Influence of sulfation on CeO2-ZrO2 catalysts for NO reduction with NH3

CeO2-ZrO2 (CeZr) and sulfated CeO2-ZrO2 (S-CeZr) catalysts were prepared for the selective catalytic reduction of NO with NH3. The CeZr catalysts exhibited higher activity at low temperatures (< 200 °C) and lower activity at high temperatures (> 200 °C) than the S-CeZr catalysts. The sulfation of CeZr was studied in terms of surface acidity, redox properties and NO adsorption-desorption by temperature-dependent experiments and in situ infrared spectroscopy. S-CeZr displayed high concentrations of acidic sites and increased surface acidities, but poor reducibility compared with CeZr. The high acidity of S-CeZr was attributed to the presence of Brønsted acid sites, arising mainly from the surface sulfates. Because the surface was covered with sulfate species, S-CeZr showed lower NO adsorption and weaker oxidation ability than CeZr. The adsorption of NH3 on the Brønsted acid sites restricted the reaction with NO at low temperatures, but the selective catalytic reduction cycle occurred easily at relatively low temperatures (150 °C), and the weakly bound nitrite was partially activated on the S-CeZr catalyst at relatively high temperatures (300 °C). The catalytic mechanisms for the CeZr and S-CeZr catalysts at 150 and 300 °C were also studied.