Systematic Inorganic Promoter Screening over MOF-Derived Fe@C Catalysts for CO2 Hydrogenation to Long-Chain Hydrocarbons

CO2 hydrogenation to liquid hydrocarbons (C5+) via the reverse water–gas shift/Fischer–Tropsch (RWGS-FTS) tandem route represents a compelling strategy for carbon-neutral e-fuel production. Iron-based catalysts are intrinsically bifunctional for this chemistry, yet secondary inorganic promoter selection beyond potassium remains largely empirical. Here, we construct a 30-member inorganic promoter library (including noble metals, transition metals, alkaline earth metals, rare-earth lanthanides, post-transition metals, and nonmetals) coimpregnated with potassium onto a MIL-100(Fe)-derived carbon-encapsulated iron catalyst (Fe@C) and evaluate them under identical conditions. Promoter identity decisively governs iron phase evolution: high-performing promoters (Pr, Pt, Co, Mg(acetate)) facilitate the formation of Hägg carbide (χ-Fe5C2) alongside magnetite (Fe3O4), while RWGS-selective promoters (Sn, Bi, Mo, N) stabilize cementite (Fe3C). Among all tested elements, praseodymium emerges as the most effective non-noble promoter, delivering C5+-hydrocarbon performance comparable to the platinum benchmark at equal nominal loading. Complementary characterization establishes three recurring structural prerequisites for high C5+ productivity: well-dispersed nanoparticles, CO2/H2 adsorption at moderate-strength basic sites, and the Fe3O4–Fe5C2 phase combination. Notably, the promoter precursor anion (acetate, nitrate, or carbonate) is associated with substantial, cation-dependent differences in iron-phase composition and particle dispersion, identifying promoter salt chemistry as an important catalyst-design variable.