(Invited) Pt/Rh/SnO2 Catalysts for Selective Ethanol Oxidation Reaction to CO2
Authors: Inoue Hiroshi , Chiku Masanobu , Higuchi Eiji 
Published: February 2020

Introduction

Direct ethanol fuel cells (DEFCs) using bioethanol are eco-friendly devices like polymer electrolyte fuel cells because they do not increase CO2. Ethanol is nontoxic, and superior in transportation, storage, and handling to hydrogen. However, ethanol is slowly and incompletely oxidized due to the hardness of C-C bond cleavage. Binary PtSn alloy and Pt/SnOx catalysts are known to be the best anode catalysts for ethanol oxidation reaction (EOR). In these catalysts Pt facilitated the adsorption and dissociation of ethanol, while Sn provided OH species for the oxidation of CO on Pt. The main product, however, was not CO2, but acetaldehyde and acetic acid, the partial EOR products. PtRh was effective for the C-C bond cleavage to form CO2, although its EOR activity was low. The combination of Pt, Rh and Sn can enhance the EOR activity in addition to the merit of the PtRh catalyst. The improvement of EOR activity and CO2 selectivity with ternary Pt-Rh-Sn catalysts has been reported so far. The atomic arrangement at active sites, however, has not been discussed except for only a few papers. In this study we used a Pt electrode modified partly with SnO2, and preferentially deposited Rh on Pt adjacent to SnO2 by a limited CO stripping technique to realize selective EOR to CO2. Moreover, based on the results with the model electrodes, we prepared Pt/Rh/SnO2 nanoparticle catalysts, in which the position of the Rh atoms was controlled, and evaluated the CO2 selectivity.

Experimental

The ternary Pt/Rh/SnO2 electrode was prepared as follows.1 Sn was deposited on a Pt disk electrode at -0.2 V vs. RHE for 30 s in an Ar-saturated 0.2 mM SnCl2/0.1 M H2SO4 solution. The resultant Sn-deposited Pt (Pt/SnO2) electrode was moved into a CO-saturated 0.1 M H2SO4, to adsorb CO on the exposed Pt surface. After removing dissolved CO by Ar bubbling, CO on the exposed Pt atoms adjacent to SnO2 was removed by a potential sweep from 0.05 to 0.5 V. After that, an aliquot of 50 mM RhCl3/0.1 M H2SO4 was added to the solution, following by depositing Rh on the exposed Pt atoms at -0.2 V for 30 s. The resultant Rh-deposited Pt/SnO2 (Pt/Rh/SnO2) electrode was thoroughly washed with ultrapure water.
Pt/SnO2(3:1) nanoparticle-loaded carbon black was prepared according to ref. 2. The Pt/SnO2/C powder was sonicated in an Ar-saturated 0.5 M H2SO4 solution at 30 oC for 1 h to get the well-dispersed suspension. CO was adsorbed on the Pt surface of the Pt/SnO2 nanoparticles. Then the adsorbed CO on Pt neighboring to Sn was selectively desorbed while the potential was set at 0.5 V vs. RHE for 15 min. After adding 0.1 mM RhCl3 solution, Rh was deposited on the exposed Pt surface of the Pt/SnO2 nanoparticles with hydrogen.3

Results and Discussion

The X-ray photoelectron spectroscopy showed the main valence of Sn for the Pt/Rh/SnO2 electrode was IV (SnO2), whereas the valences of Pt and Rh were 0 (metallic). Cyclic voltammograms exhibited that the Pt/Rh/SnO2 electrode was about 4 and 1.5 times higher in EOR activity than the Pt and Pt/SnO2 electrode, respectively. In-situ infrared refraction-absorption spectroscopy (IRAS) elucidated that CO2 was selectively formed on Pt/Rh/SnO2, and acetaldehyde and acetic acid, which were mainly produced on Pt/SnO2, were scarcely formed, demonstrating that the arrangement of Rh atoms separating Pt and SnO2 is important for selective CO2 formation.
For the Pt/Rh/SnO2/C nanoparticle catalyst, the mean particle size was 2.8 nm, which is a little bit larger than that of Pt/SnO2C (2.5 nm). For EOR activity, the former was higher than the latter. In-situ IRAS CO2 and acetaldehyde were main products although acetic acid was the main product for the Pt/SnO2/C. The CO2 production increased with potential from 0.2 V, whereas the acetaldehyde did not depend on potential.

References

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M. P. Tu, M. Chiku, E. Higuchi, and H. Inoue, ECS Trans., 77(11), 1937 (2017).