Nitrogenation and hydrogenation characteristics of transition metal-iron intermetallic compounds

Nitrogenation for the intermetallic compounds such as MFe2 (M = Ti, Zr, Nb, and Mo) or M'Fe (M' = Ti and V) and the subsequent hydrogenation for the r esulting interstitial metal nitrides were carried out to characterize their nitrogen storage properties by heating in a NH3-H-2 mixed gas or N-2 gas a t 450-500 degrees C for the former reaction and in H-2 gas at 450 degrees C for the latter one, respectively. Among the intermetallic compounds, the L aves-type MFe2 and the CrFe-type VFe compounds absorbed large amounts of ni trogen into their crystal lattices to transform into amorphous-like a-MFe2N x and a-VFeNx. They reversibly desorbed nitrogen by the following hydrogena tion with maintaining the amorphous-like structure. Qualitative and quantit ative analyses of the nitrogen species generated in the desorption step rev ealed that most of the nitrogen released from the metal nitrides readily re acted with hydrogen to produce ammonia owing to the high reactivity of the atomic nitrogen stored in the intermetallic compounds. Furthermore, the amo rphous-like a-TiFe2Nx-delta derived by the nitrogenation in the NH3-H-2 mix ed gas and the subsequent hydrogenation in H-2 gas absorbed nitrogen even i n N-2 gas at 500 degrees C as loaded with a Ru/Al2O3 catalyst on the surfac e. 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/Al2O3/a-TiFe2Nx)/(Ru /Al2O3/a-TiFe2Nx-delta).