Hydride-based heterogeneous catalysts for ammonia synthesis: a case study of oxyhydride YHO and metal hydride YH 2 Feiyang Tian 1 , Jin-Kun Li 1 , Pei-Kun Wang 2 , Ya Tang 1 , Liang Tang 1 , Ming-Hong Wu 1 1 Shanghai University, China, 2 Chemistry and Chemical Engineering Guangdong Laboratory, China Hydride-based heterogeneous catalysts such as oxyhydrides (BaTiO 2.5 H 0.5 and BaCeO 3- x H x ) and metal hydrides (LiH, VH 2 , and CaH 2 ) have attracted great attention for ammonia synthesis due to their high activity and different reaction mechanism in contrast to the conventional Fe-based catalysts. However, the role of the lattice hydride anion in the reaction remains unclear. The oxyhydride YHO (orthorhombic) and metal hydride YH 2 (cubic) with the same metal provides an ideal platform to study the role of hydrides in ammonia synthesis. In this work, we investigated the ammonia synthesis activity of Ru/YHO, Ru/YH 2 , and Ru/Y 2 O 3 . Lattice hydride anion in different host materials seem shown a different effect on reactions. The lower nitrogen reaction order ( α = 0.47) in Ru/YHO indicates that N≡N bond dissociation is not the rate-determining step of the reaction. For Ru/YH 2 , also possessing a low nitrogen reaction order ( α = 0.38). For Ru/Y 2 O 3 , the unit nitrogen reaction order ( α = 1.03), suggesting that N≡N bond dissociation remains the rate-determining step (Figure 1). For the hydrogen reaction order, Ru/YHO is β = 0.54, illustrating the absence of hydrogen poisoning and the presence of a hydrogen spillover pathway. In contrast, Ru/Y 2 O 3 exhibited severe hydrogen poisoning ( β = -0.9). It is a very interesting study that hydride anion exhibits different kinetic characteristics in different supports and affects the properties of the supports. Figure 1 Comparison of reaction orders of 2wt% Ru/YHO, 2wt% Ru/Y 2 O 3 and 2wt% Ru/YH 2 at 1MPa, 500 and 5MPa, 500; 450 for 2wt% Ru/YH 2 . References 1. Kitano, M.; Kujirai, J.; Ogasawara, K.; Matsuishi, S.; Tada, T.; Abe, H.; Niwa, Y.; Hosono, H. Low-Temperature Synthesis of Perovskite Oxynitride-Hydrides as Ammonia Synthesis Catalysts. J. Am. Chem. Soc. 2019 , 141 (51), 20344–20353. 2. Kobayashi, Y.; Tang, Y.; Kageyama, T.; Yamashita, H.; Masuda, N.; Hosokawa, S.; Kageyama, H. Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis. J. Am. Chem. Soc. 2017 , 139 (50), 18240–18246. 3. Feng, J.; Liu, L.; Zhang, X.; Wang, J.; Ju, X.; Li, R.; Guo, J.; He, T.; Chen, P. Ru Nanoparticles on Y 2 O 3 with Enhanced Metal–Support Interactions for Efficient Ammonia Synthesis. Catal. Sci. Technol. 2022 . 4. Kitano, M.; Inoue, Y.; Ishikawa, H.; Yamagata, K.; Nakao, T.; Tada, T.; Matsuishi, S.; Yokoyama, T.; Hara, M.; Hosono, H. Essential Role of Hydride Ion in Ruthenium-Based Ammonia Synthesis Catalysts. Chem. Sci. 2016 , 7 (7), 4036–4043. 5. Wang, P.; Chang, F.; Gao, W.; Guo, J.; Wu, G.; He, T.; Chen, P. Breaking Scaling Relations to Achieve Low-Temperature Ammonia Synthesis through LiH-Mediated Nitrogen Transfer and Hydrogenation. Nat. Chem. 2017 , 9 (1), 64–70.
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