Alkali metal promotion on Fe-Co-Ni trimetallic catalysts for CO₂ hydrogenation to light olefins

The catalyst obtained by calcination of the precursor Fe-Co-Ni trimetallic MOFs has exhibited great CO2 hydrogenation performance. To further improve the selectivity of target product light olefins and the catalyst stability, alkali metal doping is used to modify the Fe-Co-Ni trimetallic catalyst, and the effects of different types of alkali metals and different doping methods are also studied. After alkali metal doping, the increased electron concentration around Fe atoms, the enhanced alkalinity of the catalyst surface and the weakened H-2 activation can improve CO2 uptake while decreasing H-2 affinity, thereby inhibiting the alkane formation. Among all the catalysts doped by alkali metal, FCN-K(a) obtained by potassium citrate impregnation after calcining has the largest specific surface area, the highest microporosity and the most alkaline sites on its surface, showing the best CO2 hydrogenation performance with a CO2 conversion of 47.5 % and selectivity of light olefins 46.8 %. Meanwhile, the catalyst stability is greatly improved after alkali metal doping during similar to 50 h test. The characterization results of spent catalysts show that FCN-K(a) has the largest amount of active phase Fe5C2 for C-C coupling. Doping with a proper amount of K can promote the carbonization of Fe and improve the stability of the active phases.