A Kinetic Investigation of the Sustainable Electrocatalytic

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A Kinetic Investigation of the Sustainable Electrocatalytic Hydrogenation of Benzaldehyde on Pd/C. Effect of Electrolyte Composition and Half-cell Potentials Juan A. Lopez-Ruiz, Udishnu Sanyal, Jonathan D Egbert, Oliver Y. Gutiérrez, and Jamie Holladay ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02637 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Sustainable Chemistry & Engineering

A Kinetic Investigation of the Sustainable Electrocatalytic Hydrogenation of Benzaldehyde on Pd/C. Effect of Electrolyte Composition and Half-Cell Potentials

Juan A. Lopez-Ruiz, Udishnu Sanyal, Jonathan Egbert, Oliver Y. Gutierrez, and Jamie Holladay* Institute for Integrated Catalysis. Energy and Environment Directorate. Pacific Northwest National Laboratory 902 Battelle Blvd., Richland, WA 99352, USA

Corresponding author: Jamie Holladay Energy and Environment Directorate Pacific Northwest National Laboratory 902 Battelle Blvd., MSIN: K2-12 Richland, WA 99354, USA Tel.: +1 (509) 371-6692 E-mail address: [email protected]

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Highlights •

Selective electrocatalytic hydrogenation of benzaldehyde to benzyl alcohol on Pd/C.

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Electrocatalytic hydrogenation competes with the H 2 evolution reaction.

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Benzaldehyde coverage decreases as a function of cathodic potential.

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High benzaldehyde concentrations enable high coverages and high Faradaic efficiencies.

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A kinetic model following an Eley-Rideal type proton addition as rate determining step fits the kinetic data.

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Abstract Electrocatalytic reduction of benzaldehyde to benzyl alcohol on Pd supported on carbon felt was conducted in the aqueous phase using a continuous flow fixed-bed reactor at room temperature and atmospheric pressure. Methanol, ethanol, or isopropanol were added to the electrolyte to study the impact of alcohol type and concentration on the rates of benzaldehyde electrocatalytic hydrogenation (ECH) and H 2 evolution, which is the prevalent side reaction. Whereas the ECH rates and Faradaic efficiency decreased with increasing alcohol concentrations, H 2 evolution rates remained constant. The impact of the alcohol on hydrogenation was greater as the length of the alcohol’s hydrocarbon chain increased. Increasing the benzaldehyde concentration allows for high ECH rates, and high Faradaic efficiency. The reaction order increased from ~0.13 to ~0.66 with half-cell potential increasing from −650 mV to −1150 mV (vs. Ag/AgCl). A kinetic analysis reveals that the changes in reaction order are due to changes in benzaldehyde (and H) surface coverages as a function of half-cell cathodic potential. Thus, the results shown here reveal how the performance of the continuous electrocatalytic operation is affected by the electrolyte composition and half-cell cathodic potential. Keywords: electrocatalytic reduction; H 2 evolution; flow electrocatalytic cells; benzaldehyde reduction; Pd supported on carbon.

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Introduction The conversion of biomass into fuels and chemicals via pyrolysis or hydrothermal liquefaction generates organic streams containing high concentrations of oxygenated organic molecules. Upgrading compounds present in the organic stream is commonly done by hydrogenation at high temperatures and pressures.1-6 However, hydrogenation of biomass-derived organic molecules at high temperatures is challenging because of carbonaceous species formation by condensation of reactive compounds such as sugars, phenolics, aldehydes, and ketones. This coke formation results in catalyst deactivation and eventually to reactor plugging.4, 7 Hydrogenation catalysts can be regenerated by combustion followed by re-dispersion and re-impregnation of metals;8-9 however, the regeneration process is expensive and energy intensive. Additionally, the H 2 required for the hydrogenation reactions is usually produced from natural gas through endothermic reforming processes, which are not sustainable. For these reasons, technoeconomic analysis have consistently shown that bio-oil hydrogenation is the most energy and capitalintensive step in the transformation of bio-liquids to biofuels and bio-products.1,

5-6, 10-14

Therefore, to

make biofuels and bio-products economically viable, it is necessary to find alternative routes to hydrogenate the organic oxygenated molecules. Electrocatalytic hydrogenation (ECH) has been proposed as a promising, sustainable route for the reduction of biomass-derived organic molecules at normal conditions without the need to supply molecular hydrogen in situ hydrogen production is driven by electric potential.15 Because ECH operates at low temperatures and pressures, it eliminates much of the capital cost associated with conventional hydrogenation and has the potential to be an economically viable process. During ECH, protons are generated at the anode, which then transport to the surface of the cathode catalyst. The electrical energy can be provided from renewable energy sources such as solar and wind, thus making the ECH process sustainable. All biomass liquefaction technologies generate aqueous as well as immiscible organic

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streams both of which contain organic molecules requiring reduction. While most of the carbon content is in the organic phase, the aqueous phase contains up to 50% carbon.1, 5-6, 10-14 Recently, Weber and Holladay proposed a new process in which both organic and aqueous streams will be co-processed in the same cell;16 however, the use of a solvent such as an alcohol is required to ensure miscibility of the two phases. While some studies have focused on ECH of organic compounds in organic electrolytes,15,

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most published literature reports on ECH of organic molecules in aqueous

solutions. Because of their low solubility in pure water electrolytes, these studies have been carried out at low concentrations of organic molecules.18-21 Reported catalysts range from base metals such as copper and nickel to platinum group metals, which have superior performance.21-22 Song et al. showed that Pd is the most active metal, compared to Rh and Pt, and exhibited a Faradaic efficiency (FE, percentage of the current passed used for hydrogenation) of ≈100% for the reduction of benzaldehyde to benzyl alcohol when using an aqueous electrolyte.21 Further, they showed that the ECH rate increases as a function of the cathodic potential with minimal changes to the FE under those conditions. However, studies performed using electrolytes with high concentrations of alcohol and acids have shown FE and activity dependence with respect to the bulk concentration of benzaldehyde.23-31 For example, Polcaro et al.26, 31 reported that the FE for the ECH of benzaldehyde over Pd catalyst supported on carbon felt (CF) was below 10% when performed at low concentrations of benzaldehyde in an electrolyte containing ethanol.26 Further, Polcaro et al. showed that acidity of the hydro-alcoholic electrolyte affects the catalytic performance and reported an increase in catalytic activity by a factor of ≈2 when using an alkaline electrolyte instead of an acid electrolyte.31 Evidently, changes in reaction environment affect catalytic performance; however, such changes have been reported to also change the observed reaction mechanism from zero to positive order. For example, Song et al. reported that benzaldehyde ECH on Pd followed zero-order kinetics in an aqueous electrolyte,21 while Pt and Rh followed positive-order kinetics. However, Polcaro et al. reported that benzaldehyde ECH on Pd followed first-order kinetics in a hydro-alcoholic electrolyte, assuming

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Langmuir-Hinshelwood kinetics.26 The change in ECH kinetics could be associated with the difference in reaction environment, but substantial differences in electrode morphology and operation conditions make a direct comparison difficult. Therefore, if we are to advance in the implementation of the envisaged continuous electrocatalytic process, the response of catalytic performance to variations of wide ranges of electrolyte compositions and operational conditions must be studied and rationalized. To that end, we systematically studied the effect of several alcohols as co-solvents in the catholyte on the performance of Pd during the electrocatalytic reduction in continuous operational mode. Benzaldehyde was selected as the probe molecule because it is the simplest motif for aromatic aldehydes. The only reactions occurring at the cathode are ECH of the carbonyl group and H 2 evolution (Scheme 1). The effect of alcohol type and concentration of benzaldehyde on reaction rates, FE, and reaction order were systematically studied as a function of cathodic cell potential. O

OH

+ 2H+ + 2e

-

Benzaldehyde

Benzyl Alcohol +

2H + 2e

-

H2

Scheme 1. Reactions taking place during electrocatalytic reduction on Pd/CF at the studied conditions (room temperature, atmospheric pressure, and pH 3)

Materials and Methods Catalyst synthesis The cathode catalyst was prepared by incipient wetness impregnation of palladium (II) acetate, Pd(O₂CCH₃)₂, (Sigma-Aldrich, 98%) dissolved in tetrahydrofuran (Sigma-Aldrich, anhydrous, ≥99.9%) on a graphitic CF (Alfa-Aesar #43200). Prior to impregnation, CF coupons (~3 cm × 6 cm) were calcined in air at 673 K for 4 h at a heating rate of 7 K min-1. After cooling, the incipient wetness point was measured with tetrahydrofuran at 9 g of tetrahydrofuran per 1 g of CF. Following wet impregnation with

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the precursor, the CF was dried for 4 h at room temperature and then placed in a furnace for calcination under air. The catalyst was heated to 483 K at 1.5 K min-1 and kept isothermally for 1 h. Then it was heated to 673 K at 4.8 K min-1 and held isothermally for 2 h. Finally, the catalyst was allowed to cool to room temperature and stored. Catalyst characterization Powder X-ray diffraction (XRD) analysis from 2Ө = 20 to 80° was carried out on a Rigaku MiniFlex II X-Ray Generator with monochromatic Cu Kα-radiation (λ = 1.54056 Å) using a step size of 0.05°. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed on a Perkin Elmer Optima 7300DV. H 2 chemisorption was performed on a Micromeritics ASAP 2020. The adsorption isotherms were measured from 1 to 40 kPa at 313 K as described by Taylor et al.32 Scanning electron microscope (SEM) imaging was performed on a JEOL 5900LV FE-SEM with a Bruker XFlash 6|60 EDS detector. Transmission electron microscope (TEM) imaging was performed on a FEI Tecnai F20.

Figure 1. Diagram of fixed-bed electrocatalytic flow cell (A), compartment plates (B), and microchannel configuration of the compartment plate (C).

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Electrocatalytic reduction of benzaldehyde Electrocatalytic reduction of benzaldehyde (Sigma-Aldrich, 99%) was performed in a 10 cm2 fixedbed continuous flow electrocatalytic cell (Figure 1). The anode and cathode plate consist of a Cu lead, graphite plate (SGL, TF-6), and a polyether ether ketone frame with fluid porting (Figure 1B). The graphite plate separates the Cu lead from the reaction chamber to avoid contact of Cu with the liquid phase. The reaction chamber is designed to accommodate 6mm thick felt (Figure 1C) giving the reactor bed a volume of 6cm3. A Teflon gasket is used to seal the graphite and polyether ether ketone. NafionTM 117 was used as the membrane and was placed between the anode and cathode plates. The anode electrode was 0.1 g of Pt paper (Fuel Cell Store, 2mg Pt cm-2) and the cathode electrode was 0.50 g of 4wt% Pd/CF. Two Cole-Parmer gear pumps were used to feed anolyte and catholyte to the system at constant flow rates of 2cm3 min-1. For these experiments, the cathode is defined as the working electrode (W) and the anode the counter electrode (C). An AMETEK® VersaSTAT 4 Potentiostat Galvanostat was used to operate the cell and monitor the potential between the working and counter electrodes, which we refer as the full-cell (WvC) potential. A leak-free, 1 mm outer-diameter Ag/AgCl reference (R) electrode (Harvard Apparatus 6237653) was placed in the anode compartment to monitor the potential between the working and reference electrodes, which we refer as the half-cell (WvR) potential. When no samples were being taken, the reactor outlets were connected to a waste reservoir. During sampling, the cathode exit was connected to a 20 mL glass vial capped with a septum to separate liquid and gas products. The formation rate of the gas products was measured by connecting a bubble gas flow meter to the glass vial. A schematic of the system is depicted in Figure 2.

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Figure 2. Schematic of reaction system. Peristaltic pump photo copyright © Cole-Parmer. VersaSTAT 4 Potentiostat photo copyright AMETEK.

Before each experiment, the catalyst was reduced in situ under galvanostatic operation at −220mA (WvC= −1.80V, WvR= −1.60V) for 2 h using a catholyte composed of methanol:H 2 O (50:50 by weight) with 5 wt% acetic acid (Sigma-Aldrich, Glacial) and an anolyte composed of methanol:H 2 O (50:50 by weight) with 5 wt% acetic acid (Sigma-Aldrich, Glacial) and 5 wt% sodium acetate, CH 3 COONa, (Sigma –Aldrich, anhydrous ≥99%). After in situ reduction, the anolyte, catholyte, and current were adjusted to the desired reaction conditions. Steady-state operation was verified by monitoring the process parameters such as WvC, WvR, and reaction rate for 1 h after changing the reaction conditions. The system usually reached steady state within 15 min. To ensure steady-state operation, samples were typically taken 25 to 30 min after changing reaction conditions. Thus, 25 to 30 min after changing of parameters, samples were taken over a 10-min period. The experimental protocol was repeated for each current tested (−50, −75, −100, −125, and −150mA). The initial conditions were re-visited after all currents were tested. No catalyst deactivation was observed under the reaction conditions explored in this study.

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Several catholytes were used with varying compositions of alcohol, deionized (DI) water, and acetic acid. The alcohols used were methanol (Sigma-Aldrich, for high-performance liquid chromatography (HPLC), ≥99.9%), ethanol (Sigma-Aldrich, 200 proof, ACS reagent grade, ≥99.5%), and isopropanol (Sigma-Aldrich, ≥99.7%, FCC, FG). The different catholytes were prepared by keeping the molar ratio of H 2 O to acetic acid constant at 32. The relative composition between H 2 O and alcohol is reported using the mole fraction of H 2 O, which is defined as the moles of H 2 O divided by the sum of the moles of H 2 O and moles of alcohol. Catholytes with H 2 O mole fractions of 0.77, 0.90, 0.96, and 1.00 were used to perform the experiments. Table 1 summarizes the compositions and properties of the catholytes used. The addition of alcohols as co-solvents allowed us to explore benzaldehyde concentrations well above the solubility limit of benzaldehyde in H 2 O (28.3 mM at 293 K). Different concentrations of benzaldehyde (Sigma-Aldrich, ≥99.5%) were used, ranging from 20 to 180 mM. A minimum concentration of 20 mM was chosen as a lower limit to be consistent with previously published work using aqueous electrolytes. The maximum concentration of 180 mM was chosen as it equals the composition of individual oxygenated organic molecules found in bio-oils of ≈2.0 wt%.33-34 Diglyme (Sigma-Aldrich, ≥99%) was used as an internal standard in the catholyte at 40 mM. The anolyte was composed of 1 M KOH (SigmaAldrich, ACS reagent, ≥85%, pellet) in 10 wt% methanol (Sigma-Aldrich, HPLD, ≥99.9%) and a 90 wt% DI water solution. The gas products were analyzed with an Agilent GC 3000 A equipped with Mol Sieve and Plot U columns. A thermal conductivity detector was used to detect He, H 2 , O 2 , CO, CO 2 , and N 2 . The liquid phase was analyzed by HPLC equipped with a Waters 2414 refractive index detector. A Bio-Rad Aminex HPX-87H ion exclusion column (300 mm × 7.8 mm) was used for analyte separation. Sulfuric acid (0.005 M) was used as the eluent at a flow rate of 0.55 cm3 min-1. Under the reaction conditions used in this work, H 2 and benzyl alcohol were the only reduction products observed in the cathode.

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Table 1. Properties of catholyte as a function of alcohol type and alcohol content. The mole fraction of H 2 O to acetic acid was kept constant at 32. The concentration of benzaldehyde was 20 mM.

Catholyte DI H 2 O Methanol:H 2 O

Ethanol:H 2 O

Isopropanol:H 2 O

H 2 O mole fraction mol H2O mol (H2O+Alcohol) -1

Conductivity mS cm-1

1.00 0.96 0.90 0.77 0.96 0.90 0.77 0.96 0.90 0.77

1340 1140 667 218 1040 513 145 911 369 95.5

Measured pH 2.4 2.2 2.4 2.8 2.2 2.4 2.9 2.2 2.5 3.1

No conversion of benzaldehyde was observed in control experiments performed on a blank CF (in the absence of Pd) at WvR potentials below −1400mV or on the 4.0 wt% Pd/CF in the absence of an applied potential. Further, no catalyst deactivation was observed during our experiments even after exposing the catalysts to multiple reduction-operation cycles and different electrolyte compositions. Calculations of rates Under the reaction conditions operated in this study (298 K, 101 kPa, 2.0 cm3 min-1 anolyte and catholyte liquid feed rate, and currents between −50 and −150mA), the only observed products were H 2, and benzyl alcohol. Therefore, the rate calculations were done as follows: Equation 1

Equation 2

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Equation 3

Equation 4

Equation 5

The molar flow rate of benzaldehyde (F Benz ) was calculated using the benzaldehyde concentration measured by HPLC, Eq.1. The rate of benzaldehyde conversion (r Benz ) was calculated using the difference in concentration of benzaldehyde between the electrochemical reactor inlet (F Benz ,IN) and the outlet (F Benz ,OUT), Eq.2. Similarly, the rate of benzyl alcohol formation was calculated using the concentration of benzyl alcohol measured by HPLC in the electrochemical reactor outlet, Eq.3. Because the benzaldehyde ECH was 100% selective towards benzyl alcohol formation, the mole balance calculation was simplified as a ratio between (r Benz OH) and (r Benz ). Under the reaction conditions the mole and mass balances, Eq. 4 and 5 respectively, were between 98 and 102%. The turnover frequency (TOF) for the ECH of benzaldehyde (TOF ECH ) was calculated as a ratio between the rate of benzyl alcohol formation and the moles of Pd counted by H 2 chemisorption, Eq. 6.

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Equation 6

Equation 7

Equation 8

Equation 9

Faraday’s law was used to calculate the theoretical hydrogenation reaction current as shown in Eq.7. This was used to determine the FE for the benzaldehyde ECH, Eq.8. Under the reaction conditions used in this study, the FE ranged between 25 and 100%. The overall FE (i.e., electron balance) of the process was between 98 and 102% for all the conditions, therefore, the turnover frequency for the H 2 evolution reaction (HER) (TOF HER ) was calculated as a ratio between TOF ECH and FE as shown in Eq. 9. An alternative calculation for the TOF HER and the calculation for the overall FE can be found in the Supporting Information.

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Calculations of external mass transfer limitations Because of the negligible porosity of the CF used as a catalyst support, only the external mass transport limitations are discussed. As shown by Mears et al.,35 an analog to the Weisz-Prater36 criterion can be used to assess external transport limitations, Eq. 10. Equation 10

where r obs is the observed rate, R p is the radius of the carbon fibril (≈7.5 µm), ρ P is the density of the CF (0.14 g cm-3), k A is the external mass transfer coefficient, C AB is the bulk benzaldehyde concentration (20 to 180 mM), and n is the reaction order with respect to benzaldehyde bulk concentration. The approach used to calculate the external mass coefficient following González-García et al. is described in the Supporting Information.37 The reaction order in benzaldehyde changed from 0.13 to 0.66 as a function of the benzaldehyde and isopropanol (IPA) concentrations. Therefore, the assessment for external mass transfer limitations was calculated using an n of 0.66 (0.99. The dashed lines represent the data points operated under the same current.

As shown in Figure 7 for the catholyte composed of H 2 O:IPA with a H 2 O mole fraction of 0.96, there is a correlation between benzaldehyde concentration, TOF ECH , TOF HER , and the WvR potential. For example, at low WvR potential, the TOF ECH at the four different benzaldehyde concentrations (20 to 130 mM) were similar, suggesting that the reaction is near zero order with respect to benzaldehyde concentration. At this condition, the TOF HER was zero. However, as the WvR potential increased, the response of TOF ECH to increasing benzaldehyde concentration became stronger. That is, the reaction order changed from near zero to positive as a function of WvR potential. Further, the TOF HER had an inverse relationship with the benzaldehyde concentration; that is, the TOF HER decreased as the benzaldehyde concentration increased (FE is proportional to benzaldehyde concentration), suggesting that benzaldehyde and H compete for the same sites, therefore, the benzaldehyde surface coverage (θ Benz ) increases at the expense of θ H . Similar to the results shown in the previous section, the TOF ECH and

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TOF HER increase as the WvR potential increases, however, the FE decreased because the TOF HER increases at a faster rate than the TOF ECH . As shown in Figure 8, the TOF ECH and FE also increased as the benzaldehyde concentration increased when the catholyte had a H 2 O mole fraction of 0.77. The increase in benzaldehyde concentration from 20 to 180 mM completely inhibited HER and resulted in an FE increase from 25 to 100% when operating at WvR potentials between −900 and −1100mV. This result further supports the observation that the θ Benz increased as the benzaldehyde concentration increased, thus inhibiting θ H and TOF HER . Effect of bulk concentration of IPA and WvR on the reaction order. We have shown that the FE of the ECH of benzaldehyde is ≈100% at moderate WvR potentials (−650mV) and in the absence of alcohols (even in 20 mM benzaldehyde solutions) are in agreement with the results shown by Song et al.21 At these conditions, the TOF ECH does not depend on benzaldehyde concentration. However, when operating at WvR potentials from −650mV and −1200mV, we observed a dependence of TOF ECH and TOF HER on benzaldehyde concentration (Figures 7 and 8). We have attributed these observations to changes in θ Benz as a function of WvR potentials induced by competitive adsorption between benzaldehyde and H, which highlights the competition between HER and ECH reactions. We turn to the rate expressions derived for some possible ECH mechanisms shown in the Supporting Information. Because of the dependence of TOF ECH and TOF HER on the WvR potential and benzaldehyde concentration, we assumed that the observed rates are neither adsorption nor desorption limited. As our results are not influenced by mass transfer limitations, we assume that we are kinetically limited and that the adsorption of benzaldehyde and H+ are equilibrated. Tables 2 and 3 compare the dependence of reaction rates on benzaldehyde concentration for three mechanisms. A Langmuir-Hinshelwood (L-H) mechanism with competitive adsorption between benzaldehyde and H predicts negative orders at high benzaldehyde coverages regardless of the RLS for ECH. An L-H mechanism with non-competitive adsorption (benzaldehyde does not compete with protons for reduction sites) and an Eley-Rideal (E-R)

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mechanism predict the zero and first orders in benzaldehyde observed experimentally regardless of the RLS for the ECH reaction. We cannot differentiate between the non-competitive L-H and the E-R mechanisms because the rate expressions are very similar. However, our results suggest that the L-H with non-competitive adsorption is not a likely mechanism under our reaction conditions because we have observed complete inhibition of HER when operating at high concentrations of benzaldehyde (Figures 7 and 8). Thus, benzaldehyde and H most likely occupy the same kind of active site. This analysis suggest that E-R is the most likely reaction mechanism occurring under our working conditions, which is in agreement with the electrochemical step known as proton-coupled electron transfer.43 We note that this conclusion contrasts the accepted mechanism for hydrogenation of aromatic rings, where competitive L-H models are consistent with experimental observations.19-20, 44 There are several alternative mechanisms for the electrochemical hydrogenation of the polar carbonyl groups of aldehydes. If the addition of charges (H+ and e-) occur sequentially, the following additions could be considered: H+→e-→H+→e-, H+→e-→e-→ H+, e-→H+→e-→ H+, e-→H+→H+→e-.45 We ignore pathways that imply the accumulation of either two protons or two electrons as those should have very high energy states. None of those alternatives can be a priori ignored because charged intermediates such as protonated benzaldehyde and radical species (formed upon one e- addition) could be stabilized by the metal surface. However, we consider that a charged species should be very unstable and its neutralization must occur very rapidly so that, kinetically, the hydrogenation step can be modeled as a coupled, concurrent addition. This view is in agreement with a recent report by Cantu et al., in which a simultaneous proton-electron transfer following an E-R type proton addition was shown to be an energetically favorable pathway for ECH of aldehydes.46 Also, it is known that concerted proton-coupled electron pathways have lower energy barriers than sequential transfers.43

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Table 2.

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Summary of the effect of surface coverage of benzaldehyde on the reaction order for the three different mechanisms discussed assuming the RLS for ECH is the first H addition where k T is the kinetic constant of the hydrogenation step, K Benz is the absorption equilibrium constant of benzaldehyde, [Benz] is the concentration of benzaldehyde, K H + is the absorption equilibrium constant of H 3 0+, and a H3O + is the activity of H 3 0+.

High Surface Coverage of Benzaldehyde

Low Surface Coverage of Benzaldehyde

Negative Order

Positive Order

Zero Order

Positive Order

Rate Expressions Competitive Adsorption Langmuir-Hinshelwood

Non-Competitive Adsorption LangmuirHinshelwood

Eley-Rideal Zero Order

Positive Order

Regardless of the ECH reaction mechanism, the empirical kinetic power law expression shown in Eq. 11 allows us to infer the reaction order with respect to benzaldehyde concentration at different reaction conditions. This expression will be used as guidance to discern between the possible reaction mechanism described in Tables 2 and 3. Equation 11

where

is the normalized kinetic constant,

is the bulk concentration of benzaldehyde,

reaction order.

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is the

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Table 3.

Summary of the effect of surface coverage of benzaldehyde on reaction order for the three different mechanisms discussed assuming the RLS for ECH is 2nd H addition where k T is the kinetic constant of the hydrogenation step, K Benz is the absorption equilibrium constant of benzaldehyde, [Benz] is concentration of benzaldehyde, K H+ is the absorption equilibrium constant of H 3 0+, and a H3O + is the activity of H 3 0+.

Rate Expressions

High Surface Coverage of Benzaldehyde

Low Surface Coverage of Benzaldehyde

Negative Order

Positive Order

Zero Order

Positive Order

Competitive Adsorption LangmuirHinshelwood

Non-Competitive Adsorption LangmuirHinshelwood

Eley-Rideal Zero Order

Positive Order

As shown in Figure 9, the reaction order (n) changed from 0.13 to 0.61 as the WvR potential shifted from −650 to −1150mV for the electrolyte with 1600 mM IPA. However, when the IPA content in the electrolyte was increased to 7300 mM, the reaction order remained constant at ≈0.66 as the WvR potential changed from −950 to −1150mV. These results show that the reaction order with respect to benzaldehyde concentration changes from near zero to 1 under our reaction conditions as a function of WvR potentials, as the derived E-R expression showed. Because the L-H mechanism with competitive adsorption predicted a negative order with respect to benzaldehyde at high benzaldehyde coverages, we speculate that L-H with competitive adsorption is not a likely mechanism.

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Figure 9.

Relationship between the WvR potential and reaction order (n) of benzaldehyde to benzyl alcohol at two different IPA concentrations. Symbol legend: ▲ represents 1600 mM IPA, ● represents 7300 mM IPA, and ∆ represents extrapolated data at IPA concentrations of 1600 mM. The activity results used to determine the reaction order as a function of WvR potential were interpolated and extrapolated using the relationship found between TOF ECH and the WvR potential shown in Figures 7 and 8. Calculations of the interpolated and extrapolated results can be found in the Supporting Information. The determination of the reaction order using the empirical kinetic power law expression as a function of WvR is shown in Figures S7 and S8.

According to the rate expressions for the E-R mechanisms (Tables 2 and 3), the increase in reaction order in benzaldehyde with increasing concentration of IPA and WvR potential are most likely due to a decrease in the contribution of the surface concentration of benzaldehyde (K Benz ∙ [Benz]) relative to the contribution of the surface concentration of H (K H+ · a H3O+ ). Interestingly, the TOF ECH at low benzaldehyde concentrations (20 mM) when using a catholyte with a H 2 O mole fraction of 1 and 0.96 (Figures 6 and 7, respectively) is the same for all the WvR potentials tested, supporting our hypothesis that IPA does not compete for surface sites. However, decreasing the H 2 O mole fraction from 0.96 to 0.77 (i.e., increasing the IPA concentration by 4.6 times from 1600 mM to 7300 mM) decreases the TOF ECH by a factor of 4.5. This suggests that the increase in IPA concentration either 1) changes the

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transition state of the RLS for ECH (so that the rate constant k T decreases), or 2) decreases a H3O+ . Performing a kinetic analysis of the experimental results will determine which explanation is correct.

Scheme 3.

Possible elementary steps involved in ECH of benzaldehyde following an Eley-Rideal type H addition.

As shown in Tables 2 and 3 and in Scheme 3, the obtained rate expressions for the E-R mechanism have the same dependence on benzaldehyde concentration regardless of the RLS for ECH; therefore, we cannot discern which H addition step is the RLS for ECH. However, we can illustrate the effects of WvR potential on the benzaldehyde surface coverage (θ Benz ) and rate constant (k’). With the simple mathematical treatment shown in Eq. 12, the rate expression can be used to obtain a linear form. The full derivation can be found in the Supporting Information. Equation 12

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where:

, n = reaction order with respect to

Page 30 of 37

,

, and

Figure 10 summarizes the effect of WvR potential on k’ and k’’ and reveals that k’, which is a direct function of k T and (α H2O+ ), increases linearly with more negative WvR potential. Further, the k’ obtained in the electrolyte with low IPA concentration is ≈4 times higher than that obtained in the electrolyte with high IPA concentration under all the WvR tested. However, k’’, a reciprocal function of θ Benz (and θ H ), is the same regardless of the IPA concentration as a function of WvR potential and decreases with increasingly negative WvR potential. Thus, the increase in reaction order in benzaldehyde shown in Figure 9 and concomitant decrease in FE with increasingly WvR potential are due to the surface competition between benzaldehyde and H (i.e., benzaldehyde being replaced by H at the surface).

Figure 10. Determination of k’ (A) and k’’(B) as a function of the WvR potential for a catholyte composed of IPA and H 2 O with a H 2 O mole fraction of 0.96 (●)and 0.77 (■) using an E-R rate expression shown in Eq. 11. The O symbol represents the extrapolated results for a H 2 O mole fraction of 0.96. The activity results used to determine the k’ and k’’ values as a function of WvR potential were interpolated and extrapolated using the relationship found between TOF ECH and the WvR potential shown in Figures 7 and 8. The calculation of the interpolated and extrapolated results can be found in the Supporting Information. The k’ and k’’ values reported here were calculated using the first-order kinetic analysis shown in Figure S12.

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The surface competition between benzaldehyde and H is quantitatively described in Figure 11, which shows how θ Benz decreases at the expense of θ H as a function of WvR potential. However, increasing concentrations of benzaldehyde partially compensate for the competitive adsorption of H+ and decreases

θ H . Therefore, as shown in Figure 10A, the differences in TOF ECH values observed under different electrolyte compositions at a constant WvR potential and benzaldehyde concentration are not a result of differences in θ Benz . Rather, they are due to differences in k T and (a H3O+ )n, with the latter being the most likely reason due the decrease in x H3O+ caused by the addition of higher concentrations of alcohols (Table 1). These findings highlight that the alcohols used in this study as co-solvents do not compete for the active sites; however, they negatively affect the ECH of carbonyl groups by decreasing a H3O+ .

Figure 11. Surface coverage of benzaldehyde, θ Benz (A), and surface coverage of H, θ H (B), as a function of benzaldehyde concentration and the WvR potential for the reduction of benzaldehyde to benzyl alcohol over 4 wt% Pd/CF catalyst under normal conditions. The catholyte was composed of isopropanol and H 2 O at H 2 O mole fraction of 0.96. Different concentrations of benzaldehyde were used where ● represents 20 mM, ■ represent 45 mM, ♦ represents 90 mM, ▲ represents 130 mM, and × represents 180 mM. The activity results used to determine θ Benz and θ H as a function of WvR potential and benzaldehyde concentration are shown in Figure 10.

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Conclusions This study addresses the impact of electrolyte composition and half-cell potential on ECH, which is a promising technology for the reduction of the most unstable compounds in bio-oil. We investigated the reduction of benzaldehyde, a motif of aromatic aldehydes, in the presence of alcohols as they will be used as a co-solvents to enable bio-oil processing. Under the conditions of our investigation, ECH of benzaldehyde competes with the HER on carbonsupported Pd. The presence of alcohol has a negative impact on hydrogenation rates and FEs, which we concluded originated from a decrease in the activity of H 3 O+ ions. That is, the presence of alcohol dilutes the H 3 O+ concentration, thereby slowing down ECH rates. Accordingly, increasing alcohol concentrations decreases ECH rates while HER rates remained constant when operating at constant halfcell cathodic potential. Increasing half-cell cathodic potentials increases ECH rates, but also enhances HER rates to a greater extent, which in turn decreases FE. A first-order kinetic analysis following the Eley-Rideal mechanism shows that changes in ECH and HER rates as a function of half-cell cathodic potentials are due to changes in benzaldehyde and H surface coverages. Therefore, the findings reported in this work allow responses the prediction of ECH and HER rates and FEs on carbon-supported Pd catalysts to varying electrolyte composition.

Acknowledgements The research described in this paper is part of the Chemical Transformation Initiative at Pacific Northwest National Laboratory (PNNL), conducted under the Laboratory Directed Research and Development Program at PNNL. We gratefully acknowledge the help of Miroslaw Derewinski, Sebastian Prodinger, Katherine Koh, and Nathan Canfield at PNNL for their help with the catalyst synthesis and characterization. We also acknowledge the helpful discussion with Asanga Padmaperuma, Abhi Karkamkar, Donald Camaioni, Johannes A. Lercher, Michael Lilga, Nirala Singh, and Robert Weber at PNNL. PNNL is a multi-program national laboratory operated by Battelle for the U.S. Department of Energy.

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Supporting Information The Supporting Information is available free of charge on the website at DOI: •

Additional explanation of calculation of TOF and Faradaic efficiency as a function of WvR potential.

•

Additional plot showing catalyst characterization by XRD.

•

Additional plot showing Tafel slopes.

•

Additional derivation of rate expressions, rate constants, and surface coverages.

•

Additional plots showing determination of reaction orders and rate constants.

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For Table of Contents Use Only

Synopsis. This work evaluates the electrocatalytic, H 2 -free hydrogenation of biomass-derived oxygenates as a sustainable upgrading process compared to traditional thermal hydrotreating.

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A Kinetic Investigation of the Sustainable Electrocatalytic

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