ZIF-8 nanocatalyst was found to be superior over other Earth-abundant nanocatalysts and supports. With 85.7 molH2·molcat-1·min-1 at 25 °C, this Ni nanocatalyst surpassed the results of previous Earth-abundant nanocatalysts. The presence of NaOH accelerated the reaction, and a remarkable pH-dependent "on-off" control of the H2 production was established. Bimetallic nanoalloys Ni-Pt@ZIF-8 showed a dramatic volcano effect optimized with a nanoalloy containing 2/3 Ni and 1/3 Pt. The rate reached 600 molH2·molcat-1·min-1 and 2222 molH2·molPt-1·min-1 at 20 °C, which much overtook the performances of both related nanocatalysts Ni@ZIF-8 and Pt@ZIF-8. Next, hydrogen production was also researched via hydrolysis of sodium borohydride (4 mol H2 per mol NaBH4) using nanocatalysts in ZIF-8, and, among Earth-abundant nanocatalysts, Co@ZIF-8 showed the best performance, outperforming previous Co nanocatalysts. For exoreceptors, "click" dendrimers containing triazole ligands on their tripodal tethers were used as supports for homogeneous (semiheterogeneous) catalysis of both AB and NaBH4 hydrolysis. For both reactions, Co was found to be the best Earth-abundant metal, Pt the best noble metal, and Co1Pt1 the best nanoalloy, with synergistic effects. Based on kinetic measurements and kinetic isotope effects for all of these reactions, mechanisms are proposed and the hydrogen produced was further used in tandem reactions. Overall, dramatic triple synergies between these nanocatalyst components have allowed hydrogen release within a few seconds under ambient conditions. These nanocatalyst improvements and mechanistic findings should also inspire further nanocatalyst design in various areas of hydrogen production.

Space confined synthesis of highly dispersed bimetallic CoCu nanoparticles as effective catalysts for ammonia borane dehydrogenation and 4-nitrophenol reduction

Bimetallic CoCu nanoparticles (NPs) are successfully embedded within the cage-type mesopores of COOH functionalized mesoporous silica SBA-16 by double agent chemical reduction method. The presence of the COOH groups in SBA-16 control the size and dispersion of the CoCu NPs. CoCu@S16LC-20 is used as the catalyst for the hydrolysis of ammonia borane (AB) and reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The catalytic activity of the CoCu@S16LC-20 outperforms that of monometallic counterparts Co and Cu. In particular, Co40Cu60@ S16LC-20 can release H2 at a rate of 6570 mL min−1 g−1 and reveals superb catalytic activity with a turn over frequency (TOF) of 16.36 mol H2 min−1 molcatalyst−1. The superior catalytic activity of the bimetallic CoCu@S16LC-20 could be ascribed to the strong synergistic effects emerging from the modified electronic structure, smaller particle size, and 3-dimensional (3D) mesoporous silica support. The smaller particle size renders more active sites, and the mesoporous silica support provides easier paths for transfer of reactants to the catalyst. The catalyst magnetic nature permits straightforward recovery from the aqueous solution after reaction completion which enhances the reusability. The bimetallic Co40Cu60@S16LC-20 can degrade 4-NP to 4-AP at rapid rate of 3.85 × 10−2 s−1 in presence of reducing agent NaBH4.

Pd-doped Cu nanoparticles confined by ZIF-67@ZIF-8 for efficient dehydrogenation of ammonia borane

To exploit the low-cost and efficient catalyst for the hydrolytic dehydrogenation of ammonia borane, trace amount of palladium-doped transition metal (Cu, Ni, Co, Fe and Zn) nanoparticles have been incorporated into the interior of metal-organic frameworks named ZIF-8, ZIF-67, ZIF-67/ZIF-8 and core-shell ZIF-67@ZIF-8 by a double-solvent approach. The optimized catalyst of CuPd0.01@ZIF-67@ZIF-8 composite exhibited an excellent activity with a turnover frequency (TOF) value of 30.15 mol H2 (molmetal)−1 min−1 at 298 K and a relatively low activation energy of 38.78 kJ mol−1. It might attributed to both ultrafine size of metal nanoparticles (~3 nm) induced by the confinement of core-shell ZIF-67@ZIF-8 and the synergistic effect between Pd and Cu. Moreover, the detailed kinetic study has manifested that this catalytic reaction is first-order in regards to the catalyst amount while zero-order as for the concentration of ammonia borane. In addition, the durability and recyclability of CuPd0.01@ZIF-67@ZIF-8 have been demonstrated to be great.