Experimental Microkinetic Approach of De-NOₓ by NH₃ on V₂O₅/WO₃/TiO₂ Catalysts. 6. NH₃–H₂O Coadsorption on TiO₂-Based Solids and Competitive Temkin Model

The present study is a part of an experimental microkinetic approach (EMA) of the selective reduction of NOₓ to N₂ with NH₃ in excess O₂ on V₂O₅/WO₃/TiO₂ catalysts (NH₃-SCR process). In the temperature range of interest for NH₃-SCR (T ≥≈ 473 K) and for three TiO₂-based solids (sulfated and sulfate-free TiO₂ supports and a sulfated 0.7% V₂O₅/9% WO₃/TiO₂ catalyst), FTIR spectroscopy and volumetric measurements with a mass spectrometer are used to study the impacts of the NH₃–H₂O coadsorption on the coverages of (a) the molecularly adsorbed NH₃ species and (b) the molecularly and dissociated H₂O species on Lewis and Bronsted sites. Whatever the solid, it is shown that NH₃ dominates the molecular coadsorption on the Lewis sites. However, this does not prevent the dissociative H₂O chemisorption on a small amount of Lewis acidic sites, leading to an increase in the amount of OH groups. On the two sulfated solids, these OH groups increase the amount of adsorbed NH₄⁺ species as compared to the NH₃ adsorption equilibrium. For the sulfate-free TiO₂ solid having weak Bronsted sites, the switch between the NH₃ adsorption equilibrium to the NH₃–H₂O coadsorption equilibrium is associated to the production of a small amount of NH₃ due to the displacement of NH₃ₐdₛ₋L species by H₂O dissociation (competitive adsorption). It is shown that these experimental data are consistent with an original development of a competitive Temkin model (named Temkin-C), taking into account the individual heats of adsorption of NH₃ and H₂O species at different coverages in the absence of competition. The EMA and Temkin-C model developed in the present study can be applied to all solids having a significant IR transmission 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.