Bulk alloy formation during Ge adsorption onto a Pd-powder catalyst

Abstract

The modification of Pd or Pt by a second, inactive metal component is a well-known method in catalysis by which to improve the selectivity at the expense of the activity in the hydrogenation of organic substrates. The promotion of Pd with Ge is one of the few exceptions where the bimetallic system can exhibit a higher activity than that of the noble metal alone [l-4]. However, the structure of the catalysts and the role of Ge is far from being understood. This prompted us to undertake a systematic study of the Ge + Pd system. The adsorption and electrocatalytic activity of Ge on Pt has been thoroughly investigated [5-91 but there are only few data available on the Ge + Pd bimetallic system [lo]. We have reported in a previous paper that the oxidation of Ge adatoms adsorbed on a palladized Pt electrode results in two peaks, i.e. for the ionization of weakly (about 0.5 V) and strongly (about 0.65 V) bound forms [lo]. Moreover, indirect evidence for bulk alloy formation during Ge adsorption has also been found. The hydrogen adsorption is suppressed by Ge deposition and a hindering effect on the hydrogen transport from the bulk of Pd has been observed at higher coverages. In general, only the outer two or three atomic layers take part in the catalytic and electrocatalytic processes. In the bulk processes, such as the hydrogen dissolution in the p phase or Ge + Pd alloy formation, the outer surface layer behaves like a ‘gate’, which has a strong effect on the rate of the process. It was supposed that a relatively high surface-to-volume ratio (high dispersion of Pd) is necessary for the study of Ge dissolution into the lattice of Pd. The electrodes usually contain several thousands of atomic layers, which corresponds to a dispersion of 0.001 or less. The dispersion of commercial carbon-supported Pd catalysts is