Document Type dissertation Author Name Zhu, Guanghui URN etd-0128104-105806 Title Kinetics of Complete Methane Oxidation on Palladium Model Catalysts Degree PhD Department Chemical Engineering Advisors Fabio H. Ribeiro, Advisor William M. Clark, Committee Member Richard D. Sisson, Committee Member Keywords deactivation hysteresis kinetics structure sensitivity methane oxidation on palladium Date of Presentation/Defense 2004-01-22 Availability unrestricted
The catalytic combustion of methane in excess of O2 over Pd catalysts was studied on model catalysts, including polycrystalline palladium foil and palladium single crystals. The kinetics of this reaction could be measured at conditions not accessible to supported catalysts and, thus, the issues of structure sensitivity, mechanism, hysteresis on oxidation, and deactivation could be studied in detail.
Methane oxidation on PdO was insensitive to the original metal surface structure which PdO grew from, with turnover rates in the range of 1.3-4.7 s-1 on (111), (100) and (110) single crystals at 160 Torr O2, 16 Torr CH-4, 1 Torr H2O and 598 K. Methane oxidation on Pd metal was also insensitive to the original surface structure, with the turnover rate in the range of 2.0-2.8 s-1 on the three single crystals at 2.3 Torr O2, 0.46 Torr CH4, 0.05 Torr H2O and 973 K. Since there is no support effect and the surface purity could be certified, these turnover rates for this reaction can be used as a benchmark.
The turnover rate for methane oxidation was found to decrease 95% when PdO decomposed to Pd metal at 888 K, showing that PdO was more active than Pd metal for methane combustion at this temperature. Water inhibition to the reaction was not observed at a temperature above 813 K on both PdO and Pd metal, while it was observed at 598 K on PdO. The activation energy on PdO was 32 kJ mol-1 in the range of 783-873 K, while it was 125 kJ mol-1 in the range of 568-623 K. The activation energy on Pd metal was 125 kJ mol-1 in the range of 930-980 K. The change of reaction orders and activation energies suggests that the reaction mechanism is a function of temperature and palladium chemical states. We propose that adsorbed water, the most abundant surface intermediate at 598 K, was not present in significant quantities at temperatures above 783 K. This change in surface inhibition by water is the reason for lower activation energy at temperatures above 783 K.
Interaction between the catalyst and support, or presence of impurities, is one of the factors for catalyst deactivation. The interaction between oxidized silicon and palladium was investigated on a polycrystalline palladium foil and on supported Pd/SiO2 catalysts. During methane oxidation, oxidized silicon covered the palladium oxide surface as observed by TEM on Pd/SiO2 catalysts and by XPS on palladium foil. On Pd foil, the source of silica was a silicon impurity, common on bulk metal samples. The migration of oxidized silicon onto PdO deactivated the catalysts by blocking the active sites for methane oxidation. Silicon oxide overlayers were also observed covering the Pd surface after reduction of Pd/SiO2 by H2 at 923 K.
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