Simultaneous Electrosynthesis of Syngas and an Aldehyde from CO2

Oct 11, 2018 - Cyclic voltammograms (CVs) of the electrode are shown in. Figure 1a with (red line) and without (black line) 10 mM. PhCH2OH in 0.5 M ac...
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Simultaneous Electrosynthesis of Syngas and an Aldehyde from CO2 and an Alcohol by Molecular Electrocatalysis Ying Wang, Sergio Gonell, Ulaganatha Raja Mathiyazhagan, Yanming Liu, Degao Wang, Alexander J. M. Miller, and Thomas J. Meyer* Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States

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S Supporting Information *

ABSTRACT: A tandem cell for artificial photosynthesis with CO2 and water as the oxidants and an organic alcohol as the reductant is described. The use of molecular catalysts with high activity and selectivity, in an appropriate cell configuration, leads to electrochemical reduction of CO2 and water to CO and H2 (syngas) in tandem with benzyl alcohol oxidation to benzaldehyde. A faradaic efficiency (FE) of ∼70% for the formation of benzaldehyde was obtained with simultaneous syngas generation with varying ratios of H2 and CO at the cathode. The maximum energy efficiency obtained for the electrochemical cell was 17.6%. KEYWORDS: CO2 reduction, alcohol oxidation, electrocatalysis, artificial photosynthesis, molecular catalyst, electrochemical synthesis

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ethanol,5 methanol,6 syngas,7,8 formate,9,10 and acetate.11,12 Most cells couple the CO2RR half-cell reaction with the oxygen evolution reaction (OER). In a previous publication from this group,13 it was demonstrated that CO2 reduction and water oxidation could occur with a single bifunctional, singlesite Ru molecular electrocatalyst (1, Scheme 1), [RuII(tpy)(Mebim-py)(H2O)]2+ (tpy = 2,2′:6′,2″-terpyridine; Mebim-py = 3-methyl-1-pyridylbenzimidazol-2-ylidene). On the anodic side, alcohol oxidation was targeted as a prototype reaction. Alcohols are widely available, both from naturally occurring biomass sources and from petroleum refining, acting as precursors to a range of valuable chemicals. Oxidation of alcohols to aldehydes is particularly promising due to the considerable value of aldehydes in organic synthesis and the fragrance industry. The lower potentials for alcohol oxidation, compared to water oxidation, also help to avoid

lectrochemical syntheses typically target the product(s) generated at a “working” electrode, while the ill-defined or unwanted products generated at the “counter” electrode are discarded. An alternative approach, one that is more atomeconomical and energy efficient, emphasizes the value of all products of electrolysis, aiming to produce multiple high-value products simultaneously from an anode and a cathode while minimizing waste and unwanted byproducts. Only a handful of well-defined systems that produce valuable chemicals at both electrodes have been reported, however. A notable recent example comes from Berlinguette and co-workers, who reported CO2 reduction in tandem with TEMPO-mediated organic oxidation.1 In this work, molecular catalysts are employed in a tandem cell for CO2 reduction and alcohol oxidation, taking advantage of the ability of molecular systems to carry out well-defined reactivity at discrete potentials.2−4 In considering different reactions that could be performed in a tandem cell, we targeted reactions that would shift the formal oxidation states of abundant organic feedstocks based on successful molecular catalysis in individual half-reactions. Electrolyzers for the carbon dioxide reduction reaction, CO2RR, are a potential source of carbon-containing products relevant to the energy sector and chemical industry, including © XXXX American Chemical Society

Special Issue: New Chemistry to Advance the Quest for Sustainable Solar Fuels Received: September 28, 2018 Accepted: October 11, 2018

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DOI: 10.1021/acsaem.8b01616 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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The catalyst was immobilized on high surface area nano-ITO electrodes with a 2.4 μm thick layer of 15−20 nm diameter ITO nanoparticles. The resulting nano-ITO electrodes were loaded with a surface coverage of ∼17 nmol/cm2 (geometric area = 1.2 cm2), as determined by peak current measurements of the RuIII/II catalyst wave (E1/2 = 0.8 V vs NHE in acetonitrile, 0.1 M in NaClO4), Supporting Information Figure S1 and eq S1.15 Cyclic voltammograms (CVs) of the electrode are shown in Figure 1a with (red line) and without (black line) 10 mM PhCH2OH in 0.5 M acetate buffer at pH = 3.8, 100 mV s−1 at room temperature. The oxidative wave at E1/2 = 0.75 V vs NHE is assigned to the RuIII−OH2+/RuII−OH22+ couple on the basis of the known pKa values for RuII−OH22+ (11.3) and RuIII−OH23+ (2.5).16 A second oxidation with a small peak current, Ep,a ∼ 1.1 V, is attributed to partial oxidation of RuIII− OH2+ to RuIVO2+, which is kinetically limited by slow proton loss.14 On the reverse scan, proton-coupled reduction of RuIVO2+ to RuIII−OH2+ (Ep,c = 0.93 V) is followed by a second electron/proton transfer which returns the catalyst to RuII−OH22+ (E1/2 = 0.75 V).16 The onset potential for electrocatalytic benzyl alcohol oxidation appears near ∼1.1 V at pH 3.7 (as shown in Figure 1a), the potential at which RuIII−OH2+ is oxidized to RuIV O2+. Water oxidation, on the other hand, requires further oxidation to RuVO3+, which does not occur until ∼1.65 V.16−18 Thus, alcohol oxidation is thermodynamically favored over water oxidation, providing a potential window for selective oxidation of the organic alcohol, eq 2.14 The standard potential for the PhCH2OH/PhCHO couple, eq 2, is E2o'= −0.1 V at pH = 5 (E2o = 0.21 V vs NHE at pH = 0), which is much lower than the standard potential for the 2H2O/O2 couple, E°′ = 0.94 V at pH = 5 (E° = 1.23 V vs NHE at pH = 0). 13 The calculations are shown in the Supporting Information.

Scheme 1. Tandem Cell Catalysts for Anodic and Cathodic Reactionsa

a Compound 1, [(tpy)(Mebim-py)Ru II (OH 2 )] 2+ , catalyzes CO 2 reduction and compound 2, [Ru(bis-Mebimpy)(4,4′((OH)2OPCH2)2-bpy)(OH2)]2+, catalyzes Ph−CH2OH oxidation.

possible degradation pathways caused by intermediates in the oxygen evolution half-reaction.1,2 In the experiments described here, we have coupled reduction of CO2 to CO and H2 by electrocatalyst 1 with oxidation of benzyl alcohol (PhCH2OH) to benzaldehyde (PhCHO) by the surface-bound molecular catalyst 2 (Scheme 1), [Ru II (Mebim-py)(4,4′-(OH) 2 OPCH 2 ) 2 -2,2′-bpy](OH)2]2+, (bis-Mebimpy = 2,6-bis(1-methylbenzimidazol-2yl)pyridine, (4,4′-(OH)2PO−CH2)2-2,2′-bpy) = 4,4′-bismethlylenephosphonato-2,2′-bipyridine).14 Selective alcohol oxidation to benzaldehyde was observed without overoxidation to the carboxylic acid or water oxidation to O2. For oxidation of the alcohol, the required potential at the anode was lower than the potential for water oxidation under the conditions by ∼400 mV, and the applied cell voltage is about 1 V less than the voltage required to couple CO2RR with water oxidation.13 The molecule-based electrochemical cell achieves simultaneous electrosynthesis of benzaldehyde and syngas, CO/H2, shown as a 1:1 mixture in eq 1.

PhCH 2OH → PhCHO + 2H+ + 2e−

CO2 + 2PhCH 2OH → CO + 2PhCHO + H 2O + H 2

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(2)

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(1)

REDUCTION OF CO2 Electrochemical reduction of CO2 was investigated in both water and in anhydrous acetonitrile using the previously reported catalyst 1.7,19 Figure 2a shows CVs for catalyst 1 in MeCN, 0.1 M in NaClO4. Figure 2b shows CVs for catalyst 1

OXIDATION OF BENZYL ALCOHOL The anodic alcohol oxidation was modeled after prior work that demonstrated selective oxidation of PhCH2OH to PhCHO by a nano-ITO electrode modified with catalyst 2.14

Figure 1. (a) Cyclic voltammograms (CVs) of nano-ITO|2 with (red line) and without (dark line) 10 mM Ph−CH2OH in 0.5 M acetate buffer (pH = 3.76) at 100 mV s−1, room temperature, and electrode surface area of 1 cm2. (b) Structure of intermediate oxo, RuIV=O2+, formed upon oxidation at 1.1 V. B

DOI: 10.1021/acsaem.8b01616 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 2. Cyclic voltammograms for catalyst, 1, (a) in 0.25 mM MeCN and 0.1 M NaClO4 and (b) in aqueous 0.5 M KHCO3 with 1 atm CO2 (red line) under N2 (black line). The CVs were recorded at freshly polished glassy carbon electrodes (0.071 cm2) at 100 mV s−1 at room temperature. (c) Summary of reactivity for 1 ([Ru(tpy)(Mebim-py)(H2O)]2+) illustrating pathways for water reduction to hydrogen (black) and CO2 reduction to CO (red).7,8

in water and 0.5 M aqueous KHCO3. The electrochemical response for solutions of 1 were compared under an atmosphere of N2 (dark line) and under 1 atm of CO2 (red line) at freshly polished glassy carbon electrodes. In both solvents, there is clear evidence for interactions with added CO2, as shown by the appearance of catalytic waves at ∼−1.3 V in the CVs under an atmosphere of CO2. In organic solvents, even with added H2O, the catalyst is very selective for the formation of CO.7,19 However, in aqueous solutions, catalyst 1 produces syngas mixtures of H2 and CO, with the ratio of products precisely tuned by control of solution pH and applied potential.7 This difference in reactivity in water arises because both reactions share a common intermediate, a Ru(0) complex that can either react with CO2 to give CO or with water to give an intermediate hydride that releases H2 upon protonation (Figure 2c).7 The competing half-reactions are shown as reductions in eqs 3 and 4 with E°3 = −0.12 V vs NHE and E°4 = 0 V vs NHE under standard conditions. At pH 7, E3°′ = −0.53 V and E4°′ = −0.41 V.

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CO2 + 2H+ + 2e− → CO + H 2O

(3)

2H+ + 2e− → H 2

(4)

Scheme 2. Schematic Diagram for the Two Compartment, Anion Exchange Membrane (AEM) Separated Electrochemical Cell for Simultaneous Production of H2/ CO and benzaldehydea

a

The cathode is carbon cloth (A = 1 cm2) with 1 mM 1 and 1 atm of CO2 in 0.5 M KHCO3 with 0.5 M Na2SO4 (pH = 7.2). The anode is nano-ITO|2 (A = 1.2 cm2), with 0.1 M PhCH2OH in 0.5 M acetate buffer at pH = 5, 0.5 M in Na2SO4.

charge carrier across the cell. The anode consisted of the modified electrode, nano-ITO|2 in 0.1 M PhCH2OH, in a 0.5 M acetate buffer at pH = 5.0, in 0.5 M Na2SO4. The cathode was a carbon cloth electrode, with 1 mM catalyst 1 in CO2 saturated 0.5 M KHCO3 solution at pH = 7.2 and 0.5 M Na2SO4. In the overall reaction in eq 1, shown with CO and H2 as the products, the distribution between CO and H2 varies with reaction conditions. The pH was constant at both compartments over the time scale of the experiments. The anodic potential was monitored by a multimeter, referenced to a Ag/AgCl (saturated KCl) electrode, throughout the experiment to establish an operating window. For the

ELECTROLYSIS The two half-reactions discussed above, reduction of CO2/CO to 2H+/H2 and oxidation of benzyl alcohol to benzaldehyde, were carried out in tandem in a single electrochemical cell to achieve the net chemical reaction of eq 1 (above) wherein CO2 oxidizes benzyl alcohol to produce benzaldehyde, carbon monoxide, and dihydrogen. The cell configuration used in the electrolysis experiments is shown in Scheme 2. The two compartments were separated by an anion exchange membrane (FAA-3-PK-75) that enables SO42− to act as the C

DOI: 10.1021/acsaem.8b01616 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 3. (a) Amount of CO (red) and H2 (black) produced at the cathode and (b) faradaic efficiency for benzaldehyde during bulk electrolysis for 5400 s with the cell configuration shown in Scheme 2 at different cell voltages. (c) Bulk electrolysis of the anodic current at a cell voltage of 2 V for 5400 s. (d) Energy efficiency calculated on the basis of eq 5 at different operating voltages.

electrode reactions, the potential differences were dictated by the half-reactions at the electrodes. With the metal−solution interface used for the electrodes, it was difficult to obtain fixed electrode potentials in the cell configuration used. The absolute voltage at each electrode is not reported since it varied during the experiment. Figure 3a shows the aqueous CO2 reduction products, CO (red) and H2 (black), produced at the cathode with the cell voltage varied from 1.6 to 2.2 V over a period of 1.5 h. Operating potentials less than 2.6 V were used to avoid background water oxidation at the anode and hydrogen production at the cathode, Scheme 2. The applied cell voltage is about 1 V smaller compared to the cell of CO2 tandem with water oxidation.13 The faradaic efficiency (FE) for benzaldehyde formation, Figure 3b, was ∼70% at the operating potentials used. For CO2 reduction, the relative yields of CO and H2 were dictated, in part, by background hydrogen production with the highest ratio of CO to H2 reached at 0.5:1 at 1.6 V. Syngas production was enhanced as the total cell potential increased, with the ratio H2/CO varying from 0.5:1 at 1.6 V to 2:1 at 2.2 V. The total FE for the cathodic products was ca. 30−40% with losses due to cell design or insufficient driving force for the release of the gas phase products. The variation in syngas ratio as the potential was increased due to background hydrogen production as the potential of the cathode was increased. The potential of the electrode was ∼−1.1 V at the cathode at the end of a bulk electrolysis applying a 2 V cell voltage.

ε=

E 0 FE × 100% Vcell

(5)

The energy efficiency (ε) can be calculated based on eq 5 suggested by Whipple and Kenis.20 As shown in Figure 3d, the energy efficiency for the cell varied from 8.0% to 17.6% depending on the operating potential (detailed calculation in the Supporting Information). The most efficient performance was obtained at 1.8 V with energy efficiency of 17.6% (Table S4). As stated, a higher voltage more than 2.2 V is avoided to prevent water oxidation occurring at the anode. Considering both the anode and cathode reactions, the best performance was found at 1.8 V giving a syngas (H2/CO) ratio of 0.5:1, and faradaic efficiency and energy efficiency to benzaldehyde of 70.8% and 17.6%, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01616. Experimental details, synthetic procedures, cyclic voltammograms, and calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alexander J. M. Miller: 0000-0001-9390-3951 D

DOI: 10.1021/acsaem.8b01616 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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(16) Chen, Z.; Vannucci, A. K.; Concepcion, J. J.; Jurss, J. W.; Meyer, T. J. Proton-coupled electron transfer at modified electrodes by multiple pathways. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, E1461−E1469. (17) Concepcion, J. J.; Tsai, M.-K.; Muckerman, J. T.; Meyer, T. J. Mechanism of Water Oxidation by Single-Site Ruthenium Complex Catalysts. J. Am. Chem. Soc. 2010, 132, 1545−1557. (18) Chen, Z.; Concepcion, J. J.; Jurss, J. W.; Meyer, T. J. Single-Site, Catalytic Water Oxidation on Oxide Surfaces. J. Am. Chem. Soc. 2009, 131, 15580−15581. (19) Chen, Z.; Chen, C.; Weinberg, D. R.; Kang, P.; Concepcion, J. J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J. Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes. Chem. Commun. 2011, 47, 12607−12609. (20) Whipple, D. T.; Kenis, P. J. A. Prospects of CO2 Utilization via Direct Heterogeneous Electrochemical Reduction. J. Phys. Chem. Lett. 2010, 1, 3451−3458.

Thomas J. Meyer: 0000-0002-7006-2608 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This materials is based upon work solely supported as part of the Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001011.

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DOI: 10.1021/acsaem.8b01616 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX