Catalytic and electro-catalytic oxidation of formic acid on the pure and Cu-modified Pd(111)-surface

The electrooxidation of formic acid, potentially important for future fuel cells, was investigated by means of cyclic voltammetry (CV) and in situ infrared reflection absorption spectroscopy (IRRAS) both on a pure and on a copper-modified Pd(111)-electrode in sulphuric acid solutions. In situ IR spectra recorded under open-circuit conditions exhibit several vibrational bands in the carbonyl region characteristic of "free" and adsorbed formic acid as well as of decomposition intermediates. A detailed analysis of the intensity of the bands as a function of time leads to a reaction mechanism for the mere "catalytic" oxidation of formic acid at the Pd(111)/electrolyte interface. The "electro-catalytic" oxidation of formic acid under potential control starting above 0.35 V is investigated by following the evolution of carbon dioxide as a function of electrode potential. IR measurements at different but constant potentials point to an electronic structure of the solid/liquid interface in the open-circuit HCOOH/Pd(111)-H2SO4 system which resembles that at an electrode potential of about 0.4 V. Modification of the Pd(111) surface is achieved by copper deposition from a Cu2+/HCOOH/H2SO4 solution. Simultaneously, the influence of the foreign metal on the electro-catalytic oxidation of formic acid is studied in dependence on the potential scan direction. In the positive-going scan copper deposited on Pd(111) at cathodic potential inhibits the electro-catalytic oxidation of formic acid below 0.45 V by blocking catalytically active Pd sites. Conversely, during the negative-going scan the Cu2+/HCOOH/Pd(111)H2SO4 system shows an increased oxidation activity between 0.6 V and 0.2 V due to a direct redox reaction between copper ions and formic acid. The amount of copper deposited on the surface was determined by ex situ Auger Electron Spectroscopy after a contamination free sample transfer into UHV. (c) 2008 Elsevier B.V. All rights reserved.