Catalytic hydrogenation of CO2 from flue gas

Abstract

The thesis focuses on the catalytic hydrogenation of CO2 from flue gas. This process aims at CO2 recycling to obtain valuable compounds and requires very active catalysts which are resistant against contaminants present in the flue gas. Flue gas contains about 10 – 15 vol. % CO2, nitrogen, oxygen, water and traces of sulfur containing substances like H2S. A common catalyst for CO hydrogenation to methanol is Cu/ZnO/Al2O3 which is active also for CO2 hydrogenation but deactivates fast during the reaction. Co-Mo-S catalysts are hydrogenation catalysts resistant against sulfur. In this work Co-Mo-S catalysts were tested for CO2 hydrogenation and compared with a commercial Cu/ZnO/Al2O3 catalyst. The Co-Mo-S catalysts were synthesized in four different formulations. For this, activated carbon and alumina have been used as support. For each support, catalysts with and without potassium promoter were prepared. For comparison unsupported MoS2 without addition of promoters (Co, K) was additionally prepared and tested. Characterization of the catalysts was done by TPD (temperature programmed desorption), TPR (temperature programmed reduction), TPO (temperature programmed oxidation), DRIFTS (diffuse reflectance infrared fourier transform spectroscopy), XRD (X-ray diffraction), N2 physisorption, ATR-IR (attenuated total reflection infrared spectroscopy) and Raman spectroscopy. The materials were characterised before and after reaction. The catalytic properties were tested in a fixed-bed plug flow reactor varying reaction temperatures (260 – 330 °C), pressures (6 - 21 bar) and space velocities. Apparent activation energies and reaction orders were calculated. Under these conditions the main products on Co-Mo-S catalysts were CO and small amounts of CH4. Both promoters, K and Co, decreased the selectivity to CH4 and increased the selectivity to CO production. The nature of the support did not affect the catalytic properties. The Cu/ZnO/Al2O3 catalyst produced no CH4, more CO at lower temperatures and small amounts of methanol under these conditions. In particular at lower temperatures, the Cu-based catalyst was more active and exhibited a lower apparent activation energy. Under reaction conditions, Co-Mo-S proved to be very stable. In H2 atmosphere catalyst started to release H2S above 500 °C.