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

Abstract

The present study is a part of an experimental microkinetic approach of the selective reduction of NO x 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.

Acidic Properties of Alkaline-Earth Phosphates Determined by an Experimental-Theoretical Approach

“…The experimental spectra contained one band at cm -1 typical of Lewis coordination and/or H-bonding ((NH 3 ) vibration). 48 This vibration mode is doubly degenerated in free NH 3 but nonsymmetric environment slightly split the band in the computed spectra. It was predicted to be not sensitive to the type of interaction since it was computed at 1630, 1611 cm -1 when NH 3 was Hbonded to P-OH up ( Figure 8A) and at 1639, 1609 cm -1 when NH 3 was in interaction with a LAS (Figure 8B and D).…”

“…The accuracy of the method to determine NH 3 heats of adsorption has been evidenced for TiO 2 support and V 2 O 5 /WO 3 /TiO 2 catalysts. [17][18][19] Acidity measurements in presence of water vapor 20 were achieved at adsorption equilibrium flowing 1%NH 3 -3%H 2 O-He mixture upon heating and cooling between RT and 200 °C.…”

Tungsten-Based Catalysts for Environmental Applications

“…Hydrogen migration is involved in both acidity conversion for Lewis to Brønsted acid site transformation at T > 300 • C and reoxidation for the redox loop of the DeNO x reaction [139]. In addition, coordination of ammonia on Lewis acid sites does not prevent the dissociative H 2 O chemisorption and the formation of Brønsted acid sites, leading to adsorbed NH 4 + species [140]. Surface acidity and redox properties, as well as their effect on SCR activity, were investigated by Zhao et al on a series of V n W-TiO 2 with various vanadium loadings (1-13 wt.%), while keeping the tungsten loading constant (8 wt.% WO 3 ) [141].…”

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

“…This shows that the NH 3 −H 2 O coadsorption constitutes one of the key points of the debate and explains our interest for its EMA using TiO 2 -based solids. In Part 5, 10 different experiments have been qualitatively discussed considering the NH 3 −H 2 O coadsorption on a sulfated 0.7% V 2 O 5 /9% WO 3 /TiO 2 catalyst and its TiO 2 support. In line with the concept of the EMA, in the present study, these experimental data are compared to a competitive adsorption model based on the Temkin formalism as performed recently for the CO hydrogenation on a 2.9% Pt/ Al 2 O 3 catalyst.…”