Experimental Microkinetic Approach of De-NOₓ by NH₃ on V₂O₅/WO₃/TiO₂ Catalysts. 5. Impacts of the NH₃-H₂O Coadsorption on the Coverage of Sulfated TiO₂-Based Solids

The present study is a part of an experimental microkinetic approach of the selective reduction of NOₓ to N₂ with NH₃ in excess of O₂ on V₂O₅/WO₃/TiO₂ catalysts (NH₃-selective catalytic reduction (NH₃-SCR) reaction). Water is always present either in the reactive gas mixtures representative of industrial processes or produced by the reaction. This suggests that H₂O may modify the coverage of the pivotal adsorbed NH₃ intermediate of the reaction by either a competitive adsorption or reactions (i.e., formation of NH₄⁺). In the temperature range of interest for NH₃-SCR (T ≥ ≈423 K), Fourier transform infrared spectroscopy and volumetric measurement using a mass spectrometer are used to study the impacts of the NH₃-H₂O coadsorption on the coverages of adsorbed NH₃ (molecular adsorption) and H₂O (molecular and dissociative adsorption) species on two sulfated solids: a 0.7% V₂O₅/9% WO₃/TiO₂ NH₃-SCR catalyst and its TiO₂ support. Regardless of the solid, it is shown that at the NH₃-H₂O coadsorption equilibrium, (a) NH₃ dominates the adsorption on the Lewis sites (i.e., the introduction of NH₃ at the H₂O adsorption equilibrium displaces H₂Oₐdₛ₋L species at the benefit of NH₃ₐdₛ₋L species) and (b) the introduction of H₂O at the NH₃ adsorption equilibrium increases significantly the amount of adsorbed NH₄⁺ species. This is ascribed to the H₂O dissociation, which is operant on a small number of sites forming new Brønsted sites without a strong impact on the amount of Lewis sites. The surface composition of the solids has a limited impact on the coverages during the NH₃-H₂O coadsorption except on the fact that the NH₄⁺ species is more stable on the NH₃-SCR catalyst. In Part 6 of the present study (10.1021/acs.jpcc.8b05847), it is shown that the present experimental data are consistent with the mathematical formalism of a competitive Temkin model (named Temkin-C) developed without major approximations. The experimental procedure (present study) and the mathematical Temkin-C formalism (Part 6) can be applied for all solids having a significant IR transmission, thus offering a method to study the surface acidity during realistic experimental conditions (in the presence of H₂O), which is of interest for different catalytic processes such as NH₃-SCR and alcohol dehydration.