Yu-Te
Chan
and
Ming-Kang
Tsai
*
Department of Chemistry, National Taiwan Normal University, Taipei, 11677, Taiwan. E-mail: mktsai@ntnu.edu.tw
First published on 18th October 2017
In this work, using density functional theory, we have characterized the CO2 reduction capabilities of a series of nine transition-metal-chelated nitrogen-substituted carbon nanotube models (TM-4N2v-CNT). Each of the chelated models consists of a four-N-substituted and one vacancy framework to mimic square planar homogeneous catalysts, and is coordinated to Fe, Ru, Os, Co, Rh, Ir, Ni, Pt or Cu. The results are further investigated to search for the possible electrochemical intermediates along the CO2 reduction pathway. We’ve found that all of the tested elements are predicted to favor the hydrogen evolution reaction over CO2 reduction energetically (with the exception of Cu), and that only Group 8 elements are predicted to bind CO effectively and other cases prefer HCOOH formation. The observed CO binding preference could be rationalized via ligand field theory based on the molecular orbitals of the square planar complexes. With a suitable applied voltage to stabilize all of the adsorbed CO intermediates, Ru and Os are predicted to produce CH4, whereas Fe is predicted to produce CH3OH. Increasing the curvature of the CNT could reduce the required potential in the potential-determining step substantially. However, the predicted catalytic sequence is subject to only the selection of a metal center.
The functionality of these TM–polypyridyl complexes depends on properly aligning the d orbitals of the central metal with the frontier valence orbitals or low-lying unoccupied orbitals of the absorbed substrates. For oxidative catalysis, withdrawing t2g orbitals are helpful in facilitating the charge transfer process from the valence electron density of the substrate. For reductive catalysis, the backdonation from the low-lying d electrons of the metal to the antibonding orbitals of the substrate plays a key role in weakening the bond order of the absorbent. Both types of metal–ligand interactions require good overlap energetically and symmetrically between d orbital wavefunctions and the targeting occupied or virtual MO of the ligands. The orientation of the d orbitals was synthetically controlled by the design of the chelated ligands. However, the procedure of designing and synthesizing ideal ligands for the purpose of carrying out the particular activation process is scientifically challenging.
The fragility of coordinated ligands presents a scientific challenge as it hinders the application of such organometallic complexes. During the catalytic cycle, reactive intermediates form and could subsequently attack the supporting ligand framework. As a result, the previously working ligand field would be non-functional or even destroyed. Thus, using heterogeneous catalysis to replace the functionality of homogeneous catalysis for mass production is of industrial interest. Nonetheless, the cost of using such expensive transition metals hinders its chemical application. Reducing the number of noble metals or introducing earth abundant replacement becomes the primary focus of catalyst development. Alternatively, designing a durable ligand framework with single catalytic metal centers, being chemically resistant to the reactive catalytic-intermediate, would also be a potential solution.
Carbon based materials including carbon nanotube (CNT), graphene, and other carbon nanostructures have been extensively investigated to overcome the limits of the conventional organometallic ligand framework due to its supreme redox stability and electrical conductivity for electrocatalysis. The pioneering experiment reported by Gong et al. introduced nitrogen-substituted carbon nanotubes (N-CNT) as a metal-free electro-catalyst for the oxygen reduction reaction (ORR) where the operational stability of the catalyst was significantly maintained up to 7000 seconds in the presence of reactive oxygen derivatives.5 The chemical nature of the ORR on these N-substituted carbon materials was subsequently investigated in many experimental and theoretical studies.6–12 Recently, the CO2 reduction reaction (CO2RR) using these metal-free N-CNT materials was also reported.13 Alternatively, Lee et al. synthesized Fe–porphyrin-like carbon nanotube catalysts in an attempt to replace Pt catalysts in the oxygen reduction reaction.14 The introduction of a metal center combining with the ligand field of N-CNT promoted early TMs like Fe to be as reactive as Pt catalysts for the ORR. Other transition metal elements like Co and Mo were also synthesized with these N-substituted carbon materials for energy applications like the ORR, hydrogen evolution reaction (HER) and CO2RR.15–19
From the theoretical perspective, Stoyanov et al. reviewed various types of metal doped carbon nanostructures where the presence of metal dopants was concluded to stabilize buckled carbon materials.20 Lee et al. predicted that the formation energy of Fe–N-CNT could be the most favourable synthetic route if the chemical potential of the N-dopant increased,14 which was followed by several theoretical studies in exploring the nature of ORR reactivity on various metal centers.21–35 Tripkovic et al. explored CO and CO2 reduction with a metal-functionalized graphene model where Rh was identified as the most active candidate.36 Cheng et al. designed a graphene model with an additional axial ligand to increase the preference for CO2 reduction over the normally more competitive hydrogen evolution pathway.37 Note, however, that even though this introduction of axial ligands allows for further tuning of ligand fields, the related synthetic challenge could be non-trivial. Recently, Chai and Guo used ab initio molecular dynamics simulations to explore the interplay between the curvature of N-substituted graphene and CO2 reduction activity,13 in which the curvature of graphene was also shown to be an influential factor in the predicted overpotentials.
In the present study, we extend the theoretical exploration using a series of N-CNT models chelating with various transition metal elements, including Fe, Ru, Os, Co, Rh, Ir, Ni, Pt, and Cu. We examine the electronic structures generated using N-CNT frameworks of different CNT radii to the chelated TM. We also compare the electronic structures of these TM-N-CNT models with the case of classic square planar complexes through the lens of ligand field theory. Finally, we analyse the redox thermodynamics for the CO2 reduction vs. hydrogen evolution reaction in terms of CNT radii.
(1) |
ΔEfyv = Eyv + yEC − EP | (2) |
In order to assemble TM-3N1v-CNT or TM-4N2v-CNT structures, one has to remove 4 or 6 carbon atoms from the pristine CNT, respectively. The formation energies of such models without TM chelation can be expressed as ΔEfxNyv by
(3) |
(4) |
We observe that more N-substitution substantially causes ΔETotFxNyv (for y = 0 cases) to increase from 0.33 to 4.03 eV due to the poor orbital incorporation of the valence orbitals of N atoms into the conjugated C–C bond network of CNT. Among the three types of 2N0v models, ΔETotF2N0v is estimated to be 1.41 to 2.76 eV, and 2NP0v is the predicted lowest-energy 2N0v structure. We notice that ΔETotF2N0v is almost greater than 2 × ΔETotF1N0v, which indicates that the infinitely separated N-substitution would still be favourable. However, clustering N-substituents could be favourable for metal binding under high N doping conditions.14 For instance, ΔETotFxNyv of Ru-4N2v is substantially lower than that of 4N2v for both CNT(6,6) and CNT(12,12) models.
In the cases of 3N- and 4N-substitution, the introduction of vacant sites modestly reduced ΔETotFxNyv as shown by the comparison of 3N0v with 3N1v and 4N0v with 4N2v in Table 1. Metal binding to xNyv models (ΔETotFMxNyv as defined in eqn (5)) is not able to compensate for the endothermic formation energies of the xNyv models as shown in Table 1. For instance, the formation energies of the Ru-3N1v-CNT and Ru-4N2v-CNT models are predicted to be between 2.79 and 3.35 eV following
ΔETotFMxNyv = ΔETM + ΔETotFxNyv | (5) |
ΔETM = ΔEemb − ΔEcoh | (6) |
ΔEemb = EMxNyv − (ExNyv + EisoM) | (7) |
ΔEcoh = Ebulk − EisoM | (8) |
ΔETotFxNyv, ΔETM, and ΔETotFMxNyv of the Ru-4N2v-CNT models using CNT(n,n), n = 5–8, 10 and 12, at various site densities (ρs = 1/3–1/8) are summarized in Fig. 2. We chose Ru chelation (due to its versatile catalytic ability reported in the literature)1–3 as an example to investigate the balance between the bulk cohesion energy and the metal chelation energy. ΔEemb − ΔEcoh ranges from 0.1 to −0.6 eV as shown in Fig. 2(top-right) and its negative value suggests that the exothermic metal binding to the 4N2v-CNT model can compensate for the formation of the isolated metal atom from the bulk state. Both ΔETotFxNyv and ΔETotFMxNyv decrease generally as ρs decreases at each CNT radius. As the radii of CNT increase, the formation of the Ru-4N2v-CNT model is shown to be more favourable.
In Fig. 2, we observe that ΔETotFMxNyv is dominantly affected by ΔETotFxNyv and all predicted ΔETotFMxNyv values are greater than 2.6 eV. However, the formation of TM-xNyv-CNT may be significantly reduced if ΔEcoh is disregarded, e.g. using the atomic form of metal sources during the material synthesis. Several experiments have reported the observation of TM-4N2v structures, TM = Fe, Co, and Zn, through various synthetic pathways.14,44–46 Therefore, exploring more theoretical implications using the current TM-4N2v-CNT models would provide more chemical insights into the experiments.
The total spin angular momentum (S) and d-band center values are tabulated in Table S2 (ESI†). The partial densities of states (PDOS) of the Ru-4N2v-CNT models at various ρs and the contribution of each Ru d-orbital are summarized in Fig. S2 (ESI†). As we emphasized earlier, the ligand field of the 4N2v-CNT model is tunable subject to the curvature of CNT. The calculated d-band center (by averaging over all d valence bands) shows a systematic shift toward the Fermi level as the CNT radii increase. The up-shifting of the d-band center also aligns consistently with ΔETM, in which the stronger ligand field that resulted from the lower CNT curvature causes stronger metal binding energy with the 4N2v structure as summarized in Table S3 (ESI†).
The x*- and y*-axes for the Ru-4N2v-CNT models are rotated counterclockwise by 45 degrees from the conventional square planar (Sq) complexes as compared in Fig. 3, where the z*-axis of the Ru-4N2v-CNT model still maintains the same orientation as the z-axis for the Sq complexes. Although the d-orbital energetics for TM-Sq complexes typically have the dx2−y2 orbital at the highest energy level, the dxy* orbital is predicted at the highest energy due to the symmetry alignment of the N-substitution for the TM-4N2v-CNT models, and the dx2−y2* orbital is calculated to be the lowest energy orbital because of the stabilization effect introduced by the CNT π* orbitals along the axial direction. The change of radii of CNT is expected to break the degeneracy of dxz*/dyz*, and the latter is repelled to higher energy levels as the CNT radii decrease.
In order to understand the conductivity of Ru-4N2v-CNT models, which is a crucial factor affecting the catalytic overpotential, we thus examine the band structures of the selected Ru-4N2v-CNT(5,5) and Ru-4N2v-CNT(12,12) at ρs = 1/3 and 1/8, respectively, as shown in Fig. 4. All of the Ru models are found to maintain the conductor characteristic as same as the pristine armchair CNT, and that is beneficial to preserve the low overpotential character. It should be noted that most of the 3d TM binding at 4N2v models using zigzag CNT were found to be semi-metallic.47
Fig. 4 Band structures of Ru-4N2v-CNT(n,n) models: (a) (5,5) at ρs = 1/3, (b) (12,12) at ρs = 1/3, (c) (5,5) at ρs = 1/8, and (d) (12,12) at ρs = 1/8. |
Increasing the CNT radii generally shifts the d-band center of Ru toward the Fermi level, and consequently enhances the adsorption energies of small molecules. The interaction between the gaseous molecules and metal could be tuned for the specific small molecule activation catalysis through the selection of a metal center and a suitable ligand field in terms of CNT radii. In the following section, we comprehensively compare the CO2 reduction reaction (CO2RR) using Fe, Ru, Os, Co, Rh, Ir, Ni, Pt and Cu for different CNT radii.
In order to characterize the CO2RR mechanism, we chose CO2(aq) + bare TM-4N2v-CNT as the reference state to calculate the stepwise relative energetics. For the 1st electrochemical step, a hydrogen atom binding with the TM-4N2v-CNT model (denoted as *H to represent hydrogen binding on the TM site) is predicted to be the most thermodynamically favourable intermediate over the electrochemical-reduction-equivalent *COOH and *OCHO for all of the models, except for TM = Cu, as shown in Table S5 (ESI†). Such electrochemical reduction preference of forming *H favours the hydrogen evolution reaction (HER) instead of the CO2RR. All of the calculated chemical-bound intermediates are schematically shown in Fig. S3 (ESI†). However, it has been demonstrated experimentally and theoretically that hydrogenation at the adsorbed CO2 (*COO) intermediate can be enhanced, and the HER can be suppressed, in the presence of a bias electrode potential (U). Hori et al. observed dominant Faraday efficiency in CH4 formation on the Cu surface using U < −1.3 V vs. NHE.48 Such observed selectivity was elaborated by Peterson et al. using Density Functional Theory (DFT),49 in which the applied bias potential enhances CO2 adsorption on the Cu surface and facilitates the subsequent hydrogenation steps. In this study, all of the calculated TM-4N2v-CNT models, which were predicted to favour *H formation, could follow the CO2 hydrogenation pathway if the suitable voltage is applied.
Along the CO2 electrochemical reduction pathway, *C(O)OH is predicted to be the favourable intermediate over *OCH(O) for all of the calculated models in step 1, except for TM = Cu. In step 2, three potential intermediates could be formed, i.e. HCOOH(aq), *CO + H2O(aq) and *C(OH)(OH). For TM = Fe, Ru and Os cases, *CO is the favourable and important intermediate for the subsequent hydrogenation steps, while other metal elements prefer to form HC(O)OH(aq) thermodynamically. The recent experiment reported by Zhang et al. with cobalt phthalocyanine (CoPc) molecules anchored on carbon nanotubes shows the CO production from CO2 electrochemical reduction.19 Such experimental observation could fairly align with this theoretical study – the 2nd lowest-energy reaction channel of generating CO and H2O is only ∼0.1 eV higher than the formation of HCOOH. The CO(aq) production could be exothermic if a higher bias potential is applied, and enhanced by the temperature effect. The strong M–CO coordination bonds for the TM = Fe, Ru and Os models can be elaborated using ligand field theory as the comparison of Ru with Rh models in Fig. 5. For the conventional square-planar TM–porphyrin complexes, the d orbital energetics of the TM = RuII(d6) and RhII(d7) metal centers are schematically represented in RuIIP and RhIIP where the dx2−y2 orbital is the highest unoccupied orbital, and dxz and dyz are the degenerate lowest-energy orbitals. With CO adsorption, the complexes turn to experience the square-pyramidal ligand field, and the dz2 orbital is repelled to higher energy levels.
Unlike the conventional square planar complexes, dxz and dyz are no longer the lowest energy orbitals due to repulsion caused by the CNT curvature. Thus, M-4N2v-CNT models, TM = RuII and RhII, are predicted to be triplet and doublet, respectively. Upon introducing the 5th coordination (CO) along the z-axis, dz2 is destabilized over dxz and dyz orbitals. Thus, a d6 configuration like RuII would be favoured by the square pyramidal coordination, while the d7 case like RhII would populate one electron on the anti-bonding character dz2 orbital. The substantial CO binding with TM = Fe, Ru and Os could be attributed to the ligand field stabilization effect.
Scheme 1 The predicted electrochemical minimum energy pathway (ecMEP) of the CO2 reduction mechanism. |
In the 1st step of the CORR, *CHO is predicted to be the dominant intermediate for all of the calculated models instead of *COH. The 2nd hydrogenation step leads to the favourable formation of *CHOH for TM = Fe, Ru, Os and Ir, while other metal centres prefer the formation of *CH2(O). The interactions of *CH2(O) with the TM = Co, Rh, Ni, Pt and Cu models are almost negligible, which implies the subsequent desorption of CH2O(aq) as the dominant route. We also characterize the CORR mechanism using TM-4N2v-graphene, TM = Ru and Rh, models as shown in Fig. S4 (ESI†). We observe the same tendency of CH2O(aq) desorption as the non-Group-8 cases instead of proceeding with the subsequent hydrogenation steps as listed in Table S5 (ESI†).
In the 3rd hydrogenation step, the formation of *OCH3via the re-adsorption of oxygen on CH2O is considered to be unlikely due to the repulsive interaction between the long pairs of CH2O and the negative electric field of the cathode electrode. Therefore, the CORR of the TM = Co, Rh (with CNT or graphene), Ni, Pt and Cu models would stop at CH2O(aq) formation. Only three metal centres (M = Fe, Ru and Os) could continue along the 3rd hydrogenation pathway to form *CH2OH instead of forming *CH + H2O and the latter is a higher-energy intermediate as shown in the ESI.†
In the 4th step, the prior *CH2OH intermediate could be hydrogenated to either release CH3OH(aq) or produce *CH2 + H2O. The TM = Fe model favours CH3OH(aq) generation and the TM = Ru and Os models would favour the *CH2(g) formation (Scheme 2).
The *CH2 intermediate could undergo two more hydrogenation steps to eventually form CH4(g).
In Table 2, we summarize the major product and potential-determining step (PDS) for the CORR. Rh is predicted to be the catalyst required for the lowest potential of −0.124 V for HCOOH generation. We predict the potentials required for overcoming the PDS to be −1.255 V and −0.283 V using TM-4N2v-graphene models, TM = Ru and Rh, respectively. These predicted potentials are in good agreement with the early results obtained by Tripkovic et al.36 where their prediction was at −1.22 V for Ru and −0.19 V for Rh. The minor difference between this study and early report can be attributed to the selection of functional and site density. In this study, only the Fe-4N2v-CNT(5.5) model is found to show CH3OH formation at −0.741 V, and the previous results using the graphene model was, however, to generate CH4 at −0.93 V. The ligand field effect introduced by the curvature of CNT seems to enhance the predicted reactivity of the chelated metal atoms. In this study, we include three types of carbon materials, i.e. CNT(5,5), CNT(12,12) and graphene, to build different ligand field effects for TM = Ru and Rh cases. All three TM = Ru and Rh models are predicted to generate CH4 and HCOOH, respectively, regardless of the curvature of carbon materials. However, the required potential for overcoming the PDS becomes more negative as the curvature of carbon materials decreases.
TM | Product | Potential (V) | PDS |
---|---|---|---|
a CO(aq) or *CO, whichever gives a lower reference energy for the prediction of the PDS, is chosen. | |||
Fe | CH3OH | −0.741 | *CO → *CHO |
Ru | CH4 | −0.801 | *CO → *CHO |
Os | CH4 | −0.948 | *CO → *CHO |
Co | CH2O | −0.137 | CO(aq) → *CHO |
Rh | CH2O | −0.124 | *CHO → CH2O(aq) |
Ir | CH2O | −0.367 | *CHO → CH2O(aq) |
Ni | CH2O | −1.692 | CO(aq) → *CHO |
Pt | CH2O | −1.703 | CO(aq) → *CHO |
Cu | CH2O | −1.907 | CO(aq) → *CHO |
Ru | CH4 | −0.801 | *CO → *CHO |
Ru-12 | CH4 | −0.993 | *CO → *CHO |
Ru-G | CH4 | −1.255 | *CO → *CHO |
Rh | CH2O | −0.124 | *CHO → CH2O(aq) |
Rh-12 | CH2O | −0.146 | CO(aq) → *CHO |
Rh-G | CH2O | −0.283 | *CHO → CH2O(aq) |
Fig. 6 The scaling relation between the formation energies of the calculated intermediates vs. G(*H) and G(*CHO). |
In Fig. 6a and b, only the formation of *COOH scales linearly with that of *H for the CO2RR, while such proportional relationships for the other intermediates are insignificant. Notably, the formation energy of *COOH vs. *H can be described by the linear formula y = 1.03x + 0.18, which suggests that the overpotential to suppress the HER and activate the CO2RR could be a constant around 0.18 V regardless of the metal atoms embedded in the TM-4N2v-CNT models. The exothermic *H formation also leads to favourable *CO formation, and this enables the possibility for the subsequent hydrogenation steps, otherwise HCOOH would be the end-point product for the cases of G(*H) > 0 as shown in Fig. 6b.
In Fig. 6c–f, the formation energies of *CH2OH and *CH3 align almost linearly (scaling slope at ∼0.9) with that of *CHO. All these three intermediates can undergo one-step hydrogenation to stable chemicals – methanol, methane and formaldehyde. The formation energies of *COH, *CHOH and *CH2, in which these intermediates can potentially form two-coordination bonds (σ donation and π backdonation), align less linearly (0.8 < R2 < 0.9) with that of *CHO with a scaling slope >1.2. The scaling slope of *CH is predicted to be 1.65 (R2 = 0.83) and *CH could potentially form three-coordination bonds (one σ donation and two π backdonations) with the TM-4N2v-CNT models. The scaling slope between the formation energies of *CHO and the other intermediates is found to increase with the number of possible coordination bonds, and such a trend could roughly described as 0.9 vs. 1.2 vs. 1.6 at the slope for the comparison of 1σ and 1σ1π and 1σ2π coordination. The interaction resulted from π backdonation is estimated to be less than half of σ donation, given the change in scaling slopes. Similar analysis using the scaling slope for the CO2 reduced intermediates on a metal surface was pointed out by Rossmeisl and coworkers, where the change in the scaling slope was 1, 2 and 3 as the intermediates were adsorbed on atop, bridge and hollow sites.36 As shown in Fig. S6 (ESI†), the d-band centers of the TM-4N2v-CNT models decrease as the σ donation from *H and *CHO becomes reduced. Thus, the π backdonation to form multiple coordination bonds would be further hindered by the hardness that resulted from the increase of d-band centers.
Footnote |
† Electronic supplementary information (ESI) available: Bond length, bond angle, total spin angular momentum, d-band center, partial density of states and graphical representation of each TM-4N2V-CNT model and the formation energy of each intermediate. See DOI: 10.1039/c7cp06024f |
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