Kinetic and Oxygen-Poisoning Study of Ammonia Synthesis and Decomposition over Supported MXene Mo2C Tx

Abstract The synthesis and decomposition of NH3 have gained renewed attention in the context of using NH3 as a renewable energy carrier. Both reactions require active catalysts based on earth-abundant metals that are also stable against common poisons. Here, we dispersed two-dimensional Mo2CTx MXene nanosheets onto SiO2 to improve the accessibility of their active sites. In doing so, we achieved a ca. 10-fold improvement in Mo utilization for both reactions relative to an unsupported 3D β-Mo2Ν. A comparison with this reference catalyst showed that SiO2 strongly affects the Mo active sites and alters the kinetic parameters of both reactions. For the NH3 decomposition reaction, the support effect is stronger at lower Mo loadings, whereas at higher Mo loadings, a partial restacking occurs, and such Mo2CTx/SiO2 catalysts behave similarly to bulk β-Mo2Ν. Interaction with the SiO2 support increases the susceptibility of Mo2CTx to H2 poisoning but simultaneously enhances its stability to steam (at 8000 ppm). Additionally, SiO2 limits the reducibility of Mo2CTx, thereby impeding the activation of N2 and H2 under NH3 synthesis conditions. Unlike β-Mo2Ν, Mo2CTx/SiO2 loses its activity at a CO2 concentration of 210 ppm during NH3 synthesis, explained by O* blocking of the active sites. CO2 poisoning is only partially reversible due to the formation of inactive Mo0.