Enhanced NH3-SCR activity at low temperatures over MnOx 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 MnOx catalysts supported on commercially available anatase TiO2 (TiO2(G); G5 and TiO2(H); Hombikat 8602). A large portion of MnOx was introduced into the micropores of Mn/TiO2(H) and Mn/TiO2(G), whereas MnOx 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 MnOx played an essential role in the low-temperature NH3-SCR reaction. These bridged nitrate species were likely formed over the MnOx clusters rather than the highly dispersed MnOx. Because TiO2(Z) exhibited a high external surface area, MnOx 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.