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. 2021 Apr 14;1(5):536–542. doi: 10.1021/jacsau.1c00044

Non-Faradaic Promotion of Ethylene Hydrogenation under Oscillating Potentials

Chia Wei Lim 1, Max J Hülsey 1, Ning Yan 1,*
PMCID: PMC8395646  PMID: 34467316

Abstract

graphic file with name au1c00044_0007.jpg

The acceleration of Faradaic reactions by oscillating electric potentials has emerged as a viable tool to enhance electrocatalysis, but the non-Faradaic dynamic promotion of thermal catalytic processes remains to be proven. Here, we present experimental evidence showing that oscillating potentials are capable of enhancing the rate of ethylene hydrogenation despite no promotion effect being observed under static potentials. The non-Faradaic dynamic enhancement reaches up to 553% on a Pd/C electrode when cycling between −0.25 and 0.55 VNHE under optimized conditions with a frequency of around 0.1 Hz and a duty cycle of 99%. Under those conditions, the catalytic reaction rates were promoted beyond the rate of charge transfer to the electrode surface, confirming the non-Faradaic nature of the process. Experiments in different electrolytes reveal a good correlation between the catalytic enhancement and the double-layer capacitance, a measure for the interfacial electric field strength. Preliminary kinetic data is consistent with cyclic removal of adsorbates from the surface at negative potential and the subsequent adsorption of H2 and C2H4 and hydrogenation reaction at the positively polarized surface.

Keywords: non-Faradaic electrochemical modification of catalytic activity (NEMCA), electrochemical promotion of catalysis (EPOC), dynamic catalysis, catalytic promotion, alkene hydrogenation, pulsed potentials, polarized catalytic surfaces, double-layer capacitance

1. Introduction

Non-Faradaic promotion of heterogeneous catalysis, an approach devised by Huggins, Mason, and co-workers,1,2 and later developed by Vayenas and co-workers,3−5 is a tool to enhance the activity and selectivity of reactions on catalyst surfaces by means of applying electric potentials. The unique feature about non-Faradaic promotion is the enhancement of catalytic reaction rates beyond the rate of steady state charge carrier transfer between the electrode surface and the substrate (e.g., C2H4 or H2) or reaction intermediates. It is therefore applicable for a wide range of thermal catalytic reactions.5,6 The application of a static electric potential on a catalyst will induce the buildup of positive or negative interfacial charge as well as the adsorption of ionic compounds on the catalyst surface, thus affecting the reaction and transition state energies. In most cases, this non-Faradaic promotional effect was observed for a catalyst particle interfaced with ion conductors for gas phase reactions, such as the ethylene and methane oxidation as well as the hydrogenation of CO2.4,7,8,10,12−15 It was later also shown that non-Faradaic promotion could occur with a liquid electrolyte, for example, for the H2 oxidation reaction11,16 as well as the non-Faradaic CO2 hydrogenation.9,17

The enhancement of catalytic performance by applying dynamic potentials on catalyst surfaces has been reported by Pamić et al. as early as 1978, where they noticed an enhancement in the formic acid electro-oxidation rate by almost 2 orders of magnitude by “pulsated potentials”.18 The broader applicability was later proven for the methanol oxidation reaction.19 Kakekhani and Ismail-Beigi claimed based on DFT calculations that dynamic potentials on ferroelectric catalyst surfaces are capable of overcoming the Sabatier limit.20,21 For the electrocatalytic Faradaic adiponitrile synthesis, rapidly switching between the resting and cathodic potential enhanced both activity and selectivity.22 A thorough microkinetic analysis was provided by Dauenhauer and co-workers, demonstrating the possibility of enhancing catalytic rates by orders of magnitude by the application of time-variant external stimuli corroborated by DFT.23−27 Experimental proof for the efficiency of dynamic catalytic processes has so far been provided solely for dynamic photocatalytic28 and Faradaic electrocatalytic reactions.29−32

Herein, we demonstrate the enhancement of the ethylene hydrogenation reaction by applying dynamic electric potentials on a Pd/C electrode. Based on reference experiments and Faradaic efficiency calculations, the reaction enhancement indeed appears to be non-Faradaic with enhancement factors of the activity up to 553% under optimized conditions. Of note, the reaction was not accelerated under static electric potentials in our study. Preliminary kinetic experiments suggest that the enhancement effect observed under switching potentials may be due to the evacuation of adsorption sites at low potential followed by the adsorption and hydrogenation of ethylene at higher potentials.

2. Results and Discussion

In order to establish the electrochemical window for the non-Faradaic ethylene hydrogenation reaction, cyclic voltammetry (CV) curves were recorded under different gas atmospheres (Figure S4) between −1 and 1 V (in reference to the normal hydrogen electrode, NHE). Noticeable currents were only obtained at relatively large negative potentials of around −0.7 V, consistent with the reported onset of hydrogen evolution reaction activity in neutral electrolytes. At high positive potentials, a relatively small current was observed only in the presence of hydrogen, possibly associated with the oxidation of hydrogen. Therefore, we conducted the ethylene hydrogenation reaction under static potentials between −0.75 and 0.75 V. The activity was found to follow a volcano shape with the maximum located at −0.25 V, almost coinciding with the open circuit potential (OCP) (Figure 1a and Table 1). This indicates that the ethylene hydrogenation activity for Pd/C electrodes cannot be improved under static potential conditions (vide infra).

Figure 1.

Figure 1

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(a) Ethylene hydrogenation activity under static and dynamic potentials over Pd/C electrodes. Data points (orange) for activity under static conditions resemble a “volcano” plot. Horizontal arrows (blue and pink) indicate the enhanced activity under the two dynamic potential ranges investigated in this study (b) Square-wave oscillating potentials and a qualitative representation of the corresponding current response. For this study, the two potential ranges are Vlow = −0.65 V, Vhigh = 0.15 V (pink arrow in (a)) and Vlow = −0.25 V, Vhigh = 0.55 V (blue arrow in (a)). tlowV = 0.01–10 s, thighV = 0.001–1000 s.

Table 1. Ethylene Hydrogenation Reactivity under Various Conditions.

entry potential (V) tlow (s) thigh (s) TOF (10–2 molC2H6 molsurfacePd/Pt–1 s–1) Δ (%)a FE (%)b
1c nonef     4.2    
2d OCP (static)     <0.1    
3c OCP (static)     4.2    
4c –0.25 to 0.55 (dynamic) 0.1 10 22.9 553 2159
5c –0.65 to 0.15 (dynamic) 0.01 1 13.1 322 703
6e –0.25 to 0.55 (dynamic) 0.1 10 43.9 168 1894
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a

Δ = enhanced rate/OCP rate.

b

Faradaic efficiencies (FEs) were calculated based on the average negative current during the tlowV (for corresponding current responses, see Figure S5).

c

Standard reaction conditions: Pd/C electrode, 0.5 mol L–1 Na2SO4, 60 °C, 800 rpm, 25 mL min–1 H2, 2.5 mL min–1 C2H4, and 2.5 mL min–1 Ar (OCP = −0.28 V).

d

25 mL min–1 N2, 2.5 mL min–1 C2H4, and 2.5 mL min–1 Ar (without H2, OCP = 0.13 V).

e

Pt plate electrode (in place of Pd/C, OCP = −0.27 V).

f

The potentiostat was turned off.

We then attempted the ethylene hydrogenation reaction under oscillating square wave potentials with two different ranges but sharing the same peak-to-peak amplitude of 0.8 V, one centered around the optimum static condition of −0.25 V (−0.65 to 0.15 V) and one further shifted toward higher potentials (−0.25 to 0.55 V) (Figure 1b). The tlowV and thighV were independently varied across up to 7 orders of magnitude to determine optimum conditions for the promotion of the hydrogenation reaction (Figure 2 and Table 1). Two trends become immediately obvious from Figure 2: for efficient enhancement, thighV needs to be around 100 times longer than the tlowV, while the window centered at higher potential appears to offer larger enhancement factors (Δ). This is slightly against what has been proposed before, where optimum enhancement should be observed when external stimuli (i.e. binding energies) are cycled around the static optimum.28,29 Meanwhile, oscillating potentials did not always positively influence the reaction; under certain conditions, the activity dropped up to four times as compared to the static case under open circuit potentials. Finally, the same enhancement effect was also detected for Pt plate electrodes, demonstrating the dynamic, non-Faradaic promotion is not limited to Pd/C electrodes (Table 1).

Figure 2.

Figure 2

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Optimization of the thighV and tlowV for two different potential ranges: (a) – 0.65 to 0.15 V and (b) – 0.25 to 0.55 V. z-axis is rate enhancement factor (Δ) in %, with the same color scale used for (a) and (b). Each full decade intersection between thighV and tlowV represents one data point. The highest points at the outer boundaries of (a) were confirmed to be local maxima (for further information, see Table S1).

In principle, two competing ethylene hydrogenation reaction pathways may be proposed to occur simultaneously on the Pd surface:

2. 1
2. 2

The Faradaic reaction 1 is not commonly observed, and both the high Faradaic efficiencies as well as the lack of conversion under gas feed conditions without H2 (Table 1) suggest that this reaction does not take place to an appreciable extent. The non-Faradaic reaction 2 thus should be the dominant pathway for the ethylene hydrogenation.

To determine which factor(s) govern the improved hydrogenation activity under oscillating potentials, the effect of electrolytes was investigated. We compared the ethylene hydrogenation reaction in 0.5 mol L–1 Na2SO4, K2SO4, Li2SO4, (NH4)2SO4, MgSO4, NaOH, H2SO4, NaCl, and HClO4. The performance under static −0.25 V and under optimized dynamic conditions (t–0.25 V = 0.1 s and t0.55 V = 10 s) was measured, and the enhancement factors (Δ) were found to vary widely between 71 and 553%. Electrochemical impedance spectroscopy fitting of a Randles circuit was used to determine the double-layer capacitance of the Pd/C electrode under reactive atmosphere in all of the electrolytes (Figure S6). A good linear correlation was established between the double layer capacitance and Δ (Figure 3a), although factors like salt-dependent gas solubility and other components determining mass transfer at the liquid–solid interface might impact variability across electrolytes as well. In order to understand possible further differences in the electrochemical behavior for the electrolytes, we recorded the CV spectra of the catalyst under H2 and C2H4 (Figures 3b and S7). For Na2SO4, HClO4, (NH4)2SO4, H2SO4, and NaOH electrolytes, bearing varied electrochemical behavior and providing different Δ values, no pronounced anodic or cathodic currents were observed between −0.25 and 0.55 V. Only when the potential goes below around −0.25 V, cathodic currents likely due to hydrogen evolution became apparent, especially in acidic electrolyte. Enhancing the interfacial electric field strength by means of improving the double layer capacitance provides a method to further promote non-Faradaic (dynamic) electrocatalytic processes in situations where charged double layers can form. We also investigated the influence of anion binding strength on enhancement factors. Although the anion binding strengths to the Pd surface were reported to follow the order ClO4– < SO42– < Cl–,33 Na2SO4, NaCl, H2SO4, and HClO4 were found to obey the same linear correlation as all other electrolytes used, suggesting that surface cleaning of adsorbed ions does not significantly affect enhancement factors.

Figure 3.

Figure 3

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Electrolyte effects on the dynamic non-Faradaic promotion for ethylene hydrogenation on Pd/C. (a) Correlation between the double layer capacitance and the enhancement factor for different 0.5 mol L–1 electrolytes (for further information, see Table S2). The inset shows the electrochemical bilayer formed between the Pd surface and the electrolyte. (b) CV curves for five selected electrolytes at concentrations of 0.5 mol L–1 between −1.0 and 1.0 V with 10 mV s–1. The upper and lower boundaries for the dynamic oscillating potentials are indicated by vertical dotted lines.

The ethylene hydrogenation on noble metal surfaces is commonly assumed to follow a Horiuti–Polanyi mechanism. This entails the coadsorption of H2 and C2H4 on the surface followed by the stepwise addition of H atoms to the C-containing intermediates to eventually form ethane. Another common assumption is that both adsorbates compete for the same Pd surface sites.34

A comparison to the catalytic activity observed in the gas phase implies that there is no significant catalyst poisoning by the liquid phase (for details, see page S3 in the Supporting Information). Thus, the adsorption of H2 and C2H4, instead of water molecules, is likely to be kinetically relevant. Both H2 and C2H4 are electrophobic and thus bind more strongly on more positively polarized surfaces. Negatively polarized surfaces, in contrast will lead to low binding energies of both adsorbates.35−38 Under dynamic conditions, it may be assumed that the short pulse at low potential leads to the removal of both H2 and C2H4, thus liberating adsorption sites. Upon switching to positive potentials, both H2 and C2H4 will adsorb. Due to the excess of H2, only a certain fraction of the surface will be covered by the olefin with a presumably higher H2 coverage. This coadsorption leads to the hydrogenation of C2H4, after which the alkane will be removed and predominantly replaced by H2. After this, the catalyst is cycled back to negative potential to allow for surface cleaning and the subsequent adsorption of H2 and C2H4 after switching back to positive potential (Figure 4). This sweeping mechanism is consistent with the fact that a very short low potential pulse is required while the high potential needs to be significantly longer by a factor of around 100. Based on first-principles modeling, it was shown that the activation barriers as well as the reaction energies for unsaturated bond hydrogenation become more favorable when the surface coverage with H atoms is large.39 Therefore, achieving a continuously high H coverage while periodically allowing the surface to adsorb C2H4 might be responsible for the overall higher activity. The observation of higher activity in the higher dynamic potential window (−0.25 to 0.55 V) may suggest that some steps directly related to the hydrogenation reaction are affected to a different extent as compared to the lower potential window (−0.65 to 0.15 V).

Figure 4.

Figure 4

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Putative mechanism for the non-Faradaic enhancement of the ethylene hydrogenation reaction on dynamic Pd surfaces oscillating between a positively and negatively charged surface state.

Kinetic investigation was conducted to gain insights into the surface phenomena on the electrode under different potentials (Figure 5). Ethylene reaction orders were obtained at the lower and upper limits of the oscillating potentials (−0.25 and 0.55 V) as well as under the optimized dynamic conditions. H2 was supplied in large excess (PH2 ranged from around 4 to 10 times of PC2H4). The reaction orders were similar under these three conditions, with values between 0.9 and 1.1. Although ethylene reaction orders are often reported to be negative for gas phase reactions, positive orders have been shown for electrochemical systems before in solution.40,41 We also noticed slight pressure buildup inside the cell of around 1.6 bar, which might affect the gas solubilities in the electrolyte. Therefore, the precise measurement of reaction order is difficult, and the errors were quantified by repeating each measurement three times and representing the corresponding standard deviations as error bars in Figure 5.

Figure 5.

Figure 5

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Ethylene hydrogenation reaction kinetics under different static (−0.25 and 0.55 V) and dynamic potential (t–0.25V = 0.1 s, t0.55V = 10 s) conditions. Reaction conditions: Pd/C electrode, 0.5 mol L–1 Na2SO4, 60 °C, 800 rpm, 20 mL min–1 H2, 2.5–5 mL min–1 C2H4, 2.5–5 mL min–1 Ar, total gas flow rate = 35 mL min–1 (balance N2). Standard deviations are determined from three individual measurements.

The formation of PdH phases under H2 atmosphere was reported before, in particular under reducing electrode potential.42,43 Since the holding time at reducing potentials is very low, this hydride formation may not play a significant role for the non-Faradaic dynamic promotion of ethylene hydrogenation. Furthermore, Pt plate electrodes exhibit enhancement under dynamic potential cycling as well while the formation of Pt hydride from polycrystalline Pt electrodes is less probable. The removal of surface poisons has been proposed to govern the activity enhancement of electrocatalysts under oscillating potentials.18,19 We further conducted a dynamic-static switching experiment to evaluate the system’s reversibility. The conversion was first allowed to achieve a steady value under the optimal dynamic condition and then the system was switched to the static condition at OCP. A sharp drop in activity was recorded (Figure S8), with a response time that matches the residence time of the system (Figure S9). This procedure was repeated another three times, and each time when the system was switched from dynamic to static conditions, a sharp decrease in ethylene conversion was observed. This demonstrates that the catalyst returns to OCP condition immediately following periodic oscillations of potential. It also rules out that permanent alteration of the catalyst structure under dynamic potentials plays a significant role in the enhancement mechanism.

Under certain reaction conditions, the formation of alkylidene species has been revealed on Pd surfaces during the ethylene hydrogenation. This dehydrogenation reaction is not very likely in our case since it usually occurs with an excess of olefin in the gas feed44 and the barriers for the dehydrogenation are usually significantly higher than for the hydrogenation reaction.45 Although the desorption of alkane is often not rate-determining, the desorption of ethane was indicated to be more favorable at lower potential, suggesting that the quick sweep from positive to negative potential may impact overall hydrogenation kinetics.46,47 Possible additional effects of electric fields on the hydrogenation reaction energies and barriers remain to be investigated in detail.

3. Conclusions

We have demonstrated here that oscillating potentials can promote the ethylene hydrogenation activity, although no promotional effect was observed under static potentials. This promotional effect was non-Faradaic and not limited to the Pd/C electrode with a pronounced impact of the electrolyte. An initial assumption about the mechanism of the promotion was provided, but more work is necessary to completely understand the origin of this effect.

Acknowledgments

This research is supported by the NUS Flagship Green Energy Program (R-279-000-553-646 and R-279-000-553-731). M.J.H. thanks the SINGA scholarship for supporting his PhD studies. The authors thank Prof. Yongjun Gao for providing equipment for the electrochemical experiments.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.1c00044.

  • Materials and methods, comparison of gas- and liquid-phase TOFs, SEM image and particle size distribution, open circuit potential measurements, cyclic voltammetry curves, current responses, electrochemical impedance spectroscopy, long-time behavior, residence time distributions, and tables of TOF values (PDF)

Author Contributions

‡ C.W.L. and M.J.H. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

au1c00044_si_001.pdf (785.6KB, pdf)

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