Effect of Support Acidity and Cu Surface Structure on Cu Surface Site Quantification and Non-Oxidative Ethanol Dehydrogenation
- Author(s): Hanukovich, Sergei
- Advisor(s): Christopher, Phillip
- et al.
In this dissertation, we have utilized a multi-faceted approach, combining carefully controlled synthesis techniques, qualitative and quantitative surface characterization, as well as rigorous reaction kinetics techniques to elucidate key insights for heterogeneous catalytic processes involving Cu catalysts. Specific focus was placed on developing a comprehensive analysis of the current state-of-the-art Cu surface characterization technique as well as studying the impact of Cu-support interfacial effects on the reaction activity and mechanism of ethanol dehydrogenation.
For Cu surface characterization, the widely accepted assumption that N2O titration of Cu at mild temperatures only oxidizes metallic Cu to Cu+ lead to significant overestimation of Cu particle dispersion due to the over-oxidation of Cu sites to Cu2+. As expected, with increasing oxidation temperature, the extent of over-oxidation (relative concentration of Cu2+ sites) increased. This trend was most prominent on smaller Cu particles which were composed mainly of under-coordinated sites. On the other hand, the larger particles, which were more difficult to oxidize required larger oxidation temperatures compared to smaller particles to induce complete titration of the Cu surface. This highlights the difficulty in predicting an optimal N2O oxidation temperature, as it is Cu particle size dependent. In response to this, a novel technique was proposed, in which NO chemisorption is used to exclusively probe the quantity of Cu2+ sites generated after oxidation, as a means of correcting the initial dispersion estimates.
For the second part of this dissertation, it was demonstrated that metal oxide supports with strong Lewis acid sites promoted ethanol dehydrogenation activity over Cu by not only modifying interfacial Cu sites, but also by potentially facilitating the initiation of the reaction mechanism. The results in this work provide a potential bridge to explain the discrepancy in literature about the rate limiting step for ethanol dehydrogenation, but also elucidated the role of support acid sites, specifically at the metal-support interface, in promoting reaction rates. This study may work as a model system for carefully using reaction kinetics to determine the reaction active site, rate limiting step, and corresponding mechanism for future studies of small nanoparticles loaded on active metal oxide supports.