XAS-Investigations of Colloidal Precursors for Pt/Rh Bimetallic Catalysts

Innovative transition metal catalysts prepared on the basis of preformed colloidal nano-particles and stabilized by surfactants are of special interest with regard to industrial applications. The physical characterization of the metal colloid precursors, i.e. the determination of their geometric and electronic structure prior to supporting, has been shown to be crucial for the optimization of new catalysts. Improved activity, selectivity and resistence against deactivation in case of bimetallic systems have been ascribed to 'synergetic effects' of the two metals present in the catalytically active components. The improved catalytic performance at a certain range of Pt:Rh atomic ratio in the precursor can be interpreted on the basis of a geometric lattice structure effect - rather than an electronic band structure effect. This has been shown by an investigation of both the near edge structure and the EXAFS-region of Pt L 111 X-ray absorption measurements, performed on a series of ammonium salt stabilized bimetallic Pt/Rh resp. monometallic Pt colloids.