Carla E. Hori

Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system

Abstract CeO 2 and CeO 2 /ZrO 2 supports were prepared by two low temperature (500°C) routes: calcination of hydroxide precipitates and firing mixtures of acetates. The supports were loaded with 0.5% Pt and characterized both fresh and aged (at 800 and 1000°C). X-ray diffraction analyses show that a CeO 2 /ZrO 2 solid solution is formed in the samples prepared by precipitation of the hydroxides and calcination at 500°C. To our knowledge this is the lowest temperature chemical route (not including high energy ball milling) to CeO 2 /ZrO 2 solid solutions yet reported. Samples prepared by firing the acetate mixtures had almost exclusively CeO 2 and ZrO 2 in separated phases. Oxygen storage measurements showed that the addition of zirconia increased the oxygen storage capacities (on a per gram of catalyst basis) over ceria alone for both preparation methods. In the phase separated materials the amount of reversibly stored oxygen was 1.7–2.5 times more per gram catalyst than that of ceria only. The beneficial effects of zirconia are most pronounced in the solid solutions which had oxygen storage 3–5 times higher than ceria. When the Zr concentration is optimized, the solid solution materials aged at 1000°C showed higher oxygen storage than fresh CeO 2 . For both preparation methods, the optimal (per gram catalyst) Zr concentration was found to be 25 mol% Zr for samples aged at 1000°C, however, the performance of the solid solution materials was somewhat insensitive to Zr loading between 15 and 50xa0mol% Zr.

Studies of the oxygen release reaction in the platinum-ceria-zirconia system

Abstract The relative rates of the CO+oxygen storage material (OSM)⇒CO2 reaction (RCO2) and the amounts of rate enhancement obtained upon Pt promotion were examined for ceria and ceria–zirconia OSM. The effect of Pt surface area on RCO2 was de-coupled from metal–oxide surface area by pre-sintering the oxides prior to Pt deposition. We find that RCO2 is linearly dependent on Pt area over Pt/CeO2, but over Pt/Ce0.75Zr0.25O2, the rate is independent of Pt surface area above a threshold surface area. Although Pt sinters more readily on the Ce0.75Zr0.25O2 support, the dispersion effect is more than compensated by the enhanced availability of “bulk” O2− within the Ce0.75Zr0.25O2 particles. Furthermore, this study demonstrates that, on a unit surface area basis, the ceria–zirconia support is at least two times more active for the oxygen release reaction than the pure ceria support when the materials are slightly reduced. It is also shown that O2− diffusion is not rate-limiting for the ceria reduction reaction when the reaction is carried out in the kinetically limited regime at 500°C with CO as the reductant.

The effects of aging temperature and aging time on the oxygen storage capacity of Pt-Rh/CeZrO2 catalysts

The effects of aging temperature and time on the oxygen storage capacity (OSC) of Pt-Rh-promoted Ce0.75Zr0.25O2 solid solutions were measured and correlated with the BET surface area and noble metal (NM) surface area in the catalysts. The NM surface area is better correlated with OSC than is with the BET surface area. On a practical level, our results demonstrated that, even when operating at 900°C with alternating oxidizing and reducing conditions, these materials deactivate slowly with a near t-1 time dependence. Deactivation rates for these catalysts are dependent on the NM loading with the highest loaded catalysts deactivating roughly half as fast as the lowest loaded catalysts. As the aging temperature is increased from 900°C to 1000°C, the deactivation rate becomes two to four2-4 times higher for all three properties (BET surface area, NM surface area and OSC). The lowest NM loaded samples are more sensitive to aging temperature than the highest loaded ones.

Cobalt supported on different zeolites for fischer-tropschsynthesis

Abstract In this work, the textural and chemical properties as well as the catalytic behavior of 10 wt Cobalt supported on H-USY, H-Beta, H-Mordenite and H-ZSM-5 zeolites in the Fischer-Tropsch Synthesis (FTS) were studied. X-Ray diffraction analyses showed that the characteristic patterns for each zeolite structure were maintained after the metal addition. The average Co particle size increased in the following sequence: Co/H-Beta

The effect of pt loading and space velocity on the performance of Pt/CeZrO2/Al2O3 catalysts for the partial oxidation of methane

Abstract In this study, the perfomance of Pt/CeZrO 2 /Al 2 O 3 with Pt loadings of 0.5, 1.0 and 1.5 wt on the partial oxidation of methane was evaluated. The results showed that the increase of metal content caused a decrease of the metal dispersion, but it did not affect the catalysts reducibility properties. The methane conversion and CO selectivity increased with the use of higher Pt content. The deactivation observed for the lower Pt contents could be due to the unbalance between methane decomposition and oxygen transfer process. The increase of space velocity decreased the catalyst stability and selectivity for H 2 and CO.