Energetics of Small Molecules and Molecular Fragments on Model Catalyst Surfaces: Adsorption Calorimetry on Pt(111) and Cu(111)
Ruehl, Griffin Michael
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Heterogeneous catalysis is essential for the development and support of modern society, with the vast majority of chemical production processes reliant on catalysts. New catalysts and catalytic reactions constitute promising pathways forward in combatting the effects of climate change and transitioning human society off of our reliance on fossil fuels. However, there is an absence of a complete fundamental understanding of observed differences and trends in catalytic behavior that impedes the rapid, strategic development of new catalytic processes. Computational modeling methods, such as Density Functional Theory (DFT), constitute powerful tools for the rapid screening of catalyst materials, but these methods have large errors in energy accuracy which severely limit their quantitative predictive abilities. These methods are dependent on experimentally determined benchmarks to guide modifications for improving their energy accuracy. The technique of single crystal adsorption calorimetry (SCAC) is uniquely able to study the energetics of irreversible adsorption processes on well-defined surface sites. SCAC can therefore provide these key benchmarks and fundamental understandings of the energetics of molecular and dissociative adsorption into molecular fragments and other key surface reaction intermediates commonly seen in industrial catalytic applications. This dissertation presents experimental SCAC results for the study of the energetics of adsorption of small molecules and molecular fragments on model catalyst surfaces, namely Pt(111) and Cu(111). This work builds upon previous efforts from the Campbell group to develop a systematic understanding of trends and observed differences in catalytic behavior on late-transition metal catalysts. Additionally, by employing models recently developed by this group, we are able to estimate the adhesion energies of liquid solvents to clean, single-crystal metal surfaces from the experimental calorimetry results. This allows for the estimation of the effects of each solvent on the energetics of adsorption and desorption for surface reactants and intermediates of interest. The study of the energetics of acetonitrile and nÂ¬-decane adsorption on Pt(111), two solvents of particular interest, are reported here. Acetonitrile an important solvent due to its unique, desirable properties which make it of particular interest for electrochemical applications and the engineering of mixed solvent environments. n-Decane is similarly of interest in catalysis as linear alkanes of that and similar size are commonly used as solvents in catalytic reactions over Pt-group metals. From the experimentally determined heat of adsorption versus coverage we estimate adhesion energies of these liquid solvents to the Pt(111) surface to be Eadh = 0.198 J/m2 for acetonitrile and Eadh = 0.148 J/m2 for n-decane. Additionally, the adhesion energy of liquid formic acid to Cu(111) is estimated to be Eadh = 0.271 J/m2. These values can be used to quantify the solvent effects of these species on the local surface reaction environment. The calorimetrically measured heats of adsorption versus coverage are reported here for acetonitrile on Pt(111) at 100 K and 180 K, n-Â¬decane adsorption on Pt(111) at 150 K, azulene adsorption on Pt(111) at 150 K, and for both the molecular and dissociative adsorption of formic acid on clean and oxygen-precovered Cu(111). In combination with previously reported experimental results and DFT simulations of these systems, a number of important fundamental insights are drawn. The analysis of the n-decane heats of adsorption in comparison to a previous TPD study of shorter linear alkanes extends the observed trends to larger species such as n-decane that desorb irreversibly. Namely, we report that the adsorption energy increases nearly proportionally to carbon number, and the adhesion energy remains nearly constant (for a given surface). Naphthalene and azulene are of particular interest as representative molecules for the regular structure of graphene and the most common defect found in graphene sheets, respectively. Therefore the study of their adsorption energetics can inform experimental and computational systems involving graphene more broadly. Comparison of the heats of adsorption for azulene on Pt(111) first presented here with previous results for naphthalene and DFT simulations of both show that azulene binds significantly stronger to Pt(111) (by ~100 kJ/mol) than its isomer naphthalene. We show that DFT accurately predicts the adsorption energy of azulene but overestimates the binding energy of naphthalene, indicating that DFT is not accurately modeling the energy differences between these two systems. We report here the dissociative adsorption of formic acid on oxygen-precovered Cu(111), which results in the formation of adsorbed bidentate formate and gaseous water at 240 K. Formic acid and formate are common intermediates in a variety of reactions on late transition metals, ranging from well-established industrial reactions to emergent clean energy technologies. From the heats of this dissociative adsorption reaction, we extract a bond enthalpy of bidentate formate to Cu(111) of 335 kJ/mol, and an enthalpy of formation of bidentate formate on Cu(111) of -465 kJ/mol. We show that these enthalpies are slightly greater than those on Ni(111) (by ~15 kJ/mol) and significantly greater than those on Pt(111) (by ~85 kJ/mol). This is in opposition to the predicted order of bond strength from DFT, where Ni is predicted to bind formate more strongly than Cu, and indicates that DFT is not accurately modeling this trend in adsorption between these three surfaces. This study also constitutes the first experimental measurement of the energetics of any adsorbed molecular fragment on any Cu surface. In comparison to previous results on Pt(111) and Ni(111) this allows for the direct comparison of a single molecular fragment on all three surfaces for the first time. This forms a suite of key experimental benchmarks for improving the energy accuracy of computational models like DFT, as well as crucial fundamental insights into trends and observed differences in catalysis on late-transition metal surfaces. Lastly, we report a detailed kinetics study of the aqueous-phase hydrogenation of phenol and benzaldehyde on Pt, Pd, and Rh using small-scale thermal and electrocatalytic reactors. These molecules represent common intermediates in the process of breaking down biomass and converting its constituents into biofuels and other value-added chemicals. This work shows that the observed catalytic behavior is well fit by a Langmuir-Hinshelwood mechanism with competitive adsorption (organic versus hydrogen adsorption) on terrace, or (111)-like, sites. Additionally, we report that adsorbed benzaldehyde inhibits the formation of a bulk Pd-hydride whereas phenol does not, explaining the extreme differences in observed catalytic activity between these two systems. This work informs efforts to correlate molecular structure of biomass intermediates of interest with catalytic activity on late-transition metal catalysts.
- Chemical engineering