Catalytic decomposition of acid hydrogen peroxide solutions on platinum, iridium, palladium and gold surfaces

Abstract

The catalytic decomposition of acid H2O2 solutions (10−4 to 10−1M) in presence of platinum, iridium, palladium and gold has been studied. In H2SO4 solution, platinum, iridium and palladium are catalytically active; gold is not active. In HCl solution, none of the above metals is active. The H2O2 decomposition is a first order reaction with the following kinetic constants: k = 6·4 × 10−5/sec for platinum; k = 1·2 × 10−5/sec for iridium; k = 0·34 × 10−5/sec for palladium; all at 25°C and referred to a 10 cm2 metal surface. The surface of the electrode becomes practically covered by a monomolecular layer of oxide during the catalytic H2O2 decomposition in acid solution, as shown by anodic and cathodic polarization curves, anodic decay curves and anodic and cathodic charging curves. The metal loses its catalytic activity when it cannot be covered by an adsorbed oxide layer, as when it forms complexes with the solution: for instance, Pt and Ir in HCl solutions or Au in H2SO4 solutions. The different rate of oxide formation by H2O2 and of the reaction between oxide and H2O2 to develop O2 explains the differences of catalytic activity observed between platinum, iridium and palladium. The process of catalytic decomposition of H2O2 acid solutions necessarily involves the intervention of oxides formed on the electrode surface. A kinetic mechanism is proposed to explain the process of H2O2 decomposition, taking into account the independence of the static potential of the H2O2 concentration.