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_3CH_2OH, 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

Dalleska, Nathan F.

Koval, Carl A.

Luca, Oana R.

McCrory, Charles C. L.

Caltech Authors - Main

Supporting Information for 1 
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The Selective Electrochemical Conversion of Preactivated CO2 to Methane 4 
Oana R. Luca, a§ Charles C. L. McCrory, a Nathan, F. Dalleska, b Carl A. Koval a § 5 
a Joint Center for Artificial Photosynthesis, Division of Chemistry and Chemical Engineering, 6 
California Institute of Technology, Pasadena, California 91125, United States. 7 
b Environmental Analysis Center, California Institute of Technology, Pasadena California 8 
91125, United States. 9 
§corresponding author 10 
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S1. Electrochemical Methods 12 
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S2. Product Separation and Quantification 14 
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S3. X-ray Photoelectron Spectroscopy of Electrode Materials 16 
S4. Labeling Experiments 17 
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S1. Electrochemical Methods 39 
All electrochemical experiments were performed using either a Bio-Logic VSP-300 multichannel 40 
potentiostat/galvonostat or a BioLogic VSP-400 potentiostat/galvonostat.   All electrochemical 41 
data was recorded using the Bio-Logic EC Lab Express (5.53) software package. 42 
The reference electrodes for all measurements was a Ag/AgNO3 (0.5 mM)/CH2Cl2 nonaqueous 43 
reference electrode (also contained 0.1 M nBu4NBF4) separated from the solution by a Vycor frit 44 
(Bioanalytical Systems, Inc.) and externally referenced to ferrocene (at 410 mV vs the reference).  45 
We report the potentials vs Ag/AgNO3, but we also provide an alternate Fc/Fc+ scale for 46 
reference in each of the cyclic voltammogram figures. 47 
Cyclic voltammograms in methylene chloride (0.1 M nBu4NBF4) were collected at a glassy 48 
carbon working electrode (3 mm diameter, BASi) with a C rod counter electrode (99.999%, 49 
Strem) in 5 mL of solution.   All CVs were recorded after rigorous exclusion of air via nitrogen 50 
purge. Data workup was performed on OriginPro v8.0988. 51 
Diagnostic cyclic voltammograms and backgrounds (Figures 2 and M1) show that a high-current 52 
reduction response is observed at -1.5 V vs Ag/AgNO3 in the presence of catalyst and 53 
carboxylate (Figure 2), whereas low currents are observed in the absence of carboxylate (M1).  54 
-2.5 -2.0 -1.5 -1.0 -0.5
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
Potential (V) vs Fc/Fc+
 
 
Cu
rre
nt 
(m
A)
Potential (V) vs Ag/AgNO3
  trifluoroethanol
  NHC-CO2 + trifluoroethanol
  NHC-CO2 + trifluoroethanol + CO2
  GC Background
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5
 55 
Figure M1. Background cyclic voltammetry experiments for the diagnostic of CO2 reduction in 56 
0.1 M nBu4NBF4 in CH2Cl2.  The scan rate is 100 mV/s. 57 
 58 
Controlled-potential electrolysis experiments were performed at ambient pressure in a sealed, 59 
two-chamber H-cell where the solutions in the two chambers were separated by a fine ceramic 60 
frit, but the two chambers shared a common headspace.  The first chamber contained the 61 
reference electrode and working electrode, a 6 cm x 1 cm x 0.3 cm glassy carbon plate (HTW 62 
Hochtemperatur-Werkstoff GmbH), in 40 mL solution.  During the experiment, the working 63 
electrode was submerged such that about 75% the electrode was submerged in solution.  The 64 
second chamber contained the high-surface area reticulated vitreous carbon electrode counter 65 
electrode, in 20 mL solution.  The electrolyte solution was 0.2 M nBu4NBF4 in a 4:2 (v:v) 66 
methylene chloride/trifluoroethanol mixture.  The total volume of the electrolysis cell was 142 67 
mL (for electrolyses preceding GC-MS experiments) or 188 mL (for electrolyses preceding GC-68 
TCD measurements), leading to total headspace volumes of 82 mL and 128 mL, respectively.  69 
For the GC-TCD measurements, 10 mL of headspace were sampled and analyzed. 70 
S2. Product Separation, Identification and Quantification Methods 71 
 72 
GC-TCD measurements were performed using an Agilent Technologies 7890A GC system with 73 
two TCD detectors.  The oven was held at 50° C for 9 minutes, then the temperature was 74 
increased 8°C/min to 80°C with the following column configuration: G3591-81143 0.5m x 1/8”, 75 
Ultimetal, Hayesep T 80/100 mesh ,  G3591-81144 0.5m x 1/8”, Ultimetal, Hayesep Q 80/100 76 
mesh, G3591-81145 1.5m x 1/8”, Ultimetal, Molsieve 13X 80/100 mesh , G3591-81146 1m x 77 
1/8”, Ultimetal, Hayesep Q 80/100 mesh , G3591-81149 1m x 1/8”, Ultimetal and a Molsieve 5A 78 
60/80 mesh.  79 
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Figure M2. GC-TCD trace output for headspace sampling of catalytic electrolysis run. No H2 82 
product observed (expected at 2min on the Back Signal channel, low pane), exclusive methane 83 
formation observed. 84 
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GC-MS-TOF measurements and exact mass analyses were performed on an Agilent 6890 gas 86 
chromatograph interfaced to a Waters GCT Premier time-of-flight mass spectrometer with an 87 
electron impact ionization source.  Separation of methane from other headspace gasses was 88 
achieved on a 30m x 0.320 mm PLOT-Q capillary column (Agilent Technologies) operated with 89 
helium carrier gas flowing at a linear velocity of 35 cm/s. The oven temperature was held at 40 ͦ 90 
C for 1 minute and then raised to 45  ͦ C at 5 ͦ /min. After 1 minute at 45 the temperature was 91 
increased to 200 ͦC at a ramp of 20 ͦ C/min and held constant for 4 minutes. 50 µL gas samples 92 
were injected into a split-splitless inlet operating at 150 C and a split ratio of 1:5 and with a 1 93 
mm ID liner installed.  The GCT Premier was operated in electron impact ionization mode, with 94 
low mass cutoff parameters adjusted to allow transmission as low as 14 m/z.  Mass resolution 95 
was 7000 at 131 m/z, and approximately 900 at 17 m/z.    96 
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GC-MS nominal mass data were acquired on an HP 5890 GC 5972 MSD quadrupole mass 98 
spectrometer.  Gas samples were manually injected with a 50 uL gastight syringe into a split-99 
splitless inlet operated in split mode and held at 125 C.  Gases were separated on a 0.25 micron x 100 
30m PLOT-Q GS column (Restek Corporation) using helium carrier gas flowing at 0.6 mL/min.  101 
An oven temperature ramp was employed to ensure complete elution of all sample components 102 
after each injection.  The GC column transfer line was held at 300 C in order to keep the electron 103 
impact source hot (approximately 180 ºC).  The electron impact ionization source was operated 104 
with 70 eV electron energy.  The ion focusing lenses were retuned on residual nitrogen (28 m/z) 105 
and argon (40 m/z) in the helium carrier gas and the 131 m/z fragment of perfluorotributyl amine 106 
so as to maximize sensitivity for low mass ions such as methane. 107 
For the UPLC-MS analyses, the catholyte contents were added to a 250 mL round bottom flask 108 
and the volatiles were removed via rotary evaporation. The solid residue was then extracted three 109 
times with 75 mL diethylether. The combined organic fractions were dried over Na2SO4. After 110 
the removal of the ether phase, we recovered 640 mg white crystalline solid. The workup did not 111 
entirely remove the supporting electrolyte, but that remaining did not interfere with obtaining 112 
UPLC-MS data. The solid sample was dissolved in a minimum amount of methylene chloride 113 
and then diluted with a mixture of methanol, ethanol, isopropoanol and acetonitrile. The analysis 114 
of 4µL of the sample allowed us to identify 1,3-bis(2,6-diisopropylphenyl)imidazolium at 389 115 
m/z as the major organic species in solution aside from supporting electrolyte (242 m/z). (Figure 116 
M3). Instrument details above. 117 
UPLC-MS analysis of this catholyte sample was performed using an Acquity I-Class UPLC 118 
coupled to a Xevo G2-S time-of-flight mass spectrometer (Waters Corporation).  Samples were 119 
diluted sufficiently in 5% acetonitrile in water such that saturation of the fast electron multiplier 120 
detector did not occur.  Separation was performed on a BEH C18 column (2.1 x 50 mm, 1.7 121 
micron particle size, Waters Corporation) using a 5 minute gradient from 5% to 90% acetonitrile 122 
in water with 0.1% formic acid.  Masses from 100 to 1000 m/z in positive ion mode were 123 
recorded.  The mass axis was calibrated with sodium formate cluster peaks and spectra were 124 
referenced to the mass of singly charged leucine encephalin. Measured m/z are considered 125 
accurate to 2 millidaltons. Electrospray source conditions were capillary voltage, 2.5 kV; 126 
sampling cone, 50V; cone gas, 40 L/hr; and desolvation gas 800 L/hr at 500º C. 127 
 128 
Figure M3.  UPLC-MS base peak ion chromatogram of extracted catholyte. 129 
 130 
Major peaks in the UPLC chromatogram:  The species observed at 2.16 minutes and 389.296 131 
m/z corresponds to the protonated imidazolium core of NHC-CO2. The peak at 1.57 minutes and 132 
242.285 m/z corresponds to the tetrabutylammonium cation of the supporting electrolyte.  We 133 
also observe peaks at 2.08 minutes, 419.305 m/z and  2.22 minutes, 403.310 m/z. These masses 134 
are consistent with the structures we propose in M4.   135 
 136 
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Figure M4. Tentative structural assignments for organic fragments observed at 2.08 min and 138 
2.22 min. 139 
S3. X-ray Photoelectron Spectroscopy of electrode materials 140 
The surface speciation of the carbon electrode before and after electrolysis was determined via 141 
XPS on a Kratos Axis Nova spectrometer with DLD (Kratos Analytical; Manchester, UK). The 142 
excitation source for all analysis was monochromatic Al Kα1,2 (hv = 1486.6 eV) operating at 30 143 
mA  and 15 kV. The angle of incident x-rays with respect to the surface normal is 54.5 degrees. 144 
A base pressure of 1×10-9 Torr is maintained in the analytical chamber, which rises to 1×10-8 145 
Torr during spectral acquisition. All spectra were acquired using the hybrid lens magnification 146 
mode and slot aperture, resulting in an analyzed area of 700 µm × 400 µm.  Survey scans were 147 
conducted using 160 eV pass energy, and narrow region high resolution scans used 20 eV.  The 148 
following sequence of scans was performed: Survey (0-1200 eV), Cu 2p (925-970 eV), N 1s 149 
(410-389 eV), Sn 3d (479-500 eV), F 1s (695-674 eV), P 2p (126-140 eV), Ni 2p/Fe 2p/Co 2p 150 
(701-885 eV), O 1s (525-543 eV), C 1s/Ru 3d (276-292 eV), and Ir 4f (56-71 eV).  151 
Compositional analysis was performed using CasaXPS version 2.3.16 (Casa Software Ltd; 152 
Teignmouth, UK).  Energy scale corrections for the survey and narrow energy high resolution 153 
regions was done by setting the large component in the C 1s spectrum to 284.3 eV, the expected 154 
C 1s peak for graphitic carbon in glassy carbon.  155 
Survey scans and narrow-region high-resolution scans of the Ni 2p/Fe 2p/Co 2p regions are 156 
shown in Figures M4 and M5, respectively, for a pristine glassy carbon plate and working 157 
electrodes from three independent catalytic electrolyses.  After electrolysis, the working 158 
electrodes were removed from the electrolyte solution and gently rinsed in CH2Cl2 prior to 159 
analysis.  In general, the survey scans and high-resolution scans both confirmed that there was no 160 
Ni, Fe, Co, or other transition metal deposits on the surface within the detection limits of XPS (~ 161 
0.1 atom%). 162 
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Figure M5. XPS survey scans of (a) a pristine glassy carbon plate as received and (b)-(d) 164 
working electrodes from three independent controlled-potential electrolyses after rinsing with 165 
acetone.  XPS and Auger peaks are assigned as labeled.  The presence of Si is likely due to trace 166 
amounts of silicone grease.  Small peaks barely above background in (b) and (c) at 346 and 437 167 
eV are not labeled, but are assigned as Ca 2p and Ca 2s peaks, respectively. 168 
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Figure M6. XPS high-resolution scans of (a) a pristine glassy carbon plate after polishing and 171 
(b)-(d) working electrodes from three independent controlled-potential electrolyses in the Ni 172 
2p/Co 2p/Fe 2p region.  There are no peaks evident in this region. 173 
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S4. Labeling experiments 175 
Isotopically labeled experiments were performed with an added 466 mg of the 1,3-bis(2,6-176 
diisopropylphenyl)imidazolium-13C-carboxylate, NHC-13CO2, (MW: 433.5 g/mol) and 3.4 mg 177 
[NiCyclam]Cl2 (320 g/mol).  Experiments done with CF3CH2OD as the proton source and 178 
CH2Cl2 as the solvent were analyzed by GC-MS with unit mass resolution using the HP 5890 GC 179 
5972 MS as described above. 50 µL of the gas sample was manually injected in the GC inlet.  180 
Experiments done with CF3CH2OH as the proton sources and CD2Cl2 as the solvent were 181 
analyzed with the GC-MS-TOF system as described above.  For all analyses 5 mL of headspace 182 
gas was sampled from the reactor after 2h of electrolysis at -1.5 V (vs Ag/AgNO3).   183 
 184 
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Figure M7. GC-MS-TOF (accurate mass) extracted ion chromatograms for a 500µL headspace 187 
sampling of an electrolysis run with 13C-labeled imidazolium carboxylate with 0.5 mol% [Ni 188 
(cyclam)]2+ in deuterated methylene chloride under standard conditions.  The first three traces 189 
qualitatively show the expected decrease in intensity from CH4+ to CH3+ to CH2+.  The bottom 190 
trace is the mass of OH+, demonstrated chromatographic resolution of water from labeled 191 
methane. 192 
13
CH
2
 
13
CH
3
+
 
13
CH
4
+
 
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Figure M8. GC-MS nominal mass extracted ion chromatograms of  17, 19, 21, and 16 m/z 194 
corresponding to 13CD2, 13CD3, 13CD4 and H2O.  Areas under the peaks in the EICs decrease in 195 
the order 21, 19, 17, consistent with the spectrum expected for labeled methane.  The spike 196 
observeable in the top and bottom traces is an artifact associated with the polymeric stationary 197 
phase.  Particles detached from the stationary phase produce spikes at nearly all masses when 198 
they arrive in the ion source.  The spike is not visible in the middle traces as a consequence of 199 
normalization to the larger analyte peaks.  Note that we observe water eluting after methane with 200 
the column used in the present data, but before methane in the data of M7.  We attribute this to 201 
sourcing the columns from different manufactures and the columns having been subjected to a 202 
differing array of analyses. 203 
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13
CD2 
+
 
13
CD3
+
 
13
CD4
+
 
O
+
 
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13CD4+
13CD3+
13CD2+
(a)
(b)
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Figure M9. a. Reaction scheme for the 8e- reduction of NHC13CO2  in the presence of 212 
deuterated proton source.b. Mass spectrum at 1.57 min of the M8 GC trace. 213 
 214 


The Selective Electrochemical Conversion of Preactivated CO_2 to Methane

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The Selective Electrochemical Conversion of Preactivated CO_2 to Methane

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