Quantitative detection of hot electrons excited in surface chemical and relationship between hot electron and selectivity

Abstract

The intrinsic correlation between an enhancement of catalytic activity and the flow of hot electrons generated at metal–oxide interfaces suggests an intriguing way to control catalytic reactions and is a significant subject in heterogeneous catalysis. Thus, understanding fundamental mechanisms of energy conversion and dissipation at solid-gas or solid-liquid interfaces are crucial issues for the field of heterogeneous catalysis. Catalytic selectivity, or the production of only one desired molecule that may be used as a fuel or chemical out of several thermodynamically possible molecules, is the foundation of surface chemistry. During catalytic reactions, electronic excitation taking place on the surface creates energetic electrons called “hot electrons” that have a significant impact on catalytic reactions. Charge transfer at metal-oxide interface is an important parameter for enhancing the selectivity of desirable reaction, thus, fundamentally identifying the electronic excitation at metal–oxide interfaces is crucial for designing heterogeneous catalyst structures with the desired activity and selectivity. Despite its importance in fundamentally understanding electronic excitation on the surface, no reports show the relation between hot electron flow and catalytic selectivity. Motivated from this point, I have studied extensive research of the hot electron dynamics under the gas-phase surface chemical reaction (e.g., CO oxidation, methanol oxidation, and hydrogen oxidation) with hybrid nanostructures (e.g., metal-semiconductor or metal-oxide interface). In this dissertation, Chapter 1 introduces the research background of hot electron generation on metal surfaces by depositing external energy. In Chapter 2, I demonstrate the intrinsic relation between hot electron flow and catalytic selectivity using a Pt/n-type $TiO_2$ Schottky nanodiode. On the Pt thin film, hot electron flow was generated by methanol oxidation exhibiting a two-path reaction of either full oxidation to $CO_2$ or partial oxidation to methyl formate

a steady-state chemicurrent was detected. We show that hot electron generation is more effective in the reaction pathway that produces methyl formate. Based on these results, we conclude that the selectivity for methyl formate production correlates well with hot electron generation because of the higher exothermicity of generating the intermediate, as was confirmed using theoretical calculations based on the density functional theory. In Chapter 3, for finding the hot electron dynamics at the metal-oxide interface, I fabricated a catalytic Schottky nanodiode by placing Pt nanowrie arrays on $TiO_2$ to form a Pt-$TiO_2$ interface. We observed that when Pt nanowire arrays were supported on $TiO_2$ than Pt thin films, they showed higher partial oxidation selectivity due to the Pt-$TiO_2$ interface, which also increased the chemicurrent yield indicating the efficiency of hot electron generation. Based on a comparative analysis of results for Pt nanowire arrays forming Pt-$TiO_2$ interface and Pt thin film which does not form an interface, we showed that the metal-oxide interface can promote the partial oxidation reaction, and enhance the efficiency of hot electron generation, which was also proved by theoretical calculations based on the density functional theory. In Chapter 4, I demonstrate surface plasmon-induced catalytic enhancement by the peculiar nanocatalyst design of hexoctahedral (HOH) Au nanocrystals (NCs) with $Cu_2O$ clusters. We found that this inverse catalyst comprising a reactive oxide for the catalytic portion and a metal as the source of electrons by localized surface plasmon resonance exhibits a change in catalytic activity by direct hot electron transfer or plasmon-induced resonance energy transfer when exposed to light. Finite-difference time domain calculations show that a much stronger electric field was formed on the vertex sites after growing the $Cu_2O$ on the HOH Au NCs. These results imply that catalytic activity is enhanced when hot electrons, created from photon absorption on the HOH Au metal and amplified by the presence of surface plasmons, are transferred to the reactive $Cu_2O$. In Chapter 5, I report the effect of metal–oxide interfaces on CO oxidation catalytic activity with inverse $TiO_2$-nanostructured Au catalysts. CO oxidation activity increases as the Ti content increases up to 0.5 wt% likely due to active $TiO_2$–Au interface sites enhancing CO oxidation via the supply of adsorption sites or charge transfer from the $TiO_2$ to the Au. However, higher titania content (i.e., 1.0 wt% $TiO_2$) resulted in decreased activity caused by high surface coverage of $TiO_2$ decreasing the number of $TiO_2$–Au interface sites. These results imply that the perimeter area of the metal–oxide interface plays a significant role in determining the catalytic performance for CO oxidation.