Adhesion and Adsorption Energetics of Late Transition Metal Nanoparticles on Oxide and Carbon Supports for Predicting Catalyst Behavior and Enabling Catalyst Design

Abstract

Heterogeneous catalysts are a critical part of many important industries and fields and are used in the production of most of today’s chemicals worldwide. They also play a critical role in enabling clean technologies, reducing our reliance on fossil fuels, and environmental remediation. Most heterogeneous catalysts (including electrocatalysts) consist of metal nanoparticles on a high surface area catalytic support, which is usually an oxide or carbon-based material. This dissertation seeks to understand fundamental metal nanoparticle / catalytic support system energetics to provide information that can accelerate catalysis research and improve catalyst design. A key descriptor for metal / support system energetics is the metal atom chemical potential versus particle size, because the chemical potential determines catalyst reactivity and stability. The chemical potential also plays a key role in governing the sintering resistance of these systems, which is crucial because sintering poses one of the greatest challenges to preserving the activity of industrial catalysts over time. As shown here, the adhesion energy of a bulk metal / support system directly gives the chemical potential versus particle size of a system, and the adsorption energies of the metals to the support versus metal coverage also directly provide their chemical potential versus size. The best method for measuring metal adsorption and adhesion energies to well-defined solid surfaces is single-crystal adsorption calorimetry (SCAC), because it directly measures the heat of adsorption of metal atoms on an ordered support surface with known atomic structure. In this work, SCAC is used to measure the adhesion and adsorption energies of multiple systems, in a directed effort to predict the energetics of metal / support systems and provide benchmarks for improving theoretical methods. The first system studied here is Pd / graphene / Ni(111). Although most preceding SCAC measurements were made on systems involving oxide surfaces, carbon-based supports have gained much attention and have become very important in industry as well as very promising in catalysis research for a large variety of applications. The adsorption energies and chemical potential as a function of Pd nanoparticle size are reported, and the adhesion energy of Pd / graphene / Ni(111) is found to be 3.5 J/m2. The growth morphology of the Pd nanoparticles was investigated using He+ low-energy ion scattering spectroscopy (LEIS). Because the adhesion energy of two other metals, Ag and Ni, had been previously measured using SCAC, this Pd data reveals a new trend correlating the adhesion energies of metals to graphene / Ni(111). A linear trend is observed between the adhesion energy of a metal to graphene / Ni(111) and the carbophilicity of the metal, which is estimated here based on DFT calculations. The next system studied here is Cu / rutile-TiO2(100). The Cu adsorption energies and Cu chemical potential versus Cu nanoparticle size are reported. The Cu particle size versus coverage was measured using He+ LEIS. The growth of the particles was initially modeled using the hemispherical cap model (HCM), and then that model was converted to the recently developed spherical cap model (SCM), which can determine the contact angle of metal nanoparticles. The contact angle of the Cu nanoparticles on rutile-TiO2(100) is 67°. We also show that the recently-published mathematical shortcut, by which the SCM can be determined from the HCM, is accurate. The adhesion energy of Cu / rutile-TiO2(100) is found to be 2.5 J/m2, from the SCM. The adhesion energy is also compared to previous measurements on rutile-TiO2(100), and the adhesion energy of metals to this surface tends to linearly correlate with the metal oxophilicity. For oxides, linear proportionalities between the adhesion energy of the metal to a given surface and the oxophilicty of the metal have been previously observed for two surfaces: MgO(100) and CeO2(111). In the next part of this work, we show that this proportionality between the oxophilicity of a metal and the adhesion energy of that metal to the oxide surface also holds for rutile-TiO2(100), a third proportionality of this kind. Although these proportionalities are great for predicting the adhesion energies of late transition metals on the three surfaces mentioned, there was still lack of knowledge when it comes to predicting adhesion energies of these metals across other oxide supports. This work shows that the slope of the adhesion energy versus oxophilicity proportionality for a given oxide surface can be predicted based on its surface oxygen vacancy formation energy and/or the heat of reduction of the bulk oxide to its next lower oxidation state. The ability to predict these adhesion energy values is a valuable tool for catalyst design, since it allows predicting metal chemical potential versus particle size for different oxide supports.