Adsorption of hydrogen in nickel and rhodium exchanged zeolite X

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Adsorption of hydrogen in nickel and rhodiumexchanged zeolite X

K.P. Prasantha, Renjith S. Pillaia, H.C. Bajaja, R.V. Jasraa,�, H.D. Chungb,T.H. Kimb, S.D. Songc

aDiscipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI),

G.B. Marg, Bhavnagar 364 002, IndiabHydrogen System Center, KIER, Daejon, Republic of KoreacHanseo University, Chungnam, Republic of Korea

a r t i c l e i n f o

Article history:

Received 12 July 2007

Received in revised form

26 October 2007

Accepted 30 October 2007

Available online 3 December 2007

Keywords:

NiNaX

RhNaX

NaX

Hydrogen adsorption

Grand Canonical Monte Carlo

simulation

a b s t r a c t

Adsorption of hydrogen in zeolite NaX and its nickel and rhodium exchanged forms were

investigated at 77.4 K using a static volumetric adsorption system up to 1 bar, and at 303

and 333 K in a gravimetric adsorption system up to 5 bar. Hydrogen adsorption at 77.4 K for

NaX and the nickel and rhodium exchanged zeolite X was found to be reversible with

pressure. However, chemisorption of hydrogen was observed at 303 and 333 K. The highest

adsorption capacities for hydrogen in NaX, NiNaX and RhNaX were observed to be 24.28,

34.51 and 51.85 molecule/u.c at 333 K and 5 bar. Hydrogen uptake capacities of the nickel

and rhodium exchanged zeolite X at 303 and 333 K were found increasing initially up to 70%

of nickel and 55% of rhodium exchange, beyond which it showed a decreasing trend. The

observed decrease in adsorption capacity at higher Ni and Rh cation exchange is explained

in terms of partial degradation of the zeolite structure during cation exchange and the high

temperature vacuum dehydration process. Grand Canonical Monte Carlo simulations were

also performed to study the adsorption of hydrogen in NaX as well as nickel and rhodium

exchanged zeolites X at 77.4, 303 and 333 K and the simulated data were compared with

experimentally measured values.

& 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

The decreasing oil reserves and growing environmental

pollution awareness have necessitated search for alternate

fuels and zero emission vehicles. Fuel cell driven car with the

proton exchange membrane (PEM) fuel cell and hydrogen as a

fuel seems to be a promising alternate [1]. Though hydrogen

is a clean fuel giving water as the only by-product on its

combustion in air, on-board storage of hydrogen is still a

challenge. Various methods for storing hydrogen including

gaseous, liquid and solid state storage have been suggested

[2,3]. The important methods for storing hydrogen in

solid materials include: (i) storage in metals and alloys;

(ii) storage in complex hydrides (alanates, borides);

(iii) storage by trapping (e.g., clatharates); and (iv) storage in

microporous materials (carbons, metal-organic frameworks,

polymers, zeolitic materials, etc.). Among the best known

microporous solids, the zeolites have the advantage of high

stability, low cost and a variety of porous structures. Zeolites

are highly crystalline alumino-silicate materials with crystal-

lographically well-defined channels and cavities [4,5].

A general chemical formula of zeolites is represented as

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�Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2567562, +91 278 2566970.E-mail address: [email protected] (R.V. Jasra).

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Mx=n½ðAlO2ÞxðSiO2Þy� �wH2O, where M is an exchangeable

cation with valency n, w is the number of water molecules

and ratio y=x is having a value X1. Mainly alkali and alkaline

earth cations are present in the zeolite in the form of

exchangeable extra framework cations.

Recent studies have shown that zeolites can be potential

materials for reversible hydrogen storage [6–15]. Du and Wu

reported hydrogen adsorption capacity of 2.55 wt% for NaX at

77 K and 70 bar pressure [7]. Langmi et al. reported the

hydrogen storage capacities of 1.54, 1.79, 1.81 and 2.19 wt%

obtained at 77 K and 15 bar for zeolites NaA, NaX, NaY and

CaX respectively [8]. Zecchina et al. obtained a hydrogen

capacity of 1.28 wt% on H-SSZ-13 zeolite at 77 K and 0.92 bar

[9]. Vitillo et al. predicted a theoretical maximal hydrogen

storage capacity of 2.86 wt% for FAU and RHO zeotypes [10].

However, experimental data on hydrogen adsorption in

zeolites at room temperature are sparse. Weitkamp et al.

reported hydrogen uptake capacity of 0.023 and 0.030 wt% in

NaX, NaY and NaA zeolites, respectively, at 573 K and 100 bar

[11]. Kayiran et al. reported hydrogen uptake capacity of 0.12

and 0.10 wt% for KA and NaA zeolites at 293 K and 10 bar

pressure [12]. It has been reported that the amount of

hydrogen adsorbed on zeolites depends on its framework

structure, composition and acidity [13,14]. The cations in the

framework are considered as the adsorption sites for hydro-

gen. Hydrogen adsorption studies in zeolites are largely

confined to alkali and alkaline earth metal cations as

exchangeable extra framework cations with few studies

reported in transition metal exchanged zeolites [15,16]. In

order to examine the role of framework structure and

exchangeable cations in hydrogen adsorption, we report

hydrogen adsorption in zeolite X exchanged with Ni2+ and

Rh3+ ions in this paper. Nickel and rhodium are known to

have affinity for hydrogen and form hydrides on reaction with

hydrogen and are used as catalysts for various reactions such

as hydrogenation and hydroformylation [17,18].

Molecular simulation of adsorption phenomena in zeolite is

emerging as a rapid, cost-effective method for the evaluation

of potential adsorbents. The grand canonical Monte Carlo

simulation technique is particularly adapted to calculate the

equilibrium adsorption isotherm measurements. For exam-

ple, Razmus and Hall [19] used Monte Carlo simulation to

reproduce the experimental single component equilibrium

adsorption isotherms of nitrogen, oxygen and argon in zeolite

5A. Watanabe et al. [20] used grand canonical Monte Carlo

simulation method to study air separation properties of

zeolite A, X and Y. Song et al. [21] used grand canonical

Monte Carlo procedure for the study of the effect of cation

type, available volume, surface area, temperature and pres-

sure on the hydrogen uptake on model zeolites. Berg et al. [22]

studied the hydrogen uptake isotherms in the microporous

sodalite-type structures by means of grand canonical Monte

Carlo calculations. Grand canonical Monte Carlo simulations

are suitable for establishing a correlation between the

microscopic behavior of the zeolite and adsorbate system

with the macroscopic properties such as adsorption iso-

therms which are measured experimentally [23]. In the

present paper, grand canonical Monte Carlo simulations were

performed to study the adsorption of hydrogen in NaX as well

as nickel and rhodium exchanged zeolites X at 77.4, 303 and

333 K and simulated data were compared with experimentally

measured values.

2. Experimental

2.1. Materials

Zeolite X powder procured from Zeolites and Allied Products,

Bombay, India, has a chemical composition Na88Al88

Si104O384 � 220H2O. Nickel chloride and rhodium chloride

purchased from E Merck India Ltd., Bombay, India, were used

as the starting materials for the adsorbent preparation.

Hydrogen with 99.9995% purity was procured from Hydrogas

India Pvt. Ltd., Bombay, India. Hydrogen was further purified

by passing through a molecular sieve trap prior to its use for

the adsorption isotherm measurements.

2.2. Preparation of cation exchanged zeolite X

Nickel and rhodium cations were introduced into sodium

form of zeolite X by the conventional cation exchange

method from aqueous solution. Typically, the zeolite NaX

was treated with 0.05 M aqueous solution of nickel chloride or

rhodium chloride with solid/liquid ratio of 1:80 at 353 K for

4 h. The residue was filtered, washed with hot distilled water,

until the washings were free from chloride ions as tested by

silver nitrate solution and dried in air at room temperature.

Zeolite samples having different amounts of nickel and

rhodium were prepared by subjecting repeated ion exchange

into the zeolites. The extent of nickel and rhodium ions

exchanged on different zeolite X samples were determined by

atomic emission spectroscopy (AES) and inductively coupled

plasma (ICP) analysis using Perkin-Elmer optical emission

spectrometer (Optima 2000 DV). The following terminology is

used to describe the ion exchanged samples: the first letters

show the exchanged cation and the number at the end shows

the percentage of sodium cation present in NaX exchanged

with this cation. For example, NiNaX-49 means 49% of Na+

ions present in NaX are replaced with Ni2+ ions and RhNaX-29

means 29% of Na+ ions present in NaX are replaced with Rh3+

ions.

3. Characterization

3.1. X-ray powder diffraction

The X-ray powder diffraction measurements at ambient

temperature were carried out using a PHILIPS X’pert MPD

system in the 2y range of 5–651 at a scan speed of 0.11 s�1

using CuKa1 radiation ðl ¼ 1:54056 AÞ. The diffraction patterns

of the starting material shows it is highly crystalline showing

reflections in the 2y range 5–351 typical of zeolite X.

Percentage crystallinity of the transition metal ion exchanged

zeolites were determined from the X-ray diffraction pattern

by summation of the intensities of 10 major peaks at 2yvalues 6.1, 10.0, 15.5, 20.1, 23.4, 26.7, 29.3, 30.5, 31.0 and 32.1.

The sodium form of the zeolite X was considered as an

arbitrary standard for the calculations.

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3.2. Diffuse reflectance infrared Fourier transform (DRIFT)spectroscopy

DRIFT spectroscopic studies of adsorbed H2 at 303 K in nickel

and rhodium exchanged zeolite X samples were carried out in

a Perkin-Elmer Spectrum GX FT-IR instrument equipped with

the selector DRIFT accessory (Graseby Specac, P/N 199900

series) incorporating an environmental chamber (EC) assem-

bly (Graseby Specac, P/N 19930 series). The zeolite samples

were activated in situ at 673 K under a vacuum of 5�

10�3 mmHg at a heating rate of 10 K min�1 using an automatic

temperature controller (Graseby Specac, P/N 19930 series)

connected with the EC. The sample was kept at 673 K for

60 min and then cooled to 303 K, and the IR spectra were

collected under vacuum. H2 was purges on the sample at

1 atm pressure, kept for 60 min, and the IR spectra were

collected. Typically 60 scans were co-added at a resolution of

4 cm�1.

3.3. Adsorption isotherm measurements

Hydrogen adsorption measurements at 77.4 K were done by

static volumetric system (Micromeritics Instrument Corpora-

tion, USA, model ASAP 2010) up to 1 bar pressure. The

samples were activated by heating at a rate of 1 K min�1 to

673 K under vacuum (5� 10�3 mmHg) and the temperature

and vacuum were maintained for about 8 h before the

sorption measurements. The amount of activated sample

was determined from the weight of the samples before as

well as after activation and prior to start of adsorption

measurements.

Hydrogen adsorption measurements at 303 and 333 K were

performed in a gravimetric adsorption analysis system

(Gravimetric Sorption Analyzer, VTI Co., USA) up to 5 bar

pressure. The samples were activated in situ by controlled

heating up to 673 K (heating rate 1 K min�1) under high

vacuum (1� 10�3 mmHg) for 8 h before the sorption measure-

ments. After activation, the samples were allowed to cool

down to the desired temperature and the temperature was

maintained during the analysis using an external water

circulator (PolyScience, USA). Hydrogen with a definite

pressure steps was introduced into the sample chamber.

The increase in the weight of the sample due to hydrogen

adsorption was accurately measured using a microbalance

(Cahn D-200, USA) connected to the sample holder. Deso-

rption of hydrogen on heating the zeolite samples was

performed in gravimetric sorption analyzer. The samples

were heated at a rate of 1 K min�1 up to 423 K after the

adsorption analysis, without removing the samples from

sample holder.

3.4. Surface area measurements

The Langmuir surface area of the cation exchanged zeolites

was calculated by fitting the hydrogen adsorption isotherm

data at 77.4 K in Langmuir isotherm model. The cross-

sectional area of one hydrogen molecule is taken as

0:142 nm2 [24] for the calculation of Langmuir surface area

based on dry weight of adsorbent samples.

4. Computational methodology

NaX, NiX and RhX were modeled using the recently refined

structure reported by Zhu and Seff [25], Bae and Seff [26] and

Busch et al. [27], respectively. Four aluminum atoms of the

unit cell were converted randomly to silicon atoms in order to

get the desired number of 88 aluminum atoms per unit cell

close to Si/Al ratio of 1.2 of the sample used for experimental

measurements. The zeolite framework was built in accor-

dance with Lowenstein’s Al–O–Al avoidance rule. For NaX, the

distribution of the extra framework cations was taken from

Zhu et al., with 28 Na+ ions in SI0 sites, 28 Na+ ions in SII sites,

28 in SIII0 sites, 6-ring and 12-ring, respectively. The distribu-

tion of extra framework cations for NiX was taken as follows:

Ni2+ ions are present at two different SIII0 sites, 20 six

coordinate octahedral Ni2+ ions are found in first SIII0 sites

and 10 Ni2+ ions which are three coordinate occupy second

SIII0 sites, 8 Ni2+ ions in SI0 sites, 4 Ni2+ ions in SII sites and 2

Ni2+ ions in SI sites. In case of RhX the extra framework

cations are located in SIII sites. All structures were energy

minimized for starting simulation with low energy structures

by Cerius2 minimizer using the force field by Ramsahye et al.

[28] with the constraint that the unit cell remains cubic. Only

extra framework cations were considered to be movable

during energy minimization.

The force field used for the gas adsorption simulations was

a modified version of the Cerius2 Watanabe–Austin potential

energy model [20,29]. The total energy of the zeolite frame-

work and adsorbed molecules (U) is expressed as the sum of

the interactions energy between the adsorbate and zeolite

(UAZÞ and that between the adsorbates ðUAAÞ molecules.

U ¼ UAZ þ UAA. (1)

Both UAZ and UAA are written as the sums of pair-wise

additive potentials, uij, in the form

uij ¼ 4�ijsij

rij

!12

�sij

rij

!624

35þ qiqj

rij

!, (2)

where the first term is the repulsion–dispersion LJ potential,

with �ij and sij corresponding to the parameter sets for each

interacting pair that is obtained from �i and si of each species

by using the Lorentz Berthelot mixing rule (i.e., a geometric

combining rule for the energy and an arithmetic one for the

atomic size: �ij ¼ ð�i�jÞ1=2 and sij ¼ ðsi þ sjÞ=2). The second term

is the coulombic contribution between point charges qi and qj

separated by distance rij.

Electric multipoles of hydrogen molecules are described by

a linear electric quadrupole corresponding to the distribution

of three effective charges. For hydrogen, we used a three-

point charge model [20,29,30], where the two outer sites that

are separated by a distance of l ¼ 0:7414 A have a charge q ¼

�0:21e [31] and the third midpoint has a point charge of �2q.

The hydrogen molecule is also considered as the LJ interac-

tion site and the LJ potential parameters are taken from the

literature as given in Table 1 [30,32]. The zeolite framework is

assumed to be semi-ionic, with the partial charges for silicon

and oxygen fixed as þ2:4e and �1:2e, respectively, as usually

considered values [33,34]. The charges on sodium were taken

as þ0:85e to reproduce the heat of adsorption reported [7] at

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lower pressure range of the adsorption at 77.4 K. We chose

þ1:60e and þ2:35e charges for Ni2+ and Rh3+, respectively [33].

The values of partial charges on aluminum in sodium form of

zeolite X and the cation exchanged zeolites were taken as

þ1:55e and þ1:60e, respectively, in order to take into account

the partial charge transfer from the framework, with the

constraint of a global charge of zero for the faujasite system.

The force field LJ parameters [20,33,35] used for the simula-

tions for zeolite framework atoms are given in Table 1.

Absolute adsorption isotherms were then computed using a

grand canonical Monte Carlo calculation algorithm, as

implemented in the sorption module of Cerius2 software

suite [36]. The evolution of the total energy over the Monte

Carlo steps was plotted in order to monitor the equilibration

conditions. The zeolite structure was assumed to be rigid

during the sorption process and the extra framework cations

were maintained fixed in their initial optimized positions.

The Ewald summation method [30] was used for calculating

the electrostatic interactions and the short range interac-

tions, with a cutoff distance of 12 A. The Lennard-Jones

potential for the adsorbate–zeolite interactions and both the

Lennard-Jones and coulombic terms of the adsorbate–adsor-

bate interactions were calculated using the minimum image

convention [29] with a real space potential cut-off distance of

12.0 A. The simulations were performed at a temperature of

77.4, 303 and 333 K using one unit cell of each zeolite for 3–5

million Monte Carlo steps.

5. Results and discussion

Powder X-ray diffraction patterns of nickel and rhodium

exchanged zeolite X samples are shown in Fig. 1. The XRD

patterns show the retention of the zeolite structure after ion

exchange. But loss of crystallinity was observed during the

ion exchange and the dehydration process; the loss of

crystallinity increases with increase in ion exchange levels.

The percentage crystallinity of the ion exchanged zeolite

samples was determined from the X-ray powder patterns and

the values are given in Table 2.

It was reported that the stability of the nickel exchanged

zeolite decreases with increase in nickel ion exchange [25].

The hydrated Ni2+ and Rh3+ ions hydrolyze within the zeolite

cavity, with de-alumination of the zeolite framework [37]. The

crystal structure of the fully dehydrated nickel exchanged

zeolite X shows hydronium ions in the secondary coordina-

tion spheres of hydrolyzed Ni2+ ions. This hydronium ion

acidity is likely to be responsible for the loss of crystallinity up

on further dehydration of both nickel and rhodium ex-

changed zeolite X.

The Langmuir surface area values, crystallinity and chemi-

cal composition of the ion exchanged zeolite X samples are

shown in Table 2. The surface areas of the nickel and rhodium

exchanged zeolite X decreases continuously as the percen-

tage of cation exchange level increases. The decrease in

surface areas may be attributed to the loss of crystallinity of

the zeolite samples as the cation exchange level increases.

Fig. 2 shows the relation between the percentage of cation

exchange, Langmuir surface area and hydrogen adsorption

capacity at 77.4 K for both nickel and rhodium exchanged

zeolite X samples.

Zeolite X is a synthetic aluminum rich analogue of the

naturally occurring mineral faujasite. The 14-hedron with 24

vertices known as the sodalite cavity or b-cage may be viewed

as its principal building block. These b-cages are connected

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10 20 30 40 50 60

NiNaX-75

NaX

Inte

nsity (

a.u

)

2θ

NiNaX-70

NiNaX-65

NiNaX-73

NiNaX-49

10 20 30 40 50 60

RhNaX-73

2θ

RhNaX-55

RhNaX-69

RhNaX-44

Inte

nsity (

a.u

)

RhNaX-29

NaX

Fig. 1 – X-ray powder diffraction pattern of (a) NaX and NiNaX and (b) NaX and RhNaX zeolites.

Table 1 – Lennard-Jones parameters used for adsorba-te–adsorbate and adsorbate–zeolite interactions

Atom type s (A) � ðkcal mol�1Þ References

H 2.81 0.0171 [30]

Si 0.076 0.0370 [20]

Al 1.140 0.0384 [20]

O 3.040 0.3342 [20]

Na 1.746 0.1008 [33]

Ni 2.833 0.015 [35]

Rh 2.929 0.053 [35]

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tetrahedrally at six-rings by bridging oxygen to give double

six-rings (D6Rs, hexagonal prisms) and concomitantly, an

interconnected set of even larger cavities (super cage)

accessible in three dimensions through 12-ring (24-mem-

bered) windows. The Si and Al atoms occupy the vertices of

these polyhedra. The oxygen atoms lie approximately mid-

way between each pair of Si and Al atoms but are displaced

from those points to give near-tetrahedral angles about Si and

Al. Single six rings (S6Rs) are shared by sodalite and super

cage and may be viewed as the entrances to the sodalite units.

Each unit cell has eight sodalite units, eight super cages, 16

D6Rs, 16 12-rings and 32 S6Rs.

Exchangeable cations that balance the negative charge of

the aluminosilicate framework are found within the zeolite

cavities. These are usually found at the following sites shown

in Fig. 3, site I at the center of the D6R, I0 in the sodalite cavity

on the opposite side of one of the D6Rs six-rings from site I, II0

inside the sodalite cavity near an S6R, II at the center of the

S6R or displaced from this point into a super cage, III in the

super cage on a twofold axis opposite to a four-ring between

two 12-rings and III0 somewhat or substantially off III (off the

twofold axis) on the inner surface of the super cage. During

the cation exchange process, two sodium ions are replaced by

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0

100

200

300

400

500

600

700

0 49 65 70 73 75

% of ion exchange

Langm

uir s

urf

ace a

rea (

m2/g

)

0

20

40

60

80

100

120

140

160

180

200H

2 A

dsorp

tion (

mole

cule

/u.c

)Langmuir Surface area

H2 Adsorption at 1bar,77K

0

100

200

300

400

500

600

700

0 29 44 55 69 73

% of ion exchange

Langm

uir s

urf

ace a

rea (

m2/g

)

-10

10

30

50

70

90

110

130

150

H2 a

dso

rptio

n (

mo

lecu

le/u

.c)

Langmuir Surface area

H2 Adsorption at 1bar, 77K

Fig. 2 – Correlation between percentage of ion exchange, Langmuir surface area and hydrogen adsorption capacity of

(a) nickel exchanged zeolite X and (b) rhodium exchanged zeolite X.

Table 2 – Crystallinity, Langmuir surface area and chemical composition of nickel and rhodium exchanged zeolite X

Zeolite sample % Crystallinity Langmuir surface area ðm2 g�1Þ Unit cell formula (on dry basis)

NaX 100 665 Na88Al88Si104O384

NiNaX-49 74 515 Ni21.7Na44.5Al88Si104O384

NiNaX-65 60 398 Ni28.7Na30.6Al88Si104O384

NiNaX-70 45 322 Ni30.8Na26.2Al88Si104O384

NiNaX-73 12 268 Ni32.5Na22.9Al88Si104O384

NiNaX-75 10 256 Ni33.1Na21.7Al88Si104O384

RhNaX-29 54 465 Rh8.5Na62.5Al88Si104O384

RhNaX-44 34 251 Rh12.9Na49.2Al88Si104O384

RhNaX-55 25 116 Rh16.1Na39.5Al88Si104O384

RhNaX-69 18 84 Rh20.2Na27.3Al88Si104O384

RhNaX-73 14 51 Rh21.4Na23.6Al88Si104O384

Fig. 3 – The framework structure of zeolite X, near the center

of the each line segment is an oxygen atom. The numbers

1–4 indicate the different oxygen atoms. Silicon and

aluminum atoms alternate at the tetrahedral intersections,

except that Si substitutes for Al at about 4% of the Al

positions. Extra framework cation positions are labeled

with roman numerals.

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one nickel ion and three sodium ions are replaced by one

rhodium ion.

Hydrogen adsorption isotherms on nickel and rhodium

exchanged zeolite X were measured at 77.4 K and are shown

in Fig. 4. All the hydrogen adsorption isotherms obtained at

77.4 K are Type I in nature, showing adsorption of hydrogen

inside microporous zeolite cavities. The hydrogen adsorption

capacity was found decreasing as the ion exchange levels of

nickel and rhodium increase. At 77.4 K NaX shows a

maximum hydrogen adsorption capacity of 114.7 molecule/

u.c (1.7 wt%) which is comparable to the previous literature

data. For example Kazansky et al. [14] have reported 1.22 wt%

of hydrogen adsorbed at 77.4 K and 0.61 bar. Langmi et al. [8]

have reported hydrogen adsorption capacity of 1.79 wt% and

Du and Wu [7] have reported 2.28 wt% for NaX at 77.4 K and

15 bar. The adsorption of hydrogen at 77.4 K in all the nickel

and rhodium exchanged NaX was highly reversible with

decrease in pressure. Fig. 5 shows the simulated adsorption

isotherms of hydrogen at 77.4 K up to 1 bar in fully nickel and

rhodium exchanged zeolite NaX and also the experimental

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0.0 0.2 0.4 0.6 0.8 1.0 1.20

20

40

60

80

100

120

NaX NiNaX-49 NiNaX-65

NiNaX-70 NiNaX-73 NiNaX-75

mole

cule

/u.c

Pressure (bar)

0.0 0.2 0.4 0.6 0.8 1.0 1.20

20

40

60

80

100

120

mole

cule

/u.c

Pressure (bar)

NaX

RhNaX-69 RhNaX-73RhNaX-55

RhNaX-44RhNaX-29

Fig. 4 – Experimental hydrogen adsorption isotherms at 77.4 K up to 1 bar for (a) nickel exchanged zeolite X and (b) rhodium

exchanged zeolite X.

0.0 0.2 0.4 0.6 0.8 1.0 1.20

20

40

60

80

100

120

140

160

180

200

220

240

Closed symbol - Adsorption

Open symbol - Desorption

RhX simNiX simNaX sim

RhNaX-44 expNiNaX-70 expNaX exp

mole

cule

/u.c

Pressure (bar)

Fig. 5 – Simulated and experimental hydrogen adsorption isotherms of NaX, NiNaX and RhNaX at 77.4 K up to1 bar.

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adsorption–desorption isotherms in nickel and rhodium

exchanged NaX with higher cation exchange levels. We used

force field based simulation for the adsorption isotherm

generation in which nonbonded interactions like van der

Waals and coulombic interactions were considered.

The simulated hydrogen adsorption capacities at 77.4 K up

to 1 bar are higher compared to that of the experimentally

measured values in all the three zeolite samples. The unit cell

structures used for simulation studies are perfectly crystal-

line. However, the experimentally used Ni and Rh exchanged

zeolite samples are not always perfectly crystalline. This

could be the main reason for observance of higher hydrogen

adsorption capacity from simulation calculations compared

to experimental results. Probably a completely ion exchanged

nickel or rhodium zeolite X without any loss of crystallinity

could have shown hydrogen adsorption capacities compar-

able to that of simulated data. However, preparation of such a

sample is not practically feasible as zeolite samples lose

crystallinity on cation exchange. Another possible reason for

higher simulated capacities for hydrogen may be the poor

estimation of LJ parameters used for the simulation of

hydrogen adsorption in the Ni and Rh exchanged zeolites

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0 2 50

1

2

3

4

5

6

7

8

9

10

11

12

NaX

NiNaX-49

NiNaX-65

NiNaX-70

NiNaX-73

NiNaX-75

mole

cule

/u.c

Pressure (bar)

0

5

10

15

20

25

30

35

NaX

NiNaX-49

NiNaX-65

NiNaX-70

NiNaX-73

NiNaX-75

mole

cule

/u.c

Pressure (bar)

1 3 4 60 2 51 3 4 6

Fig. 6 – Experimental hydrogen adsorption isotherms of nickel exchanged zeolite X (a) at 303 K up to 5 bar and (b) 333 K up to

5 bar.

0

5

10

15

20

25

30

35

40

45

50

55

mole

cule

/u.c

Pressure (bar)

00

5

10

15

20

25

30

35

40

45

50

55

RhNaX-73RhNaX-69RhNaX-55

RhNaX-44RhNaX-29NaX

RhNaX-73RhNaX-69RhNaX-55

RhNaX-44RhNaX-29NaX

mole

cule

/u.c

Pressure (bar)

1 2 3 4 5 60 1 2 3 4 5 6

Fig. 7 – Experimental hydrogen adsorption isotherms of rhodium exchanged zeolite X (a) at 303 K up to 5 bar and (b) 333 K up

to 5 bar.

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that were also taken from literature in order to match the

heat of adsorption of hydrogen in zeolite samples

ð�10 KJ mol�1Þ with that of literature data.

The experimental hydrogen uptake capacities at 303 and

333 K up to 5 bar pressure for various amounts of nickel and

rhodium exchanged zeolite X samples are shown in Figs. 6

and 7 , respectively. The figures show that NaX is having

hydrogen adsorption capacities of 7.5 and 25.3 molecule/u.c at

303 and 333 K, respectively, at 5 bar. The hydrogen adsorption

capacity increases with increase in cation exchange levels up

to 70% for nickel and 55% for rhodium loading inside zeolite

X, and then shows a decreasing tendency. This may be due to

the structural breakage of the zeolite occurring at higher

cation loading levels as evidenced from XRD studies. As the

temperature increases from 303 to 333 K hydrogen adsorption

capacity in NaX increases. In case of nickel exchanged zeolite

X the hydrogen adsorption capacity increases from 303 to

333 K but rhodium exchanged zeolite X shows not much

variation in hydrogen uptake capacity at 303 and 333 K. The

experimental hydrogen uptake values in nickel and rhodium

exchanged zeolite X at 77.4 K and 1 bar and also at 303 and

333 K temperatures and at 5 bar are given in Table 3. The

hydrogen adsorption studies in nickel and rhodium ex-

changed zeolite X have been done by repeated measurements

of the hydrogen isotherms at 303 K. The isotherms are not

completely reversible showing the chemisorption of hydrogen

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Table 3 – Experimental hydrogen uptake values of nickeland rhodium exchanged zeolite X

Zeolite sample Maximum hydrogen uptake values(molecule/u.c)

77 K, 1 bar 303 K, 5 bar 333 K, 5 bar

NaX 114.65 7.52 24.28

NiNaX-49 88.80 8.42 28.72

NiNaX-65 72.07 9.12 30.97

NiNaX-70 59.82 11.31 34.51

NiNaX-73 49.31 9.21 26.96

NiNaX-75 50.68 8.82 25.36

RhNaX-29 79.11 27.23 34.85

RhNaX-44 43.63 42.91 38.66

RhNaX-55 22.19 49.29 51.85

RhNaX-69 16.81 35.61 30.07

RhNaX-73 10.40 29.40 27.07

0 40

5

10

15

20

25

30

35

40

45

50

55

RhNaX-55(I)

RhNaX-55(II)

RhNaX-55(III)

NiNaX-70(I)

NiNaX-70(II)

NiNaX-70(III)

RhNaX-55 desorption

NiNaX-70 desorption

mole

cule

/u.c

Pressure (bar)

1 2 3 5 6

Fig. 8 – Repeated hydrogen adsorption isotherms of NiNaX-70 and RhNaX-55 at 303 K up to 5 bar pressure. Desorption

isotherm of hydrogen during the first run is also shown. The roman numerals in bracket denote the number of runs.

0 2 4 50

10

20

30

40

50

mole

cule

/u.c

Pressure (bar)

RhNaX expt 303K

RhNaX expt 333K

NiNaX expt 303K

NiNaX expt 333K

NiX sim 303K

NiX sim 333K

RhX sim 303K

RhX sim 333K

1 3

Fig. 9 – Simulated and experimental hydrogen adsorption

isotherms of nickel exchanged zeolite X and rhodium

exchanged zeolite X at 303 and 333 K up to 5 bar.

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in these samples (Fig. 8). Furthermore, a decrease in the

hydrogen uptake was observed in second hydrogen adsorp-

tion run. This decrease could be due to the hydrogen

consumed for initial reduction of nickel/rhodium ion to

metal. The first hydrogen adsorption run was followed by

the activation of the sample under vacuum at 673 K for 6 h.

Thus activated sample was brought to 303 K under vacuum

and the second hydrogen adsorption run was carried out.

Following this, the third adsorption run was carried out after

activating the sample as done in the second run. Hydrogen

adsorption data obtained in the second and third adsorption

runs show comparable values for both nickel and rhodium

exchanged zeolites (Fig. 8). The data in Fig. 8 show that the

hydrogen consumed for reduction of metal ions is limited and

small compared to chemisorbed hydrogen.

Simulation studies of the adsorption of hydrogen in zeolite

NaX, NiX and RhX were also carried out at 303 and 333 K up to

5 bar pressure and the adsorption isotherms are shown in

Fig. 9. The simulation studies at 303 and 333 K for NaX, NiX

and RhX showed negligible hydrogen adsorption capacities as

compared to experimental data which means our potential

energy functions based on nonbonded van der Waals and

coulombic forces fail to explain adsorption behavior in NaX,

nickel exchanged and rhodium exchanged zeolite X at 303

and 333 K. For simulation studies, we calculated the hydrogen

adsorption isotherms of NaX, NiX and RhX at 303 and 333 K

using the same potential functions as used at 77.4 K. The

negligible hydrogen adsorption capacities at 303 and 333 K

predicted by simulation studies compared to the experimen-

tal results show that interactions other than nonbonded

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2200 2000 1800 1600 1400 1200

1690 cm-1 B

A

Tra

nsm

itta

nce (

a.u

.)

wavenumber (cm-1)

2200 2000 1800 1600 1400 1200

Tra

nsm

itta

nce (

a.u

.)

wavenumber (cm-1)

B

A1933 cm-1

Fig. 10 – (a) DRIFT spectra of nickel exchanged zeolite X, A: activated at 673 K and cooled to 303 K under vacuum, B: H2

adsorbed in A at 1 bar and 303 K. (b) DRIFT spectra of rhodium exchanged zeolite X, A: activated at 673 K and cooled to 303 K

under vacuum, B: H2 adsorbed in A at 1 bar and 303 K.

20 40 60 80 100 12035

30

25

20

15

10

5

0

H2 D

eso

rptio

n (

mo

lecu

le/u

.c)

Temperature °C40 60 80 100 120

50

40

30

20

10

0

H2 D

esorp

tion (

mole

cule

/u.c

)

Temperature °C

Fig. 11 – Hydrogen desorption from (a) NiNaX-70 hydrogen absorbed at 333 K and 5 bar and (b) RhNaX-55 hydrogen absorbed

at 333 K and 5 bar in heating up to 393 K.

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interactions between hydrogen molecule and zeolite struc-

ture are predominant. Hydrogen molecules are known to

have chemical interactions with nickel and rhodium and the

observed hydrogen sorption at 303 and 333 K is due to the

chemisorption of hydrogen on nickel and rhodium metal

which are generated by the reduction of nickel and rhodium

cations inside the zeolite framework on hydrogen adsorption.

The DRIFT spectra of the hydrogen adsorbed on NiNaX-70

and RhNaX-55 at 303 K are shown in Fig. 10. The M–H

stretching frequencies at 1690 and 1933 cm�1 confirm the

formation of hydride in nickel exchanged zeolite X and

rhodium exchanged zeolite X, respectively. The mechanism

of hydride formation inside the zeolite framework could be as

follows:

Mnþþ nH2$M2Hn þ nHþ. (3)

In accordance with this scheme, the dissociation of hydrogen

results in the formation of H+ ions and metal hydride species.

Indeed, the vibration frequencies of the metal–hydrogen bond

observed in IR and Raman are usually located in the

frequency region 160022300 cm�1. A similar type of mechan-

ism was proposed by Baba et al. [38] for the dissociative

adsorption of hydrogen on silver exchanged zeolites.

The H+ formed during hydride formation would compensate

the total framework charge of the zeolite structure. The

hydrogen adsorbed as hydrides inside the zeolites can be

desorbed fully by heating the zeolites up to 393 K. Fig. 11

shows the desorption of hydrogen from NiNaX-70 and

RhNaX-55.

6. Conclusions

Hydrogen adsorption measurements were carried out at

77.4 K up to 1 bar and 303 and 333 K up to 5 bar in NaX and

nickel and rhodium exchanged zeolite X using volumetric and

gravimetric equipments, respectively. The highest hydrogen

adsorption was obtained for NaX than for nickel and rhodium

exchanged zeolite X at 77.4 K up to 1 bar. Physisorption of

hydrogen was obtained at 77.4 K but chemisorption of

hydrogen takes place at 303 and 333 K forming hydrides with

nickel and rhodium. The chemisorbed hydrogen as hydrides

can be desorbed by heating the zeolites up to 393 K. The grand

canonical Monte Carlo simulation studies also support the

physisorption of hydrogen at 77.4 K and chemisorption of

hydrogen at 303 and 333 K, respectively.

Acknowledgments

This work was supported by the Korea Research Foundation

Grant funded by the Korean Government (MOEHRD, Basic

Research Promotion Fund) (KRF-2006-D00003). The authors

express their thanks to Korea Research Foundation, South

Korea, for financial support and Director’s KIER and CSMCRI

for encouragement.

R E F E R E N C E S

[1] Schlapbach L. Hydrogen as a fuel and its storage for mobilityand transport. MRS Bull 2002;27:675–9.

[2] Schlapbach L, Zuttel A. Hydrogen storage materials formobile applications. Nature 2001;414:353–8.

[3] Zuttel A. Materials for hydrogen storage. Mater Today2003;6:24–33.

[4] Breck DW. Zeolite molecular sieves. New York: Wiley-Inter-science; 1978.

[5] Barrer RM. Zeolites and clay minerals as sorbents andmolecular sieves. London: Academic Press; 1978.

[6] Palomino GT, Carayol MR, Arean CO. Hydrogen adsorption onmagnesium-exchanged zeolites. J Mater Chem2006;16:2884–5.

[7] Du X, Wu E. Physisorption of hydrogen in A, X and ZSM-5types of zeolites at moderately high pressures. Chin J ChemPhys 2006;19:457–62.

[8] Langmi HW, Book D, Walton A, Johnson SR, Al-Mamouri MM,Speight JD, et al. Hydrogen storage in ion exchanged zeolites.J Alloys Compd 2005;404–406:637–42.

[9] Zecchina A, Bordiga S, Vitillo JG, Ricchiardi G, Lamberti C,Spoto G, et al. Liquid hydrogen in protonic chabazite. J AmChem Soc 2005;127:6361–6.

[10] Vitillo JG, Ricchiardi G, Spoto G, Zechina A. Theoreticalmaximal storage of hydrogen in zeolitic frameworks. PhysChem Chem Phys 2005;7:3948–54.

[11] Weitkamp J, Fritz M, Ernst S. Zeolites as media for hydrogenstorage. Int J Hydrogen Energy 1995;20:967–70.

[12] Kayiran SB, Darkim FL. Synthesis and ionic exchanges ofzeolites for gas adsorption. Surf Interface Anal 2003;34:100–4.

[13] Regli L, Zechina A, Vitillo JG, Cocina D, Spoto G, Lamberti C,et al. Hydrogen storage in chabazite zeolite frameworks. PhysChem Chem Phys 2005;7:3197–203.

[14] Kazansky VB, Borovkov VY, Serich A, Karge HG. Lowtemperature hydrogen adsorption on sodium forms offaujasites: barometric measurements and drift spectra.Micropor Mesopor Mater 1998;22:251–9.

[15] Langmi HW, Walton A, Al-Mamouri MM, Johnson SR, Book D,Speight JD, et al. Hydrogen adsorption in zeolites A, X, Y andRHO. J Alloys Compd 2003;356:710–5.

[16] Nishimiya N, Kishi T, Mizushima T, Matsumoto A, TsutsumiK. Hyperstoichiometric hydrogen occlusion by palladiumnanoparticles included in NaY zeolite. J Alloys Compd2001;319:312–21.

[17] Bauer HJ, Wagner FE. Hydride formation, magnetic andtransport properties of nickel and nickel based alloys. Pol JChem 2004;78:463–514.

[18] Wang X, Andrews L. Infrared spectra of rhodium hydrides insolid argon, neon, and deuterium with supporting densityfunctional calculations. J Phys Chem A 2002;106:3706–13.

[19] Razmus DM, Hall CK. Prediction of gas adsorption in 5Azeolites using Monte Carlo simulation. AIChE J1991;37:769–79.

[20] Watanabe K, Austin N, Stapleton MR. Investigation of the airseparation properties of zeolite types A, X and Y by MonteCarlo simulations. Mol Simul 1995;15:197–221.

[21] Song MK, No KT. Molecular simulation of hydrogen adsorp-tion in organic zeolite. Catal Today 2007;120:374–82.

[22] Berg AWC, Bromley ST, Wojdel JC, Jancen JC. Adsorptionisotherms of H2 in microporous materials with the SODstructure: a grand canonical Monte Carlo study. MicroporMesopor Mater 2006;87:235–42.

[23] Fuchs AH, Cheetham AK. Adsorption of guest molecules inzeolitic materials: computational aspects. J Phys Chem B2001;105:7375–82.

ARTICLE IN PRESS

I N T E R N A T I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y 3 3 ( 2 0 0 8 ) 7 3 5 – 7 4 5744

Author's personal copy

[24] Zuttel A, Sudan P, Mauron P, Wenger P. Model for hydrogenadsorption on carbon nanostructures. Appl Phys A2004;78:941–6.

[25] Zhu L, Seff K. Reinvestigation of the crystal structure ofdehydrated sodium zeolite X. J Phys Chem B 1999;103:9512–8.

[26] Bae D, Seff K. Crystal structure of zeolite X nickel (II)exchanged at pH 4.3 and partially dehydrated, Ni2(NiOH)35

(Ni4AlO4)2(H3O)46Si101Al91O384. Micropor Mesopor Mater2004;40:219–32.

[27] Busch F, Jaeger N, Ekloff GS. Growth of rhodium dispersionsin zeolite NaX: identification of a precursor state by Rietveldrefinement. Zeolites 1996;17:244–9.

[28] Ramsahye NA, Bell RG. Cation mobility and the sorption ofchloroform in zeolite NaY: molecular dynamics study. J PhysChem B 2005;109:4738–47.

[29] Cerius2 User Guide: Forcefield-based simulations. MolecularSimulations Inc, San Diego, April 1997.

[30] Allen MP, Tildesley DJ. Computer simulation of liquids.Oxford, UK: Clarendon; 1987.

[31] Weinberger B, Darkrim FL, Kariyan SB, Gicquel A, Levesque D.Molecular modeling of H2 purification on Na–LSX zeolite andexperimental validation. AIChE J 2005;51:142–8.

[32] Murad S, Gubbins KE. Molecular dynamics simulation ofmethane using a singularity free algorithm. In: Lykos P,editor. Computer modeling of matter ACS symposium series.Washington: American Chemical Society; 1978. p. 62–71.

[33] Maurin G, Llewellyn P, Poyet T, Kuchta B. Influence ofextraframe work cations on the adsorption of X-faujasitesystems: micro calorimetry and molecular simulations.J Phys Chem B 2005;109:125–9.

[34] Kramer GJ, Farragher NP, van Beeset BWH, van Santen RA.Interatomic forcefields for silicas, aluminophosphates, andzeolites derivation based on ab initio calculations. Phys Rev B1991;43:5068–80.

[35] Rappe AK, Casewit CJ, Colwell KS, Goddard III WA, Skiff WM.UFF, a full periodic table forcefield for molecular mechanicsand molecular dynamics simulations. J Am Chem Soc1992;114:10024–35.

[36] Cerius2, v.4.2. Accelrys, Inc., San Diego, CA; 1999.[37] Olson DH. Crystal structure of the zeolite nickel faujasite.

J Phys Chem 1968;72:4366–73.[38] Baba T, Komatsu N, Sawada H, Yamaguchi Y, Takahashi T,

Sugisawa H, et al. 1H magic angle spinning NMR evidence fordissociative adsorption of hydrogen on Ag+-exchanged A andY- zeolites. Langmuir 1999;15:7894–6.

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