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Ni anchored C2N monolayers as low-cost and efficient catalysts for hydrogen production from formic acid
Journal of Power Sources 413 (2019) 399–407
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Ni anchored C2N monolayers as low-cost and efficient catalysts for hydrogen production from formic acid
T
Qiming Binga, Wei Liub,c, Wencai Yia, Jing-yao Liua,∗ a
Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023, China Beijing Computational Science Research Center, Beijing, 100193, China c Department of Physics and Astronomy, University of California, Irvine, CA, 92697, USA b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
C N monolayer acts as stable support • for nickel single atom and clusters. N catalysts achieve better cat• Nialytic@Cactivity and selectivity than 2
x
2
pure palladium.
Nitrogen atoms of C N participate and • facilitate catalytic reactions via sy2
nergetic effect.
A R T I C LE I N FO
A B S T R A C T
Keywords: Formic acid Hydrogen production Density functional theory C2N
Noble-metal-free catalysts are highly desirable for hydrogen generation from formic acid dehydrogenation. Herein, using first-principles density functional theory calculations, we design a series of nickel-anchored nitrogenated holey two-dimensional carbon structures (Nix@C2N, x = 1–3) as formic acid dehydrogenation catalysts. For all Nix@C2N surfaces, the formic acid dehydrogenation preferably proceeds via the formate pathway. The effective barrier continuously decreases for formic acid dehydrogenation while increases for hydrogen formation from Ni1@C2N to Ni3@C2N. The side reaction producing carbon monoxide and water via the carboxyl or formyl pathway cannot occur on Ni1@C2N or Ni2@C2N and is not preferred on Ni3@C2N, and thus, the Nix@C2N catalysts possess excellent selectivity of hydrogen. Notably, the unsaturated nitrogen atom of substrate also participates in the reaction and exhibits synergetic effect with the nickel component in Ni1@C2N and Ni2@C2N. The Gibbs free energetic span analysis predicts that the order of reactivity is Ni2@C2N (0.79 eV) > Ni1@C2N (0.87 eV) > Ni3@C2N (1.23 eV), and the turnover frequency of Nix@C2N is evaluated. The results are compared with the experimental and theoretical reports of some palladium-based catalysts. The present work suggests that the Nix@C2N may be promising noble-metal-free catalysts for formic acid dehydrogenation with high performance and low cost.
1. Introduction Hydrogen energy is an important alternative energy source for its renewable and non-polluting characteristics [1]. The H2 produced from
∗
hydrogen-rich compounds is a feasible solution for effective utilizing the hydrogen energy [2–4]. In different types of hydrogen-rich compounds, the liquid organic hydrogen carriers (LOHCs) are the more appropriate hydrogen carriers for their high hydrogen content,
Corresponding author. E-mail address: [email protected] (J.-y. Liu).
https://doi.org/10.1016/j.jpowsour.2018.12.063 Received 9 July 2018; Received in revised form 2 December 2018; Accepted 22 December 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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simulations were carried out to verify the thermal stability of the catalysts. Using first-principles density functional theory (DFT), we performed a systematic investigation of HCOOH decomposition on Nix@C2N. The mechanism was determined and the catalytic performance of Nix@C2N was evaluated and compared with some of Pd-based catalysts.
moderate dehydrogenation conditions and commercial availability. Among various LOHCs such as cycloalkanes, N-Heterocycles compounds, hydrazine, alcohols and formic acid (HCOOH), the HCOOH is regarded as a promising hydrogen carrier for its nontoxic, inexpensive and regenerable natures and has been studied extensively [5–7]. H2 can be released directly from the catalytic decomposition reaction of HCOOH using homogeneous or heterogeneous catalysts in the liquid formic acid or its aqueous solution. In recent years, most of the heterogeneous catalysts applied in HCOOH decomposition are based on noble metals, such as Pd, Pt and their alloys, which possess both good catalytic activity and selectivity [8–13]. Nevertheless, the high cost as well as the scarcity of noble metals limits their practical application. Thus, it has always been the focus of research to find the alternative non-noble metal or non-metal catalysts to replace the costly noble metals catalysts used in HCOOH dehydrogenation reaction. In heterogeneous catalysis, downsizing the catalytic active metal component into isolated single atom or small cluster of few atoms is a desirable way to promote the catalytic performance and reduce the cost [14,15]. Many studies have proved that the single-atom catalyst (SAC) or the supported metal cluster can achieve better activity and selectivity with remarkably reduced consumption of metal component than the traditional supported metal nanoparticle catalysts [16–18]. In heterogeneous catalysis, the two-dimensional (2D) materials, such as graphene [19–21], hexagonal boron nitride (h-BN) [22–24], graphitic carbon nitride (g-C3N4) [18] and so on [25–29], are widely used as catalyst support for their good electrical and thermal conductivities, large specific surface areas, abundant surface adsorption sites and other unique physical or chemical characteristics. Up to now, several researches by applying the single Pt or Pd atom supported on the TiN or N-doped carbon materials in HCOOH decomposition reaction have been reported [30–32]. However, the SACs using non-noble metal as the catalytic active component with 2D-material support are still lacking. Therefore, further investigation of novel HCOOH decomposition catalyst combining non-noble metal with 2D-material support is highly desirable to be explored. Recently, Baek et al. have successfully prepared a novel 2D layered crystal, C2N holey 2D (C2N-h2D), via a simple bottom-up wet-chemical method [33]. The C2N monolayer contains periodically distributed hexagonal vacancies and abundant nitrogen atoms at the edge of these vacancies, which can provide ideal coordination sites for transition metal atoms [34–36]. Recent study by Jiang et al. also suggests that the C2N can bind strongly with one or two transition metal atoms at the vacancies with improved catalytic activity [37]. Nickel is a cheap and readily available non-noble transition metal element, which has been widely applied in homogeneous and heterogeneous catalysis. However, the bulk Ni heterogeneous catalysts are not commonly used in hydrogen production from formic acid dehydrogenation due to the insufficient catalytic activity and selectivity compared with noble metals such as Pd and Pt. Previous theoretical studies by Jiao et al. suggested that the energy barriers of rate-determining step for HCOOH dehydrogenation on the Ni(111) and Ni(221) surfaces (1.03 and 1.43 eV, respectively) are both much higher than those on the Pd(111) and Pd(221) surfaces (0.76 and 0.96 eV, respectively) [38,39]. In addition, pure Ni-based catalysts possess poor selectivity to the desired H2 and CO2 products. For example, recent theoretical investigation by Yang et al. showed that the most preferable pathway for HCOOH dehydrogenation on the Ni (111) surface is to produce the CO and H2O products via the COOH intermediate [40]. The comprehensive study of HCOOH dehydrogenation happened on the (111) and (100) surfaces of transition metals by Mavrikakis et al. also suggested that Ni has the less selectivity to produce H2 than Pd and Pt [41]. In this work, taking advantages of the cheapness of Ni and the structural characteristics of C2N, we designed a series of heterogeneous non-noble metal catalysts, the Nix@C2N (x = 1–3), with the Ni single atom, diatomic and triatomic cluster embedded at the C2N monolayer for HCOOH decomposition. Ab initio molecular dynamics (AIMD)
2. Computational details In this work, all DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) [42]. The projector augmented wave (PAW) pseudopotentials [43,44] were employed, and the exchange-correlation energy was treated by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional [45]. The spin-polarization was considered in all calculations. The energy cutoff was 400 eV and the electronic self-consistent convergence was 10−4 eV. The (2 × 2) supercell model of C2N monolayer was constructed based on the optimized primitive unit cell structure. A vacuum region of 15 Å was used to remove the interactions between the periodic images. The isolated metal atoms were calculated in a 15 × 15 × 15 Å3 unit cell with 5 × 5 × 5 Monkhorst-Pack meshes. The 5 × 5 × 1 Monkhorst-Pack k-points were used for structure optimizations and transition state searches, and 27 × 27 × 1 for density of states (DOS) calculations. The geometries of minima and transition states were converged when the residual force was lower than 0.01 eV/ Å. In structure optimizations, all the internal coordinates of atoms were allowed to fully relax. The transition states were located by the climbing-image nudged elastic band (CI-NEB) method with eight inserted images [46,47]. The adsorption energy (Eads) was defined as: Eads = Etotal – Eadsorbate – Esurface
(1)
in which the Eadsorbate, Esurface, and Etotal represent the total energy of the adsorbate, the surface, and the complex of surface and adsorbate, respectively. According to this formula, the more negative Eads indicates the better thermodynamic stability. Detailed calculation methods of Gibbs free energy are discussed in the Supplementary Material. Ab initio molecular dynamics (AIMD) simulations, as implemented in the VASP, were performed under the NVT ensemble with the time step of 1 fs. The effective charges (QX) are calculated by Bader's charge population analysis with the equation: QX = ZX – qBader,X, where ZX and qBader,X are the number of valence electrons and the calculated Bader charge of X atom, respectively. 3. Results and discussion 3.1. Structural and electronic properties of Nix@C2N catalysts The optimized unit cell of C2N monolayer contains 12 C and 6 N atoms with the lattice parameter of 8.32 Å (Fig. 1a), which matches well with the experimental value of 8.30 Å [33]. The calculated energy band gap of C2N monolayer is 1.66 eV under the GGA-PBE approach (Fig. 1b), in good agreement with the previously reported theoretical values of 1.66–1.67 eV [48,49]. The effective charge of N atom in C2N is −0.09 |e|, suggesting that the N atoms can act as ideal coordination sites for transition metal owing to the negative charges. The Nix@C2N (x = 1–3) catalysts were constructed based on a (2 × 2) supercell model of C2N monolayer with the Nix component embedded at the surface vacancies. To identify the most stable configurations of Nix@C2N, we firstly calculated the adsorption of Ni single atom at different binding sites on C2N monolayer (Fig. 1c and Table 1) and found that single Ni atom prefers to adsorb at in-plane H1 site in the vacancy of C2N monolayer with a large adsorption energy (Eads) of −4.63 eV to form the Ni1@C2N catalyst (Fig. 2a) rather than adsorb at other sites. Adding second Ni atom to Ni1@C2N would result in two different configurations of Ni2@C2N catalyst (Fig. 2b): the Ni2-I@C2N and Ni2-II@C2N, in which the two Ni atoms anchor at the adjacent or 400
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Fig. 1. (a) Optimized C2N monolayer. (b) Energy band and total density of states (TDOS) of C2N monolayer. (c) Surface adsorption sites of C2N monolayer (h1 and h2: on-surface hollow sites; B1 and B2: on-surface bridge sites; H1, H2 and H3: in-plane sites at the vacancy).
bonding with Ni atoms. We further carried out AIMD simulations (5 ps, 1000 K) to verify the thermal stability of the designed Nix@C2N catalysts. The C2N support can preserve the 2D planar conformation after the relaxations, with the Ni atoms binding at the vacancy without dispersion (Fig. S1). These results suggest that the C2N monolayer can act as a suitable support to firmly capture the Ni atoms and that the Nix@C2N catalysts are durable under the practical ambient operating temperature for HCOOH decomposition reaction. The total density of states (TDOS) suggest that all Nix@C2N catalysts exhibit metallic properties (Fig. S2). The charge density difference (CDD) plots of Nix@C2N catalysts in Fig. 1 reveal the electron transfer between the Nix component and the C2N support. The Bader charge analysis was performed to give the effective charges on the Ni and N atoms of Nix@C2N catalysts. As listed in Table 2, the total effective charges of the Ni single atom, Ni2 and Ni3 cluster are +0.78, +1.12 and + 1.23 |e|, respectively. The increased positive accumulate charges of Nix component may play an important role in adsorption and reactivity in the HCOOH decomposition process.
Table 1 Eads of Ni single atom at different surface adsorption sites of C2N monolayer. The initial and the optimized sites indicate the placement positions of Ni atom before and after the optimizations. Initial site
Optimized site
Average NieC Bond Length (Å)
Average NieN Bond Length (Å)
Eads (eV)
h1 h2 H1 H2 H3 B1 B2
h1 h2 H1 H1 H1 H1 h2
2.09 2.14 / / / / 2.14
2.13 / 1.95 1.95 1.95 1.95 /
−1.46 −1.80 −4.63 −4.63 −4.63 −4.63 −1.80
opposite H1 sites with total Eads of −8.64 and −8.75 eV, respectively. The optimized structures of Ni1@C2N and Ni2-II@C2N are consistent with those reported by Jiang et al. [37]. For the case of Ni3@C2N (Fig. 2c), the Ni triatomic cluster completely fills the vacancy with the total Eads of −13.02 eV, in which all six N atoms of the vacancy are
Fig. 2. Optimized structures and charge density difference (CDD) plots of (a) Ni1@C2N, (b) Ni2-I@C2N and Ni2-II@C2N, and (c) Ni3@C2N catalysts with labeled Ni-N and Ni-Ni bond lengths (presented in Ångstrom). The green isosurfaces in CDD plots correspond to charge gain while the red ones correspond to charge lost. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 401
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Clearly, the final adsorption structures after AIMD relaxations are well consistent with the structural optimization results. These results also indicate that the embedded Ni and unsaturated N atoms of Ni1@C2N and Ni2@C2N both act as catalytic sites by synergistically adsorbing the intermediates, and thus promote the adsorption capacity via the synergetic effect. On the Ni3@C2N surface, HCOOH firstly adsorbs at the Ni3 cluster via NieO bond without dissociation, which is also confirmed by the AIMD result (Fig. 5b). The dehydrogenation of adsorbate AD1 (HCOOH*) into IM1 is facilitated with a very low barrier of 0.03 eV (TS1), indicating that the Ni3@C2N catalyst owns the better capability to adsorb and activate the reactant than traditional metal catalysts. After the dehydrogenation of HCOOH*, the produced HCOO* species on the Nix@C2N surfaces will undergo a molecular rotation step via TS2 to transform into the intermediate IM2 (HCOO’* + H*) with low transformation barrier of 0.02, 0.18 and 0.20 eV for Ni1@C2N, Ni2@C2N and Ni3@C2N, respectively. Then the second dehydrogenation step takes place via TS3 to yield the gas-phase CO2 and the intermediate IM3 (HNi* + HN* for Ni1@C2N and Ni2@C2N; 2HNi* for Ni3@C2N) by overcoming the barrier of 0.87, 0.43 and 0.07 eV, respectively. The dissociated HNi* species in IM3 adsorbs at the Ni-top site on Ni1@C2N, whereas it binds at the bridge site on the Nix cluster of Nix@C2N catalysts (x = 2, 3). It is seen from above results that for the HCOOH decomposition reaction via the formate pathway on the Nix@C2N surfaces, the effective energy barriers for HCOOH dehydrogenation (HCOOH → CO2 + 2H*) follow the order: Ni1@C2N (0.87 eV) > Ni2@C2N (0.57 eV) > Ni3@C2N (0.23 eV), which means HCOOH dehydrogenation becomes more likely to occur as the increase of the anchored Ni atoms on the C2N monolayer.
Table 2 Calculated effective charges (|e|) of the Ni and N atoms of Nix@C2N and the total charge transfer (|e|) from the C2N support to the Nix component. Catalyst
QNi
QN (Bonding to Ni)
QN (Unbonding to Ni)
Charge transfer
Ni1@C2N Ni2-I@C2N Ni2-II@C2N Ni3@C2N
+0.78 +0.56 +0.57 +0.41
−0.12 −0.13, −0.16 −0.15 −0.15
−0.12, −0.16 −0.14 −0.16 /
0.78 1.12 1.13 1.23
Fig. 3. Gibbs free energy profiles for HCOOH decomposition reaction via the formate pathway on the Nix@C2N (x = 1–3) surfaces. In the reaction coordination, RE: gas-phase HCOOH; AD1: HCOOH*; IM1: HCOO* + H*; IM2: HCOO’* + H*; IM3: HN* + HNi* for Ni1@C2N and Ni2@C2N; IM4: HNi* + HNi* for Ni3@C2N.
3.2.2. HCOOH dehydrogenation and dehydration through carboxyl (COOH) and formyl (HCO) pathways On the metal catalyst surfaces such as Pd(111) or Pt(111), HCOOH decomposition reaction can proceed through the carboxyl (COOH) intermediate to produce H2 (HCOOH* → COOH* + H* → CO2 + H2) or the by-product CO (HCOOH* → COOH* + H* → CO* + H2O). Moreover, the direct dehydration of HCOOH through the formyl (HCO) intermediate (HCOOH* → HCO* + OH* → CO* + H2O) is another major pathway leading to the CO product. However, in the case of Ni1@C2N and Ni2@C2N, the COOH or HCO intermediates are not favored to exist since the preferential dissociative adsorption of HCOOH producing the HCOO intermediate is a barrierless process. Thus, here we only study the dehydrogenation and dehydration of HCOOH via the COOH or HCO pathways on the Ni3@C2N surface. Fig. 6 depicts the corresponding energy profiles HCOOH decomposition through intermediates COOH and HCO. It can be seen that, in the dehydrogenation pathway via the COOH intermediate (Path I) to produce the desired H2 and CO2 products, adsorbate AD1 (HCOOH*) firstly transforms into the AD2 (HCOOH’*) via TS5 with the barrier of 0.87 eV. Then, the dehydrogenation of AD2 into intermediate IM5 (COOH* + H*) passes through TS6 with the barrier of 0.72 eV. The dissociated COOH* species binds at the top site of Ni3 cluster with single NieC bond, followed by a rotation to give COOH’* (as shown in IM6), and the H* species binds at the bridge site on Ni3 cluster. Finally, COOH’* undergoes the OeH bond cleavage to yield CO2 via TS8, and H2 is released from IM4 with an endothermic process. There are two dehydration pathways (Path II and III) on the Ni3@C2N surface, which would result in the undesired CO and H2O products. The Path II proceeds from the COOH* species in IM5 via the CeO bond breaking and OeH bond formation, leading to the CO* and H2O products; and the Path III passed through the HCOOH decomposition via the CeO cleavage, yielding the HCO* and OH* intermediates which then produce the CO* plus H2O. From the energy profiles shown in Fig. 5, the effective energy barriers for Path I-III on the Ni3@C2N surface are 1.06, 1.34 and 0.73 eV,
3.2. HCOOH decomposition mechanism on Nix@C2N surfaces 3.2.1. HCOOH dehydrogenation through formate (HCOO) pathway Based on the optimized models of Nix@C2N catalysts, we next studied the adsorption and dehydrogenation of HCOOH through the formate (HCOO) intermediate on the Nix@C2N surfaces. The Gibbs free energies reaction profiles are presented in Fig. 3, and the structures of intermediates and transition states are shown in Fig. 4. The results show that the dissociative adsorption of gas-phase HCOOH reactant (RE) into intermediates IM1 (HCOO* + H*) is a barrierless process on the Ni1@C2N, Ni2-I@C2N and Ni2-II@C2N surfaces, on which the HCOO* species adsorbs at the top site of Ni and the H* species binds to the unsaturated N site (denoted as HN*). It is noted that the Ni2-I@C2N and Ni2-II@C2N have the similar Eads of Ni2 cluster and only a low barrier of 0.19 eV is needed for the transformation of the two structures (Fig. S3). The HCOOH decomposition via HCOO pathway on both Ni2-I@C2N and Ni2-II@C2N surfaces (Fig. S4) suggests that Ni2-I@C2N has the lower free energy barriers (0.57 eV for HCOOH → CO2 + 2H* and 0.64 eV for 2H* → H2) than those for Ni2-II@C2N (0.63 and 0.81 eV, respectively). Therefore, Ni2-I@C2N is more feasible for HCOOH decomposition reaction and adopted as the catalyst model in the following discussion (presented as Ni2@C2N). We further performed AIMD simulations to verify and understand the dissociative adsorptions of HCOOH on the Ni1@C2N and Ni2@C2N surfaces. In the simulations, HCOOH molecule was initially placed above the surface with the height of 2.5 Å between Ni adsorption site and carbonyl O atom. The evolution of the distances between hydroxyl O and H atoms (DO-H) and between surface unsaturated N and hydroxyl H atoms (DN-H) are shown in Fig. 5a. It is seen that the DO-H elongates from 0.9 Å to more than 2.0 Å, while the DN-H simultaneously decreases as the molecule approaching the surfaces until the NeH bond is formed.
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Fig. 4. The structures of intermediates and transition states of the formate pathway on the Nix@C2N (x = 1–3) surfaces. Bond length in Ångstrom (Å).
formate pathway on the Ni3@C2N surface (0.23 eV), it is clear that the Path III is energetically unfeasible and the formation of undesired CO product is thus inhibited.
respectively. By comparing these energy barriers, it can be concluded that the dehydration pathway through the intermediate HCO (Path III) is more favorable than the other two pathways (Path I and Path II) for that the Path III has the lowest barrier (0.73 eV) than the other two pathways. However, compared with the effective energy barrier of the 403
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Fig. 5. The evolution of (a) DN-H and DO-H on the Ni1@C2N and Ni2@C2N surfaces and (b) DNi-O and DO-H on the Ni3@C2N surface for the dissociative adsorption of HCOOH during AIMD simulations (3 ps, 298.15 K).
polarization charges of Nix component, and a linear relationship between them is observed. It is seen that the larger positive accumulate charges have a promotion on the HCOOH dehydrogenation activity but is unfavorable for H2 evolution. Besides, based on PDOS shown in Fig. 8a, the order of the d-band center (εd) is Ni1 (−0.93 eV) > Ni2 (−1.17 eV) > Ni3 (−1.29 eV). It is found that as the εd shifts from the low-energy level to Fermi-level, the effective energy barrier increases for the HCOOH dehydrogenation while decreases for the H2 formation (Fig. 8c). That is to say, the accumulate polarization charges on the embedded Ni atoms and the εd can be used as the descriptors for evaluating the catalytic activity of Nix@C2N catalysts, and the active metal sites with moderate charges and εd value may be conducive to improve the whole reaction activity. Also, these relationships between the εd and reaction energy barriers of Nix@C2N provide a clue for designing a wider range of HCOOH dehydrogenation catalysts with transition metal atoms anchored on the C2N monolayer (TMx@C2N). For example, the one with moderate εd value among a series of designed TMx@C2N catalysts may possess the ideal catalytic performance for its effective reaction barrier of neither the HCOOH dehydrogenation nor H2 formation reactions is too high. The catalytic activity can also be conveniently described by the Gibbs free energetic span (δGspan, at 298.15 K), which is defined with the method by Shaik and coworkers [50]. The δGspan for HCOOH decomposition via the HCOO pathway (HCOOH → HCOO → CO2 + H2) as shown in Fig. 3 are calculated as 0.87, 0.79 and 1.23 eV for Ni1@C2N, Ni2@C2N and Ni3@C2N, respectively. To evaluate the catalytic performance of the designed Nix@C2N, here we compare our calculated results with the theoretical and experimental reports published in the literature. Since the Pd-based catalysts are most widely used in H2 production from HCOOH for their adequate activity and good selectivity, here the comparison is made between the designed Nix@C2N and Pd-based catalysts. There are a number of theoretical studies for HCOOH decomposition on Pd-based catalysts, especially on the Pd(111) surface [13,38,51–55]. In one recent paper, Sautet et al. reported the δGspan of 1.23 eV for the preferred COOH mediated pathway and 1.42 eV for the HCOO mediated pathway leading to CO2 and H2 (with the inclusion of implicit solvent model in their study). To keep consistence of the calculated results for comparison, we also calculate the potential energy profiles on Pd(111). The calculated δGspan of the HCOO and COOH pathways are 1.60 and 1.40 eV, respectively. It is clear that the δGspan for Nix@C2N are all lower than that for Pd(111), indicating the designed Nix@C2N catalysts possess much better catalytic activity than Pd(111). Furthermore, the turnover frequency (TOF) is used to determine the efficiency of a catalyst in experiment [50,56]. For purpose of comparison with experiments, we evaluate the TOF for
3.2.3. H2 release on Nix@C2N surfaces As shown in Fig. 3, after the dehydrogenation of HCOOH and the release of CO2, the HN* species will transfer onto the Ni single atom or Ni2 cluster via TS4 to form the intermediate IM4 (2HNi*) with the barriers of 0.29 and 0.57 eV for Ni1@C2N and Ni2@C2N, respectively. On the Ni1@C2N surface, the co-adsorbed HNi* have a longer HeH distance (0.82 Å) than the bond length of H2 molecule (0.75 Å), and are inclined to form the H2 product (PR) via an exothermic process of 0.10 eV. On the Ni2@C2N surface, the two HNi* species, which separately adsorb at top and bridge sites of Ni2 cluster, have a HeH distance of 1.74 Å and undergo an endothermic process of 0.37 eV to form H2. On the Ni3@C2N surface, the co-adsorbed HNi* species adsorb at adjacent bridge sites of Ni3 cluster with the HeH distance of 1.97 Å, and the H2 release process is endothermic of 1.17eV. The effective energy barriers for the H2 formation reaction (H* + H* → H2) on the Ni1@C2N, Ni2@C2N and Ni3@C2N surfaces are 0.29, 0.64 and 1.17 eV, respectively, which are nearly multiply rising with the increase of anchored Ni atoms. These results show that the adsorbed H* species is more readily to release from the single Ni site of Ni1@C2N than from the Ni2 and Ni3 cluster of Ni2@C2N and Ni3@C2N, respectively. 3.3. Discussions on electronic structure and catalytic property of Nix@C2N catalysts To gain further insights on the catalytic mechanism, charge population analysis of Nix@C2N catalysts is performed (Fig. 7) and the partial density of states (PDOS) for the 3d orbitals of Nix@C2N are calculated with the located d-band center (εd) (Fig. 8a). As shown in Fig. 7, the Ni atoms in Nix@C2N lie in low oxidation states. The accumulate polarization charges of Nix component rise higher upon the adsorbing of intermediates (Ni1: +0.78 |e| to +0.94 |e|; Ni2: +1.12 |e| to +1.30 |e|; Ni3: +1.23 |e| to +1.74 |e|) and decrease as the releasing of product, whereas the charges of the N atoms bonding with Ni have little change throughout the whole reaction. These results suggest that the Nix component serves as the electron reservoir, which is capable of electron receiving and releasing during the reaction. Moreover, the charge neutralization between the two H* species, which possess distinct charges due to the inducement from surface polarized charges of Ni and N sites, has facilitated the H2 release (from IM3 to PR on Ni1@C2N and Ni2@C2N) and the α-hydrogen dehydrogenation (from IM1 to IM4 on Ni3@C2N) processes. From these results, it can be concluded that both Ni and N in the Nix@C2N catalysts have played roles in promoting the adsorption of reactant and thus accelerate the elementary reactions. Fig. 8b plots the effective energy barriers relative to the accumulate 404
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Fig. 6. Gibbs free energy profiles for HCOOH dehydrogenation and dehydration reactions via the COOH and HCO intermediates on the Ni3@C2N surface.
HCOO + H* → CO2 + 2H*) and 0.29, 0.57, and 1.17 eV for H2 formation (2H* → H2), respectively. It is found that the Nix@C2N catalysts possess 100% selectivity of H2 since the dehydration reaction leading to undesired CO* through the COOH or HCO-mediated pathway is unlikely to occur on Nix@C2N. Based on the Gibbs free energy span (δGspan), the catalytic activity of Nix@C2N for HCOOH dehydrogenation is predicted to follow the order of Ni2@C2N (0.79 eV) > Ni1@C2N (0.87 eV) ≫ Ni3@C2N (1.23 eV) > Pd(111) (1.40 eV). The calculated TOFs for H2 production indicate that compared with some of Pd-based catalysts, Ni2@C2N possesses even better catalytic performance and Ni1@C2N also has the comparable one. Moreover, in the Nix@C2N catalysts, the C2N monolayer not only serves as a durable support to trap the Ni atoms but also participates in the catalytic reaction with the unsaturated N site. The Ni and N sites of the Ni1@C2N and Ni2@C2N surfaces synergistically adsorb the reaction intermediates and thus facilitate the reaction via the synergetic effect. The insights into the relationships between electronic structure and catalytic performance of Nix@C2N indicate that the accumulate polarization charges and the εd can be adopted as the descriptors to evaluate the catalytic activity of the
H2 production from catalytic HCOOH decomposition on the Nix@C2N surfaces. Based on the DFT derived δGspan values, the TOFs are calculated as 569, 12080 and 6.2 × 10−4 h−1 for Ni1@C2N, Ni2@C2N, and Ni3@C2N, respectively. Compared with the experimental TOFs of some of Pd-based catalysts with various supports (ranging from 250 to 3500 h−1 [57–61]), the TOF for H2 production of Ni1@C2N is at the equal order of magnitude and that of Ni2@C2N is even few times larger. These results indicate that both Ni1@C2N and Ni2@C2N are expected to have the comparable or even better catalytic performance than the Pd-based catalysts. 4. Conclusions In this work, we have designed a series of novel efficient and economic HCOOH decomposition catalysts, Nix@C2N (x = 1–3), by anchoring non-noble metal in the cavity of the 2D porous materials C2N. The hydrogen generation from HCOOH proceeds via the formatemediated pathway on the Nix@C2N surfaces, with the effective barriers of 0.87, 0.57 and 0.23 eV for HCOOH dehydrogenation (HCOOH → 405
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Fig. 7. Effective charges on the selected atoms of intermediates and Nix@C2N surfaces. Color scheme: Nix (green filled square), hydroxy H atom (orange hollow triangle), α-hydrogen atom (orange filled triangle), N atom (adsorption site for H, blue hollow circle), and N atoms (bonding with Nix, blue filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
part of the computational time.
catalysts. The present results suggest that the Nix@C2N catalysts may be promising candidates for HCOOH decomposition reaction and can act as adequate substitutions of traditional noble-metal catalysts.
Appendix A. Supplementary data Acknowledgments
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.12.063.
This work was supported by the National Natural Science Foundation of China (Grant No. 21773083). We also acknowledge the High Performance Computing Center of Jilin Province for supporting
Fig. 8. (a) PDOS of Nix@C2N catalysts. The Fermi level is set as zero and the εd is marked with red dashed line. (b and c) Linear relationships between the effective barriers against the accumulate polarization charges on Nix component and the εd of Nix@C2N catalysts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 406
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Ni anchored C2N monolayers as low-cost and efficient catalysts for hydrogen production from formic acid
T
Qiming Binga, Wei Liub,c, Wencai Yia, Jing-yao Liua,∗ a
Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023, China Beijing Computational Science Research Center, Beijing, 100193, China c Department of Physics and Astronomy, University of California, Irvine, CA, 92697, USA b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
C N monolayer acts as stable support • for nickel single atom and clusters. N catalysts achieve better cat• Nialytic@Cactivity and selectivity than 2
x
2
pure palladium.
Nitrogen atoms of C N participate and • facilitate catalytic reactions via sy2
nergetic effect.
A R T I C LE I N FO
A B S T R A C T
Keywords: Formic acid Hydrogen production Density functional theory C2N
Noble-metal-free catalysts are highly desirable for hydrogen generation from formic acid dehydrogenation. Herein, using first-principles density functional theory calculations, we design a series of nickel-anchored nitrogenated holey two-dimensional carbon structures (Nix@C2N, x = 1–3) as formic acid dehydrogenation catalysts. For all Nix@C2N surfaces, the formic acid dehydrogenation preferably proceeds via the formate pathway. The effective barrier continuously decreases for formic acid dehydrogenation while increases for hydrogen formation from Ni1@C2N to Ni3@C2N. The side reaction producing carbon monoxide and water via the carboxyl or formyl pathway cannot occur on Ni1@C2N or Ni2@C2N and is not preferred on Ni3@C2N, and thus, the Nix@C2N catalysts possess excellent selectivity of hydrogen. Notably, the unsaturated nitrogen atom of substrate also participates in the reaction and exhibits synergetic effect with the nickel component in Ni1@C2N and Ni2@C2N. The Gibbs free energetic span analysis predicts that the order of reactivity is Ni2@C2N (0.79 eV) > Ni1@C2N (0.87 eV) > Ni3@C2N (1.23 eV), and the turnover frequency of Nix@C2N is evaluated. The results are compared with the experimental and theoretical reports of some palladium-based catalysts. The present work suggests that the Nix@C2N may be promising noble-metal-free catalysts for formic acid dehydrogenation with high performance and low cost.
1. Introduction Hydrogen energy is an important alternative energy source for its renewable and non-polluting characteristics [1]. The H2 produced from
∗
hydrogen-rich compounds is a feasible solution for effective utilizing the hydrogen energy [2–4]. In different types of hydrogen-rich compounds, the liquid organic hydrogen carriers (LOHCs) are the more appropriate hydrogen carriers for their high hydrogen content,
Corresponding author. E-mail address: [email protected] (J.-y. Liu).
https://doi.org/10.1016/j.jpowsour.2018.12.063 Received 9 July 2018; Received in revised form 2 December 2018; Accepted 22 December 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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simulations were carried out to verify the thermal stability of the catalysts. Using first-principles density functional theory (DFT), we performed a systematic investigation of HCOOH decomposition on Nix@C2N. The mechanism was determined and the catalytic performance of Nix@C2N was evaluated and compared with some of Pd-based catalysts.
moderate dehydrogenation conditions and commercial availability. Among various LOHCs such as cycloalkanes, N-Heterocycles compounds, hydrazine, alcohols and formic acid (HCOOH), the HCOOH is regarded as a promising hydrogen carrier for its nontoxic, inexpensive and regenerable natures and has been studied extensively [5–7]. H2 can be released directly from the catalytic decomposition reaction of HCOOH using homogeneous or heterogeneous catalysts in the liquid formic acid or its aqueous solution. In recent years, most of the heterogeneous catalysts applied in HCOOH decomposition are based on noble metals, such as Pd, Pt and their alloys, which possess both good catalytic activity and selectivity [8–13]. Nevertheless, the high cost as well as the scarcity of noble metals limits their practical application. Thus, it has always been the focus of research to find the alternative non-noble metal or non-metal catalysts to replace the costly noble metals catalysts used in HCOOH dehydrogenation reaction. In heterogeneous catalysis, downsizing the catalytic active metal component into isolated single atom or small cluster of few atoms is a desirable way to promote the catalytic performance and reduce the cost [14,15]. Many studies have proved that the single-atom catalyst (SAC) or the supported metal cluster can achieve better activity and selectivity with remarkably reduced consumption of metal component than the traditional supported metal nanoparticle catalysts [16–18]. In heterogeneous catalysis, the two-dimensional (2D) materials, such as graphene [19–21], hexagonal boron nitride (h-BN) [22–24], graphitic carbon nitride (g-C3N4) [18] and so on [25–29], are widely used as catalyst support for their good electrical and thermal conductivities, large specific surface areas, abundant surface adsorption sites and other unique physical or chemical characteristics. Up to now, several researches by applying the single Pt or Pd atom supported on the TiN or N-doped carbon materials in HCOOH decomposition reaction have been reported [30–32]. However, the SACs using non-noble metal as the catalytic active component with 2D-material support are still lacking. Therefore, further investigation of novel HCOOH decomposition catalyst combining non-noble metal with 2D-material support is highly desirable to be explored. Recently, Baek et al. have successfully prepared a novel 2D layered crystal, C2N holey 2D (C2N-h2D), via a simple bottom-up wet-chemical method [33]. The C2N monolayer contains periodically distributed hexagonal vacancies and abundant nitrogen atoms at the edge of these vacancies, which can provide ideal coordination sites for transition metal atoms [34–36]. Recent study by Jiang et al. also suggests that the C2N can bind strongly with one or two transition metal atoms at the vacancies with improved catalytic activity [37]. Nickel is a cheap and readily available non-noble transition metal element, which has been widely applied in homogeneous and heterogeneous catalysis. However, the bulk Ni heterogeneous catalysts are not commonly used in hydrogen production from formic acid dehydrogenation due to the insufficient catalytic activity and selectivity compared with noble metals such as Pd and Pt. Previous theoretical studies by Jiao et al. suggested that the energy barriers of rate-determining step for HCOOH dehydrogenation on the Ni(111) and Ni(221) surfaces (1.03 and 1.43 eV, respectively) are both much higher than those on the Pd(111) and Pd(221) surfaces (0.76 and 0.96 eV, respectively) [38,39]. In addition, pure Ni-based catalysts possess poor selectivity to the desired H2 and CO2 products. For example, recent theoretical investigation by Yang et al. showed that the most preferable pathway for HCOOH dehydrogenation on the Ni (111) surface is to produce the CO and H2O products via the COOH intermediate [40]. The comprehensive study of HCOOH dehydrogenation happened on the (111) and (100) surfaces of transition metals by Mavrikakis et al. also suggested that Ni has the less selectivity to produce H2 than Pd and Pt [41]. In this work, taking advantages of the cheapness of Ni and the structural characteristics of C2N, we designed a series of heterogeneous non-noble metal catalysts, the Nix@C2N (x = 1–3), with the Ni single atom, diatomic and triatomic cluster embedded at the C2N monolayer for HCOOH decomposition. Ab initio molecular dynamics (AIMD)
2. Computational details In this work, all DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) [42]. The projector augmented wave (PAW) pseudopotentials [43,44] were employed, and the exchange-correlation energy was treated by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional [45]. The spin-polarization was considered in all calculations. The energy cutoff was 400 eV and the electronic self-consistent convergence was 10−4 eV. The (2 × 2) supercell model of C2N monolayer was constructed based on the optimized primitive unit cell structure. A vacuum region of 15 Å was used to remove the interactions between the periodic images. The isolated metal atoms were calculated in a 15 × 15 × 15 Å3 unit cell with 5 × 5 × 5 Monkhorst-Pack meshes. The 5 × 5 × 1 Monkhorst-Pack k-points were used for structure optimizations and transition state searches, and 27 × 27 × 1 for density of states (DOS) calculations. The geometries of minima and transition states were converged when the residual force was lower than 0.01 eV/ Å. In structure optimizations, all the internal coordinates of atoms were allowed to fully relax. The transition states were located by the climbing-image nudged elastic band (CI-NEB) method with eight inserted images [46,47]. The adsorption energy (Eads) was defined as: Eads = Etotal – Eadsorbate – Esurface
(1)
in which the Eadsorbate, Esurface, and Etotal represent the total energy of the adsorbate, the surface, and the complex of surface and adsorbate, respectively. According to this formula, the more negative Eads indicates the better thermodynamic stability. Detailed calculation methods of Gibbs free energy are discussed in the Supplementary Material. Ab initio molecular dynamics (AIMD) simulations, as implemented in the VASP, were performed under the NVT ensemble with the time step of 1 fs. The effective charges (QX) are calculated by Bader's charge population analysis with the equation: QX = ZX – qBader,X, where ZX and qBader,X are the number of valence electrons and the calculated Bader charge of X atom, respectively. 3. Results and discussion 3.1. Structural and electronic properties of Nix@C2N catalysts The optimized unit cell of C2N monolayer contains 12 C and 6 N atoms with the lattice parameter of 8.32 Å (Fig. 1a), which matches well with the experimental value of 8.30 Å [33]. The calculated energy band gap of C2N monolayer is 1.66 eV under the GGA-PBE approach (Fig. 1b), in good agreement with the previously reported theoretical values of 1.66–1.67 eV [48,49]. The effective charge of N atom in C2N is −0.09 |e|, suggesting that the N atoms can act as ideal coordination sites for transition metal owing to the negative charges. The Nix@C2N (x = 1–3) catalysts were constructed based on a (2 × 2) supercell model of C2N monolayer with the Nix component embedded at the surface vacancies. To identify the most stable configurations of Nix@C2N, we firstly calculated the adsorption of Ni single atom at different binding sites on C2N monolayer (Fig. 1c and Table 1) and found that single Ni atom prefers to adsorb at in-plane H1 site in the vacancy of C2N monolayer with a large adsorption energy (Eads) of −4.63 eV to form the Ni1@C2N catalyst (Fig. 2a) rather than adsorb at other sites. Adding second Ni atom to Ni1@C2N would result in two different configurations of Ni2@C2N catalyst (Fig. 2b): the Ni2-I@C2N and Ni2-II@C2N, in which the two Ni atoms anchor at the adjacent or 400
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Fig. 1. (a) Optimized C2N monolayer. (b) Energy band and total density of states (TDOS) of C2N monolayer. (c) Surface adsorption sites of C2N monolayer (h1 and h2: on-surface hollow sites; B1 and B2: on-surface bridge sites; H1, H2 and H3: in-plane sites at the vacancy).
bonding with Ni atoms. We further carried out AIMD simulations (5 ps, 1000 K) to verify the thermal stability of the designed Nix@C2N catalysts. The C2N support can preserve the 2D planar conformation after the relaxations, with the Ni atoms binding at the vacancy without dispersion (Fig. S1). These results suggest that the C2N monolayer can act as a suitable support to firmly capture the Ni atoms and that the Nix@C2N catalysts are durable under the practical ambient operating temperature for HCOOH decomposition reaction. The total density of states (TDOS) suggest that all Nix@C2N catalysts exhibit metallic properties (Fig. S2). The charge density difference (CDD) plots of Nix@C2N catalysts in Fig. 1 reveal the electron transfer between the Nix component and the C2N support. The Bader charge analysis was performed to give the effective charges on the Ni and N atoms of Nix@C2N catalysts. As listed in Table 2, the total effective charges of the Ni single atom, Ni2 and Ni3 cluster are +0.78, +1.12 and + 1.23 |e|, respectively. The increased positive accumulate charges of Nix component may play an important role in adsorption and reactivity in the HCOOH decomposition process.
Table 1 Eads of Ni single atom at different surface adsorption sites of C2N monolayer. The initial and the optimized sites indicate the placement positions of Ni atom before and after the optimizations. Initial site
Optimized site
Average NieC Bond Length (Å)
Average NieN Bond Length (Å)
Eads (eV)
h1 h2 H1 H2 H3 B1 B2
h1 h2 H1 H1 H1 H1 h2
2.09 2.14 / / / / 2.14
2.13 / 1.95 1.95 1.95 1.95 /
−1.46 −1.80 −4.63 −4.63 −4.63 −4.63 −1.80
opposite H1 sites with total Eads of −8.64 and −8.75 eV, respectively. The optimized structures of Ni1@C2N and Ni2-II@C2N are consistent with those reported by Jiang et al. [37]. For the case of Ni3@C2N (Fig. 2c), the Ni triatomic cluster completely fills the vacancy with the total Eads of −13.02 eV, in which all six N atoms of the vacancy are
Fig. 2. Optimized structures and charge density difference (CDD) plots of (a) Ni1@C2N, (b) Ni2-I@C2N and Ni2-II@C2N, and (c) Ni3@C2N catalysts with labeled Ni-N and Ni-Ni bond lengths (presented in Ångstrom). The green isosurfaces in CDD plots correspond to charge gain while the red ones correspond to charge lost. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 401
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Clearly, the final adsorption structures after AIMD relaxations are well consistent with the structural optimization results. These results also indicate that the embedded Ni and unsaturated N atoms of Ni1@C2N and Ni2@C2N both act as catalytic sites by synergistically adsorbing the intermediates, and thus promote the adsorption capacity via the synergetic effect. On the Ni3@C2N surface, HCOOH firstly adsorbs at the Ni3 cluster via NieO bond without dissociation, which is also confirmed by the AIMD result (Fig. 5b). The dehydrogenation of adsorbate AD1 (HCOOH*) into IM1 is facilitated with a very low barrier of 0.03 eV (TS1), indicating that the Ni3@C2N catalyst owns the better capability to adsorb and activate the reactant than traditional metal catalysts. After the dehydrogenation of HCOOH*, the produced HCOO* species on the Nix@C2N surfaces will undergo a molecular rotation step via TS2 to transform into the intermediate IM2 (HCOO’* + H*) with low transformation barrier of 0.02, 0.18 and 0.20 eV for Ni1@C2N, Ni2@C2N and Ni3@C2N, respectively. Then the second dehydrogenation step takes place via TS3 to yield the gas-phase CO2 and the intermediate IM3 (HNi* + HN* for Ni1@C2N and Ni2@C2N; 2HNi* for Ni3@C2N) by overcoming the barrier of 0.87, 0.43 and 0.07 eV, respectively. The dissociated HNi* species in IM3 adsorbs at the Ni-top site on Ni1@C2N, whereas it binds at the bridge site on the Nix cluster of Nix@C2N catalysts (x = 2, 3). It is seen from above results that for the HCOOH decomposition reaction via the formate pathway on the Nix@C2N surfaces, the effective energy barriers for HCOOH dehydrogenation (HCOOH → CO2 + 2H*) follow the order: Ni1@C2N (0.87 eV) > Ni2@C2N (0.57 eV) > Ni3@C2N (0.23 eV), which means HCOOH dehydrogenation becomes more likely to occur as the increase of the anchored Ni atoms on the C2N monolayer.
Table 2 Calculated effective charges (|e|) of the Ni and N atoms of Nix@C2N and the total charge transfer (|e|) from the C2N support to the Nix component. Catalyst
QNi
QN (Bonding to Ni)
QN (Unbonding to Ni)
Charge transfer
Ni1@C2N Ni2-I@C2N Ni2-II@C2N Ni3@C2N
+0.78 +0.56 +0.57 +0.41
−0.12 −0.13, −0.16 −0.15 −0.15
−0.12, −0.16 −0.14 −0.16 /
0.78 1.12 1.13 1.23
Fig. 3. Gibbs free energy profiles for HCOOH decomposition reaction via the formate pathway on the Nix@C2N (x = 1–3) surfaces. In the reaction coordination, RE: gas-phase HCOOH; AD1: HCOOH*; IM1: HCOO* + H*; IM2: HCOO’* + H*; IM3: HN* + HNi* for Ni1@C2N and Ni2@C2N; IM4: HNi* + HNi* for Ni3@C2N.
3.2.2. HCOOH dehydrogenation and dehydration through carboxyl (COOH) and formyl (HCO) pathways On the metal catalyst surfaces such as Pd(111) or Pt(111), HCOOH decomposition reaction can proceed through the carboxyl (COOH) intermediate to produce H2 (HCOOH* → COOH* + H* → CO2 + H2) or the by-product CO (HCOOH* → COOH* + H* → CO* + H2O). Moreover, the direct dehydration of HCOOH through the formyl (HCO) intermediate (HCOOH* → HCO* + OH* → CO* + H2O) is another major pathway leading to the CO product. However, in the case of Ni1@C2N and Ni2@C2N, the COOH or HCO intermediates are not favored to exist since the preferential dissociative adsorption of HCOOH producing the HCOO intermediate is a barrierless process. Thus, here we only study the dehydrogenation and dehydration of HCOOH via the COOH or HCO pathways on the Ni3@C2N surface. Fig. 6 depicts the corresponding energy profiles HCOOH decomposition through intermediates COOH and HCO. It can be seen that, in the dehydrogenation pathway via the COOH intermediate (Path I) to produce the desired H2 and CO2 products, adsorbate AD1 (HCOOH*) firstly transforms into the AD2 (HCOOH’*) via TS5 with the barrier of 0.87 eV. Then, the dehydrogenation of AD2 into intermediate IM5 (COOH* + H*) passes through TS6 with the barrier of 0.72 eV. The dissociated COOH* species binds at the top site of Ni3 cluster with single NieC bond, followed by a rotation to give COOH’* (as shown in IM6), and the H* species binds at the bridge site on Ni3 cluster. Finally, COOH’* undergoes the OeH bond cleavage to yield CO2 via TS8, and H2 is released from IM4 with an endothermic process. There are two dehydration pathways (Path II and III) on the Ni3@C2N surface, which would result in the undesired CO and H2O products. The Path II proceeds from the COOH* species in IM5 via the CeO bond breaking and OeH bond formation, leading to the CO* and H2O products; and the Path III passed through the HCOOH decomposition via the CeO cleavage, yielding the HCO* and OH* intermediates which then produce the CO* plus H2O. From the energy profiles shown in Fig. 5, the effective energy barriers for Path I-III on the Ni3@C2N surface are 1.06, 1.34 and 0.73 eV,
3.2. HCOOH decomposition mechanism on Nix@C2N surfaces 3.2.1. HCOOH dehydrogenation through formate (HCOO) pathway Based on the optimized models of Nix@C2N catalysts, we next studied the adsorption and dehydrogenation of HCOOH through the formate (HCOO) intermediate on the Nix@C2N surfaces. The Gibbs free energies reaction profiles are presented in Fig. 3, and the structures of intermediates and transition states are shown in Fig. 4. The results show that the dissociative adsorption of gas-phase HCOOH reactant (RE) into intermediates IM1 (HCOO* + H*) is a barrierless process on the Ni1@C2N, Ni2-I@C2N and Ni2-II@C2N surfaces, on which the HCOO* species adsorbs at the top site of Ni and the H* species binds to the unsaturated N site (denoted as HN*). It is noted that the Ni2-I@C2N and Ni2-II@C2N have the similar Eads of Ni2 cluster and only a low barrier of 0.19 eV is needed for the transformation of the two structures (Fig. S3). The HCOOH decomposition via HCOO pathway on both Ni2-I@C2N and Ni2-II@C2N surfaces (Fig. S4) suggests that Ni2-I@C2N has the lower free energy barriers (0.57 eV for HCOOH → CO2 + 2H* and 0.64 eV for 2H* → H2) than those for Ni2-II@C2N (0.63 and 0.81 eV, respectively). Therefore, Ni2-I@C2N is more feasible for HCOOH decomposition reaction and adopted as the catalyst model in the following discussion (presented as Ni2@C2N). We further performed AIMD simulations to verify and understand the dissociative adsorptions of HCOOH on the Ni1@C2N and Ni2@C2N surfaces. In the simulations, HCOOH molecule was initially placed above the surface with the height of 2.5 Å between Ni adsorption site and carbonyl O atom. The evolution of the distances between hydroxyl O and H atoms (DO-H) and between surface unsaturated N and hydroxyl H atoms (DN-H) are shown in Fig. 5a. It is seen that the DO-H elongates from 0.9 Å to more than 2.0 Å, while the DN-H simultaneously decreases as the molecule approaching the surfaces until the NeH bond is formed.
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Fig. 4. The structures of intermediates and transition states of the formate pathway on the Nix@C2N (x = 1–3) surfaces. Bond length in Ångstrom (Å).
formate pathway on the Ni3@C2N surface (0.23 eV), it is clear that the Path III is energetically unfeasible and the formation of undesired CO product is thus inhibited.
respectively. By comparing these energy barriers, it can be concluded that the dehydration pathway through the intermediate HCO (Path III) is more favorable than the other two pathways (Path I and Path II) for that the Path III has the lowest barrier (0.73 eV) than the other two pathways. However, compared with the effective energy barrier of the 403
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Fig. 5. The evolution of (a) DN-H and DO-H on the Ni1@C2N and Ni2@C2N surfaces and (b) DNi-O and DO-H on the Ni3@C2N surface for the dissociative adsorption of HCOOH during AIMD simulations (3 ps, 298.15 K).
polarization charges of Nix component, and a linear relationship between them is observed. It is seen that the larger positive accumulate charges have a promotion on the HCOOH dehydrogenation activity but is unfavorable for H2 evolution. Besides, based on PDOS shown in Fig. 8a, the order of the d-band center (εd) is Ni1 (−0.93 eV) > Ni2 (−1.17 eV) > Ni3 (−1.29 eV). It is found that as the εd shifts from the low-energy level to Fermi-level, the effective energy barrier increases for the HCOOH dehydrogenation while decreases for the H2 formation (Fig. 8c). That is to say, the accumulate polarization charges on the embedded Ni atoms and the εd can be used as the descriptors for evaluating the catalytic activity of Nix@C2N catalysts, and the active metal sites with moderate charges and εd value may be conducive to improve the whole reaction activity. Also, these relationships between the εd and reaction energy barriers of Nix@C2N provide a clue for designing a wider range of HCOOH dehydrogenation catalysts with transition metal atoms anchored on the C2N monolayer (TMx@C2N). For example, the one with moderate εd value among a series of designed TMx@C2N catalysts may possess the ideal catalytic performance for its effective reaction barrier of neither the HCOOH dehydrogenation nor H2 formation reactions is too high. The catalytic activity can also be conveniently described by the Gibbs free energetic span (δGspan, at 298.15 K), which is defined with the method by Shaik and coworkers [50]. The δGspan for HCOOH decomposition via the HCOO pathway (HCOOH → HCOO → CO2 + H2) as shown in Fig. 3 are calculated as 0.87, 0.79 and 1.23 eV for Ni1@C2N, Ni2@C2N and Ni3@C2N, respectively. To evaluate the catalytic performance of the designed Nix@C2N, here we compare our calculated results with the theoretical and experimental reports published in the literature. Since the Pd-based catalysts are most widely used in H2 production from HCOOH for their adequate activity and good selectivity, here the comparison is made between the designed Nix@C2N and Pd-based catalysts. There are a number of theoretical studies for HCOOH decomposition on Pd-based catalysts, especially on the Pd(111) surface [13,38,51–55]. In one recent paper, Sautet et al. reported the δGspan of 1.23 eV for the preferred COOH mediated pathway and 1.42 eV for the HCOO mediated pathway leading to CO2 and H2 (with the inclusion of implicit solvent model in their study). To keep consistence of the calculated results for comparison, we also calculate the potential energy profiles on Pd(111). The calculated δGspan of the HCOO and COOH pathways are 1.60 and 1.40 eV, respectively. It is clear that the δGspan for Nix@C2N are all lower than that for Pd(111), indicating the designed Nix@C2N catalysts possess much better catalytic activity than Pd(111). Furthermore, the turnover frequency (TOF) is used to determine the efficiency of a catalyst in experiment [50,56]. For purpose of comparison with experiments, we evaluate the TOF for
3.2.3. H2 release on Nix@C2N surfaces As shown in Fig. 3, after the dehydrogenation of HCOOH and the release of CO2, the HN* species will transfer onto the Ni single atom or Ni2 cluster via TS4 to form the intermediate IM4 (2HNi*) with the barriers of 0.29 and 0.57 eV for Ni1@C2N and Ni2@C2N, respectively. On the Ni1@C2N surface, the co-adsorbed HNi* have a longer HeH distance (0.82 Å) than the bond length of H2 molecule (0.75 Å), and are inclined to form the H2 product (PR) via an exothermic process of 0.10 eV. On the Ni2@C2N surface, the two HNi* species, which separately adsorb at top and bridge sites of Ni2 cluster, have a HeH distance of 1.74 Å and undergo an endothermic process of 0.37 eV to form H2. On the Ni3@C2N surface, the co-adsorbed HNi* species adsorb at adjacent bridge sites of Ni3 cluster with the HeH distance of 1.97 Å, and the H2 release process is endothermic of 1.17eV. The effective energy barriers for the H2 formation reaction (H* + H* → H2) on the Ni1@C2N, Ni2@C2N and Ni3@C2N surfaces are 0.29, 0.64 and 1.17 eV, respectively, which are nearly multiply rising with the increase of anchored Ni atoms. These results show that the adsorbed H* species is more readily to release from the single Ni site of Ni1@C2N than from the Ni2 and Ni3 cluster of Ni2@C2N and Ni3@C2N, respectively. 3.3. Discussions on electronic structure and catalytic property of Nix@C2N catalysts To gain further insights on the catalytic mechanism, charge population analysis of Nix@C2N catalysts is performed (Fig. 7) and the partial density of states (PDOS) for the 3d orbitals of Nix@C2N are calculated with the located d-band center (εd) (Fig. 8a). As shown in Fig. 7, the Ni atoms in Nix@C2N lie in low oxidation states. The accumulate polarization charges of Nix component rise higher upon the adsorbing of intermediates (Ni1: +0.78 |e| to +0.94 |e|; Ni2: +1.12 |e| to +1.30 |e|; Ni3: +1.23 |e| to +1.74 |e|) and decrease as the releasing of product, whereas the charges of the N atoms bonding with Ni have little change throughout the whole reaction. These results suggest that the Nix component serves as the electron reservoir, which is capable of electron receiving and releasing during the reaction. Moreover, the charge neutralization between the two H* species, which possess distinct charges due to the inducement from surface polarized charges of Ni and N sites, has facilitated the H2 release (from IM3 to PR on Ni1@C2N and Ni2@C2N) and the α-hydrogen dehydrogenation (from IM1 to IM4 on Ni3@C2N) processes. From these results, it can be concluded that both Ni and N in the Nix@C2N catalysts have played roles in promoting the adsorption of reactant and thus accelerate the elementary reactions. Fig. 8b plots the effective energy barriers relative to the accumulate 404
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Fig. 6. Gibbs free energy profiles for HCOOH dehydrogenation and dehydration reactions via the COOH and HCO intermediates on the Ni3@C2N surface.
HCOO + H* → CO2 + 2H*) and 0.29, 0.57, and 1.17 eV for H2 formation (2H* → H2), respectively. It is found that the Nix@C2N catalysts possess 100% selectivity of H2 since the dehydration reaction leading to undesired CO* through the COOH or HCO-mediated pathway is unlikely to occur on Nix@C2N. Based on the Gibbs free energy span (δGspan), the catalytic activity of Nix@C2N for HCOOH dehydrogenation is predicted to follow the order of Ni2@C2N (0.79 eV) > Ni1@C2N (0.87 eV) ≫ Ni3@C2N (1.23 eV) > Pd(111) (1.40 eV). The calculated TOFs for H2 production indicate that compared with some of Pd-based catalysts, Ni2@C2N possesses even better catalytic performance and Ni1@C2N also has the comparable one. Moreover, in the Nix@C2N catalysts, the C2N monolayer not only serves as a durable support to trap the Ni atoms but also participates in the catalytic reaction with the unsaturated N site. The Ni and N sites of the Ni1@C2N and Ni2@C2N surfaces synergistically adsorb the reaction intermediates and thus facilitate the reaction via the synergetic effect. The insights into the relationships between electronic structure and catalytic performance of Nix@C2N indicate that the accumulate polarization charges and the εd can be adopted as the descriptors to evaluate the catalytic activity of the
H2 production from catalytic HCOOH decomposition on the Nix@C2N surfaces. Based on the DFT derived δGspan values, the TOFs are calculated as 569, 12080 and 6.2 × 10−4 h−1 for Ni1@C2N, Ni2@C2N, and Ni3@C2N, respectively. Compared with the experimental TOFs of some of Pd-based catalysts with various supports (ranging from 250 to 3500 h−1 [57–61]), the TOF for H2 production of Ni1@C2N is at the equal order of magnitude and that of Ni2@C2N is even few times larger. These results indicate that both Ni1@C2N and Ni2@C2N are expected to have the comparable or even better catalytic performance than the Pd-based catalysts. 4. Conclusions In this work, we have designed a series of novel efficient and economic HCOOH decomposition catalysts, Nix@C2N (x = 1–3), by anchoring non-noble metal in the cavity of the 2D porous materials C2N. The hydrogen generation from HCOOH proceeds via the formatemediated pathway on the Nix@C2N surfaces, with the effective barriers of 0.87, 0.57 and 0.23 eV for HCOOH dehydrogenation (HCOOH → 405
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Fig. 7. Effective charges on the selected atoms of intermediates and Nix@C2N surfaces. Color scheme: Nix (green filled square), hydroxy H atom (orange hollow triangle), α-hydrogen atom (orange filled triangle), N atom (adsorption site for H, blue hollow circle), and N atoms (bonding with Nix, blue filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
part of the computational time.
catalysts. The present results suggest that the Nix@C2N catalysts may be promising candidates for HCOOH decomposition reaction and can act as adequate substitutions of traditional noble-metal catalysts.
Appendix A. Supplementary data Acknowledgments
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.12.063.
This work was supported by the National Natural Science Foundation of China (Grant No. 21773083). We also acknowledge the High Performance Computing Center of Jilin Province for supporting
Fig. 8. (a) PDOS of Nix@C2N catalysts. The Fermi level is set as zero and the εd is marked with red dashed line. (b and c) Linear relationships between the effective barriers against the accumulate polarization charges on Nix component and the εd of Nix@C2N catalysts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 406
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