Experimental Microkinetic Approach of De-NO by NH3 on V2O5/WO3/TiO2 Catalysts. 2. Impact of Superficial Sulfate and/or VxOy Groups on the Heats of Adsorption of Adsorbed NH3 Species
Résumé
The present article is dedicated to the impacts of sulfatation and/or VxOy deposition on TiO2 supports on the individual heats of adsorption of adsorbed NH3 species. The S and V loadings are similar to those of TiO2 based commercial catalysts for the selective catalytic reduction of NOx by NH3: NH3-SCR. Two original experimental procedures are used, namely, adsorption equilibrium infrared spectroscopy (AEIR) and temperature-programmed adsorption equilibrium (TPAE). They have been developed in previous works and adapted in Part 1 for the species formed by the adsorption of NH3 on a sulfate free TiO2 support (P25 from Degussa). In agreement with the literature, Raman and FTIR spectroscopies show that the impregnation of the sulfate-free and sulfated TiO2 supports by the vanadium precursor leads to well-dispersed VxOy species, which are involved in the adsorption of NH3 in the temperature T-a range of 300-673 K and for adsorption P-a < 0.5 IcPa. Sulfate and VxOy groups favor the amount of NH4+ species without modifying their heats of adsorption as compared to the TiO2, support: they are not detected for temperatures of interest for NH3-SCR reaction. In these experimental conditions and whatever the solids, two main adsorbed NH3 species on Lewis sites (denoted NH3ads.L1 and NH3ads.L2) are present, characterized by two delta(s) IR bands below 1300 cm(-1) and a common delta(as), IR band at approximate to 1600 cm(-1). By comparison with TiO2, P25; it is shown that sulfate groups have strong impacts neither on the proportion, x(1) and x(2),, of the two adsorbed species nor on their heats of adsorption (E-L1 (theta) and E-L2(theta) with theta the coverage of the NH3ads.L1 and NH3ads-L2 species: x(1) = 0.65, x(2) = 035, E-L1 (1) = 56 kJ/mol, E-L1(0) = 102 kJ/mol, E-L2(1) = 110 kJ/mol, E-L2(0) = 140 kJ/mol. The deposition of V-containing species on the sulfate-free and sulfated TiO2 surfaces leads to similar conclusions: in the experimental conditions (Ta and Pa) of the NH3-SCR, two adsorbed NH3 species on Lewis sites are detected with comparable proportions and heats of adsorption to those observed on the TiO2 supports.