Date

2016

Author

Kaya, Deniz

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The oxygen exchange kinetics of precious metal added ceria and ceria-alumina oxides were investigated by temperature programmed analyses and microcalorimetric measurements. 1 wt. % Pd/CeO2 and 1 wt. % Pd/Al2O3 oxides were synthesized by the incipient wetness method. 1 wt. % Pd/CeO2-Al2O3 oxides with 20 wt. %, 15 wt. % and 10 wt. % loadings of ceria were synthesized by the sequential impregnation method. Temperature programmed reduction (TPR) analysis was performed to analyze the reducibility of prepared oxides. At temperatures below 200 °C, surface ceria atoms were totally reduced for all prepared oxides and the total reduction of CeO2 to Ce2O3 was observed for alumina-added oxides. Hence, there are Pd CeO2 interactions in all prepared oxides and Pd increases the reducibility of ceria at lower temperatures via the spillover of hydrogen in the presence of Pd atoms. Temperature programmed thermal decomposition (TPtD) and temperature programmed oxidation (TPO) cycles were performed to understand the oxygen release/uptake temperature, capacity and kinetics of prepared oxides. It is clearly seen that addition of Pd to CeO2 decreases the decomposition temperature of ceria by nearly 940 °C via the reverse spillover of oxygen atoms in ceria in the presence of Pd. Additionally, increase in alumina amount for prepared oxides results in an increase in oxygen release amount per ceria basis. Activation energies were calculated for TPR by the Kissinger method. It can be concluded that reduction process is in the pore diffusion regime and mass transfer limited for while the reduction process is in the kinetic regime for 1 % Pd/CeO2. Activation energies were calculated for TPtD by Redhead and differential methods. Volumetric chemisorption measurement was performed for 1 wt. % Pd/CeO2 to analyze the hydrogen adsorption amount and to calculate the dispersion. Microcalorimetric measurements were performed for prepared oxides using H2, O2 and H2O vapor in order to investigate the adsorption amounts of gases and heat of adsorption data. It is seen that H2, O2 and H2O adsorption capacity increases with the addition of alumina and heat of O2 adsorption is almost two times higher than heat of H2O adsorption for all prepared oxides which proves that prepared oxides can be oxidized with water in addition of oxygen. 

Subject Keywords

Cerium oxides., Calorimeters., Nanoparticles., Cerium compounds.

URI

http://etd.lib.metu.edu.tr/upload/12620405/index.pdf
https://hdl.handle.net/11511/26094

Collections

Graduate School of Natural and Applied Sciences, Thesis