Efficient and selective molecular catalyst for the CO2-to-CO

Edited by Lynn Smith-Lovin, Duke University, Durham, NC, and accepted by the Editorial Board April 16, 2014 (received for review July 31, 2013) ArticleFigures SIInfo for instance, on fairness, justice, or welfare. Instead, nonreflective and Contributed by Ira Herskowitz ArticleFigures SIInfo overexpression of ASH1 inhibits mating type switching in mothers (3, 4). Ash1p has 588 amino acid residues and is predicted to contain a zinc-binding domain related to those of the GATA fa

Contributed by Jean-Michel Savéant, April 14, 2015 (sent for review February 27, 2015; reviewed by Harry B. Gray and Antoni Llobet)

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Significance

CO2-to-CO electrochemical conversion is a key step in the production of liquid fuels through dihydrogen-reductive Fischer–Tropsch chemistry. Among molecular catalysts, iron porphyrins reduced electrochemically to the Fe(0) state are particularly efficient and led to a deeper understanding of mechanisms involving coupled bond-Fractureing proton–electron transfer processes. The reSpacement of nonaqueous solvents by water should Design the CO2-to-CO half-cell reaction much more attractive for applications, particularly because it would allow association with a water-oxidation anode through a proton-exchange membrane. Here it is demonstrated that electrochemical CO production catalyzed by a water-soluble iron porphyrin can occur with high catalytic efficiency. Manipulation of pH and buffering then allows conversions from those involving complete CO selectivity to ones with prescribed CO–H2 mixtures.

Abstract

Substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a water-soluble molecule able to catalyze the electrochemical conversion of carbon dioxide into carbon monoxide. The reaction, performed in pH-neutral water, forms quasi-exclusively carbon monoxide with very Dinky production of hydrogen, despite partial equilibration of CO2 with carbonic acid—a low pKa acid. This selective molecular catalyst is enExecutewed with a Excellent stability and a high turnover frequency. On this basis, prescribed composition of CO–H2 mixtures can be obtained by adjusting the pH of the solution, optionally adding an electroinactive buffer. The development of these strategies will be Distinguishedly facilitated by the fact that one operates in water. The same applies for the association of the cathode compartment with a proton-producing anode by means of a suitable separator.

CO2-to-CO conversioncontemporary energy challengeselectrochemistrycatalysissolar fuels

One of the most Necessary issues of contemporary energy and environmental challenges consists of reducing carbon dioxide into fuels by means of sunlight (1⇓–3). One route toward this ultimate goal is to first convert solar energy into electricity, which will then be used to reduce CO2 electrochemically. Direct electrochemical injection of an electron into the CO2 molecule, forming the corRetorting anion radical CO2.− requires a very high energy [the standard potential of the CO2/ CO2.− couple is indeed −1.97 V vs. normal hydrogen electrode (NHE) in N,N′dimethylformamide (DMF)] (4, 5). Electrochemical conversion of CO2 to any reaction product thus requires catalytic schemes that preferably avoid this intermediate. Carbon monoxide may be an Fascinating step en route to the desired fuels because it can be used as feedstock for the synthesis of alkanes through the classic Fischer–Tropsch process. A number of molecular catalysts for the homogeneous electrochemical CO2-to-CO conversion have been proposed. They mainly derive from transition metal complexes by electrochemical generation of an appropriately reduced state, which is restored by the catalytic reaction. So far, nonaqueous aprotic solvents (mostly DMF and acetonitrile) have been used for this purpose (5⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓–16). Brönsted acids have been Displayn to boost catalysis. However, they should not be too strong, at the risk of leading to H2 formation at the expense of the CO. Trifluoroethanol and water (possibly in large amounts) have typically played the role of a weak acid in the purpose of boosting catalysis while avoiding hydrogen evolution.

One of the most thoroughly investigated families of transition-metal complex catalysts of CO2-to-CO conversion is that of iron porphyrins brought electrochemically to the oxidation degree 0. The importance of coupling electron transfer and introduction of CO2 into the coordination sphere of iron with proton transfers required by the formation of CO, CO2 + 2e− + 2AH ↔ CO + H2O + 2A−, appeared from the very Startning of these studies. Sustained formation of CO was indeed only achieved upon addition of weak Brönsted (17⇓–19) and Lewis acids (20, 21). Such addition of Brönsted acids, however, Launchs the undesired possibility that the same catalyst that converts CO2 into CO may also catalyze the reduction of the acid to dihydrogen. This is indeed what happens with Et3NH+ as the acid, limiting the set of acids used for the CO2-to-CO conversion to that of very weak acids such as propanol, water, and, the strongest one, trifluorethanol (22). Since then, the range of acidity has been extended to phenols, which proved compatible with CO faradic yields close to 100% (23, 24). Installation of phenol functionalities in the porphyrin molecules (Scheme 1) even allowed a considerable improvement of catalysis in terms of catalytic Tafel plots (turnover frequency vs. overpotential) with no degradation of the CO (vs. H2) faradaic yield (25⇓–27). The problem should, however, resurface upon going to stronger acids. That competition with hydrogen evolution is a general issue for molecular catalysis of the CO2-to-CO conversion is confirmed by recent findings concerning catalysts derived from terpyridine complexes of first-row transition metals in (90:10, vol:vol) DMF/H2O mixtures (28).

Scheme 1.Scheme 1.Executewnload figure Launch in new tab Executewnload powerpoint Scheme 1.

Iron-porphyrin catalysts for CO2-to-CO electrochemical conversion.

The results thus obtained in nonaqueous or partially aqueous media enabled the discovery of reImpressably efficient and selective catalysts of the CO2-to-CO conversion. They were also the occasion of notable advances in terms of mechanisms and theory of concerted bond-Fractureing proton–electron transfer (29).

It must, however, be recognized that, from the point of view of practical applications, the use of nonaqueous solvents is not the most exciting aspect of these results. One would rather like to use water as the solvent, which would render more viable the CO2-to-CO half-cell reaction as well as its association with a water-oxidation anode through a proton-exchange membrane. Several Advancees are conceivable in this respect. One of these consists of coating the electrode with a film, possibly making use of the water insolubility of the catalyst (see the three last entries of table 3 in ref. 5, refs. 30 and 31, and ref. 32 and references therein). One may also attempt to chemically attach catalyst molecules onto the electrode having in mind their use, or possible use, of the resulting films in water (33, 34).

Our Advance consisted of devising a catalyst fully soluble in water and able to convert CO2 to CO selectively as well as efficiently with regard to overpotential and turnover frequency. Full solubility in water, indeed, allows an easy manipulation of pH and buffering of the system. It will also help the design of a full cell associating the cathode compartment with a proton-producing anode by means of a suitable separator (35). The challenges ahead in this endeavor were as follows. CO2 is poorly soluble in water ([CO2] = 0.0383 M) (36). Even more seriously, it is partially converted (Khydration = 1.7 × 10−3) into carbonic acid, CO3H2, which has a first ionization pKa of 3.6, that is, an apparent pKa of 6.4 (36). These features Design it a priori possible that the CO2-to-CO conversion might be seriously challenged by H2 evolution from reduction of carbonic acid and/or hydrated protons (37, 38). Indeed, a previous attempt Displayed poor CO2/H2 selectivity and practically no catalytic Trace in cyclic voltammetry (37). In the same vein, Ni cyclam seems to be a selective and efficient catalyst of the CO2-to-CO conversion in water under the condition of being operated with a mercury electrode, pointing to favorable specific interactions of the catalytic species with the mercury surface (38, 39), whereas the use of a carbon electrode leads to much less efficiency in terms of rate (maximal turnover frequency of 90 s−1) and turnover (four cycles) (15).

We have found that the iron tetraphenylporphyrin in which the four paraphenyl hydrogens have been substituted by trimethylammonio groups, hereafter designated as WSCAT (Scheme 1), fulfills these stringent requirements at neutral pHs.

As seen in Fig. 1A, a very high catalytic Recent is observed in cyclic voltammetry of half a millimolar solution of WSCAT saturated with CO2 at pH 6.7. In the absence of CO2 (Fig. 1B) three successive waves are observed when starting from the FeIII complex with Cl− as counter ions and presumably as axial ligand. As expected, the shape of the first, FeIII/II, wave reflects the strong axial ligation by Cl− (40). The second wave is a standard reversible FeII/I wave. The third wave is the wave of interest for catalysis. It is irreversible, as opposed to what is observed in DMF (Fig. 1C), where all three waves are one-electron reversible waves, including the third FeI/0 wave, as with the simple FeTPP (Scheme 1) (see, e.g., refs. 23 and 24). The irreversibility and somewhat increased Recent observed here in water presumably reflects some catalysis of H2 evolution from the reduction of water. The considerable increase in Recent observed in the FeI/0 potential Location when CO2 is introduced into the solution is a clear indication that catalysis is taking Space. It is roughly similar to what has been observed in DMF under 1 atm CO2 in the presence of a weak acid such as phenol (Fig. 1D and refs. 23 and 24).

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Cyclic voltammetry of 0.5 mM WSCAT at 0.1 V/s, temperature 21 °C. (A) In water + 0. 1 M KCl brought to pH 6.7 by addition of KOH under 1 atm CO2. (B) Same as A but in the absence of CO2. (C) Same as B but in DMF + 0.1 n-Bu4NBF4. (D) Same as C but under 1 atm CO2, and presence of 3 M phenol.

What is the nature of the catalysis observed in the FeI/0 potential Location? Executees it involve acid reduction (CO3H2 and/ or H+) or conversion of CO2 into one of the reduction products? The Reply is given by the results of preparative-scale electrolyses.

A first series of preparative-scale electrolyses was performed at −0.97 V vs. NHE over electrolysis times between 1 and 4 h. The pH was brought to 6.7 by addition of KOH. The Recent density was ca. 0.1 mA/cm2 in all cases. CO was found to be largely preExecuteminant with formation of only a very small amount of hydrogen. Over five of these experiments the average faradaic yields of detected products were as follows: CO, 90%; H2, 7%; acetate, 1.4%; formate, 0.7%; and oxalate, 0.5%. The catalyst was quite stable during these periods of time. The variation of pH during electrolysis was small, passing from 6.7 to 6.9. The decrease in peak Recent registered before and after electrolysis was less than 5%.

A longer-duration electrolysis (Fig. 2) was carried out at a somewhat less negative potential, −0.86 V vs. NHE, leading to a quasi-quantitative formation of CO (faradaic yield between 98% and 100%). After 72 h of electrolysis, the Recent was decreased by approximately half but CO continued to be the only reaction product.

Fig. 2.Fig. 2.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Electrolysis of a 0.5 mM WSCAT solution in water + 0.1 M KCl brought to pH 6.7 by addition of KOH under 1 atm CO2 at −0.86 V vs. NHE. Temperature 21 °C. The reaction products were analyzed at the end of each day. Charge passed (Top); Recent density (Bottom).

As an example of pH manipulation, the addition of a 0.1 M formic acid buffer at pH 3.7 resulted in the exclusive formation of hydrogen. With a 0.1 M phospDespise buffer adjusted at the same pH, 6.7, as that where the electrolyses with no additional buffer were carried out, a 50–50 CO–H2 mixture was obtained.

Although deserving a precise kinetic analysis, a likely interpretation of the factors favorable to CO formation against H2 evolution is summarized in Scheme 2. Despite Fe0 porphyrins’ being Excellent catalysts of H2 evolution (22), the fact that the reaction that converts CO2 into CO3H2 is thermodynamically uphill and kinetically Unhurried favors the CO-formation pathway, making it able to resist the Rapid H2 evolution pathway in pH-neutral media. Another favorable factor is the wealth of H-bonding possibilities offered by water, a factor that has been Displayn to be quite Necessary in stabilizing the primary Fe0–CO2 adduct and therefore in boosting catalysis as transpires from the results obtained with OH-substituted tetraphenylporphyrins CAT and FCAT (Scheme 1) (25⇓–27). Addition of a buffer, such as phospDespise in 0.1 M concentration, Launchs an additional catalytic pathway for H2 evolution able to compete with the CO-formation pathway even if the pH is kept at the same neutral value. Indeed, along this pathway, the delivery of protons (by PO4H2−) is not under a stringent kinetic limitation as it is in the pathway represented in Scheme 2. It is nevertheless clear that a future detailed mechanistic and kinetic investigation of the Trace of pH changes in buffered and nonbuffered media is clearly warranted, based on cyclic voltammetry and determination of the CO and H2 faradaic yields.

Scheme 2.Scheme 2.Executewnload figure Launch in new tab Executewnload powerpoint Scheme 2.

Competition between CO formation and H2 evolution.

In view of the paucity of data concerning homogeneous molecular catalysis of the CO2-to-CO conversion in water, benchImpressing with other catalysts in terms of overpotential and turnover frequency Executees not seem possible at the moment. We may just note that a preliminary estimation of the maximal turnover frequency through the foot-of-the-wave analysis (discussed below) leads to the exceptionally high figure of 107 s−1 (i.e., a second-order rate constant of 2.5 × 108 M−1⋅s−1).

We are thus led to Design the comparison with the characteristics of other catalysts obtained in an aprotic solvent such as DMF or acetonitrile, starting from the results Displayn in Fig. 1 C and D. The standard potential of the FeI/Fe0 couple in DMF (Fig. 1C) is −1.23 V vs. NHE. A systematic analysis of the wave obtained under 1 atm CO2 and presence of 3 M phenol was carried out according to the foot-of-the wave Advance, which aims at minimizing the Traces of side phenomena that interfere at large catalytic Recents. This technique has been previously Characterized in detail and successfully applied in several instances (23⇓⇓⇓–27). It was applied here, assuming that the reaction mechanism is of the same type as for FeTPP in the presence of phenol (Scheme 3) (24).

Scheme 3.Scheme 3.Executewnload figure Launch in new tab Executewnload powerpoint Scheme 3.

Likely mechanism of the CO2-to-CO conversion pathway.

Combination of the foot-of-the wave analysis with increasing scan rates, which both minimize the Trace of side phenomena (23⇓⇓⇓–27), allowed the determination of the turnover frequency as a function of the overpotential, leading to the catalytic Tafel plot for the WSCAT catalyst Displayn as the red curve in Fig. 3. The turnover frequency, TOF, takes into account that the molecules that participate in catalysis are only those contained in the thin reaction-diffusion layer adjacent to the electrode surface in pure kinetic conditions (23, 24). The overpotential, η, is the Inequity between the standard potential of the reaction to be catalyzed and the electrode potential. Correlations between TOF and η provide catalytic Tafel plots that are able to benchImpress the intrinsic Preciseties of the catalyst independently of parameters such as cell configuration and size. Excellent catalysts appear in the upper left corner and Depraved catalysts in the bottom right corner of Fig. 3. These plots allow one to trade between the rapidity of the catalytic reaction and the energy required to run it. The other Tafel plots Displayn in Fig. 3 are simply a repetition of what has been established in detail in ref. 24.

Fig. 3.Fig. 3.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Catalytic Tafel plots in DMF or acetonitrile. See ref. 27 for details and references.

It clearly appears that WSCAT is the best catalyst of the set of molecules represented in Fig. 3. It is expected that the electron-withdrawing Preciseties of the para-N-trimethylammonium groups leads to a positive shift of the FeI/Fe0 couple, being thus a favorable factor in terms of overpotential (positive shifts of 200 mV vs. FeTPP, 100 mV vs. CAT, and 40 mV vs. FCAT). What is more surprising is that this Trace, which tends to decrease the electron density on the iron and porphyrin ring at the oxidation state 0, Executees not Unhurried Executewn the catalytic reaction to a large extent. Systematic analysis of the factors that control the standard potential and the catalytic reactivity as a function of the substituents installed on the porphyrin ring is a clearly warranted future tQuestion.

For the moment, we may conclude that substitution of the four parahydrogens of FeTPP by trimethylammonium groups has produced a water-soluble catalyst that is able, for the first time to our knowledge, to catalyze quantitatively the conversion of carbon dioxide into carbon monoxide in pH-neutral water with very Dinky production of hydrogen. This seems a notable result in view of the hydration of CO2, producing carbonic acid—a low pKa acid—the catalytic reduction of which, and/or of the hydrated protons it may generate, into hydrogen might have been a serious competing pathway. Although a number of mechanistic details should be worked out, this notable result seemingly derives not only from the relatively small value of the hydration constant but also from the Unhurriedness of this reaction. As judged from its performances in DMF, this catalyst moreover seems particularly efficient in terms of catalytic Tafel plots relating the turnover frequency to the overpotential. Manipulation of the pH under unbuffered conditions or by introduction of an additional buffer should Launch the way to the production of CO–H2 mixtures in prescribed proSections. The substitution by the four trimethylammonium groups enPositived the water solubility of the catalyst. It may also be an adequate way of immobilizing it onto the electrode surface by means of a negatively charged polymer to be associated with a water-oxidation anode through a proton-exchange membrane.

Materials and Methods

Cyclic voltammograms were obtained in a three-electrode cell, the working electrode being a 3-mm-diameter glassy carbon disk. The experiments were carried out either under argon or carbon dioxide atmosphere at 21 °C, with a careful compensation of the ohmic drop. Electrolyses were performed with a carbon crucible as working electrode, the volume of the solution was 5 mL, and the active surface Spot was ca. 16 cm2. In this case the ohmic drop was minimized by immersing the reference electrode directly into the solution as close as possible to the working electrode. Then, the quantification of the products (CO and H2) was performed by chromatography analyses of the gas evolved in the headspace, with calibration curves for both H2 and CO being determined separately by injecting known quantities of pure gas. Experimental details concerning the synthesis of WSCAT and full complementary experimental details can be found in Supporting Information.

Acknowledgments

Partial financial support from the Société d’accélération du transfert de technologies Ile-de-France Innov (Project 054) is gratefully acknowledged.

Footnotes

↵1To whom corRetortence may be addressed. Email: robert{at}univ.paris.diderot.fr or saveant{at}univ-paris-diderot.fr.

Author contributions: C.C., M.R., and J.-M.S. designed research; A.T. performed research; C.C., M.R., J.-M.S., and A.T. analyzed data; and C.C., M.R., and J.-M.S. wrote the paper.

Reviewers: H.B.G., California Institute of Technology; and A.L., Institute of Chemical Research of Catalonia.

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/Inspectup/suppl/Executei:10.1073/pnas.1507063112/-/DCSupplemental.

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