The Deactivation Mechanizm of SO₂ Treated Cu/MFI Catalyst during Selective Reduction of NO

Abstract

NO_x removal catalysts from the combustion exhaust gas are recently the subject of intense study. The Cu/MFI catalyst is supposed to be the most probable catalyst because its activity is high even in the O₂ rich conditions. The durability against heat and catalyst poisons in the exhaust gas is important to adopt the catalysts in the actual combustion exhaust gas. Especially, the exhaust gas from Diesel engines generally contains ca.50 ppm of SO₂. To know the level and the mechanism of SO₂ deactivation is important. In this study, we discussed the deactivation mechanism on the Cu/MFI catalyst. In the catalysts evaluations after 300 °C treatments, the catalyst deactivated only after the SO₂ treatment. The XPS analysis revealed that the Cu species in the catalyst changed into sulfate form. We also tried to c larify the local structure of copper by XAFS (XCANES and EXAFS) technique. Cu species in the fresh catalyst were nearly regular tetrahedral coordination. The bond length was estimated at 1.94 Å. After the SO₂ treatment, the local structure of Cu changed to distorted octahedral symmetry. The Cu-0 (zeolite framework) bondings loosened from 1.94 Å to 1.99 Å. New bondings at 2.34 Å formed. The formation of Cu-O (SO₄^2-) bonding would result in the creation of longer bondings. In the catalysts evaluation after 500 °C and 700 °C treatment, the catalytic performance of the samples was almost the same in each temperature. The migration of Cu species from the active sites and the dealumination of zeolite framework would be dominant at these temperatures.