Tunability of propane conversion over alumina supported Pt and Rh catalysts

Propane conversion over alumina supported Pt and Rh (1 wt% metals loading) was examined under fuel rich conditions (C3H8:O-2:He = 1:2.25:9) over the temperature range 450-650 degrees C. Morphological characteristics of the catalyst materials were varied by calcining at selected temperatures between 500 and 1,200 degrees C. X-ray diffraction and BET analysis showed the treatment generated catalyts metals with particle sizes in the range of < 10 to > 500 nm, and support surface areas in the range of 20-240 m(2)/g. Remarkably, both Rh and Pt yielded product compositions close to equilibrium values (with high H-2 and CO selectivity, complete oxygen conversion and almost complete propane conversion) so long as the metal particle size was sufficiently low, <= 10-15 nm. In cases where the particle size was large, primarily complete oxidation rather than partial oxidation products were observed, along with unreacted C3H8, indicative of a direct oxidation pathway in which gas-phase CO and H-2 are not present as intermediate species. It is proposed that the high resistance of Rh to coarsening is largely responsible for the observation of a higher selectivity of this material for syngas products when prepared by procedures similar to those for Pt. Overall, the tunability of the product composition obtained over Rh and Pt via processing steps has direct significance for the incorporation of such catalyts into the anodes of solid oxide fuel cells.