N-BUTANE HYDROGENOLYSIS AS A PROBE OF SURFACE SITES IN RHODIUM METAL PARTICLES - CORRELATION WITH SINGLE-CRYSTALS

Rh metal was supported on alumina and silica supports and the particle size was varied (from <1 to >5 nm) by changing the metal loading from 0.03 to 7.2 mumol of Rh/m2 of support. Metal particle sizes were determined by transmission electron microscopy. Hydrogenolysis of n-butane was used as a probe reaction. The effect of particle size and preoxidation on the hydrogenolysis activity and product selectivity was studied. The ethane selectivity of the highly dispersed Rh was very different from that of the low index single crystal surfaces of Rh. As particle size was increased, the selectivity of the catalysts approached that of Rh(111). Annealing the catalysts at 773 K in H-2 led to better agreement between the supported Rh and single-crystal Rh(111). None of the catalysts after initial H-2 reduction resembled the (100) or (110) surfaces in their hydrogenolysis selectivity. Since the (111 ) close packed surface would be favored during particle growth, it is understandable that these high temperature treatments lead to better agreement between the behavior of small particles and single crystals.