Heterogeneous catalyst preparation by liquid-phase atomic layer deposition

Heterogeneous catalyst preparation has been performed for close to a century but has been rapidly evolving spurred by the evolution in synthetic techniques and growing needs in energy, chemicals and materials. An increasing number of catalyst synthesis strategies and tools have emerged in the past decades, including Atomic Layer Deposition (ALD). This technique allows the deposition of thin films with unmatched accuracy. Originally designed for flat substrates, this method has recently begun to be used for the preparation of heterogeneous catalysts. The development of ALD, both in academic research and industrial processing, is inhibited by in-herent high investment and operating costs of the technique. The global aim of this doctoral project was to design new ALD processes specifically designed for heterogeneous catalyst preparation. This aim is achieved in three steps: transposing gas-phase layer-by-layer growth to the liquid phase, matching the quality and versatility of gas-phase ALD in the liquid phase, and finally going beyond what is technically possible with gas-phase ALD set-ups. The first chapter of this manuscript describes current state-of-the-art methods used both in hetero-geneous catalyst preparation and atomic layer deposition on particles. This chapter does not aim to be a comprehensive review but to present the main research developments necessary to understand this thesis work in a broader context. The second chapter is based on a published article in which I describe a first step toward liquid-phase ALD. A procedure was developed for stoichiometrically limited layer-by-layer deposition of alumina in solution onto a copper-based catalyst. A conformal porous layer was obtained which al-lowed the diffusion of chemicals to the copper below. Additionally, the deposited layer was an ef-ficient barrier against irreversible deactivation during the catalytic hydrogenation of a biosourced molecule in a continuous flow reactor. The third chapter, based on a published communication, presents the fully optimized method for liquid-phase ALD. In this work, I demonstrate the self-limiting nature of the studied chemical reac-tions and use this process to deposit various type of inorganic layers (oxide, phosphate and sulfide) on a broad range of substrates relevant for catalysis. This technique also allows a direct monitoring of surface reaction, highlighting phenomena such as nucleation of clusters and their coalescence to form films. The materials were extensively characterized by electronic microscopy to confirm that the quality of the coating matched that of coatings obtained via gas-phase ALD. The fourth chapter describes soon to be submitted work where deposition is used during the early stages of nucleation on a surface to form targeted atomic clusters for catalysis. The goal was to build a catalytic cluster atom-by-atom to carefuly tune its chemical environment. Starting from a single Zn atom deposited on silica, I demonstrate the positive effect of Al on dehydrogenation catalytic activ-ity and the enhancement of stability brought by Si anchoring. These effects demonstrate that liquid-phase ALD can be more than just a tool for overcoating catalysts but also a method for the rational design of new catalysts. Finally, the thesis concludes with my perspective on new challenges and opportunities in the further development of liquid-phase ALD.