Understanding of Electrochemical Mechanisms for CO2 Capture and

ACS Nano 2017.11:10825-10833. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/11/18. For personal use only.

Understanding of Electrochemical Mechanisms for CO2 Capture and Conversion into Hydrocarbon Fuels in Transition-Metal Carbides (MXenes) Neng Li,*,†,‡ Xingzhu Chen,† Wee-Jun Ong,∥ Douglas R. MacFarlane,§ Xiujian Zhao,† Anthony K. Cheetham,‡ and Chenghua Sun*,¶ †

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Hubei, 430070, China Department of Materials Science & Metallurgy, University of Cambridge, Cambridge, CB3 0FS, U.K. ∥ Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore § ARC Centre of Excellence for Electromaterials Science (ACES), School of Chemistry, Faculty of Science, Monash University, Clayton, VIC 3800, Australia ¶ Department of Chemistry and Biotechnology, Faculty of Science, Engineering & Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia ‡

S Supporting Information *

ABSTRACT: Two-dimensional (2D) transition-metal (groups IV, V, VI) carbides (MXenes) with formulas M3C2 have been investigated as CO2 conversion catalysts with well-resolved density functional theory calculations. While MXenes from the group IV to VI series have demonstrated an active behavior for the capture of CO2, the Cr3C2 and Mo3C2 MXenes exhibit the most promising CO2 to CH4 selective conversion capabilities. Our results predicted the formation of OCHO• and HOCO• radical species in the early hydrogenation steps through spontaneous reactions. This provides atomic level insights into the computer-aided screening for high-performance catalysts and the understanding of electrochemical mechanisms for CO2 reduction to energy-rich hydrocarbon fuels, which is of fundamental significance to elucidate the elementary steps for CO2 fixation. KEYWORDS: transition-metal carbides, MXene, CO2 capture and conversion, electrochemical mechanisms, density functional theory metal-free meshes including functionalized graphene oxide10 or graphite-like carbon nitride (g-C3N4),11 and others12−15 are also good candidates. However, as indicated by Whipple et al.,16 many advances in the field are still needed to improve energy efficiency and reaction rates, among other limitations. From a mechanistic point of view, the CO2 conversion process through electrochemical approaches consists of successive electroreductions by inclusion of a set of H+/e− pairs. Depending on the total even number (from two to eight) of H+/e− couples transferred through the complete reaction, different hydrocarbon compounds, such as carbon monoxide (CO), formic acid (HCOOH), formaldehyde (H2CO),

Large-scale anthropogenic carbon dioxide (CO2) emissions into the atmosphere as a consequence of heavy reliance on nonrenewable energy sources1 have triggered an intensification of climate change effects.2 The magnitude of this issue confronts us with an unprecedented challenge: significant reductions in CO2 emissions and realistic alternatives to energy sources based on fossil fuels are critical. In this regard, CO2 conversion,3 for example, into hydrocarbon-based “green fuels” that can be reburned for energy generation with a net-zero balance of greenhouse emissions, is potentially a sustainable energy storage mechanism. Since CO2 conversion technology was profiled as one of key solutions to the so-called “CO2 problem”, a vast and rich literature has emerged in recent years, with a diverse selection of catalytic materials being tested and optimized including titanium-based (TiO2) semiconductors,4−6 Cu, and photocatalytically active Cu2O.7−9 Moreover, 2D © 2017 American Chemical Society

Received: May 28, 2017 Accepted: September 11, 2017 Published: September 11, 2017 10825

DOI: 10.1021/acsnano.7b03738 ACS Nano 2017, 11, 10825−10833

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ACS Nano methanol (CH3OH), or methane (CH4), can be obtained.17 In this regard, an advanced understanding of the reaction mechanism is essential for the design of innovative catalysts, especially when specific products are targeted. It is well known that catalysis cannot occur if there is no effective physicochemical contact between gas molecules and the catalyst surface. A noncovalent interaction, normally of the OC Olp···Y, with “Y” being an electropositive atom, is required to adsorb CO2 onto the surface. However, CO2 fixation is often thermodynamically unfavorable under mild conditions (i.e., ambient temperatures and pressures), and additional energy/ pressure is required to enhance such contact. Once the CO2 is adsorbed, the next challenge is the first hydrogenation step: the single-electron CO2 + e− → CO2•− process. This represents a large impediment within the overall process because a significant amount of energy is required. As a result, a strongly negative reduction potential of −1.90 V vs NHE has been estimated,18 making this the limiting step of the whole electrochemical reaction, even when catalysts come into play. In addition, reduction potentials vs NHE at neutral pH indicate very poor selectivity for individual transformations into particular potential hydrocarbon products. To overcome these issues, alternative strategies using catalysts must be found. Recent work carried out by our group has provided ingenious insights into the early stages of the CO2 capture and conversion mechanism on two-dimensional (2D) boron nitride nanosheets or meshes (BNs) doped with beryllium.19 Insertion of electrondeficient Be atoms into the 2D network produced significant decreases in the energy barriers for the CO2 fixation and first hydrogenation steps. The very deep π-hole generated on the Be-doped surface environment led to spontaneous CO2 adsorption (in terms of the Gibbs free binding energy). More remarkably, the reactions to form the radical HOCO• and OCHO• species as a result of the first H+/e− gain also exhibited spontaneous thermodynamics, with Gibbs free reaction energies at room temperature of −0.45 and −0.98 eV, respectively. For classical semiconductors, the first proton− electron transfer often constitutes the limiting step of the whole reaction,18,20,21 but our density functional theory (DFT) results indicate that this can be effectively changed through introducing special active sites on the surface. However, despite the significance of such insights and the predicted value from the computer-aided design of catalysts perspective, the synthetic difficulties involving Be-doping of BNs, as well as the high toxicity of beryllium,22 limit its potential applicability. In this context and following similar features and mechanisms, 2D transition-metal carbides, also known as “MXenes”, emerge as promising candidates for such purposes. Recent investigations by Naguib et al.23 have demonstrated the exfoliation of solids containing strong primary bonds, such as MAX phase powders of Ti3AlC2 into Ti3C2; these 2D materials have been denoted “MXenes” because of their similarity to graphene. As has been recently summarized by Gogotsi and co-workers,24 etching out the “A” layers from MAX phases [Mn+1AXn, in which “M” is an early transition metal, “A” an atom from the triel (icosagen) or tetrel (crystallogen) groups, “X” = C or N, and n = 1, 2, or 3] produces both their respective normal MXenes and monolayer MXene25 with formulas Mn+1Xn, prior to sonication. As well as Ti3C2, this procedure has been used to synthesize Ti2C, Nb2C, V 2 C, (Ti 0 . 5 Nb 0 . 5 ) 2 C, (V 0 . 5 Cr 0 . 5 )C 2 , (Nb 0 . 8 Ti 0 . 2 ) 4 C 3 , (Nb0.8Zr0.2)4C3, Ti3CN, Ta4C3, (Mo2Ti)C2, (Mo2Ti2)C3, or (Cr2Ti)C2, among others.23,26−29 Many of these materials show

the large specific surface areas required to obtain large activity in N2 fixation30 and the hydrogen evolution reaction.31 MXenes have generated recent interest because of their outstanding electronic properties, which can be exploited for various industrial and biomedical applications.24,32,33 Multilayer MXenes are conductively similar to multilayer graphene. However, unlike graphene, MXenes are hydrophilic and can be easily dispersed in aqueous solution. Also, the inherently metallic MXenes can become semiconducting once their surfaces are −F or −OH terminated.26 In terms of real applications, MXenes have already been demonstrated to be promising candidates for energy storage applications such as Liion batteries,34−39 non-Li-ion batteries,40 electrochemical supercapacitors,41−43 and fuel cells.44 Apart from energy storage, MXenes have also been tested as photocatalytic materials,45 gas sensors,46 biosensors,47 and transparent, conductive electrodes.48 In recent years, some literature studies have emerged describing the use of metal carbides as catalytic substrates for hydroprocessing and water splitting.49−51 Metal carbides tend to have high specific surface areas and to form clean surfaces;52 they are good electrical conductors, are stable, and are hydrophilic.26 Very recently, research concerning the function of transition-metal carbides as CO2 capture materials has been reported by Zhou’s group,53 which demonstrated M2CO2 with an oxygen vacancy as a good CO2 reduction catalyst. On the basis of this background, we hypothesize that group IV−VI MXenes, i.e., transition-metal carbides with formulas Mn+1Cn (n = 2), may function as CO2 capture and conversion catalysts, based on two considerations: (i) MXenes have extensive metal-terminated (M-terminated) surfaces, which may favor CO2 capture; and (ii) MXenes exhibit an inherent metallic character, which is beneficial for electrochemical reduction of CO2. We test this hypothesis via state-of-the-art DFT calculations with dispersion corrections. In particular, we calculate that chemisorption of CO2 will be preferred over chemisorption of H2O on these surfaces. In addition, we expect that the hydrophilic nature and high electrical conductivity will facilitate electroreduction of CO2, in terms of both H+ attachment and providing electrons.

RESULTS AND DISCUSSION Transition-metal carbides (MXenes) with the formula M3C2 and M = Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo are strongly bonded graphene-like 2D materials comprising five layers of atoms. The carbons in the inner layers are octahedral-coordinated (labeled as “C” in Figure 1) with two kinds of transition metals (labeled as “M” in Figure 1): those constituting the central inner layer (also octahedral-coordinated) and the three-coordinated terminal metals that are especially reactive due to their empty d-like orbitals and are therefore where the catalytic activity will take place. As indicated in Figure 1, the CO2 may interact with the M-terminated surface of the MXenes either through physisorption, where CO2 is attached to the surface through a noncovalent interaction of OCOlp···M nature (Figure 1, center), or through chemisorption, in which CO2 is formally bound (Figure 1, right). Gibbs free energies for binding at room temperature (ΔGb), which indicate thermodynamic stability, are different for each case. The CO2 chemisorbed step is characterized by spontaneous binding energies, of between −3.19 and −1.29 eV, as summarized in Table 1. Moreover, the MXenes seem to be active toward CO2 chemisorption, exhibiting spontaneous binding energies 10826


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Table 2. Gibbs Free Energies of Reaction (in eV), Calculated at the PBE/DFT-D3 (See Methods) Computational Level (ZPE Corrections Included) for Group VI Cr3C2 and Mo3C2 Materialsa

Figure 1. Proposed path for interaction of CO2 with M3C2 MXene surfaces: clean surface (left), CO2 physical adsorption (center), and CO2 chemisorption (right). “M” and “C” labels refer to octahedralcoordinated transition metal and carbon atoms, respectively; * and ** symbols refer to physisorbed and chemisorbed species, respectively.

species/group-VI M3C2

Cr3C2

Mo3C2

**CO2 **OCHO• **•OCH2O• **HOCH2O• **H2CO **CH3O• **O **OH• **H2O

−1.28 −1.42 −1.51 −0.87 −1.01 −1.45 −3.14 −2.97 −1.27

−0.86 −1.50 −1.46 −0.69 −1.06 −1.23 −3.10 −3.03 −0.07

a

All values are referenced to the clean surface and the isolated reactive gases (0.00 eV). Note: ** symbols refer to chemisorbed species.

decreasing in strength as we move along the group; that is, surfaces for group IV MXenes are stronger capture materials than group V or group VI MXenes. At this point, the question arises, does the process of CO2 adsorption occur directly by physical adsorption or does it occur shortly after passing the barrier imposed by the CO2 chemisorption? Comparing both PBE/DFT-D2 test results and state-of-theart calculations with explicit dispersion corrections via the PBE/DFT-D3 method (Tables 1 and 2), it seems that as a consequence of the force convergence settings (see Methods section), chemisorbed CO2 minima appear as DFT artifacts, allowing us to conclude that CO2 directly interacts with the surface through a spontaneous and exothermic process that leads to its capture. More remarkable are the notable differences in the Gibbs free energies of binding computed for the chemisorption process at both the PBE/DFT-D2 and PBE/DFT-D3 levels, but correcting the errors derived from overestimated interactions between CO2 and the M-levels of

theory. As shown in Figure 2 and Table 2, the difference of ΔGb in the Cr3C2 and Mo3C2 with the PBE/DFT-D3 correction is −1.28 and −0.86 eV, providing further evidence for spontaneous interaction of the CO2 with the terminated MXene surface. The calculated adsorption energies imply that group IV−VI M3C2 MXenes are capable of spontaneously capturing CO2, which is critical because CO2 capture often requires higher temperatures or pressure to enhance the contact on the surface.54 Critically, PBE/DFT-D3 calculations of the group VI MXene Cr3C2 and Mo3C2 materials for the capture of H2O predict spontaneous Gibbs free binding energies, but always less negative than those for CO2 chemisorption (Figure 2). In other words, group IV−VI M3C2 MXenes interact preferentially with CO2 over H2O, which is promising for CO2 capture in the presence of moisture. Our system appears to be able to overcome some major limitations of previous CO2 conversion

Table 1. Gibbs Free Energies of Reaction (in eV), Calculated at the DFT-D2 Computational Level (ZPE Corrections Included)a group IV

group V

species/M3C2

Ti3C2

Zr3C2

Hf3C2

V3C2

Nb3C2

*CO2 **CO2 **OCHO• **HOCO• **•OCH2O• **HCOOH **CO **HOCH2O• **HOCH2OH **H2CO **CH2OH• **CH3O• **CH2 **CH3OH **O **CH3 **OH• **CH4 **H2O

−0.59 −3.01 −2.04 −2.06 −3.51 −1.01 −1.18 −2.47 −1.09 −2.43 −1.58 −2.93 −1.81 0.06 −4.80 −2.04 −4.59 −1.18 −3.03

0.17 −3.19 −2.25 −2.49 −4.08 −2.19 −1.11 −2.82 −1.07 −3.21 −2.05 −3.12 −1.88 0.07 −5.24 −2.37 −4.65 −0.70 −2.96

0.18 −3.05 −2.89 −2.79 −4.31 −2.47 −1.54 −3.11 −3.56 −3.31 −1.89 −3.33 −1.21 0.01 −5.23 −2.86 −4.80 −0.94 −3.04

0.29 −1.47 −1.40 −1.41 −1.93 −0.15 −1.45 −1.59 −0.61 −1.81 −1.26 −2.20 −0.97 0.33 −3.45 −2.26 −3.70 −2.15 −2.46

0.35 −1.60 −1.71 −1.54 −2.22 −0.12 −1.39 −1.88 −0.68 −2.16 −1.36 −2.36 −1.47 0.08 −3.95 −2.47 −3.90 −0.77 −2.37

group VI Ta3C2

Cr3C2

Mo3C2

−2.30 −1.58 −1.92 −2.86 −0.32 −1.80 −2.56 −1.10 −2.39 −1.94 −2.98 −2.33 0.22 −4.27 −3.27 −4.35 −0.82 −2.64

0.25 −1.29 −1.61 −1.74 −1.60 0.01 −2.00 −1.85 −0.69 −1.78 −1.51 −2.12 −1.65 0.33 −3.53 −2.52 −3.73 −0.55 −2.55

0.15 −2.11 −1.74 −1.91 −1.64 −0.78 −2.27 −2.15 −0.90 −1.86 −1.64 −2.53 −2.11 0.17 −3.57 −2.98 −3.91 −0.70 −2.88

All values are referenced to the clean surface and isolated reactive gases (0.00 eV). Note: * and ** symbols refer to physisorbed and chemisorbed species, respectively. a

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binding is strengthened by the strong attraction between the O lone pairs of CO2 and the aforementioned 3-fold metal atoms (proximal distances between 1.94 and 2.05 Å). By comparison, chemisorbed H2O molecules (Figure 2) bind through their O lone pairs to the metal atoms of the MXenes, also showing strong interactions, with ΔGb between −0.27 and −0.07 eV. This is experimentally corroborated by the measured hydrophilic behavior of MXenes.26 The bond angle ∠O−C−O is 133.31° when CO2 is chemisorbed on Mo3C2 and 118.82° on Cr3C2, indicating that CO2 is activated by the MXene substrate. Selectivity is crucial and strongly depends on how the material catalyzes the successive elementary electroreductions. In this regard, test calculations employing classical DFT including dispersion via the PBE/DFT-D2 method (Table 1) predict nonspontaneous Gibbs free energies of reaction at room temperature (hereafter simply referred to as reaction energies) for the first H+/e− pair gain, which leads to the chemisorbed HOCO• and OCHO• radical species. However, hydrogenation of the C atom of the captured CO2 molecule is more thermodynamically preferred than hydrogenation of one of the terminal O atoms of CO2. This implies that CO and HCOOH formation would be minimal compared to • OCH 2 O • formation, which is followed by the third H+/e− transfer onto one of the still unreacted O atoms to obtain the HOCH2O• intermediate species. As result of the fourth H+/e− gain, H2CO is produced (with the release of one H2O molecule), although the release of this captured H2CO requires a large amount of energy as a consequence of the aforementioned strong interactions between sorbate and surface. The formation of CH3O• as a fifth-reduced radical species, which is the precursor of CH3OH, exhibits spontaneous reactions over all catalysts examined, with the exception of

Figure 2. Left panel: Captured CO2 minima. Selected distances in dark gray and red indicate the proximal C−M and O−M distances, respectively, in Å. ∠O−C−O are 133.31° and 118.82° for CO2 chemisorbed on Mo3C2 and Cr3C2. Right panel: H2O chemisorption steps with proximal O−M distances in Å. Gibbs free energies of binding at room temperature calculated at the PBE/ DFT-D3 computational level, in their respective M3C2 MXenes.

catalysts due to the competitive reactions between CO2 reduction and H2O reduction. In our calculations of CO2 binding to MXene surfaces, the low-coordinated metal atoms interact with the carbon of CO2, potentially via electron donation from the carbides. The

Figure 3. Side view of minimum energy path (PBE/DFT-D3 calculations) followed for the CO2 conversion mechanism into *CH4 and **H2O catalyzed by Mo3C2. Note: Gray, lilac, red, and white spheres refer to C, Mo, O, and H atoms, respectively; ** refers to chemisorbed species. Selected distances are indicated in Å. The gray number is Mo−C bond length, while the orange numbers are Mo−O bond lengths; the blue numbers are energy spontaneously released by reactions, while the red numbers represent the energy required to carry out a reaction step (in eV). 10828


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Figure 4. Minimum energy path for the CO2 conversion into *CH4 and **H2O catalyzed by Cr3C2 calculated with PBE/DFT-D3. The intermediates and transition states (TSs) are indicated. Gibbs free energies for reaction (black line) and activation (purple line) are given, in eV. Different shadings indicate spontaneous (blue) versus nonspontaneous (brown) reactions and the barrier (purple) of activation for each reaction.

Hf3C2. However, the sixth hydrogenation at the CH3O• radical requires the radical to be released from the surface so that the H+/e− pair can physically access it, leading to energy barriers of 3 eV or higher for group IV MXenes and Ta3C2 MXenes and around 2.5 eV for M3C2 when M = V, Nb, Cr, and Mo. This limiting step eliminates this path, as well as the path toward the formation of CH4, which also requires the injection of an important amount of energy for the release of the seventh hydrogenated CH3• radical. However, the reactive nature of the transition-metal carbides establishes an alternative path, as also found in our theoretical model of highly reactive berylliumdoped BNs,19 for CO2 electroreduction to CH4. Such a mechanism involves the sixth H+/e− pair gain on the CH3O• radical taking place on the CH3 moiety, leading first to the release of CH4 and an O atom inserted on the material and, second, continuing with two successive hydrogenations that produce an −OH•-doped solid and finally a captured H2O molecule. Despite the H2O product contaminating the material, a relatively small reaction energy (with respect to the limiting **OH•/**H2O step) is needed for water desorption, and besides that, potential CO2 molecules will thermodynamically displace the captured water. In light of the well-known DFT limitations, more accurate DFT calculations with explicit dispersion corrections via the PBE/DFT-D3 method were carried out for the most promising Cr3C2 and Mo3C2 materials. As shown in Table 2, spontaneous reactions are predicted on both Cr3C2 and Mo3C2 MXenes for the first H+/e− gain to reach the **OCHO• intermediate species (−1.42 and −1.50 eV, respectively). These results for the first electron-reduction process are of paramount importance, highlighting an impressive outcomespontaneous production of the OCHO• radicaland overcoming the limitation imposed by this step.18 As hypothesized, the CO2 conversion mechanism catalyzed by group IV−VI M3C2 MXenes follows a common route in which the minimum energy path involves successive hydrogenations on the C and O atoms to reach OCHO•, •OCH2O•, HOCH2O•, and H2CO as fourth-order reduced species in the form of a highly distorted and captured product. Since the

chemisorbed CH3O• radical is more thermodynamically preferred than the chemisorbed H2COH• one during the fifth H+/e− pair gain, our PBE/DFT-D3 results also predict that the classical route toward the formation of the CH3OH final product is not favored with respect to the sixth electroreduction on the CH3 moiety of the CH3O• radical. As happened in the HOCH2O• to H2CO step, group V MXenes are characterized as producing **O···CH4 in a spontaneous process (Figure 3 and Table 2). Finally, the O-doped moiety on the surface appears to be highly reactive, as shown by the highly spontaneous reactions in the seventh hydrogenation to reach the **OH• radical species, which has the largest release of energy of all the elementary reactions (even greater than the earlier CO2 capture process). Unlike the classical materials in which the classical limiting step is imposed by the first hydrogenation step, our proposed materials present dramatically different behavior; that is, the limiting step becomes the release of such **OH• radical species in the form of a relatively strongly chemisorbed H2O molecule once the eighth and final H+/e− transfer comes into play. In this regard, Cr3C2 and Mo3C2 MXenes exhibit reaction energy values of 1.05 and 1.31 eV, as shown in Figures 3 and 4, which are hypothesized as the best alternatives for the catalytic reduction of CO2 based on transition-metal carbides at the present time. For comparison, the limiting steps of the Cr3C2 outcomes are in the order of some tested and/or theoretically studied materials, for instance, Cu surface (Peterson et al., CO2 to CH4, limiting step of 740 mV)20 or graphene-supported amorphous MoS2 (Li et al., CO2 to CO, overpotential of 540 mV when acting at maximum faradaic efficiency).55 In general, the first hydrogenation of CO2 to form the **OCHO• radical demands a substantial energy input in an isolated transformation. However, in the case of MXenes, we instead observe a spontaneous reaction energy of −0.14 eV for Cr3C2. Our DFT-D3 calculations have also identified the transition states (TSs) and their structures and energies. The results, as shown in Figure 4 and detailed in Table S1 (Supporting Information), indicate an activation barrier of 0.38 eV for the first transition state (TS1), which connects CO2 and 10829


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ACS Nano **OCHO•. In addition, Cr3C2 exhibits a smooth reaction profile along the rest of the CO2 electrochemical conversion mechanism. For instance, TS1 and TS2, the transition states leading to the **OCHO• and **•OCH2O• intermediate species, respectively, exhibit activation barriers of only 0.38 and 0.83 eV. Also interesting is the case of TS3, in which the electrochemical reduction of ** • OCH 2 O • to reach **HOCH2O• is found to have an increased energy barrier (1.04 eV). However, H2O is then spontaneously released from **HOCH2O• to give **H2CO without an energy barrier. TS4, the electrochemical reduction of **H2CO to reach **CH3O•, shows similar behavior to TS3, with a significant barrier (1.01 eV). Finally, the release of the chemisorbed CH4 as a result of the eighth H+/e− pair gain demands the injection of 1.05 eV (see Figure 4). However, in a flowing CO2 environment the CH4 product would be steadily removed by constant equilibration with the gas phase. Given the major energy demand for both Cr3C2 and Mo3C2 MXenes comes from the regeneration of a bare surface from OH termination, it would be highly necessary to examine the stability and performance of O- and OH-terminated MXenes. We start from the evaluation of and the possibility of etching −O or −OH group from O- or OH-covered MXenes in acidic conditions (Figure 5). For Mo3C2O2, as shown in Figure 5a,

displays a nonspontaneous Gibbs free reaction energy at 298.15 K (ΔG298, hereafter simply referred to as reaction energy) of 0.35 or 0.23 eV, respectively, which are larger than that on bare Mo3C2 (−0.86 eV). This implies that the OH or O group will render Mo3C2 for CO2 capture. However, the first hydrogenation step (−CO2 → HOCO•) displays a reaction energy amounting to −0.92 eV for Mo3C2(OH)2, which is smaller than that for bare Mo3C2 (−0.64 eV). Therefore, OH groups will facilely accelerate hydrogenation of CO2 and exhibit a nontrivial behavior as a promising CO2 conversion catalyst. Once the CO2 molecule is captured, the formation of neutral products such as *COOH, *HCOOH, *CH2OOH, *CH2O (with the release of one H2O molecule), *CH3O, *O (with the release of one CH4 molecule), and *OH along the subsequent reaction steps, respectively, is characterized by the release of energy, indicating higher activity than bare Mo3C2. In the end, the last product is H2O after the formation of *OH with absorbed energy of 1.17 eV. Meanwhile, for Mo3C2O2, a CO2 molecule is captured with an absorbing energy of 0.49 eV to form *COOH (Figure 7). Subsequently, the thermodynamic profile indicates that the formation of neutral products such as *HCOOH, *CHO (with the release of one H2O molecule), *CH2OH, *CH3, and *CH4 along the third, fourth, sixth, seventh, and eighth steps, respectively, is exothermic. On the contrary, the formation of radicals in the hydrocarbon compounds demands energy input, as 0.54 and 0.35 eV for *CH2O and *CH3OH, respectively. Overall, O or OH termination, as a key strategy to stabilize MXene in real samples, can actually promote CO2 conversion with respect to the bare case.

CONCLUSION In conclusion, our theoretical calculations predict that 2D group IV, V, and VI transition-metal carbides (MXenes) with formulas M3C2 are capable of catalyzing CO2 conversion into hydrocarbons, being selective toward the formation of CH4, with MXenes Cr3C2 and Mo3C2 as the most promising candidates. In the bare case, energy input of 1.05 and 1.31 eV is required for the CO2 → CH4 conversion, and the energy cost can be further reduced to 0.35 and 0.54 eV when the MXene surface is terminated with −O or −OH, respectively. Thus, we offer encouraging perspectives for the experimental testing of these materials in a moist environment, profiling Cr3C2 and Mo3C2 as the best alternatives of these series of MXenes with limiting reaction energy for CO2 conversion.

Figure 5. Deoxidation/dehydroxylation process of one of the O/ OH functional groups on (a) Mo3C2O2/(b) Mo3C2(OH)2, respectively. Gibbs free energy change for the protonation and removal of H2O, vs SHE, is shown in eV. Different shadings indicate spontaneous (blue) and nonspontaneous (red) reactions. The O atom participating in the reaction is marked with bright red.

METHODS

*O can convert to *OH spontaneously with a release energy of 0.45 eV, followed by the formation and release of *H2O, demanding an energy input of 0.70 and 0.50 eV, respectively, while if the surface is fully hydroxylated, the energy input is reduced remarkably. As shown in Figure 5b for Mo3C2(OH)2, the release of water needs an energy of only 0.22 eV, suggesting that Mo3C2 may be refreshed without hydroxylation. From the above calculation, it is reasonable to assume that MXene may be bare or OH-terminated, depending on the working conditions (such as pH value); therefore, it is necessary to further evaluate the effect of functional groups on MXene toward CO2 capture and conversion. Herein, the CO2 conversion mechanism catalyzed by Mo3C2T2 (T = −O or −OH), which is a good candidate for CO2 reduction based on the bare Mo3C2 MXene electrochemical analysis, has been studied as shown in Figures 6 and 7 as an example. At the beginning, CO2 physisorption on Mo3C2 (OH)2 or Mo3C2O2

Theoretical Modeling and Calculations. The well-resolved density functional theory was performed by the Vienna ab Initio Simulation Package (VASP, version 5.3.5).56−59 To describe the ion− electron interactions, the projector-augmented wave (PAW)60 potentials were adopted. The exchange−correlation interactions were described by the generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE).61 Taking account of the van der Waals (vdW) interactions, the dispersioncorrected DFT-D262 and DFT-D363 schemes are employed for the DFT calculations with comparison. The kinetic energy cutoff for the plane-wave basis was set to 450 eV in the current work. The Brillouin zones of the supercells were sampled by 5 × 5 × 1 k-point meshes with a spacing of 0.03/Å. All atomic structures were fully optimized until the forces were smaller than |0.02| eV/Å and energy change smaller than 10−4 eV. In addition, the climbing-image nudge elastic band (CINEB) method59 was used to search for transition states between each reaction step. The energy barriers were calculated to systematically evaluate the highest energy needed in this reaction path. To evaluate 10830


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Figure 6. Minimum energy pathway for the CO2 conversion into CH4 and H2O over Mo3C2(OH)2. Gibbs free energies along the pathway, vs SHE, are shown in eV (top). Side view and some selected distances are shown in Å (middle). The corresponding formula with Gibbs free energy change is shown in eV. Blue and red texts represent spontaneous and nonspontaneous reactions (in eV), respectively (bottom).

Figure 7. Minimum energy pathway for the CO2 conversion into CH4 and H2O over Mo3C2O2. Gibbs free energies along the pathway, vs SHE, are shown in eV (top). Side view and some selected distances are shown in Å (middle). The corresponding formula with Gibbs free energy change is shown in eV. Blue and red texts represent spontaneous and nonspontaneous reactions (in eV), respectively (bottom). the zero point energy (ZPE)20 as well as the thermal correction terms, additional calculations over the Γ points were carried out.

Thermochemistry analysis, Gibbs free energies, density of states (HSE06 functional),64−66 intermediate and

ASSOCIATED CONTENT

transition state structures, and Cartesian coordinates

S Supporting Information *

allowing unambiguous reproduction of the theoretical

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03738.

outcomes discussed in the present work (PDF) 10831


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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Neng Li: 0000-0001-9633-6702 Wee-Jun Ong: 0000-0002-5124-1934 Xiujian Zhao: 0000-0002-2517-2605 Anthony K. Cheetham: 0000-0003-1518-4845 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS N.L. would like to thank the financial support from National Natural Science Foundation of China with No. 51461135004, China Scholarship Council (CSC) under project number 201606955033, Natural Science Foundation (NSF) of Hubei Province with No. 2015CFB227, Fundamental Research Funds for the Central Universities (Nos. 2017IVB020, 2017IIGX47), and the research board of the State Key Laboratory of Silicate Materials for Architectures. D.R.M. and C.S. acknowledge the Australian Research Council (ARC) for its support through the ARC Centre of Electromaterials Science (ACES), Discovery Project (DP130100268, C.S.), Future Fellowship (FT130100076, C.S.), and Laureate Fellow (D.R.M.) schemes. We also thank the Shanghai Supercomputer Center for providing computing resources. REFERENCES (1) Maginn, E. J. What to Do with CO2. J. Phys. Chem. Lett. 2010, 1, 3478−3479. (2) Karl, T. R.; Trenberth, K. E. Modern Global Climate Change. Science 2003, 302, 1719−1723. (3) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazábal, G. O.; Pérez-Ramírez, J. Status and Perspectives of CO2 Conversion into Fuels and Chemicals by Catalytic, Photocatalytic and Electrocatalytic Processes. Energy Environ. Sci. 2013, 6, 3112−3135. (4) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon Fuels. Nano Lett. 2009, 9, 731−737. (5) Liu, G.; Hoivik, N.; Wang, K.; Jakobsen, H. Engineering TiO2 Nanomaterials for CO2 Conversion/Solar Fuels. Sol. Energy Mater. Sol. Cells 2012, 105, 53−68. (6) Zhao, C.; Liu, L.; Zhang, Q.; Wang, J.; Li, Y. Photocatalytic Conversion of CO2 and H2O to Fuels by Nanostructured Ce−TiO2/ SBA-15 Composites. Catal. Sci. Technol. 2012, 2, 2558−2568. (7) Graciani, J.; Mudiyanselage, K.; Xu, F.; Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D. J.; Liu, P.; Hrbek, J.; Sanz, J. F.; Rodriguez, J. A. Highly Active Copper-Ceria and Copper-Ceria-Titania Catalysts for Methanol Synthesis from CO2. Science 2014, 345, 546− 550. (8) Lim, D. H.; Jo, J. H.; Shin, D. Y.; Wilcox, J.; Ham, H. C.; Nam, S. W. Carbon Dioxide Conversion into Hydrocarbon Fuels on Defective Graphene-Supported Cu Nanoparticles from First Principles. Nanoscale 2014, 6, 5087−5092. (9) Li, H.; Zhang, X.; MacFarlane, D. R. Quantum Dots: Carbon Quantum Dots/Cu2O Heterostructures for Solar-Light-Driven Conversion of CO2 to Methanol. Adv. Energy Mater. 2015, 5, 1401077. (10) Hsu, H. C.; Shown, I.; Wei, H. Y.; Chang, Y. C.; Du, H. Y.; Lin, Y. G.; Tseng, C. A.; Wang, C. H.; Chen, L. C.; Lin, Y. C.; Chen, K. H. Graphene Oxide as a Promising Photocatalyst for CO2 to Methanol Conversion. Nanoscale 2013, 5, 262−268. (11) Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial 10832


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