K-Promotion Enhances the Intrinsic Kinetics of Fe/ZrO ₂ Catalysts for the RWGS Reaction

The reverse water gas shift (RWGS) reaction offers a promising pathway for selectively transforming CO2 into syngas, thereby enabling carbon mitigation and facilitating the integration of renewable resources. Accordingly, advancing this technology toward industrial deployment requires the development of catalysts that are highly active, intrinsically selective, and economically viable. Here, we investigate unpromoted and K-promoted Fe catalysts supported on ZrO2, Fe/ZrO2, and Fe–K/ZrO2, to elucidate the promotional role of K and to assess their potential for efficient RWGS performance. We synthesized and characterized these two catalysts and then examined their activity in the RWGS reaction. Both catalysts are prepared with 2.5 wt % Fe, and the Fe–K/ZrO2 sample additionally contains 0.1 wt % K. The characterization studies show that the presence of K increases the amount of reducible Fe species and enhances CO2 adsorption. These two factors are responsible for the promotional effect. Kinetic measurements of the RWGS reaction are performed over the two catalysts at ambient pressures and temperatures ranging from 427 to 467 °C. The CO formation rates (rCO) are accurately captured by a power-law kinetic expression. The Fe/ZrO2 catalyst exhibits an apparent activation energy of 192 kJ/mol, which decreases to 162 kJ/mol upon K promotion, reflecting the enhanced CO2 activation imparted by the basic promoter. The reaction orders increase from 0.46 (CO2) and 0.36 (H2) for Fe/ZrO2 to 0.50 and 0.46 for Fe–K/ZrO2, respectively, indicating a stronger rate dependence on reactant partial pressures in the presence of K. Several mechanistic models were also evaluated. Although some of these models yielded statistically acceptable parameters, the parameters were thermodynamically inconsistent. These findings demonstrate that K promotion enhances the intrinsic RWGS kinetics by lowering the activation barrier and strengthening reactant adsorption, thereby positioning K-promoted Fe catalysts as promising, earth-abundant materials for cost-effective RWGS catalysis.