Song Yi Moon

The effect of hot electrons and surface plasmons on heterogeneous catalysis.

Hot electrons and surface-plasmon-driven chemistry are amongst the most actively studied research subjects because they are deeply associated with energy dissipation and the conversion processes at the surface and interfaces, which are still open questions and key issues in the surface science community. In this topical review, we give an overview of the concept of hot electrons or surface-plasmon-mediated hot electrons generated under various structural schemes (i.e. metals, metal-semiconductor, and metal-insulator-metal) and their role affecting catalytic activity in chemical reactions. We highlight recent studies on the relation between hot electrons and catalytic activity on metallic surfaces. We discuss possible mechanisms for how hot electrons participate in chemical reactions. We also introduce controlled chemistry to describe specific pathways for selectivity control in catalysis on metal nanoparticles.

Photocatalytic activity of metal-decorated SiO2@TiO2 hybrid photocatalysts under water splitting

We report the fabrication of a metal-decorated hybrid nanocomposite with TiO2 encapsulation (Metal/SiO2@TiO2, Metal=Pt or Ru) using a simple surface-modification chemical process. Metal nanoparticles capped with polyvinylpyrrolidone were successfully assembled on functionalized SiO2 via electrostatic interactions, after which a thin layer of TiO2 was coated on the surface by the sol-gel process to avoid agglomeration of the coated silica spheres. Transmission electron microscopy studies confirmed that the metal nanoparticles were uniformly distributed throughout the surface of the SiO2 with a thin layer of TiO2. In addition, X-ray diffraction was employed to ensure the crystal structure of the uniformly coated thin TiO2 layer. Even after calcination at 500 °C, the structure remained intact, confirming high thermal stability. The photocatalytic activity of the metal-decorated SiO2/TiO2 nanocomposites was evaluated using the H2 evolution reaction. The Metal/SiO2@TiO2 catalysts show the photocatalytic water splitting efficiency for H2 generation (i.e., 0.14% for Pt/SiO2@TiO2 and 0.12% for Ru/SiO2@TiO2), while there is no generation of H2 on the Metal/SiO2 without a coating layer. These results indicate that the anatase crystalline coating layer has good thermal and chemical stability and plays a significant role in photocatalytic H2 production.

Photocatalytic H2 generation on macro-mesoporous oxide-supported Pt nanoparticles

A series of photocatalysts were prepared with crystalline macro-mesoporous oxides and Pt nanoparticles (Pt–TiO2, Pt–Ta2O5, Pt–Nb2O5, Pt–ZrO2, and Pt–Al2O3). Transmission electron microscopy, X-ray diffraction, and N2 adsorption–desorption isotherms reveal that the oxides have prominent macro- and mesoporosity in the crystalline walls. The high surface area and crystalline walls of the oxides play significant roles in photocatalytic H2 production. Pt–TiO2 catalysts show enhanced photocatalytic water splitting efficiency for H2 generation (solar energy conversion efficiency of 1.06%) under a 150 W and 1.5 AM solar simulator. Pt–Ta2O5 and Pt–Nb2O5 also generate noticeable photocatalytic activities of 0.21% and 0.16%, respectively. The enhanced photocatalytic activity is attributed to correct band alignment of the porous oxides with absorption in the UV-visible range, and ordered macro- and mesoporosity of the crystalline oxides for efficient charge transfer.

Hot Electrons at Solid–Liquid Interfaces: A Large Chemoelectric Effect during the Catalytic Decomposition of Hydrogen Peroxide

The study of energy and charge transfer during chemical reactions on metals is of great importance for understanding the phenomena involved in heterogeneous catalysis. Despite extensive studies, very little is known about the nature of hot electrons generated at solid-liquid interfaces. Herein, we report remarkable results showing the detection of hot electrons as a chemicurrent generated at the solid-liquid interface during decomposition of hydrogen peroxide (H2 O2 ) catalyzed on Schottky nanodiodes. The chemicurrent reflects the activity of the catalytic reaction and the state of the catalyst in real time. We show that the chemicurrent yield can reach values up to 10(-1) electrons/O2 molecule, which is notably higher than that for solid-gas reactions on similar nanodiodes.

Liquid-phase catalytic reactor combined with measurement of hot electron flux and chemiluminescence

Understanding the role of electronically nonadiabatic interactions during chemical reactions on metal surfaces in liquid media is of great importance for a variety of applications including catalysis, electrochemistry, and environmental science. Here, we report the design of an experimental apparatus for detection of the highly excited (hot) electrons created as a result of nonadiabatic energy transfer during the catalytic decomposition of hydrogen peroxide on thin-film metal-semiconductor nanodiodes. The apparatus enables the measurement of hot electron flows and related phenomena (e.g., surface chemiluminescence) as well as the corresponding reaction rates at different temperatures. The products of the chemical reaction can be characterized in the gaseous phase by means of gas chromatography. The combined measurement of hot electron flux, catalytic activity, and light emission can lead to a fundamental understanding of the elementary processes occurring during the heterogeneous catalytic reaction.

Iron-doped ZnO as a support for Pt-based catalysts to improve activity and stability: enhancement of metal–support interaction by the doping effect

In heterogeneous catalysis, the role of the interface between a metal and a metal oxide in deciding catalytic performance has remained a long-standing question. Out of many molecular-scale factors that affect the properties of metal–oxide interfaces, doping or impurities in the oxides can result in excess charge carriers or oxygen vacancies on the oxides, which lead to a change in catalytic activity. For a model system with a tunable dopant, we employed Pt nanoparticles with Fe doping. We synthesized a series of Fe-doped ZnO with different Fe loadings (i.e., 0, 1, and 4%) using the co-precipitation method, and then deposited Pt nanoparticles onto these supports. The Pt-based catalysts were employed to investigate the effect of the dopant to promote the catalytic performance for the CO oxidation reaction. The 4% Fe loading sample showed the highest catalytic activity among the catalysts, with a turnover frequency of 5.37 s−1 at 126 °C. The dopant was found to enhance the interaction between the Pt nanoparticles and the catalyst support, including the prevention of metal sintering, which resulted in an improvement of catalytic activity.

Probing surface oxide formations on SiO2-supported platinum nanocatalysts under CO oxidation

Formations of an ultrathin oxide layer on noble metal catalysts affect the characteristics of fundamental molecular behaviours such as adsorption, diffusion, and desorption on their surfaces. That is directly correlated to enhancement of catalytic activity under operating conditions because the kinetics of catalytic reactions are also simultaneously influenced. Especially, a sub-monolayered surface oxide is known as having a key role for improving catalytic activity, but revealing its existence in catalysis is challenging due to their fast chemical conversion. Herein, we report the first evidence of surface oxide formations on platinum (Pt) nanocatalysts under CO oxidation probed with a diffuse reflectance infrared Fourier transform (DRIFT) technique. Spectroscopic information demonstrates that the abrupt blue shift of adsorbed CO molecules vibrational frequencies of CO stretching mode on the reduced Pt nanocatalyst surface is initiated prior to aggressive CO conversion to CO2 gas molecules. Site-specific replacements of the adsorbed CO molecule with dissociative oxygen occur just before the ignition temperature that is supposed to be an important reaction step for CO oxidation over a Pt nanocatalyst. Density functional theory (DFT) calculation results support this phenomenon as a function of relative atomic fractions between CO and O on a Pt model surface and consistently show a similar trend with experimental evidence.

Hot-Electron-Mediated Surface Chemistry

Hot electrons and surface-plasmon-mediated surface chemistry have drawn much attention in the field of surface and interfacial chemistry because they are intrinsically associated with energy dissipation and conversion processes at the surface and at interfaces. In this chapter, we give an overview of the concept of hot-electron generation under various structural schemes, including Schottky diodes, and the role of hot electrons in the catalytic activity of surface chemistry. We highlight recent studies on the relation between hot electrons and catalytic activity on metallic surfaces and discuss possible mechanisms for how hot electrons participate in chemical reactions.