Selective catalytic reduction of nitric oxide by ammonia on pillared clay catalysts: Possible mechanism and promoters

The delaminated Fe₂O₃-pillared clay shows high activities for selective catalytic reduction (SCR) of NO by NH₃. Temperature program desorption (TPD) studies show that large amounts of NO_x are adsorbed on the pillared clay catalyst at the SCR reaction temperatures (i.e. near 400°C). This result indicates that a Langmuir-Hinshelwood type mechanism (for reaction between chemisorbed NO_x and NH_x on the surface to form N₂) is operative for the pillared clay catalyst, which is in contrast to the SCR reaction on the commercial vanadia-based catalysts. The SCR activities for the delaminated Fe₂O₃-pillared clay catalyst are higher than that of a commercial-type V₂O₅ + WO₃/TiO₂ catalyst under SO₂+H₂O free conditions, but became lower in the presence of SO₂ + H₂O. However, when promoted by doping 1-3% Cr₂O₃, the pillared clay catalyst exhibits higher SCR activities than the commercial-type catalyst in the presence of SO₂ + H₂O at all practical SCR reaction temperatures.