Isotopic Apportioning of Hydrogen/Deuterium on the Surface of an Activated Iron Carbide Catalyst

Abstract

Fischer–Tropsch (FT) synthesis continues to receive widespread attention. Even after 90 years of investigation, the mechanistic route has yet to be fully defined. FT, as a polymerization process, uses CO and H2 as the reactants to produce a broad spectrum of hydrocarbons. Since the conception of the FT synthesis, several different isotopic routes have been employed for mechanistic studies. Various isotopes, such as 13C, 14C, 18O, and 2H, have been utilized through different types of experiments to shed light on the active site(s), the rate-limiting step, and the catalytic pathways. Direct evidence in the FT mechanism has been uncovered by utilizing experiments such as H2/D2 switching trials, as these experiments attempt to shed light on the rate-limiting step of CO hydrogenation. However, before hydrogen participates in the mechanism of CO hydrogenation, it may first dissociatively adsorb on the catalyst surface. The aim of this work is to ascertain to what extent H and D partition on the surface. This is accomplished by passing an equimolar H/D gas mixture over the activated FT catalyst, followed by a TPD method to determine if the active carbide surface displays a partitioning preference toward one of the isotopes. If a preference is observed, then the interpretation of kinetic isotopic effect (KIE) data ascertained in the CO hydrogenation switching experiments could potentially be affected. However, only a very slight isotopic preference toward deuterium was observed, and it is deemed not significant enough to affect an interpretation of the KIE based on H/D switching.