ISSN 1003-9953CN 21-1484/O4

天然气化学

Editors-in-Chief:Xinhe BAO Alexis T. BELL

Vol.19 No.5Sep. 2010

Journal of Natural Gas Chemistry Vol. 19 No. 5 2010

CONTENTS

453 The production of carbon nanotubes from carbondioxide: challenges and opportunities

Geoffrey S. Simate, Sunny E. Iyuke, Sehliselo Ndlovu,Clarence S. Yah, Lubinda F. Walubita

461 Continuous conversion of methanol to higher hydro-carbons at ambient pressure

William W. Porterfield, Gordon M. Zrelak, L. Av-ery Moncure, Matthew D. Huff

463 NF3 decomposition over some metal oxides in the ab-sence of water

Xianjun Niu, Liang Sun, Yanan Wang, Haipeng Wu, Xi-ufeng Xu

468 Catalytic performance of iron carbide for carbonmonoxide hydrogenation

Minglin Xiang, Juan Zou, Qinghua Li, Xichun She

471 Facile synthesis of microporous carbon through a soft-template pathway and its performance in desulfuriza-tion and denitrogenation

Bo Sun, Gang Li, Xiaoxing Wang

477 Autothermal reforming of biogas over a monolithiccatalyst

Sadao Araki, Naoe Hino, Takuma Mori, Susumu Hikazu-dani

482 Cross metathesis of butene-2 and ethene to propeneover Mo/MCM-22-Al 2O3 catalysts with differentAl2O3 contents

Shenglin Liu, Xiujie Li, Wenjie Xin, Sujuan Xie,Peng Zeng, Lixin Zhang, Longya Xu

487 Effect of preparation methods of aluminum emulsionson catalytic performance of copper-based catalysts formethanol synthesis from syngas

Lili Wang, Wen Ding, Yingwei Liu, Weiping Fang, Yi-quan Yang

493 Forecasting China’s natural gas consumption basedon a combination model

Gang Xu, Weiguo Wang

497 Selective CO methanation over amorphous Ni-Ru-B/ZrO 2 catalyst for hydrogen-rich gas purification

Qihai Liu, Zili Liu, LieWen Liao, Xinfa Dong

503 Fischer-Tropsch synthesis over ruthenium-promotedCo/Al2O3 catalyst with different reduction proce-dures

Ali Karimi, Ali Nakhaei Pour, Farshad Torabi,Behnam Hatami, Ahmad Tavasoli, Moham-mad Reza Alaei, Mohammad Irani

509 Catalytic properties of Ni/ceria-yttria electrode mate-rials for partial oxidation of methane

Shaohua Zeng, Lei Wang, Maochu Gong, Yaoqiang Chen

515 Characterization and catalytic behavior of Na-W-Mn-Zr-S-P/SiO2 prepared by different methods in oxida-tive coupling of methane

Wen Zheng, Dangguo Cheng, Fengqiu Chen, Xiaoli Zhan

522 Product oriented oxidative bromination of methaneover Rh/SiO2 catalysts

Zhen Liu, Wensheng Li, Xiaoping Zhou

530 Synthesis, characterization and hydrodesulfurizationactivity of silica-dispersed NiMoW trimetallic cata-lysts

Di Liu, Lihua Liu, Guangci Li, Chenguang Liu

534 Direct oxidation of methyl radicals in OCM processdeduced from correlation of product selectivities

Zhiming Gao, Yuanyuan Ma

539 Prediction and measurement of pollutant emissions inCNG fired internal combustion engine

M. Mansha, A. R. Saleemi, Javed S. H, Badar M. Ghauri

548 Syntheis of SWNTs over nanoporous Co-Mo/MgOand using as a catalyst support for selective hydro-genation of syngas to hydrocarbon

A. M. Rashidi, A. Karimi, H. R. Bozorgzadeh,K. Kashefi, M. Zare

552 A new and reliable model for predicting methane vis-cosity at high pressures and high temperatures

Ehsan Heidaryan, Jamshid Moghadasi, Amir Salarabadi

www.jngc.orgwww.elsevier.com/locate/jngc

CONTENTS

Review

453The production of carbon nanotubes from carbondioxide: challenges and opportunities

Geoffrey S. Simate, Sunny E. Iyuke, Sehliselo Ndlovu,Clarence S. Yah, Lubinda F. Walubita

Communications

461Continuous conversion of methanol to higher hydro-carbons at ambient pressure

William W. Porterfield, Gordon M. Zrelak, L. Av-ery Moncure, Matthew D. Huff

463NF3 decomposition over some metal oxides in the ab-sence of water

Xianjun Niu, Liang Sun, Yanan Wang, Haipeng Wu, Xi-ufeng Xu

468Catalytic performance of iron carbide for carbonmonoxide hydrogenation

Minglin Xiang, Juan Zou, Qinghua Li, Xichun She

Articles

471Facile synthesis of microporous carbon through asoft-template pathway and its performance in desul-furization and denitrogenation

Bo Sun, Gang Li, Xiaoxing Wang

477Autothermal reforming of biogas over a monolithiccatalyst

Sadao Araki, Naoe Hino, Takuma Mori, Susumu Hikazu-dani

482Cross metathesis of butene-2 and ethene to propeneover Mo/MCM-22-Al 2O3 catalysts with differentAl2O3 contents

Shenglin Liu, Xiujie Li, Wenjie Xin, Sujuan Xie,Peng Zeng, Lixin Zhang, Longya Xu

487Effect of preparation methods of aluminum emul-sions on catalytic performance of copper-based cat-alysts for methanol synthesis from syngas

Lili Wang, Wen Ding, Yingwei Liu, Weiping Fang, Yi-quan Yang

493Forecasting China’s natural gas consumption basedon a combination model

Gang Xu, Weiguo Wang

497Selective CO methanation over amorphous Ni-Ru-B/ZrO 2 catalyst for hydrogen-rich gas purification

Qihai Liu, Zili Liu, LieWen Liao, Xinfa Dong

503Fischer-Tropsch synthesis over ruthenium-promotedCo/Al2O3 catalyst with different reduction proce-dures

Ali Karimi, Ali Nakhaei Pour, Farshad Torabi,Behnam Hatami, Ahmad Tavasoli, Moham-mad Reza Alaei, Mohammad Irani

509Catalytic properties of Ni/ceria-yttria electrode mate-rials for partial oxidation of methane

Shaohua Zeng, Lei Wang, Maochu Gong, Yao-qiang Chen

515Characterization and catalytic behavior of Na-W-Mn-Zr-S-P/SiO2 prepared by different methods inoxidative coupling of methane

Wen Zheng, Dangguo Cheng, Fengqiu Chen, Xi-aoli Zhan

522Product oriented oxidative bromination of methaneover Rh/SiO2 catalysts

Zhen Liu, Wensheng Li, Xiaoping Zhou

530Synthesis, characterization and hydrodesulfurizationactivity of silica-dispersed NiMoW trimetallic cata-lysts

Di Liu, Lihua Liu, Guangci Li, Chenguang Liu

534Direct oxidation of methyl radicals in OCM processdeduced from correlation of product selectivities

Zhiming Gao, Yuanyuan Ma

539Prediction and measurement of pollutant emissions inCNG fired internal combustion engine

M. Mansha, A. R. Saleemi, Javed S. H, Badar M. Ghauri

548Syntheis of SWNTs over nanoporous Co-Mo/MgOand using as a catalyst support for selective hydro-genation of syngas to hydrocarbon

A. M. Rashidi, A. Karimi, H. R. Bozorgzadeh,K. Kashefi, M. Zare

552A new and reliable model for predicting methane vis-cosity at high pressures and high temperatures

Ehsan Heidaryan, Jamshid Moghadasi, Amir Salarabadi

Journal of Natural Gas Chemistry 19(2010)453–460

Review

The production of carbon nanotubes from carbondioxide: challenges and opportunities

Geoffrey S. Simate1∗, Sunny E. Iyuke1, Sehliselo Ndlovu1,Clarence S. Yah1, Lubinda F. Walubita2

1. School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, P/Bag 3, Wits 2050, South Africa;2. TTI-Texas A&M University System, College Station, TX, USA

[ Manuscript received February 11, 2010; revised April 9, 2010 ]

AbstractRecent advances in the production of carbon nanotubes (CNTs) arereviewed with an emphasis on the use of carbon dioxide (CO2) asa sole source of carbon. Compared to the most widely used carbonprecursors such as graphite, methane, acetylene, ethanol,ethylene,and coal-derived hydrocarbons, CO2 is competitively cheaper withrelatively high carbon yield content. However, CNT synthesis fromCO2 is a newly emerging technology, and hence it needs to be ex-plored further. A theoretical and analytical comparison ofthe cur-rently existing CNT-CO2 synthesis techniques is given including areview of some of the process parameters (i.e., temperature, pres-sure, catalyst, etc.) that affect the CO2 reduction rate. Such analysisindicates that there is still a fundamental need to further explore thefollowing aspects so as to realize the full potential of CO2 based CNTtechnology: (1) the CNT-CO2 synthesis and formation mechanism,(2) catalytic effects of transitional metals and mechanisms, (3) uti-lization of metallocenes in the CNT-CO2 reactions, (4) applicabilityof ferrite-organometallic compounds in the CNT-CO2 synthesis reac-tions, and (5) the effects of process parameters such as temperature,etc.

Key wordssupercritical CO2; CO2 reduction; chemical vapour decomposition(CVD); carbon nanotubes; ferrite catalysts

1. Introduction

Carbon dioxide (CO2) is one of the main greenhousegases. With the increasing CO2 concentration in the atmo-sphere, the “green house effect” becomes more and more se-rious. The increase in the concentration of CO2 in the atmo-sphere has stimulated significant global research and develop-mental efforts regarding the reduction of CO2 emissions fromall point and non-point sources [1]. In order to mitigate the

effect of CO2 on the global warming, catalytic reduction ofCO2 was studied by chemical [2], biological [3] and photo-chemical methods [4,5].

Furthermore, interest in the use of CO2 as a raw materialin the synthesis of organic molecules and carbon materials has

Geoffrey S. Simate is a lecturer and researcher inthe School of Chemical and Metallurgical Engi-neering, University of the Witwatersrand, Johan-

nesburg in South Africa. His research interests arein the fields of nanotechnology and biotechnology.His main research focus is in the production of

carbon nanomaterials for water treatment and pu-rification, cell immobilization, brewing, drug de-livery, and road materials. He has more than tenyears of industrial experience as a Process Engi-

neer. He is a Member of The Southern African Institute of Mining and Met-allurgy, and an Associate Member of The Institution of Chemical Engineers,UK.

Professor Sunny E.Iyuke holds a PhD in Chemicaland Process Engineering. He is the Head of the

School of Chemical & Metallurgical Engineering,University of the Witwatersrand, Johannesburg inSouth Africa, and holds the Chairmanship of theAPV-SPX Flow Technology Professor of Biochem-

ical Engineering. His research interests lie in theproduction of carbon nanomaterials for variousapplications such as proton exchange membrane

fuel cell components, microbrewery, biomedicaldevices, water treatment and purification, energy storage,etc. He has pro-

duced several patents in nanotechnology. He is a registeredProfessional

Engineer (Pr Eng) with the Engineering Council of South Africa, a CouncilMember of The South African Institution of Chemical Engineers (SAIChE),and a Chartered Engineer (CEng) with UK Engineering Council.

∗ Corresponding author. Tel: +27-11-717-7570; Cell: +27-76-112-6959; Fax: +27-11-717-7591; E-mail: simateg@yahoo.com (G.S.Simate)

Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(09)60099-2

454 Geof frey S. Simate et al./ Journal of Natural Gas Chemistry Vol. 19 No. 5 2010

Dr. Sehliselo Ndlovu holds a PhD in Mineralsand Mining Engineering, and a Diploma of Impe-

rial College (DIC) in Hydrometallurgy from Im-perial College, UK. She is a senior lecturer in theSchool of Chemical and Metallurgical Engineer-

ing, University of the Witwatersrand, Johannes-burg in South Africa. Her main research areas in-clude nanobiotechnology and biohydrometallurgy

with principal focus on bioleaching of ores andbioremediation processes involving the reclaiming

of effluent streams and toxic residue wastewaters for economic and sustain-able development. She is a Member of The Southern African Institute of

Mining and Metallurgy.

Dr. Clarence S. Yah is a Research Fellow inthe School of Chemical and Metallurgical Engi-

neering, University of the Witwatersrand in SouthAfrica. He holds a PhD in Medical Microbiol-ogy and is a Member of the American Society for

Microbiology (ASM), and South African Societyfor Microbiology (SASM). His research interestsare in the fields of nanobiotechnology, biotechnol-ogy, brewing technology, genetic engineering and

bioinformatics. His main research focus is in theproduction of carbon nanomaterials for cell immobilization, drug deliveries,toxicological studies of carbon nanotubes, and water treatment.

Lubinda F. Walubita holds a PhD in CivilEngineering from Texas A&M University, USA.His area of specialization and primary research

interests are roads, materials, and pavementengineering. His other area of research interestis nanotechnology, particularly its application to

road materials.

also increased over the last decade [6–8]. As a result, mucheffort has also been made to explore several possibilities ofthermally splitting of CO2 [1].

With the above background, the objective of the paper wasto review the current status and potential applications whereCO2 has been used solely as a carbon source for the produc-tion of carbon nanotubes (CNTs). In particular, the focus ofthe review is on the current existing CNTs production tech-niques that are based on the CO2 source. Additionally, theshortfalls of the current existing CNT-CO2 production tech-niques have also been highlighted including areas needing fur-ther research.

As regards to the scope and content reviewed, the proper-ties and current applications of carbon nanotubes along withpotential uses will not be discussed herein as they have beenextensively covered in other reviews [9–15]. Furthermore,the production of carbonaceous products from CO2 other thanCNTs is outside the scope of this review. The review is or-ganized as follows. Section 2 briefly discusses the synthesisof CNTs. Section 3 focuses on the results of different growthtechniques used in the production of CNTs from CO2. Section4 discusses the pertinent findings of the different techniques

where CO2 was employed in the production of CNTs. Section5 outlines some of the challenges and gives an overview of thefuture prospects.

2. Synthesis of carbon nanotubes

The remarkable properties of CNTs give promise to adiverse array of revolutionary technologies and applications.However, synthesis remains the key to their development.Based on the literature reviewed, there are at present threeba-sic methods which are in widespread usage for the synthesis ofboth single-walled and multi-walled CNTs: laser vaporizationof metal-doped carbon targets [16], arc-discharge of metal-doped carbon electrodes [17], and catalytic decompositionofhydrocarbons [18,19], which is more commonly referred toas the chemical vapor decomposition or CVD method. Morerecently, catalytic reduction of carbon monoxide was also re-ported [20]. In the arc discharge method, a vapour is createdby an arc discharge plasma between two carbon electrodeswith or without a catalyst. In the laser ablation method, apiece of graphite target is vapourized by laser irradiationun-der an inert atmosphere [15,21]. In the CVD approach, thebasic premise involves the pyrolysis of gas-phase carbon-richmolecules (e.g., C2H2, CH4) in the presence of a transitionmetal catalyst at elevated temperatures (800–1000◦C), andthe subsequent conversion of the carbon-fragments into nan-otubes. CVD method is widely used for carbon nanotube syn-thesis due to its high product yield and scale-up capability[22]. Though the pyrolysis of hydrocarbon precursors for syn-thesizing carbon nanotubes is very useful and is used widely,there are some disadvantages with these methods. Most hy-drocarbons used in these methods are hazardous chemicals,and for most cases, the pyrolysis temperatures are around1000◦C, which are impractical for large scale industrial pro-duction [23]. One approach to tackling this problem is bythe use of CO2 which is a cheap, non-toxic, low-energy, andabundant molecule on the earth [23]. CO2 is easily formedby the oxidation of organic molecules during combustion orrespiration. Furthermore, CO2 can be acquired from naturalreservoirs or recovered as a by-product of industrial chemi-cal processes, so, no new production of CO2 is necessary andthere will be no addition to greenhouse gases [24]. However,CO2 is a kinetically and thermodynamically stable moleculethat is difficult to reduce [25].

3. Growth results

3.1. Supercritical CO2

Using CO2 molecule as the carbon source for the prepara-tion of various materials has long been a goal for the synthesischemists and material scientists. When CO2 is heated beyondits critical point (>31◦C and>73 atm), the gas and liquidphases merge into a single supercritical phase (scCO2) whichserves as a novel media for chemical reactions because it com-bines the characteristics associated with gas and liquid phases,

Journal of Natural Gas Chemistry Vol. 19 No. 5 2010 455

such as miscibility with other liquids, high mixing rates andrelatively weak molecular association as compared to ordinarycondensed phases, resulting in high reactivity of the reactant[23,26].

Since the “rediscovery” of carbon nanotubes in 1991 [27],several researchers worldwide cutting across all disciplineshave embarked on stimulating research to find ways of massproduction of CNTs. However, there was no evidence thatcarbon nanotubes can be synthesized from CO2 until Motieiet al. [28] succeeded in 2001 as depicted in Figure 1. Motieiet al. prepared CNTs by the reaction of scCO2 with mag-nesium (Mg) at about 1000◦C and 10 kbar. In this process,CO2 was reacted with metallic Mg in an autoclave for 3 hat 1000◦C. The reaction produced MgO, CNTs and nestedfullerenes. Motiei et al. interpreted that the chemical reac-tions occurred at the operating temperature of 1000◦C basedon the work of Shafirovich and Goldshleger [29], which sug-gested that the first step is a homogeneous gas phase reaction,followed by a heterogeneous reaction occurring on the sur-face of the liquid magnesium as expressed in Equations 1 and2, respectively.

Mg(g) +CO2(g) −→ MgO(s) +CO(g) (1)

Mg(l) +CO(g) −→ MgO(s)+C(graphite) (2)

Magnesium melts at 650◦C, and its normal boiling tem-perature is 1090◦C. The vapor pressure of magnesium at1000◦C is 350 mmHg. The CO2 is in its supercritical state,and the calculated pressure at 1000◦C is approximately 10kbar. Afterwards, other researchers also demonstrated thepro-duction of CNTs from CO2 using similar methods [30–32].

Figure 1. High resolution transmission electron microscopy (HRTEM)pic-ture of an individual nanotube after HCl treatment [28]

Lou et al. [30] used metallic Li as a reductant at an oper-ating temperature of 550◦C for 10 h. In the light of the factthat Li2CO3 was detected by XRD analysis, the chemical re-actions which occurred at the operating temperature of 550◦Cwere proposed as follows:

CO2 +4Li −→ C(graphite) +2Li2O (3)

CO2 +Li2O−→ Li2CO3 (4)

The XRD patterns of the products indicated the formationof well crystallized graphite. The low ratios of theID/IG (in-tensity ratio of the D and G bands) which are characteristicsof a graphite lattice with perfect two-dimensional order alsoshowed that the nanotubes grew perfectly.

The work reported by Lou and others in 2005 [31] in-volved the use of scCO2 as a carbon source and the alkalimetals (Li or Na) as the reductants to synthesize CNTs underreaction temperatures of 600−750◦C. CNTs with differentkinds of morphologies ranging from double helical, rope-like,porous, and bamboo-like structures were formed dependingon the reductant as shown in Figure 2.

The Y-junction CNTs with bamboo-like structures wereproduced when scCO2 was used as the carbon source andsodium borohydride (NaBH4) as the reductant [32]. Tempera-ture played a key role here. The conventional CNTs formed at600◦C, whereas the Y-junction CNTs formed when the tem-perature was further increased to 700◦C. The presence of hy-drogen seems to be crucial for the formation of Y-junctionCNTs [33], as no Y-junction CNTs were reported in the ear-lier work [30]. These Y-junction CNTs were well crystallized,with nearly parallel lines depicting graphite atomic planes andseparated by 0.34 nm.

3.2. Reduction of CO2 over ferrite catalysts

The reduction of gaseous oxides such as CO2 and H2Ohas long been an important concern in industrial processesand pollution control. Tamaura and Tabata [34] reported thecomplete reduction of CO2 to carbon at 290◦C using oxygen-deficient ferrites (ODF) represented by the general formulaMxFe3−xO4−δ, where M is a bivalent metal of Fe, Ni, Co,Cu, Zn, Mg and Mn;δ is the reduction degree of ferrite [35].The ODF is formed by hydrogen reduction as follows:

MxFe3−xO4 + δH2 −→ MxFe3−xO4−δ + δH2O (5)

The ODF is reactive and decomposes CO2 to carbon andoxygen as follows:

MxFe3−xO4−δ + δ/2CO2 −→ MxFe3−xO4 + δ/2C(s) (6)

In Reaction (6), oxygen in the CO2 is transferred in theform of O2− to the oxygen deficient MxFe3−xO4−δ. The car-bon in the CO2 is reduced to carbon by the addition of an elec-tron donated from MxFe3−xO4−δ so as to maintain electricalneutrality during the transfer of O2− to MxFe3−xO4−δ [34].However, the form in which carbon existed was not reportedhere.

As regards Reaction (6) which is considered to be thebasis of CO2 reduction to carbon over ferrites, several re-search groups got involved. Khedr and co-workers foundthe presence of carbon nanotubes during the reoxidation ofdifferent types of freshly reduced oxygen deficient ferrites[35−38]. In these experiments, nano-crystallized metallicphases produced from the reduction of nano-crystallized fer-rites were reoxidized in flowing CO2 in temperatures rangingfrom 400−600◦C. Khedr and Farghali [36] found that both

456 Geof frey S. Simate et al./ Journal of Natural Gas Chemistry Vol. 19 No. 5 2010

Figure 2. Images of CNTs. (a) The field emission scanning electron mi-croscopy (FESEM) image of the sample produced by the reaction of 8.0 gCO2 with 0.5 g metallic lithium at 720◦C; double helical CNTs and the tubu-lar structure with an open cap can be clearly observed. (b) Magnified imageof the boxed areas in (a), depicting clearly the pitting surface of CNTs. (c)TEM image of a typical CNT grown in 0.5 g metallic lithium and 8.0 g CO2system at 600◦C. (d) TEM image of a large CNT grown in the system of8.0 g CO2 and 0.3 g metallic lithium at 700◦C; the graphitic layers of CNTwere eroded, forming porous structure. (e)–(g) TEM images of the CNTsgrown with the addition of 1 ml, 2 ml and 4 ml CCl4, respectively, exhibitingbamboo-like structures. (h) TEM image of a typical CNT obtained in metallicNa-CO2 system at 650◦C, exhibiting bamboo-like structures [31]

single- and multi-walled CNTs with diameters rangingfrom 40–90 nm were formed at low reduction temperatures(400◦C) over a CuFe2O4 catalyst and high reoxidation tem-perature (600◦C). In the case of the reduced CuFe2O4 cata-lyst [37], CO2 was reduced on the surface of metallic phasesto form single- and multi-walled CNTs which were dependenton the particle sizes of the catalysts, i.e., single-walledCNTswere formed with smaller particle sizes (19.5 nm) and multi-walled CNTs were formed with larger particle sizes (38.5 nm).The extent of reduction was also higher for smaller particles.In the case of Fe2O3, CNTs with average diameters of 100–200 nm and average lengthes of 4–5µm were formed [35].Both single- and multi-walled CNTs with diameters rangingfrom 29–40 nm were observed when CO2 was reduced duringthe reoxidation of freshly reduced Cu0.5Zn0.5Fe2O4 [38].

Other studies also showed the formation of CNTs dur-ing the reduction of CO2 over reduced metallic bearing fer-rites [39]. Ling at al. [39] observed that with the increaseof the reaction cycle number, the carbon deposited becamemore and more regular with CNTs diameters varying from

2–30 nm and lengthes ranging from 20–200 nm. It was alsofound that CO2 reduction efficiency is dependent on the com-position of ferrites, and that Mn-Zn ferrites are better than Mn-Ni ferrites [2].

These studies showed that the reactions, in which CO2

is reduced over oxygen deficient metal bearing ferrites, werecontrolled by the interfacial chemical reaction mechanismwith some contribution form the gaseous diffusion mechanismat the initial stages. The mechanism was found to be the solidstate diffusion at the final stages of the reaction [35–38].

3.3. Reduction of CO2 over supported and unsupported tran-sition metal catalysts

Several transition metal catalysts show activity for thegeneration of CNTs [40,41]. The most common catalysts usedare iron, nickel or cobalt [41]. CVD is the most common tech-nique in which these catalysts are used for the production ofCNTs. Usually, as the carbon source decomposes, carbon de-posits onto the catalyst which is supported by a material suchas alumina, CaCO3 and CaO. Carbon has a low solubility inthese metals at high temperatures and thus carbon will pre-cipitate to form nanotubes [42]. In fact the most acceptedgrowth model suggests that after the decomposition of the car-bon source, carbon diffuses into the metal particles until thesolution becomes saturated. Carbon saturation in the metaloc-curs either by reaching the carbon solubility limit in the metalat a given temperature or by lowering the solubility limit viatemperature decrease [43]. Supersaturation of the saturatedsolution then results in precipitation of solid carbon fromthemetal surface [43,44].

Xu and Huang [45] prepared multi-walled CNTs by thecatalytic reduction of CO2 using the CVD technique. In thisprocess, CO2 was reacted with iron supported on CaO underH2 environment in a horizontal quartz tubular reactor at 790–810◦C. The CNTs produced had a variety of morphologies(bamboo-like, open ended, straight and crooked). The reac-tion temperature seems to play an important role in the syn-thesis process producing branched CNTs as the temperatureincreased to 810◦C.

In these experiments, the effects of catalysts support weretested against Al2O3, SiO2 and MgO, but no CNTs were pro-duced. This shows that catalyst support played a key role inthe reduction of CO2. Unlike the other catalyst supports, CaOhas a stronger interaction with CO2. When CO2 meets CaOcatalyst support, it carbonates thus enhancing the adsorptionof CO2 on the surface of the catalyst, which may acceleratethe reduction of CO2 [45].

Using a vertically orientated CVD technique, Moothi [46]and Maphutha [47] compared the structures of carbon nano-materials produced from CO2 and methane (CH4) over an un-supported nickel alloy catalyst (LaNi5). Moothi et al. foundthat CH4 produced well defined CNTs at all the tested tem-peratures (650–850◦C) as shown in Figure 3.

Journal of Natural Gas Chemistry Vol. 19 No. 5 2010 457

Figure 3. Images of (a) CNT produced at 650◦C, (b) CNT produced at 750◦C, with inside diameter and outside diameter of 6.6 nm and 16.6 nm respectively,(c) CNT produced at 750◦C with inside diameter and outside diameter of 10 nm and 23.2 nm respectively, and (d) CNT produced at 850◦C [46,47]

A mixture of carbon nanofibres (CNFs) and carbon nan-otubes was produced at lower temperatures (650–700◦C) us-ing CO2, whereas at higher temperatures (750–850◦C) onlyCNFs were produced as shown in Figure 4. CNFs are cylin-

dric nanostructures with graphene layers arranged as stackedcones, cups or plates. Furthermore, CNFs with graphenelayers wrapped into perfect cylinders are called carbon nan-otubes.

Figure 4. Images of (a) CNF/CNT produced at 650◦C growing from large catalyst particles, (b) CNF/CNT produced at 700◦C, (c) CNF produced at 750◦Cwith catalyst particles attached at the ends, and (d) CNF produced at 850◦C with a circular structure and no catalyst embedded [46,47]

4. Discussion

The results showed that the reduction of scCO2 to CNTswas sensitive to the reductant used, and temperature and pres-sure of the system. This was seen in the facts that at 550◦Cno CNTs were formed in the presence of K or Na, and that inthe temperature range of 600–750◦C different morphologies(helical, rope-like) of CNTs were produced in the presence ofLi reductant. Only bamboo-like CNTs were formed with Na.It was also seen that higher temperature resulted in decreased

yield, but the crystallinity of CNTs was improved, and thelength of CNTs also increased significantly. Higher tempera-tures also resulted in the corrosion of graphitic layers of CNTs[48], leading to the erosion of CNTs. This is because CO2 is amild oxidising agent that could react with graphitic layersofCNTs, leading to the erosion of CNTs. At temperatures lowerthan 600◦C the oxidation reaction could not occur [31].

The bamboo-like structures formed with both Na andNaBH4 reductants seem to suggest that Na atoms play a piv-otal role in their formation. Though Lou and co-workers sug-

458 Geof frey S. Simate et al./ Journal of Natural Gas Chemistry Vol. 19 No. 5 2010

gested that a root-growth process was involved in their stud-ies, much work is still required to understand the mechanismssuggested here.

The results for the reduction of CO2 over oxygen deficientmetal bearing ferrites showed that the reactions are sig-nificantly different with processing methods and composi-tions. It was found that the particle size (or structure) ofthe catalyst, the reduction temperature at which the metallicphases were formed, and the reoxidation temperature affectthe reoxidation of the oxygen deficient metallic bearing fer-rites. At low reduction temperatures, the surface of the sam-ple is highly porous, facilitating the diffusion of CO2, whileat high temperatures the surface is sealed against the diffusionof CO2 gas due to the partial sintering of the formed metal-lic phase. As a result, the reoxidation of higher-temperature-reduced samples showed lower extents, whereas higher ex-tents were obtained for samples reduced at lower tempera-tures. For example, Figure 5 shows the photomicrographs ofnanocrystalline Fe2O3 compacts completely reduced at 450and 600◦C, respectively. It is observed that at 450◦C thesample contains macro- and micropores. By increasing thereduction temperature, the macropores decrease and the com-pacts contain greater micropores while dense metallic struc-ture is formed gradually as shown in Figure 5(b), where thecompacts appear to be of low porosity. So, it is clarified thatthe porosity of the compacts decreases by increasing the re-duction temperature, which certainly affects the reoxidationrate of these compacts.

Figure 5. Photomicrographs of nanocrystalline Fe2O3 completely reduced at(a) 450◦C, and (b) 600◦C (1000x) [35]

Also, higher oxidation temperatures for the samples re-duced at lower reduction temperatures cause partial sinteringand coalescence for the grains, hindering the gas diffusionandits contact to the inner grains. The structure of the catalystsafter reduction affected the extent to which oxygen deficientmetallic bearing ferrites was reoxidized. Higher reoxidationrates were observed for particles with high porosity. Thisis because highly porous structures facilitate the diffusion ofCO2 gas. Particles with dense matrix of metallic phases withlimited amounts of micro-pores had low reduction rates.

On the other hand, CO2 was almost reduced into carbonby active nanometallic phases. The oxygen in CO2 is trans-ferred to the surface of metals in the form of O2− ions andthese ions react with metals to form spinel structures. In or-der to maintain the electrical neutrality of the oxide, activenanometals release electrons, which move to the surface andare donated to the carbon in CO2 [34].

The role of the catalyst support is not only to maintainthe catalytically active phase in a highly dispersed state,butmay actually contribute to the catalytic activity [41,49].WhenCO2 meets the catalyst support such as CaO, it will carbon-ate, therefore, the CaO system would enhance the adsorptionof CO2 on the surface of the catalyst, accelerating the reduc-tion of CO2 [45]. It is also expected that other supports suchas CaCO3 and other transition metals and bimetallic metalsmight work in the decomposition of CO2 to carbon nanotubes.

The presence of hydrogen can either accelerate or sup-press the synthesis of CNTs [50,51]. In the experiments by Xuand Huang [45], H2 was considered beneficial as it providedadditional carbon atoms through the CO hydrogenation. Thisfollowed the thermal splitting of CO2 as follows [1]:

CO2 −→ CO+O2 (7)

and the reaction of CO and H2 (hydrogenation) is as follows:

H2(g) +CO(g) −→ C(s) +H2O(g) (8)

However, other researchers have observed that H2 has a sup-pressing effect on the synthesis of CNTs [52,53]. The sup-pressing effect has been reported to be due to the surface hy-drogenation reactions and also hydrogasification of carbon toform methane [50]. In most cases, H2 is used to reduce thecatalyst [54].

5. Summary: challenges and future prospects

In this review, it has been shown that CO2 could be suc-cessfully used as a carbon source for the synthesis of CNTs us-ing different methods. However, there are several challenges.The use of scCO2 requires abnormally high pressure. For ex-ample, the pressure is autogenic depending on the amountof reactants added and reaction temperature [23], therefore,proper equipment that can withstand pressures up to 10 kbaris required. According to the experimental data, the use ofmetal bearing ferrites has been limited to the conditions wherereduction of the catalyst is possible. These catalysts are used

Journal of Natural Gas Chemistry Vol. 19 No. 5 2010 459

freshly to avoid reoxidation. The use of freshly reduced cata-lysts requires a proper control of the process parameters, andtheir productions need an inert environment.

In the CVD technique in which either a classical catalystwith supports or a floating catalyst is used, the reduction ofCO2 can be represented as:

CO2(g) −→ CO(g) +O2(g) (9)

and the produced carbon monoxide (CO) will be dispropor-tionated as:

CO(g) −→ C(s) +CO2(g) ∆H = −171 kJ·mol−1 (10)

which results in carbon liberation.However, the disproportionation reaction is exothermic

and proceeds towards the initial products at high temperatures.The kinetic and thermodynamic factors limit the effective COdisproportionation reaction in the temperature range of 520–800◦C at normal pressure, and this temperature range may notbe the optimal for carbon dissolution and precipitation fromthe metal particles [43]. Increasing the CO pressure shiftstheeffective CO disproportionation reaction temperature range tohigher temperatures. It has been observed, however, that in-creasing the pressure favors the production of CNTs [20].

Further work to optimize the reaction processes for the re-alization of industrial processes is necessary, and several im-portant issues which are listed below need to be understood.

• The formation mechanism of CNTs with regard to theuse of CO2 as a carbon source.

• Catalyst effects in terms of the transition metal and cat-alyst support choices.

• Is it possible to use metallocenes?• Under what conditions can metal bearing ferrites be

used as organometallic compounds?• What is the effect of temperature? What are the effects

of other process parameters?Understanding of the issues listed above is important for

further progress in this field. While the CNT literature is fullof recipes, growth results and application potential, the pro-duction of CNTs from CO2 using CVD techniques is almostnonexistent. It is hoped that the enormous potential of CNTs,and the much publicised greenhouse effects would stimulateinterest in such complementary studies.

DisclaimerThe contents of this paper reflect the views of the authors who

are responsible for the facts and accuracy of the data presented hereinand do not necessarily reflect the official views or policies of anyagency or institute. This paper does not constitute a standard orspecification, nor is it intended for design, construction, bidding, orpermit purposes. Trade names were used solely for information andnot for product endorsement.

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