Supported CuO+Ag/Partially Stabilized Zirconia Catalysts for the Selective Catalytic Reduction of NOx under Lean Burn Conditions: 2. Catalytic Properties

Abstract

The catalytic performances of copper oxidic species supported on mesoporous zirconia supports stabilized by alkaline earth cations were studied for the reactions of NO oxidation to NO2 and the selective catalytic reduction (SCR) of NOx in excess oxygen. Three different types of hydrocarbon reductants were studied, namely, propane, propylene and the long-chain hydrocarbon decane. For comparison, the performance of CuO and CuO+Ag supported on alumina for the SCR of NOx by propane was also studied. This data was analyzed and discussed with reference to the information concerning the surface properties of these systems presented in Part 1 of this series. In general, for the same system, the maximum level of NOx conversion and temperature of maximum NOx conversion is strongly dependent upon the type of reducing agent. No universal relation was found between the surface density of coordinatively unsaturated cations able to activate hydrocarbons and the activity of the catalysts for the SCR reaction. In part, it can be explained by blocking the surface sites in reaction media either by strongly bound nitrate species or by coke. Addition of silver was found to be of significance only in the case of strong interaction between the metallic and oxidic components. Such a factor as the strength of oxygen and nitrate complexes bonding with the surface copper/copper–silver oxidic species depending upon the type of support and methods of samples synthesis appears to be the most important for performance of the systems studied in NOx-selective reduction by hydrocarbons in the presence of excess oxygen.