SELECTIVE REDUCTION OF NOX WITH PROPENE UNDER OXIDATIVE CONDITIONS - NATURE OF THE ACTIVE-SITES ON COPPER-BASED CATALYSTS

The chemical changes that occurred in two copper-based catalysts (Cu-Z SM-5 and Cu-Al2O3) during the selective reduction of NO with propene i n the presence of oxygen were studied using in situ X-ray absorption n ear edge structure (XANES). For the quantitative analysis of the XANES spectra, a mathematical procedure based on evolving factor analysis ( EFA) has been applied. Correlation with catalytic data shows that copp er is fully oxidized in both systems when the conversion of propene is complete and the conversion of NO to N-2 reaches its maximum value. T he XANES analysis together with comparison of the catalytic behavior o f Cu-ZSM-5 with Cu-Al2O3 and Cu-SiO2 systems indicates that the rate l imiting step of the reaction takes place on cupic oxides (although oth er species may also participate). This conclusion is of general signif icance because it establishes the physical basis for the role of Cu in selective catalytic reduction of NO by hydrocarbons; this explains th e recently recognized fact that zeolitic supports do not play an essen tial role in the reaction and that the mechanism may not involve a Cu2 +/Cu1+ redox cycle or, if it does, the cycle is dominated by the oxidi zed species. In situ XANES analysis coupled with EFA is demonstrated t o be a useful technique to obtain information about the active sites p resent on a catalyst during reaction.