Synthesis of Nanoparticle Model Systems for Sustainable Catalysis by Gas Aggregation

Abstract

The overall goal of this thesis is to develop better catalysts for chemical reactions used in sustainable energy storage and environmental protection. Specifically, the thesis presents research on well-defined catalyst model systems of nanoparticles synthesized by magnetron sputtering, gas-aggregation, and subsequent massfiltering. The thesis opens with a presentation of the broader context of the research, particularly focusing on the societal importance of catalysis, followed by an introduction to the fundamentals of the science of catalysis. Three research projects are then described in individual chapters, summarized in the following: Platinum Catalysts for TW-Scale H2 Production: Platinum has the highest activity of known catalysts for the hydrogen evolution reaction (HER), but due to scarcity and price it is often assumed to by infeasible for photoelectrochemical water splitting on the terawatt-scale that is needed for significant global impact. This study investigates the relationship between catalytic activity for the HER and platinum catalyst loading using well-defined model systems with different loadings of mass-selected 5nm Pt nanoparticles. Using the knowledge gained on these systems, a technoeconomic analysis is carried out, showing that photoelectrochemical HER at a current density of 10 mA/cm2 and an overpotential of 50mV could be obtained with a catalyst consumption of 54 tons of Pt per TW energy stored in H, corresponding to ∼ 1/4 of the global, annual Pt production. Synthesis of Ni−Mo−S Nanoparticles by Reactive Gas Aggregation: In this project, a method was developed for synthesizing in-flight sulfided Ni-Mo-S nanoparticles by aggregation of sputtered metal from a Mo75Ni25 target in a reactive atmosphere of Ar and H2S. The resulting particles are undersulfided with a stoichiometry of Mo0.8Ni0.2S1.1, and the particles exhibit high-surface area morphologies such as platelets, very different from the spherical morphologies observed for metal nanoparticles. The particles are mass-filtered before deposition, and it is shown that different masses results in significantly different particle morphologies. Using a microreactor platform, the catalytic activity of the nanoparticles is assesed for hydrodesulfurization (HDS) of dibenzothiophene, relevant for e.g. production of diesel with ultra-low sulfur content. It is found that in-flight sulfided Ni-Mo-S nanoparticles have more than twice as high HDS activity as Ni-Mo-S nanoparticles produced by inert gas aggregation and post-sulfidation. This points towards the potential of  engineering nanoscale catalysts for HDS by reactive gas aggregation synthesis of nanoparticles. Dynamic Effects of Surface Oxygen in CO Electroreduction: One of the keys to developing better catalysts for energy-storage by electrolysis of CO2 is to understand the principles behind electroreduction of the reaction intermediate CO. This study reports the discovery of a high, transient production of methane at the onset of electroreduction of CO on mass-selected copper nanoparticles produced by inert gas-aggregation, investigated with a newly developed system for electrochemical mass-spectrometry. This "dynamic methane" is only observed when the nanoparticles have been exposed to O2 before the electrode potential is stepped to CO-reduction potentials. Based on analysis of experimental data and density functional theory, it is proposed that the dynamic methane is formed on nanoparticle kink-sites, which are activated by adsorbed oxygen; the transient nature of the dynamic methane is attributed to the fact that the adsorbed oxygen is only metastable. The results contribute to the understanding of the role of oxygen in CO-electroreduction, and could potentially be used on industrial scale if metastable active sites were cyclically regenerated. The presented research demonstrates the insights that can be gained by studying well-defined model systems in catalysis, which can both contribute to fundamental scientific understanding and guide the development of catalysis technology.