Elsevier

Electrochimica Acta

Volume 43, Issue 24, 21 August 1998, Pages 3631-3635
Electrochimica Acta

Electrooxidation of H2, CO and H2/CO mixtures on a well-characterized Pt–Re bulk alloy electrode and comparison with other Pt binary alloys

https://doi.org/10.1016/S0013-4686(98)00120-0Get rights and content

Abstract

We report new results for the electrooxidation of H2, CO and H2/CO mixtures on a Pt75Re25 bulk alloy using surface preparation in a UHV chamber and characterization by X-ray photoelectron spectroscopy (XPS). Sputter-cleaned but not-annealed surfaces having the bulk composition were used. This alloy surface has unique properties for CO electrooxidation: finite current densities below 0.2 V (rhe), a positive reaction order in CO partial pressure of +0.5, but an unusually high slope in the log i vs E curve, about 220 mV per decade. Anodic stripping of the irreversibly adsorbed CO on this surface also occurs at an unusually low potential, initiating at about 0.15 V, but only about 1/3 of the total amount of CO on the surface is oxidized at 0.15–0.4 V, the remainder being oxidized at 0.6–0.8 V. The polarization curves for the oxidation of H2/CO mixtures containing 0.1–2% CO are, however, very Pt-like. It appears that the low-potential oxidation of both dissolved CO and adsorbed CO on this surface occurs exclusively at Re sites, so that with the H2/CO mixture the Pt sites remain fully covered (and thus deactivated) by COads.

Introduction

A major problem in the development of low-temperature fuel cells for transportation applications is the deactivation of the Pt anode catalyst by even trace levels, e.g. 10–100 ppm of carbon monoxide (CO). Consequently, in order to use hydrogen generated on board the vehicle by steam reforming a hydrocarbon fuel, there must be development of alternative catalysts that are tolerant to CO in hydrogen at these levels (and preferably higher). In recent studies in this laboratory1, 2, 3, 4, 5, we have investigated the oxidation of H2, CO and their mixtures on Pt, Ru, Pt–Ru alloys2, 3, Pt3Sn1, 4and Pt70Mo30[5]all as solid electrodes prepared and characterized in UHV and transferred into a rotating disk electrode assembly for kinetic evaluation of the reaction rates. In the present communication, we report new results for the electrooxidation of H2, CO and H2/CO mixtures on a Pt75Re25 bulk alloy using identical methodology, and compare the results with those from the other alloy surfaces. Although the study is by no means definitive, the results point to uniquely different and surprising properties of this alloy surface for the oxidation of irreversibly adsorbed CO. Unfortunately (from a technological perspective), these properties do not appear to impart enhanced CO-tolerance. However, understanding the chemistry behind the unique property of this surface for the oxidation of adsorbed CO could lead to the discovery of new catalysts that do impart enhanced CO-tolerance.

Section snippets

Experiment

The polycrystalline Pt–Re alloy was prepared as bulk crystal by arc-melting of the pure elements in an argon atmosphere and a homogenizing heat treatment. Details of the preparation procedure, which was the same as used for Pt–Ru alloys, are given in Ref.[2]. The nominal composition of the alloy from the weight of the pure metals was 25.0 at.% Re. X-ray diffraction analysis confirmed the formation of a single-phase fcc structure with lattice constant 0.3895 nm, in agreement with the lattice

Cylic voltammetry

The cyclic voltammetry of the sputter-cleaned alloy in de-aerated 0.5 M sulfuric acid at 333 K is shown in Fig. 1. The voltammetry with the positive potential kept below 0.4 V exhibits Pt-like hydrogen adsorption/desorption pseudocapacitance, but without the resolution of “weakly” and “strongly” adsorbed states. Significantly, the Pt75Re25 alloy surface does not have the large pseudocapacitance in the 0.3–0.45 V region characteristic of the Pt75Mo25 alloy surface[5]; in the latter case, this large

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Advanced Automotive Technologies, of the U.S. Department of Energy under contract No. DE-AC03-76SF00098.

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