Multilayered Molybdenum Carbonitride MXene: Reductive Defunctionalization, Thermal Stability, and Catalysis of Ammonia Synthesis and Decomposition

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)Tx, 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)Tx under H2 flow to be ca. 575 C. Exceeding this temperature results, under H2, in a transformation of the predominantly defunctionalized Mo2(C,N)Tx to a 3D Mo2(C,N) phase, which prevents the complete defunctionalization of Mo2(C,N)Tx while retaining its 2D morphology. Before this phase transformation occurs, the remaining Tx species reside in the interior layers of the mostly defunctionalized Mo2(C,N)Tx nanoplatelets, with the amorphous exterior being free from Tx groups, rendering the Mo2(C,N)Tx 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)Tx 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)Tx, 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)Tx (500 or 575 C), the gravimetric ammonia formation rate increases appreciably with H2 pretreatment, viz., Mo2(C,N)Tx pretreated at 575 C outperforms by ca. four times both the reference β-Μο2Ν catalyst and Mo2(C,N)Tx 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.