Promotional Effects on NH3-SCR Performance of CeO2–SnO2 Catalysts Doped by TiO2: A Mechanism Study

A novel Ti-doped Ce– Sn mixed oxide (Ce–Sn–Ti) catalysts were developed for the selective catalytic reduction (SCR) of NOx with NH3. The addition of titanium could improve SCR activity at a low-temperature window. By optimizing the element proportions, the NOx conversion of Ce0.6Sn2.4Ti2 catalysts could reach more than 90% in a wide-operating temperature window (200 –450 °C). The appropriate doping Ti formed Sn–O-Ti, Sn–O-Ce and Ti–O-Ce structures, which could increase the content of Ce3+ through electrons transfer from Sn or Ti to Ce (Ce3+  + Ti4+  ↔ Ce4+  + Ti3+ and 2Ce4+  + Sn2+ ↔ 2Ce3+  + Sn4+). The solid solution structure increased specific surface areas, active sites (Ce3+), and Lewis acid sites over the Ce0.6Sn2.4Ti2 catalysts. According to DRIFTS results, only bidentate nitrate could react with adsorbed NH3 species. However, adsorbed NH3 species could quickly react with the gaseous NO. The NH3-SCR reaction mechanism over Ce0.6Sn2.4Ti2 catalysts mainly followed the Eley –Rideal mechanism. The electron transfer between tin, cerium, and titanium elements led to generate more active sites (Ce3+) used for NH3 and NOx activation/oxidation, which improved SCR activity. The NH3-SCR reaction mechanism over Ce0.6Sn2.4Ti2 catalyst mainly followed the Eley−Rideal mechanism, in which adsorbed NH3 species could quickly react with the gaseous NO to generate N2 and H2O.