The Adsorption Isobar of Dioxygen on Rhodium Powders

Abstract

Isobar oxidations of rhodium powders with dioxygen in the temperature range 77–473 K have been studied gravimetrically. The uptake of oxygen was found to increase with oxidation temperature. However, levelling off of the O/Rh^s stoichiometry at a value of 1.0 was found between 232 K and 300 K, indicating monolayer chemisorption of oxygen on the rhodium surface. At higher temperatures, the stoichiometry increased above unity because of sublayer and bulk oxidations. Dioxygen chemisorption at 300 K was subsequently used to estimate the dispersion of rhodium catalysts assuming O/Rh^s = 1.0, the value obtained agreeing well with that evaluated from measurements of the broadening of the X-ray diffraction line for a 10 wt.% Rh/Al₂O₃ catalyst and for the modified deuterium uptake of a 1.5 wt.% Rh/Al₂O₃ sample. Such agreements suggest that dioxygen chemisorption provides a direct and dependable method for estimating the particle size of supported rhodium catalysts over a wide dispersion range.