The oxidation of ethylene over evaporated palladium-rhodium alloy films: I. Film structure with special reference to phase-separation

Abstract

Recent thermodynamic data for Pd-Rh alloys confirm earlier experimental observations that annealing for very prolonged periods below 850 °C causes separation into two FCC phases over a wide composition range. Palladium-rhodium alloy films, prepared by slow simultaneous evaporation of the component metals on to a glass substrate at 400 °C, show good bulk homogeneity (expected lattice constants, symmetrical X-ray diffraction profiles) at the ends of the composition range (0–30 and 80–100% Rh) but tend to phase-separation at intermediate compositions. After use as catalysts for ethylene oxidation, Pd-Rh alloy films with 30–80% Rh show clear evidence of two “phases” contributing to the diffraction profile; Phase I varied in composition and was deficient in Rh compared with the corresponding solid solution and Phase II had a composition of 88 ± 5% Rh in accord with the solubility diagram. Palladium-rich films showed strong (111) preferred orientation, weakening with increasing Rh content so that the crystallites in almost pure Rh films had approximately random orientation. The formation of Pd-Rh alloy films is considered with regard to the interpretation of the catalytic results according to composition: 0–30% Rh; diffusion during film formation and use is limited and inhibits the phase-separation indicated, giving a homogeneous alloy. Evidence was found for hydrogen solubility in alloy films with <10% Rh after carrying out ethylene oxidation with partial consumption of the reactants, indicating a subdivision of this composition range. (Films with up to ~30% Rh could be charged with hydrogen if all the oxygen was consumed, leaving unreacted ethylene.) 40–80% Rh; a mechanism is proposed involving the preferential nucleation of Rh whereby Phase II can form in conditions of limited diffusion as a Rh-rich kernel to the crystallite, surrounded by a Rh-deficient solid solution of variable composition. 80–100% Rh; the process continues but the “phases” are sufficiently close in composition to remain undetected.