Nitrogenation and Hydrogenation Characteristics of Transition Metal−Iron Intermetallic Compounds

Nitrogenation for the intermetallic compounds such as MFe 2 (M = Ti, Zr, Nb, and Mo) or M‘Fe (M‘ = Ti and V) and the subsequent hydrogenation for the resulting interstitial metal nitrides were carried out to characterize their nitrogen storage properties by heating in a NH 3 −H 2 mixed gas or N 2 gas at 450−500 °C for the former reaction and in H 2 gas at 450 °C for the latter one, respectively. Among the intermetallic compounds, the Laves-type MFe 2 and the CrFe-type VFe compounds absorbed large amounts of nitrogen into their crystal lattices to transform into amorphous-like a-MFe 2 N x and a-VFeN x . They reversibly desorbed nitrogen by the following hydrogenation with maintaining the amorphous-like structure. Qualitative and quantitative analyses of the nitrogen species generated in the desorption step revealed that most of the nitrogen released from the metal nitrides readily reacted with hydrogen to produce ammonia owing to the high reactivity of the atomic nitrogen stored in the intermetallic compounds. Furthermore, the amorphous-like a-TiFe 2 N x - δ derived by the nitrogenation in the NH 3 −H 2 mixed gas and the subsequent hydrogenation in H 2 gas absorbed nitrogen even in N 2 gas at 500 °C as loaded with a Ru/Al 2 O 3 catalyst on the surface. The nitrogen stored was also reversibly desorbed as ammonia, suggesting that the molecular nitrogen was converted to ammonia under ambient pressure via the nitrogen absorption−desorption cycle of (Ru/Al 2 O 3 /a-TiFe 2 N x )/(Ru/Al 2 O 3 /a-TiFe 2 N x - δ ).