THE NET ADSORPTION OF HYDROGEN ONPALLADIUM NANOPARTICLES

DEBJYOTI SAHU*, PRASHANT MISHRA†, NITUN DAS†,ANIL VERMA*,† and SASIDHAR GUMMA*,†,‡

*Centre for Energy, Indian Institute of Technology Guwahati,Guwahati, Assam 781039, India

†Department of Chemical Engineering,Indian Institute of Technology Guwahati,

Guwahati, Assam 781039, India‡s.gumma@iitg.ernet.in

Received 25 September 2013Revised 5 December 2013

Accepted 17 December 2013Published 23 January 2014

In this paper, we report the synthesis of polymer coated palladium (Pd) nanoparticles through asingle stage reduction of Pd2þ ions by ethylene glycol. Polyvinyl pyrrolidone (PVP, MW 25,000) isused as a stabilizer. Self-assembled Pd nanoparticles (10–40 nm) were used in hydrogen adsorptionstudies. Gravimetric adsorption measurements were carried out in a pressure range of 0–26 bar at293, 324, 364 and 392K. Saturation for all isotherms was obtained within a few bars of pressure at alltemperatures. Maximum hydrogen storage capacity observed was 0.58wt.% at 324K and 20 bar.Net adsorption calculations indicated that required tank volume (for storing a particular amount ofhydrogen) can be signi¯cantly reduced by using a tank ¯lled with Pd nanoparticle.

Keywords: Hydrogen storage; hydrogen storage material; nanostructured materials.

1. Introduction

Hydrogen storage is one of the key challenges in

realizing fuel cell propelled automobiles. Such an

automobile requires on-board storage capacity of

5–13 kg of H2 to run at least 300 miles before refuel-

ing.1 Adsorption based hydrogen storage is one of the

potential techniques to store hydrogen. The US

DOE's adsorption based storage target for 2015 is

about 9wt.% gravimetric capacity and 81 g L�1 vol-

umetric capacity.2,3 However, the future generation

cars will be lighter in weight and more fuel e±cient;

therefore, a lesser value than the target is also of

interest. A typical storage tank of 100L volume can

only contain 1.22 kg of hydrogen at 150 bar pressure

and at room temperature. If hydrogen is stored as

liquid, temperature of about 20K needs to be

achieved, which is highly energy incentive. However,

a storage tank ¯lled with solid adsorbent can be a

potential candidate to store su±cient hydrogen at an

elevated temperature and at a relatively lower pres-

sure. Adsorbents can be porous materials like metal

organic frameworks (MOFs),4 carbon,5,6 zeolite7 or

hydride forming metals like magnesium,8 palladium

(Pd), etc. New materials are being investigated for

vehicular onboard hydrogen storage. Synthesis of new

metal nanoparticles is of interest, as they are more

versatile from their bulk analogs.9–12 Hydrogen stor-

age on metal nanoparticles is of recent interest. While

adsorption on carbon and other porous adsorbents

like MOFs, zeolites show better results at higher

Surface Review and Letters, Vol. 21, No. 2 (2014) 1450022 (6 pages)°c World Scienti¯c Publishing CompanyDOI: 10.1142/S0218625X1450022X

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pressure and cryogenic temperature, complex metal

hydrides on the other hand show better adsorption

capacity at low pressure. However, the major pro-

blems with these materials include slow absorption/

desorption kinetics, high binding energies (> 30 kJ

mol�1), etc.13

Pd is a classical hydrogen-adsorbing metal. The

bulk properties of Palladium–Hydrogen (PdHx) sys-

tem have been studied and reported in literature.

Higuchi et al. reported that thin ¯lm of Pd can store

only 0.15–0.3 wt.% of hydrogen.14 However, nano-

structures show di®erent behavior than their bulk

analogs. Preparation of Pd–PVP nanoparticles by

single stage reduction using polyol process, which

were subsequently used for Suzuki coupling reaction,

was reported by Radha et al.15 After glycol reduction,

separation of the nanoparticles from the viscous so-

lution is very crucial and few methods are reported in

the literature. Xiong et al. used inorganic membrane16

to separate the Pd nanoparticle, whereas, Yamauchi

et al. separated these particles by precipitation.17

Hydrogen adsorption capacity on Pd nanoparticles

reported in literature varies over a wide range, since it

depends on the size of the particles. Theoretical

maximum loading for �-PdHx system varies between

0:58 < x < 1.18 Yamauchi et al. reported a maximum

loading of about 0.7 wt.% for 2.6 nm Pd particles and

about 0.88wt.% for 7 nm particles at 393K and

10 bar. Kishore et al. reported marginally better

loading for 70 nm particles (0.53 wt.%) than 4 nm

particles (0.46 wt.%) at 323K and 10 bar.19 Narehood

et al. reported maximum loading for 2–3 nm particles

to be 1.08wt.% at 323K and 0.8 wt.% at 363K and

5 bar.20 Harinouchi et al. reported loading of 0.6wt.%

at 373K and 10 bar for a similar particle size.21 Most

of the literature report better loading of hydrogen

on Pd nanoparticles than that on the bulk at low

pressure (< 100mbar).

In this work, we report hydrogen adsorption iso-

therms for Pd nanoparticles at several temperatures

between 293 and 392K over a wide pressure range.

Adsorption isotherms at several temperatures will

help in understanding the e®ect of temperature on

adsorption characteristics of synthesized nano-

particles. This work will also help in understanding

the potential of nanoparticles for storing hydrogen at

ambient conditions. Measurement of hydrogen ad-

sorption is a relatively simple task. However, error

associated in calculation of impenetrable volume of

solid and impenetrable portion of adsorbent may in-

troduce considerable uncertainty. Such errors can be

avoided if adsorbed quantities are reported in terms of

net adsorption measurement.22 Therefore, in this

work, we also report net adsorption for hydrogen

which is more suitable to evaluate the actual adsorp-

tion capacity of a solid adsorbent ¯lled in a storage

tank. In addition, the loss of tank volume by impen-

etrable solid adsorbent can also be accounted readily

in this method. Net adsorption isotherm directly gives

the additional amount of hydrogen that can be stored

in a tank containing adsorbent compared to that of an

empty tank at the same temperature and pressure

conditions.4,22

2. Experimental Details

2.1. Synthesis of Pd nanoparticles

Palladium (II) chloride (Spectrochem), ethylene gly-

col (Merck), and polyvinyl pyrrolidone (CDH Lab.)

were used without further puri¯cation. Ethylene

glycol was heated up to 393K for 2 h in three separate

conical °asks to remove moisture. In the ¯rst °ask,

900mg of PdCl2 was added to 252ml ethylene glycol

and 0.1ml conc. HCl at 333K and ¯ltered subse-

quently. In the second °ask, 5.64 g PVP was is added

to 252ml ethylene glycol at 393K. In the third °ask,

480ml of ethylene glycol was kept at 393K. During

the reaction, solutions from the ¯rst and second °ask

were added simultaneously at a rate of 2.1ml min�1

to the third °ask maintained at 398K. The contents

of the third °ask were stirred at 1200 rpm during this

process at atmospheric pressure. The contents in the

°ask were allowed to react for about 7 h. Proper

precautions were taken to ensure that no temperature

gradient exists in the reaction medium. As the reac-

tion proceeded, the pale orange color solution pro-

gressively turned black, due to formation of Pd

nanoparticles.

Initially, several preliminary experiments were

conducted to understand the e®ect of reaction time,

temperature and concentration of Cl� ions.23 The

particle size and yield gradually increase with the

reaction time. After several trials, about 6 to 7 h of

reaction time was found to be suitable to obtain

particle size around 50 nm. After the desired reaction

time, methanol was added to the reactant solution in

1:1 ratio and centrifuged for 45min at 10,000 rpm to

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separate the nanoparticles. The solids thus separated

were transferred into a ceramic crucible and air dried

at 353K for about 6 h.

2.2. Characterization

The puri¯ed Pd nanoparticles were examined at room

temperature using X-ray di®ractometer (Rigaku

TTrax III) equipped with a Cu–K radiation source.

FESEM (Sigma), TEM and selected area electron

di®raction (SAED) (Jeol) to determine the size, mor-

phology and crystallinity of the particles. The sample

for TEM was prepared by depositing a drop of Pd

nanoparticles dispersed in 2-propanol onto a carbon

coated Cu grid (300 mesh), followed by evaporation of

the solvent at room temperature. BET surface area

analysis of Pd nanoparticle was done using a surface

area analyzer (autosorb-iq, Quantachrome) to mea-

sure N2 isotherms at 77K. In the surface area mea-

surement, prior to the analysis, the sample was

degassed under vacuum at 523K for about 3 h.

2.3. Pressure composition isotherm

First, the sample (Pd nanoparticles) was degassed at

473K for 3 h under vacuum (and a helium purge

°ow) to remove the solvent and other impurities. Iso-

therms of hydrogen uptake on sample were measured

gravimetrically using a magnetic suspension balance

(RubothermTM, Germany). For the measurement of

the buoyancy volume, heliumwas injected from 1 to 20

bar and correlation with pressure was established with

nonadsorbing assumption for helium. Buoyancy cor-

rection or the measured weight was incorporated to

calculate excess adsorption.24 Adsorption measure-

ments were carried out at 293, 324, 364 and 392K and

in the pressure range 0–26 bar. High purity hydrogen

gas (99.999% purity) was used for the sorption studies.

3. Results and Discussion

3.1. Characterization

Crystalline nanoparticles formation starts at 30–

40min of the polyol reaction. Initially, a few small

particles of 2–3 nm are formed. These particles are

known as seeds.25 As the reaction proceeds, these

seeds grow to form bigger size crystals. This phenom-

enon is called \Ostwald ripening".26 Homogeneous

distribution of Pd nanoparticles of 10–20 nm was ob-

served by TEM (Fig. 1) if the reaction is allowed for

9 h. After 18–20 h, 200–300 nm particles were formed

as shown in FESEM image (Fig. 2). Several trials were

performed to check the repeatability. The particles

shown in both Figs. 1 and 2, were synthesized in very

small batches (�30–50mg Pd nanoparticles per batch)

to understand the e®ect of reaction time.

The characterization of particles used in adsorp-

tion measurements, is shown in Figs. 3 and 4. These

particles were synthesized in a larger batch (� 250mg

sample) and for about 7 h of reaction time.

The X-ray di®raction pattern of puri¯ed Pd

nanoparticles is presented in Fig. 3. These patterns

are characterized by several intense peaks between

the di®raction angles of 30� and 90�. The peaks can

Fig. 1. TEM image of Pd nanoparticle synthesized in 9 h.

Fig. 2. FESEM image of Pd nanoparticle synthesized in18 h.

The Net Adsorption of Hydrogen on Palladium Nanoparticles

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be indexed to those re°ection from (111), (200),

(220), (311), and (222) planes of nano-crystalline Pd

as reported in Ref. 19. Scherrer formula based on the

full width of half maximum (FWHM) can approxi-

mate the average crystal size. Individually each peak

can be selected to determine the average particle size

and the intense peak at 40� yield a particle size of

about 19 nm.

TEM image shown in Fig. 4(a) con¯rm that most

of the particles are in nanometer size. Three shapes

that can be distinctively ¯gured out are icosahedrons

(looks like a hexagon from top), cubes (looks like a

square or rhombus from top) tetrahedron (looks like a

triangle from the top). HRTEM was used to spot one

such icosahedron and is shown in Fig. 4(c). SAED

patterns are a projection of the reciprocal lattice, with

lattice re°ections showing as sharp di®raction spots.

SAED pattern is shown in Fig. 4(b). There are ¯ve

planes for which d-spacing can be calculated from

SAED as 0.3 nm (111), 0.3 nm (200), 0.19 nm (220),

0.19 nm (311) and 0.16 nm (222). These values are

close to 0.2 nm that is reported as the average lattice

distance in Pd.16

BET surface area of Pd nanoparticles (synthesized

in 7 h) is 2.5m2g�1 which is close to 10m2g�1 as re-

ported by Kishore et al.19 These particles are non-

porous material and the hydrogen loading would be

mainly due to chemisorption.

3.2. Excess adsorption

Most important aspect in the measurement of the

hydrogen adsorption isotherm data is in achieving the

equilibrium,27 which can be as long as few hours for

H2 adsorption on Pd. Su±cient equilibrium time is

necessary to allow hydrogen to completely penetrate

Fig. 3. Powder XRD pattern of Pd nanoparticle synthe-sized in 7 h (color online).

Fig. 4. (a) TEM image of nanoparticles synthesized in 7 h. (b) SAED shows di®raction from ¯ve re°ection planes sur-rounding the core. (c) An icosahedrons particle detected by HRTEM.

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the inter lattice planes of the adsorbent particles. The

thermodynamic aspects of metal–hydrogeninterac-

tions are usually understood from pressure composi-

tion isotherm. Figures 5 and 6 show the pressure

composition (PC) isotherms of Pd nanoparticles.

Each isotherm exhibits three distinct regions.

Surface adsorption due to weak van der Waal's force

attained within 0.4 bar, denoted as � phase in Fig. 5

(for all the measured temperatures). Sub-surface ad-

sorption happens within 0.6 bar denoted as �þ �

phase. Hydrogen adsorption is a continuous process;

within the lattice planes hydrogen will form bonds

with Pd atom making way for more hydrogen mole-

cules to get adsorbed at its surface this is denoted as

the � phase in Fig. 5. The maximum loading we have

observed corresponds to a value of x ¼ 0:55 for

�-PdHx system.

Hydrogen uptake at di®erent temperatures upto

26 bar is shown in Fig. 6. Adsorption at 293K and 1

bar pressure can be seen as 0.38wt.%. This value is

signi¯cantly higher than those for porous adsorbents

like graphene,28 carbon nanotube,5 zeolite,29 common

MOFs like MIL 101,30 HKUST-1,31 etc. At 324K and

1 bar, even higher loading is observed (0.5 wt.%).

Similarly, 0.48wt.% loading is observed at 364K

and 1 bar. But loading decreases to 0.35wt.% at

392K and 1 bar. MOFs, zeolite and other porous

material predominantly exhibit physisorption.32 The

adsorbent surface having chemical a±nity toward

hydrogen leads to chemisorptions.33,34 At favorable

temperature and pressure, hydrogen molecules reach

the inter-lattice planes of Pd crystals to form the

hydride.

3.3. Net adsorption

In addition to excess adsorption, hydrogen loading

can also be reported in terms of net adsorption.22

Since net adsorption can give the extra amount of

hydrogen that can be stored in a tank ¯lled with solid

Pd nanoparticles compared to that in an empty tank,

bene¯t of using the adsorbent to enhance the storage

capacities are better represented by this quantity.

Figure 7 compares the volume of a tank (with and

without Pd nanoparticles) used to store H2 at same

condition. For example, a 664 L tank would be re-

quired to store 1 kg of H2 at 20 bar when the tem-

perature is 324K. In contrast, the tank volume

reduces to 65 L when it is ¯lled with Pd nanoparticles.

Fig. 5. Hydrogen uptake isotherm at low pressure.

Fig. 6. Hydrogen uptake isotherm at high pressure.Fig. 7. Tank volume required to store 1 kg of hydrogen at20 bar (color online).

The Net Adsorption of Hydrogen on Palladium Nanoparticles

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4. Conclusion

Pd nanoparticles were synthesized using polyol pro-

cess. Ethylene glycol was used as reducing agent and

PVP as stabilizer. The size of the Pd nanoparticles

mainly depends on the reaction time. It was found

that about 7 h of reaction time at 398K yielded

nanoparticles in the desired range of 10–40 nm. Hy-

drogen storage capacity of these Pd nanoparticles was

measured at 293, 324, 364 and 392K in the pressure

range of 0–26 bar. Maximum adsorption of 0.58wt.%

was observed at 324K and 20 bar. Net adsorption

framework was suggested to highlight the bene¯t of

storing hydrogen in a tank containing Pd particles as

compared to using a tank without an adsorbent.

Acknowledgments

Authors are thankful to Centre for Scienti¯c and In-

dustrial Research, New Delhi, India for ¯nancial

support through project grant 01(2522)/11/EMR-II

and the Central Instruments Facility at Indian

Institute of Technology, Guwahati for BET Surface

Area analysis.

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