Study on quantitative measurement and manipulation of hot electrons for catalytic surface reactions

Abstract

Catalysis is a core task for developing the next-generation energy technology as well as traditional chemical industry because catalysts are used in more than 85 % of the practical chemical processes. For innovations in heterogeneous catalysis, molecular-level understanding and control of surface reaction are of underlying interest as well as of tremendous technological relevance. In recent year, ‘nanocatalyts’, combined with nanotechnology have been intensively studied to improve catalytic activity and selectivity. With intensive development of in-situ techniques including high-pressure XPS and STM, many unprecedented surface phenomena under chemical reactions have been proved during the past decade, however, it still remains a critical challenge to demonstrate ultrafast kinetics and ambiguous electronic effect on surface reactions, especially, both under condition of real applications and on nanocatalysts. Elucidation of the electronic effect on catalytic activity requires direct detection of hot electrons, which is the energetic electrons generated on the surface of catalysts during chemical reactions, on the nanocatalysts

however, this is challenging because of quick thermalization of hot electrons via electron？electron scattering, electron？phonon coupling, and the electrical disconnection of the nanocatalysts. Motivated from this, I have studied the hot electrons dynamics on the nanocatalysts under real reaction conditions with advanced catalytic devices. In this dissertation, Chapter 1 and 2 introduce the research background and experimental methods and techniques, repectively. In Chapter 3, I demonstrate the direct detection of hot electrons induced by exothermic hydrogen oxidation on Pt nanoparticles of two different size (1.7nm and 4.5nm) and its size effect using an $Au/TiO_2$ Schottky nanodiode. For this purpose, Pt nanoparticles are deposited on $Au/TiO_2$ nanodiodes, which allow detection of hot electrons excited during the chemical reaction. It is shown that Pt nanoparticles of smaller size lead to higher chemicurrent yield, which is associated with the shorter travel length for the hot electrons, compared with their inelastic mean free path. I also show the impact of capping on charge carrier transfer between Pt NPs and their support. In Chapter 4, a novel graphene-based catalytic nanodiode (i.e., Pt nanoparticles/graphene/$TiO_2$ nanodiode) is introduced, where Au film is replaced to a layer of graphene which is promising catalytic support due to its unique electronic, thermal, and chemical properties. By making a comparative analysis of data obtained from measuring the hot electron current and turnover frequency, we demonstrate that graphene’s unique electronic structure and extraordinary material properties allow improved conductivity at the interface between the catalytic Pt nanoparticles and the support. Thereby, graphene-based nanodiodes offer an effective and facile way to approach the study of chemical energy conversion mechanisms in composite catalysts with carbon-based supports. Lastly in Chapter 5, I show hot electrons excited on PtCo bimetallic nanoparticle during $H_2$ oxidation as a chemicurrent in a catalytic nanodiode, whose signal is directly related to its catalytic activity of the PtCo NPs. Through many research, it has been elucidated that the catalytic performance of the bimetallic nanocatalysts is significantly enhanced, however, the clear origin of these phenomena has been wrapped in a veil. I reveal that the presence of CoO/Pt interface enables efficient transport of the electrons, resulting in outstanding catalytic performance of the PtCo nanoparticles using a real-time quantitative detection of hot electrons induced by a chemical reaction on catalysts.