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 N-2 with NH3 in excess of O-2 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 >= approximate to 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 Bronsted 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.