Characterization of rhodium oxide nanoparticles in MCM-41 and their catalytic performances for NO–CO reactions in excess O 2

Abstract

Nanoparticles of rhodium oxides were prepared in the MCM-41 mesoporous molecular sieve and were characterized by XRD, TEM, H2-TPR and EXAFS. It was found that the particle size, location and structure of the rhodium oxide particles were greatly influenced by the hydrothermal synthesis conditions. The Rh K-edge EXAFS study revealed that the nanoparticles of RhO x with the size <3 nm has a hexagonal structure with a RhRh bond distance of 0.273 nm, while the nanoparticles of RhO x with 6–8 nm size possessed a distorted orthorhombic structure without any long range order. The RhO x nanoparticles were employed as catalysts for NO–CO reactions. The rhodium oxides with <3 nm particles located in the mesopore channels of MCM-41 exhibited a promoting effect of excess O 2 on the conversion of NO to N 2 and N 2O in the catalytic NO–CO reaction. The rhodium oxides of 6–8 nm size located in the bulk of MCM-41 and a supported RhO x /MCM-41 catalyst showed poisoning effects of excess O 2 on the NO and CO conversions. The promotional effects of O 2 observed for the catalyst with RhO x nanoparticles in the mesopores were attributed not to the small size of RhO x , but to the location of RhO x in the MCM-41 channel.