HREM analysis of the Nanostructure/Oxygen Buffering Capacity relationship in a Rh/CeTbO_x catalyst

摘要

A HREM investigation of the nanostructure of a 0.5% Rh/Ce_(0.8)Tb_(0.2)O_2 catalyst after a series of Oxygen Buffering Capacity (OBC) essays performed in the temperature range 473K- 773K has been carried out. The chemical changes suffered by the nanosized particles containing the noble metal, under the action of the pulsing 5% O_2/He atmosphere employed to run the OBC tests, have been monitored. Results indicate that in the experiments run at low temperature, 473K, rhodium is present in the catalyst as metallic nanoparticles. OBC experiments at 573K induce the transformation of the metal into an orthorrombic-Rh_2O_3 phase. In the HREM images obtained on the catalysts tested at the highest temperature, 773K, particles of both orthorrombic-Rh_2O_3 and tetragonal-RhO_2 have been detected. The stepwise increase in the oxidation state of rhodium revealed by the HREM study correlates quite well with the increase in the ability of the catalyst to damp down sudden changes in oxygen pressure. Further HREM results obtained from a sample coming out from an OBC experiment at 473K performed with a catalyst preoxidised at 773K suggest, nevertheless, that the bulk composition of the rhodium particles is not the unique factor to obtain high OBC values.