Surface-Sensitive Spectroscopy of the Catalytic Hydrogenation of CO and CO2

Abstract

This PhD Thesis described surface-sensitive spectroscopy as a promising analytical tool for the better understanding of reaction and deactivation mechanisms in heterogeneous catalysis. Chapter 1 introduced the basic principles of spectroscopy and catalysis, as well as how fundamental insights are obtained through spectroscopic characterization under realistic reaction conditions. We set the stage for investigating the cobalt-catalyzed CO and CO2 hydrogenation reactions. A two-pronged approach was formulated: firstly, we aimed to provide new or improved insights into the catalytically active phase. Secondly, we targeted to uncover the reaction mechanisms during the formation of (un)desired reaction products. Chapter 2 introduced nanoscale spectroscopy techniques, which are promising tools to achieve the goals formulated above. Although the academic world strives to understand the fundamental phenomena that occur on the catalysts’ surfaces, the most sensitive analytical tools are often unable to operate under the harsh reaction conditions that industrial catalysis requires. This chasm was discussed and a more realistic model system consisting of cobalt nanoparticles on titania islands was introduced. In Chapter 3, infrared and Raman spectroscopy were used to decipher the reaction mechanisms involved in hydrocarbon product formation during the CO hydrogenation with cobalt-titania catalysts. By modulating the concentration of the reactant CO, infrared spectroscopy was turned into a surface-sensitive method and consequently active species were observed during the reaction. Hydrogen-containing oxygenated species responded to the CO stimulus and thus provided evidence for the hydrogen-assisted C-O bond scission. Gold nanoparticles were used to enhance the Raman spectroscopy signal and evidence was observed for the direct C-O bond scission mechanism. The reactant modulation idea recurred in Chapter 4 to investigate the catalytically active phase and reaction mechanisms during CO2 hydrogenation with cobalt-based catalysts. We also refute the general consensus that metallic cobalt is the only active phase in heterogeneous catalysis, as cobalt oxide nanoparticles on titania performed better in terms of long-chain hydrocarbon yield compared to the metallic cobalt variant. While metallic cobalt dissociated CO2 into CO directly, cobalt oxide required hydrogen atoms to split the C-O bonds. In Chapter 5, we explored potassium promoter effects in the CO and CO2 hydrogenation reactions with cobalt-titania catalysts. The addition potassium to the cobalt-titania catalyst appeared interesting for renewable energy applications that aim to convert CO2 into long-chain hydrocarbons. Potassium made the cobalt surface slightly positively charged and enabled the conversion of CO2 into the more reactive CO molecule via the reverse water-gas shift reaction. Besides, the amount of hydrogen on the catalyst surface diminished upon the addition of potassium. In Chapter 6, a solid mineral residue from industrial biomass gasification was repurposed as a solid catalyst material. The residue contained around 15 different elements, of which iron was the most important one. Iron enabled the conversion of a gas feed mixture of CO, CO2, H2, and N2 into methane and olefins. By means of operando X-ray diffraction, the transformation of metallic iron into an iron carbide phase was associated with an increase in total carbon conversion and an improved selectivity towards the desired lower olefins.