Structural effects on the oxidation of formic acid on the high index planes of palladium

Electrochemical oxidation of formic acid has been studied on the stepped and kinked-stepped surfaces of Pd in 0.1 M HClO₄ containing 0.1 M formic acid with the use of voltammetry. The surfaces examined are Pd(S)-n(1 0 0) × (1 1 0)], Pd(S)-n(1 1 1) × (1 0 0)] and Pd(S)-n(1 1 1) × (1 1 1)] series (n = 2–9). The results are compared with those of Pd(S)-n(1 0 0) × (1 1 1)] series reported previously. All the electrodes give maximum currents of formic acid oxidation j_P between 0.5 and 0.8 V (RHE). The values of j_P plotted against the density of step (kink) atoms d_S depend on the surface structure remarkably. Pd(S)-n(1 1 1) × (1 0 0)] surfaces provide maximum of j_P at n = 5, whereas Pd(S)-n(1 0 0) × (1 1 0)] and Pd(S)-n(1 1 1) × (1 1 1)] do not give maximum of j_P. The values of j_P have the following order: Pd(S)-n(1 1 1) × (1 1 1)] < Pd(S)-n(1 1 1) × (1 0 0)] < Pd(S)-n(1 0 0) × (1 1 0)] < Pd(S)-n(1 0 0) × (1 1 1)]. The anodic current at more negative potential 0.20 V (RHE) shows different activity series: Pd(1 1 1) and Pd(1 1 0) have the highest rate for formic acid oxidation at 0.20 V (RHE).