The oxidation of ethylene over evaporated palladium-rhodium alloy films: II. Variation of activity with film composition and structure

Abstract

Rates of CO 2 formation when ethylene was completely oxidized in a static system over evaporated PdRh alloy films showed a complex variation with alloy composition. This activity pattern generally reflected the variations in activation energy, E, with composition; values of E and log (frequency factor) fell on a linear plot, indicating a compensation effect, with the exception of pure Rh which is discussed. The activity pattern can be interpreted in terms of the film structure and properties occurring in the three composition ranges distinguished as follows: 0–30% Rh; increased activity compared with pure Pd, expected from bulk electronic properties and previous results with PdAg alloy films, was not observed. Instead the activity passes through a minimum at 10–15% Rh. It is believed that at this composition the alloy has a reasonable capacity to dissolve hydrogen (from C 2H 4) which would reduce the activity by filling d-band holes, combined with a decreased ability to effect H 2O 2 combination. An alternative explanation, based on the onset of phase-separation at the surface at lower Rh contents than expected, was tested by CO oxidation. 30–80% Rh; the activity decreased continuously with increasing Rh content in alloys composed of two “phases,” the Rh-rich Phase II is believed to form the kernel of the crystallites, surrounded by a variable-composition Phase I. The activity results were re-plotted assuming that the surface composition can be equated with the composition of Phase I given by X-ray data in Part I. Apparently anomalous results fitted into place and a maximum in activity below 30% Rh, which would occur in the absence of hydrogen solubility, was indicated. 80–100% Rh; some enrichment of the surface by Pd may occur; the decrease in activity between 98% and pure Rh is discussed, as well as its very low absolute activity.