Elucidating the Role of Mo Vacancies in a Two-Dimensional Molybdenum Carbide MXene on Fischer−Tropsch Catalysis

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.