Carbon Nanofibers as Catalyst Support for Noble Metals

Abstract

In the quest for new and well-defined support materials for heterogeneous catalysts we explored the potential of carbon nanofibers (CNF). CNF belongs to the by now extensive family of synthetic graphite-like carbon materials with advantageous and tunable physico-chemical properties. Aim of the work described in this thesis has been the exploration of the potential of CNF as catalyst support material, notably for platinum and ruthenium, and its role in the performance of these catalysts in hydrogenation reactions. A prerequisite for such a study was to make well-defined CNF-supported catalysts available. In Chapter 2 research aiming at a further extension of our knowledge of the formation of uniform CNF using various nickel catalysts is described. It turned out that the nickel particle size as well as the nature of the carbon-containing gas significantly affect the CNF growth process. Before applying the active phase the inert and non-polar CNF have to be activated in boiling nitric acid or an HNO3/H2SO4 mixture in order to introduce oxygen-containing groups on the CNF surface, i.e. to introduce polar sites. Chapter 3 deals with the effect of this acid treatment on the structure and texture of CNF and both the total number of oxygen groups and the number of acidic oxygen-containing groups introduced are determined. In Chapter 4 a literature review on the synthesis of supported palladium catalysts is given. This review focuses on the chemistry of catalyst synthesis, relevant support properties and case studies for oxide- and notably carbon-supported catalysts. In the Chapter 5, the preparation of CNF-supported platinum and ruthenium catalysts by two different ion exchange techniques, one at constant pH of ? 6 (ion exchange) and one in which the pH is gradually increased from 3 to 6 by the hydrolysis of urea (HDP method) is presented. The work described in Chapter 6 demonstrates the surprisingly strong influence of oxygen-containing surface groups on the activity and selectivity of CNF-supported ruthenium catalysts in the hydrogenation of cinnamaldehyde. The overall activity strongly increases with a decreasing number of oxygen-containing groups, along with a shift in selectivity from cinnamyl alcohol to hydrocinnamaldehyde. In Chapter 7 we extended this work to CNF-supported platinum catalysts in the hydrogenation of cinnamaldehyde. The observed overall catalytic activity increased by a factor of 25 with decreasing amount of oxygen on the fibers. The differences in intrinsic activity are even larger, as internal diffusion limitations slow down the reaction rate of the most active catalyst. With XPS and H2-chemisorption no clear evidence has been found for oxygen in the support indirectly influencing the catalytic behavior by changing the electronic properties of the platinum particles. A model including both Langmuir-Hinshelwood kinetics and mass transfer effects is presented in Chapter 8 to establish whether differences in the number of oxygen support groups can be related with changes in certain kinetic parameters. Results suggest that hydrogenation becomes assisted by adsorption of the benzene ring of cinnamaldehyde on the non-polar CNF support surface after removal of the oxygen surface groups. The results described in this thesis have shown the potential of CNF as catalyst support material for liquid-phase reactions. We succeeded in the production of well-defined CNF and the development of a method to reproducibly apply small, uniform and thermally stable ruthenium or platinum particles. These CNF-supported catalysts displayed unique high activities in the selective hydrogenation of cinnamaldehyde. Moreover, spectacular effects of support oxygen have been observed on catalysis; the intrinsic reaction rates increase with a factor of up to 120 with the removal of the oxygen-containing groups from the CNF surface probably by substrate adsorption on the support.