CO2-FT activity of Fe7C3 in FeZnK/ZrO2 catalysts synthesized by using citric acid: Effect of pretreatment gas

Abstract

Here in, FeZnK/ZrO2 catalysts were synthesized by using citric acid (CA) chelation method. These catalysts were pretreated under H2(g) or CO(g) or CO/H2(g) for 2 h at 350 °C and subsequently tested in CO2-FT reaction at 300 °C and 2.0 MPa. The resultant fresh or spent samples were characterized by using BET-SA, XRD, TPD, TPR, XPS, HR-TEM, SEM-mapping-EDX and Mossbauer techniques. Improved textural properties (SA/pore width) and uniformly dispersed iron particles were obtained in FeZnK/ZrO2 catalysts through CA chelating method as established by BET-SA and SEM-mapping results respectively. The carbidization under CO/H2(g) produced Fe7C3 phase due to the ease reduction of dispersed iron oxides in CA catalyst as evidenced by XRD, HR-TEM and Mossbauer results. On the other hand, CA catalyst carbidized under CO(g) exhibited Fe7C3/Fe phases suggests inadequate carbidiztion of reduced iron species in agreement with H2-TPD results. Among the CO/H2 gas pretreatment ratio (0.45, 0.93 and 1.25), Fe7C3 with optimum particle size of 30.0 nm was achieved at CO/H2(g) = 0.93. Two fold increase in the –C2-C4= olefins yield (7.29 mmol gcat−1 h−1) over conventional impregnation catalyst (3.24 mmol gcat−1 h−1) was observed for CA catalyst. It was associated with stable Fe7C3 phase existed in a greater extent on the surface (Fe-C = 0.21 atom.%) in close vicinity to K (0.25 atm.%) and resulted in dissociative activation of adsorbed CO2/H2(g). In the present study, citric acid chelating method followed by CO/H2 gas pretreatment at 350 °C for only 2 h generated stable Fe7C3/ZrO2 unlike, literature reports (5–10 h pretreatment) and has the potential for further developments in CO2-FT process.