Active Tetrahedral Iron Sites of γ-Fe2O3 Catalyzing NO Reduction by NH3

Maghemite (gamma-Fe2O3) with a spinel structure, consisting of tetrahedral Fe3+ (Fe-Td(3+)) and octahedral Fe3+ (Fe-oh(3+)) sites, has been intensively investigated as an environmentally benign catalyst for selective catalytic reduction (SCR) of NOx with NH3. In most cases, Fe-Oh(3+), sites were regarded as catalytically active sites (CASs). Here we identify the CASs in SCR by substituting Fe-oh(3+) or Fe-Td(3+) sites of gamma-Fe2O3 with catalytically inactive Ti4+ or Zn2+, respectively. The SCR activity of gamma-Fe2O3 is preserved after Ti4+ doping but drastically decreases when the catalyst is doped with Zn2+, demonstrating that Fe-Td(3+) sites serve as CASs in SCR.