Structure-Controlled Rh/Sn/Pt Ternary Catalysts for Complete Oxidation Reaction of Ethanol to Carbon Dioxide

Introduction

            In direct ethanol fuel cells (DEFCs), the combination of Pt, Sn, and Rh in a ternary catalyst has been proved to be efficient for the ethanol oxidization reaction (EOR).¹ However, the arrangement of the three elements is significant for the complete EOR to produce CO₂. In this study, we prepared the two kinds of Rh/Sn/Pt electrodes in which each component was finely arranged, and evaluated their EOR activity and CO₂ selectivity by electrochemical and spectroscopic methods.

Experimental

            The preparation of Rh/Sn/Pt electrodes was conducted in a three-electrode cell with a Pt plate (surface area: 1 cm²) as the counter electrode (CE), reversible hydrogen electrode (RHE) as reference electrode, and a Pt rod substrate (diameter 5 mm, height 5 mm) as the working electrode (WE). In an Ar-saturated 0.02 mM SnCl₂/0.1 M H₂SO₄ solution, a potential of -0.2 V was applied to the cell for 30 s to obtained Sn/Pt. Then the cell was rinsed off with ultrapure water and moved to a CO-saturated 0.1 M H₂SO₄ solution.

In the first method, Rh was directly deposited on the Sn/Pt electrode by H-upd method. At first, atomic hydrogen was adsorbed on the bare Pt area, and then RhCl₃ solution was injected to the cell, following by thoroughly stirring, which is named Rh/Sn/Pt(1).

In the second one, Rh deposition site was controlled by CO stripping in a limited potential range. CO was first adsorbed on the Pt surface of Sn/Pt, and CO adsorbed on Pt neighboring to Sn was selectively desorbed by the limited CO stripping. Then Rh was deposited on the catalyst by the same process as in the first method, which is named Rh/Sn/Pt(2). The resultant electrodes were washed thoroughly with ultrapure water.

Cyclic voltammetry was performed to evaluate the electrochemical surface area (ECSA) of each electrode in 0.1 M H₂SO₄, and linear sweep voltammetry (LSV) in 1 M C₂H₅OH/0.1 M HClO₄ was done to evaluate EOR activity. Products and intermediates of EOR were identified by infrared reflection absorption spectroscopy (IRRAS).

Results and discussion

            The X-ray photoelectron spectroscopy showed the main valence of Sn in the Sn/Pt electrode was IV (SnO₂), whereas the valences of Pt and Rh were 0 (metallic). From cyclic voltammograms of the Sn/Pt, Rh/Sn/Pt(1) and Rh/Sn/Pt(2) electrodes, the coverage of Sn on the Pt surface was 0.23; and the Rh coverage was 0.09 and 0.04 for Rh/Sn/Pt(1) and Rh/Sn/Pt (2), respectively. Figure 1 illustrates that Rh/Sn/Pt(1) gave the highest current density toward EOR, followed by Rh/Sn/Pt(2) and Sn/Pt, which have quite similar EOR performance, and the Pt electrode had low activity. Figure 2 shows the IRRAS spectra of the Sn/Pt, Rh/Sn/Pt(1) and Rh/Sn/Pt(2) electrodes. The Rh/Sn/Pt(1) electrode facilitated the formation of acetic acid (1277 cm^-1) as well as CO₂ (2343 cm^-1). In contrast, the Rh/Sn/Pt(2) electrode exhibited the peak intensity of formic acid and CO₂ quite similar to the Sn/Pt electrode.

References

1. A. Kowal, M. Li, M. Shao, K. Sasaki, M.B. Vukmirovic, J. Zhang, N.S. Marinkovic, P. Liu, A.I. Frenkel and R.R. Adzic. Nature Materials, 8, 325 (2009).