Characterization of rhodium oxide nanoparticles in MCM-41 and their catalytic performances for NO–CO reactions in excess O2 Original Research Article

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 RhOx with the size <3 nm has a hexagonal structure with a RhRh bond distance of 0.273 nm, while the nanoparticles of RhOx with 6–8 nm size possessed a distorted orthorhombic structure without any long range order. The RhOx 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 O2 on the conversion of NO to N2 and N2O in the catalytic NO–CO reaction. The rhodium oxides of 6–8 nm size located in the bulk of MCM-41 and a supported RhOx/MCM-41 catalyst showed poisoning effects of excess O2 on the NO and CO conversions. The promotional effects of O2 observed for the catalyst with RhOx nanoparticles in the mesopores were attributed not to the small size of RhOx, but to the location of RhOx in the MCM-41 channel.