Formation of small rhodium metal particles on the surface of a carbon support

Abstract

A rhodium catalyst supported on a Sibunit graphitized carbon carrier was studied by in situ XAFS spectroscopy. A comparative study of the reduction of rhodium was performed for the following two samples: Rh/C(120) dried at 120°C and Rh/C(350) calcined at 350°C. EXAFS data showed an absence of carbon atoms within the nearest environment of rhodium atoms in the Rh/C(120) uncalcined sample, which implies the absence of direct interaction between rhodium and the carbon support. In the course of the reduction of this sample (200°C), coarse particles with small metal cores were initially formed. These metal particles rapidly agglomerated upon the complete reduction of rhodium (350°C). These reduction of the Rh/C(350) calcined sample at 100–500°C resulted in the formation of small metal particles early in the reduction (100°C). The high dispersity of these particles was retained as the temperature of treatment in hydrogen was increased to 500°C due to metal-support interaction. The conversion of benzene into cyclohexane on the Rh/C(350) catalyst containing small rhodium particles was much higher at the same temperature of hydrogenation.