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The present study is a part of an experimental microkinetic approach of the selective reduction of NOxto N2with NH3in excess of O2on V2O5/WO3/TiO2catalysts (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 NH3intermediate 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/TiO2NH3-SCR catalyst and its TiO2support. Regardless of the solid, it is shown that at the NH3-H2O coadsorption equilibrium, (a) NH3dominates the adsorption on the Lewis sites (i.e., the introduction of NH3at the H2O adsorption equilibrium displaces H2Oads-Lspecies at the benefit of NH3ads-Lspecies) and (b) the introduction of H2O at the NH3adsorption 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.

The present study is a part of an experimental microkinetic approach (EMA) of the selective reduction of NOxto N2with NH3in excess O2on V2O5/WO3/TiO2catalysts (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 TiO2supports and a sulfated 0.7% V2O5/9% WO3/TiO2catalyst), 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 NH3species and (b) the molecularly and dissociated H2O species on Lewis and Brønsted sites. Whatever the solid, it is shown that NH3dominates 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 NH3adsorption equilibrium. For the sulfate-free TiO2solid having weak Brønsted sites, the switch between the NH3adsorption equilibrium to the NH3–H2O coadsorption equilibrium is associated to the production of a small amount of NH3due to the displacement of NH3ads-Lspecies 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 NH3and 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.

The present study is a part of an experimental microkinetic approach of the removal of NOxfrom coal-fired power plants by reduction with NH3on V2O5/WO3/TiO2catalysts (NH3-selective catalytic reduction, NH3-SCR). It is dedicated to the characterization of the heats of adsorption of molecularly adsorbed H2Oadsspecies formed on sulfate-free and sulfated TiO2supports. Water, which is always present during the NH3-SCR, may be in competition and/or react (formation of NH4+) with the adsorbed NH3species controlling the coverage of the adsorbed intermediate species of the reaction. Mainly, an original experimental procedure named adsorption equilibrium infrared spectroscopy (AEIR) previously used for the adsorption of NH3species on the same solids is adapted for the adsorption of H2O. At Ta= 300 K and for PH2O≤ 1 kPa, three main H2Oadsspecies are formed (associated with a minor amount of dissociated H2O species) on the two TiO2solids. The species are identified by the positions of their IR bands in the 3750–3000 cm–1range. Considering the decreasing order of stability, they are (a) coordinated to strong (L2) and weak (L1) Lewis sites and denoted H2Oads-L2and H2Oads-L1, respectively, and (b) hydrogen bonded to the H2Oads-Lspecies and on O2–/OH sites of the solids (denoted H2Owads). The three species have a common well-defined δH2OIR band at a position in the range 1640–1610 cm–1according to the total coverage of the surface. According to the AEIR method, the evolution of the intensity of this IR band during the increase in the adsorption temperature Tain isobaric condition provides the evolution of the average coverage of the three species and then to their individual heats of adsorption as a function of their coverage. It is shown that there are no significant differences on the two TiO2solids. In particular, the heat of adsorption of the H2Oads-L2species varies from ∼114 to 61 kJ/mol at low and high coverages respectively, indicating that it can be present in the experimental conditions of the NH3-SCR. In a forthcoming article, the competitive chemisorptions and reaction between adsorbed H2O and NH3species are studied and modeled on the TiO2supports and model and commercial V2O5/WO3/TiO2catalysts.

The present article is dedicated to the measurement of the individual heats of adsorption of adsorbed NH3species on WO3/TiO2and V2O5/WO3/TiO2(a catalyst for the selective catalytic reduction of NOxby NH3on stationary sources, briefly NH3-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. C2014, 118, 15664] and 2 [Giraud et al. J. Phys. Chem. C2014, 118, 15677] for the adsorbed NH3species on TiO2and V2O5/TiO2solids. In agreement with the literature, Raman and Fourier transform infrared spectra indicate the presence of well-dispersed VxOyand WOzentities on the different solids. For NH3adsorption at pressure Pa< 0.5 kPa and temperature Tain 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 NH3, forming NH3ads-Land NH4+species on Lewis and Brønsted sites, respectively. For Ta= 300 K, it is shown that four adsorbed NH3species are formed on the WOz-containing catalysts: two are adsorbed on Lewis sites, named NH3ads-L1and NH3ads-L2, and two are adsorbed on Brønsted sites, named NH4+-1 and NH4+-2 (“1” and “2” indicate the increasing order of stability of the different species). Using the δasIR band characteristic of the NH3ads-L(∼1600 cm–1) and NH4+(∼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% V2O5/6% WO3/TiO2catalyst 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 NH3ads-L2and 78 and 135 kJ/mol for the NH4+-2 species, respectively. These values indicate that the presence of WOzincreases significantly the heat of adsorption of the NH4+species, as compared to TiO2and V2O5/TiO2, explaining that both NH3ads-L2and NH4+-2 species can be present in the experimental conditions of NH3-SCR.

The present article is dedicated to the impacts of sulfatation and/or VxOydeposition on TiO2supports on the individual heats of adsorption of adsorbed NH3species. The S and V loadings are similar to those of TiO2based commercial catalysts for the selective catalytic reduction of NOxby NH3: NH3–SCR. Two original experimental procedures are used, namely, adsorption equilibrium infrared spectroscopy (AEIR) and temperature-programmed adsorption equilibrium (TPAE). They have been developed in previous works and adapted in Part 1 for the species formed by the adsorption of NH3on a sulfate free TiO2support (P25 from Degussa). In agreement with the literature, Raman and FTIR spectroscopies show that the impregnation of the sulfate-free and sulfated TiO2supports by the vanadium precursor leads to well-dispersed VxOyspecies, which are involved in the adsorption of NH3in the temperature Tarange of 300–673 K and for adsorption Pa< 0.5 kPa. Sulfate and VxOygroups favor the amount of NH4+species without modifying their heats of adsorption as compared to the TiO2support: they are not detected for temperatures of interest for NH3–SCR reaction. In these experimental conditions and whatever the solids, two main adsorbed NH3species on Lewis sites (denoted NH3ads-L1and NH3ads-L2) are present, characterized by two δsIR bands below 1300 cm–1and a common δasIR band at ≈1600 cm–1. By comparison with TiO2P25; it is shown that sulfate groups have strong impacts neither on the proportion, x1and x2, of the two adsorbed species nor on their heats of adsorption (EL1(θ) and EL2(θ) with θ the coverage of the NH3ads-L1and NH3ads-L2species: x1= 0.65, x2= 0.35, EL1(1) = 56 kJ/mol, EL1(0) = 102 kJ/mol, EL2(1) = 110 kJ/mol, EL2(0) = 140 kJ/mol. The deposition of V-containing species on the sulfate-free and sulfated TiO2surfaces leads to similar conclusions: in the experimental conditions (Taand Pa) of the NH3–SCR, two adsorbed NH3species on Lewis sites are detected with comparable proportions and heats of adsorption to those observed on the TiO2supports.

The present study is dedicated to the development of experimental procedures allowing the measurement of the individual heats of adsorption of adsorbed NH3species on a sulfate-free TiO2solid (P25 from Degussa). This solid has been selected because it is frequently used as support of V2O5– or/and WO3–TiO2model catalysts for the understanding of the surface processes implicated in the selective catalytic reduction of NOxby NH3(NH3-SCR). Two original analytical procedures denoted adsorption equilibrium infrared spectroscopy (AEIR) and temperature-programmed adsorption equilibrium (TPAE) (developed in previous works) were applied. These methods are based on (a) the experimental measurement of the change in the adsorption equilibrium coverage of the individual adsorbed species in isobar conditions and (b) the comparison of the experimental data to an adsorption model. It is shown that in the ranges of the ammonia partial pressures and reaction temperatures of the NH3-SCR process, only two adsorbed NH3species on Lewis sites (Ti+δ) are detected on the solid dehydrated at 673 K. These species noted NH3ads-L1and NH3ads-L2are differentiated by their δsNH3IR bands at 300 K (1149 and 1228 cm–1, respectively), whereas their δasIR bands are at the same position (1596 cm–1). The AEIR and TPAE methods indicate that the heats of adsorption of the NH3ads-L1and NH3ads-L2species (noted EL1(θ) and EL2(θ) accuracy ± 5 kJ/mol) vary linearly with their respective coverages θ from EL1(1) = 56 kJ/mol to EL1(0) = 105 kJ/mol and from EL2(1) = 105 kJ/mol to EL2(0) = 160 kJ/mol. These values are compared to (a) isosteric heat of adsorption provided by the Clausius–Clapeyron method and (b) literature data using temperature-programmed desorption, microcalorimetry, and DFT calculations. Forthcoming articles show that the simplicity of the analytical procedures allows studying the impact of the presence of sulfate and VOx/WOydepositions over TiO2on the nature and heats of adsorption of adsorbed NH3species.