Structural changes of Rh-Mn nanoparticles inside carbon nanotubes studied by X-ray absorption spectroscopy

Supported Rh-based catalysts such as Rh-Mn nanoparticles (NPs) have potential use in the synthesis of ethanol from syngas. The structure of Rh-Mn NPs in multi-walled carbon nanotubes under different atmospheres and temperatures was studied by X-ray absorption spectroscopy (XAS). TEM images showed that the NPs dispersed in the carbon nanotubes had a uniform size of 2 nm. XAS data revealed that the Rh-Mn NPs before reduction were composed of Rh2O3 clusters and mixed Mn oxide species. After reduction in a 10% H-2-90% He atmosphere, the mixed Mn oxides were converted into nearly pure MnO. In contrast, the Rh2O3 clusters were easily decomposed to metallic Rh clusters even under a He atmosphere at 250 degrees C. The Rh clusters remained in the metal state under the next reduction atmosphere, but their dispersion in the Rh-Mn NPs increased with increasing temperature. No significant Mn-Rh or Mn-O-Rh interaction in the reduced NPs was observed in the extended X-ray absorption fine structure analysis. The results showed that there was no interaction between the MnO particles and Rh clusters and the role of the Mn promoter was mainly to improve Rh dispersion. (C) 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.