Multilayered molybdenum carbonitride MXene: Reductive defunctionalization, thermal stability, and catalysis of ammonia synthesis and decomposition

Abstract

Harnessing two-dimensional (2D) materials for catalytic applications is promising due to the high site utilization. Here, we synthesized a 2D molybdenum carbonitride of the MXene family, Mo₂(C,N)T_x, and applied it as a catalyst for ammonia synthesis and decomposition, the essential reactions to establish NH₃ as an energy vector. We determine the thermal stability limit of Mo₂(C,N)T_x under H₂ flow to be ca. 575 °C. Exceeding this temperature results, under H₂, in a transformation of the predominantly defunctionalized Mo₂(C,N)T_x to a 3D Mo₂(C,N) phase, which prevents the complete defunctionalization of Mo₂(C,N)T_x while retaining its 2D morphology. Before this phase transformation occurs, the remaining T_x species reside in the interior layers of the mostly defunctionalized Mo₂(C,N)T_x nanoplatelets, with the amorphous exterior being free from T_x groups, rendering the Mo₂(C,N)T_x nanoplatelets chemically anisotropic in the direction orthogonal to the basal plane. The effect of this structure on catalytic properties is highlighted in the thermocatalytic synthesis and decomposition of NH₃. In the latter reaction, Mo₂(C,N)T_x shows similar gravimetric rates to a reference bulk β-Μο₂Ν catalyst, which is ascribed to the presence of too narrow 2D pores (ca. 5.2 Å) with irregular shapes due to a disorder in the stacking of nanosheets in Mo₂(C,N)T_x, limiting interlayer diffusion. A deactivation pathway in Mo-based MXenes was identified, and it relates to a precipitation of carbon vacancies to metallic molybdenum under NH₃ decomposition conditions. While the ammonia decomposition reaction shows no dependence of the reaction rate on the specific H₂ pretreatment of Mo₂(C,N)T_x (500 or 575 °C), the gravimetric ammonia formation rate increases appreciably with H₂ pretreatment, viz., Mo₂(C,N)T_x pretreated at 575 °C outperforms by ca. four times both the reference β-Μο₂Ν catalyst and Mo₂(C,N)T_x pretreated at 500 °C, explained by a smaller molecule size of the reactants H₂ and N₂ relative to NH₃, and an increased accessibility and utilization of the interlayer space for ammonia synthesis. Overall, our study highlights the importance of addressing limitations due to small pore sizes in multilayered MXenes and the stability of carbon vacancies while simultaneously using optimized pretreatment conditions for surface defunctionalization to uncover the full potential of MXene-based heterogeneous catalysts.