Tailoring Metalloporphyrin Frameworks for an Efficient Carbon Dioxide

Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand. Inorg. Chem. , 2017, 56 (12), pp 72...
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Tailoring Metalloporphyrin Frameworks for an Efficient Carbon Dioxide Electroreduction: Selectively Stabilizing Key Intermediates with H‑Bonding Pockets Sippakorn Wannakao, Watthanachai Jumpathong, and Kanokwan Kongpatpanich* Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand S Supporting Information *

ABSTRACT: The electrocatalytic reduction of carbon dioxide (CO2ER) is a great challenge within the field of energy and environmental research. Competing reactions, including hydrogen evolution reactions (HER) and surface oxidation, limit the conversion of CO2ER at low overpotentials. This is because these competing reactions produce intermediates (adsorbed H and OH) with chemical bonds similar to those formed in CO2ER (adsorbed COOH and OCHO). Here, we report the adsorption free energies of CO2ER and competitive intermediates within H-bonding functionalized metalloporphyrin frameworks using first-principles calculations. The functionalized frameworks shift the scaling relation of adsorption free energies to favor the CO2ER intermediates rather than the HER. Inspired by molecular catalysts, we proposed and studied H-bonding interfaces that specifically stabilize the target intermediates of the CO2ER. The selective H-bonding stabilization reduced the limiting potential for CO2ER by up to 0.2−0.3 V. Our results agree with previous experiments that found that cobalt- and iron-based metalloporphyrins exhibited the most promising catalytic activity in CO2-to-CO reduction, with small potential barriers for the adsorbed COOH intermediate. In addition, embedding the functionalized metalloporphyrin moieties in a rigid framework structure acted to enhance the CO2ER selectivity by preventing the porphyrin from stacking and keeping H-bonding interfaces in close proximity to only CO2ER intermediates. Improved selectivity to the desired CO2ER was achieved through three steps: first by systematically screening for metal centers, second by creating an ideal H-bonding environment, and finally by using a rigid macrocycle ring structure.

1. INTRODUCTION Conversion of the greenhouse gas carbon dioxide (CO2) is a research topic of great importance for two reasons. First, it can be used to produce alternative energy sources such as CO, HCOOH, alcohols, and hydrocarbons. Second, it can be used to reduce the excessive amount of CO2 in the atmosphere that has led to global warming. The electrocatalytic reduction of carbon dioxide (CO2ER) attracts a great deal of interest because it can be easily operated under ambient conditions by controlling the electrode potential and chemical conditions of electrolytes.1−3 However, the CO2ER process requires efficient catalysis at the electrode to improve CO2ER activity and maximize the selectivity while minimizing the required applied potential. Many materials have been reported recently as potential catalysts for the CO2ER, e.g., metals,4−8 alloys,9,10 carbides,11−14 sulfides,15−17 and molecular and carbon-based nanostructures.18−22 In all cases, the efficiency of CO2ER is limited by several competing reactions including CO poisoning, surface oxidation, and the thermodynamically favorable hydrogen evolution reaction (HER). Each competitive reaction occurs via different surface intermediates, and the correlation © 2017 American Chemical Society

between the binding energies of these intermediates on metal surfaces can be described by a scaling relation.23 These scaling relations highlight the difficulty to obtain a high activity at low overpotentials for metal surfaces.24,25 Oxophilic surfaces, including carbides, exhibit a deviation from this scaling;12 but the strong oxygen binding energy can cause surface blockage from water oxidation (*OH and *O, where the asterisk indicates a surface adsorbing intermediate) at low electrode potentials.13,18 HER is the major competitive reaction to the CO2ER at low overpotentials as the *H intermediates of HER are more stable than the *COOH and *OCHO intermediates of the CO2ER. This reduces the selectivity of the desired CO2ER.8,26−28 It has been shown previously that enzymes,25 covalent metals,16,29,30 and metal alloys18,31 can break the scaling relation between the binding energies of *COOH and *CO intermediates, overcoming the challenge of CO poisoning. However, the availability of catalysts that can selectively stabilize *COOH over *H is limited because of the similarity in the nature of the chemical bond of both intermediates.19,30 Received: March 31, 2017 Published: June 1, 2017 7200

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Figure 1. Illustration of the catalyst design concept in this work. The structure of the M-Por-R frameworks with an edge-to-edge distance of the porphyrin of ∼10 Å. The Rs are the functionalized H-bonding sites, and the *COOH intermediate is sat on top of the coordination metal site. Selectivity of the products depends on the difference between the H-bond length from the functional group to the starting intermediate of CO2ER (*COOH) and HER (*H). Side view structures are trimmed to clearly illustrate the H-bond lengths.

Recently, Back et al.21 demonstrated that transition metals embedded on vacancies of graphene prefer CO2ER intermediates over HER intermediates. Such single-site catalysts enhance the activity and selectivity of the metals when compared to bulk metal surfaces. However, the active sites must be located on defects within the material, which is difficult to experimentally control. Metalloporphyrin and metallophthalocyanine-based electrocatalysts have been realized as customizable platforms32 for efficient use of single transition metal atom catalysts. The compounds can be processed in various forms including dispersion in solution33−35 or immobilization onto electrodes.36,37 An improved catalyst may be produced by using metalloporphyrins as the building blocks within extended network structures. This can be in the form of a covalent organic framework (COF)38 or a metal organic framework (MOF).39−44 The scaling relationship between binding energy, of the initial intermediate for HER (*H) and the initial intermediate of CO2ER (*COOH) on metalloporphyrin-like graphene structures, has previously been investigated theoretically.19,22 The scaling relationship favored HER over CO2ER regardless of the metal centers and functionalized axial ligands. To achieve maximum activity at low overpotentials, the stability of the *COOH intermediate must be improved over that of the *H intermediate.18,30 Ultimately, it is difficult to transcend the thermodynamically favorable HER, as both of the key intermediates react on the same active sites, even when accounting for the curvature strain in porphyrin nanotubes.20 Based on molecular lengths, the reaction intermediates at the electrode can be divided into two groups: polyatomic length (*COOH and *OCHO) and monatomic length (*OH, *CO, and *H). The former are the key intermediates of the desired CO2ER while the latter represent the competitive intermediates. The difference in dimension of the surface intermediates can be utilized to design efficient catalysts for CO2ER. The specific interaction between enzymes and their substrate allows a catalyst design with improved catalytic selectivity to the CO2ER. Tetraphenylporphyrin (TPP) is commonly used as a homogeneous catalyst with specific edge-to-edge length of ∼10 Å (see Figure 1). This allows it to be chemically manipulated to have a restricted length of intermediate stabilizing groups, reducing the energy barrier in a similar fashion to enzymatic catalysis.25 Previously, hydrogen bonding groups in M-Por have been used to trap the key CO2ER intermediates for

spectroscopic study.45 Moreover, Constentin et al. demonstrated that the introduction of phenolic groups in all ortho positions of TPP improved catalytic activity.33−35 These substitution groups increase local proton concentration,33 as well as facilitating H-bonding which stabilizes the primary CO2 adduct.46 Inspired by molecular catalysts, we propose to exploit the advantage of MOFs as a framework for a heterogeneous catalyst, tailored to specifically stabilize the key CO2ER intermediates, while leaving the competitive intermediates’ stabilities unchanged. Within this work, H-bonding groups with different atomic lengths have been functionalized in metalloporphyrin frameworks (M-Por-R where M denotes the active metal centers) to create H-bonding pockets tailored to the CO2ER intermediates. To do so, density functional theory (DFT) calculations were used to investigate the thermodynamic stabilization of relevant intermediates on the M-Por-R surface. We preliminarily screened the metals of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt, and Au in M-Por. Subsequently, the phenyl linkers were functionalized by the two ortho-substituent groups (R = −OH and − CH2OH groups) to produce an H-bonding pocket to stabilize the molecules adsorbed on the porphyrin metal center. These appended interacting interfaces from functionalized groups can overcome the competitive HER. The concept of the catalyst design is summarized in Figure 1. The well-established synthesis process of metalloporphyrins allows the macrocycles to be customized with several functional groups including the H-bonding pocket for the CO2ER.32,44,47 Such intramolecular interfaces cannot be easily accomplished in metal-based surfaces or other single-site catalysts. Also, placing the molecular catalysts within a heterogeneous catalyst framework allows it to be deployed within an aqueous environment.37,45 Several types of substituted metalloporphyrins have been synthesized successfully by condensation of a pyrrole with a corresponding aldehyde derivative under acidic conditions.44,48,49 From these substituted metalloporphyrins, it is feasible to create new types of frameworks with specifically tailored H-bonding pockets. These H-bonding pockets pave a new pathway toward achieving a high activity CO2ER at low overpotentials by breaking the thermodynamic limitation arising from metal active sites. 7201

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However, the CHE model is restricted to a concerted proton−electron transfer (CPET) process and cannot account for a sequential proton− electron transfer (SPET) process. A recent study has shown that the regioselectivity between both mechanisms is pH dependent, specifically that the CPET and SPET are likely to occur at the lower and higher pH ranges, respectively.59 The computed equilibrium potential of the *COOH formation on molecular Co-Por agrees well with the experimental onset potential, indicating the possibility of a CPET pathway.60 In this work we assume that CPET (pH = 0) is the sole process when studying the thermodynamic stability of reaction intermediates, as consistent with similar previously reported systems.19,20,22 The reaction intermediates of negatively charged systems of Co-Por and Fe-Por, the active catalysts used in experiments, were also investigated to briefly describe the SPET process for the catalysts. We investigated the free energies of absorbed intermediates on Co-Por-R systems using van der Waals corrected DFT-D261,62 implemented in the Quantum Espresso package. The free energies of the intermediate adsorptions were stabilized by −0.30 eV for *COOH and −0.08 eV for *H (see Table S3), regardless of the H-bonding groups. It has been shown that the GGA-PBE functional provides intermediate binding energies ∼−0.2 eV stronger than those obtained from the revised Perdew−Burke−Ernzerhof (RPBE) functional, but the trends of free energies among different metals remain unchanged.13 In this work, we discuss the basis of GGA-PBE results in order to trendwise compare our results with those obtained from previous reports on similar systems.19,20,22

2. METHODOLOGY To imitate porphyrin framework structures in a COF or MOF, while maintaining a reasonable computational cost, we modeled the metalloporphyrin frameworks as two-dimensional (2D) structures linked with phenyl rings in the x and y directions of the unit cell (Figure 1 and Figure S1). A vacuum layer of 15 Å was set between each 2D sheet. The 2H+ in the middle of porphyrin were substituted by divalent metals, M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt, and Au. Spin analysis also confirms the +2 oxidation state of M-Por consistent with previous reports.50,51 To obtain selective H-bonding pockets, we substitute H atoms at the ortho position of the phenyl rings with −OH and −CH2OH as shown in Figure 1. Experimentally, a similarly structured porphyrin polymer has been successfully synthesized, demonstrating a high adsorption capacity for CO2.48,49 We employed simple models to represent the network structure of MOF-525, due to MOF-525’s previous use in several CO2ER experiments.39,40 To simplify the model further, only two of the four possible phenyl groups have been functionalized by the H-bonding groups to initially prove our hypothesis that the H-bonding could stabilize the reaction intermediates. All periodic spin-polarized DFT calculations were performed with the planewave Quantum Espresso52 (QE) package. The generalized gradient approximation of Perdew−Burke−Ernzerhof53 (GGA-PBE) was employed as an exchange-correlation functional. Ultrasoft pseudopotentials, available in the pslibrary 1.0,54 were used in this study. We performed all planewave calculations with wave function kinetic energy cutoffs at 45 Ry (612 eV) and used the Monkhorst− Pack grids of k-point sampling of 2 × 2 × 1. The cell parameter on the porphyrin plane (xy) was optimized (vc-relax) at a fixed vacuum axis (z) of 15 Å for all metal−porphyrin frameworks. Then the lattices were kept fixed while studying all adsorbing species. The convergence thresholds for the total energy and forces were set at 10−4 and 10−3 au, respectively. Gaussian smearing with the width of kBT = 0.002 Ry (0.027 eV) was also applied. The stability of reaction intermediates on molecular TPP structures was investigated as a control to compare with the framework structures. The calculations were performed with Q-Chem 4.4 software55 with the VTZ basis set for Co atom and the 6311G(d,p) basis set for all other atoms. The solvent effect (water) was treated with a conductor-like screening model (COSMO).56 The free energies (G) were calculated by correction of electronic energies by including the zero-point energy (ZPE) and the contributions from thermal (Cp dT) and entropic (−TS) terms: G DFT = E DFT + ZPE +

3. RESULTS AND DISCUSSION 3.1. Investigation of Reaction Intermediate Free Energy Relationships on M-Pors. To obtain the details of the reaction and the interaction energy of intermediates during electrocatalysis, the free energy was calculated for the key surface intermediates in the following elementary steps: (ΔG COOH) *

* + CO2 + [H+ + e−] → *OCHO

(ΔG OCHO) *

* + [H+ + e−] → *H

(ΔG H) *

* + H 2O → *OH + [H+ + e−]

∫ Cp dT − TS

(ΔG OH) *

(ΔG CO) * The ΔG*COOH and ΔG*OCHO represent the limiting potential for the CO2ER; while the free energies of ΔG*H, ΔG*OH, and ΔG*CO are the undesired competing surface species. The free energies of all reaction intermediates were computed at pH 0 and U = 0 V. Most of the electroreduction from *COOH to *CO + H2O occurs exergonically (i.e., there is a negative free energy). Therefore, the binding free energy of CO (ΔG*CO), which is a non-electrochemical process, is considered the desorption limiting factor. According to previous experimental work, further reduction of CO can occur, yielding methane as a product.36,60 However, methane production requires higher overpotentials, and the formation efficiency of methane is poor when compared to that of CO. To improve the activity and maximum selectivity at minimized overpotentials, we focused our study on the production of CO and HCOOH from two proton−electron electroreductions, both of which are used extensively within the chemical industry.29 First we screened 14 metal centers on the pristine porphyrin framework to evaluate the effect of metal species on the reaction. All of the free energy values calculated in this work are summarized in Table S2. A competition between CO2ER and HER on pristine M-Pors is apparent within the plots of ΔG*H * + CO → *CO

We applied free energy corrections obtained from ref 19 (Tables S1 and S2) to all adsorbing molecules and gas molecules. Since these terms are small when compared to the electronic energy, they can be considered as constants for all materials.28 For periodic framework systems, the solvent effect was accounted for by an additional stabilization of 0.25 eV for *COOH, 0.50 eV for *OH, and 0.1 eV for *CO as recommended by Peterson et al.28 This approximated solvent effect used for the framework systems compared well with the COSMO treatment for the molecular M-Por-R system when calculating the free energies of the intermediates. Water was chosen as the solvent as aqueous conditions are desirable for practical applications. A −0.25 eV correction was added to CO as suggested by Calle-Vallejo et al.7 This correction is justified as it yields equilibrium potentials of −0.20 and −0.12 V for CO2 + 2[H+ + e−] → HCOOH and CO2 + 2[H+ + e−] → CO + H2O respectively, which are consistent with experimental values. The free energy for each electronic step can be related to the electrode potential using the computational standard hydrogen electrode28,57 (CHE) approach: ΔGA*→ AH *(U ) = GAH * − GA * −

* + CO2 + [H+ + e−] → *COOH

1 G H2 + neU 2

where U is the applied potential and n is the number of electron− proton pairs involved in the elementary step. This method has been successfully used for many electrochemical catalyst screenings.10,13,15,58 7202

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Inorganic Chemistry versus ΔG*COOH and ΔG*H versus ΔG*OCHO in Figure 2. The ΔG*H = ΔG*COOH,OCHO line (black dashed line in Figure 2)

A strong correlation was observed between the oxo species, such as *O and *OCHO, with respect to ΔG*OH. These relationships and their regression fits are illustrated in Figure S2. The *OH and *O from water oxidation would block the active site at low negative potentials and reduce the CO2ER efficiency.13,18 Fortunately, surface blockage is not an issue for the M-Por frameworks, since all ΔG*OH are positive except for the *OH on the Ru-Por. From the relationships between ΔG*OCHO and ΔG*OH for each M-Por, the oxygen affinity, based on the binding energies of *OH, can be utilized to predict the activity for HCOOH formation and a trend for surface oxidation (with *O and *OH).24 3.2. Stability of the Reaction Intermediates on Negatively Charged Co- and Fe-Por Systems. In section 3.1, we found that the Co- and Fe-Pors could undergo CO2 reduction with low thermodynamic barriers. However, the ΔG*COOH for both catalysts fell into the H2 selective area. This is at odds with experimental evidence that shows that CO was produced efficiently.37−39 This led us to investigate the stability of the intermediates from the reaction on the [M-Por]−n (n = 0, −1, and −2) charged systems where the MII active site was reduced to MI and M0 for the purpose of comparing our theoretical observation with the expermental results. In general, CO2 reduction proceeds via a CO2•− radical anion intermediate adsorbed on an immobilized metal complex in a charged catalyst.36,46,63−66 Therefore, for CO2 reduction to occur, Co(II)-Por and Fe(II)-Pors must receive an electron to generate the reactive species.37−40,46,63,67 The free energies of the intermediates formed are summarized in Table 1 while the

Figure 2. Relationships between free energy of *H (ΔG*H) and *COOH (ΔG*COOH) (gray circles) as well as *H (ΔG*H) and *OCHO (ΔG*OCHO) (red diamonds) on M-Pors. Also included is the regression fit for ΔG*H vs ΔG*COOH illustrated as a dotted gray line (fitting results top left). The x = y line (black dashed) separates results that prefer HER by being H2 selective from those that prefer CO2ER by being CO or HCOOH selective.

indicates the selectivity between HER (above the line) and CO2ER (below the line). The catalysts can be divided into three groups on the basis of their binding strength: the weakly binding (group 10, 11 metals and Zn), the moderately binding (Cr, Mn, Fe, and Co) and the strongly binding (Ru, Rh, Ir) active sites. A strong linear relationship of ΔG*H and ΔG*COOH was observed (gray dotted trendline) with a correlation coefficient, r2, of 0.9971, though no similar relationship between ΔG*H and ΔG*OCHO is apparent. These findings were similar to the case of graphitic porphyrin systems.19,22 Compared to the HER, the CO2ER to CO via *COOH is less favorable as all of ΔG*COOH values are above the selectivity line (Figure 2). Co-Por served as the most promising catalyts among all the metals screened for the CO2ER. This is because it could produce CO via *COOH as a main product and HCOOH as a minor product that required higher potential, which is consistent with experimental results.36,60 In addition, the ΔG*COOH calculated for Co-Por was at a low potential point of 0.28 eV, close to the values obtained from previous studies on graphitic19,22 and nanotube20 structures of Co-Por. The CO desorption free energy was also low (0.14 eV), which allowed the product CO to leave the active site easily. The findings were consistent with previous experimental results of Co-Por frameworks that exhibited higher activity of the CO2ER catalysis than any other catalysts at low overpotentials.36,38 The strongly interacting sites, Ru, Rh, and Ir, led to negative free energies for *COOH formation. However, the electroreduction from *COOH to *CO + H2O required 0.4−0.5 eV for Rh and Ir, while the −2.03 eV of *CO binding free energy could limit the reaction on the Ru-Por by CO poisoning. The low potential HCOOH formation via *OCHO could be obtained by using Cr and Mn as the metal centers, while higher potential more than 1 V is required for Zn-, Ag-, and Au-Por even if they prefer the *OCHO adsorption to the *H.

Table 1. Free Energies of Intermediates on the Negatively Charged Co- and Fe-Por Systems system

ΔG*CO2

ΔG*H

ΔG*COOH

ΔG*CO

[Co(II)-Por]0 [Co(I)-Por]−1 [Co(0)-Por]−2 [Fe(II)-Por]0 [Fe(I)-Por]−1 [Fe(0)-Por]−2

−0.02 0.04 −0.12 −0.02 0.12 −0.43

0.11 0.86 0.82 0.50 0.21 0.21

0.28 1.06 1.03 0.58 0.20 0.19

−0.14 0.56 0.44 −0.88 −0.41 −0.49

adsortion geometries of CO2 on the charged Co-Por and FePor systems are shown in Figure S3. It is important to note that CO2 adsorption is not an electrochemical process, therefore its free energy is unaffected by the electrode potential. On the contary, ΔG*H becomes more negative with increasingly negative electrode potential. Because ΔG*H is dependent on the electrode potential, the HER predominantly occurs at low pH via CPET as demonstrated in previous experimental work on immobilized Co-porphyrin.36,67 When the pH is raised to 7, the sequential proton−electron transfer (SPET) becomes the preferred pathway, via the generation of [Co-Por]−1 and CO2•−. Under these conditions, ΔG*H becomes more positive (0.86 eV) along with ΔG*COOH and ΔG*CO and the adsorption of the CO2•− is more favorable than that of *H. The more positive ΔG*CO value allows CO to leave the surface rapidly, therefore decreasing the possibility of undergoing the further reduction. This is consistent with studies that demonstrated high CO production efficiency at low electrode potentials, and also explains why the yield for the HER increased with increasingly negative potential.39,67 The Fe-Por framework catalyst was reduced from FeII to Fe0 at high negative potentials (>−1 V).40 Although the values of ΔG*COOH and ΔG*H (∼0.2 7203

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Figure 3. Effect of H-bonding functional groups on free energies of differenct intermediates: (a) *COOH and *OCHO on pristine M-Por and functionalized M-Por-R where R = −OH and −CH2OH and (b) *CO, *H, and *OH on M-Por and M-Por-R where R = −OH and −CH2OH. The x = y lines (black dashed lines in panels a and b) are used to indicate how the ΔG change after functionalizations. Also shown is (c) the geometries of each intermediate on Co-Por-R, where R = −H, −OH, and −CH2OH, including the length (in angstroms) of the H-bond between R and the interacting intermediate.

V for both) are higher than the value for ΔG*CO2 (−0.4 V), a large applied negative potential of ∼−1.3 V could reduce the values of ΔG*H and ΔG*COOH such that they become more negative than the value of ΔG*CO2. This explains the similar turnover number and Faradaic efficiency for CO2ER and HER in the previously reported Fe-MOF-525 experiment by Hod et al.40 To conclude this section, competition between the formation of *H and *COOH is inevitable in a negatively charged unmodified M-Por framework. In the next section, we propose a stategy to stabilize the *COOH intermediate by functionalizing the phenyl groups, of the M-Por, with H-bonding groups. As both *H and *COOH intermediates occur via one electron−proton transfer, selective stabilization of *COOH will lead to improved CO production.

3.3. Functionalized H-Bonding Groups for Selective Stabilizing CO2ER Intermediates. We functionalized the MPor systems with the H-bonding groups, −OH and −CH2OH, to study the selectivity of the interaction with CO2ER intermediates. From the previous screening, the weakly interacting noble metal active sites (Pd, Ag, Pt, and Au) required a potential lower than ∼−2 V. Hence, only Ni, Cu, and Zn were conisdered as the representatives for the weak binding active sites in the H-bonding modification. The aim of H-bonding modification is to stabilize only the CO2ER intermediates, while leaving the stability of competitive intermediates unchanged. The effect of changing the Hbonding functional group and its interaction with different adsorbing species are summarized in Figure 3 including the following: the free energy of the desired *COOH and *OCHO intermediates before and after H-bonding modification (Figure 7204

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Inorganic Chemistry 3a); the free energy of the competitive *CO, *H, and *OH intermediates before and after H-bonding modification (Figure 3b); and examples of H-bonding lengths from the functional group to the intermediate of Co-Por-R (Figure 3c). As expected, changing the H-bonding functional group stabilized the CO2ER key intermediates, *COOH and *OCHO, as all ΔG*COOH and ΔG*OCHO after functionalization lay below the x = y line (dashed black line in Figure 3a). The *COOH intermediates were more stable by 0.1 and 0.2 eV with the −OH and −CH2OH respectively when compared to the unmodified M-Por. Previous studies on molecular catalysts have demonstrated that −OH groups act to enhance the local proton concentration (by the functional group) and provide H-bonds that stabilize the CO2ER intermediate.33,46 Figure 3b confirms that the functionalization of H-bonding groups does not encourage competitive reactions. The H-bonding length (Figure 3c) of the *COOH intermediate with the Co-Por-R shortens from ∼4 Å in the unmodified framework to 2.9 Å in the Co-Por-OH framework, while the H-bonding length of the *COOH intermediate in the case of the Co-Por-CH2OH framework was 2.0 and 2.7 Å. The reduction in H-bonding length leads to increased stabilization of the *COOH intermediate and is consistent with the free energy results of Figure 3a. From Figure 3c it is observable that the *OCHO intermediate can be positioned closer to the functional groups than the *COOH intermediate. This leads to an increase in ΔG*OCHO stabalization of ∼0.3 eV, which is greater than the stabilization for the *COOH intermediate, regardless of the functional groups. Most of the free energies for *H, *CO, and * OH were not significantly changed upon functionalization of the phenyl groups, as shown by the fact that the free energies all lie along the x = y line (dashed black line in Figure 3b). The Hbonding lengths for these intermediates were mostly found to be greater than 4 Å. Only the weakly interacting metal sites showed an increase in the *OH binding energies with less positive ΔG*OH; this is because the interaction from the −CH2OH group was stronger than that from the metal sites. The scaling relationship between ΔG*COOH and ΔG*H for MPor-R and the free energy diagram showing the H-bonding effects for Co-Por-R are shown in Figure 4. We focused on the moderate and strong interacting metals with the aim of improving low potential CO2ER (U < 1 V). Because of the seletive stabilization for *COOH when adding the −OH and −CH2OH groups, the scaling relation line was shifted down to cross the ΔG*COOH = ΔG*H line (dashed black line Figure 4a). The regression fits for all systems are similar, but the offsets are 0.167, 0.080, and 0.007 eV, reflecting the drop in potential barrier for M-Por-H, −OH, and − CH2OH, respectively. In general, breaking of the ΔG*COOH > ΔG*H limit, which controls the CO2ER efficiency, is difficult and cannot be achieved by axial ligand19 selection. On the other hand, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) have been shown to improve upon changing the axial ligands.68 The projeted density of state (PDOS) of the metal sites demonstrated that the electronic structures are not altered by functionalization of phenyl group (Figure S4). In particular, the PDOSs of the metal center remained at the same position for both spins after the functional groups were modified, both before and after intermediate adsorptions. Therefore, the increased stabilization can be attributed soley to the interaction of the interface H-bonds, rather than a change in the electronic structure of the metal. The Co-Por-CH2OH is the most promising catalyst from our study, because the ΔG*COOH was

Figure 4. (a) Free energy scaling relationships of *H versus *COOH intermediates for M-Por (gray), M-Por-OH (red), and M-Por-CH2OH (blue). The trendlines and regression information are included, as well as the line ΔG*COOH = ΔG*H (dashed black line). (b) Energy diagram of the effect of H-bonding stabilization of ΔG*COOH on Co-Por (gray), Co-Por-OH (red), and Co-Por-CH2OH (blue).

reduced from 0.28 to 0.09 eV without significant change in the value of ΔG*H. This limiting free energy barrier is lower than that for previously reported Co-Por nanotubes.20 Upon modification of Fe-Por to Fe-Por-CH2OH, the ΔG*COOH decreased from 0.58 to 0.40 eV, while the ΔG*H remained constant 0.46 eV, making it less favorable after functionalization. Since Co-Por and Fe-Por are already extensively used as the CO2-to-CO catalysts, this feasible organic functionalization provides an excellent pathway for enhancing the reaction. From the slopes of ΔG*COOH and ΔG*H relations, Cr-, Mn-, and FePor-CH2OH can become more selective for CO2ER. On the other hand, the strong interacting sites (Ru, Rh, and Ir) continue to be selective toward *H adsorption even after functionalization. The ΔG*OCHO for Cr and Mn active sites decreased from 0.5 to 0.2 eV upon modification, increasing the HCOOH selectivity rather than the H2 or CO products as the ΔG*OCHO < ΔG*H and ΔG*OCHO < ΔG*COOH (see Figures S5 and S6). Although the −CH2OH group can stabilize the *COOH better than the −OH group, the −OH group exerts an advantage by increasing the local proton concentration by 6 orders of magnitude.33,46 This is due to the lower pKa of phenol 7205

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Inorganic Chemistry (9.95) compared with that of benzyl alcohol (15.4). We also calculated ΔG*COOH for a −CH2NH2 functional group as the pKa of benzylamine (9.34) is close to that of phenol. However, the H-bonding of the −CH2NH2 group is weaker than that of the −CH2OH group, which in turn yielded the higher ΔG*COOH value of 0.25 eV. The H-bonding stabilization and the proton concentration are both important factors in terms of thermodynamic and kinetic perspectives. Because of this, we suggest that the combination of the functional groups −CH2OH and −OH on asymmetric porphyrins could improve activity and product selectivity of the CO2ER. 3.4. The Effect of the Catalyst Structures on the CO2ER Selectivity. Structural factors affecting the catalytic properties, such as the distance between the M-Por-R sheets and restricted flexibility of the porphyrin ring, were also investigated. Modeling the vacuum distance as 15 Å produces a good representation of MOFs and other open frameworks. However, in the case of 2D covalent organic frameworks, the interlayer distance could be closer to 3−6 Å depending on the specific structure.69 To investigate confinement effects, we reduced the distance between the sheets by reoptimizing the vacuum axis of the Co-Por and Co-Por-OH structures to mimic the interlayer spacing expected in COF crystals or 2D porphyrin polymers. After lattice relaxation, the vacuum layer spacing for the Co-Por and Co-Por-OH was calculated as 6.4 and 7.0 Å respectively. The decreased interlayer spacing reduces the values of ΔG*COOH and ΔG*H (Table S4), but the trends of stability remain unchanged. In the case of Co-PorOH, the H-bond groups still only stabilizes the *COOH intermediate. We further reduced the interlayer spacing to 5 Å to calculate the stability of intermediates within an amorphouslike structure, where the interlayer spacing could be decreased past that of a crystalline structure. The restricted space destabilizes the *COOH intermediate, while the stability of the *H is unchanged, due to its smaller size (Figure 5). Moreover, decreased interlayer spacing forced the −OH groups to tilt away from the *COOH plane. This decreases the stabilizing effect of the H-bond groups. According to our results, distances between the M-Por rings play a crucial role in intermediate selectivity. We conclude that using MOF or COF structures, as platforms to immobilize the M-Por-R, will allow for greater CO2ER selectivity. This is due to their ability to be designed to purposefully prevent porphyrin stacking.41,44,70 Finally, to assess the effect of framework rigidity on the CO2ER and competitive reactions, we investigated the free energies of intermediates on molecular Co-Por-R (Figure 6 and Table S5) in order to compare them with our previous results for Co-Por-R within a framework. The free energies of reaction intermediates on molecular Co-Por were almost equal to those obtained from the framework calculations. But, the flexibility of the molecular porphyrins allows the structure to be distorted into nonplanar conformations.71−74 These distortions result in the H-bonding groups getting closer to all reaction intermediates including competitive intermediates, as observable by comparison of Figure 6 and Figure 3c. The H-bonding distances of the *COOH intermediate on the molecular CoPor-OH were nearly as short as those on Co-Por-CH2OH within a framework, resulting in a similar value of ΔG*COOH of 0.1 eV. The trend for free energies of *COOH, *H, and *CO on the molecular catalysts is similar to their free energy trends on the framework catalysts. The ΔG*OH, however, dramatically decreases from 0.89 eV on molecular Co-Por to 0.39 eV on

Figure 5. Geometries of the *COOH intermediate with a reduced vacuum layer spacing of 5 Å for Co-Por and Co-Por-OH sheets. The ΔG*COOH and ΔG*H corresponding to both structures are included in the bottom right corners.

molecular Co-Por-CH2OH due to the shortening of the Hbonding length from 4.56 to 1.8 Å (Figure 6). This indicates that the H-bonding acts to stabilize reaction intermediates on both molecular M-Por and their corresponding frameworks. However, the large increase in ΔG*OH stabilization (−0.5 eV), derived from flexibility of the molecular structure, could lead to negative values of ΔG*OH for Cr, Mn, Fe, Ru, Rh, and Ir active sites. Subsequently a greater overpotential will be required to remove the *OH from the catalyst surface. From this, we conclude that the rigidity of the framework structure can prevent the H-bonding groups and the competitive intermediates from arranging themselves too close in proximity. In other words, restricted framework structures, such as MOF or COF, enhance specificity for the H-bonding pockets toward desired CO2ER intermediates.

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CONCLUSIONS In summary, we investigated the selectivity of CO2ER and HER intermediates on 14 metal sites of porphyrin frameworks using first-principles calculations. Via initial screening Co-Por and FePor were proven to exhibit low limiting potentials for the *COOH intermediate formation. By investigating the negatively charged systems, we found that the selective CO production was more likely to occur via the SPET rather than the CPET pathway. However, the calculation results showed that the HER competed with the CO2ER at higher potentials, which is consistent with previous experimental observations. We proposed a catalyst design concept to promote the CO2ER, over the thermodynamically favorable HER, by tailoring the Hbond interfaces on metalloporphyrin-based frameworks. We 7206

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Figure 6. Geometries of *COOH and *OH intermediates on the molecular Co-Por-Rs with H-bonding distances (in angstroms). Free energies of the intermediates on molecular Co-Por-Rs (black) are compared with those obtained from the periodic frameworks (red).

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simulated H-bonding pockets in M-Por frameworks that exploit the difference in the size of the CO2ER intermediates (*COOH and *OCHO), when compared to the competitive intermediates (*H, *CO, and *OH). These pose as promising CO2ER catalysts as the functionalized frameworks can shift the scaling relation in favor of the CO2ER. The required overpotential of CO2ER processes could be reduced by 0.2−0.3 V by stabilization of the intermediates using H-bonding groups. Our results propose that Co-PorCH2OH is the most promising catalyst for the CO2-to-CO reaction, with a limiting free energy of only 0.09 eV. The HCOOH product could be obtained by Cr-Por and Mn-PorRs, at low potential, regardless of the H-bonding groups (−OH or −CH2OH). We also found that structural factors, such as interlayer distance and framework rigidity, played a role in the intermediate selectivity. In particular, reduced interlayer distance in framework catalysts destabilized the *COOH intermediate, decreasing CO2ER efficiency, while the flexibility of molecular catalysts allowed the H-bonding groups to come extremely close to the *OH intermediate, leading to an undesirable increase in its stability. By embedding the specific H-bonding sites in a framework structure with an adequate interlayer spacing, it is possible to control the competitive reactions, such as HER and surface oxidation. In conclusion, MPor-R frameworks provide a customizable and modular platform for the production of specific H-bonding interfaces. These can be uniquely tuned to enhance specific reaction intermediates, hence reducing unwanted competitive reactions. Further design and development of the M-Por-R platform may help to overcome the limited activity and selectivity of the CO2ER at low overpotentials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sippakorn Wannakao: 0000-0003-2613-5184 Kanokwan Kongpatpanich: 0000-0002-4353-7057 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Postdoctoral Fellowship from Vidyasirimedhi Institute of Science and Technology (to S.W. and W.J.). K.K. acknowledges grants from Thailand Research Fund (MRG 6080278). Support of computing resources by the Frontier Research Center, Vidyasirimedhi Institute of Science and Technology, is gratefully acknowledged. The authors thank Alexander D. Mottram for his kind consultation.

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REFERENCES

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* Supporting Information S

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