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C: Surfaces, Interfaces, Porous Materials, and Catalysis x

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Experimental Microkinetic Approach of De-NO by NH on VO/WO/TiO Catalysts. 6. NH-HO Coadsorption on TiO Based Solids and Competitive Temkin Model 2

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Francois Giraud, Julien Couble, Christophe Geantet, Nolven Guilhaume, Stéphane Loridant, Sébastien Gros, Lynda Porcheron, Mohamed Kanniche, and Daniel Bianchi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05847 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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The Journal of Physical Chemistry

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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

AUTHOR NAMES François Giraud,1,2 Julien Couble1, Christophe Geantet,1 Nolven Guilhaume,1 Stephane Loridant,1 Sébastien Gros,2 Lynda Porcheron,2 Mohamed Kanniche,2 and Daniel Bianchi1*

AUTHOR ADDRESS 1

Institut de Recherche sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256

CNRS, Université Claude Bernard Lyon I, Bat. Chevreul, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne-France.

2

EDF- Fluid Dynamics, Power Generation and Environment Department, 6 Quai Watier,

78401, Chatou-France.

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2 ABSTRACT: 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 Brønsted 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 Brønsted site 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 .


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3 Text 1. INTRODUCTION The present study is the continuation of the experimental microkinetic approach (abbreviation EMA) of the catalytic reduction by NH3 of NOx (NO and NO2) into N2 and H2O (named NH3-selective catalytic reduction: NH3-SCR) considering the experimental conditions of flue gas denitrification in coal-fired power plants (T≥ 473 K)1 using x% V2O5/y% WO3/sulfated-TiO2 (weight %) catalyst with x and y in the ranges 0.7-2 and 9-13 respectively.2,3 The aims of the EMA are related to the identification of the surface elementary steps controlling the rate of the reaction in particular (a) the nature and the coverage of the pivotal adsorbed species and (b) their kinetic/thermodynamic parameters. This must lead to the modeling of the catalytic activity (i.e. in TOF s-1) in large range of experimental conditions. According to literature data

4,5

there is a debate on the pivotal NH3 intermediate

of the NH3-SCR reaction considered as either NH3ads-L species formed by adsorption on Lewis sites associated to a competition with H2O,4,6,7 or NH4+ species formed by adsorption on Brønsted sites without8 and with9 competition with H2O. This shows that the NH3-H2O coadsorption constitutes one of the key points of the debate and explains our interest for its EMA using TiO2 based solids. In Part 510 different experiments have been qualitatively discussed considering the NH3-H2O coadsorption on a sulfated 0.7% V2O5/9% WO3/TiO2 catalyst and its TiO2 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/Al2O3 catalyst.11,12 As discussed,11,12 modeling the rate of a catalytic reaction taking into account the heterogeneity of the adsorption sites via the Temkin model has been rarely made in the literature13,14 and in these few cases rough mathematical approximations have been used limiting the modeling to short range of experimental conditions. Boudart15 has discussed this



4 situation as the “paradox of surface kinetics”: briefly, whatever the heterogeneity of the surface, the rate of a gas/solid catalytic reaction can be modeled using the Langmuir formalism. This “paradox” has limited significantly the interest for the development of others formalism for the modeling of surface processes, such as catalytic reactions and the competitive adsorption of two gases. However, Boudart15 noted that the values of the different thermodynamic/kinetic parameters associated to the Langmuir formalism on heterogeneous sites may have no physical meaning. This justifies the interests for the development of adsorption/reaction formalisms taking into account the heterogeneity of the active sites, even if the interest of these efforts constitutes an open scientific debate.16-22 Previous works have shown that the Temkin model (based on a representation of the heterogeneity of the sites by a linear decrease in the heats of adsorption with the increase of the coverage) is representative of the adsorption equilibrium coverages of different adsorbed species such as (a) linear and bridged CO species on supported metal particles23-27 and (b) NH3ads-L, NH4+ and molecular H2Oads species on different TiO2 based solids in particular model and commercial x% V2O5/y% WO3/TiO2 catalysts.28-31 For these last solids, this clearly indicates that in large ranges of adsorption temperatures and partial pressures, experimental data on the NH3-H2O coadsorption can only be modeled accurately according to the Temkin formalism. This was the topic of previous works dedicated to the CO-H2 coadsorption11 (T in the range 300-473 K) and reaction12 (CH4 production for T> 473 K) on a 2.9% Pt/Al2O3 catalyst. For T< 473 K the formalism of a competitive Temkin model (named Temkin-C)11 has been developed using the experimental evolutions of the coverages of adsorbed CO and hydrogen species. For T> 473 K, this model has been developed taking into the experimental rate of the CO hydrogenation to CH4 reaction leading to a competitive Temkin model with reaction (named Temkin-C.R model).12 For broad ranges of experimental conditions, these works11,12 have shown the clear advantage of the models based on the Temkin formalisms as


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5 compared to the Langmuir formalism. The mathematical formalism of the Temkin-C and Temkin-C.R models can be applied to any gas/solid catalytic system in particular to the surface processes associated with the NH3-SCR reaction on x% V2O5/y% WO3/TiO2 catalysts such as the NH3-H2O coadsorption. The modeling to this coadsorption by the Temkin-C formalism must take into account that, regardless of the composition of TiO2-based solids (sulfated or not, with and without VxOy and/or WOz groups), NH3 is adsorbed, in the temperature range 300-713 K and for PNH3 consistent with the NH3-SCR process, on two types of Lewis sites named L1 and L2 in the increasing order of stability.28-30 These sites can be situated at different locations of the nanocrystallites such as different planes, terraces and defects (steps and corners). Using the AEIR method (see supporting information) it has been shown that the NH3ads-L1 and NH3ads-L2 species have different heats of adsorption increasing linearly with the decrease in their coverages according to the Temkin adsorption model.28,29 In parallel, for the VxOy or/and WOz containing TiO2 solids, NH3 is adsorbed on two types of Brønsted sites forming, NH4+-1 and NH4+-2 species with different heats of adsorption according to the Temkin adsorption model.29,30 On TiO2 solids (without and with sulfate groups), NH4+ species are also formed at 300 K in parallel to the NH3ads-L species, however their stabilities are lower than on the V2O5 and/or WO3 containing solids.28 Similarly, using the AEIR method is has been shown that the adsorption of H2O on TiO2 based solids in the temperature range of the NH3-SCR reaction leads to the formation of two molecularly adsorbed species on the L1 and L2 Lewis sites consistent with the Temkin model.10,31 In parallel to the molecular adsorption, there is a dissociative H2O chemisorpton leading to the formation of hydroxyl groups which interact with the adsorbed H2O species via hydrogen bonds.10,31 The aim of Part 510 and of the present study is to show how experimental data associated with the NH3-H2O coadsorption can be accurately modeled according to the



6 Temkin-C model taking into account the diversity of the adsorbed NH3 and H2O species formed on TiO2 based solids. Part 510 was dedicated to the experimental study (using FTIR spectroscopy and mass spectrometry), at Ta > 423 K, of the NH3-H2O coadsorption on sulfated 0.7% V2O5/9% WO3/TiO2 and TiO2 solids. It has been shown that, whatever the solids, (a) NH3 dominates the adsorption on the Lewis sites and (b) this does not prevent H2O dissociation on a small amount of sites forming new OH groups and (c) these OH act as Brønsted sites leading to the increase in the amount of NH4+ species, as compared to the adsorption of NH3 in the absence of H2O.10 The aim of the present study is to compare these experimental data to theoretical curves obtained from the Temkin-C model,11 particularly for the competitive adsorption on the Lewis sites and the formation of Brønsted sites. Supplementary experiments on the impacts of H2O and NH3 partial pressures during coadsorption on the 0.7% V2O5/9% WO3/ TiO2 solid are presented to confirm the interest of the Temkin-C model for the accurate modeling of data obtained under broad range of experimental conditions. Moreover, the experiments of Part 510 on the NH3-H2O coadsorpion have been performed on a sulfate-free TiO2 solid having weak Brønsted sites. This provides key experimental data supporting the conclusions of the present EMA. The present EMA of the NH3-SCR reaction has a second broader intrinsic interest: the comparison of the surface acidity of a solid in the absence and in the presence of H2O via the change in the amounts of the adsorbed NH3 species on the Lewis and Brønsted sites. In particular, it is shown that the Temkin-C model provides a quantification of the water-tolerant Lewis sites on metal oxides as qualitatively discussed in the literature.32-34

2. EXPERIMENTAL SECTION The sulfated solids: 0.7% V2O5/9% WO3/TiO2 catalyst and its TiO2-DT51 support, their pretreatments, the microreactor IR cell35 and the volumetric measurement using a mass


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7 spectrometer28-31,36 are identical to those in Part 510 (see more details in Supporting information). The sulfate free solid is TiO2- P25 from Degussa (55 m2/g, 80% anatase, 20% rutile). Its pretreatment before adsorption is similar to the sulfated solid (see ref 10).

3. RESULTS AND DISCUSSION 3.1 NH3-H2O Coadsorption on 0.7% V2O5/9% WO3/TiO2: Impacts of the Ta, PH2O and PNH3 The impacts of these parameters on the composition of the adsorbed species are clearly revealed using FTIR spectroscopy. To facilitate the presentation some results obtained in previous works10, 28-31 on the adsorption and coadsorption of NH3 and H2O at 473 K are briefly summarized (particularly the assignment of the IR bands using literature data). Adsorption and Coadsorption of PH2O= 250 Pa and PNH3=100 Pa at 473 K. At 473 K, the pretreated solid presents two IR bands at 1378 and 3640 cm-1 (Figure 1 spectrum a) ascribed to the ν(S=O) vibration of SO4 groups and bridged OH groups respectively.31 3250 3323 3379

Absorbance

0.1

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3024

a

3262

3648

d

3656

b

b 3640

3800

0.2

1365 1334

3326

c a 3400

3000

Wavenumbers (cm-1)

1615

1800

1378

3168

c, d 1660

1423 d b 1605

1321

c

1600 1400 Wavenumbers (cm-1)

1200

Figure 1: Comparison of the FTIR spectra at the H2O and NH3 adsorption equilibrium and the NH3-H2O coadsorption equilibrium on 0.7% V2O5/9% WO3/TiO2 at Ta= 473 K: (a) solid before adsorption; (b) in 0.25% H2O/He; (c) in 0.1% NH3/He and (d) in 0.1% NH3/0.25% H2O/ He.



8 The 0.25% H2O/He adsorption equilibrium (Figure 1 spectrum b) leads to the following modifications: 10 (a) the ν(S=O) vibration decreases and shifts to 1365 cm-1 due to long range interactions with the adsorbed H2O species, (b) the IR band of the OH groups increases and shifts slightly to 3648 cm-1 indicating a dissociative H2O chemisorption and (c) the new IR band at 1615 cm-1 corresponds to the δH2O IR band of molecularly adsorbed H2O species on Lewis sites (named H2Oads-L species) associated with the broad IR band in the range 3600-3000 cm-1 due to hydrogen bonds (hereafter named H-bond) between H2Oads-L species and/or with O2-/OH groups (these species may contribute to the IR band at 3648 cm-1 via dangling hydrogen, abbreviation d-H )37,38. It has been shown using the AEIR method31 that two species named H2Oads-L1 and H2Oads-L2 formed on different Lewis sites contribute to the δH2O IR band at 473 K. Their individual heats of adsorption and proportions at saturation of the sites (see AEIR method in the Supporting Information) are indicated in Table 1. At the adsorption equilibrium using 0.1% NH3/He (spectrum c in Figure 1) the IR bands are ascribed as follows:10,30 (a) 1605 cm-1 is the δas IR band of adsorbed NH3 species on Lewis sites (named NH3ads-L), (b) 1423 cm-1 and the shoulder at 1660 cm-1 are the asymmetric (δas-NH4+) and symmetric (δs-NH4+) deformation of NH4+ species formed on Brønsted sites, (c) the IR bands in the 3500-3000 cm-1 range are due to the overlaps of the stretching vibrations of the NH3ads-L and NH4+ species and their overtones, (d) 3656 cm-1 is the IR band of the OH groups decreased as compared to spectrum a due to the formation of NH4+ species (maybe with the contribution of H-bonds with NH3ads-L species) and (e) 1334 cm-1 is the ν(S=O) IR band which is decreased and shifted (compare spectra a and c) due to long range interactions with adsorbed NH3 species. It has been shown30 that (a) the IR band at 1605 cm-1 is due to the contribution of two molecularly adsorbed species on different Lewis sites named, NH3ads-L1 and NH3ads-L2 and (b) the IR band at 1423 cm-1 is due to the contribution of two NH4+-1 and NH4+-2 species on different Brønsted sites. The heats of adsorption of the


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9 different NH3ads-L and NH4+ species and there proportions at saturation of the sites are provided in Table 1. Solids

TiO2-P25a TiO2-DT51

NH3ads-L2

NH3ads-L1

b

0.7% V2O5/9% WO3/TiO2c

E(1) kJ/mol

E(0) kJ/mol

x1

E(1) kJ/mol

E(0) kJ/mol

x2

56

112

0.7

104

160

0.3

56

102

0.65

110

140

0.35

59

97

0.65

100

142

0.35

NH4+-1

0.7% V2O5/9% WO3/TiO2c

NH4+-2

E(1) kJ/mol

E(0) kJ/mol

x1

E(1) kJ/mol

E(1) kJ/mol

E(0) kJ/mol

57

90

0.65

75

135

0.35

NH4+ E(1) kJ/mol

E(0) kJ/mol

E(1) kJ/mol

43 56 H2Oads-L1 E(0) kJ/mol

TiO2-P25d

55

TiO2-DT5d 0.7% V2O5/9% WO3/TiO2

TiO2-P25 TiO2-DT51

x1*

E(1) kJ/mol

80 124 H2Oads-L1 E(0) kJ/mol

61

0.28

61

110

0.3

54

60

0.3

61

114

0.35

56

61

0.26

58

117

0.34

x2*

OH E(1) kJ/mol

E(0) kJ/mol

TiO2-P25

95

180

TiO2-DT51

100 OH-1 E(0) kJ/mo

135 OH-2 E(0) kJ/mol

E(1) kJ/mol

x1

E(1) kJ/mol

x2

0.7% V2O5/9% 96 98 0.55 90 165 0.45 WO3/TiO2 * the proportions include a third weakly adsorbed species present at 300 K (named H2Owads) a: ref 26, b: ref 29, c: ref 30, d: ref 31 Table 1: Heats of adsorption E(θ) at low θ= 0 and high θ= 1 coverages and proportions at 300 K (xi,) of the adsorbed species formed at the adsorption equilibrium for NH3 and H2O.



10 The data in Table 1 and the Temkin model (eqs ES4-ES5) provide the evolutions with Ta of the coverages of the different molecularly adsorbed H2O and NH3 species on 0.7% V2O5/9% WO3/TiO2 for PH2O= 250 Pa and PNH3= 100 Pa (see Figure S1). For Ta ≥ 473 K, Figure S1 shows that the three adsorbed species: NH3ads-L2, NH4+-2 and H2Oads-L2 have significant coverages (1, 0.75 and 0.51 respectively) and can therefore be involved in the NH3-H2O coadsorption. The NH3ads-L1 species is present with a lower coverage: ≈ 0.2 at 473 K. However, the L1 sites represent ≈ 65% of the Lewis sites (Table 1) which imposes considering their role in the experimental data of the NH3-H2O coadsorption. Spectrum d in Figure 1 is obtained at the coadsorption equilibrium of 0.1% NH3/0.25% H2O/He at 473 K. The comparison with spectra b and c leads to the following comments and conclusions (more details in ref 10): the IR band of the NH3ads-L species at 1605 cm-1 is not strongly modified (slight decrease) by the presence of H2O while that of the H2Oads-L species at 1615 cm-1 is not observed during coadsorption (this is consistent with the absence of the broad IR band in the range 3500-3000 cm-1). This indicates that the competitive adsorption of molecular species on the Lewis sites is dominated by NH3. The intensity of the IR band at 1423 cm-1 is higher in spectrum d than spectrum c which can be ascribed to an increase in the amount of NH4+ species due to the formation of new Brønsted sites by the dissociation of H2O in the presence of NH3ads-L species. Note that the difference in the position of the ν(S=O) IR band in spectra d and c confirms that the natures and/or amounts of the adsorbed species are different for NH3-H2O and NH3 adsorption equilibriums. The data in Figure 1 are used as support of the Temkin-C model. Impacts of Ta, PNH3 and PH2O at the NH3-H2O Coadsorption Equilibrium Figure 2 (Parts A-D) compares the IR bands of the NH3ads-L, NH4+ species and adsorbed H2O species at Ta= 423, 473, 573 and 673 K for three NH3-H2O coadsorption


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11 equilibriums obtained under 0.1% NH3/0.25% H2O/He (spectrum a), 0.5% NH3/0.25%

c 1429

3660

c 1426 a b

3640

0.02

d

a b

a

Absorbance

Absorbance

Absorbance

H2O/He (spectrum b) and 0.1% NH3/1% H2O/He (spectrum c).

3700 3600 3500 Wavenumber (cm-1)

0.1 1634

1605

a b

1670

0.1 1636 1605 1670

A

b a

T= 473 K

1600 1500 1400 Wavenumber (cm-1)

1700

1600 1500 Wavenumber (cm-1)

1424

Absorbance

1605

b a

1670

1400

c1424 a,b

ca b 0.1

B

c

T= 423 K

c 1700

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1 1605

b

a,c

c

C

D T= 673 K

T= 573 K 1700

1600 1500 1400 Wavenumber (cm-1)

1700

1600 1500 1400 Wavenumber (cm-1)

Figure 2: Comparison of the FTIR spectra at the NH3-H2O coadsorption equilibrium on 0.7% V2O5/ 9% WO3/TiO2 for different partial pressures and adsorption temperatures: (a) 0.1%NH3/0.25% H2O/He, (b) 0.5% NH3/0.25% H2O/He, (c) 0.1% NH3/1% H2O/He. Inset Part A: IR bands of the OH bonds for 0.1% NH3/1% H2O/He at different adsorption temperatures: (a)–(d) Ta= 423, 473, 573 and 673 K. Regardless of Ta, spectra a and b in Figure 2 show that increasing PNH3 for PH2O= 250 Pa leads to the increase in the IR band of the NH3ads-L species at 1605 cm-1. This is consistent with the heats of adsorption of the two NH3ads-L species: for PNH3= 100 Pa, Figure S1 shows that at 423 K the coverage of the NH3ads-L2 and NH3ads-L1 are 1 and 0.46 indicating that increasing PNH3 increases the coverage of the NH3ads-L1 species without modification of that of NH3ads-L2 while for Ta≥ 473 K, the coverages of the two NH3ads-L species must increase with PNH3. Moreover, spectra a and b in Figure 2 show that the IR band of the NH4+ species at ≈



12 1425 cm-1 decreases slightly with the increase of PNH3 whereas the heats of adsorption of the two NH4+ species (Figure S1 curves c and d) suggest an impact similar to the NH3ads-L species. This indicates that the amount of NH4+ formed during the coadsorption is controlled by different equilibriums (i.e. the dissociative adsorption of H2O and the NH4+ adsorption equilibrium). This is confirmed by the comparison of spectra b and c in Figure 2 showing that, regardless of Ta, the increase in PH2O for PNH3= 100 Pa increases significantly the IR band of the NH4+ species at ≈ 1425 cm-1 with a slight decrease in the IR band at 1605 cm-1. This suggests that the increase in PH2O is associated to the formation of more Brønsted sites due to a dissociative H2O equilibrium via a competition with the NH3ads-L species on a small amount of the Lewis sites.10 The differences in the IR extinction coefficients of the two species εNH4+>> εNH3ads-L (ratio≈ 6.7),10 contribute to explain that a small decrease in the IR band at 1605 cm-1 can be associated to a significant increase in that at 1425 cm-1. At Ta= 423 and 473 K, Figure 2A and 2B show that for PH2O= 1 kPa, an IR band at 1634 cm-1 overlaps the IR bands of NH3ads-L (1605 cm-1) and NH4+ (≈ 1670 cm-1) species. This IR band seems due to the presence of a molecularly adsorbed H2O species. However, the inset of Figure 2A (range of 3700-3600 cm-1) showing the evolutions of the IR bands of the isolated OH bonds during the increase in Ta for 0.1% NH3/1% H2O/He reveals that at Ta= 423 K (spectrum a, inset Figure 2A), there is no IR band for OH bonds (no isolated OH groups and no d-H bonds associated with an H2Oads species). This indicates that the adsorbed H2O species providing the IR band at 1634 cm-1 is in interaction via H-bonds with the adsorbed NH3 species and/or O2-/OH groups. It is shown below that according to the Temkin-C model and literature data this IR band is probably due to H2O adsorbed on the NH3ads-L species.

3.2 NH3-H2O Coadsorption on 0.7% V2O5/9% WO3/TiO2 and Temkin-C Formalism


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13 According to the aims of EMA of the surface processes, the qualitative conclusions obtained from the experimental data on the NH3-H2O coadsorption on 0.7% V2O5/9% WO3/TiO2 (Part 510 and Figures 1 and 2) must be supported by an adsorption model and its mathematical formalism. Temkin-C model for Competitive Adsorption of Two Gases. Previous studies,28-31 have shown (Table 1 and Figure S1) that the coverages of the different adsorbed species formed by the adsorption NH3 and H2O on the two sulfated NH3SCR catalyst and its TiO2-DT51 support and the sulfate free TiO2-P25 are consistent with the Temkin adsorption model (abbreviation T in the figures). This constitutes the driving force for the development of a competitive adsorption model based on this formalism. The Temkin formalism39 is a proposal to take into account the heterogeneity of adsorption sites in the modeling of experimental data obtained at the adsorption equilibrium. The heterogeneity of adsorption sites can be due to either a difference in the adsorption properties of the sites (biographical or intrinsic heterogeneity such as different planes, terraces, steps and corners) or to interactions between adsorbed species (induced heterogeneity).39 Temkin39 noted that the two types of non-uniformity can be simultaneously operant and that a single mathematical expression (eq ES3) must be representative of this situation to prevent an excessive mathematical complexity in the adsorption formalism: its proposal was that the heterogeneity in a group of sites can be represented by a linear decrease in the heats of adsorption of a gas with the increase in its coverage.39 This means that experimental data in agreement with the Temkin formalism do not allow one to conclude on the origin of the heterogeneity. Moreover, for highly dispersed solids constituted of nanocrystallites such as the present TiO2-based solids (mainly of anatase structure) an intrinsic heterogeneity can be relevant of the exposed planes. For instance, we have suggested28 that the presence of two types of Lewis sites L1 and L2 on TiO2-P25 can be relevant of different planes in lines with literature data on DFT



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14 calculations.40,41 This leads to the following view of the heterogeneity of the present solids: the L1 and L2 groups of Lewis sites (Table 1) can be situated on different planes each having an intrinsic and/or induced heterogeneity consistent with the Temkin model. The same view can be applied to the two Brønsted sites (Table 1). Modeling the competitive chemisorption between two gases according to the Temkin formalism has been described in a previous work using experimental data on the competition between CO (forming a linear CO species) and H2 (dissociative chemisorption) on the Pt° sites of a Pt/Al2O3 catalyst.11,12 This competitive Temkin-C model (abbreviation T-C in the Figures) has been developed11 by extension of the integral equation (IE) approach used to obtain the generalized Temkin equation (eq ES3).42-45 For the competitive adsorption of a gas A adsorbed without dissociation and a gas B adsorbed either without or with dissociation on the same sites (i.e., A= NH3 and B= H2O for the NH3-H2O coadsorption on 0.7% V2O5/9% WO3/TiO2), the IE approach leads to a set of two equations (see more details in the Supporting Information and in refs 11, 12) providing the coverage of each adsorbed species at the coadsorption equilibrium:

EA0 θA = ∫

dE A EA0 = ∫ f (E A ) 1 / α ∆E A EA1   ∆E B EA1 ∆ E  B  1 + K A ( E A ) PA +  K B ( EA − EA0 − EB0 ) PB    ∆E A  ∆E A    K A ( E A ) PA

EB0

dE A (1) ∆E A

( K B ( E B ) PB )1 / α dE B EB0 = ∫ g ( EB ) EB1 1 + K A ( ∆E A E B −  ∆E A EB0 − EA0  ) PA + [K B ( E B ) PB ]1 / α ∆E B EB1  ∆E B  ∆E B

θB = ∫

dE B (2) ∆E B

where α= 1 or 2 for molecular or dissociative adsorption of B, EX0 and EX1 are the heats of adsorption of the X= A or B adsorbed species at coverage 0 and 1 respectively (with ∆EX= EX0- EX1), EX and KX are the heats of adsorption and the adsorption coefficient at different coverages and PX the partial pressures for X=A or B during the coadsorption. At each


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15 temperature Ta, eqs 1 and 2 are solved by a numerical method (using the Mathcad software in the present study, calculations in the 200-800 K range) considering: (a) the expression of the adsorption coefficients for A and B for localized adsorbed species (eq ES4); (b) the values of PA, PB and Ta used during the experiments in Figure 2 and (c) the individual heats of adsorption of the Aads and Bads species at low and high coverages measured in the absence of competitive chemisorption (Table 1 for the heats of adsorption of the different NH3 and H2O adsorbed species). These calculations provide the theoretical evolutions of the coverages of the Aads and Bads species in competitive chemisorption according to the Temkin-C model with the increase in Ta for constant partial pressures PA and PB (isobaric conditions). These theoretical curves are compared to the experimental data on the NH3-H2O coadsorption on the L1 and L2 groups of Lewis sites of the 0.7% V2O5/9% WO3/TiO2 catalyst to valid or not the qualitative conclusions of Part 5,10 such as considering Figures 1 and 2: (a) the adsorption equilibrium coverage of the NH3 on the Lewis sites is not strongly modified by coadsorption while (b) the H2Oads-L species are displaced from the surface in the presence of NH3. Temkin-C Model Between H2Oads-L and NH3ads-L for PH2O≈ 250 Pa and PNH3= 100 Pa According to Figure S1, it is considered that the experimental data of the NH3-H2O coadsorption for Ta ≥ 423 K involve the L1 and L2 types of Lewis sites. Curves a and b in Figure 3A correspond to the evolution of the coverages of the NH3ads-L2 and NH3ads-L1 species during the adsorption of 0.1% NH3/He according to the Temkin model (eqs ES3-ES4, see Figure S1 and ref 10 for more details). Similarly, curves c and d in Figure 3A provide the coverage of the H2Oads-L2 and H2Oads-L1 species during the adsorption of 0.25 % H2O/He according to the Temkin model (see Figure S1). Curves e, f, g and h in Figure 3A provide the coverages of the NH3ads-L2, NH3ads-L1, H2Oads-L2 and H2Oads-L1 species respectively according to the Temkin-C model (eqs 1 and 2 for α=1, using the heats of adsorption in Table 1) during the



16 coadsorption of 0.1% NH3/0.25% H2O/He considering competitions between (a) NH3ads-L1 and

Coverage of the adsorbed species

PNH3=100 Pa, PH2O=250 Pa

1

e

0.8

a

c 0.6

d b NH3-L1 (T)

h

H2OL1 (T-C)

200

300

c ×100

g H2OL2 ×100 (T-C)

f

0.2

NH3-L2 (T)

H2OL2 (T)

H2OL1 0.4 (T)

0

A

NH3-L2 (T-C)

NH3-L1 (T-C) 400

500

600

Temperature (K)

700

PNH3=100 Pa and P1H2O=250 Pa, P2H2O=1 kPa

1 0.8 0.6

f

NH3-L2 (T-C, P2 )

NH3-L1 (T-C, P2 )

NH3-L2 (T-C, P1 )

C

e

NH3-L1 (T-C, P1 )

0.4 0.2

a

b

H2OL1 H2OL1 (T-C, P1) (T-C, P2)

g

c

d

h

0 200

H2OL2 ×10 (T-C, P2)

H2OL2 ×10 (T-C, P1)

300

400

500

600

Temperature (K)

700

1 PH2O=250 Pa and P1NH3=100 Pa, P2NH3=500 Pa

e

800

b

a

3-L1 0.8 NH (T-C, P1 )

NH3-L1 (T-C, P2 )

H2OL2

H2OL2

×100

×100

(T-C, P1)

0.4

B

NH3-L2 (T-C, P2 )

NH3-L2 (T-C, P1 )

f

0.6

(T-C, P2)

d

c

2OL1 HO 0.2 H(T-C, P1) 2 L1

(T-C, P2)

g

h

0 200

800



H2Oads-L1 on the L1 sites and (b) NH3ads-L2 and H2Oads-L2 on the L2 sites.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 54

300

400

500

Temperature

600

(K)

700

800

1 PNH3=100 Pa, PH2O=10 kPa

b

e

NH3-L1 (T)

0.8

NH3-L2 (T-C)

D

f

NH

3-L1 0.6 (T-C)

c g H2OL1 (T)

0.4

H2OL2 (T)

0.2

h 0 200

d

H2OL1 (T-C) 300

400

500

a NH3-L2 (T) H2OL2 (T-C) 600

Temperature (K)

700

800

Figure 3: Comparison of the theoretical coverages of the molecularly adsorbed NH3 and H2O species on the L1 and L2 Lewis sites of 0.7% V2O5/ 9% WO3/TiO2 without (Temkin model, curves named T) and with (Temkin-C model curve named T-C) coadsorption for different experimental conditions. Part A: Comparison between Temkin and Temkin-C for PNH3= 100 Pa and PH2O= 250 Pa. Part B: Comparison of Temkin-C for PH2O= 250 Pa and either P1NH3= 100 Pa or P2NH3= 500 Pa. Part C: Comparison of Temkin-C for PNH3= 100 Pa and either P1H2O= 250 Pa or P2H2O= 1 kPa. Part D: Comparison between Temkin and Temkin-C for PNH3= 100 Pa and PH2O= 10 kPa. For the two NH3ads-L species, the overlap of the curves obtained from the Temkin and Temkin-C models for T> 423 K leads to the conclusion that NH3 dominates the adsorption on the Lewis sites. This is associated with the strong decrease in the coverages of the H2Oads-L species (curves g (with a factor 100) and h in Figure 3A) particularly at low temperatures. However, for the coadsorption, the coverage of the H2Oads-L2 species (curve g Figure 3A) increases at high temperatures because a fraction of the L2 sites are made available by the


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17 decrease in the adsorption equilibrium coverage of the NH3ads-L2 species (curve a). Moreover for Ta< 340 K, the coverage of the NH3ads-L1 species is slightly decreased in the presence of H2O (i.e. from 1 (curve b) to 0.96 (curve f) at 300 K). The theoretical curves obtained by the Temkin-C model in Figure 3A are consistent with the experimental data obtained at T≥ 423 K on the model NH3-SCR catalyst for PNH3=100 Pa and PH2O=250 Pa. Particularly, this concerns the facts (Figures 1 and 2) that (a) the presence of H2O has no strong impact on the intensity of the common δas IR band of the NH3ads-L species at 1606 cm-1 (there is a slight decrease which is discussed below in relationship with the increase in the amount of NH4+ during the coadsorption) and (b) in the presence of NH3 the IR bands of the H2Oads-L species are not detected. Moreover, these theoretical curves are consistent with the M.S data of Part 510 indicating that the switch 0.1% NH3/He → 0.1% NH3/0.2% H2O/He does not disturb significantly the NH3ads-L adsorption equilibrium while the switch 0.2% H2O/He → 0.1% NH3/0.2% H2O/He leads to the desorption of a large fraction of the adsorbed H2O species. Temkin-C model and Impacts of PNH3 and PH2O on the Coadsorption on the Lewis Sites Figure 3B compares the theoretical coverages of the NH3ads-L and H2Oads-L species from the Temkin-C model for two experimental conditions: PH2O= 250 Pa and PNH3 is equal to either 100 Pa or 500 Pa. Curves a and b show that the coverage of the NH3ads-L2 species does not depend on PNH3 at Ta< 470 K (full coverage for 100 Pa). For higher temperatures, the coverage decreases for the two NH3 partial pressures, however at Ta, the higher PNH3 the higher the coverage: i.e., 0.93 and 0.82 at 550 K for PNH3= 500 Pa and 100 Pa respectively. For the NH3ads-L1 species (curves e and f), the increase in PNH3 favors its coverage whatever Ta. These theoretical data are consistent with the IR spectra of Figure 2 (spectra a and b) showing, whatever Ta, an increase in the IR band at 1606 cm-1 common to the NH3ads-L species with the increase in PNH3 for PH2O= 250 Pa. Note in Figure 3B that the coverage of the H2Oads-



18 L2 (curves

c and d) and H2Oads-L1 (curves g and h) species decreases for PH2O= 250 Pa with the

increase in PNH3. Similarly, Figure 3C compares the theoretical coverage of the NH3ads-L and H2Oads-L species according to the Temkin-C model for two experimental conditions: PNH3= 100 Pa and PH2O is equal either to 250 Pa or 1 kPa. The coverage of the NH3ads-L2 species (curves a and b) is not significantly disturbed by the increase in PH2O whereas that of the NH3ads-L1 (curves e and f) is modestly decreased (by ≈ 0.05) for T< 360 K. This is consistent with the experimental data in Figure 2 (compare spectra a and c) indicating that the increase in PH2O from 250 Pa to 1 kPa for PNH3= 100 Pa has no significant impact on the intensity of the IR band at 1606 cm-1. For the H2Oads-L2 species its theoretical coverage (Figure 3C) increases with PH2O but remains very low (note the factor 10 for curves c and d). There is a significant increase in the coverage of the H2Oads-L1 species with the increase in PH2O (compare curves g and h in Figure 3C) for Ta< 400 K: i.e. from 0.037 to 0.1 at 320 K for PH2O= 250 Pa and 1 kPa. Figure 2 indicates that the increase in PH2O for Ta in the range 423-473 K leads to the detection of an IR band 1634 cm-1 which can be ascribed to molecularly adsorbed H2O species. Considering Figure 3C, this cannot be ascribed to the increase in the coverage of one on the H2Oads-L species (their coverages are ≈ 0 in the temperature range 423-473 K). This IR band must be ascribed to a new adsorbed species such as formed by the adsorption of H2O on the NH3ads-L species (without competition with NH3) in line with Soria et al.46 The authors consider that at room temperature H2O can be adsorbed via H-bond to strongly adsorbed H2Oads species (i.e., on Lewis sites) with a heat of adsorption slightly higher than physisorbed species.46 This is also consistent with the interpretation by Hadjiivanov et al.47 of FTIR data associated with the NH3-benzene coadsorption on TiO2-P25. The authors47 justify an interaction between benzene and NH3ads-L species as follows: (a) the NH3ads-L species form weak Brønsted-acid centers due to the weakening of the N-H bonds in the adsorbed state and


Page 18 of 54

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19 (b) these centers interact with others molecules via H-bonding (i.e., benzene). The experimental study of the coverage of this weak adsorbed H2O species on NH3ads-L and its theoretical modeling via the AEIR method is complex because the evolution of the IR band at 1634 cm-1 during the increase in Ta is due at least to three adsorption equilibriums: (a) those of the two NH3ads-L species which determines the amount of adsorption sites and (b) that of H2O on the NH3ads-L species. However, Figure 2 shows that this species is not present on the surface for Ta> 473 K and PH2O= 1 kPa which are of interest for the NH3-SCR reaction. Due to experimental limitations, high H2O partial pressures representative of the conditions of the NH3-SCR process for coal fired power plants (i.e., PH2O≈ 10 kPa) were not obtainable in the present study. The Temkin-C model allows to simulate these conditions as shown in Figure 3D which compares the coverage on the Lewis sites from the Temkin and Temkin-C model for PNH3= 100 Pa and PH2O= 10 kPa. It can be observed (curves a and b) that the presence of H2O decreases modestly (≈ 0.08) the coverage of the NH3ads-L2 species for T> 400 K. Curves e and f in Figure 3D, indicate that H2O decreases significantly the coverage of the NH3ads-L1 species for T< 380 K (by ≈ 0.22 at 330 K) whereas it is roughly not modified at higher temperatures. This shows that assuming that the pivotal intermediate of the NH3-SCR is the NH3ads-L2 species, its coverage is slightly decreased in the presence of PH2O≈ 10 kPa for temperatures of interest for the NH3-SCR process (T> 473 K). This is consistent with the view that the rate of the NH3-SCR is controlled by NH3ads-L species in competition with H2O. 4,6,7 However, Figures 3A-3C show that low H2O partial pressures have no significant impacts on the coverage of the NH3ads-L species. This situation corresponds to NH3-SCR reaction performed either in the absence or with low partial pressures of H2O in the reactive mixtures which are often those used for the kinetic studies at laboratory scale.

3.3 Applications of Temkin-C Model to the NH3-H2O Coadsorption on Metal Oxides



20 Temkin-C model and Water-Tolerant Lewis Sites The Temkin-C model provides a quantitative interpretation of the presence of watertolerant Lewis sites on metal oxides33 such as anatase TiO2,34 TiO4 tetrahedra deposited on mesoporous silica48 and anatase TiO2,34 Nb2O5,33 Nb32, Ta32 and Sn49 on beta zeolite and ceria.50 For instance, the water-tolerant Lewis sites are evaluated as follows by Nakajima et al.:34 “the total amounts of Lewis acid sites (measured by pyridine adsorption at 298 K) on the dehydrated TiO2 (homemade solid) and the water-saturated TiO2 (water-tolerant Lewis acid site density) were 0.26 and 0.24 mmol g−1, respectively”. These two similar amounts of sites are consistent with the present FTIR measurements at Ta≥ 473 K showing that the adsorption of NH3 on the Lewis sites is not strongly disturbed by the presence of H2O for PH2O≤ 1 kPa. Taking into account that metal oxides may have different types of Lewis sites, the Temkin-C model provides a quantification of the water-tolerant Lewis sites as follows: a Lewis adsorption site for a specific reactant is water-tolerant if the heat of adsorption of the H2Oads-L species, at different coverages, is lower than the heat of adsorption of the reactant. However, this quantification must be completed by the competitive adsorption between molecular adsorption of NH3 and dissociative adsorption of H2O as discussed below. Temkin-C Model and NH3-H2O Competition on TiO2 Based solids. The heats of adsorption of the molecularly adsorbed NH3ads-L and H2Oads-L species on different TiO2 based solids28-31 in particular those of the present study are not strongly different (Table 1) leading to theoretical curves from the Temkin-C model similar to those in Figure 3 (see Figure S2 for TiO2-P25). This explains that the experimental data (FTIR and volumetric measurements) related to the NH3-H2O coadsorption on the Lewis sites are similar on the two sulfated solids 0.7% V2O5/9% WO3/TiO2 and its TiO2-DT51 support.10 It is shown below that similar experimental data are observed for the NH3-H2O coadsorption on sulfate free TiO2-P25. However, one of the interests of this solid for the development of the


Page 20 of 54

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21 EMA of the NH3-H2O coadsorption on TiO2 based solid is that in the absence of the ν(S=O) IR band, the impacts of H2O on each NH3ads-L1 and NH3ads-L2 species can be studied via their different δs IR bands in the range 1300-1100 cm-1 (see references cited in ref 28). Moreover TiO2-P25 presents a second interest for the EMA: it provides key experimental data on the H2O dissociation in the presence of NH3ads-L species. On the sulfated solids, this H2O dissociation and the NH4+ formation are concomitant processes (due to the high heats of adsorption of the NH4+ species, Table 1) leading to difficulties in the interpretation of experimental data.10 TiO2-P25 is characterized by weak Brønsted sites leading to adsorbed NH4+ species of low stability (see ref 28 and references therein). This means that for Ta≥ 473 K, the absence of formation of NH4+ on TiO2-P25 simplifies significantly (as compared to the sulfated solids) the EMA of the processes involved in the dissociative chemisorption of H2O. However, in line with the concept of the EMA, the modeling of the experimental data has imposed obtaining, via the AEIR method, the thermodynamic parameters of different adsorption equilibriums particularly the heats of adsorption of NH4+ species on TiO2-P25 and TiO2-DT51 and an estimation of the heats of adsorption of dissociated H2O species on the sulfated and sulfate free solids (Table 1).

3.4 Heat of Adsorption of the NH4+ Species on Sulfate free and Sulfated TiO2 The AEIR procedure described in ref 30 for 0.7% V2O5/9% WO3/TiO2 using the δas IR band of the NH4+ has been applied to TiO2-P25 and TiO2-DT51. Heat of Adsorption of the NH4+ Species on TiO2-P25 using the AEIR method After adsorption at 300 K, Figure 4 shows the evolution of the δas IR band of the NH4+ species at 1435 cm-1 with the increase in Ta for 0.1% NH3/He. The IR band disappears for Ta≈ 450 K indicating weakly adsorbed species. This explains that the saturation of the Brønsted sites cannot be obtained in the range of PNH3 available with the experimental setup. The values



22 of A(M) in eq ES1 has been obtained via eq ES2 (inset A in Figure 4) using the area of the IR band at 300 K for three adsorption pressures according to the switches 0.1% NH3/He  0.5% NH3/He  1% NH3/He (results not shown). The value of AM indicates that the coverage of

A

1

1435

1/A

B

1

0.8

0.8

a

0.6

0.6 0

Coverage of the NH4+ species

the NH4+ species at 300 K varies from 0.61 for 0.1% NH3/He to 0.93 for 1% NH3/He.

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 54

0.4

0.005 0.01 1/Pa (Pa-1)

0.2

a

0

300 500 700 Temperature (K)

1431

0.002 e 1460

1440

1420

Wavenumber (cm-1)

1400

Figure 4: Measurement of the heats of adsorption of the NH4+ species on TiO2-P25 using the AEIR method. Evolution of the δas IR band of NH4+ during the increase in Ta for 0.1% NH3/He: (a)-(e) 300, 335, 353, 423 and 448 K. Inset A : Estimation of the area of the IR band at saturation of the sites using eq. ES2 (see the text for more details). Inset B :  : evolution of the experimental coverage of the NH4+ species for the FTIR spectra using Eq. ES1, (a) theoretical evolution of the coverage of the NH4+ species according to the Temkin model eqs ES3 and ES4 considering heats of adsorption of 80 and 43 kJ/mol at low and high coverages. The evolution of the area of the IR bands in Figure 4 provides that of the coverage of the NH4+ via eq ES1, as shown in inset B of Figure 4 (black square symbols). Curve (a) fitting the experimental data is obtained using eqs ES3-ES4 for E(1)= 43 kJ/mol to E(0)= 80 kJ/mol which are roughly similar and significantly lower than those of the NH4+-1 and NH4+-2 species of the model NH3-SCR catalyst respectively (Table 1). Heat of Adsorption of NH4+ Species on TiO2-DT51 using the AEIR method Figure 5 shows the evolution with Ta of the IR bands of the adsorbed NH3 species for 0.1% NH3/He. At 300 K (a) the IR band at 1605 cm-1 is the common δas IR band of the two


Page 23 of 54

23 NH3ads-L species adsorbed on the Lewis sites and (b) the IR band at 1451 cm-1 is δas IR band of NH4+ species formed on the Brønsted sites. As discussed in previous works dedicated to sulfated solids,29,30 only half of the IR band of the NH4+ species in the high wavenumbers range can be exploited for quantitative applications. This is due to the strong shifts of the IR band of the ν(S=O) vibration (at 1371 cm-1 before NH3 adsorption) to lower wavenumbers

Coverage of the NH4+ species

after NH3 adsorption.29,30 1 0.8

1605

1451

A a

0.6

0.02

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.2

a

0


a

k 1600

1430

k 1550

1500

Wavenumber (cm-1)

1450

Figure 5: Measurement of the heats of adsorption of the NH4+ species on TiO2-DT51 by using the AEIR method. Evolution of the δas IR band of NH4+ during the increase in Ta for 0.1% NH3/He: (a)-(k) 300, 310, 335, 353, 393, 433, 473, 513, 573, 613 and 473 K. Inset A :  : evolution of the experimental coverage of the NH4+ species for the FTIR spectra using Eq. ES1, (a) theoretical evolution of the coverage of the NH4+ species according to the Temkin model eqs ES3 and ES4 considering heats of adsorption of 125 and 56 kJ/mol at low and high coverage. The impact of the adsorption pressure at 300 K on the IR band at 1451 cm-1 according to the switches 0.1% NH3/He  0.5% NH3/He  1% NH3/He indicates that the Brønsted adsorption sites are saturated at 300 K under PNH3= 100 Pa (A(M) in eq ES1 is that of spectrum a in Figure 5). The black square symbols in inset A of Figure 5 give from the spectra in Figure 5, the experimental evolution with Ta of the coverage of the NH4+ for PNH3= 100 Pa and curve (a) fitting the experimental data is obtained by using eqs ES3-ES4 considering E(1)= 56 kJ/mol to E(0)= 124 kJ/mol. Similarly to TiO2-P25 only one type of



24 Brønsted sites is present on TiO2-DT51 however the heats of adsorption on the NH4+ species are significantly higher on DT51 than P25 (Table 1) probably due to the sulfate groups.

3.5 Experimental approach of NH3-H2O coadsorption on TiO2-P25 The experiments are identical to those described in Part 510 on the sulfated solids (see Figure 1): they consist in studying how the NH3 and H2O adsorption equilibriums are disturbed by the NH3-H2O coadsorption equilibrium using FTIR spectroscopy and volumetric measurements with a mass spectrometer. The absence of sulfate groups permits obtaining key experimental data on the dissociative H2O chemisorption in the presence of adsorbed NH3ads-L species. Impacts of H2O on the NH3 Adsorption Equilibrium for Ta≥ 473 K Study Using FTIR Spectroscopy At 473 K, the IR spectrum of the solid before adsorption (Figure 6, spectrum a) presents IR bands and shoulders in the range 3600-3750 cm-1 due to isolated OH groups. The IR bands at 3712 and 3670 cm-1 can be ascribed according to literature data to either different TiO2 faces and/or terminal and bridging OH (see references cited in ref 31).

Absorbance

3670 3656

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 54

1167

0.04 3383 3712

3415 3152

d b

c

b a

3667 3800

0.04

3264

3401

a

3600

1620

3273

3400 3200 Wavenumber (cm-1)

1218 1601

d

b

b c

c

a,d

a 1800

1600 1400 Wavenumbers (cm-1)

1200

Figure 6: Comparison of the FTIR spectra at the H2O and NH3 adsorption equilibria and the NH3H2O coadsorption equilibrium on TiO2-P25 at Ta= 473 K: (a) solid before adsorption; (b) 0.1% NH3 /He, (c) 0.1% NH3/0.25% H2O/ He and (d) 0.25% H2O/He.


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25 The IR bands observed after the NH3 adsorption equilibrium at 473 K according to the switch He  0.1% NH3/He (Figure 6, spectrum b) are ascribed as follows (see references cited in ref. 28 ): 1167 and 1218 cm-1 are the δs IR bands of two NH3ads-L1 and NH3ads-L2 species on two groups of Lewis sites with their common δas IR band at 1601 cm-1 and their NH stretching and overtone IR bands in the range 3600-3100 cm-1 (i.e., 3383, 3264 and 3152 cm-1 in Figure 6). The IR bands of isolated OH groups of the solid are strongly decreased by the NH3 adsorption (compare spectra a and b in Figure 6). In the absence of the IR bands of NH4+ species, this must be ascribed to either (a) a competitive coadsorption between dissociated adsorbed H2O (see the impact of H2O traces in the Supporting Information) and NH3 species or/and (b) interactions with the NH3ads-L species (a small broad H-bond IR band can be overlapped with the NH stretching in the range 3500-3100 cm-1). Note that the displacement of dissociated H2O species by NH3 is consistent with literature data51,52 indicating that adsorbed ammonia species promote the proton migration and favor the dehydroxylation of solids. The switch 0.1% NH3/He  0.1% NH3/0.25% H2O/He at 473 K leads at the coadsorption equilibrium to spectrum c in Figure 6 indicating by comparison with spectrum b a slight decrease in the IR at 1601 cm-1 due to a decrease in the amount of the NH3ads-L species (as observed on the sulfated solids, Figure 1 and ref 10). However, Figure 6 provides new experimental data: the comparison of the δs IR band of the NH3ads-L species in spectra b and c indicates that only the NH3ads-L2 species (IR band at 1218 cm-1) decreases significantly in the presence of H2O (the IR band of the NH3ads-L1 species at 1161 cm-1 is not modified). This indicates that the slight decrease in the common IR band at 1601 cm-1 must be ascribed to the NH3ads-L2 species. Moreover spectra b and c in Figure 6 show that at the NH3H2O coadsorption equilibrium, the IR bands of the isolated OH groups are significantly increased as compared to NH3 adsorption equilibrium leading to two strongly overlapped IR bands at 3670 and 3656 cm-1. This indicates that the dissociative chemisorption of H2O is



26 operant in the presence of NH3ads-L species probably via the displacement of a small fraction of the NH3ads-L2 species (Figure 6). Note that the increase in the OH IR bands due to the NH3H2O coadsoprtion is significantly higher on TiO2-P25 than on the NH3-SCR catalyst (Figure 1) because on TiO2-P25 the heats of adsorption of NH4+ are low (Table 1) and the new Brønsted sites formed by the H2O dissociation are not disturbed. The experiments in Figure 6 have been repeated at Ta = 523 and 573 K (see Figures S3A and S3B respectively) leading to similar conclusions: i.e., the switch 0.1% NH3/He  0.1% NH3/0.25% H2O/He (a) has a limited impact on the IR bands of the NH3ads-L species: there is a very slight decrease in the common δas IR band which is associated with that of the δs IR band of the NH3ads-L2 species at ≈1224 cm-1 and (b) increases the IR bands of the isolated OH groups (however, the higher Ta the lower the increase in this IR band as a priori expected for the impacts of Ta on adsorption equilibriums). Note that at the NH3-H2O coadsorption equilibrium for 473, 523 and 573 K, they are formations of neither NH4+ species (consistent with its heats of adsorption (Table 1) and Figure 4) nor H2Oads-L (consistent with the Temkin-C model for molecularly adsorbed NH3 and H2O species on the Lewis sites, Figure S2). Study Using Mass Spectrometry Figure 7 shows the evolution of the molar fractions of the gases during the adsorption of NH3 at 473 K on TiO2-P25 during the switch He  0.1% NH3/0.1%Ar/He (A B): the total amount of NH3 adsorbed is 86 µmol/g of catalyst.

0.4

Molar fractions of the gases

Ta= 473 K

Molar fractions of the gases

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

A

Ar ×100 H2O×100

(Ar- 2 10-3) ×100

0.1 0

NH3×100

0

A 0.1

C

0.3

0.3 0.2

0.2



B

500

 Ar ×100

1000

Time (s)

(Ar- 1 10-3) ×100

1500

B



NH3×100 0

0

400ACS Paragon 800Plus Environment 1200

Time (s)

C

H2O×100 1600

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27

Figure 7: Evolutions of the molar fractions of the gases during adsorption of NH3 (or H2O) and then NH3-H2O on TiO2-P25 at 473 K. A: in helium; B: in 0.1% NH3/0.1% Ar/He and C: in 0.1% NH3/0.2% H2O/0.3% Ar/He. Inset: A: in helium; B: in 0.2% H2O/He and C: in 0.1% NH3/0.2% H2O/0.3% Ar/He.

The switch 0.1% NH3/0.1% Ar/He 0.1% NH3/0.2% H2O/0.3% Ar/He (BC in Figure 7) is associated with the adsorption of 11 µmol of H2O /g (as observed on the sulfated solids, see Table 2). This is due to the dissociative chemisorption of H2O leading to the increase in the OH IR bands (compare spectra b and c in Figure 6). M.S. Experiments

P25

DT51§

At 473 K (amount in µmol/g) Impact of NH3 on H2O equilibrium He → A* 53 83 QadsH2O A→ B* 81 180 QadsNH3 40 73 QdesH2O QadsH2O with NH3 53-40= 13 83-73= 10 Impact of H2O on NH3 equilibrium He → C* 86 165 QadsNH3 C→ B 11 10 QadsH2O 6 0 QdesNH3 Impact of H2O on QadsNH3 81-86= -5 180-165= 15 At 523 K (amount in µmol/g) Impact of NH3 on H2O equilibrium He → A 32 51 QadsH2O A→ B 50 120 QadsNH3 25 45 QdesH2O QadsH2O with NH3 32-25= 7 51-45= 6 Impact of H2O on NH3 equilibrium He → C 58 113 QadsNH3 C→ B


Model catalyst§

79 197 66 79-66= 13

179 12 0 197-179= 18

32 113 22 32-22= 10

105


28 QadsH2O 9 8 9 QdesNH3 9 0 0 Impact of H2O on QadsNH3 50-58= -8 120-113= 7 113-105= 8 *A: 0.2% H2O/0.2% Ar/He, B: 0.1%NH3/0.2% H2O/0.3% Ar/He, C: 0.1% NH3/0.1% Ar/He §

from ref 10

Table 2: Amounts of NH3 and H2O adsorbed/desorbed on three TiO2 based solids during adsorption and coadsorption of NH3 and H2O. At the difference of the sulfated solids,10 in Figure 7 there is a net NH3 production associated to the H2O consumption during the switch BC: 6 µmol/g This indicates that a fraction of the NH3ads-L species desorbs concurrently with the dissociative H2O adsorption in agreement with the decrease in the IR bands of the NH3ads-L2 species evidenced in Figure 6 after the switch 0.1% NH3/ He 0.1% NH3/0.25% H2O/He. The experiments of Figure 7 have been repeated at Ta= 523 K (see Figure S4) leading to similar qualitative observations. However, Table 2 shows that the higher Ta, the lower the amount of adsorbed and desorbed species except for the amount of desorbed NH3 (slight increase) at the switch NH3 adsorption → NH3-H2O coadsorption equilibriums (9 µmol/g). The data on TiO2-P25 show clearly that a small amount of L2 sites is involved in a competitive adsorption between molecularly adsorbed NH3 species and dissociated H2O species. This interpretation is discussed below with the support of the Temkin-C model. Impacts of NH3 on the H2O Adsorption Equilibrium for Ta≥ 473 K Study Using FTIR spectroscopy The IR bands observed after adsorption of 0.25% H2O/He at 473 K on TiO2-P25 (Figure 6, spectrum d) are ascribed as follows (see references cited in ref 31): 1620 cm-1 is the δH2O deformation IR band of H2Oads-L species associated with those at 3415 and 3273 cm-1, which overlaps a broad IR band in the range of 3500-3000 cm-1 due to H-bonds between H2Oads-L and/or O2-/OH groups. The adsorption of H2O increases the amount of isolated OH groups of the solid (compare spectra a and d in Figure 6) leading to IR bands at 3670 and


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29 3656 cm-1. This increase can be ascribed to two contributions (a) the formation of new bridged OH groups by dissociative adsorption of H2O and (b) the presence of dangling hydrogen (d-H) in the H2Oads-L species (see references cited in ref 31). The switch 0.25% H2O/He  0.1% NH3/ 0.25% H2O/He leads at the coadsoption equilibrium (Figure 6 spectrum c) to the disappearance of the IR bands of H2Oads-L species at the benefit of the IR bands of the two NH3ads-L species: 1601 cm-1, 1218 and 1167 cm-1. This is consistent with the conclusions of the Temkin-C model for the molecularly adsorbed NH3 and H2O species on the Lewis sites of TiO2-P25 (Figure S2). Moreover, in the presence of NH3, the two IR bands of isolated OH bonds at 3670 and 3656 cm-1 decrease (compare spectra d and c). In the absence of NH4+ species, this decrease can be ascribed to different origins (a) a competitive adsorption between the dissociative adsorption of H2O and the molecular adsorption of NH3ads-L species, (b) the interaction of isolated OH groups with NH3ads-L species and (c) maybe the disappearance of the d-H bonds associated with the displacement of the H2Oads-L species. Figures S3A and S3B show the IR spectra after similar experiments at Ta = 523 and 573 K leading to the same qualitative observations: i.e., (a) the IR bands of the H2Oads-L species disappear at the introduction of NH3 at the benefit of the IR bands of the NH3ads-L species, (b) the IR band of the NH4+ species are not detected and (c) the IR bands of the bridged OH groups decrease. Study using mass spectrometry The inset of Figure 7 shows the evolution of the molar fractions of the gases during the adsorption of H2O at 473 K on TiO2-P25 according to the switch He  0.2% H2O/0.2% Ar/He: the total amount of H2O adsorbed is 53 µmol/g of catalyst. The switch 0.2% H2O/0.2% Ar/He  0.1% NH3/0.2% H2O/0.3% Ar/He is associated with the desorption of 40 µmol of H2O /g (13 µmol/g of H2O remain adsorbed as OH groups) and the adsorption of 81 µmol/g of NH3 which is slightly lower than in the absence of H2O: (81-86= -5 µmol/g) (Table



30 2). The experiments of Figure 7 have been repeated at T= 523 K (see Figure S4) leading to similar qualitative observations. However, the higher is Ta, the lower are the amounts of adsorbed and desorbed species (Table 2). The surface elementary steps involved in the H2O dissociation on the sulfated NH3SCR catalyst and its TiO2-DT51 support are discussed below in light to the experimental data on the sulfate free TiO2-P25

3.6 H2O Dissociation During NH3-H2O Coadsorption on TiO2 Based Solids On the sulfated 0.7% V2O5/9% WO3/TiO2 catalyst (Figures 1 and 2) and its TiO2DT51 support10 the switch 0.1% NH3/He → 0.1% NH3/0.25% H2O/He at Ta≥ 473 K leads to the increase in the intensity of the IR bands of the NH4+ species (i.e., ≈ 1425 cm-1 in Figures 1 and 2). This increase can be ascribed to either (a) the formation of new Brønsted sites or/and (b) the increase in the heat of adsorption of the NH4+ species (if its coverage was < 1 before the introduction of H2O) or/and (c) the increase in the IR absorption coefficient of NH4+ species without modification of its amount on the surface. This last proposal can be discarded since the M.S data (see Table 2) at Ta≥ 473 K indicate, for the same partial pressure of NH3, a higher adsorption of NH3 for the NH3-H2O coadsorption than the NH3 adsorption. The increase in the heats of adsorption of the NH4+ species, due for instance to the dissociative adsorption of H2O during coadsorption, is not supported by the experimental data. For instance, in Figure 6, the absence of the IR bands of NH4+ species during NH3-H2O coadsorption at Ta= 473 K on TiO2-P25 (as observed in the absence of H2O, Figure 4) indicates that there is no major increase in the heats of adsorption of this species. Similarly, on the model NH3-SCR catalyst for PNH3= 100 Pa and Ta= 473 K, the introduction of PH2O= 250 Pa increases the IR band at 1428 cm-1 by a factor 2.9 (Figure 1). Figure S1 indicates that in the absence of H2O the coverage of the NH4+-2 and NH4+-1 species are 0.75 and 0.08


Page 30 of 54

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31 respectively at 473 K for PNH3= 100 Pa. This indicates that the increase in the amount of NH4+ during the coadsorption via a modification of the heats of adsorption must concern mainly the NH4+-1 species. Simple calculations using the Temkin model (eq ES3-ES4) and assuming that the coverage of this species increases from 0.08 to 0.24 by addition of H2O, reveal that its heats of adsorption at low coverage should change from 90 to 100 kJ/mol. This is not a large difference whereas the calculations do not take into account the increase in the amount of OH groups in the presence of H2O. Considering these different remarks, in the following, it is assumed that (a) the heats of adsorption of the NH4+ species on the three solids are not significantly modified by NH3-H2O coadsorption and (b) it is mainly the increase in the amounts of Brønsted sites due to the dissocitive adsorption of H2O in the presence of NH3ads-L species which explains the increase in the IR band of the NH4+ species. However, on the sulfated solids there are no decisive experimental data justifying that this H2O dissociation is operant either via a competitive adsorption with the adsorbed NH3ads-L species or without competition on free Lewis sites (i.e. a large fraction of the L1 sites are free at Ta≥ 473 K, for PNH3= 100 Pa, Figure 3A). Assuming that the same processes are operant on the sulfated and sulfate free solids concerning this H2O dissociation, the experiments on TiO2P25 provide key experimental data validating the competition between dissociated adsorbed H2O species and NH3ads-L2 on a small amount of the L2 Lewis sites (Figure 6). This competition explains the NH3 production during the switch from the NH3 adsorption equilibrium to the NH3-H2O coadsorption equilibrium in parallel to the dissociative adsorption of H2O (Figure 7 and S4). These data are not accessible on the sulfated solids because (a) the ν(S=O) IR band overlaps the δs IR bands of the NH3ads-L species while (b) the NH3 produced by the displacement of the NH3ads-L2 species are not detected in the gas phase (using the M.S, see ref 10) because it is readsorbed as NH4+ on the new Brønsted sites formed



Page 32 of 54

32 by the H2O dissociation due to the high heats of adsorption of the NH4+ species on the sulfated solids (see Table 1) . In line with the concept of the EMA of surface process, the above qualitative interpretation of the impact of the H2O dissociation on the coverages of the Lewis and Brønsted sites during NH3-H2O coadsorption on the three TiO2 based solids are supported below by an accurate modeling using the Temkin-C model after the measurement of new thermodynamic parameters such as the heats of adsorption of dissociated H2O species in the absence of NH3.

3.7 Mechanism of the Dissociative Adsorption of H2O in the Absence of NH3. As discussed in more details in Part 5,10 according to literature data, two mechanisms can be considered for H2O dissociation on TiO2 based solids in the absence of NH3. In line with the views of Henderson53 it is can be considered a two steps process with a molecularly adsorbed precursor state on a Lewis site followed by its dissociation if the site is in adapted environment according to: TiS+δ-O-Tis+δ + H2O ↔ TiS+δ-O-TiS+δ (H2O)ads↔ TiS+δ-OHB-TiS+δ-OHT

(3)

where OHT and OHB are terminal and bridging hydroxyl species respectively. A second mechanism40,54 considers that the hydrolysis of one of the Ti-O-Ti bonds leads to two terminal pseudo bridging hydroxyls according to the global reaction: TiS+δ-O-Ti+δ + H2O ↔ 2 TiS+δ OHPB

(4)

A similar mechanism has been proposed for the H2O dissociation on TiO2/SiO2 solid considering Ti-O-Si bridges,55 and can be applied to the present solids considering different X+δ-O-Y+δ’ bridges with X and Y being either Ti, or V or W according to the composition of the solids. The two H2O dissociation mechanisms support the interpretation of the increase in the amount of OH groups in the presence of adsorbed NH3 species. According to eqs (3) and


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33 (4), the result of the dissociative H2O adsorption is that superficial X+δ or Y+δ’ sites of the pretreated solids are transformed into OH groups. These M+δ sites can be Lewis sites adsorbing NH3ads-L species leading to the view that during the NH3-H2O coadsorption there is a competition between NH3ads-L and the dissociated H2O species on a small amount of Lewis sites which can be modeled using the Temkin-C model (eqs 1-2) after estimation of the heats of adsorption of dissociated adsorbed species on the different TiO2 based solids.

3.8 Heats of Adsorption of Dissociated H2O Species using the AEIR Method According to eqs 3-4, dissociated H2O species on the present solids, leads to the formation of isolated OH groups characterized by IR bands in the range ≈ 3700-3600 cm-1 which can be used for the AEIR method. However, during the adsorption of H2O at low temperatures (i.e. 300-400 K for 0.2% H2O/He), the intensity of these IR bands are modified by interactions with the weakly adsorbed H2O species via hydrogen bonds.31 These weakly adsorbed species disappears at high temperature,31 explaining that the IR spectra used in the present application of the AEIR method have been recorded at Ta in the range ∼400-673 K for 0.2% H2O/He. Dissociated H2O Species on TiO2-P25 The spectrum a in the inset of Figure 8 shows that the adsorption equilibrium of 0.2% H2O/He at 423 K leads to two overlapped IR bands at 3673 and 3655 cm-1 ascribed to bridging OH groups (see reference cited in ref 31). The increase in Ta leads to the progressive decrease in the IR bands providing a main IR band at 3662 cm-1 at 673 K (spectrum g). The comparison of spectrum g with spectrum h recorded at 673 K in helium shows that the coverage of the dissociated adsorbed H2O species for PH2O= 200 Pa is significant at this temperature indicating strongly adsorbed species (the intensity of the IR band of OH groups in helium is commented below).



34

1 c 3673

a

NH3ads-L2 (T)

3655

0.8

d

0.6

NH3ads-L2 (T-C)

3662

0.04 Absorbance


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 54

b OH (T) from ref 40

g

a

h

0.4

OH (T)

3702 3669

0.2 3800 3700 3600 Wavenumbers (cm-1)

0

300

400

500

e OH-2 (T-C) 600

700

Temperature (K)

800

900

Figure 8: Measurements of the heats of adsorption of dissociated H2O species on TiO2-P25 using the AEIR method and competitive adsorption between molecular adsorption of NH3 on the L2 sites and the dissociated H2O species according to the Temkin-C model. : experimental coverage of the dissociated H2O species for 0.2% H2O/He; (a) theoretical coverage according to eqs ES3-ES4 for 180 kJ/mol and 95 kJ/mol at low and high coverages, (b) theoretical coverage using the DFT calculations in ref. 40 (see the Text); (c) theoretical coverage of the NH3ads-L2 species using eq ES3-ES4 for 0.1% NH3/He (Table 1); (d) and (e) coverages of the NH3ads-L2 and dissociated H2O species considering the TemkinC model (PNH3= 100 Pa and PH2O= 200 Pa) with the heats of adsorption used for curves a and c. Inset: Evolution of the FTIR spectra of the OH IR bands during the increase in Ta for 0.25% H2O/He: (a)-(g) Ta= 423, 473, 523, 573, 598 and 673 K and (h) in helium at 673 K.

The average coverage of the two OH species (the two IR bands at 423 K are not differentiated) is obtained using eq ES1. The marginal difference between spectra a and b in inset of Figure 8 indicates that the strongly adsorbed species are at full coverage at 423 K and the area of the IR band of spectrum a is used as A(M) in eq ES1. This provides the experimental evolution with Ta of the coverage of the dissociated H2O species for 0.2% H2O/He (red square symbols in Figure 8). Curve a in Figure 8 fitting the experimental data is obtained using the Temkin model for dissociated adsorbed species (eqs ES3 with α= 2 and


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35 ES4) considering that the heats of adsorption are EOH(1)= 95 kJ/mol and EOH(0)= 180 kJ/mol at high and low coverages respectively. These values are consistent with an estimation of the activation energy of desorption of dissociated adsorbed H2O species obtained from temperature programmed desorption (TPD) in helium at 23 K/min after adsorption of H2O at 300 K (see inset of Figure 3 in ref 31). Three H2O TPD peaks have been detected at Tm= 368 K, 427 K and 523 K ascribed to H2Oads-L1, H2Oads-L2 and dissociated H2O species respectively.31 The third peak provides an activation energy of desorption of ≈ 147 kJ/mol, using classical equations of the TPD method in the absence of readsorption (assuming a frequency factor of the rate constant of desorption of 1013 s-1).56 This approximate value appears as an average of the heats of adsorption at low and high coverages provided by the AEIR method. Moreover, EOH(0) and EOH(1) are consistent with different literature data on DFT calculations dedicated to the heats of adsorption of dissociated H2O species on TiO2 surface: (a) ≈ 155 kJ/mol at coverages ≤ 0.5 according to Vittadini et al.54 and (b) 165 kJ/mol and 101 kJ/mol (quasi linear variation) at low and high coverages on the (001) surface of TiO2 anatase nanocrystallites according to Arrouvel et al.40 These last values are particularly consistent with those determined in the present study, as shown by curve b in Figure 8 which gives the theoretical evolution (eqs ES3-ES4) of the coverage of dissociated species for PH2O=200 Pa with the heats of adsorption of ref 40. It must be noted that the heats of adsorption of the dissociated H2O species are consistent with the impact of H2O traces (i.e., in the range 1-10-4 Pa) on the FTIR spectra of TiO2-P25, in particular during the cooling stage following the pretreatment procedure.31 Figure S5 of the supporting information section shows clearly that the coverage increases during cooling even for PH2O= 10-4 Pa. The comparison of curves a and b in Figure 8 underscores the agreement between the present heats of adsorption and the DFT calculations of Arrouvel et al.40 However, these authors show that the heats of adsorption of dissociated H2O species are strongly dependent



36 on the TiO2 surface. They indicate the following values at low and high coverages according to the exposed surface: 138 and 109 kJ/mol on (001) surface, 84 and 65 kJ/mol on (100) and 79 and 69 kJ/mol on (101). This shows that the competitive adsorption between NH3ads-L species and dissociated H2O species can be strongly dependent on the exposed surface of the TiO2 particles. This point is considered in the discussion of the experimental data with the support of the Temkin-C model. Dissociated H2O Species on TiO2-DT51 The spectrum a in the inset A of Figure 9 shows that the adsorption equilibrium of 0.2% H2O/He at 423 K leads to an IR band at 3661 cm-1 ascribed to bridging OH groups.31 The increase in Ta to 673 K (spectrum f) leads to the progressive decrease in the IR band without any shift. Similarly to TiO2-P25, the comparison of spectra f and g recorded at 673 K in helium shows that the coverage of the dissociated adsorbed H2O species is significant at this temperature indicating strongly adsorbed species. The coverage of the OH species (black square symbols in Figure 9) is obtained using for A(M) in eq ES1 the area of the IR band of spectrum a (note the small difference between spectra a and b in inset A of Figure 9). Similarly to TiO2-P25, curve a in Figure 9 fitting the experimental data is obtained considering the Temkin model (eqs ES3-ES4) for dissociative chemisorption (α= 2) considering EOH(1)= 100 kJ/mol and EOH(0)= 135 kJ/mol. EOH(0) is significantly lower for TiO2-DT51 than for TiO2-P25 (180 kJ/mol). However these values are consistent with the DFT calculations of dissociated H2O species on the (110) plane of nanocrystallites on anatase TiO2 : 138 kJ/mol and 109 kJ/mol.40 1

B a

0.8 A 0.6

0.4

a

3653

Absorbance

a

3661

c

Absorbance


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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i 0.05 3642

f 0.04

3700

3600

Wavenumbers (cm-1)

g

0.2 3700

b

3600

Wavenumbers (cm-1)

ACS Paragon Plus Environment 0 300

400

500

600 700 Temperature (K)

800

900

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37 Figure 9: Measurements of the heats of adsorption of dissociated H2O species on the sulfated TiO2-DT51 and 0.7% V2O5/9% WO3/TiO2 solids using the AEIR method.  and  : experimental coverages of the dissociated H2O species for 0.2% H2O/He on TiO2-DT51 and 0.7% V2O5/9% WO3/TiO2; (a) and (b) theoretical coverage according to eq. ES3 with α=2 and ES4 using E(0)= 135 kJ/mol and E(1)= 100 kJ/mol and E(0)= 110 kJ/mol and E(1)= 95 kJ/mol respectively,(c) theoretical coverage of the NH3ads-L2 species using eq ES5 considering the presence of two types of dissociated H2O species OH-1 and OH-2 using 98 kJ/mol and 96 kJ/mol for OH-1 and 165 kJ/mol and 90 kJ/mol for OH-2 with x1= 0.55 and x2= 0.45. Inset A: Evolution of the FTIR spectra of the OH IR band on TiO2-DT51 during the increase in Ta for 0.2% H2O/He: (a)-(f) Ta= 423, 473, 523, 573, 623 and 673 K, g) in helium at 673 K. Inset B: Evolution of the FTIR spectra of the OH IR band on 0.7% V2O5/9% WO3/TiO2 during the increase in Ta for 0.2% H2O/He: (a)-(i) Ta= 398, 423, 448, 473, 498, 523, 573, 623 and 673 K. Dissociated H2O Species on 0.7% V2O5/9% WO3/TiO2 The spectrum a in the inset B of Figure 9 shows that the adsorption equilibrium of 0.2% H2O/He at 398 K leads to IR band at 3653 cm-1 ascribed to bridging OH groups.31 The increase in Ta leads to the progressive decrease in the IR band with a shift to 3642 cm-1 at 673 K (spectrum i). At the difference of the TiO2 supports, we have considered that the full coverage of the dissociated H2O species is obtained at 398 K (note the small difference between spectra a and b in inset B of Figure 9) and A(M) in eq ES1 is the area of the IR band of spectra a. The red circle symbols in Figure 9 give the experimental evolution of the coverage of the dissociated OH species with the increase in Ta for PH2O= 200 Pa. There are no sets of E(0) and E(1) values allowing fitting the experimental data using eqs ES3-ES4. For instance, curve b in Figure 9 consistent with the experimental coverages > 0.6 is obtained using EOH(1)= 95 kJ/mol and EOH(0)= 115 kJ/mol. This indicates that two or more dissociated H2O species contribute to the IR band displayed in inset B of Figure 9. Curve c in Fig. 9 fitting the experimental data is obtained considering the presence of two types of dissociated adsorbed H2O species (named OH-1 and OH-2 in the increasing order of stability) according to eqs ES3-ES5 using EOH-1(1)= 96 kJ/mol and EOH-1(0)= 98 kJ/mol for the OH-1 species and



38 EOH-2(1)= 90 kJ/mol and EOH-2(0)= 165 kJ/mol for the OH-2 species and considering that the contributions in fraction of each species to the OH IR band at saturation of the sites are xOH-1= 0.55 and xOH-2= 0.45 respectively. The E(0) and E(1) values for the OH-1 species indicate that its coverage is roughly consistent with the Langmuir adsorption model. Considering the heats of adsorption of each dissociated H2O species which are reported in Table 1 and those of the NH3ads-L2 species (Table 1) on the different solid, the Temkin-C model provides the theoretical evolutions of the coverages of the two adsorbed species which can be compared to the experimental data on the NH3-H2O coadsorption. In a first step, this is performed for TiO2-P25 because the absence of NH4+ species simplifies the interpretations.

3.9 Temkin-C Model for Dissociated H2O and NH3ads-L Species on TiO2-P25 The data in Figures 6 and 7 lead to the conclusion that the dissociated H2O species are formed via a competitive adsorption with the NH3ads-L2 species. Curve c in Figure 8 give the evolution of the coverage of the NH3ads-L2 species for PNH3= 100 Pa according to the Temkin model (eq ES3-ES4, with the heats of adsorption cited in Table 1). Curves d and e give the coverage of the NH3ads-L2 and OHads species during NH3-H2O coadsorption for PH2O= 250 Pa and PNH3= 100 Pa according to the Temkin-C model (eqs. 1 and 2 with α= 2) with the heats of adsorption in Table 1. For the NH3ads-L2 species, the comparison of curves c and d shows that the competitive adsorption has no impact on its coverage at Ta< 450 K whereas it is moderately decreased at higher temperatures (i.e., at 480 K from 1 to 0.95, at 525 K from 0.97 to 0.89 and at 573 K from 0.85 to 0.68 which represent decreases by 5, 8 and 20% respectively). The theoretical impacts of H2O dissociation on the coverage of the NH3ads-L2 species are consistent with different experimental data on TiO2-P25.


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39 For the adsorbed NH3 species, curves c and d in Figure 8 are consistent with (a) the limited decrease in the δs IR band at 1218 cm-1 in Figure 6 and S3 and (b) the NH3 production during the switch from the NH3 to the NH3-H2O adsorption equilibrium (Figures 7 and S4). On this last point the comparison of curves c and d in Figure 8 indicate that the higher Ta the higher the impact of H2O on the coverage of the NH3ads-L2 species consistent with the increase in the amount of NH3 displaced with the increase in Ta: 6 µmol/g (Figure 7) and 9 µmol/g (Figure S4) at 473 K and 535 K respectively (Table 2). Considering that only the L2 Lewis sites are involved in the competitive adsorption between dissociated H2O and NH3ads-L species (Figure 6), a quantitative comparison of the experimental and theoretical data (provided by the Temkin and the Temkin-C model) can be developed after the estimation of the contributions of the L1 and L2 Lewis sites of TiO2-P25 to the total amount of NH3 adsorbed at different adsorption temperatures. This imposes an estimation of the amounts (in µmol/g) of the L1 and L2 sites on TiO2-P25 which is obtained as follows using the M.S data in Table 2. Taking into account the absence of the NH4+ species for Ta≥ 450 K (Figure 4) the amount of NH3 adsorbed at Ta is only due to the two NH3ads-L species. Table 2 indicates that the switch He → 0.1% NH3/He at Ta= 473 and 523 K leads to the adsorption of QNH3(473 K)= 86 µmol/g and QNH3(523 K)= 58 µmol/g respectively of NH3. Figure S2 provides the coverages of the two adsorbed species at 473 and 523 K: θNH3ads-L2(473 K)= 0.99, θNH3ads-L2(523 K)= 0.956, θNH3ads-L1(473 K)= 0.27 and θNH3ads-L1(523 K)= 0.08. This gives the following set of equations: QNH3(473 K)= NL2 θNH3ads-L2(473 K) + NL1 θNH3ads-L1(473 K)

(5)

QNH3(523 K)= NL2 θNH3ads-L2(523K) + NL1 θNH3ads-L1(523 K)

(6)

where NL2 and NL2 are the amounts of L2 and L1 Lewis sites. Equations (5) and (6) lead to NL2≈ 49 µmol/g and NL1≈ 138 µmol/g. Note these values indicates that the proportions of L1 and L2 sites are PL1= 0.73 and PL2=0.27 which are consistent (taking into account the



40 accuracy of the different measurements) with the proportion of NH3ads-L1 and NH3ads-L2 species determined from their contributions to their common δas IR band at 300 K at full coverage of the Lewis sites: 0.7 and 0.3 at 300 K (see Table 1).28 These calculations lead to the following conclusion: on TiO2-P25, the amount (6 µmol/g) of NH3 displaced at 473 K from the surface during the switch 0.1% NH3/He → 0.1% NH3/0.2% H2O/He represents ≈(6×100/49)≈ 12% of the L2 sites (at 523 K ≈ 18%) showing that only a small fraction of those sites are implicated in the competitive adsorption between the molecular adsorption of NH3 and the H2O dissociation. Note that these values are moderately higher than those provided by the TemkinC model ∼5 and ∼ 8% respectively (considering that the coverage of the L2 sites is not strongly different of 1 at 473 and 523 K). These differences can be ascribed to the accuracy of the M.S data and AEIR method. For the OH groups formed by H2O dissociation, curves a and e in Figure 8 show that the coverage of the OH species is strongly decreased by the NH3-H2O coadsorption (i.e., from ≈1 to 0 at T< 450 K, from 0.96 to 0.03 at 480 K, from 0.91 to 0.09 at 525 K and from 0.83 to 0.20 at 573 K). These theoretical data are consistent with (a) the FTIR spectra in Figure 6 and S3 showing a strong decrease in the intensity of the isolated OH bonds at the switch 0.25% H2O/He (spectrum d) → 0.1% NH3/0.25% H2O/He (spectrum c) whereas the OH species are not consumed by the NH4+ formation and (b) the authors who consider that the adsorption of NH3 favors the dehydroxylation of the surface.51,52 Figures 6 and S3 indicate that the NH3ads-L1 species on TiO2-P25 are not disturbed by the presence of H2O (the δs IR band at 1167 cm-1 is not modified). In line with the DFT calculations of Arrouvel et al.40 indicating that the heats of adsorption of the dissociated H2O species are significantly different according to the exposed faces of nanocrystallites, it can be considered that the L1 and L2 sites are on different faces. For instance, using the heats of adsorption obtained by Arrouvel et al.40 for the face (101) of anatase: 79 kJ/mol and 69 kJ/mol


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41 at low and high coverages and the heats of adsorption of the NH3ads-L1 species (Table1) the Temkin-C model indicates that H2O has no impact on the coverage of the NH3ads-L1 species for PH2O= 250 Pa and Ta> 450 K (result not shown).

3.10 Temkin-C Model for Dissociated H2O and NH3ads-L Species on the Sulfated Solids Similarly to TiO2-P25, it is assumed that the competition between dissociated H2O and NH3ads-L species concerns the L2 Lewis sites of the 0.7%V2O5/9% WO3/TiO2 and TiO2-DT51 solids. Therefore, the Temkin-C model has been applied to the competition between OHads and NH3ads-L2 species. Moreover, as compared to TiO2-P25, the comparison between theoretical and experimental data is strongly limited on the sulfated solids due to the formation of NH4+ (this limits the exploitation of the M.S data) and the presence of the ν(S=O) IR band overlapping the δs IR band of the NH3ads-L species (Figure 1 and ref 10). Sulfated TiO2-DT51 Solid Figure 10A compares the coverages of the OHads and NH3ads-L2 species from the Temkin (curves a and b) and Temkin-C (curve c and d) models considering their heats of adsorption in Table 1. The different curves are similar to those on TiO2-P25 (Figure 8). However, the lower heats of adsorption of the OHads species on TiO2-DT51 than on TiO2-P25 explain the strong decrease in their coverages (i.e., at 523 K the coverage of the OH species decrease from 0.94 to 0.05). In parallel this limits the decrease in the coverage of the NH3ads-L2

1

A

0.8

PNH3= 100 Pa PH2O= 250 Pa

b OH (T)

b NH3ads-L2 (T)

c NH3ads-L2 (T-C)

0.6 0.4 0.2 0 300

d OH (T-C) 400


800


species (i.e. from 0.98 to 0.84 at 523 K ). Coverage of the adsorbed species

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


900

PNH3= 100 Pa

1 B

b 0.8 OH-1 (T) 0.6 0.4 0.2

d NH3ads-L2 (T-C) OH-2

c PH2O= 250 Pa NH3ads-L2 (T) f NH3ads-L2 (T-C) OH-1 a OH-2 (T)

e OH-2 (T-C) g OH-1 (T-C)

0 300

400

500 600 700 800 Temperature (K) ACS Paragon Plus Environment

900


42 Figure 10: Comparison of the coverages from Temkin and Temkin-C models of the dissociated adsorbed H2O and molecularly adsorbed NH3 species on the L2 Lewis sites of TiO2-DT51 (Part A) and 0.7% V2O5/ 9% WO3/TiO2 (Part B) for PNH3= 100 Pa and PH2O= 250 Pa (see the heats of adsorption in Table 1). The small decrease in the theoretical coverage of the NH3ads-L2 species in Figure 10A is consistent with the small decrease in the δas IR band of the NH3ads-L species after a switch 0.1% NH3/He→ 0.1% NH3/0.25% H2O/He.10 Similarly the strong decrease in the coverage of the OHads species is consistent with the strong decrease in the IR band of the OH groups at the switch 0.25% H2O/He → 0.1% NH3/0.25% H2O/He at 473 K (see Figure S7 in ref 10) Sulfated 0.7% V2O5/9% WO3/TiO2 Catalyst For the 0.7% V2O5/9% WO3/TiO2 catalyst the competition between the dissociated H2O species and the NH3ads-L2 species according to the Temkin-C model must consider the presence of two types of OHads species. Curves a and b in Figure 10B give the coverages of the OH-1 and OH-2 species for PH2O= 250 Pa and curve c that of the NH3ads-L2 species for PNH3= 100 Pa in the absence of competition according to the Temkin model (eqs ES3-ES4). Curves d and e in Figure 10B give the coverages of the OH-2 and NH3ads-L2 species in competition for PH20= 250 Pa and PNH3= 100 Pa according to the Temkin-C model (eqs 1 and 2). These two curves are similar to those on TiO2-P25, with differences mainly due to the lower heats of adsorption of the NH3ads-L2 species at low coverages on the model catalyst (see Table 1): the coverage of the NH3ads-L2 species is roughly not modified in the range 300-423 K whereas that of the OH-2 species decreases strongly (from 1 to ≈ 0). At higher temperatures, the coverage of the NH3ads-L2 species is slightly lower than that during NH3 adsorption (i.e., decrease of 0.99 to 0.95 at 473 K and 0.95 to 0.77 at 523 K) whereas in parallel the coverage of the OH-2 species increases, without attaining its coverage in the absence of coadsorption (i.e., the two coverages are 0.94 and 0.05 at 473 K and 0.85 and 0.16 at 523 K). Regarding the competition between the NH3ads-L2 species and the OH-1 species, curves c and f show that the


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43 coverage of the NH3ads-L2 decreases slightly during the coadsorption in the range of 440-600 K (i.e., form 0.96 to 0.93 at 500 K). The conclusions of the Temkin-C model on the competitive adsorption of dissociated H2O and NH3ads-L species for 0.7% V2O5/9% WO3/TiO2 (Figure 10B) are consistent with different experimental data such as in Figure 1 for Ta= 473 K (a) the limited decrease in the δas IR band of the NH3ads-L species after the switch 0.1% NH3/He (spectrum c) → 0.1% NH3/0.25% H2O/He (spectrum d) and (b) the strong decrease in the IR band of the OH groups at the switch 0.2% H2O/He (spectrum b) → 0.1% NH3/0.2% H2O/He (spectrum d) even if on this solid the formation of the NH4+ species may contribute to the decrease in this IR band. Finally, as discussed in more detail in Part 5,10 two points must be remembered. Firstly, regardless of the solid (including TiO2-P25 as compared to ref 10) a small amount of Lewis sites are implicated in the H2O dissociation in the presence of adsorbed NH3. For instance, the M.S data in Table 2 (line 14) indicating that the amount of adsorbed H2O species (dissociated) during the switch 0.1% NH3/0.1% Ar/He → 0.1% NH3/0.2% H2O/0.3% Ar/He is ≈ 10 µmol/g on the three solids. Considering that the dissociative chemisorption creates two OH groups (eqs 3 and 4), their amounts: ≈ 1.5 1017 OH/m2 for the sulfated solids and ≈ 2.6 1017 OH/m2 for TiO2-P25 represent ≈1.5% and ≈ 2.6 % of the total amount of sites (≈1019 sites/m2) on a surface (a similar value is obtained considering the concentration of Ti4+ per m2 on different exposed faces of TiO2 in the range 5-7.7 1018 m-2).57,58 This means that the L2 sites implicated in the competition can be relevant of defects (steps, corners) in line with literature on TiO2 based solid.53,59-61 The second point is that on 0.7% V2O5/9% WO3/TiO2, the increase in the amount of adsorbed NH3 in the presence of H2O at 473 K: ≈ 18 µmol/g (see Table 2 line 16) is slightly lower than the amount of OH groups (≈ 26 µmol/g) due to the H2O dissociation, indicating a high coverage of OH groups by NH4+ species. This is consistent with the view that H2O dissociation on the L2 sites creates the Brønsted sites for the NH4+-2



44 species, which is at coverage 0.75 at 473 K for PNH3= 100 Pa as compared to 0.08 for the NH4+-1 species (curves c and d in Figure S1 respectively). The situation is similar for TiO2DT51. Temkin-C Model and NH3-SCR Reaction on the 0.7% V2O5/9% WO3/TiO3 NH3-SCR Catalyst The different experimental setups cannot be used with the high partial pressures of H2O (i.e., 10 kPa) used in industrial applications of the NH3-SCR. However, the Temkin-C model provides the theoretical coverage of the different adsorbed species for these experimental conditions, as shown in Figure S7: curves (d and e) and (curve f and h) give the coverages of the NH3ads-L2 and OH-2 species according to the Temkin-C model for PNH3= 100 Pa and PH2O= 250 Pa and 10 kPa respectively using the heats of adsorption of the different species in Table 1. Considering that NH3ads-L2 species is the intermediate of the NH3-SCR, it can be observed that considering a gas mixture representative of the NH3-SCR process with 10% of H2O, the decrease in the coverage of the NH3ads-L2 species becomes significant: at 473 K the coverages are 1, 0.83 and 0.7 for PH2O= 0, 250 Pa and 10 kPa, respectively and at 523 K the coverages are of 0.91, 0.71 and 0.34 for PH2O= 0, 250 Pa and 10 kPa respectively. This impact of PH2O is consistent with a decrease in the rate of the NH3-SCR reaction ascribed to a competition between the NH3ads-L intermediate and H2O in line with Forzatti et al.5-7

3.11 Temkin models and Impacts of H2O Traces on the Study of Adsorbed NH3 Species. Considering the presence of H2O traces in the different gases used during an experimental study on surface processes involved in the NH3-SCR reaction such as the adsorption of NH3 on TiO2-based solid, the Temkin-C model permits an estimation of their impacts on different measurements. Impact of H2O Traces on the heats of adsorption of adsorbed NH3 species.


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45 Assuming that the presence of a low partial pressures of H2O (i.e., 1 Pa) in the different gas mixtures, the theoretical curves for the NH3ads-L species (Table 1) obtained from the Temkin and Temkin-C models, considering H2O dissociation, for PNH3= 100 Pa (using the heats of adsorption in Table 1) are not significantly different. Clearly, H2O traces have no impact on the values of the heats of adsorption obtained in previous studies using the Temkin model.28-30 Increase of in the NH4+ species in the presence of H2O For the sulfated DT51 and 0.7% V2O5/9% WO3/TiO2 solids, the adsorption of NH3 is associated with the formation of NH4+ species on the Brønsted sites (isolated OH groups) in parallel to the NH3ads-L species (see Figure 1 and 2 and Part 510) in amounts depending on their heats of adsorption, Ta and PNH3. However, the amount of OH groups before NH3 adsorption is not an intrinsic property of the solids since these groups are mainly due to the dissociative adsorption of H2O traces (see Figure S5 and ref 31). This means that the increase in the amount of NH4+ species during the switch 0.1% NH3/He → 0.1% NH3/0.25% H2O/He (Figures 1 and ref 10) is strongly dependent on experimental parameters associated with the experimental system, such as the level of the H2O traces, the adsorption temperature Ta and the time on stream in helium before adsorption: the higher the amount of H2O traces adsorbed before NH3 adsorption, the lower the increase in the amount of NH4+ at the introduction of the NH3/H2O containing gas mixture (at the limit the increase can be below the accuracy of the measurement). This situation may lead to significantly different conclusions on the NH3H2O coadsorption according to the experimental setup.

4- CONCLUSION The present study dedicated to the NH3-H2O coadsorption on a model sulfated 0.7% V2O5/9% WO3/TiO2 catalyst and two TiO2 supports (sulfated DT51 and sulfate free P25) is a



46 part of an experimental microkinetics approach (EMA) of the De-NOx by NH3 (NH3-SCR process) in presence of O2 in the experimental conditions of coal fired power plants. On the three solids, using FTIR and M.S data, it has been shown that NH3 dominates the molecular NH3-H2O coadsorption on the Lewis sites in the experimental conditions of the NH3 SCR process (i.e. Ta ≥ 473 K, PNH3 and PH2O in the ranges of 100-500 Pa and 0.2-1000 Pa respectively). A competitive Temkin model (named Temkin-C)11,12 shows that this is consistent with the heats of adsorption of molecularly adsorbed NH3ads-L and H2Oads-L species on two types of Lewis sites (named L1 and L2 in the increasing order of strength) measured in previous works in the absence of coadsorption.28-31 This model indicates that the H2Oads-L species are present in very small amounts (below the accuracy of the experimental procedures) at the coadsorption equilibrium for NH3-SCR conditions as compared to the H2O adsorption equilibrium under the same PH2O adsorption pressure. On the sulfated solids, the NH3-H2O coadsorption is associated to the increase in the amount of NH4+ species as compared to the NH3 adsorption for the same PNH3 partial pressure. This is ascribed to the formation of new Brønsted sites via H2O dissociation associated with the high heats of adsorption of the NH4+ species. On the sulfate free TiO2-P25, the NH4+ species have low heats of adsorption whatever their coverage and cannot be present on the surface at Ta> 473 K. For this solid the switch from NH3 to NH3-H2O adsorption equilibrium studied by M.S at Ta≥ 473 K leads to the adsorption of H2O and the desorption of a small amount of NH3 due to the displacement of a fraction of the NH3ads-L species. This indicates that a competitive adsorption between dissociated H2O species and molecular NH3ads-L species is operant on the Lewis sites and FTIR data lead to the conclusion that only a fraction of the L2 Lewis sites is concerned. On the sulfated solid, the switch from the NH3 to the NH3-H2O adsorption equilibrium indicates that the H2O adsorption is not associated with a NH3 production. This is ascribed to the fact that the NH3 displaced from the surface is readsorbed


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47 on the new Brønsted sites. After the measurement of the heats of adsorption of dissociated H2O species on the three solids, it is shown that the Temkin-C model provides theoretical data consistent with the experimental data linked to the competitive adsorption on the L2 Lewis sites. The present study provides key data for the modeling of the catalytic activity of the model NH3-SCR catalyst. For instance, in a forthcoming article it is shown how the identification of the pivotal adsorbed NH3 species of the reaction must take into account the impact of H2O (produced by the reaction and introduced in the reactive mixture) on the amounts of the NH3ads-L species (decrease) and NH4+ species (increase). Moreover, the present modeling via the Temkin-C model of the coverage of the different adsorbed species formed by the reactant NH3 in large ranges of experimental conditions (T, PNH3, PH2O) is a key step of the EMA of the N2 production during the NH3-SCR reaction. The development of this model by taking into account the modification of the coverages of the adsorbed NH3 species by the surface elementary steps controlling the N2 production leads to a competitive Temkin model with reaction (named T-C.R)11 allowing an accurate representation of the catalytic activities in large range of experimental conditions for NOx conversions < 20%.



48 AUTHOR INFORMATION Corresponding Author *: To whom correspondence should be addressed. Tel:0033472431419 E-mail: [email protected]

SUPPORTING INFORMATION AVAILABLE: The procedure and equations of the AEIR method, the theoretical coverages of the NH3 and H2O adsorbed species on 0.7% V2O5/9% WO3/TiO2 catalyst in the absence of coadsorption, the mathematical formalism of the competitive Temkin model (Temkin-C) with limited mathematical approximations, the coverages of the NH3ads-L and H2Oads-L species on TiO2-P25 from Temkin and Temkin-C models, the modifications of the NH3 and H2O adsorption equilibrium by the coadsorption on TiO2-P25 at Ta> 473 K using FTIR, the perturbations of the NH3 and H2O adsorption equilibrium on TiO2-P25 by the coadsorption equilibrium at 523 K studied by M.S, the heats of adsorption of the H2O species and the impacts of H2O traces on TiO2-P25, the competitive adsorption between dissociated H2O species and NH3ads-L1 and H2Oads-L2 species on TiO2-P25, the impact of PH2O on the competition between NH3ads-L and dissociated H2O species. This information is available free of charge via the Internet at http://pubs.acs.org

ACKNOWLEDGMENT: The authors from IRCELYON would thank the financial support of the EDF- Fluid Dynamics, Power Generation and Environment Department during the Ph.D thesis of F.G


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54

NH3-H2O Coadsorption on 0.7% V2O5/9% WO3/TiO2 Molecular 1 adsorption on NH3-L2 (T) Lewis acidic sites = 100 Pa P NH3 0.8 Adsorption Model: H2OL2 (T) Temkin → T NH3-L2 (T-C) 0.6 PH2O= 250 Pa


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Coadsorption Model: Temkin → T-C competitive

0.4 PNH3= 100 Pa PH2O= 250 Pa

0.2 0

200

H2OL2 (T-C) ×100 300

400

500

600

Temperature (K)

700

800

TOC graphic


Experimental Microkinetic Approach of De-NOx by ... - ACS Publications

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