Understanding the nature of Pt-Rh synergy for three-way conversion catalysis

A series of Al2O3 supported Pt, Rh and Pt-Rh catalysts was investigated for TWC catalysis. Profound synergy between Pt and Rh was found on all Pt-Rh compositions relative to their single metal references with significantly reduced light-off temperatures. The nature of this synergy was probed using various characterization techniques. CO chemisorption, CO-DRIFTS and XPS results show that the surface Rh concentrations on aged Pt-Rh catalysts are much higher than that of the Rh reference. In addition, on the bimetallic catalysts the Rh species is in a more reduced state as indicated by XPS and XANES. AC-STEM/EDS results clearly illustrate that there are two types of Rh species on an aged Pt-Rh catalyst in terms of its location: one sitting on Pt particles as Rh nanoparticles and the other associated with the Al2O3 support as sub-nano Rh moieties. The Pt-Rh bimetallic particles contain a certain degree of Pt-Rh alloy as evidenced by the upshift of the metal XRD peaks and the Pt-Rh coordination from the EXAFS measurement. All structural and microscopy analyses suggest that Rh is surface enriched on the Pt-Rh particles, and it is the surface enriched Rh species that plays a primary role in enhancing the activity of Pt-Rh catalysts.