Reply to Inoue et al. Comment on “Weber et al. Mayenite-Based Electride C12A7e⁻: A Reactivity and Stability Study. Catalysts 2021, 11, 334”

With gratitude, we would like to thank Yasunori Inoue, Masaaki Kitano and Hideo Hosono for their comments on our recently published article and express our appreciation for the critical discussion of our results. We would like to state in the first place that all the research and results obtained were presented to the best of our knowledge and all the work was completed according to best scientific practice and we are confident that the results and data that were presented in the discussed original work are justified [1].

Based on the comments on our paper, we think there are two points that we would like to react to. It may be a subject of scientific debate whether the plasma synthesized C12A7e⁻ materials indeed show different properties regarding samples from conventional synthesis, such as the electron concentration as discussed in the comments; therefore, the results from our work have to be discussed carefully in comparison to results obtained from materials synthesized with other synthesis procedures. This might specifically include influences of the synthesis on the surface structure of the material, etc., which, from our point of view, is a complex matter, currently possibly not the focus of most groups. Nevertheless, as your group and also other groups generally report the possibility of formation of hydrided mayenite forms, specifically under ammonia synthesis conditions over Ru/C12A7e⁻ under low pressure conditions below 1 MPa [2,3,4], we are firmly convinced that our reported deactivation pathway (over catalysts from the plasma synthesis route) is the prominent deactivation pathway for our tested materials at higher pressures. This can be also rationalized via favored formation of the C12A7:H⁻ form according to the principle of Le Chatelier at high pressures. This is indeed accompanied with the reduction of the electron concentration and loss of the unique promoting properties of the electride and thus deactivation of the Ru/C12A7e⁻ catalyst, as discussed in our work, mentioned in the comment and previously reported in [2,3,4,5]. Based on the data we collected, we cannot completely exclude that the synthesis of the material has a pivotal influence on the final stability and ageing properties of the material. However, based on our results, we think the formation of Ru/C12A7:H⁻ has to be considered at high reaction pressures (>1 MPa) causing the deactivation of the catalysts based on the loss of the unique electride properties—a fact that shouts out for the identification of a more stable material for industrial applications.

Secondly, as the decomposition of the pure C12A7e⁻ in H₂O already takes place under ambient conditions, we generally expect from an industrial perspective that for the industrial application of C12A7e⁻-based materials in reactions or even handling procedures involving H₂O, reactions producing H₂O or with H₂O traces being present has to be carefully avoided as severe deactivation phenomena can be expected due to irreversible phase transformation phenomena. This again limits an industrial application of the material.

Finally, as we also stated in the original work: “… Therefore, the future challenge is to better understand the nature of the promoting mechanism of electride materials and to identify alternative material classes with similar promoting properties to the C12A7e⁻ electride support, but with increased stability towards the found deactivation mechanisms, i.e., irreversible hydride transformation of the bulk phase at higher hydrogen pressures and improved hydrothermal stability. …” [1], we are more than happy to see that by modification of the Ru/C12A7e⁻ catalyst, the Hosono group seems to be able to tackle the stability issues we observed and discussed. We congratulate the commentators and Hideo Hosono personally for achieving this goal. Therefore, we recommend that for the benefit of the scientific community, a detailed synthetic, characterization and testing study is presented illustrating how a mayenite electride material can be stabilized to withstand ammonia synthesis conditions at 5 MPa for over two years. While to best of our knowledge such a study is not presented in the literature so far, we are of the firm opinion that this would definitely be a most remarkable step forward in the application of electrides in the field of heterogeneous catalysis and can certainly be considered of greatest importance to the field of catalysis.