PtâNi Octahedra as Electrocatalysts for the Ethanol Electro-Oxidation

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Pt-Ni Octahedra as Electrocatalysts for Ethanol Electro-Oxidation Reaction Jordy Evan Sulaiman, Shangqian Zhu, Zelong Xing, Qiaowan Chang, and Minhua Shao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01435 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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Pt-Ni Octahedra as Electrocatalysts for Ethanol Electro-Oxidation Reaction Jordy Evan Sulaiman†, 1, Shangqian Zhu†, 1, Zelong Xing†, Qiaowan Chang†, Minhua Shao†, #, * †

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science

& Technology, Clear Water Bay, Kowloon, Hong Kong #

Energy Institute, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong *

[email protected]

1

Equal contribution.

Abstract Alloying Pt electrocatalysts with late transition metals (e.g. Ni, Co and Fe) is an effective strategy to lower the catalyst cost and improve their tolerance towards CO in the anode of direct ethanol fuel cells (DEFCs). In this study, shape-controlled octahedral Pt-Ni/C nanocrystals with uniformly exposed (111) facet and an average edge length of 10 nm were synthesized. The octahedral PtNi/C nanocatalyst was at least 4.6 and 7.7 times more active than conventional Pt-Ni/C and commercial Pt/C catalysts, respectively. In situ infrared spectroscopic results showed that the acetic acid/CO2 absorbance peak intensity on octahedral Pt-Ni/C was 7.6 and 1.4 times higher as compared to commercial Pt/C and conventional Pt-Ni/C, respectively at 0.75 V. This result suggests that ethanol oxidation on octahedral Pt-Ni produces more acetic acid than on other surfaces. The synergistic electronic and facet effects may explain the superior EOR activity of octahedral Pt-Ni/C. Further surface modification with Ru significantly lowered the onset potential for CO2 production by ~100 mV and resulted in a higher selectivity on CO2 as compared to unmodified one, which further boosted the ethanol utilization efficiency. Keywords: Pt alloy, ethanol electrooxidation, FTIR, reaction intermediates, adatoms

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1. Introduction In recent years, direct alcohol fuel cell (DAFC) as an alternative power source for various applications has attracted great attention due to its high efficiency under ambient operating conditions. Ethanol is more favored as the fuel than methanol due to its higher energy density (8 vs 5 kWh kg-1), lower toxicity, and larger mass-production capacity.1-4 However, the high-cost, low activity, and easily-poisoned anode (commonly Pt-based) catalysts by strongly adsorbed intermediates such as CO have hindered the wide adoption of this technology.4-6 To solve these problems, the development of advanced catalysts that can minimize the CO poisoning and improve the catalytic performance is needed. Alloying platinum with late transition metals, such as Fe, Co and Ni is an attractive strategy to alleviate the poisoning issue, reduce catalyst cost7-9, and improve the electrocatalytic activity towards ethanol oxidation reaction (EOR).10 Among metals with high electronegativity, Ni has been proven to effectively enhance the EOR activity of Pt.11-12 It has been reported that the catalytic activity of conventional Pt-Ni alloy nanoparticles is around two times higher than that of Pt towards EOR11. Wang et al. found that the peak current density during the EOR could be further increased by ~1.5 times after the incorporation of Ni into Pt-Ru alloys12. The bifunctional mechanism was widely adopted to explain the improved EOR activity on Pt-Ni alloy catalysts.1315

Ni would activate water molecules and provide sites for OH adsorption (H2O → OHads + H+ +

e-) at lower potentials as compared to Pt and the presence of OHads is essential for the complete oxidation of CO intermediates to avoid catalyst poisoning. In addition, the modified electronic structure caused by the incorporation of Ni atoms into Pt lattice would also weaken Pt-intermediate bonding strength (such as Pt-CO), and favor the oxidation reaction.16-19 Besides the adoption of alloy catalysts, crystalline facet effects on EOR are also insensitively studied.20-25 In situ FTIR study by the Feliu group found that the reaction pathway and products selectivity were highly dependent on the crystalline orientations of single crystal Pt electrodes.26 Tian et al. observed that tetrahexahedral (THH) platinum nanocatalysts with high-index facets have a significantly improved EOR activity (up to 400%) as compared to commercial Pt/C.27 It is interesting that attempt on the combination of this two strategies is rather lacking. In this study, ~10 nm octahedral Pt-Ni nanocatalyst with uniformly exposed (111) facets was successfully synthesized and its catalytic activity was compared with commercial Pt/C and conventional Pt-Ni alloy. Further activity improvement was also achieved by surface modification with Ru adatoms. In situ FTIR Page 2 of 22 ACS Paragon Plus Environment

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measurements indicated that the product selectivity was highly dependent on the structure and composition of catalysts.

2. Experimental section 2.1 Material synthesis A modified impregnation method was employed to synthesize the conventional Pt2Ni/C.28 In a typical synthesis, 0.0186 g of Ni(NO3)2·6H2O (Aldrich) was dissolved in 10 mL of water (Milli-Q® UV-plus). Then 0.05 g of Pt/C (46%, TEC10E50E, TKK) was added into the solution resulting in a Pt:Ni atomic ratio of 2:1. The mixture was sonicated for 2 hrs and dried in a vacuum oven at 60°C. The dried mixture was then sintered under a 93% Ar + 7% H2 atmosphere at 700°C for 1 hr. The composition of the synthesized conventional Pt-Ni nanocatalyst was Pt2Ni as determined by inductively coupled plasma optical emission spectrometry ICP-OES. In the synthesis of the sub-10 nm octahedral Pt−Ni, 20 mg Pt(acac)2 and 10 mg Ni(acac)2 were mixed with 2 mL of oleylamine (Aldrich), 1 mL of oleic acid (Aldrich) and 7 mL of benzyl ether (Aldrich) in a three-neck flask with reflux. Besides assisting the formation of octahedral shape, the introduction of benzyl ether as a solvent also reduced the residuals of both oleylamine and oleic acid surfactants on the surface of octahedral Pt-Ni catalyst as shown by Choi and coworkers.29 The mixture was heated to 130 °C under N2 protection. It was then followed by the addition of 60 mg of W(CO)6 while the N2 purging was stopped at the same time. The mixture was then heated to 225 °C, and the 10 nm octahedral Pt−Ni nanoparticles were obtained after keeping the temperature for 40 min. The synthesized Pt-Ni octahedra were ultrasonically dispersed on carbon black and washed by toluene and ethanol for several times to remove the solvents and capping agents. The composition of the synthesized Pt-Ni octahedra was Pt2.3Ni as determined by ICP-OES, and the Pt loading was 18 wt.%.

2.2 Electrochemical Measurements Approximately 5 mg of catalyst was ultrasonically dispersed in a solvent consisting of 4 ml of water, 1 ml of isopropanol (VWR Chemicals) and 20 µl of 5% Nafion (Aldrich) for 20 min. 10 µl of suspension was deposited on the pre-cleaned glassy carbon electrode and allowed to dry in air. A leak-free Ag/AgCl electrode (with sat. KCl solution) calibrated by a reversible hydrogen


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electrode (RHE) and a Pt wire were used as reference and counter electrodes, respectively. All the potentials were referred to RHE if not otherwise mentioned. The working electrode was cycled between 0.02 (0.05 V for Pt-Ni) and 1.2 V for 10 cycles in an Ar-saturated 0.1 M HClO4 (GFS Chemicals) solution at 100 mV s-1. Then a stable cyclic voltammetry (CV) curve was recorded at 50 mV s-1. CO striping experiments was conducted to measure the electrochemical surface area (ECSA) of the catalysts. CO was adsorbed on the precleaned catalyst by holding the potential at 0.02V for Pt/C, and 0.05V for conventional Pt2Ni/C and octahedral Pt2.3Ni/C for 10 min in a CO-saturated 0.1 M HClO4 solution.29 The CO stripping curves were recorded after purging Ar for 30 min by sweeping the potential anodically to 1.2 V. The ECSA was calculated from the charge of CO oxidation peak after subtracting the background charge in the same potential range. The surface charge densities of 0.42 mC cm-2 for Pt/C and conventional Pt2Ni/C, and 0.48 mC cm-2 for octahedral Pt2.3Ni/C were used in the calculations.30 In the EOR activity measurement, the working electrode was cycled between 0.02 V (0.05 V for Pt-Ni) and 1.2 V at 50 mV s-1 in an Ar-saturated 0.2 M ethanol (Aldrich) + 0.1 M HClO4 solution. The EOR peak current density was reported with normalized to the ECSA of the corresponding catalyst.

2.3 Surface modification by Ru adatoms The pre-cleaned working electrodes loaded with catalysts were dipped into an Ar-saturated 10 mM RuCl3 (Alfa Aesar) + 50 mM H2SO4 (Aldrich) solution for 1 min. The electrode was then washed thoroughly with water. Electrochemical evaluation was conducted following the procedures described in Section 2.2.

2.4 In situ infrared absorption studies A Nicolet iS50 FTIR spectrometer equipped with a MCT detector cooled with liquid nitrogen was employed for the in situ spectroelectrochemical FTIR studies. All spectra are shown in absorbance (-log(R/R0)), with R and R0 representing the sample and reference spectra, respectively. The spectral resolution was 4 cm-1 for all the measurements if not otherwise mentioned. The working electrodes were prepared by following the same procedure described in Section 2.2. After a stable CV was obtained by cycling the working electrode in an Ar-saturated 0.1 M HClO4 solution, the working electrode was pressed tightly to a hemispherical CaF2 prism Page 4 of 22 ACS Paragon Plus Environment

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(20 mm in diameter, MTI Corporation) and assembled into a homemade spectroelectrochemical cell. An Ag/AgCl electrode and a Pt mesh were used as the reference and counter electrodes, respectively. Then, the potential was kept at 0.1 V for 10 min in the Ar-saturated 0.2 M ethanol + 0.1 M HClO4 solution before the reference spectrum was collected. The potential was then anodically swept to 1.20 V at a scan rate of 5 mV s-1. The real-time spectroscopic measurement was started at the same time with a collection duration of 10 s (25 interferograms) for each spectrum. The spectra were denoted according to the end potential during the collection period.

3. Results and discussions 3.1 Physical characterization of catalysts X-ray diffraction patterns of conventional and octahedral Pt-Ni catalysts were recorded and compared with Pt in Fig. 1. The diffraction peaks of Pt-Ni were slightly shifted to higher angles as compared to Pt, indicating the successful incorporation of Ni atoms into the Pt fcc lattice leading to a smaller lattice constant.23, 31 No peaks associated with other Ni-containing compounds were observed. The morphology of all the three catalysts were characterized by TEM (Fig. 2). The commercial Pt/C (Fig. 2a) and conventional Pt2Ni/C (Fig. 2b) nanoparticles were typical nanospheres without the preferential exposure of certain facets. A slightly increased average particle size (~ 4 nm) was observed on conventional Pt2Ni/C alloy from Pt (2.5 nm) due to the particle aggregation and addition of Ni atoms during the synthesis. The octahedral Pt2.3Ni/C showed a uniform octahedral shape with an average edge length of 10 nm as shown in Fig. 2c and d. The average inner plane distance (d-spacing) for two adjacent fringes measured on highresolution TEM images was ~2.20 Å (Fig. 2d). This value is smaller as compared to Pt (111) (2.27 Å) due to the successful incorporation of Ni atoms into the Pt lattice and was consistent with the XRD result.


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Fig. 1. X-ray diffraction patterns of Pt/C (a), conventional Pt2Ni/C (b), and octahedral Pt2.3Ni/C (c).

Fig. 2. TEM images of Pt/C (a), conventional Pt2Ni/C (b), octahedral Pt2.3Ni/C (c), and HRTEM images of octahedral Pt2.3Ni with 10 nm edge length (d).


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3.2 Electrochemical evaluation Fig. 3 compares the CVs (solid lines) of Pt/C, conventional Pt2Ni/C and octahedral Pt2.3Ni/C. The hydrogen adsorption/desorption and oxide formation/reduction features could be clearly observed. The CO stripping curves (dashed lines) of these catalysts are also compared in Fig. 3. The onset potentials for CO oxidation on both Pt-Ni samples were lower as compared to that on Pt/C, which might be caused by the weaker CO adsorption strength due to the change of electronic structure after the incorporation of Ni atoms into the Pt lattice16-19, and the presence of OH groups at lower potentials.13-14 In addition, a narrower CO stripping peak was observed on octahedral Pt2.3Ni/C surfaces. As reported by Arenz et al., the removal/oxidation of adsorbed CO on Pt/C surface was sensitive to the surface structures.32 Octahedral Pt2.3Ni/C surfaces with uniformly exposed (111) facets have a narrower bonding strength distribution towards CO, which led to a corresponding narrower CO oxidation peak as compared to conventional Pt and Pt-Ni alloy that have mixed facets. The peak potentials of CO stripping were 0.885, 0.845, 0.835 V for Pt/C, conventional Pt2Ni/C and octahedral Pt2.3Ni/C, respectively. The calculated ECSA for Pt/C, conventional Pt2Ni/C, and octahedral Pt2.3Ni/C based on CO stripping were 89.9, 32.9, and 43.7 m2 g-1, respectively.

Fig. 3. CO stripping curves (dashed lines) and CVs (solid lines) in an Ar-saturated 0.1 M HClO4 solution at a scan rate of 50 mV s-1 for Pt/C (black), conventional Pt2Ni/C (blue) and octahedral


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Pt2.3Ni/C (red). The Pt loadings on the RDE tip are 23.5, 21.9, and 9.2 µg cm-2 for Pt/C, conventional Pt2Ni/C and octahedral Pt2.3Ni/C, respectively. EOR activities of all the three catalysts were measured in an Ar-saturated 0.1 M HClO4 + 0.2 M ethanol solution. As shown in Fig. 4, all three catalysts had a similar forward peak current potential around 0.9 V. The peak current density of octahedral Pt2.3Ni/C (1.46 mA cm-2) was 2.4 and 3.7 times higher than that of conventional Pt2Ni/C (0.62 mA cm-2) and Pt/C (0.39 mA cm-2), respectively, implying that the octahedral shape and alloying effect would synergistically improve the electrocatalytic activity toward EOR. In the backward scan, the highest peak current density was also observed on octahedral Pt2.3Ni/C (3.04 mA cm-2), which was 4.6 and 7.7 times higher than conventional Pt2Ni/C (0.65 mA cm-2) and Pt/C (0.39 mA cm-2), respectively. Even though there is no difference in peak current potential for these three samples, the onset potential of EOR is much lower for Pt-Ni alloys (~0.45 V) than that for Pt/C (~0.7 V). Several possible reasons may explain the improved catalytic performance on the octahedral Pt2.3Ni/C nanocatalyst. It should be noted that the above-mentioned bifunctional effect may not work here, since Ni was prone to dissolution in acid during ethanol oxidation reaction.33 Instead, the electronic property change of Pt with the presence of Ni in subsurface may be the key factor, which weakened Pt-intermediate bonding strength (such as Pt-CO) to ease their oxidation.16-19 The further improved activity on octahedral Pt2.3Ni/C as compared to conventional Pt2Ni/C indicates the facet effect on ethanol oxidation reaction is also significant, which needs further studies.


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Fig. 4. CVs of the Pt/C (black line), conventional Pt2Ni/C (blue line) and octahedral Pt2.3Ni/C electrocatalyst (red line) in an Ar-saturated 0.2 M ethanol + 0.1 M HClO4 solution at a scan rate of 50 mV s-1. The currents are normalized to the electrochemical surface areas.

It was worth noting that the backward (jb) to forward (jf) peak current ratio was much higher on octahedral Pt2.3Ni/C (2.07) as compared to those on conventional Pt2Ni/C (1.05) and Pt/C (1.00), which was widely adopted as an indicator for the amount of carbonaceous species accumulated on the catalyst surfaces due to its incomplete oxidation.13, 34 However, CO stripping curves (Fig. 3) showed the completion potential of CO oxidation/removal was the lowest on octahedral Pt2.3Ni/C, thus it was expected to have the lowest jb/jf ratio, which was distinct to the results observed. Recent electrochemical impedance spectroscopy (EIS)35 and in situ surface enhanced FTIR studies36 on methanol oxidation reaction also indicated that the backward peak current was exclusive related with the freshly adsorbed molecules instead of the accumulated surface CO during forward scanning. In our study, the activity test was conducted with the cathodic scan as the initial direction where the amount of possibly accumulated carbonaceous species (formed in low potential during anodic scan) at the beginning of the test was negligible. The results (Figure S1) demonstrated that the jb/jf ratio was independent on the initial polarization direction, indicating negligible contribution from residual CO (formed during forward scanning) to the backward scanning current.


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We suspect that the jb/jf ratio is related to the status of the catalyst surface. At high potentials, the Pt surfaces are highly oxidized resulting in a smaller current density. Once the PtOx was reduced to Pt in the following cathodic scan, active sites were regenerated which were directly available for ethanol adsorption and oxidation. As shown Fig. 3, the PtOx reduction region in the cathodic scan was the narrowest on octahedral Pt2.3Ni/C (from 0.579 to 1 V) as compared to both Pt/C (0.424 to 1 V) and conventional Pt2Ni/C (from 0.536 V to 1 V). This trend is also consistent with the literature.33 This implied that the Pt surface of octahedral Pt2.3Ni/C could be quickly recovered in the cathodic scan, leading to more active sites for EOR as compared to the other two catalysts. Thus, we propose that the faster PtOx reduction on the octahedral Pt2.3Ni/C surface in the cathodic scan is the main reason for a higher jb/jf ratio. In order to evaluate the catalytic activity of the catalysts and the possible poisoning under continuous operation conditions, chronoamperometric test was conducted by fixing the potential at 0.63V for 30 min. As shown in Fig. 5, there was a sharp reduction in current during the first several minutes, which was caused by the dissociative adsorbed intermediates, i.e., CO and CHx, etc., formed at the beginning of the oxidation reaction that poisoned the active sites. As the reaction proceeded for a prolonged duration, the intermediates adsorbing and oxidizing rates reached a balance, thus the current was more stable. A rapid activity decay was frequently reported on Pt catalysts due to its difficulty in removing CO intermediate37, which was consistent with our results as shown in Fig. 5. By comparing with Pt/C, the octahedral Pt2.3Ni/C had a better ability to overcome catalytic poisoning, hence had a continuous higher current density in prolonged duration.


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Fig. 5. Chronoamperometric curve of Pt/C (black line) and octahedral Pt2.3Ni/C (red line) electrocatalysts in an Ar-saturated 0.2 M ethanol + 0.1 M HClO4 solution by keeping the potential at 0.63V for 30 minutes.

3.3 Surface modification by Ru adatoms By alloying with Ru, the activity of Pt toward methanol and ethanol oxidation reactions can be significantly approved mainly due to the bifunctional effect as Ru-OH can be easily formed at low potentials. In this study, we also tested the effect of Ru on Pt-Ni alloys via a surface modification approach. Instead of incorporating Ru in Pt-Ni to form a ternary alloy during the synthesis, Ru atoms was deposited on the catalyst surface by a modified self-deposition process. 38

The pre-cleaned Pt/C, Pt-Ni/C alloys were dipped into an Ar-saturated 10 mM RuCl3 + 50 mM

H2SO4 solution for 1 min. As shown in Fig. 6a, the H adsorption/desorption and oxide reduction regions of Pt/C after Ru modification did not change significantly after Ru modification. On the other hand, the hydrogen and oxide regions of Pt-Ni/C (Fig. 6b-c) were clearly attenuated caused by the partial occupation of active sites by Ru atoms. The CV results imply that Ru coverage is higher on Pt-Ni/C than that on Pt/C. One possible reason is that (sub-)surface Ni in Pt-Ni alloys might participate in the reaction: 3Ni + 2Ru3+  2Ru + 3Ni2+ resulting in more Ru deposition. The EOR activity was recorded for the Ru-modified catalysts (dashed lines) and compared with the pristine samples (solid lines) in Fig. 7. The forward peak current densities showed negligible Page 11 of 22 ACS Paragon Plus Environment

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change after Ru modification for all samples. However, the current densities at potentials lower than the peak potential were increased. In addition, the onset potentials for EOR were negatively shifted by ~100 mV for both Pt-Ni/C samples, and 150 mV for Pt/C after Ru modification. These results are in consistence with previous study by Colle et al. on Pt single crystal surfaces.38 The EOR chronoamperometric testing also demonstrated better performance of the Ru modified PtNi/C than the pristine one (Fig. S2).

Fig. 6. CVs of Pt/C & Ru modified Pt/C (a), conventional Pt2Ni/C & Ru modified conventional Pt2Ni/C (b), octahedral Pt2.3Ni/C& Ru modified octahedral Pt2.3Ni/C (c) in an Ar-saturated 0.1 M HClO4 at a scan rate of 50 mV s-1.


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Fig. 7. Anodic scan of CVs of Pt/C (black solid line), Ru modified Pt/C (black dashed line), conventional Pt2Ni/C (blue solid line), Ru modified conventional Pt2Ni/C (blue dashed line), octahedral Pt2.3Ni/C (red solid line), and Ru modified octahedral Pt2.3Ni/C (red dashed line) in an Ar-saturated 0.2 M ethanol + 0.1 M HClO4 solution at a scan rate of 50 mV s-1.

3.4 In situ FTIR study In order to get more insights into the reaction mechanisms and product selectivity during EOR on different catalysts, in situ FTIR studies were carried out. The real-time spectrum of ethanol oxidation on Pt/C, conventional Pt2Ni/C, octahedral Pt2.3Ni/C, and Ru/Pt2.3Ni/C catalysts by sweeping the potential from 0.10 V to 1.20 V are shown in Fig. 8. Several absorption bands between 1200 and 2700 cm-1 were observed in the investigated potential window. The bipolar band at around 2044 cm-1 was attributed to the linearly adsorbed CO (COL) (Table 1)39, and the COL band features were similar for four samples. An upward peak near 2342 cm-1 was attributed to the asymmetric stretch vibration of CO2.40 This peak reflects the cleavage of the C-C bond of ethanol, and their further oxidation to the final product. Another peak located at around 1705 cm-1 was assigned to the stretch vibration of C=O bond in acetaldehyde or acetic acid.41 However, the C=O bands in acetaldehyde and acetic acid cannot be well separated in IR spectra as they are very close (1713 cm-1 for acetaldehyde and 1715 cm-1 for acetic acid).42 The two upward peaks at around


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1270 cm-1 and 1369 cm-1 belong to the C-O stretch and O-H deformation of acetic acid in solution.43 The intensity of the absorption band located at ~1270 cm-1 was observed and continuously increased from about 0.45 V on Pt/C and conventional Pt2Ni/C. Interestingly, this vibration feature was observed at a much lower potential (0.25V) on octahedral Pt2.3Ni/C, indicating that acetic acid was produced at significantly lower potentials on the shape controlled catalysts. The evolution of band intensities as a function of potential is shown in Fig. 9. It can be seen that both octahedral Pt2.3Ni/C and conventional Pt2Ni/C had a higher acetic acid selectivity (according to the relative band intensities between ~1270 and ~2340 cm-1) as compared to Pt/C in a wide potential region. It should be mentioned that the comparison on real faradaic selectivity between these products requires external calibration under conditions similar to the ones during spectra collection as well as considering different number of electrons transferred44-45. Herein, only the relative peak intensity was calculated to give a rough estimation. The acetic acid/CO2 relative peak intensity on octahedral Pt2.3Ni/C at 0.75 V was 7.6 and 1.4 times higher as compared to Pt/C and conventional Pt2Ni/C, respectively. It is obvious that acetic acid was more preferentially produced on Pt-Ni alloys, especially on the shape-controlled one. A higher acetic acid selectivity implies that breaking C-C bonds on octahedral Pt2.3Ni/C is more difficult. Next, the effect of surface modification by Ru adatoms on product selectivity was analyzed. The onset potential of acetic acid production was similar to that on the pristine octahedral Pt2.3Ni/C. However, the formation of C=O containing species and CO2 was observed at much lower potentials. The C=O band at around 1705 cm-1 did not appear until at 0.55 V on octahedral Pt2.3Ni/C, while it was negatively shifted to 0.45 V after Ru modification. This indicates that the C2 products (acetaldehyde and acetic acid) are more easily formed when the surface is partially covered with Ru adatoms. The CO2 absorption band appeared at 0.45 V, which is also 100 mV more negative as compared to unmodified octahedral Pt2.3Ni/C, indicating that the oxidation of CO could occur at more negative potentials with Ru on the surface. Thus, one may conclude that the negatively shifted onset potentials of EOR on Ru-modified catalysts shown in Fig. 7 correspond to earlier formation of both C1 and C2 products. Moreover, the ratio of acetic acid/CO2 peak intensities was decreased by more than 50% after Ru modification on octahedral Pt2.3Ni/C at 0.75 V, implying that more C-C bonds were cleaved and the ethanol utilization efficiency was improved. Our results clearly demonstrated that Ru can further reduce the overpotentials of EOR Page 14 of 22 ACS Paragon Plus Environment

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on Pt-Ni alloys by providing reactive OH species to oxidize ethanol to acetaldehyde/acetic acid and CO to CO2 (bifunctional effect).46 More interestingly, our FTIR data suggested that the CO2 selectivity can also be further improved by surface Ru atoms. The mechanism of facilitating C-C bond breaking by Ru adatoms on Pt-Ni alloy surface requires further study.

Fig. 8. Real-time in situ FTIR spectra recorded during ethanol electrooxidation on Pt/C (a), conventional Pt2Ni/C (b), octahedral Pt2.3Ni/C (c), and d) Ru modified Pt2.3Ni/C in an Ar-saturated 0.2 M ethanol + 0.1 M HClO4 solution. Reference potential was 0.1V.


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Fig. 9. IR band intensities of CO, CO2, C=O, and O-H deformation of acetic acid on Pt/C (a), conventional Pt2Ni/C (b), octahedral Pt2.3Ni/C (c), Ru modified octahedral Pt2.3Ni/C (d) during oxidation of ethanol at different potentials (data derived from Fig. 9).

Table 1. Assignments of the absorption bands in Fig. 9. Wavenumber/cm-1

Assignment

Ref

2342

CO2 asymmetric stretching

40

2036-2045

COL (linearly adsorbed CO)

39

1705-1706

C=O stretching of acetaldehyde/acetic acid

41, 42

1366-1369, 1265-1273

C-O stretching and O-H deformation in CH3COOH

43, 42


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Conclusion In summary, we successfully synthesized octahedral Pt2.3Ni/C nanocatalyst with an average size of 10 nm as electrocatalysts for ethanol electro-oxidation. The synthesized octahedral Pt2.3Ni/C has a much higher catalytic activity as compared to commercial Pt/C and conventional Pt2Ni/C (around 3.7 and 2.4 times, respectively) in terms of forward peak current density. In situ FTIR measurements were conducted to monitor the reaction intermediates and product selectivity. It was found that Pt/C has a high CO2 selectivity as compared to acetic acid. However, it possibly suffers from CO poisoning, which leads to both low activity and durability. Meanwhile, the CO poisoning issue was significantly alleviated on octahedral Pt2.3Ni/C due to its higher preference on C2 reaction pathways. The faster kinetics on C2 pathways as compared to that in C1 pathway may also increase the catalytic activity of octahedral Pt2.3Ni/C. These results demonstrated that the simultaneous adoption of alloying and catalyst morphology controlling technique could significantly improve the catalyst activity for EOR. Further catalysts surface modification with ruthenium demonstrated the energy utilization efficiency of ethanol could be boosted according to the doubled CO2/acetic acid peak intensity ratio, indicating future attempts via using combined strategies in catalyst development are promising to achieve higher EOR activities. Overall, our results may shed lights on further development of more active and inexpensive electrocatalysts for EOR.

Supporting Information Additional electrochemical analysis results are provided in the Supporting Information.

Acknowledgement The authors acknowledge the support from Research Grant Council (IGN13EG05 and 26206115), Innovation and Technology Fund (ITS/323/14) of the Hong Kong Special Administrative Region, and a startup fund from the Hong Kong University of Science and Technology. S. Zhu thanks HKJEBN Group for providing the PhD scholarship.


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References (1)

Lamy, C.; Belgsir, E.; Leger, J. J. Appl. Electrochem. 2001, 31, 799-809.

(2)

Li, G.; Pickup, P. G. J. Power Sources 2006, 161, 256-263.

(3)

Ribeiro, J.; Dos Anjos, D.; Léger, J.-M.; Hahn, F.; Olivi, P.; De Andrade, A.; Tremiliosi-

Filho, G.; Kokoh, K. J. Appl. Electrochem. 2008, 38, 653-662. (4)

Wang, Y.; Zou, S.; Cai, W.-B. Catalysts 2015, 5, 1507-1534.

(5)

Zhou, W.; Zhou, Z.; Song, S.; Li, W.; Sun, G.; Tsiakaras, P.; Xin, Q. Appl. Catal., B 2003,

46, 273-285. (6)

Lamy, C.; Rousseau, S.; Belgsir, E.; Coutanceau, C.; Léger, J.-M. Electrochim. Acta 2004,

49, 3901-3908. (7)

Hsieh, C.-T.; Lin, J.-Y.; Wei, J.-L. Int. J. Hydrogen Energy 2009, 34, 685-693.

(8)

Lopes, T.; Antolini, E.; Colmati, F.; Gonzalez, E. R. J. Power Sources 2007, 164, 111-114.

(9)

Spendelow, J. S.; Wieckowski, A. Phys. Chem. Chem. Phys. 2007, 9, 2654-2675.

(10)

Yang, B.; Lu, Q.; Wang, Y.; Zhuang, L.; Lu, J.; Liu, P.; Wang, J.; Wang, R. Chem. Mater.

2003, 15, 3552-3557. (11)

Soundararajan, D.; Park, J.; Kim, K.; Ko, J. Curr. Appl. Phys. 2012, 12, 854-859.

(12)

Wang, Z.-B.; Yin, G.-P.; Zhang, J.; Sun, Y.-C.; Shi, P.-F. J. Power Sources 2006, 160, 37-

43. (13)

Hsieh, C.-T.; Lin, J.-Y. J. Power Sources 2009, 188, 347-352.

(14)

Park, K.-W.; Choi, J.-H.; Kwon, B.-K.; Lee, S.-A.; Sung, Y.-E.; Ha, H.-Y.; Hong, S.-A.;

Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 1869-1877. (15)

Zhang, Z.; Xin, L.; Sun, K.; Li, W. Int. J. Hydrogen Energy 2011, 36, 12686-12697.


Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(16)

Neto, A. O.; Dias, R. R.; Tusi, M. M.; Linardi, M.; Spinacé, E. V. J. Power Sources 2007,

166, 87-91. (17)

Kitchin, J.; Nørskov, J. K.; Barteau, M.; Chen, J. J. Chem. Phys. 2004, 120, 10240-10246.

(18)

Antolini, E.; Salgado, J.; Gonzalez, E. J. Electroanal. Chem. 2005, 580, 145-154.

(19)

Simões, F.; Dos Anjos, D.; Vigier, F.; Léger, J.-M.; Hahn, F.; Coutanceau, C.; Gonzalez,

E.; Tremiliosi-Filho, G.; De Andrade, A.; Olivi, P. J. Power Sources 2007, 167, 1-10. (20)

Koper, M. T. Nanoscale 2011, 3, 2054-2073.

(21)

Lee, Y. W.; Kim, M.; Kim, Z. H.; Han, S. W. J. Am. Chem. Soc. 2009, 131, 17036-17037.

(22)

Ozoemena, K. I. RSC Adv. 2016, 6, 89523-89550.

(23)

Solla-Gullon, J.; Vidal-Iglesias, F.; Feliu, J. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem.

2011, 107, 263-297. (24)

Zhao, T.-T.; Wang, H.; Han, X.; Jiang, K.; Lin, H.; Xie, Z.; Cai, W.-B. J. Mater. Chem. A

2016, 4, 15845-15850. (25)

Lai, S. C.; Koper, M. T. Faraday Discuss. 2009, 140, 399-416.

(26)

Busó‐Rogero, C.; Herrero, E.; Feliu, J. M. ChemPhysChem 2014, 15, 2019-2028.

(27)

Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732-735.

(28)

Shao, M.; Sasaki, K.; Liu, P.; Adzic, R. Phys. Chem. 2007, 221, 1175-1190.

(29)

Choi, S.-I.; Xie, S.; Shao, M.; Odell, J. H.; Lu, N.; Peng, H.-C.; Protsailo, L.; Guerrero, S.;

Park, J.; Xia, X. Nano Lett. 2013, 13, 3420-3425. (30)

Okada, J.; Inukai, J.; Itaya, K. Phys. Chem. Chem. Phys. 2001, 3, 3297-3302.

(31)

Yang, H.; Coutanceau, C.; Léger, J.-M.; Alonso-Vante, N. J. Electroanal. Chem. 2005,

576, 305-313.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(32)

Page 20 of 22

Arenz, M.; Mayrhofer, K. J.; Stamenkovic, V.; Blizanac, B. B.; Tomoyuki, T.; Ross, P. N.;

Markovic, N. M. J. Am. Chem. Soc. 2005, 127, 6819-6829. (33)

Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković,

N. M. Science 2007, 315, 493-497. (34)

Hu, F. P.; Wang, Z.; Li, Y.; Li, C.; Zhang, X.; Shen, P. K. J. Power Sources 2008, 177, 61-

66. (35)

Chung, D. Y.; Lee, K.-J.; Sung, Y.-E. J. Phys. Chem. C 2016, 120, 9028-9035.

(36)

Hofstead-Duffy, A. M.; Chen, D.-J.; Sun, S.-G.; Tong, Y. J. J. Mater. Chem. 2012, 22,

5205-5208. (37)

Brandalise, M.; Verjulio-Silva, R.; Tusi, M.; Correa, O.; Farias, L.; Linardi, M.; Spinacé,

E.; Neto, A. O. Ionics 2009, 15, 743. (38)

Colle, V. D.; Giz, M. J.; Tremiliosi-Filho, G. J. Braz. Chem. Soc. 2003, 14, 601-609.

(39)

Lu, G.-Q.; Sun, S.-G.; Cai, L.-R.; Chen, S.-P.; Tian, Z.-W.; Shiu, K.-K. Langmuir 2000,

16, 778-786. (40)

De Souza, J.; Queiroz, S.; Bergamaski, K.; Gonzalez, E.; Nart, F. J. Phys. Chem. B 2002,

106, 9825-9830. (41)

Zhou, Z. Y.; Huang, Z. Z.; Chen, D. J.; Wang, Q.; Tian, N.; Sun, S. G. Angew. Chem. Int.

Ed. 2010, 49, 411-414. (42)

Iwasita, T.; Rasch, B.; Cattaneo, E.; Vielstich, W. Electrochim. Acta 1989, 34, 1073-1079.

(43)

Leung, L. W. H.; Weaver, M. J. J. Phys. Chem. 1988, 92, 4019-4022.

(44)

Zhou, Z.-Y.; Wang, Q.; Lin, J.-L.; Tian, N.; Sun, S.-G. Electrochim. Acta 2010, 55, 7995-

7999.


Page 21 of 22

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ACS Catalysis

(45)

Zhou, Z.-Y.; Chen, D.-J.; Li, H.; Wang, Q.; Sun, S.-G. J. Phys. Chem. C 2008, 112, 19012-

19017. (46)

Antolini, E. J. Power Sources 2007, 170, 1-12.


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PtâNi Octahedra as Electrocatalysts for the Ethanol Electro-Oxidation

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