Fischer--Tropsch Synthesis: Effect of the Promoter's Ionic Charge and Valence Level Energy on Activity.

In this contribution, we examine the effect of the promoter's ionic charge and valence orbital energy on the catalytic activity of Fe-based catalysts, based on in situ synchrotron X-ray powder diffraction (SXRPD), temperature-programmed-based techniques (TPR, TPD, CO-TP carburization), and Fischer--Tropsch synthesis catalytic testing studies. We compared the promoting effects of K (a known promoter for longer-chained products) with Ba, which has a similar ionic radius but has double the ionic charge. Despite being partially "buried" in a crystalline BaCO₃ phase, the carburization of the Ba-promoted catalyst was more effective than that of K; this was primarily due to its higher (2+) ionic charge. With Ba²⁺, higher selectivity to methane and lighter products were obtained compared to the K-promoted catalysts; this is likely due to Ba's lesser capability of suppressing H adsorption on the catalyst surface. An explanation is provided in terms of a more limited mixing between electron-filled Ba²⁺ 5p and partially filled Fe 3d orbitals, which are expected to be important for the chemical promotion, as they are further apart in energy compared to the K⁺ 3p and Fe 3d orbitals. [ABSTRACT FROM AUTHOR]