Encapsulation studies of hydrogen on cadmium exchanged zeolite rho at atmospheric pressure

Temperature programmed diffusion (TPDi) has been used to study the encapsulation of hydrogen in cadmium exchanged Cs-rho zeolite. The amount encapsulated after 2 h has been observed to be about 71 μmol/g at 50°C and 1 atm. This amount is over 30 times the amount of hydrogen encapsulated with NaX or NaA at 37°C for the same time and pressure. Upon increasing the encapsulation temperature to 100°C, the amount encapsulated increased to 161 μmol/g (2 h of encapsulation). At 200°C, the encapsulate is about 620 μmol/g, for the same pressure and time. With increasing temperature, more than one peak is seen in the TPDi spectra, revealing the availability of more than one site1 for the encapsulation. 3 peaks are observed in TPDi spectra for the encapsulation at 200°C - at 107, 295 and 345°C. Large encapsulated amounts of hydrogen arise from blocking effects caused by the presence of cations (cadmium and/or cesium). Experiments for encapsulation of hydrogen on H-rho (hydrogen exchanged zeolite rho) show negligible uptake of hydrogen, proving that the presence of either the cadmium ion (5.05 Cd2+ per unit cell) or the cesium ion (1.87 Cs+ per unit cell) or both is directly responsible for the encapsulation of hydrogen.

For encapsulation at 200°C, possible migration of the encapsulate among the sites seems to occur with encapsulation time. This could explain the relative changes in the intensities of the 3 peaks in the TPDi spectra for the encapsulation at 200°C.