Abstract
The continuous increase of carbon dioxide (CO2) level, nowadays, has become a knotty problem and life-threatening which is envisaged as the next pandemic. The question is: Can we use it as a plentiful feedstock instead of leaving it as a waste gas? The answer is Yes. Recently, more and more scientists and researchers put their efforts into this topic, converting CO2 into a valuable source of chemicals and fuels. Within current technologies, reverse water-gas shift (RWGS) reaction has attracted wide attention because the produced carbon monoxide (CO) is the versatile compound within the field of C1 chemistry, having the potential to be applied in large-scale when coupled to downstream processes (e.g. Fischer-Tropsch and methanol synthesis). However, due to its endothermic nature, RWGS requires high temperatures and it has kinetic limitations in terms of CO2 conversion at lower temperature regions. Besides, methane (CH4) production from the competitive methanation process is produced at low and moderate temperatures. Therefore, further efforts must be put into the development of active catalysts to overcome the slow kinetics and improve the production distribution towards carbon monoxide.
In this present research work, firstly, a comprehensive literature review was done in order to guide the catalyst selection. Following this, Ni, Fe, and Cu oxides have been chosen as active metals, supported on the homemade CeO2-Al2O3 mixed oxide matrix for catalysts synthesis. Catalysts were tested in fixed-bed continuous flow reactors and multiple characterisation techniques were applied. Results indicate that Ni-based catalysts present outstanding CO2 activation, but they also favour CH4 formation and thermal sintering. Effects of selected promoters FeOX and CrOX were studied. Results show that FeOX promoted Ni/CeO2-Al2O3 has better activity/stability balance under critical reaction conditions due to Ni-Fe interaction, as well as the textural property/thermal stability supplied by FeOX. As for Fe-based catalysts, they show less activity, especially at lower temperatures. But their catalytic performance can be largely improved by adding CuOX and NiOX as promoters. Among promoted catalyst systems, Fe-Cu/CeO2-Al2O3 catalyst reveals a considerable CO2 conversion at 500oC for about 40% with nearly 100% CO selectivity. Then, we focused on the study of Fe/Cu oxides ratio effects, developing a bimetallic Fe-Cu catalytic system. Results indicate that when the CuOX increases to 75% (based on the total amount of active metals), 0.25Fe0.75Cu shows the best CO selectivity. And the electronic interaction between Fe and Cu has proven to be important for CO2 activation and H2 dissociation. Besides, the Cu species are found to be partially incorporated into CeOX lattice, which is the key to produce CO. Based on these findings, a new series of FeOX promoted Cu/CeO2-Al2O3 (Fe2O3:CuO = 0.25:0.75) was developed. Catalysts were calcined at three different temperatures (400oC, 600oC, and 800oC) and tested in direct CO2 hydrogenation to methanol. To unravel the reaction pathway of this reaction over our prepared catalysts, advanced technique in-situ Diffusive Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) was applied. CO and formate species were detected simultaneously, evidencing the co-existence of the redox pathway, the RWGS + CO-hydro pathway, and the formate pathway during the reaction. In any case, the calcination temperature has a great impact on the product distribution. Besides, the final experimental chapter suggests a Cu-Fe/CeO2-Al2O3 catalyst calcined at 600oC, which shows the highest CO2 conversion and considerable methanol selectivity due to highly dispersed CuOX and FeOX particles, and the existence of CuCeOX structure.
Overall, this thesis is based on rational design and reactive studies to produce highly effective catalysts for CO2 conversion via RWGS. It is hoped that this work will inspire more investigation in this field.