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. 2020 Jan 9;5(2):1221–1228. doi: 10.1021/acsomega.9b03757

Disposition of Iridium on Ruthenium Nanoparticle Supported on Ketjenblack: Enhancement in Electrocatalytic Activity toward the Electrohydrogenation of Toluene to Methylcyclohexane

Yuta Inami †, Shoji Iguchi †, Shinichi Nagamatsu ‡, Kiyotaka Asakura ‡, Ichiro Yamanaka †,*
PMCID: PMC6977208  PMID: 31984280

Abstract

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Electrohydrogenation of toluene (TL) to methylcyclohexane (MCH) has been recognized as a promising technology for the hydrogenation process in the organic hydride hydrogen storage system. Recently, we found that the Ketjenblack (KB)-supported Ru–Ir alloy electrocatalyst showed a high electrocatalytic activity for the electrohydrogenation of TL to MCH, and there was the synergy of Ir electrocatalysis for the formation of adsorbed hydrogen species (Had) (H+ + e– → Had) and Ru catalysis for hydrogenation of TL (6Had + TL → MCH). In this paper, the Ir-modified Ru nanoparticle supported on KB (Ir/Ru/KB) electrocatalyst was synthesized by using a modified spontaneous deposition method. The method enables to control the surface structure of Ru–Ir nanoparticles. The Ir/Ru/KB cathode showed higher electrohydrogenation activity than the Ru−Ir alloy/KB cathodes even at lower loadings of precious Ir. Characterization studies using a scanning–transmission electron microscope with an energy-dispersive X-ray spectrometer and X-ray absorption fine structure proved selective and uniform modification of Ru nanoparticle with Ir. Cyclic voltammetry measurements in H2SO4 aqueous solution indicated higher electrochemical active surface areas of Ir at the Ir/Ru/KB electrocatalysts than that at the Ru–Ir alloy/KB electrocatalysts, which is the reason for the strong synergy of Ru and Ir for the electrohydrogenation of TL to MCH.

Introduction

Researches on electrolysis have attracted great attentions in recent years as a promising technology to convert renewable energy into chemical energy, and many studies on water electrolysis,1,2 CO2 electroreduction,3−5 N2 electroreduction,6,7 and so on8−13 are addressed to develop a sustainable society. Electricity derived from renewable energy can be used directly to operate these electrolyzers. In addition, the rates of electrochemical reactions would be easily controlled by the electrolysis current in accordance with the energy supply situation; therefore, electrolyzers can follow daily and seasonal fluctuation of renewable energy unlike the conventional thermal reactions. In recent years, it is known that electrohydrogenation of toluene (TL) to methylcyclohexane (MCH) is one of the promising technology as the hydrogenation process in an organic hydride hydrogen storage system.14−32 Water is oxidized on an anode (eq 1), and TL is reduced to MCH with electrons and protons on a cathode (eq 2) which is widely known as a transportable hydrogen carrier.33−36 As presented by eq 3, the theoretical electrolysis voltage of the reaction (1.07 V) is lower than that of the water electrolysis (1.23 V). Thus, the direct electrohydrogenation process has significant advantages in energy conversion efficiency for the hydrogenation of TL to MCH to compare with a conventional 2-step hydrogenation process, which consists of the water electrolysis and the catalytic hydrogenation of TL.16 In accordance with the demand of energy, hydrogen is extracted by a catalytic dehydrogenation of MCH to TL and used as an energy source.33−36

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To achieve a high efficiency of the energy conversion for the hydrogenation process, it is necessary to develop an active electrocatalyst for the electrohydrogenation of TL to MCH which can suppress the side-reaction on a cathode, that is, hydrogen evolution reaction (eq 4). Cathodes supported Pt with higher loadings for the electrohydrogenation of TL to MCH were widely studied, and it was reported that 50 wt % Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo) and 54 wt % Pt–Ru/C (TEC61E54, Tanaka Kikinzoku Kogyo)-coated gas-diffusion layer (GDL) cathodes were active for the reaction.16,26

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Recently, we reported that 10 wt % Ru–Ir alloy supported on Ketjenblack (Ru–Ir alloy/KB) electrocatalysts showed high electrocatalytic activity for the electrohydrogenation of TL to MCH.27,29 The Ru–Ir alloy/KB cathode showed a comparable electrocatalytic activity to the 50 wt % Pt/KB cathode even at lower loadings of noble metals. We proposed that the electrohydrogenation of TL on Ru–Ir alloy proceeded as below; (i) adsorbed hydrogen species (Had) formed on Ir at the surface of Ru–Ir alloy by the electrochemical reduction of a proton (H+ + e– + Ir → Had–Ir), (ii) Had on Ir migrated to Ru surface (Had–Ir + Ru → Ir + Had–Ru), and (iii) Had on Ru reacted with TL adsorbed on Ru and MCH formed (TLad–nRu + 6Had–Ru → MCH + (n + 6)Ru). Ir and Ru in Ru–Ir alloy worked as an electrocatalyst for Had formation and a catalyst for hydrogenation of TL, respectively. The high catalytic activity of Ru for the hydrogenation and the high electrocatalytic activity of Ir for the Had formation synergistically catalyze the electrohydrogenation of TL to MCH. The surface structure and surface composition of RuIr particle must be critical for the electrohydrogenation activity; however, it is almost impossible to control those properties of Ru–Ir alloy independent from the bulk composition because Ru–Ir alloy is random solid solutions.

The spontaneous deposition is known as a specific adsorption of metal ions (MAn+) on another metal surface (MB). It occurs when MB contacts with MAn+ solution containing acid electrolyte.37−44 We considered that it could be possible to control the surface structure of RuIr particles independent from the bulk composition by using the spontaneous Ir-deposition on Ru nanoparticles. Fundamental studies on the spontaneous depositions of Ru,38,39 Rh,40 Pd,41 Ir,42 Pt,42,43 and Au44 on precious metal substrates were extensively studied; however, there were limited works applying the spontaneous deposition to supported catalysts.45−49 Furthermore, to the best of our knowledge, there was no work on the spontaneous Ir-deposition on Ru nanoparticles. In the present study, we developed Ir-modified Ru nanoparticles supported on KB (Ir/Ru/KB) electrocatalyst by using a modified spontaneous deposition method. The addition of acid electrolyte was not necessary in this method. We found that the Ir/Ru/KB cathode exhibited a superior electrocatalytic activity toward the electrohydrogenation of TL to MCH at lower loadings of precious Ir than the Ru–Ir alloy/KB cathode. Purposes of this work are (i) to prove excellent electrocatalytic activity of the Ir/Ru/KB cathode, (ii) to characterize the structure of the Ir/Ru/KB electrocatalyst, and (iii) to propose well-designed surface of the electrocatalyst for the electrohydrogenation of TL to MCH.

Result and Discussion

Electrohydrogenation of TL to MCH

Figure 1 shows effects of Ir loadings (X) on working cathode potentials and faradic efficiencies of the MCH formation [FEs(MCH)] in the galvanostatic electrohydrogenation of TL at 0.20 A cm–2 using the Ir(X)/Ru(10)/KB (X wt % Ir-modified 10 wt % Ru nanoparticles supported on KB) cathodes and the Ru(10 – X)–Ir(X) alloy/KB cathodes29 which has been reported previously. For all cathodes, sum of FE(MCH) and FE(H2) was ∼100%. At an Ir loading of 0 wt % [i.e., Ru(10)/KB cathode], a high FE(MCH) of 95% was obtained; however, the working potential was markedly negative as −0.330 V corresponding to a large overpotential of 0.490 V. On the Ru(10 – X)–Ir(X) alloy/KB cathodes, the working potential shifted positively with the increase in the Ir loading. The working potential and the FE(MCH) were, respectively, −0.183 V and 86% at the Ru(5)–Ir(5) alloy/KB cathode.29 The positive shift of the cathode potential was due to the synergy of Ru and Ir, that was caused by the acceleration of the electroreduction of H+ to Had species on Ir. In addition, the working potential at the Ir(10)/KB cathode was relatively positive (−0.171 V) under the same reaction conditions, though the FE(MCH) was only 24%.29 In the case of the Ir(X)/Ru(10)/KB cathodes, the working potential drastically shifted to positive potentials at Ir loadings of 0.5 and 1 wt %. The working potential at the Ir(1)/Ru(10)/KB cathode was −0.190 V which was very close to −0.183 V for the Ru(5)–Ir(5) alloy/KB cathode. Furthermore, the FE(MCH) on the Ir(1)/Ru(10)/KB cathode was 94%, which was higher than 86% on the Ru(5)–Ir(5) alloy/KB cathode. At the Ir(5)/Ru(10)/KB cathode, the working potential was −0.147 V where the overpotential was only 0.307 V with a high FE(MCH) of 92%. The synergy of Ru and Ir in the Ir/Ru/KB electrocatalysts was stronger than that in the Ru–Ir alloy/KB electrocatalysts. Figure S1 shows results of a long-time electrolysis using the Ir(1)/Ru(10)/KB and Ir(5)/Ru(10)/KB cathodes. The cathode potentials and FEs(MCH) were very stable during 10 h electrolysis on the both cathodes. This indicated the steady activity of Ir(X)/Ru(10)/KB cathodes under the electrohydrogenation conditions.

Figure 1.

Figure 1

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Effects of Ir loadings on (a) working cathode potentials and (b) FEs(MCH) under galvanostatic electrohydrogenation of TL to MCH at j = 0.20 A cm–2 on (i) Ir(X)/Ru(10)KB and (ii) Ru(10 – X)–Ir(X) alloy/KB cathodes.

Figure 2 shows effects of current densities (j) of the galvanostatic hydrogenation on the working cathode potentials and the FEs(MCH). In spite of a lower loading of Ir, the working potentials and the FEs(MCH) of the Ir(1)/Ru(10)/KB cathode were very similar to that of the Ru(10)–Ir(5) alloy/KB cathode at j values of 0.20–0.40 A cm–2. Furthermore, the Ir(5)/Ru(10)/KB cathode showed a higher electrocatalytic activity at 0.40 A cm–2 to compare with the Ru(10)–Ir(5) alloy/KB cathode and the 50 wt % Pt/KB16,26 cathode which was well known as the active cathode. It is to be noted that the loadings of catalysts on the GDL were the same as 2.0 mg-catal. cm–2 for all cathodes; therefore, the sum of metal loadings were 0.27 and 1.00 mg-metals cm–2 on the Ir(5)/Ru(10)/KB and 50 wt % Pt/KB cathodes, respectively. Though the metal loading of the former was less than one-third of the later, the former was superior. Accordingly, we have succeeded to develop the stable and effective Ir(X)/Ru(10)/KB cathodes for the electrohydrogenation of TL to MCH.

Figure 2.

Figure 2

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Effects of current densities (j) of galvanostatic electrohydrogenation of TL to MCH on (a) working cathode potentials and (b) FEs(MCH) at (i) Ir(5)/Ru(10)/KB, (ii) 50 wt % Pt/KB (TEC10E50E), (iii) Ir(1)/Ru(10)/KB and (iv) Ru(10)–Ir(5) alloy/KB cathodes.

Characterization Studies of Ir/Ru/KB Electrocatalysts

Characterization studies were conducted to reveal structures of the Ir(X)/Ru(10)/KB electrocatalysts. As shown in Figure S2 of transmission electron microscopy (TEM) images, nanoparticles with 1.5–3 nm diameters (2.3 nm average) were observed on (a) the Ru(10)/KB electrocatalyst which was the base material. The average particle diameters of (b) Ir(1)/Ru(10)/KB and (c) Ir(5)/Ru(10)/KB electrocatalysts were 3.1 and 2.5 nm, respectively. These data indicated the modification of Ru with Ir increased the particle diameters slightly. Figures 3, S3, and S4 show scanning–transmission electron microscopy–energy-dispersive X-ray spectroscopy (STEM–EDS) images of the Ir(1)/Ru(10)/KB, Ru(10)/KB, and Ir(5)/Ru(10)/KB electrocatalysts, respectively. In the case of the Ru(10)/KB electrocatalyst (Figure S3), Ru nanoparticles were observed. In the case of Ir(1)/Ru(10)/KB electrocatalyst (Figure 3a–d), strong signals of Ru were detected from all particles, and weak Ir signals were detected from the Ru particles. Any isolated Ir nanoparticles were not detected. Moreover, a high magnification high-angle annular dark-field (HAADF)–STEM image of one particle (Figure 3e) and the line-scan profiles (Figure 3f) indicated a uniform distribution of Ru and Ir. These results proved that selective modification of Ru nanoparticle with Ir was succeeded by the present spontaneous deposition method. For the Ir(5)/Ru(10)/KB electrocatalysts (Figure S4), much stronger Ir signals were detected from Ru nanoparticles, and isolated Ir nanoparticles were not observed.

Figure 3.

Figure 3

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(a) HAADF–STEM image of the Ir(1)/Ru(10)/KB electrocatalyst and EDS mapping of (b) Ru and Ir, (c) Ru, (d) Ir, (e) a high magnification HAADF–STEM image and (f) EDS line-scans of Ru and Ir intensities across the positions.

Figure S5 shows X-ray diffraction (XRD) patterns of the Ir(X)/Ru(10)/KB electrocatalysts. Diffraction lines were assigned to hcp-structured Ru metal for all patterns. Crystal structures of Ru did not change before and after the Ir modification. Figure 4a shows Ru K-edge X-ray absorption near-edge structure (XANES) spectra. The Ir(X)/Ru(10)/KB (X = 0–5) electrocatalysts exhibited similar XANES spectra to that of Ru metal as a reference material; however, the more intense peaks around 22 133 eV in the Ir(X)/Ru(10)/KB electrocatalysts suggested a presence of partially oxidized state of Ru. Figures S6 and 4b show k3-weighted extended X-ray absorption fine structure (EXAFS) oscillations and Fourier-transform of k3-weighted EXAFS spectra, respectively. In Figure 4b, peaks at 1.5 and 2.4 Å were mainly originated from coordination of O and Ru around Ru, respectively. Curve-fitting results of the EXAFS spectra are indicated in Table 1. The coordination numbers and the bond lengths of Ru–Ru and Ru–O for the Ir(X)/Ru(10)/KB electrocatalysts were almost the same with regardless of Ir loadings. In addition, the coordination of Ir around Ru was not observed for all Ir(X)/Ru(10)/KB electrocatalysts. In the case of the Ru(5)–Ir(5) alloy/KB electrocatalysts, the coordination of Ir around Ru was observed.29 The differences in the coordination clearly indicated the Ru and Ir structure estimated by EXAFS in the Ir(X)/Ru(10)/KB electrocatalyst so differed from that in the Ru–Ir alloy/KB electrocatalyst.

Figure 4.

Figure 4

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Ru K-edge (a) XANES spectra and (b) Fourier-transform of k3-weighted EXAFS spectra (solid line: observed, and dashed line: fitting line) of (i) Ru(10)/KB, (ii) Ir(1)/Ru(10)/KB, (iii) Ir(3)/Ru(10)/KB, (iv) Ir(5)/Ru(10)/KB electrocatalysts, and (v) Ru metal as a reference material.

Table 1. Curve-Fitting Analysis of Fourier-Transform of Ru K-Edge EXAFS Spectra of the Ir(X)/Ru(10)/KB Electrocatalysts and Ru Metal Reference Material.

  Ru–Ru
 Ru–O
 Ru–Ir
  R/Åa C.N.b R/Åa C.N.b R/Åa C.N.b
Ru(10)/KB 2.67 5.5 1.97 1.4    
Ir(1)/Ru(10)/KB 2.67 6.1 1.96 1.5    
Ir(3)/Ru(10)/KB 2.67 6.3 1.96 1.4    
Ir(5)/Ru(10)/KB 2.67 6.9 1.96 1.3    
Ru metal 2.68 12        
Ru(5)–Ir(5) alloy/KBc 2.67 3.7 1.97 1.0 2.66 2.3
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a

Bond length.

b

Coordination number.

c

Referred from our previous paper.18

Figure 5a shows X-ray photoelectron spectroscopy (XPS) spectra of Ru 3p3/2 region at the Ir(X)/Ru(10)/KB electrocatalysts and reference materials. The spectra of the Ir(X)/Ru(10)/KB electrocatalysts were deconvoluted using reference spectra of Ru metal and RuO2·nH2O materials. For all spectra of the Ir(X)/Ru(10)/KB electrocatalysts, oxidized Ru species were observed, which was consistent with the results of XAFS analysis. No major differences in oxidized states of Ru were observed regardless of Ir loadings. The oxidized Ru species should form by contacting with Ir3+ during the spontaneous deposition process and air after the catalyst preparation. Figure 5b shows XPS spectra of an Ir 4f region at the Ir(X)/Ru(10)/KB electrocatalysts and reference materials. The spectra were varied with regard to Ir loadings. The peak position at the spectrum of Ir(1)/Ru(10)/KB electrocatalyst was similar to that of reference spectra of Ir metal, indicating that the major species of Ir at the Ir(1)/Ru(10)/KB electrocatalyst was Ir metal. This proved that Ir3+ was reduced and deposited on the catalyst during the spontaneous deposition process. When the Ir loadings increased, full widths at half-maximum of Ir 4f peaks increased. This indicated an increase in the ratio of oxidized Ir species. The oxidized species should form by the incomplete reduction of Ir3+ during the spontaneous deposition process. We observed partial oxidized states of Ru and Ir in the Ir(X)/Ru(10)/KB electrocatalysts; however, the oxidized Ru and Ir species should be electrochemically reduced to the metallic states during the electrohydrogenation reaction due to the negative working potential. The working states of Ru and Ir in the electrohydrogenation reaction must be metallic states.29

Figure 5.

Figure 5

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XPS spectra of (a) Ru 3p3/2 and (b) Ir 4f region of the (i) Ru(10)/KB, (ii) Ir(1)/Ru(10)/KB, (iii) Ir(3)/Ru(10)/KB, (iv) Ir(5)/Ru(10)/KB electrocatalysts, (v) Ru metal, (vi) RuO2·nH2O, (vii) Ir metal and (viii) IrCl3·nH2O reference materials.

The HAADF–STEM and EDS results indicated uniform and selective Ir modification of the Ru nanoparticles. The XRD and Ru K-edge EXAFS results proposed that bulk structures of Ru particles were not affected by the modification with Ir. Ir should be immobilized only at the surface of Ru nanoparticles in the Ir(X)/Ru(10)/KB electrocatalysts. The XPS results proved that Ir3+ reduced and deposited on the catalyst surface during the spontaneous deposition process. The strong synergy of Ru and Ir observed in the electrohydrogenation of TL using the Ir/Ru/KB cathode should be due to the unique surface structure of Ir-modified Ru nanoparticles.

Electrochemical Studies of the Ir/Ru/KB Electrocatalysts

The surface structures of the Ir-modified Ru nanoparticles at the Ir(X)/Ru(10)/KB electrocatalysts were analyzed using cyclic voltammetry (CV) measurements. The CV in H2SO4 aqueous solution between 0 and 0.8 V (SHE) are shown in Figure 6. We confirmed that this potential range was suitable because the maximum hydrogen adsorption/desorption current was observed as shown in Figure S7. At the CV of the Ir(10)/KB electrocatalysts (Figure 6f), a redox couple was observed at 0.0–0.3 V which was attributed to electrochemical adsorption/desorption of Had (adsorbed hydrogen species) on Ir. On the other hand, a redox couple of Had was not observed at the Ru(10)/KB electrocatalyst (Figure 6a). Therefore, the redox couple at 0.0–0.3 V presented in CVs of Ir(X)/Ru(10)/KB electrocatalysts (Figure 6b–e) were electrochemical adsorption/desorption of Had on the Ir surface at the Ir-modified Ru nanoparticles.29 The charges passed for the oxidation of Had (QH, shaded area in Figure 6) could be converted to the electrochemical active surface area of Ir (ECSAIr) using a conversion factor of 179 μC cmIr–2.51,52Figure 7 shows the QH and ECSAIr values per unit amount of catalyst as a function of Ir loading. Those of Ru–Ir alloy/KB electrocatalysts are also shown in Figure 7 for comparison.29 The ECSAIr value of the Ir(X)/Ru(10)/KB electrocatalysts were higher than that of the Ru(10 – X)–Ir(X) alloy/KB electrocatalysts with the same Ir loadings. These results indicated that the surface area exposed Ir at the Ir(X)/Ru(10)/KB electrocatalysts were more than that at the Ru(10 – X)–Ir(X) alloy/KB electrocatalysts. This strongly suggested a surface modification with Ir on the Ru nanoparticle. Furthermore, the shape of the peaks for the Had desorption at the Ir(X)/Ru(10)/KB electrocatalysts changed as the Ir loadings. This difference reflected the difference in the surface structure of Ir.29 The Had desorption at more positive potentials as observed in the CV of the Ir(10)/KB electrocatalyst indicated a presence of a larger Ir ensemble. When the size of Ir ensemble decreased, the peak width decreased and the peak potential shifted negatively. The narrow Had desorption peaks at the Ir(X)/Ru(10)/KB electrocatalysts indicated a small ensemble size of Ir. Ir dispersed highly on the Ru surface at the Ir(X)/Ru(10)/KB electrocatalysts.

Figure 6.

Figure 6

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CVs of (a) Ru(10)/KB, (b) Ir(0.5)/Ru(10)/KB, (c) Ir(1)/Ru(10)/KB, (d) Ir(3)/Ru(10)/KB, (e) Ir(5)/Ru(10)/KB, and (f) Ir(10)/KB electrocatalysts in 0.5 M H2SO4 aqueous solution.

Figure 7.

Figure 7

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Effects of Ir loadings on QH and ECSAIr values at the (i) Ir(X)/Ru(10)/KB and (ii) Ru(10 – X)–Ir(X)/KB electrocatalysts.

Figure 8 shows an image for the electrohydrogenation of TL to MCH on the Ir/Ru/KB electrocatalyst. The Had species formed by the electrochemical reduction of H+ over the Ir atomic site and spilled over the Ru surface. The Had species on Ru atoms hydrogenated the TL adsorbed species activated by the Ru surface and MCH formed. The higher ECSAIr value accelerated the formation of Had; as the result, the Ir/Ru/KB cathode exhibited lower overpotentials for the electrohydrogenation of TL even at lower loadings of Ir. By tuning the surface structure of the RuIr nanoparticle, the synergy of Ru and Ir toward the electrohydrogenation of TL to MCH was drastically enhanced.

Figure 8.

Figure 8

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Conceptual diagram of the reaction model for the electrohydrogenation of TL to MCH on the Ir/Ru/KB cathode.

Conclusions

We developed the Ir-modified Ru nanoparticle supported on KB electrocatalysts using the modified spontaneous deposition method. The Ir/Ru/KB cathode exhibited the strong synergy of Ru and Ir toward the electrohydrogenation of TL to MCH even at lower loadings of Ir and superior electrocatalytic activities to the Ru-Ir alloy/KB cathode previously reported. Characterization studies revealed that Ir selectively and uniformly immobilized on the Ru nanoparticles. The CV measurements demonstrated higher ECSAIr values at the Ir/Ru/KB electrocatalysts than that at the Ru–Ir alloy/KB electrocatalysts, which was the reason for the stronger synergy toward the electrohydrogenation of TL to MCH. In other words, the synergy of the Ir electrocatalysis for the electroreduction of H+ and the Ru catalysis for the catalytic hydrogenation of TL was effectively performed on the Ir/Ru/KB electrocatalyst. This work suggests that the controlling surface structure of electrocatalyst, the specializations of catalysis and their synergy are essential for realization of other difficult electrochemical reactions.

Experimental Section

Detailed information of reagents and materials is described in the Supporting Information.

Catalyst Preparation

Ru Suppoted on KB (Ru/KB) Electrocatalysts

The Ru/KB electrocatalyst was prepared as reported in our previous paper.29 A powder of KB (90 mg) and ion-exchanged water (100 mL) were mixed in a PTFE beaker (200 mL). The suspension was ultrasonicated to disperse KB in water. Aqueous solutions of RuCl3·nH2O (20 mM, 4.95 mL) was added to the solutions and heated to 353 K with stirring by a magnetic spin-bar. Then, aqueous solutions of NaOH were added to adjust at pH = 12. Aqueous solutions (20 mL) of NaBH4 (37.5 mg) and NaOH (10 mg) were added dropwise and have been stirred overnight. This mixture was filtered, washed with ion-exchanged water (100 mL) and 2-propanol (20 mL), and dried in vacuum at 343 K for 1 h, and a catalyst powder, denoted as Ru(10)/KB, was obtained, where the number inside brackets was the mass loading of Ru in preparation (wt %).

Ir-Modified Ru/KB Electrocatalysts

The Ir modification was conducted using a Personal Organic Synthesizer ChemiStation PPS-2511 (EYELA). The Ru(10)/KB electrocatalyst (50 mg) was put into a tube type reactor (I.D. 26.4 mm, O.D. 30.0 mm, L 200 mm) and reduced with H2 (101 kPa, 100 mL min–1) at 298 K for 1 h to remove oxidized phase on the surface of Ru nanoparticles. Subsequently, the gas phase was replaced by Ar (101 kPa, 100 mL min–1), and 10 mL of 0.13–1.3 mM of IrCl3·nH2O aqueous solutions was added using a syringe without contacting with air. Before the addition, the IrCl3·nH2O aqueous solution was deoxygenated by bubbling Ar (101 kPa, 50 mL min–1) for 30 min. The suspensions were stirred for further 1 h in an Ar flow. During stirring the suspension, the oxidation of Ru (eq 5) and reduction of Ir3+ (eq 6) should proceed spontaneously43 because the standard redox potentials of eq 5 [i.e., −0.050 V (SHE) for RuOH/Ru and +0.788 V (SHE) for RuO2/Ru] are more negative than that of Ir3+/Ir (eq 6).

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After that, the suspension was filtered, washed using deionized water and 2-propanol, and dried under vacuum at 298 K for 1 h. The obtained powder was denoted as a Ir(X)/Ru(10)/KB electrocatalyst, where X is a mass loading of Ir in preparation (wt %). Actual loadings of Ir and Ru were confirmed using inductively coupled plasma optical emission spectroscopy (ICP–OES) analysis and were almost equal to the loadings in preparation (see Table S1).

50 wt % Pt/KB Electrocatalyst

An anode electrocatalyst of KB supported Pt (50 wt %) was prepared by the impregnation method. Aqueous solutions of H2PtCl6·6H2O (0.1 M) and the KB support were well mixed by using a magnetic spin-bar, and the mixtures were dried up on a hot plate at 393 K with mixing by a glass stick. The precursor powder in a quartz reactor was reduced in a stream of H2 (101 kPa) at 573 K for 2 h.

Preparation of MEA

Electrode and membrane-electrode assembly was prepared as follows: an ink of the electrocatalyst was made from mixtures of electrocatalyst powder (4.0 mg), 10 wt % Nafion-dispersion (20 μL), and acetone (250 μL). The ink was prepared by ultrasonicating the mixture for 10 min. The ink was casted on GDL (SIGRACET GDL-25BC, 2.0 cm2), and the electrocatalyst-ink/GDL was dried in vacuum at 323 K for 10 min. The electrocatalyst/GDL cathode has been prepared. Loadings of the electrocatalyst were 2.0 mg-catal. cm–2 for all cathodes. The anode of 50 wt % Pt/KB/GDL (5.0 mg-catal cm–2) was prepared by similar procedures. The cathode and the anode were connected on both sides of the Nafion 117 membrane by hot-pressing at 413 K and 50 MPa.8 This electrocatalyst/GDL|Nafion 117|Pt/KB/GDL unit as the MEA completed.

Electrohydrogenation of TL to MCH

The MEA was set in a two-compartment batch electrolysis cell.27 Pure TL (25 mL) was added to the cathode compartment, and Ar (101 kPa, 20 mL min–1) was flowed. Ion-exchanged water (15 mL) was soaked to the anode compartment and a pure H2 gas (20 mL min–1) was supplied. We replaced the water oxidation to the H2 oxidation for the anode reaction; however, we confirmed that the same results of the cathode potential and the FE of products were obtained, regardless of the anode reaction.27 An electrode of Ag/AgCl saturated KCl aq [+0.199 V (SHE)] was connected to a Nafion 117 via 0.5 M H2SO4 aq solutions. A pure Ar and pure H2 were, respectively, supplied to the anode and cathode compartments for 20 min and a galvanostatic electrolysis was conducted under 0.10–0.40 A cm–2 by the IviumStat.XR (Ivium Technology) for 2 h at 298 K. MCH and H2, were respectively, analyzed by GC-2025-FID [Shimadzu, HP-1 capillary column (0.25 mmϕ × 100 m, Agilent Co.)] and GC-8A-TCD [Shimadzu, active carbon column (3 mmϕ × 2.5 m)]. The cathode potentials were measured by using the reference electrode; however, the potentials were denoted against SHE without an iR-correction. The faradic efficiency to the MCH formation [FE(MCH)] and cathode potential were employed to evaluate catalytic activities. The FE is defined as follows: FE = (yield × F × n/Q) × 100%, where F is the Faraday constant (96 485 C mol–1), n is the number of electrons involved in the reaction, and Q is the charge passed.

Characterization Studies

ICP–OES Analysis

The filtrate after the preparation of the Ru(10)/KB and Ir(X)/Ru(10)/KB electrocatalysts was analyzed using the Agilent 5100 VDV ICP-OES instrument. The concentration of Ru and Ir in the filtrates was determined. Actual loadings of Ru and Ir in the electrocatalysts were calculated subtracting the amounts in filtrates from charged amounts.

TEM and STEM Measurement

TEM images were measured using the JEM-2010F (JEOL, 200 kV). STEM images and energy-dispersive X-ray spectroscopy (EDS) spectra were measured using the JEM-ARM200F-B (JEOL, 200 kV) equipped with dual SDD-type EDS spectrometers.

XRD Measurement

XRD patterns were measured using the MiniFlex-600 XRD instrument (Rigaku) with Cu Kα radiation (40 kV, 15 mA).

XAFS Measurement

Ru K-edge XAFS spectra were measured in transmission mode at the beamlines NW10A of Photon Factory Advanced Ring (PF-AR) of Institute for Materials Structures Science, High Energy Accelerator Research Organization (KEK-IMSS), Tsukuba, Japan. The X-ray energy was calibrated using Ru metal (E0 = 22 119.3 eV). XAFS samples were prepared by mixing the electrocatalyst and BN powders and pressing into a pellet (10 mm diameter). The XANES and EXAFS spectra were analyzed using the REX2000 software (Rigaku). The backscattering amplitudes and the phase shifts of Ru–Ru and Ru–O were calculated using the FEFF program.50

XPS Measurement

The electrocatalyst was mechanically mixed with Au powder as reference for calibration. XPS spectra were measured using the JPS-9010MC instrument (JEOL) with Al Kα radiation (12 kV, 25 mA) and an analyzer pass energy of 30 eV. The binding energy was calibrated using a Au 4f7/2 peak as 84.0 eV. Spectra at the Ru 3p3/2 region were analyzed using the linear-combination fitting with reference spectra of Ru metal and RuO2·nH2O materials. For spectra at the Ir 4f region, the intensities were normalized by the maximum intensity of respective spectra.

CV Measurement

CVs were measured using an H-type cell divided by a glass filter and the Ivium Stat. XR potentio/galvanostat. An electrocatalyst ink is made from the electrocatalyst (2.0 mg), 10 wt % Nafion-dispersion (20 μL), and acetone (200 μL) by ultrasonication. A working electrode was a grassy carbon electrode (I.D. 5.0 mm, 0.19 mg-catal. cm–2) that casted the electrocatalyst ink (4.0 μL). The working electrode, a Pt-black/Pt-wire counter-electrode, and the Ag/AgCl reference electrode were soaked in H2SO4 aq (0.5 M) solutions of the two compartments, respectively. The electrolyte solution was deoxygenated by bubbling Ar for 15 min. CVs were measured between 0.0 and +0.8 V (SHE) with 50 mV s–1 for several cycles until stable CVs were obtained. The stabilized CVs were reported. The data of CVs were normalized by a geometric area of GC electrode (0.20 cm2) and electrode potentials were converted to potentials on the basis of SHE without an iR-correction.

Acknowledgments

This article is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). STEM–EDS measurement was supported by NIMS microstructural characterization platform as a program of “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. XAFS measurements were performed at Photon Factory Advanced Ring (PF-AR) of Institute for Materials Structures Science, High Energy Accelerator Research Organization (KEK-IMSS), Tsukuba, Japan with the proposal number 2016G546. TEM and ICP–OES measurements were supported by Ookayama Materials Analysis Division, Tokyo Institute of Technology.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03757.

  • Detailed information of reagents and materials; ICP–OES analysis; long-time electrolysis; TEM images; HAADF–STEM images and EDS mappings; XRD patterns; EXAFS oscillations; and CVs (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b03757_si_001.pdf (541.5KB, pdf)

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