CO2 Electroreduction Performance of Phthalocyanine Sheet with Mn

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CO2 Electroreduction Performance of Phthalocyanine Sheet with Mn Dimer: A Theoretical Study Haoming Shen, Yawei Li, and Qiang Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00317 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 4, 2017

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The Journal of Physical Chemistry

CO2 electroreduction performance of phthalocyanine sheet with Mn dimer: A Theoretical Study Haoming Shen a, Yawei Li a and Qiang Sun *ab a

Department of Materials Science and Engineering, Peking University, Beijing 100871, China.

b

Center for Applied Physics and Technology, Peking University, Beijing 100871, China

1

ACS Paragon Plus Environment


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Abstract

Due to the high surface ratio and dispersed metal sites, organometallic sheets provide a very special platform for catalysis. Here we investigate the CO2 electroreduction performance of expanded phthalocyanine sheets with different transition metal dimers using density functional theory. We have screened out Mn dimer to be the best active center, and the reaction path CO2 → COOH* → CO* → CHO* → CH2O* → CH3O* → CH3OH is identified as the preferable one with the overpotential of 0.84 eV. Electronic structures analyses show that σ bonding-π backbonding mode exists when COOH* adsorbed on Mn2-Pc, which is different from the bonding mode on Mn-Pc counterpart. Our study indicates that the introduction of metal dimer in porous covalent organic frameworks provides a new strategy for the design of catalytic materials for CO2 electroreduction.

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1. Introduction Carbon dioxide is a major industrial by-product and finding an energy-efficient way to turn CO2 back into chemical fuels has been a hot issue for a long time. Electrocatalytic CO2 reduction has gained a lot of attention due to its high energy efficiency and high product selectivity as well as its mild react conditions.1-3 The developing characterization techniques and first-principles calculations make it possible to find out the catalytic mechanisms.4-7 Based on these mechanisms, a good catalyst should possess the following properties: (1) active in CO2 adsorption, showing low reduction overpotentials; (2) easy dissociation to CO or low overpotentials turning into other hydrocarbons; and (3) thermodynamically stable and limited side reactions like hydrogen evolution reaction (HER). Throughout the past few decades, both experimental and theoretical studies focused primarily on the CO2 electroreduction on electrodes composed of extended metal surfaces.8 It was claimed that the adsorption energies of the intermediates with the same binding atom attached to the metal surfaces exhibit linear scaling relations.1, 9-12

For example, on metal surfaces, the adsorption energies of COOH* and CO* scale

linearly with each other, as well as other carbon-end intermediates such as CHO* and COH*.5 Consequently, the expected current density and the overpotential cannot lie in the satisfactory range for both CO2 electroreduction to CO and CO further reduction to hydrocarbons on metal electrodes.9, 12

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To break the linear scaling relations and optimize the catalyst performance, one of the practice is to introduce dimer active sites to enhance the adsorption of intermediates with bridge adsorption modes2, 12-13, such as CO2, *CHO or *COOH. For instance, by introducing metal oxide to the metal surfaces,1 CO2 can form a bridge adsorption around the interface between metal and metal oxide. Graciani et al.14 found that the CO2 reduction rate raise rapidly when CeOx nanoparticles were introduced to Cu surfaces. In situ IR confirmed that CeOx bonded with O atom in CO2 and increased the adsorption energy. Similar improvements have also been applied in homogeneous catalysts by using molecules with two adsorption sites. Raebiger et al.15 reported a bimetallic Palladium catalyst with two active sites exhibited a high catalytic rate of 104 M-1s-1 while isolated sites showed much lower catalytic rates of 10-300 M-1s-1. Bridge adsorption also appeared in Carbon Monoxide Dehydrogenase, one of the most high-efficient CO2 reduction catalyst.12 A Ni2+-COOH-Fe2+ bridge was characterized by 13C NMR.16 Getting inspiration from these works, we believe a modified catalyst with two adjacent active sites is able to be a possible candidate for a good CO2 reduction catalyst. Inspired by the experimental progress in synthesizing graphene-supported metal dimers17, theoretical investigations suggested that several kinds of metal dimers may convert CO2 into desired products selectively.6 However, to further unveil the roles of metal dimer and substrates, it is highly intriguing to study other systems with different substrates that can incorporate metal dimers. Several works have been done on homogeneous catalysts with N coordinated structure like metal porphyrin.18-28 4


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Recently,

Matsushita

et

al.29

synthesized

rectangular-shaped

expanded

phthalocyanines (Pc) with two nearby metal sites, forming a metal dimer containing Pc-sheet. These metal dimer containing Pc-sheets contain two active sites with a 2-3 Å distance in between, make it possible to form a bridge adsorption.30 Therefore, this kind of organometallic sheet proves to be another excellent platform that includes metal dimers. The choice of metal ions was based on the ability to form porphyrin-based stable structure with variable oxidation state and low cost of synthesis.

2. Models and methods Experimentally,

rectangular-shaped

expanded

phthalocyanines

can

be

synthesized from phthalic anhydride or phthalimide with urea heated in the presence of metal chlorides.29 Therefore, phthalocyanines with two identical metal ions as coordination

centers

are

viable.

To

get

a

single

layer

of

polymeric

Metal-phthalocyanine, phthalic anhydride can be replaced by multidentate ligands like tetracyanobenzene or naphthalenetetracarboxylic acid dianhydride. Polymerization can also be observed when self-assembled metal-Pc molecules on metal surfaces are heated.31 Therefore, we design the structure as was shown in Figure 1. In our work, we mainly focus on first-row transition metals. Systems with two different metal atoms as dimer center are ruled out because of their high difficulty in experiment synthesis.

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Fig. 1. Geometric structure of 2D Mn2-Pc sheet. The primitive unit cell is marked by red dashed line, whereas the rectangular unit cell is marked by green dashed line.

Structure relaxations and single-point energies are calculated using density functional

theory

as

implemented

in

the

Vienna

ab

initio

Simulation

Package(VASP);32 the projector-augmented-wave (PAW)33 pseudopotential is utilized to

treat

the

core

electrons,

while

the

Perdew−Burke−Ernzerhof

(PBE)34

exchange-correlation functional of the generalized gradient approximation (GGA) is used for describing the electron interactions. A plane-wave cutoff energy of 400 eV is adopted for all the calculations. The Monkhorst-Pack 8 × 8 × 1 special k-point meshes35 are used for a primitive unit cell and the vacuum space in z-direction is set as 20 Å to prevent the interaction between layers. The criteria for convergence in energy and force are 1 × 10−4 eV and 0.01 eV / Å. Various possible structures are also simulated to investigate the microkinetics on the surfaces. The computational

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electrode model (CHE)36 is applied in calculating the free energies of the proton-electron transfer steps. Further details are listed in the Supporting Information.

3. Results and discussion 3.1 Screening Procedure of Catalyst Candidates. In order to find out a suitable catalyst for CO2 electroreduction, we investigate the first few steps of CO2 electroreduction to screen out the possible catalyst. The optimized structural parameters of all the catalyst candidates are shown Table S2. The structure of Mn2-Pc sheet is shown in Figure 1. The lattice parameters of the primitive cell are 14.93 × 14.93 Å with an angle of 76.86° between two adjacent sides. All the catalyst candidate structures are planar and possess the symmetry of cmmm. Our calculation shows that the trend of the lattice parameters of the catalysts varies with the metal atomic number from Cr to Cu as an inverted volcanic curve. The minimum lattice parameters are around Fe and Co. Meanwhile, the distance between metal atoms in the dimer shows a significant increase in Co2-Pc and Ni2-Pc, while Mn2-Pc, Fe2-Pc and Co2-Pc have shorter metal-metal distances. The magnetic coupling of these systems is also considered in our calculation. The most stable spin state of all the catalyst candidates as well as their magnetic moments after intermediate adsorption are listed in Table S3.

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Fig. 2. Geometric structure of COOH* adsorbate on Mn2-Pc.

Consequently, here we first investigate the COOH* adsorption. By considering different bonding configurations (see Table S6 and Figure S3), we find that the adsorbate prefers to form a carbon-end bonding as shown in Fig. 2, and the structure information for other catalysts are listed in Table S4. The carbon-end bonding structure indicates that the reduction follows a carboxyl mechanism with CO* being the next key adsorbate, while formate mechanism which might lead to formic acid formation are not favorable.37 We find that COOH* adsorption energy increases with the atomic number from Cr to Cu, where the adsorption energy on Cr2-Pc is -0.11 eV, much lower than others (Table S7). From the structure information, we can find that the carbon-metal distance in Cr2-Pc-COOH is shorter than those in the others and its oxygen-metal distance is also much shorter. In contrast, the carbon-oxygen distance in Cr2-Pc-COOH is 2.994 Å, showing no chemical bonding between carbonyl and hydroxyl, which means that COOH adsorbate spontaneously decomposes. The adsorption energy of COOH* is 0.84 eV on Mn2-Pc. The Mn2-Pc-COOH structure has a Carbon-metal distance of 1.965 Å and an Oxygen-metal distance of 2.701 Å. The COOH* adsorption energy

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significantly increases from Co to Ni, which can be attributed to the increase in the distance between two metal atoms in unadsorbed catalysts (Table S2). Different from COOH*, CO* shows a distinct variation in the adsorption energy, which increases with the atomic number from Cr to Ni and reaches the maximum of 0.58 eV on the Cu2-Pc sheet. The carbon-metal distances in Ni2-Pc and Cu2-Pc sheets are over 3.6 Å, suggesting the weak interaction between CO and Ni2-Pc and Cu2-Pc sheets. The C-O bonds also match the length of that in carbon monoxide. On the other hand, carbon-metal distances in Mn2-Pc, Fe2-Pc and Co2-Pc are around 1.7~1.8 Å with a longer C-O bond around 1.16~1.17 Å (Table S4). The adsorption energy of CO on Fe2-Pc and Co2-Pc sheets is 0.15 and 0.22 eV, respectively, so in the reduction progress, the CO* intermediate can change to carbon monoxide or other hydrocarbon.

Fig. 3. Energy diagrams of CO production on metal dimers.

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The energy diagrams of different catalysts for CO2 electroreduction to CO are shown in Fig. 3. Catalyst with Cr dimer is ruled out due to its incapability to reduce COOH* to CO* while the sheet with Cu dimer is also not considered for the high stability of CO* adsorbate. The activation energy of CO production on Mn dimer containing Pc-sheet is the lowest among those on all the metal dimer containing Pc-sheets due to the lowest free energy barrier for COOH* generation. To see the improvement in activation energy by the dimers, a single Mn containing Pc-sheet with similar structure is also considered (Fig. S4). The results indicate that Mn dimer reduces the COOH* adsorption energy by 0.20 eV, showing the difference between Mn2 and Mn in catalysis.

3.2 Product Distribution Analyses and Overpotential Reduction. By judging the adsorption energy of COOH* and CO*, we have screened out Mn2-Pc sheet to be the most potential catalyst candidate. In order to determine final product of CO2 electroreduction on these catalysts, we examine all possible paths for CO further reduction to methanol or methane. The chemical potential of each intermediate is calculated and the zero-point energy, vibrational entropy and solvent correction are all considered. The free energy diagram of CO2 electroreduction is shown in Fig. 4 as well as the most favorable path.

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Fig. 4. Energy diagrams of intermediates on Mn2-Pc sheet. The pathway is shown at the indicated potential of -0.84 V. The dashed line is the most favorable pathway.

Fig. 4. shows the free energy diagram of the full reaction path for CO2 electroreduction on Mn2-Pc sheet. The applied potential is -0.84 V because this is the minimum potential needed to render the free energy of the whole reaction path negative. As CO* exhibit a lower free energy than gaseous CO, CO further hydrogenation is more preferred. We find that the CO* → CHO* → CH2O* → CH3O* → CH3OH(l) path is the most favorable one. The potential- limiting step is still CO2 → COOH*, the overpotential of which is higher than any other steps. For other cases like Fe2-Pc and Co2-Pc, the step with the most positive free energy change still is CO2 → COOH*. However, Mn2-Pc has the lowest reaction energy barrier among all these catalysts. 11



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On the Mn dimer containing phthalocyanine sheet, an aldehyde adsorbate is most stable when the proton-electron pair transfer number is 3. The C-Mn distance is 1.918 Å and O-Mn distance is 2.436 Å while the C-O bond is 1.227 Å in length. Another similar structure can be found with a slightly longer C-Mn and O-Mn distance (1.944 Å, 2.848 Å), and the C-O bond length is 1.217 Å. The structure is 0.09 eV less stable than the previous one. The increase in C-Mn and O-Mn distance shows a weaker interaction between the aldehyde and the catalyst while the C-O bond length decreases due to the weaker π backbonding. The energy difference indicates that the second active site helps to stabilize the adsorbates and the O-Mn interaction also helps to activate the C-O bond. We also test the *COH adsorbate. However the chemical potential is 1.64 eV higher than the aldehyde adsorbate. Therefore, the aldehyde adsorbate is likely to be the key intermediate for further hydrogenation reaction. Interestingly, the overpotential from CO species to CHO species is lower than the overpotential of the CO2 to *COOH step. This is significantly different from the cases on extended metal surfaces, where CO reduction to CHO/COH is the rate-determining step.4 The different proton-electron transfer behavior suggests that CO* poisoning is less likely on the catalyst while CO2 activation kinetics needs to be further taken into consideration in order to improve the Pc sheet catalytic performance.

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Fig. 5. Geometric structures of (a) *COOH, (b) *CO, (c) *CHO, (d) *CH2O and (e) *OCH3 adsorbed on Pc-sheet

When the reduction process continues, a formaldehyde adsorbate can be observed on Mn2-Pc sheet. Different from the previous structures, O-Mn interaction is strong with O-Mn distance of 1.843 Å, while the C-O bond is 1.380 Å larger than the value of 1.10 Å in a free state of formaldehyde molecule. Comparing to previous adsorbates, the O-Mn bond stabilizes the formaldehyde adsorbate and the influence cannot be ignored. Further hydrogenation reaction continues with a methoxyl adsorbate. In the reaction process from *CO to *OCH3, the O atom associates with the active site next to the original site and eventually form an O-Mn bond. The mechanism is similar to CO2 electroreduction on Cu (211). Considering all possible 13



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desorption products like HCOOH, CH2O, CH3OH and CH4, we find the CH3OH to be the most favorable product throughout the reaction path.

3.3 Side reaction analyses. The electroreduction of CO2 is proceeded in aqueous solution and the proton is provided by the solution. In the CHE model we used, the pH of the solution is set to be 0. The competing reaction like hydrogen evolution reaction (HER) cannot be ignored due to its effect on the Faradaic efficiency at low pH region. To find out the influence, we investigate the free energy changes of HER. For a catalyst with two nearby active sties, there are a few different adsorption states of the hydrogen atom. On graphene supported metal dimers6, DFT calculation results suggest that hydrogen atoms prefer to adsorb on the bridge sites of the transition metal dimers. However, similar to CO adsorption, an H-atom adsorbate on single metal site is more favorable here. Due to the singly adsorption mode of H* and the relatively large distance of Mn-Mn, the Tafel mechanism is unfavorable. Therefore, here we take Volmer-Heyrovsky steps into consideration, namely, the hydrogen evolution reaction contains proton adsorption and proton coupling to adsorbed H*. The reaction can be shown in Fig. 6.

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Fig. 6. Schematics of the hydrogen evolution reaction. (a)Mn2-Pc, (b) H* adsorbate.

The overpotential of the hydrogen evolution reaction is calculated to be 0.35 eV. The activation energy is lower than that of CO2 reduction. However, compared to the activation energy of hydrogen evolution on typical metal electrode (0.05~0.15 meV)38, the activation energy is rather high. By adjusting the electrolyte pH, the activation energy can be even higher due to the computational electrode model applied at pH = 0. The change in pH have different influences on reaction rate of HER and CO2 reduction due to the CO2 diffusion in electrolyte39, which can lead to an increase in the CO2 electroreduction Faradaic efficiency in neutral or basic solutions. Nonaqueous solutions are also capable of suppressing HER. The solubility of CO2 increases and the concentration of water is limited. Some nonaqueous solvents have been used in experiments such as propylene carbonate (PC), acetonitrile (AN), DMF, and dimethyl sulfoxide (DMSO)39. Furthermore, other experimental adjustments like tuning the partial pressure of the CO2 gases and temperature can also 15



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influence the reaction rates and reduce the HER side reaction. Therefore, it is possible to minimize the influence of the side reaction under real experimental adjustments.

3.4 Electronic structures analyses The density of states (DOS) of the Mn2-Pc sheet with COOH* adsorbate is compared to the Mn-Pc sheet (adsorption structure is shown in Fig. S5). The adsorbate-metal binding can be referred to two localized states according to similar adsorption on metal surfaces.40 The metal-ligand bonding theory of organometallic catalysts indicates that the σ bonding and π backbonding are the main interactions between the catalyst and the adsorbate. The σ bonding state energy is slightly lower than that of π backbonding state, while both bonding states are located around − = −6.0~ − 8.0 eV. Therefore, by analyzing the projected density of states (PDOS), we can see that the adsorbate-to-metal σ bonding and metal-to-adsorbate π backbonding is at − = −6.70/−6.12 eV for COOH* adsorption on Mn-Pc sheet (Fig. 7a and Fig. 7c) and at − = −6.47/−5.90 eV on Mn2-Pc sheet (Fig. 7b and Fig. 7d). Projected density of states analysis suggests the σ bonding is contributed by carbon pz orbital and Mn dz2 orbital (Fig. 7a and Fig. 7b), while the π backbonding is by carbon px / py orbitals and Mn dxz / dyz orbitals (Fig. 7c and Fig. 7d). The relatively higher energy range on Mn2-Pc sheet indicates that the COOH*-Mn2-Pc sheet interaction is strong, resulting in a lower overpotential.

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Fig. 7. Projected density of states of COOH* adsorbed on Mn2-Pc sheet (a, c) and Mn-Pc sheet (b, d). Lower energy peaks A and A’ are the pz-dz2 σ bonding (a, b) and the higher energy peaks B and B’ are the px/y-dxz/yz π backbonding (c, d).

The energy peaks in PDOS indicate the bonding between the COOH* adsorbate and the metal atom. We visualize these states with wavefunction density maps. The map slices are chosen alongside the COOH plane to show the detailed density information of the adsorption bonding. The wavefunction density maps are shown in Fig. 8. Both Mn2- and Mn-Pc sheets are illustrated. In Fig. 7, on both catalysts, the carbon px / py orbitals also contribute to the lower energy peak A and A’ corresponding to adsorbate-to-metal σ bonding. This phenomenon is due to px / py orbitals interacting with p orbitals of O atoms as indicated by analyzing the wavefunction density maps of the states for the σ bonding energy peak. Another important factor is both Mn atoms contribute to the bonding in Mn2-Pc sheet. As for the adsorbate-to-metal σ bonding, a slight interaction between O atom and Mn atom can be detected while a much more 17



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obvious metal-to-adsorbate π backbonding interaction can be seen. The enhanced π backbonding leads to the stabilization of COOH* adsorption on Mn2-Pc sheet, which also matches the C-Mn bond length (1.965 Å in Mn2-Pc sheet, 2.019 Å on Mn-Pc sheet).

Fig. 8. Wavefunction density maps of the states as the peaks A, A’, B and B’ in Fig. 7. The pz-dz2 σ bonding on Mn2-Pc sheet (a) and Mn-Pc sheet (b) and the px/y-dxz/yz π backbonding on Mn2-Pc sheet (c) and Mn-Pc sheet (d). The density map slices are chosen alongside the COOH* plane. Atoms near the density map planes are shown.

4. Conclusions We investigate the CO2 electroreduction performance of expanded phthalocyanine sheet with 3d transition metal dimers as active centers, and find Mn2-Pc sheet to be the best electrocatalyst for CH3OH production among all the candidates, and the 18


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corresponding reduction path is identified as CO2 → COOH* → CO* → CHO* → CH2O* → CH3O* → CH3OH, where the COOH* adsorption is the rate-determining step in this reaction with an overpotential of 0.84 V, lower than similar catalyst with single metal center by 0.20 V. The introduction of Mn dimer causes a significant decrease of activation energy, the underlying reason can be disclosed from electronic structures, which shows that the two Mn atoms in the dimer contribute to the bonding between COOH* adsorbate and catalyst, and the bridge adsorption of Mn-C-O-Mn enhances the metal-to-adsorbate π backbonding, resulting in an easy transition between C-end adsorbate and O-end adsorbate with lowered energy cost in CH3OH desorption. These findings open a door to design metal dimer based organic sheets with outstanding catalytic performance for CO2 electroreduction.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is partially supported by grants from the National Natural Science Foundation of China (NSFC-21573008), and the National Key Research and Development Program of China (2016YFB0100200). 19



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References 1.

Li, Y.; Sun, Q., Recent Advances in Breaking Scaling Relations for Effective Electrochemical

Conversion of Co2. Advanced Energy Materials 2016, 6, 1600463-n/a. 2.

Aresta, M.; Dibenedetto, A., Utilisation of Co 2 as a Chemical Feedstock: Opportunities and

Challenges. Dalton Transactions 2007, 2975-2992. 3.

Lichtfouse, E.; Schwarzbauer, J.; Robert, D., Co2 Sequestration, Biofuels and Depollution; Springer,

2015; Vol. 5. 4.

Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K., How Copper Catalyzes

the Electroreduction of Carbon Dioxide into Hydrocarbon Fuels. Energy & Environmental Science 2010, 3, 1311-1315. 5.

Peterson, A. A.; Nørskov, J. K., Activity Descriptors for Co2 Electroreduction to Methane on

Transition-Metal Catalysts. The Journal of Physical Chemistry Letters 2012, 3, 251-258. 6.

Li, Y.; Su, H.; Chan, S. H.; Sun, Q., Co2 Electroreduction Performance of Transition Metal Dimers

Supported on Graphene: A Theoretical Study. ACS Catalysis 2015, 5, 6658-6664. 7.

Li, Y.; Chan, S. H.; Sun, Q., Heterogeneous Catalytic Conversion of Co 2: A Comprehensive

Theoretical Review. Nanoscale 2015, 7, 8663-8683. 8.

Mikkelsen, M.; Jørgensen, M.; Krebs, F. C., The Teraton Challenge. A Review of Fixation and

Transformation of Carbon Dioxide. Energy & Environmental Science 2010, 3, 43-81. 9.

Montemore, M. M.; Medlin, J. W., Scaling Relations between Adsorption Energies for

Computational Screening and Design of Catalysts. Catalysis Science & Technology 2014, 4, 3748-3761. 10. Abild-Pedersen, F.; Greeley, J.; Studt, F.; Rossmeisl, J.; Munter, T.; Moses, P. G.; Skulason, E.; 20


Page 21 of 25

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Bligaard, T.; Nørskov, J. K., Scaling Properties of Adsorption Energies for Hydrogen-Containing Molecules on Transition-Metal Surfaces. Physical review letters 2007, 99, 016105. 11. Montemore, M. M.; Medlin, J. W., Scaling Relations between Adsorption Energies for Computational Screening and Design of Catalysts. Catal. Sci. Technol. 2014, 4, 3748-3761. 12. Hansen, H. A.; Varley, J. B.; Peterson, A. A.; Nørskov, J. K., Understanding Trends in the Electrocatalytic Activity of Metals and Enzymes for Co2reduction to Co. The Journal of Physical Chemistry Letters 2013, 4, 388-392. 13. Miller, G.; Esser, T. K.; Knorke, H.; Gewinner, S.; Schöllkopf, W.; Heine, N.; Asmis, K. R.; Uggerud, E., Spectroscopic Identification of a Bidentate Binding Motif in the Anionic Magnesium–Co2 Complex ([Clmgco2]−). Angewandte Chemie International Edition 2014, 53, 14407-14410. 14. Graciani, J.; Mudiyanselage, K.; Xu, F.; Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D. J.; Liu, P.; Hrbek, J.; Sanz, J. F., Highly Active Copper-Ceria and Copper-Ceria-Titania Catalysts for Methanol Synthesis from Co2. Science 2014, 345, 546-550. 15. Raebiger, J. W.; Turner, J. W.; Noll, B. C.; Curtis, C. J.; Miedaner, A.; Cox, B.; DuBois, D. L., Electrochemical Reduction of Co2 to Co Catalyzed by a Bimetallic Palladium Complex. Organometallics 2006, 25, 3345-3351. 16. Seravalli, J.; Ragsdale, S. W., 13c Nmr Characterization of an Exchange Reaction between Co and Co2 Catalyzed by Carbon Monoxide Dehydrogenase†. Biochemistry 2008, 47, 6770-6781. 17. He, Z.; He, K.; Robertson, A. W.; Kirkland, A. I.; Kim, D.; Ihm, J.; Yoon, E.; Lee, G.-D.; Warner, J. H., Atomic Structure and Dynamics of Metal Dopant Pairs in Graphene. Nano letters 2014, 14, 3766-3772. 18. Lin, S.; Diercks, C. S.; Zhang, Y.-B.; Kornienko, N.; Nichols, E. M.; Zhao, Y.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M., Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic Co2 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

Reduction in Water. Science 2015, 349, 1208-1213. 19. Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P., Nickel (Ii)-Cyclam: An Extremely Selective Electrocatalyst for Reduction of Co 2 in Water. Journal of the Chemical Society, Chemical Communications 1984, 1315-1316. 20. Beley, M.; Collin, J. P.; Ruppert, R.; Sauvage, J. P., Electrocatalytic Reduction of Carbon Dioxide by Nickel Cyclam2+ in Water: Study of the Factors Affecting the Efficiency and the Selectivity of the Process. Journal of the American Chemical Society 1986, 108, 7461-7467. 21. Collin, J. P.; Jouaiti, A.; Sauvage, J. P., Electrocatalytic Properties of (Tetraazacyclotetradecane) Nickel (2+) and Ni2 (Biscyclam) 4+ with Respect to Carbon Dioxide and Water Reduction. Inorganic Chemistry 1988, 27, 1986-1990. 22. Bhugun, I.; Lexa, D.; Savéant, J.-M., Catalysis of the Electrochemical Reduction of Carbon Dioxide by Iron (0) Porphyrins: Synergystic Effect of Weak Brönsted Acids. Journal of the American Chemical Society 1996, 118, 1769-1776. 23. Varela, A. S.; Ranjbar Sahraie, N.; Steinberg, J.; Ju, W.; Oh, H.-S.; Strasser, P., Metal-Doped Nitrogenated Carbon as Efficient Catalyst for Direct Co2 Electroreduction to Co and Hydrocarbons. Angewandte Chemie International Edition 2015, 6, 2136-2144. 24. Zhou, J.; Sun, Q., Magnetism of Phthalocyanine-Based Organometallic Single Porous Sheet. Journal of the American Chemical Society 2011, 133, 15113-15119. 25. Zhu, G.; Sun, Q., Recent Advances in Computational Studies of Organometallic Sheets: Magnetism, Adsorption and Catalysis. Computational Materials Science 2016, 112, 492-502. 26. Li, Y.; Sun, Q., The Superior Catalytic Co Oxidation Capacity of a Cr-Phthalocyanine Porous Sheet. Scientific reports 2014, 4, 4098. 22


Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. Zhu, G.; Li, Y.; Zhu, H.; Su, H.; Chan, S. H.; Sun, Q., Curvature-Dependent Selectivity of Co2 Electrocatalytic Reduction on Cobalt Porphyrin Nanotubes. ACS Catalysis 2016, 6, 6294-6301. 28. Tripkovic, V.; Vanin, M.; Karamad, M.; Björketun, M. r. E.; Jacobsen, K. W.; Thygesen, K. S.; Rossmeisl, J., Electrochemical Co2 and Co Reduction on Metal-Functionalized Porphyrin-Like Graphene. The Journal of Physical Chemistry C 2013, 117, 9187-9195. 29. Matsushita, O.; Derkacheva, V. M.; Muranaka, A.; Shimizu, S.; Uchiyama, M.; Luk'yanets, E. A.; Kobayashi, N., Rectangular-Shaped Expanded Phthalocyanines with Two Central Metal Atoms. J Am Chem Soc 2012, 134, 3411-8. 30. Zhu, G.; Kan, M.; Sun, Q.; Jena, P., Anisotropic Mo2–Phthalocyanine Sheet: A New Member of the Organometallic Family. The Journal of Physical Chemistry A 2013, 118, 304-307. 31. Sun, Q.; Zhang, C.; Cai, L.; Xie, L.; Tan, Q.; Xu, W., On-Surface Formation of Two-Dimensional Polymer Via Direct C–H Activation of Metal Phthalocyanine. Chemical Communications 2015, 51, 2836-2839. 32. Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Physical Review B 1996, 54, 11169. 33. Blöchl, P. E., Projector Augmented-Wave Method. Physical Review B 1994, 50, 17953. 34. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Physical review letters 1996, 77, 3865. 35. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Physical Review B 1976, 13, 5188. 36. Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H., Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. The Journal of Physical 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

Chemistry B 2004, 108, 17886-17892. 37. Cheng, D.; Negreiros, F. R.; Apra, E.; Fortunelli, A., Computational Approaches to the Chemical Conversion of Carbon Dioxide. ChemSusChem 2013, 6, 944-965. 38. Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y., First-Row Transition Metal Dichalcogenide Catalysts for Hydrogen Evolution Reaction. Energy & Environmental Science 2013, 6, 3553-3558. 39. Hori, Y., Electrochemical Co2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry, Vayenas, C. G.; White, R. E.; Gamboa-Aldeco, M. E., Eds. Springer New York: New York, NY, 2008; pp 89-189. 40. Kim, K.-S.; Kim, W. J.; Lim, H.-K.; Lee, E. K.; Kim, H., Tuned Chemical Bonding Ability of Au at Grain Boundaries for Enhanced Electrochemical Co2 Reduction. ACS Catalysis 2016, 6, 4443-4448.

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CO2 Electroreduction Performance of Phthalocyanine Sheet with Mn

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