EXAFS Determination of the Change in the Structure of Rhodium in Highly Dispersed Rhodium/gamma-Aluminum Oxide Catalysts after Carbon Monoxide and/or Hydrogen Adsorption at Different Temperatures

Abstract

Extended X-ray absorption spectroscopy (EXAFS) has been applied to study the Rh K-edge of two ultradisperse Rh/Al{2}O{3} catalysts containing 0.47 and 1.04 wt.% rhodium respectively. The structural properties of the Rh crystallites were determined after reduction with H{2}, evacuation at elevated temperatures, CO admission at room temperature, CO desorption, CO admission at 523 K (at which temperature the Boudouard reaction takes place) and after hydrogenation of CO at 523 K.During reduction of the catalyst, metallic rhodium crystallites are formed which are attached to the support via Rh}0{-O}2{}-{ bonds, implying zero-valent rhodium. The Rh-Rh nearest-neighbour distance in the particles is contracted to 2.63 @9 in vacuum and relaxes to 2.68 @9 (similar to the bulk value) upon hydrogen chemisorption. Adsorption of CO at room temperature onto the reduced catalysts results in a partial disruption of the small Rh crystallites, ultimately leading to rhodium geminal dicarbonyl species. A subsequent desorption of CO leads to rearrangement of the metal clusters. The oxidation state of rhodium alters from 0 to 1+ upon CO chemisorption, and becomes 0 again after CO desorption. After CO admission at 523 K, the rhodium particles break up completely, probably because of intercalation of carbon and/or oxygen atoms. The highly dispersed Rh/Al{2}O{3} catalysts change to poorly dispersed systems after reaction between CO and H{2} at 523 K, due to sintering of the metallic crystallites.