N2‐to‐NH3 conversion get into the bowl

The N₂-to-NH₃ conversion is a fundamental chemical process to supply nitrogen source for modern industry and agriculture. Enormous efforts have been made since the invention of Haber-Bosch process, but it is still a challenging task to deliver N₂-to-NH₃ conversion at mild conditions. A general issue arises from the scaling relations, which impose an apparent contradiction between the capabilities of a catalyst to activate N₂ and to release NH₃. This results in a volcano curve of the catalytic activity for the N₂-to-NH₃ conversion, and thus poses a limit on the catalytic performance by the optimal catalyst design. This is the Sabatier principle for the catalyst design, which states that the adsorption of a relevant intermediate on the optimal catalyst should be neither too strong nor too weak. In other words, the optimal catalyst should be a compromise. Therefore, it is appealing to identify and elucidate the catalytic processes that are not dictated by the scaling relations, in order to design highly efficient heterogeneous catalysts beyond the compromise.

Recently, a research team led by Prof. Hai Xiao at Tsinghua University, China investigated the thermocatalytic mechanisms for N₂-to-NH₃ conversion on the intermetallic electride LaRuSi by first-principles calculations. They find that a bowl active site, composed of surface La cations and negatively charged subsurface Si atom originated from the electride nature, is key to the efficient catalysis of N₂-to-NH₃ conversion. The electrostatic and orbital interactions between this bowl active site and reaction intermediates greatly enhance the N₂ activation that results in negatively charged N₂ for facile cleavage of NN bond, while destabilize the adsorptions of NH_x (x = 1, 2, 3) species that contain positively charged H atoms, which facilitates the desorption of the final NH₃ product. It is this particular bowl active site composed of f-block La cations and electride Si anion that breaks the scaling relations for highly efficient N₂-to-NH₃ conversion.

By comparison with other intermetallic electride catalysts isostructural to LaRuSi, they explicitly confirm the breaking of scaling relations between the adsorptions of NH_x species and that of N. The adaptive electrostatic interactions exerted by the bowl active site play the key role in breaking the scaling relations for N₂-to-NH₃ conversion. They also identify the possible presence of similar bowl active sites in other types of highly efficient heterogeneous catalysts. Thus, they propose this bowl active site with adaptive electrostatic interactions as a design concept, which shed novel insights on the design of highly efficient heterogeneous catalysts for the N₂–to–NH₃ conversion, as well as other catalytic reactions that are dictated by the scaling relations. The results were published in Chinese Journal of Catalysis (https://doi.org/10.1016/S1872-2067(22)64129-9).

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About the Journal

Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top two journals in Applied Chemistry with a current SCI impact factor of 12.92. The Editors-in-Chief are Profs. Can Li and Tao Zhang.

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