Carbon nanotubes-promoted Co–B catalysts for rapid hydrogen generation via NaBH4 hydrolysis

In this work, multiwalled carbon nanotubes (MWCNTs) promoted Co–B catalysts for NaBH₄ hydrolysis have been designed and synthesized. The structural features of as-prepared catalysts have been investigated and discussed as a function of MWCNTs contents by X-ray diffraction, X-ray photoelectron spectra, N₂ adsorption/desorption isotherms, scanning electron microscope. The results show that the catalysts still maintain an amorphous structure with the addition of carbon nanotubes promoter. However, the appropriate amount of MWCNTs promoter in Co–B catalysts leads to large specific surface area, fine dispersion of active components, increased active sites and high electron density at active sites. Moreover, hydrogen spillover on the catalyst is promoted, which contributes to regeneration of active sites and accelerating catalytic cycle. Among all the experimental samples, it is found that the Co–B catalyst promoted by 10 wt% carbon nanotubes exhibits optimal catalytic activity with remarkably high hydrogen generation rate of 12.00 L min⁻¹·g_catalyst⁻¹ and relatively good stability.     

Partial phosphorization of porous Co–Ni–B for efficient hydrogen evolution electrocatalysis

Design of cost-effective and high-efficient electrocatalysts for hydrogen evolution reaction (HER) is of vital significance for the current renewable energy devices — fuel cells. Herein, we report a facile strategy to prepare partial phosphorization of Co–Ni–B material with porous structure via a water-bath boronizing and subsequent phosphorization process at moderate temperature. The optimal atomic proportion of Co to Ni is investigated via physical and electrochemical characterization. As a result, Co₉–Ni₁–B–P exhibits the best HER activity, which require an lower overpotential of ~192 mV to deliver a current density value of 10 mA cm⁻² and a smaller Tafel slope of 94 mV dec⁻¹ in alkaline media, relative to P-free Co–Ni–B catalysts, Co₉–Ni₁–B–P with other Co: Ni proportion and mono metallic borides The excellent electrocatalytic performance of Co₉–Ni₁–B–P is mainly ascribed to the three-dimensional (3D) porous structure and the coordinate functionalization between the borides and phosphides. This work provides a promising strategy for the exploration of quaternary composites as efficient and cost-effective electrocatalysts for HER.     

Polypyrrole supported Co–W–B nanoparticles as an efficient catalyst for improved hydrogen generation from hydrolysis of sodium borohydride

Effective and reusable catalysts with high performance are essentially necessary for NaBH₄ based on-demand hydrogen generators to the widespread use for energy conversion in fuel cell power systems. Herein, we report a facile synthesis of surfactant-directed polypyrrole-supported Co–W–B nanoparticles as a robust catalyst for efficient hydrolysis of NaBH₄ reaction. This non-noble metal catalyst provides much higher catalytic activity than a conventional cobalt boride catalyst. By incorporating tungsten to catalyst composition and tuning molar ratio of W/(Co + W), about a four-fold higher hydrogen generation rate was attained compared to bare Co–B. Among the all catalysts tested, Co–W–B/PPy with 7.5% W possessed the remarkable catalytic performance of 9.92 L min^?1 g^?1 and high stability over five cycles with the apparent activation energy of 49.18 kJ mol^?1.     

Electroless-deposited Co–P catalysts for hydrogen generation from alkaline NaBH4 solution

Cobalt–phosphorus (Co–P) catalysts, which were electroless deposited on Cu sheet, have been investigated for hydrogen generation from alkaline NaBH₄ solution. The microstructures of the as-prepared Co–P catalysts and their catalytic activities for hydrolysis of NaBH₄ are analyzed in relation to pH value, NaH₂PO₂ concentration, and the deposition time. Experimental results show that the Co–P catalyst formed in the bath solution with pH value of 12.5, NaH₂PO₂ concentration of 0.8 M, and the deposition time no more than 6 min presents the highest hydrogen generation rate of 1846 mL min⁻¹ g⁻¹. Furthermore, the as-prepared catalyst also shows good cycling capability and the corresponding activation energy is calculated to be 48.1 kJ mol⁻¹. The favorable catalytic performance of the electroless-deposited Co–P catalysts indicates their potential application for quick hydrogen generation from hydrolysis of NaBH₄ solution.     

Accurately measuring the hydrogen generation rate for hydrolysis of sodium borohydride on multiwalled carbon nanotubes/Co–B catalysts

Multiwalled carbon nanotubes supported cobalt–boron catalysts (Co–B/MWCNT) were developed via the chemical reduction of aqueous sodium borohydride with cobalt chloride for catalytic hydrolysis of alkaline NaBH₄ solution. The hydrogen generation (HG) rates were measured on an improved high-accuracy, low-cost and automatic HG rate measurement system based on the use of an electronic balance with high accuracy. The HG of Co–B/MWCNT catalyst was investigated as a function of heat treatment, solution temperature, Co–B loading and supporting materials. The catalyst was mesoporous structured and showed lower activation energy of 40.40 kJ mol⁻¹ for the hydrolysis of NaBH₄. The Co–B/MWCNT catalyst was not only highly active to achieve the average HG rate of 5.1 l min⁻¹ g⁻¹ compared to 3.1 l min⁻¹ g⁻¹ on Co–B/C catalyst under the same conditions but also reasonably stable for the continuous hydrolysis of NaBH₄ solution.     

Graphene modified Co–B catalysts for rapid hydrogen production from NaBH4 hydrolysis

Graphene oxide (GO) modified Co–B catalysts for NaBH₄ hydrolysis have been synthesized by the chemical reduction in this work. The structural features and catalytic performance of as-prepared samples have been investigated and discussed as a function of amounts of GO. According to structure characterization, the catalysts still retain the amorphous structure of Co–B alloy with the addition of GO, while GO exists as reduced GO (r-GO). The textural analysis and morphology observation indicate that the appropriate amount of GO in Co–B catalyst results in the obvious increase of specific surface area and uniform clustered morphology, which contributes to improve active surface area for catalytic reactions. The results of surface species characterization show that the electron density at active Co sites increases due to an electron transfer from B to Co facilitated by r-GO. It has been found that 50 mg GO modified Co–B catalyst exhibits especially high activity with a hydrogen generation rate of 14.34 L min⁻¹·g_catalyst⁻¹ and much lower activation energy of 26.2 kJ mol⁻¹ for hydrolysis reaction of NaBH₄. Meanwhile, the reusability evaluations show that the catalyst preserves high stability which can still maintain 81.5% of its initial activity after 5 catalytic cycles.     

Mesoporous Co–B nanocatalyst for efficient hydrogen production by hydrolysis of sodium borohydride

Two types of mesoporous Co–B nanocatalysts were prepared by the reduction of cobalt chloride with Sodium Borohydride (SBH) in the presence of cationic and non-ionic surfactant templates, namely n-cetyl-trimethyl-ammonium bromide (CTAB) and Pluronic (P123) respectively. Nitrogen adsorption–desorption isotherms revealed the presence of slit-like pores on the catalyst surface which provide high effective surface area. These surface enhanced catalysts were tested for hydrogen production by hydrolysis of sodium borohydride. The mesoporous Co–B catalysts showed much higher activity (4 times) in comparison to the non-porous Co–B, which can be attributed to the higher surface area of the mesoporous structures. Co–B/P123 catalyst showed the highest hydrogen generation rate owing to the presence of wide uniform pores which facilitated easier interaction of the reactants to release hydrogen. The lack of stability in the pore structure is observed at elevated temperatures for both the mesoporous Co–B catalyst.     

A MOF-templated approach for designing ruthenium–cesium catalysts for hydrogen generation from ammonia

Ammonia is regarded as a safe hydrogen energy carrier due to its high hydrogen content and narrow flammable range. Herein, we report a novel metal-organic framework (MOF) templated approach to synthesize highly active catalysts for the hydrogen production from ammonia. Calcination of ruthenium-impregnated UiO-66(Zr)-NH₂ leads to formation of small ruthenium nanoparticles (<3 nm) with a strong interaction with the resulting mesoporous zirconia support. These catalysts present a turnover frequency considerably higher than Ru/CNTs catalysts, defying the established believe of 3–5 nm as optimum Ru size for this reaction. Moreover, cesium promoter acts as an electronic modifier of Ru and also as a molecular spacer enhancing the stability under reaction conditions. Despite the exciting potential of this approach, the collapse of MOF framework structures during the calcination process limits the accessibility with an estimated ~16% of the ruthenium accessible, underestimating the actual turnover frequency (TOF) values.     

Sodium borohydride hydrolysis on highly efficient Co–B/Pd catalysts

Cobalt–Boron (Co–B) catalysts are prepared on the nickel foam substrate (NiFS) by in situ reduction of Co²⁺ ions in sodium borohydride (NaBH₄) solution for the catalytic generation of hydrogen from NaBH₄. The formation of Co–B catalysts on the substrate is much faster by using a dip-coating and extended drying (“dry-dip-coating method”) followed by chemical reduction as compared to that prepared by a conventional dip-coating method followed by chemical reduction. The dry treatment results in a significant reduction in the re-dissolution of the dip-coated Co–B catalysts during the following dipping processes. Co–B catalysts on Pd modified NiFS have also been prepared using dry-dip-coating method. The factors affecting the performance of the catalysts such as dipping time, calcination temperature, Co–B loadings, Pd formation time and operating temperature, are studied. The best catalytic activity and stability is obtained on Co–B on Pd modified NiFS.     

Optimization of catalyst preparation conditions for hydrogen generation in the presence of Co–B using taguchi method

Efficient hydrogen generation is a significant prerequisite of future hydrogen economy. Therefore, the development of efficient non-noble metal catalysts for hydrolysis reaction of sodium borohydride (NaBH₄) under mild conditions has received extensive interest. Since the transition metal boride based materials are inexpensive and easy to prepare, it is feasible to use these catalysts in the construction of practical hydrogen generators. In this work, temperature, pH, reducing agent concentration, and reduction rate were selected as independent process parameters and their effects on dependent parameter, such as hydrogen generation rate, were investigated using response surface methodology (RSM). According to the obtained results of the RSM prediction, maximum hydrogen generation rate (53.69 L. min^?1g_cat^-1) was obtained at temperature of 281.18 K, pH of 5.97, reducing agent concentration of 31.47 NaBH₄/water and reduction rate of 7.16 ml min^?1. Consequently, after validation studies it was observed that the RSM together with Taguchi methods are efficient experimental designs for parameter optimization.     

MgFe and Mg–Co–Fe mixed oxides derived from hydrotalcites: Highly efficient catalysts for COx free hydrogen production from NH3

Mg–Fe Layered Double Hydroxide (LDH) with M²⁺: M³⁺ 3:1 stoichiometric ratio was synthesized and employed as catalyst precursor for CO_x-free hydrogen production from ammonia. The resulting catalyst showed good catalytic activity. A series of Mg/Co–Fe layered double hydroxides were synthesized by replacing Mg²⁺ with Co²⁺ without disturbing M²⁺:M³⁺ ratio. The influence of nature and extent of Co(II) substitution on structure, morphology and surface properties were studied. A systematic study was carried out using these materials as catalyst precursors for ammonia decomposition. BET, XRD, TPR, XPS, CO₂-TPD and TEM techniques were used to characterize the synthesized catalysts. These Fe-based catalysts are highly active, highly stable and not promoting any stable surface nitridation during the ammonia decomposition reaction. Among all catalysts, the Mg₃Co₃Fe₂ catalyst showed the highest activity i.e. 100% conversion at 6,000 h⁻¹ and 60% at 50,000 h⁻¹ space velocities at 550 °C. The registered superior catalytic activity was result of the formed specific catalyst's properties like high surface area, high surface Co and Fe atomic concentration and suitable basicity. These Fe-based materials are, cost-effective, easily synthesize and highly stable, thus attractive for large-scale operation.     

Promoting effect of W doped in electrodeposited Co–P catalysts for hydrogen generation from alkaline NaBH4 solution

Amorphous Co–W–P catalysts were prepared on Cu substrates by electrodeposition, which have been investigated as the catalyst for hydrogen generation from alkaline NaBH₄ solution. The surface morphology and chemical composition of the as-prepared Co–W–P catalysts were analyzed in relation to the cathodic current density and the electrodeposition time. The hydrogen generation rate for the optimized Co–W–P catalyst is measured to be 5000 mL (min g-catalyst)⁻¹ at 30 °C. From hydrogen generation tests in solutions with the various concentrations of NaBH₄ and NaOH, there were optimum concentrations for both NaBH₄ and NaOH, above or below which the hydrogen generation rate decreased. Furthermore, the as-prepared catalyst also showed good cycling capability and the activation energy for hydrolysis of NaBH₄ by the Co–W–P catalyst was calculated to be 22.8 kJ/mol, which was lower than other reported Co-based catalysts.     

Novel Ni–Co–B hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride

Ni–Co–B hollow nanospheres were synthesized by the galvanic replacement reaction using a Co–B amorphous alloy and a NiCl₂ solution as the template and additional reagent, respectively. The Ni–Co–B hollow nanospheres that were synthesized in 60 min (Ni–Co–B-60) showed the best catalytic activity at 303 K, with a hydrogen production rate of 6400 mL_hydrogenmin^?1g_catalyst^?1 and activation energy of 33.1 kJ/mol for the NaBH₄ hydrolysis reaction. The high catalytic activity was attributed to the high surface area of the hollow structure and the electronic effect. The transfer of an electron from B to Co resulted in higher electron density at Co sites. It was also found that Ni was dispersed on the Co–B alloy surface as result of the galvanic replacement reaction. This, in turn, facilitated an efficient hydrolysis reaction to enhance the hydrogen production rate. The parameters that influenced the hydrolysis of NaBH₄ over Ni–Co–B hollow nanospheres (e.g., NaOH concentration, reaction temperature, and catalyst loading) were investigated. The reusability test results show that the catalyst is active, even after the fifth run. Thus, the Ni–Co–B hollow nanospheres are a practical material for the generation of hydrogen from chemical hydrides.     

Graphene supported bimetallic G–Co–Pt nanohybrid catalyst for enhanced and cost effective hydrogen generation

A highly active and stable bimetallic nano-hybrid catalyst Graphene–Cobalt–Platinum (G–Co–Pt) is proposed for the enhanced and cost effective generation of hydrogen from Sodium Borohydride. Three different nano-hybrid catalysts namely Graphene–Cobalt (G–Co), Graphene–Platinum (G–Pt) and Graphene–Cobalt–Platinum (G–Co–Pt) are synthesized, characterized using XRD, FTIR, SEM, HRTEM, EDAX and Cyclic voltammetry (CV) analysis and tested for hydrogen generation. The activity and stability of the catalysts are analyzed by estimating the turnover frequency (TOF), the electrochemically active surface area (ECSA), the percentage decay of current density over ten cycles of CV and the decay in the rate of hydrogen generation with the age of catalyst. Among the three catalysts G–Co–Pt exhibits the highest catalytic activity (TOF = 107 min⁻¹, ECSA = 75.32 m²/gm) and stability. The evaluated value of activation energy of the catalytic hydrolysis using G–Co–Pt is 16 ± 2 kJ mol⁻¹.     

Ammonia decomposition over SiO2-supported Ni–Co bimetallic catalyst for COx-free hydrogen generation

NH₃ decomposition over non-noble catalyst to generate CO_x-free H₂ has attracted great attention in recent years. In this work, fumed SiO₂-supported Ni, Co and Ni–Co bimetallic catalysts are synthesized by using a co-impregnation method and evaluated for NH₃ decomposition, which shows that the bimetallic catalysts exhibit better catalytic activity than the monometallic ones. This enhanced activity observed on bimetallic catalyst can be largely attributed to the more appropriate catalyst metal-N binding energy resulting from the synergistic effect between Ni and Co in the formed Ni–Co alloy. Among the synthesized catalysts, Ni₅Co₅/SiO₂ synthesized with the Ni/Co molar ratio of 5:5 achieves 76.8% NH₃ conversion under a GHSV of 30,000 mL h⁻¹ g⁻¹_cat at 550 °C and shows the best catalytic activity, which can be further improved by doping with K (78.1% NH₃ conversion at 30,000 mL h⁻¹ g⁻¹_cat), and the obtained Ni₅Co₅/SiO₂–K also shows excellent catalytic stability.     

Nickel versus cobalt catalysts for hydrogen production by ethanol steam reforming: Ni–Co–Zn–Al catalysts from hydrotalcite-like precursors

The urea hydrolysis method allowed to prepare well-crystallized Ni–Co–Zn–Al Layered Double Hydroxides to be used as precursors of mixed oxide catalysts for the Ethanol Steam Reforming (ESR) reaction. The calcination of the layered precursors gives rise to high surface area mixed oxides, being actually a mixture of a rock salt phase (NiO), a wurtzite phase (ZnO) and a spinel phase.     

Phase-separated Ce–Co–O catalysts for CO oxidation

As a major gas pollutant, the control of CO gas emission from chemical industry and vehicle exhausts has aroused global attention in the past years and it is extremely essential to develop efficient, low-cost and environmental friendly catalysts for CO conversion and removal. In this work, we report the facile synthesis of Ce–Co–O catalysts through a simple ultrasonic spray pyrolysis process and investigate their application for low-temperature CO oxidation. The Ce–Co–O catalysts comprising of separated CeO₂ and Co₃O₄ phases show superior CO oxidation capability below 473 K (200 °C) without the assistance of any other co-catalyst or noble metal. With the increase of Co₃O₄ concentration, the CO oxidation temperature of Ce–Co–O catalysts decreases quickly and reaches a complete conversion temperature of 410 K (137 °C) in the case of the optimized Co content. Microstructure analysis using high-resolution transmission electron microscope reveals that the tiny CeO₂ and Co₃O₄ phases assembled into porous particles are well crystallized and show high chemical purity. The porous feature of Ce–Co–O catalysts synthesized from feasible ultrasonic spray pyrolysis makes them more competitive and promising towards gaseous environmental pollution processing including CO oxidation, TWCs, SCR, etc.     

Agglomerated Co–Cr/SBA-15 catalysts for hydrogen production through acetic acid steam reforming

Hydrogen produced from renewable resources is becoming interesting as an alternative to conventional fossil fuels. Co-based catalysts have been reported for their active role in steam reforming of acetic acid as the main model compound of bio-oil aqueous fraction. In the present work, a series of Co–Cr/SBA-15 extrudates were prepared by varying the binder (bentonite) content and particle size in order to get catalyst particles suitable to be used in a steam reformer at industrial scale. Catalysts were characterized by N₂ physisorption, ICP-AES, TEM, SEM, XRD and H₂-TPR. The physicochemical characterization results showed that no remarkable changes occurs after the extruding process of the powdered sample, except for the particle size and mechanical strength. Acetic acid steam reforming tests were done at 600 °C and WHSV = 30.1 h⁻¹ varying the feed flow rate and the catalysts particle size in order to study the influence of internal and external diffusion limitations. Extruded particles with an effective diameter of 1.5 mm and 30 wt% of bentonite get similar conversion and hydrogen selectivity than powder sample. Besides, the agglomerated catalysts are also stable up to 12 h of TOS.     

Platinum–ceria–zirconia catalysts for hydrogen production in sulfur-iodine cycle

In this study, Pt/Ce_1−xZr_x catalysts with different Zr mole concentration (x = 0, 0.2, 0.5, 0.8, 1) have been tested to evaluate their effects on hydrogen iodide (HI) decomposition for hydrogen production in the sulfur-iodine (SI or IS) cycle at various temperatures. The Pt/Ce_1−xZr_x catalysts strongly enhanced the HI conversion to H₂ by comparison with blank test, especially the Pt/Ce_0.8Zr_0.2 catalyst. BET, XRD, TEM, EDS, TPR were performed for catalysts characterization. It was found that, through introducing ZrO₂ into Pt/CeO₂, a synergistic effect between Pt and CeO₂-ZrO₂ solid solution was different from Pt and CeO₂ yield, such as improvement of the thermal stability and increase of Pt-O-Ce reducibility. Among the three samples containing Zr, the one with 20 mol% displayed the best activity for hydrogen production.