Activity and selectivity in oxidation catalysis

In oxidation catalysis, a fine balance must be struck between catalyst activity and selectivity. This research investigates the challenges associated with achieving high activity and selectivity in the oxidative dehydrogenation of ethane and the preferential oxidation of carbon monoxide in hydrogen rich streams.;In this work, the use of N2O as oxidant, rather than O 2, in ethane ODH was investigated. A 10%Mo/Si:Ti catalyst was tested for ethane oxidative dehydrogenation (ODH) activity using either O2 or N2O as the oxidant. Ethane ODH activity was tested at contact times varying from 0.46 to 1.0 mg s cm-3. N2O gives superior ethylene selectivities at a given ethane conversion outperforming oxygen at all contact times tested. Ethylene selectivities decrease with contact time at a far slower rate when using N2O as the oxidant. XPS experiments demonstrate that molybdenum in the catalyst is fully oxidized to Mo(VI) during ethane ODH using O2 while the reduction state decreases to an average of +5.8 when using N2O as the oxidant. Temperature programmed oxidation experiments of pre-reduced 10%Mo/Si:Ti were carried out at different temperature ramp rates. The activation energy of reoxidation when using N2O is 98 kJ/mol while that of O2 is 41 kJ/mol leading to a reoxidation rate of at least 1700 times faster when O2 is the oxidant. This difference in rates accounts for the less oxidized state of molybdenum during ethane ODH with N2O and explains the behavior observed during reaction experiments. The concept of site isolation provides a satisfactory framework for understanding the steady state reaction results. Some carbon deposition was observed when using N2O as oxidant during ethane ODH as determined from post ODH TPO and XPS experiments but it did not affect ODH activity.;Proton exchange membrane (PEM) fuel cells promise to be clean and efficient alternatives to combustion of fuels for power generation. Unfortunately, the catalysts used in PEM anodes are easily poisoned by trace amounts of carbon monoxide. Reduction of the carbon monoxide concentration to a level of approximately 10 ppm is currently necessary to prevent this poisioning. Preferential oxidation of carbon monoxide (PROX) offers an economic and simple method of CO removal but in high concentrations of hydrogen maintaining a high catalyst activity and selectivity simultaneously can be problematic due to unselective H 2O oxidation and CO methanation.;This work demonstrates the preparation of a 10%CoOx/CeO 2 catalyst that is highly effective for the preferential oxidation of carbon monoxide in a hydrogen rich feed. The CoOx/CeO2 catalyst had a high surface area of 78m2/g and was able to maintain a near 100% CO conversion while maintaining a selectivity of 58% during PROX experiments in high concentrations of hydrogen. The catalyst is able to obtain high CO conversions under a wide range of weight hourly space velocities. Introducing H2 into the feed has a negative effect on the CO consumption rate and decreases O2 selectivity to CO 2. This shows that the reaction rates for CO oxidation and H2 combustion are not independent and suggests that hydrogen may competitively adsorb on sites responsible for CO oxidation. Separate H2 and CO oxidation experiments give activation energies of 74 and 52 kJ/mol, respectively. Additionally, the preexponential factor for H2 is 25 times higher than that for CO under the reaction conditions employed. It is possible that there are more sites available for H2 oxidation but this reaction requires high temperatures to occur at appreciable rates. Temperature programmed PROX reaction experiments show three temperature regions where different reactions are important. Below 175°C, CO oxidation is dominant but above this temperature, CO oxidation and H2 combustion compete with one another. The temperature necessary to obtain high activity in the PROX reaction occur near this transition at 175°C. At even higher temperatures, irrelevant to the PROX reaction, methanation begins to occur. Raman spectroscopy and X-ray diffraction experiments have demonstrated that the cobalt takes the form of Co3O4 and no CoO was detected under any experimental conditions. It is likely that the octahedral Co3+ sites are the active sites for oxidation of carbon monoxide. (Abstract shortened by UMI.)...