Potential-Dynamic Surface Chemistry Controls the Electro-catalytic Processes of Ethanol Oxidation on Gold Surfaces

Transcription

1 Supporting Information Potential-Dynamic Surface Chemistry Controls the Electro-catalytic Processes of Ethanol Oxidation on Gold Surfaces Yanyan Zhang, a,b,c Jun-Gang 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 , China. b Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, USA c University of Chinese Academy of Sciences, Beijing , China d National Centre for Mass Spectrometry in Beijing, Beijing , China S-1

2 Table of Contents Experimental procedures.page S-3 Results and Discussion..Page S-7 1. Electrochemical performance of the microfluidic three-electrode device..page S-7 2. In situ liquid SIMS analysis of electrode-electrolyte interface.page S Reconstructed in situ liquid SIMS spectra at different electrode potentials.page S Illustration of chemical changes on the gold electrode surface at different potentials.page S Additional discussions on the mechanism of ethanol electro-oxidation on gold electrode surfaces.page S-23 References Page S-31 S-2

3 Experimental Procedures Fabrication of the microfluidic electrochemical device A microfluidic electrochemical cell for in situ liquid SIMS analysis was designed and fabricated using a polyether ether ketone (PEEK) block, which consisted of three electrodes including gold (Au) film working electrode (WE), platinum counter electrode (CE) and platinum pseudo reference electrode (RE). A gold film working electrode with a thickness of 50 nm was sputter-coated on an adhesion layer of 5 nm Cr which was pre-sputter coated on the SiN membrane window (100 nm thick, 0.5 mm 0.5 mm) on a silicon frame (200 µm thick, 7.5 mm 7.5 mm) for the better attachment. A narrow strip of the gold film was extended to the edge of the silicon frame for the connection to a copper wire which further connected with an electrochemical workstation. The silicon frame was placed on the top of the liquid chamber which is 6.0 mm in length, 5.2 mm in width and 1.0 mm in height and fixed to the PEEK block with an epoxy. Two platinum wires of 0.5 mm in diameter served as counter and reference electrodes, respectively, which were inserted through two punched holes (0.55 mm in diameter) in the PEEK block and fixed on the bottom of a liquid chamber inside the block. The other ends of the wires were exposed outside the block for connecting with the electrochemical workstation. Aqueous electrolyte solutions of 0.1 M potassium hydroxide (KOH) and 0.1 M ethanol in 0.1 M KOH were prepared using ultrapure water. The electrolyte was then introduced into the liquid chamber by using two PEEK tubes inserted through two punched holes in the block, after that the two tubes were sealed with unions and fittings. The assembly of the microfluidic electrochemical cell was shown in Movie S1, and a schematic diagram of a side view of the microfluidic electrochemical cell was shown in Figure 1a. S-3

4 Movie S1. An animation of the assembly of the microfluidic three-electrode device for in situ liquid SIMS measurements. From top to bottom, blue: a silicon piece with a window in the centre; green: a silicon nitride membrane; pink: T-shaped Cr/Au thin film; orange: a curved copper line for conductive connection; yellow: a PEEK basement; grey: two platinum wires as reference and counter electrode, respectively; light blue: two PEEK tubes for introduction of electrolyte. The copper line and the two platinum wires could connect with the external circuit of the electrochemical workstation. S-4

5 Electrochemical measurements A CHI660C electrochemical workstation (CH Instruments, TX, USA) was used for applying electrode potentials to the microfluidic three-electrode device. Firstly, to verify the electrochemical performance of the device under high vacuum condition, cyclic voltammetry (CV) scanning was performed in the potential range of -0.4 V to 0.6 V at a scan rate of 25 mv/s at after the cell was mounted onto the sample holder and transferred into the analysis chamber of the TOF-SIMS instrument. For comparison, CV curves were also obtained in a traditional three-electrode electrochemical system with a gold wire as WE, silver/silver chloride electrode or a Pt wire as RE and a Pt wire as CE, respectively. To investigate the electro-catalytic oxidation mechanism of ethanol at the gold film electrode surfaces, different step potentials were set 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, respectively. At the same time when a specific constant potential value was applied to the gold film WE, in situ liquid SIMS analysis was conducted. In situ liquid SIMS analysis The electrochemical cell was mounted onto the ToF-SIMS sample holder and transferred into the analysis chamber of a ToF-SIMS 5 instrument (IONTOF GmbH, Münster, Germany) under high vacuum. In situ liquid SIMS experimental settings were similar to those described in our previous paper 1 with some minor improvements. In brief, a pulsed 10 khz 25 kev Bi3 + primary ion beam was focused to ~350 nm in diameter, and rastered on a round area with 2 µm in diameter on SiN membrane to drill an aperture for in situ liquid SIMS measurements in the negative ion mode. A potential at a specific value was applied right before the starting of ToF-SIMS measurements. At first, a 500 ns Bi3 + pulse was used for about 36 s (for the negative ion mode), and then it was changed to 100 ns (100 ns pulsed beam current is about 0.24 pa). After ~20-30 seconds more, the Bi3 + beam sputtered through the S-5

6 SiN membrane and the Au-related signals (e.g., Au -, m/z 197; AuOH -, m/z 214; and Au3 -, m/z 591) appeares as shown at the 0 s in the Figure 1b. The 500 ns pulse was used to reduce total sputtering-through time, and the 100 ns pulse was used for better mass resolution. The penetration of the SiN membrane was associated with a sharp increase in signals of H - (m/z 1) and O - (m/z 16), indicating the Au film electrode was porous and a small amount of liquid could diffuse to the SiN-Cr/Au interface. After 15 s of the punching-through of the SiN membrane, a new increasing step of H - and O - appeared with decreasing of Au-related signals, indicating punching-through of the Au film to reach the liquid. With further sputtering, the aperture became larger and reached a relatively stable state (e.g., after 31 s in Figure 1b). After collecting data of more s, ToF-SIMS measurement was stopped, and a mass spectrum could be reconstructed (Figure 1c) from the final s period (shown in the shadowed area in Figure 1b). The spectra were mass calibrated using C - (m/z 12), OH - (m/z 17), C2 - (m/z 24), Si - (m/z 28) and Au3 - (m/z 591) ions. Generally, for each microfluidic electrochemical device, 3-5 apertures could be drilled (at least 100 microns between them to avoid cross-contaminations) and measurements at 3-5 electrode potential values could be conducted. For each potential value, at least two measurements were performed using different microfluidic electrochemical devices to ensure data repeatability. The vacuum pressure in the main chamber during measurements was about mbar. S-6

7 Results and Discussion 1. Electrochemical performance of the microfluidic three-electrode device Figure 2 shows the CV curves obtained from 0.1 M KOH solution in the absence (black line) or presence (red line) of 0.1 M ethanol in the fabricated electrochemical microfluidic cell with Au film as WE and two Pt wires as RE and CE under high vacuum of the TOF-SIMS analysis chamber. The oxidation and reduction waves shown in the CV curves in Figure 2 are of high-consistency with those in traditional electrochemical systems in Figure S1 and the previous studies 2, based on which the proposed mechanisms were discussed as follows. For the 0.1 M KOH solution, during the anodic sweep from -0.4 V to 0.6 V, an oxidation peak appeared in the range of -0.2 V to 0.1 V (black line in Figure 2) contributed by the chemisorption of OH - ions on the gold electrode via a partial charge transfer forming the adsorbed hydroxide intermediate Au(OH)ads species which in previous reports were believed to be the catalytic component of the gold electrode in alkaline solutions. 3-9 Here, it should be noted that the electrolytes we used in our experiments were not deoxygenated. We compared the CV curves of the gold electrode in 0.1 M KOH system before (black line) and after (red line) being deoxygenated by bubbling with N2 into the electrolyte for 20 min to remove O2 as shown in Figure S2. The large negative current at the start during the anodic direction in the presence of the dissolved O2 was significantly decreased after the electrolyte being deoxygenated. This indicated that the reduction of O2 occurred at the begining. In the range of 0.1 V to 0.6 V, the another wide anodic wave was observed (black line in Figure 2) indicated the formation of a superfacial gold oxide layer, which was reduced in the range of 0.3 V to V during the cathodic sweep. 4-9 When the potential became more negative, another cathodic wave appeared from V to V S-7

8 with a peak at V, which was partially contributed by desorption of OH - ions from the gold electrode. Besides that, the reduction of oxygen also occurred as the reduction wave was markedly decreased after deoxygenating the electrolyte. The CV curve of the gold film electrode in the 0.1 M ethanol in 0.1 M KOH solution was shown in red line in Figure 2, which presented a typical electrocatalytic oxidation feature of ethanol at the gold electrode in an alkaline solution during the anodic sweep. Similar to the situation where only 0.1 M KOH was introduced into the cell, during the first oxidation wave from -0.1 V to 0.1 V Au(OH)ads species were formed as a result of the chemisorption of hydroxide ions onto the gold surface. As the potential swept more positively, the current of the second anodic wave rose significantly with the increase of potential and reached a maximum of about A at about 0.34 V, which was much larger than that in the ethanol-free solution (about A). The significantly enhanced anodic current was due to the occurrence of the electrooxidation of ethanol molecules at the gold film electrode surface. While, after 0.34 V the anodic current began to decrease dramatically. Compared with the CV curve of the ethanol-free solution, the formation of gold oxide layer in ethanol-containing solution occurred in the similar potential range, leading to the gradual consumption of the Au(OH)ads species. Therefore, it was proposed that the reduced Au(OH)ads sites may retard the electro-oxidation of ethanol, implying that the electro-oxidation activity of ethanol strongly depends on the quantity of Au(OH)ads. What s more interesting, during the cathodic sweep, a third oxidation wave was observed in the range of 0.3 V to 0.1 V. As the reduction of the surface gold oxides occurred at the this potential range as shown in black line in Figure 2, surface active sites of Au(OH)ads were regenerated on the gold electrode surface, reinitiating the ethanol electro-oxidation. As a result, a corresponding oxidation current regained immediately after the potential reached 0.3 V. When the potential became lower, a reduction wave appeared as a result of S-8

9 desorption of the chemisorbed OH - ions, which is consistent with the phenomenon in the ethanol-free situation. Figure S1. Cyclic voltammograms (CVs) of 0.1 M KOH solution with (red line) or without 0.1 M ethanol (black line) which were performed on a traditional three-electrode electrochemical system with an Au electrode as a working electrode, a Pt wire as a counter electrode, and a Ag/AgCl electrode (a) or a Pt wire (b) as a reference electrode under ambient conditions. Potential range: -0.4 V to 0.6 V. Scan rate: 25 mv/s. The CV curves were of high-consistency with those obtained in the fabricated microfluidic three-electrode device. S-9

10 Figure S2. Cyclic voltammograms (CVs) of 0.1 M KOH solution before (black line, a) or after (red line, b) being deoxygenated by bubbling with N2 for 20 min which were performed on a traditional three-electrode electrochemical system with an Au electrode as a working electrode, a Pt wire as a counter electrode, and a Pt wire as a reference electrode under ambient conditions. Potential range: -0.4 V to 0.6 V. Scan rate: 25 mv/s. S-10

11 2. In situ liquid SIMS analysis of electrode-electrolyte interface Figure S3. A schematic illustration of in situ liquid SIMS analysis of electrode electrolyte interface. A primary ion beam (e.g., a Bi3 + beam in this research) was used to drill an aperture and liquid (electrolyte) surface was exposed. The primary ion beam continuously sputtered the exposed liquid to generate secondary ion species for mass spectrometric analysis. This novel approach allows simultaneous analysis of electrode surface, reactants, intermediates, as well as products at a molecular level. In details, chemical species (e.g., reactants, intermediates and products) in the liquid can diffuse to the exposed liquid surface to be ionized and detected. At the same time, due to the continuous erosion of SiN/Au film by the high ion dose of the primary ion beam, the side wall of the SiN/Au film around the aperture and the corresponding Au electrode-liquid interface (such interface is a circle) can be simultaneously analyzed, too. More description can be seen in our previous papers S-11

12 3. Reconstructed in situ liquid SIMS spectra at different electrode potentials Figure S4a. Negative ion SIMS spectra within the range of m/z of 0.1 M KOH solution in 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, and their intensities were normalized to H - ion. Peaks assignments: H -, m/z 1; O -, m/z 16; OH -, m/z 17; SiO2 -, m/z 60; SiO2H -, m/z 61; SiO3 -, m/z 76; SiO3H -, m/z 77. S-12

13 Figure S4b. Negative ion SIMS spectra within the range of m/z of 0.1 M KOH solution in 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, and their intensities were normalized to H - ion. Peak assignments: Au -, m/z 197; AuOH -, m/z 214; Au(OH)2 -, m/z 231. S-13

14 Figure S4c. Negative ion SIMS spectra within the range of m/z of 0.1 M KOH solution in a microfluidic cell when no potential or 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, and their intensities were normalized to H - ion. Peak assignments: Au2 -, m/z 394; Au2OH -, m/z 411; Au2(OH)2 -, m/z 428. S-14

15 Figure S4d. Negative ion SIMS spectra within the range of m/z of 0.1 M KOH solution in a microfluidic cell when no potential or 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, and their intensities were normalized to H - ion. Peak assignments: Au3 -, m/z 591; Au3OH -, m/z 608; Au4 -, m/z 788. S-15

16 Figure S5a. Negative ion SIMS spectra within the range of m/z of 0.1 M ethanol in 0.1 M KOH solution in a microfluidic cell when no potential or 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, and their intensities were normalized to H - ion. Peak assignments: H -, m/z 1; O -, m/z 16; OH -, m/z 17; SiOH - and C2H5O -, m/z 45; CH3COO -, m/z 59; SiO2 -, m/z 60; SiO2H -, m/z 61; SiO3 -, m/z 76; SiO3H -, m/z 77. S-16

17 Figure S5b. Negative ion SIMS spectra within the range of m/z of 0.1 M ethanol in 0.1 M KOH solution in a microfluidic cell when no potential or 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, and their intensities were normalized to H - ion. Peak assignments: Au -, m/z 197; AuOH -, m/z 214; Au(OH)2 -, m/z 231. S-17

18 Figure S5c. Negative ion SIMS spectra within the range of m/z of 0.1 M ethanol in 0.1 M KOH solution in a microfluidic cell when no potential or 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, and their intensities were normalized to H - ion. Peak assignments: Au2 -, m/z 394; Au2OH -, m/z 411; Au2(OH)2 -, m/z 428. S-18

19 Figure S5d. Negative ion SIMS spectra within the range of m/z of 0.1 M ethanol in 0.1 M KOH solution in a microfluidic cell when no potential or 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, and their intensities were normalized to H - ion. Peak assignments: Au3 -, m/z 591; Au3OH -, m/z 608; Au4 -, m/z 788. S-19

20 4. Illustration of chemical changes on the gold electrode surface at different potentials Figure S6. Representative normalized negative SIMS spectra in the mass ranges of 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 Au - (m/z 197) ions, respectively. Peak assignments: Au -, m/z 197; AuOH -, m/z 214; Au(OH)2 -, m/z 231. S-20

21 Figure S7. The trends of the normalized signal intensities of Aux(OH)y - as function of step potential which was 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. (a) 0.1 M KOH only, and (b) 0.1 M ethanol in 0.1 M KOH electrolyte. The signal intensities of AuOH - (m/z 214), Au(OH)2 - (m/z 231) were normalized to Au - ion (m/z 197) and those of Au2OH - (m/z 411), Au2(OH)2 - (m/z 428) were normalized to that of Au2 - ion (m/z 394). S-21

22 Figure S8. Schematic diagram of chemical changes on the gold electrode surface in 0.1 M KOH solution when a constant electrode potential of (a) -0.4 V, 0 V, 0.4 V or 0.6 V was applied during the anodic direction and (b) 0.2 V, -0.1 V or -0.4 V was applied during the cathodic direction. Note: in (a) at -0.4 V, the two hydroxide ions are representatives of hydroxide ions in the 0.1 M KOH electrolyte for brevity. S-22

23 5. Additional discussions on the mechanism of ethanol electro-oxidation on gold electrode surfaces 5.1 SIMS signals at m/z 45 Figure S9. Negative ion SIMS spectra of ions at m/z 45 (SiOH - and C2H5O - ) 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. S-23

24 Figure S10. The normalized intensities of ions at m/z 45 (SiOH - and C2H5O - ) with the H - ion as a reference during the anodic step potential measurements at -0.4 V, 0 V, 0.4 V, and 0.6 V and the cathodic step potential measurements at 0.2 V, -0.1 V and -0.4 V when 0.1 M ethanol was absent (black line) or present (red line) in 0.1 M KOH solution. S-24

25 In this work, we also used C2H5O - ions (m/z 45) with H - ion as a reference to monitor the electrocatalytic reactions of ethanol at gold electrode surfaces. It should be pointed out that the m/z values of C2H5O - (fragment ion of the reactant ethanol) and SiOH - (possible interference from the Si frame and/or SiN membrane) ions are both 45. We cannot distinguish them due to the unit mass resolution of ToF SIMS instrument. In the 0.1 M KOH solution, although no ethanol existed, the signal intensity of the ion at m/z 45 (SiOH - ) normalized to H - also varied with the change of the electrode potential. However, considering the trends of the signal intensities of the ions at m/z 45 as a function of potential in both ethanol-free and ethanol-containing KOH solutions were totally different, it s reasonable to discuss the adsorption and electro-oxidation of ethanol molecules based on the changes in signal intensity of ions at m/z 45 in the ethanol-containing solution. If we compare the signal intensities of ions at m/z 45 in both electrolyte systems when the same step potentials were applied (Figure S10), we noticed that at 0.4 V and -0.1 V the signal intensity of ions at m/z 45 in ethanol-containing system was distinctly smaller than that in the ethanol-free system. These are in well consistence with the two maximum values of CH3COO - ion intensity (red line in Figure 5b), indicating a great number of adsorbed ethanol molecules were transformed into their oxidation products. Here, we also need to note that ethanol interacting with gold surfaces and thus preventing OH - ions from interacting with gold has the very low possibility to be the reason behind the significantly decreased Au(OH)ads related signals in the ethanol-containing system at potentials of 0.4 V, 0.2 V and -0.1 V (red line in Figure 5a) as compared to those in the ethanol-free system (black line in Figure 5a). First, because pka of water is lower than pka of ethanol, OH - ions should be more easily to be adsorbed onto the gold electrode surface at relatively positive potentials in comparison with ethanol molecules or even ethoxy ions. More importantly, Figures S9 and S10 show that when potential increased from -0.4 V to 0 S-25

26 V, more ethanol signal can be observed, indicating adsorption of ethanol. However, when potential continuously increased to 0.4 V, ethanol signal sigificantly decreased, assoiated with dramatical increasing of CH3COO - signal, suggesting adsorption of enthol actually decreased. Similar situation was observed for -0.1 V in the cathodic direction. Therefore, the decreasing of Au(OH)ads should not result from the interaction of ethanol with the Au electrode. S-26

27 5.2 The effects of dissolved oxygen As we mentioned above, the electrolytes we used in this work were not deoxygenated. However, it should be noted that the presence of oxygen wouldn t affect the main observations and conclusions of this work. First, from the Figure S2, we can see two major differences between the CV curves with and without O2: (1) the staring current (at -0.4 V) in the anodic direction with O2 is much more negatively larger than that without O2, and (2) there is a clear reduction peak at about -0.2 V in the cathodic direction with O2. Both differences suggest reduction of O2 occurs at the low potentials, but showed little effects on the processes at higher potentials. Second, Au(OH)ads related signals in the SIMS spectra at -0.4 V with O2 are very low (Figure 3 and Figure 5a), indicating that such a by-reaction does not lead to interference of dection of Au(OH)ads species. Also, Figure 5b shows that this reduction reaction cause little interference of detetion of CH3COO - signal. Moreover, the potential role of oxygen as well as its reduction product hydrogen peroxide on the electrochemical oxidation activity of glycerol on gold in alkaline solutions was previously reported, which revealed no major influence on the rate or products of alcohol electro-oxidation on gold in alkaline media. 8 DFT calculations also suggested that oxygen in the electrolytes would not incorporate into the acetate acid product. 13 Besides, they revealed that the reduction of oxygen with water produces hydrogen peroxide (HOOH*) intermediates as shown in Eqs. (1)-(3) (* represents a site on gold surfaces), which are hard to occur decomposition to hydroxide on gold surfaces (Eqs (4)) due to the large activation barrier as compared to the situation on Pt and Pd. 13 O2* + H2O* OOH* + *OH (1) OOH* + H2O* HOOH* + *OH (2) *OH + e - OH - + * (3) S-27

28 HOOH* + * *OH + *OH (4) 5.3 Whether Pt nanoparticles were formed Figure S11. ToF-SIMS spectra of the gold electrode surface before (a) and after (b) electrochemical measurements in 0.1 M KOH electrolyte which were performed on a traditional three-electrode system containing the gold electrode as the working electrode, and two platinum wires as a reference electrode and a counter electrode, respectively. The peak at m/z 197 was assigned to Au - ion. Pt related peaks at m/z 194, 195, and 196 were too weak to be seen. S-28

29 In some references, formation of Pt nanoparticles on the Au electrode was reported when using Pt as a counter electrode. 14, 15 However, there are specific conditions required. First, a platinum counter electrode is employed in acidic electrolytes, such as 1 M H2SO4. Second, a sufficiently negative potential is applied at the working electrode and lasts for a sufficient time, for example, a controlled potential electrolysis at -1.2 V for 6 h. In this way, a rather positive potential would be developed at the Pt counter electrode to corrode some Pt metal which would in turn electrodeposit at the negative working electrode. In our experiments, first, a basic solution of 0.1 M KOH was used as the electrolyte instead of the acidic solution. Second, the range of the potential range was from -0.4 V to 0.6 V, among which the most negative value was -0.4 V. Besides, the experiment only lasted for several minutes. Therefore, the formation of Pt nanoparticles on the Au electrode should not happen in our experimental conditions. This statement was evidenced by the undetected Pt related signals at m/z 194, 195 and 196 in the SIMS spectra of the Au electrode surface after electrochemical measurements in the traditional three-electrode system using 0.1 M KOH as the electrolyte (as shown in Figure S11). Therefore, the results and conclusions reported in this work should not be influenced by the formation of Pt nanoparticles on the Au electrode. S-29

30 5.4 The difference of the electro-oxidation products of ethanol in alkaline and acidic solutions The product of electro-oxidation of ethanol we detected in our work using a basic solution of 0.1 M KOH containing 0.1 M ethanol was acetate ions, instead of acetaldehyde. Under a positive electrode potential, hydroxide ions would be chemisorbed onto the gold electrode surface, forming the adsorbed hydroxide intermediates AuOHads. This intermediate species would act as a catalytic site for the electro-oxidation of ethanol by nucleophilic attack of the activated hydroxyl groups on the electrode surface into the adjacent absorbed ethanol molecules, which leads to acetate ion as the final oxidation product as shown in the reaction pathway in Figure 6a. However, a different situation might occur in an acidic solution as shown in Figure S12. As adsorbed hydroxide intermediates were difficult to be formed on the gold electrode surface, the chemisorbed ethanol molecules would be oxidized but with no nucleophilic attack by hydroxyl groups, leading to the primary oxidation product of acetaldehyde which might be further oxidized into acetic acid as the secondary product. 4 Figure S12. The reaction mechanism for the oxidation of ethanol on gold film WE surface in an acidic solution. The primary oxidation product of ethanol is acetaldehyde which is further oxidized to acetic acid as a secondary product. S-30

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