Carbon Dioxide Conversion Over Carbon-Based Nanocatalysts

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Copyright © 2013 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 13, 4825–4837, 2013

Carbon Dioxide Conversion OverCarbon-Based Nanocatalysts

Mehrnoush Khavarian1, Siang-Piao Chai2, and Abdul Rahman Mohamed1�∗1School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia

2School of Engineering, Monash University, Jalan Lagoon Selatan, 46150 Bandar Sunway, Selangor, Malaysia

The utilization of carbon dioxide for the production of valuable chemicals via catalysts is one of theefficient ways to mitigate the greenhouse gases in the atmosphere. It is known that the carbon diox-ide conversion and product yields are still low even if the reaction is operated at high pressure andtemperature. The carbon dioxide utilization and conversion provides many challenges in exploringnew concepts and opportunities for development of unique catalysts for the purpose of activating thecarbon dioxide molecules. In this paper, the role of carbon-based nanocatalysts in the hydrogenationof carbon dioxide and direct synthesis of dimethyl carbonate from carbon dioxide and methanol arereviewed. The current catalytic results obtained with different carbon-based nanocatalysts systemsare presented and how these materials contribute to the carbon dioxide conversion is explained.In addition, different strategies and preparation methods of nanometallic catalysts on various carbonsupports are described to optimize the dispersion of metal nanoparticles and catalytic activity.

Keywords: Carbon Nanocatalyst, Carbon Dioxide, Catalytic Activity, Hydrogenation, DimethylCarbonate.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48252. CO2 Hydrogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4827

2.1. Methanol-Mediated Reaction . . . . . . . . . . . . . . . . . . . . . . . 48272.2. Non-Methanol-Mediated Reaction . . . . . . . . . . . . . . . . . . . 4828

3. Synthesis of Dimethyl Carbonate from CO2 and Methanol . . . . 48313.1. Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4832

4. Preparation of Carbon-Supported Nanocatalysts . . . . . . . . . . . . 48335. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48346. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4834

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4835References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4835

1. INTRODUCTION

The emissions of greenhouse gases, including NOx, SOx,carbon dioxide (CO2� and methane (CH4�, from fuels com-bustion and manufacturing plants in various energy sys-tems have caused global warming and climate changes.1

Various strategies have been carried out to mitigatethe emission of CO2 through separation and capture,2–4

storage,5 adsorption,6 transformation,7�8 and direct utiliza-tion of CO9–11

2 as a cheap and highly abundant C1 feed-stock. However, direct utilization of CO2 is challengingbecause CO2 is thermodynamically stable. Indeed, using

∗Author to whom correspondence should be addressed.

CO2 as chemical feedstock is limited to a few processes inthe industry, including synthesis of urea and its derivatives,salicylic acid, and carbonates.5

Substitute CO2 for CO as feedstock in the productionof methanol (CH3OH) and dimethyl carbonate (DMC)have received serious attention. CO2 hydrogenation hasbeen more intensively investigated recently, due to thefundamental and practical significance in the context ofcatalysis, surface science, nanoscience and nanotechnol-ogy, biology and environmental science. Both homoge-neous and heterogeneous catalysts have been used in thehydrogenation of CO12–15

2 and direct synthesis of DMCfrom CO2 and methanol.16�17 Although hydrogenation ofCO2 and direct synthesis of DMC from CO2 and methanolthrough the heterogeneous catalytic routes are the mostpromising methods, the product yield is relatively lowowing to CO2 being thermodynamically stable and kineti-cally inert. Many attempts have been carried out to solvethese problems and nanocatalysts with high performanceare investigated thoroughly.Trends in technological and scientific progress in

nanocatalysis lead to the development of high efficiencyand selectivity catalysts. For this reason, nanosized cat-alytic particles have been the recent focus.18 Traditionalmetal nanoparticles in CO2 conversion tend to sinter athigh reaction temperature. The catalytic activity of nickelor nickel alloy nanoparticles lost in a relatively short

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Carbon Dioxide Conversion Over Carbon-Based Nanocatalysts Khavarian et al.

period of time due to their quick aggregation. Furthermore,some of the determined substrate particles may react withother substrate particles or the catalysts. Therefore, choos-ing a suitable catalyst support for metal nanoparticles playsa vital role in the catalytic activity and deactivation of thecatalyst in the reaction.19

In the last decades, the use of carbon materials as sup-ports for heterogeneous catalysts has been very attractiveas they suit most of the characteristics required for thecatalytic reactions. The catalytic activity of carbon mate-rials is strongly correlated to the surface area and sur-face functional groups present, such as oxygen surfacegroups, functional acids and the OH groups.20�21 Interac-tions of these functional groups on the surface of carbonmaterial with the metal cations can potentially result indispersion of the active metal nanoparticles. Modificationof the morphology and pore structure of carbon materialscan be done by manipulating the preparation conditions.Impregnation,22–25 deposition precipitation,25�26 sol–gel27

and electroless deposition28�29 are amongst methods that

Mehrnoush Khavarian earned her B.S. degree (2006) from University of Kashan (Iran)and M.S. degree (2010) from Universiti Sains Malaysia in the field of Chemical Engi-neering. Currently, she is pursuing her Ph.D. degree at Universiti Sains Malaysia. Herresearch interests are conversion of carbon dioxide technology, synthesis and developmentof nanomaterials.

Siang-Piao Chai received his B.E. and Ph.D. degrees in Chemical Engineering from Uni-verisiti Sains Malaysia. He is currently working as a senior lecturer at the School of Engi-neering in Monash University, Sunway Campus. His research interests include catalysisand reaction engineering, membrane technology, natural gas processing technology, surfaceengineering and development of nanomaterials.

Abdul Rahman Mohamed received his Ph.D. degree in chemical engineering from theUniversity of New Hampshire, USA, in 1993. Currently, he is a Professor at the Schoolof Chemical Engineering and the Director of the Centre for Engineering Excellence at theUniversiti Sains Malaysia. His research interests focus on catalysis and reaction engineer-ing, air pollution and wastewater control engineering, fuel technology, nanoscience, andnanotechnology.

have been extensively studied in the development of car-bon supported nanocatalysts.There are few advantages associated with using carbon

materials rather than typical oxide supports. They include(1) being inert in most reaction conditions;(2) the metal oxide precursor can be easily reduced to ametallic state on the carbon support;(3) providing high dispersion of active metal in nano-scalewithin its small pore;(4) being resistant to acidic/basic media;(5) having a stable structure at high temperature (evenabove 750 �C) and(6) it can be prepared to have a different physical appear-ance such as fibers, pellets and granules.

Nevertheless, the disadvantages of carbon supports aretheir ease of gasification and poor reproducibility.30 Thecatalytic properties of carbon can be modified to satisfythe specific application of a proper treatment. Two variantsof carbon materials have been used to prepare carbon sup-ported nanocatalyst: amorphous carbons such as activated

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Khavarian et al. Carbon Dioxide Conversion Over Carbon-Based Nanocatalysts

carbon (AC) and crystalline carbons such as diamond,graphite and carbon nanotubes (CNTs).31

This review focuses on the developments of carbon-based nanocatalysts in CO2 utilization for environmentaland energy applications. The applications of the catalystsin the field of hydrogenation of CO2 and direct synthesisof DMC from CO2 are discussed. The catalytic activitybehavior and the physical and chemical properties of thecarbon nanocatalysts as well as the reasons of deactivationduring the mentioned reactions are investigated. The chal-lenges and potential applications of these carbon-supportedcatalysts are also discussed. In addition, different strate-gies and preparation methods of nanometallic catalysts onvarious carbon supports are described to optimize the dis-persion of metal nanoparticles and catalytic activity.

2. CO2 HYDROGENATION

One of the economical and effective methods to utilizegreat amounts of emitted CO2 is hydrogenation of CO2.The main products of CO2 hydrogenation can fall intotwo categories of fuels and chemicals (Fig. 1). The prod-ucts of CO2 hydrogenation such as CH3OH, dimethyl ether(DME) and hydrocarbons are important fuels in internalcombustion engines. Furthermore, CH3OH and formic acidare raw and intermediate materials for many chemicalindustries.Production of hydrocarbons from CO2 hydrogenation

is essentially a modification of the Fischer–Tropsch (FT)synthesis, where CO2 is replaced by CO. Catalyst com-ponent for CO2 hydrogenation is comparable to that ofFT synthesis but it is altered to maximize the productionof hydrocarbons. The catalytic synthesis of light hydro-carbons (C1–C6� from synthesis gas (syngas) in FT reac-tions has been studied over CNTs,32–37 and AC-supportedplatinum (Pt), ruthenium (Ru), iron (Fe) and cobalt (Co)catalysts, and multicomponent catalysts.6�38–40 The stud-ies on the hydrogenation of CO2 can be divided intothe categories of methanol-mediated and non-methanol-mediated reactions.12 In the methanol-mediated approach,CO2 and H2 react over Cu–Zn-based nanocatalysts to pro-duce CH3OH, and then methanol is transformed into otherhydrocarbons such as gasoline.41 Unfortunately, these sys-tems are unfavorable for commercial applications becauseof the high cost and the regeneration/disposal operations.

Fig. 1. Products from CO2 hydrogenation.12

In spite of the efforts made in nanocatalysts develop-ment, the major products of the target reactions are usu-ally light alkenes because of the catalytic hydrogenation ofalkenes.42 In the case of non-methanol-mediated process,CO2 hydrogenation proceeds through initial reduction ofCO2 to CO in the reverse water-gas shift (RWGS) (Reac-tion 1) followed by the conversion of CO to hydrocarbonsin a FT reaction (Reaction 2).Reverse water-gas shift reaction

CO+H2O= H2+CO2 �H298 K =−41�2 kJ/mol (1)

Fischer–Tropsch reaction

CO+2H2=−CH2−+H2O �H298 K=−166 kJ/mol (2)

The hydrogenation of CO2 takes place through the dis-sociation of CO2 along with hydrogenation of CO (Reac-tion 2).43�44 Carbon materials are known to have tunablepore sizes and surface areas. It is due to these couple offacts that carbon materials have been chosen to be usedin synthesizing catalysts via FT reactions. For instance, Feand Co, supported on AC and CNT have been tested fortheir effectiveness for FT synthesis.31�32�34�35�45–51 Copper-based catalysts which are mostly studied in catalytic sys-tems have also been tested for the RWGS reaction. Thus,the results of these reactions have challenged researchersto use carbon nanocatalysts in the hydrogenation of CO2.The metal oxide supported catalysts in comparison withcarbon supported catalysts generally have less interactionof metal with its support, hence improving the catalyticactivity significantly. Furthermore, a good dispersion ofnanometal species is achieved due to functional groups,particularly acidic functional groups on the surface ofcarbon.52

2.1. Methanol-Mediated Reaction

Production of methanol through CO2 hydrogenation hasattracted continuous worldwide research interest in the past15 years. Methanol synthesis from atmospheric reactionof CO2 with hydrogen under suitable condition is consid-ered the most economical way to reduce the greenhouseeffect in the atmosphere.9�53�54 The major reaction in theprocess of CO2 hydrogenation is the methanol formation(Reaction 3):

CO2+3H2 = CH3OH+H2O

�H298 K =−49�3 kJ/mol(3)

The drop in temperature or the rise of pressure wouldbe thermodynamically favorable whereas the methanol for-mation is an exothermal reaction. The RWGS reactionconsumes more hydrogen and reduces methanol forma-tion. Thus, methanol synthesis and the RWGS reactionsresult in generation of large amounts of water. The deac-tivation of the catalyst is due to the effect of an inhibitor

J. Nanosci. Nanotechnol. 13, 4825–4837, 2013 4827



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on the active metal. On the other hand, methanol forma-tion is usually correlated with other hydrogenated productssuch as higher alcohols (C2+− alcohols) and hydrocarbons.Therefore, a more selective catalyst is required for produc-tion of methanol from CO2 hydrogenation to prevent theformation of undesirable by-products.9

In fact, the development of highly active catalysts forCO2 hydrogenation for methanol synthesis has been thekey objective in the recent R&D efforts. It was demon-strated for the first time by Ishikawa et al.55 that sur-faces of the graphitized carbon had the capability of rapidCO2 and H2 mixture equilibration. The researchers work-ing on the highly active CNT-supported nanocatalyst forCO2 hydrogenation to methanol have demonstrated thatthe high performance of this developed nanocatalyst isrelated to unique structure, texture and surface of the CNTsas nanocatalysts carrier. Table I summarizes the carbonmaterials supported catalyst used for the CO2 hydrogena-tion to methanol. AC supports generally have a dominantmicroporous structure which hinders mass transfer pro-cesses and imposes additional transport resistances in com-parison with CNT supports.37�51 Although CNTs possesssimilar properties as AC, according to the higher thermalstability and superior electronic conductivity of the former,in most cases CNT outperforms AC.56 Their high capac-ity of hydrogen uptake is important to be used as supportmedia for nanocatalysts.57�58 The electronic conductivityof the supports is vital for the chemical reactions whichare affected by an electronic factor. The ease or abilityto exchange cation or anion on the surface of the cata-lyst support determines the stability of the reaction media.In addition, based on the type of surface moieties, theactivity/performance of the catalyst can be assessed. Highsurface area with respect to graphite and better electronicconductivity compared to AC makes CNT a suitable can-didate as support materials in nanocatalysis field.59

Recently, Kong et al.61 studied the development ofPd-decorated CNT-promoted Pd-Ga catalysts and reportedexcellent performance for CO2 hydrogenation to methanol.The stability of the developed catalyst, 3%Pd/CNTs-promoted Pd/Ga with 1/10 molar ratio, in the CO2 hydro-genation to methanol can take up to 200 h under thereaction conditions of 5 MPa and 250 �C with no obvi-ous deactivation observed. The formed products includeCH3OH (95.7%), DME (4.2%) and CH4 (0.1%). Thespace-time-yield (STY) of methanol of the catalyst washigher than the level of the host (Pd1Ga10� and the CNT-promoted catalyst (Pd1Ga10-12.6%CNT). They reportedthat the Pd-decorated CNTs as a promoter of Pd–Ga2O3

catalyst increased the molar percentage of active Pd0

species and enhanced adsorption capability and acti-vation of hydrogen. Transfer of those active adsorbedH-species to Pd/Ga active sites via hydrogen spilloverresulted in increasing the CO2 reaction rate.60�61 The inves-tigation of H2-TPD indicated the possibility of adsorp-tion of hydrogen on the CNTs at ambient temperatures

and atmospheric pressure. It is also shown that the H2

was only produced at temperatures lower than 450 �C,while CH4, C2H2 and C2H4in addition of H2 were pro-duced at temperatures of 500 �C and above.66 Moreover,the Raman spectroscopy has revealed that adsorption ofhydrogen on the CNTs may forms as associative anddissociative (molecular and atomic state).67 The same sce-nario was explained for CO2 hydrogenation over CNTs-promoted Pd/ZnO63 and for hydrogenation of CO2/COover Cu ZnO Al2O3.

60

The dispersed aligned CNTs arrays into Cu/Zn/Al/Zrnanocatalyst showed a high activity for methanol synthe-sis. The stability of the catalyst was improved due to thehigh thermal conductivity of CNTs. Zhang et al.68 exper-imentally found that the carrier could significantly affectthe catalytic activity in methanol synthesis. The STY ofmethanol over the CNT-supported catalyst at 230 �C was1.95 and 2.57 times as high as those of the correspondingcatalysts supported on AC and �-Al2O3 at the operatingtemperatures of 250 and 270 �C, respectively.Pd–ZnO nanocatalysts supported on multi-walled car-

bon nanotubes (MWCNTs) displayed high performancefor CO2 hydrogenation to methanol, owing to higherconcentration of the active Pd0 species.63 The reversibleadsorption and storage of greater amount of hydrogen hasresulted in using MWCNTs-supported Pd/ZnO nanocat-alysts. The CO2 hydrogenation catalyzed by CNTs sup-ported PdZn catalysts compared to that of other supportssuch as AC and �-Al2O3. It was found that an increase inthe relative content of the catalytically active Pd0–speciesat the CNT surface is related to the methanol genera-tion. The turnover-frequency (TOF) of AC and �-Al2O3

supported nanocatalysts was reported to be relatively low.On the contrary, the TOF for the CNT-supported nanocat-alyst was much higher. Thus, it shows that CNT is playinga key role in the system by not only acting as a catalystsupport, but also as a promoter. AC and �-Al2O3 on theother hand, just performed as a support. It has also beeninvestigated that the molar ratio of two metals in a bimetal-lic catalyst such as Pd/Ga and Pd/ZnO61 has influenced thecatalytic activity of CO2 hydrogenation to methanol.

2.2. Non-Methanol-Mediated Reaction

2.2.1. Synthesis of Formic Acid

Formic acid can be synthesized by CO2 hydrogenationusing nanocatalysts on a carbon-based support. AC appearsto be a suitable catalyst support in the synthesis of formicacid because it can easily form functionalized oxygen-bonded surface groups.20 In addition, the high concentra-tion of hydroxyl groups on the activated surface of supportenhances the capability of CO2 adsorption.

69 Immobilizingmore hydroxyl groups on the Ru over AC catalyst may besuitable for the formic acid formation. Hao et al.70 inves-tigated the novel heterogeneous supported Ru catalysts in




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TableI.

Summaryof

carbon

-based

nano

catalystsused

inCO

2hydrogenation.

Reactioncond

ition

Reactionresults

Catalyst

Temp(�C)

Press(M

Pa)

GHSV

Feed

ratio

H2/CO/CO

2/N

2SS

AM-SA

XCO%

XCO2%

TOF

CH

3OH

(C%)

DME(C

%)

CH

4(C

%)

STY

CNT–

prom

oted

Cu–

ZnO

–Al 2O

3

(Cu 6Zn 3Al 1−1

2.5%

CNTs)

60

220

580

00a

62/30/5/3

61.4

39�4

36�8

e–

––

––

–

Cu 6Zn 3Al 1–1

2.5%

CNTs6

022

02

3000

a62

/30/5/3

61.4

39�4

42�4

––

98�0

––

–Cu 6Zn 3Al 1–1

5%CNTs6

022

02

3000

a62

/30/5/3

60.3

35�5

35�1

––

––

––

Cu 6Zn 3Al 1–1

0.0%

CNTs6

022

02

3000

a62

/30/5/3

60.2

36�8

38�1

––

––

––

Cu 6Zn 3Al60 1

220

230

00a

62/30/5/3

50.3

33�9

25�3

––

––

–Cu 5Zn 2

�5Al 1–1

2.5%

CNTs6

022

02

3000

a62

/30/5/3

55.7

38�3

39�8

––

––

–Pd

1Ga 1

0–1

2.6%

(3%

Pd/CNTs)

(h–typ

e)61

250

518

000

69/0/23/8

–2�3

–9�8

–95

�74.2

0.1

555c

Pd1Ga 1

0–1

2.6%

CNTs

(h-typ

e)61

250

518

000

69/0/23/8

–2�3

–9�0

–96

�23.7

0.1

512c

Pd1Ga6

1 1025

05

1800

069

/0/23/8

–2�7

–8�6

–94

�55.4

0.1

480c

Pd1Ga 1

0–1

2.6%

CNTs

(p-typ

e)61

250

518

000

69/0/23/8

–2�3

–7�5

–96

�63.3

0.1

428c

3%Pd

/CNTs(h-typ

e)61

250

518

000

69/0/23/8

––

–3�6

––

–10

0–

Cu 1Zr 1–1

0%(4.3%

Co/CNT)6

224

05

8000

69/0/23/8

–23

�5–

7�30

2�89

92�0

–8.0

176c

Co 3Cu 1–1

1%CNT(h-typ

e)62

300

572

0045

/45/5/5

––

––

–Cu 1Zr 1–1

0%CNT62

250

318

0069

/0/23/8

–23

�5–

6�04

2�40

97�5

–2.5

155c

Cu 1Zr6

2 125

03

1800

–23�0

–5�83

2�36

99�1

–0.9

152c

16%Pd

0�1Zn 1/CNT(h-typ

e)63

250

318

0069

/0/23/8

121

1�3

–6�30

1�15

99�6

––

37.1

c

22%

Pd0�1Zn 1/CNT(p-typ

e)63

250

318

0069

/0/23/8

133

1�4

–6�22

1�08

95�2

––

35.0

c

35%

Pd0�1Zn 1/AC

6325

03

1800

69/0/23/8

515

1�2

–4�93

0�98

96�5

––

28.1

c

20%

Pd0�1Zn 1/c–A

l 2O

63 325

03

1800

69/0/23/8

110

1�1

–4�44

0�97

92�1

––

24.2

c

Cu/C

6426

02

1000

a4:0:1:1.7

200

––

5�5

0�08

b–

––

Cu/AC

6426

02

1000

a4:0:1:1.7

300

––

0�3

0�02

b–

––

Cu–

ZnO

/C

6426

02

1000

a4:0:1:1.7

210

––

5�6

1�55

b–

––

Cu–

ZnO

/AC

6426

02

1000

a4:0:1:1.7

270

––

1�4

0�48

b–

––

Com

mercial

catalyst

6524

04

6000

3:0:1:0

––

0�4

0�16

3–

––

–15

1d

Cu 6Zn 3Al 0�5Zr6

5 0�5

240

460

003:0:1:0

––

0�5

0�20

5–

––

–26

1d

Cu 6Zn 3Al 0�5Zr 0

�5–1

5%CNT65

240

460

003:0:1:0

––

0�5

0�21

5–

––

–28

2d


�5–

15%CNT+A

l 2O

65 3

270

460

003:0:1:0

––

0�8

––

––

–67

5d


�5–1

5%CNT+H

ZSM

–565

270

460

003:0:1:0

––

–0�26

––

––

220d

Notes:SS

A:specificsurfacearea

(m2g−

1�,

M-SA:metal

surfacearea

(m2g−

1�,

XCO:CO

conversion,ST

Y:Sp

ace-tim

e-yield(m

gg−

1h−

1�,

TOF:

Turnover-frequency,

GHSV

:Gas

hourly

spacevelocity

(mlg−

1h−

1�,

H-type:

Herringbone-typeMWCNTs,p-type:

Paralleltype

MWCNTs(a)h−

1�

(b)Yield

ofmethanol(w

t.%),(c)ST

Yof

yieldof

CH

3OH

and(d)ST

Yof

DME(m

gg−

1h−

1�.




REVIEW


the synthesis of formic acid from the CO2 hydrogenation.The effects of experimental parameters have been studiedand optimized at reaction temperature of 80 �C, stirringrate of 300 rpm and total pressure of 13.5 MPa where H2

pressure was set to be 5 MPa. To compare the activity ofAC with �-Al2O3 supported Ru, the yield of formic acidwas found to be much higher for the later, due to the highernumber of hydroxyl groups on the support surface. In fact,the hydroxide–Ru species formed from the interaction ofhydroxyl groups and Ru promoted the CO2 hydrogenation.

2.2.2. Synthesis of Higher Alcohols

Amongst the chemical feedstock and coal-based clean syn-thetic fuels species, the higher alcohols (C2+−alcohols)along with CH3OH and DME have been considered asthe most important ones. In a recent work,71 CNTs weredecorated with transition metals such as Co, Cu, Pd,Zn, Mo and Zr. It was found that their presence couldimprove the adsorption of hydrogen and promote theH-adspecies spillover for CO or CO2 hydrogenation toalcohols, CH3OH and DME.62�71�72 Interestingly, in con-trast to the higher alcohol synthesis using CO only,71–74

the flow of a suitable amount of CO2 in feed gas overthe CNT-promoted Co–Cu catalyst was found to be highlyselective regarding formation of the oxygenated prod-ucts in CO hydrogenation. The Co–Cu-doped AC cata-lyst was not contributing to the catalytic performance anddecreased the conversion of CO and STY of higher alco-hol and DME.62 The CNTs used as a catalyst promoterwere reported to increase the concentration of surface-species and the improvement of the H2 adsorption capa-bility was observed. Zhang et al.62 studied the catalyticactivity of MWCNT-promoted Co–Cu and Co decoratedMWCNT-promoted Co–Mo–K catalysts in the synthesisof higher alcohol.62 In addition, CO2 hydrogenation overCo decorated MWCNT-promoted Cu–Zr and Pd decoratedMWCNT promoted Pd-Zn were investigated. It was indi-cated that the decoration of the CNTs by Co or Pd stronglyincreased the CNT capability of adsorbing H2.

2.2.3. Synthesis of Dimethyl Ether

Dimethyl ether (DME) is considered as a potential sub-stitute for diesel oil as it shows a better combustion per-formance with a high cetane number and low emissionsof NOx.

12 There are four independent stoichiometric reac-tions (Reactions 1, 3, 4 and 5) with regards to DME directsynthesis from hydrogenation of CO/CO2.

65

Methanol synthesis from CO hydrogenation:

CO+2H2 = CH3OH �H298 K =−90�4 kJ/mol (4)

DME synthesis from methanol dehydration:

2CH3OH= CH3OCH3+H2O

�H298 K =−23�4 kJ/mol(5)

In contrast to CO hydrogenation, more water is pro-duced in the CO2 hydrogenation from the RWGS reaction.However, formation of water decreased the activity of thecatalyst due to the high adsorption capability of wateron the catalyst acid sites. A kinetic model has been pro-posed for the synthesis of DME in a one-step reactionover Cu–Zn–Al/�-Al2O3 bifunctional catalysts by Aguayoet al.75 HZSM-5 zeolite is also an active material to beused for transformation of DME into hydrocarbons. It wasfound that the addition of a suitable concentration ofNa to HZSM-5 prevented the formation of hydrocarbonsfrom DME and there was no irreversible deactivationobserved.76 Zhang et al.65 developed Cu/Zn/Al/Zr oververtically aligned CNT arrays catalyst for production ofDME and CH3OH through CO/CO2 hydrogenation. Theaddition of CNTs resulted in increasing specific surfacearea, especially for Cu. The CNT promoted catalystsshowed a high performance for methanol synthesis withthe effect of phase separation and ion doping. Hydrogenspillover is promoted by reversible adsorption and storageof hydrogen on CNTs.65 The STY of methanol (0.94 and0.28 g/(gcath)) for CO/CO2 hydrogenation on Cu/Zn/Al/Zrcatalyst with CNTs increased to 7% and 8% comparedto that of Cu/Zn/Al/Zr catalyst without CNTs (0.87 and0.26 g/(gcath)). When �-Al2O3 and HZSM-5 were added tothe Cu/Zn/Al/Zr catalyst with CNTs, DME was obtainedwith a STY of 0.90 and 0.077 g/(gcath).

2.2.4. Methanation of CO2

One of the important processes in CO2 hydrogenation ismethanation of CO2 (Reaction 6) which called Sabatierreaction. This reaction has a range of applications one ofwhich is the purification of the synthesized gas from theproduction of ammonia.

CO2+4H2=CH4+2H2O �H298 K=−252�9kJ/mol (6)

It was illustrated that the surface-based activities ofmetal are related to the type of support and the degreeof metal dispersion. For instance, the following order ofTOFs for the reaction was obtained by Kowalczyk et al.77

Ru/Al2O3 (16.5) > Ru/MgAl2O4 (8.8) > Ru/MgO (7.9) >Ru/Carbon (2.5) for high metal dispersion. The catalyticproperties of Ru nanoparticles were strongly affected bythe interactions of metal with its support. CO2 hydrogena-tion on AC-supported Fe catalysts was carried out at anatmospheric pressure to produce CO and CH4 whereasthe rate of CO formation was much higher than methaneformation.43�44 The principal product of the CO2 hydro-genation was CH4 when Co, Ni, Fe and Ru were supportedon carbon due to the oxygen produced from the dissocia-tion of CO2.

43�78�79

Sakata et al.64 showed that the catalytic activity ofcarbon supported Cu and Cu–ZnO impregnated on saw-dust in CO2 hydrogenation was high. The conversion of




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CO2 was negligible when the AC (obtained from the car-bonization of sawdust) was used as the catalyst. The prod-ucts of CO2 hydrogenation over Cu/C were mainly COwith very small amounts of methanol and methane. It isindicated that Reaction 4 occurred predominantly whenthe catalytic component was copper (Cu). The addition ofZnO to Cu-based catalyst greatly improved the produc-tion of methanol from CO2 hydrogenation and the RWGSreactions.54�64�80�81 The higher activity has been achievedwhen sawdust was used as the catalyst support due to thelarger dispersion of catalytically active species.The physical and chemical properties of CNTs such

as impressive mechanical strength, high electrical con-ductivity and thermal stability make them very unique.The high purity of CNTs omits self-poisoning in catalyticreactions. In addition, the nanosized channel, sp2-C con-structed surface and graphite-like structure tube-wall ofCNTs make them excellent in adsorbing hydrogen andelectron transport. The considerable amount of adsorbedhydrogen species on the CNT support would create amicro-environment with higher concentration of hydrogen-adspecies. The active adsorbed hydrogen species weretransferred to active metal sites via the CNT-promotedhydrogen spillover. Thus, the rate of CO2 hydrogenationwas increased for the synthesis of higher alcohols andmethanol over the CNTs-promoted Co–Cu,82 Co–Mo–K71

and Cu–ZnO–Al2O60�833 catalysts. Furthermore, the possi-

bility of macroscopic support formation, tuning the spe-cific interactions of metal and support, the effects ofconfinement on the inner cavity as well as high accessi-bility of the active phase are important factors that leadto CNTs being used as support materials in catalytic reac-tions. There are few reasons that CNTs have a high flex-ibility for the dispersion of the active phase compared toconventional supports. CNTs have the possibility of(a) modulating their specific surface area or internaldiameter (50–500 m2 g−1 and 5–100 nm for MWCNTsrespectively);(b) easily functionalizing their surfaces;84

(c) changing their chemical composition and(d) depositing the catalytic phase on their outer surface oreven inner cavity.85

Catalytic performance of CNT-supported and con-fined CNTs catalysts for applications such as syn-gas conversion,33�36�87 hydrogenation/dehydrogenation,86

NH3 decomposition,87 fuel cells,88 hydroformylation,89

photocatalysis,90 and selective oxidation91 have beeninvestigated.33�36�92

3. SYNTHESIS OF DIMETHYL CARBONATEFROM CO2 AND METHANOL

Dimethyl carbonate (DMC) is a non-toxic chemical prod-uct widely used in industry such as fuel additives,alkylating agents, polar solvents and the synthesis ofpolycarbonate and polyurethane.93–95 The synthesis of

DMC directly from CO2 and methanol is more attrac-tive compared to other commercial processes includingthe oxidative carbonylation of methanol,96 methanolysisof phosgene95 and transesterification route.97 The advan-tage of this approach is utilization of CO2 without usingtoxic chemicals such as phosgene.98 Direct synthesis ofDMC from CO2 and methanol (Reaction 7) is favorable fordevelopment of a new carbon source and also for reduc-tion of greenhouse gas emissions. Recently, referring to thesynthesis of DMC from CO2 and methanol as “Sustain-able Society” and “Green Chemistry” has received muchattention.

2CH3OH+CO2 → �CH3O�2CO+H2O �H298 K

= −27�9 kJ/mol (7)

Using metals such as Cu and Pd in the catalytic reac-tions are known to be very active for CO2-involved reac-tions. CO2 adsorption on Cu-based catalyst surfaces hasbeen the main concern in several research reports.99–101

The major issue associated with metal catalysts in largescale is their high cost, making them economically unfa-vorable in some cases. Even in the presence of dehydratingagents and additives, the activities of the catalytic directsynthesis of DMC systems are quite low due to the limitedreaction thermodynamics. The previous works in this fieldconfirmed that the reaction temperature and pressure hadgreat influence on the direct catalytic synthesis of DMCfrom CO2 and the DMC yield.102�103

The carbon materials as novel host for Cu Ni bimetal-lic nanoparticles offer advantages of low-cost and beinghighly efficient catalysts in direct synthesis of DMC pro-posed by Bian’s research group.59�98�104–107 It was notedthat graphite oxide (GO), MWCNTs and AC were inertto the reaction, when Cu or Ni monometallic carbon-based nanocatalysts were catalytically active, althoughthe activity was relatively low.108 The highest activitywas achieved in the case of Cu Ni bimetallic overcarbon materials. The additional metal can improve thesize and the morphology of active nanoparticles as wellas the catalyst selectivity.109 Furthermore, Cu Ni/GOshowed higher catalytic activity than Cu Ni/MWCNTsand Cu Ni/AC catalysts.106 The low catalytic activityof Cu Ni/MWCNTs is probably due to the aggregationof catalyst. The moderate Cu Ni–graphite interactions,metal naoparticles dispersion, and particular character ofgraphite, contributed to the catalytic activity. The resultsof stability test for all mentioned carbon catalysts showedthat there was no obvious deactivation after 10 h reaction.The existence of graphite sheets on such carbon-based

catalysts creates a high electronic conductivity and elec-tron transport properties which induce electron perturba-tions in the metallic phase. The functional groups of GOfacilitate the dispersion of metal (Cu and Ni) atoms. More-over, release of electrons as the important factor of CO2




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Table II. Results of DMC synthesis over Cu–Ni/C bimetallic composite catalysis.

Reaction condition Reaction results

Calcination Nanoparticle Press Temp Time on DMC DMCCatalyst temp (�C) size (nm) (MPa) (�C) stream (h) XCH3OH

(%) yield (%) selectivity (%)

Cu–Ni/G105 400 5–25 1.2 105 10 10�13 9�02 88.0Cu–Ni/MWCNT59 450 5–30 1.2 120 3 4�3 – 85.0Cu/MWCNT106 500 Na 1.2 110 – 1�27 1�19 93.4Ni/MWCNT106 500 Na 1.2 110 – 0�13 0�12 93.0Cu–Ni/MWCNT106 500 5–40 1.2 110 10 4�32 3�82 88.5V–Cu–Ni/AC107 500 10–30 1.2 110 3 7�76 – 89.9CuCl2/AC

104 400 80–120 1.2 120 4 – – 90.1Cu/AC106 500 Na 1.2 110 – 2�16 1�99 92.5Ni/AC106 500 Na 1.2 110 – 1�43 1�31 91.6Cu–Ni/AC106 500 5–50 1.2 110 10 6�10 5�31 87.0Cu–Ni/TEG98 500 10–50 1.2 100 10 4�97 – 89.3Cu/GO106 500 Na 1.2 110 – 2�51 2�32 92.3Ni/GO106 500 Na 1.2 110 – 0�79 0�72 91.1Cu–Ni/GO106 773 6–26 1.2 383 10 7�34 6�33 86.2Cu–Ni/GO108 873 24.8 1.2 378 10 10�12 – 90.2

Note: TEG: Thermally expanded graphite.

activation, is obtained from low carbonic electro-negativityof graphite.106 The results of DMC synthesis over Cu Nisupported on the different tested carbon material are shownin Table II.DMC formation could reach 5.5 mmol with DMC selec-

tivity of 87.0% over Cu Ni/V2O5–SiO2 catalyst at 140�C

and 0.6 Mpa.110 Wang et al.111 indicated DMC selec-tivity of 87.2% and CH3OH conversion of 4.83% overthe catalyst of Cu (Ni,V,O)/SiO2 combined with photo-assistance. Bian et al.107 presented the effect of vanadium-doped Cu Ni/AC catalyst system for the direct synthesisof DMC. Systematical change of the size and/or the elec-tronic structure of the catalyst are obtained by the additionof one or two metal components of a catalyst. The advan-tages of this addition consist of modifying the absorptioncharacteristics, changing the reducibility or altering thedeactivation behavior of the catalyst.107�112 The effects ofvanadium element on the catalytic activity and the selectiv-ity of carbon-based catalysts were studied. The addition ofvanadium element to Cu Ni/AC catalysts could improvethe activity of catalysts and the selectivity of DMC yieldat low V loading. However, the gradual decrease in DMCselectivity was resulted in excessive vanadium loading. Itwas also found that the reaction temperature had a greateffect on the reaction. At low temperature, increasing tem-perature favored the DMC formation, while at high tem-perature led to DMC decomposition.107

Cu nanoparticles supported on AC in the forms ofCuCl/AC113�114 and [Cu(OCH3�(pyridine)Cl]2/AC

115 is thecatalysts currently under investigating for production ofDMC by oxidative carbonylation of methanol (CH3OH+O2+CO). High methanol conversion with 80–90% DMCselectivity in CH3OH+O2+CO route has been achievedover developed CuCl2/AC.116 It is noted that the addi-tion of palladium (Pd) or potassium (K) could improve

the catalytic activity.117 However; the catalyst has a seri-ous deactivation problem. Direct catalytic DMC synthesisfrom CO2 and methanol has been investigated over ACsupported Cu-based catalysts by Bian et al.104 The rateof DMC formation on the AC supported CuCl2 catalystsenhanced with the reaction time at the initial stage. Thereaction rate was too slow when the reaction temperaturewas below 80 �C and too much by-products were pro-duced when it was above 140 �C. The presence of Cu0,Cu+ and Cu2+ with micro-crystallinity forms played therole of active species on the catalyst in the formation ofDMC. The major by-products were CO, CH2O, DME andH2O. Apparently, the cleavage of the C O bond of theCO2 formed CO and the activation of methanol lead toformation of CH2O and DME. However, the reports aboutthe development of carbon materials as a catalyst supportin the direct synthesis of DMC are scarce.

3.1. Reaction Mechanisms

Figure 2 shows a catalytic cycle using Cu Ni carbon-based nanocatalyst in the direct synthesis of DMC fromCO2 and methanol.59�105 The proposed mechanism illus-trated activation of CO2 and CH3OH on a metal or bimetal-lic alloy surface to form CH3O– and –C O species. Thereactions between these species then resulted in the for-mation of DMC and regeneration of metal or bimetallicalloy. One of the important factors is electron transfer inthe activation of the reactants and formation of DMC, thusthe effective electronic transport properties are consideredfor choosing the suitable support. The characteristics andstructure of carbon support in addition of the synergeticeffects of metal or bimetallic alloy could be very importantin activation of CO2 and CH3OH. Furthermore, extraor-dinary electronic transport properties and high electronic




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Fig. 2. Catalytic mechanism for direct synthesis of DMC from CO2 andmethanol over Cu–Ni/C nanocomposite catalyst59�105 (M is Cu or Ni orCu and Ni alloy).

conductivity of MWCNTs change the electronic balancein the metallic phases. This electronic noise improves theinteractions of metal and support and thus the reactiv-ity and selectivity of chemical reactions are affected.105

MWCNTs may facilitate the small and highly dispersedmetal nanocatalyst formation. The results of direct synthe-sis of DMC from CO2 and methanol reactions on variouscarbon-based nanocatalysts suggest that the interactionsof metal nanoparticles and support and the morphologicalproperties of carbon-based nanocatalysts greatly influencethe catalytic activity.

4. PREPARATION OF CARBON-SUPPORTEDNANOCATALYSTS

Carbon supported nanocatalysts have been studied in CO2

conversion to yield valuable products such as methanol,hydrocarbon and DMC. The extensive knowledge is essen-tial for carbon support material characteristics with regardto their porous texture and surface chemical properties.The relative inertness of the carbon surface of carbon sup-port materials is a limitation for catalyst preparation inCO2 hydrogenation or direct synthesis of DMC catalyticreactions. The carbon based catalysts were usually activewith metal phase, mostly bimetallic and even promoted

metal or metal oxides. In fact, a variety strategies andtechniques have been employed to investigate the prepa-ration of different active phases including metals, metalalloys and carbides over catalyst support. Carbon sup-ported Cu catalysts are among the most studied systems.Sakata et al.64 showed that the high dispersion of Cu overcarbon support (Cu/C) could be achieved on carbons withhigh surface area. The nature of metal and the precur-sor solution evidenced a substantially different behaviorfor the preparation of metal nanoparticles over the carbonsupports. In the case of CO2 reforming of methane cata-lyst which is prepared by different precursors, the higherH2/Co ratio (syngas production) obtained follows the orderof chloride > nitrate> acetate.12�118

Performance of CO2 hydrogenation and direct synthesisof DMC from CO2 and methanol reactions is improvedby the addition of the bimetallic and promoter particles.Bian et al.106 observed that the methanol conversion andyield of DMC were the highest for Cu Ni bimetalliccatalyst on GO, CNTs and AC as compared to that ofeither of these two monometallic catalysts. The combinedeffect of bimetallic (Cu Ni) alloy coupled with the loca-tion of these nanoparticles on the carbon support wasthe cause of lack of steric hindrance for the substrate onthe outer surfaces of carbon support as compared to theother catalysts.119 Bian et al.107 reported that the additionof vanadium element resulted in a decrease of catalystsurface acidity which was indicated by the temperature-programmed reduction (TPR) analysis.Applying pretreatment on the carbon support can mod-

ify the metal dispersion. Surface oxidation of carbon sup-port is the key nucleation site on the surface of supportfor the metal nanoparticles deposition. The strong inter-actions between the metal and support cause well deposi-tion and dispersion of metal nanoparticles over the surfaceof support.120 For example, in the case of the depositionof precipitated nickel hydroxide onto carbon nanofiberssupport, nickel ions may adsorb onto the acidic func-tional groups.121 The presence of acidic oxygen groupson the carbon support surface enhances the interactionbetween active metal and support. The surface chem-istry of carbon materials can be modified by oxidation toincrease their hydrophilicity to favor ionic exchange.30 Forexample, modification of structure in MWCNTs mainlyoccurs on the nanotubes tip which results in their open-ing the cap and the formation of edges on the graphitesheets. The common methods such as nitric acid treat-ments provide the surface oxygen functionalities such ascarboxylic groups on the outer and even inner cavity ofCNT. It is demonstrated that nitric/sulfuric acid treatmentsare impressive for the carboxylic and carboxylic anhy-dride group formation. Figure 3 illustrates the synthesisapproach for MWCNTs and graphite supported Cu Ninanocatalysts prepared by conventional incipient wetnessimpregnation.60�106




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Fig. 3. Schematic illustration of composite catalyst synthesis.60�106

The preparation method and modification of carbon-based nanocatalyst are effective in catalytic performance.The deposition precipitation method with respect to thepore volume of the support is done often in an excessof solution. CuZnAl over different amounts of CNTswas prepared by a constant pH co-precipitation method.60

In this method, the hydrolysis of Na2CO3 increased pH andoccurred uniformly throughout the solution upon heating.It was noted that a mixed compound formation betweenmetal hydroxide and support which provided a thermody-namic driving force for nucleation on the surface of mostsupport were not expected in view of inertness propertiesof carbon support materials.Other preparation steps include drying, calcination and

reduction condition which have significant effects on theproperties of carbon supported nanocatalysts.122 It is com-mon to carry out the drying technique in conventionalovens. Microwave drying also offers certain advantagessuch as a shorter drying time and a more uniform heating.It is noted that the metal loading and calcination temper-ature stages were affected strongly by the surface speciesof carbon-based nanocatalysts in the catalyst prepara-tion. The Cu loading and calcination temperature forCuCl2/AC catalyst were optimized at 7 wt.% and 400 �C,respectively.107 The presence of Cu+ and Cu2+ in the cat-alyst was maximized at this condition and the catalyticactivity was increased. The surface structure of the catalystchanged with time-on-stream during a catalytic reaction.In the reduction stage, the right reduction temperature isan important factor to avoid sintering which forms cata-lysts with poor metal and large metal particle. For exam-ple, the Co nanoparticles presented a much smaller sizeand a higher dispersion on the oxidized MWCNTs sup-port after oxidation and reduction steps.121 It has beenreported that when the catalyst was pretreated under reduc-tive atmosphere of hydrogen, initial catalytic activity waslower than those obtained after pre-treated under flow ofhelium.22

5. FUTURE WORK

There are limited studies on the effects of different met-als over carbon-based nanocatalysts prepared by a varietyof precursors and techniques on the catalytic activity andproduct selectivity in CO2 hydrogenation and direct syn-thesis of DMC from CO2. It is expected that different pre-treatments and methods need to be investigated to achievesuitable dispersion and interaction between the supportsand the nanocatalysts. The review revealed that the carbon-based nanocatalysts are promising supported catalysts foractivation of either CO2 or co-reactant under mild con-ditions. The fundamental process at hand during prepara-tion method has not been fully understood and the studieson different graphitized and structured carbon materialsin CO2 hydrogenation and synthesis of DMC from CO2

and methanol are very limited. The preparation conditions,the surface structure and evaluating parameters are theprinciple experimental variables to investigate the efficientcatalytic activity and catalytic mechanism of CO2 hydro-genation and direct synthesis of DMC. For the practicalCO2 utilization, however, the developments of high perfor-mance nanocatalysts need a deep understanding followedby the reaction mechanisms in the future. This work canspeed up the scientific progress in CO2 hydrogenation anddirect synthesis of DMC from CO2 and facilitate betterunderstanding the mechanism of these reactions.

6. CONCLUSION

This survey of the literature has shown that the cat-alytic activity of metallic nanoparticles in CO2 hydrogena-tion and direct synthesis of DMC from CO2 reactionsis greatly influenced by the carbon-based support materi-als. The catalytic activity and deactivation of carbon-basednanocatalysts relates to the physical and chemical prop-erties as well as the texture and structure of the carbonmaterials as support in the chemical reactions. Catalytic




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studies over carbon-based nanocatalyst have shown attrac-tive results in view of activity and selectivity of methanol,non-methanol mediated materials and DMC from CO2

hydrogenation and direct synthesis of DMC from CO2.The studies of bimetallic nanocatalysts on carbon based-support revealed that the selectivity and conversion isenhanced by the presence of a secondary metal. The welldispersed bimetallic nanoparticles are likely alloyed underthe preparation conditions. The mechanism of the catalyticreaction was presented regarding to the synergetic effectsof metal nanoparticles in the activation of CO2 which per-form a significant role in catalytic activity. Further devel-opment of a wide new class of carbon-based nanocatalystswith modified properties in CO2 hydrogenation and directsynthesis of DMC are required. Catalytic studies directedon carbon-based supports have shown that the extraordi-nary electronic transport properties are very important tobe considered for choosing the most suitable support. It isexpected that carbon-based nanocatalysts with the meritssuch as the cheap nature of used materials, readily prepara-tion and high efficiency in the catalytic reaction processesmay refer to green chemistry in the CO2 conversion. Thehigh effectiveness of the nanocatalysts is the main focusof the current and future catalysis research area. Futurework should be directed towards the establishment of suit-able methods in designing the nanocatalysts that possessimproved activity for the utilization and conversion of CO2

in the synthesis of more valuable products.

Acknowledgments: The authors gratefully acknowl-edge the financial support provided by the Universiti SainsMalaysia (USM) under the Research University Team(Project A/C No. 854001) and Fellowship scholarshipand the Ministry of Higher Education under Long TermResearch Grant Scheme (203/PKT/6723001).

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Received: 6 October 2012. Accepted: 19 December 2012.


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