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

Abstract

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 (Rhx/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 Rhx/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 Rhx/Pt electrodes exhibited noteworthy CO-stripping and CO-adsorption 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 Rhx/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 Rhx/Pt electrodes, which reflected the Pt 5d electronic structure, shifted downward as the Rh coverage was increased.