WO3

Article pubs.acs.org/JPCC

Experimental Microkinetic Approach of De-NOx by NH3 on V2O5/WO3/ TiO2 Catalysts. 3. Impact of Superficial WOz and VxOy/WOz Groups on the Heats of Adsorption of Adsorbed NH3 Species François Giraud,†,‡ Christophe Geantet,† Nolven Guilhaume,† Stéphane Loridant,† Sébastien Gros,‡ Lynda Porcheron,‡ Mohamed Kanniche,‡ and Daniel Bianchi*,† †

Institut de Recherche sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256 CNRS, Université Claude Bernard Lyon I, Bâtiment Chevreul, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne, France ‡ Power Generation and Environment Department, Fluid Dynamics, EDF, 6 Quai Watier, 78401, Chatou, France S Supporting Information *

ABSTRACT: The present article is dedicated to the measurement of the individual heats of adsorption of adsorbed NH3 species on WO3/TiO2 and V2O5/WO3/TiO2 (a catalyst for the selective catalytic reduction of NOx by NH3 on 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. C 2014, 118, 15664] and 2 [Giraud et al. J. Phys. Chem. C 2014, 118, 15677] for the adsorbed NH3 species on TiO2 and V2O5/TiO2 solids. In agreement with the literature, Raman and Fourier transform infrared spectra indicate the presence of welldispersed VxOy and WOz entities on the different solids. For NH3 adsorption at pressure Pa < 0.5 kPa and temperature Ta in 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‑L and NH4+ species on Lewis and Brønsted sites, respectively. For Ta = 300 K, it is shown that four adsorbed NH3 species are formed on the WOzcontaining catalysts: two are adsorbed on Lewis sites, named NH3ads‑L1 and 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 δas IR 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/TiO2 catalyst 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‑L2 and 78 and 135 kJ/ mol for the NH4+-2 species, respectively. These values indicate that the presence of WOz increases significantly the heat of adsorption of the NH4+ species, as compared to TiO2 and V2O5/TiO2, explaining that both NH3ads‑L2 and NH4+-2 species can be present in the experimental conditions of NH3-SCR.

1. INTRODUCTION The present study is part of an experimental microkinetic approach (abbreviation EMA) of the catalytic reduction of NOx (NO and NO2) into N2 and H2O by NH31 (denoted NH3selective catalytic reduction, NH3-SCR) considering the experimental conditions of flue gas denitrification in coal-fired power plants. Stationary sources use mainly x wt % V2O5/y wt % WO3/sulfated-TiO2 catalysts with x and y in the ranges 0.7− 2 and 9−13, respectively.2,3 It is known that different adsorbed NH3 species can be formed on the surface of these catalysts, and the first step of the EMA was an in-depth kinetic/ thermodynamic characterization of these species considering particularly their (a) nature (adsorbed on Lewis and Brønsted sites) and numbers, (b) surface concentrations, and (c) individual heats of adsorption taking into account the catalyst © XXXX American Chemical Society

composition from pure TiO2 supports (sulfate-free and sulfated) to V2O5/WO3/TiO2 catalysts. The identification of the pivotal adsorbed species of NH3-SCR process constitutes one of the aims of the EMA (ref 4 and references therein) to obtain the mathematical expression providing the impact of the different experimental and kinetic parameters on the rate of the reaction. In part 14 of the present study, two analytical procedures, denoted adsorption equilibrium IR spectroscopy (AEIR)5−8 and temperature-programmed adsorption equilibrium (TPAE),9 developed in previous works, were adapted to measure the individual heats of adsorption of two adsorbed Received: April 30, 2015 Revised: June 10, 2015

A

DOI: 10.1021/acs.jpcc.5b04143 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C NH3 species on Lewis sites (denoted NH3ads‑L) present simultaneously at 300 K on a sulfate-free TiO2 (P25 from Degussa) in experimental conditions: partial pressure of NH3 (PNH3) and adsorption temperature (Ta), representative of the NH3-SCR (a small amount of NH4+ species is detected for Ta in the range 300−490 K). In part 2,10 the analytical procedures have been applied to study the modifications of the NH3 species and their heats of adsorption (a) on a sulfated TiO2 solid (DT51 from Millennium Inorganic Chemicals) and (b) on V2O5 supported on sulfate-free (P25) and sulfated (DT51) TiO2, at loadings of interest for the NH3-SCR. The adsorption of NH3 on these solids, for Ta in the 300−673 K range, (a) forms two NH3ads‑L species and (b) disturbs the sulfate- and/or V-containing superficial groups considering the modification the their characteristic IR bands at 1375 (SO bond) and 2050 cm−1 (VO overtone), respectively.10 This indicated that the nature of the Lewis adsorption sites depends on the composition of the solids. However, it has been shown that this has no significant impact on the individual heats of adsorption of the NH3ads‑L species.10 Moreover, superficial sulfate and VxOy entities increase the amount of NH4+ species, without modifying significantly their heats of adsorption as compared to TiO2-P25. Considering model and commercial NH3-SCR catalysts, the present study focuses on the impact of the deposition of WOz and VxOy groups on sulfate-free and sulfated TiO2 supports on the nature and the individual heats of adsorption (at different coverages) of adsorbed NH3 species. It is shown that the deposition WOz (a) maintains the presence of two strongly adsorbed NH3ads‑L species and (b) leads to the formation of a strongly adsorbed NH4+ species detected for adsorption pressure PNH3 and temperature Ta relevant to NH3-SCR. The measurement of the individual heats of adsorption of the NH4+ species in parallel to those of the two NH3ads‑L species constitutes an original contribution to the kinetic/microkinetic approach of the NH3-SCR on V2O5/WO3/sulfated-TiO2 catalysts.

Table 1. Heats of Adsorption at Low and High Coverages Ex(θ) of the Adsorbed NH3 Species Formed on Two Lewis Sites and the Proportions of Sites on the Surface as a Function of the Solid Composition NH3ads‑L1 solid P25a 0.5% V2O5 /P25b 2% V2O5 /P25b 6% WO3 /P25b 0.5% V2O5/ 6% WO3/ P25 DT51b 0.6% V2O5 /DT51b DT52c 0.6% V2O5/ DT52 COM1 COM2 a

NH3ads‑L2

E1(1) (kJ/mol)

E1(0) (kJ/mol)

58 55

102 104

58

94

55 57

56 55

E2(1) (kJ/mol)

E2(0) (kJ/mol)

x1

160 155

0.7 0.7

0.3 0.3

99

146

0.5

0.5

105

110

147

0.65

0.35

95

105

148

0.63

0.37

S-Containing Solids 102 110 150 98 102 148

0.65 0.65

0.35 0.35

S-Free Solids 102 105

x2

59 59

94 97

98 100

139 142

0.65 0.65

0.35 0,35

57 56

85 84

93 93

145 140

0.6 0.68

0.4 0.32

From part 1.4 bFrom part 2.10 c9% WO3/TiO2.

Two samples (∼50 g) of fresh commercial honeycomb TiO2 based NH3-SCR catalysts (cell density of ∼15 cpsi in geometry of L = 80 cm and section 13 × 13 cm) from two major industrial companies in the field have been studied (denoted COM1 and COM2) after being finely crushed (similar BET area, ∼60 m2/g). Elementary analysis (inductively coupled plasma atomic emission spectroscopy, ICP-AES) indicated slight differences in solid compositions: 0.9% V2O5 and 9% WO3 for COM1 and 0.7% V2O5 and 8.5% WO3 for COM2. Moreover Si (∼6%), Al (∼1.4%), Ca (∼1.3%), and S (∼0.9%) were present in the two solids. This was consistent with the elementary analysis of a commercial NH3-SCR catalyst studied by Lietti et al.11 X-ray diffraction (XRD) indicated that these elements are poorly crystallized, whereas TiO2 was pure anatase phase. Scanning electron microscopic (SEM) analysis (see Supporting Information, Figure S1) revealed the presence of cylindrical fibers (diameter ∼ 8 μm) which have been ascribed to ceramic additives (silicoaluminates and fiberglass) to improve the mechanical properties of the catalyst.12 Assuming SiO2, Al2O3, and CaO metal oxides, their amounts were estimated to ∼10% of a monolith, which was consistent with the ICP-AES characterizations of a BASF monolith of composition 1% V2O5/8% WO3/TiO2.13 It was impossible to separate these fibers from the catalyst, and they were present during the different characterizations. However, it is shown below that their contributions to the NH3 adsorption properties of the solids can be neglected. Fourier transform infrared (FTIR) spectroscopy revealed the presence of an IR band at 1375 cm−1 in COM1 and COM2 indicating that sulfur was present as sulfate in the catalysts similarly to DT51.10 Model catalysts have been prepared, to mimic commercial NH3-SCR catalysts, by deposition of V2O5 on WO3/TiO2 by the incipient wetness method using an aqueous solution of

2. EXPERIMENTAL SECTION 2.1. Solids, Pretreatments, and Structure/Texture Characterizations. In the following the compositions of the solids are mentioned in weight percent considering V2O5 and WO3 even if the species formed on the supports have different structures (denoted VxOy and WOz with x and y integer and z integer/fractional ≥ 1). The choice of the loadings (see Table 1, column 1) has been made considering literature data on the composition of (a) commercial NH3-SCR catalysts and (b) TiO2 based solids with a monolayer of VxOy and/or WOz groups (see Supporting Information for more details). Model and Commercial WO3/TiO2 Catalysts. The deposition of WO3 on sulfate-free (P25 Degussa, ∼55 m2/g, 80% anatase, 20% rutile) and sulfated (DT51, Millennium Inorganic Chemicals, ∼80 m2/g, 100% anatase, 0.56% of S) TiO2 solids has been made by using the incipient wetness method with aqueous solutions of ammonium metatungstate followed by a slow drying procedure and calcination in air at 713 K (see Supporting Information for more details). The solids are named 6% WO3/TiO2-P25 and 9% WO3/TiO2-DT51. A 9% WO3/TiO2 sulfated solid from Millenium Inorganic Chemicals (DT52, pure anatase, 85 m2/g, 1.35% S) has been also studied for comparison with model 9% WO3/TiO2-DT51. B


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The Journal of Physical Chemistry C ammonium metavanadate dissolved in the presence of oxalic acid (see Supporting Information and ref 10 for more details). Note that Vuurman et al.14 have shown by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) that the molecular structure of V2O5/WO3/TiO2-P25 solids was not influenced by the sequence of impregnation of the support. The Raman spectra of the different solids (see Supporting Information) showed that model 0.7% V2O5/DT52 (80 m2/ g) and commercial catalysts present strong similarities. It must be noted that Kleemann et al.15 have observed that the NH3 adsorption capacity at different temperatures of a V2O5/DT52 model catalyst and a commercial catalyst were similar. The measurement of the heats of adsorption of the adsorbed NH3 species on model NH3-SCR catalysts has been made on V2O5/ DT52 and 0.5% V2O5/6% WO3/P25 as examples of sulfated and sulfate-free catalysts. Before NH3 adsorption, the model and commercial solids were treated on the different analytical systems as follows: O2 (713 K, 10 min) → He (713 K, 5 min) → He (300 K). The same sample of solid was used to perform a series of experiments, and it was pretreated as above before each experiment. 2.2. Analytical Methods. The different solids have been characterized by conventional analytical methods: (a) ICP-AES for elemental analysis, (b) N2 physisorption at 77 K for the BET area, (c) XRD for phase identification, and (d) Raman and IR spectroscopies for the dispersion of VxOy and WOz species on the TiO2 supports (see Supporting Information and refs 4 and 10). The individual heats of adsorption of adsorbed species (which constitute the original contribution of the present study) have been obtained using FTIR spectroscopy (AEIR method).5−8 The quantification of the amount of adsorbed species has been obtained by use of transient experiments with a mass spectrometer (MS) detector.9 IR Cell in Transmission Mode. FTIR characterizations have been performed using a Nicolet-6700 FTIR spectrometer equipped with a small internal volume (∼2 cm3) stainless steel IR cell in transmission mode using CaF2 windows previously described in more detail.4,16 Briefly, it allowed in situ treatments of a compressed disk of solid (Φ = 1.8 cm, m ≈ 40−80 mg), in the 293−800 K temperature range with a controlled gas flow rate in the 150−2000 cm3/min range at atmospheric pressure selected using different valves and purified by different traps in particular cold traps for H2O impurities (77 K for helium and ∼200 K for NH3 containing gas mixtures). The slow cooling rate, ∼1 h from 713 to 300 K due to thermal inertia of the IR cell, led to the readsorption of tiny traces of H2O on the solids.4,10 Volumetric Measurements Using Mass Spectrometry. To support the exploitation of the IR spectra, the amounts of adsorbed NH3 species on the different TiO2 solids have been quantified with an analytical system for transient experiments using a quartz microreactor and a mass spectrometer as described previously,4,17 providing the rates of either formation of a product or consumption of a reactant with time on stream: R(t) (see Supporting Information for more details). The low thermal inertia of the microreactor which allowed a fast cooling from 713 to 300 K (∼4 min) and the amount of solid (200− 300 mg) limited the impact of H2O traces on the different measurements. 2.3. Heats of Adsorption of Adsorbed NH3 Species Using the AEIR and TPAE Methods. The procedures for the measurement of the individual heats of adsorption of NH3

species adsorbed on Lewis sites (denoted NH3ads‑L) using the AEIR5−8 and TPAE9 methods have been described in part 14 (see Supporting Information, eqs ES1−ES4). The impact of the presence of sulfate and/or VxOy species on the heats of adsorption of NH3ads‑L species has been described in part 2.10 The original contribution of the present study concerns the measurement of the individual heats of adsorption of (a) two NH3ads‑L species and (b) two NH4+ species on Lewis and Brønsted sites, respectively, of model and commercial WO3containing solids.

3. RESULTS AND DISCUSSION The AEIR and TPAE methods have been used to determine the heats of adsorption of the adsorbed NH3 species present on the different WO2/TiO2 and V2O5/WO3/TiO2 solids in the experimental conditions of interest for the NH3-SCR taking into account the presence or not of sulfate groups. XRD and N2 chemisorption (results not shown) revealed that the structure and textural properties of the TiO2 supports are not modified by the deposition of WOz and VxOy groups. 3.1. Characterization of WOz and VxOy Groups on TiO2 Based Solids. Raman Spectroscopy. All the dehydrated Wcontaining solids show a Raman band at ∼1010 ± 2 cm−1 ascribed to ν(WO) stretching vibrations of well-dispersed WOz species (different from WO3 crystallites) indicating limited interactions with the SO4 and VxOy species (see Supporting Information, Figure S2, for more details). The addition of vanadium to WOz/TiO2 leads to a new Raman band at 1030 cm−1 ascribed to ν(VO) stretching vibrations of well-dispersed VxOy species similar to that observed on the binary V2O5/TiO2 solids10 confirming limited interactions between W- and V-containing species (see Supporting Information, Figure S2). On the different W-containing commercial solids (a) DT52, (b) 0.7% V2O5/DT52, (c) COM1, and (d) COM2, a third Raman band is observed at 977 cm−1 (see Supporting Information, Figure S2) which seems associated with a second type of WO bond because of the catalyst preparation method (see Supporting Information). For instance, it is not detected on the homemade 9% WO3/DT51 solid, indicating that it is associated with a W-containing species formed by the industrial preparation method of DT52. The position of this third Raman band suggests an interaction with the sulfate groups of the solids (see Supporting Information). FTIR Spectroscopy: Overtone IR Bands of VxOy and WOz Species and OH Groups. Literature data indicate that FTIR spectroscopy provides additional material on the VxOy and WOz dispersions using the first overtone (range 2100−2000 cm−1) of their fundamental IR bands (P25 and DT51 have no IR bands in this wavenumber range). All the IR spectra are recorded at 300 K, after pretreatment of the solids at 713 K in O2 followed by cooling in helium (there is no significant difference by cooling in O2). Whatever the W-containing solid, an IR band is detected at ∼2012 cm−1 (see Supporting Information, Figure S3, ascribed to the overtone (2 × 1010 cm−1) of the IR band of the WO stretching12,18 of welldispersed WOz species indicating limited interactions with the sulfate group and the VxOy species. The overtone of the IR band at 977 cm−1 on the commercial W-containing solids is probably too weak to be detected. The V-containing solids present an IR band at 2050 cm−1 on the S-free solids and 2060 cm −1 on sulfated solids, respectively (see Supporting Information, Figure S2) ascribed to the VO overtone of well-dispersed VxOy species (ref 10 and references therein). In C


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The Journal of Physical Chemistry C

Figure 1. IR bands of adsorbed NH3 species after adsorption of 0.1% NH3/He on 6% WO3/TiO2-P25 pretreated at 713 K in helium as a function of adsorption temperature Ta: (a−e) Ta = 300, 373, 498, 573, and 673 K. (inset) Evolution of the overtone IR band of the WOz species with the adsorption of NH3: (a) in helium at 300 K; (b−d) in 0.1% NH3/He at Ta = 300, 573, and 673 K; (e) in helium at 673 K.

addition to the IR bands of the WO and VO bonds, the commercial solid COM1 presents an IR band at 1880 cm−1 (the IR transmission of COM2 is very low in this wavenumber range) similar to that observed at 1890 cm−1 on commercial catalysts19 and ascribed to overtone/combinations IR bands of the silica/silicate compounds mixed with the SCR catalyst for industrial applications. For COM1, the IR band at 1880 cm−1 must be linked to the glass fiber fraction of the monolith. The good dispersions of the WOz and VxOy species over TiO2 have been confirmed by studying the modification of the OH groups of the support. The 6% WO3/TiO2-P25 solid leads to very small and broad IR bands at 3735 and 3645 cm−1 (see Supporting Information, Figure S4) indicating strong perturbations of the TiO2 surface. The decrease in the IR bands of the OH group of the TiO2 support by the WO3 deposition is probably due, similarly to the deposition of V2O5 (see part 2 and references therein),10 to the reaction with the Wcontaining precursor of the more basic OH groups (IR band at high wavenumbers) and then of the less basic OH groups (IR band at low wavenumbers) at low and high W loadings, respectively. The deposition of 0.7% V2O5 on 6% WO3/TiO2P25 decreases the IR band of the OH group at 3735 cm−1, whereas the one at 3645 cm−1 is not significantly modified (Supporting Information, Figure S4). This is consistent with the reactivity of the more basic OH groups at low V2O5 loading.10 On V2O5/TiO2 an intense IR band was observed at 3660 cm−1 ascribed to V−OH species.10 This IR band is not observed in Figure S4 in the Supporting Information, suggesting that the nature of VxOy species on V2O5/WO3/ TiO2 can be partially different from that on V2O5/TiO2. DT52 is characterized by an IR band at 3666 cm−1 with a significantly lower intensity than DT51 (Supporting Information, Figure S4). The impregnation of 0.7% V2O5 on DT52 leads to an IR band of similar intensity slightly shifted to 3656 cm −1 which is also observed on COM 1 (Supporting Information, Figure S4). The main difference between the model 0.7% V2O5/DT52 and COM1 is the presence of the IR band at 3742 cm−1 for COM1 (Supporting Information, Figure S4) that is probably due to terminal OH groups on the SiO2 fraction of the glass fibers.12 A similar IR band is observed on P25 (3720 cm−1) ascribed by different authors to SiO2 traces.4

However, on P25, this IR band disappears after VxOy and/or WOz depositions, whereas it is detected on the commercial catalyst (glass fibers are probably inserted after the catalyst preparation). Finally, Raman and FTIR spectra of the different solids confirm that (a) the model WOz-containing solids are similar to those described in the literature, (b) the preparation methods lead to well-dispersed VxOy and WOz species on P25 and DT51 supports, without formation of V2O5 or WO3 crystallites and with limited interactions between these species and the sulfate groups, (c) DT52 and commercial catalysts have two types of WOz species because of the solids preparation, and (d) the Raman spectra of the model catalyst 0.7% V2O5/DT52 and COM1 are similar. This means that the heats of adsorption of the adsorbed NH3 species determined in the present study can be representative of both model and commercial catalysts, allowing their use for kinetic/microkinetic approaches of the NH3-SCR. The impacts of the presence of WOz species on the properties of adsorbed NH3 species on the different solids are presented as follows: first the sulfate-free P25 based solids and second the sulfated solids (including the commercial catalysts). 3.2. Nature of Adsorbed NH3 Species at 300 K on TiO2-P25 Based Solids. Figure S5 of the Supporting Information provides a detailed comparison of the IR spectra of the adsorbed NH3 species formed at 300 K using 0.1% NH3/ He on the P25 based solids. The adsorption of NH3 on the TiO2 support (see more details in part 14) leads to the detection of a strong IR band at 1142 cm−1 and a shoulder at 1215 cm−1 ascribed to the symmetric deformation (δs NH3) of two NH3 species adsorbed on Lewis sites designated NH3ads‑L1 and NH3ads‑L2 species, respectively (the notation NH3ads‑L is used without distinction of the two adsorbed species). The asymmetric deformation IR bands δas of the two NH3ads‑L species are overlapped at 1596 cm−1.4 Small IR bands 1680 and 1477 cm−1 are ascribed to the symmetric and antisymmetric deformation of a small amount of the NH4+ species.20 The adsorption of NH3 at 300 K on 0.5% V2O5/TiO2 leads to the detection of an additional shoulder at 1180 cm−1, indicating that NH3 is adsorbed on both TiO2 and VxOy species (Supporting Information, Figure S5).10 The adsorption of D


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The Journal of Physical Chemistry C NH3 on 6% WO3/TiO2 leads to the detection of (a) two δs IR bands at 1198 and 1280 cm−1 and a common δas NH3 IR band at 1603 cm−1 (Supporting Information, Figure S5) due to two NH3ads‑L species and (b) IR bands at 1445 (δas) and 1675 cm−1 (δs) due to NH4+ species.20 The ratio of the intensities of the IR bands at 1445 and 1602 cm−1 denoted the B/L ratio is strongly higher than on TiO2-P25. The increase in the amount of NH4+ species is confirmed by the detection of IR bands at 3031 and 2775 cm−1 on 6% WO3/TiO2.20 NH3 adsorption on WO3/TiO2 is associated with the decrease in the broad IR band of the OH groups leading to negative IR bands in the range 3700−3550 cm−1 (Supporting Information, Figure S5). The deposition of 0.5% V2O5 on 6% WO3/TiO2 does not modify significantly the IR bands of the different adsorbed NH3 species (Supporting Information, Figure S5). The originality of the present study is the measurement of the individual heats of adsorption of both the NH4+ (with are more strongly adsorbed on the W-containing solids) and NH3ads‑L species. 3.3. Heats of Adsorption of NH3 Species Adsorbed on 6% WO3/TiO2-P25. The procedures, for the measurement by use of the AEIR method of the individual heats of adsorption of the NH3ads‑L and NH4+ species on 6% WO3/TiO2-P25, are described in detail. Then they are applied to the other solids to reveal (a) the impacts of the sulfate groups and the deposition of V2O5 and (b) the differences between model and commercial catalysts. Adsorption of 0.1% NH3/He in the 300−673 K Temperature Range. After a first increase in Ta to 673 K followed by cooling to 300 K in the presence of 0.1% NH3/He (see below the impacts of this procedure on the IR spectra), Figure 1 shows the evolutions of the IR bands of the adsorbed NH3 species during the second increase in Ta from 300 to 673 K. After the adsorption equilibrium at 300 K, the inset of Figure 1 shows a shift of the WO overtone from 2013 (spectrum a) to 1940 cm−1 (spectrum b), associated with a broadening. This indicates clearly that WOz species are involved in NH3 adsorption.18 Figure 1 shows that the increase in Ta (range 300−673 K) for Pa = 0.1 kPa leads to a progressive decrease in the intensity of (a) the δas IR band at ∼1600 cm−1 associated with a slight shift to higher wavenumbers (1602 and 1606 cm−1 at 300 and 673 K, respectively) and (b) the main δs NH3ads‑L IR band that shifts more significantly from 1198 cm−1 at 300 K to 1227 cm−1 at 673 K whereas a shoulder due to the second δs IR band is detected at 1280 cm−1. In parallel, the main IR band of the NH4+ species decreases with the increase in Ta and shifts to lower wavenumbers (1418 cm−1 for Ta ≥ 573 K). The significant difference with the P25 and V2O5/P25 solids is that the IR bands of the NH4+ species are still observed at 673 K (on V2O5/P25, they disappear for Ta ≥ 523 K), indicating that WOz entities lead to more strongly adsorbed NH4+ species which can be present with NH3ads‑L species at the temperature and partial pressure of NH3 relevant to NH3-SCR (this situation is observed on all the W-containing solids). The inset in Figure 1 shows that the decrease in the amount of adsorbed NH3 species with Ta leads to a shift to the WO overtone to higher wavenumbers 1972 (spectrum c) and 1992 cm−1 (spectrum d) at 573 and 673 K, respectively, which becomes progressively sharper than at 300 K. At 673 K, the IR band is still disturbed by the remaining fraction of the adsorbed NH3 species (Figure 1), as shown by the position of the IR band at 2004 cm−1 after the pretreatment of the solid at 673 K (spectrum e).

Heats of Adsorption of NH3ads‑L Species on 6 wt % WO3/ TiO2-P25. In part 1,4 it has been shown that the δs NH3 IR bands on the TiO2-P25 support (1142 and 1215 cm−1 at 300 K) can be used to measure the individual heats of adsorption of two NH3ads‑L species. However, theses IR bands cannot be used for quantitative applications after either V10 or W deposition because of (a) the low IR transmission of 598 K (see the text for more details). (inset A) Impact of the number of cycle H(673 K)/ C(300 K) in 0.1% NH3/He on the intensity of the IR bands of the NH3ads‑L and NH4+ species at 300 K: (a) after the pretreatment of the solid, (b and c) after the first and second cycles. (inset B) Impact of the NH3 adsorption pressure on the IR bands of NH3ads‑L and NH4+ species: (a) 0.5% NH3/He and (b) 0.1% NH3/He. E


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Heat of Adsorption of NH4+ Species on 6 wt % WO3/TiO2. Symbols ▲ and ■ in Figure 3 give the evolution of the

of 0.1% NH3/He on the intensity of the IR bands of the adsorbed NH3 species at 300 K. After the first cycle the intensity of the NH4+ IR band (1445 cm−1) is decreased whereas that of the NH3ads‑L (1603 cm−1) is increased. These data are similar to those observed on TiO2-P254 and V2O5/ TiO2-P25.10 This can be explained considering that (a) tiny H2O traces are readsorbed during the cooling stage in He after the pretreatment procedure favoring the NH4+ species whereas this readsorption is significantly decreased in the presence of NH3 because of the competitive chemisorption and (b) it has been shown that the presence of adsorbed NH3 increases the proton mobility favoring the dehydration of the solid which leads to a decrease in the amounts of Brønsted sites and a parallel increase in Lewis sites.21,22 Inset A of Figure 2 shows that the second cycle has no impact on the intensity of the IR bands. The AEIR method has been applied after a first H(673 K)/C(300 K) cycle. The saturation of the different adsorption sites must be attested, for instance by checking that the increase of either Pa at a constant temperature (i.e., Ta = 300 K) or Ta at a constant adsorption pressure has no impact on the intensity of the IR bands of the adsorbed species. For instance, inset B in Figure 2 shows that, after reaching the steady state in 0.5% NH3/He, the intensities of the IR bands at 1602 (NH3ads‑L species) and 1445 cm−1 (NH4+ species) are identical before and after the switch 0.5% NH3/He → 0.1% NH3/He. This justifies that the L and B sites are saturated under 0.1% NH3/He and that the intensities of the IR bands at 1602 and 1445 cm−1 at 300 K can be used as AXM values (Supporting Information, eq ES1) for the determination of the coverages of the NH3ads‑L and NH4+ species, respectively. This was not the case for the NH4+ species on TiO2-P254 and V2O5/TiO2-P25,10 indicating that this species is more strongly adsorbed on 6% WO3/TiO2. For this reason, the AEIR method has been applied using PNH3 = 0.1 kPa, allowing the use of cool traps at the lowest temperature to limit the impact of H2O traces. Heat of Adsorption of NH3ads‑L Species on 6 wt % WO3/ TiO2. Symbols ● in Figure 2 give the evolution of the total coverage of the NH3ads‑L species (Supporting Information, eq ES1) using the IR band at 1602 cm−1 during the increase in Ta under Pa = 0.1 kPa. Curve a is obtained considering in Supporting Information, eqs ES2−ES4, the following: (a) 55 and 105 kJ/mol for NH3ads‑L1 at coverages 1 and 0 respectively, (b) 110 and 147 kJ/mol for NH3ads‑L2 species at coverages 0 and 1 respectively, and (c) x1 = 0.65 and x2 = 0.35 for the proportions of sites of the NH3ads‑L1 and NH3ads‑L2 species, respectively. Curves b and c give the theoretical evolutions of the coverages of NH3ads‑L1 and NH3ads‑L2 according to Supporting Information, eq ES2, considering the heats of adsorption obtained from curve a. Symbols ■ are experimental coverage values obtained as follows: the coverage of the NH3ads‑L1 species is very small at Ta > 573 K (curve b in Figure 2); this allows the assumption that (a) the δas NH3 IR band for Ta > 573 K (Figure 2) is only due to the NH3ads‑L2 species and (b) at 573 K the area of the IR band corresponds to the theoretical coverage provided by curve b: 0.9. It can be observed that the experimental data (symbols ■) are reasonably consistent with curve c for Ta > 573 K. Table 1 summarizes the heats of the adsorption of NH3ads‑L1 and NH3ads‑L2 species to facilitate the comparison between the different solids.

Figure 3. Heats of adsorption of the adsorbed NH4+ species on 6% WO3/TiO2-P25 using the AEIR method. (■ and ▲) Average coverage of the NH4+ species during the increase in Ta for the second and third cycles in 0.1% NH3/He: (a) theoretical curve from Supporting Information, eqs ES2 and ES3, assuming the presence of a single NH4+ species with the heats of adsorption E1 = 60 and E0 =121 kJ/mol; (b) theoretical curve from Supporting Information, eqs ES2−ES4, assuming the presence of two NH4+ species with the parameters of Table 2. (inset) ■, experimental average coverage of the two NH4+ species using the area of the half IR band at 1445 cm−1 at high wavenumbers; b; same theoretical curve as curve b in the main figure.

average coverage of the NH4+ species (Supporting Information, eq ES1) during the second and third increases in Ta for Pa = 0.1 kPa by using the IR band at 1445 cm−1 (Figure 1). Curve a in Figure 3 is obtained using Supporting Information, eqs ES2 and ES3, considering the presence of a single NH4+ species with the following heats of adsorption: E1 = 60 and E0 = 121 kJ/mol at coverages 1 and 0, respectively. These values allow obtaining an overlap between experimental and theoretical curves in the 1 to ∼0.2 range. An improvement is obtained considering the presence of two types of NH4+ species (noted NH4+-1 and NH4+-2) according to the procedure used to obtain the heats of adsorption of the two NH3ads‑L species (Figure 2). Curve b in Figure 3 is obtained by using in Supporting Information, eqs ES2−ES4, the following parameters: (a) 59 and 105 kJ/mol for the heats of adsorption at coverages 1 and 0 respectively for the NH4+-1 species, (b) 75 and 140 kJ/mol at coverages 1 and 0 respectively for NH4+-2 species, and (c) x1 = 0.6 and x2 = 0.4 for the proportions of sites of the two species. The good overlap between experimental data and theoretical curves strongly supports the presence of two adsorbed NH4+ species on the 6% WO3/TiO2-P25 surface. On the sulfated solids, the SO IR band is strongly shifted to lower wavenumbers after NH3 adsorption. Then, the decreases in the coverages of the NH3ads‑L and NH4+ species lead to a progressive shift to higher wavenumbers.10 These evolutions induce a partial overlap with the main δas IR band of the NH4+ species (at 1445 cm−1) in the wavenumber range < 1420 cm−1. This means that the overall IR band cannot be used to quantify the coverage of the NH4+ species on W- and Scontaining solids (see below). Symbols ■ in the inset of Figure F


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exploitation of the TPAE can only provide an average of the heats of adsorption of the different species and not their individual heats of adsorption, similarly to literature data on temperature programmed desorption (TPD) characterizations of NH3 adsorption on TiO2 based NH3-SCR catalyst.23,24 However, symbols ■ in the inset of Figure 4 give the evolution of the average coverage of NH3 (Supporting Information, eq ES5) during the second TPAE, whereas curve a of the inset is obtained (Supporting Information, eqs ES2−ES4) considering the presence of the two main adsorbed species NH3ads(1) and NH3ads(2) with the following heats of adsorption and proportions of sites: 55 and 98 kJ/mol at coverages 1 and 0 with x1 = 0.65 for NH3ads(1) and 98 and 137 kJ/mol with x2 = 0.35 for NH3ads(2). These values are consistent with the AEIR method considering that NH3ads(1) and NH3ads(2) are the averages of weakly and strongly adsorbed NH3ads‑L and NH4+ species, respectively. 3.5. Heats of Adsorption of NH3ads‑L Species on 0.5% V2O5/6% WO3/TiO2-P25. The inset of Figure 5 shows that the overtone IR bands of the VxOy and WOz species at 2050 and 2012 cm−1 respectively on the pretreated solid (spectrum a) are shifted to lower wavenumbers 2005 and 1927 cm−1 after adsorption of 0.1% NH3/He at 300 K (spectrum b) on the model NH3-SCR catalysts. This is consistent with literature data18 indicating that these superficial groups are involved in the adsorption of NH3. Figure 5 shows the evolutions of the IR bands of the adsorbed NH3 species during the increase in Ta in the presence of 0.1% NH3/He after a first H(673 K)/C(300 K) cycle. The impacts of this cycle (result not shown) on the IR bands of the NH4+ and NH3ads‑L species at 300 K are similar to those on W/TiO2-P25 (inset A of Figure 2). The comparison of Figures 2 and 5 shows that the deposition of 0.5% V2O5 over 6% WO3/TiO2 has no significant impact on the positions of the IR bands. The main difference is due to the detection at 300 K of an IR band at 1360 cm−1 after the pretreatment procedure on the V2O5/WO3/TiO2 solid (see Supporting Information, Figure S5) as observed previously on the V2O5/TiO2 samples,10 because of the presence of a small amount of sulfate groups arising from S impurities (see Supporting Information for more details). This SO IR band leads to a partial overlap with the IR band of NH4+ species as shown in Figure 5, imposing using the half area of the IR band of NH4+ (in the 1535−1445 cm−1 range) for quantitative application. Figure 5 shows that the increase in Ta leads to the progressive decrease in the different IR bands of the adsorbed species. This is associated with (a) a large shift from 1445 cm−1 at 300 K to 1421 cm−1 at 673 K for the IR band of the NH4+ species and (b) a small shift from 1602 cm−1 at 300 K to 1605 cm−1 at 673 K for the IR band of the NH3ads‑L species. The presence of the two species at 673 K (Figure 5g) confirms that WOz groups increase significantly the heat of adsorption of NH4+ species as compared to TiO2-P254 and V2O5/TiO2-P25.4,10 The decrease in the coverage of these adsorbed species leads to a shift of the VxOy and WOz overtone IR bands which are detected at 2039 and 1995 cm−1, respectively, at 673 K (spectrum c in the inset of Figure 5) in the presence of 0.1% NH 3/He, whereas after the pretreatment procedure in helium they are located at 2040 and 2006 cm−1, respectively (spectrum d). These data show that a small amount of adsorbed NH3 species disturbs the totality of the two VxOy and WOz groups. Similarly to WO3/TiO2, we have verified that the IR bands of NH3ads‑L and NH4+ species at 300 K for PNH3 = 0.1 kPa can be

3 give, for the 6% WO3/TiO2-P25 solid, the evolution of the coverage of NH4+ species using, in Supporting Information, eq ES1, the area of the left half IR band at 1455 cm−1 (Figure 1) from 1535 cm−1 to the maximum of the band (this wavenumber range is allowed for quantitative application on the all W- and S-containing solids). Curve b is the same theoretical curve as Figure 3. The good agreement between the experimental and theoretical coverages shows that the coverages obtained from the half IR band are consistent with those obtained in Figure 3 using the full IR band. This allows us to apply this procedure on the W- and S-containing solids. However, the accuracy on the experimental data is slightly decreased because the IR band of NH4+ is broad, leading to some uncertainties on the position of the maximum. 3.4. Adsorption of NH3 on 6% WO3/TiO2-P25 Using the MS System and TPAE Method. After the pretreatment of the solid, Figure 4A gives the evolution of the molar fraction

Figure 4. Adsorption of 0.5% NH3/0.5% Ar/He and TPAE procedure with the 6% WO3/TiO2-P25 solid. Part A: adsorption of NH3. Part B: TPAE procedure. Part C: readsorption of NH3 during cooling. Part D: second TPAE. (inset) ■, experimental evolution of the coverage by NH3 of the solid (four adsorbed NH3 species) using the second TPAE; a, theoretical curve from Supporting Information, eqs ES2− ES4, considering the presence of two strongly and weakly adsorbed species denoted NH3ads(1) and NH3ads(2) (see the text for more details).

of the gases at the outlet of the microreactor during the switch He → 0.5% NH3/0.5% Ar/He, indicating an adsorption of 317 μmol of NH3/g. Figure 4B gives the evolution of the molar fractions during the increase in Ta in the presence of NH3 (TPAE). A broad NH3 peak is detected starting as soon as Ta increases and which is not ended at 660 K. At this temperature the H2O and N2 productions start probably due to NH3 oxidation/dissociation via the oxygen of the solid. Note that there is no shoulder at low temperatures as observed on TiO2 and V2O5/TiO210 confirming the higher heat of adsorption of NH4+ species. The amount of NH3 desorbed before the NH3 consumption is 297 μmol/g. The amount readsorbed in Figure 4C during cooling in the presence of NH3 is 283 μmol/g, in good agreement with the TPAE data. The second TPAE (in Figure 4D) leads to a slightly lower amount of NH3, 269 μmol/ g, probably because of the modification of the surface during the first TPAE. According to the AEIR data, the NH3 production during the TPAE is due to the desorption of two NH3ads‑L species and two NH4+ species. This means that the G


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Figure 5. IR bands of adsorbed NH3 species after adsorption of 0.1% NH3/He on 0.5% V2O5/6% WO3/TiO2-P25 pretreated at 713 K in helium as a function of adsorption temperature: (a−g) Ta = 300, 373, 473, 523, 573, 623, and 673 K. (inset) Evolution of the overtone IR bands of the VxOy and WOz entities with the adsorption of NH3: (a) in helium at 300 K; (b and c) in 0.1% NH3/He at Ta = 300 and 673 K; (d) in helium at 673 K.

used as AXM values in Supporting Information, eq ES1, for the determination of the evolution of the coverages of the two species with Ta. Due to the SO IR band of the sulfate group, the calculations are performed using the evolution of the area of the left half of the IR band of the NH4+ species. Symbols ■ in Figure 6 give the evolution of the total coverage of NH3ads‑L species during the increase in Ta for Pa = 0.1 kPa using eq ES1 in the Supporting Information, and the area of the IR band at 1602 cm−1 at 300 K as the AXM value. Curve a is obtained considering in Supporting Information, eqs ES2−ES4, the following: (a) 57 and 95 kJ/mol for the NH3ads‑L1 at coverages 1 and 0 respectively, (b) 105 and 148 kJ/mol for the NH3ads‑L2

species at coverages 1 and 0 respectively, and (c) x1 = 0.65 and x2 = 0.35 for the proportion of sites of the NH3ads‑L1 and NH3ads‑L2 species, respectively. Curves b and c give the theoretical evolutions of the coverage of the NH3ads‑L1 and NH3ads‑L2 according to Supporting Information, eqs ES2 and ES3, considering the heats of adsorption obtained from curve a. Symbols ▲ overlapping curve c are experimental data obtained as follows: (a) curve b in Figure 6 shows that the coverage of the NH3ads‑L1 species is very small at Ta > 600 K indicating that the evolution of the δas NH3 IR band for Ta > 600 K is only due to the NH3ads‑L2 species and (b) the area of the δas NH3 IR band at 598 K is assumed equal to the theoretical coverage of 0.71. Symbols ● in Figure 6 give the evolution of the average coverage of the NH4+ species on V2O5/WO3/TiO2-P25 (Supporting Information, eq ES1) using the area of the left half IR band between 1540 cm−1 and its maximum (i.e., 1445 cm−1 at 300 K). Curve d, which overlaps the experimental data, is obtained from Supporting Information, eqs ES2−ES4, considering the presence of two NH4+ adsorbed species with the following parameters: (a) 59 and 80 kJ/mol for the NH4+-1 species at coverages 1 and 0 respectively, (b) 78 and 135 kJ/ mol for the NH4+-2 species at coverages 1 and 0 respectively, and (c) x1 = 0.67 and x2 = 0.33 for the proportions of sites of the two species. Curves a and d in Figure 6 show clearly that NH3ads‑L2 and NH 4+-2 species can be present in the experimental conditions of the NH3-SCR. Their reactivity with NO will determine their respective roles in the reaction. 3.6. Adsorption of NH3 on TiO2-Sulfated Based Catalysts. After the pretreatment procedure at 713 K and the cooling stage in helium to 300 K, all the sulfated model and commercial solids present an IR band in at ∼1378 cm−1 corresponding to the SO vibration of sulfate groups (i.e., 1380 cm−1 on COM1 in spectrum a of Figure 7). The adsorption of tiny traces of H2O during the cooling stage after the pretreatment procedure at 713 K leads to a small IR band at ∼1615 cm−1 (i.e., 1611 cm−1 in spectrum a of Figure 7 for COM1). The adsorption of 0.1% NH3/He leads to (a) the detection of the IR bands of the adsorbed NH3 species of NH4+ (3309 (tentative assignment), 3178, 3030, 1675, 1445 cm−1) and NH3ads‑L (3380, 3340, 3257, 1607 cm−1) and (b) a strong shift of the SO IR band of the sulfate groups to lower wavenumbers, as observed on the TiO2-DT51 support,10

Figure 6. Heats of adsorption of the adsorbed NH3ads‑L and NH4+ species on 0.5% V2O5/6% WO3/TiO2-P25 using the AEIR method. (■) Average coverage of the NH3ads‑L species for PNH3 = 0.1 kPa using the area of the IR band at 1602 cm−1: (a) theoretical curve from Supporting Information, eqs ES2−ES4, considering two adsorbed species on the Lewis sites NH3ads‑L2 and NH3ads‑L1 with the parameters of Table 1; (c) theoretical curves for NH3ads‑L1 and NH3ads‑L2 species respectively from Supporting Information, eqs ES2 and ES3, and the parameters of Table 1. (▲) Experimental coverage of the NH3ads‑L2 species from the IR band at 1602 cm−1 for Ta > 598 K (see the text for more details). (●) Average coverage of the NH4+ species with Ta using the half area of the IR band at 1445 cm−1: (d) theoretical curve from Supporting Information, eqs ES2−ES4, considering two NH4+ species with the parameters of Table 2. H


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Figure 7. Evolution of the IR bands of commercial catalyst COM1 and of the adsorbed NH3 species during adsorption of 0.1% NH3/He at different temperatures: (a) at 300 K after the pretreatment procedure; (b−d) after adsorption of NH3 at Ta = 300, 473, and 673 K; (e) in helium at 673 K.

Figure 8. Evolutions of the δas IR band of the NH3ads‑L species on the four S- and W-containing solids (A, DT52; B, V2O5/DT52; C, COM1; D, COM2) during the adsorption of 0.1% NH3/He at different temperatures: (a−f) Ta = 300, 373, 423, 473, 573, and 673 K.

overlapping the δs NH3 IR bands that are hardly detected at 300 K. This shift (which indicates that the sulfate groups are disturbed by the adsorption of NH3) is strongly dependent on the coverage of NH3 species. In particular, the increase in Ta leads to a progressive shift of the SO IR band to higher wavenumbers allowing the progressive detection of the disturbed sulfate groups first as a shoulder (i.e., at 1327 cm−1 for Ta = 473 K in Figure 7, spectrum c) and then as a distinct IR band (i.e., at 1354 cm−1 for Ta = 673 K, Figure 7, spectrum d). At this temperature, the remaining fraction of NH3 species still disturbs the SO IR band which is detected at a higher position after the pretreatment procedure (i.e., at 1375 cm−1, Figure 7, spectrum e). Similarly to TiO2-DT51,10 the strong dependence of the shift of the SO IR band on the coverage of the NH3 species prevents using the δs NH3 IR bands for the measurement of the heat of adsorption of the NH3ads‑L species. Only the δas NH3 IR band at ≈1600 cm−1 (i.e., 1607 cm−1 for COM1, spectrum a of Figure 7) is used for this purpose. Figure 8 shows the evolution of the δas IR band of the NH3ads‑L species with the increase in Ta in the presence of 0.1% NH3/He, for the different model and commercial S-containing solids. Figure 8 indicates strong similarities in the evolutions of the coverage of the adsorbed NH3ads‑L species with the increase in Ta. In particular, there is a small shift to higher wavenumbers and the full coverage of the sites is obtained at 300 K using a partial pressure of NH3 of 0.1 kPa. 3.7. Heats of Adsorption of NH3ads‑L Species on Different S- and W-Containing Solids. The symbols ●, ■, ▲, and ○ in Figure 9 give the experimental evolutions of

Figure 9. Heats of adsorption of the adsorbed NH3ads‑L species on DT52, V2O5/DT52, COM1, and COM2 using the AEIR method. (●, ■, ▲, and ○) Average coverage of the NH3ads‑L species for PNH = 0.1 3 kPa using the area of the IR band at ≈1602 cm−1 on DT52, V2O5/ DT52, COM1, and COM2 respectively: (a and b) theoretical curves from Supporting Information, eqs ES2−ES4, considering two adsorbed species on the Lewis sites NH3ads‑L2 and NH3ads‑L1 for V2O5/DT51 and COM2 with the parameters of Table 1; theoretical evolution of the coverage of the NH3ads-L1 and NH3ads-L2 species on (c−d) V2O5/DT52 and (e−f) COM1 from Supporting Information, eqs ES2 and ES3, and the parameters of Table 1.

I


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Figure 10. Evolutions of the IR band of the NH4+ species on three S- and W-containing solids (A, DT52; B, V2O5/DT52; C, COM1) during the adsorption of 0.1% NH3/He at different temperatures: (a−d) Ta of 300, 373, 423, and 673 K. (inset) Overlap of the IR bands of the NH4+ species and sulfate groups at 300 K.

progressive decrease in the intensity associated with a shift to lower wavenumbers up to Ta ≈ 473 K (on COM1 the temperature is of 523 K) and then the IR band decreases without any shift. Using the evolution of the area of the left half IR band of the NH4+ species (from 1540 cm−1 to the maximum of the bands), the IR spectra of Figure 10 can be used to obtain the evolution of the average coverage of the NH4+ species with Ta in isobaric conditions. Symbols ●, ■, and ▲ in Figure 11

the average coverage of the NH3ads‑L species (Supporting Information, eq ES1) on DT52, 0.6% V2O5/DT52, COM1, and COM2, considering the δas IR band and using eq ES1 in the Supporting Information. Figure 9 shows clearly the limited differences in the evolution of the coverage of the NH3ads‑L species with Ta in isobaric conditions on the different solids. This leads to similar heats of adsorption and proportions of sites of the NH3ads‑L1 and NH3ads‑L2 species. For instance, curves a and b in Figure 9 give the theoretical evolutions of the average coverage of the NH3ads‑L species for the two limit situations corresponding to the model catalyst 0.6% V2O5/DT52 (symbols ■) and COM2 (symbols ○) using eqs ES2−ES4 in the Supporting Information with the heats of adsorption and proportions of sites provided in Table 1. The differences in the experimental data on the two solids are due to minor modifications of the parameters such as a higher amount of the more weakly adsorbed NH3ads‑L1 species of 0.68 and 0.65 on COM2 and 0.6% V2O5/DT52, respectively, and lower heats of adsorption of the NH3ads‑L2 species at high coverage of 93 and 100 kJ/mol on COM2 and 0.6% V2O5/DT52, respectively. Table 1 shows clearly that the differences in the surface composition of the different solids (including the presence or the absence of sulfate groups) have limited impacts on (a) the number, (b) the heats of adsorption at different coverages, and (c) the proportion of sites of the two adsorbed NH3ads‑L species. 3.8. Heats of Adsorption of NH4+ Species on Different S- and W-Containing Solids. Figure 10 shows the evolutions of the main IR band of NH4+ species on DT52, V2O5/DT52, and the commercial catalyst COM1 during the increase in Ta for PNH3 = 0.1 kPa (the IR transmission of COM2 below 1500 cm−1 is

WO3

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