Improving Ni Catalysts Using Electric Fields: A DFT and Experimental

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Improving Ni Catalysts Using Electric Fields: A DFT and Experimental Study of the Methane Steam Reforming Reaction Fanglin Che, Jake Gray, Su Ha, and Jean-Sabin McEwen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02318 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016

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Improving Ni Catalysts Using Electric Fields: A DFT and Experimental Study of the Methane Steam Reforming Reaction Fanglin Che,a Jake T. Gray,a Su Ha,a Jean-Sabin McEwen*abc a

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

Department of Physics and Astronomy, Washington State University, WA, 99164 c

Department of Chemistry, Washington State University, WA, 99164

Abstract This work demonstrates the benefits of applying an external electric field to the methane steam reforming reaction (MSR) in order to tune the catalytic activity of Ni. Through combined DFT calculations and experimental work, we present evidence for the usefulness of an electric field in improving the efficiency of current MSR processes – namely by reducing coke formation and lowering the overall temperature requirements. We focus on the influence of an electric field on: (i) the MSR mechanisms; (ii) the ratelimiting step of the most favorable MSR mechanism; (iii) methanol synthesis reaction during the MSR reaction; and (iv) the formation of coke. Our computational results show that an electric field can change the most favorable MSR mechanism as well as alter the values of the rate constants and equilibrium constants at certain temperatures, and hence significantly affect the kinetic properties of the overall MSR reaction. Both computational and experimental results also suggest that a positive electric field can impede the formation of coke over a Ni catalytic surface during the MSR reaction. Moreover, the presence of a negative electric field notably increases the rate constant and the equilibrium constant for the methanol synthesis reaction, which suggests a possible direct route from methane to methanol. Finally, a field-induced Brønsted-Evans-Polanyi (BEP) relationship was developed for C-H cleavage, C-O cleavage, and O-H formation over a Ni catalytic surface. Overall, this investigation strengthens our understanding of the effect of an electric field on the Ni-based MSR catalytic system and highlights the benefits of designing heterogeneous reactions under applied electric fields. Keywords: Electreforming; Methane Steam Reforming; Electric Field-Induced BEP correlations; Transition State Theory; Coking Formation; Methanol Synthesis. *

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

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1. Introduction According to the Annual Energy Outlook for 2015, natural gas production is dominant in the US and is projected to continue rising through 2040 [1]. In order to make the most of this abundant natural resource and at the same time reduce emissions of harmful greenhouse gases, it is imperative that we fully understand the catalytic reactions that are used in methane processing – particularly in the case of the methane steam reforming (MSR) reaction. MSR produces around 98% of the world’s hydrogen gas supply. The current infrastructure, which is set up for the transportation of natural gas, oil, and gasoline, presents a serious economic disadvantage for the use of hydrogen gas as an alternative fuel source such as in hydrogen fuel cell-based cars [2-6]. Understanding the MSR reaction at a more fundamental level can guide us in developing more efficient hydrogen-producing processes and perhaps even lead to the development of small-scale methane reformers which can be used to upgrade the current natural gas infrastructure for on-site reforming. There are two significant issues facing MSR: (i) the formation of coke which can rapidly deactivate the Ni catalysts [7]; (ii) the highly endothermic MSR reaction which is both energy inefficient and requires reactor materials with high thermal stability [2, 4, 8]. There are two possible routes to improve the catalytic performance of Ni in the MSR reaction that would suppress the formation of coke and lower the operating temperature. One route is by modifying a pure Ni catalyst via doping with other metals or adding metal oxide supports. There are several examples of these approaches in the literature. Bengaard et al. [9] found that a Au/Ni bimetallic catalyst was active for this reaction and improved its coke resistance as compared to pure Ni catalysts. Nikolla et al. [7] found


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that the addition of Sn can significantly reduce the formation of coke over Ni/YSZ catalytic surfaces. One explanation for this improvement is that the addition of Sn reduces the number of Ni atom ensembles, which suppresses the nucleation and growth rates of coke [10-12]. In addition, the 5p electrons of the Sn atoms can interact with the 3d electrons of the Ni atoms, which further prevents the surface from coking [7, 12, 13]. On the other hand, Roh et al. showed that dispersed Ni particles (Ni particle size < 3 nm) on a Ce1-xZrxO2 support, in particular NiCe0.8Zr0.2O2, exhibited the highest conversion of methane at 873 K in the MSR reaction [14], which is lower than the normal operating temperature of 973 K. Instead of using bimetallic Ni-M catalysts or using a metal oxide support, another possible route to alter the selectivity and activity of a Ni catalyst in the MSR reaction is by performing the MSR reaction in the presence of an external electric field. MSR is one of the main reactions that occurs on Ni-based anodes used in direct methane-fueled solid oxide fuel cells (SOFCs) [15, 16] where the conversion of methane to syngas via internal reforming affects the SOFCs' overall performance [17, 18]. One reason for the significant improvement of the methane conversion under internal reforming conditions over a Nibased anode compared to that of a conventional heterogeneous catalyst could be due to the presence of an electric field in a SOFC under normal operating conditions [2, 8, 19]. Due to the potential differences between the electrode and the electrolyte, a large potential drop could occur inside their Helmholtz layer. Such a potential drop can generate a large uniform electric field, on the order of ±1 V/Å, which can alter adsorbatesurface interactions by rearranging the potential energy states of molecular orbitals and


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directly changing the overall electrocatalytic selectivity and activity of the catalyst [2029]. Several groups have shown experimentally that one can apply an external electric field to modify catalyst performance. Sekine et al. [2, 8, 19] used an external electric field to increase the hydrogen production rate and lower the temperature requirements for the MSR processes. Gorin et al. [30, 31] presented how an induced interfacial electric field, which is generated from the application of a voltage between the two Si electrodes, significantly changed the selectivity of a Rh porphyrins-TiO2 catalyst for the carbine reaction to favor the cyclopropanation product rather than the insertion product. A number of theoretical works have modeled the effect of an external electric field as well. In particular, Lozovoi and Alavi modeled the influence of an electric field on the stretching frequency of CO on a Pt(111) surface from first principles [32]. The importance of their work is that the dvC-O/dF slope changes in the CO frequency (vC-O) as a function of electric field strengths (F) closely matched with ultra-high vacuum measurements [32-34]. Additionally, from our previous work, we have examined the influence of an electric field on methane and water dehydrogenation over two Ni catalytic surfaces and the reaction energy profiles of the MSR reaction by using density function theory calculations (DFT) [35-37]. We concluded that the electric field has a more significant effect on the water group or methanol group than that on the methane group. Also, a positive electric field can lower the overall energy profiles of the MSR reaction as well as strengthen the adsorption of methane and water. However, in order to better understand the electric field effects on the overall MSR mechanism, the field effects on the MSR rate-limiting steps need to be determined.


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In this work, we provide a comprehensive kinetic study of: (i) the most favorable mechanism for the MSR reaction (shown in Scheme. 1) in the presence and absence of an electric field; (ii) the competitive rate-limiting steps of the most favorable reaction pathways under experimental operating conditions with different electric fields; (iii) the influence of an electric field on the synthesis of methanol and the formation of coke over a Ni(111) surface during the MSR reaction and to compare our results with corresponding experimental work. Recently, Nørskov and coworkers suggested that the application of an external electric field results in a circumvention of a pure-metal scaling relation and a considerably better catalyst for N2 activation.[38] Here we present the effects of an external electric field on catalytic MSR reaction, where a field-induced Brønsted-Evans-Polanyi (BEP) relationship of C-H/C-O bond cleavage and O-H bond formation during the Ni-catalytic MSR reaction is presented. All of the analyses are done using a combination of our ab initio quantum chemical computational results and experimental work. This information can provide guidance for designing novel Ni-based catalytic steam reforming operations with improved coke-resistant catalysts and lower operating temperatures. Furthermore, this work can also deliver some insights into the effects of an electric field on other heterogeneous reaction mechanisms.


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Scheme 1. Possible mechanisms and intermediates of MSR on the Ni(111) surface.

2. Methods 2.1 Computational Setup We performed our DFT calculations with the Vienna Ab Initio Simulation Package (VASP) code [39, 40], wherein a plane wave basis set was used under periodic boundary conditions. To solve the ion-electron interactions in the Kohn-Sham equations, we used the projector-augmented wave (PAW) method [41]. The exchange correlation energy was calculated using the Generalized Gradient Approximation [42, 43] with the Perdew-Wang 91 functional (GGA-PW91) [44]. The electronic wavefunctions at each kpoint were expanded with a plane-wave energy cutoff of 400 eV. We considered the flat Ni(111) surface in our calculations. From our previous work, we concluded that the electric field effects on the methane dissociation and water/Ni interactions are similar on Ni(111) as compared to the stepped Ni(211) surface [37]. As a result, we assume that the electric field effects on the kinetic properties of the MSR reaction over Ni(111) can be extended to that on a Ni(211) surface. The Ni(111) surface was constructed using a four-layer periodic slab separated by a vacuum layer of


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11 Å. We fixed the two bottom layers at their bulk equilibrium positions and only allowed the two top layers to relax away from their bulk positions. Each layer had 9 Ni atoms with a (3×3) periodicity, allowing for the study of an adsorbate coverage as low as 1/9 Monolayer. The Ni lattice constant was calculated and found to be 3.521 Å (3.522 Å [45]), which is in agreement with the experimental value of 3.524 Å [46]. All selfconsistent field (SCF) calculations were converged to 10-4 eV and the forces were smaller than 0.03 eV/Å. The Brillouin zone integration was approximated by a sum over k-points chosen using a Monkhorst-Pack mesh [47] with a grid of (4×4×1). Due to the ferromagnetic properties of Ni, all the calculations included spin-polarization. Using a higher energy cutoff (450 eV), a finer k-points mesh of (6×6×1), and a 5-layer slab for Ni(111) surface resulted in adsorption energy differences of only ~0.01 eV [35, 36]. Also, van der Waals corrections were examined by calculating the reaction energy of water dehydrogenation with both the PW91 and the optB88-vdW [48-52] functionals, and the reaction energies were found to change by less than ~0.1 eV when an external electric field is applied. Therefore, the calculations presented here were performed with the PW91 functional. More details about the computational setup are given in our previous work [35-37, 53]. DFT calculations are able to simulate a uniform electric field, which uses the same approach as proposed by Neugebauer and Scheffler [54]. This method generates uniform electric fields without adding or removing any charge from the supercell [34]. With regard to the ‘field emissions’ effect [55], our modeling has less charge density in the vacuum than the magnitude of the Gibbs’ oscillations (0.001 e/Å3) associated with plane-wave cutoffs. More details can be found in our previous work [35].


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2.2 Transition State Theory For the transition state (TS) calculations of each examined elementary step, we applied the nudged elastic band (NEB) to get a better initial guess for the TS and used the climbing image nudged elastic band (CINEB) methods to find the true TS, which only has one imaginary mode from our vibrational frequency calculations [56, 57]. We define the field-dependent activation (Ea(F)) and the field-dependent reaction energy (∆Hrxn(F)) as:

Ea ( F ) = ETS ( F ) − EIS ( F )

(1)

∆H rxn ( F ) = EFS ( F ) − EIS ( F )

(2)

where EIS(F), ETS(F), and EFS(F) are the field-dependent total energies of the initial states (IS), transition states (TS), final states (FS) of an examined elementary reaction. Moreover, the field-dependent forward reaction rate constant (kf(F)), reverse reaction rate constant (kr(F)), and the equilibrium constant (K(F)) at experimental conditions for the elementary steps can be identified by harmonic transition state theory [58-60]: *

k T q ( F ) − Ea 0 ( F ) / k BT k f ( F ) = ( B )( TS )e h q IS ( F )

K (F ) =

k f (F ) kr (F )

(3)

(4)

*

k T q ( F ) −( Ea 0 ( F )− ∆H rxn 0 ( F )) / k BT k r ( F ) = ( B )( TS )e h q FS ( F )

(5)

where T is the absolute temperature; kB and h are the Boltzmann's and Planck's constants, respectively; Ea0(F) and ∆Hrxn0(F) represents the zero-point energy (ZPE) corrected activation energy and reaction energy of the examined elementary reaction; qIS(F), qTS*(F),


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and qFS(F) are the field-dependent harmonic vibrational partition functions of the IS, the TS and the FS, as given by:

q( F ) = ∏ i

1 1− e

−hvi ( F ) / kBT

(6)

where vi(F) is the real vibrational frequency of each vibrational mode of the adsorbed intermediate from our DFT calculations. It is worth to mention that temperature and entropic effects are taken into account within the normal mode approximation (harmonic oscillation approximation) [58, 59, 61, 62]. More details on the derivation of the rate constants in the transition state theory in the harmonic oscillator approximation are given in the supporting information (SI) Section 1. In addition, when we examined the vibrational frequencies, both the adsorbates and the same number of the uppermost bonded Ni atoms were allowed to relax for the IS, TS, and FS for the same examined elementary reaction. The five largest vibrational frequencies along with the TS’s imaginary frequency, for the IS, TS, and FS for all possible MSR-related elementary reactions are given in Table S2. 2.3 Experimental Setup The MSR reaction was performed using a 2:1 H2O:CH4 ratio at 1073 K for all experimental tests. A quartz tube reactor (Quartz Scientific, 1 cm inner diameter) was loaded with a catalyst consisting of Ni foam (95%, GoodFellow) augmented with Ni microparticles (90 µm diameter, 99.8%, Alfa Aesar). The catalyst was prepared by sintering the microparticles to the foam at 850°C under H2/Ar for several hours. An electric potential was delivered to the catalyst via silver wires (99.9%, Alfa Aesar) attached to the top and bottom of the catalyst and connected to a DC power source (VOLTEQ HY20010EX). Reactions were performed under no applied field and under


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electric field conditions by applying 200V across the catalyst. The reactor temperature was controlled using a ceramic furnace from Watlow connected to a PID temperature controller (Barnant). A K-type thermocouple (Omega) was placed in direct contact with the catalyst to ensure precise control of catalyst temperature and to ensure no Joule heating of the catalyst occurred during application of the potential. For surface composition tests, the catalyst was removed from the reactor after four hours of continuous operation and the upper surface composition was obtained using x-ray photoelectron spectroscopy (XPS, Kratos AXIS-165, AlKα X-ray anode, calibrated using Au 4f7/2 at peak at 84.0 eV and Ag 3d5/2 peak at 368.3 eV). The O 1s and C 1s peaks were recorded at room temperature and deconvoluted using CasaXPS to determine individual species on the surface. 3. Results and Discussions 3.1. Enhancing Ni’s coke-resistance by an electric field One of the main issues for industrial MSR operations is that carbon-carbon bonds (coking) are easily formed over the Ni active sites and deactivate Ni catalysts. From our previous simulation work, we concluded that the adsorption of the carbon atom is weakened when an external positive electric field is applied. In order to arrive at a Nibased catalyst that is coke-resistant for the MSR reaction, we investigated the electric field effects on the kinetic properties of the CH dissociation as well since the carbon atoms are from the dissociation of CH during the MSR reaction [36, 37]. Thus, here, we mainly present how the simulated electric fields alter the rate constant at the experimental temperature of 1073 K for CH dissociation over a Ni(111) surface.


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From Figure 1(a), it is apparent that the activation energy and the reaction energy of the CH * → C * + H * reaction with a tunable electric fields falls within a BrønstedEvans-Polanyi (BEP) [63-66] relationship. That is, the activation energy (Ea) for a single elementary reaction scales linearly with the reaction energy (∆Hrxn(F)) of the

CH * → C * + H *

reaction

under

different

applied

electric

values

(

Ea ( F ) = α∆H rxn ( F ) + β ). Our identified field-induced BEP correlation for this C-H bond cleavage has an α value of 0.68 and a β value of 1.00 eV, which is similar to Wang’s BEP relationship [67] for the CH * → C * + H * reaction over different catalysts in the absence of an electric field, in which the mean absolute value (MAE) is 0.05 eV when using Wang’s BEP linear correlations to predict the activation energies of the CH * → C * + H * reaction in the presence of an electric field. This information rationally reveals the fieldinduced C-H cleavage can fit in a BEP linear relationship in the absence of a field. Thus, we can estimate the energy requirements of the CH cleavage over different catalysts with different electric fields based on the field-altered reaction energies.


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*

*

*

Figure 1. (a) Brønsted-Evans-Polanyi (BEP) correlations for the CH → C + H reaction, and (inset) rate constants (kf) at 1073 K of CH dissociation; (b) Graphitic C as a fraction of the total surface C from X-ray photoelectron spectroscopy (XPS) analysis of post-reacted Ni surface.

Moreover, Figure 1(a) illustrates that a positive electric field increases both the activation energy and the reaction energy of the CH * → C * + H * reaction, while a negative electric field had the opposite effect. As a result, both the rate and the equilibrium constants at 1073 K of the CH * → C * + H * reaction were decreased by a factor of 10 as a positive electric field value was applied (Table 1). That is, the

CH * → C * + H * reaction over Ni(111) is largely suppressed by the presence of a positive electric field. Consequently, a positive electric field will impede the formation of carbon deposits and reduce Ni’s coking. Experimental XPS data of the post-MSR Ni catalysts indicates a clear suppression of coking under a positive electric field. A Ni catalyst subjected to MSR conditions for four hours under the influence of a positive electric field developed only 3.7% of the surface coke that an identical catalyst in the same conditions developed with no fields (1.1 atom% compared to 30 atom%; Figure 1(b)). Therefore, both the theoretical and the experimental results suggest that the presence of a positive electric field can impede coke formation over a Ni surface in the MSR reaction. Negative field data is omitted due to unexpected side reactions occurring which are still under investigation. A brief discussion is provided in the SI (Section 6). 3.2. Methane steam reforming (MSR) mechanism 3.2.1 The MSR mechanism in the absence of an electric field


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Besides coke formation, one of the other main issues for the Ni-based MSR reaction is the high operating temperature of 900 K or higher. In order to investigate the role of an electric field in changing the activity and temperature requirements of the Nibased catalysts in the MSR reaction, we first need to understand the Ni-catalytic MSR mechanism in the absence of an electric field. We have investigated the kinetic and thermodynamic parameters of the possible mechanisms (Scheme 1) of the MSR reaction in the absence of an external electric field. The structures and the minimum energy pathways of the proposed elementary reactions of the MSR reaction with different electric fields can be found in Figure S1 to Figure S25. All the reaction energies and activation energies of the possible MSR elementary reactions with and without an electric field are given in Table S1. Here, we present the most favorable and second-most favorable MSR mechanism (as shown in Eq. (7) and (8)) with their corresponding structures at each elementary step in Figure 2. Rate constants and equilibrium constants at 1073 K and 873 K of the competitive rate-limiting steps in the most favorable MSR mechanisms are given in Table 1. The reason for the examined temperature of 1073 K is that it is convenient to compare our theoretical data with experimental work since the operating temperature used was 1073 K. In order to compare the rate constants of the possible MSR-involved elementary steps at a lower temperature with different electric fields to the ones with a higher temperature in the absence of an electric field, we also chose a lower operating temperature of 873 K. In this way, the results presented here could provide guidance for the future design of a Ni-catalytic methane electro-reforming system (the reforming


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reaction in the presence of an electric field) with a lower operating temperature requirement. *

CH 4 + H 2 O * → CH * + OH * + 4 H * → CHOH * + 4 H * → COH * + 6 H * → CO ( g ) + 3 H 2 ( g )

CH 4* + H 2O* → CH * + O* + 5H * → CHO* + 5H * → CO* + 6H * → CO(g) + 3H 2(g)

(7) (8)

Figure 2. The most (a) and second-most (b) favorable MSR reaction pathway in the absence of an external electric field. The configurations and intermediates highlighted by the blue color represent the ones that were most significantly altered after applying the simulated electric fields.


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As shown in Figure 2(a), the rate-limiting step during this most favorable mechanism is the CHOH synthesis via hydroxyl oxidation of a CH fragment over a Ni(111) surface, with a Ea0 value of 1.17 eV, a rate constant of 3.5 × 107 s-1, and an equilibrium constant of 2.4 × 10-4 at 1073 K. Comparing the rate-limiting step of the

CH * + OH * → CHOH*

reaction,

the

H 2O* → OH * + H *

reaction

and

the

*

CH 4 ( gas) → CH 3 + H * reaction we find that all three reactions have similar high activation energies. Thus, these two elementary reactions can also have competitive ratelimiting steps. The second kinetically most favorable mechanism is highlighted in Figure 1(b), which includes the intermediate CHO instead of the COH species. According to the rate constants and the equilibrium constants at 1073 K, the two competitive rate-limiting steps that occur in the second most favorable mechanism are the CH * + O* → CHO* and *

the CH 4 ( gas) → CH 3 + H * reaction step. While it is difficult to distinguish between CHO and COH species formed on the surface, XPS can be used to determine the ratio of CHO and COH to other C species on the sample. These tests show that 1.75% of the no-field sample’s surface carbon is CHO/COH. XPS results are shown in Figure S31. This confirms that CHO/COH species could be one of main reaction intermediates formed during MSR as suggested by Equations (7) and (8). Additionally, Bengaard et al. [9] predicted that one of the competitive rate-limiting steps of the Ni-based MSR reaction was the C-H bond breaking of CH4, while the other one is the C * + O* → CO* reaction. However, based on our DFT calculations, the activation energy of the C * + O* → CO* reaction is up to 2.25 ± 0.08 eV in the absence and in the presence of an electric field (Table S1), which is much more unfavorable than the one for the CH * + O* → CHO* and


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the CH * + OH * → CHOH* reactions in our predicted favorable pathways for the methane steam reforming reaction. Therefore, our predicted reaction pathways are more favorable for the MSR reaction as compared to the path with the C * + O* → CO* elementary step. Our predicted favorable reaction paths also correlate well with Blaylock’s microkinetic modeling [68] that the decomposition of CH4 and the oxidation of the CH fragment by surface OH or O species are the rate dominating steps during the MSR reaction under realistic conditions. The predicted most favorable pathway and its rate-limiting step from our DFT results also agrees with the experimental results from Wei and Iglesia where CH4 activation was indicated to be rate limiting for the dry reforming of methane with CO2 and the MSR reaction [69]. Table 1. Electric Field Effects on the Kinetic Parameters (Including ZPE-Corrected Activation (Ea0) and Reaction Energies (∆Hrxn0), along with Forward Rate Constants (kf) and Equilibrium Constants (K)) for the Competitive Rate-Limiting Steps, Methanol Synthesis, and Carbon Formation in the MSR Mechanisms Studied in Section 3.1 - 3.3. Competitive Ratelimiting steps H2O* ⇌ OH* + H*

OH* ⇌ O* + H*

CH4* ⇌ CH3* + H*

CH* + O* ⇌ CHO* CH* + OH* ⇌ CHOH*

1 0 -1 1 0 -1 1 0 -1 1 0 -1 1 0 -1

Ea0 (eV) 1.02 0.69 0.74 1.25 0.97 0.72 0.62 0.71 0.87 1.13 1.14 1.15 1.44 1.17 1.13

∆Hrxn0 (eV) -0.30 -0.42 -0.07 0.07 -0.27 -0.58 -0.16 -0.11 0.09 0.06 0.55 0.11 0.73 0.73 0.40

1 0 -1

1.81 1.92 1.38

0.19 0.44 -0.08

F (V/Å)

873 K kf (s-1) K 1.4E+07 4.7 5.0E+08 8.0 9.9E+07 3.1E-02 3.0E+06 5.8E-01 1.2E+08 15.0 4.0E+09 2.1E+03 1.1E+07 5.0E-03 1.5E+08 7.7E-02 1.8E+08 6.9E-02 5.8E+06 3.0 4.8E+06 1.8 3.2E+06 1.6 1.9E+05 3.8E-04 1.8E+06 4.2E-05 3.7E+06 1.2E-02

1073K kf (s-1) K 2.0E+08 2.2 3.2E+09 2.6 7.0E+08 2.4E-02 7.9E+07 7.0E-01 1.5E+09 7.3 2.7E+10 4.8E+02 1.1E+08 6.5E-03 9.6E+08 5.8E-02 9.2E+08 4.4E-02 1.1E+08 3.6 9.0E+07 2.5 6.0E+07 2.1 7.3E+06 2.2E-03 3.5E+07 2.4E-04 6.4E+07 3.0E-02

3.5E+03 3.4E+02 4.4E+06

3.6E+05 4.5E+04 1.7E+08

Methanol Synthesis CH3* + OH* ⇌ CH3OH*

6.5E-01 5.5E-03 43.0


2.4E-03 4.1E-02 3.0E-02

16

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Carbon Formation CH* ⇌ C* + H*

1 0 -1

1.37 1.23 1.14

0.63 0.45 0.34

3.2E+05 4.3E+06 6.6E+06

4.9E-04 1.3E-02 1.3E-02

1.1E+07 1.1E+08 1.3E+08

1.8 × 10-3 1.4 × 10-2 3.9 × 10-2

Figure 3. The most (a) and second-most (b) favorable MSR reaction pathway with a positive electric field (red color). The blue color represents the most (a) and second-most (b) favorable MSR reaction pathway in the absence of an electric field. The configurations highlighted in red are the species that are most largely influenced by the presence of a positive field. All the geometries presented in this figure are optimized in the presence of a positive electric field.

3.2.2 The MSR mechanism in the presence of a positive electric field


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In order to improve our understanding of the electric field effects on the kinetic properties of the MSR reaction and work toward the goal of lowering the MSR temperature requirements, we also investigated all the possible mechanisms for the MSR reaction in the presence of a positive and a negative electric field, as shown in Table S1. By comparing the energy profiles of all the possible mechanisms, the kinetically most favorable MSR mechanisms and the corresponding structures in the presence of a positive field are presented in Figure 3(a). After applying a positive electric field in this system, the kinetically and thermodynamically most favorable MSR mechanism passes through the same intermediates as the ones in the absence of an electric field. Interestingly, as shown in Figure 3(a), the configurations of the water reactant and the CHOH intermediate are significantly changed in the presence of a positive electric field. The O-H bond of both H2O and CHOH become H-up structures rather than lying parallel to the flat Ni surface, as seen in the absence of an electric field. Due to the significant changes in the H2O and CHOH configurations, their corresponding transition states, TS1 and TS5 (Figure 3(a)), are largely altered by a positive electric field as well. Consequently, for both the

H 2O* → OH * + H * and the CH * + OH * → CHOH* reactions, the corresponding activation energy barriers are ~0.3 eV higher in the presence of a positive electric field than in the absence of an electric field since a positive electric field strongly stabilizes the adsorption of the H2O molecule and the co-adsorption of CH and OH species resulting in significant changes of their corresponding transition state structures. Since a positive electric field can alter the interactions between the Ni surface and the intermediates along the pathway (especially the molecules that have permanent dipole moments such as H2O,


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CHOH), a positive field can subsequently change the kinetic properties of the overall MSR mechanism. By comparing activation energies, rate constants, and equilibrium constants at 1073 K, the rate-limiting step of the most favorable MSR mechanism in the presence of a positive electric field is the CH * + OH * → CHOH* reaction. In the presence of a positive electric field, this reaction has the highest activation energy barrier of 1.44 eV and the smallest rate constant at 1073 K (7.3 × 106 s-1) with an equilibrium constant being much smaller than 1 (2.2 × 10-3). Additionally, for the H 2O* → OH * + H * reaction, the rate constant at 1073 K is 2.0 × 108 s-1 (two orders of magnitude smaller than in the absence of an electric field) with an equilibrium constant of 2.2, which can also be one of the rate-limiting steps during the methane steam reforming reaction in the presence of a positive electric field. In Figure 3(a), we can see that a positive electric field can slightly lower the overall energy scale for forming COH intermediates. Also, the equilibrium constant of the CH * + OH * → CHOH* reaction is 10 times higher than in the absence of a field at the same temperature. Thus, we predict that a positive field will generate more COH intermediates as compared with the reaction in the absence of an electric field. XPS results show that in the presence of a positive electric field, 8.61% of sample’s surface carbon is CHO/COH, which is ~five times higher than that with no electric fields (Figure S31). Using temperature-programmed oxidation to quantify these ratios reveals that the positive field sample forms 30-fold more CHO/COH species during the reaction than a similar catalyst without an electric field. In addition, in the presence of a positive electric field, the adsorption energies of the reactants (the CH4 and H2O species) over the Ni surface are significantly larger (~0.4


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eV) as compared to when the field is absent. Furthermore, a positive electric field can also lower the CO desorption energy by ~0.3 eV as compared to when a field is absent, which can assist in the formation of the products during the MSR reaction and leave more active Ni surface sites. As a result, the overall energy profile (as shown in Figure 3) of the MSR reaction in the presence a positive electric field is lower than the one without the presence of a field. Therefore, we conclude that a positive field can potentially decrease the operating temperature requirements by reducing the overall energy scale of the MSR reaction by stabilizing the adsorption of methane and steam and lowering the syngas desorption energy. More details regarding this conclusion can be found in our previous work [53]. 3.2.3 The MSR mechanism in the presence of a negative electric field We provide the most and second-most favorable MSR mechanism and their corresponding configurations in the presence of negative field in Figure 4. Interestingly, the presence of a negative electric field changes the kinetically and thermodynamically most favored MSR mechanism (as shown in Eq. (8)).


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Figure 4. The most (a) and second-most (b) favorable MSR reaction pathway in the presence of a negative electric field (black solid line). We compare this MSR pathway to the corresponding one in the absence of a field shown by the blue line. The intermediates highlighted in black are the species largely influenced by a negative field value. All the geometries presented in this figure are optimized in the presence of a negative electric field.

As shown in Figure 4(a), the most favorable reaction pathway in the presence of a negative electric field proceeds through the CHO intermediate instead of the CHOH or the COH intermediates. The main reason for the changed mechanism is that a negative electric field largely reduced the activation energy of the OH * → O* + H * reaction. On


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the other hand, in the presence of a positive field (Figure 3(b)) or in the absence of a field (Figure 2(b)) the activation energies of this elementary reaction are higher than the corresponding rate-limiting step. Also, at 873 K, the rate constant of the

OH * → O* + H * reaction in the presence of a negative electric field is ~3 times larger than in the absence of field at 1073 K. Such an example reveals that in the presence of an electric field, the kinetic properties of the elementary reactions involved in the heterogeneous reaction at a certain temperature can drastically change and directly change the overall selectivity and activity of the catalysts. Moreover, the rate-limiting step in the presence of a negative field is the CH * + O* → CHO* reaction due to its high activation energy barriers of 1.15 eV and a low rate constant of 6.0 × 107 s-1 at 1073 K. The rate constant of the CH * + O* → CHO* reaction at 1073 K is lower by around two orders of magnitude as compared with the other elementary reactions during this MSR *

mechanism in the presence of a negative electric field. The CH 4 ( gas) → CH 3 + H * reaction can be a competitive rate-limiting step for this MSR mechanism when a negative field is applied since the equilibrium constant (1073 K) of this step is only 4.4 × 10-2 and the adsorption of methane is weaker as compared to when a positive field is applied or when there is no applied field. Overall, an electric field can alter the kinetic properties of the catalytic reaction, such as changing the most favorable mechanism, alter the ratelimiting steps, altering the rate constants and equilibrium constants of the proposed elementary reactions, and subsequently influencing the Ni catalytic activity of the MSR reaction. It is also worth noting that the experimental conditions can play a more significant role in the energy profiles for the phase change steps (i.e. CH4 dissociative adsorption,


22

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H2O adsorption, H2 desorption, and CO desorption) than the ones only occurring over the surface ( A* + B* → C * + D* , where ‘*’ stands for the species adsorption over the Ni surface). However, the electric field trends on the overall MSR energy profile will not change even when we consider the corresponding entropy corrections. For surface *

*

reactions such as CH x → CH x −1 + H * , the entropic effects on the reaction energies or activation energies under the experimental conditions are all less than 0.2 eV, which will not largely influence the overall energy profiles that we present here. More details about the calculations relative to the entropy corrections for the phase changed steps or surface reactions are given in our previous work [53]. In addition, we also aware that we didn't include the effect of the configuration entropy. In the current work, we only considered one particular configuration when examining the effect of the electric field on the reaction barriers. However, in a more realistic model, an ensemble average of all possible reaction pathways would need to be taken into account. We will do this in part within a mean field model in our subsequent work where the influence of the field on the activation barriers, reaction energies and rate constants will be incorporated into a microkinetic model while assuming no lateral interactions between the adsorbed species. Furthermore, when discussing the kinetic properties of the MSR reaction, we do not take the adsorption of H2O and CH4 into account. In the other words, we assume that the reactants (CH4* and H2O*) are already adsorbed on the surface and more attention is paid to possible surface reactions. From experimental data, the initial sticking coefficient of CH4 is less than 10-7 [70] and the initial sticking coefficient of H2O is less than 10-4 [71] *

which might make their associated reaction pathways ( CH 4 ( gas) → CH 3 + H * and

H 2O( gas ) → H 2O* ) the rate-limiting steps of all presented favorable pathways in the


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presence of an electric field. Therefore, as a future step, in order to fully understand how experimental conditions (temperature and pressure) of the MSR reaction affect its kinetic properties, it is very important to establish a microkinetic model that takes into account the influence of a field on the underlying pathways. At high temperatures (i.e., 873 K and 1073 K), we also aware that the intermediates are highly mobile due to the fact that the diffusion rate constants of the MSR-related intermediates are much higher than the ones for the bond cleavage elementary steps. As such, the MSR-related possible elementary reaction pathways can occur when the initial states or the final states are not sitting at their most favorable adsorption sites. This can also lead to a change of the field-induced kinetic properties (e.g., activation energies, rate constants) for the MSR-related possible elementary steps. However, it is extremely difficult and nearly impossible to consider the rate constants of all possible diffusion steps or the minimum energy pathways of the MSR-related elementary steps when the initial states are at different adsorption sites. More details regarding the diffusion effects are presented in the SI Section 5. As a summary for Section 3.2, we can conclude that an electric field, on the order of -1 V/Å to 1 V/Å, can result in different favorable catalytic MSR mechanisms with different competitive rate-limiting steps as well as different overall energy profiles. Particularly, when the catalytic reaction mechanism includes intermediates or reactants that have permanent dipole moments (such as H2O and CHOH), the configurations of the transition states, the activation energies, and the reaction energies of elementary steps were affected to a larger degree than the ones with no permanent dipole moments. Furthermore, a positive electric field could lower the overall MSR energy profiles by


24

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stabilizing the adsorption of reactants and assist in the desorption of products, while a negative electric field had an opposite effect. Consequently, the presence of a simulated electric field can alter the catalytic performance of the MSR reaction over a Ni catalyst. This information will prove invaluable when designing new and improved Ni-based electro-catalytic MSR systems with lower operating temperatures. 3.3. Methanol synthesis in the presence of an electric field In Section 3.2, we predicted that the molecules that have permanent dipole moments (such as H2O and CHOH) will experience significant electric field effects. In this section, we provide one more example of how the electric field influences on the minimum energy pathway and the configurations along the pathways of the *

CH 3 + OH * → CH 3OH * reaction. As shown in Figure 5, since a methanol molecule has a permanent dipole moment similar to the H2O and the CHOH species, an electric field has important effects on its synthesis and dehydrogenation with respect to both its energetics and adsorption conformation. In the presence of a negative electric field, the CH3OH molecule becomes a H-down structure, while it becomes an H-up structure in the presence of a positive electric field [53]. Similar structural changes arise in a CH2OH molecule in an electric field (shown in Figure S12, S16, and S20). It is clearly seen that the configuration of the transition state (C3 in Figure 5) of the CH 3* + OH * → CH 3OH * reaction is significantly changed when a negative electric field is applied as compared to when a positive electric field is present or in the absence of a field. Such variation in the structures results in a significant drop (~ 0.5 eV) in the activation energy of methanol synthesis when a negative electric field is applied as compared to the activation energy in the absence of a


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field. Based on our DFT results, even though applying a negative electric field largely reduces the activation energy of methanol synthesis, the Ea0 value (1.38 eV) of this reaction is still very high. Thus, even in the presence of a negative electric field, methanol production from the MSR reaction over a Ni surface will not be possible. Methanol byproducts are simply too unfavorable given the high activation energy on Ni to allow the MSR to be a feasible method for methanol formation as a byproduct. However, such information could possibly be used for the formation of methanol over other transition metal catalysts, such as Cu [72, 73], Pd or Au [74] in the presence of a negative electric field. Therefore, it provides some insights into possible methanol reaction pathways in the presence of an electric field.

Figure 5. Field effects on a methanol reaction pathway from the hydroxyl oxidation of the CH3 fragment. To highlight the activation energy differences, the energies of the IS with different fields were referenced to the co-adsorption energy of CH3 and OH fragments in the absence of a field.


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3.4. The Brønsted-Evans-Polanyi (BEP) correlations with different electric fields As described in Section 3.1-3.3, applying a simulated electric field significantly alters the thermodynamic and kinetic properties of the Ni catalytic MSR reaction. Thus, for such heterogeneous catalytic reactions taking place inside a fuel cell or a reverse fuel cell, the role of an electric field must be taken into account since such a high electric field can exist inside these cells. Also, using an electric field for heterogeneous reactions over transition metals can be a novel design consideration in the future since the electric field can be easily controlled. In order to predict the field effects on complex heterogeneous catalytic reactions over different transition metals, the linear energy correlations have proven to successfully simplify and reduce the theoretical calculations for non-field induced reactions. Therefore, in order to develop new catalysts rapidly, the significance of establishing field-induced linear energy correlations is self-evident. The widely used linear energy correlations, so-called BEP correlations, shows a linear relationship between the reaction energy and the activation energy [66, 75, 76]. However, until now, the influence of the environmental factors, such as the electric fields, on the BEP correlations have not been understood. The electric field effects on the reaction energies of the CH dissociation linearly correlate to its corresponding activation energies in Figure 1(a). This information suggests that a broader field-induced BEP relationship might be established as well. Therefore, in this section, we analyzed the activation energies and the reaction energies for 78 hydrogenation/dehydrogenation reactions during the Ni-catalytic MSR reaction (Scheme 1) in the presence and absence of an electric fields by using DFT calculations. The field-induced BEP correlations of C-H cleavage, C-O cleavage, and OH bond formation are observed for all examined reactions.


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The *

elementary

reactions

related

to

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the

C-H

cleavage

include

the

*

CH x → CH x −1 + H * and CH x OH * → CH x−1OH * + H * reactions. The linear scaling slope and constant of the BEP correlations for this C-H cleavage in the absence of an electric field are 0.64 and 0.72 eV, respectively (given in Figure S26). Similar linear BEP relationships without an electric field can be found in previous publications [67, 75, 7779]. After applying an electric field, the slope and the constant of the BEP relationship of the C-H cleavage are very similar to the corresponding ones in the absence of an electric field. Moreover, the mean absolute error for this BEP linear correction is only 0.14 eV. Thus, we conclude that the field-induced kinetic properties (activation energies) of the CH cleavage can be predicted to some degree by their field-induced thermodynamic properties (reaction energies). Similarly, applying a simulated electric field does not significantly alter the BEP relationship for the C-O bond cleavage. In other words, we can also predict the activation energies of C-O cleavage based on the reaction energy of this reaction in the presence of an electric field. Therefore, in the presence of different electric fields, the larger the reaction energies of the C-H cleavage and the C-O cleavage, the larger their activation energies will be. This information is important as it marks the first time that such field-induced BEP correlations for electro-catalysis have been examined.


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Figure 6. The BEP correlations for C-H breaking (reaction NO. 4~7, 21~25 in Scheme 1), C-O breaking (reaction NO. 8~15), and O-H formation (reaction NO. 1~3, 16~19) elementary reactions during for a number of elementary processes involved in the MSR reaction with and without the presence of an electric fields. Here, the red, blue, and black dots represent the energy with positive, negative, and no fields. Note: The MAE [67, 75, 80] represents the mean absolute error of each individual line.

For *

O-H

bond

formation,

the

calculated

reactions

include

the

*

OH x + H * → OH x+1 and the CH x O * + H * → CH x OH * reactions. From Figure 6(c), the activation energies and the reaction energies of the O-H bond formation didn't fit well in the BEP correlations. Additionally, the slopes and the constants of the BEP correlations


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of the O-H bond formation are significantly altered by the presence of a large simulated electric field (both negative and positive) as compared to when an electric field is absent (Figure S26(c)). Vallejo and Sautet have proposed that the current BEP linear relationships did not incorporate structure sensitivity and consequently cannot adequately describe the overall activity of realistic catalytic particles [81]. By examining twelve different low-indexed, stepped, and kinked surfaces of different transition metals, they investigated the effects of the adsorbate-site geometries on the linear energy scaling correlations. They concluded that the adsorption-energy scaling relationships are not only influenced by the metal activity, but also determined by the valence electrons of the adsorbate, which is established by the different coordination environments of the active sites. From our previous work, we have already shown that the adsorption configurations and the valence electrons of the adsorbate (i.e. HxO and CHxOH) were significantly changed by such a large electric field [35, 53]. Therefore, the BEP correlations of the OH bond formation reactions are significantly altered by the presence of an electric field due to the fact that the electric fields changed the configurations and the valence electrons of the adsorbate over the Ni(111) surface. The corresponding structures along the minimum energy pathways of all the O-H bond formation-related elementary reactions with different electric fields are also given in Figures S1-3 and Figures S12-21. Since the structures of the initial states, transition states, and the final states during the O-H bond cleavage or formation can be significantly altered by a large field, establishing a fieldinduced energy scaling linear or non-linear relationship of such polarized systems can greatly facilitate the computational design of electro-catalysts. 4. Conclusions


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A comprehensive kinetic study on the Ni-catalytic methane steam reforming (MSR) reaction in the presence and the absence of an electric field has been performed by both DFT calculations and corresponding experiments. On one hand, a positive electric field can significantly decrease the rate constant and equilibrium constant at 1073 K of the CH * → C * + H * reaction, suppressing carbon deposits on the surface. The corresponding experimental XPS data also shows that the carbon deposits are largely reduced over the post-reacted Ni surface during the catalytic MSR process in the presence of a positive electric field. Similarly, a positive electric field can also lower the energy profiles of the most favorable MSR mechanism. Furthermore, simulated electric fields are able to alter the most favorable MSR mechanism as well as influence the rate constants and equilibrium constants of the competitive rate-limiting steps. In the absence of an electric field and in the presence of a positive electric field, the most favorable MSR mechanism had CHOH and COH intermediates, while with a negative electric field the most favorable MSR mechanism had a CHO intermediate. By examining all possible MSR mechanisms with different electric fields strengths, we conclude that an electric field can have a greater effect on the kinetic properties of the reactions when one includes molecules with a permanent dipole moment (i.e. CHxOH and HxO). For example, a negative

electric

field

largely

decreased

the

activation

energy

of

the

*

CH 3 + OH * → CH 3OH * reaction by ~0.5 eV and increased the forward rate constant at 1073 K by a factor of 104 as compared to corresponding pathways in the absence of an electric field. This information suggests that a novel catalytic methanol synthesis method directly from methane using a negative electric field could be feasible over other transition metals.


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Additionally, a field-induced linear energy scaling BEP correlation for the C-O and the C-H cleavage reactions are observed, which is similar to the ones found in the absence of an electric field. The linear BEP correlations of the O-H bond forming reactions is notably changed when examining the effect of a simulated electric field, which is due to the large changes in the structural configurations and the valence electrons of the adsorbate via the presence of a field. Overall, this paper provides us a better understanding of the electric field effects on the MSR reaction over a Ni surface and gives us a new tool to use in the design of heterogeneous catalytic reactions through the application of electric fields. 5. Supporting Information Theory: harmonic transition state theory derivation, minimum energy pathways and geometries of elementary reactions, vibrational frequency analysis, and field-induced diffusion; Experiment: additional XPS and XRD data. 6. Acknowledgments This work was supported by institutional funds provided to JSM from the Voiland School of Chemical Engineering and Bioengineering and was partially funded by USDA/NIFA through the Hatch Project #WNP00807 titled: “Fundamental and Applied Chemical and Biological Catalysts to Minimize Climate Change, Create a Sustainable Energy Future, and Provide a Safer Food Supply”. Our thanks also go to the donors of the American Chemical Society Petroleum Research Fund for partial support. Parts of the computational resources were provided by the Center for Nanoscale Materials at Argonne National Laboratory. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences


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under Contract No. DE-AC02-06CH11357. We also acknowledge Dr. Alyssa J. Hensley and Dr. Renqin Zhang for their helpful comments.

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Improving Ni Catalysts Using Electric Fields: A DFT and Experimental

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