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M. T. M.; Ager, J.; Xu, R.; Zheng, H. A Spongy Nickel- organic CO2 Reduction Photocatalyst for Nearly 100%. Selective CO Production. Sci. Adv.2017, 3 ...
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Reduction of CO to CO by an Iron Porphyrin Catalyst in the presence of Oxygen Biswajit Mondal, Pritha Sen, Atanu Rana, Dibyajyoti Saha, Purushottam Das, and Abhishek Dey ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00529 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Reduction of CO2 to CO by an Iron Porphyrin Catalyst in the presence of Oxygen Biswajit Mondal#, Pritha Sen#, Atanu Rana, Dibyajyoti Saha, Purusottom Das, Abhishek Dey* School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Kolkata, 700032, WB, India [email protected] Supporting Information Placeholder ABSTRACT: Reduction of CO2 to value added chemicals is a

logical way of fixing it. Activation and reduction of CO2 requires low-valent transition metals as catalysts. A major challenge in this chemistry is sensitivity of these low valent metal sites to more abundant O2. Since O2 is a stronger oxidant than CO2 and apart from the obvious competitive inhibition of CO2, partial reduction of O2 leads to formation of reactive oxygen species like O2- and H2O2 which are deleterious to the catalyst itself. An iron porphyrin complex appended with four ferrocene groups in its distal site is demonstrated to reduce CO2 unabated in the presence of O2 as it can reduce O2 to benign H2O under the same conditions. Further investigations reveal that iron porphyrins, in general, reduce CO2 selectively in the presence of O2. The aforementioned selectivity is derived from a 500 times faster rate of reaction of CO2 with Fe(0) porphyrin relative to O2 despite a higher driving force for the later.

This idea is translated into non-porphyrin systems as well.21,22 So far turnover frequencies of 106 s-1 and low overpotentials of 220 mV have been recorded for iron porphyrin based catalysts having suitably oriented trimethylanilinium groups in orthoposition of the meso- substituted phenyl porphyrin (Fe-o-TMA) in DMF with PhOH as external acid source.23 Additionally, when the catalyst (Fe-p-TMA) is subjected to photo irradiation in the presence of Ir or organic dye CH4 was obtained in trace amounts.24,25.Recently Warren et. al has demonstrated that iron porphyrin bearing 2-hydroxyphenyl as pendant group has superior activity in acetonitrile compared to DMF.26 Our group has recently shown that either in acetonitrile or DMF the rate of CO2 reduction can be tuned by controlling the second sphere H-bonding residues on iron porphyrin.20

Key words Iron porphyrin, Oxygen tolerance, CO2 reduction, Electrochemistry, Kinetics Introduction Chemical storage of energy in the form of reduced forms of CO2 is a logical way to address the present concerns of global warming and rising fossil fuel crisis simultaneously.1-3 Among these, the 2e-/2H+ reduction of CO2 to CO is a lucrative option for a carbon neutral energy cycle.4 The CO gas produced can be easily separated and used as feedstock for the industrial Fischer-Tropsch process. CO2 is a very stable molecule and its one electron reduction is quite demanding (E0CO2/CO2-. = -1.98V vs NHE in DMF). Alternatively, activating CO2 for reduction to its stable fixed forms like CO, HCOOH, CH4 etc. by an appropriate catalyst could facilitate the process.5 Recently, there have been a flurry of activity in the development of catalysts for the conversion of CO2 to its reduced products.6-12 Ni-cyclam is one of the early examples of molecular CO2 reduction catalyst.13-15 Examples of efficient molecular catalysts include Savéant’s tetra-o,o’-dihydroxyphenyl iron porphyrin which under homogeneous conditions in organic solvent converts CO2 to CO with a rate of 104 s-1. 16-18 The phenolic O-H groups in this catalyst act as proton transfer relay and provides kinetic advantage for the catalysis.17 Subsequent work has demonstrated that several different 2nd sphere residues can be used to abate the CO2 reduction process.19,20

Scheme 1: The basis of oxygen sensitivity in low-valent catalysis. In general, CO2 binds a low valent (formally “0” and “-1”) metal in the first step of catalysis. The high electron density on the metal leads to activation of the bound CO2, weakening the C-O bond by backbonding, which is essential for reduction of CO2.27 In iron porphyrins, CO2 binding to a formally Fe(0)porphyrin leads to a Fe(II)-CO22- species in the first step of catalysis.28 Molecular complexes of low-valent transition metal centres, such as Fe(0), are prone to oxidation by O2 which has a much higher thermodynamic reduction potential (0.83 V vs NHE at pH 729) relative to CO2. The competitive inhibition of CO2 reduction by oxygen is currently a severe limitation in practical implementation of CO2 reduction technologies as O2 is 500 times more abundant than CO2 in the atmosphere. O2 is mostly reduced to form O2- and H2O2 as its complete reduction to H2O is, in itself, quite challenging.30,31 These partially reduced

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oxygen species (PROS) are very reactive and, in the presence of transition metals, catalyze oxidization/degradation of the ligand backbone of the catalyst, irreversibly damaging them (Scheme 1). For example, in bio-inspired models of iron only hydrogenases, the H2O2 produced from partial reduction of oxygen by the redox state responsible for H+ activation, oxidizes the bridging thiolate to sulfoxides leading to degradation of this, otherwise promising, HER catalysts.32 Yet, this limitation needs to be addressed and any potent catalyst for the reduction of CO2 needs to be “oxygen tolerant” keeping their eventual practical application in mind.

Fe(0) state. The preferred selectivity for O2 reduction to H2O makes FeFc4 a better choice over other iron porphyrins. Furthermore, kinetic investigations reveal that “formally” Fe(0) iron porphyrins (Fig. 2) react with CO2 500 times faster than O2, under the same experimental conditions, offering considerable selectivity for CO2 reduction is achieved in the presence of O2. Results CO2 Reduction under anoxic conditions The FeFc4 complex shows four cyclic voltammetric responses due to: Fc+/Fc, Fe(III/II), Fe(II/I) and Fe(I/0) at 0.0 V, -0.67V, 1.47V and -2.05V, respectively, in acetonitrile solution (Fig. 1A (inset): green), vs Fc/Fc+. The polarographic current (ip) for Fc+/Fc process is ~4 times greater than the cyclic voltammetric response from the iron in the porphyrin as there are four ferrocenes in the FeFc4 molecule and only one iron in the porphyrin. The fully reduced complex can effectively provide up to six electrons albeit at different potentials (four from the four ferrocenes in addition to three from the Fe porphyrin where Fe can be oxidized from Fe(II) to Fe(V) systems) to catalyze multi-electron catalytic processes (as has been demonstrated for ORR).42,43 Cyclic voltammetry in the presence of 3M phenol in CO2 saturated acetonitrile solution shows a catalytic wave that starts growing in at -1.90 V and saturates at -2.50 V (Fig. 1A: purple). The onset of the catalysis overlays with the formal Fe(I/0) potential and catalytic current increases as a function of CO2 concentration (Fig. S1A) indicating that the Fe(0) state of the porphyrin is responsible for CO2 reduction consistent with the previous literature reports.16 Bulk electrolysis experiments indicate that CO2 is

Scheme 2: (top) Electrocatalytic and chemical CO2 reduction17,28mechanism for CO2 reduction. (bottom) Schematic representation of A). FeFc4 and B) FePf. (X =bromide axial ligand). There are three conceivable ways of making a catalyst oxygen tolerant.33-37 First, a co-catalyst may be used to scavenge any partially reduced oxygen species produced before they can damage the catalyst for CO2 reduction.38 Second, the CO2 reduction catalyst may be designed such that it can reduce oxygen to water by itself. And the third possibility, albeit challenging, is the reduction of CO2 selectively in the presence of O2. Some of these approaches have been successfully demonstrated in hydrogen evolution reaction which requires low valent transition metals as well.33-36,38,39 For example, a bioinspired iron only hydrogenase model has been designed to enable complete reduction of oxygen to water rescinding formation of ROS and allow air tolerant H2 evolution.40 Similarly, ammonium tetrathiomolybdate catalyzed HER proceeds via a ligand centred proton coupled electron transfer (PCET) mechanism allowing it to selectively reduce protons in the presence of O2 with greater than 90% efficiency.41 In this paper, an iron porphyrin complex, with four ferrocene groups appended to it, is (Scheme 2A) shown to reduce CO2 unabated in the presence of O2. This bifunctional catalyst can efficiently catalyze both the 4H+/4e- reduction of O2 in its Fe(II) state to benign H2O as well as the 2H+/2e- reduction of CO2 to CO in its

selectively reduced to CO with a Faradaic efficiency > 92%. Fig. 1 (A) The cyclic voltammetry of FeFc4 in acetonitrile under Ar atmosphere (orange) and in CO2 saturated solution with 3M PhOH as external proton source (purple); Inset: CV of FeFc4 in Ar (B) The cyclic voltammetry of FeFc4 in acetonitrile with O2 (yellow); O2 and CO2 mixture (1:10) (green) with PhOH as external proton source; oxygen (red) and Ar (blue); using glassy carbon as working electrode, Pt as counter electrode and AgCl/Ag as reference electrode. Scan rate = 100 mV/s. 100 mM tetrabutyl ammonium perchlorate (TBAP) is used as supporting electrolyte. Potential is adjusted with respect to Fc+/0 (C) The charge v/s time plot at different O2:CO2 partial

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ACS Catalysis pressure (D) A plot of % CO2 (in CO2 and O2 mixture) vs % Faradaic yield (FY%).

CO2 reduction in the presence of O2. The CV data obtained in an aerated electrolytic solution show electrocatalytic O2 reduction by the Fe(II) state at -0.80 V vs Fc/Fc+(Fig. 1B, red). The electrocatalytic current is enhanced in the presence of a proton donor like PhOH (Fig. 1B, yellow). However, the data also show that the Fe(I/0) couple is intact even in the presence of oxygen (Fig. 1B red). Of course an intact Fe(I/0) process in the presence of O2 may imply a) lack of reactivity of the Fe(0) state towards oxygen and/or b) depletion of O2 in the diffusion layer due to its removal by reduction by the catalyst in its Fe(II) state. This raises the possibility of selective CO2 reduction, which is catalysed by the Fe(0) state, under aerobic conditions. A logical way of addressing the second possibility is to perform the O2 reduction using forced convection methods like rotating disc electrochemistry (RDE) where the concentration of the substrate remains the same during potential sweep. Unfortunately, the large background O2 reduction by the graphite electrode itself compromises the RDE experiments at Fe(0) level (Fig. S2 & S3). Alternatively, mixtures of CO2 and O2 gases (having different partial pressures) can be used to assess relative contributions of CO2 reduction and O2 reduction to the overall electrocatalysis. The data indicate that the catalytic current resulting from CO2 reduction remains unchanged in the presence of oxygen (Fig. S4 & S5). In fact, the CV of the catalyst after bulk electrolysis has similar CO2 electroreduction current and has no trace of the oxygen reduction current at -0.8 V indicating that all the O2 present in the diffusion layer before electrolysis has been reduced (Fig. S6). As the pressure of O2 is increased, the electrocatalytic O2 reduction current increases at -0.8 V, expectedly, but the CO2reduction current stays unchanged thereby establishing stable CO2 reduction in the presence of O2 (Fig.S4). Bulk electrolysis in solution containing both O2 and CO2 at – 2.50 V affords a FY of 92%, 84%, 60% and 43% in CO2/O2 mixtures where the partial pressures of CO2 are 100%, 75%, 50% and 25%, respectively (Fig. 1C & D). Bulk electrolysis with the catalyst in air (20% O2) under homogeneous conditions shows only 5-6% PROS (H2O2) in xylenol orange assay indicating the O2 is reduced by the FeFc4 complex in its Fe(II) state selectively to benign H2O, consistent with previous reports, allowing O2 tolerant CO2 reduction to be catalyzed by the FeFc4 complex. Considering the much larger driving force for the oxidation of Fe(0) porphyrin (Eo = -2.05 V) by O2 (Eo = 1.29 V) relative to CO2 (Eo = -0.12 V), a FY of 43% for CO2 reduction even in a 1:3 mixture of CO2:O2 suggests that the reaction of Fe(0) porphyrin with O2 must be inherently slower than its reaction with CO2. Intrigued, the reactivity of an iron porphyrin without the internal ferrocene group is investigated. Iron picket fence porphyrin (FePf, Scheme 2B) is demonstrated to selectively reduce CO2 to CO under these conditions at very facile rates.20 A similar experiment using CO2 and O2 mixtures reveal remarkable selectivity for CO2 reduction in the case of FePf as well (Fig. S7). These results strongly indicate preferential binding of CO2 over O2 for Fe(0) porphyrins. This is independently verified by investigating the reaction of chemically produced Fe(0) porphyrin with O2 and CO2.

The observed preference of a formally Fe(0) porphyrin for CO2 over O2 could either have a thermodynamic or kinetic origin. It may be anticipated that the thermodynamics of O2 binding to Fe(0) porphyrin is favored over CO2 binding. Indeed, geometry optimized density functional theory (DFT) calculations (Gaussian 03, BP86 functional)43 suggest that Fe(II)-O22formation by the reaction of Fe(0) with O2 is thermodynamically more favorable than the formation of Fe(II)-CO22- adduct by 22.68 kCal/mol (absolute free energies of CO2 and O2 (triplet) adducts are -28.29 and -50.96 kCal/mol, respectively) (Fig. S8). While these calculations don’t model the distal environment and are very approximate they clearly suggest that the preference of Fe(0) porphyrin for CO2 over O2 is not due to thermodynamic reason rather the, experimentally observed, enhanced affinity of Fe(0) porphyrin for CO2 relative to O2 is likely due to the kinetic barrier involved in the reaction of O2 with Fe(0) porphyrin.

Fig. 2 UV-Vis kinetic trace of the disappearance of the 433 nm band as recorded in the reaction of Fe(0)Fc4 with CO2 (green dots) and O2 (blue dots); kinetic fit is shown in red dashed line (inset) The UV-Vis spectra of Fe(0)Fc4 (green); Fe(III)Fc4 (red). Chemically produced Fe(0) porphyrin (by the reduction of Fe(III) porphyrin with three equivalent of Na-anthracenide) is reacted separately with CO2 and O2 and the reaction is followed by absorption spectroscopy.28,44 A Fe(0) porphyrin has characteristic Soret at 433 nm which blue shifts upon oxidation. While there is hardly any immediate shift of the Soret when O2 is reacted with Fe(0) porphyrin, the Soret immediately shifts to 424 nm when CO2 is reacted suggesting the oxidation of Fe(0) porphyrin (Fig.2 inset) by CO2 is substantially faster than its reaction with O2 under the same conditions. The kinetics of the reaction of Fe(0) porphyrin with O2 and CO2 is monitored. The data is simulated using a pseudo 1st order kinetics and the rates obtained clearly show that the reaction of CO2 with Fe(0) porphyrin is 500 times faster than that of O2 (Fig. 2). Taking into consideration the solubility of CO2 (0.28M) and O2 (0.01M) in the acetonitrile medium the second and first order rate constants of the reaction between Fe(0) porphyrin is estimated to be 42.8 M-1 s-1 and 0.12 s-1 with CO2 and 2.01 M-1 s-1 and 0.0002 s-1 with O2,respectively. Thus, the rate of CO2 binding to the formal Fe(0) porphyrin is an order of magnitude higher than O2 consistent with the selectivity for CO2 reduction over O2 reduction.

Oxidation of a formal Fe(0) porphyrin with CO2/O2 in solution.

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The authors acknowledge the Department of Science and Technology grant (SERB/EMR/008063), India. B.M. and P.S. acknowledge CSIR-SRF. We acknowledge Dr. Subhra Samanta for his generous help in synthesis and Ranjana Barman for editorial help. Scheme 3: General reaction scheme of oxygen tolerant activity of FeFc4. Conclusion The FeFc4 complex can reduce CO2 to CO in the presence of O2. The complex reacts with CO2 only when it is reduced to a formal Fe(0) level. This complex can reduce oxygen to water under the same reaction conditions (Scheme 3) in its Fe(II) state.42,45 During O2 reduction, three of the four ferrocenes are oxidized along with the Fe(II) center in the porphyrin and thus providing a total of four electrons required for the ORR.45 The protons come from the externally added phenol. The O2 tolerance in CO2 reduction in the FeFc4 is derived from two distinct attributes of the complex. First, the rate of CO2 binding to Fe(0) state is 500 times faster than that of O2. The slow rate of O2 reaction with Fe(0) porphyrin is not unique to FeFc4 but true for any iron porphyrin in general. This provides a distinct kinetic advantage to CO2 reduction over O2 reduction by Fe(0) porphyrins. Note that, a slow O2 binding rate does not automatically ascertain selective CO2 reduction in the presence of oxygen. This is because CO2 reduction by Fe(0) porphyrin involves several intermediates where the formal oxidation state of iron is +2 (Scheme 2 & 3). Simple iron porphyrins (e.g. FeTPP, FePf) bind O2 in their Fe(II) state fast and generate ROS (Table S1) that lead to catalyst decay. The second attribute of FeFc4 that allows it to reduce CO2 in the presence of oxygen is its ability to reduce O2 selectively to H2O in organic solvents in its Fe(II) state. As a result, any oxygen present gets mitigated by being reduced to water and bulk electrolysis experiments in aerated organic solutions with the FeFc4 shows little PROS generation (~ 5%). Thus, FeFc4 will have greater long-term stability when reducing CO2 in an oxygen atmosphere than simpler iron porphyrins like FePf. Overall, the 500 times faster CO2 binding rate to Fe(0) porphyrin relative to O2 and the ability of FeFc4 to reduce O2 to H2O in organic solvent under conditions relevant to CO2 reduction result in the first instance of CO2 reduction by a low-valent transition metal catalyst in the presence of O2.

AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions #B.M. and P.S. contributed equally. Notes The authors declare no competing financial interests.

ASSOCIATED CONTENT Supporting Information

The experimental details and optimized co-ordinates are available free of charge on the ACS Publication website. ACKNOWLEDGMENT

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