Investigation of the heterogeneously catalysed gas phase CO2 hydrogenation reaction: Development of analysis methods and reaction analysis on pristine metal catalysts

The aim of this thesis is to contribute to the development of analysis methods that enable an improved investigation of the CO2 hydrogenation reaction such as quantitative mass spectrometry and infrared thermography. Furthermore, the contribution is in the characterization of the activity of basic catalyst building elements such as Fe, Co, Ni, Cu and Ru. To achieve these aims, we have developed and build a gas-controlling and analysis system to carry out the experiments in a wide range of conditions. In the frame of setting up the instruments and working on the analysis scripts, a rapid and quantitative gas analysis method by means of mass spectrometry was developed. In a second step, the CO2 hydrogenation reaction over Fe, Co, Ni and Cu in their pristine form was investigated with a particular focus on the catalytic activity and the activation energies. An Al2O3 supported Ru catalyst was used as reference catalyst. We showed that the reaction goes through a similar rate limiting step on Co, Ni and Ru/Al2O3, while the reaction rates are significantly different. Pristine Co showed minor activity in the formation of C2+ hydrocarbons. Fe was mostly active in the Carbon Monoxide formation. Cu exhibited no catalytic activity without a supporting metal-oxide phase. Based on these findings, we carried out an in-depth kinetical analysis of the catalytic CO2 hydrogenation over pristine Ni, Co and Ru/Al2O3 was carried out. A reaction model describing the CO2 hydrogenation over a broad temperature range, taking kinetically, diffusional and thermodynamic limitations into account, was developed. Diffusional limitation occurs at temperatures where the reaction kinetics is already rapid and diffusion of educts -and products to -and from the catalyst and its pores becomes rate limiting before the thermodynamic limitations are approached. The turnover rate of the CO2 conversion depends critically on the partial pressure of CO2 in the educts gas stream: With a high CO2 partial pressure, the catalyst surface saturates and the reaction order approaches zero. Out of the unsupported pristine metals Fe shows the lowest activation energy, but also the lowest activity while Co showed the highest activity and the highest activation energy for the overall CO2 conversion. Therefore, we have synthesized bimetallic nanosized pristine Fe-Co catalysts to investigate the influence of alloying Fe to Co and how this influences the activation energies of the formation of methane and C2-C5 hydrocarbons. We found clear trends towards higher activation energies with increasing Fe content- attributed to the stronger binding energy of CO2 to Fe. Furthermore, a new experiment for thermography was built with the aim to visualization the exothermic reaction dynamics. The dedicated reaction cell with an IR transparent window is coupled with the mass spectrometer. With the rapid gas sampling, the gas composition can directly be linked to the evolution of the thermal signal, contributing to the deeper understanding of the reaction start-up. We found and could prove that a CO2 covered surface has a strong inhibiting effect on the reaction startup while an H2 covered surface leads to a linear acceleration of the reaction front in the form of a thermal runaway.