IR spectroscopic study of NOx adsorption and NOx-O-2 coadsorption on Co2+/SiO2 catalysts

Adsorption of nitrogen oxides (NO, NO2) and their coadsorption with oxygen on Co2+/SiO2 samples has been investigated by IR spectroscopy with a view to elucidating the mechanism of selective catalytic reduction (SCR) of NOx with hydrocarbons. A Co2+/SiO2 sample synthesized by ion exchange is characterized by a highly dispersed cobalt and a very weak surface acidity: CO is adsorbed only at low temperature (100 K) forming Co2+-CO carbonyls [v(CO) = 2180 cm(-1)]. Adsorption of NO on Co2+/SiO2 leads to the formation of Co2+(NO)(2) dinitrosyl complexes (1872 and 1804 cm(-1)) which are decomposed upon evacuation. Adsorption of NO2, as well as coadsorption of NO and O-2, produce NO2 species weakly bound to the support (a band at 1681 cm(-1)) and N2O4 (a band at 1744 cm(-1) with a shoulder at 1710 cm(-1)), the latter being adsorbed reversibly on both the support and the Co2+ ions. In the second case N2O4 is transformed into surface monodentate nitrates of Co2+ (a band at 1550-1526 cm(-1)) and partly into bridged nitrates (a band at ca. 1640 cm(-1)). The monodentate nitrates are stable with respect to evacuation up to 125 degrees C and act as strong oxidising agents: they are reduced by NO, even at room temperature, and by methane at 100 degrees C. In the latter case, organic nitro-compounds and isocyanate groups are registered as reaction products (probably intermediate compounds in SCR). The Surface species obtained after NO and NO2 adsorption on Co2+/SiO2 prepared from cobalt acetate (active SCR catalyst) are essentially the same as those observed with the ion-exchanged sample. No monodentate nitrates, however, are formed during NO2 adsorption on a Co2+/SiO2 Sample synthesized by impregnation with cobalt nitrate, which accounts for the lack of activity of this sample in the SCR.