Operando study of CO$_2$ hydrogenation over Cu-Zn nanoparticle-based catalysts

Methanol is one of the most widely produced chemicals globally, valued for its importanceas both a chemical feedstock and a potential fuel in the transition toward a carbon-neutraleconomy. The industrial synthesis of methanol from syngas mixtures of CO, CO₂, and H₂is facilitated by the use of the Cu-ZnO-Al2O3 (CZA) catalyst. Despite decades of research,the exact reaction mechanism of the CZA catalyst is still debated. Various theories havebeen proposed regarding the active sites of the catalyst, including copper-zinc alloys,copper decorated with zinc atoms, or a ZnOx overlayer with oxygen vacancies at the Cu-ZnO interface.Possibly the most important challenge to be overcome in order to advance theunderstanding of this catalytic system is the so-called pressure gap. This gap refers to thediscrepancy between the ultra-high vacuum to ambient pressure conditions under whichsurface science techniques typically operate, and the high-pressure conditions (50 barand above) encountered in real-world industrial processes. This thesis focuses onbridging the pressure gap by employing surface-sensitive X-ray diffraction (XRD) at bothambient and high pressures to gain structural insights into the CZA catalyst system. Theprimary goal was to investigate the dynamic behaviour of copper and zinc duringmethanol synthesis under various reaction gas mixtures and high pressures that simulateindustrial conditions. For this purpose, two model systems of the catalyst were preparedusing molecular beam epitaxy (MBE) under ultra-high vacuum (UHV) conditions: (1) asimplified model system comprising epitaxial Cu nanoparticles supported on single-crystal Al2O3 substrates, and (2) a ZnO-supported model system in which metallic Zn wasfirst deposited onto the Al2O3 support, oxidized under UHV conditions, and then followedby the deposition of Cu. A custom MBE chamber was designed and commissioned for theprecise deposition of metallic Zn, allowing precise control over the amount of Znincorporated into the samples.Structural analysis of the catalyst under operando conditions was enabled by thedevelopment and improvement of a special high-pressure surface XRD (SXRD) setup. Thisadvanced system was utilized to gather novel structural information about the Cu andCu2O phases of the catalyst under various reaction gas mixtures, operating at a systempressure of 30 bar.One of the major questions addressed in this thesis was whether a CuOx-type structure orpartially oxidized copper phase forms under reaction conditions. No evidence of such aphase above 575 K was found, either at ambient pressure or operando pressure. Instead,copper remained in its metallic state during exposure to various reaction gas mixtures,including CO, CO₂, and H₂. For both model systems, oxidation was observed only underpure Ar gas flow containing trace amounts of oxygen and water (around 1 ppm). Thesample preparation yielded epitaxial (111)-oriented Cu nanoparticles, which were foundto retain their crystal orientation with respect to the sapphire substrate under variousreaction gas mixtures at ambient pressure, with a particle size ranging between 8-10 nm.Upon exposure to air at room temperature, these Cu nanoparticles formed core-shell Cu-Cu2O structures, where the Cu2O shell maintained the same crystal orientation as the Cucore. Even under high pressure reducing conditions, the Cu2O shell remained stable below575 K, with full reduction of the Cu occurring only at higher temperatures. Additionally,the formation of (110)-oriented Cu particles was observed if the sample temperature wasincreased stepwise to 575 K. These (110)-oriented particles displayed faster oxidationand reduction dynamics compared to (111)-oriented particles, despite comparableparticle size.A central focus of this work was the highly debated issue of Cu-Zn alloy formation duringcatalytic reactions. The study revealed the presence of two distinct Cu-Zn alloy phases:Cu0.2Zn0.8 and Cu0.64Zn0.36. The data suggest a dynamic interplay between these two alloyphases, dependent on the amount of hydrogen in the reaction gas mixture. Partialoxidation of the Cu0.2Zn0.8 phase occurred in reaction gases with little to no hydrogen,resulting in a transition to the more Cu-rich Cu0.64Zn0.36 phase. This transition wasaccompanied by the diffusion of zinc into the metallic copper particles and the potentialformation of ZnOx overlayers. These observations challenge the conventionalunderstanding that the formation of ZnO/ZnOx and Cu-Zn alloys phases is solely governedby the oxidizing or reducing nature of the gas environment. Instead, this work suggeststhat the amount of hydrogen in the gas mixture plays a crucial role in determining theformation and oxidation states of the Cu-Zn alloy phases.Finally, the Al2O3 substrate was observed to remain stable across all investigated reactiongas mixtures and at both ambient and operando pressures. This confirms its stabilizingrole in the catalytic system, without any indication of active participation in the catalyticprocess itself.