Experimental Microkinetic Approach of De-NOx by NH3 on V2O5/WO3/TiO2 Catalysts. 3. Impact of Superficial WOz and VxOy/WOz Groups on the Heats of Adsorption of Adsorbed NH3 Species
Résumé
The present article is dedicated to the measurement of the individual heats of adsorption of adsorbed NH3 species on WO3/TiO2 and V2O5/WO3/TiO2 (a catalyst for the selective catalytic reduction of NOx by NH3 on stationary sources, briefly NH3-SCR) model and commercial solids by using an original experimental procedure (adsorption equilibrium infrared spectroscopy, AEIR) developed in parts 1 [Giraud et al. J. Phys. Chem. C 2014, 118, 15664] and 2 [Giraud et al. J. Phys. Chem. C 2014, 118, 15677] for the adsorbed NH3 species on TiO, and V2O5/TiO2 solids. In agreement with the literature, Raman and Fourier transform infrared spectra indicate the presence of well-dispersed VxOy and WOz entities on the different solids. For NH3 adsorption at pressure P-a < 0.5 IrPa and temperature T-a in the range 300-673 K, the modifications of the V=O and W=O overtone IR bands indicate that these entities are involved in the adsorption of NH3, forming NH3ads-L, and NH4+ species on Lewis and Br-misted sites, respectively. For T-a = 300 K, it is shown that four adsorbed NH3 species are formed on the WOz- containing catalysts: two are adsorbed on Lewis sites, named NH3ads-L1 and NH3ads-L2, and two are adsorbed on Bronsted sites, named NH4+-1 and NH4+-2 ("1" and "2" indicate the increasing order of stability of the different species). Using the delta(as) IR band characteristic of the NH3ads-L (similar to 1600 cm(-1)) and NH4+ (similar to 1445 cm(-1)) species, it is shown that the AEIR method provides the individual heats of adsorption of the four adsorbed species at low and high coverages of their adsorption sites. For instance, on a model 0.5% V2O5/6% WO3/TiO2 catalyst the heats of adsorption of the two more stable species (which slightly change with the exact composition of the solid) at low and high coverages of the sites are 105 and 148 kJ/mol for NH3ads-L2 and 78 and 135 kJ/mol for the NH4+-2 species, respectively. These values indicate that the presence of WOz increases significantly the heat of adsorption of the NH4+ species, as compared to TiO2 and V2O5/TiO2, explaining that both NH3(ads-L2) and NH4+-2 species can be present in the experimental conditions of NH3-SCR.