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A Combined Experimental and Theoretical Study on the Activity and Selectivity of the Electrocatalytic Hydrogenation of Aldehydes David C. Cantu, Asanga B. Padmaperuma, Manh-Thuong Nguyen, Sneha A Akhade, Yeohoon Yoon, Yang-Gang Wang, Mal-Soon Lee, Vassiliki-Alexandra Glezakou, Roger Rousseau, and Michael A. Lilga ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00858 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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A Combined Experimental and Theoretical Study on the Activity and Selectivity of the Electrocatalytic Hydrogenation of Aldehydes David C. Cantua#&, Asanga B. Padmaperumab#, Manh-Thuong Nguyena, Sneha A. Akhadea, Yeohoon Yoona, Yang-Gang Wanga, Mal-Soon Leea, Vassiliki-Alexandra Glezakoua, Roger Rousseaua*, Michael A. Lilgab* a

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland WA, 99352

b

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland WA,

99352

ACS Paragon Plus Environment

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ABSTRACT

A detailed mechanistic study of the electrochemical hydrogenation of aldehydes is presented toward the goal of identifying how organic molecules in solution behave at the interface with charged surfaces and what is the best manner to convert them. Specifically, this study focuses on designing an electrocatalytic route for ambient temperature post-pyrolysis treatment of bio-oil. Aldehyde reductions are needed to convert biomass into fuels or chemicals. A combined experimental and computational approach is taken toward catalyst design to provide testable hypotheses regarding catalyst composition, activity, and selectivity. Electrochemical hydrogenation mechanisms for benzaldehyde and pentanal reduction are found to proceed by a coupled proton-electron transfer process. Initial results show that Au, Ag, Cu, and C catalysts exhibit the highest conversion to alcohol products. These catalysts are suitable because they show high cathodic onset potentials for H2 formation and low cathodic onset potentials for organic reduction. Conversion of aromatic aldehydes is found to be appreciably higher than aliphatic ones. Classical molecular dynamics simulations of solvent and substrate mixtures in an electrolytic cell were performed to assess how species concentrations vary at the solid/liquid interface and in the bulk as a function of applied voltage. Results show that an increase in voltage in the electrolytic cell decreases organic and increases water mole fractions at the solid/liquid interface. In this current study, charged cathodic surfaces result in carbonyl orientations at the surface that do not favor electron transfer. Repulsion of organic substrates to the bulk must be compensated by strong adhesion to the electrode surface. Implications on catalyst choice and process design are discussed.


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KEYWORDS Pyrolysis Oil upgrading, Atomistic Simulations, Electrocatalysis, Aldehydes, Electrochemical reduction, benzaldehyde, pentanal 1. INTRODUCTION A great challenge in today’s energy economy is eliminating carbon and environmental footprints associated with the use of petroleum, while satisfying an expanding demand for fuels, power, and chemical products. One approach is to replace or supplement petroleum-based carbon sources with non-traditional renewable and carbon-neutral sources, such as biomass or municipal wastes. The United States Department of Energy estimates that well over a billion tons of biomass (including agricultural, forestry, waste, and algal materials) will be available in the contiguous United States1. If converted to biofuel, biopower, and bioproducts, this amount of biomass is estimated to be sufficient to displace approximately 30% of the U.S. petroleum consumed in 2005. There are many obstacles to realizing the use of biomass for energy applications. For use in modern infrastructure, solid biomass must be converted to liquid form. In addition, biomass feeds are oxygen rich and hydrogen poor, yet high energy fuels are oxygen poor and hydrogen rich. The amount of oxygen that can be tolerated depends on the fuel or power application, but invariably the oxygen content of raw biomass must be reduced to be viable. Therefore, multi-step processes that deconstruct, liquefy, deoxygenate, and increase the hydrogen content are central to successful biomass utilization for energy purposes. Besides these chemical transformations, the distributed nature of biomass is an added complication. Unlike petroleum, which is pumped from a point source and easily transported via pipelines, biomass is distributed over landscape scales and transportation constitutes a large economic and logistical factor. Small-scale, distributed processing that liquefies, densifies, and


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stabilizes biomass would help solve many logistical issues, but economies of scale are lost. For example, while hydrogen generation and hydrotreating are most economically conducted at a large scale2, wide distribution of small hydrogen generation plants and hydrogenation facilities is economically and operationally unlikely. In addition, a small-scale distributed process cannot rely on the same degree of efficient heat integration which underlies the overall efficiency of large scale processes such as those in refining. One approach to liquefaction and densification of biomass is fast pyrolysis.

Fast pyrolysis

(450 – 520 °C; 30 ms residence time) and the rapid condensation of vapors formed, creates a liquid bio-oil (or pyrolysis oil) product as a precursor to biofuels and chemicals3. Bio-oil is a complex micro-emulsion of aqueous and non-aqueous phases containing hundreds of organic compounds. Compared to hydrocarbons derived from petroleum, bio-oils have higher oxygen, moisture, and acid contents4-5 and require upgrading to remove oxygen before use in the conventional transportation fuel infrastructure4, 6-10. Nuclear magnetic resonance analyses show that bio-oils contain carboxylic, carbonyl, phenolic, aromatic/olefinic, etheral, alcoholic, and aliphatic carbons11-13. Several of these functional groups, such as acids, phenolics, and aldehydes are reactive, which complicates bio-oil storage, transportation, and downstream processing, as reviewed by Diebold and co-workers14. Aldehydes are believed to be particularly troublesome, being able to react with phenolics to form resins, or oligomerize to higher molecular weight compounds via aldol condensation reactions.

Organic acids and chloride salts may act as

catalysts for some of these reactions. Bio-oil upgrading is limited by polymerization reactions that affect process stability and longevity15. Its thermal instability forms coke, which blocks catalyst sites and plugs reactors during treatment at elevated temperatures. For example, when bio-oil is heated to 100 °C during


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upgrading or to remove water, solids rapidly form that contain about 50 wt% of the original biooil. In addition, aging occurs during storage due to secondary condensation and polymerization reactions that increase bio-oil viscosity and cause solids to form. To minimize coking, a multistep process using pressurized hydrogen is often employed in which bio-oil is first stabilized by hydrogenation of the most reactive functionalities at relatively low temperatures, followed by further upgrading and deoxygenation under successively more harsh conditions. For example, in a three-stage method [SOT]16,17 bio-oil is first stabilized at relatively mild conditions of 140 °C and 1200 psig H2 over a Ru-based catalyst, which reduces species that cause downstream fouling. Additional species reduction occurs in the second bed at 252 °C and 2000 psig H2, also over a Ru catalyst. Near complete deoxygenation occurs in the third bed operated at 400 °C and 2000 psig H2 over a Mo-based catalyst. As an alternative to high pressure and temperature processing, we are studying electrochemical hydrogenation (ECH) for bio-oil stabilization under low or ambient temperature and pressure conditions. Bio-oil ECH might allow more economical processing by replacing precious metal catalysts. It may also enable distributed processing18-19 where bio-oil is stabilized closer to the widely disbursed biomass resources, allowing stabilized bio-oils to be transported for further upgrading at centralized facilities for production of transportation fuels or chemicals. Distributed hydrogen plants would not be needed and handling of high pressure hydrogen would be avoided. In addition, the scale of electrochemical facilities is appropriate for distributed processing where economies of scale are not possible15. Electrochemical hydrogenation has a long history and its application to biomass-related compounds is increasing20-25. Electrocatalytic upgrading of biomass derivatives such as lignin and lignin derived phenolic compounds into chemical grade hydrocarbons has been previously


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attempted26-29. Sugar hydrogenation, in particular conversion of glucose to sorbitol, has been demonstrated via electrolysis20,

30-31

. Electroreduction of activated organic acids has been

conducted to yield aldehydes, which reduces more easily to the alcohol32-33. For instance, lactic acid reduction on carbon-supported Ru using an aqueous electrolyte under mild operating conditions predominantly produced lactaldehyde33. Benzoic acid serves as an example of an aromatic carboxylic group that can be reduced all the way to the alcohol, aided by electron density from the benzene ring34. The reduction of organic acids attached to fully saturated molecules is difficult32. Esters can be reduced to aldehydes or ethers depending on the reaction conditions35. Aldehyde and ketone reductions are shown to be generally more facile35-39. In all cases, the ability of the organic to undergo hydrogenation strongly depends on the functional group, the nature of the electrocatalyst, and the surrounding chemical environment (electrolyte composition, pH). ECH reactions occur primarily by way of surface phenomenon at a solvated interface. And so the extent of charge transfer to the organic species and consequently the efficiency and selectivity towards hydrogenation, is reliant on the (i) adsorption of the organic species to the catalyst surface and (ii) their interaction with the electrolyte. Several studies have demonstrated the role of surface modifications and/or functionalization of electrocatalyst supports in altering the electrode surface properties to promote or impede organic adsorption22,40-42. St. Pierre and coworkers reported that reticulated glassy carbon cathodes modified with silica particles and vapor-deposited Ni nanoparticles served to increase the conversion of cyclohexanone to cyclohexanol and was attributed to stronger adsorption of cyclohexanone on the electrodes22. Curtiu et al. demonstrated higher efficiencies in hydrogenation of benzophenone with aluminasupported Pd compared to finely divided unsupported Pd catalysts41. In situ grafting of the


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alumina surface with organic functionalization using aliphatic monocarboxylic acids showed a strong variation in the ECH efficiency with the acid chain length that reportedly served as modulators for the adsorption of phenol40. Li et al. recently reported that Ru supported on activated carbon cloth displayed higher activity towards reduction of phenol, guaiacol and syringol when synthesized via cation exchange than incipient wetness owing to the support surface functionalization by oxidation pretreatment42. These studies strongly demonstrate how the nature of the cathode materials alter the ECH efficiency, emphasizing the need to develop a molecular scale understanding of the interfacial factors that govern the adsorption and the extent of charge transfer between the electrode and the target organic molecules. This sets up a critical knowledge gap that needs to be filled in order to rationally design catalysts and optimize the operating reaction conditions. The nature and type of solvent can also critically impact the catalyst activity and selectivity towards hydrogenation reactions43-48. While many investigations are primarily conducted for aqueous phase thermal hydrogenation, in an electrochemical setting, the polarity and pH of the solvent are likely to have an exaggerated impact under the influence of interfacial charging. The distribution and H-bonding behavior of the solvent molecules will strongly vary with the electrode potential and alter the interactions and rate of charge transfer between the target organic molecules and the electrode surface. Several previous studies have investigated the impact of the solvent on the electroreduction process of aromatic aldehydes49-61. In aqueous and aquo-alcoholic media, two successive one-electron waves during reduction are observed in the presence of acid: the first wave corresponding to the formation of a pinacol (fast process k ~ 0.1 cm s−1)53, with the second wave corresponding to the formation of the alcohol49-52. The proton transfer mechanism, whether concerted or antecedent was correlated to the solution pH. In


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aprotic solvents, including dimethylformamide (DMF) or acetonitrile, the acid serves to protonate the aromatic radical species formed via one-electron reduction, making subsequent additions of electrons easier54-58. Unlike protic solvents, ECH of aromatic aldehydes using aprotic solvents leads to dimerization products like pinacol56-57, 59-61. These studies offer robust evidence that the solvent strongly influences the ECH reaction mechanism and selectivity. Nonetheless, there is a clear lack of atomic-level insight on how the solvent impacts the reactivity and selectivity by way of molecular interactions. Using average continuum level parameters including, dielectric constant or solvatochromic factors, tend towards a static interpretation of the solvent and fail to capture non-uniform, localized solvent contributions to the ECH reactivity and selectivity. We emphasize the need to identify parameters that accurately represent interfacial solvation properties that can provide a clear interpretation of the solvent influence. In this study, we use a combined experimental-theoretical approach to identify critical atomistic level features and parameters that impact the selectivity and reactivity of electrocatalysts towards ECH of organic compounds under ambient reaction conditions. For this we choose the specific test reaction of the conversion of aldehydes (RCHO) to primary alcohols (RCH2OH) as shown in (Scheme 1 Eq 1) (where * denotes organic adsorbed on the electrode surface). We specifically focus on the ECH of an aromatic aldehyde, benzaldehyde (BZY) (Scheme 1 Eq 2), and an unsaturated aldehyde, pentanal (PAL) (Scheme 1 Eq 3).

(R − CHO)* +2H + + 2e− → ( R − CH2OH )*

Eq 1

Eq 2


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Eq 3

Scheme 1. Reduction reactions of aldehydes considered in this study

Aromatic aldehydes can typically undergo electrochemical hydrogenation to the corresponding alcohols under mild conditions62. The hydrogenation can proceed via reduction of the carbonyl moiety or reduction of the aromatic ring. In contrast, aliphatic aldehydes are appreciably harder to reduce under the same reaction conditions62. These model compounds make for important test cases that allow us to examine the relationship between molecule adsorption and charge transfer at the electrode-electrolyte interface and investigate the influence of applied electrode potential (or current density) and complex solvent interactions. This study aims to realize an atomic-level description of two phenomena: (i) what catalysts are suited for ECH of aldehydes and how does the ECH mechanism proceed? (ii) What is the role of the electrolyte/solvent at the interface and how does it impact adsorption, desorption and charge transfer between the organic species and electrocatalyst? We utilize electrochemical conversion experiments and atomistic scale simulations and theory to address this challenge. While the work targets aldehyde species, one of the most critical bad actors in pyrolysis oil hydrotreating, the adopted approach and implications of this study can be generally applied towards the electrochemical hydrogenation of other organic molecules.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Experimental electrochemical reduction potential for aldehyde reduction


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Linear sweep voltammetry measurements were made using a Princeton Applied Research (PAR) 263A potentiostat. A three-electrode assembly was employed, Ag/AgCl electrode was used as the reference, and a platinum wire was used as the counter electrode. The Ag/AgCl electrode was made in-house according to the method described by Sawyer63-64 using saturated KCl/AgCl as the filling solution. Experiments were performed with Au, Cu, Pd, Ag, and Pt wires (1 mm diameter) served as the working electrode. For C, a glassy carbon electrode (4 mm diameter) was used as the working electrode. Aldehyde ECH was conducted in a jacketed glass H-cell custom fabricated by Adams & Chittenden Scientific Glass, Berkeley, California. A constant current was applied to the reactor using the Princeton Applied Research (PAR) 273A potentiostat. The applied current and measured potential were recorded using CorrWare software (Scribner Associates Inc.). A Pt foil was used as the anode. Nafion117 was used as the cation exchange membrane. The film was hydrated in water overnight, cleaned by soaking in 3% H2O2 overnight, followed by soaking in 1 M H2SO4 overnight. The cleaned and hydrated membranes were stored in de-ionized water. The composition of catholyte and anolyte was kept constant for all reduction experiments and consisted of a 90% (1:1 w/w) ethanol-water mixture, containing 5% (w) acetic acid and 5% (w) sodium acetate as the supporting electrolyte. The gases evolved at both the anode and the cathode were collected using gas burettes and analyzed at the conclusion of the experiment using an Inficon 3000 micro gas chromatograph (micro-GC). Gas analyses showed no oxygen evolution at the anode. This, and the presence of CO2, ethylene, and methane in the gas phase suggested that Kolbe-type oxidation controlled the anodic reactions. Liquid phase analyses of the catholyte and anolyte were performed using a high performance liquid chromatograph (HPLC) equipped with a Waters 2414 refractive index


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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 eluent at a flow rate of 0.55 mL/min. Since the work conducted in this study is focused exclusively on cathodic processes, the dominant reactions under investigation involved hydrogen production and organic reduction reactions. 2.2. Density functional theory calculations Periodic density functional theory (DFT) calculations were performed within the generalized gradient approximation with the exchange correlation functional of Perdew, Burke and Ernzerhoff

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as implemented in the CP2K package66-67. Grimme’s third-generation corrections

were used to take into account dispersion forces or van der Waals interactions to describe energies more precisely.68 The norm-conserving pseudopotentials69 were used to represent the core electrons, while the valence wave functions were expanded in terms of double-zeta quality Gaussian basis sets70 optimized for condensed systems to minimize linear dependencies and superposition errors. Electrostatic terms were calculated using an additional auxiliary plane-wave basis set with a 400-Ry cutoff. The Γ-point approximation was employed for the Brillouin zone integration. For cluster calculations, a dielectric continuum function (ε0 = 72) was used to represent water solvation71-72. Reaction energetics across different electrocatalyst candidates were evaluated using 1 nm (corresponding to 50 atom clusters) metal nanoparticles of Ru, Pt, Au, Ag, and Cu as well as a graphene sheet to represent carbon support, C. Similar to our previous work73-76, all 50-atom electrocatalyst clusters were prepared with an ab initio molecular dynamics protocol that involved heating the cluster to 2000 K, propagating the system for ~3 ps, and annealing for ~10 ps until the temperature is below 1 K. A final geometry optimization was used to remove


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residual forces. Although the protocol does not guarantee a global energy minimum cluster conformation, it does result in a stable low energy cluster. 2.3. Classical molecular dynamics for speciation at the solvent/cathode interface Classical molecular dynamics simulations were performed on two cathode surfaces: Au modeled as a planar (111) surface, and carbon modeled as a graphene sheet. All atoms of the electrode were fixed in position. The OPLS all atom force field77 was used for all solvent molecules, and the TIP3P78 parameters for water molecules. The GROMACS package79 was used for all classical molecular dynamics (MD) simulations. Electrostatic potential (ESP) charges for solvent molecules were obtained from optimized structures with Gaussian80 using the M06 functional81. The force field’s non-bonded interactions (van der Waals and electrostatics) parameters for classical MD simulations of graphene and Au atoms were fit to match the DFT (with Grimme’s D3 correction68) binding energies of benzaldehyde, pentanal, and water and Au (111) and a graphene sheet to appropriately incorporate short-range bonded and long-range nonbonded interactions between the organic, solvent and electrode molecules. All gas phase binding energies computed with the classical force field are within U0) (see Table 2). Figure 9(d) reveals that at more reductive potentials, the surface adsorbed aldehyde molecules are less likely to experience electron transfer. This is attributed to the lower population (P) of aldehyde molecules at more reductive potentials (Figures 9(a) and (b)) and the unfavorable orientation of the C=O bond (Figure 8(d)


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and Section S10 of the SI) that decreases the percentage of molecules susceptible to undergo electron transfer (integral of L and S). In all cases, the drop in population of the aldehydes nearest to the electrode surface occurred concurrently with an increase in water and ethanol near the surface. Thus, while an increasing negative electrode charge may increase the likelihood of electron transfer, a too negative value will cause the aldehydes to diffuse away from the interface into the bulk. As the cathode becomes increasingly polarized, it will attract more water species to compensate the surface charge. This will influence the aldehyde to diffuse into the bulk electrolyte. Irrespective of the cathode material (graphene or Au), and the solvent mixture (benzaldehyde or pentanal), our simulations reveal that a balance in terms of the cathode charge and aldehyde population is necessary to optimize the probability of aldehyde reduction. The number of pentanal molecules susceptible to undergo electron transfer at the interface is lower than the number of benzaldehyde molecules (Figure 9(c) and (d)). Thus, pentanal is likely to have a lower conversion than benzaldehyde, even though benzaldehyde and pentanal have similar surface binding energies: on graphene 46.5 and 47.6 kJ/mol and on Au (111), 97.2 and 92.9 kJ/mol for benzaldehyde and pentanal respectively. We attribute this to the orientation and the extent of molecular overlap between the aldehyde and the surface as discussed in Section S10 of SI. In the case of benzaldehyde, the overlap of the frontier molecular orbital orients the molecule to allow for interfacial charge transfer even under increasing reduction potentials. The extent of overlap and orientation are not maintained for the case of pentanal (see Figure S11 of the SI), thereby lowering the concentration of pentanal molecules susceptible to direct charge transfer. Therefore, the experimental observation that more aromatics are reduced than aliphatics cannot be trivially rationalized using the relative binding energies of these species to the surface,


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but also need to account for the potential-dependent binding configuration. This suggests that for aliphatic aldehydes, use of either a surfactant or a pulsed voltage technique could be used to enhance the concentration of species near the surface. The solvent and aldehyde reorganization predicted by theory was observed experimentally. Figure 10 shows the experimental current measured during a potential square wave pulse using a three electrode setup with the electrolyte containing benzaldehyde or pentanal. The intermittent hold time (thold) and the initial voltage (Vhold) was varied during the course of the experiment and the maximum current (Imax) was measured (details of these experiments are reported in Section S11 of the SI). For both pentanal and benzaldehyde, more reductive potentials (U changed from −0.25 V to −1.00 V) lead to a lower current flow through the cell, or lower concentration of aldehydes near the cathode, regardless the solvents in use. The addition of acetic acid to the solvent leads to the increase of Imax. After a hold time of 100 second, Imax values show little change in the presence and absence of acetic acid and is lower at the more reductive applied potential value of −1.00 V. These experiments suggest that a lower concentration of aldehydes can be observed at the cathode at more reductive potential (U changed from −0.25 V to −1.00 V).


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(a)

(b)

Figure 10. Current measured (Imax) in mA as a function of the hold time (thold) in secs for (a) benzaldehyde and (b) pentanal in the presence (solid markers) and absence (hollow markers) of acetic acid at U values of −0.25 V (blue squares) and −1.00 V (brown triangles) using a Ag/AgCl reference electrode. Electrolyte solution contains a 1:1 ethanol water (w/w) mixture with sodium acetate.

We can proportionally correlate the current flow through the cell I(U) shown in Figure 10 to the extent of electron transfer F(U) shown in Figure 9(d). We refer the reader to Section S12 of the SI for details on this correlation. The decrease in the likelihood of the aldehyde molecule to undergo electron transfer as a function of voltage F(U) as predicted by our theoretical framework and MD simulations and that observed from the potential square wave pulse experiments are in excellent agreement.

CONCLUSIONS


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In this study, we identified the electrochemical hydrogenation mechanism of aldehydes, and its dependence on reaction conditions, including the charged interface and multicomponent electrolyte. The electrochemical hydrogenation of aldehydes is a required step toward electrocatalytic bio-oil stabilization. We have found that the most energetically and electrochemically efficient mechanism for this process is a concerted electron-proton transfer process. The process, however, is reliant on the availability and proximity of the aldehyde to protons within the electrochemical double layer. Aldehydes need to be within the first solvent layer of the cathode to undergo electron transfer. At higher reduction potentials (or more negative cathode charge), this is less likely to occur because the cathode becomes more hydrophilic with increasing negative charge and drives the organics to the bulk attracting water to the surface. Additionally, the orientation of the aldehyde molecules due to surface charging lowers the favorability of electron transfer, implying that a non-monotonic dependence of conversion on current density is likely to exist. The concurrence between measured and predicted conversion serves to prove the need for a fundamental understanding of molecular scale interactions that go beyond routine energetic estimations in order to accurately account for the factors that control ECH reactivity and selectivity of aldehydes and other target organic molecules.

AUTHOR INFORMATION #

Equal contributions

&

Current Address: Chemical and Materials Engineering, University of Nevada, Reno, Reno NV

89557


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* Corresponding Authors Roger Rousseau: [email protected] Michael A. Lilga: [email protected] Notes The authors declare no competing financial interests. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. ASSOCIATED CONTENT Supporting Information This Supporting information is available free of charge on the ACS Publication website http://pubs.acs.org. The following topics are presented in detail: (1) Adsorption energy vs surface charging level. (2) Determining theoretical reduction potential for organic reduction. (3) Overpotentials for acid and alcohol reductions. (4) Computation of ECH Mechanisms. (5) Energy profiles for ECH mechanisms. (6) Concerted electron and proton transfer. (7) Experimental measurement of aldehyde conversion. (8) Determining the cutoff distance of the 1st layer close to the surface. (9) Derivation of the wavefunction overlap integral. (10) Analyses from classical MD simulations. (11) Potential Square Wave Experiments. (12) Derivation of F(ρ) and the current.


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ACKNOWLEDGMENTS Work performed by D.C.C., A.B.P., Y.Y., Y.G.W., R.R. and M.A.L. was supported by the United States Department of Energy (U.S. DOE), Office of Energy Efficiency and Renewable Energy and the Bioenergy Technologies Office. Work done by M.T.N., S.A.A., M.S.L. and V.A.G. was supported by the Pacific Northwest National Laboratory’s (PNNL) Laboratory Directed Research Development (LDRD) project through the Chemical Transformation Initiative. PNNL is operated by Battelle for the United States Department of Energy under Contract DEAC05-76RL01830. The authors would like to thank Dr. Harsha Annapureddy for his insightful discussions. Computational resources were provided by PNNL Institutional Computing (PIC) and the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility supported by the Office of Science of the U.S. DOE. REFERENCES 1. Langholtz, M.; Stokes, B.; Eaton, L. 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy; EERE Publication and Product Library: 2016. 2. Padro, C. E. G.; Putsche, V. Survey of the Economics of Hydrogen Technologies; National Renewable Energy Lab., Golden, CO (US): 1999. 3. Bridgwater, A. V., Review of Fast Pyrolysis of Biomass and Product Upgrading. Biomass Bioenergy 2012, 38, 68-94. 4. Zacher, A. H.; Olarte, M. V.; Santosa, D. M.; Elliott, D. C.; Jones, S. B., A Review and Perspective of Recent Bio-Oil Hydrotreating Research. Green Chem. 2014, 16, 491-515. 5. Ragland, K.; Aerts, D.; Baker, A., Properties of Wood for Combustion Analysis. Bioresour. Technol. 1991, 37, 161-168. 6. Elliott, D. C., Historical Developments in Hydroprocessing Bio-Oils. Energy Fuels 2007, 21, 1792-1815. 7. Bridgwater, A. V., Upgrading Biomass Fast Pyrolysis Liquids. Environmental Progress & Sustainable Energy 2012, 31, 261-268. 8. Wang, H. M.; Male, J.; Wang, Y., Recent Advances in Hydrotreating of Pyrolysis BioOil and Its Oxygen-Containing Model Compounds. ACS Catal. 2013, 3, 1047-1070. 9. Mortensen, P. M.; Grunwaldt, J.-D.; Jensen, P. A.; Knudsen, K.; Jensen, A. D., A Review of Catalytic Upgrading of Bio-Oil to Engine Fuels. Appl. Catal. A: General 2011, 407, 1-19. 10. Al-Sabawi, M.; Chen, J., Hydroprocessing of Biomass-Derived Oils and Their Blends with Petroleum Feedstocks: A Review. Energy Fuels 2012, 26, 5373-5399.


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