Effect of Rhodium Modification on Activity of Platinum Nanoparticle-Loaded Carbon Catalysts for Electrochemical Toluene Hydrogenation

Abstract

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.

Effect of Rh Coverage on CO-Adsorption and -Stripping Behaviors of Rhodium-Adlayer-Modified Platinum Electrodes

“…To check whether Cu adatoms are completely exchanged with Rh atoms in the present galvanic replacement conditions, the amount of Rh substituted for Cu adatoms deposited at an E upd of 0.35 V was quantified by atomic absorption spectroscopy according to the previous Rh quantification method . Consequently, it was determined as 1.8 nmol cm –2 .…”

“…62,63 To check whether Cu adatoms are completely exchanged with Rh atoms in the present galvanic replacement conditions, the amount of Rh substituted for Cu adatoms deposited at an E upd of 0.35 V was quantified by atomic absorption spectroscopy according to the previous Rh quantification method. 54 Consequently, it was determined as 1.8 nmol cm −2 . Since the amount of Cu adatoms evaluated from the charge for Cu-upd was 2.7 nmol cm −2 , the mole ratio of the replaced Rh to Cu adatoms is 2/3, indicating that the Cu adatoms on the Pt electrode were quantitatively replaced with rhodium.…”

“…Bimetallic Pt–Rh catalysts have also been used to catalyze the oxidation of CO, ,− formic acid, ,, and methanol − and to catalyze hydrogenation. − Their catalytic activities were often evaluated on the basis of mass activity, defined as the current per total mass of Pt and Rh, because it was difficult to evaluate the ECSAs of Rh and Pt individually using hydrogen adsorption/desorption peaks. We recently found that the catalytic activity of carbon-black-supported Pt nanoparticles toward the electrochemical hydrogenation of toluene was significantly enhanced by Rh modification . Moreover, we succeeded in determining the ECSA and coverage of Rh on Pt nanoparticles using the CO-stripping technique, which gave a more quantitative understanding of the synergistic effect of using Rh together with Pt.…”

“…We recently found that the catalytic activity of carbon-black-supported Pt nanoparticles toward the electrochemical hydrogenation of toluene was significantly enhanced by Rh modification. 54 Moreover, we succeeded in determining the ECSA and coverage of Rh on Pt nanoparticles using the CO-stripping technique, which gave a more quantitative understanding of the synergistic effect of using Rh together with Pt. Consequently, it was found that an increase in the Rh coverage resulting from an extension of the Rh surface in two dimensions contributed to an increased rate of electrochemical toluene hydrogenation, which was attributed to an increase in the rate of addition of H ads generated on Rh to toluene adsorbed on Pt at the extended interface between Rh and Pt, but the growth of Rh deposits in three dimensions did not contribute to an increased rate of electrochemical toluene hydrogenation, which has not yet been explained.…”

One‐Pot Synthesis of Tensile‐Strained PdRuCu Icosahedra toward Electrochemical Hydrogenation of Alkene

“…[8] Subsequently, H ads are added to unsaturated bonds (e. g., C=C) or consumed through hydrogen evolution reaction (HER) path. [9][10][11] The key to improve ECH performance is to intensify the adsorption of unsaturated chemical bonds on catalyst surface as well as weaken the PdÀ H binding to facilitate H ads transfer and addition to substrate. Previous studies have indicated that tensile strain could upshift the d-band center to intensify the species adsorption (e. g., C=C) to speed up ECH.…”