Effect of zeolite topology on NH3-SCR activity and stability of Cu-exchanged zeolites

Abstract

The catalytic activity and stability of Cu-exchanged zeolites Cu-ZSM-5 (MFI topology), Cu-TNU-9 (TUN), Cu-FER (FER) and steamed Cu-Y (FAU) were evaluated in the selective catalytic NO reduction by ammonia (NH3-SCR). The NH3-SCR activity of the investigated catalysts strongly depended on the feed composition, i.e., the presence of H2O (5.0 Vol.-% of H2O). The catalysts revealed high stability during 24 h of NH3-SCR reaction, also in the presence of water vapour. DR UV–vis and FT-IR with probe molecules (CO or NO) showed the presence of isolated Cu+/Cu2+ and aggregated copper oxide species. Although Cu-ZSM-5 is less active than Cu-TNU-9 in catalysing NO oxidation to NO2, both catalysts revealed similar activity in the NH3-SCR reaction. Applying Rapid-Scan FT-IR spectroscopy and 2D COS analysis the reaction mechanism of NH3-SCR on Cu-exchanged zeolites was investigated. It is proposed that NH3-SCR over the studied Cu-exchanged catalysts proceeds via [Cu(OH)(NH3)2(NO)]+ intermediates followed by the reduction of Cu(II) to Cu(I). The differentiated speciation of copper sites is reflected in their various susceptibility for the reduction and finally affects the catalytic activity and stability of the zeolitic catalysts. The formation of the [Cu(OH)(NH3)2(NO)]+ mixed ligand species is governed by the competition between the O2− and NH3 ligands. Also the stability of the forms initiating NH3-SCR, i.e., [Cu(NH3)4]2+, [Cu(OH)(NH3)3]+ and [Cu(NH3)2]+, is ruled by the confine interaction of ammonia molecules, which can be both adsorbed on a protonic site and ligated to copper sites with the framework oxygen atoms.