Title:Metal-freegraphene-basedcatalyst—insightintothecatalyticactivity:Ashortreview
Author:HuawenHuJohnH.XinHongHuXiaowenWangYeeyeeKongPII:DOI:
Reference:Toappearin:Receiveddate:Reviseddate:Accepteddate:
S0926-860X(14)00747-9
http://dx.doi.org/doi:10.1016/j.apcata.2014.11.041APCATA15134
AppliedCatalysisA:General21-825-1127-11
Pleasecitethisarticleas:H.Hu,J.H.Xin,H.Hu,X.Wang,Y.Kong,Metal-freegraphene-basedcatalystmdashinsightintothecatalyticactivity:Ashortreview,AppliedCatalysisA,General(2014),http://dx.doi.org/10.1016/j.apcata.2014.11.041
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1Metal-free graphene-basedcatalyst―insight into the catalytic activity: A short 2review
34Huawen Hu*, John H. Xin*, Hong Hu, Xiaowen Wang, Yeeyee Kong
t5pi6The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
rc7E-mail: tcxinjh@polyu.edu.hk; Fax: +86-852-2766-6474
s8u9Highlights
10A review for response to the rapidly increasing trend of research on carbocatalysis.
n11Structure and catalytic properties of oxygenated graphene and reduced graphene oxide.
a12The active sites on graphene and its related materials for various catalytic reactions.M 13Mechanistic study on the catalytic performance of graphene-baseddcarbocatalysts. 14Towards real-world applications of metal-free graphene-based carbocatalysts.
e15t16p17e18c19c20A212223Table of Content
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For responding to the rapidly increasing trend of research on carbocatalyst, an emerging metal-free all carbon-based catalyst, we present here a timely short review article to summarize some relevant state-of-the-art contributions, to find thebottleneck of the research in the current stage, and to offer a guideline for rational exploration and design of ahigh-performance graphene-based carbocatalyst for a specific catalytic reaction.Special emphasis is brought to the aspects of what make graphene-basedcarbocatalysts active for many kinds of synthetic transformations, such as reduction and oxidation reactions. Due to the importance of active species identification for graphene materials in the field of carbocatalysis, the possible active sites on the surfaces of graphene and itsrelated materials are summarized and presented schematically as a general model, which can shed light on the mechanistic study on the catalytic performance of graphene-basedcarbocatalysts. Moreover, the difference between oxygenated graphene and reduced graphene oxide are specially reviewed and analyzed in terms of the relationship between their respective unique structures and catalytic performances. Finally, a conclusive model map is provided for combining the respective catalytic reaction models with the corresponding probable active sites on the surfaces of graphene-based carbocatalysts. The article presented here is expected to advance research on the biocompatible, sustainable, low-cost, scalable and green graphene-based carbocatalysts as promising alternatives to the metal-based catalysts that are being under excessive exploitation despite their scarcity,
Accepted M2
A B S T R A C T
anPage 2 of 33
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high cost and relative environmental unfriendliness, especially the noble metallic materials.
Keywords:
1.Introduction
at certain production stage [1], reflecting the pivotal role that the catalytic materials is playing in various industries. The catalytic materials have already made significant contributions to the human development, and will continue to bring increasing scientific, economic, environmental and even social benefits with the advance of science and technology. To date,the most widely explored catalytic materials have been concentrated on the transition metals and metal oxides, hybrid materials or composite structures with the active metallic centers, and organometallic compounds [2-13]. It is worth pointing outthat active metallic particles, especially the nano-structured ones, are ready to aggregatedue to their high surface energy, which leads to the decrease of the accessible active
3
AcceptUp to 90% of all commercially available chemical products involve using catalysts
ed ManPage 3 of 33
uscriptcarbocatalyst; graphene oxide; reduced graphene oxide; metal-free; catalytic activity; active site
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surfaces and hence degradation of the catalytic activity. Besides, particle leaching is another serious problem for the hybrid structures containing the active transition metal and metal oxide particles [14]. Such leaching not only decays the catalytic properties of
be recognized that many metallic resources on earth are becoming scarce, especially the
developing modern industries. It is herein highly desirable to explore promisingcatalytic alternativesbeing metal-free, low-cost,scalable and sustainable in orderto alleviate the
help to balance the exploitation, to stabilize the metallic resources on earth, to reduce the
a result, increasing attention has turned to metal-free catalysts, particularly the carbon materials because they are biocompatible, inexpensive, stable, environmentally friendly and readily available [1, 15, 16].
Since the ground-breaking work by Novoselov and Geim who provided a simple yet
explored its unique electrical properties, graphene and other two-dimensional sp2-hybridized carbon nanomaterials have become tremendously popular in the areas of modern physics, chemistry, and materials science and engineering [17, 18].The significant impact of these materials on different scientific disciplines is attributed to their extraordinary physical and chemical properties such as the huge specific surface area (up to ~2,600 m2 g-1), and remarkable electrical, thermal and mechanical properties of graphene materials [19-22].
4
Acceeffective micromechanical exfoliation method for peeling off graphene from graphite and
pted Menvironmental pollution caused by heavy metals, and to maintain ecological balance. As
anheavy dependence on the metallic resources for many catalytic reactions, which can thus
uscrnoble metallic materials, caused by the excessive exploitation from the rapidly
iptthe catalytic materials but causes secondary environmental pollution. Moreover, it should
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12345Accompanied by the exploration of graphene properties, many feasible routes have been developed to prepare various kinds of graphene materials, which can be classified roughly into top-down and bottom up methods [23]. Among them, the top-down graphite
production of graphene material because it uses naturally abundant, low-cost graphiteas
ptoxide exfoliation method is regarded as the most feasible one for industrial scale
6the raw material, and has much higherproduction yield as compared to other top-down
ir7methods such as micromechanical cleavage [17] and liquid-phase cexfoliation [24]. 8Additionally, the fascinating oxygenated graphene or sgraphene oxide (GO) before
9converted to reduced graphene oxide (r-GO) is 10rather than simply regarded as a precursor to r-GO and nan equally uimportant product to r-GO,
its derivatives [25], due to the
11abundant active sties on GO surfaces a12various kinds of modification, functionalization Mthat can be harnessed in a versatile manner for
and applications. Even though some 13fascinating properties of GO are dseverely impaired such as the electrical conductivity, 14thermal stability and chemical eresistance, the abundant oxygen-containing functional 15groups on the GO surfaces provide rich chemical properties, te.g., affording an attractive
16platform to tether p17[26]. These indicate a great adaptability of GO to many promising applications such as eother catalytically active species such as amine and sulfonic groups
c18high-performance ccomposites [27], drug-release control materials [28, 29], adsorbents 19[30], catalyst supports [31, 32], and intrinsic catalysts on its own [33-36].
A20On another hand, the graphene materialconverted from GO has been demonstrated 21to possess much more structural defects and remaining oxygen functionalities [37, 38] as 22compared to those prepared by bottom-up methods such as chemical vapor deposition 23
(CVD) and epitaxial growth [39], and by some othertop-down methods such as
5
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micromechanical delamination [17] and liquid-phase exfoliation of graphite [24]. Owingto such unique structureswith many beneficial active sites, the graphene derived fromGO is much more suited as the carbocatalysts for many kinds of synthetic
and mechanically cleaved graphene [16, 40]. More importantly, considering the
carbocatalystshold great potential for real-world applicationsin many industries involving catalytic reactions.
graphene as the supports for carrying various semiconductor and metal nanoparticles
applications in catalysis[44-47, 49-53, 55], the graphene moiety in these hybrid or composite materials is not considered as the catalytically active component in the presence of the catalytically active nano-crystals intimately covered on graphene surfaces. It is also worth pointing out that employing graphene materials as the metal-free
enormous potential [18]. In particular, exploration of the catalytic mechanism for a variety oftransformation reactions with metal-free graphene-based catalysts is still in its infancy and much work remains to clarify the active sites forspecific catalytic reactions. Moreover, the non-stoichiometric and inhomogeneous nature of GO poses a substantial challengeto unravel the underlying mechanism ofthe catalytic activity of GO. This also indicatesthe sensitivity and uncertainty of the counterpart r-GO, making it difficult to establish the relationship between the structure and catalytic properties since theyare
6
Accecarbocatalyst to accelerate synthetic transformations is a relatively new area with
pted Mhave been explored extensively and is still a hot topic [41-55], especially for the
anAlthough using GO and its related materialssuch as r-GO and functionalized
uscrsuperiority of low-cost and large-scale production, the GO- and r-GO-based
ipttransformations relative to other kinds of high-quality graphenessuch as CVD-graphene
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varied with reduction conditions. Specifically, the final structure, morphology and properties of r-GO can be affected by the reduction method adopted, the type of reducing agent, addition ratio and concentration of reactants, reaction temperature and pressure,
structure and properties of r-GO, the parameter of the precursor GOshould also not be
In these regards, big challenges but also opportunities are presented for research communities to explore and deeply understand the mechanism of carbocatalysis that is an
deep mechanistic understanding can subsequently facilitate furtherwork on controllable
the undesired active sites from the surfaces of graphene in a controlled manner, and flexible creation of desired active sites on the graphenesurfaces using chemical, physical, thermal, electrical, electrochemical, optical, acoustic method, or thecombined two or more thereof. High-performancegraphene-based carbocatalystscan thereby be designed
that the importance of active site isolation on the heterogeneous catalyst surfaces has also been emphasized by Védrine in his latest review article, with the detailed case studies of metal oxide catalysts to exemplify the concepts [3]. All in all,more effort needs to be investedin the research and development of the sustainable, eco-friendly, cost-effective and scalable graphene-based carbocatalysts for various synthetic reactions in order to realize the real-world applications of the carbocatalysts.If so, at least part of precious
Accefor specific catalytic reactions with high efficiency and selectivity. It is worth mentioning
pted M7
and effective isolation of the active sites on the graphene surfaces, selective removal of
anintriguing and promising research direction in the area of graphene-based catalysis. The
uscrignored, such as oxidation extent, surface area and density of specific oxygen groups [25].
iptpH condition, sonication condition, reaction time, etc. As another variableaffecting the
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123
metalliccatalysts will be saved from the excessive exploitation, and environmentalpollution caused by heavy metals will bereduced.
5Here, we highlight some important work dedicated to exploring graphene-based
pt42.Graphene and its related materials as metal-free catalyst
6materials as carbocatalysts, including GO, r-GO, and functionalized graphene, as well as
ir7graphene allotropes including graphite, fullerene and carbon nanotubes. cMany catalytic 8reduction, oxidation, addition reactions, etc., are covered. The contribution made herein
s9is highly expected to shed light on the mechanistic study on the catalytic activity of the
u10graphene-based carbocatalysts, as well as to offer a guideline for rational exploration and
n11design of a high-performance metal-free carbocatalyst for a specific catalytic reaction.
a12Before reviewing thework on graphene-based carbocatalysts, it isMnecessary to point 13outthat the graphene-basedmaterial,dexplored as carbocatalyst, should betrulymetal-14freeor metallic residues (if any) have negligible influence on the catalytic performance. e15Otherwise, thereports ont“carbocatalyst”becomemeaningless and misleading. In this
16regard, during exploration of a graphene-based carbocatalyst, close attention needs to be
p17paid to themetallicecimpurities even in trace quantities that might be present within the 18graphene-based materials, especiallycthose prepared using graphite as a startingmaterial19[56]. It has been reported thatAthevacancy defect in graphene can strongly stabilize the 20metallic adatoms [57],indicative ofthe difficulties inthorough removal of the metallic 21impurities. This also reveals that to study graphene-based carbocatalystsis a difficult and 22challenging task, especially forclarification of the mechanism for carbocatalysis.
23
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2.1. Graphene allotrope-based catalyst
Because graphite is the parent material of various kinds of graphenes prepared by the top-down method, natural flake graphite is always used as a control to compare its
much lower activity of the parent graphite due to its markedly smaller specific surface
atomthickness, respectively. The shortage of the active species such as oxygen groups, defect holes and graphene edges in graphite is also an important reason for the much
catalytic capability for conversion of a variety of substituted nitrobenzenes to the
harsh conditions were typically required for reasonable production yield [58] (Table 1). Larsen et al. extended to investigate the mechanism of the carbon-catalyzed reduction and found that the graphite could serve as an adsorbent and electrical conductor to enable the reaction to occur [59]. Oh et al. also made a similar conclusion that graphite
conductors for catalytic reduction of nitro compounds [60].
were demonstrated toexhibit catalytic activities for certain reactions. For instance, fullerene was explored as a nonmetal hydrogenation catalyst to activate molecular hydrogen, but the exact mechanism was not clearly explained [61]. Guo et al. suggested that the vacant orbital of fullerene could accept the electron from the bonding orbital of azo dyes, which led to the N=N bond activation and consequent effective catalytic
9
AcIn addition to graphite, other graphene allotropes and their modified derivatives
cecontained graphenic domains, which could act as both sorption sites and electron
pted Mcorresponding anilines (using hydrazine as the terminal reducing agent), even though
anlower activity. Nevertheless, natural graphite has early been demonstrated to have a
uscrarea as compared to that of the single- or few-layer graphenematerials with one- or few-
iptcatalytic activity with that of graphene objectives, with the common results showing the
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reduction of azo groupsin basicaqueous solution under UV irradiation[62]. Additionally, surface-modified carbon nanotubes were verified as efficient heterogeneous catalyst in oxidative dehydrogenation of n-butane to butenes, with the ketonic C=O groups
information about the graphene allotrope-based carbocatalysts is given in Table 1.
2.2. Doped graphenecatalyst
Heteroatom doping has also been extensively employed to functionalize and activate
(ORR),e.g., doping with B and N atoms, which can lead to p- and n-type semiconductors,
For example,satisfactory activitiesfor ORRwere achieved inN-doped carbon materialslikely originated fromthe conjugation between the N lone-pair electrons and the graphene π systems[66, 67]. In addition, a relatively high positive charge density on adjacent carbon atomscanprobablybeafforded by the electron-accepting N atom,
metal-free electrocatalyst for ORR [68, 69]. Moreover, the graphite-like or graphitic Nseems to play a more important role in the enhanced ORR activity than other types of embedded N [65, 66, 70]. In addition to the main contribution from nitrogenfunctionalities, the oxygen groups and structural defects were also considered as contributions to the enhanced electrocatalytic activity of an N doped graphene [71]. It should also be mentioned thatthe electrocatalytic ORR performance of an ideal metal-free doped graphene catalystwassuccessfully predictedtheoretically, with the values of
10
Acceenabling the activation of the carbon atoms and consequently leading toan efficient
pted Mrespectively, is well employed to modify the electrocatalytic activity of graphene [64, 65].
angraphene materials for catalytic reactions, especially the oxygen-reduction reaction
uscrPage 10 of 33
iptdemonstrated as the catalytic active center for the dehydrogenation reaction [63]. More
12345the typicalelectrochemical descriptors comparable to or even higher than those of the state-of-the-art Pt catalyst, by effectivelycombining the density functional theory calculation and experimental study [72].Following this, much more efficient metal-free
level. More information about doped graphene as metal-free catalyst for ORR can be
ptelectrocatalysts for reactions beyond ORRare expected to be designed at the molecular
6found in the recentreview articles [73, 74].
ir7In addition to ORR, reduction of 4-nitrophenol to 4-aminophenol was reported to be c8catalyzed effectively by an N-doped graphene carbocatalyst.sThiscatalytic activity was
9conjectured to stem from the carbon atoms next to the doped N atoms on the N-doped
u10graphene surfaces.In detail,a relatively high positive charge density could be imparted
n11on the adjacent carbon atomsinduced by12weakened conjugation and consequent activation ofMthe electron-accepting N atoms, which led to the
atheadjacent carbon atoms[1].Some 13typical work devoted to exploration of doped graphene as metal-free catalysts, especially d14N doped graphene, is summarized ein Table 2, which contains the comparisonof the 15catalytic performancesof the metal-free doped graphene catalyst andtthe corresponding
16metal-basedcatalyst.
p17ec182.3.
cReactionscatalyzed by GO
19AGiven the rich chemical properties of GO as mentioned before, GO is likely to
20function as the oxidant for some oxidation reactions, and the acidic nature of GO allows 21it to serve as a solid acid for hydration as well. Additionally, even catalytic reduction 22reaction can be mediated by GO. For instance, GO was adopted as a solid acid for 23
synthesis of dipyrromethane and calix[4]pyrrole [75] and for the room temperature ring
11
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opening of epoxides [76]. Interestingly, the sulfonated GO seemed to be much better solid acid catalyst for the dehydration of xylose to furfural in water, as a result of the introduced aryl sulfonic acid groups that were verified to be more acidic and thermally
attached to the GO surfaces [77]. The efficient oxidative coupling of amines to imines
be essential for activation of the catalytic species in GO. The synergistic effect of carboxylic acid groups and unpaired electrons at the edge defects was found to account
demonstrated as the plausible catalytic species on GO for the oxidative dehydrogenation
groups were located around the epoxides leading to the drastically enhanced activation of C–H bond in propane [78].On the other hand, carboxyl and epoxy groups seemedunfavorable for the catalytic conversion of 4-nitrophenol to 4-aminophenol, whereas the hydroxyl and alkoxy radicals, as well as hole defects were demonstrated to facilitate this
was found to have much higher catalytic activity for the aza-Michael addition of amines with activated alkenes to yield the corresponding β-amino compounds. The authors speculated that the oxygen functionalities dispersed on the GO surfaces could beresponsible for thecatalytic feasibility, but the definitive functional groups among the rich chemical functionalities on GO surfaces (the specific active functionalities for the activity) were not investigated in depth [33]. GO was also found to act as an auto-tandem oxidation–hydration–aldol coupling catalyst in facilitating formation of chalcones in a
12
Accecatalytic reduction reaction [79]. Additionally, as compared to the counterpart r-GO, GO
pted Mof propane to propene by first-principles calculations, especially when the hydroxyl
anfor the activity [15]. In addition, epoxy and hydroxyl functional groups were
uscrcould be catalyzed by GO after appropriate base and acid treatments which appeared to
iptstable under the catalytic reaction conditions as compared to other functional groups
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12345single reaction vessel with alkynes or alcohols as the starting materials, but the factors that contributed tothe remarkable reactivity of GO was as yet unknown[25], Future effortscanthusfocus on exploring the factors includingoxidation extent and surface area
GO is summarized in Table 3, including the insight into the catalytic activity and the
ptof GO,density of specific functional groups, etc. More information of the carbocatalyst
6comparison of the catalytic performance of carbocatalyst GO and otherirrelevant catalyst
7available in the articles.
c8s92.4. r-GO-mediated catalytic reactions
u10Concerning r-GO, it is worth mentioning that nits chemical resistance and thermal
11stability should be much higher than the parent GO since the labile oxygen functionalities
a12are largely removed from GO surfaces Mafter the reduction processing. This is rather 13beneficial for utilizing r-GO as a more stable catalyst, in particular for the reactions under d14harsh reaction conditions esuch as those with reductive and high-temperature 15environments. Moreover, tit can be easier to explore the mechanism of the catalytic
16activity due to the fewer variables on the surfaces of r-GO as compared to those on the
p17GO surfaces. However, ecthe lack of the active oxygen-carrying groups on the r-GO 18surfaces cmight have a great influence on the catalytic activity for certain synthetic 19reactions. It is undoubtedly true by considering the big structural difference between GO A20before and after effective reductionin terms ofthe concentration of sp2-hybridized carbon 21domains, graphene edges, oxygenfunctionalities and others, perhaps indicative of the22marked difference in catalytic properties,e.g., as mentioned before, the catalytic activity 23
for the aza-Michael addition of amines to activated alkenes was found to be largely
13
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12345depressed in r-GO as compared with that in GO [33]. Nevertheless, other structural features useful for certain catalytic reactions can be generated after effective reduction of GO. The sp2 carbon domains on GO sheets can be greatly restored and recovered, which
graphene edges can be increased due to removal of carbonyl and carboxyl groups
ptleads to the improved π-system of r-GO. In addition, the number of active crystalline
6decorated at the periphery of the GO planes [80, 81]. The split rof igraphene sheet into
7smaller pieces during the preparation for r-GO is likely to occur as wellc[82], which also 8probablyresults in the increase of graphene edges [83] and formation of new structural
s9defects [20]. Besides, thedecrease of the average u10increased numbershas usually beendetected for nsize of sp2 carbon domains with
r-GO in comparison with GO [38],
11whichis also a possible indication of increase of graphene edges, particularly considering
a12that the sp2 carbon domains can besurrounded Mby defect holes with internal graphene
13edges. Such restoration and average size reduction of the spd2 carbon domains can be well
14confirmed by measuring the eRaman D band to G band intensity ratio from the Raman 15spectra of GO and r-GO. tFurthermore, as mentioned previously, since chemical and
16thermal reduction p17structure such aseof GO can hardly yield a r-GO possessingan ideal graphene-like
cisfound in the graphene produced by micromechanical exfoliation [17]18or CVD [84] method, it is undeniable that some structural defects still remain on the r-c19GO Asurfaces [37]. These structural defects can be:(i) the recalcitrant oxygen 20functionalities,(ii) the topological defects like in-plane distortions and strain either 21initially generated in oxidation process but without effective restoration in the reduction 22process or likely caused by deoxygenation reaction along with the removal of functional 23
groups and release of the yielded gases, and (iii) the defect holes formed within the
14
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restored sp2 carbon domains. The reducing agent might bring some doping effect on r-GO [85], which is also a possible contribution to the catalytic activity of r-GO.
To find whether the described factors relating to the intrinsic activity of r-GO can be
information is given in Table 3). The π-system ofanr-GO provided bothactivation sites
benzene to phenol [86]. In addition, an r-GOwas reported as a catalyst for nitrobenzene
on the r-GO were hypothesized as catalytically activecenters for activation and
believed to serve as the catalytically active sites, and the graphene basal planes(with assistance of thequinone moieties at the edges) acted as the conductor for the electron transfer, which resulted in the effective reduction of nitroaromatic compounds [88]. Tan et al. found that preconcentration of monomers and oxidant on their surfaces makes both
only ther-GO was able to catalyzeelectrochemical polymerization due to the restoredelectronic structure and properties of the r-GO such as π-conjugated structure and electrical conductivity [89]. Likewise, the sorption and electron transfer sites, i.e., graphene π-system, on an r-GO was hypothesized to be responsible for the catalytic activity for reduction of 2,4-Dinitrotoluene [90]. In addition, the metal-like nature of anr-GO, as well as the structural defects and graphene clusters, was supposed as the contribution to the catalytic feasibilityof r-GO for hydrogenation of ethylene at high
15
AcceGO and r-GO active forin situ polymerization of 3-aminophenylboronic acid, whereas
pted Mconsequent reduction of nitrobenzene [87]. Similarly, the zigzag edges ofanr-GO were
anreduction, and the unsaturated carbon atoms at the zigzag edges ofthe r-GO and defects
usinteractions, thus leading toan efficient graphene-catalyzed low-temperature oxidation of
crfor oxidant H2O2 and the adsorption surfaces for benzene through intermolecularπ-π
iptcorroborated by the literature, some typical work is summarized as follows (More
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temperatures [91]. Furthermore, an r-GO prepared by microwaveand thermal reduction method was demonstrated to exhibit significant catalytic effect on reversible dehydrogenation and rehydrogenation properties of LiBH4, but the definitive active
[92].
2.5. Catalytic reactions over functionalized graphene
Benefiting from thesulfonic acid groups, sulfonated graphenes were explored as
reaction [94], and esterification reaction between benzoic acid and isoamylol [95]. A
oxidation of organic dyelikely resulting from the reduction sites,much defect and sp3state of carbon on the hydrogenated graphene that were surmised to facilitate the degradation of H2O2 to yield hydroxyl radicals [96]. Moreover, based on ab initio molecular dynamics simulations, carbon vacancy defects decorated by oxygen groups in
nitromethane and its derivatives through the reaction pathways involving the exchange of protons or oxygens between the oxygen groups and nitromethane and its derivatives[97]. More information of the catalytic reactions over functionalizedgraphene is presented in Table 3.
2.6.
Ac The active sites on graphene and its related materials
cea functionalized graphenewere found to accelerate the thermal decomposition of
pted M16
hydrogenated graphene was also reported as an effective catalyst for Fenton-like
ansolid acid catalyst for efficient acid-catalyzed liquid reactions[93], ester-exchange
uscrPage 16 of 33
iptspecies on the graphene surfaces, responsible for this catalytic activity, were not provided
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We use a general model to schematically illustrate some important active sites on graphene and its related materials, whichlikely act as the active centers for a varietyof catalytic reactions according to the literature, asshown in Scheme 1.
3.Conclusion
carbocatalysts based on graphene and its related materials has been summarized in this short review article, especially for the insights into the mechanisms of catalytic reactions
accelerated by graphene-basedcarbocatalysts, including but not limited to ester-exchange
epoxides, polymerization, reduction of nitroaromatic compounds such asnitrobenzene and 4-nitrophenol, activation of molecular hydrogen, reduction of dyecompounds, oxidative dehydrogenation, ORR, oxidation of benzene to phenol, bionic oxidation and aza-Michael addition. Some typical synthetic transformations catalyzed by specific active
in Scheme 2, with the respective reaction model placed around the corresponding possible active site based on the literature. The oxygen functionalities such as carboxyl, hydroxyl, epoxy, alkoxyl, ketonic and quinone groups, graphene π-system, graphene edges especially the zigzag edges, structural defects like holes on graphene surfaces and structural distortion and strain in graphene networks, vacant orbitals, doped heteroatoms such as N and B, grafted moieties such as sulfonic group, or the combined two or more active sites thereof are anticipated to facilitate a specific synthetic transformation with
17
Accespecies on graphene and its related materials are summarized and schematically presented
pted Mreaction,condensation reactions (such as esterification reaction), ring opening of
andescribed in the literature. There are many kinds of synthetic transformations that can be
uscrIn summary, some state-of-the-art work devoted to exploration of metal-free
iptPage 17 of 33
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high yield, selectivity and recyclability provided that the beneficial sites for the reaction are identified and utilized sufficiently. At the same time, other invalid and catalyticallyunfriendly sites are screened by either removing from the graphene surfaces or converting
Given the high demands of the modern industry in environment, economy and
catalysts will show great potential and significance toreplace part of the existing precious metallic catalystsin the real-world industries, especially when the following aspects can
containingsufficient beneficial active species for corresponding useful catalytic reactions;
how to remove the undesired active species from graphene surfaces in a controlled manner, 4) how to flexibly create new beneficial sites on the graphene surfaces through technologically feasible routes, and 5) how to stabilize the graphene carbocatalyst during the catalytic reaction process. As a consequence, it is highly expected that the
of increasing scientific and engineering interest in the coming years.
Acknowledgements
The funding from RGC of the Hong Kong SAR Government (funding code: PolyU 5316/10E) is greatly acknowledged.
Reference
18
Accecarbocatalysis, particularly for this relating to the graphene-based carbocatalysts, will be
pted M2) how to controllably and effectively isolate and expose the active sites on graphene; 3)
anbe realized: 1) how to select and exploitthe appropriate available graphene material
uscrsustainable development, the affordable, sustainable and green metal-free graphene-based
iptto beneficial sites.
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Table 1.Summary of some work involvinggraphene allotrope-based carbocatalysts.CatalystReaction Typegraphitereduction MTables
anReactanthydrazine hydrate,nitro compound,ethanolhydrazine,nitro compound,isopropanoldithiothreitol,nitro compound,HEPES bufferbusReaction Conditionrefluxing,N2 protectionrefluxing,N2 protectionInsight into ActivityNAcrRemark aRef.[58]edNAgraphitereductiongraphite served as adsorbentNAand electrical conductorgraphite contained graphenicdomains that were bothNAsorption sites and electronconductorsNAwith catalytichydrogenationcapability comparableto noble metal catalystgraphitereductionptorbital shaking,N2 protectionfullerenefullereneSurface-Modified11121314151617
CNTscH2 (1 atm.) with UVmolecular hydrogen,irradiation or H2 (4-5 MPa)hydrogenationnitro compoundsat 120-160 °C without lightirradiationNaBH4,stirring,azo derivatives,reductionUV irradiationNaOH,distilled wateratmospheric pressure,O2,400−450 °C,oxidativebutane,total flow rate ofdehydrogenationHe (as balance)–110−15 mL minceAcvacant orbital of fullerenceenabled the activation of theNAN=N bondketonic carbonyl groupactivated the butane;phosphorus improved theselectivityenhanced selectivity toC4 alkenes ascompared to Mg3V2O8and Mg3V2O7Note: NA denotes not available from the literature; a comparison of the catalytic performances of the graphene allotrope-based carbocatalyst and other relevantcatalyst; b N-[2-hydroxyethyl]-piperazine-N’-[ethanesulfonicacid]; c carbon nanotubes.
26
ipt[59][60][61][62][63]3945-3954.
Page 26 of 33
1Table 2. Summary of some typical work dedicated to the study of doped graphene as metal-free 2catalyst.
CatalystReaction TypeInsight into ActivityRemark aRef.B dopedThe electron-deficient B atoms may function as(relative to Pt catalyst) similargrapheneORRactive sites for oxygen adsorption and activatingperformance, superior long-term stability[64]the O–O bond cleavageand CO tolerancemuch poorer ORR activity with higherPyridinic NC atom adjoining a N atom has a reduced energyonset potential as compared to Pt; thedoped grapheneORRbarrier to ORRpyridinic N may not be an effectivet[65]promoter for ORR activity as previouslyexpectedN dopedelectron-accepting ability of N atoms creates a netbetter electrocatalytic activity, long-termgrapheneORRpositive charge on adjacent carbon atoms enablingoperation stability, and tolerance topready attraction of electrons from the anodecrossover effect than Pti[69]N dopedN functional groups, as well as oxygen groups andmuch higher durability and selectivitygrapheneORRstructural defectsthan Ptr[71]N dopedreduction of 4-a relatively high positive charge density could bewith activity comparable to3
graphenenitrophenolimparted on the adjacent carbon atoms induced bynanostructured Co, Ni, and CoNi alloyc[1]the electron-accepting N atomsfilms reported previously4Note: a comparison of the catalytic performances of doped graphene suand metal-basedcatalyst 5such as the commonly used Pt catalyst for ORR.
67n8Table 3. Summary of some work devoted to ainvestigation of GO, r-GO and functionalized 9
graphene as metal-free catalysts.
M detpeccA27
Page 27 of 33
CatalystReaction TypeInsight into ActivityRemark aRef.[75]123456
comparable with Zeolite HY, HZSM-5(30), Al-condensation of pyrrolesGOsolid acid catalyst (acidic nature of GO)TMwith dialkylketonesMCM-41, and Amberlyst-15comparable to p-CH3–C6H4–SO3H andring opening of epoxidesH2SO4, much better than Norit A andGOwith methanol and othersolid acid catalyst (acidic nature of GO)CH3COOH in terms of conversion andprimary alcoholsselectivitysolid acid catalyst,a significant yield and short reaction time at adehydration of xylose tosulfonated GOaryl sulfonic acid groups were the key activelow catalyst loading in comparison tofurfuralsitesconventional liquid and solid acid catalystssynergistic effect of carboxylic acid groupschemicallyand unpaired electrons at the edge defectsoxidative coupling ofNAtreated GOamines to iminesin traping molecular oxygen and the aminemoleculeshydroxyl and alkoxy radicals, as well aswith reaction rate constant comparable tohydrothermallypreviously reported values for Co and Nireduction of 4-nitrophenolholes as beneficial sites, while epoxy andtreated GOcarboxyl groups as unfavorable sitescatalyststhe presence of hydroxyl groups around theoxidative dehydrogenationcatalytically active epoxides can remarkablyGONAof propaneenhance the C–H bond activation ofpropaneaza-Michael addition ofattributed roughly to the presence of oxygenGOamines to activatedfunctionalities on GO surfaces for theNAalkenesactivityauto-tandem oxidation–GOhydration–aldol couplingNANAreactionsπ system of r-GO offers appropriate H2O2improved benzene coversion and phenolactivation rate and good benzeneselectivity as compared to modified titaniumone-step oxidation ofr-GOadsorption ability, consequently affordingsilicalite; activity towards the formation ofbenzene to phenolthe balanced kinetic control over thephenol is not observed over Pt/SiO2oxidation reactiondefects/edges on r-GO as the catalyticallyr-GOreduction of nitrobenzenecomparable to the Pt/SiO2 catalystactive siteszigzag edges of r-GO activate thenitrobenzene molecule; basal planes ofr-GOreduction of nitrobenzener-GO (with assistance of the quinoneNAmoieties on the edge) enhance electrontransferpreconcentration of monomers and oxidanton the huge surfaces of GO and r-GOfavorable for in situ polymerization of ABA;bGO, r-GONApolymerization of ABAπ system of r-GO facilitates electrontransfer for electrochemical polymerizationof ABAreduction of 2,4-sorption and electron transfer sitesr-GONAdinitrotolueneprovided by graphene π systemmetal-like nature, as well as structuralr-GOhydrogenation of ethyleneNAdefects and graphene clustersmore effective for promoting thedehydrogenation andenormous specific surface area andr-GOrehydrogenation reaction than transition metalrehydrogenation of LiBH4suitable pore size(fluoride and hydroxides)more active than the conventional solid acidheterogeneous solid acid catalyst,sulfatedesterification, hydrationalmost no limitation of mass transfer due tocatalysts (Amberlyst 15, OMC-SO3H, SBA-15-grapheneand Peckmann reactionsits unique sheet structureSO3H)ester-exchange reactionssulfonatedsolid acid catalyst,catalytic activity is obviously superior to that ofof various alcohols withgraphenestrong acidity of the sulfonic acid groupsthe conventional Dowex 50 W × 2ethyl acetatehydrogenatedFenton-like degradation ofreduction sites, much defect and sp3 stateNAgrapheneorganic dyeof carbon supposed as active speciesoutperform more conventional catalysts suchfunctionalizedthermal decomposition ofcarbon vacancy defects decorated byas aluminum monohydroxide and silicaGraphenenitromethaneoxygen functionalitiesnanoparticles[76][77]crusan MedptceAcNote: NA denotes not available from the literature; a comparison of the catalytic performances of the graphene-based carbocatalyst and other relevant catalyst; b3-aminophenylboronicacid.
Figure legend
28
ipt[15][78][79][33][25][86][87][88][89][90][91][92][93][94][96][97, 98]Page 28 of 33
12345678Scheme 1. General schematic model illustrating the possible active sites on graphene and its related materials for a variety of catalytic reactions.Black, red, magenta, blue and green balls represent carbon, oxygen, boron, nitrogen, and sulfur atoms, respectively.Scheme 2. Schematic model map illustrating some typical synthetic transformations using graphene and its related materials as metal-free catalyst. Note that the possible active sites on graphene and its related materials have been marked in Scheme 1.Following this, in the model map, the respective catalytic reaction modelsare placed pt9around the correspondingpossible active sites existing on the surfaces of graphene and its i10related materials based on the literature.Black, red, magenta, rblue and green balls 11represent carbon, oxygen, boron, nitrogen, and sulfur atoms, crespectively.The smaller 12gray ball denotes hydrogen atom.
13su14
naM detpeccA29
Page 29 of 33
*Graphical Abstract (for review)
Accepted ManuPage 30 of 33
scriFigure
Accepted ManPage 31 of 33
uscriptFigure
Accepted ManuPage 32 of 33
scriTable 1.Summary of some work involvinggraphene allotrope-based carbocatalysts.Note: NA denotes not available from the literature; a comparison of the catalytic performances of the graphene allotrope-based carbocatalyst and other relevant catalyst; b N-[2-hydroxyethyl]-piperazine-N’-[ethanesulfonicacid]; c carbon nanotubes.
Table 2. Summary of some typical work dedicated to the study of doped graphene as ptmetal-free catalyst.
iNote: a comparison of the catalytic performances of doped graphene rand metal-basedcatalyst such as the commonly used Pt catalyst for ORR.
csuTable 3. Summary of some work devoted nto investigation of GO, r-GO and functionalized graphene as metal-free catalysts.
Note: NA denotes not available from aMthe literature; a comparison of the catalytic performances of the graphene-based carbocatalyst and other relevant catalyst; b3-aminophenylboronicacid.
detpeccAPage 33 of 33
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