Palladium Nanoparticle Multi-walled Carbon Nanotube Composite as

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Palladium Nanoparticle Multi-walled Carbon Nanotube Composite as Catalyst for Hydrogen Production by the Hydrolysis of Sodium Borohydride Clay Huff, Julia Madeline Long, Austin Heyman, and Tarek Abdel-Fattah ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00748 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Energy Materials

Palladium Nanoparticle Multi-walled Carbon Nanotube Composite as Catalyst for Hydrogen Production by the Hydrolysis of Sodium Borohydride Clay Huff1, Julia M. Long1, Austin Heyman1 and Tarek M. Abdel-Fattah1,2* 1

Applied Research Center at Thomas Jefferson National Accelerator Facility and Department of Molecular Biology and Chemistry at Christopher Newport University, Newport News, VA 23606, USA 2

Faculty of Sciences, Alexandria University, P.O. Box 426, Ibrahimia, 21321, Alexandria, Egypt *Corresponding Author email: [email protected]

Abstract A palladium nanoparticle/multi-walled carbon nanotube composite (Pd/MWCNT) was produced, characterized, and tested for catalytic activity in a hydrogen evolution reaction. The characterization included TEM, which confirmed the presence of the nanoparticles on the walls of the MWCNTs, SEM, and SEM-EDS. TEM characterization showed the 20 to 35 nm diameter of the MWCNTs and the 2-4 nm diameter of the palladium nanoparticles. The composite outperformed the precursor MWCNTs and Pd nanoparticles and performed best at pH 7 at 295K with 835 µmoles producing hydrogen at a rate of 23.0 mLmin-1gcat-1. Variation of the temperature of the reaction allowed the calculation of the activation energy, which is 62.66 kJ/mol, showing its promise as a heterogeneous catalyst for hydrogen evolution reactions.

Keywords: Palladium Nanoparticles, Catalysts, Carbon Nanotubes, Nanocomposites, Hydrogen Evolution, Hydride Precursors

Introduction Carbon based fuels are and have been the main fuel sources used in transportation and energy for centuries. Many of the more heavily used carbon-based fuels, such as natural gas, oil, and coal, are nonrenewable and are being consumed at a growing rate due to increasing

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population and energy demands. Recent research has focused on discovering and applying new fuel and energy sources that are renewable, environmentally friendly, and cost effective.1 One such candidate for an alternative energy source is hydrogen gas. While it is the most abundant element in the universe, making up approximately 75% of the universe’s total mass, on earth, hydrogen gas is not found naturally due to its rising up into the atmosphere.2 Rather, it is often derived from other hydrogen containing compounds such as water, petroleum, or coal, through electrolysis or steam reforming.3,4 Hydrogen synthesized from the decomposition of fossil fuels is currently the most popular and energy efficient process in industry.5 However, these processes are still dependent on non-renewable fuels and produce carbon dioxide as a byproduct. Another method of producing hydrogen gas is through electrolysis of water; however, this is an inefficient and impractical method of producing alternative renewable fuel.6 Despite these shortcomings, the infrastructure to use hydrogen gas is already being developed as it is used as a rocket fuel in its liquid form, and the applications of hydrogen gas in fuel cells and other smaller scale systems are continuously being studied.7,8 Storage of hydrogen gas also presents a problem as pressurization is used to take advantage of the energy density of hydrogen gas, but this presents a safety issue when applied to motor vehicles. Thus, research has turned to systems that can provide hydrogen gas as needed, and methods of storing this fuel in safer ways. This has led to the study of hydride precursor materials, one such material being sodium borohydride (NaBH4), which contains 10.8% hydrogen by weight and is environmentally safe.9,10 The hydrolysis reaction that sodium borohydride undergoes to release hydrogen does not proceed rapidly and would need a catalyst to be brought to an acceptable rate for such technologies. Much research has gone into the synthesis and testing of materials as catalysts for the hydrogen evolution reactions of metal hydride precursors. In the case of NaBH4, studies have


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shown promising results in using various transition metal materials, such as ruthenium, cobalt, nickel, platinum, gold, and palladium, to catalyze the production of hydrogen gas.9,11-21 Many of these catalysts have dimensions within the nanoscale as the large surface area to volume ratio provided by these materials. Increasing this ratio would provide an increase of catalytically active sites on the material, which is important for the catalysts made from more expensive metals. One such metal, palladium is known to be an efficient metal in catalyzing various reactions, such as the widely known Suzuki reaction.22-26 Unfortunately, many nanomaterials are colloidal and difficult to separate from the reaction mixture after catalysis. Thus, carbon nanotubes (CNTs) are highly attractive supports for metal nanoparticles, including palladium due to their stability and hydrophobic nature that allow them to be easily reclaimed from aqueous reaction mixtures.25,26

Experimental 2.1 Synthesis The multi-walled carbon nanotubes (MWCNTs) were synthesized by chemical vapor deposition of methane over iron-based catalysts.9 The nanoparticles were created by adding 1 mM aqueous precursor palladium solutions from palladium II chloride (Sigma Aldrich) to a 10 mM aqueous beta-cyclodextrin (Sigma Aldrich) solution. The resulting 135 µM palladium solution was stirred for ten minutes before adding 250 µL of 180 mM aqueous sodium borohydride to reduce the metal ions. The produced solution was then stirred for two hours to facilitate the formation of nanoparticles. The composites were synthesized via incipient wetness impregnation of the MWCNT support with the produced aqueous palladium colloids. The mixture was then stored at 333K for 48 hours to evaporate the excess water.9,21,27 2.2 Characterization



High Resolution Transmission electron microscopy (TEM) was run on the produced nanomaterial to assess the dimensions of the component materials, as well as to confirm the functionalization of the MWCNTs (JEM-2100F). TEM sample grids were prepared with one microliter of the composite material suspended in methanol and dried in an oven overnight. Scanning electron microscopy (SEM) was run to determine the overall morphology of the composite at the microscale. Energy dispersive spectroscopy (EDS) was also used to determine the quantity of palladium present within the composite. This characterization was completed using a FE-SEM (Hitachi 4700) with EDAX attachment. 2.3 Catalysis The composite was tested for catalytic activity for the hydrolysis of sodium borohydride using a gravimetric water displacement system.9,21,27 The reactions were run using 10 mg of the produced composite, 100 mL of dionized water at pH 7 and 295K, and using 835 µmoles of NaBH4 (J.T. Baker). The temperature, pH, and concentration of reactant conditions were altered individually using an ice water bath or hot plate (273K, 303K), HCl or NaOH (pH 6, 8), and 635 or 1035 µmoles of NaBH4 (J.T. Baker). All reactions were stirred using a magnetic stir bar for the full two hours of the trial except in the cases of the temperature increased and decreased trials, which required insulation that interfered with the magnetic stir plate. The gravimetric water displacement system relied on an Ohaus Pioneer Balance (Pa124) and the automatic mass logging software. Rate was determined as a product of the overall hydrogen gas produced, the inverse of the duration of the trials (2 hours), and the mass or volume for homogenous of catalyst used.


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Results and Discussion

Figure 1: TEM micrographs of the Pd/MWCNT composite. 1A shows an overview of the composite at the nanoscale. 1B displays the functionalization of the palladium nanoparticles within the MWCNTs. 1C depicts the composite at high magnification to allow the dimensions of the original component materials to be determined. 1D shows the d-spacing of the nanoparticles, confirming the presence of metallic palladium. The micrographs 1A, B, and C have scale bars of 100 nm, 50 nm, and 20 nm, respectively. Due to the tendency of nanoparticles to agglomerate, the use of a capping agent is necessary. The stabilization effect of beta-cyclodextrin, resulting from its unique confirmation of a smaller primary hydroxyl ring (0.78 nm) within a larger secondary ring (1.54 nm), can be used to control the final morphology of the particles.28 It acts to restrict excessive agglomeration of



the nanoparticles resulting in the small, uniform particles as seen in Figure 1.28 Figure 1 shows the TEM micrographs of the produced Pd/MWCNT composite at differing magnifications. Figure 1A shows the presence of the Pd nanoparticles throughout the MWCNTs. Figure 1B resolves that these palladium particles are occurring in clusters, rather than evenly across the surface of the MWCNTs. Figure 1C allows the measurement of the components of the materials, depicting the 20 to 35 nm diameter of the MWCNTs and the 2-4 nm diameter of the individual palladium nanoparticles. Figure 1D depicts the 0.22 nm d-spacing of the nanoparticles, which was determined as the average spacing across 10 measurements. This spacing correlates to the 0.22 nm d-spacing found in the 111 plane of Palladium metal, and thus confirms that the nanoparticles are Palladium, and were successfully reduced.29


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Figure 2: SEM micrograph of the Pd/MWCNT composite and the corresponding EDS analysis of the imaged section (inset). Nanoparticles were not detected by this method as depicted in the micrograph and by the lack of a palladium peak in the EDS spectrum. SEM micrograph in Figure 2 show the Pd/MWCNT composite as produced at 50k magnification with the EDS analysis of the area shown in the micrograph. Carbon makes up the majority of the composite as 95.9 % wt, followed by oxygen as 4.1 % wt. Palladium was not quantified by EDS as the Pd loading of the composite was 0.0525%, well below the detection limits. The high carbon percentage is expected because of the composition of the support material, and the presence of oxygen would be from the beta-cyclodextrin capping agent used, or present as a contaminate within the composite. 0.84 0.79 0.74 0.69 0.64 0.59 0.54

Transmission

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MWCNT Pd/MWCNT

0.49 0.44 0.39 3500

3000

2500

2000

1500

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500

0

Wavenumber (cm-1)

Figure 3: FTIR Spectrum of the Pd/MWCNT composite and the precursor MWCNTs Figure 3 displays the FTIR spectra of the composite material as well as the precursor MWCNTs. The precursor MWCNT shows the expected aromatic peaks with the multi-band C=C stretch at 1618 cm-1, 1475 cm-1 and 1425 cm-1. The Pd/MWCNT shows the same peaks, and some



additional peaks, all of which can be associated to the beta-cyclodextrin capping agent which displays a broad O-H stretch at 3100 cm-1 and a weak peak at 1000 cm-1 caused by coupled CO/C-C stretching and O-H bending vibration.30 30

Pd/MWCNT Hydrogen Gas Generated (mL)

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25

PdNP 20 15

MWCNT

MWCNT

PdNP 10

Pd/MWCNT

5 0 0

20

40

60

80

100

120

140

Time (min)

Figure 4: Comparison of the volume of hydrogen gas generated versus time by the Pd/MWCNT composite and its component materials. The reactions were run at 295 K, pH 7, and with 835 µmoles of NaBH4. The Pd/MWCNT material increased the production of hydrogen gas from the reaction of sodium borohydride by 25% when compared to the precursor palladium nanoparticles and 53% when compared to the original MWCNTs (Figure 4). Hydrogen gas was produced at a rate of 21.6 mL•min-1•gcat-1 as catalyzed by the composite material, 0.86 mL•min-1•mLcat-1 for the Pd nanoparticles, and only 14.1 mL•min-1•gcat-1 for the original MWCNTs.


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30

835 µmoles 25

Hydrogen (mL)

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1035 µmoles

20

635 µmoles

15

635 μmol 835 μmol

10

1035 μmol

5 0 0

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Time (min)

Figure 5: Comparison of the volume of hydrogen gas generated versus time by the hydrolysis reaction of three concentrations of sodium borohydride (635 µmoles, 835 µmoles, 1035 µmoles) as catalyzed by the Pd/MWCNT. For the sodium borohydride concentration adjusted trials, the reaction of 835 µmoles produced hydrogen at the greatest rate of 21.7 mL•min-1•gcat-1. This is significantly higher than the 15.8 mL•min-1•gcat-1 and 10.5 mL•min-1•gcat-1 for the 1035 and 635 µmoles of sodium borohydride respectively (Figure 5). The data showed that 835 µmoles performed best, and the decrease in reaction rate with higher concentrations has been observed in other studies and is often attributed to greater solution viscosity.10,31



30

pH 7

25

Hydrogen (mL)

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20

pH 6 pH 6

15

pH 7

pH 8 10

pH 8

5 0 0

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Time (min)

Figure 6: Volume of hydrogen gas generated versus time as catalyzed by Pd/MWCNTs under the pH conditions (pH 6, 7, 8). Modified pH catalytic activity tests showed that pH 7 was the best condition for the catalysis of the reaction producing hydrogen at a rate of 23.0 mL•min-1•gcat-1 greatly outperforming pH 6 and 8 conditions which evolved hydrogen with rates of 16.0 mL•min-1•gcat-1 and 12.6 mL•min-1•gcat-1 respectively (Figure 6). This alludes to a pH sensitivity of the composite material, as sodium borohydride is known to react faster at lower pHs.32 The production of BO2ions at the completion of the hydrolysis is the cause of pH sensitivity, as OH- ion shifts the equilibrium of the reaction to the reactants, and H+ shifting to the products (Equation 1). BH4- + 2H2O → BO2- + 4H2

(1)

Similar results have been observed in previous studies where produced Au/MWCNT and Ag/MWCNTs composites perform better at neutral pH.21,27


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30

295 K 25

303 K Hydrogen (mL)

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20 273 K

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Time (min)

Figure 7: Volume of hydrogen gas generated by the hydrolysis of NaBH4 versus time as catalyzed by the Pd/MWCNT composite at 273K, 295K, and 303K. In the three temperature conditions, the reaction produced hydrogen the fastest at 295K at a rate of 21.7 mL•min-1•gcat-1. This was 1 mL•min-1•gcat-1 higher than the rate of the reaction at 303K, this is not typical of catalyzed sodium borohydride reactions. Shivhare et al. reported that larger molecules surrounding the metal nanoparticle can inhibit the diffusion of species, but Huang et al. demonstrated that cyclodextrin as a capping agent does not inhibit the metal catalysis of NaBH4.33,34 Moreover, Devi et al. reported an increase in susceptibility of the nanoparticles to be attacked by other species in solution (altered capping effect).35 In the Devi et al. study, it was shown that hydroxypropyl cyclodextrin displayed strong electrostatic repulsion in increased pH systems, reducing the capping effect.35 The nanoparticles in this study were capped using β-cyclodextrin. Therefore, an alteration of capping effect would not directly affect the catalysis but could increase the ability of other negatively charged species in the reaction mixture such as chloride and hydroxyl ions to bind to the catalytically active sites, which have been reported to decrease catalytic activity.36 An alteration of the capping effect of β-



cyclodextrin is believed to be the cause of the lower rate at the higher temperature. At 273K, the reaction produced hydrogen at a rate of 1.7 mL•min-1•gcat-1 of hydrogen gas (Figure 7). From the temperature data, the activation energy of the catalyzed reaction was calculated using an Arrhenius plot (Figure 8) and the Arrhenius equation (Equation 2). lnK = lnA – Ea/RT

(2)

The activation energy was determined to be 62.66 kJ/mol, which is higher than other similar catalysts (Table 1). The activation energy may be improved by increasing the loading of palladium in the composite as the loading in the synthesized composite was 0.052 %, which is lower than that of other similarly supported catalysts.21 The composite was collected after the reaction, and minimal changes were observed to the overall composite (Figure 9A) or to the nanoparticles (Figure 9B) from TEM analysis. The image (Figure 9A) of the reclaimed catalysts shows unchanged morphology and dimensions of the Palladium nanoparticle and the Pd/MWCNT. 4 3.5

ln(r, mL•min-1•g-1)

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3 2.5 2 1.5 1 y = -7.537x + 28.202 R² = 0.9362

0.5 0 3.25

3.3

3.35

3.4

3.45

3.5

3.55

3.6

1000/T (K)


3.65

3.7

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Figure 8: Arrhenius Plot used to determine the activation energy of the hydrolysis of NaBH4 as catalyzed by the Pd/MWCNT composite.

Figure 9: TEM images of the composite after catalysis. Figure 9A shows the composite with intact MWCNTs and 2-4 nm Pd nanoparticles. Figure 9B shows a single, spherical 3 nm intact Pd nanoparticle after the reaction. Table 1: Reported activation energies for NaBH4 hydrolysis by catalyst Catalyst

Ea (kJ/mol)

Temperature (K)

Time (min)

Reference

Ru/Graphite

61.1

298-318

30

14

Ru intrazeolite

49.0

293-313

6

15

Ru nanoclusters

41.0

298-318

5

16

Co intrazeolite

57.0

298-318

20

17

Co nanoclusters

39.0

293-313

9

18

Pt-Pd-CNTs

19.0

302-332

120

19

Pd/C

28.0

283-328

20

20



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Au/MWCNTs

21.1

273-303

120

21

Ag/MWCNTs

44.5

273-303

120

27

Pd/MWCNTs

62.66

273-303

120

This Work

Scheme 1: Proposed mechanism for the hydrolysis of NaBH4 hydrolysis as catalyzed by Pd/MWCNTs. Scheme 1 depicts the proposed mechanism of the catalyzed hydrolysis of aqueous borohydride.19,21,27 The mechanism shows that the initial formation of the palladium-borohydride complex is reversible, and this complex is stabilized by an adjacent unoccupied catalytic site receiving a hydride ion. These two adjacent sites continue to benefit the reaction as the adjacent hydride reacts with a hydrogen from water in the aqueous environment to produce H2 gas, while the remaining hydroxide ion attacks the metal-borohydride complex displacing another hydride ion to the adjacent catalytic site. This process repeats until there are no longer hydride ions to be released by the palladium-borohydride complex, and the tetrahydroxyborate is formed as the complex is broken. The importance of the adjacent unoccupied site further stresses the


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importance of catalytic surface area for the reaction and may explain the trend of increasing reactant concentration negatively affecting hydrogen generation rate as seen in Figure 5.

Conclusion The Pd/MWCNT composite was successfully produced as determined by SEM-EDS and TEM analysis. The composite outperformed its component materials, and performed best at pH 7 at 295K with 835 µmoles producing hydrogen at a rate of 23.0 mL•min-1•gcat-1. The temperature adjusted conditions allowed for the activation energy to be calculated, which is comparable to other reported catalysts at 62.66 kJ/mol; however, it was much higher than previous works with Au/MWCNT composites that had higher loading of noble metal within the material.21 This may indicate that a higher loading of palladium into the MWCNT support is necessary to achieve the level of catalytic activity needed for the application of these materials. The Pd/MWCNT composites show promise as catalysts for hydrogen generation from the hydrolysis of sodium borohydride but require optimization to be competitive with other catalysts in this rapidly developing field.

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Abstract Graphics

Pd Nanoprticle


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Abstract Graphics

Single Pd Nanoparticle


Palladium Nanoparticle Multi-walled Carbon Nanotube Composite as

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