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Chemical Engineering Journal 187 (2012) 10– 15

Contents lists available at ScienceDirect

Chemical Engineering Journal

jo ur n al homep age: www.elsev ier .com/ locate /ce j

alladium nanoparticles decorated graphite nanoplatelets for room temperaturearbon dioxide adsorption

shish Kumar Mishra, Sundara Ramaprabhu ∗

lternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras,hennai 600036, India

r t i c l e i n f o

rticle history:eceived 29 September 2010eceived in revised form2 December 2010ccepted 5 January 2011

eywords:alladium nanoparticles

a b s t r a c t

In order to counteract the greater level of green house gases including CO2, adsorption process hasbeen found as one of the solution. Present work focuses on the high pressure CO2 adsorption studyof nanocomposite comprising of Pd nanoparticles decorated graphite nanoplatelets (GNP). Graphitenanoplatelets (GNP) were prepared by acid intercalation followed by thermal exfoliation. Function-alized graphite nanoplatelets (f-GNP) were prepared by further treatment of GNP in acidic medium.Palladium (Pd) nanoparticles were decorated over f-GNP surface by chemical method. Nanocompositewas characterized by electron microscopy, X-ray powder diffraction pattern, BET measurement, Raman

raphite nanoplateletsarbon dioxide adsorption

spectroscopy and FTIR spectroscopy techniques. The CO2 adsorption capacity was measured using highpressure Seiverts’ apparatus by incorporating van der Waals corrections and adsorption of CO2 was con-firmed by FTIR spectroscopy. A remarkable enhancement of 15–20% is obtained in CO2 adsorption bydecorating Pd nanoparticles over functionalized graphite nanoplatelets. Dubinin–Radushkevitch (DR)equation is applied to the adsorption isotherm at room temperature and the results have been discussed.

. Introduction

The increasing demand of fossil fuel energy poses a great chal-enge in the control of CO2 emissions in Earth’s atmosphere. Amongll the means to maintain optimum CO2 level in the atmosphere, theeological sequestration and storage options of CO2 are found envi-onmentally and economically beneficial and thus have attractedorldwide attention among researchers [1,2]. As an alternativeethod for CO2 capture, adsorption can be considered to be one

f the more promising methods, offering potential energy sav-ngs compared to absorbent systems, especially with respect toompression costs. Pressure swing adsorption (PSA) using solidorbents has gained interest due to its low energy and capitalnvestment costs [3–6]. In terms of achieving high adsorptionapacities, activated carbons (ACs), zeolite-based molecular sievesnd chemically modified porous silica have shown much promise.Cs are often preferred over zeolites because of their relativelyoderate strengths of adsorption for gases, which facilitates easier

esorption [7–10]. CO2 adsorption capacities of activated carbons

epend not only on their pore structure but also on the surfacehemistry properties [11,12]. Recently, enhanced CO2 adsorptionapacity is reported in nitrogen treated AC [13]. Along with ACs,

∗ Corresponding author.E-mail address: ramp@iitm.ac.in (S. Ramaprabhu).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.01.024

© 2011 Elsevier B.V. All rights reserved.

one dimensional carbon based nanostructures like single walledand multi walled carbon nanotubes also provide a good alternativefor CO2 adsorption due to their large surface area and high porosity[14]. Some metal based complexes such as metal organic frame-works, titanium covered graphene and palladium based complexesand metal oxides like Fe2O3, Al2O3 have been recently investigatedfor CO2 adsorption [15–19].

Metal based complexes can be good adsorbent for CO2 dueto the affinity of metals towards polarized CO2 molecules. Butmetal nanoparticles have greater tendency to agglomerate andthereby reducing their performance. Hence to avoid this prob-lem, Pd nanoparticles were decorated over graphite nanoplatelets(GNP) in the present work. This Pd decorated GNP nanocompositeincludes the adsorption property of GNP and Pd nanoparticles. Costeffective and easy preparation process of GNP, leads to the utiliza-tion of this Pd decorated GNP nanocomposite for industry. This lowcost nanocomposite can be used as CO2 adsorbent at high pres-sures, especially for the storage of CO2 coming out from thermalpower plants and cement industries by compressing the exhaustwith some means. In addition, desorption of adsorbed CO2 at hightemperatures suggests the possible utilization of adsorbed CO2 forfood packaging. CO2 adsorption capacity of Pd-GNP was studied

using high pressure Seiverts’ apparatus at three different temper-atures (25, 50 and 100 ◦C). Adsorption capacity was calculated byincorporating van der Waals corrections in gas equation and roomtemperature isotherm was treated with Dubinin–Radushkevitch

A.K. Mishra, S. Ramaprabhu / Chemical Engineering Journal 187 (2012) 10– 15 11

d d) E

ev

2

2

HsgatwfTwd

dianfiaw

Fig. 1. (a) SEM, (b) TEM images and (c an

quation. Desorption of CO2 was performed at 150 ◦C under highacuum (∼10−9 bar).

. Experimental

.1. Preparation of Pd-GNP nanocomposite

Graphite was vigorously stirred with conc. HNO3 and conc.2SO4 in 1:3 ratios for 3 days. Vigorous stirring of graphite under

trong acidic medium may cause the formation of acid intercalatedraphite. This intercalated graphite was further thermally exfoli-ted at 1000 ◦C. This thermal shock may lead to the destacking ofhe graphite plates and hence formation of GNP [20,21]. This GNPas further treated with conc. HNO3, which introduces hydrophilic

unctional groups (–COOH, –C O, and –OH) at the surface of GNP.hese functionalized graphite nanoplatelets (f-GNP) were furtherashed several times with water to achieve pH 7 followed byrying.

Decoration of Pd nanoparticles over the f-GNP surface wasone by chemical technique. Functionalized GNP was suspended

n de-ionised water by ultrasonication method. Functional groupst the surface of GNP provide the anchoring sites for metal

anoparticles and hence better decoration of nanoparticles at the

-GNP surface. Pd-GNP nanocomposite was prepared by reduc-ng aqueous acidic solution of PdCl2 and f-GNP with 0.1 M NaBH4nd 1 M NaOH solutions followed by washing to neutralize withater [22].

DAX analysis of Pd-GNP nanocomposite.

2.2. Characterization techniques

Pd-GNP nanocomposite was characterized by FEI QUANTA3D scanning electron microcope (SEM) and Philips JEOL CM12transmission electron microscope (TEM). X-ray powder diffrac-tion analysis was performed by X’ Pert Pro PANalytical X-raydiffractometer. Raman analysis was performed by using HORIBAJOBIN YVON HR800UV Confocal Raman spectrometer, while FTIRstudy was performed by using PERKIN ELMER Spectrum One FT-IR spectrometer. Adsorption studies for CO2 gas was performedusing Seivert’s apparatus. Surface texture study was performed byN2 adsorption–desorption using Micromeritrix ASAP 2020 surfacearea analyzer. Surface acidity measurement was performed by NH3adsorption using Micromeritrix Autochem II 2920 analyzer.

2.3. Adsorption studies

Adsorption studies were carried out using high pressureSeivert’s apparatus, which has been used well for high pressurehydrogen sorption studies [23]. The experimental setup consistsof stainless tubes, tees, elbow joints and needle valves procuredfrom NOVA, Switzerland. They can withstand up to 1000 bar pres-sure. The pressure transducers procured from Burster, Germany are

used to monitor the gas pressure in the range 0–50 bar. Numbers ofcycles were performed to recheck the adsorption capacity and thevalues were found to be consistent within the experimental error.Samples were degassed at 150 ◦C under high vacuum (10−9 bar) to

12 A.K. Mishra, S. Ramaprabhu / Chemical Engineering Journal 187 (2012) 10– 15

20 40 60 80

C (

002

)

Inte

nsi

ty (

a.u

.)

2θ (degree)

f-GNP

C (

002)

Pd

(31

1)

Pd

(22

0)

Pd

(20

0)

Pd

(11

1)

Pd -GNP

rur

3

3

wpr(5peepPto

op6cnP

3

ptia

Ntstwp

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

Qu

anti

ty A

dso

rbed

(cm

³/g

ST

P) N

2-Adso rption

N2- Desorption

to the higher values (279.4, 428.3, 629 cm−1), while no shift wasobserved in G-band. Shift in D-band corresponds to the attachmentof CO2 at functional groups [26]. Peak shift at lower wave num-ber may correspond to the strong interaction between Pd metal

0.00

0.01

0.02

0.03

TC

D s

ign

al (

a.u

.)

Pd-GNP

Fig. 2. X-ray diffraction pattern for f-GNP and Pd-GNP nanocomposite.

egain their adsorptive sites for CO2 and the same sample againsed for adsorption study and adsorption capacity was found to beepeatable.

. Results and discussion

.1. Morphological and structural studies

The morphological studies of the as-prepared nanocompositeere carried out by electron microscopy techniques. SEM and TEMhotographs of nanocomposite clearly suggest the uniform deco-ation of Pd nanoparticles over the surface of f-GNP (Fig. 1(a) andb), respectively). The particle size of Pd nanoparticles ranges from

to 8 nm. Energy dispersive X-ray (EDAX) analysis confirms theurity of the sample by only indicating the presence of C and Pdlements (Fig. 1(c)). A small fraction of Oxygen is due to the pres-nce of functional groups. EDAX analysis was performed at differentortion of the sample and over large area to identify the loading ofd nanoparticles. Histogram of loading of Pd in different regions ofhe sample is plotted in Fig. 1(d). This suggests the average loadingf around 25 wt% for Pd over graphite nanoplatelets.

XRD pattern of f-GNP (Fig. 2) exhibits a carbon peak at 2� valuef 26.2◦ which corresponds to the graphitic layered structure. XRDattern of Pd decorated GNP shows extra peaks at 40.28◦, 46.86◦,7.95◦ and 82.27◦. These peaks correspond to the face centeredubic structure of Pd nanoparticles. Thus XRD pattern of Pd-GNPanocomposite suggests the formation of two phases, one of cubicd nanoparticles and other of graphite structure of GNP [24].

.2. BET surface area and surface acidity measurement

BET surface area measurement of Pd-GNP nanocomposite waserformed with N2 adsorption–desorption isotherm. Fig. 3 showshe N2 adsorption–desorption isotherm for Pd-GNP nanocompos-te. It suggests the surface area of 10.32 m2/g with pore diameterround 20 nm and pore volume 0.054 cm3/g.

Surface acidity measurement of Pd-GNP was performed withH3 gas adsorption. Surface acidity measurement in terms of

hermal conductivity detector signal is shown in Fig. 4. This

hows almost negligible surface acidity of nanocomposite. In addi-ion, 5 mg of nanocomposite is dispersed in 10 ml of de-ionizedater with pH 6.4. This nanocomposite dispersed solution showsH 5.8, suggesting very slight acidic nature of nanocomposite.

Relative Pre ssure (P/Po)

Fig. 3. N2 adsorption desorption isotherm of Pd-GNP nanocomposite.

Thus negligible acidity of nanocomposite and low surface area ofnanocomposite suggests that the adsorption of CO2 is associatedwith interaction of CO2 molecule with Pd nanoparticles along withporous adsorption in nanocomposite.

3.3. Raman spectroscopy analysis

Fig. 5 shows the Raman spectrum of Pd-GNP nanocomposite.Along with the D-band (1338.2 cm−1) and G-band (1567.2 cm−1)corresponding to the defects and graphitic structure of GNP respec-tively, other peaks (274.7, 423.7 and 624.5 cm−1) were observed atlower wave number in the case of Pd-GNP nanocomposite [25].These extra peaks arise due to the formation of Pd nanoparticlesover the surface of GNP and may be attributed to the vibrations ofPd–C bond at the surface of GNP.

In the case of CO2 adsorbed Pd-GNP nanocomposite D-band shiftto 1342.3 cm−1 and the peaks corresponds to the Pd–C bonds shift

0 200 400 60 0

Tempera ture (°C)

Fig. 4. Surface acidity measurement of Pd-GNP nanocomposite.

A.K. Mishra, S. Ramaprabhu / Chemical Engineering Journal 187 (2012) 10– 15 13

400 800 120 0 16 00 200 0

Pd-GNPPd-GNP+CO2

274.7

Inte

nsi

ty (

a.u

.)

Wave number (cm-1)

624.5 1338. 2

1567.2

428.3

Fn

na

3

s1([ot

ttwipmn

Fp

0 3 6 9 12 150

2

4

60

2

4

60

2

4

6

1000C

Peq

(bar)

500C

CO

2 ad

sorp

tio

n (

mo

le/g×10

-3)

250C

Functionali zed GN P Pd -GN P

ig. 5. Raman spectra of Pd-GNP nanocomposite and CO2 adsorbed Pd-GNPanocomposite.

anoparticles and polarized CO2 molecule, which leads to the highdsorption of CO2 in nanocomposite compared to the GNP [18].

.4. Fourier transform infrared spectroscopy analysis

Fig. 6 shows the FTIR spectrum of Pd-GNP nanocomposite. FTIRtudy of nanocomposite confirms the presence of >C C (1633,720 cm−1), >C O (1086 cm−1), –CH2 (2852, 2923 cm−1) and –OH3435 cm−1) functional groups on the surface of nanocomposite27]. Peak at 1384 cm−1 corresponds to the in-plane OH bendingf carboxylic group, while peak at 780 cm−1 may corresponds tohe interaction of Pd with carbon of GNP.

In the case of CO2 adsorbed Pd-GNP nanocomposite, an addi-ional peaks was noticed at 2330 cm−1, which may be attributedo the asymmetric stretching of CO2 molecules [16,21,28]. Alongith an extra peak, shifts were observed with peaks correspond-

−1 −1

ng to >C C (1633 cm ), >C O (1086 cm ) and Pd–C interactioneak (784 cm−1) to the higher frequency. This suggests that CO2olecules have more affinity towards functional groups and Pd

anoparticles attached to the GNP surface [26]. The CO2 molecule is

500 10 00 1500 200 0 25 00 3000 35 00 40000

25

50

75

100

125

780

1086

1384

1633

2852

2923

784

10741384

1636 3435

2923

2852

2330Inte

nsi

ty (

a.u

.)

Wave nu mber (cm-1)

Pd-GNPPd-GNP+CO2

ig. 6. FTIR spectra of Pd-GNP nanocomposite and CO2 adsorbed Pd-GNP nanocom-osite.

Fig. 7. Adsorption isotherms at three different temperatures for f-GNP and Pd-GNPnanocomposite.

linear with negative partial charge on the oxygen atoms and a posi-tive partial charge on the carbon center. This polarization allows fordifferent interactions with potential coordination centers in metalcomplexes [18]. Thus more CO2 adsorption and the correspond-ing shift in Pd–C peak may be attributed to the good interactionbetween polarized CO2 molecule and less electronegative Pd metalnanoparticles.

3.5. Adsorption isotherm studies

High pressure adsorption capacity was calculated using van derWaals corrections in the gas equation. Adsorption of CO2 over Pd-GNP nanocomposite was studied at three different temperatures(25, 50 and 100 ◦C) and high pressures (3–12 bar). Amount of CO2adsorbed in mole was measured by following equations:

�nadsorbed = ni − (n′ + n′′) (1)

where ‘ni’ is the number of mole of CO2 in the initial volume ‘Vi’at the known initial pressure ‘Pi’. n′ is the number of moles in ‘Vi’at equilibrium pressure ‘Peq’ and n′′ is the number of moles in cellvolume Vc at equilibrium pressure Peq. Where ni, n′ and n′′ can becalculated by using following equations:

abni3 + aVini

2 + (RT + Pib)Vi2ni − PiVi

3 = 0 (2)

abn′3 + aVin′2 + (RT + Pib)Vi

2n′ − PiVi3 = 0 (3)

abn′′3 + aVin′′2 + (RT + Pib)Vi

2n′′ − PiVi3 = 0 (4)

where ‘T’ is the cell temperature and ‘R’ is the universal gas constant.‘a’ and ‘b’ are the van der Waals coefficient for CO2 gas.

Fig. 7 shows the comparative adsorption behavior of Pd-GNPnanocomposite and functionalized GNP (f-GNP) at three dif-ferent temperatures. Maximum adsorption capacities of 0.0051and 0.0043 mol/g were found for Pd-GNP nanocomposite and f-GNP respectively, at 11 bar and room temperature (25 ◦C). At11 bar and 50 ◦C maximum adsorption capacities were 0.0045and 0.0038 mol/g for Pd-GNP nanocomposite and f-GNP. At 11 bar

and 100 ◦C maximum adsorption capacities were 0.0041 and0.0033 mol/g for Pd-GNP nanocomposite and f-GNP. It is evidentthat at each temperature the adsorption capacity increases withthe increase in equilibrium pressure, which may be attributed to

14 A.K. Mishra, S. Ramaprabhu / Chemical Engineering Journal 187 (2012) 10– 15

0 25 50 75 100 125 15 01.0

2.0

3.0

4.0

5.0

6.0

Ad

sorb

ed C

O2 (

mo

le/g×1

0-3)

T (ºC)

4 ba r 6 ba r 8 ba r 10 bar 12 bar

F

tNcGbwtp

3

utea1Clthlwlctsc

3

ttwmmitiptt

2.0x105 4.0x105 6.0x1 05 8.0x 105 1.0x1 06-6.4

-6.2

-6.0

-5.8

-5.6

-5.4

-5.2

-5.0

-4.8

ln (

N)

(N in

mo

l/g)

(T ln P /P)2 (K2)

Pd-GNP DR fit

out the possibility for agglomeration of Pd nanoparticles. A large

ig. 8. Temperature dependence for adsorption capacity of Pd-GNP nanocomposite.

he condensation of CO2 molecules in pores at high pressures.early 15–20% enhancement was observed in CO2 adsorptionapacity of Pd-GNP nanocomposite compared to functionalizedNP at the same temperature and equilibrium pressure. This maye attributed to the strong interaction of polarized CO2 moleculeith Pd metal nanoparticles (interaction between partially nega-

ive charge oxygen atoms of polarized CO2 molecule and partiallyositive Pd metal due to its low electronegativity).

.6. Comparison with other solid sorbents

Zhang et al. have also reported around 20% enhancement in CO2ptake by modifying the activated carbon with nitrogen at roomemperature [13]. High pressure CO2 adsorption study on differ-nt metal organic frameworks by Millward and Yaghi exhibits CO2dsorption capacity ranging from 0.002 to 0.008 mol/g at nearly1 bar pressure [15]. The present study also shows comparableO2 adsorption capacity at same pressure. Ease of preparation and

ow production cost give an edge to Pd-GNP nanocomposite overhese metal organic frameworks as CO2 adsorbent. Cavenati et al.ave reported around 0.0032 mol/g of CO2 adsorption in 13X zeo-

ite at 12 bar pressure and room temperature [29]. The presentork shows higher (0.0051 mol/g) CO2 adsorption capacity at even

ower pressure (11 bar). This nanocomposite sustains its adsorptionapacity up to 0.0041 mol/g even at 100 ◦C, while in zeolites it dras-ically decreases with increase in temperature. These results clearlyuggest the superiority of Pd-GNP nanocomposite over other lowost sorbents like zeolites.

.7. Temperature dependence

Fig. 8 clearly shows the CO2 adsorption dependence on tempera-ure for Pd-GNP nanocomposite. This suggests that with increase inemperature the adsorption capacity of nanocomposite decreases,hich may be attributed to the higher kinetic energy of CO2 gasolecules at higher temperatures. At lower temperatures, the CO2olecules possess lower kinetic energies and therefore a good

nteraction between CO2 molecules and nanocomposite increaseshe adsorption capacity. At low pressures CO2 adsorption capac-ty increase linearly with decrease in temperature, while at higher

ressures it does not follow linear behavior. This may be attributedo the large condensation of gas at high pressure and low tempera-ure due to the interaction between the gas molecules themselves.

0

Fig. 9. Dubinin–Radushkevitch fit for Pd-GNP nanocomposite.

3.8. Dubinin–Radushkevitch (DR) equation

The CO2 adsorption isotherms at room temperature for Pd-GNPnanocomposite was treated by the Dubinin–Radushkevitch (DR)equation [30,31]. DR equation can be represented as follows:

ln W = ln W0 +(

R

E

)2[T ln

(P0

P

)]2(5)

where ‘W’ is the amount of adsorbed CO2, ‘W0’ is the microporousvolume and ‘E’ is the characteristic adsorption energy. ‘W’ is givenby-

W = �n × M

�(6)

where ‘�n’ is the amount of CO2 adsorbed in mol/g, ‘M’ is the molec-ular weight (M = 44 g) and ‘�’ is the density of the CO2 adsorbate atthe temperature T (� = 0.85 g/cc at 298 K). D.R. equation is based onthe postulate that the mechanism for adsorption in micropores isthat of pore filling rather than a layer-by-layer formation of a filmon the walls of the pores.

Fig. 9 shows the DR equation fit for Pd-GNP nanocomposite atroom temperature (25 ◦C). It clearly demonstrates that the mea-sured adsorption isotherms obey the DR equation over the widerange of pressure and slightly deviates at higher pressure. Thissuggests that adsorption in Pd-GNP nanocomposite is mainly dueto micropores filling along with some order of mesoporous con-densation. Good interaction between polarized CO2 molecule andPd metal nanoparticle may induce this effect and hence more CO2adsorption was observed for Pd-GNP nanocomposite compared tof-GNP. Micropore volume for Pd-GNP nanocomposite was found tobe 0.515 cc/g and characteristic adsorption energy was 5.5 kJ/mol.Micropore volume is found to be high with DR fit, which may beattributed to the additional CO2 adsorption through mesoporouscondensation, interaction of CO2 with Pd nanoparticles and func-tional groups along with micropores filling.

4. Conclusion

The present work demonstrates uniform decoration of Pdnanoparticles over functionalized graphite nanoplatelets ruling

enhancement of 15–20% in CO2 adsorption capacity was achievedby Pd-GNP compared to the f-GNP. Raman and FTIR spectroscopyclearly suggests the greater affinity of CO2 molecules towards Pd

cal En

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A.K. Mishra, S. Ramaprabhu / Chemi

anoparticles, which may be attributed to the interaction of polar-zed CO2 molecule with Pd nanoparticles. Low cost and desorptiont 150 ◦C under high vacuum suggests the effective reutilization ofd-GNP nanocomposite for CO2 adsorption and hence can be useds CO2 adsorbent at high pressure for the exhaust of thermal powerlants, cement industries and in food packaging industries.

cknowledgements

The authors acknowledge the supports of IIT Madras and DST,ndia. One of the authors (Ashish) is thankful to DST India for pro-iding the financial support. Authors are also thankful to SAIF, IITadras for helping in FTIR analysis.

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