Elucidating the Roles of Electric Fields in Catalysis ... - ACS Publications

Perspective Cite This: ACS Catal. 2018, 8, 5153−5174

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Elucidating the Roles of Electric Fields in Catalysis: A Perspective Fanglin Che,† Jake T. Gray,† Su Ha,† Norbert Kruse,†,∇ Susannah L. Scott,‡,§ and Jean-Sabin McEwen*,†,∥,⊥,#,∇ †

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States ∇ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡ Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States § Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States ∥ Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, United States ⊥ Department of Chemistry, Washington State University, Pullman, Washington 99164, United States # Department of Biological Systems Engineering, Washington State University, Pullman, Washington 99164, United States 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 pulsed 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 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.12 to assess the effect of dielectric absorption on reaction equilibria in liquid solutions. 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.

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 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. © XXXX American Chemical Society

Received: August 27, 2017 Revised: March 25, 2018 Published: April 2, 2018 5153

DOI: 10.1021/acscatal.7b02899 ACS Catal. 2018, 8, 5153−5174

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ACS Catalysis Kreuzer established the theoretical basis for such high-field surface chemistry using quantum mechanical methods,13 providing insight into the effect of the local electric field strength on individual atoms14 as well as a molecular orbital analysis of the influence of the field 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 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 counter electrode 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-modeled 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 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 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 (PBP) reactors,33−39 in continuous-circuit (CC) reactors,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 as a result of the opposite charges at each

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 study the influence of the field, an arbitrary electric field (the “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 the field pulse amplitudes are adjusted so as to keep the total field, FD, constant.

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 ±0.1−0.4 V/Å is produced within this thin interfacial layer on the order of 3 to 8 Å. The radius of curvature of a metal surface greatly affects the magnitude of the local electric fields (F0 = V0/R for a spherical particle, where F0, V0, 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, Liu et al.44 showed that the current density for electroreduction of CO2 is greatly enhanced when the local electric field strength is increased. 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. We also note that the electric field 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 as a function of position between the apex of the tip and its shank (see Appendix A): 5154


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F=

examples of electric field influences on catalyst selectivity in pulsed field desorption mass spectroscopy (PFDMS) experiments. 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 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 and enzymes as well as how the experimental techniques used in such investigations might aid in the quantification of electric field strengths in other types of catalysts.

F0 2

1+

r R2

(1)

where F0 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, and the distance between the tip and the screen. Further, r denotes the position of a point on the tip surface as a function of its radial distance. At the apex of the tip, r = 0, while at its shank, r = ∞. Thus, an electric field on the order of 1 V/Å present at the apex of the tip will gradually decreases to zero 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, which presents STM images of O on

2. EXTERNAL ELECTRIC FIELD EFFECTS STUDIED BY PULSED FIELD DESORPTION MASS SPECTROMETRY 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 (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 home-built version of the PFDMS setup, time resolution between 100 μs and about 1 s can be achieved at temperatures between room temperature 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 a 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 adsorbed 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 for 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 low activation barrier. However, the influence of the field on the activation barrier was not significant. It remains to be seen whether the presence of an external electric field can 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” 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

These values agree well with results obtained for macroscopic Pt(111) single-crystal surfaces using molecular beam relaxation spectroscopy (MBRS).50,51 In particular, the dominant role of steps as trapping sites for NO molecules was demonstrated. The ability to examine field-dependent processes provides insight not readily accessible by other experimental methods, as demonstrated in Figure 3. A steady electric field was varied at a

Figure 3. Field-induced decomposition of NO on Pt. The probe hole selects about 150 atomic sites (including steps) on the (111) pole of the Pt tip. The steady electric field, FR, is varied while the pulsed field, FP, is adjusted so as to keep the total desorption field constant at FD = 24 V/nm. Under these conditions, adsorbed NO molecules are desorbed quantitatively by the field pulses. A reaction time tR = 10 ms (pulse repetition frequency f = 100 Hz) was chosen for convenience (short data acquisition times) at T = 296 K. The device was operated with a continuous supply of NO at P = 6.7 × 10−5 Pa.

constant total desorption field strength (FD = 24 V/nm) sufficient to achieve complete field desorption of NO adsorbed on Pt(111) during reaction cycles of tR = 10 ms. Only molecular NO+ ions are field desorbed for FR = 0, but various other species (N2O+, N2+, and O+) appear in the mass spectrum when the field strength exceeds FR = 4 V/nm. Thus, adsorbed NO molecules decompose in high electric fields at room temperature. This may be a significant finding with respect to low-temperature deNOx strategies. However, the simple dissociation 2NOad → 2Nad + 2Oad does not account for the underlying mechanism of field-induced NO decomposition. While the presence of N2+ in the mass spectrum could result from recombination of Nad, our understanding of the formation N2O+ and O+ remains incomplete. First, reaction stoichiometry dictates that equal amounts of N2O+ and O+ should be produced, which is not the case according to Figure 3. Second, while high N2O+ intensities demonstrate that field pulses can desorb the entire surface population of N2Oad, the low O+ counts reflect the displacement of Oad from higher-field regions toward lower-field regions (essentially, the shank of the tip). It should be noted that without this field-induced mobility, Oad would accumulate within the Pt(111) area monitored during the measurements, resulting in self-poisoning of the reaction due to blocking of adsorption sites as well as possible formation of an oxide. Formation of N2O in the thermal catalytic reduction of NO over Pt-based catalysts is a well-known and undesired side 5156


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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 in 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.

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 density is 0.003 e/bohr3. The yellow and blue areas represent gain and loss of electrons, respectively.

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 as FR was varied. 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.

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 firstprinciples-based calculations on Ni(111) by Che et al.56 Their work shows that external electric fields also weaken the adsorption strength of methoxy, whose calculated charge distribution is shown in Figure 5b. 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. Since the appearance of the CH3+ signal indicates fieldinduced 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” PFDMS are powerful tools to investigate field effects under controlled and reproducible reaction conditions.

< 2 V/nm), the mass spectra are dominated by reaction products. In Figure 4, the ion intensity ratio (CO+/∑nHn+) = 0.5 at low fields reflects the overall reaction stoichiometry CH3OH → CO + 2H2. 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. It should be noted that in kinetic measurements with low time resolution, product species like COad and atomic hydrogen would accumulate at the catalyst surface, and thus, their mean residence times are long at room temperature. 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. As the steady electric field increases, the field desorption mass spectra in Figure 4 undergo dramatic changes. In order to confirm the assignments, experiments with isotopically labeled 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 values, 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 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). 5157


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3. A THEORETICAL VIEW OF FIELD-INDUCED ELECTROCHEMICAL SYSTEMS 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 in a solid oxide fuel cell (SOFC). A related question is the origin of the increased H2 yield under the lowertemperature operating conditions compared with 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 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., 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. 3.2. Computational Methods for Studying Electric Fields. 3.2.1. Modeling of 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 code64,65 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 ΔD(r) = ε0

∫ εnl (r, r′)·ΔF(r′) dr′

(2)

where ε0 is the permittivity of vacuum, εnl(r, r′) is the nonlocal dielectric tensor, ΔD(r) is the change in the displacement field at position r, and ΔF(r′) is the change in the local electric field at position r′. 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, field-assisted 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 coworkers16 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 of 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 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 layer, as schematically illustrated in Figure 6. 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

Figure 6. Schematic illustration of how one applies an external electric field in a periodic system using the Neugebauer and Scheffler method. The external field is applied by inserting a dipole layer in the middle of the vacuum layer between two slabs. The system on the left depicts a slab model in the absence of an external electric field, while the slab on the right depicts the system under consideration in the presence of an electric field. Reproduced from ref 70. Copyright 2009 American Chemical Society. 5158


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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 eq 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. The 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 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 in order to ensure that the charge density in the vacuum is less than the magnitude of the Gibbs oscillations (0.001 e/Å3) associated with plane-wave 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 field-dependent 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 the 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 are also featured in phenomena such as electrochemical corrosion as well as in electrochemically deposited electronic and magnetic metallic films.89 The double reference method, developed by the Neurock group,77,90 models the aqueous− metal interface by introducing explicit water 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.

surface, as originally described by Kreuzer and co-workers.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 shown in eq 3:57,71−74 E (F ) = E 0 +

⎛ ∂E ⎞ 1 ⎛ ∂ 2E ⎞ ⎜ ⎟ · F + ⎜ 2 ⎟ F2 ⎝ ∂F ⎠ F = 0 2 ⎝ ∂F ⎠ F = 0

(3)

where all of 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 (eq 4): Ead(F) = Ead0 + ΔdF = 0·F +

1 ΔαF = 0 F2 2

(4)

where Ead0 is the adsorption energy in the absence of a field and ΔdF=0 (or ΔαF=0) is the difference in dipole moment (or polarizability)71,76 between the metal slab with adsorbates and the corresponding noninteracting 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 (ΔdF=0) between the initial and final states 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 effective dipole moment analysis can be found in previous publications.5,40,56,78−81 For an elementary reaction step, A + B → AB⧧ → C + D, the field-induced activation energy (Ea(F)) 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 annealing82 as well as in reactions of enzymes, such as ketosteroid isomerase.25,83,84 In those studies, the effect of the electric field on the activation energy for a given reaction was argued to depend on the dipole moment dif ference 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 undissociated CO molecule has a shorter CO bond length relative to the transition state. The change in dipole moment 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 effect of the electric field on the activation energy depends on the magnitude of the difference 5159


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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 ΔV 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 statisticalmechanical techniques. The calculated ΔG(F = 0) and Ea(F = 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. Currently, three common computational methods can be implemented when an external electric field is applied: the Neugebauer−Scheffler method (for vacuum−metal systems), the double reference method (for solvation−metal systems), and the linear free energy relationship (for roughly predicting the Gibbs free energy and activation energy for an electrochemical cell). The Neugebauer−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 be applied only to simple reactions, such as water splitting. The use of linear free energy relationships can save large amounts 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 the following: (1) geometry changes that arise because of 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 methods 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. 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

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, an optimized solvated system in which a ca. 20 Å vacuum thickness is inserted 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 at their bulk positions 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, ϕNHE(q), is given by eq 5: ϕNHE(q) = −4.85 V − [ϕFermi(q) − ϕW (q) + ϕW (0)] (5)

where −4.85 V is the experimentally determined electrode potential of the NHE, ϕW(0) is the work function at zero charge (from the first reference calculation), and ϕFermi(q) and ϕW(q) 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 because of the 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 Heyden94 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 (ΔG(F)) and the activation energy barrier (Ea(F)) in the presence of an electric field (F) are defined as ΔG(F ) = ΔG(F = 0) − neΔV

(6)

Ea(F ) = Ea(F = 0) − nαeΔV

(7)

where ΔG(F = 0) and Ea(F = 0) represent the change in the Gibbs energy and the activation energy barrier, respectively, for 5160


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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 e/bohr3. The yellow and blue areas represent gain and 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.

vibrational frequencies of adsorbed CO on Pt(111) using DFT calculations. IR measurements showed from the slope (dνCO/ dF) that the CO frequency (νCO) 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 because of 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. 3.3.2. Thermodynamic and Kinetic Properties of Reversible Electrocatalytic Reactions. As described in section 3.3.1, a high electric field can alter the 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

and change their electronic interactions with a metal surface. Figure 7 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 when the electric field is absent 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 Neurock77 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 a Hdown structure when a negative field is applied but switches to a 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 moment (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. 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 Ptgroup metals (Pt, Ir, Pd, Rh, and Ru), which were modeled using a cluster model. According to DFT calculations, negative fields and low CO coverages cause Pt and Rh to switch preferences from a top to hollow binding site, 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 solid oxide fuel cell) can be influenced by the field strength and orientation for certain metals. Lozovoi and Alavi103 and Deshlahra and co-workers70,104 simulated the influence of an electric field on the

2H+

2H+

from dissociative (O2 → O2,ads ⎯⎯⎯→ 2OHads ⎯⎯⎯→ 2H 2O) to 5161


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Figure 8. (a) Brønsted−Evans−Polanyi (BEP) correlations for the CH dissociation reaction on Ni(111). (b) Graphitic carbon as a fraction of total surface carbon from XPS analysis of the post-reacted Ni surface.40 H+

H+

workers110,111 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. Only a limited number of studies have combined DFT calculations with experiments to explore external (or local) electric field effects on heterogeneous reactions. For example, a few studies have elucidated electric field effects on water ionization in electrochemistry. Both DFT calculations and ramped field desorption experiments showed that the field requirements for water ionization decrease as the size of the water cluster increases and that 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 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 and co-workers 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 post-reacted Ni surface 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 fielddependent 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 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 syngas. We further consider the influence of water, which has been argued to influence the underlying reaction mechanism for the

2H+

⎯ OOHads ⎯→ ⎯ 2OHads ⎯⎯⎯→ 2H 2O). associative (O2 → O2,ads ⎯→ Similarly, the McEwen group’s DFT calculations predicted 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 and co-workers27,48 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 0.5VNHE and will be accompanied by the formation of OH− ions. Likewise, when the electrode potential exceeds 1.1VNHE, the electrooxidation of water is predicted to occur and to be accompanied by the formation of H3O+ ions.108 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 Brønsted−Evans−Polanyi (BEP) 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 Nørskov109 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 downward (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 O−H 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 for this 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, Sautet, and co5162


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

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 eq 8 (following previous work):56

aldehyde and ketone products by factors of up to 10. 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 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 STM probe nanoreactors, PBP reactors, integrated circuit reactors, and 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. To date, no single approach satisfies both of these criteria particularly well. Because of 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 an STM tip124 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 these theoretically increase the 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

E = ECHx* + E H yO* + (6 − x − y)E H* − ECH4(gas) − E H2O(gas) − (8 − x − y)E Ni_slab

(8)

Adsorption energies for step 5 were obtained similarly using eq 9: E = ECH3OH* + 2E H* − ECH4(gas) − E H2O(gas) − 3E Ni_slab (9)

For step 6, the influence of the external electric field was computed for methanol in the gas phase according to eq 10: E = ECH3OH(gas) + 2E H* − ECH4(gas) − E H2O(gas) − 2E Ni_slab (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. 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 affects the rate of methane activation, while the Neese group121 stated that higher metal oxidation states facilitate C− H bond cleavage. Changes in adsorption and/or reaction energies in the 1 presence of an electric field (ΔU = Δd·F + 2 Δα F2 ) are related to the dipole moment (Δd) and the polarizability (Δα) 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 5163


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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 Supporting Information.122 Modeling of 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.127 in another STM tip reactor study involving the isomerization of azobenzene under varying electric field conditions. Here the main effect was molecular configurational changes of products depending on the applied voltage. Above a certain “threshold voltage”, the molecule switches from the trans isomer 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 fewmolecule 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 single-molecule 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 electricfield-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 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 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

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 (PBP) reactors, in which a catalyst bed is placed in the gap between two probes; (c) continuouscircuit (CC) reactors, in which the catalyst bed is integrated into an electric circuit (the color gradient in the wire represents the surface charge gradient); and (d) capacitor-type reactors. Red arrows indicate the general field structure; R and R′ denote adsorbed reactants.

generate such high electric fields with such a low applied voltage. Thus, the current approach is to model an idealized version of whichever reactor is in use and assume that the result will be slightly larger in reality because of the presence of defects, particles, and pores. 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 high electric fields predicted by theory to be necessary to affect molecules significantly (0.1−1.0 V/Å).4 It is wellknown that very high electric fields develop at sharp points in charged conductors, which is the principle by which lightning rods, field ion microscopes, and STMs 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 and their relative ease of use and measurement, STM probes are well-suited as “nanoreactor” systems for testing the effects of electric fields on singlemolecule reactions. Such an experiment involving a Diels− Alder reaction was recently reported by Aragonès et al.122 A diene covalently bonded to a gold STM tip was brought into contact with the dienophile-covered substrate (R and 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 the tip was withdrawn ∼0.2 nm from the surface, and the reactivity was highly dependent on both the strength and direction of the applied electric field as a result of 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 Supporting Information of that study. 5164


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ACS Catalysis Table 1. Overview of PBP-Type Reactor Work Carried Out by Sekine and Colleagues year 128

1999 2001129 2003130 2003131 2003132 2003133 2004134 2004135 2004136 2005137 2005138 2008139 2008140 200937 2010141 2011142 2012144 2012143 2013145 201335 201439 2016148 2016147 2016146 2017156 2017157 2017158 2017159

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 2.1 NR 6.0 NR 6.0 3.0−5.0 5 5 NR 10−60e NR 3.0−6.5 NR 1.1 1.0 NR NR bed height NRf 4.0

temp. (K)a ambient 453 300−773 ambient ambient ambient 420−800 420−960 393 ambient ambient 423−623 ambient 425−700 423−523 423−753 423 423−1650 423 423 423−723 398−823 423 423 423−1073 423 541−584 473−823

current (mA)a 4 3 2.0−4.0 0.5−8.0 1.0, 2.0 2.0−4.0 20000 NR NR NR 2.0−15.0 2.0−3.0 2 3.0−8.0 3 3 3 1−9 3.0−7.0 3.0 3.0−7.0 5 3.0−13 1.5−20 1.5−7.0 3−10 3−12 5

voltage (kV)a d

6.25 1.07−8.0d 1.8−4.2 0.3−7.4 NR 1.8−4.2 5.0 NR NR NR 4−7 NR 4−10 0.15 0.13−0.60 0.46−0.77d 0.9−3.2 0.4−2.0 0.6−2.3 0.43−1.16 0.87−1.65 NR 0.1−0.9 0.1−1.3 0.1−1.7 1.2−2.5 0.4−1.2 0.03−0.29

reactionb

catalyst

phenomenonc

OCM MSR DDCM OCM OCM, DDCM POxM DDCM MSR, DDCM, OCM MSR EtOH-SR EtOH-SR dec. EtOH OM dec. EtOH EtOH-SR, dec. EtOH, WGS, MSR MSR OCM OCM OCM MSR rev. WGS MSR WGS OCM OCM OCM DRM MSR

none none none Pt/SiO2, Ni/C none none none Lindlar, Ni/C none none none Pt on CeO2, ZrO2, TiO2 MgO, ZrO2, TiO2, CeO2 Pt/CeO2 Pd, Pt, Rh on CeO2 Pt, Rh, Pd, Ni on CeO2 La2O3, Sr−La2O3 Sr−La2O3 La−ZrO2 Pt−CeO2 metals on La−ZrO2 Pd−CeO2 Pt/La−ZrO2 WO3/CeO2 mixtures PW12O40/CeO2 LaCaAl perovskites Ni/La−ZrO2 Pd/CeO2

DCPD DCPD DCPD CD, DCPD DCPD CD, DCPD CD, SD CD, SD DCPD DCPD DCPD DCPD DCPD EF, DkC EF EF, DkC EF, DCPD EF, DkC EF EF EF EF EF EF EF EF EF EF

a NR = not reported. bOCM = 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. cDCPD = direct current pulsed discharge; CD = corona discharge; SD = spark discharge; EF = electric field; DkC = dark current. dCalculated from reported power values (P = IV). e Calculated from the “effective contact time (ECT)” value given in the report. ECT = (gap (mm))/(feed rate (mmol/min)). fReported as “contiguous” but actually 1−10 mm according to the authors (personal correspondence).

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 the catalysts with Sr/La = 1/ 200 and 1/20. Corona discharges are often called “lowtemperature 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 to 10−2 A flows between the electrodes and is associated with glows whose color depends on the surrounding atmosphere.160,161 Kado et al.130 reported that the pulse discharge (an intermittent spark discharge occurring at currents of ∼1 to 102 A) in methane glowed a blue color, while coronas in methane are typically red. However, Liu et al.151 reported a pink glow during corona discharge in methane/oxygen over Sr−La2O3. Images of the reactor published by Oshima and Tanaka et al.143 show the reactor operating under “electric field” conditions at ambient temperature with a dark-red glow clearly visible within the catalyst bed. 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 versus with a pure electric field. A reasonable criterion would be to consider any

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 co-workers.35,37,39,128−148 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 reported on the formation of plasmas, or coronal discharges, based on the current flowing between the two disconnected probes. Other groups working with PBP-style reactors reported similar phenomena.149−155 From 2009 onward, however, the focus shifted to studying the “electric field” phenomenon in the same reactor setup, with current flowing between the probes. An example of the confusion currently surrounding the PBP reactor is provided by a comparison of the experiments conducted by Marafee et al.149 to those conducted by Tanaka et al.144 and Oshima et al.143 (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 Sr−La2O3 catalysts for oxidative coupling of methane (Marafee used Sr/La ≈ 0.11, while Tanaka and Oshima used Sr/La = 1/2000−1/20), and both used currents between 3.0 and 3.5 mA with similar 5165


Perspective

ACS Catalysis

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 nature 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. Applying an electrical bias to a conductive wire establishes surface charges that provide the driving force for current flow.125,162,163 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 the 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.,162 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 (e.g., 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 source. From 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 0: 5168


Perspective

ACS Catalysis Tip:

F0 =

x2 + y2 R z= − 2 2R

(A4)

■

Norbert Kruse: 0000-0002-6528-0627 Susannah L. Scott: 0000-0003-1161-0499 Jean-Sabin McEwen: 0000-0003-0931-4869 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS F.C. thanks The American Chemical Society Petroleum Research Fund for partial support. J.-S.M. was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences (Award DE-SC-0014560). S.L.S. acknowledges the support of the Department of Energy, Office of Basic Energy Sciences, under the Catalysis Science Initiative (DE-FG-02-03ER15467). J.-S.M. also acknowledges P. Gaspard and L. Grabow for fruitful discussions. J.-S.M. also thanks Juan Manuel Arce-Ramos for his input on the TOC image and Shawn Kathmann for the preliminary quantum mechanical calculations of the local electric field strength within a zeolite. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. DOE.

(A6)

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

(A7)

which has the solution Φ(ξ) = A ln ξ + B. The constants of integration A and B can be fixed according to the values of the potential at the tip and the screen. Since

■

x 2 + y 2 + z 2 , we obtain

ξ=z+

Φ = Φtip −

ΔΦ ln

ln

2L + R R

z+

(A8)

with the potential difference ΔΦ = Φtip − Φscreen. The electric field is perpendicular to the metallic surface and proportional to the surface density of charges via the vacuum permittivity: F = −n ·∇Φ|tip = −2

ξ ∂Φ ξ + η ∂ξ

= ξ= R

σ ϵ0

(A9)

where we have expressed the gradient in parabolic coordinates. We note here that n is the unit vector normal to the surface. On the isopotential surface ξ = R, the other coordinates vary as η = (x2 + y2)/R and φ = arctan(y/x). Accordingly, the electric field at the surface of the tip takes the value F=

2ΔΦ ln

2L + R R

1 2

R + x2 + y2

(A10)

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

x2 + y2

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, 1990; pp 133−158. (2) Kreuzer, H. J. Chemical Reactions in High Electric Fields. ACS Symp. Ser. 1992, 482, 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, 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, 187−200. (6) Stuve, 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. Interface 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 AdsorbateAdsorbate Interactions of Na and K Adlayers on Al(111). Phys. Rev. B: Condens. Matter Mater. Phys. 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; Chemical Catalog Company: 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. (13) Kreuzer, H. J. Physics and Chemistry in High Electric Fields. In Atomic and Nanometer-Scale Modification of Materials: Fundamentals

x2 + y2 + z2 R

AUTHOR INFORMATION

ORCID

4 ∂ ⎛ ∂Φ ⎞ 4 ∂ ⎛ ∂Φ ⎞ 1 ∂ 2Φ ⎜ξ ⎟+ ⎜η ⎟+ ξ + η ∂ξ ⎝ ∂ξ ⎠ ξ + η ∂η ⎝ ∂η ⎠ ξη ∂φ 2

∂ ⎛ ∂Φ ⎞ ⎜ξ ⎟=0 ∂ξ ⎝ ∂ξ ⎠

(A12)

*E-mail: [email protected] (J.-S. McEwen). Phone: (+1) 509-335-8580.

(A5)

=0

2L + R R

Corresponding Author

The screen is far enough from the tip (L ≫ R) that it can be considered to be flat. In the vacuum between the needle and the screen, the Laplacian of the electric potential Φ vanishes. In parabolic coordinates,188 the Laplacian of Φ is given by ∇2 Φ =

R ln

we find that the electric field normal to the surface varies as given in eq 1.

where R is the radius of curvature of the tip. The screen is taken as the isopotential surface ξ = 2L + R at a distance L from the apex of the needle: Screen: x2 + y2 R z=L+ − 2 2(2L + R )

2ΔΦ

(A11)

and the electric field at the apex of the tip in the direction of its axis pointing outside by 5169


Perspective

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