Model Nanocatalysts with Tunable Reactivity: Tailoring the Structure and Surface Chemistry of Nanomaterials for Energy and Alternative Fuels Catalysis

One of the most pressing challenges of our time is meeting growing global energy demands while limiting human impact on the environment. To meet this challenge, new catalysts are needed to enable carbon neutral energy production processes and low cost synthesis of alternative fuels. In order to design new catalysts for such processes, fundamental understanding is needed on how the structural and chemical properties of nanostructured materials influences their surface chemistry. In this dissertation, I explore how the properties of nanoparticles, such as particle size, shape, composition, and chemical state, can be used to tune their reactivity. For this work, model nanoparticles were synthesized with well-defined structural and chemical properties, and a variety of surface science approaches were used for catalyst characterization. In particular, emphasis was placed on understanding the changes which may occur in the catalyst structure and chemical state during a reaction using advanced in situ techniques and correlating these changes to reactivity. After exploring how nanostructuring the catalyst surface can be used to tune reactivity and how dynamic changes can occur to nanocatalysts in reactive environments, these general principles are applied to a model reaction, the electroreduction of carbon dioxide, which is a promising process for synthesizing valuable products using renewable energy while consuming waste carbon dioxide. I explore the mechanisms behind how catalyst particle size, composition, and oxidation state can be used to improve activity and tune selectivity towards different carbon dioxide reduction products. Such fundamental mechanistic insights are critically needed to design efficient catalysts for this reaction

Observations of Late-Type Stars with the Infrared Spatial Interferometer

The Infrared Spatial Interferometer (ISI) has been conducting mid-IR measurements of red supergiant and AGB stars for about 20 years. This paper reviews the ISI system and results with an emphasis on measurements of changes in stellar sizes and shapes, and those of the surrounding dust shells, over time scales of weeks to decades. The long-term measurement record provides important new observables for stellar theory. A new spectrometer-correlator system is discussed where this system will measure visibilities on-and-off individual molecular spectral lines

Size-Dependent Reactivity Of Gold-Copper Bimetallic Nanoparticles During Co2 Electroreduction

New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5 nm. Product analysis showed formation of H2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying

Dynamic Changes In The Structure, Chemical State And Catalytic Selectivity Of Cu Nanocubes During Co2 Electroreduction: Size And Support Effects

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size- and shape-controlled ligand-free Cu nanocubes during CO2 electroreduction (CO2RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuOx species observed were found to lead to a suppression of the selectivity for multi-carbon products (i.e. C2H4 and ethanol) versus CH4. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of CuI species under CO2RR in the former samples. Both factors are held responsible for the higher C2/C1 product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO2RR selectivity

Operando Phonon Studies Of The Protonation Mechanism In Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts

Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction