Energetics of Small Molecules and Molecular Fragments on Model Catalyst Surfaces: Calorimetric Adsorption and Adhesion Energies on Pt, Ni and Cu(111)

Abstract

Heterogenous catalysts are essential for our society. From the production of food and energy to the manufacture of chemicals and the mitigation of pollution, catalysts touch every facet of our lives. In order to maintain our way of life and improve our energy systems it is critical to seek out new and better catalysts. We need continual improvement in our catalysts and processes to continue supporting our society’s growth. Modeling methods such as Density Functional Theory (DFT) do great work in exploring new catalyst materials. By using quantum mechanical first principles and experimentally determined adsorbate bond enthalpies of catalytic intermediates, DFT can predict the energetics of individual steps of complex reactions on catalyst surfaces. However, DFT suffers from errors in absolute energy accuracy, which in turn leads to large errors in predicted reaction rates. It’s imperative that we improve these models. By experimentally measuring the energetics of catalytically relevant species on catalysts surfaces, critical benchmarks for DFT and other modeling methods can be provided. Of the available experimental methods, Single-Crystal Adsorption Calorimetry (SCAC) is the only technique capable of directly measuring the binding energies of adsorbates on surfaces that do not adsorb and then desorb reversibly. This dissertation details the use of SCAC to measure the heats of adsorption of a variety of catalytically relevant molecular fragments on the Pt(111), Ni(111) and Cu(111) surfaces, including the first direct measurement of the heat of adsorption of any molecular fragment on a Cu surface. These measurements continue a decades-long effort by the Campbell group to supply critical benchmarks of experimentally determined energetics of small molecules and molecular fragments on model catalyst surfaces. This work also details the calculated adhesion energies of several small molecules on these transition metal surfaces using a method recently developed by the Campbell group. In the following chapters, SCAC is used to measure the heats of adsorption of methanol and methoxy on Ni(111). Ni is a common catalyst for steam reforming and methanol synthesis, while methoxy is a common intermediate in those processes. SCAC is also used to make the first measurement of the energetics of adsorption of any molecular fragment on a Cu surface, here the dissociative adsorption of formic acid on oxygen-precovered Cu(111) to form bidentate formate and gaseous water. Cu is a promising monometallic catalysts for the reduction of CO2, where formate is an intermediate. The adhesion energy of molecular formic acid on Cu(111) is also presented. The energetics of several species to Pt(111) are measured via SCAC. We use azulene as an analogue for defects in graphene, and its isomer naphthalene for well ordered graphene, to examine their energetic differences on Pt(111). The heat of adsorption of n-decane on Pt(111) is measured and compared to previous work via Temperature Programmed Desorption (TPD). The adhesion energy of n-decane is calculated and compared to the adhesion energy of shorter linear alkanes on Pt(111) to show that adhesion energy remains nearly constant as chain length increases due to the per unit area definition of adhesion energy. Finally, the heat of adsorption and adhesion energy of acetonitrile on Pt(111) are reported. Acetonitrile is a common solvent in electrocatalysis, where Pt is a common electrode material, in addition to being the simplest organic nitrile. These fundamental energetics provide important benchmarks for DFT modeling, especially on Cu(111) where there is a lack of experimental measurements.