Thermodynamics and Kinetics of Elementary Surface Reaction Steps in Catalysis by Single Crystal Adsorption Calorimetry
Silbaugh, Trent Leonard
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Many chemical technologies rely on the interaction of gas phase molecules with solid surfaces. One of the most important application fields among these technologies is heterogeneous catalysis, which includes chemical manufacturing, energy generation, conversion and storage and environmental technology. Many of the related processes include one or more steps catalyzed at solid interfaces or involve adsorption of gaseous molecules. For the rational design of new catalytic and other functional materials, a detailed knowledge of the energetics of the adsorbate-surface interaction and the thermodynamics of surface reaction intermediates is required. Recent results have revealed that state-of-the-art computational methods based on density functional theory have far greater energy errors than originally believed, so they are insufficient for this task. The data set of experimental benchmarks needed to improve these is also still far too limited. Thus, more measurements of adsorption energies are badly needed. Because many of these important intermediates exist in metastable states, traditional experimental techniques that rely on reversible desorption of adsorbed species to get adsorption energies (i.e. temperature programmed desorption and equilibrium adsorption isotherms) cannot be used. In this thesis, single crystal adsorption calorimetry (SCAC), which allows for the direct measurement of heat deposition during surface adsorption and reaction processes, is utilized to determine the energetics of several simple adsorbed molecular fragments on Pt(111). A new data analysis method is introduced here that allows SCAC to be used to simultaneously probe the thermodynamics as well as kinetics of surface reactions. This analysis method is used to provide the rate barriers for elementary steps in the decomposition of formic acid on oxygen precovered Pt(111). A review of all SCAC studies of molecular adsorption and reaction is also provided, and important recent results from this body of literature is discussed. A correlation of bond enthalpies determined from SCAC data to gas phase bond enthalpies has provided a linear relationship that allows for the prediction of surface bond strengths from gas phase data. Also, using energetics from several SCAC and TPD studies, a complete energy landscape for the oxidation of methanol and formic acid on oxygen precovered Pt(111) has been generated which provides insight into the mechanism of this process. A comparison of experimental bond energies to values obtained from density functional theory (DFT) calculations shows that errors in standard DFT can be quite large, particularly for systems with large van der Waals interactions, and that these errors are not systematic.
- Chemical engineering