Potential-Dynamic Surface Chemistry Controls the Electrocatalytic

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Potential-Dynamic Surface Chemistry Controls the Electrocatalytic Processes of Ethanol Oxidation on Gold Surfaces Yanyan Zhang, Jungang Wang, XiaoFei Yu, Donald R Baer, Yao Zhao, Lanqun Mao, Fuyi Wang, and Zihua Zhu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02019 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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ACS Energy Letters

Potential-Dynamic Surface Chemistry Controls the Electro-catalytic Processes of Ethanol Oxidation on Gold Surfaces Yanyan Zhang,a,b,c Jungang Wang,b Xiaofei Yu,b Donald R. Baer,b Yao Zhao,a Lanqun Mao,a,c Fuyi Wang,*,a,c,d and Zihua Zhu*,b

a

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Analytical

Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

b

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

Richland, WA 99354, USA

c

University of Chinese Academy of Sciences, Beijing 100049, China

d

National Centre for Mass Spectrometry in Beijing, Beijing 100190, China.

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AUTHOR INFORMATION

Corresponding Author *Email: [email protected]; [email protected]


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ABSTRACT

Electro-catalysis has various important applications especially fuel cells. As a key element in electro-catalysis, the surface chemistry of electro-catalysts may strongly influence the catalytic activity and reaction mechanism, the fundamental understanding of which would provide guidance for designing high-efficiency catalysts. Herein, we utilized our recently developed in situ liquid SIMS approach to investigate the electrocatalytic oxidation of ethanol on gold surfaces in alkaline environments involved in direct alcohol fuel cells. Formation of adsorbed hydroxide intermediates on the gold surfaces upon electro-oxidation was molecularly witnessed under operando condition, the evolution of which was revealed to govern the electro-catalytic processes. Moreover, the hydroxide intermediates as active sites participated in the reaction through transferring nucleophilic hydroxyl groups into the adjacent ethoxy molecules. This work brings new light into electro-catalytic research and will facilitate the improvement of catalytic systems on the basis of surface chemistry-catalytic performance relationship.

TOC GRAPHICS


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Electro-catalysis, a heterogeneous catalysis that occurs on or near the surface of solid electro-catalysts,1 has important applications such as electro-synthesis,2 electrode based sensing,3 electrolysis,4 and especially the efficient operation of fuel cells5-8. Changes in surface chemistry of electro-catalysts during electro-catalytic reactions may influence both the catalytic efficiency and the reaction mechanism.9-15 General strategies of electro-catalysts development focus on surface active sites such as by enhancing the intrinsic activity of each active site, increasing their total number, or shortening the distance of two kinds of active sites on multicomponent catalysts.5, 11, 16, 17

Therefore, the fundamental understanding of active sites on electro-catalysts as well

as their relationship with the electro-catalytic processes would facilitate the design of high-efficiency catalysts.5, 9, 18, 19


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Direct alcohol fuel cells (DAFCs) involve classic surface electro-catalytic reactions utilizing small organic molecules such as methanol and ethanol as fuels to be directly oxidized at metal anode surfaces, which have been recognized as a promising future power source with advantages of environmental compatibility, facile fuel storage, easy refilling and high power density.8, 19-22 Gold has been reported to be a good catalyst for alcohol electro-oxidation in alkaline environments, but poor in acidic solutions. Much of our understanding towards the difference comes from electrochemical studies which proposed that surface adsorbed hydroxide intermediates (abbreviated as Au(OH)ads) were formed on gold electrode surfaces in basic environments during electrooxidation,23-29 suggesting a vital role of electrode surface chemical structures in catalytic activity. In situ scanning tunnelling microscopy (STM) has demonstrated changes in surface morphology of gold electrodes during electrochemical processes.30, 31 In a more chemical level of insights, in situ Fourier-transform infrared spectroscopy (FTIR) has been devoted to this topic.32-35 However, it is difficult to obtain the direct vibrational information on a molecule-metal bond which is located at low wavenumbers below 800 cm-1 due to the absorption of infrared light by the thin solution layer and optical window.


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Recently, shell-isolated nanoparticle-enhanced Raman spectroscopy was developed to tackle the challenge in obtaining the detailed description of the intermediate species on gold surfaces during electro-oxidation.9,

36

However, no establishment of the

fundamental link of such surface chemical evolution with the dynamic electro-catalytic mechanism of ethanol oxidation and the complete picture of the catalytic processes still remains to be further depicted. Recently, we have developed an in situ liquid secondary ion mass spectrometry (SIMS) method37-40 which possesses uniqueness of in situ obtaining molecular evolution information of electrode surfaces, reactants, intermediates, and products simultaneously at electrode-electrolyte interfaces during electrochemical reactions, offering the possibility to explore the underlying relationship between surface chemistry and electrocatalytic mechanism. In this work, with the electro-catalytic oxidation of ethanol on gold electrode surfaces as an objective system, we utilized in situ liquid SIMS as molecular “eyes” to attempt to in situ molecularly illustrate the vital role of surface chemistry during electro-catalysis, which would greatly impact the mechanism investigations in the catalysis fields.


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In brief, we designed and fabricated a high vacuum compatible microfluidic electrochemical cell for liquid SIMS analysis, employing a gold film working electrode (WE) of 50 nm in thickness which was sputter-coated beneath the silicon nitride (SiN) membrane window, a platinum counter electrode (CE) and a platinum pseudo reference electrode (RE). The assembly of the cell is shown in Movie S1 and a side view in Figure 1a. After introducing an electrolyte of interest, the cell was sealed and then transferred into the analysis chamber of a ToF-SIMS instrument under high vacuum.

Figure 1. (a) A schematic diagram of a side view of the microfluidic electrochemical device fabricated on a PEEK block with the gold film sputter-coated beneath the SiN membrane as a working electrode (WE) and two platinum wires fixed at the bottom of


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the liquid chamber as a reference electrode (RE) and a counter electrode (CE), respectively. (b) The ToF-SIMS depth profiles in negative ion mode with a potential of 0.4 V in the anodic direction applied to the gold working electrode immersed in 0.1 M KOH electrolyte containing 0.1 M ethanol. The time point when H-, O- and Au related signals were suddenly increased was reset to 0 s, which indicated the punching through of SiN membrane and reaching the gold film working electrode beneath it by the primary ion beam. (c) A representative spectrum was reconstructed from the shadowed area in (b) where signals especially those of Au related species were relatively steady.

To validate the electrochemical behavior of the microfluidic cell, cyclic voltammetry (CV) scans were performed on a 0.1 M KOH aqueous solution in the absence (black line) or presence (red line) of 0.1 M ethanol (Figure 2), and compared with those on traditional electrochemical systems (Figures S1 and S2). Although minor differences, there is a high degree of consistency of the electrochemical responses in three systems. On the basis of the CV curves and previous studies, the detailed electrochemical processes are discussed in the Supporting Information. Briefly, from the CV curve in 0.1


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M KOH electrolyte, two oxidation waves were observed in the anodic direction, which was proposed to be the chemisorption of OH- ions and the further formation of gold oxide, respectively. In the cathodic direction, the gold oxide was electrochemically reduced and then the desorption of OH- ions occured. When ethanol molecules were present, the second oxidation wave markedly increased which was assigned to the oxidation of ethanol. What’s especially interesting is that during the cathodic sweep a third oxidation wave was observed due to the reinitiation of ethanol oxidation in the same potential range where gold oxide was reduced. These phenomenon suggest the vital role of the gold electrode surface chemistry on the electro-oxidation processes of ethanol.


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Figure 2. CV curves of 0.1 M KOH solution with (red line) or without 0.1 M ethanol (black line) in a microfluidic cell. The blue dots on the CV curves refer to the chosen step potentials including -0.4 V, 0 V, 0.4 V, and 0.6 V during the anodic scans, and 0.2 V, -0.1 V and -0.4 V during the cathodic scans, at which further in situ liquid SIMS analysis were conducted.

In situ liquid SIMS measurements were then performed at potentials of -0.4 V, 0 V, 0.4 V, 0.6 V in the anodic direction and 0.2 V, -0.1 V, -0.4 V in the cathodic direction based on features marked with blue dots on the CV curve of the ethanol-containing electrolyte (red line in Figure 2) to examine the occurring molecular processes. During in situ liquid SIMS measurements, a focused Bi3+ primary ion beam was used to drill an aperture of ~2 µm in diameter in the SiN membrane as soon as the indicated electrode potential was applied, enabling analysis of the Au and the Au-electrolyte interface (Figure 1a). Data collected from a region (the shadowed area in Figure 1b) where the Au-related signals became relatively stable following penetration of the membrane and a transition state were used to reconstruct mass spectra at each potential as shown by an example


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in Figure 1c. It is notable that the spectra reflect information not only from the electrolyte solution but also from the side wall of the aperture through the SiN/Au film as well as the electrode-electrolyte interface of several nanometers thickness (Figure S3).37, 39, 40 Spectra in the absence or presence of 0.1 M ethanol in a microfluidic cell with H- ion (m/z 1) as a reference were generated for each potential as shown in Figures S4 and S5, respectively. A series of characteristic Aux- (x=1 – 4) and Aux(OH)y- (x=1 – 3, y=1 – 2) ion signals dominated the spectra. Here, to illustrate the surface chemical changes of the gold electrode as the potential varied, we normalized the intensities of Aux(OH)yions (AuOH-, m/z 214; Au(OH)2-, m/z 231; Au2OH-, m/z 411; and Au2(OH)2-, m/z 428) to those of corresponding Aux- ions (Au-, m/z 197; and Au2-, m/z 394). The resulting spectra in the related mass range in both systems are shown in Figures 3 and S6. Moreover, the spectra highlighting the signals of acetate ions (CH3COO-, m/z 59), the oxidation product of ethanol in the system24,

27, 41,

at different potentials were also

compared in Figure 4.


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Figure 3. Representative normalized negative SIMS spectra in the mass ranges of 390435 highlighting the chemical evolution of the gold electrode surface in 0.1 M KOH solution in the absence (a) or presence (b) of 0.1 M ethanol within a microfluidic cell when different step potentials were applied to the gold working electrode at -0.4 V, 0 V, 0.4 V, 0.6 V in the anodic direction and 0.2 V, -0.1 V, -0.4 V in the cathodic direction. The masses of all secondary ions were calibrated by using C- (m/z 12), OH- (m/z 17), C2- (m/z 24), Si- (m/z 28) and Au3- (m/z 591) ions. Signal intensities were normalized to those of Au2- (m/z 394) ions, respectively. Peak assignments: Au2-, m/z 394; Au2OH-,

m/z 411; Au2(OH)2-, m/z 428.


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To make the comparison more straightforward, we extracted normalized Aux(OH)y- ion intensities from the spectra at each potential, and depicted their changes as a function of the applied WE potential in Figure S7. As all Aux(OH)y- ion intensities showed similar trends as a function of applied potentials in both systems, for brevity the normalized Au2OH- ion intensities to Au2- (Figure 5a) are selected as a representative for the following discussion. In the ethanol-free KOH solution, Figure 5a (black line) shows that as the potential went from -0.4 V to 0 V, the normalized Au2OH- ion intensity slightly increased, which is well consistent with the chemisorption of OH- ions to the gold electrode surface during oxidation forming the long proposed adsorbed hydroxide intermediate species Au(OH)ads9,

23-28.

At 0.4 V, a sharp increase was observed for the Au2OH- intensity,

revealing that a larger amount of OH- ions were chemisorbed onto the positively charged gold film surface forming more Au(OH)ads species. The intensity fell sharply at 0.6 V, indicating at this high oxidation potential Au(OH)ads species were electro-oxidized to a surface gold oxide layer9, 24-28, resulting in a decrease of Au(OH)ads species. Upon shifting the potential in the cathodic direction by lowering the voltage from 0.6 V to 0.2 V,


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the normalized Au2OH- intensity recovered at some extent. This change was ascribed to reduction of the surface oxide, which reproduced Au(OH)ads species on the electrode surface. When the potential further decreased to -0.1 V, the Au2OH- signal intensity markedly increased, indicating a significant increase in the amount of Au(OH)ads species due to further reduction of the surface gold oxide layer. As the potential went back to 0.4 V, the relative Au2OH- intensity dramatically dropped due to deficiency of Au(OH)ads species arising from desorption of OH- from the electrode surface. These data provide direct molecular evidence of the dynamic formation and evolution of Au(OH)ads species on the gold electrode surface in an alkaline solution at different electrode potentials as illustrated in Figure S8.


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Figure 4. Negative ion SIMS spectra of ions at m/z 59 (CH3COO-) with H- ions as a reference from 0.1 M KOH solution in the absence (a) or presence (b) of 0.1 M ethanol when different step potentials were applied to the gold working electrode at -0.4 V, 0 V, 0.4 V, 0.6 V in the anodic direction and 0.2 V, -0.1 V, -0.4 V in the cathodic direction. The ion at m/z 60 is assigned to SiO2-.

Figure 5. The normalized intensities of interesting ions at electrode-electrolyte interfaces measured at chosen electrode potentials including -0.4 V, 0 V, 0.4 V, and 0.6 V during the anodic scans, and 0.2 V, -0.1 V and -0.4 V during the cathodic scans. (a) Au2OHion intensities to Au2- ions and (b) CH3COO- ion intensities (m/z 59) to H- ions with 0.1 M


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KOH in the absence (black line) and presence (red line) of 0.1 M ethanol as electrolytes.

In order to reveal the significance of Au(OH)ads species during electro-catalytic oxidation of ethanol, we also depicted the trends of signal changes of the oxidation product of ethanol, acetate ions (CH3COO-, m/z 59), from Figure 4 with the variation of the applied potenital as shown in Figure 5b. Compared with results in the ethanol-free system (black line in Figure 5b), the remarkable changes of CH3COO- ion intensities in the ethanol-containing system at various electrode potentials (red line in Figure 5b) directly indicate occurrence of the ethanol electro-oxidation with different efficiency. What’s especially interesting is that the CH3COO- ion intensity in the ethanol-containing system changed with the applied potential (red line in Figure 5b) in the same trend as that of Au2OH- ions in KOH alone system (black line in Figure 5a). This reveals that the catalytic efficiency strongly depends on the number of Au(OH)ads species on the gold electrode surface, interpreting the previous electrochemical observations that the electro-catalytic oxidation of ethanol in this system is surface-controlled29. Furthermore,


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in the ethanol-containing solution Au2OH- signal intensities decreased significantly at potentials of 0.4 V, 0.2 V and -0.1 V (red line in Figure 5a) relative to those in the ethanol-free system (black line in Figure 5a). Given that the CH3COO- ion intensities at these three potentials greatly increased, it is evident that the ethanol electro-oxidation occurred efficiently through the interaction of adjacent Au(OH)ads species on the gold electrode surface. Density functional theory calculations indicated that Au(OH)ads intermediates significantly lower the barrier of β-H elimination (highlighted by red in Figure 6a) to accelerate the electro-oxidation of ethanol.41 Therefore, Au(OH)ads species act as active sites for the electro-catalytic oxidation of ethanol by nucleophilic attack of the activated hydroxyl groups on the electrode surface into the adjacent absorbed ethoxy, causing instantaneous consumption of the Au(OH)ads species as illustrated in Figure 6a. Other molecular signals help provide a more complete picture of the processes occurring at different potentials (detailed in the Supporting Information). A slight increase of the CH3COO- signal (red line in Figure 5b) along with a significant increase of C2H5O- signal (m/z 45, Figures S9 and S10) was observed when the potential went


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from -0.4 to 0 V. This suggests at 0 V ethanol molecules were already adsorbed onto the gold electrode surface, some of which could be electro-oxidized by interaction with the available Au(OH)ads active sites nearby. As the number of active sites at 0 V was not large in comparison to that at 0.4 V (black line in Figure 5a), a significant portion of chemically adsorbed ethoxy remained unreacted. The results claimed that the oxidation of ethanol starts at the potential where Au(OH)ads species begins to form.


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Figure 6. Schematic diagrams of the electro-catalytic processes. (a) The reaction mechanism for the oxidation of ethanol to acetate ion on gold film WE surface in an alkaline solution. (b, c) Illustration of chemical changes on the gold WE surface in 0.1 M ethanol in 0.1 M KOH solution when chosen electrode potentials were applied in the (b) anodic, and (c) cathodic directions, respectively.

On the basis of the above observations, we depict schematic diagrams (Figure 6) to illustrate how the surface chemical changes on the gold electrode govern the electrocatalytic oxidation of ethanol. As shown in the step (1) in Figure 6a, at the high pH of 13, a small amount of ethanol molecules deprotonate forming ethoxy anions, more reactive species42. At an original potential of -0.4 V (Figure 6b), not only negative OH- and ethoxy anions but also neutral ethanol molecules with interactive ends of oxygen atoms with negative polarity are repulsed away from the negatively charged electrode. With the potential increasing to 0 V, a large quantity of ethanol molecules is electrochemically adsorbed onto the gold electrode surface apart from the formation of Au(OH)ads species due to the chemisorption of OH- ions via a partial charge transfer, resulting in the


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occurrence of electro-oxidation of ethanol via interaction with nearby Au(OH)ads species. At 0.4 V, more OH- ions could be chemisorbed onto the electrode surface, providing more active sites and thus greatly accelerating the electro-catalytic reaction. However, an higher potential of 0.6 V gave rise to formation of surface gold oxide by oxidation of Au(OH)ads9,

24-28,

resulting in the deficiency of Au(OH)ads species and the restrained

electro-catalytic oxidation of ethanol. In the cathodic direction (Figure 6c), when the potential was set to 0.2 V, it’s interesting that the reduction of the surface gold oxide occurred, reproducing a number of active sites for reactivating the electro-oxidation of ethanol. At -0.1 V, more active sites are reproduced via further reduction of the surface gold oxide layer, accelerating oxidation of ethanol to the maximum level. This surprising finding may be helpful for designing the best process flow in practical application of DAFCs. When applying -0.4 V again, desorption of OH- ions leads to the dramatically decreased coverage of active sites on the electrode surface, making the electro-catalytic reaction stopped. Koper et al. reported that the reactivity of alcohols on gold electrodes in alkaline solution could be higher if deprotonation in the step (1) in Figure 6a became easier.


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Such a result suggests this base catalysis should be the main driver behind to dominate the reactivity, although the interaction of gold surfaces with hydroxide ions is still necessary but less important.42 We agree with the opinion that the higher concentration of hydroxide anions in alkaline solutions and lower pKa of alcohols indeed facilitates the initial deprotonation leading to higher oxidation activity of alcohols. However, the sluggish step during the whole reaction mechanism still remains debating, which would determine the overall reaction efficiency. In previous FTIR measurements, the further reaction of the adsorbed ethoxy was proposed to be very fast on the basis of the undetected ethoxy.34 While, in a recent study the catalytic efficiency was greatly improved when shortening the distance between the active sites that adsorb these two kinds of surface species respectively in novel ternary catalysts and the combination of the adsorbed hydroxide and reactant was indicated to be the rate-determining step for ethanol oxidation reaction.17 Here, our in situ liquid SIMS observations justified with solid molecular evidence that the potential-dynamic formation and evolution of the adsorbed hydroxide intermediates Au(OH)ads on gold surfaces played a mandatory catalytic role and controlled the electro-oxidation of ethanol in alkaline solutions.


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Moreover, it was reported that in the case of electro-oxidation of carbon monoxide (CO) on gold surfaces the cooperative process of CO and OH- adsorption led to enhanced reactivity.33,

43-46

The adsorption of both the negatively charged species on the gold

surface would not lead to a repulsive interaction, but instead results in the upward shifting of the Au d band toward the Fermi level, which facilitates the charge transfer between the adsorbed species and metal surfaces.47 This might also explain the catalytic mechanism of ethanol electro-oxidation in this system as discussed above. To conclude, our in situ liquid SIMS technique served as molecular “eyes” into the nature of catalytic activity of gold electrode surfaces towards the electro-oxidation of ethanol in an alkaline solution. For the first time, we provided direct molecular evidence to witness the formation of adsorbed intermediate Au(OH)ads species on the gold electrode surfaces due to chemisorption of OH- ions during electro-oxidation, and show that the potential-dynamic evolution of the Au(OH)ads species controls the catalysis processes. More importantly, the Au(OH)ads species on the gold electrode surface were molecularly identified as surface active sites which were revealed to participate in the oxidation reaction of ethanol via transferring the nucleophilic hydroxyl groups into the


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ajacent absorbed ethoxy. Furthermore, our data show that more Au(OH)ads species are available at the cathodic direction than that in the anodic direction, prividng useful information for opimization of production processes. By providing fundamental insights into what species on electro-catalyst surfaces act as active sites during various electrocatalysis reactions and how they exert effects on the underlying catalytic mechanisms, our work may faciliate the design of high-efficiency catalytic systems on the basis of surface chemistry-catalytic performance relationship.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

The assembly of the cell (AVI)

Experimental procedures, additional results and discussions (PDF)

AUTHOR INFORMATION


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by a FY2016 open call LDRD fund of the Pacific Northwest National Laboratory (PNNL). The work was performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL. Fuyi Wang, Yanyan Zhang and Yao Zhao thank the NSFC (Grant Nos. 21127901, 21575145, 21621062) for support.

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