Monodisperse Metal Nanoparticle Catalysts: Synthesis, Characterizations, and Molecular Studies Under Reaction Conditions

Abstract

We aim to develop novel catalysts that exhibit high activity, selectivity and stability under real catalytic conditions. In the recent decades, the fast development of nanoscience and nanotechnology has allowed synthesis of nanoparticles with well-defined size, shape and composition using colloidal methods. Utilization of mesoporous oxide supports effectively prevents the nanoparticles from aggregating at high temperatures and high pressures. Nanoparticles of less than 2 nm sizes were found to show unique activity and selectivity during reactions, which was due to the special surface electronic structure and atomic arrangements that are present at small particle surfaces. While oxide support materials are employed to stabilize metal nanoparticles under working conditions, the supports are also known to strongly interact with the metals through encapsulation, adsorbate spillover, and charge transfer. These factors change the catalytic performance of the metal catalysts as well as the conductivity of oxides. The employment of new in situ techniques, mainly high-pressure scanning tunneling microscopy (HPSTM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) allows the determination of the surface structure and chemical states under reaction conditions. HPSTM has identified the importance of both adsorbate mobility to catalytic turnovers and the metal substrate reconstruction driven by gaseous reactants such as CO and O-2. APXPS is able to monitor both reacting species at catalyst surfaces and the oxidation state of the catalyst while it is being exposed to gases. The surface composition of bimetallic nanoparticles depends on whether the catalysts are under oxidizing or reducing conditions, which is further correlated with the catalysis by the bimetallic catalytic systems. The product selectivity in multipath reactions correlates with the size and shape of monodisperse metal nanoparticle catalysts in structure sensitive reactions.