Diffusion and reaction of H2 gas for reducing Eu3+ ions in glasses

Abstract We have recently developed new Eu 2+ -doped Na 2 O–Al 2 O 3 –SiO 2 glasses by heating precursors in H 2 gas, in which Al 3+ ions play an important role in reducing Eu 3+ to Eu 2+ . However, the pathway by which the Eu 3+ ions are reduced was not established. To address this question, the structural changes involving Al 3+ and Eu 3+ ions have been elucidated from Magic-Angle Spinning Nuclear Magnetic Resonance (MAS NMR) in solid state and X-ray absorption fine-structure spectroscopies. In the glasses with Al/Na >1, the Al 3+ ions form AlO 4 units without incorporating Na + ions for charge compensation, and the Eu 3+ ions are coordinated by a network structure comprising AlO 4 and SiO 4 . When heated in H 2 gas, the H 2 gas molecules diffuse and react with the Eu 3+ ions, reducing them to Eu 2+ and forming O–H bonds. The diffusion rate of H 2 molecules was analyzed from the formation process of O–H bonds; 3.37×10 −12 m 2 /sec at 700 °C and 39.5 kJ/mol for diffusion coefficient and activation energy, respectively.