Graphene-Based Nanomaterials for Catalysis - American Chemical

Review pubs.acs.org/IECR

Graphene-Based Nanomaterials for Catalysis Maocong Hu, Zhenhua Yao, and Xianqin Wang* Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, United States ABSTRACT: Graphene is considered as one of the most promising materials in a wide range of applications because of its outstanding electronic, optical, thermal, and mechanical properties. Given its large specific surface area, two-dimensional structure, facile decoration, and high adsorption capacity, numerous graphene-based nanomaterials with unprecedented characteristics have been designed, prepared, and applied in catalysis. In this article, we first reviewed common synthetic methods to prepare graphene-based catalysts followed by critical comments and possible solutions. We then briefly summarized the characterization techniques that were relevant to catalysis applications and their applications in energy conversion, environmental protection, and several other typical fields. Finally, we discussed the challenges and opportunities for the future development of graphene-based nanomaterials in sustainable catalysis. This review provides key information to the catalysis community to design and fabricate graphenebased novel nanomaterials with great performance.

1. INTRODUCTION Since the groundbreaking experiments done by Novoselov and Geim in 2004,1 graphene (G), the amazing nanostructured carbon allotrope,2 has attracted extensive interests in both the scientific and engineering communities in past decade.3−7 Graphene is a single-layer two-dimensional sheet of carbon atoms chemically bonded in a sp2 hybridized configuration with a hexagonal pattern (benzene ring). It has shown many attractive properties, such as quantum hall effect (QHE),8 large theoretical specific surface area (2630 m2 g−1),9 high intrinsic electron mobility (∼200 000 cm2 V−1 s−1),10 high Young’s modulus (∼1 TPa),11 good optical transparency (∼97.7%)11 and excellent thermal conductivity (3000−5000 W m−1 K−1).12 The unique nanostructure and properties make it very promising for potential applications over a wide range of areas, such as separation,13,14 environment,15−17 memory devices,18,19 transistors,20 transparent conducting electrodes,21,22 optical modulator,23 surface-enhanced Raman spectroscopy, 2 4 −2 6 sensor, 27 , 2 8 dye-sensitized solar cell (DSSC),29−32 supercapacitor,33,34 batteries,35−38 fuel cell,39,40 catalysis,41,42 and even medicine.43−45 Given its large specific surface area, good biocompatibility and high adsorption capacity for certain molecules,46,47 graphene and its derivatives can be used as valuable substrates to interact with various species, which makes it possible to serve as catalyst support or catalyst by its own.48 Thus, far, various preparation routes such as micromechanical exfoliation,49 chemical vapor deposition (CVD),50 exfoliation of graphite oxide51−54 and on-surface chemical reactions55−57 have been used for their synthesis. These well designed and well prepared graphene-based catalysts have been employed in numerous catalytic reactions, such as oxygen reduction reaction (ORR), 58,59 Fischer−Tropsch synthesis (FTS), 60,61 CO2 reduction,62,63 water splitting,64,65 selective hydrogenation,66−68 © 2017 American Chemical Society

NOx abatement,69−71 catalytic purification of VOCs,41,54 and wastewater treatment.70,72 The promising and exciting results make the graphene-based catalysts to be considered as the future revolutionary materials in catalysis or chemistry community.73−76 In this review article, we cover recent advances in the preparation, characterization, and application of the graphenebased nanomaterials in catalysis, focusing on energy conversion, environmental protection, and several other typical fields. The major challenges and opportunities for the future development are discussed. This review provides key information to the catalysis community to design and fabricate graphene-based novel nanomaterials with great performance.

2. GRAPHENE-BASED NANOMATERIALS PREPARATION Since graphene’s discovery in 2004,1 many feasible, reliable, and versatile routes have been developed to prepare various kinds of graphene-based nanomaterials, which can be categorized roughly into top-down and bottom-up methods.77,78 This session covers the graphene-based catalytic materials synthesis methods including micromechanical cleavage, exfoliation of graphite oxide (top-down), chemical vapor deposition (CVD), and other novel bottom-up methods. The whole picture of the top-down and bottom-up synthetic strategies to prepare graphene-based nanomaterials for catalysis is illustrated in Figure 1. 2.1. Top-down Routes. The micromechanical cleavage of highly ordered pyrolytic graphite (HOPG) to obtain graphene Received: Revised: Accepted: Published: 3477

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29, 2016 2017 2017 2017 DOI: 10.1021/acs.iecr.6b05048 Ind. Eng. Chem. Res. 2017, 56, 3477−3502

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Industrial & Engineering Chemistry Research

of the prepared pristine graphene by the micromechanical cleavage, which makes it difficult to decorate and affects its applications in the catalysis field. One of the most developed methods to obtain higher yields of graphene-based nanomaterials is the graphite oxide exfoliation followed by post-treatment with catalytically active species under varying reduction approaches.87 It is considered as the most feasible method within the top-down family for the production of graphene material in the industrial scale, because its raw material is naturally abundant low-cost graphite, and has much higher production yield as compared to micromechanical cleavage top-down or CVD bottom-up methods.88 In the first step, graphite is oxidized to graphite oxide with various oxygen functional groups such as hydroxyl, epoxy, alkoxy, carboxyl, and carbonyl on the graphite surface and within the layers. In the following step, the obtained graphite oxide is exfoliated by ultrasonic, mechanical/chemical, or thermal method to produce graphene oxide (GO). These oxygen functionalities increase interlayer distance (d-spacing, d002)89 of graphite oxide and GO since the hybridization of the oxidized carbon atoms is changed from planar sp2 to tetrahedral sp3. This makes the graphite oxide layers hydrophilic, readily intercalating water molecules between the layers, whereas the pristine graphite is hydrophobic. This transformation of solubility makes the uniform dispersion of a catalyst active species on the graphene surface possible and easy. Owing to the unique structures with many beneficial active sites (oxygen functional groups), catalysts or their precursors could deposit on the GO sheets by anchoring with the oxygen functional groups of GO. They are then reduced by a wide range of techniques such as thermal, chemical, and hydro/solvothermal treatment and photoreduction. Moreover, the intrinsic oxygen-doped surface of GO holds a much higher binding energy toward metal atoms than that of pristine graphene, which would increase the stability of supported catalytically active species and their resistance to sintering.90 Furthermore, the redox reaction between graphene oxide and catalytically active species precursors allows for the spontaneous deposition of the catalysts nanoparticles on the basal plane of graphene, which subsequently enhances the dispersion of the catalytically active species.54,91 Finally, GO is cheaper and more readily prepared and available compared to pristine graphene.92 Therefore, the graphene derived from GO is much more suitable as the catalyst support compared to other kinds of graphene from CVD or mechanically cleaved methods. Though the graphite oxide exfoliation method showed excellent adaptability for catalyst preparation, several challenges still remain for its extended application, not the least of which is the existence of residual defects even after so-call “complete” reduction of graphene. The preparation of graphene-based catalysts with this method starts from the synthesis of the highquality 2-dimensional (2D) graphene oxide sheets by strong oxidation. However, it is difficult to completely remove these functional groups in the post-treatment at the current technical stage. The residual defects showed great effects on catalytic performance especially in electrocatalysis and photocatalysis due to their severe influence on the properties including electrical conductivity, charge carrier mobility, surface roughness, electronic band gap, and charge carrier separation. The crucial role of graphene in electrocatalysis is to provide a network with high electron conductance to minimize ohmic losses associated with electron transport.92 However, the presence of residual defects will significantly reduce the

Figure 1. Top-down and bottom-up synthetic strategies for the preparation of graphene-based catalysts.

is well-known because the groundbreaking process harvests single-layer graphene (SLG). Compared to curved structures, such as soot, fullerenes, and nanotubes,79−83 SLG was considered to be thermodynamically unstable in the free state. The success of this simple method convinced the scientific community of the existence of SLG by direct observation of graphene.1 The single layers were obtained by repeated peeling of small mesas of highly oriented pyrolytic graphite with tape (Figure 2). Though this approach was

Figure 2. Groundbreaking experiments for graphene discovery. (A) photograph and (B, C) AFM images of the obtained FLG and SLG; (D) SEM micrograph and (E) schematic view of the experimental devices. Reprinted with permission from ref 1. Copyright 2004, AAAS, Washington, DC.

suggested to be highly reliable and allowed one to prepare a large area (up to 10 μm in size) of few-layer graphene (FLG, less than five layers including SLG) films,1 it is obviously not very efficient and thus was not widely used later except in the study of field effect transistor (FET) after its invention in 2004.84 The micromechanical cleavage route requires a high purity condition, and the SLG randomly locates on the substrate, which may limit its application.85 Furthermore, the SLG only is a small portion among the large quantities of thin graphite flakes (few- to multilayer graphene). Thus, it is not controllable and not suitable for large-scale fabrication processes because of its low reproducibility.2,86 Moreover, it is hard to deposit catalytically active species on the inert surface 3478

DOI: 10.1021/acs.iecr.6b05048 Ind. Eng. Chem. Res. 2017, 56, 3477−3502

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Figure 3. Schematic preparation mechanism of Ag-rGO in the absence and presence of PDDA. Reprinted with permission from ref 98. Copyright 2015, American Chemical Society.

achieve unique properties and benefit catalytic performance. Men et al.98 used polydiallyldimethylammonium chloride (PDDA) as the protective agents to prepare Ag-functionalized graphene electrocatalyst by simultaneous reduction of Ag[(NH3)2]+ and GO (Figure 3). The morphology characterization indicated that Ag nanoparticles uniformly dispersed on the surface of RGO while in turn the graphene aggregation was effectively prevented. Hui et al.54 proposed a one-pot method for MnO2/graphene synthesis through a redox reaction between the carbons of GO and potassium permanganate and through the reduction of GO sheets to graphene sheets at hydrothermal conditions. With suitable loading percentage, the aggregation could be hindered and the well dispersed MnO2 (on graphene sheet surface) was achieved. Another concern is from the oxidation procedure. The insufficient oxidation would result in incomplete exfoliation of graphite to the level of individual graphene sheets and thus showed negative effect on the quality of the graphene-based catalysts. One practical technique to measure the degree of oxidation is the thermogravimetric analysis (TGA) with which the mass loss of the function groups produced in the oxidation procedure can be determined quantitatively. Moreover, much more accurate calculation on the oxidized carbon may be achieved by combining TGA and FTIR or MS or DSC, which provides useful information for decomposed residual components. In particular, attention should be paid to possible residual metals in the produced graphene-based nanomaterials. Many of graphene-based catalysts were considered as “‘metal-free”’ based on the fact that no metal precursors have been intentionally added prior to the thermal, chemical, or hydro/ solvothermal reduction step. However, this claim must be carefully testified. In the initial oxidation step of graphite oxide exfoliation, with either the original99 or modified Hummers’ method,100 the strong oxidant KMnO4 was always introduced. Even with many washings, a trace amount of impurities within the synthesized graphite oxide and GO would still remain. And

electron mobility resulting into limited electron transfer with the catalytically active species, which finally lowers the electrocatalytic performance.92−96 In a typical photocatalytic system with graphene-based catalysts, graphene is expected to play the role as an electron acceptor to enhance separation between the photogenerated electrons and holes formed on the supported catalysts after illumination, which accordingly suppresses the recombination of electron and holes, and consequently enhances the photocatalytic efficiency. The disadvantages mentioned above due to the presence of residual defects will lead to a sluggish separation and undesired reaction performance. This actual “partially reduced” graphene is generally far inferior to chemical vapor deposited (CVD) graphene due to the existence of high disorder originating from the initial strong oxidation. Thus, an efficient reduction process for high-quality graphene production should be modeled. One most recent work done by Voiry et al.53 may be an alternative. Voiry et al. used a simple and rapid method to reduce GO into pristine graphene by 1−2 s long pulses of microwaves. The microwave-reduced GO (MW-rGO) exhibited pristine CVD graphene-like features measured by Raman spectrum. XPS and HRTEM suggested highly a ordered structure in which oxygen functional groups are almost entirely removed. Furthermore, the MW-rGO showed exceptionally high activity for the oxygen evolution reaction (OER). The other issue related to the reduction procedure is that the chemical, electrochemical, or thermal reduction of GO into graphene would lead to irreversible restacking or aggregation of graphene sheets due to the π−π interaction and further partly restore its graphitic structure.97 It may be retarded by intercalating catalytically active species within the graphene layers via the in situ synthesis in which the reduction of GO and the deposition of the catalytically active species occurred simultaneously for the fabrication of graphene-based catalysts. With this method, the formation of a stacked graphitic structure would decrease. Furthermore, incorporation of the catalytically active species onto the graphene or rGO surface with good distribution may 3479


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Figure 4. Illustration of the preparation process of N and S codoped nanoporous graphene: (a) preparation of nanoporous NS-doped graphene by CVD; (b) potential defect structures in NS-doped nanoporous graphene; (c) expected reaction mechanism on nanoporous graphene. Reprinted with permission from ref 111. Copyright 2015, John Wiley & Sons, Inc.

chemical vapor deposition (PECVD)107 and microwaveassisted CVD.108 Furthermore, due to its flexibility, it could produce not only large-area graphene sheets, but also various chemically tuned graphene-based materials, such as heteroatom-doped graphene by depositing the carbon atom and heteroatom simultaneously.109,110 Ito et al.111 successfully prepared N and S codoped nanoporous graphene by the nanoporous-metal-based CVD method with pyridine and thiophene as carbon, nitrogen, and sulfur sources, respectively, and further used in the oxygen reduction reaction with promoted performance (Figure 4). The heteroatom-doped graphene-based photocatalyst prepared by the CVD route with enhanced electrical conductivity, environmental stability, and photocatalytic properties was also reported.112 Even though the breakthrough in making large area SLG and FLG was achieved, there are still several concerns remaining with the CVD route. First, there is still the absence of comprehensive understanding of the growth mechanisms, which results in a lack of guidelines for the optimization of the deposition process. Thus, in most cases the empirical dose of the carbon source and the experimental control of the process temperature profile are the only two approaches one can have. Zangwill et al.113 proposed a simple rate theory of epitaxial graphene growth on close-packed metal surfaces for understanding of the complex atomic assembly kinetics of graphene layers. However, the rate theory and its refinement was developed at the macroscopic level, which means that the revealing underlying mechanisms would be out of its range at atomic level. Moreover, the quantitative account of the measured carbon atom was based on the assumption that graphene islands grew homogeneously via the attachment of five carbon atom clusters (C5) while it is not true in the case of graphene growth on the copper foil using methane as a carbon source under hydrogen atmosphere with different partial pressure.114 The full understanding of the mechanism is still challenging. The other issue that may increase one’s doubt on its application is from the required post-treatment, named chemical etching of the metal substrate, in order to separate and purify the produced graphene from the catalytic metal substrate. However, even with the time-consuming chemical etching the complete removal of the metal substrate cannot be guaranteed.88 Moreover, the residual metallic impurities showed important influence on the electronic and electro-

studies showed that the metal impurities might have a beneficial impact on ORR activity.101 Therefore, it is suggested, more practically, to use X-ray fluorescence (XRF, nondestructive) or inductively coupled plasma mass spectrometry (ICP-MS, destructive) to clarify the actual metal concentration. 2.2. Bottom-up Methods. Chemical vapor deposition (CVD), a typical bottom-up method, was considered as one attractive alternative to produce graphene-based nanomaterials in large scale due to its low cost and ability to produce a high quality FLG films.102 CVD is a chemical process used to produce high quality and high-performance solid materials, which is often used in the semiconductor industry to produce thin films. With the CVD method, few or even single layer graphene with low-defect can be achieved and further used as the support for catalytically active species. Furthermore, the heteroatom-doped graphene, which was widely used as catalyst support and showed intrinsic catalytic activity originating from doping (so-called “metal-free” catalysts), can be prepared by depositing the carbon atom and heteroatom simultaneously. Subsequently, the catalytically active sites can be introduced to the pristine graphene (or doped-graphene) support from the CVD by the traditional catalyst preparation methods such as wetness impregnation and deposition precipitation,103 though the challenge still remains as mentioned in the above graphite oxide exfoliation route. Therefore, scientists in the graphene community adopted it to make the individual graphene by the epitaxial growth of graphene films on single crystalline and polycrystalline transition metal surfaces such as Ni, Cu, Co, Pt, Ir, and Ru.78,104−106 The metal substrate played the role as the seed for the growth of graphene. Under low pressure or ultrahigh vacuum conditions, carbon atoms from the gaseous carbon precursor would deposit on the exposed transition metal surfaces in the sp2 bonding arrangement to form graphene. Therefore, it became possible to manipulate the graphene synthesis at the atomic level and further provided excellent homogeneity. After etching off the metals, the obtained large area epitaxial graphene films (up to a few micrometers in size) could be subsequently transferred to other substrates for application. CVD has been viewed as the most versatile and commercially viable technique for the manufacturing of continuous graphene films to meet the industrial demand for “electronic-grade” materials. Several enhanced techniques based on CVD were also developed such as plasma enhanced 3480


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Figure 5. Growth of graphene on Cu foils assisted by a continuous oxygen supply: (a, b) schematic and side view of experimental design; (c) AES of the SiO2/Si substrate; (d−g) optical images of graphene domains on the back or front surfaces of the Cu foils using different supporting substrate. Reprinted with permission from ref 123. Copyright 2016, Nature Publishing Group.

Figure 6. Schematic drawing of the synthesis route and characterization results of boron-doped graphene nanoribbons (B-GNRs):55 (a) reaction schematic drawing; (b) STM of molecules, polymerized by annealing at 180 °C and (c) after the tip-induced manipulation; (d) B-GNR, synthesized by annealing at 400 °C, scale bar 30 nm; (e−g) high-resolution STM topographies (left) and corresponding simulated STM images (right) of BGNR; (h) chemical structure of B-GNR. Black and pink balls indicate carbon and boron atoms.

copper foils can be at 60 μm s−1. The high growth rate was achieved by placing the copper foil above an oxide substrate which provided a continuous supply of oxygen to the surface of the copper catalyst during the CVD growth and further significantly lowered the energy barrier to the decomposition of the carbon feedstock and increased the growth rate. One recent work reported by Kawai et al.55 demonstrated a novel bottom-up method for graphene-based nanomaterial synthesis, named on-surface chemical reaction growth, which is completely different from CVD. Here, two pristine anthracene units and one 9,10-dibora-9,10-dihydroanthracene moiety were employed as precursors. They were covalently bonded while two bromine atoms were located at both termini as shown in Figure 6. Then, the bromine atoms were removed by surface catalytic dehalogenation on the Au surface by annealing at 180 °C, leading to linearly polymerization of the precursor molecules. Finally, the boron-doped graphene nanoribbons

chemical properties of graphene which may affect the reaction performance of the graphene-based catalysts.115,116 Therefore, in order to identify the source of promoted effect when the involved graphene-based catalyst was obtained by CVD, X-ray fluorescence (XRF, nondestructive) or inductively coupled plasma mass spectrometry (ICP-MS, destructive) can be used to determine the residual metal concentration of the graphenebased catalyst prior to further investigation. Another issue with the CVD method is the efficiency. The rates of growth for graphene by traditional CVD on copper were low (typically in 0.03−0.36 μm s−1),117−119 resulting in that the synthesis of large scale of SLG and FLG domains takes at least a few hours which limited its potential application in industry. One most recent work reported by Xu et al.120 may partially solve this problem (Figure 5), where the authors proposed one novel ultrafast growth of SLG with CVD assisted by a continuous oxygen supply and the growth rate of SLG on 3481


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Figure 7. Main techniques for graphene-based catalysts characterization.

Figure 8. DFT calculation for mechanism exploration of electrochemical reactions (ORR, OER, and HER) over trifunctional defective graphene catalyst. (a) 5-8-5 defect; (b) edge pentagon; (c) 7-55-7 defect. (d−f) Schematic energy profiles for the ORR pathway, the OER pathway, and the HER pathway on defective graphene in alkaline/acidic media. Reprinted with permission from ref 151. Copyright 2016, John Wiley & Sons, Inc.

“molecular assembly” route of this method, while in a typical CVD deposition species on the substrate are atomic level and more energy is consumed to decompose the hydrocarbons precursors to carbon atom. The other advantage is the extensive potential applications of the easily produced BGNRs (or extended to heteroatom-doped graphene). Though

(B-GNRs) were achieved by surface-assisted cyclodehydrogenation at an elevated temperature of up to 400 °C. The first advantage of this method superior to CVD is energy conservation. The temperature required in the process is much lower than that of CVD (∼1000 °C or even higher104) which would save lots of energy. It is attributed to the 3482


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defect, which were plotted, labeled, and highlighted with green color in Figure 8a−c. Finally, the minimum energy pathways for the ORR, OER, and HER were calculated, and five defect atomic sites (5-1, 585-1, 585-3, 7557-1, and 7557-4) with the highest catalytic activity were shown in Figure 8d−f. By comparing the smallest activation barrier, minimum potential, and lowest free energy, the authors successfully identified the most active sites for the ORR, OER, and HER, respectively, which were consistent with the reported results. DFT already shows the great potential on graphene-based catalysis with such short time of its application. Moreover, it is highly advised to combine DFT computational results with experimental observation. 3.2. Raman Spectroscopy. As a quick and facile way to investigate the structure and quality of carbon-based materials, Raman spectroscopy is a powerful analytical technique for qualitative and quantitative analysis of graphene and graphenebased nanomaterials. Two recent review articles focused on Raman’s application in graphene characterization were separately presented.152,153 Generally, the D peak (at 1320− 1350 cm−1), G peak (at 1570−1585 cm−1), and 2D or named G′ peak (at 2640−2680 cm−1) bands are the predominant features in the spectrum of graphene and its derivatives. In some cases,154,155 the D′ peak will appear at ∼1602−1625 cm−1. The D peak represents the defects and disordered carbon in the graphite layers while the G and 2D bands are from twodimensional hexagonal lattice. Some structure information obtained from Raman is very helpful in understanding the catalytic performance of graphene-based nanomaterials. For example, the intensity ratio of the D to G band (ID/IG) is usually used to qualitatively compare the disorder and defects among different graphene-based nanomaterials, which are related to their surface properties and adsorption capability.156−158 The 2D band always seen in the Raman spectra of graphene can be used to determine the number of graphene layers (particularly sensitive for few layer graphene, FLG) which greatly affects the surface area. This technique was originally proposed by Ferrari et al.159 and had been widely adopted.155,160−163 After curve fitting, the multipeak 2D band could be deconvoluted to two or several subpeaks. Then the number of layers could be determined based on the value of difference in frequencies of the two dominant subcomponents Δ21 (Figure 9). Moreover, in-plane crystallite sizes (La) could also be determined quantitatively from the Raman spectra of the graphene according to the Tuinstra-Koenig (TK) relation, La(nm) = (2.4 × 10−10) λ4(ID/IG)−1 (λ is the Raman excitation wavelength).156 It is worth noting that some graphene derived from the graphite oxide exfoliation method may show extensive defects due to overoxidation or inappropriate reduction, which resulted into that some of the above-mentioned techniques may not be carried out on these samples, such as the layer number determination by 2D band deconvolution. 3.3. X-ray Photoelectron Spectroscopy (XPS). XPS is one of the best tools for surface, especially modified surface, study which is crucial for catalysis. However, it was not regarded as a major technique for characterizing graphenebased catalysts164 until the recent great emergence of the heteroatom-doped graphene nanomaterials for catalysis.165,166 Now it is considered as the standard technique for the study of this kind of catalysts because it allows investigation of the distribution and bonding of the heteroatom dopants.167,168 The XPS spectrum could confirm the presence of the dopants within the graphene structure and further reveal the types of

the future applications indicated by the authors are for dynamic control and transistor, use as catalyst may be a promising option. The excellent Lewis acidity of the B-GNRs allowed NO molecule adsorption on it by bounding the N atom to the B site of BC3 moieties in B-GNRs,55 which may be further used for NOx abatement. Moreover, it was also found that the BC3 sites were doped uniformly at designated positions with a fixed doping ratio according to the precursor. Therefore, it may be employed as an attractive strategy for synthesizing the emerging boron and other heteroatom-doped graphene catalysts which can be further used in oxygen reduction and other reactions.121−124

3. CHARACTERIZATION TECHNIQUES FOR GRAPHENE-BASED CATALYSTS Lots of review articles were presented with highlights in the unique physical, chemical, and mechanical properties of graphene-based nanomaterials while the characterization was always one of the most important parts among the whole text.125−132 Among tens of characterization techniques included in current literature, several that are highly related to catalysis are briefly introduced in this review (Figure 7). In practical application, coupling more than two tools is usually recommended since an individual technique often provides partial information and is not able to accurately characterize intrinsic properties of graphene-based catalysts. In particular, Density Functional Theory (DFT) computations are also introduced in this section due to their extensive applications when combined with the mentioned characterization techniques to rationalize experimental results in graphene-based catalysis.133−135 3.1. Density Functional Theory (DFT) Computations. DFT can describe and predict the chemical and physical properties of pure and functionalized materials by investigating the electronic structure.136−138 With DFT computations, many graphene-based nanomaterials have been explored and designed, and fantastic properties are disclosed. Two main classes of standard DFT, plane-wave DFT (such as VASP,139 SIESTA,139 CASTEP,140 ABINIT,141 and Quantum ESPRESSO142) and local orbitals DFT (such as Gaussian,143 ADF,144 and TURBOMOLE145), have been implemented for graphenebased catalysis. The results derived from DFT over graphenebased catalysts have been widely adopted for determining the rate-limiting step146 and active sites,147 investigating adsorption and activation mechanisms,148 activation energy calculations,149 and catalytic pathway discussions,150 which cover almost all catalysis related topics. For example, Jia et al.151 used DFT to understand the underlying catalytic mechanisms displayed by the defect graphene for three different electrochemical reactions, ORR, OER, and HER (Figure 8). Four DFT computational models were first developed based on the experimentally observed types of defect sites by aberrationcorrected high-resolution transmission electron microscopy (AC-HRTEM), that is, edge pentagon, 585, 7557, and 5775 defects where the Arabic numerals represent the shape of the defects. After the analysis of the frontier molecular orbitals, the edge atoms around edge pentagon, 585 defect, and 7557 defect were found to contribute to the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) while those from 5775 defect showed negligible effect. Since the catalytic reaction is highly correlated with HOMO/LUMO orbital distributions, the calculation process was further limited to the edge pentagon, 585 defect, and 7557 3483


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Figure 9. Raman spectroscopy of graphene, mechanism, spectrum, and layers number determination: (a) scattering mechanisms of various Raman modes; (b) typical Raman spectrum of graphene; (c) G′ (i.e., 2D) band Raman spectra deconvolution for layers number determination. Reprinted with permission from ref 155. Copyright 2007, AIP Publishing LLC.

Figure 10. STM images of nitrogen doped graphene: (a) isolated bilayer N-doped graphene; (b,c) high resolution images with defects arranged in different configurations with different Vbias; (d) simulated STM image for image c. The inserted schematic structures represent N-doping graphene. (Reprinted with permission from ref 154. Copyright 2011, American Chemical Society).

the presence qualitatively based on the different binding energies after curve fitting (i.e., deconvolution). For example, three kinds of nitrogen were identified in nitrogen-doped graphene (NG) by deconvoluting the N 1s spectrum. They were assigned to pyridinic N (398.1−399.3 eV), pyrrolic N (399.8−401.2 eV), and quaternary N (401.1−402.7 eV).169 Moreover, the level of dopant on the surface can be determined quantitatively by the ratio of peak intensity. For instance, the sulfur content of sulfur-doped graphene was determined to 0.26−4.27 atom % by the ratio of peak intensity between S 2s and C 1s.170 Furthermore, XPS also helps with detection of the active sites and further reveals the related reaction mechanism. Xing et al.171 performed synchrotron-based XPS to compare the amount of the oxygen reduction intermediate OH(ads) on NG catalysts before and after ORR. And then they concluded that the carbon atoms close to pyridinic nitrogen were the main active sites among the different nitrogen doping configurations based on the XPS observations. 3.4. Scanning Tunneling Microscopy (STM). STM images are useful in determining morphology and the presence of defects on graphene when catalytically active species are doped or due to its preparation method. This technique is based on the probe of charge density at the Fermi level. By applying different bias voltage (Vbias) between the tip and sample, the electrons moving direction could be changed and then the lowest unoccupied or highest occupied states of the specimen will be probed. Atomic resolution images of graphene-based nanomaterials could be obtained due to the sharp tip used. The application of this technique keeps increasing, such as the investigation on the electronic properties of doped graphene performed by both theoretical simulation172 and experimental study (Figure 10).154 3.5. Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS). Brønsted and Lewis acid sites, two of the most important properties of catalysts, can be distinguished by differentiating vibrational bands of the adsorbed basic probe molecules using Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS). Wang et al.173 used GO as a facile acid catalyst for the conversion of carbohydrates into 5-ethoxymethylfurfural (EMF), where the

types of acid sites in GO played a crucial role and was investigated by using NH3 as a probe molecule. The Brønsted acid sites in GO were identified based on the characteristic bands at ∼1410 and ∼1730 cm−1, which are formed from NH4+ because of the reaction between NH3 molecules and Brønsted acid sites. Lewis acid sites in GO were determined according to the band at ∼1600 cm−1 attributed to NH3 coordinated to the Lewis acid sites. The authors compared the signal intensity of catalyst before and after use and observed that Brønsted acid sites were reduced while Lewis acid sites were increased. And they concluded that it could be ascribed to the partial thermal reduction during the catalytic reaction, leading to a minor decrease in the acid strength of GO and finally resulting into the slight drop in the catalytic activity of the catalyst for the conversion of fructose to EMF. One recent work done by Xiao et al.174 investigated the condition of ruthenium active sites with the DRIFTS, where few-layer graphene supported ruthenium catalysts were employed for the conversion of levulinic acid (LA) to γ-valerolactone (GVL). CO was used as the probe molecule. Only one single CO adsorption peak at 1977 cm−1 was observed on Ru supported on graphene (Ru/ FLG), while two peaks were observed at 1977 and 2038 cm−1, respectively, over commercial catalyst Ru supported on activated carbon (Ru/C). Combining these results with the results of XPS, the authors assigned the peak at 1977 cm−1 to linear CO adsorbed on defect sites on the Ru nanoparticle surface, which indicated that Ru/FLG possessed more Ru(0) atoms on the surface in comparison with Ru/C. Furthermore, they concluded that these electron-rich Ru(0) atoms could be the reason for the superior catalytic properties of Ru/FLG in LA to GVL conversion. 3.6. XANES and XAFS. X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (XAFS) spectroscopy were also employed by many researchers to characterize graphene-based catalysts. For example, El-Sawy 3484


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Figure 11. Several techniques for graphene-based catalyst characterization. (a) AFM image of black dye N749 covalently functionalized reduced graphene oxide (rGO-N749). Reprinted with permission from ref 270. Copyright 2015, American Chemical Society; (b) XRD patterns of different graphene-based samples and (d) SEM of magnetically separable graphene oxide supported molybdenum (Fe3O4/GO-Mo). Reprinted with permission from ref 271. Copyright 2017, Elsevier; (c) HRTEM and particle size histogram of Pt(0)/DPA@GO nanoparticles. Reprinted with permission from ref 272. Copyright 2016, Elsevier.

et al.175 integrated the sulfur K-edge by XANES and scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) to confirm the increased incorporation of heterocyclic sulfur in sulfur-doped carbon nanotubes. Moreover, the authors proved that the doped sulfur sites were the active sites for the oxygen evolution reaction (OER) and revealed the correlation of the superior activity and stability of the synthesized catalyst with the increase of sulfur doping based on the results from XANES. XAFS analysis combined with other characterization methods were used by Xing et al.176 to obtain the detail information on iron/graphene-cobalt(II) interactions at different initial pH values. Coordination numbers and interatomic distances of both Fe and Co were determined. Furthermore, the adsorption mechanism of Co(II) was proposed to be the inner-sphere complexation and dissolution/reprecipitation of the substituted metal oxides based on the results of XAFS spectra and other characterization techniques. 3.7. Other Techniques. Owing to the number of the graphene layers that showed a critical effect on the properties of graphene-based nanomaterials and their further performance for catalysis, several tools were utilized to characterize this predominant feature. Atomic force microscopy (AFM) is considered as the leading method for the layer number counting due to its convenience and reliability. AFM uses the tip to scan across the surface and analyze the topography of the sample. Thus, the number of the layers could be counted by differential height measurements at the edge. With the X-ray diffraction (XRD) pattern, the number can be determined by using the Scherrer equation combined with Lorentzian fitting of the (002) reflection.177 Moreover, the crystalline structure of the graphene-based catalyst, another important feature for

catalysis, can also be evaluated by XRD. The peak at 2θ = 26° is for pristine graphite, while that at 2θ = 11° is attributed to the graphene oxide. The high resolution transmission electron microscopy (HRTEM) also provides information on the number of graphene layers based on TEM image of the folds formed at the edge. Furthermore, HRTEM can be employed to characterize the crystalline character of graphene by its electron diffraction pattern.178 A Brunauer−Emmett−Teller (BET) surface obtained from nitrogen adsorption−desorption at 77 K could offer some indirect support for the layer number identification qualitatively. Theoretical calculations indicate that the highest surface a single layer graphene can hold is 2630 m2/ g. Because of the random agglomeration and overlap of exfoliated graphene sheets, the actual area of graphene-based catalysts is usually much lower than the value. However, one may compare the layer number of samples by looking at the surface area data.54 For metal catalysts supported on graphene, temperature-programmed reduction (TPR) is usually used to investigate the oxidation state of the metal species and the related metal−support (graphene and its derivatives) interactions.179,180 Moreover, CO chemisorption can be adopted to further elucidate the nature of active sites (dispersion and amount) in the catalysts.181−183 NH3/CO2-TPD analyses are considered as the reliable and convenient techniques to characterize the acidity/basicity of graphene-based catalyst and were widely used in the related studies over graphenebased catalysts.184−186 In addition to the above tools, scanning electron microscope (SEM),187 selected area electron diffraction (SAED),188 energy dispersive spectroscopy (EDS),189 inductively coupled plasma mass spectrometry (ICP-MS),190 UV−vis diffuse reflectance spectroscopy,191 Fourier transform infrared spectroscopy (FTIR),192 and thermogravimetric 3485


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Industrial & Engineering Chemistry Research Table 1. Preparation, Characterization, and Application of Graphene-Based Nanomaterials for Catalysis catalystc

preparation method

characterization techniques

application

ref

G PtM/rGO Pt/G-PCNF Ag/N-rGO NG N−S/G N−P−O/G N−S/G Pt/CNT-G lanthanides/G N−P/G Co−N/G N-PC@G N-GQDs/g-C3N4 Cu2O/FLG g-C3N4/rGO Pt/TiO2-rGO rGO/BiOCl Fe/rGO Ru/G CZZA/rGO Cu/p-NG G(3D) Z-scheme Cu2O/G/TNA Pt-rGO ZnO/G Mn-MGO NRGO@Co NG CN/RGO/ZS CN-G TiO2/G/Cu2O MnO2/G ZnO/G TiO2/G Pd/N-rGO G Co/NG NG Au/G Au3/(N)G B, N-G

mechanical cleavage hydrothermala thermal annealinga thermal annealinga thermal annealinga CVD gas exfoliated CVD thermal annealinga thermal annealinga thermal annealinga thermal annealinga thermal annealinga hydrothermala thermal annealinga b photoreductiona hydrothermala hydrothermala CVD chemical reductiona thermal annealinga CVD photoreductiona electrochemicala Hydrothermala b hydrothermala thermal annealinga thermal annealinga bath precipitation thermal annealinga CVD hydrothermala hydrothermala hydrothermala hydrothermala pyrolysis exfoliation thermal annealinga b b b thermal annealinga

TEM, AFM, XPS, Raman, DFT HRTEM, XRD, ICP-MS, XPS XRD, SEM, TEM, XPS XPS, EDS, Raman, TEM, XRD SEM, BET, XRD, Raman, XPS HRTEM, SEM, Raman, XPS, BET Raman, XPS, SEM, HRTEM, DFT HRTEM, XPS, DFT, SEM, Raman, BET HRTEM, XPS, ICP, Raman, DFT, BET SEM, TEM, XPS, BET, Raman SEM, XPS, XRD, FTIR, Raman SEM, TEM, BET, XRD, XPS XRD, SEM, TEM, Raman, XPS, BET SEM, XRD, UV−vis, BET, FTIR, XPS ICP, XRD, UV−vis, AFM, TEM, XPS DFT HRTEM, UV−vis, EPR, TA XRD, SEM, TEM, XPS, UV−vis HRTEM, XPS, Raman, TEM, XRD, XPS, UV−vis BET, XRD, SEM, TPR, TPD HRTEM, SEM, XPS SEM, HRTEM, XPS, Raman, DFT XRD, XPS SEM, XRD, Raman, XPS, UV−vis XRD, XPS, TEM, AFM, SEM DFT SEM, TEM, XRD, XPS HRTEM, SEM, XRD, Raman, TGA XRD, FTIR, TGA, BET, XPS, Raman XRD, HRTEM, XPS, FTIR, UV−vis TEM, XPS, XRD, BET, FTIR, UV−vis XRD, TEM, XPS, UV−vis TEM, XPS, XRD, BET, ICP, FTIR SEM, HRTEM, XRD, SAED, XPS XRD, SEM, TEM, UV−vis, FTIR XRD, FT-IR, XPS, TEM, Raman TEM, AFM, TPD, XPS, Raman, XRF XRD, SEM, XPS, FTIR, Raman, BET DFT DFT DFT TEM, XPS, TPD, TGA, Raman, DFT

ORR ORR ORR ORR ORR ORR ORR HER HER HER, ORR HER, ORR OER, ORR OER, ORR water splitting water splitting water splitting water splitting water splitting F-T synthesis F-T synthesis CO2 reduction CO2 reduction CO2 reduction CO2 reduction CO2 reduction CO2 reduction CO2 reduction water treatment water treatment water treatment water treatment water treatment water treatment air purification air purification air purification hydrogenation hydrogenation oxidation oxidation hydrochlorination hydrochlorination hydrochlorination

194 198 199 200 205 208 209 111 217 218 219 222 224 212 214 215 216 225 231 238 242 243 244 245 63 246 243 248 249 250 251 252 253 54 254 255 67 48 257 258 268 269 124

a

Prepared based on graphite oxide exfoliation with different reduction methods. bDFT computation only research. cFull name of the catalysts refer to Nomenclature section.

analysis (TGA)193 were also used to characterize graphenebased catalysts (Figure 11).

Some other important applications are also introduced, such as selective hydrogenation, oxidation, and acetylene hydrochlorination. Table 1 is a summary of the reactions on graphenebased catalysts. 4.1. Energy-Related Reactions. 4.1.1. Oxygen Reduction Reaction. One emerging application of graphene-based nanomaterial in catalysis is for ORR, the invariable cathode reaction and the main rate-determining step in polymer electrolyte membrane fuel cells (PEMFC). The attractive nature of graphene for ORR includes highly graphitic features leading to resistivity to carbon corrosion during fuel cell operation, high surface areas, and electronic conductivity serving as ideal support for uniformly distributing catalytically active nanoparticles. Accordingly, the role of graphene was involved as

4. APPLICATION OF GRAPHENE-BASED NANOMATERIALS IN CATALYSIS Graphene and graphene-based nanomaterials are of considerable interest in catalysis due to unique structural and electronic properties of graphene. Over the past decade, these materials have been used in numerous reactions and shown exciting and promising performance. In this session, the particular attention would be paid to energy conversion and environmental protection reactions including oxygen reduction reaction (ORR), water splitting, Fischer−Tropsch synthesis (FTS), CO2 reduction, water treatment, and air purification. 3486


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Figure 12. Preparation and application of N−P−O codoped 3D graphene with enhanced electrochemical activity for a supercapacitor and ORR. Reprinted with permission from ref 209. Copyright 2016, Elsevier.

catalyst on its own,194 support for the most used platinum (Pt) and nonprecious metals,195 and the substrate for heteroatom doped catalysts.196 Pristine graphene was used as an efficient catalyst for ORR because DFT calculations indicated zigzag edges with lots of defects which are chemically active.194,197 Moreover, by cutting into small nanosheets with the ball milling device (micromechanical cleavage method), the zigzag edge density and the ratio of edge atoms to bulk atoms of graphene would increase, which led to the significant promotion of the ORR activity.194 However, the application with pristine graphene on ORR may be limited by low reproducibility since controlling the ratio of edge atoms to bulk atoms is still challenging at the current technical stage. The application of graphene as the support for Pt is a widely adopted approach to promote the performance of platinumbased catalysts for ORR. It can be roughly classified into pristine graphene (or rGO) and modified graphene supports. Li et al.198 prepared a series of PtM(M = Co, Ni)/rGO nanocomposites by a graphite oxide exfoliation method followed by a facile hydrothermal reduction treatment. The low loss of the electrochemical active surface area (ECSA) value for the PtNi/rGO indicated that stability was significantly improved. The authors proposed that the interaction between PtNi with rGO may be the main reason for the high performance. Fu et al.199 used graphene-porous carbon nanofiber (G-PCNF) and graphene-carbon nanofiber (GCNF) hybrid materials as unique Pt nanoparticles supports for ORR. The modified graphene supports were prepared by the graphite oxide exfoliation method followed by thermal annealing reduction. The ORR results showed that the onset potential of Pt/G-PCNF (38 mV) was higher than that of the commercial catalyst Pt/C (JM20) and the mass activity (at 0.8 V) was twice higher than that of JM20. Moreover, better stability was also achieved over Pt/G-PCNF; ECSA maintained 50.4% after accelerated durability testing (ADT) while JM20 only retained 17.0%. The authors attributed the superior performance to the incorporation of 1D PCNF and CNF to 2D graphene as spacers. With this structure, the high intrinsic surface area of graphene could be fully utilized and the Pt nanoparticles could be deposited on the hybrid supports uniformly resulting in high activity and long stability. In consideration of the high cost of platinum, the nonmetalbased catalysts were developed for ORR and graphene was

considered as an alternative support. Soo et al.200 proposed a silver-incorporated nitrogen-doped reduced graphene oxide (Ag/N-rGO) synthesized from the simple annealing of metal salts with graphene oxide and melamine. XPS and EDS were used to confirm the presence of silver and nitrogen atoms in Ag/N-rGO. Compared to that of the rGO, more Ag were loaded on the N-rGO surface. Electrochemical results suggested that N-rGO was a better support for Ag than rGO. Moreover, the ORR performance, including diffusion-limited current density (−4.37 mA cm−2, at −0.5 V vs Ag/AgCl), ECSA, the number of transferred electrons (four electron pathways) and stability (methanol tolerance), were comparable or even better than those from the commercial Pt/C catalyst. The authors ascribed the superior performance to the increased number of active sites from the formation of N-doped sites and higher Ag loading in Ag/N-rGO. Another emerging route for replacing the platinum-based catalyst in ORR is using the so-called “metal-free” carbon-based materials including heteroatom-doped graphene.201 Numerous results202−204 have been published on this field since the first work was reported by Qu et al. using nitrogen-doped graphene (NG) as an ORR electrocatalyst.165 Heteroatoms including nitrogen,205 boron,123 sulfur,206 selenium,206 and halogen207 have been widely studied. Bayram et al.205 reported the synthesis of NG in a two-step solution-based scalable procedure by direct annealing of GO/melamine gel mixture. The pregelation of melamine on GO sheets served as a spacer resulting into the increase in specific surface area of GO. Moreover, compared with the commercial Pt/C, NG exhibited higher current density and better stability in both acidic and alkaline electrolytes due to the synergetic effect of surface graphitic N groups and larger surface area of NG. The two or three heteroatoms codoped graphene catalysts were also investigated.208 Zhao et al. reported a universal and readily scalable strategy to produce an N−P−O codoped free-standing 3D graphene through a one-pot red phosphorus-assisted “cutting-thin” technique.209 The continuously 3D hierarchical porous (3D-HPG) structure with good quality (ID/IG = 0.4) was formed with this approach. DFT calculations suggested that the charge delocalization was significantly enhanced, which benefited the electrochemical activity in subsequent supercapacitor and ORR tests (Figure 12). Although excellent progress has been achieved on the application of graphene involved catalysts for ORR, challenges 3487


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Industrial & Engineering Chemistry Research still remain. Stability is the first obstacle to industrial applications. Lifetime tests under practical situations are required in the future study. Second, the role of graphene in the reaction mechanism of the oxygen reduction reaction (ORR) has been extensively studied but is still inconclusive. New detection techniques or experimental methods and theoretical calculations may be proposed. In particular, more attention should be paid to the doped graphene catalysts which were deemed as “metal-free”. As mentioned before in the section 2 (Graphene-Based Nanomaterials Preparation), some residual metal may still be retained in the produced graphene. And even a trace of metal impurities would have a beneficial impact on ORR activity as proven by Wang et al.101 Therefore, special caution should be exercised when a “metal-free” catalyst is claimed. It is suggested, more practically, to use ICP-MS or XRF to clarify the actual metal concentration concern before making such a claim. 4.1.2. Water Splitting. Hydrogen from water splitting is an attractive solution for meeting future energy demand. The reaction consists of two half reactions, that is, water reduction or hydrogen evolution reaction (HER) and water oxidation or oxygen evolution reaction (OER). The two most studied lowest-cost routes are via either direct use of sunlight (i.e., photocatalysis) or electricity generated from renewable resource (i.e., electrocatalysis). In the photocatalytic route, sacrificial reagents (such as methanol) are usually employed to decouple these two reactions since the challenge of designing efficient photocatalysts for the overall reaction still remains. In the electrocatalytic route, optimization of the design of HER and OER catalysts individually is possible because the two reactions occur at the cathode and anode separately.76 Many catalysts have been proposed and intensively studied for both photocatalytic and electrocatalytic routes in the past few decades. In this session, we will discuss the recent progress in the application of graphene-based nanomaterials for the two routes separately due to their completely different requirements for the catalysts. 4.1.2.1. Water Splitting with Photocatalysis. The fast recombination of electron−hole pairs and the mismatch between the band gap energy and solar radiation spectrum are the two main issues for the photocatalysis since its discovery including its application in water splitting.210 Owing to its high specific surface area and superior electron mobility, graphene is considered as not only a high quality support, but also a circuit board to retard such recombination. In this section, the roles of graphene played in photocatalytic water splitting including an electron acceptor and transporter, a cocatalyst, a photocatalyst, and a photosensitizer are elaborated, respectively. The most extensively investigated role of graphene for enhancing the photocatalytic hydrogen production is as an electron acceptor and transporter. This is attributed to its high work function (4.42 eV) and high intrinsic electron mobility (∼200 000 cm2 V−1 s−1). The former feature allows graphene to accept photogenerated electrons from the conduction bands of most semiconductors or the LUMOs of dyes with no barrier leading to efficient suppression upon recombination, while the latter one has the accepted electrons migrating rapidly across its 2D plane to reactive sites for H2 evolution.211 Zou et al.212 combined g-C3N4 with nitrogen-doped graphene quantum dots (N-GQDs) to form N-GQDs/g-C3N4 catalysts to overcome the high recombination rate of photogenerated electrons and holes and further improved the photocatalytic performance of g-C3N4. The PL spectra of N-GQDs irradiated under 600−800

nm light indicated that N-GQDs exhibited good upconversion properties. When excited by 600−800 nm light, short wavelength light in the range of 400−600 nm was generated. This emitting light together with UV from an environmental light source would be adsorbed by g-C3N4 to generate electrons (e−) and holes (h+). The electrons transferred to the surface of N-GQDs, whereas the holes reacted with sacrificial reagents (Figure 14). Accordingly, there is effective charge separation, leading to a high H2 production activity (with a H2 evolution rate of 2.18 mmol h−1 g−1) and quantum efficiency (5.5% at 420 nm). Graphene used as a cocatalyst was also proposed by many groups.213 Mateo et al.214 prepared Cu2O nanoplatelets from a Cu2+-chitosan precursor with preferential 2.0.0 facet orientation supported on few-layer graphene (FLG) as films in a single step by pyrolysis at 900 °C under inert atmosphere. It held unexpected superior photocatalytic activity with the overall water splitting of 19.5 mmol/gCu+Gh. The value was about 3 orders of magnitude higher than the activity for Cu2O nanoparticles.214 The experimental results revealed that the role of graphene as a cocatalyst, strong Cu2O−graphene grafting, and preferential 2.0.0 facet orientation derived from the pyrolysis preparation procedure, together with the efficient charge separation between Cu2O and graphene, benefited the H2 production. Graphene (or rGO) also can serve as the active material for photocatalytic water splitting. Xu et al.215 presented a DFT theoretical calculation result over rGO and g-C3N4/rGO composites. The interaction between the g-C3N4 and rGO sheets was systematically explored. They found that the O atom played a crucial role in the rGO-based composites. At the higher O concentration, the negatively charged O atoms in the rGO were confirmed to be active sites leading to high H2 production activity. Wang et al.216 reported using rGO as photosensitizer together with a membrane protein bacteriorhodopsin (bR) to sensitize the Pt/TiO2 photocatalyst for harvesting visible light. Introduction of the rGO boosted the nanobio catalyst performance leading to hydrogen production rates of approximately 11.24 mmol of H2 (μmolprotein)−1 h−1. The interfacial charge transfer from the photoexcited rGO to Pt/ TiO2 under visible light was indicated by electron paramagnetic resonance (EPR) and transient absorption (TA) spectroscopy and considered to be the main reason for activity promotion. Despite that great success was achieved, the study over graphene-based nanomaterials for photocatalytic H2 production is still at the initial stage and challenges remain including the scale-up synthesis of graphene-based catalysts and understanding the mechanism of the promotion effect by graphene. 4.1.2.2. Water Splitting with Electrocatalysis. As mentioned in section 4.1.1, graphene is considered as a promising material for electrochemical applications because of its high surface area, good mechanical strength, and excellent electrical conductivity. However, the basal plane of graphene shows chemical inertness leading to poor electrocatalytic activity. Two effective approaches for enhancing the electrocatalytic performance of graphene-based catalysts are creating more active sites and improving the reactivity of active sites.76 In particular, though preparing graphene-based catalysts with significantly exposed edge sites is still complex, the fact that the edge sites of graphene have a lower contact resistance and a faster electron transport than that of the basal plane tempts scientists to develop such catalysts for electrochemical applications such as 3488


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Industrial & Engineering Chemistry Research HER. Odedairo et al.217 synthesized a new type of Pt/CNT-G electrocatalyst based on a three-dimensional hybrid consisting of horizontally aligned carbon nanotubes and graphene (CNTG) with abundant active edge sites on graphene and a large specific surface area (863 m2 g−1) using a fluidization process. Both the metal-free CNT-G and Pt/CNT-G exhibited superior HER activity compared to NG and commercial Pt/C, respectively. The excellent performance was concluded to be originated from the abundant active edge sites and the large, ion-accessible surface area confirmed by combination of experimental observations with DFT calculations. The nonprecious metal (Pt-free)-based catalysts were also investigated for HER in recent years. Shinde et al.218 presented a facile method to fabricate lanthanides (L = La, Eu, Yb)-doped graphene bifunctional catalysts for both HER and ORR. The Eugraphene catalyst showed excellent HER performance with small values of onset potential (81 mV), overpotential (160 mV), Tafel slope (52 mV dec−1), and a high current density (7.55 × 10−6 A cm−2) as well as long-term stability. The authors attributed excellent electrocatalytic performance to the synergistic effect of abundant edges and doping sites, high electrical conductivity, large active surface areas, and fast charge transfer. The metal-free heteroatom-doped graphene nanomaterials were also employed for HER. Zhang et al.219 first reported the carbon-based metal-free ORR-HER bifunctional catalyst recently. They prepared N, P-co-doped carbon network by self-assembling melamine and phytic acid into melaminephytic acid supermolecular aggregate (MPSA) in the presence of graphene oxide (MPSA/GO). The authors attributed the excellent performance from both ORR and HER reactions to the N,P codoping and the graphene edge effect based on their previous DFT calculation results.220 Iridium and ruthenium are regarded as the most active catalysts for the OER.221 However, their high cost as well as the poor durability severely hinders their potential industrial application. Recently, Qiao et al.222 reported a cobalt and nitrogen codoped graphene with inserted carbon nanospheres as an efficient bifunctional electrocatalyst, exhibiting not only excellent OER performance but also promoted ORR performance. The authors ascribed the unbeatable outcome to the versatile role of the nanospheres and further proposed three possible mechanisms: (i) increasing the graphene’s accessible surface area by acting as “spacers”, (ii) providing abundant electrolyte channels facilitating the diffusion of reactive species, and (iii) guaranteeing the material’s good conductivity by serving as “shortcuts” to interplanar electron transport. Heteroatom-doped carbon nanomaterials were also considered as alternative catalysts for OER.223 Liu et al.224 prepared sandwich-like structured N-doped porous carbon@graphene composites (N-PC@G) with excellent performance for both OER and ORR by involving metal−organic framework and GO as precursors. N-PC@G with suitable GO in precursors obtained at 900 °C possessed high surface area (1094.3 m2 g−1), bimodal-pore structure (micropore and mesopore), and high graphitization degree. The synergistic effect between porous carbon and graphene as well as the structure and composition of N-PC@G boosted the catalytic active sites, mass transport, and electron transfer and was proposed as the mechanism behind for the performance promotion (Figure 13). Photoelectrocatalytic (PEC) water splitting, which combines photocatalytic and electrocatalytic technology, has also been explored. Graphene-based nanomaterials were employed in this system as promising catalysts. Li et al.225 used a phosphate-

Figure 13. Schematic illustration of a synthetic process of sandwichlike structured N-doped porous carbon@graphene (N-PC@G) catalyst for OER and ORR reactions. Reprinted with permission from ref 224. Copyright 2016, Elsevier.

Figure 14. Proposed photocatalytic mechanism for hydrogen evolution over N-GQDs/g-C3N4 under visible light irradiation. Reprinted with permission from ref 212. Copyright 2016, Elsevier.

modified rGO/BiOCl catalyst for this purpose. The catalyst was prepared by a two-step route: coupling BiOCl with reduced graphene oxide (rGO) first and then modifying with phosphoric acid. With the optimal amount of phosphatemodified rGO/BiOCl, 3.8× greater activity could be achieved compared to that of the bare BiOCl. The authors attributed the improved activities to the unique structure of the catalyst. Chemically coupling BiOCl with rGO enhanced charge separation and prolonged carrier lifetime to favor electron collection, while the modification of phosphate groups formed a negative field on the surfaces to benefit the positive holes trap. Though excellent progress was made in the past few years, big challenges are still remaining in the application of graphenebased catalysts for water splitting. First, the efficiencies of graphene-based catalysts (except the Pt ones) for both routes are still in the low range. More work is still required for the design of novel catalysts with higher activity. Second, the fundamental understanding of the role played by graphene is still unclear. One example is the debate on which sites of the graphene lattice are more active, edge or basal plane ones.217,220 More in-depth techniques and theoretical calculations may be proposed to advance the study as mentioned in section 4.1.1. 4.1.3. Fischer−Tropsch Synthesis (FTS). Graphene involved in FTS was not pursued in the beginning since one of the possible catalyst deactivation mechanisms in the FTS was carbon deposition in the form of a graphene overlayer, which was proven by both DFT computation and experimental observations.60,226−230 However, in 2013 Sun et al.231 first 3489


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in graphene, tuning graphene surface properties to stabilize catalytic active sites, and further enhancement of the reaction performance. Since most of the graphene supports are actually partially reduced, the incorporation of carbonate from these supports into products may exist and must be clarified by suitable techniques such as temperature-programmed oxidation (TPO) and in situ FTIR.239,240 4.2. Environment-Related Reactions. 4.2.1. CO2 Reduction. The conversion of greenhouse gases into valuable fuels and chemicals by CO2 reduction is considered as an attractive strategy for solving both energy and environmental problems on our planet.241 Currently, it is mainly realized by using thermal, electrochemical, photochemical, and photoelectrocatalytic routes. The development of high performance catalysts is critical. In this section, we will discuss the recent advance in the application of the graphene-based catalysts for all three routes. Attention will be paid to the role of graphene or modified graphene in the catalysts or for the reactions. Fan et al.242 used reduced graphene oxide (rGO) as a novel support for the CuO-ZnO-ZrO2−Al2O3/rGO (CZZA/rGO) catalyst in forming methanol by carbon dioxide hydrogenation in a fixed-bed reactor. With comparison to the control experiments on CZZA, CZZA/rGO showed superior activity (methanol yield, 11.6% vs 9.8% under the reaction conditions of 2 MPa and 513 K). The authors ascribed the excellent performance to large surface area (improved the reducibility of the Cu active component) and high H2 and CO2 adsorption capacity of CZZA/rGO, which prevented the catalyst sintering and led to a higher CO2 conversion and methanol selectivity. Unlike the conventional thermal catalysis where high pressure and high temperature are required, the electrochemical reduction reaction can proceed under ambient environments, which makes it much more attractive. Li et al.243 presented the selective electrochemical reduction of CO2 to ethylene (C2H4) over monodisperse Cu nanoparticles (NPs) assembled on a pyridinic-N rich graphene (p-NG) support. At −0.9 V, the highest selectivity toward C2H4 in all Cu-catalysts reported previously was achieved. The role of graphene was investigated in details. And the characterization results suggested that p-NG itself could catalyze CO2 reduction to formate. However, after combining with Cu, the pyridinic-N served as a CO2 and proton absorber, facilitating hydrogenation and carbon−carbon coupling reactions on Cu for the formation of C2H4. The electrocatalytic reduction of CO2 to energy-rich chemicals is considered as another strategy for energy storage and utilization. Wu et al.244 reported the three-dimensional (3D) graphene foam incorporated with nitrogen defects as a metalfree catalyst for CO2 reduction into carbon monoxide (CO). The N-defects were found to be the active sites for catalyzing the reaction at low overpotentials with high selectivity and long durability. Furthermore, a combination study of experimental and theoretical investigations revealed that the pyridinic-N defects exhibited the highest catalytic activity in all N functionalities by lowering the free energy barrier to form adsorbed COOH*, eventually leading to CO formation. This work indicated that intentional incorporation high pyridinic-N in graphene nanostructures as a metal-free catalyst might be a promising strategy for electrochemical CO2 reduction. Graphene-based catalysts are also employed in photocatalytic CO2 reduction, which converts harmful greenhouse gases into valuable energy-rich chemicals such as CH4 and CH3OH by using solar energy. Iwase et al.245 demonstrated a powdered Zschematic system with CuGaS2 and rGO(5 wt %)-CoOx-

proposed to use rGO as the support for iron-based FTS catalysts and achieved positive results,231 which was an interesting exploration. Sun et al. synthesized the iron oxide/rGO nanocatalyst via a facile one-pot hydrothermal hydrolysis−reduction (HHR) strategy which is able to uniformly disperse Fe2O3 nanoparticles (NPs) that are sub-3 nm in size. The conversion of CO could remain constant at approximately 78% and showed long stability, which was attributed to the small but thermally robust iron NPs on the Fe/rGO catalyst anchored by the defects on rGO. In particular, the unexpected superior selectivity for the gasoline fraction (48%) was considered to be due to the acidic oxygen-containing groups on rGO leading to cracking heavier hydrocarbons to the gasoline fraction. The results from the control experiment done over Fe/Vulcan XC-72 activated carbon (AC) with similar NPs size were far inferior to those of the Fe/rGO catalyst, suggesting that the nature of the rGO also played an important role in enhancing the FTS performance. This work validated for the first time, the feasibility of graphene-based nanomaterials in very challenging high temperature applications such as FTS. The promotion effect of graphene was reported by another group at almost the same time where the authors used iron oxide nanoparticles supported on pyrolytic graphene oxide as model catalysts for FTS.232 In the following years, several graphene-supported iron-233−235 or cobalt-236,237 based catalysts were reported with liquid fuels or olefins as the desired products. Guo et al.238 proposed a photocatalytic FTS route at mild conditions (150 °C, 2.0 MPa H2, and 1.0 MPa CO) by using graphenesupported worm-like ruthenium nanostructures as catalysts. The control experiments on Ru/SiO2 showed a lower lightenhanced activity, which indicated that graphene as support improved the reaction activity. The bound electrons in Ru nanostructures, together with those transferred from graphene due to a different work function between graphene and Ru, can absorb irradiation light, leading to efficient solar energy harvest, and become energetic. Then the chemical reactions would be promoted by these energetic electrons. Therefore, the high reaction temperature could be avoided and thus energy consumption and CO2 emission could be lowered (Figure 15). The primary results from the limited attempts using graphene-based catalysts for FTS have already indicated their great potential application in the future. Issues still need to be addressed including understanding the underlying mechanism of the promotion effect of graphene, controlling the defect sites

Figure 15. Photocatalytic Fischer−Tropsch synthesis on Ru/graphene. Reprinted with permission from ref 238. Copyright 2015, American Chemical Society. 3490


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Figure 16. Variation of the binding energies of B1-I and B2 as a function of the charge on the ZnO cluster by DFT computations. Reprinted with permission from ref 243. Copyright 2016, American Chemical Society.

be addressed before realizing the practical use of graphenebased catalyst for CO2 reduction, including the clear role of graphene in the reaction, poor stability in some applications such as the photocatalytic route, and general low activity. However, the partially reduced graphene oxide supports cause the similar issues as mentioned above in FTS and need further investigations.239,240 4.2.2. Water Treatment. Owing to the rapidly growing population, it is the responsibility of the scientific community to develop new materials, techniques, and devices to provide safe potable water. Graphene-based nanomaterials have been widely studied as adsorbents for this purpose.247 In this section, the recent progress on their roles as catalysts in advanced oxidation processes (AOPs), photocatalysis, and photoelectrocatalysis for wastewater treatment will be reviewed. A heterogeneous manganese/magnetite/graphene oxide (Mn-MGO) hybrid catalyst was reported by Du et al.248 for wastewater treatment. The Mn-MGO catalyst exhibited high efficacy and long-term stability in activating peroxymonosulfate (PMS) to generate sulfate radicals for the removal of bisphenol A (BPA) from water. The production of sulfate radicals and hydroxyl radicals was validated. The catalytic PMS activation was attributed to involved electron transfer from MnO or Mn2O3 to PMS with the generation of sulfate radicals, protons, and MnO2, as well as the simultaneous reduction of MnO2 by PMS. GO served as a platform to immobilize the composite and provide better aqueous dispersibility and a higher rate of particle distribution. Doped graphene may also serve as the support for AOPs. Zent et al. 249 fabricated a robust heterogeneous sulfate radical-based advanced oxidation (SRAOPs) catalyst with cobalt nanoparticles (NPs) embedded in nitrogen-doped reduced graphene oxide matrix (NRGO@Co). Because of the high porosity and surface area of the NRGO scaffold, the NPs could be stabilized, and the accessibility and adsorption of substrates to the active sites were greatly facilitated. The synergistic effect within the NRGO and Co NPs led to the NRGO@Co hybrid catalyst being considered as the contributor of the enhanced catalytic activity for the activation of peroxymonosulfate (PMS) to degrade organic pollutants in water. Even metal-free graphene-based catalysts also showed extraordinary activity for wastewater treatment. Duan et al.250 synthesized N-doped graphene (NG) nanoma-

Loaded BiVO4 photocatalyst for CO2 reduction using water as the sole electron donor under visible light irradiation. BiVO4 served as an O2 evolving photocatalyst upon CoOx-loading for a Z-scheme water-splitting system with CuGaS2 as an H2evolving photocatalyst bridged by rGO electron mediator. The authors also confirmed a strong relation between the present Zscheme system with an rGO electron mediator and photoelectrochemical properties. Li et al.63 prepared the monolithic two-side Cu2O/graphene/TiO2 nanotube array (TNA) heterostructure with anodic TNA as the substrate followed by sequential electrochemical deposition of graphene and Cu2O. It was used as a separated oxidation and reduction catalyst for photocatalytic CO2 conversion to methanol under visible light irradiation (λ > 400 nm). The methanol generation rate (45 μmol cm−2 h−1) achieved was much higher than those obtained from reported existing TNA-based photocatalysts. The superior performance was attributed to the combined heterojunction in the ternary composite which increased light absorption, prevented electron−hole recombination, and facilitated electron transfer across the heterojunction interfaces. Graphene-based catalysts were also reported in photoelectrocatalytic (PEC) reduction of CO2. Cheng et al.246 proposed a novel PEC reactor system by combining Ptmodified reduced graphene oxide (Pt-rGO) with Pt-modified TiO2 nanotubes (Pt-TNT) as cathode and photoanode catalysts, respectively, to convert CO2 into valuable chemicals such as CH3OH, C2H5OH, HCOOH, and CH3COOH. A high carbon conversion rate up to 1130 nmol/h·cm2 was observed. The high reactant adsorptivity and efficient charge transportation of Pt-rGO were considered as the two main reasons for the outstanding catalytic activity. The fundamental understanding of the mechanistic role of graphene in the CO2 reduction reaction was studied by using DFT computations as implemented in the Vienna Ab Initio Simulation Package (VASP) program.243 The authors used ZnO/graphene as model catalyst and concluded that singleheteroatom doping in graphene had a significant impact on the catalytic activity of ZnO. Furthermore, n-type doping helped to facilitate CO2 reduction on ZnO, whereas p-type doping enhanced reductions occurring on graphene (Figure 16). Despite the excellent progress made for using graphene in CO2 reduction, several theoretical and fundamental issues must 3491


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Figure 17. Schematic illustrations of the possible charge transfer over g-C3N4/reduced graphene oxide/ZnS for photocatalytic degradation of RhB under visible light irradiation. Reprinted with permission from ref 251. Copyright 2016, American Chemical Society.

yphenyl) propane, BPA) under visible light irradiation. Bamboo-shaped graphene was observed in the TiO2 nanotubes because of strong wall stress inside the nanotubes. The superior photoelectrocatalytic activity was attributed to the high conductive and interconnected three-dimensional channels inside the TiO2/G/Cu2O mesh.253 One main challenge with using graphene-based catalyst in water treatment is the active sites stripping due to the liquid environment. Another issue is the recycle of catalysts from the liquid. 4.2.3. Air Purification. VOCs degradation and NO x abatement is of importance for human health. Many catalysts have been investigated in these reactions. The emergence of unique graphene may open a new door for this research. Hui et al.54 proposed a series of MnO2/graphene with different MnO2 loading for catalytic ozonation of gaseous toluene. MnO2 nanoparticles were uniformly distributed throughout the surface of the graphene at a suitable MnO2 loading, whereas aggregation or erosion of graphene sheets occurred at low or high loading of MnO2. The highest toluene degradation rate (7.89 × 10−6 mol min−1 g−1) over the 64.6 wt % MnO2/graphene sample was attributed to the synergetic effect of graphene. Furthermore, based on the reaction data on different samples they concluded that graphene not only served as a support for MnO2/graphene catalysts but also functioned as an adsorption site for toluene due to the π−electron coupling between graphene and toluene. Moreover, the authors concluded that the planar structure of graphene as support played a positive role for the reaction by promoting mass transfer during the reaction. In the following work,41 they performed a detailed kinetic modeling based on the Langmuir− Hinshelwood mechanism. However, the single-site (denoted as L-Hs) mechanism did not fit the experimental data, suggesting that the reaction was nonsingle-site governed. The derived kinetic equation based on the dual-site (denoted as L-Hd) hypothesis could converge well with the experimental observations. Moreover, the physical criterion was in accordance with both enthalpy and entropy of toluene adsorption constraints. Fulfillment of mathematical and physical criteria indicated the catalytic ozonation of toluene over MnO2/graphene can be well described by the L-Hd mechanism. This mechanism was based on the hypothesis that MnO2 was responsible for ozone decomposition and

terials by directly annealing GO with the nitrogen precursor of melamine. The N-doping level was 8−11 atom % achieved at a moderate temperature. The sample obtained at a calcination temperature of 700 °C, showed the highest efficiency in degradation of phenol solutions by catalytic activation of peroxymonosulfate (PMS). The catalytic activity of NG-700 was 18.5 times higher than that of the most popular metalbased catalyst of nanocrystalline Co3O4 in PMS activation. Theoretical calculations by DFT illustrated that quaternary N was able to dramatically reduce the adsorption energy and facilitate electron transfer for PMS activation on graphene sheets. The application of graphene-based catalysts for photocatalytic degradation of wastewater is much more common and has been widely investigated. Shao et al.251 for the first time used the ternary nanocomposite of g-C3N4/reduced graphene oxide/ZnS (CN/RGO/ZS) for the degradation of Rhodamine B under visible light irradiation. The CN/RGO/ZS composite demonstrated distinctly enhanced photocatalytic activities toward the target pollution (about 97% after 60 min) which was much higher than the control group CN/RGO, CN/ZS, and RGO/ZS. The enhanced photocatalytic performance of RGO/CN/ZS was ascribed to the synergistic effects of three components: effective charge separation, multistep transfer between CN/ZS heterojunction and RGO nanosheets, and enhanced visible-light absorption (Figure 17). Metal-free graphene-based catalysts were also used in the photocatalytic degradation of wastewater especially for organic pollutants. Ai et al.252 reported photo-oxidation of methylene blue (MB) and phenol in water solutions under various light irradiations by metal-free graphitic carbon nitride (g-C3N4) and rGO (CN-G) composites. Both the adsorption capacity and optical absorption in visible light radiation were modulated due to the introduction of graphene, which accordingly promoted the visible light photodegradation of MB on CN-G samples. The efficient degradation of phenol solutions by hybrid photocatalysts was also observed. An emerging technology for water treatment is photoelectrocatalysis, in which graphene-based catalysts also show attractive performance. Yang et al. synthesized TiO2/graphene/ Cu2O mesh by combining chemical vapor deposition of graphene and electrochemical deposition of Cu2O for photoelectrocatalytic degradation of bisphenol A (2,2-bis(4-hydrox3492


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reliable and facile preparation methods still need to be developed. 4.3. Other Reactions. Graphene-based nanomaterials were also employed as catalysts in other reactions such as selective hydrogenation. Nie et al.256 used N-doped reduced graphene oxides (N-rGO) as support for ultrafine Pd nanoparticles (NPs) (Pd/N-rGO) in selective hydrogenation of CC bonds of unsaturated hydrocarbons. It was found that Pd/N-rGO was highly active and selective for the hydrogenation of the CC bond in cinnamaldehyde and hydrogenation of phenol to cyclohexanone under mild conditions. The superior performance came from the nitrogen doping. N in N-rGO could enhance interaction between Pd and the graphene surface resulting in the uniform dispersion of Pd. Moreover, Raman analysis of adsorbed cinnamaldehyde indicated that N species in the skeleton of graphene could anchor the single bond −CHO group and activated the CC bond in the same molecule, which improved the selectivity. Primo et al.48 used pristine graphene (G) in the absence of metals as high performance catalysts for the hydrogenation of CC bonds, both in the gas phase and in the liquid phase. The authors attributed the unexpected activity and selectivity to the existence of a similar type of frustrated Lewis acid−base pairs on the G layer surface. The independent acid and basic pairs located on the G sheet at an adequate distance would promote the splitting of H2 molecules leading to formation of H+-like and H−-like sites, which would be subsequently transferred to the alkene and promote the reactions. Graphene also showed great potential in organic chemical reactions such as selective oxidation of alcohol to aldehyde. Yang et al.257 reported a nitrogen-doped magnetic Co-based catalyst with excellent catalytic activity (Co/NG) (89.5% conversion and 97.3% selectivity to benzaldehyde) for selective oxidation of benzyl alcohol with molecular oxygen. The doped nitrogen species in the graphene support played a crucial role in this reaction. The nitrogen doping enhanced the electrondonor property and further promoted the activation of oxygen molecules. Moreover, Co−N sites would form in the carbon matrix by coordination nitrogen atoms with cobalt ions, which reduced the oxygen adsorption energy resulting into the reduction of oxygen easily. Metal, such as iron, -doped graphene was also developed as the catalyst support for selective oxidation of benzyl alcohol.184 The Au/Fe-G-K2CO3 catalysts showed excellent activity with precious selectivityswitchable property. With incorporation of iron to the graphene support, the separation of catalyst was more simple and convenient. The metal-free nitrogen-doped graphene (NG) was used as model catalyst in DFT computations to investigate the mechanism of the selective oxidation of alcohols to aldehydes and ketones.258 It was found that NG can activate dioxygen to form different activated oxygen species (AOS) depending on the position of N-doping. The oxidation reaction can carry out on all three AOS. However, the overall barrier of the reaction over aromatic alcohols was found lower at ketonic oxygen (subK) while that over aliphatic alcohols was at the center (subC) (Figure 19). It is well-known that mass transfer is one of the most important issues in heterogeneous catalytic reactions affecting the reaction rate and selectivity of targeted products. Furthermore, it usually becomes the predominant factor in the liquid phase where the diffusion coefficient is much smaller than it is in the gas phase.92 Due to its unique 2D planar structure, which is expected to promote mass transfer during

toluene adsorption on graphene; these two types of adsorption were coupled by an adjacent attack. Chen et al.254 proposed a facile, green one-pot hydrothermal method to prepare ZnO/rGO composites with controllable rGO content. The separated photoexcited charge carriers increased and were available for photocatalysis utilization with rGO addition. ZnO/rGO was used the first time for gaseous acetaldehyde (CH3CHO) degradation. The suitable rGO contents (1.0 and 3.0 wt %) displayed significantly enhanced photocatalytic activity in both CH3CHO degradation and CO2 generation. The promoted carrier utilization efficiency of ZnO via ready acceptance by rGO was considered to contribute the enhanced activity, which led to abundant photogenerated holes on the ZnO for CH3CHO photodegradation (Figure 18).

Figure 18. ZnO/rGO for photocatalytic degradation of CH3CHO and OER. Reprinted with permission from ref 254. Copyright 2015, Elsevier.

The TiO2/(surfactant-stabilized graphene, ssG and rGO) was used as photocatalyst for photocatalytic NOx oxidation under UV and visible light irradiation.255 The TiO2/(ssG and rGO) exhibited higher photocatalytic efficiency than pure TiO2 especially under visible light irradiation. It was found that the photocatalytic performance of the composites in NOx removal displayed two trends. First, the efficiency of the composite photocatalysts was slightly improved under UV light in both (ssG- and rGO-containing TiO2) cases, which was attributed to the interaction between TiO2 nanoparticles and graphene sheets. Also, the sheets acted as electron traps in the case of ssG, while they acted as photosensitizer in the case of rGO. Furthermore, the enhancement was more prominent under visible light in which the rGO was activated. Second, with the addition of ssG and rGO, low NO2 release was achieved, which was related to the high affinity of graphene sheets to NO2 molecules. In both trends, graphene had a critical effect. The advantages of the graphene addition to the catalysts for air purification can be concluded as follows: (1) In photocatalysis, the charge recombination is suppressed by transferring the excited electrons from a catalytically active species to graphene. (2) In photocatalysis, new chemical bond are formed to narrow the band gap of the catalytically active species and extend the photo response range; (3) In general, the adsorption of target pollution molecules on catalysts is enhanced by the interactions between molecules and the aromatic regions of graphene. However, attention should be paid to following comments: (1) the promotion effect by graphene on the charge transfer is limited and the more general pathway should be focused on catalytically active species; (2) the underlying mechanism still needs to be investigated in-depth ; (3) more 3493


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mechanism of Au/graphene-based catalysts for acetylene hydrochlorination. The graphene and N-doped graphene support Au3 clusters demonstrated extremely similar reaction mechanisms with activation energy values of 23.26 and 23.89 kcal/mol, respectively. The graphene support could obviously enhance the adsorption of reactants. It suggested that Au3/Ngraphene could be a potential catalyst for acetylene hydrochlorination. Dai et al.124 studied the boron and nitrogen codoped graphene (B,N-G) metal-free catalysts for acetylene hydrochlorination by both experimental investigations and DFT calculations. The B, N-G catalyst showed significantly high activity (∼95% conversion), which was attributed to the synthetic effect of B and N doping. The dopants promoted HCl adsorption confirmed by DFT theory calculations and temperature-programmed desorption experiments. The roles of graphene in the graphene-based catalysts for these reactions are similar to above-mentioned reactions: platform for catalytically active species, active sites due to adsorption between reactants molecular and graphene aromatic regions or doping atoms. More applications of graphene involved reactions are still to be explored. In summary, owing to its unique properties, graphene was employed as a potential catalyst support or catalyst by its own for different reactions and showed promising results. However, the detailed or accurate mechanisms behind its superior activity in catalysis is still unclear. In this review, we summarized the recent studies and outlined the main advantages from graphene or modified graphene including large surface area for the possibly high dispersion of catalytically active sites, abundant anchoring sites for high loading capacity of catalytic species from the oxidized graphene platform, high intrinsic electron mobility to minimize ohmic losses associated with electron transport, planar structure of graphene as support to promote mass transfer during reactions, excellent thermal conductivity to reduce diffusion resistance, high adsorption capacity for certain molecules to enhance reaction rate, and great mechanical strength for its practical applications.

Figure 19. Generalized reaction mechanism for the selective aerobic oxidation of aryl and alkyl alcohols over nitrogen-doped graphene by DFT computations. Reprinted with permission from ref 258. Copyright 2015, American Chemical Society.

the reaction, the application of graphene in the liquid organic reactions was explored.259−265 Zhao et al.266 immobilized a copper (salen) complex on GO by an in situ synthesis strategy to remedy the drawbacks of homogeneous catalysis for catalytic epoxidation of olefins. The two-dimensional sheet character of GO was observed in the obtained copper (salen) complex [Cu(salen)-f-GO]. Thus, the reactive species can readily reach or leave the catalytic active sites immobilized on GO leading to limited mass transfer resistance and excellent reaction performance. Fu et al.267 prepared CuB23/graphene catalyst by the coreduction of GO and Cu(NH3)42+ with the assistance of solution plasma. The well dispersed CuB23/graphene catalyst demonstrated exceedingly high activity to the Heck-type coupling of inactivated alkyl halides with alkenes and the liquid phase hydrogenation of levulinic acid to γ-valerolactone. The authors ascribed the excellent catalytic performance to the combined effect of the high dispersion of CuB23 nanoparticles and the interaction between graphene and CuB23 nanoparticles, which not only facilitated electron transfer and mass transport but also improved thermal stability during the catalytic reaction. Though the application of its considerably high mass transfer feature in the liquid organic reactions is still in the very early stage, the promising results already indicate the great potential of graphene for the organic industry. More wide and in-depth applications in organic catalysis are anticipated. Another potential application of graphene-based nanomaterials for catalysis is in acetylene hydrochlorination. Gu et al.268 established a big graphene cluster model of C110H28 to investigate the effect of different nitrogen-doped carbon supports on three kinds of gold species models of Au dimers, Au2Cl2 and Au2Cl6 through DFT calculations, which aimed on disclosing the reason that the N-doped carbon support can enhance the stability of Au-based catalysts for acetylene hydrochlorination. The authors found that the N-doped support GRN-I (the pyridinic N-doped graphene) exhibited the highest adsorption energies of the Au dimer, Au2Cl2, and Au2Cl6 among the different supports. Moreover, the stabilized gold species Au2Cl6 on GRN-I enhanced interaction between Au2Cl6 and HCl and inhibited the reduction of Au3+, which finally increased the long-term stability of Au-based catalysts. Zhao et al.269 also utilized DFT computations to investigate the

5. SUMMARY AND OUTLOOK Graphene has come a hot topic since it was first discovered in 2004. The emerging of graphene and its derivatives has opened a new door for catalytic supports or catalyst by their own. In this review, we covered the preparation, characterization, and application of graphene-based nanomaterials for catalysis. Two preparation routes (top-down and bottom-up) were critically reviewed and possible solutions were discussed. The frequently used techniques and DFT computations method were briefly summarized. Their applications focusing on energy conversion, environmental protection, and several other typical fields were briefly summarized while the challenges for each type of reaction were raised. It is clear that graphene-based nanomaterials can serve as promising catalysts for a variety of reactions due to their huge specific surface area, easy modification, high electrical and thermal conductivities, great mechanical strength, and adsorption capacities. All of these are beneficial to the catalysis community and allow us to design and develop countless novel graphene-based catalysts with extraordinary performance. Despite the remarkable progress made in the preparation, characterization, and catalytic applications of the graphenebased nanomaterials, it should be noted that the use of graphene catalysts is only at its very early stage. Several general issues still exist, such as unclear mechanism or fundamental 3494


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Industrial & Engineering Chemistry Research understanding on the exact role of graphene in the reactions, difficulty in large scale preparation of graphene and graphenebased catalysts, and limited stability in certain reactions such as photocatalysis. Therefore, it is still a long road ahead before their commercial applications. In our opinion, the following directions may be helpful and should be paid more attention in the future research. First, new synthetic strategies other than the most used graphite oxide exfoliation and CVD method still need to be developed with an aim to produce high-quality graphene-based nanomaterials in large-scale, reduce the production cost, precisely control doping type and amount, and improve the catalytic performance. Especially the composites composed of graphene and traditional catalysts or supports may show some unexpected properties and deserve further investigation. Second, novel characterization techniques are still expected to be explored. In situ or other enhanced Raman spectroscopy may be a promising direction in consideration of its high sensitivity to carbon-based materials. Third, combining theoretical computations such as DFT with experimental observations is a useful approach to fully understand the structure of the novel graphene-based nanomaterials as well as their roles in the reactions. Theoretical calculations could provide a conceptual framework to help understand how graphene and loaded catalytical species interact, which type of doping site is more active, and the activation energies, as well as how the elementary reaction pathways process. Finally, though there are doubts on the metal-free graphene-based catalyst, it is still one of the most attractive areas in the future for sustainable catalysis. More reactions may be explored for its application considering that most of the current investigations focus on the electrochemical and photocatalytic processes. These endeavors would help our community with the design and fabrication of novel graphenebased nanomaterials with greater performances for future practical catalytic processes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 973 596 5707. Fax: +1 973 596 8436.

QHE = the quantum hall effect FTS = Fischer−Tropsch synthesis ORR = oxygen reduction reaction OER = oxygen evolution reaction CVD = chemical vapor deposition XRF = X-ray fluorescence TGA = thermogravimetric analysis MS = mass spectrometry XRD = X-ray diffraction AFM = atomic force microscope CV = cyclic voltammetry LSV = linear sweep voltammetry CZZA = CuO-ZnO-ZrO2−Al2O3/rGO p-NG = pyridinic-N rich graphene GO = graphene oxide MW-rGO = microwave-reduced GO FLG = few-layer graphene NG = nitrogen doped graphene N−P−O/G = nitrogen, phosphorus, oxygen codoped G N-PC = nitrogen-doped porous carbon PECVD = plasma enhanced chemical vapor deposition HOPG = highly ordered pyrolytic graphite HER = hydrogen evolution reaction SAED = selected area electron diffraction DFT = Density Functional Theory computations XPS = X-ray photoelectron spectroscopy HR-TEM = high-resolution transmission microscopy FTIR = Fourier transform infrared spectroscopy EDS = energy dispersive spectroscopy PEMFC = polymer electrolyte membrane fuel cells N-GQDs = nitrogen-doped graphene quantum dots EPR = electron paramagnetic resonance spectroscopy TA = transient absorption spectroscopy TPR = temperature-programmed reduction TPD = temperature-programmed desorption Mn-MGO = manganese/magnetite/graphene oxide

REFERENCES

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ORCID

Xianqin Wang: 0000-0003-1056-7214 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This contribution was identified by Prof. Qingyan Cheng (Hebei University of Technology, China) as the Best Presentation in the session “ENFL: Progress in Coal to Liquids & Gases” of the 2016 ACS Fall National Meeting in Philadelphia, PA. This work was supported by Grant ACSPRF 53582-ND10.

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NOMENCLATURE G = graphene rGO = reduced graphene oxide SLG = single-layer graphene GNRs = graphene nanoribbons N−S/G = nitrogen−sulfur doped G FET = field effect transistor PCNF = porous carbon nanofiber DSSC = dye-sensitized solar cell 3495


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