Elucidating the Roles of the Electric Fields in Catalysis: A Perspective

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Elucidating the Roles of the Electric Fields in Catalysis: A Perspective Fanglin Che, Jake T. Gray, Su Ha, Norbert Kruse, Susannah L Scott, and Jean-Sabin McEwen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02899 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Elucidating the Roles of the Electric Fields in Catalysis: A Perspective Fanglin Che,a Jake T. Gray,a Su Ha,a Norbert Kruse, a Susannah L. Scott,b,c Jean-Sabin McEwen*a,d,e,f,g a

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States b

Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States c

Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States

d

Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, United States e

Department of Chemistry, Washington State University, Pullman, Washington 99164, United States

f

Department of Biological Systems Engineering, Washington State University, Pullman, Washington 99164, United States g

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

*

Corresponding author: Email: [email protected] (J.-S. McEwen); Phone: (+1)509-335-8580

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Abstract This Perspective illustrates how the presence of internal and external electric fields can affect catalytic activity and selectivity, with a focus on heterogeneous catalysts. Specifically, experimental investigations of the electric field influence on catalyst selectivity in pulse field mass desorption microscopes, scanning-tunneling microscopes, probe-bed-probe reactors, continuous-circuit reactors, and capacitor reactors are described. Through these examples, we show how the electric field, whether externally applied or intrinsically present, can affect the behavior of a wide number of materials relevant to catalysis. We review some of the theoretical methods that have been used to elucidate the influence of external electric fields on catalytic reactions, as well as the application of such methods to selective methane activation. In doing so, we illustrate the breadth of possibilities in field-assisted catalysis.

Keywords: Field-Assisted Catalysis, External Field, Pulsed Field Mass Desorption Spectroscopy, Probe-Bed-Probe Reactor, Continuous Circuit Reactor, Density Functional Theory

1. Introduction Electric fields can be used to alter the thermodynamics of chemical reactions in much the same way as we have traditionally used temperature and pressure. A van’t Hoff equation can therefore be formulated in which the variation of the equilibrium constant with the electric field strength is described as a function of the changing electric moments (e.g., electric dipole moment, polarizability). Since catalytic reactions usually occur far from equilibrium, we must also identify energy transfer mechanisms and derive time-dependent equations in accordance with the appropriate kinetic descriptors. Since electric fields over 0.1 V/Å alter the energies of


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molecular orbitals of adsorbates, they can also change the activation barriers for reactions and hence their associated kinetic parameters, making electric fields relevant to the field of catalysis.1-5 Recent advances in the synthesis, characterization, and computational modeling of catalytic materials have made the measurement and prediction of field influences more readily accessible. Consequently, field-assisted catalysis is a forefront area of research,4, 6-9 as we will attempt to convey in this Perspective. In discussing electric field effects, it is convenient to distinguish between the short-range effects of covalent or polar covalent chemical bonding, and longer-range electrostatic effects. With regard to the latter, Oppenheimer10 predicted the dissociation of a hydrogen atom into a proton and an electron at a field strength of about 1 V/Å. At lower field strengths (e.g., 0.01 V/Å), molecules remain intact but are oriented and polarized according to their dipole moment and polarizability. The origins of such effects are explained in the work of Debye.11 Thermodynamic consequences were investigated by Bergmann et al. to assess the effect of dielectric absorption on reaction equilibria in liquid solutions.12 However, such classical considerations cannot be applied to reactions in fields as high as those considered by Oppenheimer, because atomic and molecular orbital energy levels shift dramatically and their occupation can change, eventually giving rise to field dissociation/association or the appearance of new reaction pathways that are not accessible in the absence of electric fields. Kreuzer established the theoretical basis for such high-field surface chemistry using quantum mechanical methods,13 providing insight into the effect of local electric field strength on individual atoms14 as well as a molecular orbital analysis of the field influence on a given reaction pathway.15 Since these pioneering studies, local electric fields as well as their fluctuations have also been studied quantitatively in the condensed phase using classical molecular dynamics simulations. For


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example, in an aqueous NaCl solution, the calculated local electric field of O and H atoms had a mean value of 2 V/Å, with a non-Gaussian field distribution over these sites.16 Classical molecular dynamics simulations were also performed in several other liquid solutions (n-hexane, dibutyl ether, tetrahydrofuran, valeronitrile, acetonitrile, dimethyl sulfoxide and water), where local electric field values ranging from 0.1 to 0.7 V/Å were obtained.17 However, the field distribution around a water molecule calculated using classical molecular dynamics was also shown to differ significantly from that obtained using quantum mechanical methods,16 further demonstrating the need for a quantum mechanical description of such systems. Historically, long-range electrical fields on the order of 1 V/Å have been generated in a controlled manner using field emission microscopes and “atom-probes”, as first developed by Müller.18-20 In his original work, Müller used metal tips electrochemically conditioned to have a small apex at the extremity of a thin metal wire, which is biased with either a negative or positive potential on the order of a kilovolt relative to a counterelectrode at a short distance in front of the tip. The electrode system with the tip, whose radius of curvature is on the order of 100 nm, and the counter-electrode, also operated as a screen, can then be used to trigger either field electron emission or, at reversed polarity, field ionization. In the latter case, atom-resolved images of the tip apex are obtained. Figure 1a shows an example of a Pt tip imaged by Field Ion Microscopy (FIM). A ball model (Figure 1b) demonstrates that the nearly hemispherical morphology of the tip is well-modelled by a metal nanoparticle, thereby establishing the relevance to heterogeneous catalysis. Müller’s scientific achievements also include the design of the first “atom-probe”, in which he combined the atomic resolution of FIM with mass spectrometry for chemical identification of field-ionized species.21 Subsequently, Block22 used a one-dimensional setup with a probing hole in the screen to investigate electric field effects in surface reactions and


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heterogeneous catalysis. This research area has recently been the subject of renewed interest in the catalysis community, when it was realized that electric fields can be used to tune catalytic and electrocatalytic behavior. An overview of the experimental possibilities was provided by Kruse and Visart de Bocarmé.23

Figure 1. a) Field ion micrograph of a (001)-oriented Pt sample, with the main planes indicated by Miller indices (conditions: temperature T = 60 K, gas pressure P(Ne) = 2×10−3 Pa, static electric field intensity F ~ 3.5 V/Å). b) Ball model of a face-centered cubic crystal shaped as a quasi-hemisphere, in which the most strongly protruding atoms, corresponding to kinks and step positions, appear as bright spots. c) Time scheme of field pulses in Pulsed Field Desorption Mass Spectrometry (PFDMS). The instrument combines time-of-flight mass spectrometry with Field Ion Microscopy in an “atom-probe” device. About 100 to 150 atomic surface sites are selected for kinetic analysis. The field pulse repetition frequency is varied between 10 kHz and 1 Hz, corresponding to reaction times of 100 µs to 1 s between pulses. To


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study the field influence, an arbitrary electric field (“reaction” field, FR) is applied between pulses. FR can be varied from the onset of field electron emission (at reversed polarity) to the occurrence of field evaporation of the tip metal, while field pulse amplitudes are adjusted so as to keep the total field, FD, constant.

Field-induced chemistry can occur with or without an applied field. For example, natural field-induced chemistry, which occurs in the presence of a large local electric field, has been observed in zeolite cavities24 and within the binding sites of enzymes.25 The field can also be generated artificially in a variety of reactor designs, such as in a field emission microscope (as mentioned above),26-27 as well as at electrode/electrolyte interfaces in electrochemical cells,6, 28-29 in scanning tunneling microscope (STM) probe ''nanoreactors'',30-32 in probe-bed-probe reactors (PBP),33-39 in continuous-circuit reactors (CC),5, 40 and in capacitor reactors41-42 (to be discussed below). In an electrochemical cell, a large potential drop (ca. ±1.2 V) is generated due to the opposite charges at each side of the interface layer between the electrode and the electrolyte (the so-called ''Helmholtz layer''). As a result, a very high electric field on the order of ± (0.1 to 0.4) V/Å is produced within this thin interfacial layer (ca. 3 to 8 Å). The radius of curvature of a metal surface greatly affects the magnitude of the local electric fields ( =

for a spherical particle, where , , and R represent the electric field

strength, the applied electric potential, and the radius of curvature, respectively).43 Small r values can lead to high electric fields, on the order of ±1 V/Å at the interface between the electrode and electrolyte in an electrochemical cell. To illustrate this principle, Min et al. showed that the current density for electro-reduction of CO2 is greatly enhanced upon increasing the local electric field strength. The field strength can be increased by decreasing the radius of curvature of the electrode tip or by adsorbing K+ ions on the tip surface.44 We also note that the electric field


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around a field emitter tip will be heterogeneous, and will depend on the exact geometry of the tip. For example, if one approximates the curvature of the nanotip as a paraboloid, one can solve for the field dependence precisely as a function of position between the apex of the tip and its shank (see Appendix A): =

(1)

where is the electric field at the apex of the tip, which depends on the potential difference between the tip (electrode) and the screen (counter-electrode), the radius of curvature of the tip, as well as the distance between the tip and the screen. Further, denotes the position of a point on the tip surface as a function of its radial distance. At the apex of the tip, = 0, while at its shank, = ∞. Thus, an electric field of the order of 1 V/Å present at the apex of the tip will gradually decreases to 0 as one approaches the shank. The electric field can also unintentionally influence the experimental observations as well. An example of such an effect is shown in Figure 2, where a STM image of O on Fe(100) is taken when the tip is at two different distances from the surface.31 As can be seen from Figure 2, the corresponding STM image of the adsorbed O adatom depends on the distance of the tip to the surface. When the tip distance is greater than 4 Å, the O atom appears as a protrusion for which one would concludes (correctly) that the O adatom is at a hollow site, which is well known from DFT calculations to be the preferred adsorption site of O.45 However, when the tip distance is less than 4 Å, the O adatom appear as a depression, for which one would conclude (incorrectly) that an O adatom is adsorbed on a top site. The reason for this observed behavior is that the electric field of the STM tip affects the vacuum barrier of the surface electronic structure (and thus the decay length of the tunneling current). Corresponding theoretical calculations by Hofer and coworkers have shown that, as the tip approaches the surface, the surface layer gradually


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loses its surface characteristics, which leads to a loss of surface states. As such, by taking into account the influence of the field on the corresponding simulated STM image, they were able to reproduce the observed experimental results. One therefore also needs carefully interpret the experimental results when interpreting a measured STM image or, for that matter, any other experimental observation for which an electric field is present.

Figure 2. STM image of Fe(100) with an adsorbed oxygen atom. Oxygen appears to be at the hollow site of the surface for distances above 4 Å (correct), but it appears to be on the on-top site for distances below 4 Å (incorrect). The conclusion from the experiments is a reversal of surface corrugation in the low distance regime. Reprinted with permission from Ref 31. Copyright Elsevier, 2003.

This Perspective will illustrate how electric fields can affect catalytic activity and selectivity, focusing on examples from heterogeneous catalysts. A review of oriented electric fields was recently published,32 but was limited to STM investigations of the influence of electric fields on single molecules. Such a viewpoint is rather limited in the broader context of catalysis, where it is desirable to investigate electric field effects under more realistic reaction conditions. We present examples of electric field influences on catalyst selectivity in pulsed field desorption mass spectroscopy (PFDMS) experiments in Section 2. In Section 3, we review some of the theoretical methods used to elucidate the influence of external electric fields on catalytic reactions, as well as the application of such methods to methane activation. In Section 4, we


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discuss the experimental investigation of field influences. We end the perspective with some thoughts on future research directions, by briefly discussing related effects in microporous catalysts as well enzymes, and by discussing how the experimental techniques used in such investigations might aid in the quantification of electric field strengths in other types of catalysts.

2. External Electric Field Effects Studied by Pulsed Field Desorption Mass Spectrometry (PFDMS)

2.1 Introduction Two examples will illustrate how electric field effects can be described quantitatively via experiments based on the “atom-probe” principle. The first involves the field-promoted decomposition of nitric oxide (NO) over Pt model catalysts conditioned as tips.46 Low temperature NO decomposition is relevant to catalytic air pollution control. The second example relates to direct methanol fuel cells (DMFCs), since strong electric fields alter the rate of methanol decomposition at Rh tips.27, 47-48 The issue of field-induced molecular fragmentation is also considered. To study the influence of electrostatic field effects on surface reactions in a controlled and reproducible manner, field strengths must be varied systematically, from the onset of field electron emission to the occurrence of field evaporation (e.g., the removal of metal surface atoms from kink sites). In PFDMS, this procedure is combined with field pulse variation. According to Figure 1c, experiments are usually conducted by varying the steady electric field, FR (the “reaction field”), while adjusting the pulsed field (FP) so as to keep the total desorption field strength (i.e., the sum of FR and FP) constant. Under conditions of quantitative field desorption


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(in which the entire surface layer is field-desorbed with each field pulse), the measured total ion intensity reflects the surface coverage prior to the field pulse. Under these conditions, kinetic investigations become possible by varying the reaction time, tR, between field pulses. In a homebuilt version of the PFDMS setup, time resolution between 100 µs and about 1 s can be achieved at temperatures between RT and about 1000 K (although there is no technical upper limit except the melting temperature of the metal tip). A number of studies have demonstrated the validity of this approach.27, 46-48

2.2 Field-induced NO Decomposition The possibility of determining kinetic parameters for catalytic reactions occurring on selected crystallographic facets of metals using the “atom-probe” principle of PFDMS was demonstrated for NO decomposition on Pt tips.49 This is useful as a benchmark system, given the detailed information available from (field-free) studies of the reaction on extended Pt single crystals.50 Mean surface lifetimes were determined as a function of surface temperature by varying the reaction time in a continuous supply of gaseous NO at low overall pressure ( 6 V/nm (a value remarkably similar to the field at which N2O formation is observed experimentally, see Figure 3), because of a significant reduction in the activation barrier for NOad dissociation. In the proposed mechanism, Nad resulting from dissociation of NOad combines with intact NOad to form N2O. Additional DFT calculations might provide further insight into the N2O formation mechanism. Such studies should consider the possibility of pathways involving dinitrosyl, (NO)2,ad, a minor product found in experimental studies involving Pt (not shown in Figure 3), and a major product in studies with Au.52 Indeed, work by de Vooys et al.53 on the electrocatalytic reduction of NO at positive potentials (likely to cause electric fields similar to the experiments of Kruse et al.)49 suggests a serial pathway of N2 formation from NO via (NO)2 and N2O. Such a reaction mechanism was considered in a more recent modeling investigation by McEwen et al.,54 in which (NO)2 decomposition on a stepped surface was found to have a small activation barrier. However, the field influence on the activation barrier was not significant. It remains to be seen whether the presence of an external electric field could influence the barrier at other types of adsorption sites, such as kinks, which may bind (NO)2 more strongly than step sites. Further, the effect of surface coverage on the underlying energetics should be considered, since earlier modeling was performed only for low NO coverages.

2.3 Field-induced Methanol Decomposition The electrocatalytic decomposition of methanol over a Rh tip differs from the NO/Pt reaction (for which a steady electric field accelerates decomposition and opens a new reaction pathway, leading to N2O formation), in that CH3OH decomposition over Rh is slowed by the presence of an electric field. The deceleration is caused by stabilization of intermediate “CH2O”


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species. It may be possible to exert more control over the rather complex phenomena that occur during the reaction once the fundamental mechanisms of the field-interaction are better known. Figure 4 shows PFDMS results obtained under conditions of quantitative field desorption at room temperature, using a total desorption field strength of 28 V/nm and a reaction time tR = 250 µs between field pulses (more than 105 cycles overall). Generally, the mass spectra in Figure 4 suggest hydrogen abstraction from adsorbed methanol: CH3OHad → CH3-xOad + (x+1) Had (x = 0 - 3). The parent species CH3OH+ is formed either by gas-phase ionization or by field desorption from a molecularly-adsorbed state. The final reaction products, CO and H2, appear as CO+/RhCO+ and H+/H2+. Evidence for stepwise hydrogen abstraction is found in the variable yields of CH3-xO+ (x = 0 - 3) species. For low, steady electric fields, FR < 2 V/nm, the mass spectra are dominated by reaction products. In Figure 4, the ion intensity ratio (CO+ / ΣnHn+) = 0.5 at low fields reflects the overall reaction stoichiometry CH3OH → CO + 2 H2. Thus decomposition is complete after 250 µs at room temperature, and since at 298 K COad desorption is rate limiting, one can assume first-order kinetics with a rate constant greater than 4 × 103 s-1 under essentially field-free conditions. Note that in kinetic measurements with low time resolution, product species like COad and atomic hydrogen would accumulate at the catalyst surface, thus their mean residence times are long at RT. Further, since COad binds significantly more strongly than hydrogen the rate-limiting step of the overall process at this temperature is clearly thermal desorption of CO.


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Figure 4. Effect of varying the steady electric field, FR, during methanol decomposition on a Rh tip. The total desorption field strength was a constant FD = 28 V/nm, and the field pulse amplitudes were adjusted while varying FR. The probe hole monitors reaction processes on about 150 atomic sites close to the (001) pole of the Rh tip. The reaction time tR = 250 µs was adjusted and then kept constant during the continuous impingement of methanol molecules at P = 1.3×10-5 Pa and T = 298 K.

As the steady electric field increases, the field desorption mass spectra in Figure 4 undergo dramatic changes. In order to confirm assignments, experiments with isotopicallylabeled methanol, CHD2OH, were performed using Rh as well as other tip materials.27 The most intriguing observation is the steep increase in CH2O+ for steady electric fields FR > 2 V/nm. Moreover, high amounts of CH3+ were detected; this ion actually dominates the mass spectra for the highest fields reported in Figure 4. CH3+ is also observed at low field, albeit in smaller amounts, while CH2O+ is completely absent under these conditions. At the same time, the intensities of the CO+ and H+/H2+ signals decrease by more than an order of magnitude as the


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electric field strength is increased above 1 V/nm. The electric field apparently impedes methanol decomposition by stabilizing adsorbed reaction intermediates, in particular, CH2O and methoxy (see below). The high abundance of CH3+ in the mass spectra can be explained by the charge distribution in adsorbed methoxy, CH3Oad, which is adsorbed at the metal surface through its oxygen atom.55 Adsorption is associated with a decrease of the work function, suggesting net electron transfer from the methoxy group to the metal. Field pulses of sufficient amplitude can then lead to C-O bond cleavage, as indicated in Figure 5. Indeed, the CH3+/CH3O+ intensity ratio is always high (ca. 100) for temperatures below ca. 460 K. More insight into the C-O bond cleavage process can be obtained from recent first principles-based calculations by Che et al. on Ni(111). Their work shows that external electric fields also weaken the adsorption strength of methoxy, whose calculated charge distribution is shown in Figure 5b.56 Moreover, since the reaction CH3O+ → CH3+ + O is exothermic (by ca. -0.2 eV on Ni), the methoxy species readily dissociates into CH3+ and O adatoms. The presence of a positive electric field repels the CH3+ species from the positively charged metal surface, leading to a high abundance of CH3+ as observed in the PFDMS experiments on a Rh surface. However, such a picture does not fully explain the experimental observations. For example, H2+ and H3+ species are also generated at Rh tips, suggesting the need for further ab initio investigations to unravel the details of the reaction mechanism.


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Figure 5. (a) Schematic showing field-induced fragmentation of adsorbed methoxy by preferential C-O bond cleavage, leading to large yields of CH3+; (b) DFT-calculated differential charge density for the methoxy species on a Ni(111) surface. The isosurface level of the differential charge densities is 0.003 e/bohr3. The yellow and blue areas represent the gain and loss of electrons, respectively.

Since the appearance of the CH3+ signal indicates field-induced methoxy fragmentation, water formation must occur simultaneously in order to avoid accumulation of surface oxygen (which would lead to self-poisoning of methanol decomposition). Indeed, large amounts of H2O+/H3O+ are observed throughout the experiment. Their signal intensities increase with FR because of the strong interaction of the electric field vector with the dipole moment of water, as demonstrated in other investigations.5, 26, 57-58 The discussion above shows that new reaction patterns can appear under high electric field conditions. Such field-induced chemistry can play an important role in electrocatalytic reactions, where local fields at electrode surfaces may be high. Field ion microscopy and, in particular, “atom-probe” pulsed field desorption mass spectrometry (PFDMS), are powerful tools to investigate field effects under controlled and reproducible reaction conditions.

3. A Theoretical View of Field-Induced Electrochemical Systems


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3.1 Introduction Electrocatalytic reactions in fuel cells involve a variety of redox reactions, including hydrocarbon steam reforming, H2 oxidation, and O2 reduction, while electrolysis cells (reverse fuel cells) use reactions such as water splitting and CO2 reduction. There has been much debate on how applied electric fields create local electric field strength effects in electrochemical devices. For example, an open question is the role of the local electric field strength in the conversion of methane in the methane steam reforming reaction (MSR) in a solid oxide fuel cell (SOFC). A related question is the origin of the increased H2 yield under the lower temperature operating conditions, compared to conventional thermal reforming.59-60 Another interesting example is that of the reforming of light alkanes (e.g., methane) or alcohols (e.g., ethanol) in probe-bed-probe (PBP) reactors, where conversion is also greatly enhanced at lower temperatures relative to the purely thermal reforming process.33-35, 37-39 Such rate enhancements in electrochemical devices may be caused by high electric fields that can alter the thermodynamics as well as the kinetics of the rate-limiting step for heterogeneous reactions. To better understand the role of electric fields, theoretical studies at an atomic level (e.g., density functional theory, DFT) are a powerful tool. Section 3.2 presents various computational approaches for applying DFT calculations to electrocatalytic reactions using a periodic representation of the electrode surface. Section 3.3 combines theoretical studies and experimental observations to provide more examples of how electric fields in electrochemical devices can directly change the conversion and selectivity of reversible electrocatalytic reactions. 'Bottom-up' fundamental insight on electric field effects in reversible reactions is provided at the end of Section 3.3.


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3.2 Computational Methods for Studying Electric Fields

3.2.1 Modeling the Vacuum-Metal Interface in the Presence of an Applied Electric Field To describe the effect of electric fields on the thermodynamic and kinetic properties of electrocatalytic reactions, we start with an overview of some of the computational methods that can be employed to treat field-induced interactions between gas phase adsorbates and catalytic metal surfaces. The methods presented here can be implemented when a plane wave basis set is used to model a system with periodic boundary conditions, as is done in the VASP code. We note that the effect of an electric field can also be modeled using other methods when a cluster model is employed, or using other types of basis sets (e.g., Gaussian-type or Slater-type orbitals). Such approaches are implemented in the Gaussian, ADF and DMol3 software packages.61-63 Although one can more easily simulate higher electric field values using such codes, their main drawback is that they cannot easily represent a surface using a slab model. Another recent development is an implicit self-consistent description of the electrolyte using plane-wave density functional theory, as implemented in the VASPsol code,64-65 which can be used to model the effects of an external electric field in a solvent. Although many possible choices for the implicit model exist, one widespread standard uses Poisson-Boltzmann theory, which has been immensely successful. This standard has also been implemented in the VASPsol code

64-65

and

the FHI-aims code.66 In such a model, the local electric field consists of two contributions: the displacement field emanating from the charged objects, and the polarization field stemming from the dielectric medium. A linear relationship between the displacement field and the local electric field vectors is assumed:66-67


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() = (, ′) ∙ (′) ′ (2) where is the permittivity of vacuum, (, ! ) is the nonlocal dielectric tensor, () is the change in the displacement field at position , and (′ ′) is the change in the local electric field at position ′. Although this approach can be very useful, the assumption of a linear relationship begins to break down when the electric field strength exceeds ~0.2 V/Å.67-68 However, as we showed in the previous section and will further illustrate in this one, fieldassisted catalysis often involves higher field strengths. Moreover, implicit solvation models do not take into account the role of the solvent when it actively participates in the reaction mechanism (e.g., proton shuttling), which is the case for some catalytic reactions in condensed phases (e.g., hydrodeoxygenation of phenolic compounds).69 Finally, an implicit solvation model cannot account for local electric field fluctuations, as quantified recently by Kathmann and coworkers

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in an aqueous NaCl electrolyte. Such local electric field fluctuations can play an

important role for the system under consideration. For example, Kathmann and co-workers showed that field fluctuations are linked to the long-lived emission of visible light during the crystallization of certain salts. Therefore, we will limit our discussion here to modeling electric field effects using the VASP code, and will not discuss implicit solvation models further for conciseness. The computational models described below are directly relevant to experiments conducted in STM probe ''nanoreactors'', FIM, FEM, SOFC or SOEC, PBP, CC, and capacitor reactors. The computational method, as developed by Neugebauer and Scheffler,9 simulates an external or internal electric field in VASP-implemented DFT calculations. A dipole sheet that is


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negatively (positively) charged at one end of the sheet and positively (negatively) charged at the other end is created in the middle of the vacuum, as schematically illustrated in Figure 6.

Figure 6. Schematic illustration of how one applies an external electric field in periodic system using Neugebauer and Scheffler method. The external field is applied by inserting a dipole layer in the middle of the vacuum layer in between two slabs. The system on the left depicts a slab model in the absence of external electric while the slab on the right depicts the system under consideration in the presence of an electric field. Reproduced with permission from Ref 70. Copyright American Chemical Society, 2009.

Although the applied electric field is uniform, the resulting local electric field can be enhanced locally at step sites, kinks and other defects on the catalyst surface, as originally described by Kreuzer and coworkers.14 Using this method, we can investigate electric field effects on adsorbate-surface interactions, such as configuration optimization, electronic structures, adsorption energies, vibrational frequencies, and so on. In the presence of a uniform electric field (F), the total energy (E) can be expanded as a Taylor series, as given in Equation (3):57, 71-74 ( "(#) = " $ (%"&%#) ' ∙ # $ (% "&% ( ) ' #( (


(3)

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where all the derivatives are evaluated at F = 0 V/Å and E0 is the total energy in the absence of an electric field.75 With these definitions, the dependence of the adsorption energy of adsorbates on the electric field strength can be written as a Taylor series (Equation (4)):

")* (#) = ")* $ + ' ∙ # $ , ' #( (

(4)

where Ead0 is the adsorption energy in the absence of a field and ∆dF=0 (or ∆αF=0) is the difference in dipole moment (or polarizability)71, 76 between the metal slab with adsorbates and the corresponding non-interacting system (i.e., clean surface and gas phase molecules). Specifically, when a molecule interacting with a surface has a permanent dipole moment (e.g., water), the third-order derivative of the energy (i.e., the electrochemical change of the Hessian matrix) must be included in order to describe the field dependence of the energy of the polarized molecule.71, 77 Equations (3) and (4) describe the interaction energy between an adsorbate and the surface, which can be altered by the presence of an applied electric field. As a result, an electric field can alter the reaction energy (i.e., the energy difference between the initial and final states). When the effective dipole moment difference (+ ' ) between the initial and final state is aligned with the field direction, the field can further decrease the reaction energy, and vice versa. More details regarding field effects on reaction energies via the effective dipole moment analysis can be found in previous publications.5, 40, 56, 78-81 For an elementary reaction step, - $ . → -. ‡ → 1 $ 2 , the field-induced activation energy (") ()) is defined as the energy difference between the initial state and the transition state when an electric field is applied. The effect of an electric field on the transition state energy has been discussed in the literature in the context of electrochemical annealing,82 as well as in reactions of enzymes, such as ketosteroid isomerase.25, 83-84 In those studies, the effect of the


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electric field on the activation energy for a given reaction was argued to depend on the dipole moment difference between the initial state and the transition state. In general, the dipole moment at the transition state will differ from that in the ground state conformation since their geometries will be different. For example, consider the dissociation of a CO molecule on a metal surface in the presence of an electric field. The dipole moment of the CO molecule depends on the C—O bond distance, and the initial state of the un-dissociated CO molecule has a shorter CO bond length relative to the transition state. The change in dipole moments between the initial and transition states can therefore lead to stabilization of the transition state, since the external electric field affects the initial and transition state energies differently. Naturally, the electric field effect on the activation energy depends on the magnitude of the difference between these states. Thus, an electric field can significantly alter the rates of a given heterogeneous reaction. Finally, it is important to note that the polarizability of adsorbates can also affect field-induced activation energies. Indeed, for certain adsorbates, such as water,5 the field dependence of the adsorption energy, as given in Equation (4), depends more strongly on its polarizability than its dipole moment. Thus, for some reactions, it is important to consider changes in polarizability between the initial state and the final state as well. This Neugebauer and Scheffler method is suitable for applying an external electric field with a specific field strength to a vacuum-metal system. An advantage of the method is that the neutrality of the system of the system is maintained, without adding or removing charge from the supercell, even in the presence of electric fields on the order of 1 V/Å. However, when the electric field strength is around 0.5 V/Å, the vacuum thickness must be chosen carefully, since 'field emission' effects85 can easily occur as the vacuum thickness increases. Thus, the optimal vacuum width needs to be determined beforehand, to ensure that the charge density in the


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vacuum is less than the magnitude of the Gibbs' oscillations (0.001 e/Å3) associated with planewave cutoffs. Besides simulating a high electric field in a charge-neutral system, this method can also accurately capture the effects of high electric fields on configuration optimizations, adsorption energies, reaction energies, activation energies, and vibrational frequencies. The fielddependent rate constant at a given temperature can therefore be calculated directly using transition state theory,86-88 and a field-dependent microkinetic model can be constructed. However, the method does not work well for metal surfaces in the presence of solvents. Indeed, since the artificial dipole sheet is inserted in the middle of the vacuum layer, solvent molecules cannot be nearby since they will induce field emission effects via electrons located in the vicinity the dipole layer sheet. This is an inherent limitation of the underlying method. Finally, the electrode potential in an electrochemical cell (e.g., SOFC or SOEC) is the energy difference between the calculated work function of the charged system and the experimentally measured work function of the absolute normal hydrogen electrode (NHE) (VNHE = 4.4 - 4.8 eV). Since the vacuum energy in the presence of an external electric field changes as a function of distance from the metal surface, it is extremely difficult to compare computed electrode potentials with observed values.

3.2.2 Double Reference Method for Simulating Solvent-Metal Interfaces Solvent-metal interfaces occur in a range of electrochemical devices, such as Li ion batteries, aqueous electrochemical capacitors, electrolytic cells for water splitting or cells for coelectrolysis of carbon dioxide and water to methanol, and fuel cells (e.g., direct-methanol fuel cells) They also feature in phenomena such as electrochemical corrosion, as well as in electrochemically-deposited electronic and magnetic metallic films.89 The double reference


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method, developed by the Neurock group,77,

90

models the aqueous-metal interface by

introducing explicit water gas phase molecules into the vacuum region between metal slabs . In this method, a charged slab is generated and is charge-compensated with additional background charges, which polarize the water region and thereby simulate the electrochemical double layer. The double reference method has two reference potentials.89 The first is a vacuum reference, which determines the work function of the aqueous-metal interface at zero external charge. Since there is no vacuum layer in an aqueous/metal model, the work function is determined by a single-point DFT calculation by considering a different computational setup, where an optimized, solvated system where a ca. 20 Å vacuum thickness is inserted in between the water layers that interact with the metal surface at zero charge. The second reference is an aqueous reference that relates the vacuum reference to a tunable potential that is varied by addition or removal of background electrons from the unit cell. Since the background charges create an electric field at the interface, the vacuum reference potential cannot be determined. Instead, when the aqueous region is large enough, the region in the middle of the water layers (i.e., between the upper and lower slab surfaces) is assumed to be unaffected by the charged system. Thus, some of the water molecules distant from the metal slab are fixed in their bulk position at zero charge, while the rest of the system is relaxed with the applied charges. The electrode potential in the middle of the aqueous region far from the electrode is then used as the second reference potential. The electrochemical potential of a charged system (3456 (7)) is given by Equation (5): 3456 (7) = −4.85 − [3 >?@A (7) − 3B (7) $ 3B (0)]

(5)

where -4.85 eV is the experimentally-determined electrode potential of the normal hydrogen electrode (NHE); 3B (0) is the work function at zero charge (from the first reference


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calculation); and 3 >?@A (7) and 3B (7) represent the Fermi level and the work function of the aqueous-metal interface system, respectively, with charges (q) obtained from the second reference calculation. The double reference method is suitable for accounting for both solvent and electric field effects in electrochemical devices. An advantage of this approach is that the calculated potential can be directly compared to a measured potential in an electrochemical cell. Similar to the Scheffler-Neugenbauer method described above, the double reference method can also accurately capture the effect of an electric field on the underlying structure of the solvent molecules that interact with the metal surface, as well as the influence of a field on the energetics of the solvent-metal interface. However, the approach is very sensitive to the size of the unit cell used to model the interface, due to variation of the electrostatic potential throughout the continuum countercharge. It is important to adopt an appropriate water structure so that the computed potential conforms to a physically reasonable result. Furthermore, simulations of such aqueous-metal systems are computationally expensive, limiting the underlying complexity of the electrode/electrolyte systems that can be modeled.

3.2.3 Linear Free Energy Relationships Elementary reaction free energies and activation barriers for electrocatalytic systems are influenced by the electrode potential in an electrochemical cell. The groups of Nørskov,91 Janik,92 Nikolla,93 and Heyden,94 have employed linear free energy relationships to perform field-dependent calculations of reaction and activation energies. In a fuel cell or an electrolysis cell environment, the change in Gibbs energy (∆E()) and the activation energy barrier (") ()) in the presence of an electric field (F) are defined as:


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ACS Catalysis

∆E() = ∆E( = 0) − FG∆

(6)

") () = ") ( = 0) − F,G∆

(7)

where ∆E( = 0) and ") ( = 0) represent the change in the Gibbs energy and the activation energy barrier, respectively, for an elementary step under experimentally relevant temperatures and pressures in the absence of an electric field; n stands for the number of electrons involved in the elementary step; α is the transfer coefficient, which ranges from 0.3 to 0.7 for most electrochemical systems; and ∆ is the overall potential, which is the electric potential difference between the electrode and electrolyte. In the absence of an electric field, computations are performed by combining DFT calculations with statistical mechanical techniques. The calculated ∆E( = 0) and ") ( = 0) values are then used to determine the rate and equilibrium constants for each elementary reaction to establish a field-dependent microkinetic model. However, these DFT computational studies do not take into account electric field effects on the corresponding adsorbate geometries. The linear Gibbs energy relationship, developed by the Nørskov group, predicts the catalytic activity of a transition metal or alloy surface in an electrochemical cell via standard DFT calculations for heterogeneous reactions. Such an approach can save a large amount of computational time as the complexity of the system increases. However, in contrast to the two methods described above, it neglects the effect of an electric field on the structures of adsorbates in the underlying DFT calculations. Therefore, such an approach is suitable for roughly predicting the reactivity of a system in an electrochemical cell, but is not appropriate for quantitative comparisons to experimental results, such as those obtained from STM, FIM, FEM, PBP, CC, and capacitor reactors. 3.2.4 Computational Method Summary and Future Directions


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Currently, three common computational methods can be implemented when an external electric field is applied: the Neugebauer and Scheffler method (for a vacuum-metal system), the double reference method (for solvation-metal system), and the linear free energy relationship (for roughly predicting the Gibbs free energy and activation energy for an electrochemical cell). The Neugebauer and Scheffler method can take changes in adsorbate/surface geometries into account, but it is extremely hard to relate the applied electric field directly to the experimentally applied potential in an electrochemical cell. The double reference method of applying an electric field can include solvation effects, as well as the interaction between water and adsorbates. In addition, the output can be compared with experimental results involving an applied potential. However, this method can be very time-consuming and can only be applied to simple reactions, such as water splitting. The use of linear free energy relationships can save large amount of computational power, but cannot capture geometry changes in adsorbates that occur when an external electric field is applied. Future computational efforts in field-assisted catalysis should take into account: (1) geometry changes that arise due to the application of an external electric field; (2) solvation effects in electrochemical devices that go beyond an implicit solvation model; (3) quantum effects of the phenomena under consideration that are currently not captured in classical models.16 Future method should produce results that are readily comparable to experimental electrocatalysis results in which an external electric field is applied. To develop such a method, one could begin with a reactive force field model (ReaxFF)95-99 and use machine learning methods to incorporate quantum-mechanical effects as needed.100-101

3.3 Electric Field Effects on Catalytic Reactions: A Theoretical View


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3.3.1 Electronic Interactions between Adsorbates and Metal Surfaces As mentioned in Section 3.1, an electric field can alter the electronic structure of adsorbates and will change their electronic interactions with a metal surface. Figure 5 shows the post-reaction-determined water coverage for a Ni methane steam reforming (MSR) catalyst, obtained by analysis of the corresponding experimental X-ray photoelectron spectra. The surface coverage of water increases monotonically with the applied electric field strength.6 This result agrees with DFT calculations that predict stronger adsorption of H2O molecules in the presence of a positive electric field relative to the absence of an electric field or for negative external field values. Figure 7a also shows that high electric fields (> ±0.4 V/Å) can significantly alter the structure of adsorbed H2O molecules on Ni(111). The method used to create an external electric field in the DFT calculations was insertion of a dipole layer in the middle of the vacuum-metal interface. Filhol and Neurock used the double reference method to apply an electric field to a water/Pd interface and found similar results: the optimized structure of the water layer on Pd(111) is an H-down structure when a negative field is applied, but switches to an H-up configuration when a positive electric field is applied. An applied electric field can also change the structures of adsorbed alcohols and alkoxides (e.g., methanol, ethanol, methoxide).27-28, 48, 102 For adsorbates with no permanent dipole moments (e.g., methyl, Had and light alkyls),57, 80 the electronic interaction with a metal surface is much less significantly affected by the presence of an external electric field.


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Figure 7. (a) Water adsorption energy on a Ni(111) surface as a function of the applied electric field strength. The inset pictures show differential charge densities for a H2O molecule on Ni(111), with an isosurface level of 0.003 electrons/Bohr3. The yellow and blue areas represent the gain or loss of electrons, respectively.5 (b) Experimental X-ray photoelectron spectroscopy (XPS) experiments reveal the dependence of the water coverage on a Ni surface after its use in the MSR reaction at different electric field strengths.

The presence of an applied electric field can also influence adsorption-site preferences and vibrational frequencies of adsorbates on metal surfaces. Koper et al.62 studied fielddependent binding energies and intramolecular vibrational frequencies for CO on the flat, hexagonal surfaces of five Pt-group metals (Pt, Ir, Pd, Rh and Ru), which were modeled using a cluster model. According to DFT, negative fields and low CO coverages cause Pt and Rh to switch from top to hollow binding-site preferences, while other metals (Ir, Pd, and Ru) exhibit uniform site preferences. Over a field range of ± 0.5 V/Å, the CO binding energy decreases by ca. 0.3 eV for adsorption at hollow sites on Pt and Rh, while adsorption energies for atop sites on Ir, Pd, and Ru change only slightly. This suggests that the rate of CO oxidation in an electrochemical cell (e.g., a low-temperature fuel cell, direct alcohol fuel cell, or direct methane


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solid oxide fuel cell) can be influenced by the field strength and orientation for certain metals. Alavi et al.70, 103-104 simulated the influence of an electric field on the vibrational frequencies of adsorbed CO on Pt(111) using DFT calculations. IR measurements showed from the slope (dυCO/dF) that the CO frequency (υCO) changes as a function of the electric field strength (F), in agreement with the DFT calculations. An applied electric field can also change the oxidation states of adsorbates and the metal surface, which can directly influence electronic interactions in the adsorbate/metal system. For example, the Grabow group105 studied methane oxidation over four model systems of increasing complexity, namely, Pd(100), Pd(211), PdO(101), and Pd10/γ-Al2O3(110), using DFT simulations and temperature-programmed oxidation (TPO) experiments. Each surface was further modified with a variety of metal promoters (Co, Pd, Cu, Ni, and Pt) to study reactivity trends. Pt was a better promoter than other transition metals, due stronger electrostatic interactions between methane and the Pt surface in the transition state. The authors proposed that changes in the localization of charge density are ultimately responsible for promoting or inhibiting CH4 activation for Pd-based catalysts. These results correspond well with those from an investigation into the field-dependence of the MSR reaction on Ni(111) by the McEwen group,40, 78 who found that the oxidation states of MSR-related species, particularly polar species (e.g., CHxOH, HxO and CHxO), were significantly altered by the presence of an external electric field. When the catalyst has a permanent dipole moment, such as in Ni/YSZ (Ni supported by yttrium-stabilized zirconia, the anode of the SOFC system)81 and Ni-doped V2O5 (the cathode in rechargeable batteries),106 an electric field can also induce substantial changes in metal oxidation states, and thereby change the energetics of heterogeneously-catalyzed reactions.


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3.3.2 Thermodynamic and Kinetic Properties of Reversible Electrocatalytic Reactions As described in Section 3.3.1, a high electric field can alter electronic interactions between adsorbates and catalytic surfaces and consequently affect the thermodynamic and kinetic properties of reversible catalytic reactions, including changing the most favorable mechanism; altering the energetic profile of the overall reaction; and modifying activation barriers, rate constants, and equilibrium constants of individual elementary reactions. Karlberg et al.107 applied the linear free energy method (discussed in Section 3.2.3) to the electrochemical reduction of O2 over Pt(111), and found that a positive electric field can change the mechanism from

dissociative

( H( → H(,)*I KLMJ 2HO)*I KLMJ 2O( H ) (5

(5

to

associative

( H( → H(,)*I

KMJ HHO)*I , HHO)*I KMJ 2HO)*I KLMJ 2O( H). Similarly, the McEwen group’s DFT calculations 5

5

(5

predict that a high external electric field can change the most favorable mechanisms for MSR and water dehydrogenation-formation reactions over Ni(111).5, 40, 78 Corresponding experimental results showed that the orientation of the electric field can alter the overall mechanism in the MSR reaction. Chuah et al.27,

48

studied methanol decomposition on Rh by pulsed field

desorption mass spectrometry. When the electric field is higher than 0.4 V/Å, methanol desorption results in deceleration of the decomposition reaction and an increase in the amount of adsorbed CH3O and CH2O species, in good agreement with the experimental PFDMS results discussed in Section 2.2. In addition, Filhol and Neurock77, 90 predicted that electroreduction of water will occur when the electrode potential is below 0.5 VNHE, and will be accompanied by the formation of OH- ions. Likewise, when the electrode potential exceeds 1.1 VNHE, the electrooxidation of water is predicted to occur and to be accompanied by the formation of H3O+ ions.108


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In predicting reactions over transition metal or alloy surfaces in the presence of electric fields, linear energy correlations can simplify and reduce the number of theoretical calculations needed for non-field-dependent reactions. Therefore, to develop new electrocatalysts rapidly, it is important to establish field-dependent BEP (Brønsted−Evans−Polanyi) correlations. A BEP correlation is a linear relationship between the reaction energy and its activation energy. However, field effects on BEP correlations are not well understood. Vojvodic and Norskov109 simulated a local electric field by promoting transition metal surfaces with alkali metal (oxides) and exploring their effect on N2 dissociation. The BEP correlations for N2 dissociation over individual transition metals parallel-shift downwards (i.e., corresponding to lower energy barriers for a given N adsorption energy) when an alkali-promoted transition metal surface is used as the catalyst. Similarly, there is a linear relationship between reaction energies and activation energies for C-H and C-O bond cleavage during the MSR-on-Ni reaction in the presence of external electric fields.40 However, the reaction energy and activation energy for OH bond cleavage on Ni(111) for the same reaction do not fit well in the BEP correlations in the presence of electric fields. The reason might be that the species involved in the elementary steps related to the O-H bond cleavage are polarized, and consequently, their structures are significantly altered by the presence of a high electric field. Calle-Vallejo and Sautet110-111 showed that BEP relationships are influenced not only by the metal activity but also by the number of metal valence electrons and the coordination modes of the adsorbates. There are only limited studies combine DFT calculations with experiments to explore external (or local) electric field effects on heterogeneous reactions. For example, few studies elucidate electric field effects on water ionization in electrochemistry. Both DFT calculations and ramped field desorption experiments showed that the field requirements for water ionization


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decrease as the size of the water cluster increases, and as the temperature increases, the field requirements for water ionization decrease.6,

112-114

These effects originate in a decreased

activation energy for water ionization with water cluster size and electric field strength. In addition, as mentioned briefly mentioned in Section 2.2, the presence of an electric field can play a significant role in determining nanopatterns over different nanofacets of Rh in the H2/O2 system under FIM conditions.26 That study indicated that reaction rates and the performance of heterogeneous catalysts can be influenced by external electric fields. In a study of the field-dependent MSR reaction,40 McEwen et al. found that both the activation energy and the reaction energy for CH dissociation decreased monotonically as the electric field strength increased from negative to positive values, as shown in Figure 8. Accordingly, the amount of surface carbon detected on the Ni surface post-reaction in the presence of a positive electric field in a continuous-circuit reactor decreased dramatically, compared to the amount found in the absence of an electric field. However, the negative field data showed unexpected side reactions which are still under investigation (Figure 8b). In addition to establishing a field-dependent microkinetic model, the calculations predicted that a positive electric field will lower the temperature requirement significantly, since the field decreases the energy requirement of the rate-limiting step.78 These findings resemble those of the Sekine group, who found that an applied electric field causes methane conversion to increase during the MSR reaction over various metal surfaces in a PBP reactor at lower operating temperatures than are typically used for such reactions.34-35, 37-38


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Figure 8. (a) Brønsted-Evans-Polanyi (BEP) correlations for the CH dissociation reaction on Ni(111), with rate constants (kf) at 1073 K (inset); (b) graphitic C as a fraction of total surface C, from XPS analysis of the post-reaction Ni surface.40

As a final example, we briefly consider how an external field can be used to facilitate the direct selective oxidation of methane to methanol. This reaction is a highly desirable alternative to the indirect route, via the formation of syn gas. We further consider the influence of water, which has been argued to influence the underlying reaction mechanism for the selective oxidation of methane significantly.115-118 The potential energy landscape is depicted in Figure 9. Adsorption energies in steps 1 to 4 were obtained by DFT calculations, considering the effect of the external electric field on isolated reactants and products on a Ni metal surface, as shown in Equation (8) (following previous work):56 " = "P5Q ∗ $ "5S T∗ $ (6 − V − W)"5 ∗ − "P5X (Y)I) − "5 T(Y)I) − (8 − V − W)"4A_I)[ (8) Adsorption energies for step 5 were obtained similarly using Equation (9): " = "P5\ T5 ∗ $ 2"5 ∗ − "P5X (Y)I) − "5 T(Y)I) − 3"4A_I)[

(9)

For step 6, the influence of the external electric field was computed for methanol in the gas phase, Equation (10):


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" = "P5\ T5(Y)I) $ 2"5 ∗ − "P5X (Y)I) − "5 T(Y)I) − 2"4A_I)[

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(10)

Figure 9 shows that the presence of an external electric field strengthens the adsorption of the reactants and lowers the desorption energy of the products. Consequently, the influence of electric fields on other selective oxidation reactions is an exciting area for future research.

Figure 9. DFT calculations on the effect of an external electric field on the partial oxidation of methane to methanol over Ni(111).56

3.3.3 Fundamental Insight into Electric Field Effects The incorporation of electric fields into the design of heterogeneous reactions requires that we elucidate their role in catalysis. In this section, we hypothesize that the oxidation state at the surface of the transition metal is altered by the presence of an external electric field, altering the catalytic activity and the reaction selectivity.119 DFT calculations reveal that the rate of the MSR reaction, which is determined by the first C-H bond cleavage in the methane molecule, is closely related to local oxidation state changes at the transition metal surface. Thus, applying a positive electric field increases the oxidation state of Ni surface atoms, which greatly facilitates the activation of methane.119 Similarly, the Bell group120 reported that the oxidation state of Pd


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affects the rate of methane activation, while the Neese group121 stated that higher metal oxidation states facilitates C-H bond cleavage. Changes in adsorption and/or reaction energies in the presence of an electric field

(∆^ = ∆+ ∙ # $ ∆, ( ) are related to the dipole moment (∆+) and the polarizability (∆,) of the ( adsorbate system. Thus, electric fields will exert larger effects on polar systems relative to nonpolar ones. For example, Gorin et al.41 applied interfacial electric fields in a capacitor reactor to alter the selectivity and activity of Al2O3 catalysts during the rearrangement of cis-stilbene oxide. The presence of an electric field increased the conversion of cis-stilbene oxide to aldehyde and ketone products by factors of up to 10 times. Furthermore, the aldehyde-to-ketone product ratio increased from 1:4 to 17:1.

4. Other Experimental Investigations of Electric Field Influences

4.1. Introduction Few experimental investigations have examined the effects of the non-electrochemical activation of chemical reactions using external electric fields, and many of those used inconsistent methods that are difficult to compare to one another and to theoretical predictions.5, 37, 122-123

Here we distinguish between internal and external electric fields by the absence or

presence of an applied electrical potential. Thus, an external electric field is defined by the utilization of an external power source to create the required surface electric field over the catalytically active metal surface. Internal electric fields will be discussed further in Section 5. In this section, we highlight some different empirical approaches, including the use of scanningtunneling microscope nanoreactors, probe-bed-probe reactors, integrated circuit reactors, and


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capacitor reactors (Figure 10). We discuss the methods, offer thoughts on the practicality of each, and compare their results to theory. The ideal experimental system is one that is readily modeled using theory and is easily scaled to realistic reaction conditions. Thus far, no single approach satisfies both of these criteria particularly well. Due to the complex geometries of the real catalyst surfaces involved, and the lack of experimental measurement techniques, it has proven to be extremely difficult to adequately define the surface electric fields generated in these reactors. This complicates comparison of the practical reactor with the virtual one. Whereas surface fields can be very nicely defined and set to any arbitrary strength in a reaction simulation, the real situation is much messier and more difficult to describe. For the purposes of this discussion we assume only the most ideal surfaces and classical physical behavior of the electric field with the understanding that the real situation will be much more complex. Fields are defined with respect to the surface, such that a positive field is one that points away from the surface as if generated by the presence of a positive charge. Field strength cannot be measured directly, but can be estimated using physical models. This is easiest and most accurate for simpler systems. Models for field strength within a capacitor are taught at an undergraduate level, and models for field strength outside a scanning-tunneling microscope tip

124

as well as

around the wires and elements in a circuit are readily available.125-126 Small defects, the addition of micro or nanoparticles, and pore structures add additional layers of complexity for which there is currently no good solution, but all of which theoretically increase field strength. Field enhancement effects are well known to occur around very small irregularities. In fact, this is how the STM tip is able to generate such high electric fields with such a small applied voltage. Thus, the current approach is to model idealized versions of whichever reactor is in use, and assume the result will be slightly larger in reality due to the presence of defects, particles, and pores.


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Figure 10. Simplified schematics of the four reactor types to be discussed in this section: (a) scanning tunneling microscope (STM) probe “nanoreactors,” (b) probe-bed-probe reactors, in which a catalyst bed is placed in the gap between two probes, (c) continuous-circuit reactors, in which the catalyst bed is integrated into an electric circuit (color gradient in wire represents surface charge gradient), and (d) capacitor-type reactors. Red arrows indicate general field structure; R/R´ denote adsorbed reactants.

4.2. STM Probe Nanoreactors One of the most difficult aspects of designing an experimental system for testing the effects of electric fields in heterogeneous catalysis is achieving the very large electric fields predicted by theory to be necessary to affect molecules significantly (0.1-1.0 V/Å).127 It is wellknown that very high electric fields develop at sharp points in charged conductors, being the principle by which lightning rods, field-ion microscopes, and scanning-tunneling microscopes (STM) work. For example, an electric field on the order of 0.1 V/Å can be generated with an applied electrical bias of 1 V if a metal tip with a 1 nm diameter is used. Because of this property


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and their relative ease of use and measurement, STM probes are well-suited as “nanoreactor” systems for testing the effects of electric fields on single-molecule reactions. Such an experiment involving a Diels-Alder reaction was recently reported by Aragonès et al.32,

122

A diene

covalently bonded to a gold STM tip was brought into contact with the dienophile-covered substrate (R/R´ in Figure 10a) using tapping mode. Reactivity was determined by measuring the electrical current “blinks” established during bonding events. Molecular junctions were found to be destroyed after withdrawing the tip ~0.2 nm from the surface, and the reactivity was highly dependent on both the strength and direction of the applied electric field due to transition state stabilization arising from variation of the adsorbate-surface interaction, as explained in Section 3.3. The effect of the electric field on the molecular configurations is described in the Supplementary Information of that study. The Diels-Alder reaction is particularly orientation-dependent: the diene and dienophile must be aligned properly (generally, in the more stable endo configuration) before electron exchange can occur. If multiple products are possible, then the specific orientation of either reactants or key intermediates can result in the selective formation of one particular product. No data were reported to validate this hypothesis in the Aragonès study, but the possibility of four Diels-Alder products is mentioned in their Supplementary Information.122 Modeling the effects of the electric field on the reaction using Gaussian 09 showed that the magnitude and direction of the electric field alters whether the exo or endo configuration is favored. This study shows that the electric field can change the behavior of a reactive process in multiple ways, all of which must be carefully considered in implementing an electric field-assisted catalytic system. Similar effects were noted by Alemani et al.128 in another STM tip reactor study involving the isomerization of azobenzene under varying electric field conditions. Here, the main


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effect was molecular configurational changes of products depending on the applied voltage. Above a certain “threshold voltage”, the molecule switches from the trans to the cis isomer. The effect is reversed if electric field pulses are applied below the threshold voltage. This work demonstrates that electric fields can influence molecular configurations, even to the point of chemical bond restructuring or isomerization. Thus, in a highly orientation-dependent reaction, the effect of the electric field could cause a significant change in reactivity and/or selectivity. For reactions with less configurational dependence (e.g., symmetrical systems), we would expect other factors like transition state (de)stabilization to play a more important role. STM-tip “nanoreactor” experiments are good systems for comparison to theoretical predictions. The single- or few-molecule scale and high-vacuum nature of the experiments closely approximate the approach taken in computational efforts, in which one or two molecules are modeled on an idealized surface in a perfect vacuum. Currents produced during STM measurements generally range from pico- to nanoamperes. Currents this small usually do not generate confounding Joule heating effects in conductive systems, but given the few- or singlemolecule nature of the measurements, the currents could have unintended side-effects. More work with these systems is expected to greatly improve our understanding of the fundamentals underpinning electric field-assisted catalysis. A significant disadvantage is that the connection to more realistic processes is problematic, since many effects are not considered in the “nanoreactor” system, such as competitive adsorption on the catalyst surface, poisoning, and elevated pressures and temperatures. Furthermore, the catalyst itself is limited to conductive materials that can serve as STM tips, and the system cannot be scaled up. STM-tip “nanoreactor” systems should therefore be considered an extremely useful tool for understanding some of the scientific principles of electric field-induced catalysis with close connections to theoretical


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predictions, but extrapolation of the results to larger-scale reactions must be done with great caution. 4.3. Probe-bed-probe Reactors The most thoroughly explored reactor systems to incorporate electric field effects in heterogeneous catalysis are probe-bed-probe (PBP)-type reactors. A traditional packed catalyst bed (e.g., supported metal nanoparticles) is loaded into a reactor and metallic probes are placed on either side (e.g., top and bottom). A high voltage is then applied across the probes, establishing an electric field between them. There is a relatively large body of literature based on this type of reactor from Sekine and coworkers.35, 37, 39, 129-149 It uses the same type of reactor setup shown in Figure 10b with various catalysts, supports, applied current conditions, electric potential, external temperatures, and catalytic reactions, as summarized in Table 1. Work conducted between 1999 and 2008 reports on the formation of plasmas, or coronal discharges, based on the current reported flowing between the two disconnected probes. Other groups working with PBP-style reactors have reported similar phenomena.150-156 From 2009 onward, however, the focus shifted to studying the “electric field” phenomenon in the same reactor setup, with current flowing between the probes. Table 1. An overview of PBP-type reactor work carried out by Sekine and colleagues. Year

Reactionb

Catalyst

Phenomenonc

None None None Pt/SiO2, Ni/C None None None Lindlar, Ni/C

DCPD DCPD DCPD CD, DCPD DCPD CD, DCPD CD, SD CD, SD

NR NR 4-7 NR

OCM MSR DDCM OCM OCM, DDCM POxM DDCM MSR, DDCM, OCM MSR EtOH-SR EtOH-SR Dec. EtOH

DCPD DCPD DCPD DCPD

4-10

OM

None None None Pt on CeO2, ZrO2, TiO2 MgO, ZrO2,

1999 129 2001 130 2003 131 2003 132 2003 133 2003 134 2004 135 2004 136

Electrode gap (mm)a 1.5 1.5, 10 1.5, 10 1.5, 2.0, 10 1.5, 5.0 1.5, 10 5.0 5.0

Temp. (K)a ambient 453 300-773 ambient ambient ambient 420-800 420-960

Current (mA)a 4 3 2.0-4.0 0.5-8.0 1.0, 2.0 2.0-4.0 20000 NR

Voltage (kV)a 6.25d 1.07-8.0d 1.8-4.2 0.3-7.4 NR 1.8-4.2 5.0 NR

2004 137 2005 138 2005 139 2008 140

2.1 NR 6.0 NR

393 ambient ambient 423-623

NR NR 2.0-15.0 2.0-3.0

2008 141

6.0

ambient

2


DCPD

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2009 37 2010 142

3.0-5.0 5

425-700 423-523

3.0-8.0 3

0.15 0.13-0.60

2011 143

5

423-753

3

0.46-0.77d

Dec. EtOH EtOH-SR, Dec. EtOH, WGS, MSR MSR

2012 145

NR

423

3

0.9-3.2

OCM

2012 144 2013 146 2013 35 2014 39

10-60e NR 3.0-6.5 NR

423-1650 423 423 423-723

1-9 3.0-7.0 3.0 3.0-7.0

0.4-2.0 0.6-2.3 0.43-1.16 0.87-1.65

OCM OCM MSR Rev. WGS

2016 149 2016 148 2016 147

1.1 1.0 NR

398-823 423 423

5 3.0-13 1.5-20

NR 0.1-0.9 0.1-1.3

MSR WGS OCM

2017 157 2017 158

NR bed height

423-1073 423

1.5-7.0 3-10

0.1-1.7 1.2-2.5

OCM OCM

2017 159 2017 160

NRf 4.0

541-584 473-823

3-12 5

0.4-1.2 0.03-0.29

DRM MSR

a

TiO2, CeO2 Pt/CeO2 Pd, Pt, Rh on CeO2 Pt, Rh, Pd, Ni on CeO2 La2O3, SrLa2O3 Sr-La2O3 La-ZrO2 Pt-CeO2 metals on LaZrO2 Pd-CeO2 Pt/La-ZrO2 WO3/CeO2 mixtures PW12O40/CeO2 LaCaAl perovskites Ni/La-ZrO2 Pd/CeO2

EF, DkC EF EF, DkC EF, DCPD EF, DkC EF EF EF EF EF EF EF EF EF EF

NR = not reported. OCM = oxidative coupling of methane, MSR = methane steam reforming, DDCM = direct dehydrogenative coupling of methane, MC = methane cracking, POxM = partial oxidation of methane, EtOH-SR = steam reforming of ethanol, Dec. EtOH = decomposition of ethanol, OM = oxidation of methane, (Rev.) WGS = (reverse) water-gas shift, DRM = dry reforming of methane. c DCPD = direct current pulsed discharge, CD = corona discharge, SD = spark discharge, EF = electric field, DkC = dark current. d Calculated from reported power values (P = IV). e Calculated from “effective contact time (ECT)” value given in report. ECT = (gap (mm))/(feed rate (mmol/min)) f Reported as “contiguous,” but actually 1-10 mm according to the authors (personal correspondence). b

As an example of the confusion currently surrounding the PBP reactor, compare the experiments conducted by Marafee et al.150 to those conducted by Tanake et al.145 and Oshima et al. (both from the Sekine group). Both sets of experiments used similar PBP configurations with stainless steel probes (rod/plate for Marafee and rod/rod for Tanaka and Oshima). Both tested SrLa2O3 catalysts for oxidative coupling of methane (Marafee used Sr/La ≈ 0.11, while Tanaka used Sr/La = 1/2000-1/20), and both used currents between 3.0 and 3.5 mA with similar voltages. Despite these similarities, Marafee described the phenomenon as a corona discharge, while Tanaka and Oshima claimed it was a pulse discharge over the catalyst with Sr/La = 1/2000 and an electric field over catalysts with Sr/La = 1/200 and 1/20. Corona discharges are often


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called “low-temperature plasmas,” meaning that the gas temperature is low while the electron temperature is high. This type of discharge occurs when a current of ~10-4-10-2 A flows between the electrodes, and is associated with glows whose color depends on the surrounding atmosphere.161-162 Kado et al. reported that the pulse discharge (an intermittent spark discharge occurring at currents of ~1-102 A) in methane glowed a blue color, while coronas in methane are typically red.131 Yet Liu et al. reported a pink glow during corona discharge in methane/oxygen over Sr-La2O3.152 Images of the reactor published by Tanaka et al. show the reactor operating under “electric field” conditions at ambient temperature with a dark red glow clearly visible within the catalyst bed.145 Given the similarities in reactor operation and observations, it is unclear why the phenomena should be attributed to discharge (whether spark or corona) in the experiments of Marafee, Kado, and Liu, but an electric field effect in the work of Tanaka and Oshima. Clearly, a standard must be set to distinguish between the operation of a PBP with plasma vs. with a pure electric field. A reasonable criterion would be to consider any opencircuit system with current flow to be a plasma or discharge rather than an electric field. After all, how else would the current arise? Furthermore, it can be difficult to distinguish between an electric field effect and the effect of Joule heating arising from current flowing through the catalyst bed, since only the external temperature is measured in many studies.


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Figure 11. Metallic particles embedded in a uniform electric field will have non-uniform surface electric fields in the reference frame of molecules adsorbed on the surface (inset).

Assuming that a PBP reactor could be constructed such that only an electric field phenomenon was produced (i.e., without plasma formation), issues would still arise with respect to comparison to theory. It is impossible to generate a consistent electric field across a particle in a PBP configuration, since the surface electric fields experienced by reactive molecules will differ depending on their location on the catalyst surface. For example, a “negative” surface field on one side of a particle corresponds to a “positive” surface field on the opposite side of the same particle, as shown in Figure 11 (note that the field strength drops to zero around the “equator” of the particle). Although theory predicts that positive and negative fields will have different effects on reactants, a PBP-type reactor cannot be used to test these effects individually. In reactions where one field type has a stronger effect than the other, a net change may be observed, but otherwise the effects should nearly cancel. It is also possible that a positive field offers certain benefits and a negative field others, such that when taken together a net effect can still be achieved. For example, in the steam reforming of methane, a positive electric field facilitates


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adsorption of water while a negative field facilitates activation of water, both being necessary steps in the reaction of water with methane.5 Thus, a mixture of fields may provide both benefits simultaneously, although molecules will have to migrate across the particle, since the effects are not available in the same region. In summary, a PBP reactor, if implemented without plasma formation, could be a useful device for testing large-scale chemical reactions in an electric field. Connection with theory is extremely difficult because of the inconsistent surface electric fields established over individual catalyst particles. This reactor type could, however, still find use in large-scale or commercial applications where mixed effects (both positive and negative fields) might be desirable. 4.4. Continuous-Circuit Reactors Establishing a uniform surface electric field is necessary to allow direct comparison of experimental results with theory. The STM probe guarantees a high-strength, uniform electric field. Another method to establish uniform fields is to incorporate a conductive catalyst bed into an electrical circuit in the so-called “integrated circuit” (IC) reactor, as shown in Figure 10c. The key difference between IC reactors and PBP reactors (discussed in Section 4.3) is that a PBP reactor relies on an external probe to create an electric field in a catalyst bed, while an IC reactor generates a surface field by taking advantage of the properties of an electrical circuit. Another feature of IC reactors is that additional circuit elements can be incorporated into the circuit to alter the natures of the electric field generated. For example, the addition of a capacitor into the circuit can change the charge distribution across the wires and make analysis of the field strength at the catalyst surface much easier.125-126 These other circuit configurations have not yet been tested, however, so only the simplest circuit designs are considered here.


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Applying an electrical bias to a conductive wire establishes surface charges that provide the driving force for current flow.125, 163-164 The color gradient in Figure 10c depicts this charge gradient graphically. If a conductive catalyst bed is incorporated into such a circuit, charges on the catalyst will establish a uniform electric field over the catalyst surface (as long as the circuit is unsymmetrical—see Figure 10c). Results from this type of reactor were compared to DFT calculations by Che et al., with very good agreement.5, 40, 78 These studies looked specifically at the decomposition of water5 and the formation of various surface species such as elemental carbon (coke)40 during MSR. Trends in experimental H2O decomposition rates corresponded very well with theoretical predictions, with water coverage increasing in a positive electric field and decreasing in a negative field; however, decomposition was facilitated in a negative field and retarded in a positive field. The correspondence between theory and experiment was close for carbon-containing species only in positive fields; investigations are still underway to determine the reasons for significant deviations in negative fields.40 While capable of establishing uniform surface electric fields and allowing for testing of more realistic reaction conditions than STM probes, IC reactors suffer from quantifiability issues. It is very difficult to determine exactly the magnitude of the surface electric field when a given voltage is applied. Using the methods of Assis et al.,163 it is possible to obtain a rough estimate by measuring the applied voltage, the overall resistance of the circuit, and the current flowing through the circuit. Current flowing through an IC reactor is limited to be as low as physically achievable in order to avoid Faraday reactions and Joule heating of the catalyst. This is achieved either by using specific circuit elements (such as capacitors) or by using a power box capable of supplying voltage with no current. The location of the catalyst bed with respect to the power supply is also important, since surface charges vary as a function of distance from the


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source. Using the experimental parameters established by Che et al. (200 V, 0.5 Ω, 1 µA, and 0.1 m), and the catalyst particle size, the surface electric fields were estimated to be on the order of 0.01-0.1 V/Å. Decreasing the catalyst particle size to q: o

Tip: l = − n

an in

(A4)

no

where o is the radius of curvature of the tip. The screen is taken as the isopotential surface c = nr $ o at the distance r from the apex of the needle:

Screen:

o

l=r$ − n

an in

(A5)

n(nr o)

The screen is far enough from the tip r ≫ o so that it can be considered to be flat. In the vacuum between the needle and the screen, the Laplacian of the electric potential t vanishes. In parabolic coordinates,189 the Laplacian is given by u nt =

v

w

wt

v

w

wt

m wn t

xc wc y $ c d wd xd wd y $ cd whn = q c d wc

(A6)

Since the needle and the screen have isopotential surfaces, the electric potential is a function of the sole coordinate c, so that Laplace’s Equation (A6) reduces to the differential equation w

wt

xc wc y = q wc

(A7)

which has the solution t(c) = z {k c $ |. The constants of integration A and B can be fixed according to the values of the potential at the tip and the screen. Since c = l $ ban $ in $ ln , we obtain:


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t = t}~ −

t

nrJo {k o

{k

Page 56 of 69

l ban in ln

(A8)

o

with the potential difference t = t}~ − t . The electric field is perpendicular to the metallic surface and proportional to the surface density of charges via the vacuum permittivity: # = − ∙ ut|}~ = −n

c wt

c d wc c'o

=

q

(A9)

where we express the gradient in parabolic coordinates. We note here that is the unit vector normal to the surface. On the isopotential surface c = o, the other coordinates vary as d = (an $ in )/o and h = ek(i/a). Accordingly, the electric field at the surface of the tip takes the value # =

nt

nrJo {k o

m

bon an in

(A10)

If we denote the position of a point on the surface by its radial distance r to the symmetry axis of the paraboloid: = ban $ in

(A11)

and the electric field at the apex of the tip in the direction of its axis pointing outside by #q =

nt

nrJo o

{k

(A12) we find that the electric field normal to the surface varies as given in Equation (1).


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7. Acknowledgments FC thanks The American Chemical Society Petroleum Research Fund for partial support. JSM was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences (Award DE-SC-0014560). SLS acknowledges the support of the Department of Energy, Office of Basic Energy Sciences, under the Catalysis Science Initiative (DE-FG-02-03ER15467). JSM also acknowledges P. Gaspard and L. Grabow for fruitful discussions.

References 1. Kreuzer, H. J., Surface Physics and Chemistry in High Electric Fields. In Chemistry and Physics of Solid Surfaces VIII, Vanselow, R.; Howe, R., Eds. Springer Berlin Heidelberg: Berlin, Heidelberg, 1990; pp 133-158. 2. Kreuzer, H. J., Chemical Reactions in High Electric Fields. In Surf. Sci. Catal., American Chemical Society: 1992; Vol. 482, pp 268-286. 3. Block, J. H., Field Desorption and Photon-Induced Field Desorption. In Chemistry and Physics of Solid Surfaces IV, Vanselow, R.; Howe, R., Eds. Springer Berlin Heidelberg: Berlin, Heidelberg, 1982; pp 407-434. 4. Kreuzer, H. J.; Wang, R. L. C., Physics and Chemistry in High Electric Fields. Philos. Mag. B 1994, 69, 945-955. 5. Che, F.; Gray, J. T.; Ha, S.; McEwen, J.-S., Catalytic Water Dehydrogenation and Formation on Nickel: Dual Path Mechanism in High Electric Fields. J. Catal. 2015, 332, 187200. 6. Stüve, E. M., Ionization of Water in Interfacial Electric Fields: An Electrochemical View. Chem. Phys. Lett. 2012, 519-520, 1-17. 7. Kreuzer, H. J., Physics and Chemistry in High Electric Fields. Surf. Sci. Anal. 2004, 36, 372-379. 8. Pacchioni, G.; Lomas, J. R.; Illas, F., Electric Field Effects in Heterogeneous Catalysis. J. Mol. Catal. A: Chem. 1997, 119, 263-273. 9. Neugebauer, J.; Scheffler, M., Adsorbate-Substrate and Adsorbate-Adsorbate Interactions of Na and K Adlayers on Al(111). Phys. Rev. B 1992, 46, 16067-16080. 10. Oppenheimer, J. R., Three Notes on the Quantum Theory of Aperiodic Effects. Phys. Rev. 1928, 31, 66-81. 11. Debye, P., Polar Molecules In Polar Molecules The Chemical Catalog Company, Inc.: New York, 1929; p 172. 12. Bergmann, K.; Eigen, M.; De Maeyer, L., Dielektrische Absorption Als Folge Chemischer Relaxation. Ber. Bunsengese. Phys. Chem. 1963, 67, 819-826.


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13. Kreuzer, H. J., Physics and Chemistry in High Electric Fields. In Atomic and NanometerScale Modification of Materials: Fundamentals and Applications, Avouris, P., Ed. Springer Netherlands: Dordrecht, 1993; pp 75-86. 14. Suchorski, Y.; Schmidt, W. A.; Ernst, N.; Block, J. H.; Kreuzer, H. J., Electrostatic Fields above Individual Atoms. Prog. Surf. Sci. 1995, 48, 121-134. 15. Kreuzer, H. J.; Wang, L. C., Field-induced Surface Chemistry of NO. J. Chem. Phys. 1990, 93, 6065-6069. 16. Sellner, B.; Valiev, M.; Kathmann, S. M., Charge and Electric Field Fluctuations in Aqueous NaCl Electrolytes. J. Phys. Chem. B 2013, 117, 10869-10882. 17. S. D. Fried, L.-P. W., S. G. Boxer, P. Ren, V. S. Pande, Calculations of the Electric Fields in Liquid Solutions. J. Phys. Chem. B 2013, 117, 16236-16248. 18. Müller, E. W., Die Sichtbarmachung einzelner Atome und Moleküle im Feldelektronenmikroskop. In Z. Naturforsch., 1950; Vol. 5, p 473. 19. Müller, E. W.; Tsong, T. T., Field Ion Microscopy; Principles and Applications. In Field Ion Microscopy; Principles and Applications, American Elsevier Pub. Co.: New York, 1969. 20. Müller, E. W., Elektronenmikroskopische Beobachtungen von Feldkathoden. Z. Phys. 1937, 106, 541-550. 21. Müller, E. W.; Panitz, J. A.; McLane, S. B., The Atom‐Probe Field Ion Microscope. Rev. Sci. Instrum. 1968, 39, 83-86. 22. Block, J. H.; Czanderna, A. W., Chapter 9 - Field Ion Mass Spectroscopy Applied to Surface Investigations. In Methods of Surface Analysis, Elsevier: Amsterdam, 1975; pp 379-446. 23. Kruse, N.; de Bocarmé, T. V., Heterogeneous Catalysis and High Electric Fields. In Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA: 2008. 24. Liu, N.; Zhang, R.; Li, Y.; Chen, B., Local Electric Field Effect of TMI (Fe, Co, Cu)BEA on N2O Direct Dissociation. J. Phys. Chem. C 2014, 118, 10944-10956. 25. Fried, S. D.; Bagchi, S.; Boxer, S. G., Extreme Electric Fields Power Catalysis in the Active Site of Ketosteroid Isomerase. Science 2014, 346, 1510-1514. . 26. McEwen, J.-S.; Gaspard, P.; de Bocarmé, T. V.; Kruse, N., Nanometric Chemical Clocks. Proc. Nat. Acad.. Sci. U.S.A. 2009, 106, 3006-3010. 27. Chuah, G. K.; Kruse, N.; Schmidt, W. A.; Block, J. H.; Abend, G., Methanol Adsorption and Decomposition on Rhodium. J. Catal. 1989, 119, 342-353. 28. Rothfuss, C. J.; Medvedev, V. K.; Stüve, E. M., Cluster Formation and Distributions in Field Ionization of Coadsorbed Methanol and Water on Platinum. Surf. Sci. 2016, 650, 130-139. 29. Rothfuss, C. J.; Medvedev, V. K.; Stüve, E. M., The Influence of the Surface Electric Field on Water Ionization: A Two Step Dissociative Ionization and Desorption Mechanism for Water Ion Cluster Emission From a Platinum Field Emitter Tip. J. Electroanal. Chem. 2003, 554–555, 133-143. 30. Hofer, W. A.; Fisher, A. J.; Wolkow, R. A.; Grütter, P., Surface Relaxations, Current Enhancements, and Absolute Distances in High Resolution Scanning Tunneling Microscopy. Phys. Rev. Lett. 2001, 87, 236104. 31. Hofer, W. A., Challenges and Errors: Interpreting High Resolution Images in Scanning Tunneling Microscopy. Prog. Surf. Sci. 2003, 71, 147-183. 32. Shaik, S.; Mandal, D.; Ramanan, R., Oriented Electric Fields as Future Smart Reagents in Chemistry. Nat. Chem. 2016, 8, 1091-1098.


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Graphical Abstract:


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Elucidating the Roles of the Electric Fields in Catalysis: A Perspective

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