Potential-Dependent Selectivity of Ethanol Complete Oxidation on Rh

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Potential-Dependent Selectivity of Ethanol Complete Oxidation on Rh Electrode in Alkaline Media: A Synergistic Study of Electrochemical ATR-SEIRAS and IRAS Chan Zhu, Bin Lan, Rui-Lin Wei, Chao-Nan Wang, and Yao-Yue Yang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00138 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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Potential-Dependent Selectivity of Ethanol Complete Oxidation on Rh Electrode in Alkaline Media: A Synergistic Study of Electrochemical ATR-SEIRAS and IRAS Chan Zhu, Bin Lan, Rui-Lin Wei, Chao-Nan Wang, Yao-Yue Yang* College of Chemistry and Environmental Protection Engineering, Southwest Minzu University, Chengdu 610041, Sichuan Province, China.

Abstract Rh-based catalysts might resolve the long-standing problem of poor C1 pathway efficiency toward ethanol oxidation reaction (EOR). To facilitate the rational designing and preparation of Rh-based EOR catalysts, here we fundamentally study ethanol adsorptive dissociation and oxidation on Rh electrode surface by electrochemical infrared absorption spectroscopy. Firstly, real-time infrared spectral results show that ethanol could be easily splitted on Rh surface into COad and CHx intermediates only in alkaline media but not in acidic media. Secondly, the onset oxidation potential of EOR on Rh is ca. 180 mV more negative than that on Pd and Pt electrode in alkaline media. The EOR Jf /Jb ratio is ca. 5.73, 1.62 and 0.35 on Rh, Pt, and Pd, respectively, suggesting that COad and/or CHx intermediates could be readily oxidized into CO2 on Rh. Accordingly, the apparent selectivity efficiency of C1 pathway (η) is estimated to be 100% when the potential is at 0.4~0.6 V vs RHE, subsequently η sharply decreases to zero at 0.65~0.8 V vs. RHE, and then η gradually rebounds to ca. 15% when the potential move positively. This work may provide some theoretical supporting for fabricating highly efficient EOR catalysts.

Keywords Rh electrode; ethanol oxidation reaction; C-C bond cleavage; C1 pathway selectivity; ATR-SEIRAS; IRAS

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1. Introduction Rhodium (Rh) recently attracted wide attention in the research field of surface electro-catalysis towards direct ethanol fuel cells (DEFCs), owing to its potentially high catalytic activity of splitting C-C bond.1-3 It is now general accepted that ethanol oxidation might follow a so-called dual-pathway mechanism (i.e., C1 and C2 reaction pathway as shown in Figure S1), and the C-C bond cleavage is the key role to improve the Faraday efficiency of ethanol oxidation reaction (EOR) and even DEFCs.4-8 So far, Pt-based and Pd-based catalysts have been the most active for EOR in alkaline media, nevertheless, they show poor capability in splitting ethanol C-C bond. Thus the Faraday efficiency for EOR C1 pathway on Pd-based and Pt-based catalysts is generally quite low (normally only 1-7.5% for Pt or ca. 2.5% for Pd)9-12 at room temperature，even if a ca. 24% Faraday efficiency was obtained on Pd-Ni(OH)2/GO in alkaline media by Huang et al.13 and a much stronger capability for C-C cleavage was observed on a non– alloyed Pt46–(SnO2)54 core–shell particles.14 Fortunately, R.R. Adzic’s and co-workers2 first reported a PtRhSnO2/C catalysts towards EOR, and they suggested that Rh could be critical in obtaining high selectivity of ethanol oxidation into CO2.3 After that, some Rh-contained catalysts for EOR have been reported successively, such as Pt9RhFex,15 PtRhCu nanoboxes,16 Pt-Rh alloy nanodendrites,17 PtRhNi alloy nano-assemblies,18 Octahedral PtNiRh Nanoparticles,19 and Pt/Rh/Sn.20 But their C1 faraday efficiency is still very low or not shown up. Meanwhile, Rh-based EOR catalysts (employing Rh as the main catalyst) are limited. Li et al.21 and Zhang et al.22 separately reported an Rh/C catalyst with a high EOR activity, He et al.23 showed that RhCeO2/C can obtain a higher EOR activity than Rh/C. Recently, Zhang et al. prepared Rh nanoplates24 and pentatwinned Rh nanobranches25 with ca. 14% C1 pathway faraday efficiency. All these results mean that Rh-based catalysts may be alternative to traditional Pt-based and Pdbased EOR catalysts in alkaline media. Nevertheless, it should be pointed out that Rh is more expensive than Pt, thus the usage amount of Rh should be controlled. Moreover,


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although the C1 pathway selectivity for EOR was obviously promoted, the activity of above-mentioned Rh-based catalysts toward EOR are still too low to meet the requirements of practical utilization of DEFCs. To create better and more practical Rh-based EOR catalysts with both high activity and C1-pathway selectivity, it’s necessary to deeply understand the ethanol adsorption and oxidation process on Rh electrode surface. When it comes to surface adsorption and reaction mechanism study, real-time electrochemical attenuated total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) has been demonstrated as an excellent method due to its high surface-sensitivity and simple spectral selection rule, which could be effective in clarifying what the adsorbed intermediates are and how they vary with electrode potentials.5,

26-28

Meanwhile,

external-reflection mode electrochemical infrared absorption spectroscopy (IRAS), being the counterpart of ATR-SEIRAS, can sensitively detect the dissolved products of electrocatalytic reactions confined in its thin-layer between the electrode and CaF2 window.29-30 Therefore, the combination of electrochemical ATR-SEIRAS and IRAS should be a complementary methodology and pretty useful to clarify the EOR reaction mechanism on Rh in alkaline media.27 Nevertheless, relative research (especially on pristine Rh electrode surface) is seriously limited so far even if some IRAS studies have been reported on PtRh31-32, PtRhSnO2 surface2-3, 33 and Rh nanocrystal surface.25, 34 Therefore, in this work, we studied ethanol adsorption and oxidation on Rh electrodes in alkaline media by ATR-SEIRAS and IRAS. The results indicate that ethanol can readily self-dissociate into C1 species such as CO and CHx on Rh only in alkaline media, meanwhile, the potential-dependent selectivity of CO2 products has also been researched, which may give a theoretical guidance for design high active EOR catalysts.

2. Experimental Section 2.1 ATR-SEIRAS measurements


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First, a ca. 60-nm thick SEIRA-active Au film was electroless deposited on the reflection plane of a semicircular cylinder Si prism according to the previously reported routine.35 Then the Au film-coated Si prism was fixed upon a home-made spectroelectrochemical cell which was designed according to a so-called Kretschmann attenuated-total reflection (ATR) configuration.26 Under this configuration, an electrochemical three electrode system can be readily assembled, employing the Au nanofilm to serve as the working electrode (WE), associated with a Pt foil counter electrode (CE) and a saturated calomel electrode (SCE) reference electrode (RE). Thus Rh overlayer on Au surface was readily prepared by galvanostatic deposition on Au surface in 5 mM RhCl3 + 0.1 M HClO4 solution, with the current of 100 μA for 100 s. The average thickness of as-deposited Rh nanofilm was estimated to ca. 25 ± 2 nm by the cathodic charge accumulated in the electro-deposition process.35 Meanwhile, the Rh overlayer was flat and pinhole-free as shown in the Atomic Force Microscopy (AFM) images in Figure S2. Subsequently, electrochemical ATR-SEIRAS measurements on Rh overlayer were performed in a home-made above-mentioned ATR mode optical system embedded in the spectroscopic chamber, which has been detailed described elsewhere.27,

35-36

The real-time ATR-SEIRA spectra were simultaneously collected

while ethanol was dosing to Rh film electrode surface or ethanol electro-oxidation reaction on Rh electrode was ongoing. Time resolution for each spectrum was 5 s, being equivalent to average co-adding ca. 40 interferograms. 2.2 IRAS measurements The IRAS measurements were similar with the procedures that were described elsewhere.10,

27

Specifically, a Փ 5 mm smooth Au disk electrode was used as the

working electrode, and thus Rh Working Electrode (RWE) could be obtained by electro-deposition on Au surface as the above-mentioned method (deposition time is 300 s). Before spectra collecting, a thin-layer with its thickness ca. 10 μm should be obtained by pressing the RWE onto CaF2 IR window, then the sample and reference spectra can be acquired at corresponding electrode potentials. Note that, the thin-layer


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solution and electrode surface were renewed after the collection of one set of singlebeam spectra at the reference potential (ER) and sample potential (ES) as the following previously reported cleaning procedures.10, 27 First, the RWE was lifted up with the applied potential of 1.4 V vs. RHE for ca. 10 s to oxidize any adsorbed species completely, and then the applied potential was stepped to 0 V vs. RHE for ca. 5 s to remove the surface oxygen species but not oxidize the ethanol; second, the RWE was pressed upon the CaF2 infrared window to form a new solution thin-layer with the thickness of ca. 10 μm by controlling the MCT response of the reflected IR beam from RWE; and third, the single-beam spectra at the ER and the next ES were collected. This lifting-repressing operation can effectively supplement the ethanol and OH— in the thin layer and keep the RWE surface under the same condition for each measurements. Every spectrum was generated by average co-adding 512 interferograms (acquisition time of ca. 100 s) to reach a better signal/noise ratio. The incident angle of the IR beam was accurately controlled to be 55⁰. 2.3 Transmission IR measurements The Transmission IR spectra of 0.1 M Na2CO3, 0.1 M CH3COONa, and 0.034 M CO2 (saturation solution) was obtained on an infrared spectroscopic flow-cell (PIKE162-1100, vide infra), which could be used to estimate the products’ concentration in the thin layer of IRA spectroscopic cell. This flow cell consists of two CaF2 disks and one of them is drilled two small holes as the inlet and outlet of liquid samples, respectively. A 25-μm-thick Teflon gasket is placed between two CaF2 disks to form a thin layer. Thus, the sample solution and pure water could be readily injected into this thin layer through the inlet hole to collect the sample and reference spectrum (both 128 interferograms co-added). 2.4 Others An Agilent Cary 660 FTIR with the liquid nitrogen cooled MCT detector was used to collect the infrared spectra. All the spectra in this work are shown in the absorbance units defined as -log(R/R0), where R and R0 represent the intensities of the


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reflected radiation at the sample and reference conditions, respectively. A CHI 660E electrochemistry workstation was employed to control the electrode WE potential and record the cyclic voltammograms (CV) and time-dependent open circuit potential (OCP-t) curves. Although SCE was used to serve as the RE, all potentials were converted to the reverse hydrogen electrode (RHE) scale to facilitate the comparison with other results. CO and N2 used in this work are all of high purity (> 99.999%), all chemicals used here are guaranteed reagents (GR), and all solutions were freshly prepared with ultrapure Mill-Q water (18.25 Mcm) before each measurement. All measurements were performed at 25 ± 2 °C.

3. Results and Discussion 3.1 Ethanol self-dissociation on Rh

Figure 1. (A) and (C) show time-dependent OCP variation curve (OCP-t) for ethanol dosing to Rh electrode surface, (B) and (D) show corresponding real-time ATR-SEIRA spectra collected on Rh electrode, taken single-beam spectrum in neat electrolyte as the reference spectrum with the spectral resolution of 8 cm-1. -◄- and -★- profiles in (C) are the time-dependent variation of COL and COB band intensities.


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Ethanol absorption and self-dissociation on catalyst’s surface under open circuit potential is an important part for understanding the EOR mechanism.5,

27

Here we

probed this surface process by monitoring the time-dependent OCP variation and the evolution of real-time ATR-SEIRA spectra in two electrolytes. As shown in Figure 1A and B, the OCP of Rh film electrode almost unchanged after injecting ethanol into the spectro-electrochemical cell, accordingly, the real-time ATR-SEIRA spectra can only detect 1433 cm-1 (δ(C-H)) and 1041 cm-1 (ν(C-OH)) assigned to the surface weakly adsorbed ethanol species.5, 37 It means that surface ethanol species barely dissociate on Rh surface in acidic media, which is in side with that on Pd.38 In contrast, as ethanol was injected into Rh electrode/NaOH interface, the OCP of Rh electrode quickly moved down to ca. 0.2 V (RHE) within 100 s and then leveled up to ca. 0.4 V. Meanwhile, in corresponding ATR-SEIRA spectra we clearly observed COB (1763-1846 cm-1) and COL species (1919-1968 cm-1), even the band of surface co-adsorbed free water (hydrogen-bond broken surface water) was also detected at ca. 3600 cm-1 (ν(O-H)) and 1630 cm-1 (δ(H-O-H)), suggesting that a high COad coverage formed on Rh surface at that moment.35 Thereby, it may conclude that surface ethanol could readily undergo self-dissociation through the C-C bond cleavage on Rh surface in alkaline media. This is totally different from that in acidic media, which could be partially explained by the increasing electro-negativity of Rh electrode in alkaline media.39 3.2 Ethanol oxidation on Rh The ethanol oxidation has also been investigated by Cyclic Voltammetry and the results are shown in Figure 2 and Figure S3-S4. It is apparent that the EOR peak current density of forward scanning (Jf) on Rh in alkaline media (Figure 2) is above 10 times higher than that in acidic media (Figure S3). This means that Rh could be more suitable to utilize in alkaline media, which is definitely similar to Pd. Nevertheless, the EOR electrochemical behaviors on Rh was found to be obviously different from that on Pd and Pt catalysts.


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Figure 2. Cyclic voltammograms of Rh in 0.1 M NaOH with and without 1 M ethanol with the scan rate of 5 mV s-1. Table 1. A summary of the voltammetric features for EOR on Rh, Pt and Pd electrode surface shown in Figure 2.

Rh Pt Pd a

Ef /V

Eb /V

Jf /Jb

0.63a/0.81b 0.8 0.86

0.43 0.75 0.83

5.73a/5.60b 1.62 0.35

obtained from peak I in figure 2; b obtained from Peak II in figure 2.

First, the onset oxidation potential of ethanol on Rh (ca. 0.4 V vs. RHE) has been brought downward at least ca. 180 mV with respect to that on Pd and Pt electrode in alkaline media (see Figure 2 and Figure S4). Second, as shown in Figure 2 and Table 1, the EOR CV on Rh exhibits two main anodic peaks with the peak potential of ca. 0.63 (Peak I) and 0.81 V vs. RHE (Peak II), respectively. And the forward scanning peak current density (Jf) is much lower than that on Pd and Pt (see the comparison in Figure S4).5 Thereby we have to say that the EOR activity on Rh should be further enhanced for meeting the practical requirements (vide infra). Third, on Rh surface, the Jf is much higher than that of backward scanning (Jb).


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This is exactly in side with previous results on Rh-based catalysts, 22-23, 25 nevertheless, being sharply different with the typical CV profiles on Pd-based and Pt-based catalysts.4-5, 13-14, 19, 40 In previous literatures, Goodenough and co-worker 41suggested that the anodic peak of reverse scanning is attributed to the removal of incompletely oxidized carbonaceous species during the forward scan. After that, it has been wellaccepted that the Jf/Jb ratio could be effectively used to define the tolerance to carbonaceous species built-up on the catalyst surface.42-45 In this work, the Jf /Jb ratio is calculated to be 5.73, 1.62 and 0.35 on Rh, Pt, and Pd (see Table 1), respectively. The tremendously high Jf/Jb ratio on Rh surface suggests that ethanol derived COad intermediates (and/or CHx intermediates, if possible) could be readily electrochemically oxidized into CO2 product during the positive scan.40-45 Nevertheless, it is worthwhile to note that COad and/or CHx species could be rapidly build up on the surface and poison further C-C bond cleavage, which may explain the low steady-state activity of pure Rh catalysts toward EOR (as described in Ref.22,

24-25).

This exactly exposures the

dominated shortcoming of Rh-based EOR catalysts, i.e., the relative poor activity for oxidizing strong-adsorbed C1 intermediates. Accordingly, Figure 3 shows the in situ ATR-SEIRA and IRA spectra corresponding to positive voltammetric scanning in 0.1 M NaOH+1 M C2H5OH, which could provide rich molecular information to further understand the EOR process on Rh. In Figure 3A, it is clearly observed that the ν(C-O) band of bridge-adsorbed COad species (COB) at 1758-1842 cm-1 and linearly-adsorbed COad species (COL) at 19211956 cm-1, varying with electrode potentials.46-47 Meanwhile, a ca. 1611 cm-1 band could be detected at ca. 0 V vs. RHE, which might be associated with surface adsorbed acetyl or acetaldehyde species.5, 10, 48 In our previous work on Pd surface, we found that adsorbed acetyl species rather than acetaldehyde species could be the pivotal intermediates for ethanol electro-oxidation through an isotope labeling ATR-SEIRAS investigation.5 Nevertheless, here we cannot simply exclude the possibility of surface


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acetaldehyde species according to the spectra in Figure 3A.

Figure 3. Real-time ATR-SEIRA spectra and IRA spectra collected in 0.1 M NaOH+1 M C2H5OH when the electrode potential positive scanning from 0 V to 1.2 V vs. RHE, taken the singlebeam spectrum at designated potential as the reference spectrum, spectral resolution 8 cm-1.

On the other hand, IRAS results (Figure 3B) can effectively help to manifest the dissolved products in the thin layer at each electrode potential. When the electrode potential was at 0.65~1.4 V vs. RHE, two apparent IR bands at ca. 1552 and 1415 cm1

associated with a small band at ca. 1348 cm-1 can be detected (they were also slightly

detected in ATR-SEIRA measurements as shown in Figure S5), which is the same as the spectrum of acetate shown in Figure 4A. And the bands at ca. 1552 and 1415 cm-1 can be respectively assigned to the asymmetric and symmetric ν(O-C-O) of acetate near Rh surface.3, 10, 25, 27, 49 It indicates that ethanol would be oxidized into acetic acid on Rh electrode when potential is higher than 0.65 V vs. RHE. When electrode potential was at 0.4~0.6 V vs. RHE, only ca. 1410 cm-1 band can be clearly observed (Figure 3B), and no obvious band at ca. 1552 cm-1 detected. Thereby, the bands at 1410 cm-1 cannot be attributed to the symmetric ν(O-C-O) of acetate any more, since the relative intensity ratio of feature vibrational bands that comes from one single species should be fixed.10, 48, 50 According to the results in Figure 4B, we found that the feature band frequency of carbonate positively shifts from 1370 cm-1 to nearly 1410 cm-1 with its concentration decreasing by IRAS measurements. Therefore, we may speculate that this 1410 cm-1 band could be owing to the carbonate product (C2 reaction pathway) at 0.4~0.6 V vs. RHE.


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Figure 4. (A) IRA spectra of 1 M Na2CO3, 1 M NaHCO3 and 1 M CH3COONa dissolved in 0.5 M NaOH solution, (B) IRA spectra of various amount of Na2CO3 dissolved in 0.5 M NaOH solution, taken the single-beam spectrum in 0.5 M NaOH as the reference spectrum. (C) Transmission IR spectra of 0.1 M Na2CO3, 0.1 M CH3COONa and 0.034 M CO2 aqueous solution collected by the transmission IR flow cell (D), taken the single-beam spectrum of ultrapure water as the reference spectrum. Spectral resolution, 8 cm-1.

Another important feature in IRA spectra should be the 2345 cm-1 band of CO2 emerged from 0.8 V vs. RHE (Figure 3B). This band detected in alkaline electrolyte suggested that pH in the thin-layer between Rh electrode and CaF2 window should be lowered down to ca. 4.0 (the pH value of saturated carbonic acid). Furthermore, we also try to subtract the spectra collected above 0.8 V by the single-beam spectra of acetate, but we can barely obtain some feature bands that can be attributed to bicarbonate and carbonate. It means that the above-mentioned pH decreasing at high potentials should resulted from the neutralization by as-produced acetic acid. Moreover, we also took previously reported semi-quantitative method to estimate the relative concentration (Cr) of products in the thin-layer at each potential.10, 27, 51 To approach that purpose, an transmission mode flow cell (PIKE-162-1100) which consists of two CaF2 disks spaced by a 25-μm-thick Teflon gasket (Figure 4D) was employed to collect the transmission IR spectra of a 0.1 M Na2CO3, 0.1 M CH3COONa


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solution and 0.034 M CO2 (saturation solution), with the single-beam spectrum of pure water as the reference spectrum. According to the Beer-Lambert Law, A = αlC, where A is the absorbance, α and l are respectively the molar extinction coefficient and the length of IR light path, C is the sample concentration. Due to α is constant for each species at a specific measurement, A is proportionate to C. Thus we can estimate the Cr of products confined in the thin layer under IRAS configuration according to the equation below, Atrans/AIRAS =l×C / l’×Cr= 0.72 C/Cr where Atrans and AIRAS are the absorbance of the feature IR band of one specific species, l=25 μm and l’≈2×10 μm×cos55⁰=34.8 μm. The corresponding feature band for each species is ca. 1390 cm−1 for CO32−, ca. 1546 cm−1 for CH3COO−, and ca. 2343 cm−1 for CO2. The relative error is within 10% by this method as previously demonstrated.10, 27, 51

Thus, the potential-dependent intermediate and product distributions can be obtained, As shown in Figure 5A and S6, the band intensities of COad species gradually increase until 0.4 V vs. RHE, and then quickly diminish within 200 mV (namely at ca. 0.60 V vs. RHE). The turning point potential (0.4 V vs. RHE) is opportunely the same as the above-mentioned onset oxidation potential of ethanol on Rh surface. In the meantime, the feature band of carbonate start to show up also at 0.4 V vs. RHE, and its band intensity initially increase and then decrease to zero at ca. 0.6 V vs. RHE. These results might lead us to reveal that the EOR Peak I should be mainly owing to the oxidation of COad species into carbonate (C1 reaction pathway). When the potential comes to 0.65 V vs. RHE, the acetate that is the product of C2 reaction pathway will generate, and the acetate is almost the only reaction product at 0.7~0.9 V vs. RHE, which indicates that EOR C2 reaction pathway might predominately contribute to the EOR Peak II. Moving the potential more positive, CO2 band intensity gradually increases from 0.9 V vs. RHE, combined with the 13C isotope-labeled investigation on


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Pt surface by Wieckowski et al.,52 CO2 product may result from the oxidation of surface CHx species at these high electrode potentials (see the asterisked small peak in Figure 5A).

Figure 5. (A) Potential-dependent variation of COad band intensity and relative concentration (Cr) of acetate and CO2 products. Solid line is the linear scan voltammogram of Rh in 1 M CH3CH2OH + 0.1 M NaOH at 5 mV s-1. (B) The apparent selectivity of C1 pathway and C2 pathway at different electrode potentials.

In addition, on basis of the above-described potential-dependent product distributions, the Apparent Selectivity Efficiency (η) of EOR C1 pathway on Rh electrode can be calculated. Here we define the parameter η as follows,10 [𝐶𝑟(CO23 ― ) + 𝐶𝑟(CO2)]/2 𝜂= 𝐶𝑟(acetate) + [𝐶𝑟(CO23 ― ) + 𝐶𝑟(CO2)]/2 since the complete oxidation of one ethanol molecule can produce two CO2 (or CO32−), their concentration should be divided by the factor of 2. Note that, the apparent selectivity efficiency is only an eclectic parameter to evaluate the ratio of C1 pathway, which is determined by the inherent semi-quantitative characteristics of infrared spectroscopy. The yield of products cannot be obtained since their concentration cannot be accurately measured by IRAS and transmission FTIR. In this perspective, previously reported on-line High Performance Liquid Chromatography (HPLC) HPLC53 and online Differential Electrochemical Mass Spectrometry (DEMS)54-55 could be more effective for the estimation of EOR C1 pathway Faraday Efficiency. As shown in Figure 5B , η value is nearly 100% when the potential is at 0.4~0.6 V vs RHE, which means that ethanol can be completely oxidized into CO2 under this


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potential range. And according to above discussion, it should be associated with the oxidation of surface CO species. It is interesting that η sharply decrease to zero when electrode potential is positively move into the range of 0.65~0.8 V vs. RHE, indicating that acetate is the EOR products under this conditions. Whereas η gradually increases again when the potential is higher than 0.8 V vs. RHE where CHx species started to perform oxidation. This ethanol oxidation reaction process on Rh surface in alkaline media is illustrated in Figure 6 below, where the so-called dual-pathway mechanism is also exhibited.

Figure 6. The schematic illustration of the reaction processes proposed in this work.

Based on the above results on ethanol self-dissociation and oxidation on Rh, we believe that Rh catalysts should be more efficient for EOR in alkaline media than in acidic media, although the famous PtRhSnO2/C2 was used in acidic media (note that its main catalytic component is Pt rather Rh). Meanwhile, Rh could have a high selectivity for splitting ethanol C-C bond, but the stead-state activity (current density) is should be further enhanced. Therefore, rational designing and preparing superior Rh-based EOR catalysts with high C1 pathway selectivity, long-term durability and excellent activity is still full of challenge. According to the discussion above, it may be associated with the strong adsorption of C1 intermediates on Rh. Thereby, it might be effective to promote the stead-state activity of C1 production by combining Rh nano-structure with the oxyphilic metallic or nonmetallic atoms (or clusters), forming the so-called nanodomain. In fact, we recently fabricate a brand-new RhPb-PbOx/C catalyst, the preliminary measurements toward EOR in alkaline media shows a C1 pathway faraday efficiency of ca. 20 % with a mass activity of ca. 3000 mA mg-1Rh.


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4. Conclusions In summary, ethanol dissociation and oxidation process on Rh film electrode have been investigated by a high-sensitivity electrochemical ATR-SEIRAS and IRAS. Ethanol is found to easily perform self-dissociation on Rh surface only in alkaline media but not in acidic media, which could be partially explained by the increasing electro-negativity of Rh electrode in alkaline media. This self-dissociation can split ethanol C-C bond to generate surface COad and CHx species, which is significantly important in enhancing C1 pathway. As for ethanol oxidation, EOR in alkaline media show at least 10 times higher than that in acidic media. Meanwhile, the onset oxidation potential of ethanol on Rh (ca. 0.4 V vs. RHE) has been brought downward at least ca. 180 mV with respect to that on Pd and Pt electrode in alkaline media. Moreover, the Jf /Jb ratio is roughly calculated to be 5.73, 1.62 and 0.35 on Rh, Pt and Pd, respectively. The tremendously high Jf/Jb ratio on Rh surface suggests that ethanol derived COad intermediates (and/or CHx intermediates, if possible) could be readily electrochemically oxidized into CO2 product during the positive scan. At last, based on electrochemical ATR-SEIRAS and IRAS results, EOR on Rh in alkaline media can be divided into three potential sections. Correspondingly, ethanol can be electro-oxidized on Rh surface through C1 pathway at a potential scope of 0.4~0.6 V, then C2 products (acetate) became the only product when electrode potential is at 0.65~0.85 V, subsequently C1 and C2 products could both produce when it was higher than 0.85 V. Thus a potential-dependent C1 pathway selectivity was estimated. Thus, based on this work, Rh could have a high selectivity for splitting ethanol CC bond especially at lower potentials, but the stead-state activity for C1 pathway is still low due to the fast building up and poisoning effect of COad and CHx species on Rh surface. It is suggested that constructing a Rh-oxyphilic species nano-domain might be effective to promote the activity of C1 production. We hope this work may provide


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some new insights for designing and fabricating new Rh-based EOR catalysts with high activity and C1 pathway selectivity from fundamental aspects.

ASSOCIATED CONTENT Supporting Information. Schematic illustration of dual-pathway mechanism of EOR; AFM images of Rh nanofilm on a Si wafer; CVs of EOR on Rh surface in acidic and alkaline media; CVs of EOR on Pt and Pd film electrodes; ATR-SEIRA spectra of EOR on Rh with the reference spectrum collected at 0 V RHE. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; ORCID iD: 0000-0002-4573-9437 ACKNOWLEDGMENT This work is supported by NSFC (grant numbers 21603177), Natural Science Foundation of Sichuan Province (grant numbers 2016JY0212), and the Fundamental Research Funds for the Central Universities (grant numbers 2018NZD04). NOTES The authors declare no competing financial interests. REFERENCES (1) Murphy, S. K.; Park, J. W.; Cruz, F. A.; Dong, V. M. Rh-Catalyzed C-C Bond Cleavage by Transfer Hydroformylation. Science 2015, 347, 56-60. (2) Kowal, A.; Li, M.; Shao, M.; Sasaki, K.; Vukmirovic, M. B.; Zhang, J.; Marinkovic, N. S.; Liu, P.; Frenkel, A. I.; Adzic, R. R. Ternary Pt/Rh/SnO2 Electrocatalysts for Oxidizing Ethanol to CO2. Nat. Mater. 2009, 8, 325-330. (3) Li, M.; Zhou, W. P.; Marinkovic, N. S.; Sasaki, K.; Adzic, R. R. The Role of Rhodium and Tin Oxide


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Potential-Dependent Selectivity of Ethanol Complete Oxidation on Rh

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