VI. Properties of different chromium oxide phases in the catalytic reduction of NO by NH₃ studied by in Situ diffuse reflectance FTIR spectroscopy

Chromia on titania catalysts containing different chromium oxide phases, i.e., CrO₂, CrOOH, and Cr₂O₃, have been prepared and tested for the low-temperature selectivecatalytic reduction of nitric oxide by ammonia (SCR). The different chromium oxide phases (10 wt% total chromium oxide content) differ in their activity and selectivity to N₂. Supported CrOOH is oxidized to CrO₂ under SCR conditions above 570 K. The highest activity paired with a high selectivity to N₂ (> 90%) is found foramorphous Cr₂O₃ supported on titania prepared by decomposition of CrOOH/TiO₂ under mild conditions. Crystallization of Cr₂O₃ induced at higher temperatures reduces the activity significantly. The surface adsorption sites for the components of the SCR feed gas were investigated using temperature-programmed desorption combined with in situ diffuse reflectance FTIR spectroscopy. NO, which is adsorbed as a nitratespecies, is most abundant on CrO₂, and seems to be responsible for the formation of undesiredN₂O at temperatures below 400 K over catalysis containing this chromium oxide.