Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Theoretical Insight into M1TPyP−M2 (M1, M2 = Fe, Co) MOFs: Correlation between Electronic Structure and Catalytic Activity Extending to Potentiality in Capturing Flue Gases Bikash Mandal, Jin Suk Chung,* and Sung Gu Kang* School of Chemical Engineering, University of Ulsan, 93 Daehakro, Nam-Gu, Ulsan 44610, South Korea S Supporting Information *
ABSTRACT: We investigated the structural, magnetic, and electronic properties of 5,10,15,20-tetrakis(4-pyridyl)porphyrin (TPyP) metal organic frameworks (MOFs). A spin-liquid state was found in FeTPyP−Fe and CoTPyP−Co MOFs. Long-range ferromagnetism was observed in FeTPyP−Co and CoTPyP−Fe MOFs. Ferromagnetic coupling in the CoTPyP−Fe (FeTPyP−Co) MOF was attributed to through-bond spin polarization (both spin delocalization and through-bond spin polarization). Our calculations revealed CoTPyP−Fe to be a half-metal. Dirac bands in the CoTPyP−Co and FeTPyP−Co MOFs might be responsible for the high reactivity of these MOFs in oxygen evolution reactions (OERs). The highest reactivity of FeTPyP−Co among the other TPyP MOFs might be related to the many frontier Dirac bands in the conduction band of FeTPyP−Co. In addition, the hazardous gas capturing ability of the most reactive MOF, FeTPyP−Co MOF, was examined. The adsorption of all gas molecules studied was energetically favorable, and the observed adsorption energies in decreasing order were NO2 > NO > SO2 > CO > O2 > PH3 > N2 > NH3 > H2S > OCS > H2O > CO2. The highest charge transfer observed was from metal (Fe and Co) to NO2, which may form nitrate species by dissociating the N−O bonds of adsorbed NO2. capacity than the individual components. Britt et al.41 studied adsorption of gaseous contaminants, such as ammonia, chlorine, benzene, etc. on a series of MOFs. Theoreticians have made considerable efforts to explore the potentiality of MOFs in capturing flue gas molecules.26,27,42−45 For example, Smith et al.26 examined the interaction of the flue gas components and hydrocarbons like CH4, C2H2, C2H4, etc. with MOF-74 for a series of metals, such as Mg, Ti, and V. They26 reported that the binding energies were changed by the electronic configuration of metals; high (low) binding energies were observed for Ti and V (Cr and Cu). From first principle calculations, Supronowicz et al.27 predicted the trend of the adsorption of gas molecules (O2, N2, H2S, etc.) in HKUST-1 MOFs. Howe et al.42 examined the capability of BDC (1,4benzenedicarboxylic acid) MOF in capturing acid gas molecules (NO2, SO2, CO2, etc.) and found that SO2 shows the strongest binding to the open metal site in Zn−BDC MOF. Using DFT calculations, Ding et al.45 explored the effect of other flue gas molecules like CO, NO, NO2, O2, and SO2, on the adsorption of CO2 using various MOFs like DOBDC, MOF-74, and Biomof-11. Balbuena et al.46,47 examined the impact of water on carbon dioxide adsorption on HKUST-1/Mg-MOF-74. They46,47 noticed that the capacity of CO2 adsorption was
1. INTRODUCTION Metal organic frameworks (MOFs) have broadly attracted substantial notice in the area of gas adsorption,1−5 storage,6−8 separation,9,10 sensing, 11,12 delivery of drugs,13−15 and catalysis.16−18 In MOFs, organic ligands connect all metal centers/clusters to construct networks that can contain a range of guest molecules.19−21 MOFs contain many open metal sites, which are able to capture industrial flue gases, such as CO, NO, CO2, NO2, SO2, etc.22−27 All these flue gas molecules have adverse effects on the environment. For example, CO2 emissions from burning of fossil fuels are major issues for the greenhouse effect and subsequent climate changes.28 NO2 and SO2 are responsible for induction of smog as well as acid rain,29 and CO and NO are asphyxiants for humans.30,31 Therefore, to achieve a healthy atmosphere, it is very important to control the release of these hazardous gases to the environment. MOFs have the potentiality to capture CO2 from power plant flue gases.32,33 MOF-74 has been studied widely owing to its high capacity of CO2 uptake at room temperature and low pressures.34−36 The carbon capture ability of MOF-74 has been reported to be higher than the carbon capture ability of zeolites.37 Millward et al.38 reported the exceptionally high CO2 storage capacity of different MOFs (MOF-177, IRMOFs, MOF-2, etc.) at room temperature. Tan et al.39 reported that Ni(bdc)(ted)0.5 adsorbed large amounts of SO2 at room temperature. Levasseur et al.40 showed that composite of HKUST-1 and graphite oxide possesses higher NO2 adsorption © XXXX American Chemical Society
Received: January 5, 2018 Revised: April 14, 2018
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DOI: 10.1021/acs.jpcc.8b00080 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C increased (decreased) by water in the matter of HKUST-1 (Mg-MOF-74). With the help of calculations and experiments, Wilmer et al.48 reported that the modification of Ni−DOBDC by pyridine molecules reduces the capacity of H2O adsorption, keeping high capacity of CO2 uptake. Using combined experimental and modeling approaches, Yazaydn et al.34 screened a set of MOFs (DOBDC, MIL-47, ZIF-8, etc.) for CO2 uptake at low pressure and found that the maximum uptake was observed in Mg-DOBDC at p = 0.1 bar. As M1TPyP−M2 MOFs (M1, M2 = Fe, Co and TPyP = 5,10,15,20-tetrakis(4-pyridyl)porphyrin)49 consist of two metal centers at two distinguished coordination environments, they are bimetallic MOFs. The MOFs become either homo- or heterobimetallic depending on M1 and M2. Considering all possibilities, four different MOFs were obtained: CoTPyP−Co, CoTPyP−Fe, FeTPyP−Co, and FeTPyP−Fe. Wurster et al.49 examined the performance of these MOFs in oxygen evolution reactions (OERs), which generate O2 from water. Thus, OERs are important for maintaining balance in the atmosphere as concentration of O2 is decreasing. This OER is associated with hydrogen evolution reactions (HERs), which induce formation of H2 at cathode.50,51 Wurster et al.49 reported that catalytic activity of heterobimetallic MOFs was higher than that of metallo-porphyrins. They observed nonlinear changes in catalytic activity because of the exchange of metal centers (Fe, Co) between two coordination spheres. The maximum catalytic performance was reported in FeTPyP−Co MOF in OERs. The first part of the manuscript explains the highest catalytic activity of FeTPyP−Co. The hazardous gas capturing abilities of the most reactive MOF are analyzed in the second part of the manuscript.
Figure 1. Optimized structure of a (2 × 2) supercell of M1TPyP−M2 MOFs (M1, M2 = Fe and Co), with a unit cell in blue box. H, C, N, M1, and M2 are demonstrated by the white, gray, blue, brown, and pink atoms, respectively.
group. The unit cell contains two metal centers; one (M1) at the center of porphyrin and the other (M2) at the periphery bonded with pyridyl groups of neighboring cells, generating a two-dimensional (2-D) sheet. M1 has a square-planar geometry, whereas M2 has a quasi-square-planar geometry, which is not in a perfect square-planar environment, as shown in Figure 1. Table 1 lists the detailed geometric parameters of these MOFs from DFT calculations. The lattice constants of CoTPyP−Fe and FeTPyP−Fe were larger than those of FeTPyP−Co and CoTPyP−Co systems because the Fe−N bond is longer than the Co−N bond at the periphery, as shown in Table 1. In this context, the metal ion at the center of porphyrin had no significant effects on the lattice constants. For example, similar lattice constants were observed in CoTPyP−Co and FeTPyP− Co MOFs, which have different metal ions at the center of porphyrin. 3.2. Magnetic Properties. First-row transition metals, such as Cr, Mn, and Fe, make novel contributions to the magnetic properties of MOFs.62−64 On the other hand, it is essential to realize the magnetic properties of these materials for a practical application in spintronics or magnetic storage devices.65,66 This study investigated the magnetic properties of these TPyP MOFs. The results showed that every system studied was magnetic in nature. To find out the magnetic ground state in these MOFs, the energies of the FM (ferromagnetic) and AFM (antiferromagnetic) states were calculated, considering a (2 × 2) supercell. The corresponding exchange energies12,67 were shown in Table 1. The positive exchange and negative exchange energies signify the AFM and FM state as the magnetic ground state, respectively.12,67 Thus, these calculations show that CoTPyP−Fe and FeTPyP−Co prefer the FM ground state, whereas FeTPyP−Fe and CoTPyP−Co favor the AFM ground state. Specifically, FM ordering was 163.61 and 26.76 meV/(2
2. COMPUTATIONAL DETAILS Density functional theory (DFT) was employed to model TPyP MOFs with the basis set of double-ζ plus polarization (DZP) and the pseudopotential of Troullier−Martins52 using SIESTA.53 In this study, the GGA-PBE functional54 was used. The Hubbard parameters (Ueff) chosen for Fe and Co were 4.055 and 3.0,56,57 respectively. The conjugate gradient geometry optimization was carried out without imposing any geometrical constraints until the force that acts on every atom is lower than 0.05 eV/Å. A cutoff of 300 Ry was utilized. The criterion for the convergence of the density matrix was set to 10−3. An electronic temperature used in this study was 300 K. The threshold for the convergence of energy was 0.001 eV. Periodic boundary conditions in both x-direction and ydirection are applied. The vacuum of 20 Å was included in the z-direction. The k-point meshes of a 6 × 6 × 1 (2 × 2 × 1) Monkhorst−Pack58 grid were employed for the unit cells (2 × 2 supercells). The dispersion corrections of the adsorption energies were added using the DFT-D3 method,59 employing the Becke-Johnson (BJ)-damping60,61 function with periodic boundary conditions. 3. RESULTS AND DISCUSSION 3.1. Geometrical Properties. Geometry optimizations were performed on M1TPyP−M2 MOFs (M1, M2 = Fe and Co). Figure 1 exhibits the fully relaxed geometry of the MOF, which represents a square-shaped unit cell as the building block of TPyP MOFs. The MOF is not planar; the pyridyl group is out of a molecular plane of the porphyrin moiety. This is due to the steric interaction between the porphyrin moiety and pyridyl B
DOI: 10.1021/acs.jpcc.8b00080 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Table 1. Geometric Parameters, Such As Lattice Constants (lc) and Metal−Nitrogen Bond Lengths (d), Exchange Energies, and Magnetic Moments (MM) of Different TPyP MOFs Using GGA-PBE Calculationsa
a
systems
lc (Å)
dM1−N (Å)
dM2−N (Å)
Eex (meV/(2 × 2) cell)
MM (μB/unit cell)
CoTPyP−Co CoTPyP−Fe FeTPyP−Co FeTPyP−Fe
13.89 13.96 13.90 13.97
2.05 2.05 2.07 2.08
1.99 2.08 2.00 2.08
63.56 −163.61 −26.76 105.07
0.15 5.01 5.06 0.31
Here, Eex = EFM − EAFM.
Figure 2. Magnetic charge density distribution with the isovalue of 0.001 of (a) CoTPyP−Fe and (b) CoTPyP−Co using GGA-PBE calculations. The red isosurface indicates net up spin density and blue isosurface indicates net down spin density.
Figure 3. Schematic diagram, showing (a) through-bond spin polarization and (b) spin frustrated magnetism. The arrows represent the spin direction. The green/red lines represent the triangular motifs in the unit cell of TPyP MOFs. In addition, the green lines connect the AFM pairs, whereas the red lines show FM coupling between the metal ions. H, C, N, Fe, and Co are denoted by the white, gray, blue, brown, and pink atoms, respectively.
× 2) cell more preferable for CoTPyP−Fe and FeTPyP−Co than AFM ordering, respectively. On the other hand, the exchange energies of the AFM ordering were 63.56 and 105.07 meV/(2 × 2) cell lower than the energies of FM ordering for CoTPyP−Co and FeTPyP−Fe systems, respectively. Therefore, the heterobimetallic systems prefer the FM ground state, whereas the AMF state is favored in the homobimetallic systems. Magnetic charge density12,65 can be analyzed to find out the origin of the favorable magnetic coupling (i.e., FM or AFM)
(Figure 2). Although the contribution of Fe and Co to the total magnetic moment is large, the contribution from the ligand is not negligible. These transition metals are too far from each other (by >7 Å) to be coupled directly.12 The induced magnetic moments on the ligand atoms control the magnetic coupling between transition metals. First, the magnetic charge density distribution of the CoTPyP−Fe system (Figure 2a) was considered. The magnetic coupling in CoTPyP−Fe MOF can be understood by through-bond spin polarization,12,68−71 which is considered as indirect coupling between the transition metals C
DOI: 10.1021/acs.jpcc.8b00080 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C through the ligand. In the case of through-bond coupling, a transition metal with net up (down) spin density induces down (up) spin polarization to the directly bonded ligand owing to AFM coupling of the ligand to the metal. Similarly, the nearest transition metal takes the up spin orientation due to the favorable AFM alignment with the ligand, resulting in longrange FM coupling between transition metals. This is illustrated in Figure 3a. In this context, ligand atoms (C and N) do not have any intrinsic spin polarization; they are polarized by the transition metals via d-p mixing.12,67 Thus, long-range ferromagnetism can be explained by d−p mixing. The FM coupling between the transition metals, which was observed in the CoTPyP−Fe system was also observed in the FeTPyP−Co system, even though the involvement of ligand atoms in the FeTPyP−Co MOF was different from that in the CoTPyP−Fe system. Two competitive mechanisms along the opposite diagonals observed for long-range ferromagnetism in FeTPyP− Co (Figure S1), are spin delocalization and through-bond spin polarization.72−76 Regarding spin delocalization, the spin density on the transition metals is delocalized over the ligand. Similar spin delocalization was observed in a Ni−Fe Hofmann MOF.15 On the other hand, the origin of AFM coupling in CoTPyP−Co and FeTPyP−Fe is different from the origin of FM coupling in FeTPyP−Co and CoTPyP−Fe systems. This can be elucidated with the aid of spin-frustrated magnetism,12,77,78 which induces the spin-liquid state formation.79−81 To understand this phenomena, magnetic charge density distribution of CoTPyP−Co MOF (Figure 2b) was investigated. TPyP MOFs consist of triangular units (Figure 3b) containing three transition metals at three corners. Two adjacent transition metals were coupled antiferromagnetically (Figure 2b and Figure 3b) on this triangle via a superexchange interaction. These two metals are designated as an AFM pair and the remaining metal on this triangle is in a frustrated state. This is due to the fact that the antiparallel alignment with the others (i.e., AFM pair) cannot be achieved at the same time. The remaining metal takes the AFM (FM) orientation with one (the other) of the AFM pair. The FM alignment that has a higher energy leads to a spin-frustrated state for the third metal, which is the remaining metal. In this way, all the local magnetic moments keep changing their orientations, resulting in a spinliquid state with a high degree of degeneracy.82 The spinfrustrated state has been observed widely in kagom lattices, which can be observed even at below absolute zero temperature.83 Previous studies showed that spin-liquid systems were useful for designing high-transition-temperature superconductors.84,85 The calculated magnetic moments of CoTPyP−Fe and FeTPyP−Co listed in Table 1 can be analyzed using “4 + 1 splitting”.12,86,87 On the basis of the “4 + 1 splitting” model,12,86,87 the d-orbitals of Fe and Co in a square-planar crystal field are split into low-lying quadruple-degenerate g1 orbitals, and a high-lying g2 orbital (Figure 4). The g2 orbital can be occupied once the g1 orbitals are filled. The electrons in the 4s orbital can be excited to 3d orbitals, resulting in 8 and 9 valence electrons in the d shell for Fe and Co, respectively. The filling of these electrons in g1 and g2 orbitals (Figure 4), based on the “4 + 1 splitting” model,12,86,87 indicates that magnetic moments of CoTPyP−Fe and FeTPyP−Co have to be 1 μB/ unit cell, which is inconsistent with the calculated values. The calculated magnetic moment can be explained only by the transfer of three electrons from each metal centers to the ligand. After donating three electrons each, Fe and Co
Figure 4. “4 + 1 splitting” for (a) Fe and (b) Co. The red (blue) arrows represent the spin up- (down- ) electrons.
contribute 3 and 2 μB/atom (Figure 4), respectively, making the magnetic moments as 5 μB/unit cell, which agrees well with the calculated values. As a consequence of electron transfer, oxidation states of Fe, Co, and TPyP become +3, + 3, and −6, respectively. The attractions between these oppositely charged ions can stabilize the MOF. Small magnetic moments of CoTPyP−Co and FeTPyP−Fe MOFs are due to the spinliquid state, which was discussed in the previous paragraph. 3.3. Electronic Structure and Catalytic Activity. For deeper insight into TPyP MOFs, the electronic properties of these MOFs were investigated. Figure 5 exhibits the spinpolarized electronic band structures of these MOFs. The (2 × 2) supercell with their respective magnetic ground states was employed in the band structure calculations. For the CoTPyP− Fe system, the down-spin valence and the down-spin conduction bands are overlapped, showing the behavior of metals,88 whereas the up-spin state possesses an energy gap of 1.12 eV, displaying semiconducting behavior.88 This material can be considered as a half-metal.89,90 In half-metallic materials, one spin-state dominates charge transport over the other.65,90 Therefore, half-metals are useful in designing spintronic devices.65,66,90 In the case of FeTPyP−Fe MOFs, the downspin (up-spin) state is semimetallic (semiconducting (gap
