High Resolution Image Download MS PowerPoint Slide Bulk molybdenum carbides catalyze the Fischer−Tropsch (FT) reaction but display low selectivity to liquid hydrocarbons. Understanding which structural features of Mo carbide-based catalysts correlate with a selectivity to C 5+ products requires detailed structure-performance studies. Here, we investigate the effect of Mo vacancies (V Mo ) on the FT performance of the two-dimensional (2D) i -MXene Mo 4/3 C T x, where T x are the surface passivating groups. We show that the initially ordered V Mo sites, constituting ca. one-third of all Mo sites in Mo 4/3 C T x, undergo clustering already at 250 °C under H 2 . Following V Mo clustering, lattice carbon is released as CH 4 and CO, creating carbon vacancies (V C ), which also form in Mo 4/3 C T x under FT conditions at 330 °C. The presence of V C sites is associated with a high selectivity towards methane and C 2 −C 4 alkanes and a reduced selectivity for higher, C 5+, alkanes. Mo 4/3 C T x converts CO at a rate of ca. 60−80 mmol CO (g cat h) −1, which is comparable to that of the T x -free 2D-Mo 2 C and 2D-Mo 2 C 1− x reference catalysts that only contain a Mo +2 (carbidic) electronic state. However, post-FT analysis of Mo 4/3 C T x shows that it contains Mo +2, Mo +4, and Mo +5 states, possibly because oxygen atoms populate the V C sites and do not block the Mo +2 active sites at the surface. Mo 4/3 C T x undergoes a phase transition to cubic α-MoC 1− x at 400 °C under H 2, which leads to a continuous deactivation under FT conditions, suggesting, more generally, that pretreatment protocols for MXenes should be carefully optimized to unlock their potential in thermal catalysis.
Kountoupi et al. (2026) studied this question.