Reversible chemisorption of nitric oxide in the presence of oxygen on titania and titania modified with surface sulfate

Adsorption of NO (with, and without O-2) on a commercial Degussa TiO2 (designated TiO2-d), and sol-gel TiO2 (TiO2-g) and their surface-sulfated forms were studied by TGA and in situ FTIR. Using TGA, these samples were studied as sorbents for selective, reversible adsorption of NO from hot combustion gases. Adsorption and desorption were measured, respectively, at 300 and 450 degrees C in the same gas atmosphere (2000 ppm NO and 4% O-2) The sulfated TiO2-g sample showed the highest reversible amount of NOx adsorbed (approximate to 14 mg/g) as compared to all known sorbents. The amount of NO adsorbed was higher on the unsulfated TiO2 samples, however, only 20% of the adsorbed NO could be desorbed at 450 degrees C. Thus, NO adsorption was weakened by the presence of surface sulfate. In situ FT-IR studies showed the following results: The unsulfated TiO2 showed only (strong) Lewis acidity, with no Bronsted acidity. Bidendate SO42- ions (most likely a bridged form) are formed on TiO2 upon sulfation (with 2000 ppm SO2 and 4% O-2), which also generated Bronsted acidity. There was more Bronsted acidity than Lewis acidity (by NH3 adsorption) at room temperature, which was reversed as the temperature increased. The adsorbed NO (in the presence of O-2) was predominantly in the form of bidendate NO3- ions bonded to Ti sites. On TiO2, the main IR band for NO3- was at 1542 cm(-1); while the same band was shifted to 1582 cm(-1) on the sulfated TiO2. Thus the TiO2 surface was modified by SO42- ions, possibly by electron transfer through the S-O-Ti bridge to the neighboring Ti sites, and consequently weakened the adsorption of NO3- on the Ti sites. Furthermore, since TiO2 is the support for V2O5 in the selective catalytic reduction of NO (by NH3), where the TiO2 surface is also sulfated in combustion gases, the results of this study support the NOx spillover mechanism (from TiO2 to V2O5) as a possible role played by TiO2. (C) 1998 Elsevier Science B.V.