Letter pubs.acs.org/JPCL
Design of Efficient Catalysts with Double Transition Metal Atoms on C2N Layer Xiyu Li,†,§ Wenhui Zhong,‡,§ Peng Cui,† Jun Li,† and Jun Jiang*,† †
Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, Hefei Science Center of CAS, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Institute of Applied Physics, Guizhou Synergetic Innovation Cen-ter of Scientific Big Data for Advanced Manufacturing Technology, Guizhou Normal College, Gaoxin Road 115, Guiyang, Guizhou 550018, P. R. China S Supporting Information *
ABSTRACT: Heterogeneous catalysis often involves molecular adsorptions to charged catalyst site and reactions triggered by catalyst charges. Here we use first-principles simulations to design oxygen reduction reaction (ORR) catalyst based on double transition metal (TM) atoms stably supported by 2D crystal C2N. It not only holds characters of low cost and high durability but also effectively accumulates surface polarization charges on TMs and later deliveries to adsorbed O2 molecule. The Co−Co, Ni−Ni, and Cu−Cu catalysts exhibit high adsorption energies and extremely low dissociation barriers for O2, as compared with their single-atom counterparts. Co−Co on C2N presents less than half the value of the reaction barrier of bulk Pt catalysts in the ORR rate-determining steps. These catalytic improvements are well explained by the dependences of charge polarization on various systems, which opens up a new strategy for optimizing TM catalytic performance with the least metal atoms on porous lowdimensional materials.
H
interactions such as replacing C atom with TM, which, however, request extra endeavor and sophisticated techniques to decorate the perfect graphene.15 Meanwhile, the optimal numbers of TM atoms for superior performance remain unknown for most cases, prohibiting the rational design or optimization of catalysts. The motif of a single TM atom was widely proposed,4,5 but it is also stated that several atoms being together is necessary to ensure high catalysis performance.2,3 Moreover, the difficulties to identify or control the actual number of TM atoms impede a thorough examination of the size dependence of catalysis performance from the experimental point of view. Very recently, Baek. et al. reported a novel 2D layered crystal with uniform hole vacancies named C2N holey 2D (C2N-h2D), easily prepared through wet chemical reaction.16 As seen in Figure 1a, its unique porous structure originated from 2D graphene layer provides ideal support for metallic active center, and the high surface-to-volume ratio property ensures sufficient exposure of TM atoms to interact with reactant molecules.17 Unlike C3N4 layer that breaks a graphene into fragments connected by nonconductive nitrogen nodes, the C2N layer holds continuous network structure so as to maintain high electric conductivity.16 More importantly, it contains well-
eterogeneous catalysis utilizing the ensemble of a small number of atoms on certain substrate often exhibits excellent catalytic performance for many important chemical reactions including oxygen reduction reaction, carbon monoxide oxidation, and hydrogen production.1−4 It provides a promising way to minimize the use of noble metals or even substitute them with a few non-noble transition metal (TM) atoms5,6 so as to meet the ultimate goal of cost-effective catalysis. Recent years have witnessed remarkable catalyst designs of several TM atoms;5,7 however, the search for appropriate support substrate remains very challenging, and the control of optimal TM atom number or size is extremely hard, both of which hindered the realistic applications. Metal substrates have been widely used to support small TM nanoclusters8,9 to offer highly active reaction sites, but their applications are often limited by the high cost of bulk metal. Nonmetallic supports can utilize carbon-based materials such as C3N4, while electrochemical reactions would be jeopardized by the low efficiency in charge delivery.10 Multi-transition-metal active sites stabilized by organic or inorganic ligands have been shown to be very promising,11−14 but their surface-to-volume ratios are not always ideal and the charge injection and transportation in ligands are not very efficient. Graphene owns high conductivity, but TM atoms often drift easily on graphene surface to form aggregated particles, which heavily reduces active catalytic sites. Strategies have been proposed to fix several TM atoms on graphene through strong chemical © 2016 American Chemical Society
Received: January 14, 2016 Accepted: April 19, 2016 Published: April 19, 2016 1750
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Figure 1. Top and side views of the structure of the monolayer C2N (h2D form) in a 2 × 2 supercell (a) and the structures of TM-N-hole in TM@ C2N (b) and TM2@C2N (c).
in Figure 1b) causes negligible structural change, while double TM atoms anchoring slightly distorts the 2D plane (TM2@C2N in Figure 1c). Each TM atom bonds strongly to two N atoms with bonding lengths of 1.93 to 2.13 Å (Figures S2 and S3). The binding energy (Eb) was calculated to examine the structural stability, where Eb = ETMn + EC2N − ETMn@C2N and ETMn@C2N, ETMn, and EC2N denote energies of the hybrid structure and isolated parts. In Table S1, the smallest Eb of TM on C2N is 3.85 eV, suggesting strong couplings and good stability. More importantly, the addition of another TM on TM@C2N to form TM2@C2N effectively reduces system energy (increases Eb) for 1.91−5.09 eV. Because the computed binding energies of the single or double TM on C2N are slightly larger than the corresponding TM bulk cohesive energies (Table S1), one can expect good stability for the anchoring of TM on C2N.25 Meanwhile, the computed diffusion barriers for TM atoms to move from one hole vacancy to the next hole site were 2.97 to 3.91 eV (Table S2), indicating strong chemical bondings between TM and C2N. This prohibits the drifting of TM atoms to aggregate. In contrast, TM atoms on other 2D layered structures such as graphene and h-BN have very small diffusion barriers (0.02 to 0.69 eV in Table S2), causing the serious problem of TM aggregation. Meanwhile, because TM atoms can drift on the nonvacancy surface part of C2N before being trapped by vacancies, one can adjust the existence of TM@C2N and TM2@C2N configurations by changing TM deposition density and later identify them with TM−TM bond measurement. Meanwhile, Bader charge analysis reveals that the C2N part can extract 0.47 to 0.96 e− electrons from TM, which are increased to 0.64 to 1.50 e− in TM2@C2N. These accumulate polarization charges in C2N, which partially occupied the previous conduction bands (Figure S4) and induced a metallic feature favoring electrochemical reactions. Here the magnetic couplings of TMs are weak (Table S3). We then move to investigate the ORR catalytic properties. It is well-established that the initial adsorption manner of O2 molecule often influences or determines the entire reaction pathway. For the pure monolayer C2N, the physical adsorption of O2 (Eb = 0.06 eV) and nearly zero polarization charge suggest very low activity of O2 on C2N (Figure S5). While for the case of TM@C2N, there are strong positive charges remaining on TM sites, favoring O2 adsorption. Starting with two configurations of O−O bond being parallel or vertical to the C2N plane, we found that O2 is always adsorbed to TM sites. The most stable configurations were achieved by maximizing the adsorption energy (Eads = ETMn@C2N + EO2 −
ordered big vacancies to anchor one or two TM atoms tightly (Figure 1b,c). The sp2-bonded nitrogen atoms at the vacancy hole edge can provide the coordination sites for TM atom owing to their lone-pair electrons, which would couple strongly with TM and thereby prohibit TM drifting and aggregating. Spectroscopic tools such as X-ray or Raman are able to measure the TM−TM bond, making it feasible to identify (and later to control) one and double TM configurations on C2N. Taking advantages of high durability and conductivity, the systems of C2N encapsulating one or double TM atoms in the vacancy (namely, TM@C2N or TM2@C2N) offer opportunities to design efficient catalysts by optimizing the number of TM atoms. Here we applied first-principles theoretical simulations to study the system of four types of TM atoms (Pt, Co, Ni, and Cu) anchored on a C2N monolayer, based on which the catalytic activity of oxygen was examined. Promoting O2 activation is of fundamental importance for chemical reactions in many biologic applications and electrochemical field, such as CO oxidation18 and ORR.19 Tremendous efforts have been devoted to exploring durable, efficient and low-cost catalysts to accelerate the sluggish kinetics of ORR in fuel cell, which critically restricts electrochemical performance. Pt-based catalyst probably exhibits the best performance in acidic and alkaline electrolyte, while its utilization is limited by the scarcity and poor durability. In our designed TM@C2N and TM2@C2N system, merits of low cost and morphological variety can be expected. The strong couplings of several TM atoms with sp2bonded N and later with reactant molecule would cause d electron redistribution, so as to promote the activities of nonnoble Co-, Ni-, and Cu-based systems to be better than (or at least as good as) those expensive Pt-based catalysts. Our simulations were carried at the spin-polarized density functional theory (DFT) level using the VASP package (see details in Support Information).20 The Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional within a generalized gradient approximation (GGA)13 and the projector augmented-wave (PAW)21 potential were employed. The GGA +U method was applied to describe partially filled d orbitals by considering coulomb and exchange corrections.22 The method of climbing image nudged elastic band (CI-NEB) was used for transition-state search.23 The optimized structure of a monolayer C2N-h2D holds a unit cell of 12 C and 6 N atoms, where four unit cells form a uniform hole (Figure 1a). Our calculated lattice parameter of 8.32 Å matches with experiment.16 The computed energy band structure (Figure S1) agrees with previous reports.24 The deposition of one TM atom of Pt, Co, Ni, and Cu (TM@C2N 1751
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Figure 2. (a) Optimized configurations of O2 adsorbed on TM@C2N (TM= Pt, Co, Ni, Cu). (b) Partial density of states (PDOS) of O2 on TM@ C2N. (c) Distribution of exchange/polarized charges between O2 and TM@C2N with the isovalue of 0.003 e Å−3 (computed as the difference charge of systems before and after O2 adsorption). Cyan and yellow bubbles represent positive and negative charges, respectively.
Table 1. Computed Adsorption Energies (Eads), O−O Bond Lengths, and Charges Donated from TM@C2N and TM2@C2N Catalysts to O2 Obtained by Bader Charge analysis Eads (eV) Pt Co Ni Cu
O2 charge (e−)
O−O bond (Å)
TM@C2N
TM2@C2N
TM@C2N
TM2@C2N
TM@C2N
TM2@C2N
1.22 1.18 0.81 0.64
1.77 2.39 1.83 1.41
1.36 1.33 1.29 1.28
1.40 1.47 1.44 1.45
−0.51 −0.51 −0.37 −0.33
−0.62 −0.81 −0.76 −0.63
(TS) in Figure S6, where the dissociating O−O bond is stretched from 1.36 to 2.10 Å. In the TS geometry, a newly formed O atom via the O−O bond scission moves to an atop position of Pt. In the final state (FS) of reaction, two O atoms locate at the atop position via Pt−O bonds. The O−O distance is extended to 2.78 Å in FS, while the Pt−O bonds are decreased from 2.00 Å in IS to 1.80 Å in FS. This process is exothermic by 0.84 eV and needs to overcome a barrier of 1.29 eV (Figure S6). Substituting the Pt atom by Co, Ni, and Cu, we identified similar reaction processes and obtained the O2 dissociation barriers of 1.56, 2.36, and 2.79 eV (Figure S6), respectively. Unfortunately, these barriers are still too high to enable efficient ORR. We thus turn to the TM2@C2N systems with double TM atoms anchoring. In Figure 3a, O2 on TM2@C2N all take the side-on adsorption configurations, where both O atoms bond to two TM atoms and pull them out the C2N plane. Such strong bindings result in high adsorption energies (1.41 to 2.39 eV in Table 1) and cause strong couplings between TM-3d and O22p orbitals, as reflected by PDOS in Figure 3b. These enable O2 to extract more polarized charges (0.62 to 0.81 e− in Table 1) from TM2 and C2N (Figure 3c), in comparing with TM@C2N (Figure 2c). The O−O bond length (1.22 Å in free O2) on
ETMn@C2N−O2), where ETMn@C2N−O2, ETMn@C2N, and EO2 represent energies of the hybrid structure and separated parts. In Figure 2a, O2 is adsorbed on Pt@C2N with a side-on configuration, in which both O atoms connect to Pt with equal distances. In contrast, the adsorption of O2 to Ni/Cu@C2N takes the endon configuration, where only one O atom bonds to TM and the other one is far from metal. O2 on Co@C2N also chooses the end-on mode, except the disconnected O is still close to Co. As a result, the order of O2 adsorption energy is Pt > Co > Ni > Cu (Table 1). Nevertheless, the high values of Eads (0.64 to 1.22 eV) confirm strong chemical adsorption. The adsorption also elongates the O−O bond length from 1.22 Å in free O2 to 1.28 to 1.36 Å in TM@C2N (Table 1). Electronic structures reflected by partial density of states (PDOS) in Figure 2b show that electronic coupling mainly exists between TM-3d and O22p orbitals. Importantly, TM@C2N always donates electrons to the adsorbed O2, accumulating negative charges (yellow bubbles in Figure 2c) that normally help trigger reduction reaction. The adsorption of O2 would then be followed by its dissociation, which is often the rate-determining step for the ORR processes.26 With negative charges in the O2 on Pt@C2N, the O−O bond scission would occur via the transition state 1752
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Figure 3. (a) Optimized configurations of O2 adsorbed on TM2@C2N (TM = Pt, Co, Ni, Cu). (b) Partial density of states (PDOS) of O2 on TM2@ C2N. (c) Distribution of exchange/polarized charges between O2 and TM2@C2N with the isovalue of 0.003 e Å−3 (computed as the difference charge of systems before and after O2 adsorption). Cyan and yellow bubbles represent positive and negative charges, respectively.
Figure 4. (a) Dependence of O2 adsorption energy on TM polarization charge (positive) in TM@C2N and TM2@C2N. (b) Dependence of O2 dissociation barrier on its polarization charge (negative) induced by adsorption on TM@C2N and TM2@C2N. (c) O2 and OOH dissociationinitiated pathways for ORR catalyzed by Co2@C2N. The reaction energy (ΔE) and activation energy (Ea) for all steps are given in parentheses with the form of (ΔE, Ea).
transition state of 1.43 Å O−O bond and arrive at the final state of 2.47 Å. The reaction is exothermic by 1.75 eV and only needs to overcome a barrier of 0.63 eV. Amazingly, the
TM2@C2N is enlarged to 1.40 to 1.47 Å (Table 1), implying high activity. Taking O2 on Pt2@C2N as an example (Figure S7), the O−O bond (1.40 Å) scission would occur via a 1753
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binding to different positions. The variation of geometries (Figure S13) and polarization charges (Tables S6 and S7) are found to be very small. The shortest distance between two TM sites at adjacent C2N vacancies is 8.32 Å, based on which the influence of adjacent TM atoms to the neighboring C2N hole should be negligible. The solvent effect is found to be insignificant to the reaction barriers and ORR pathways (Table S8, Figure S14). In summary, using double TM atoms supported by the novel 2D material C2N, we designed low-cost, highly durable, and highly efficient ORR catalysts. The Cu−Cu, Co−Co, and Ni− Ni systems, made of non-noble metals, exhibit both high adsorption and low activation barrier for O2. Specifically, Ni− Ni and Co−Co provide amazingly low O2 dissociation barrier. The C2N material serves as an ideal substrate to trap single or double TM atoms and prohibit TM aggregation, offering highly durable structures to adsorb molecules and accommodate charges. C2N also holds good electric conductivity to deliver charges for electrochemical and photocatalysis reactions. The striking improvement of catalytic O2 activity by Co−Co, Ni− Ni, and Cu−Cu catalysts is explained by the accumulation of polarization charges, which highlights the importance of atom number control in designing efficient and economic metal catalysts. With the most stable and inexpensive Co2@C2N structure, we demonstrated that the rate-determining barriers along two full ORR pathways are only 0.39 eV, which is less than half the value of the reported bulk Pt catalysts. Future work with better control over metal size by porous lowdimensional materials would further boost catalytic activities.
dissociation of O2 on Co2@C2N is exothermic by 1.25 eV with no energy barrier at all (Figure S7). This could be ascribed to the increase in d electrons in Co−Co, which, in turn, shift up the d-band center position of the single Co on C2N. The barriers of O−O bond scission on the Ni2@C2N and Cu2@ C2N are also lowered to 0.11 and 0.56 eV, respectively (Figure S7). Therefore, the system of double TMs of Co−Co, Ni−Ni, and Cu−Cu anchoring together on C2N is facile to dissociate O2. According to the above studies, the O2 activation with double TM atoms is evidently superior to those of the single TM atom, in adsorbing and dissociating O2. It was reported that O2 dissociation by single Pt on N-doped graphene,27 MoS2,28 and single Co/Fe atom embedded in nitrogen-modified graphene29 requires activation energy of 2.39, 1.44, and 1.96/1.19 eV, respectively. More importantly, with only a few metal utilizations, our TM2@C2N systems could dissociate O2 with energy barriers as low as those of the corresponding bulk metals.30,31 O2 molecule is the electron-accepting agent in ORR. Those performance improvements could thus be ascribed to the change of charge polarization, as revealed by our recent work on metal32 or semiconductor33 catalysts. It is found that metals can accumulate more positive charges in the double TM atoms (0.64 to 1.50 e+) than the single TM (0.47 to 0.96 e+), and O2 extracts more electrons from TM2@C2N (0.62 to 0.81 e−) than TM@C2N (0.33 to 0.51 e−). Putting all systems together, Figure 4a shows that except for the single Pt and double Pt−Pt cases (Pt is known for being excellent in adsorbing O2), the O2 adsorption ability increases with the increase in TM-positive polarization charges following the order of Cu < Ni < Co < Cu−Cu < Ni−Ni < Co−Co. More strikingly, the O2 dissociation barrier decreases almost linearly with the increase in negative polarization charge accumulated in O2 in Figure 4b. Therefore, it is convenient to predict the catalytic performance of several TM atoms by evaluating the induced polarization charges. The effect of external electric field induced by electrochemistry or photocatalysis was modeled by adding one extra electron, in which ∼0.25 e− electrons are trapped by O2 (Table S4) to facilitate reduction. The accumulation of polarization charges explains the order of O2 dissociation barrier (Figure S8), suggesting good catalytic O2 activity by double TMs. It is found that neither the O2 adsorption nor the extra charge can cause big structural deviation (Table S5), demonstrating C2N as an ideally stable substrate to support TM and deliver charges. As a demonstration, we have chosen the most stable system of Co2@C2N to investigate the full ORR through two pathways initiated by O2 or OOH dissociation in Figure 4c (detailed structures and reaction steps are in Figures S9−S11), which are thermodynamically viable at critical electrode potentials as suggested by free-energy diagram calculations (Figure S12). Considering the low dissociation barrier of OOH (0.09 eV), it is unnecessary to study the hydrogenation reaction of OOH and the pathway starting with HOOH dissociation. The results show that a four-electron O2 dissociation pathway would be kinetically favorable, in which the H2O formation through OH hydrogenation reaction is the rate-determining step with an activation barrier of 0.39 eV. This value is less than the half value of that in bulk Pt catalysts (∼0.80 eV).34−36 We thus achieve ORR catalytic performance as good as bulk Pt with much less expensive yet highly durable Co−Co structure. Meanwhile, the varying TM concentration was examined by modeling a 2 × 1 C2N supercell with one or two TM atoms
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00096. Optimized geometries and formation energies of TM1,2@ C2N, atomic structures of the initial state, transition state, and final state for O2 dissociation absorbed on TM1,2@ C2N. The elementary reactions along the O2 and OOH dissociation ORR pathway and DFT-calculated free energy diagrams following O2 dissociation pathway for Co2@C2N. The water-solvent effect on O2 dissociations for TM 2 @C 2 N and ORR along O 2 and OOH dissociation pathways for Co2@C2N. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions §
X.L. and W.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the 973 Program (No. 2014CB848900), NSFC (No. 21473166 and 21303027), CAS Strategic Priority Research Program B (No. XDB01020000), and Hefei Science Center CAS (2015HSC-UP011).
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DOI: 10.1021/acs.jpclett.6b00096 J. Phys. Chem. Lett. 2016, 7, 1750−1755
