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Selective Electrochemical Oxidation of Lactic Acid using Iridium-based Catalysts Chi Chen, Aaron James Bloomfield, and Stafford W. Sheehan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b05073 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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Industrial & Engineering Chemistry Research

Selective Electrochemical Oxidation of Lactic Acid using Iridium-based Catalysts Chi Chen,† Aaron J. Bloomfield†‡, and Stafford W. Sheehan§†* †

Catalytic Innovations LLC, 151 Martine Street, Fall River, Massachusetts 02723, United States. ‡

Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States.

§

Waste Hub, 11 Broadcommon Road, Suite 341, Bristol, Rhode Island 02809, United States.

ABSTRACT Catalysts that allow for selective oxidation of organic compounds in solution are useful tools in chemical synthesis, waste remediation, and renewable energy storage schemes. Of these, iridium oxide and iridium-based molecular compounds are frequently employed as catalysts to oxidize substrates ranging from small organic molecules to water. Here, we study the mechanism and evaluate the potential of iridium-based catalysts for small molecule oxidation in wastewater treatment, using lactic acid as a substrate. By investigating the reaction products, kinetic isotope effects, and electrochemical properties of iridium oxide and a molecular analogue on a high surface area antimony-doped tin oxide (ATO) support, we find evidence for a new iridiumcatalyzed pathway for lactic acid oxidation. Applying this at concentrations representative of effluent from dairy production processes, we show that iridium oxide can selectively remove small amounts of lactic acid from water by oxidizing it to CO2 with concomitant cathodic production of H2 by proton reduction. 1 ACS Paragon Plus Environment


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1. INTRODUCTION Economic, social, and environmental issues brought about by excessive use of fossil fuels have spurred researchers to search for ways to make carbon-neutral or carbon-negative renewable fuels.1, 2 In many cases, research in renewable fuels is viewed as part of an energy storage scheme for intermittent sources of renewable energy, such as solar or wind energy, and in others it is seen as a pathway to make value-added products from carbon dioxide using artificial photosynthesis.3 The ultimate goal is to develop a renewable fuel to offset fossil fuels. However, due to the vast quantity of oil and natural gas reserves, as well as the low cost of exploration and refining at scale compared to developing a renewable fuel infrastructure, renewable fuel technologies are not likely to dethrone fossil fuels in the foreseeable future.4 In order to utilize the vast knowledge and research that has been invested in renewable fuels in the present, new value propositions have to be found for the technologies that researchers in this field have developed. For example, research in both precious and nonprecious metal catalysts for water oxidation is widespread,5-7 but there are very few applications for water oxidation catalysts outside a laboratory setting. Iridium-based catalysts are being used for oxidative chemical synthesis and wastewater treatment, but do not draw the same attention as when used for renewable fuels.8, 9 In these cases, the scarcity of iridium is not a major factor because its use is restricted to niche applications or, in the case of larger-scale uses, a very small amount of the material is able to suffice due to its high catalytic activity and resistance to corrosion when compared to nonprecious metals.10 This emphasizes approaches that maximize activity of small amounts of precious metal catalysts for cost-effective utilization, a prominent research direction in this field.11, 12 Using lessons like these, that have been learned from research in renewable fuels, we can find new value for water oxidation catalysts by applying these materials to supply protons and 2 ACS Paragon Plus Environment

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electrons in other fuel-generating reactions. Primarily in the waste-to-value industry, there are numerous easily oxidizable wastewater streams,13 and many have not received as much attention as they are having impact on the environment. Therefore, waste-based renewable fuels may be a particularly promising avenue toward bringing renewable fuel research to industry. One type of wastewater that is both widespread and a major environmental problem is a byproduct of Greek yogurt production, acid whey.14 For every pound of milk used to make Greek yogurt, only 25%-33% ends up in the final product.15 The remaining byproduct is acid whey, a mixture of water, inorganic ionic species, protein, lactose, and lactic acid that has a pH too acidic for land application, degradation in anaerobic digesters, or other widely used disposal methods. Over 160 million tons of dairy whey waste is produced annually, and volumes are constantly increasing to keep up with consumer demand.14 The primary contaminant in acid whey that causes its acidity and increases its economic and environmental cost of disposal is lactic acid.16 Therefore, it is advantageous to find energy efficient, low-cost methods of oxidizing or otherwise removing lactic acid from wastewater. In this article, we examine an electrochemical method for the oxidative conversion of lactic acid to carbon dioxide. We propose a mechanism for lactic acid oxidation on the surface of iridium-based catalysts previously studied for water oxidation,17-19 to generate to CO2 and H2 by: 3H2 O + C3 H6 O3 → 3CO2 + 6H2

(1)

We tested our hypothesis using electrochemical and physical methods to determine coordination environments and analyze products. In doing so, we found that iridium oxide is capable of oxidizing lactic acid with good selectivity and at industrially-relevant rates. Electrochemical experiments provide evidence that lactic acid is oxidized on the surface of iridium oxide directly via hole transfer from the metal center, rather than through a hydroxyl radical. Furthermore, a single-site heterogeneous molecular analog of iridium oxide was used as a tool to determine 3 ACS Paragon Plus Environment


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similarities and differences between well-studied mechanisms on the surface of iridium oxide, such as water oxidation.

2. EXPERIMENTAL SECTION 2.1 Materials and Methods. High purity deionized water supplied from a Millipore purification system was used in all electrochemical experiments. KNO3 (≥99.9% purity), D2O (99.9 atom % D, glass distilled), and pyruvic acid (98%) were supplied by Sigma Aldrich and used as received. NaIO4 (99.9% purity) was purchased from Acros Organics and used as received. Lactic acid (ACS Reagent, 85.0-90.0% aqueous solution) was purchased from Alfa Aesar and used as received. Other chemicals were purchased from major suppliers and used as received. All electrochemical measurements were performed with the working and counter electrode separated by a glass frit. Three-electrode electrochemical experiments were taken on a Pine AFCBP1 Bipotentiostat with a platinum mesh counter electrode and Ag/AgCl reference electrode (BASi Inc.). Two-electrode experiments were performed using a BK Precision DC power supply and a platinum mesh counter electrode. Cyclic voltammograms were taken without stirring, so that mass transport was consistent across all samples and proportional to current passed. Tafel plot data (3 minute dwell times, average of steady state current) and chronoamperograms were taken with gentle stirring. Aliquots (400 µL) of electrolyte were removed, spiked with a solution of maleic acid in deuterium oxide (100 µL) and analyzed by 1H NMR using Agilent NMR spectrometers operating at 400.13 MHz . All resonances in the 1H NMR spectra are referenced to residual protiated water (δ 4.80 ppm). Integration of anolyte peaks were adjusted relative to the maleic acid internal standard, which was of known concentration.

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2.2 Antimony-doped Tin Oxide (ATO) Nanoparticle Synthesis. ATO nanoparticles were synthesized according to literature procedures, with minor modifications.20 To a round bottom flask equipped with a stir bar, tetradecylamine (TDA, 95.0%, 1.0 g, Sigma Aldrich) was added to an ethanol solution (EtOH 50 mL and H2O 128 mL) followed by vigorously stirring at room temperature under air for 3 hours. Tin tetrachloride (SnCl4, 99.995%, 3.8 g, Sigma Aldrich) solution in ethanol (16 mL) was added dropwise to TDA suspension and the mixture was stirred for one additional hour. The reaction mixture was then added dropwise to ammonium hydroxide solution (1.5 mmol/L, 160 mL, Sigma Aldrich). The resulting suspension was stirred at room temperature for one hour and then refluxed at 80 ˚C for 3 days. After cooling down to room temperature, the white precipitation was separated by centrifugation at 5000 rpm for 10 min and then washed with ethanol (25 mL) 5 times. The wet samples were then transferred to a glasslined stainless-steel autoclave and hydrothermally (HT) treated at 120 ˚C for 24 hours. The product was washed again with ethanol (25 mL) 5 times after HT treatment. The resulting pale tan product was then dried in a freeze dryer (-1 ˚C, 950 min, 40 mTorr) and calcined at 400 ˚C for 3 hours under air. The resulting powder was added to ethanol in a 4:1 ethanol to ATO w/w ratio, sonicated for two minutes, and stirred for 24 hours to form a slurry. 2.3 Electrode Preparation. The slurries were doctor-bladed on conductive glass slides comprised of a thin film of fluorine-doped tin oxide (FTO) coated on 2.2 mm thick glass (sheet resistance: 7 Ω/sq, TEC 7, Hartford Glass Co., Inc.). A 1 cm2 area was framed out on the slides using tape (3M brand Scotch Magic tape) and a thin layer of ATO paste was swept across the surface using a razor blade. The tape was carefully removed, and the slides were left to dry at room temperature. After drying, the electrodes were heated to 500 °C for 1 hour, with a ramp rate of 8 °C per minute in a Paragon SC-2 ceramic furnace, in order to sinter the particles 5 ACS Paragon Plus Environment


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together. Analysis of the ATO nanoparticulate films was carried out using a JEOL JSM 5610 Scanning Electron Microscope and a TriStar II Plus surface area and porosity analyzer. For GCMS experiments, electrodes were contacted with copper wire using conductive epoxy (Fast Setting Conductive Silver Epoxy, SPI) and coated using a non-conductive water resistant marine epoxy (Loctite) and allowed to cure for 24 hours. 2.4 Catalyst Preparation. IrOx was anodically deposited onto the conductive oxide nanoparticle working electrodes in a standard three-electrode electrochemical cell with a platinum counter electrode using procedures previously reported in scientific literature.21, 22 In brief, a 25 mL solution of 2.1 mM [Cp*Ir(H2O)3]SO4 in a 0.1 M KNO3 aqueous supporting electrolyte was used for IrOx electrodeposition. Electrolysis at 2.5 V vs Ag/AgCl for 60 seconds was employed to deposit a layer of IrOx on the mesoporous working electrode. For the molecular catalyst, a previously reported syntheses was used.18, 23 A 1 mM solution of homogeneous [2(pyridine-2-yl)-2-propanato]Ir(IV) dimer was synthesized by oxidation of a 5 mM solution of Cp*Ir(pyalc)OH precatalyst with 100 molar equivalents of NaIO4, followed by stirring for 30 minutes. Electrodes were immersed in this solution overnight and the [2-(pyridine-2-yl)-2propanato]Ir(IV) dimer heterogenized to form a conformal monolayer coating of the molecular iridium catalyst. Platinum metal was electrodeposited on ATO working electrodes in a standard three-electrode cell from a solution of acidic aqueous 24 mM H2PtCl6. Electrolysis at -0.5 V vs Ag/AgCl for 60 seconds resulted in formation of a thin layer of Pt black on the mesoporous conducive oxide film.24 2.5 Gas chromatography-Mass spectrometry (GC-MS) Experiments. GC-MS analyses were performed on a PerkinElmer Clarus 580 gas chromatograph coupled to a Clarus SQ 8S mass spectrometer equipped with PerkinElmer COL-VELOCITY-5 column (30 m * 0.32


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mm ID. 0.25 μm) using helium as the carrier gas. The analysis used 5 mL inj. of sample, inj. temp of 35 °C, initial inlet pressure was 7.8 psi, but varied as the column flow was held constant at 5.0 mL/min for the duration of the run, the interface temperature was held at 35 °C, and the ion source (EI, 70 eV) was held at 250 °C. Oven temperature was held at 35 °C for 9.00 min with detector on from 0.00 min. Total run time was 9.00 min.

3. RESULTS AND DISCUSSION Synthesis of an acid-stable and conductive ATO nanoparticle support followed prior reports,20 using a TDA template to achieve high surface area and maximize the utilization of iridium. Brunauer-Emmett-Teller (BET) surface area of the nanoparticles was measured to be 122 m2/g. Antimony-doped tin oxide particles are known for their use in wastewater treatment, since at high potentials (>2 V) ATO does not perform water oxidation; rather, it selectively forms hydroxyl radicals on its surface that are capable of catalyzing organic electrooxidation reactions themselves.25, 26 However, this is highly dependent on the surface properties of the ATO that is being used, since its activity toward radical generation and stability are dependent on its surface properties and processing conditions.27 Therefore, care must be taken when using ATO as a scaffold to perform control experiments that ensure that it does not complicate investigation of other species on its surface. ATO does not suffer from the same acid instability as tin-doped indium oxide (ITO) does which makes it particularly desirable for electrochemical oxidation in acidic solutions.28 Cyclic voltammograms of bare ATO electrodes in 0.1 M KNO3 with 1% and 10% v/v lactic acid present showed that these particles do not have high activity for organic oxidation in the range of 0.2 – 1.8 V vs. the standard hydrogen electrode (SHE, Figure S1). However, they do possess a high 7 ACS Paragon Plus Environment


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capacitive background current due to their high surface area. No features indicative of Sb surface segregation that lead to surface instability are detected, therefore we expect this material to be stable under electrochemical conditions.27 IrOx coated ATO electrodes, denoted IrOx/ATO consist of a small (~5 nm) layer of IrOx anodically deposited on the mesoporous ATO using previously published procedures.17, 21 This electrodeposition approach allows for controlled synthesis of high surface area IrOx, which is necessary since planar films of similar thickness do not possess high enough electroactivity to collect meaningful data (Figure S2). We found that IrOx and its molecular analogues are active catalysts for lactic acid oxidation, yet no mechanism has been suggested or proposed for iridiumbased species despite their high activity and selectivity. Unlike the oxidation of acetic acid and formic acid,29-31 mechanistic details for the electrochemical oxidation of lactic acid are not frequently studied in general. In 1981, G. Horányi proposed a mechanism involving hydroxyl radicals which can be generated on the surface of platinum electrodes by oxidizing water.32 In these prior studies, pyruvic acid was produced as an intermediate before it was further oxidized to acetic acid with carbon dioxide as a byproduct (Mechanism I in Scheme 1).


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Scheme 1. Proposed mechanistic pathways for lactic acid oxidation that are tested in this study.

We propose two other possible mechanistic pathways (Mechanism II and III) based on our understanding of Ir-catalyzed oxidation of water and organic compounds.33-35 During water oxidation, Ir accesses labile oxidation states that are capable of oxidizing coordinated organic ligands via an inner-sphere pathway.23, 36 Similarly, we propose a mechanism where lactic acid is present in the first coordination sphere and is oxidized by holes transferred through the iridium center. In Mechanism II, the Ir catalyst is first coordinated with lactic acid at a hydroxyl group, 9 ACS Paragon Plus Environment


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followed by β-hydride elimination resulting in a pyruvic acid-Ir complex. Proton-coupled electron transfer (PCET) through the Ir catalyst and isomerization of ligated pyruvic acid leads to a κ2-coordination complex, which can decarboxylate under oxidative electrochemical conditions. The resulting Ir-acetyl complex can then react with a hydroxyl radical or its equivalent to generate acetic acid. In Mechanism III, the lactic acid is first coordinated to Ir in a κ2 pattern through a carboxylate group. CO2 can then be eliminated under electrochemical conditions, giving a secondary Ir-alkyl complex. β-hydride elimination of the Ir-alkyl complex then results in an Iracetaldehyde complex. Next, in situ oxidation of Ir-aquo is used to generate a hydroxyl equivalent, which can be then transferred either in an intermolecular or intramolecular rearrangement to the Ir-acetaldehyde, forming a hemiacetal Ir complex. A final β-hydride elimination of the hemiacetal results in the desired acetic acid on Ir center. This pathway is unique from Mechanisms I and II in that it provides an explanation of electrochemical oxidation of lactic acid to acetic acid without forming pyruvic acid. To test these mechanisms, we performed electrochemical experiments on lactic acid concentrations within an order of magnitude of 1% v/v. We chose this concentration range because the concentration of lactic acid in the acid whey wastewater that we seek to remediate is typically near 1%,15 so that our experiments on a standardized lactic acid solution with 0.1 M KNO3 as a supporting electrolyte may be translatable to the end application. Furthermore, this ensures that we are probing the mechanism and collecting data using concentrations that are meaningful for wastewater treatment, to control for any mechanistic variation that may be present at lower or higher molarities. We investigated solutions at concentrations (v/v) ranging from 0.1% to 10%, which corresponds to molarities from 0.013 M to 1.342 M. Tables S1 and S2


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show conversion factors as well as measured and calculated pH values for each of these solutions. Cyclic voltammograms (CVs) were taken in each of these solutions in a two-chamber electrochemical cell separated by a glass frit, using a IrOx/ATO working electrode and Ag/AgCl reference electrode in the anode chamber, as well as a Pt mesh counter electrode. The anolyte was the lactic acid solution at varied concentrations, and the catholyte was 0.1 M KNO3 in deionized water. Over the time scale of these experiments, we found that no lactic acid diffused through the frit into the cathode chamber, therefore the cathode chamber remained free of residual organic material. Figure 1 shows the CVs of the IrOx/ATO working electrode in the lactic acid solutions of different concentration, with control CVs done on 0.1 M KNO3 solutions of equivalent pH shown in Figure S3. In both cases, the Ir(III)/Ir(IV) redox wave appears at E1/2 = 0.9 V vs SHE, is quasi-reversible for all traces (ΔEp = 125 ± 10 mV), and migrates toward higher potentials with pH as expected for Nernstian behavior. The catalytic wave, however, is different from that of water (Figure S3). The anodic and cathodic pre-features between 1.1 V vs. SHE and 1.4 V vs. SHE that indicate oxidation from Ir(IV) to catalytically active Ir(V),33 and its reduction on the return sweep both gradually disappear with increasing lactic acid concentration.



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Figure 1. (a) Cyclic voltammograms of an IrOx/ATO electrode taken at 50 mV/s in a 0.1 M KNO3 solution with increasing concentrations of lactic acid present, ranging from 0% (black), 0.1%, 0.5%, 1.0%, 5.0%, to 10% (red) v/v. Inset: Black arrows indicate disappearance of Ir(V) features, red arrow indicate appearance of lactic acid oxidation feature. (b) Plot of the lactic acid concentration versus anodic peak current at 1.7 V vs. SHE normalized to the 0% lactic acid sample for each electrode tested (red) to accentuate sharp decrease in peak current at 1% lactic acid concentration. This phenomenon is reproducible across different scan rates. Control anodic peak currents (black) were taken at equivalent pHs without lactic acid present (see Figure S3 and Table S2 for full CVs and data).

Furthermore, it would be expected that replacing a difficult-to-oxidize substrate, such as water, with a more easily oxidizable compound like lactic acid would increase the intensity of the catalytic wave starting at 1.3 V vs. SHE. While a feature does gradually build in at 1.4 V vs. SHE, the catalytic wave assigned to water oxidation is surprisingly decreased between 0.1% and


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1% v/v lactic acid concentrations, as shown in Figure 1b, before recovering at higher concentrations of lactic acid. One explanation consistent with this behaviour is coordination of lactic acid to Ir(IV) open coordination sites, as we propose in both Mechanisms II and III. Since lactic acid is a weaker electron donor than hydroxide, this would destabilize the Ir(V) state and increase the potential at which the Ir(IV)/Ir(V) transition takes place. Since Ir(V) is the active oxidation state in the Ir-catalyzed water oxidation cycle,19 destabilizing Ir(V) in this manner would inhibit water oxidation, as we see in Figure 1b. This also provides reasoning for IrOx’s selectivity for lactic acid over water when the latter is present in much higher concentration. When 5% v/v lactic acid is reached, the concentration of lactic acid is high enough to provide significant catalytic current from its oxidation, thus increasing the magnitude of the catalytic wave over that of the 1% v/v lactic acid solution. While this may be a reasonable explanation, proving conclusively that lactic acid coordinates to iridium and inhibits water oxidation on a heterogeneous catalyst surface is a challenging task. One major issue is the variety of active sites present on the surface of IrOx, therefore it would be useful to find a way in which we could probe only one active site at a time. To provide further evidence for our hypothesis, we employ a molecular analogue of IrOx, a heterogenized [2-(pyridine-2-yl)-2-propanato]Ir(IV) dimer with comparable activity for water oxidation, as a single-site IrOx analog.18 Cyclic voltammograms in Figure 2 show that lactic acid may have an effect on the reversibility of the Ir(III)/Ir(IV) redox wave that is difficult to discern using IrOx alone. Integrals of the pseudocapacitive Ir(III)/Ir(IV) waves in each cyclic voltammogram indicate that there is approximately 15 nmol/cm2 of electroactive iridium present on IrOx-containing electrodes and 64 nmol/cm2 present on electrodes functionalized with the 13 ACS Paragon Plus Environment


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molecular dimer, consistent with prior studies that predict higher electroactivity for the molecular system.15 The increase in peak separation (ΔEp) as the scan rate is varied on IrOx shows that the redox feature is indeed quasi-reversible for both bulk and (to a greater extent) molecular catalysts. Stabilization of Ir(IV) by lactic acid may contribute to hindered electron transfer kinetics, however, any attempt to explain this using only voltammetric analysis may be highly speculative and we caution reading too much into this data. Our future work will include spectroelectrochemistry and electrochemical impedance spectroscopy to further elucidate these phenomena.

Figure 2. (a) Cyclic voltammograms taken at varied scan rates of an IrOx/ATO electrode in a 1% v/v lactic acid solution with the following ΔEp, 10 mV/s: 26 mV; 20 mV/s: 45 mV; 50 mV/s: 98 mV; 100 mV/s: 166 mV. (b) A cyclic voltammogram taken at 50 mV/s of the [2-(pyridine-2-yl)2-propanato]Ir(IV) dimer on the same thickness of ATO in a 1% v/v lactic acid solution. The H/D kinetic isotope effect (KIE) can also be used to test a single-site catalyst such as the [2-(pyridine-2-yl)-2-propanato]Ir(IV) dimer. It is already documented that this molecular


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catalyst typically possesses a KIE of 1 while IrOx has a KIE > 1 for the water oxidation reaction.22 For electrooxidation of small organics, KIE studies are rarely pursued and neither of the prior studies of Pt-catalyzed lactic acid oxidation performed this investigation.32, 37 Therefore, we found it useful to investigate Pt in parallel as well. Mechanistic studies for Pt-catalyzed water oxidation,38, 39 and studies of H/D KIE for methanol C-H bond oxidation in water using a Pt electrode,40, 41 suggest that we would find a KIE > 1 for Pt-catalyzed hydroxyl radical generation consistent with Mechanism 1. For the iridium-based catalysts, Mechanism 2 allows a non-unity H/D isotope effect for both the molecular catalyst and IrOx due to the hydroxyl radical equivalent required in its final step, while Mechanism 3 avoids hydroxyl radicals altogether. In the latter case, we would expect to find a KIE of unity for the molecular catalyst and IrOx (in contrast to the metal oxide’s behavior for water oxidation). Tafel plots for these systems are shown in Figure 3 both in H2O and D2O. If the ratelimiting step for lactic acid oxidation involved the generation of a hydroxyl radical, we would expect there to be a KIE greater than 1, which is the case for electrodeposited Pt in Figure 3c as predicted by Mechanism 1. However, in contrast to their kinetics for water oxidation, where IrOx and the molecular catalyst behave differently, these two catalysts both exhibited no discernible H/D isotope effect in the presence of lactic acid. For the case of the iridium-based catalysts, this distinguishes the rate limiting step between lactic acid oxidation (which the KIE shows does not involve proton transfer) and water oxidation (which does involve proton transfer on IrOx), and provides evidence against a hydroxyl radical present in the rate limiting step.



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Figure 3. Tafel plots of (a) IrOx, (b) molecular dimer catalyst, and (c) Pt in 1% lactic acid solution in D2O (red) and H2O (black) on an ATO support. No significant KIE > 1 (within experimental error) is observed for the Ir-based catalysts at low potentials where lactic acid oxidation dominates, while Pt exhibits a KIE > 1.

The most significant difference between the mechanism proposed for lactic acid oxidation on platinum surfaces (Mechanism 1) and the experimental evidence that we have for lactic acid oxidation on iridium-based catalysts is in the products detected by NMR. Across many experiments, we do not see any evidence of pyruvic acid as an intermediate in this reaction despite detecting even trace amounts of formic acid, in contrast to the same reaction on Pt.37, 42 Using GC-MS, we are able to detect and identify CO2 evolved from the iridium catalyst in the anode chamber above background and control levels, as shown in Table 1. The gas evolved in the cathode chamber was identified to be H2 by a flame test, and since no organics diffused through the frit from the anode chamber to the cathode chamber as determined by NMR, the total volume of gas evolved from the cathode is presumably H2. Our current work using functional


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polymer electrolyte membrane (PEM) electrolyzers further explores mass balance and quantification of product gases in an industrially-relevant system.

Baseline ATO Control ATO + IrOx a

Time (hr)

Lactic Acid (mM, NMR)

Acetic Acid (mM, NMR)

N2, m/z = 27.90 (GC-MS)

CO2, m/z = 43.93 (GC-MS)

0 1.0 1.0

127.1 121.3 115.4

0 < 0.2a 0.4

100% 100% 100%

0.1% 1.3% 6.7%

Peak integration near detection limit.

Table 1. Mass spectrometry analysis of carbon dioxide in the anode chamber with concomitant NMR analysis of the solution-phase products, taken at approx. 1.8 V vs SHE using a three-electrode electrochemical cell with an Ag/AgCl reference. All reactions were performed using 1% lactic acid in D2O so that lactic acid removal could be confirmed via NMR. MS counts relative and normalized to N2 are reported. Baseline sample was collected after a 15 min N2 flush prior to the start of electrolysis. CO was not detected for any samples. A flame test was used to confirm the presence of hydrogen in the cathode chamber.

To further test our proposed mechanisms, CVs of equivalent (0.134 M) pyruvic acid, acetic acid, and formic acid solutions in 0.1 M KNO3 (Figure S4-S6) show that pyruvic acid is oxidized at lower potentials than any other components of this system, with an onset potential around 1.1 V vs SHE (compared to 1.3 V vs. SHE for acetic acid and 1.2 V vs SHE for formic acid), therefore it is difficult to fully rule out a mechanism involving very rapid generation and subsequent oxidation of pyruvic acid. However, since both formic and pyruvic acids are oxidized at potentials below that of lactic and acetic acids, and we can generate detectable amounts of formic acid without any trace of pyruvic acid, a mechanism that relies on pyruvic acid as a longlived intermediate is unlikely. 17 ACS Paragon Plus Environment


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Figure 4. NMR spectra of a 1% lactic acid solution before and after electrolysis. 28% of the lactic acid present (peaks at 1.4 ppm and 4.4 ppm) is consumed, with acetic acid (2.05 ppm) detected as a major product.

Bulk electrolysis experiments were performed to assess catalyst stability, efficiency, and search for reaction intermediates to give further insight into our mechanistic understanding. Figure 4 shows 1H NMR spectra over the course of one such experiment. Maleic acid (6.4 ppm) was used as a standard while peaks representative of lactic acid (1.4 ppm, 4.4 ppm) and acetic acid (2.05 ppm) were monitored to determine the progress of the reaction. To control for substrate-specific reactions and to generate detectable amounts of trace catalytic intermediates,


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thicker ATO films of larger particle size (Figure S7 and Figure S8), which allowed for larger catalytic currents on the 1 cm2 geometric surface area anode, were also used. Bulk electrolysis experiments show selectivity for lactic acid is retained at applied potentials up to 2.7 V vs. SHE. Faradaic efficiencies were calculated to be approximately 55% for lactic acid removal assuming lactic acid not converted to acetic acid forms CO2, however, this value may not be accurate without precise quantification of CO2 generated from lactic acid that could be achieved using isotopic labelling experiments. Still, this shows promise for further optimization of this reaction. Across repeated experiments, the major products of electrolysis were found to be CO2, followed by acetic acid, with trace formic acid, and no pyruvic acid present. Because of this, we propose that Mechanism III or one similar is the most likely route by which this reaction takes place on iridium-based catalyst surfaces.

CONCLUSIONS Selective electrooxidation of lactic acid using iridium-based heterogeneous electrocatalysts has been demonstrated, with the major end products being carbon dioxide and hydrogen gas. Electrochemical experiments at varied solution concentrations of lactic acid provide evidence for a reaction that proceeds through Ir(IV) rather than an Ir(V) pathway, the latter of which being indicative of iridium-catalyzed water oxidation. By evaluating the anolyte prior to complete oxidation of lactic acid and its derivatives, we found acetic acid and formic acid to be the primary reaction intermediates with no trace of pyruvic acid present. Using the insight gained from these experiments, we propose a new mechanism for lactic acid oxidation that proceeds via direct coordination of lactic acid to iridium rather than through the formation of



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hydroxyl radicals. The use of an acid-stable support based on mesoporous ATO to reduce the loading of precious metal catalysts on anodes for electrolysis allows for efficient utilization of iridium and helps to improve the scalability of this process.

SUPPORTING INFORMATION Lactic acid conversion tables, control experiments, SEM images, NMR spectra, and additional figures are available in the Supporting Information. This material is available free of charge at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Correspondence should be addressed to Stafford W. Sheehan (S.W.S.). E-mail: [email protected]. Tel: +1 (720) 262-9950. Notes The authors declare the following competing financial interest(s): US Patent Applications 14/317,906 and 15/283,403 by S.W.S. contain work described in this paper. ACKNOWLEDGEMENTS This contribution was identified by Prof. Aaron Fafarman (Drexel University) as the Best Presentation in the session “INOR: Environmental & Energy-Related Inorganic Chemistry” of the 2016 ACS Fall National Meeting in Philadelphia, PA. This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2016-33610-25355. We thank Prof. Paul Anastas, Prof. Robert Crabtree, 20 ACS Paragon Plus Environment

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Selective Electrochemical Oxidation of Lactic Acid ... - ACS Publications

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