Abstract :
NH3 insertion mechanism into the (2×2) tunnel structure of a hollandite-type manganese oxide (H-Hol) and the (2×∞) layered structure of a birnessite-type manganese oxide (H-Bir) were studied by direct adsorption calorimetry. It was found that H-Bir has a smaller NH3 adsorption enthalpy (−ΔHd,NH3) compared to H-Hol because of the structural flexibility of its MnO sheets. NH3 insertion into the tunnel structure of H-Hol is divided into two stages: in the initial stage, NH3 molecules interact with the H+ sites at the external surfaces and near the tunnel entrance, giving a peak in −ΔHd,NH3 curve at a low adsorption temperature due to the energy barrier required to expand the lattice structure; in the secondary stage, NH3 can access the inner H+ sites in the tunnel, giving a broad shoulder in −ΔHd,NH3 curve. H2O/H+ contents in H-Hol have an important role in NH3 insertion. Porous manganese oxides have a comparable or much higher −ΔHd,NH3 value compared to zeolites having a similar pore dimension or of a same proton form.
