Energetics of Elementary Steps in Catalysis and Their Use to Search for New Catalysts
Wolcott, Christopher A.
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We live in a society based upon the mass production of chemicals. Whether it is the fuel in a car, the fertilizers used to make food, or the plastics present in just about everything, these chemicals are so ubiquitous that it is difficult to imagine living in a world without them. Nearly all consumer chemicals are produced through a catalytic process, the vast majority of which are heterogeneous. On top of their current, massive presence, heterogeneous catalysts are also expected to play an important role in new emerging technologies such as fuel cells, hydrogen production, green chemistry, and more. Considering their ubiquity in the present and their potential uses in the future, it is no surprise that improving catalyst performance is a very active area of research. Yet despite their ubiquity, and despite their long history of active study, there remains much which is unknown about the fundamentals of catalysts on surfaces. One of the major gaps is in quantitative understanding of the energetics of elementary steps in catalytic reactions on surfaces. The stability or instability of molecules and molecular fragments adsorbed on surfaces in these elementary steps is KEY to understanding what makes one material an effective catalyst and another less effective. In general, one must use single-crystal model catalysts to produce well-defined adsorbates. Classic studies of the energetics of adsorbates on such surfaces have typically involved techniques (such as temperature programmed desorption or equilibrium adsorption experiments) which limit the types of systems which can be studied to those where adsorption is reversible. For most catalytic intermediates present in these elementary steps, this is not the case. Upon adsorption and heating many molecules fall apart and produce strongly bound adsorbates which further dissociate at higher temperatures, or will not leave the surface until they have reacted with something else. Single crystal adsorption calorimetry (SCAC) is a fairly new technique which allows one to probe the heats of formation of such adsorbates for the first time. In this thesis SCAC is used to study the dissociative adsorption of diiodomethane on Pt(111) to produce adsorbed -CH2 and -CH, and water on Fe3O4(111) and NiO(111) to produce adsorbed -OH. This work expands the library of adsorbates on transition metal surfaces which has been studied by SCAC, and is among the first ever measurements of molecules on well-defined oxide surfaces using SCAC. These results are compared to density functional theory (DFT) calculations of adsorbate energetics, and their use as computational benchmarks is discussed. A new, universally-applicable method of data analysis for SCAC is also developed which allows for the extraction of heat data even in the presence of complex surface reaction/diffusion dynamics without any need for kinetic modeling as required in previous analysis methods, thus greatly expanding the versatility of SCAC. Finally a new method of computational catalyst screening is presented which uses the concept of degree of rate control to simplify calculations compared to the standard method developed by Jens Nørskov's group. It greatly reduces the number of adsorbate energies needed to predict the reaction rate for a new catalyst, and provides greater accuracy when studying materials with similar properties to the reference catalyst used. The Nørskov method is more robust when extended to materials that are dissimilar. The new method presented here is thus expected to be an important complimentary tool to Nørskov's method for high-throughput computational screening. Taken together, the results presented in this dissertation show the importance of experimental measurements for guiding the development of fast quantum mechanical methods like DFT to more closely approach thru "chemical accuracy" in energetic prediction, and how one could use "chemically accurate" DFT energies to rapidly screen potential catalysts for computational catalyst discovery to advance energy and environmental technologies.
- Chemical engineering