INFLUENCE OF THE PREPARATION METHOD ON THE SELECTIVE REDUCTION OF NITRIC-OXIDE OVER CU-ZSM-5 - NATURE OF THE ACTIVE-SITES

Copper ions were introduced in a Na-ZSM-5 zeolite by using exchange, i mpregnation and precipitation procedures. The catalytic activity of th e three solids was measured in the reduction of NO by propane in the p resence of oxygen (0 to 10 vol.-%) in the 423-773 K temperature range. At a given temperature the activity for nitrogen formation remarkably increased with the oxygen content in the 0.5-2 vol.-% range, then sli ghtly decreased for higher amounts of oxygen. For an oxygen content ex ceeding 0.5 vol.-% and regardless of the reaction temperature, the Cu- ZSM-5 sample prepared by exchange was the most active catalyst, the ca talytic activity being expressed per gram of introduced copper. The ad sorption of carbon monoxide followed by FT-IR spectroscopy suggested t hat copper was present as isolated Cu+ ions in the three catalysts. Th e dispersion of cuprous ions was the highest for the sample prepared b y the exchange process. The catalytic activity appeared related to a h igh dispersion of isolated Cu+ ions in the MFI framework.