Enhanced NH₃-SCR activity at low temperatures over MnO_x supported on two-dimensional TiO₂ derived from ZIF-8

ZIF-8 was used as a sacrificial template to synthesize porous anatase TiO₂ (TiO₂(Z)), consisting of agglomerated 2D flake-like particles. TiO₂(Z) exhibited a high specific surface area (259 m² g⁻¹), in which the external surface area was significantly larger than that of the micropores because of its 2D flake-like structure. The catalytic performance of Mn/TiO₂(Z) in selective catalytic reduction with NH₃ (NH₃-SCR) reaction was superior to that of MnO_x catalysts supported on commercially available anatase TiO₂ (TiO₂(G); G5 and TiO₂(H); Hombikat 8602). A large portion of MnO_x was introduced into the micropores of Mn/TiO₂(H) and Mn/TiO₂(G), whereas MnO_x was mainly located on the external surface of TiO₂(Z) in the form of clusters. In particular, the cause of the high catalytic activity of Mn/TiO₂(Z) was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy, demonstrating that bridged nitrate species formed on MnO_x played an essential role in the low-temperature NH₃-SCR reaction. These bridged nitrate species were likely formed over the MnO_x clusters rather than the highly dispersed MnO_x. Because TiO₂(Z) exhibited a high external surface area, MnO_x clusters were produced mainly over the external surfaces, resulting in high catalytic activity at low temperatures in the NH₃-SCR reaction. In addition, it was demonstrated that high N₂ selectivity of Mn/TiO₂(Z) resulted from the migration of intermediate ammonium nitrate species, which was stabilized in a micropore structure.