| Popis: |
The effects of oxidation-reduction cycling on the morphology, surface composition, metal surface area, and kinetics of ethane hydrogenolysis on Rh particles on SiO 2 have been studied by hydrogen chemisorption, kinetics in a differential flow reactor, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). When ~150-A-diameter Rh particles are oxidized at 500 °C and then reduced in H 2 at 250 °C, the activity increases by up to three orders of magnitude. Heating progressively up to 650 °C produces a decrease in activity to nearly its original value. Hydrogen uptake shows only a factor of less than 3 increase in metal surface area associated with the redispersion and sintering following oxidation and reduction. XPS confirms total oxidation by 500 °C and reduction by 150 °C. TEM shows that particles spread on the SiO 2 surface upon oxidation and form clusters of smaller Rh particles upon low-temperature reduction. These large activity changes appear to be associated with morphological changes in types and distribution of active sites on the metal surface. These transformations can be produced reversibly by successive oxidation-reduction treatments, and structures can be metastable for long times under reaction Conditions. |
