In situ studies of supported rhodium catalysts

Abstract

Rhodium catalysts have been prepared, supported on γ-alumina, vanadium(III) oxide, chromia and molybdena. The activity and selectivity of these materials in the conversion of synthesis gas, (H₂/CO: 1/1, 523 K and 5 bar pressure), to methanol, ethanol and hydrocarbons has been studied. All catalysts showed similar activity except for chromia, which was almost inactive at the temperature chosen. Selectivity to higher oxygenates followed the trend: V₂O₃ > MoO₃ > Al₂O₃ > Cr₂O₃. The alumina supported catalysts had the highest selectivity to methanol. Catalysts have been characterised in situ by extended X-ray absorption fine structure (EXAFS) and the alumina-supported material has been examined in the greatest detail. On reduction at 473 K, rhodium particles with diameter < 1 nm are observed by electron microscopy, and EXAFS indicates that these contain ca. 10 rhodium atoms on average. There is good evidence that these particles may be bonded to oxygen of the support or residual chlorine from the impregnation procedure. X-ray photoelectron spectroscopy shows that the surface stoichiometry is ca. Cl/Rh: 1.3/1.

On exposure to carbon monoxide at room temperature the metal particles break up completely and form entities with the composition (CO)₂·Rh·Cl₂ and bond lengths similar to those in crystalline Rh₂Cl₂(CO)₄. On exposure to synthesis gas at 323 K metallic particles are partly reformed.

Under conditions of catalytic relevance only metallic particles are observed although some Rh—Cl bonding persists on alumina. No rhodium–chlorine bonding is observed on vanadia-supported catalysts, although the metal particles have a similar average size to those found on alumina. There is some evidence that the particle size is much less uniform on vanadia than on alumina. Exposure to synthesis gas causes small changes in particle parameters, which may be related to the chemisorption of carbon monoxide. The average particle size is much larger, > 30 Å diameter, on the chromia support.

The relation between catalyst structure and performance is discussed.