Experimental Microkinetic Approach of De-NO_x by NH₃ on V₂O₅/WO₃/TiO₂ Catalysts. 3. Impact of Superficial WO_z and V_xO_y/WO_z Groups on the Heats of Adsorption of Adsorbed NH₃ Species

The present article is dedicated to the measurement of the individual heats of adsorption of adsorbed NH 3 species on WO 3 /TiO 2 and V 2 O 5 /WO 3 /TiO 2 (a catalyst for the selective catalytic reduction of NO x by NH 3 on stationary sources, briefly NH 3 -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 NH 3 species on TiO 2 and V 2 O 5 /TiO 2 solids. In agreement with the literature, Raman and Fourier transform infrared spectra indicate the presence of well-dispersed V x O y and WO z entities on the different solids. For NH 3 adsorption at pressure P a < 0.5 kPa 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 NH 3, forming NH 3ads-L and NH 4 + species on Lewis and Brønsted sites, respectively. For T a = 300 K, it is shown that four adsorbed NH 3 species are formed on the WO z -containing catalysts: two are adsorbed on Lewis sites, named NH 3ads-L1 and NH 3ads-L2, and two are adsorbed on Brønsted sites, named NH 4 + -1 and NH 4 + -2 (“1” and “2” indicate the increasing order of stability of the different species). Using the δ as IR band characteristic of the NH 3ads-L (∼1600 cm –1 ) and NH 4 + (∼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% V 2 O 5 /6% WO 3 /TiO 2 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 NH 3ads-L2 and 78 and 135 kJ/mol for the NH 4 + -2 species, respectively. These values indicate that the presence of WO z increases significantly the heat of adsorption of the NH 4 + species, as compared to TiO 2 and V 2 O 5 /TiO 2, explaining that both NH 3ads-L2 and NH 4 + -2 species can be present in the experimental conditions of NH 3 -SCR.