Adsorption Calorimetry Measurements of the Energetics of Catalytic Intermediates: Empirical Trends and Benchmarks for Theory
Karp, Eric Michael
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Catalysts enable chemical reactions to occur with less energy input than would be required without a catalyst, simultaneously increasing the rate and selectivity of a reaction. Thus, catalysts are important industrial materials crucial for the production of commodity chemicals and fuels. In particular, solid, heterogeneous catalysts are of great interest in the chemical industry, because the reactants and products can be easily separated from the catalyst. Much effort is dedicated to the discovery and development of new catalytic materials capable of facilitating important industrial reactions, however these materials are mainly discovered through a trial and error approach, which can be a time-consuming and expensive process. A quicker and more efficient way to develop the future generation of catalysts is to understand the fundamental energetics that control catalytic activity and selectivity and understand how those energetics depend on catalyst surface structure and composition. The most important parameters that determine the activity of a catalyst material are the bond strengths with which it binds a few key chemical intermediates and transition states. There are many computational approaches (mainly based on Density Functional Theory, DFT) that calculate these parameters and how they depend on the material. This provides a wonderful opportunity for computational screening of potential new catalytic materials that has already led to the discovery of a few new catalysts. However, prior calorimetry results suggest that even the best of these methods may not yet be accurate enough to achieve anywhere near its full potential for catalyst discovery. Unfortunately, accurate experimental values are still not available for the bond strengths of even the simplest adsorbates to catalyst materials, like -OH, -OCH3, -CH and -CH3, which are very widely recognized to be key intermediates in a broad variety of catalytic reactions used in energy and environmental technology. In this dissertation, I detail the results of experimental measurements of the energetics of these important adsorbates on Pt(111), using Single Crystal Adsorption Calorimetry (SCAC). The results provide important benchmarks for assessing the accuracy of new calculational methods based on DFT that are being designed to achieve higher energy accuracy. Thus, these experimental energies are compared to published DFT results throughout these chapters. Earlier measurements of oxygen adsorption energies on Pt(111) both from TPD and calorimetry data are reexamined also in light of our group's calorimeter calibration methods, and it is shown that a calibration error was made in that calorimetry data. We present corrected values for the calorimetric adsorption energy of oxygen on Pt(111), show that it agrees with prior TPD results, and use it to amend previously-reported energetics of adsorbed OH. Finally, SCAC results for several oxygen-containing ligands on Pt(111) which bind to the Pt through an oxygen atom are shown to follow a trend, whereby their O-Pt(111) bond strength is proportional to the strength with which these ligands bind H in gas-phase molecules, with a slope of 1.0. This trend allows the prediction of the bond strengths for other adsorbates that cannot or have not been measured. This trend is identical to one observed previously for the bond strength in organometallic complexes between metal centers and ligands. Here this trend is shown, for the first time, to also hold for metal surfaces and adsorbates bound through a single bond, and is hopefully the first step in developing trends for design rules for catalytic materials based on fundamental parameters.
- Chemical engineering