Toluene (TL)/methylcyclohexane (MCH) is an attractive organic hydride couple for secure storage and transportation of large amounts of hydrogen. Electrochemical hydrogenation of TL to MCH can achieve energy savings compared with hydrogenation using molecular hydrogen generated separately, and catalyst development to improve the kinetics of electrochemical TL hydrogenation is imperative. Here, we found that using rhodium to modify Pt nanoparticles (NPs) supported on carbon black (Pt/C) significantly improved the kinetics of electrochemical TL hydrogenation despite rhodium as a single catalyst being less active than platinum. Rhodium was deposited on Pt NPs by reducing Rh3+ ions with molecular hydrogen. By optimizing the CO-stripping conditions, the electrochemical surface areas (ECSAs) of Pt and Rh for Rh-deposited Pt NP-loaded carbon (Rh x /Pt/C) catalysts were individually determined, and the Rh coverage (θRh) and the ratio of ECSA of the deposited Rh to the total ECSA (F Rh) were successfully estimated. θRh increased with the Rh/Pt mole ratio for Rh x /Pt/C and then plateaued around 0.20, whereas F Rh still increased. According to linear sweep voltammograms and Tafel plots, the Rh x /Pt/C electrodes exhibited peak hydrogenation activity 2.7 times that of the Pt/C electrode, and addition of Hads to TL was rate-determining. Increased θRh facilitated the addition of Hads generated on Rh to TL adsorbed on Pt at the extended interface between Rh and Pt, leading to increased geometric reduction current density at 0 V vs RHE (j geo,0). Plateauing of θRh was accompanied by plateauing of j geo,0, attributed to a lack of any further increase in the extent of the interface. In the electrochemical hydrogen pump hydrogenation of 30 vol % TL with the Rh x /Pt/C electrodes, MCH was the sole TL hydrogenation product. After the conversion of TL to MCH exceeded about 85%, the hydrogen evolution reaction became appreciable, occurring significantly later than with the Pt/C electrode.

Pt/Rh bimetal catalysts are often used in various reactions such as alcohol oxidation and hydrogenation. For comparison, their catalytic activity is often standardized by the real surface area or electrochemical surface area. However, the conventional method of using the electric charge for hydrogen desorption is difficult to apply to Pt/Rh bimetal catalysts because the potential regions of hydrogen desorption for Pt and Rh overlap. In this study, Rh-adlayer-modified Pt (Rh x /Pt) electrodes with different Rh coverages were prepared by underpotential depositions of Cu adatoms at different potentials followed by galvanic replacement of Cu with rhodium, and CO stripping was applied to determine the Rh coverages of these electrodes. No CO that had been adsorbed on a Pt electrode was removed by potentiostatic CO oxidation at 0.75 V vs reversible hydrogen electrode for 15 s in an Ar-saturated 0.5 M sulfuric acid solution at 5 °C, but 85% of CO on an Rh electrode was removed. The Rh coverages for the Rh x / Pt electrodes corrected on the basis of this result were in good agreement with the corresponding Rh coverages estimated from the electric charge for the stripping of the Cu adlayer. Moreover, the Rh x /Pt electrodes exhibited noteworthy CO-stripping and COadsorption behaviors. A single CO-stripping cyclic voltammetry peak was observed regardless of Rh coverage, and shifted toward lower potential as the Rh coverage was increased. This result resembled the trend of initiation potentials of chemisorbed oxygen formation, suggesting a bifunctional mechanism. In infrared reflectance−absorption spectra of the CO-adsorbed Rh x /Pt electrodes, a single asymmetric band assigned to "atop" CO was observed irrespective of the Rh coverage, and shifted to lower wavenumbers as the Rh coverage was increased. The d-band center estimated from the valence level spectra of the Rh x /Pt electrodes, which reflected the Pt 5d electronic structure, shifted downward as the Rh coverage was increased.

A couple of toluene (TL) and its hydrogenation product, methylcyclohexane (MCH), are promising high-density hydrogen carriers to store and transport large amounts of hydrogen. Electrochemical hydrogenation of TL to MCH can achieve energy savings compared with hydrogenation using molecular hydrogen generated separately, and development of highly active catalysts for electrochemical TL hydrogenation is indispensable. In this study, binary Pt3M (M = Rh, Au, Pd, Ir, Cu and Ni) alloy nanoparticle-loaded carbon catalysts were prepared by a colloidal method, and their activity for electrochemical TL hydrogenation was evaluated by linear sweep voltammetry. Each Pt3M electrode was initially activated by 100 cycles of potential sweep over a potential range of 0–1.2 or 0.8 V vs. reversible hydrogen electrode (RHE). For all activated Pt3M electrodes, the cathodic current density for electrochemical TL hydrogenation was observed above 0 V, that is the standard potential of hydrogen evolution reaction. Both specific activity, cathodic current density per electrochemical surface area, and mass activity, cathodic current density per mass of Pt3M, at 0 V for the Pt3Rh/C electrode were the highest, and about 8- and 1.2-times as high as those of the commercial Pt/C electrode, respectively, which could mainly be attributed to electronic modification of Pt by alloying with Rh. The Tafel slope for each activated Pt3M/C electrode exhibited the alloying of Pt with the second metals did not change the electrochemical TL hydrogenation mechanism.

Introduction Organic hydrides are known to be hydrogen carriers to achieve large-scale storage/transportation system of hydrogen. Especially, a couple of toluene (TL) and its hydrogenation product, methylcyclohexane (MCH), is promising because they are liquids at ambient temperature and pressure to use existing infrastructure and MCH has high hydrogen storage density (6.1 wt.%). In general, MCH is regenerated by the hydrogenation of TL with molecular hydrogen on appropriate catalysts, and another chemical or electrochemical process is required for the production of molecular hydrogen. However, electrochemical hydrogenation of TL to MCH is one-pot process, in which the electrochemical generation of atomic hydrogen by one-electron reduction of H+ and the hydrogenation of TL with atomic hydrogen consecutively proceed on the same catalyst surface. The standard potential of the electrochemical TL hydrogenation process is more positive than that of hydrogen evolution reaction, indicating the former is more energy-saving.1, 2 Pt nanoparticles loaded on carbon (Pt/C) are active for electrochemical hydrogenation of TL to MCH.1, 2 Recently, it has been found in our group that Rh-modified Pt/C (Rh/Pt/C) catalysts exhibited higher current for electrochemical hydrogenation of TL than Pt/C. In order to evaluate the specific activity and investigate the relationship between electrochemical hydrogenation activity and Rh loading or Rh coverage (θ Rh), it is necessary to separately evaluate the electrochemical surface areas (ECSAs) of Pt and Rh, but it is difficult because the potential region of hydrogen adsorption/desorption for Pt and Rh overlaps. In this study, we have succeeded in separately evaluating the ECSAs of Pt and Rh for Rh/Pt/C catalysts with different quantities of deposited Rh by using the difference in CO stripping potential between Pt and Rh, and estimated the Rh coverage (θ Rh) to discuss the relationship with catalytic activity. Experimental The commercial Pt/C (Tanaka Kikinzoku Kogyo, TEC10E50E) powder was dispersed in ultrapure water. Different concentrations (1.2, 2.4, 3.6 and 4.8 mM) of rhodium chloride aqueous solutions were added into the dispersion, followed by hydrogen bubbling for 1 h at 30 °C. The final black powder is named Rh0.19/Pt/C, Rh0.38/Pt/C, Rh0.56/Pt/C and Rh0.76/Pt/C hereafter. For the preparation of a catalyst electrode, each catalyst ink was cast on a GC substrate (diameter: 5 mm) and dried overnight to make a working electrode. The counter electrode, reference electrode and electrolyte were a platinized Pt electrode, reversible hydrogen electrode (RHE) and 0.5 M H2SO4 aqueous solution, respectively. For the ECSA evaluation, CO was saturated into the electrolyte solution at 0.30 V. After removing dissolved CO by Ar-bubbling, the electrode potential was stepped from 0.30 to 0.75 V, and kept at this potential for 15 s to preferentially desorb CO adsorbed on the Rh surface. After that, cyclic voltammogram was measured between 0.05 and 1.2 V. Results and Discussion The θ Rh increased with the Rh/Pt mole ratio for Rh/Pt/C catalysts, and stagnated as the Rh/Pt mole ratio excessed 0.38 (Fig. 1). However, the fraction of ECSA of Rh (ECSARh) to total ECSA of Pt and Rh (ECSARh/Pt/C) continued to increase with the Rh/Pt mole ratio, suggesting that Rh was preferentially accumulated on deposited Rh as θ Rh reached ca. 0.2. The Tafel slope of the electrochemical hydrogenation of TL on the Rh0.19/Pt/C, Rh0.38/Pt/C, Rh0.56/Pt/C and Rh0.76/Pt/C catalysts was ca. 34, 34, 33 and 32 mV dec-1, respectively. Each Tafel slope was close to that for the Pt/C catalyst, suggesting that electrochemical TL hydrogenation on Rh/Pt/C follows the same reaction mechanism and rate-determining step, which is the addition reaction of adsorbed TL with atomic hydrogen. The change in hydrogenation current per ECSAPt (j Pt) at 0 V vs. RHE with the Rh/Pt mole ratio showed the similar tendency to that in θ Rh (Fig. 1), suggesting that the electrochemical hydrogenation of TL preferentially occurs at the interface between Pt and Rh. Galvanostatic electrolysis of 30 vol.% TL with the Pt/C and Rh/Pt/C catalysts at −200 mA cm−2 was carried out with polymer electrolyte membrane electrolysis cell. It was found by GC/MASS analysis that sole reaction product in the electrochemical hydrogenation of TL was MCH. The faradaic efficiency for MCH production and conversion of TL to MCH were also increased with the increase of θ Rh, and the side reaction, hydrogen generation, was suppressed. These results clearly indicate that the Rh/Pt/C catalysts are superior to Pt/C. References 1 Y. Bao, T. W. Nappom, K. Nagasawa, S. Mitsushima, Electrocatalysis, 10, 184 (2019). 2 S. Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiko, Electrocatalysis, 7, 127 (2016). Figure 1