Confined electrochemical catalysis under cover: Enhanced CO2 reduction at the interface between graphdiyne and Cu surface

Applied Surface Science 479 (2019) 685–692

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Confined electrochemical catalysis under cover: Enhanced CO2 reduction at the interface between graphdiyne and Cu surface ⁎

Xi Chena, , Zheng-Zhe Lina, Ming Jub, Li-Xin Guoa, a b

T

⁎

School of Physics and Optoelectronic Engineering, Xidian University, Xi'an 710071, China School of Economics and Management, Shanghai Technical Institute of Electronics and Information, Shanghai 200018 201411, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphdiyne CO2 reduction Confined catalysis

Two-dimensional (2D) confined catalysis, in which reactions occur in restricted space between metal surface and 2D overlayer, opens up a new thread for enhancement in the performance of common catalysts. In this paper, Cu surface covered with graphdiyne (GDY) is proposed as an excellent catalyst for 2D confined electrochemical CO2 reduction. Density functional theory calculations reveal the superiority of GDY overlayer in reducing the free energies of key intermediates and onset voltage. In the confined space underneath GDY cover, reaction intermediates are stabilized due to charge transfer between adsorbates and GDY. With large holes as molecular tunnels, GDY has an advantage over graphene in 2D confined catalysis. Our findings inspire a new method to enhance the performance of catalysts through coating with porous 2D materials.

1. Introduction The growth of CO2 emission into the atmosphere leads to harmful effect on the global environment. The ever-increasing climate change triggered by CO2 is regarded as a threat to human society. To reduce the influence on environment and produce more fuels as well, CO2 reduction has recently gained massive attention [1–6]. Electrochemical and photochemical conversion of CO2 into hydrocarbons are considered to be promising green chemistry approaches [7], which provide great potential for renewable energy storage and a solution for greenhouse effect. However, the chemical inertness of CO2 renders many electrochemical and photochemical conversion processes inefficient. Therefore, feasible pathways for converting CO2 into fuels are still in demand [8]. With the development of emerging two-dimensional (2D) materials and special nanostructures, they are considered as promising catalysts for CO2 reduction [9–20] and water splitting [21–23]. In the past decades, electrochemical reduction of CO2 on various metal surfaces has been widely studied [24,25]. However, the selectivity towards desired products still remains a challenge [24–26]. Furthermore, large onset voltage is required to reach significant reaction rate. Previous reports showed that large onset voltage (> 0.5 V) occurs on polycrystalline metal catalysts [24,26]. Over the past decades, many kinds of electrocatalysts had been screened, and Cu is found to be the most effective metal for CO2 reduction [27–31]. However, the onset voltage for CO2 reduction to hydrocarbons on Cu electrode is on the order of 1 V

⁎

[32]. To guide the development of more active catalysts, theoretical criteria for effective CO2 reduction catalysts are proposed based on free energy diagram [33–36]. Computational technique was developed based on scaling relations between reaction intermediates, identified using a thermodynamic analysis of reaction pathway [33,34,37]. To put CO2 reduction into practical use, more excellent catalysts have yet to be developed, and theoretical models will provide guidance and bring benefit to the exploration of new catalysts. To achieve the ultimate limit of catalysis, noble metals are downsized to single atoms, which exhibit more superior catalytic ability than conventional metal nanoparticles [38–43]. It has been acknowledged that single-atom catalyst is one of the most effective ways to reduce the cost of noble metal. On the other hand, people also explore “large” catalysts, the opposite side of downsizing catalyst. In recent years, 2D overlayer (graphene or hexagonal boron nitride (hBN)) on metal surface was utilized to enhance the catalytic ability in terms of confinement effect [44–49]. Graphene overlayer, which is commonly considered as inhibitor for surface reactions due to its chemical inertness and physical blockage, has been found to be able to promote small molecule reactions on transition metal surface [44]. Studies have found that gaseous molecules can readily intercalate into the interface between 2D overlayer and metal surface. With low potential barriers, reactions can be enhanced in the confined “nano reactor” underneath 2D overlayer. As a novel idea, 2D confined catalysis will open a new era of catalytic applications with 2D materials.

Corresponding authors. E-mail addresses: [email protected] (X. Chen), [email protected] (L.-X. Guo).

https://doi.org/10.1016/j.apsusc.2019.02.132 Received 22 December 2018; Received in revised form 12 February 2019; Accepted 14 February 2019 Available online 16 February 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 479 (2019) 685–692

X. Chen, et al.

4 × 4 × 1 Monkhorst-Pack grid and a Gaussian smearing of σ = 0.05 eV. All the results are obtained by spin-polarized calculations. The convergence of total energy is considered to be achieved until the total energy difference of two iterated steps is < 10−5 eV. In extensive studies of heterogeneous catalysis, computational cost will often limit treatment of the exchange energy to be performed using GGA rather than higher level methods. To improve the accuracy calculations relying on functionals with GGA type exchange, Peterson et al. developed total energy correction on the RPBE functional via a fitting procedure to the systematic errors versus experimental data of a given set of reactions [30,70,71]. With a sufficiently low level of computational cost, such correction to GGA total energy has widely used in the calculations of CO2 reduction. With the RPBE functional, a sensitivity analysis based on experimentally determined reaction energies give a systematic correction of +0.45 eV to the total energies of gas-phase CO2 and HCOOH molecule [30,33,36,72]. Furthermore, solvation effect at the solvent-metal interface is approximately accounted for using previously described correction [30,33,73,74]. This effect includes a total energy correction of −0.25 eV for the COOH* intermediate, −0.5 eV for OH* and −0.1 eV for CO* and CHO*. No correction is applied to the OCHO* intermediates. We have applied above corrections in this work.

Over the last decade, 2D crystals and their possible applications in catalysis have been attracting considerable interest [50,51]. Porous 2D crystal overlayer, in which molecules can readily pass through the holes and form intercalation, is playing an important role in 2D confined catalysis. Several decades ago, graphyne family (graphyne, graphdiyne (GDY), graphyne-3 etc.) were predicted to be a series of porous 2D layered C allotropes [52–54]. In 2010, GDY was successfully synthesized on Cu surface [55]. Soon afterwards, theoretical studies indicated that the acetylenic triangle holes of GDY sheet, which small molecules can pass through at room temperature, can act as gas tunnels and molecular filters [56–59]. This property makes GDY overlayer superior over graphene or hBN in 2D confined catalysis. Further research on GDY-covered systems can benefit future development of 2D confined catalysis. In this paper, theoretical study using density functional theory (DFT) calculations reveals the superiority of 2D confined catalysis via GDY cover on Cu surface. Free energy diagram exhibits that electrochemical CO2 reduction underneath GDY overlayer is enhanced by the confinement effect. The superiority of GDY overlayer in decreasing reaction barriers and onset voltage is demonstrated via the free energy calculations of intermediate steps. Charge coupling in the confined space is analyzed to uncover the origin of stabilization effect on reaction intermediates. Moreover, molecular transit through the holes of GDY sheet is studied to reveal the advantage of GDY as an efficient 2D cover. According to the conclusion, porous 2D material coating is proposed as a possible way to enhance the performance of metal catalysts.

2.3. Free energy calculations Free energies calculations are performed on the basis of standard statistical thermodynamics. Within harmonic oscillator approximation, the vibrational modes of each species are used to determine the zeropoint energies, entropies, and heat capacities [75]. These contributions to the free energy are calculated with the RPBE functional from the relaxed geometries. At T = 300 K, the free energy is calculated according to

2. Computational methods 2.1. Simulation model GDY-covered Cu surface is simulated by a repeated slab model, in which GDY layer is adsorbed on top of Cu fcc (111) facet (Fig. 1(b)). A 1 × 1 GDY unit cell is adjusted to a five-layered 2 3 × 2 3 Cu slab (a0 = 8.90 Å) with the cell shape fixed. The lattice constant mismatch is 6.3%. In the Cu-GDY system, GDY mainly interacts with the two topmost layers of Cu atoms, and thus, the bottom three layers of Cu atoms are fixed. The replicas of simulation system are separated by a vacuum layer of at least 12 Å in the direction perpendicular to the Cu (111) surface. In lateral directions, interaction between the replicas of adsorbed molecules still exists. However, for naked and GDY-covered Cu surfaces, the same lateral size causes same intermolecular interaction to a similar extent, which might not make much influence on the comparison between the catalytic performances of naked and GDY-covered Cu surfaces. All the geometries are fully relaxed without any symmetric constrains until the Hellmann-Feynman forces are below 0.01 eV/Å. The optimized geometries are verified by means of frequency calculations. The search of reaction paths and transition states is performed using the climbing image nudged elastic band (CINEB) method [60–62] with linear interpolation between the coordinates of reactant and product as initial guess of reaction path. Seven images are inserted between the two stable states. The reaction paths are relaxed by minimizing the residual forces with quasi-Newton algorithm. More images are also tested, but it does not result in obvious change in calculated reaction barriers and transition state configurations.

G = EDFT + EZPE +

∫0

T

C p dT − TS,

(1)

where EDFT is the DFT total energy, EZPE is the zero-point energy, ∫ 0TC pdT is the integrated heat capacity, T is the temperature, and S is the entropy. For gas-phase molecules, ∫ 0TC pdT and S are obtained by standard ideal gas methods [75]. All gaseous species from CO2 reduction are assumed to have a fugacity of 101,325 Pa. The free energy of liquid H2O is calculated as ideal gas with an adjusted fugacity of 3534 Pa (the vapor pressure of H2O at 300 K). The free energies of CH3OH and HCOOH are adjusted to a fugacity of 6080 and 19 Pa, respectively (corresponding to an aqueous activity of 0.01 [31,72]). 2.4. Computational hydrogen electrode model The computational hydrogen electrode model (CHE) [36,76] accounts for the chemical potential of proton–electron pairs without explicit calculation of their free energy. CHE takes advantage of the fact that chemical potentials of electrodes in equilibrium are reported relative to reversible hydrogen electrode (RHE). At any pH value, the reaction

H+ + e− ↔

1 H2 (g) 2

(2)

is equilibrated at 0 V. At an electrode potential U (relative to RHE), the electron energy is shifted by −eU (e is the elementary positive charge), and thus, the free energy of a proton–electron pair reads

2.2. Electronic structure calculations

G (H+) + G (e−) = DFT calculations are performed using the Vienna ab initio simulation package [63–66]. The projector-augmented wave method [67,68] is used with a kinetic energy cutoff of 400 eV. Electron exchange and correlation are described with the use of revised Perdew-Burke-Ernzerhof (RPBE) functional [69] at the level of generalized gradient approximation (GGA). The Brillouin-zone integration is performed with

1 G (H2 )–eU . 2

(3)

Then, the free energy change for a general reaction (a “*” denotes surface bound species)

A∗ + H+ + e− ↔ AH∗ is written as 686

(4)

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Fig. 1. (a) Structure of free-standing GDY sheet. The primitive cell is enclosed by dashed lines. (b) The H configuration of GDY-covered Cu (111) slab. The simulation cell is enclosed by dashed lines. The lattice constant a0 = 9.18 Å. Cu, C, O and H atoms are represented in gold, gray, red and white, respectively. Binding energies Eb(COOH) (c) and Eb(CHO) (d) versus Eb(CO) (with reported data in Ref. [9]). The black and blue lines represent the scaling relations of (111) and (211) transition metal surfaces, respectively. The black and blue points denote (111) and (211) surfaces of some transition metals, respectively. The red points denote our calculation results for GDY-covered Cu (111) surface.

ΔG = G (AH∗)–G (A∗)–G (H+)–G (e−) 1 = G (AH∗)–G (A∗)– G (H2) + eU . 2

Ea = (EDFT (Cu − GDY) − EDFT (Cu) − EDFT (GDY))/N ,

(6)

where EDFT(Cu), EDFT(GDY) and EDFT(Cu-GDY) are the total energies of clean Cu slab, pristine GDY sheet and GDY-covered Cu (111) slab, respectively, and N = 18 is the number of C atoms in the GDY primitive cell. According to the calculation, the adsorption energies of the T, F and H configurations are Ea = −0.038, −0.045 and −0.052 eV, respectively. This result indicates that H is the most stable configuration (Fig. 1(b)), with the adsorption energy close to the adsorption energy of graphene on Cu (111) (−0.039 to −0.068 eV/atom [77]). The equilibrium distance from GDY to Cu (111) surface is d = 3.3 Å. It is noticeable that the GDY-Cu distance is close to graphene-Cu distance (> 3 Å) [44]. This distance provides an environment of 2D confined catalysis similar to the catalysis underneath graphene cover. The scale of triangular hole in GDY is about 5.0 Å (Fig. 1(b)), which is much larger than the size of CO2 molecule (2.4 Å) and the possible reduction products (CO 1.1 Å, CH4 1.8 Å, CH3OH 2.8 Å). Small molecules can readily pass through the GDY holes, and thus reactions can be performed underneath GDY. In previous theoretical research, Peterson et al. proposed systematic criteria for effective CO2 reduction catalysts [33]. On transition metal

(5)

Eq. (5) is used to evaluate reaction free energies for elementary CO2 reduction steps, with the DFT method in Section 2.3 used to determine the free energies of individual species. 3. Results and discussion 3.1. Activity of CO2 reduction underneath GDY cover To evaluate the catalytic activity of GDY-covered Cu surface, let us start from the structure of GDY. The primitive cell of GDY (Fig. 1(a)) contains 18C atoms, including 6C atoms in the hexagonal ring with sp2 hybridization and 12C atoms in the linear acetylenic chains with sp hybridization. To obtain the most stable structure of GDY-covered Cu (111) surface, we consider high-symmetry T, F and H configurations, in which the center of C hexagon is positioned above the surface Cu atom, the FCC hollow site and the HCP hollow site, respectively. The adsorption energy of GDY on Cu (111) surface is defined as. 687

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Fig. 2. (a) Reaction paths for electrochemical CO2 reduction on Cu surface. (b) The structures of reaction intermediates of CO2 reduction on GDY-covered Cu (111) surface. (c) Relative free energy landscape of CO2 reduction on GDY-covered Cu (111) surface, at electrode potential U = 0, −0.61 and −0.73 V versus RHE. The value of relative free energy of every intermediate is shown.

GDY-covered Cu (111) surface significantly exceeds pristine (111), and even (211) transition metal surfaces. For further reduction of CO, the catalytic activity of GDY-covered Cu (111) surface is stronger than pristine Cu (111) surface, and close to pristine Cu (211) surface (the red point in Fig. 1(d)). The results above reveal the high catalytic performance of GDY-covered Cu (111) surface in CO2 reduction.

surfaces, the key steps of CO2 reduction are determined to be.

CO2 +

H+

COOH∗

+

+

e−

H+

+

→

COOH∗

e−

CO∗

→

CO∗ + H+ + e− → CHO∗.

(7)

+ H2 O

(8) (9)

The reduction of CO2 to CO (Eqs. (7) and (8)) should be fast if the intermediate COOH* is strongly bound on metal surface, i.e. low (negative) binding energy Eb(COOH). By contrast, the leaving of CO from metal surface requires weak binding, i.e. high binding energy Eb(CO). For further reduction of CO, strong binding to the intermediate CHO* (Eq. (9)), i.e. low Eb(CHO), is required. The study of Peterson et al. [33,78] shows that the catalytic activity of pristine transition metal surfaces is limited by a linear scaling relation between Eb(COOH) and Eb(CO), and a similar relation between Eb(CHO) and Eb(CO). The solid lines in Fig. 1(c) and (d) represent the scaling lines of pristine (111) and (211) transition metal surfaces (note that the (211) surfaces are expected to be more active than the close-packed (111) surfaces). Any catalyst with significantly improved activity relative to transition metals must stabilize COOH* and CHO* more than they do CO*, so that their Eb(COOH) and Eb(CHO) versus Eb(CO) should deviate negatively from the scaling lines. Under the guidance of the above criteria, we calculate Eb(COOH), Eb(CO) and Eb(CHO) of GDY-covered Cu (111) surface (for the definition of binding energy Eb see the Supporting Information of Ref. [9]). For the reduction of CO2 to CO, the calculation result (the red point in Fig. 1(c)) indicates that the catalytic activity of

3.2. The confined catalysis of CO2 reduction underneath GDY cover Fig. 2(a) shows possible reaction paths of CO2 reduction to CH4, CH3OH and HCOOH [30,31]. The major route is from CO2 to CO and then to CH4 or CH3OH, with COOH* and CHO* as key intermediates. There is a side reaction producing HCOOH via OCHO* intermediate. All these steps involve OeH or CeH bond formation. In the following text, the reaction free energies of these steps on GDY-covered (Fig. 2(c)) and pristine (Fig. 3(b)) Cu (111) surface are compared to reveal the advantage of confined catalysis underneath GDY cover. The structures of reaction intermediates are illustrated in Fig. 2(b) and Fig. 3(a) for GDYcovered and pristine Cu (111) surface, respectively. In the initial step to CO2 reduction, the mechanism of CO2 transfer from the solvent phase to Cu surface is of great importance. CO2 molecule is stabilized underneath the GDY cover with a free energy decrease of −0.58 eV, which is much lower than that on pristine Cu (111) surface. Such stabilization effect originates from electron transfer from CO2 to GDY (see diagram for electron redistribution in Fig. 5(a)). To understand the mechanism, Fig. 5(b) plots the projected density of 688

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Fig. 3. (a) The structures of reaction intermediates for CO2 reduction on pristine Cu (111) surface. (b) Relative free energy landscape of CO2 reduction on pristine Cu (111) surface, at electrode potential U = 0 and −0.86 V versus RHE. The value of relative free energy of every intermediate is shown.

suggested that HCOOH is a “dead-end” species [79,80] with high formation barrier [81,82]. On GDY-covered Cu (111) surface, the reaction free energy of OCHO* + H+ + e− → HCOOH is ΔG = +0.97 eV, which is much larger than ΔG = +0.31 eV on pristine Cu (111) surface (at U = 0 versus RHE). This result reveals that GDY cover is disadvantageous to HCOOH formation. Furthermore, we also consider another side reaction OCHO* + H+ + e− → OCH2O*, but the calculation result ΔG = +1.19 eV denies the favorability of this path. Peterson et al. found ΔG = +1.9 eV to convert OCHO* to OCH2O* on Cu (211) surface [30], which also indicates that this path is unfeasible. Based on this, we will report the reaction paths and free energies through COOH* path in the following text. COOH* is a key intermediate of CO2 reduction. Once COOH* forms on transition metal surface, a proton-electron transfer step COOH* + H+ + e− → CO* + H2O results in the formation of CO* and H2O, and then leads to subsequent hydrocarbon products. The reaction free energy of CO* generation is found to be downhill (ΔG = −0.38/ −0.60 eV on GDY-covered/pristine Cu (111) surface, respectively). So, once the applied electrode potential U is low enough to reduce G(COOH*), CO is then produced on Cu electrode. On GDY-covered Cu (111) surface, the onset voltage of CO generation is predicted to be U = −0.61 V versus RHE (the gray arrow in Fig. 2(c)), at which the free energies from CO2(g) to CO* become downhill (see the blue diagram in Fig. 2(c)). At this voltage, the reaction from OCHO* to HCOOH is still uphill, indicating that HCOOH would not be produced on GDY-covered Cu (111) surface. By contrast, on pristine Cu (111) surface, the onset voltage of CO generation U = −0.77 V versus RHE (close to the reported value of Durand et al. [31]) is much lower than on GDY-covered Cu (111) surface (see the blue diagram in Fig. 3(b)). At this voltage, the by-product HCOOH is also generated because the free energy from OCHO* to HCOOH is downhill. Indeed, HCOOH is experimentally observed as a by-product along with CO on Cu electrode [32]. In comparison with pristine Cu (111) surface, GDY-covered Cu (111) surface has moderate onset voltage for CO production without the generation of HCOOH. Once the electrode potential U is low enough to reduce G(COOH*)

Fig. 4. Onset electrode potentials of CO, CH4, CH3OH and HCOOH predicted in this study.

states (PDOS) of GDY and adsorbed CO2 molecule. Since the Fermi level of adsorbed CO2 molecule is higher than GDY, a small number of electrons are injected into the conduction band of GDY. This interaction between CO2 and GDY leads to decrease of total energy in the Cu-CO2GDY system. In subsequent reactions, GDY cover shows similar stabilizing effect on the intermediates. The first reduction occurs through a proton-electron transfer step, which results in OeH bond formation to produce COOH*. On GDYcovered Cu (111) surface, the reaction free energy of CO2* + H+ + e− → COOH* is ΔG = +0.61 eV. There is also a possible side reaction passing through OCHO* intermediate and producing HCOOH. On GDY-covered Cu (111) surface, the reaction free energy of CO2* + H+ + e− → OCHO* is ΔG = +0.09 eV. Although OCHO* is thermodynamically more stable than COOH*, previous studies 689

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Fig. 5. (a) Isosurfaces of electron redistribution between adsorbed CO2*/COOH*/CHO* and GDY cover with values of ± 0.001 e/Å3 (cyan/yellow respectively). (b) PDOS of GDY bands and adsorbed molecular orbitals in Cu-GDY-CO2, Cu-GDY-COOH and Cu-GDY-CHO systems. The Fermi level (dashed line) is set to zero. The shadow areas denote the occupied states in the valence and conduction bands of GDY.

Fig. 6. (a) The DFT energy profile of CO2, CO and CH4 through the triangular hole of GDY cover. (b) The DFT potential energy surface of CO2 molecule underneath GDY. The color map (in eV) is plotted for the triangular region underneath GDY.

To understand the mechanism of 2D confined catalysis, the charge redistribution between GDY cover and adsorbates is investigated. Fig. 5(a) represents the electron redistribution between GDY cover and the key intermediates COOH* and CHO*. The PDOS calculations (Fig. 5(b)) show the electron transfer from the σ orbitals of COOH* and CHO* to the π*2p orbitals of GDY, which weakens the bonds in adsorbates and strengthens the adsorption of GDY to adsorbates. Similar effect has been found in amino acid adsorption on GDY [83]. On GDYcovered Cu (111) surface, the adsorbates sandwiched between Cu substrate and GDY are strongly bound by both of them. Such “dual adsorption” makes the binding energies Eb(COOH) and Eb(CHO) more negative than those on pristine Cu surface. With such stabilizing effect on the key intermediates COOH* and CHO*, GDY-cover Cu surface possesses higher catalytic activity than pristine Cu surface.

and make reaction free energy downhill, the subsequent hydrocarbon products are then formed through the CHO* intermediate, with the overall path proceeding as CO2* → COOH* → CO* → CHO* → HCHO* → OCH3* → O* + CH4 or CH3OH. For GDY-covered Cu (111) surface, the blue diagram in Fig. 2(c) represents the free energy landscape at CO onset voltage U = −0.61 V versus RHE. At this voltage, it can be seen that the key step CO* + H+ + e− → CHO* is still uphill with ΔG = +0.12 eV, which may slightly hinder the generation of CH4 and CH3OH. At an electrode potential U = −0.73 V versus RHE, the free energies from CO2 to CH4 and CH3OH (green diagram in Fig. 2(c)) are totally downhill throughout the whole path. By contrast, a much lower potential U = −0.86 versus RHE is required for pristine Cu (111) surface to make the reaction free energies downhill (the blue diagram in Fig. 3(b)). On GDY-covered Cu (111) surface, CH4 and CH3OH can be readily generated with moderate onset voltage. The above predicted onset voltages are summarized in Fig. 4. The above results reveal the superiority of confined catalysis underneath GDY cover, which leads to low onset voltages for CO, CH4 and CH3OH production without the side reaction of HCOOH production.

3.3. The transit of gas-phase molecules through GDY cover To keep CO2 reduction going well underneath GDY cover, suitable conditions for continuous molecular transit through GDY are necessary. 690

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At room temperature, the reaching of CO2 to the area underneath GDY cover and the leaving of products (CO and CH4) should have considerable rate. Fig. 6(a) plots the DFT total energy profiles of CO2, CO and CH4 along the path through the triangular hole of GDY. For the reaching of CO2 to the area underneath GDY, the CO2 molecule climbs over a barrier of about 0.15 eV. Such a low barrier is easy for CO2 passing through GDY at room temperature. Underneath the GDY cover, CO2 molecule is stabilized with an energy decrease of 0.38–0.46 eV. In Fig. 6(b), the potential energy map of CO2 underneath GDY is plotted for different positions of CO2 molecular center. Because of small fluctuation of potential energy surface (≤0.08 eV), CO2 molecule could wander around on Cu surface and perform reaction everywhere at room temperature. For the leaving of CO and CH4 through the hole of GDY, the barriers are 1.12 and 0.31 eV, respectively. The high barrier for CO escaping attributes to strong binding of CO on GDY-covered Cu surface. Peterson et al. have pointed out that large CO adsorption energy is in favor of the reduction of CO* [33]. The strong binding of CO makes the subsequent reactions of CO* to CH4 prevail over the escape of CO. The low barrier of CH4 is advantageous to CH4 escaping at room temperature. Jiao et al. have pointed out that CH4 can pass through GDY with high diffusion rate at room temperature [56]. According to above results, the reactant CO2 and the product CH4 can readily penetrate the GDY cover on Cu substrate. GDY cover is suitable for efficient CO2 reduction in confined catalysis.

and reaction on surfaces of photocatalysts, Energy Environ. Sci. 9 (2016) 2177. [4] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658. [5] Y. Sohn, W. Huang, F. Taghipour, Recent progress and perspectives in the photocatalytic CO2 reduction of Ti-oxide-based nanomaterials, Appl. Surf. Sci. 396 (2017) 1696. [6] S. Ye, R. Wang, M.-Z. Wu, Y.-P. Yuan, A review on g-C3N4 for photocatalytic water splitting and CO2 reduction, Appl. Surf. Sci. 358 (2015) 15. [7] X.C. Jiao, Z.W. Chen, X.D. Li, Y.F. Sun, S. Gao, W.S. Yan, C.M. Wang, Q. Zhang, Y. Lin, Y. Luo, Y. Xie, Defect-mediated electron-hole separation in one-unit-cell ZnIn2S4 layers for boosted solar-driven CO2 reduction, J. Am. Chem. Soc. 139 (2017) 7586. [8] D.T. Whipple, P.J.A. Kenis, Prospects of CO2 utilization via direct heterogeneous electrochemical reduction, J. Phys. Chem. Lett. 1 (2010) 3451. [9] K. Chan, C. Tsai, H.A. Hansen, J.K. Nørskov, Molybdenum sulfides and selenides as possible electrocatalysts for CO2 reduction, ChemCatChem 6 (2014) 1899. [10] X. Jiao, X. Li, X. Jin, Y. Sun, J. Xu, L. Liang, H. Ju, J. Zhu, Y. Pan, W. Yan, Y. Lin, Y. Xie, Partially oxidized SnS2 atomic layers achieving efficient visible-light-driven CO2 reduction, J. Am. Chem. Soc. 139 (2017) 18044. [11] M. Asadi, K. Kim, C. Liu, A.V. Addepalli, P. Abbasi, P. Yasaei, P. Phillips, A. Behranginia, J.M. Cerrato, R. Haasch, P. Zapol, B. Kumar, R.F. Klie, J. Abiade, L.A. Curtiss, A. Salehi-Khojin, Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid, Science 353 (2016) 467. [12] S. Kattel, P.J. Ramírez, J.G. Chen, J.A. Rodriguez, P. Liu, Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts, Science 355 (2017) 1296. [13] S. Liu, H. Tao, L. Zeng, Q. Liu, Z. Xu, Q. Liu, J.-L. Luo, Shape-dependent electrocatalytic reduction of CO2 to CO on triangular silver nanoplates, J. Am. Chem. Soc. 139 (2017) 2160. [14] D. Gao, H. Zhou, J. Wang, S. Miao, F. Yang, G. Wang, J. Wang, X. Bao, Size-dependent electrocatalytic reduction of CO2 over Pd nanoparticles, J. Am. Chem. Soc. 137 (2015) 4288. [15] D. Bhattacharjya, C.-H. Kim, J.-H. Kim, I.-K. You, J.B. In, S.-M. Lee, Fast and controllable reduction of graphene oxide by low-cost CO2 laser for supercapacitor application, Appl. Surf. Sci. 462 (2018) 353. [16] Q. Huang, J. Yu, S. Cao, C. Cui, B. Cheng, Efficient photocatalytic reduction of CO2 by amine-functionalized g-C3N4, Appl. Surf. Sci. 358 (2015) 350. [17] G. Shi, L. Yang, Z. Liu, X. Chen, J. Zhou, Y. Yu, Photocatalytic reduction of CO2 to CO over copper decorated g-C3N4 nanosheets with enhanced yield and selectivity, Appl. Surf. Sci. 427 (2018) 1165. [18] J.-y. Tang, R.-t. Guo, W.-g. Pan, W.-g. Zhou, C.-y. Huang, Visible light activated photocatalytic behaviour of Eu (III) modified g-C3N4 for CO2 reduction and H2 evolution, Appl. Surf. Sci. 467 (2019) 206. [19] H.-Z. Wu, S. Bandaru, J. Liu, L.-L. Li, Z. Wang, Adsorption of H2O, H2, O2, CO, NO, and CO2 on graphene/g-C3N4 nanocomposite investigated by density functional theory, Appl. Surf. Sci. 430 (2018) 125. [20] C. Zhu, Q. Xu, W. Liu, Y. Ren, CO2-assisted fabrication of novel heterostructures of h-MoO3/1T-MoS2 for enhanced photoelectrocatalytic performance, Appl. Surf. Sci. 425 (2017) 56. [21] W. Hu, L. Lin, R. Zhang, C. Yang, J. Yang, Highly efficient photocatalytic water splitting over edge-modified phosphorene nanoribbons, J. Am. Chem. Soc. 139 (2017) 15429. [22] Y.-R. An, X.-L. Fan, Z.-F. Luo, W.-M. Lau, Nanopolygons of monolayer MS2: best morphology and size for HER catalysis, Nano Lett. 17 (2017) 368. [23] Y.P. Zhu, C. Guo, Y. Zheng, S.-Z. Qiao, Surface and interface engineering of noblemetal-free electrocatalysts for efficient energy conversion processes, Acc. Chem. Res. 50 (2017) 915. [24] Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media, Electrochim. Acta 39 (1994) 1833. [25] K.P. Kuhl, T. Hatsukade, E.R. Cave, D.N. Abram, J. Kibsgaard, T.F. Jaramillo, Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces, J. Am. Chem. Soc. 136 (2014) 14107. [26] M. Jitaru, D.A. Lowy, M. Toma, B.C. Toma, L.J. Oniciu, The electrochemical reduction of carbon dioxide on flat metallic electrodes, J. Appl. Electrochem. 27 (1997) 875. [27] K.W. Frese Jr., B.P. Sullivan, K. Krist, H.E. Guard (Eds.), Electrochemical and Electrocatalytic Reactions of Carbon Dioxide, Elsevier, New York, 1993, pp. 145–216. [28] Y. Hori, R. Takahashi, Y. Yoshinami, A. Murata, Electrochemical reduction of CO at a copper electrode, J. Phys. Chem. B 101 (1997) 7075. [29] M. Gattrell, N. Gupta, A. Co, A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper, J. Electroanal. Chem. 594 (1) (2006). [30] A.A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, J.K. Nørskov, How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels, Energy Environ. Sci. 3 (2010) 1311. [31] W.J. Durand, A.A. Peterson, F. Studt, F. Abild-Pedersen, J.K. Nørskov, Structure effects on the energetics of the electrochemical reduction of CO2 by copper surfaces, Surf. Sci. 605 (2011) 1354. [32] Y. Hori, Modern Aspects of Electrochemistry, 42 Springer, New York, 2008, pp. 89–189. [33] A.A. Peterson, J.K. Nørskov, Activity descriptors for CO2 electroreduction to methane on transition-metal catalysts, J. Phys. Chem. Lett. 3 (2012) 251. [34] X. Liu, J. Xiao, H. Peng, X. Hong, K. Chan, J.K. Nørskov, Understanding trends in electrochemical carbon dioxide reduction rates, Nat. Commun. 8 (2017) 15438. [35] J.T. Feaster, C. Shi, E.R. Cave, T. Hatsukade, D.N. Abram, K.P. Kuhl, C. Hahn, J.K. Nørskov, T.F. Jaramillo, Understanding selectivity for the electrochemical

4. Conclusions In this paper, DFT calculations are employed to study the superiority of GDY-covered Cu surface in 2D confined catalysts. The binding energy criteria of Peterson et al. [33] reveal the excellent catalytic ability of GDY-covered Cu surface beyond ordinary metal surfaces, with the binding to the key intermediates COOH* and CHO* versus CO* surpassing the scaling of pristine transition metals. Free energy diagram exhibits that electrochemical CO2 reduction underneath GDY overlayer is enhanced by the confinement effect. As a result of the energetic stabilization of reaction intermediates, the onset voltage of CO2 reduction to hydrocarbon on GDY-covered Cu surface is predicted to be lower than the one on pristine Cu surface. Moreover, molecular transit through the holes of GDY sheet is easier than graphene cover. The low barriers for the in and out of reactants and products reveal that GDY cover is suitable for efficient CO2 reduction under confined catalysis. Our finding opens up an avenue to enhance the performance of catalysts through coating with porous 2D materials. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 11847047), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JQ1034), the Fundamental Research Funds for the Central Universities (Nos. JB180513 and XJS18038) and Postdoctoral Research Project of Shaanxi Province. Conflict of interest The authors declare that they have no conflict of interest. References [1] S. Gao, X.C. Jiao, Z.T. Sun, W.H. Zhang, Y.F. Sun, C.M. Wang, Q.T. Hu, X.L. Zu, F. Yang, S.Y. Yang, L. Liang, J. Wu, Y. Xie, Ultrathin Co3O4 layers realizing optimized CO2 electroreduction to formate, Angew. Chem. Int. Ed. 55 (2016) 698. [2] Z. Cao, D. Kim, D.C. Hong, Y. Yu, J. Xu, S. Lin, X.D. Wen, E.M. Nichols, K. Jeong, J.A. Reimer, P.D. Yang, C.J. Chang, A molecular surface functionalization approach to tuning nanoparticle electrocatalysts for carbon dioxide reduction, J. Am. Chem. Soc. 138 (2016) 8120. [3] X.X. Chang, T. Wang, J.L. Gong, CO2 photo-reduction: insights into CO2 activation

691

Applied Surface Science 479 (2019) 685–692

X. Chen, et al.

[36]

[37] [38] [39] [40]

[41]

[42]

[43]

[44]

[45]

[46] [47]

[48] [49] [50] [51]

[52]

[53] [54] [55] [56]

[57]

[58]

[59] S.W. Cranford, M.J. Buehler, Selective hydrogen purification through graphdiyne under ambient temperature and pressure, Nanoscale 4 (2012) 4587. [60] G. Mills, H. Jónsson, Quantum and thermal effects in H2 dissociative adsorption: evaluation of free energy barriers in multidimensional quantum systems, Phys. Rev. Lett. 72 (1994) 1124. [61] G. Mills, H. Jónsson, G.K. Schenter, Reversible work transition state theory: application to dissociative adsorption of hydrogen, Surf. Sci. 324 (1995) 305. [62] G. Henkelman, B.P. Uberuaga, H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (2000) 9901. [63] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47 (1993) 558. [64] G. Kresse, J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metalamorphous-semiconductor transition in germanium, Phys. Rev. B 49 (1994) 14251. [65] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169. [66] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a planewave basis set, Comput. Mater. Sci. 6 (1996) 15. [67] P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953. [68] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758. [69] B. Hammer, L.B. Hansen, J.K. Nørskov, Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals, Phys. Rev. B 59 (1999) 7413. [70] F. Studt, F. Abild-Pedersen, J.B. Varley, J.K. Nørskov, CO and CO2 hydrogenation to methanol calculated using the BEEF-vdW functional, Catal. Lett. 143 (2013) 71. [71] F. Studt, M. Behrens, E.L. Kunkes, N. Thomas, S. Zander, A. Tarasov, J. Schumann, E. Frei, J.B. Varley, F. Abild-Pedersen, J.K. Nørskov, R. Schlögl, The mechanism of CO and CO2 hydrogenation to methanol over Cu-based catalysts, ChemCatChem 7 (2015) 1105. [72] J.B. Varley, H.A. Hansen, N.L. Ammitzbøll, L.C. Grabow, A.A. Peterson, J. Rossmeisl, J.K. Nørskov, Ni-Fe-S cubanes on CO2 reduction electrocatalysis: a DFT study, ACS Catal. 3 (2013) 2640. [73] J.B. Varley, J.K. Nørskov, First-principles calculations of Fischer-Tropsch processes catalyzed by nitrogenase enzymes, ChemCatChem 5 (2013) 732. [74] V. Tripković, E. Skúlason, S. Siahrostami, J.K. Nørskov, J. Rossmeisl, The oxygen reduction reaction mechanism on Pt(111) from density functional theory calculations, Electrochem. Acta 55 (2010) 7975. [75] C.J. Cramer, Essentials of Computational Chemistry: Theories and Models, 2nd ed., Wiley, New York, 2004. [76] J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, Origin of the overpotential for oxygen reduction at a fuel-cell cathode, J. Phys. Chem. B 108 (2004) 17886. [77] T. Olsen, K.S. Thygesen, Random phase approximation applied to solids, molecules, and graphene-metal interfaces: from van der Waals to covalent bonding, Phys. Rev. B 87 (2013) 075111. [78] C. Shi, H.A. Hansen, A.C. Lausche, J.K. Nørskov, Trends in electrochemical CO2 reduction activity for open and close-packed metal surfaces, Phys. Chem. Chem. Phys. 16 (2014) 4720. [79] E. Vesselli, L.D. Rogatis, X. Ding, A. Baraldi, L. Savio, L. Vattuone, M. Rocca, P. Fornasiero, M. Peressi, A. Baldereschi, R. Rosei, G. Comelli, Carbon dioxide hydrogenation on Ni(110), J. Am. Chem. Soc. 130 (2008) 11417. [80] E. Vesselli, M. Rizzi, L. De Rogatis, X.B. Ding, A.G. Comelli, L. Savio, L. Vattuone, M. Rocca, P. Fornasiero, A. Baldereschi, M. Peressi, Hydrogen-assisted transformation of CO2 on nickel: the role of formate and carbon monoxide, J. Phys. Chem. Lett. 1 (2010) 402. [81] A.A. Gokhale, J.A. Dumesic, M. Mavrikakis, On the mechanism of low-temperature water gas shift reaction on copper, J. Am. Chem. Soc. 130 (2008) 1402. [82] L.C. Grabow, M. Mavrikakis, Mechanism of methanol synthesis on Cu through CO2 and CO hydrogenation, ACS Catal. 1 (2011) 365. [83] X. Chen, P. Gao, L. Guo, S. Zhang, Graphdiyne as a promising material for detecting amino acids, Sci. Rep. 5 (2015) 16720.

reduction of carbon dioxide to formic acid and carbon monoxide on metal electrodes, ACS Catal. 7 (2017) 4822. H.A. Hansen, J.B. Varley, A.A. Peterson, J.K. Nørskov, Understanding trends in the electrocatalytic activity of metals and enzymes for CO2 reduction to CO, J. Phys. Chem. Lett. 4 (2013) 388. X. Hong, K. Chan, C. Tsai, J.K. Nørskov, How doped MoS2 breaks transition-metal scaling relations for CO2 electrochemical reduction, ACS Catal. 6 (2016) 4428. B. Qiao, A. Wang, X. Yang, L.F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li, T. Zhang, Singleatom catalysis of CO oxidation using Pt1/FeOx, Nat. Chem. 3 (2011) 634. X.-F. Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Single-atom catalysts: a new frontier in heterogeneous catalysis, Acc. Chem. Res. 46 (2013) 1740. J. Lin, A. Wang, B. Qiao, X. Liu, X. Yang, X. Wang, J. Liang, J. Li, J. Liu, T. Zhang, Remarkable performance of Ir1/FeOx single-atom catalyst in water gas shift reaction, J. Am. Chem. Soc. 135 (2013) 15314. H. Wei, X. Liu, A. Wang, L. Zhang, B. Qiao, X. Yang, Y. Huang, S. Miao, J. Liu, T. Zhang, FeOx-supported platinum single-atom and pseudo-single-atom catalysts for chemoselective hydrogenation of functionalized nitroarenes, Nat. Commun. 5 (2014) 5634. J. Liu, M. Jiao, L. Lu, H.M. Barkholtz, Y. Li, Y. Wang, L. Jiang, Z. Wu, D.-J. Liu, L. Zhuang, C. Ma, J. Zeng, B. Zhang, D. Su, P. Song, W. Xing, W. Xu, Y. Wang, Z. Jiang, G. Sun, High performance platinum single atom electrocatalyst for oxygen reduction reaction, Nat. Commun. 8 (2017) 15938. H. Li, L. Wang, Y. Dai, Z. Pu, Z. Lao, Y. Chen, M. Wang, X. Zheng, J. Zhu, W. Zhang, R. Si, C. Ma, J. Zeng, Synergetic interaction between neighbouring platinum monomers in CO2 hydrogenation, Nat. Nanotechnol. 13 (2018) 411. Y. Yao, Q. Fu, Y.Y. Zhang, X. Weng, H. Li, M. Chen, L. Jin, A. Dong, R. Mu, P. Jiang, L. Liu, H. Bluhm, Z. Liu, S.B. Zhang, X. Bao, Graphene cover-promoted metal-catalyzed reactions, Proc. Natl. Acad. Sci. U. S. A. 111 (2014) 17023. Y. Zhang, X. Weng, H. Li, H. Li, M. Wei, J. Xiao, Z. Liu, M. Chen, Q. Fu, X. Bao, Hexagonal boron nitride cover on Pt(111): a new route to tune molecule–metal interaction and metal-catalyzed reactions, Nano Lett. 15 (2015) 3616. R. Mu, Q. Fu, L. Jin, L. Yu, G. Fang, D. Tan, X. Bao, Visualizing chemical reactions confined under graphene, Angew. Chem. Int. Ed. 51 (2012) 4856. Y. Zhou, W. Chen, P. Cui, J. Zeng, Z. Lin, E. Kaxiras, Z. Zhang, Enhancing the hydrogen activation reactivity of nonprecious metal substrates via confined catalysis underneath graphene, Nano Lett. 16 (2016) 6058. Q. Fu, X. Bao, Surface chemistry and catalysis confined under two-dimensional materials, Chem. Soc. Rev. 46 (2017) 1842. H. Li, J. Xiao, Q. Fu, X. Bao, Confined catalysis under two-dimensional materials, Proc. Natl. Acad. Sci. U. S. A. 114 (2017) 5930. S.H. Hur, J.N. Park, Graphene and its application in fuel cell catalysis: a review, Asia Pac. J. Chem. Eng. 8 (2013) 218. K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, Two-dimensional atomic crystals, Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 10451. R.H. Baughman, H. Eckhardt, M. Kertesz, Structure–property predictions for new planar forms of carbon: layered phases containing sp2 and sp atoms, J. Chem. Phys. 87 (1987) 6687. N. Narita, S. Nagai, S. Suzuki, K. Nakao, Optimized geometries and electronic structures of graphyne and its family, Phys. Rev. B 58 (1998) 11009. N. Narita, S. Nagai, S. Suzuki, K. Nakao, Electronic structure of three-dimensional graphyne, Phys. Rev. B 62 (2000) 11146. G. Li, Y. Li, H. Liu, Y. Guo, Y. Lia, D. Zhua, Architecture of graphdiyne nanoscale films, Chem. Commun. 46 (2010) 3256. Y. Jiao, A. Du, M. Hankel, Z. Zhu, V. Rudolph, S.C. Smith, Graphdiyne: a versatile nanomaterial for electronics and hydrogen purification, Chem. Commun. 47 (2011) 11843. H. Zhang, X. He, M. Zhao, M. Zhang, L. Zhao, X. Feng, Y. Luo, Tunable hydrogen separation in sp-sp(2) hybridized carbon membranes: a first-principles prediction, J. Phys. Chem. C 116 (2012) 16634. S. Lin, M.J. Buehler, Mechanics and molecular filtration performance of graphyne nanoweb membranes for selective water purification, Nanoscale 5 (2013) 11801.

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