Calorimetric study on NH3 insertion reaction into microporous manganese oxides with (2×2) tunnel and (2×∞) layered structures

Abstract NH 3 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 NH 3 adsorption enthalpy (−Δ H d,NH 3 ) compared to H-Hol because of the structural flexibility of its MnO sheets. NH 3 insertion into the tunnel structure of H-Hol is divided into two stages: in the initial stage, NH 3 molecules interact with the H + sites at the external surfaces and near the tunnel entrance, giving a peak in −Δ H d,NH 3 curve at a low adsorption temperature due to the energy barrier required to expand the lattice structure; in the secondary stage, NH 3 can access the inner H + sites in the tunnel, giving a broad shoulder in −Δ H d,NH 3 curve. H 2 O/H + contents in H-Hol have an important role in NH 3 insertion. Porous manganese oxides have a comparable or much higher −Δ H d,NH 3 value compared to zeolites having a similar pore dimension or of a same proton form.