In situ monitoring of oxide-supported platinum-group metal catalysts by energy dispersive EXAFS

Energy dispersive EXAFS (EDE), using a Laue monochromator on ID24 of the ESRF, and multiple ion monitoring mass spectrometry have been used to monitor the structure and function of alumina-supported rhodium and palladium catalysts within a microreactor. Metallic 5%-Rh/Al₂O₃ was preformed in situ by hydrogen reduction, and shown to react rapidly with NO at room temperature yielding an oxidised phase. In order for this material to catalyse the reduction of NO by H₂ (to N₂), it was shown that the metallic phase must be reformed. The "light-off" temperature at which the metallic phase was formed and catalysis commenced varied with NO : H₂ stoichiometry, this temperature lowering in more reducing conditions. N₂O formation was concomitant with the conversion from the oxidised to the metallic phase.

The EDE configuration allowed the simultaneous monitoring of structure at both the rhodium and palladium K-edges. Incorporation of a 20% palladium mole fraction into 5%-Rh-Pd/Al₂O₃ protected rhodium from oxidation and also reduced the light-off temperature, and this is reflected in low coordination number values for the composite Pd to Rh/Pd shell. This favours palladium being preferentially on the surface of the nanoparticles. At higher temperatures there are changes in coordination numbers than imply a redistribution of metal sites under catalytic conditions. Future plans to include infrared spectroscopy as a third simultaneous technique will be outlined.