The Selective Electrochemical Conversion of Preactivated CO 2 to Methane

This work reports the selective electrochemical conversion of CO 2 to methane, the reverse reaction of fossil fuel combustion. This reaction is facilitated by preactivation of the CO 2 molecule with an N-heterocyclic carbene (NHC) to form a zwitterionic species in the ﬁrst step. In the presence of Ni(cyclam) 2 + and CF 3 CH 2 OH, this species is shown to undergo further electrochemical reduction of the bound-CO 2 fragment at glassy carbon cathodes in dichloromethane electrolyte solution. Labeling studies conﬁrm the origin of the carbon and protons in the methane product are the preactivated CO 2 and triﬂuoroethanol respectively.

][18][19][20][21][22] Herein we report the direct electrochemical conversion of CO 2 to CH 4 via the reduction of 1,3-bis (2,6-diisopropylphenyl)imidazolium carboxylate, NHC-CO 2 , at a carbon electrode in the presence of a [Ni (cyclam)] 2+ mediator 23 with CF 3 CH 2 OH as a proton source.The sole observed gas product of this electrochemical conversion is CH 4 with >93% faradaic efficiency. 13C-labeling experiments show that after 13 CO 2 is first converted to NHC-13 CO 2 , it is then reduced to 13 CH 4 .Further deuterium-labeling experiments show the proton source in the observed 13 CD 4 is CF 3 CH 2 OD.CH 4 as the selective reduction product formed from CO 2 , has the advantage of maximal CH bonds and thus maximal calorific content.As a relatively inert gaseous product, it overcomes separation issues that a liquid CO 2 reduction product would bring in a solar fuel device.Our results demonstrate the viability of a preactivation strategy toward the selective, electrochemical conversion of CO 2 by 8e − .

Results and Discussion
CO 2 reduction poses significant challenges in terms of chemical reactivity.Although progress has been made in the realm of heterogeneous catalysis for the electroreduction of CO 2 , the selective reduction of CO 2 to highly-reduced (≥6 e − ) products with high faradaic efficiency remains an obstacle 14 in the technological implementation of a solar fuel device. 15n order to overcome the kinetic limitations and the consequent high overpotentials typically observed for electrochemical reduction of CO 2 , we explored the possibility of converting CO 2 into a different species, thus potentially circumventing the challenges generally encountered in the direct reduction of CO 2 .
N-heterocyclic carbenes (NHCs) are strong Brønsted bases with conjugate acids with pK a values that vary between 17 and 25, comparable to alkoxides. 24They are capable of reacting with ketenes forming [1,2]-dipolar species that can undergo subsequent reactions with electrophiles and nucleophiles (Figure 1a). 24In a directly analogous reaction, N-heterocyclic carbenes can reversibly react with CO 2 (Figure 1b), although their subsequent reactions with electrophiles and nucleophiles have not been explicitly explored.][18] N-heterocyclic carbenes (NHCs) are known to reversibly react with CO 2 to form zwitterionic imidazolium carboxylates. 25We now exploit the chemical opportunity to develop a preactivation of our desired substrate (Figure 1b).A free NHC allows us to capture, bend, and thus transform CO 2 to a different chemical entity prior to any chemical transformation.Known modes of CO 2 activation with molecular species often involve binding to low-valent nucleophillic metal centers. 23Organic activators such as frustrated Lewis Acid/Base pairs have also been reported. 22Our strategy involves the capture of CO 2 with a free NHC to form a zwitterionic NHC-CO 2 carboxylate, which is distinct from the Lewis Acid/Base methodology.
Figure 2 shows cyclic voltammograms of [Ni(cyclam)] 2+ and the proton source trifluoroethanol in CH 2 Cl 2 under various conditions.In the absence of CO 2 (black curve), very little reduction current was observed at electrode potentials positive of ca.-2.0 V. Addition of CO 2 to the electrolyte solution resulted in enhanced cathodic current at potentials negative of ca.-1.3 V (blue curve), consistent with previous reports of the electrochemistry of the Ni(cyclam) 2+ cation in nonaqueous solvents in the presence of CO 2 . 23In contrast, addition of  While it is tempting to compare the (over)potentials observed for the reduction of NHC-CO 2 with thermodynamic values for CO 2 reduction in aqueous electrolytes, or with studies conducted in other nonaqueous electrolytes, quantitative comparisons are problematic for a variety of reasons.Even in aqueous electrolytes, comparison of overpotentials reported in the literature for CO 2 reduction is complicated by inadequately defined and/or controlled electrolyte pH, reaction products not being present during the measurements, and reporting of overpotential values without an associated current den-sity.Even greater difficulties exist for comparisons involving CO 2 reduction in nonaqueous electrolytes, including different nonaqueous reference electrode conventions and their relationship to the NHE or RHE, and the fact that the activities of protons and water vary widely for different non-aqueous solvents, making it difficult to define thermodynamically relevant quantities.Nevertheless, it is certain that the reduction of CO 2 to methane reported here would require substantial overpotential if it proceeded catalytically via the electrochemical formation of the NHC-CO 2 adduct from NHC-H + , which along with methane is a product of the electrolysis.
A series of preparative-scale CPE experiments combined with analyses of the cell headspace by GC-TCD and GC-MS was used to confirm the identity and source of electrolysis products.Each electrolysis experiment was carried out for two hours at -1.5 V (vs Ag/AgNO 3 ) and the results are summarized in Table I.As expected from the CVs, electrolysis of solutions that did not contain [Ni cyclam] 2+ and carboxylate, resulted in relatively little current passed and generated only small amounts of CO and H 2 as products.In contrast, when [Ni(cyclam)] 2+ and carboxylate were present in the electrolyte, the electrolysis charge was significantly greater and the sole electrolysis process product observed in GC-TCD analysis of the headspace was methane (Figure M2).Quantitation of the methane peak and comparison with the electrolysis charge allowed calculation of a faradaic efficiency > 93% for the 8e − reduction of the NHC-CO 2 to methane.After electrolysis, the surface of the working electrode was analyzed with X-ray photoelectron spectroscopy (XPS) to test for the presence of any adsorbed Ni or other transition metal materials (Figure M5 and M6).No Ni or other transition metal materials were detected within the ∼0.1 atom% detection limit of XPS. 26,27omewhat surprisingly, in the absence of [Ni(cyclam)] 2+ in the electrolysis solution, we observed the formation of a 12/1 H 2 /CH 4 mixture with satisfactory faradaic efficiency, suggesting that glassy carbon is also capable of reducing NHC-CO 2 to CH 4 product, although with poor selectivity.There is always a possibility that small amounts of Ag + from the reference electrode may contribute to the production of CH 4 in the absence of [Ni(cyclam)] 2+ , although we see no evidence of Ag contamination on our working electrode post-electrolysis within the ∼ 0.1 atom % detection limit of XPS.Also, from the control experiment in the absence of [Ni(cyclam)] 2+ , it can be concluded that any possible contribution of trace Ag to the production of CH 4 is small, and therefore does not contribute appreciably to the observed catalysis in the presence of [Ni(cyclam)] 2+ .
The source of the carbon contained in the methane found in the headspace after CPE was determined by isotopic labeling of various reactants and GC-MS analysis of the products.By performing CPE in deuterated methylene chloride (CD 2 Cl 2 ), unlabeled trifluoroethanol and 13 CO 2 -NHC, 13 CH 4 is the exclusive product observed indicating that the source of carbon in the electrochemically produced methane is the CO 2 bound to the NHC fragment (Figure 3 for the GC-TOF-EI trace for the 13 CH 4 exact mass detection).In Figure 3 the molecular ion 13 CH 4 + is clearly resolved from OH + , and is at the expected exact n/a n/a n/a a 0.2 M nBu 4 NBF 4 solution in 60 ml of a 4:2 (v:v) methylene chloride/ trifluoroethanol mixture.The working chamber volume was 40 ml and the counter chamber volume was 20 ml within a total cell volume of 188.5 ml.The experiment was run with a CO 2 -saturated solution with an added 466 mg of the 1,3-bis(2,6-diisopropylphenyl)imidazolium carboxylate 1 (MW: 432.5 g/mol) and 3.4 mg [Ni(cyclam)]Cl 2 (MW: 320 g/mol).b,c 10 ml of the headspace volume were sampled after 2h of electrolysis at -1.5 V and analyzed by an Agilent GC-TCD instrument.d Faradaic efficiencies were calculated assuming an 8e − transformation per mol of CH 4 detected, a 2e − transformation per mol of H 2 detected, and a 2e − transformation per mol of CO detected.mass. 13CH 3 + appears at an intensity consistent with electron impact library mass spectra of methane, and is at the expected mass(16.029m/z measured, 16.027 m/z calculated).Our mass calibration is confirmed by the measured mass of O + (15.997 m/z measured, 15.995 m/z calculated).CH 4 + is 16.031 m/z (calculated) and would be expected to appear at 16.033 m/z with the present calibration.There is no evidence of a shoulder on the 13

CH 3
+ peak or of a separate peak at the position expected for unlabeled methane.We did not detect any evidence of D-incorporation in the methane detected in the headspace, thus suggesting that solvent participation is not occurring in the chemistry under our conditions.In a similar experiment, we performed the electrochemical transformation in the presence of deuterated trifluoroethanol and unlabeled methylene chloride with NHC- 13 CO 2 (Figure M8).We observed a response of mass peak 21 m/z with the appropriate isotopic distribution (21/19/17 m/z) corresponding to the formation of 13 CD 4 (Figures M8 and M9 in the SI).
In order to determine the fate of the organic fragments left behind in this transformation, we performed a standard organic workup and analyzed the sample by UPLC-MS.The data revealed that the major organic species left in solution, after workup is protonated 1,3-bis (2,6-diisopropylphenyl)imidazolium with 389 m/z (Figures M3 and  M4 in the SI).We were able to identify two other organic fragments in the liquid phase analysis (M4), possibly indicating the participation of C-bound intermediates in the observed chemistry.
In conclusion, we report the electrode-driven conversion of imidazolium carboxylates, acting as CO 2 surrogates, directly to methane.The product of this transformation is an 8 e − , high-value product that was achieved by implementing an unprecedented preactivation motif.We used [Ni(cyclam)] 2+ , a known electrocatalyst for CO 2 reduction to CO as mediator for the transformation.Further studies are under way to elucidate the mechanism of the transformation.

Methods
All reagents were received from commercial sources and used without further purification unless otherwise specified.Solvents were dried by passage through a column of activated alumina followed by storage under dinitrogen.Ni(cyclam)Cl 2 and NHC-CO 2 were prepared as previously described. 23,25C measurements were collected using an Agilent Technologies 7890A GC system with front and back TCD channels.GC-MS-TOF and exact mass analyses were performed on a GC-MS using an Agilent 6890 gas chromatograph interfaced to a Waters GCT Premier time-offlight.All electrochemical experiments were performed using either a Bio-Logic VSP-300 multichannel potentiostat/galvonostat or a Bi-oLogic VSP-400 potentiostat/galvonostat.All electrochemical data was recorded using the Bio-Logic EC Lab Express (5.53) software package.
The reference electrode for all electrochemical measurements was a Ag/AgNO 3 (0.5 mM)/CH 2 Cl 2 nonaqueous reference electrode (also contained 0.1 M nBu 4 NBF 4 ) separated from the solution by a Vycor frit (Bioanalytical Systems, Inc.) and externally referenced to ferrocene.We report the potentials vs Ag/AgNO 3 , but we also provide an alternate Fc/Fc + scale for reference in each of the cyclic voltammogram figures.
The surface speciation of the carbon electrode was determined via XPS on a Kratos Axis Nova spectrometer with DLD (Kratos Analytical; Manchester, UK).The excitation source for all analysis was monochromatic Al K α 1,2 (hv = 1486.6eV) operating at 30 mA and 15 kV. 28,29dditional details are available in the Supporting Information.

Figure 1 .
Figure 1.(a) Reversible binding of ketenes as CO 2 analogues can be achieved with by a N-heterocyclic carbene 24 (b) Reversible binding and bending of CO 2 by an NHC as preactivation motif in the reduction of CO 2 in the current work.

Figure 3 .
Figure 3. 13 C Labeling experiments for the reduction of NHC-CO 2 -13 C. (a) Chemical equation for the 13 C-labeling experiment in the selective electrochemical conversion of CO 2 to methane.(b) Water-resolved mass spectrum of an electrolysis headspace sample for the 13 C-labeling experiment.