Tuning A/B- site cations in Gd-perovskite catalysts for selective CO₂ hydrogenation to C₂-C₃ olefins: the role of Fe-Co synergy and potassium promotion

Introduction

The progressive accumulation of atmospheric CO₂ represents the primary driver of anthropogenic climate change, making chemical CO₂ upcycling a strategic priority for a green and circular economy. Light olefins are among the most industrially significant petrochemical intermediates; however, their conventional production is responsible for 180-200 million tonnes of CO₂ emissions annually. CO₂-assisted Fischer-Tropsch synthesis - integrating the reverse water-gas shift reaction with CO hydrogenation - offers a viable alternative pathway. Perovskite-type oxides (ABO₃) are particularly promising catalyst platforms, as heterovalent substitution at the A- and B-sites enables exceptional tunability of electronic structure, oxygen mobility, and redox properties.

Methods

GdCo_xFe_1-хO₃ (х=0; 0.2; 0.5; 0.8; 1) and potassium-doped analogues (K_yGd_1-yFeO₃, K_yGd_1-yCoO₃, K_yGd_1-yCo_0.5Fe_0.5O₃) were synthesized via a citrate–nitrate sol-gel route and characterized by XRD, XPS, FTIR spectroscopy, TGA-DSC, and iodometric titration.

Results

All compositions adopt a single-phase orthorhombic perovskite structure. XPS analysis revealed coexisting mixed-valence states at the B-site (Fe²⁺/Fe³⁺, Co²⁺/Co³⁺). Potassium incorporation generates ordered oxygen vacancies that serve as CO₂ activation sites, accompanied by reduction of the B-site cation oxidation states. TGA-DSC confirmed the thermal robustness of GdFeO₃, whereas cobalt-enriched phases showed partial decomposition under prolonged operation. Catalytic evaluation under CO/CO₂ hydrogenation conditions (523-723 K) yielded 80-100% conversion, correlating with oxygen non-stoichiometry. The equimolar composition GdCo_0.5Fe_0.5O₃ exhibited superior C₂-C₃ olefin selectivity, attributed to a balanced Fe²⁺/Co²⁺ ratio that governs chain propagation, olefin desorption, and suppression of secondary hydrogenation. Potassium acts as a bifunctional promoter, enhancing both vacancy ordering and olefin selectivity.

Conclusions

Catalytic performance is governed by the synergistic interplay between B-site Fe/Co composition and potassium-induced vacancy concentration - parameters that together determine active site density, redox state, and surface basicity. These findings support rational perovskite engineering as an effective strategy for sustainable CO₂ upcycling to light olefins from bio-derived syngas.