Comparative ESR Studies on the Reducibility and Adsorbate Interactions of Paramagnetic Ni(I) in Synthetic Ferrierite and Mordenite

Abstract

The formation of monovalent nickel in NiNaK-ferrierite and its interaction with several adsorbates are compared to those of Ni(I) species in NiNaK-mordenite using electron spin resonance (ESR) spectroscopy. The ESR results show higher reducibility of Ni(II) in ferrierite upon thermal and hydrogen reduction. This is due to a lesser negative charge of the ferrierite framework. Dehydration at 573 K as well as hydrogen reduction at 573 K produces one Ni(I) species in both NiNaK-ferrierite and NiNaK-mordenite. However, Ni(I) species in these two materials show noticeable differences in the way they interact with several adsorbates. This suggests that the Ni(I) ions in these two materials are in different sites. The lesser accessibility of Ni(I) in ferrierite to larger adsorbate molecules leads to the assignment that Ni(I) is in a small ferrierite cage. The contrasting greater accessibility of Ni(I) in mordenite to adsorbates suggests a Ni(I) site in the large 12-ring channel in mordenite. The reduction of Ni(I) in mordenite to Ni(0) by methanol and pyridine further supports this location assignment.