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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

Here, we present a protocol to synthesize Co nanoparticles supported on carbon nanotubes with Co- and N- dopants for hydrogen productions.

Abstract

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented herein. The novel strategy is based on a facile one-pot pyrolysis treatment of cobalt (II) acetylacetonate and nitrogen-rich organic precursors under Ar atmosphere at 800 °C, resulting in the formation of Co- and N- co-doped carbon nanotube with earthworm-like morphology. The obtained catalyst was found to have a high density of defect sites, as confirmed by Raman spectroscopy. Here, cobalt (II) nanoparticles were stabilized on the atomically dispersed cobalt- and nitrogen-doped carbon nanotubes. The catalyst was confirmed to be effective in the catalytic hydrolysis of ammonia borane, in which the turnover frequency was 5.87 mol H2·molCo-1·min-1, and the specific hydrogen generation rate was determined to be 2447 mL H2·gCo-1·min-1. A synergistic function between the Co nanoparticle and the doped carbon nanotubes was proposed for the first time in the catalytic hydrolysis of ammonia borane reaction under a mild condition. The resulting hydrogen production with its high energy density and minimal refueling time could be suitable for future development as energy sources for mobile and stationary applications such as road trucks and forklifts in transport and logistics.

Introduction

Developing low-cost and highly efficient catalysts for renewable energy production remains one of the most critical and challenging problems to relieve the energy crisis. However, it is far from practical applications due to several concerns, such as large-scale production methods with reliable performance, high production cost, and long-standing stability to extend the service life of catalysts. Industry sectors, like transport and logistics, require energy production for vehicles and equipment with long operation hours, high powered energy supply, and minimal refueling time in achieving efficient operations1,2,3. Therefore, effective strategies have been extensively exploited to address the above technical challenges. For example, by regulating the electronic structure of the metal active sites and catalyst supports, designing the specific architecture of the metal nano-catalysts, fine-tuning metal compositions, functional group modification of anchored support, and varying the morphology to increase the number of intrinsic active sites. In the past few decades, nanoparticles (NPs) have dominated the fields of various heterogeneous catalysis, and the catalytic activities can be effectively tuned by varying the size of the NPs. Only until in recent years, highly dispersed single-atom catalysts (SACs) emerged to have excellent properties towards many catalytic reactions due to their unique electronic structure and coordination environment. Particularly, SACs have already demonstrated superior performances in energy conversion such as electrochemical reactions (HER, ORR, OER) and electrochemical energy systems (e.g., supercapacitors, rechargeable batteries)4,5,6. While both NPs and SACs have their respective advantages and limitations in catalytic applications, there do exist reactions that require both NPs and SACs in order to boost catalytic reactivity. For example, Ru NPs supported on Ni- and N-co-doped carbon nanotube superstructure could facilitate the high catalytic wet air oxidation of acetic acid7. This synergistic effect was also demonstrated by Pd1+NPs/TiO2 catalysts for highly selective ketone and aldehydes hydrogenation at room temperatures8. In order to accelerate the field of synergistic NPs and SACs catalysis and explore more on their catalytic applications, a facile way of catalyst synthesis is highly desirable, and the introduction of high loadings of the atomically dispersed active site remains a challenge due to the high tendency of the aggregation of SACs9.

Several methods have been utilized to synthesize SACs for applications in the hydrogenation of nitroarenes10, oxygen reduction reaction and hydrogen evolution reaction11,12, lithium-oxygen batteries13.The most common strategy is the bottom-up approach, in which the metal precursors were absorbed, reduced, and immobilized on the defects of the corresponding support. Mononuclear metal complexes could also be first attached to the functional group of supports, followed by subsequent removal of the organic ligands, thus creating active metal sites for the catalytic process. Atomic layer deposition (ALD) is probably the most frequently used procedure for bottom-up fabrication by depositing a thin layer of film on the substrate with repeated exposure of reactants. Although the catalyst size could be precisely controlled and the reactivity could be greatly improved14, the purity of the substrate was rather demanding, and the metal loading was relatively low, thus resulting in high production costs for practical applications. Various methods such as direct impregnation, co-precipitation, and deposition-precipitation, have been employed to immobilize metal nanoparticles onto the support surfaces, such as metal oxide and nitride, through surface charging effects. However, increasing metal loading usually leads to significant agglomeration and cluster formation of the metal atoms or nanoparticles. Therefore, usually, a very diluted metal solution is required, thus leading to low SACs loadings of the catalysts15. Amine ligands such as phenanthroline have been employed to undergo pyrolysis with metal precursors to prepare atomically dispersed metal catalysts with highly active Co-Nx active sites for the selective dehydrogenation of formic acid. However, the metal loading was relatively low (2-3 wt%) due to the limited number of available N atoms in the amine precursors16.

In the past few decades, hydrogen has been regarded as a potential alternative to replace fossil fuels or hydrocarbons, such as coal, natural gas, and gasoline, due to the advantage of zero-emission of the former. Until now, about 94% of commercial hydrogen is still produced from the reforming process of fossil fuels, in which the process releases a great deal of greenhouse gas17. Therefore, hydrogen production from renewable resources such as water electrolysis is a way to solve the problem of depleted fossil resources and severe carbon emissions. However, the low hydrogen production efficiency has hindered their wider applications. Thus, to overcome this kinetic energy barrier for water-splitting, numerous efficient electrocatalysts have been discovered in the past decade18. Another issue is the storage problem due to the gaseous and explosive nature of hydrogen gas at ambient conditions. Physical storage methods such as compression will require the hydrogen to be compressed up to 700-800 bar, and cryogenic storage by liquefaction will require low temperature at -253 °C19. Although commercialized hydrogen fuel cell-powered vehicles have been successfully demonstrated, the storage problem is yet to be solved if the technology is to be used in wider applications, such as miniature devices and mini-fuel cells. Thus, storage methods of using chemical H materials have been one of the hot focuses in hydrogen energy research. Some examples of chemical H storage materials are ammonia borane (AB)20, formic acid (FA)21, ammonia gas22, sodium alanate23, and magnesium hydride24. Among these, AB has a low molecular weight (30.7 g·mol-1), high gravimetric and volumetric densities (196 gH2·kg-1 and 146 gH2·L-1, respectively). Besides, it is an air and moisture stable compound, non-toxic, and highly soluble in water. Metal nanoparticles on various supported materials have been widely used to release the three equivalents of hydrogen from AB, such as platinum- (Pt-), palladium- (Pd-), ruthenium- (Ru-), cobalt- (Co-), and nickel- (Ni-) based catalysts. Co-based heterogeneous catalysts supported on carbon materials are especially attracting much attention due to their low cost, high abundance, and ease of recovery. Several synthetic strategies have been reported, such as the Co NPs supported on branched polyethylenimine-decorated graphene oxide25. The 3D structure with a large surface area ensures the stabilization of Co NPs maintaining at the 2-3 nm size range and prevented the aggregation of NPs. Another strategy is to use N-doped carbon materials to support Co NPs with small sizes. Using Co(salen)26 and Co-MOF27 (metal organic framework) as the precursors, Co NPs of 9.0 nm and 3.5 nm supported on N-doped porous carbon materials have been prepared respectively. The stability towards AB hydrolysis are high and the reactivity can maintain over 95% of the initial activity after 10 reaction runs. Recently, catalysts with hollow micro/nanostructures have been exploited for AB hydrolysis. These materials are conventionally prepared by hydrothermal methods and have been widely used for lithium-ion batteries, supercapacitors, chemical sensors, and heterogeneous catalysis research. Thus, the copper-cobalt synergy towards AB hydrolysis has been demonstrated by the hollow CuMoO4-CoMoO428, which gives a high TOF of 104.7 min-1. Other highly structural-dependent examples include the core-shell CuO-NiO/Co3O429, the CoxCu1−xCo2O4@CoyCu1−yCo2O4 yolk-shell type30, and the Ni0.4Cu0.6Co2O4 nanoarrays31 were also found to be active towards AB hydrolysis. Another type of emerging materials known as heterostructured catalysts, such as MXenes and layered double hydroxides (LDHs), are increasingly being exploited for electrocatalytic and photocatalytic reaction32,33,34,35. These materials such as the NiFe-layered double hydroxide36,37 and the CoB-N materials having N-doped carbon-cobalt boride heterointerfaces38 are especially active for oxygen evolution and reduction reaction. In principle, they could be further exploited for hydrogen evolution reactions from hydrogen storage materials such as ammonia borane39. Maximizing the interaction between the catalysts and substrates is also another strategy for AB hydrolysis. Chiang et al. have utilized the surface oxide group of graphene oxide to form an initiated complex species with AB40, thus Ni0.8Pt0.2/GO and rGO demonstrated excellent reactivity towards AB hydrolysis. The use of α-MoC as support for Co and Ni bimetallic catalysts assisted the activation of water molecules and achieved high TOF towards AB hydrolysis, which is four times higher than the commercial Pt/C catalyst41.

Taking advantage of high N contents of the dicyandiamide and related C3N4 materials, a protocol for achieving a facile synthesis of cobalt NPs supported on highly dispersed Co- and N-doped carbon nanotubes is presented herein. The gradual in-situ formation of Co NPs from the formed atomically dispersed Co during the pyrolysis of C3N4 materials ensure that 1) Co NPs and Co dopants are highly dispersed; 2) Co NPs can be strongly anchored on the doped carbon supports and 3) Co NPs size can be carefully controlled by the temperature and time of the pyrolysis. The as-prepared Co/Co-N-CNT, as a result of the strongly anchored Co NPs and the ability of the Co dopants to lower the adsorption energy of water molecules, was found to have superior stability towards the hydrolysis of AB for hydrogen production. The details of the synthetic protocol of the catalysts and the measurement of the hydrogen production will be the focal point of this report.

Protocol

CAUTION: Readers are advised to carefully check the properties and toxicities of the chemicals described in this paper for the proper chemical handling from the relevant material safety data sheets (MSDS). Some of the chemicals used are detrimental to health, and special cares are to be taken. The impact of nanomaterials on human health is unknown and could pose safety and health risks. Inhalation and contact through the skin with these materials should be avoided. Safety precautions shall also be exercised, such as releasing the waste gas during the catalyst synthesis to the fume hood and catalyst performance evaluation with proper venting of the hydrogen gas. Personal protective equipment is advised to be worn at all times. Hydrogen is a potentially explosive gas with a very broad flammability range from 4%-74% in air. Care shall be taken to allow the hydrogen gas to vent properly to the atmosphere.

1. Synthesis of melem-C 3N4 materials

  1. Weigh out 280 g of dicyandiamide (density = 1.4 g·cm-3) into an 800 mL beaker.
  2. Place the beaker with the above solid into a muffle furnace and slowly raise the temperature from room temperature to 350 °C with a ramp of 5 °C·min-1.
  3. Keep the temperature at 350 °C for 2 h, cool down the furnace by natural cooling.
  4. Grind the obtained white solids into fine powder as the C3N4 materials in the melem form (DCD-350).
    ​NOTE: The yield is 175 g.

2. Annealing the melem-C 3N4 and Co(acac)2 mixtures at different temperature

  1. Mix 10.0 g of melem-C3N4 with 0.218 g of Co(acac)2 [Co : melem-C3N4 = 1:200 (weight ratio)]. Grind and mix the two solids until the homogeneous color is observed.
  2. Add 6 mL of citric acid solution (water: ethanol = 1:1, citric acid = 10 g·L-1) to the homogeneous mixture and further grind the materials.
  3. Dry the materials in an oven at 60 °C for 6 h.
  4. Place the materials into a square-shaped crucible and then put it into a tubular furnace.
  5. Heat the materials at a heating rate of 2.6 °C·min-1 from room temperature to 800 °C and keep for 2 h under an Ar flow of 100 mL·min-1.
  6. Slowly cool down the furnace by natural cooling.
  7. Weight out the catalyst samples. Here, the yield was 0.65 g.

3. Measuring hydrogen release from ammonia borane hydrolysis

  1. Set up the water-filled inverted cylinder system (Supplementary Figure 1).
  2. Set up the 0.1 M H2SO4 washing solution.
  3. Connect the Schlenk flask with the washing solution and the water-filled inverted cylinder.
  4. Set the water bath temperature to 40 °C. 
  5. Place 0.04 g of the catalyst into the Schlenk flask.
  6. Prepare a solution of ammonia borane in water, with 0.04 g of ammonia borane in 0.948 mL of water (concentration = 0.04 g·mL-1).
  7. Inject 1 mL of the NH3BH3 solution (40 mg·mL-1) to the reactor to initiate the hydrolysis reaction.
  8. Monitor the drop in the water level as the reaction proceeds. Carefully record the production volume at designated times, e.g., each 5 s intervals.
  9. Plot the graph of volume of H2 production vs. time in minutes.

4. Kinetic studies

  1. Determination of the activation energy
    1. Set the water bath temperature at 40 °C.
    2. Place 0.04 g of the catalyst and 10 mL of water into the Schlenk flask and immerse into the water bath. Sonicate the solution at 40 kHz in an ultrasonic bath for 6 min.
    3. Inject 1 mL of the NH3BH3 solution (40 mg·mL-1) to the reactor to initiate the hydrolysis reaction.
    4. Record the time for completion of the hydrogen release.
    5. Repeat steps 4.1.1-4.1.4 setting the water bath temperature at 35 °C.
    6. Repeat the above experiment at 30 °C and 25 °C, respectively.
    7. Plot the specific rate constant versus time on a graph using the following equation. A plot of ln k and 1/T should yield a straight line.
      figure-protocol-4558
      where ko denotes the specific rate constant (mol H2·gCo-1·min-1), R is the ideal gas constant (8.314 kJ·mol-1), T represents the reaction temperature (K), and A is the pre-exponential factor (mol H2 gCo-1·min-1).
  2. Determination of the turnover frequency and specific hydrogen generation rate
    1. Calculate the turnover frequency according to the following equation:
      figure-protocol-5170
      where nH2 is the moles of hydrogen produced, Δt is the time required for complete hydrogen release and nM is the molar amount of metal in the catalyst.
    2. Calculate the specific hydrogen generation rate according to the following equation42,43:
      figure-protocol-5594
      Where ΔVH2 is the volume of hydrogen produced, t is the time required for the initiating and stabilizing stages (e.g., the time where 70 mL of hydrogen generated for 40 mg ammonia borane, the time where 140 mL of hydrogen generated for 80 mg ammonia borane) and ω​M is the mass of metal in the catalyst.
  3. Determination of the relationship between [ammonia borane] and reaction rate
    1. Set the water bath temperature at 40 °C.
    2. Place 40 mg of the catalyst and 10 mL of water into the Schlenk flask and immerse into the water bath. Sonicate the solution at 40 kHz in an ultrasonic bath for 6 min.
    3. Inject 1 mL of the NH3BH3 solution (40 mg·mL-1) to the reactor to initiate the hydrolysis reaction.
    4. Record the time for completion of the hydrogen release.
    5. Repeat step 4.3.3 injecting 2 mL of the NH3BH3 solution (i.e., 80 mg per 2 mL) to the reactor to initiate the hydrolysis reaction.
    6. Repeat steps 4.3.1-4.3.4 with 0.5 mL and 0.25 mL of the NH3BH3 solution (40 mg/mL) respectively to record the time for completion of the hydrogen release.
    7. Calculate the reaction rate according to the following equation44:
      figure-protocol-7029
      where ΔmlH2 is the volume of hydrogen produced, Δt is the time required for 70 mL of hydrogen release.
    8. Plot the ln rate vs. ln[ammonia borane] and determine the slope of the graph.
  4. Determination of the relationship between [catalysts] and production rate
    1. Set the water bath temperature at 40 °C.
    2. Place 40 mg of the catalyst and 10 mL of water into the Schlenk flask and immerse into the water bath. Sonicate the solution at 40 kHz in an ultrasonic bath for 6 min.
    3. Inject 1 mL of the NH3BH3 solution (40 mg·mL-1) to the reactor to initiate the hydrolysis reaction.
    4. Record the time for completion of the hydrogen release.
    5. Repeat steps 4.4.1-4.4.4 varying the amount of catalyst (20 mg, 40 mg, 60 mg, 80 mg) and inject 1 mL of the NH3BH3 solution (i.e., 40 mg·mL-1) into the reactor to initiate the hydrolysis reaction.
    6. Record the time for completion of the hydrogen release for using the above various catalyst amounts.
    7. Calculate the reaction rate according to the following equation44:
      figure-protocol-8338
      where ΔmlH2 is the volume of hydrogen produced, Δt is the time required for 70 mL of hydrogen release.
    8. Plot the ln rate vs. ln[catalyst] and determine the slope of the graph.

5. Cycling performance test

  1. Set the water bath temperature at 40 °C.
  2. Place 0.04 g of the catalyst and 10 mL of water into the Schlenk flask and immerse into the water bath. Sonicate the solution at 40 kHz in an ultrasonic bath for 6 min.
  3. Inject 1 mL of the NH3BH3 solution (40 mg·mL-1) to the reactor to initiate the hydrolysis reaction.
  4. Record the time for completion of the hydrogen release.
  5. Filter off the catalyst, washed with water (5 mL) three times, and then dry the catalyst in the oven (60 °C) for 3 h.
  6. Place the catalyst in 10 mL of water and sonicate the solution at 40 kHz in an ultrasonic bath.
  7. Repeat steps 5.1.3-5.1.5 for ten times.
  8. Plot the hydrogen production volume, TOF and specific generation rate vs. cycles, respectively.

6. Leaching experiment for metal NPs to obtain pure metal SAs CNT

  1. Set the oil bath temperature at 80 °C.
  2. Place 0.15 g of the catalyst and 50 mL of 0.5 M H2SO4 into the Schlenk flask and immerse into the oil bath.
  3. Stir the reaction for 2 h.
  4. Filter off the solid using a Buchner funnel and wash the solid with deionized water (3x in 10 mL each). Dilute the leachate further to 250 mL in a 250 mL volumetric flask.
  5. Collect the metal nanoparticles-leached solids (which contain only Co-doped CNT) and dry at 60 °C in an oven.

7. Metal content determination using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)

  1. Determination of total cobalt metal content
    1. Place approximately 0.02 g of as-prepared catalyst from section 2 into 50 mL of 2 M acid solution (HCl: HNO3 = 3:1 mole ratio)45,46 in a polytetrafluoroethylene-lined stainless-steel autoclave.
    2. Place the polytetrafluoroethylene-lined container into the stainless steel bomb and secure the cap.
    3. Place the bomb into an oven, set the temperature to 180 °C, and heat the bomb for 12 h.
    4. Remove the bomb and empty the contents. Filter the solid and dilute the solute in a 250 mL volumetric flask with 200 mL of deionized water.
      NOTE: The purpose of the dilution is to adjust the concentration of the ICP samples, which will fit into the metal standard concentration range, i.e., 0-40 ppm.
    5. Run the ICP-OES test of the solution and calculate the total amount of Co in wt%.
  2. Determination of the cobalt atoms content on the CNT
    1. Place approximately 0.02 g of as-prepared catalyst from step 6.5 into 50 mL of 2 M acid solution (HCl: HNO3 = 3:1 mole ratio)45,46 in a polytetrafluoroethylene-lined stainless-steel autoclave.
    2. Place the polytetrafluoroethylene-lined container into the stainless-steel bomb and secure the cap.
    3. Place the bomb into an oven, set the temperature to 180 °C, and heat the bomb for 12 h.
    4. Remove the bomb and empty the contents. Filter the solid and dilute the solute in a 250 mL volumetric flask with 200 mL of deionized water.
      NOTE: The purpose of the dilution is to adjust the concentration of the ICP samples, which will fit into the metal standard concentration range, i.e., 0-40 ppm.
    5. Run the ICP-OES test of the solution and calculate the amount of Co atom contents in wt%.
  3. Determination of the cobalt nanoparticles (NPs) content
    1. The difference between the metal content of 7.1.5 and 7.2.5 is the wt% of Co NPs.

Results

X-ray diffraction patterns (XRD) have been obtained to determine the crystallinity and size of the cobalt NPs. As shown in Figure 1, diffraction peaks corresponding to the (111), (200) and (220) planes (at 2θ of 44.2°, 51.5°, and 75.8° respectively) of the cubic phase of metallic cobalt were present in agreement with the JCPDS (Joint Committee for Powder Diffraction Standards) power diffraction file (card # 15-0806)47. The broad peak at 2θ of ...

Discussion

The pyrolysis method has become one of the powerful strategies in the synthesis of one-dimensional nanomaterial on various heteroatom-doped solid supports with controlled sizes of NPs. For example, the nanospace-confined pyrolysis strategy was reported by Guo et al.56. Briefly, the pre-treated MWCNTs, cobalt, and phosphorus precursors were pyrolyzed at 800 °C under N2 atmosphere, and the CoP NPs supported on N-CNT can be obtained. The presence of the micro-pores can act as the...

Disclosures

We have nothing to disclose.

Acknowledgements

This work was fully funded by Hong Kong University Grants Committee - Institutional Development Scheme (IDS) Collaborative Research Grant, grant number UGC/IDS(C)14/B(E)01/19, the Faculty Development Scheme (FDS), grant number UGC/FDS25/E08/20 and partially funded by the Institutional Development Scheme (IDS), grant number UGC/IDS(R)25/20.

.

Materials

NameCompanyCatalog NumberComments
DicyandiamideSigma AldrichD76609
Borane-ammonia complexAladdinB131882-100g
Citric acid, 99%Sigma AldrichC0759
Cobalt metal standard solution, traceable to SRM from NIST Co(NO3)2 in HNO3 0.5 mol/l 1000 mg/l Co CertipurSigma Aldrich1.19785
Cobalt(II) acetylacetonate, ≥ 99%Sigma Aldrich727970
Hydrochloric acid, ACS reagentSigma Aldrich320331-2.5L
ICP-OESICP-OES with dichroic spectral combiner (Agilent 5110)
Muffle furnaceHigh Performance Hybrid Muffle furnace, Chamber: (360 x 250 x 320) mm, Exterior: (610 x 545 x 500) mm, Power(3100W), Vulcan 3-1750)
Nitric acid, puriss. p.a., 65.0-67.0%Sigma Aldrich84378
Sulphuric acid, ACS reagent 95-98%Sigma Aldrich258105
Tubular furnaceOTF-1200X with tube size of 60 mm outer diameter (Hefei Kejing)
Ultrasonic bath10L Digital Single Frequency 40 kHz Ultrasonic Cleaner (Biobase)

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