In situ FTIR study of the formation and consumption routes of nitroorganic complexes-Intermediates in selective catalytic reduction of nitrogen oxides by propene over zirconia-based catalysts

The results of this study show that the adsorption and catalytic properties of bulk ZrO₂ and nanoscale ZrO₂ pillars in pillared clay are significantly different. The NO + O₂ interaction with the surface of bulk zirconia results in the formation of three types of surface nitrate complexes (bridging, bidentate and monodentate), while over ZrO₂ pillars monodentate nitrates are not revealed. The interaction of the propene-oxygen mixture with the surface of bulk zirconia leads to the formation of the acetate complexes only. Meanwhile, the main activation form of propene over ZrO₂ pillars is the isopropoxide complex; the acetate complexes are observed only in small amounts at elevated temperatures. Complexes of three types are formed on the surface of Pt,Cu/ZrPILC after interaction with the propene-oxygen mixture flow: isopropoxide complexes, coordination-bound acetone, and acetates. Modification of ZrO₂ pillars by Pt and Cu results in a significant change in the concentration and the temperature range of existence of C-H-O surface compounds. The difference in the forms of reagents activation results in different structures of nitroorganic complexes. Over bulk zirconia the acetate and nitrate complexes form surface complexes with the structure similar to that of adsorbed nitromethane, monodentate nitrate being the most reactive species. Under reaction conditions at relatively low temperatures, over both modified and unmodified ZrO₂ pillars, interaction of isopropoxide and nitrate complexes produces surface complexes structurally similar to adsorbed dinitropropane. IR spectral kinetic studies revealed that the rates of surface nitrate and nitroorganic complexes transformation are in general close to each other and to the rate of catalytic reaction, significantly increasing due to Pt + Cu supporting on pillars. This demonstrates that these complexes are real intermediates of NO_x HC-SCR over studied catalysts.