Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide

Transcription

1 Supporting Information for Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide Michael L. Pegis, John A. S. Roberts,, Derek J. Wasylenko, #, Elizabeth A. Mader, Aaron M. Appel, *, and James M. Mayer *, Department of Chemistry, Yale University, PO Box , New Haven CT , United States Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999 (K2-57), Richland, Washington 99352, United States. # Department of Chemistry, Campus Box , University of Washington, Seattle, WA, , United States. REC Silicon, 3322 Road "N" NE, Moses Lake, WA 98837, United States FireWater Fuel Corp., 3535 Research Road N.W., Calgary, AB, T2L 2K8, Canada Contact: AMA: aaron.appel@pnnl.gov, JMM: james.mayer@yale.edu Table of Contents General Approach... 2 Experimental Details for Determining the H + /H2 Couple in DMF... 4 Thermodynamic Schemes to Determine the CO2/CO and CO2/CH4 Couples... 5 Using Vapor Pressure to Determine the Effects of Acid on Water in Organic Solvents... 6

2 General Approach Our general approach for acquiring equilibrium potentials in non-aqueous media (MeCN & DMF) relies on the equilibrium potential for H + /H2 in organic solvents (org) determined experimentally. The details of such electrochemical experiments are discussed in section 2. The equilibrium potential for O2/H2O in aqueous medium can be used in combination with other thermochemical equations to acquire the thermodynamics for O2/H2O in non-aqueous solvents. The conversion begins with the respective half reactions in aqueous media. 1 This yields the free energy for production of two molecules of water from oxygen and hydrogen in their standard state. Next, the thermodynamic potential for the H + /H2 half reaction in organic solvent can be used (see the experimental details section below). For the sake of clarity, we will assume org = MeCN. Fc +/0 represents the ferrocenium/ferrocene couple, the preferred electrochemical standard in organic solvents. 2 Finally, the free energy for transfer of H2O from (aq) to (org) must be considered. This is a minor contribution to the overall thermodynamics. The summation of the equations above yields the free energy for O2/H2O in acetonitrile. Since the H + /H2 reaction is a half reaction referenced to ferrocene in acetonitrile, summation of the free energies above and conversion back to potential via the Nernst equation yields a potential referenced to ferrocene. (1) S. G. Bratsch, J. Phys. Chem. Ref. Data, 1989, 18, (2) (a) R. R. Gagne, C. A. Koval, G. C. Lisensky, Inorg. Chem., 1980, 19, (b) G. Gritzner, J. Kůta, Pure Appl. Chem. 1984, 56,

3 The transfer free energy of H + aq->org can also be calculated using the scheme shown below. 3

4 Experimental Details for Determining the H + /H2 Couple in DMF To determine the potential of the H + (DMF)/H2(g) couple in DMF, the previously reported general approach was used. 3 The reported approach is based on measuring the open circuit potential of a platinum wire versus the ferrocenium/ferrocene couple (Fc +/0 ). Instead of using a weak acid with a known pka value, protonated DMF was added as the triflate salt, 4 in effect equivalent to using triflic acid in DMF. The concentration dependence of the open circuit potential was used to extrapolate from the linear region to 1 M solvated H +, yielding a potential of V vs. Fc +/0 in DMF, as illustrated in Figure S1, reproduced from Figure 1 from the main text for convenience. Figure S1 The potential for the H + /H2 couple in DMF based on the open circuit potential as a function of the concentration of [(DMF)H] + OTf, using the general procedure reported by Roberts and Bullock. 3 (3) J. A. S. Roberts and R. M. Bullock, Inorg. Chem., 2013, 52, (4) I. Favier and E. Duñach, Tetrahedron Lett., 2004, 45,

5 Thermodynamic Schemes to Determine the CO2/CO and CO2/CH4 Couples Scheme S1 Thermodynamic cycle for determining the CO2 to CO reduction potentials in MeCN and DMF using the analogous approach to that for O2 to H2O. Scheme S2 Thermodynamic cycle for determining the CO2 to CH4 reduction potentials in MeCN and DMF using the analogous approach to that for O2 to H2O. 5

6 Using Vapor Pressure to Determine the Effects of Acid on Water in Organic Solvents (A) To determine the effect of acid upon the transfer free energy of water into organic solvents, the vapor pressure of water in MeCN was examined using gas phase IR spectroscopy by monitoring the change in intensity of H2O absorption modes in the cm -1 region. These experiments were acquired in transmission mode on a Bruker alpha IR spectrometer inside an N2 atmosphere glovebox using an 8 cm path length quartz cell. A 20 ml solution of 1 M H2O in MeCN was prepared by combining dry MeCN (19.64 ml) with H2O (360 μl). 1 ml of this solution was added to the solution reservoir of the quartz cell and sealed. An IR spectrum was then collected (Figure S2). The absorption intensity did not change over the course of 18 hours, confirming the cell had fully equilibrated. The temperature inside the spectrometer remained constant at 25 C. 80x M H 2 O/MeCN Baseline (Glovebox Atm) 60 Absorbance Wavenumber (cm -1 ) 4000 Figure S2 Gas phase IR spectrum of 1 M H2O dissolved in MeCN, where Baseline (Glovebox Atm) represents the spectrum taken in an empty quartz cell open to the glovebox atmosphere. 6

7 (B) The cell was removed from the glove box, cleaned, and dried under an N2 stream. [HDMF]OTf or HOTf was added to aliquots of the 1 M H2O/MeCN stock solution to produce solutions containing 1 M [(DMF)H] + OTf and 1 M HOTf. These solutions will be referred to as 1 M [(DMF)H] + OTf /1 M H2O/MeCN and 1 M HOTf/1 M H2O/MeCN respectively. The gas phase IR spectra for these solutions has a significant attenuation of water vapor absorption ( 10-fold decrease in absorption, Figure S3). 1 M DMF was then added to the 1 M [(DMF)H] + OTf /1 M H2O/MeCN solution (Figure S4). The water absorption increased to nearly half the original intensity. 80x M H 2 O/MeCN Baseline (Glovebox Atm) 1 M HDMF/1 M H 2 O/MeCN 1 M HOTf/1 M H 2 O/MeCN 80x M H 2 O/MeCN 1 M HDMF/1 M H 2 O/MeCN 1 M HDMF/1 M DMF/1 M H 2 O/MeCN Absorbance 40 Absorbance Wavenumber (cm -1 ) Wavenumber (cm -1 ) 4000 Figure S3 Gas phase IR spectrum of 1 M H2O dissolved in MeCN upon addition of acids. Figure S4 Gas phase IR spectrum of 1 M H2O dissolved in MeCN in the presence of 1 M [(DMF)H] + OTf and 1 M [(DMF)H] + OTf 1 M DMF. 7

8 (C) Similar experiments have been done using a higher resolution IR spectrometer outside of the glovebox, shown in Figure S5. Overall, these experiments suggest the vapor pressure of H2O over MeCN can change by 2-10 fold upon addition of acid, resulting in an error in the O2/ H2O(solv) couple of 9-30 mv. Figure S5 Gas phase IR experiments of 1 M H2O dissolved in acetonitrile. 8