The effect of specific chloride adsorption on the electrochemical behavior of ultrathin Pd films deposited on Pt(1 1 1) in acid solution

The electrochemical behavior of thin Pd films supported on a Pt(1 1 1) electrode is investigated by cyclic voltammetry and in situ Fourier transform infrared (FTIR) spectroscopy. It is demonstrated that in perchloric acid solution underpotential deposition of hydrogen (Hupd) and hydroxyl adsorption (OHad) is in strong competition with the adsorption of Cl− anions, the latter being present as a trace impurity in HClO4. The interaction of Cl− with Pd is rather strong, controlling the adsorption of Hupd and OHad as well as the kinetic rate of CO oxidation. The microscopic insight (the binding sites) of the adsorbed CO (COad) and the rate of CO oxidation (established from CO2 production) on Pt(1 1 1) modified with a (sub)monolayer of Pd is elucidated by means of Fourier infrared (FTIR) spectroscopy. The appearance of both the characteristic Pt(1 1 1)–COad and Pt(1 1 1)–1 ML Pd–COad stretching bands on a Pt(1 1 1) surface covered by 0.5 ML Pd confirms previous findings that the Pd atoms agglomerate into islands and that the bare Pt areas and the Pd islands behave according to their own surface chemistry. The systematic increase of the Pd surface coverage results in a gradual change in the catalytic properties of Pt(1 1 1)–xPd electrodes towards CO oxidation, from those characteristic of bare Pt(1 1 1) to those which are characteristic for Pt(1 1 1) covered with 1 ML of Pd.