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. [Display omitted] •A porous TiO2, which showed aggregated 2D flake-like particles, was synthesized via ZIF-8 as a sacrificial template.•Mn/TiO2 exhibited high catalytic performance at low temperatures.•Because of its unique structure, the MnOx species on the TiO2 majorly exist on the external surface as MnOx clusters.•The DRIFTS study demonstrated that MnOx clusters induced bridged nitrate species, which are highly reactive toward NH3.