Anion vacancies activate N2 to ammonia on Ba–Si orthosilicate oxynitride-hydride
Zhujun Zhang, Kazuki Miyashita, Tong Wu, Jun Kujirai, Kiya Ogasawara, Jiang Li, Yihao Jiang, Masayoshi Miyazaki, Satoru Matsuishi, Masato Sasase, Tomofumi Tada, Hideo Hosono, Masaaki Kitano
Abstract
Anion vacancies on metal oxide surfaces have been studied as either active sites or promoting sites in various chemical reactions involving oxidation/reduction processes. However, oxide materials rarely work effectively as catalysts in the absence of transition metal sites. Here we report a Ba–Si orthosilicate oxynitride–hydride as a transition-metal-free catalyst for efficient ammonia synthesis via an anion-vacancy-mediated mechanism. The facile desorption of H− and N3− anions plus the flexibility of the crystal structure can accommodate a high density of electrons at vacancy sites, where N2 can be captured and directly activated to ammonia through hydrogenation processes. The ammonia synthesis rates reach 40.1 mmol g−1 h−1 at 300 °C by loading ruthenium nanoparticles. Although not found to dissociate N2, Ru instead facilitates the formation of anion vacancies at the Ru–support interface. This demonstrates a new route for anion-vacancy-mediated heterogeneous catalysis. N2 reduction to ammonia typically requires transition metal catalysts, proceeding via a strong metal–nitrogen interaction. Now a Ba–Si orthosilicate oxynitride–hydride has been shown to function as a transition-metal-free catalyst for ammonia synthesis through an anion-vacancy-mediated mechanism, where electrons at the vacancy sites facilitate N2 activation.