Effect of Citric Acid Dosage on Active Phase Transformation and Fischer–Tropsch Performance of Fe/ZrO2 Catalyst

A series of Fe/ZrO 2 Fischer–Tropsch synthesis catalysts were prepared via a solid-state grinding method assisted by citric acid ligands, aiming to enhance the dispersion of active metal and tailor the metal–support interfacial structure through a green, solvent-free synthesis strategy, thereby optimizing catalytic performance. In this study, citric acid (CA) was used as the ligand and a series of Fe/ZrO 2 catalysts were synthesized by varying the CA/Fe molar ratio (0, 0.25:1, 0.5:1, 1:1, and 2:1) to systematically investigate the effect of ligand dosage on the transformation of active Fe phases and Fischer–Tropsch performance. The structure–performance relationship was elucidated using XRD, TEM, BET, XPS, H 2 -TPR, and Mössbauer spectroscopy. Results revealed that an appropriate CA dosage (1:1) significantly improved the dispersion of Fe nanoparticles, promoted the formation of Fe–O–Zr interfacial structures and oxygen vacancies, lowered the reduction temperature of FeO x species, and enhanced CO adsorption capacity. These factors collectively facilitated the formation of a high content of the χ-Fe 5 C 2 active phase, resulting in the highest CO conversion and C 5+ hydrocarbon selectivity. The catalyst with a CA/Fe molar ratio of 1:1 exhibited the best performance, achieving a steady-state CO conversion of 42.6% and a C 5+ selectivity of 46.5% after 192 h on stream, which are significantly higher than those of the unmodified catalyst (CO conversion 23.2%, C 5+ selectivity 36.4%). In contrast, insufficient CA dosage (0.25:1, 0.5:1) led to nonuniform dispersion of Fe species on the support surface, reduced surface area, and limited interfacial synergy. Excessive CA (2:1), although maintaining high dispersion, could impede the reduction pathway and cause unfavorable structural evolution, ultimately reducing catalytic performance. Overall, the study demonstrates the existence of an optimal CA dosage window, with the 1:1 molar ratio condition achieving synergistic optimization of Fe electronic structure and interfacial activity, leading to high CO conversion in Fischer–Tropsch synthesis. These findings provide robust experimental evidence for the application of organic ligand-assisted green synthesis strategies in controlling the active phase transformation and performance optimization of iron-based catalysts, as well as offering new insights and approaches for the design of green and efficient Fischer–Tropsch synthesis catalysts.