Energetics of Catalytic Intermediates on Nickel(111) by Calorimetry: Empirical Trends and Benchmarks for Quantum Theory
Carey, Spencer J
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Our society depends on the use of catalysts for the manufacture of 90% of chemical industry products, for the mass production of fertilizers that grow our food supply, for the synthesis of the fuels that drive our transportation systems, and for the purification of pollutants, such as those produced by car engines. With this utility comes a huge investment of effort to understand the fundamental science behind catalysts and to improve their efficiency, durability, and selectivity. It is also important to be able to design new catalysts for changing feedstocks (e.g., replacement of coal and petroleum with methane, biomass and other renewables). In the last fifty years, new methods to study catalytic processes have been developed, which in turn resulted in an explosion of research studies that address the fundamental questions in the catalysis field. Quantum mechanical calculations using Density Functional Theory (DFT) are one such technique that has become invaluable in studying catalysts. This method allows for the efficient and inexpensive prediction of catalyst mechanisms and kinetics, structure-function relationships, and even in screening for new, more effective catalysts. However, the accuracy of these predictions depends upon reliable energetic information of adsorbed catalytic reaction intermediates, such as their heats of formation and bond enthalpies to the surface. The energies of adsorbed intermediates and transition states on surfaces are the key factors that determine the effectiveness of any given catalyst. The results of this dissertation show that the energy accuracy of these DFT methods is far less than desirable, and it provides many experimental benchmark energies that will be useful for the development of more accurate DFT functionals. This dissertation is part of a decades-long effort by our research group to compile a large database that contains the heats of formation of many adsorbates on different model catalyst surfaces. This database aims to provide valuable benchmarks that theorists can use to improve DFT functionals. To expand this database, our research group uses Single-Crystal Adsorption Calorimetry (SCAC), the only method able to directly measure the binding energies of adsorbates to model surfaces. My research is focused on expanding the adsorbate bond energy database in the areas that it is lacking. Specifically, previous to this dissertation, this database only included adsorbed molecular fragments on one metal surface, Pt(111) and only included aromatic molecules on one non-noble metal surface, again Pt(111). We have extended both of these classes of adsorbates to their energies on the Ni(111) surface, with comparisons between the energies on Pt(111) versus Ni(111) which help explain some of the differences in catalytic properties of Pt versus Ni. In this thesis, SCAC is used to study the molecular adsorption of phenol and benzene on both Pt(111) and Ni(111). Both benzene and phenol are aromatic, and their energetics are heavily influenced by van der Waal forces. SCAC is also used to study the dissociative adsorption of methyl iodide on Ni(111) to produce adsorbed methyl and iodine adatoms and the dissociative adsorption of methanol on O-precovered Ni(111) to produce adsorbed methoxy and hydroxyl. Adsorbed methyl and methoxy are important molecular fragments that are catalytic intermediates in several industrial processes. Finally, a new equation is derived that relates the sigma bond enthalpies of several molecular fragments to both Ni(111) and Pt(111). This trend allows predictions of the sigma bond enthalpies of other small molecular fragments to transition metal surfaces.
- Chemistry