Molecular and Electronic Structures of Transition-Metal Macrocyclic

Article pubs.acs.org/JPCC

Molecular and Electronic Structures of Transition-Metal Macrocyclic Complexes as Related to Catalyzing Oxygen Reduction Reactions: A Density Functional Theory Study Hui He,† Yinkai Lei,§ Chan Xiao,‡ Deryn Chu,∥ Rongrong Chen,*,† and Guofeng Wang*,§ †

Richard G. Lugar Center for Renewable Energy and ‡Department of Mechanical Engineering, Indiana University − Purdue University Indianapolis, Indianapolis, Indiana 46202, United States § Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ∥ U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States S Supporting Information *

ABSTRACT: Transition-metal (TM) macrocyclic complexes have potential applications as nonprecious electrocatalysts in polymer electrolyte membrane fuel cells. In this study, we employed density functional theory calculation methods to predict the molecular and electronic structures of O2, OH, and H2O2 molecules adsorbed on TM porphyrins, TM tetraphenylporphyrins, TM phthalocyanines, TM fluorinated phthalocyanines, and TM chlorinated phthalocyanines (here TM = Fe or Co). Relevant to their performance on catalyzing oxygen reduction reaction (ORR), we found for the studied TM macrocyclic complexes: (1) The type of the central TM is the most determinant factor in influencing the adsorption energies of O2, OH, and H2O2 (chemical species involved in ORR) molecules on these macrocyclic complexes. Specifically, the calculated adsorption energies of O2, OH, and H2O2 on the Fe macrocyclic complexes are always distinguishably lower than those on the Co macrocyclic complexes. (2) The peripheral ligands are capable of modulating the binding strength among the adsorbed O2, OH, and H2O2, and the TM macrocyclic complexes. (3) A N−TM−N cluster structure (like N−Fe−N) with a proper distance between the two ending N atoms and a strong electronic interaction among the three atoms is required to break the O−O bond and thus promote the efficient fourelectron pathway of the ORR on the TM macrocyclic complexes. test.7 As compared with the FePc/C catalyst, the CoPc/C electrode was shown to have a more negative onset ORR potential but better durability in electrochemical environments. Both the activity and the stability of the TM macrocyclic complex catalysts have been improved with a heat-treatment processing technique.8−10 For example, an enhancement in the electrocatalytic activity by a factor of 50 was observed after the carbon-supported CoPc was heat-treated at 700−800 °C.11,12 Chu et al.13 further found that the mixture of the heat-treated Co- and Fe-tetraphenylporphyrins (CoTPP/FeTPP) had even better catalytic performance for ORR than that of the respective heat-treated single components. Moreover, it was reported that nitrogen-containing carbon nanotubes produced by the pyrolysis of the FePc catalyzed an efficient four-electron ORR process with a much higher electrocatalytic activity, lower overpotential, and better long-term operation stability than that of Pt-based electrodes in alkaline electrolytes.14 Consequently,

1. INTRODUCTION Transition-metal (TM) macrocyclic complexes, such as ironphthalocyanine (FePc) or cobalt-phthalocyanine (CoPc), can be employed as nonprecious catalysts for promoting oxygen reduction reaction (ORR) that occurs on the cathodes of polymer electrolyte membrane (PEM) fuel cells. Jasinski first observed the catalytic activity of CoPc for ORR in the mid1960s.1,2 This finding stimulated extensive research on evaluating TM macrocyclic complexes as possible electrocatalysts for ORR in PEM fuel cells (to replace precious metal Pt). It has been found that many TM macrocyclic complexes with N4-TM, N2O2-TM, N2S2-TM, O4-TM, and S4-TM structures, especially the N4-chelates (porphyrins, phthalocyanines, and tetraaza annulenes) of Fe and Co, exhibit measurable levels of catalytic activity for ORR.3 In alkaline solutions, the onset ORR potential of the FePc/C electrode was found to start at the same potential (0.050 V versus Hg/HgO) as that of the Pt/C electrode, and the ORR on the FePc/C electrodes proceeds mainly via a four electron pathway.4−7 However, the catalytic performance of the FePc/C catalysts was, disappointingly, observed to deteriorate quickly in the catalyst durability © 2012 American Chemical Society

Received: April 6, 2012 Revised: July 3, 2012 Published: July 6, 2012 16038

dx.doi.org/10.1021/jp303312r | J. Phys. Chem. C 2012, 116, 16038−16046

The Journal of Physical Chemistry C

Article

activity and stability in PEM fuel cells. Moreover, it notes that all of the investigated TM macrocyclic complexes have welldefined chemical formula and the planar TM-N4 clusters as the active sites for ORRs. Heat-treatment processing could lead to the formation of new active sites (TM-Nx clusters), which may have different chemical compositions or nonplanar geometric structures, in the heat-treated TM macrocyclic complex catalysts. Hence, the attained knowledge in this study on how the molecular and electronic structures of the TM macrocyclic complexes are related to their activity of catalyzing the ORR also lays a foundation for future study to elucidate the nature of the active sites and the reaction pathway of ORR on the heattreated TM macrocyclic complex catalysts.

it is of great interest to investigate how the molecular and electronic structures of the TM macrocyclic complexes are related to the ORR processes on these catalysts. It is widely believed that the catalytic activity of TM macrocyclic complexes for ORRs is attributed to the inductive and mesomeric effects of the peripheral ligands on modifying the electronic structure of the central TM atom. In this regard, the molecular and electronic structures of some TM macrocyclic complexes have previously been studied using density functional theory (DFT). Liao et al.15 examined systematically the influence of some peripheral ligands (-porphyrin (P), -porphyrazine (Pz), and −phthalocynine (Pc)) on the electronic structure of a series of TM macrocyclic complexes (here TM = Fe, Co, Ni, Cu, and Zn). They found that replacing the CH groups in the TMP molecules by N atoms led to the stabilization of all valence molecular orbitals in the TMPz molecules while introducing the benzo rings into the TMPz molecules to form the TMPc molecules destabilized the occupied valence molecular orbitals.15 Furthermore, Liao et al. investigated how varying the central TM atom (here TM = Fe, Co, Ni, Cu, Zn, and Mg) would affect the electronic structures of the TMPc molecules.16 They concluded from their DFT calculations that the highest occupied molecular orbital (HOMO) of the FePc, CoPc, and CuPc molecules resembled the 3d orbital of the metals, whereas the HOMO of the NiPc and ZnPc molecules localized on the phthalocyanine ring.16 Specifically related to the catalytic activity of TM macrocyclic complexes for ORRs, Shi and Zhang17 have used the DFT calculations to determine the adsorption structures of O2 molecule on various TMP and TMPc complexes. They found that the central metal, ligand, and substituents are major factors in affecting the oxygen adsorption on those TM macrocyclic molecules.17 In our early DFT calculations, we found that O2 would bind to the FePc molecule much more strongly than to the CoPc molecule, and O2 could even be adsorbed on the FePc with a side-on configuration (O−O bond in parallel to the molecule).18 Furthermore, we studied the adsorption of O2, H2O, OH, and H2O2 molecules on the FePc and CoPc complexes to determine the reaction pathways of the ORR on those catalyst molecules.7 Our results indicated that the ORR pathways, twoelectron (2e−) or four electron (4e−), were primarily determined by the structures of the H2O2 adsorbed on the FePc and CoPc catalysts. The breaking of the O−O bond during the H2O2 formation process accounts for the 4e− ORR pathway on the FePc catalyst molecule, whereas the lack of the O−O bond breaking process leads to the 2e− ORR pathway on the CoPc catalyst molecule. In addition, our previous DFT study predicted that the degradation process should be more severe for the FePc than the CoPc complexes due to a larger energy gain when adsorbing OH instead of O2 on the central Fe atom of the FePc catalyst.7 In this work, we extended our previous study and used the DFT method to study the adsorption process of O2, OH, and H2O 2 molecules on the TM porphyrins (TMP), TM tetraphenylporphyrins (TMTPP), TM phthalocyanines (TMPc), TM fluorinated phthalocyanines (TMPcF16), and TM chlorinated phthalocyanines (TMPcCl16) (here TM = Fe or Co). Such a systematic investigation allows us to acquire much insight into the molecular and electronic factors that govern the catalytic performance of the TM macrocyclic complexes for promoting the ORR. These insights are useful in finding the TM macrocyclic molecular catalysts with optimal

2. COMPUTATIONAL METHODS All DFT calculations were performed using the DMol3 module of Material Studio from Accelrys (Materials Studio 4.1, DMol3, Accelrys, San Diego, CA). The generalized gradient approximation of Perdew and Wang19 was used to describe exchange and correlation, and the double numeric basis with polarization functions was chosen as the atomic basis set. Density functional semicore pseduopotentials, spin-unrestricted wave functions, and Fermi orbital occupations were employed in our calculations. The energy convergence for geometry optimization was set to be 1 × 10−5 eV. 3. COMPUTATIONAL RESULTS 3.1. Molecular Structures of TM Macrocyclic Complexes. In Figure 1, we plotted the relaxed molecular structures of the TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 molecules. After structural optimization with the DFT method, the TMP, TMPc, TMPcF16, and TMPcCl16 molecules had exactly planar molecular structures, whereas the four benzo groups turned perpendicular to the central plane in the equilibrium TMTPP molecule. Table 1 gives the calculated values of the distance (DTM‑N) between the central TM atom and its nearest neighboring N atoms. Our predictions are very close (within 2% discrepancy) to the available experimental data and other theoretical results. It appears that our calculations systematically overpredicted the D TM‑N as compared with the experimental values.17,20−23 However, this discrepancy between theory and experiment might be because the TMP, TMTPP, and TMPc complexes were in crystalline states in the experimental samples but were assumed as isolated molecular states in our calculations. Our results in Table 1 indicate that the DTM‑N of the Fe macrocyclic complex is always slightly larger than the value of the corresponding Co macrocyclic complex. However, the values of the DTM‑N of the ten TM macrocyclic complexes in this study fall into a fairly narrow range, from 1.930 to 2.003 Å. As compared with the values of the TMPc (Figure 1c) molecules, the DTM‑N of the TMP (Figure 1a) and TMTPP (Figure 1b) molecules was found to be distinguishably larger as associated with the changes of ligand groups surrounding the central TM atom. Replacing the 16 H atoms in the benzo ring of the TMPc molecules with 16 F or Cl atoms, as in the TMPcF16 (Figure 1d) and TMPcCl16 (Figure 1e) molecules, induces only a marginal change in the value of the DTM‑N. 3.2. Molecular Structure of O2 and OH Adsorbed on TM Macrocyclic Complexes. For ORR in alkaline solutions,24 O2 is a reactant, whereas OH is the reaction product. The strength of the interactions between these two 16039



Article

systems of the adsorbate and molecular catalyst, can be used to gauge the binding strength between the adsorbate and the molecular catalyst. Negative adsorption energy indicates that the adsorbate is energetically favorable to be adducted to the surface of the molecular catalyst. Consequently, the negative adsorption energy of the O2 molecule on the catalyst surfaces is a requirement for the catalyst to promote the ORR. In this work, we have optimized the configurations and determined the adsorption energy for both O2 and OH molecules adsorbed on the central TM atoms of the TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 molecules using the DFT method. In our previous DFT study,18 we identified that the most stable configurations for O2 and OH adsorbed on the FePc and CoPc molecules were the end-on adsorption configurations. Hence, we studied only the end-on adsorption configurations of O2 and OH molecules adsorbed on the TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 molecules in this work. In the end-on adsorption configurations of O2 molecule on TM macrocyclic complexes (shown in Figure 2a,b), one oxygen atom lies right

Figure 1. Molecular structures of (a) TMP, (b) TMTPP, (c) TMPc, (d) TMPcF16, and (e) TMPcCl16. In the Figure, the central ball represents the transition-metal Fe or Co atom. The dark-blue, gray, light-blue, and green balls represent N, C, F, and Cl atoms, respectively.

Table 1. Calculated Distance (DTM‑N) between the Central Transition-Metal Atom and Its Nearest Neighboring N Atom in the Equilibrium Macrocyclic Moleculesa

Figure 2. Optimized end-on configurations of (a,b) the O2 molecule and (c,d) the OH molecule adsorbed on TMPc macrocyclic molecule. The central ball represents the transition-metal Fe or Co atom, blue balls represent N atoms, gray balls represent C atoms, and red balls represent O atoms.

DTM‑N (Å) this work CoP CoTPP CoPc CoPcF16 CoPcCl16 FeP FeTPP FePc FePcF16 FePcCl16

1.985 1.981 1.930 1.933 1.944 2.003 2.000 1.949 1.952 1.945

expt 20

other works

1.950 1.94922 1.91021

1.98729 1.96617 1.91717 1.92217

1.97023 1.97223 1.92822

1.99929 1.96717 1.92317 1.92817

above the central TM atom, and the other oxygen atom tilts away from the metal atom. In the end-on adsorption configurations of OH molecules on TM macrocyclic complexes (shown in Figure 2c,d), the oxygen atom lies right above the central TM atom, and the hydrogen atom tilts away from the metal atom. Furthermore, there are two distinct end-on configurations for O2 or OH adsorbed on the macrocyclic molecules, respectively. In one configuration (denoted as “endon A”, see Figure 2a,c), the projection of the O−O bond in the O2 molecule, or the O−H bond in the OH molecule in the TM macrocyclic molecule plane is aligned with the bisecting line of the two neighboring bonds between the central metal atom and the two nearest neighboring nitrogen atoms. In the other configuration (denote as “end-on B”, see Figure 2b,d), the projection of the O−O bond in the O2 molecule or the O−H bond in the OH molecule in the TM macrocyclic molecule

a

For comparison, experimental measurements and other theoretical results are also included in the Table.

species and the macrocyclic complex molecules profoundly influences the rate of ORR on the molecular catalysts. Adsorption energy (Ead), which is defined as the energy difference between the adsorption assembly and the isolated 16040



Article

configuration “end-on B” was more energetically favorable than configuration “end-on A”. Overall, our results in Table 2 show that the adsorption energies of O2 on the Fe macrocyclic complexes were lower than those on the Co macrocyclic complexes. Moreover, the distance (DTM‑O1) between the central metal atom and the adjacent O atom of O2 on the Fe macrocyclic complexes was considerably shorter than the corresponding value of O2 on the Co macrocyclic complexes. Therefore, our results imply that the adsorbed O2 molecule would form stronger bonding with the Fe macrocyclic complexes than with the Co macrocyclic complexes. Our DFT calculation results in Table 3 indicate for OH adsorbed on the TM macrocyclic complexes that configuration “end-on B” (Figure 2d) was always more energetically favorable than configuration “end-on A” (Figure 2c). This is reasonable, considering that the N atom normally prefers bonding with the H atom and the distance between N and H atoms is shorter in “end-on B” than in “end-on A” for the OH molecule adsorbed on the TM macrocyclic complexes. Moreover, it was found that the adsorption energies of OH on the Fe macrocyclic complexes were lower by ∼0.9 eV than those values of OH adsorbed on the Co macrocyclic complexes. 3.3. Molecular Structure of H2O2 Adsorbed on TM Macrocyclic Complexes. In alkaline solution, ORR has two main possible pathways. One pathway involves the transfer of two electrons (2e−) to produce H2O2 and the other one is a direct four-electron (4e−) pathway to produce water. Thus, H2O2 is a product molecule of the 2e− ORR and might be an intermediate molecule in the 4e− route of ORR.7 Molecule H2O2 has two isomers: HOOH (hydrogen peroxide) and H2OO (oxywater). In this work, we have optimized all possible structures of HOOH and H2OO molecules adsorbed on the TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 complexes. The relaxed structures of the HOOH and H2OO molecules adsorbed on the TMPc molecules are plotted in Figure 3, which shows that when the HOOH molecule is adsorbed on the TM macrocyclic complexes, the O−O segment of HOOH could assume either a side-on adsorption configuration [in which the O−O bond extends in parallel to the catalyst molecule plane and the two oxygen atoms are in equal separation from the central TM atom (see Figure 3a,b)] or an end-on configuration [in which one oxygen atom lies right above and the other one is farther away from the central TM atom (see Figure 3c,d)]. In contrast, the H2OO molecule could adopt only the end-on configuration in its optimized adsorption structure on the catalyst molecules (see Figure 3e,f). In Table 4, we presented the predicted adsorption energy and structural geometry of the lowest-energy adsorption configuration of the H2O2 molecule on the TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 catalyst molecules. Moreover, we listed in Table 5 the calculated energetic and structural properties of the lowest-energy configuration of the HOOH molecule with the side-on configurations adsorbed on the Fe macrocyclic complexes. In this work, no stable structures were found for the HOOH molecule adsorbed on the Co macrocyclic complexes with the side-on configurations. Our results in Table 4 revealed that the H2O2 molecule would have the HOOH isomer structure in its lowest-energy adsorption configuration on the FeP, FeTPP, FePc, FePcF16, and FePcCl16 catalyst molecules but assume the H2OO isomer structure in the lowest-energy adsorption configuration on the CoP, CoTPP, CoPc, CoPcF 16 , and CoPcCl 16 catalyst molecules. Thus, it was inferred that the HOOH isomer

plane is aligned with the bond between the central metal atom and one of its nearest neighboring nitrogen atoms. We presented the calculation results of the optimized structure and the adsorption energy of O2 (in Table 2) and Table 2. Calculated Energetic and Structural Properties of O2 Molecule Adsorbed on TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 Macrocyclic Moleculesa CoP

CoTPP

CoPc

CoPcF16

CoPcCl16

configurations Ead (eV) DTM‑O1 (Å) DO1−O2 (Å) ∠O2O1TM (deg)

Figure 2b −0.591 1.900 1.271 118.7

Figure 2b −0.621 1.898 1.273 118.7

Figure 2a −0.402 1.906 1.267 117.4

Figure 2b −0.467 1.924 1.263 118.7

Figure 2b −0.602 1.918 1.264 117.9

FeP

FeTPP

FePc

FePcF16

FePcCl16

configuration Ead (eV) DTM‑O1 (Å) DO1−O2 (Å) ∠O2O1TM (deg)

Figure 2a −1.207 1.740 1.277 121.4

Figure 2a −1.257 1.740 1.279 121.5

Figure 2a −1.160 1.743 1.275 121.0

Figure 2a −1.113 1.745 1.271 120.9

Figure 2b −1.247 1.747 1.269 124.9

a

Listed properties include: the adsorption energy (Ead), the distance (DTM−O1) between the central transition metal atom and the nearest neighboring O atom (denote as O1) of the adsorbed O2 molecule, the length of the O−O bond in the adsorbed O2 molecule (DO1−O2), and the angle (∠O2O1TM) formed by the central transition-metal atom with the two O atoms of the adsorbed O2 molecule. The corresponding atomistic structures of the adsorption configurations are shown in Figure 2.

OH (in Table 3) adsorbed on the 10 TM macrocyclic complexes. From Table 2, it was found for O2 adsorbed on the CoPc, FeP, FeTPP, FePc, and FePcF16 macrocyclic complexes that configuration “end-on A” (Figure 2a) was more energetically favorable than configuration “end-on B” (Figure 2b), whereas for O2 adsorbed on the CoP, CoTPP, CoPcF16, CoPcCl16, and FePcCl16 molecules, it was found that Table 3. Calculated Energetic and Structural Properties of OH Molecule Adsorbed on TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 Macrocyclic Moleculesa configuration Ead (eV) DTM‑O (Å) DO−H (Å) ∠HOTM (deg) configurations Ead (eV) DTM‑O (Å) DO−H (Å) ∠HOTM (deg)

CoP

CoTPP

CoPc

CoPcF16

CoPcCl16

Figure 2d −2.597 1.840 0.974 104.7

Figure 2d −2.628 1.842 0.974 104.5

Figure 2d −2.364 1.843 0.974 104.9

Figure 2d −2.485 1.838 0.974 106.5

Figure 2d −2.486 1.838 0.974 106.3

FeP

FeTPP

FePc

FePcF16

FePcCl16

Figure 2d −3.477 1.794 0.976 106.3

Figure 2d −3.413 1.792 0.977 107.8

Figure 2d −3.383 1.789 0.977 108.5

Figure 2d −3.572 1.790 0.977 108.8

Figure 2d −3.442 1.791 0.977 106.5

a Listed properties include: the adsorption energy (Ead), the distance (DTM−O) between the central transition-metal atom and the O atom in the adsorbed OH molecule, the length of the O−H bond in the adsorbed OH molecule (DO−H), and the angle (∠HOTM) formed by the central transition-metal atom with the O and H atoms in the adsorbed OH molecule. The corresponding atomistic structures of the adsorption configurations are shown in Figure 2.

16041



Article

Table 4. Predicted Energetic and Structural Properties of the H2O2 Molecule Adsorbed on TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 Macrocyclic Moleculesa CoP

CoTPP

CoPc

CoPcF16

CoPcCl16

configuration Ead (eV) DTM‑O1 (Å) DO1−H (Å) DO1−O2 (Å) ∠HO1H (deg) ∠O2O1TM (deg)

Figure 3f −0.698 2.915 0.994 1.502 104.0 123.5

Figure 3e −0.675 2.322 0.990 1.507 104.1 117.8

Figure 3e −0.471 2.688 0.980 1.536 106.7 133.9

Figure 3f −0.575 2.797 0.979 1.547 106.8 107.5

Figure 3f −0.739 2.782 0.980 1.544 106.9 112.3

FeP

FeTPP

FePc

FePcF16

FePcCl16

configuration Ead (eV) DTM‑O1 (Å) DO1−O2 (Å) ∠O2O1TM (deg)

Figure 3c −1.144 1.955 1.514 116.1

Figure 3c −1.154 1.952 1.519 116.2

Figure 3d −1.009 1.861 1.647 118.0

Figure 3d −1.165 1.932 1.532 118.0

Figure 3d −1.330 1.932 1.533 116.7

a

Listed properties include: the adsorption energy (Ead), the distance (DTM−O1) between the central transition metal atom and the its nearest neighboring O atom (denote as O1), the bond length between the two oxygen atoms of the adsorbed H2O2 molecule (DO1−O2), the bond length between the O1 and H of the adsorbed H2O2 molecule (DO1−H), the angle (∠HO1H) formed by the O1 with the two H atoms of the adsorbed H2O2 molecule, and the angle (∠O2O1TM) formed by the central transition-metal atom with the two oxygen atoms of the adsorbed H2O2 molecule. The corresponding atomistic structures of the adsorption configurations are shown in Figure 3.

Table 5. Predicted Energetic and Structural Properties of the Lowest-Energy Side-On Adsorption Configuration of the HOOH Molecule on FeP, FeTPP, FePc, FePcF16, and FePcCl16 Macrocyclic Complexesa

Figure 3. Optimized structural configurations of the HOOH molecule (a−d) and the H2OO molecule (e,f) adsorbed on TMPc macrocyclic molecules. The central ball represents the transition-metal Fe or Co atom, blue balls represent N atoms, gray balls represent C atoms, red balls represent O atoms, and white balls represent H atoms.

configuration Ead (eV) DTM‑O1 (Å) DO1−O2 (Å) DO1−H(Å) ∠O2 TM O1 (deg) ∠TMO1H (deg)

would be the dominant species of the H2O2 intermediate for ORR on the Fe macrocyclic complex catalysts, whereas H2OO isomer would be the dominant species of the H 2 O 2 intermediate for ORR on the Co macrocyclic complex catalysts. The bond lengths of the O−O bond were calculated to be 1.225, 1.485, and 1.501 Å in the optimized O2, HOOH, and H2OO molecules, respectively. Hence, the O−O bond has been weakened more in the HOOH and H2OO molecules than that in the O2 molecule. Our results in Table 4 showed that the bond length of the O−O bond did not change much when the HOOH isomer was adsorbed on the Fe macrocyclic complexes with the configurations shown in Figures 3c,d as well as when the H2OO isomer was adsorbed on the Co macrocyclic complexes with the configurations shown in Figure 3e,f. However, our results in Table 5 indicate that when the HOOH molecule was adsorbed on the Fe macrocyclic complexes with the configurations shown in Figures 3a,b, the bond length of the O−O bond would increase by ∼59.5% as compared with that in the isolated HOOH molecule. This result suggests that it is possible to break the O−O bond on the Fe macrocyclic complex catalysts if the HOOH molecule is adsorbed on them assuming the configurations as in Figure 3a,b. The O−O bond broken configuration (Figure 3a) was found to be the lowest-energy adsorption structure of HOOH on FePc. In contrast, the energies of the O−O bond broken configurations (Figures 3a,b) were calculated to be 0.147 (on

FeP

FeTPP

FePc

FePcF16

FePcCl16

Figure 3a −0.997 1.814 2.230 0.973 82.1

Figure 3b −0.542 1.927 2.243 0.974 71.2

Figure 3a −1.052 1.820 2.368 0.972 81.2

Figure 3a −0.968 1.815 2.365 0.973 81.3

Figure 3a −1.319 1.817 2.383 0.973 82.0

111.2

107.3

114.0

114.8

114.0

a

Listed properties include: the adsorption energy (Ead), the distance (DTM−O1) between the central transition metal atom and its nearest neighboring O atom (denote as O1), the bond length between the two oxygen atoms of the adsorbed H2O2 molecule (DO1−O2), the bond length between the O1 and H of the adsorbed H2O2 molecule (DO1−H), the angle (∠TMO1H) formed by the O1 with the central transition-metal atom and the H atom of the adsorbed H2O2 molecule, and the angle (∠O2 TM O1) formed by the central transition-metal atom with the two oxygen atoms of the adsorbed H2O2 molecule. The corresponding atomistic structures of the adsorption configurations are shown in Figure 3.

FeP), 0.612 (on FeTPP), 0.197 (on FePcF16), and 0.011 eV (on FePcCl16) higher than those of the O−O bond unbroken configurations (Figures 3c,d) for HOOH on the other Fe macrocyclic complexes. 3.4. Electronic Structure of O2 Adsorbed on TM Macrocyclic Complexes with End-On Configurations. As pointed out in Section 3.2, the O2 molecule energetically prefers adsorbing on the TM macrocyclic complexes with an 16042



Article

end-on configuration (Figures 2a,b). It is generally believed that the electronic origin of the attractive interaction between the adsorbed O2 molecule and the TM macrocyclic complexes is the interaction of the d orbitals of the central TMs with the p orbitals of the O2 molecule. When the O2 molecule is absorbed on the TM macrocyclic complexes, an σ-type bond could be formed through the σ-rich orbital of the O2 molecule-donating electron density to an acceptor d orbital of the central TM atom of the macrocyclic complex, and a π-type bond could be formed through the π* antibonding orbital of the O2 molecule interacting with a dπ orbital of the TM atom in the macrocyclic complex (dπ orbital: dxz and dyz).24 To examine this hypothesis, we plotted the electronic interaction orbitals of the O2−CoPc system in Figure 4 and the O2−FePc system in Figure 5.

Figure 5. Some electronic orbitals of the O2−FePc adsorption system with an end-on configuration (shown in Figure 2a). These plots show the (a) dx2−y2 orbital and (b) dxz orbital of the central Fe atom in the FePc molecule at an isovalue of ±0.020 au. In the Figure, the colors blue and red are used to represent the different spin orientations of electrons.

involved in the formation of the σ-type bond between O2 and CoPc. However, we did not find any interaction between the dz2 orbital of the FePc molecule and the electronic orbitals of the adsorbed O2 molecule in this study. Furthermore, we examined the electronic structures of the adsorption systems of the O2 molecule on the other TM macrocyclic complexes. Exactly as in the O2−CoPc adsorption system, the p orbitals of the adsorbed O2 molecule overlapped with the dz2, dx2−y2, and dxz orbitals of the central Co atom in the O2−CoPcF16 system. However, in the O2−CoP, O2−CoTPP, and O2−CoPcCl16 adsorption systems, our DFT calculation results show the overlaps of only the p orbitals of the adsorbed O2 molecule with the dz2 and dxz orbitals of the central Co atom. Namely, in this work, the dx2−y2 orbitals of the Co atom were not observed to participate in the bond formation between the adsorbed O2 molecule and the CoP, CoTPP, or CoPcCl16 complexes. Consequently, our DFT results suggest that there is always one π-type bonding (p-dxz) formation and at least one σ-type bonding (p-dz2) formation during the adsorption process of the O2 molecule on the Co macrocyclic complexes. Regarding the adsorption of the O2 molecule on the Fe macrocyclic complexes, we found that the p orbitals of the adsorbed O2 molecule would overlap only with the dxz orbital of the central Fe atom in the O2−FeP, O2−FeTPP, and O2− FePcCl16 adsorption systems; however, the p orbitals of the adsorbed O2 molecule overlapped with both the dx2−y2 and dxz orbitals in the O2−FePc and O2−FePcF16 adsorption systems. Therefore, our DFT results suggest that there is always one πtype bonding (p-dxz) formation and no necessary σ-type bonding formation during the adsorption process of the O2 molecule on the Fe macrocyclic complexes. If a σ-type bonding

Figure 4. Some electronic orbitals of the O2−CoPc adsorption system with an end-on configuration (shown in Figure 2a). These plots show the (a) dz2 orbital, (b) dx2−y2 orbital, and (c) dxz orbital of the central Co atom in the CoPc molecule at an isovalue of ±0.020 au. In the Figure, the colors blue and red are used to represent the different spin orientations of electrons.

In Figure 4, it could be observed for the O2−CoPc system that the p orbital of the adsorbed O2 molecule would largely overlap with the dz2 (Figure 4a), dx2−y2 (Figure 4b), and dxz (Figure 4c) orbitals of the central Co atom in the CoPc molecule. In contrast, Figure 5 showed that the p orbital of the adsorbed O2 molecule would mainly overlap with the dx2−y2 (Figure 5a) and dxz (Figure 5b) orbitals of the Fe atom in the O2−FePc system. (For comparison, the d orbitals of the TMs in the CoPc and FePc molecules are given in the Supporting Information). Comparing the results in Figures 4 and 5, we found that for both the CoPc and FePc molecules, the dx2−y2 orbital of the TMs would interact with the p orbital of the O2 to form σ-type bond, and the dxz orbital of the TMs participated in the formation of a π-type bond between the O2 and the TMPc complexes. This implies that the dx2−y2 orbital of the TMPc molecules is an electron acceptor, whereas the dxz orbital is an electron donor in the O2-TMPc adsorption systems. Moreover, Figure 4 shows that the dz2 orbital of the CoPc molecule is also 16043



Article

4. DISCUSSION The ORR on TM macrocyclic complexes has been assumed to first involve the adsorption of O2 molecule, then the electron transfer from the central TM atom to the adsorbed O2, and followed by oxygen reductions to form H2O (4e− pathway) or H2O2 (2e− pathway). If the ORR proceeds via the 2e− pathway, then it provides only nearly half of the energy as given by the 4e− reduction. To achieve maximum energy capacity, it is highly desirable to reduce the O2 via the 4e− pathway on the TM macrocyclic complexes.25,26 In our previous work,7 we proposed the following features/facts related to the ORR mechanism on the FePc and CoPc molecule catalysts in alkaline solutions: (1) the adsorption process of O2 molecule is the limiting step that determines the onset potential of the ORR; (2) the adsorption of OH molecule affects the cycling stability of the catalysts for the ORR; and (3) the different H2O2 formation processes lead to either 4e− or 2e− routes of ORR. In this work, we will discuss further how the molecular and electronic structures of the TM macrocyclic complexes determinatively affect the above-mentioned elementary processes. 4.1. Relation of the Adsorption Energies of O2, OH, and H2O2. In Figure 7, we plotted our calculated adsorption

could be formed between the O2 molecule on the Fe macrocyclic complexes, then it must be a p-dx2−y2 type. 3.5. Electronic Structure of HOOH on Fe Macrocyclic Complexes with Side-on Configurations. Our results in Table 5 revealed that the HOOH molecule could adsorb on the Fe macrocyclic complexes with a side-on configuration in which the two O atoms of the HOOH were separated by more than 2.2 Å. Consequently, the side-on adsorption process of the HOOH molecule is responsible for splitting the O−O bonds in the ORR on the Fe macrocyclic complexes and hence distinguishes the 4e− ORR on the Fe macrocyclic complexes from the 2e− ORR on the Co macrocyclic complexes. To gain insight into this process, we plotted (in Figure 6) the electronic

Figure 6. Some electronic orbitals of the HOOH-FePc adsorption system with a side-on configuration (shown in Figure 3a). These plots show the (a) dz2 orbital and the (b) dxz orbital of the central Fe atom in the FePc molecule at an isovalue of ±0.020 au. (b) In the Figure, the colors blue and red are used to represent the different spin orientations of electrons.

Figure 7. Correlation plot showing the variation of the calculated adsorption energy of the OH (blue circles) and H2O2 (red squares) molecules as a function of the calculated adsorption energy of the O2 molecule on the Fe and Co macrocyclic complexes. The dashed lines are used to guide the eye.

structures of the HOOH molecule adsorbed on the FePc molecule with the side-on configuration shown in Figure 3a. Figure 6 shows that when the HOOH molecule is adsorbed on the FePc molecule the electronic orbitals of the HOOH molecule primarily interacted with the dz2 and dxz orbitals of the central Fe atom of the FePc. It can also be seen in Figure 6a that the dz2 orbitals of the Fe atom overlap strongly with the p orbitals of the two nearest N atoms in the FePc molecule, and the electronic orbitals of the OH groups in the HOOH are attracted by the p orbitals of these two N atoms. In contrast, no obvious interactions between the electronic orbitals of the HOOH molecule and the N atoms in the FePc complex were observed in Figure 6 b, showing the interaction of the p orbitals of the two O atoms of the HOOH molecule with the dxz orbital of the central Fe atom in the FePc. Our further analysis indicated that the electronic structures of the HOOH adsorbed on the FeP, FeTPP, FePcCl16, and FePcF16 molecules exhibit the exact same features as shown in Figure 6.

energy of OH (Table 3) and H2O2 (Table 4) molecules on the TM macrocyclic complexes as a function of the adsorption energy of the O2 molecule (Table 2) on the same TM macrocyclic complexes. As shown in Figure 7, the adsorption energy of OH or H2O2 would normally increase with the increase in the adsorption energy of O2 on the 10 studied TM macrocyclic complexes. It is worth emphasizing that the lines in Figure 7 are meant only to indicate the trend of the variation of the calculated adsorption energies. Our data in Figure 7 do not rigorously support the existence of a linear relation between the adsorption energy of OH or H2O2 and the adsorption energy of O2 on various TM macrocyclic complexes, especially when the central TM in the TM macrocyclic complexes varies. 16044



Article

bond in the reactant O2 molecule will be broken in the 4e− ORR pathway but not in the 2e− ORR pathway. Experimental measurements identified that the ORR assumed the 4e− route on Fe macrocyclic complexes but the 2e− route on Co macrocyclic complexes.26−28 From our DFT calculations, we did not find a way to split the O−O bond immediately following the O2 molecule being adsorbed on the central TM atom of the TM macrocyclic complexes. Instead, our theoretical calculations in Figure 3 suggest that the O−O bond could be split on the Fe macrocyclic complexes after the adsorbed O2 molecule is further protonated to form HOOH molecule. In contrast, we found that it was not possible to scissor the O−O bond on the Co macrocyclic complexes even if HOOH is formed. Consequently, we believe that the reason for the ORR to take the 4e− route on the Fe macrocyclic complexes is because the HOOH molecule could adopt the adsorption configurations (shown in Figures 3a,b) in which the O−O bond was broken, whereas lacking such a process on the Co macrocyclic complexes only led to the observed 2e− ORR on them. Note that we assumed the single central TM atom as the active site for O2 to adsorb on the TM macrocyclic complex catalysts in this work. Beyond this assumption, it is possible (conceivably, with much lower probability) for the O2 molecule to adsorb on the two adjacent TM macrocyclic molecules at the same time (i.e., one O adsorbs on each TM macrocyclic molecule). If the two central TM atoms have a proper distance, then the O−O bond may be directly broken following the adsorption process of the O2 molecule. However, this route of breaking O−O bond, which requires two TM macrocyclic molecules aligned at well-defined orientations and separations, is not easily achieved in the TM macrocyclic complex samples prepared with conventional catalyst fabrication methods. Moreover, our calculation results indicate that the whole linear N−Fe−N cluster structure (not just the central Fe atom) accounted for the O−O bond breaking process on the Fe macrocyclic complexes. In the Fe macrocyclic complexes, the N−Fe−N structure has a length of ∼4.0 Å (referencing to the data in Table 1). As shown in the molecular structures of Figures 3a,b, the central Fe atom could attract the two O atoms in the HOOH molecule and simultaneously the two neighboring N atoms attract the two H atoms. In this way, the N−Fe−N structure would stretch the O−O bond in the HOOH molecule to a separation of more than 2.2 Å (see Table 5) and thus break the O−O bond. Our electronic structure analysis (Figure 6) clearly showed that the d orbitals of the Fe atom overlapped with the p orbitals of the two N atoms in the N−Fe−N structure when the HOOH adsorbed on the FePc with the configuration shown in Figure 3a. Hence, our DFT results suggest that it requires the existence of linear N−Fe−N cluster structure to break the O−O bond on the Fe macrocyclic complex catalysts.

Referencing to the molecular structures of the adsorption configurations (Figures 2 and 3), we noticed that the central TM atom in the TM macrocyclic complexes was the sole active site for various chemical species (O2, OH, and H2O2) to adsorb on. It is noticeable in Figure 7 that the adsorption energies of all involved species (O2, OH, and H2O2) are distinguishably lower on the Fe macrocyclic complexes than those on the Co macrocyclic complexes. In comparison, the different ligands surrounding the central TM atom modulate the adsorption energies by a much less extent. For the ORR on the TM macrocyclic complexes in an alkaline solution, O2 is a reactant and OH is a product. Conceivably, an ideal catalyst for ORR in alkaline solutions must keep an optimal balance between retaining O2 on its surface and removing OH from its surface. If the catalyst binds to the O2 molecule too weakly on its surface, then the possibility for an ORR to occur will be limited. If the catalyst binds the OH molecule too strongly to its surface, then the availability of active sites on the catalyst surface will be reduced. Because the adsorption energies of OH increase with an increase in the adsorption energies of O2 (as shown in Figure 7), a volcano plot would be expected when plotting the catalytic activity of the TM macrocyclic complexes for ORR as a function of the calculated O2 adsorption energy on them. In experiments, it was found that the Fe macrocyclic complex catalysts led to quite high initial activity for the ORR but experienced fast performance degradation with time.3−7 This observation might be the result of the strong binding of both the O2 and OH to the Fe macrocyclic complexes. In contrast, most Co macrocyclic complex catalysts displayed stable but weak catalytic activity for the ORR.3−7 It was inferred from our calculations that the weak binding of O2 and OH to the Co macrocyclic complexes was responsible for this observation. Thus, our results in Figure 7 suggest that increasing the O2 adsorption energy on Co macrocyclic complexes or decreasing the OH adsorption energy on Fe macrocyclic complexes could lead to a catalyst with a good combination of activity and durability for ORR. Furthermore, we investigated what molecular and electronic structures of the TM macrocyclic complexes would affect the O2 adsorption energies. Our two main findings in this work are: (a) The type of the central TM is the most determinant factor in influencing the O2 adsorption energy. Our results in Table 2 indicated that the adsorption energies of O2 on the Fe macrocyclic complexes were lower by at least 0.5 eV than those on the Co macrocyclic complexes. Moreover, our electronic structure analysis in Figures 4 and 5 reveal that the electronic orbitals of the adsorbed O2 interact only with the d orbitals of the central TM Fe or Co atom. (b) Peripheral ligands could modulate the binding strength between the adsorbed O2 molecule and the TM macrocyclic complexes. The adsorption energy of the O2 molecule on FeTPP and FePcCl16 was found to differ by 0.144 eV, and the adsorption energy of the O2 molecule on CoTPP and CoPc was found to differ by 0.219 eV. Hence, our study suggests that a rational design of peripheral ligands could be an effective way to tune the O2 adsorption energy and further enhance the activity for ORR on the TM macrocyclic complex catalysts. 4.2. Splitting the O−O Bond by Using the N−Fe−N Structure. It is more desirable to promote the 4e− ORR (whose final product is H2O) than the 2e− ORR (whose final product is H2O2) on the TM macrocyclic complex catalysts. The main difference between the two routes is that the O−O

5. CONCLUSIONS We have performed first-principles DFT calculations to determine the molecular and electronic structures of the TMP, TMTPP, TMPc, TMPcF16, and TMPcCl16 (TM = Fe or Co) macrocyclic complexes as well as the lowest-energy adsorption configurations of O2, OH, and H2O2 molecules on these TM macrocyclic complexes. Moreover, we have discussed how these calculated molecular and electronic structures could be related to the ORR processes on the TM macrocyclic complex catalysts. 16045



■

ACKNOWLEDGMENTS Y.L. and C.X. were funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (grant no. DE-FG02-09ER16093). G.W. also acknowledges the research grants from EERE program of U.S. Department of Energy (grant no. DE-AC02-06CH11357). H.H. and R.C. were supported by the U.S. Army Research Lab (grant no. W911NF-10-2-0075).

In this study, we found that all adsorption energies of O2, OH, and H2O2 molecules on the Fe macrocyclic complexes were lower than those on the Co macrocyclic complexes. Namely, the O2, OH, and H2O2 molecules would bind much more strongly to the Fe macrocyclic complexes than to the Co macrocyclic complexes. In addition, we found that the adsorption energies of OH and H2O2 would increase with an increase in the adsorption energy of O2 on the TM macrocyclic complexes. Our electronic structure analysis indicated that only the central TM atoms would interact with the O2 molecule when O2 is adsorbed on these TM macrocyclic complexes. We believed that the strong binding of O2 and OH to the Fe macrocyclic complexes led to a high but unstable catalytic activity for ORR on the Fe macrocyclic complexes, whereas the relatively weak binding of O2 and OH to the Co macrocyclic complexes led to a low but stable catalytic activity for ORR on the Co macrocyclic complexes. Moreover, we found that the O−O bond in the HOOH molecule could be split on the Fe macrocyclic complexes but not on the Co macrocyclic complexes. This finding explains well why the ORR can take the 4e− reduction route on the Fe macrocyclic complexes but must assume the 2e− reduction route on the Co macrocyclic complexes. Hence, our calculation results revealed that the linear N−Fe−N structure was required to break the O−O bond through holding the two O atoms by the central Fe atom and dragging the two H atoms away by the two N atoms. Our electronic structure analysis showed a conspicuous overlap among the electronic orbitals of the central Fe atom and the two adjacent N atoms in the linear N− Fe−N structure when breaking the O−O bond in the adsorbed HOOH molecule. Our study further indicates that it is possible to tune the O2 and OH adsorption energy on the TM macrocyclic complexes through tailoring the peripheral ligand groups. To balance the activity and durability of the catalysts for ORR, the ligand modifications that weaken the adsorption of OH on the Fe macrocyclic complexes or strengthen the adsorption of O2 on the Co macrocyclic complexes are predicted to be desirable in this work. To promote the efficient 4e− route of ORR on the TM macrocyclic complex catalysts, our study indicates that the linear N-TM-N cluster structure with a proper distance between the two ending N atoms and a strong electronic interaction among the three atoms is necessary. In conclusion, we have gained much insight (as discussed above) into how the molecular and electronic structures of TM macrocyclic complexes affect their performance to catalyze the ORR in this study by using the accurate DFT computation technique.

■

■

REFERENCES

(1) Jasinski, R. Nature 1964, 201, 1212−1213. (2) Jasinski, R. J. Electrochem. Soc. 1965, 112, 526−528. (3) Wiesener, K.; Ohms, D.; Neumann, V.; Franke, R. Mater. Chem. Phys. 1989, 22, 457−475. (4) Behret, H.; Binder, H.; Sandstede, G.; Scherer, G. G. J. Electroanal. Chem. Interface 1981, 117, 29−42. (5) Tanaka, A. A.; Fierro, C.; Scherson, D.; Yeager, E. B. J. Phys. Chem. 1987, 91, 3799−3807. (6) Jiang, L.; Hsu, A.; Chu, D.; Chen, R. J. Electroanal. Chem. 2009, 629, 87−93. (7) Chen, R.; Li, H.; Chu, D.; Wang, G. J. Phys. Chem. C 2009, 113, 20689−20697. (8) Faubert, G.; Lalande, G.; Côté, R.; Guay, D.; Dodelet, J. P.; Weng, L. T.; Bertrand, P.; Dénès, G. Electrochim. Acta 1996, 41, 1689−1701. (9) Lalande, G.; Côté, R.; Tamizhmani, G.; Guay, D.; Dodelet, J. P.; Dignard-Bailey, L.; Weng, L. T.; Bertrand, P. Electrochim. Acta 1995, 40, 2635−2646. (10) Tributsch, H.; Bron, M.; Hilgendorff, M.; Schulenburg, H.; Dorbandt, I.; Eyert, V.; Bogdanoff, P.; Fiechter, S. J. Appl. Electrochem. 2001, 31, 739−748. (11) Gupta, S.; Tryk, D.; Bae, I.; Aldred, W.; Yeager, E. J. Appl. Electrochem. 1989, 19, 19−27. (12) Vasudevan, P.; Santosh, Mann, N.; Tyagi, S. Transition Met. Chem. 1990, 15, 81−90. (13) Chu, D.; Jiang, R. Solid State Ionics 2002, 148, 591−599. (14) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Science 2009, 323, 760−764. (15) Liao, M. S.; Scheiner, S. J. Comput. Chem. 2002, 23, 1391−1403. (16) Liao, M. S.; Scheiner, S. J. Chem. Phys. 2001, 114, 9780−9791. (17) Shi, Z.; Zhang, J. J. Phys. Chem. C 2007, 111, 7084−7090. (18) Wang, G.; Nitia, R.; Hsu, A.; Chu, D.; Chen, R. Mol. Simul. 2008, 34, 1051−1056. (19) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244−13249. (20) Madura, P.; Scheidt, W. R. Inorg. Chem. 1976, 15, 3182−3184. (21) Williams, G. A.; Figgis, B. N.; Mason, R.; Mason, S. A.; Fielding, P. E. J. Chem. Soc., Dalton Trans. 1980, 1688−1692. (22) Coppens, P.; Li, L. J. Chem. Phys. 1984, 81, 1983−1993. (23) Collman, J.; Hoard, J.; Kim, N.; Lang, G.; Reed, C. J. Am. Chem. Soc. 1975, 97, 2676−2681. (24) Zagal, J. Coord. Chem. Rev. 1992, 119, 89−136. (25) Rovira, C.; Kunc, K.; Hutter, J.; Ballone, P.; Parrinello, M. J. Phys. Chem. A 1997, 101, 8914−8925. (26) Yeager, E. Electrochim. Acta 1984, 29, 1527−1537. (27) Yeager, E. J. Mol. Catal. 1986, 38, 5−25. (28) Zagal, J.; Bindra, P.; Yeager, E. J. Electrochem. Soc. 1980, 127, 1506−1517. (29) Tsuda, M.; Dy, E. S.; Kasai, H. J. Chem. Phys. 2005, 122, 244717−244719.

ASSOCIATED CONTENT

S Supporting Information *

Electronic orbitals of the isolated CoPc and FePc molecules. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest. 16046


Molecular and Electronic Structures of Transition-Metal Macrocyclic

Recommend Documents