Paramagnetic metal species on highly dispersed rhodium/polyphosphine catalysts

ESR active paramagnetic species were used as a probe for the description of the metal centers in highly dispersed rhodium/polyphosphine catalysts. Magnetically dilute Rh(0) centers were observed at high P Rhs] (s = surface) ratios. Two different Rh(d0) diluted species were detected (g1 = 2.085, g2 = 2.000; g′1 = 2.045, g′2 = 2.008, g′3 = 1.975) whose relative amounts vary with the P Rhs value. At lower P Rhs values, metal crystallites play a dominant role in giving broad unresolved signals. The metal particle size and the absolute Rh% are also related to the resonance shape. It is suggested that phosphine coordinates Rh centers, where a rhombic species corresponds to a higher number of coordinated phosphines. The results of the ESR investigation are in good agreement with previous studies on the same catalysts (G. Cocco, S. Enzo, F. Pina, and G. Strukul, J. Catal. 82, 160, 1983) performed by SAXS, WAXS, TGA, TPR, and infrared study of CO adsorption.