Fundamentals of the Active Catalytic Phases of a Novel High-Performance Spinel Catalyst for Carbon Dioxide Conversion to Liquid Fuels through Hydrogenation

CO₂, one of the most significant greenhouse gases, is thought to be a major cause of global warming. The main feasible possibility for CO₂ reduction is its utilization as a carbon source for transportation fuels and chemicals production via the CO2 hydrogenation process. This process consists of two successive steps. First CO₂ is converted to CO by reverse water gas shift reaction (RWGS) which then reacts in a modified Fischer-Tropsch synthesis (FTS) to form a mixture of hydrocarbons which is an excellent feedstock for production of fungible liquid fuels and chemicals. Iron-based catalysts make excellent candidates for the single-step CO₂ hydrogenation process since they possess the unique feature of catalyzing both the RWGS reaction and the FT synthesis. This bi-functionality of Fe-based catalysts is attributed to their ability to re-organize under reaction conditions and form additional iron-based phases so that the activated catalyst comprises a complex mixture of iron phases, mainly iron carbide and iron oxide. A novel Fe-Al-O spinel catalyst was developed at the Blechner center for the CO₂ hydrogenation process which yields excellent results. The performance of this catalyst is highly dependent on potassium content which was found to be the most efficient promoter and a key-ingredient in this process. However, due to the complexity of this multi-reactions and multi-phases catalytic system, determining the promoting effect of potassium to this catalyst is extremely difficult. In this study an innovative approach for determining the promoting effect of potassium to the novel Fe-Al-O spinel catalyst was applied. The effect of potassium as a promoter to each of the catalytic iron-based phases, Fe-Al-O spinel and Fe₅C₂, separately was examined. This was accomplished under controlled experimental conditions that prevented changes in these phases such as carburization of the iron-oxide or oxidation of the iron carbide during the catalytic reaction.