Experimental Microkinetic Approach of De-NO - American

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Experimental Microkinetic Approach of De-NOx by NH on VO/ WO/TiO Catalysts. Part II: Impact of Superficial Sulfate and/or VO groups on the Heats of Adsorption of Adsorbed NH species 3

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François Giraud, Christophe Geantet, Nolven Guilhaume, Stéphane Loridant, Sébastien Gros, Lynda Porcheron, Mohamed Kanniche, and Daniel Bianchi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 Jul 2014 Downloaded from http://pubs.acs.org on July 3, 2014

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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. Part II: Impact of Superficial Sulfate and/or VxOy groups on the Heats of Adsorption of Adsorbed NH3 species.

AUTHOR NAMES François Giraud,1,2 Christophe Geantet,1 Nolven Guilhaume,1Stephane 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. KEYWORDS TiO2, sulfate, vanadia, NH3, heat of adsorption, AEIR

ACS Paragon Plus Environment


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2 ABSTRACT: The present article is dedicated to the impacts of sulfatation and/or VxOy deposition on TiO2 supports on the individual heats of adsorption of adsorbed NH3 species. The S and V loadings are similar to those of TiO2 based commercial catalysts for the selective catalytic reduction of NOx by NH3: NH3-SCR, Two original experimental procedures are used, noted adsorption equilibrium infra-red spectroscopy (AEIR) and temperature programmed adsorption equilibrium (TPAE). They have been developed in previous works and adapted in Part I for the species formed by the adsorption of NH3 on a sulfate free TiO2 support (P25 from Degussa). In agreement with literature, Raman and FTIR spectroscopies show that the impregnation of the sulfate-free and sulfated TiO2 supports by the vanadium precursor leads to well dispersed VxOy species which are involved in the adsorption of NH3 in the temperature Ta range of 300-673 K and for adsorption Pa < 0.5 kPa. Sulfate and VxOy groups favor the amount of NH4+ species without modifying their heats of adsorption as compared to the TiO2 support: they are not detected for temperatures of interest for NH3-SCR reaction. In these experimental conditions and whatever the solids, two main adsorbed NH3 species on Lewis sites (noted NH3ads-L1 and NH3ads-L2) are present characterized by two δs IR bands below 1300 cm-1 and a common δas IR band at ≈ 1600 cm-1. By comparison with TiO2 P25; it is shown that sulfate groups have strong impacts neither on the proportion: x1 and x2, of the two adsorbed species nor on their heats of adsorption (EL1(θ) and EL2(θ) with θ the coverage of the NH3ads-L1 and NH3ads-L2 species: x1= 0.65, x2= 0.35, EL1(1)= 56 kJ/mol, EL1(0)= 102 kJ/mol, EL2(1)= 110 kJ/mol, EL2(0)= 140 kJ/mol. The deposition of V containing species on the sulfate free and sulfated TiO2 surfaces, leads to similar conclusions: in the experimental conditions (Ta and Pa) of the NH3-SCR, two adsorbed NH3 species on Lewis sites are detected with comparable proportions and heats of adsorption than those observed on the TiO2 supports.


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3 TEXT: 1. Introduction The present study is a part of an experimental microkinetic approach (abbreviation EMA) of the catalytic reduction of NOx (NO and NO2) into N2 and H2O using NH3 as reducing agent1 (noted NH3-SCR), considering flue gas denitrification in coal-fired power plants. Power plant stationary sources use mainly x wt% V2O5/ y wt% WO3/ sulfated-TiO2 catalysts (conventionally, the weight loadings are provided considering V2O5 and WO3 even if the species formed on the supports have different structures) with x and y in the ranges of 0.72 and 9-13 respectively.2,3 The main aim of the EMA is the experimental evaluation of the kinetic parameters of the surface elementary steps which control the catalytic activity as developed in previous studies for different catalytic reactions.4-7 The first surface elementary steps of the NH3-SCR are the adsorption/desorption of the reactants in particular NH3 which may provide different adsorbed species according to the adsorption sites of the catalyst (i.e. NH3ads-L and NH4+ on Lewis and Brønsted sites): the identification of the pivotal adsorbed species of NH3-SCR process (Ref. 8 and references therein) constitutes one of the aims of the EMA. The parameters controlling the amount of an adsorbed species and depending on the catalyst composition are the concentration of the adsorption sites (i.e. in µmol/m2) and the adsorption coefficients of the adsorbed species which may vary with the coverage of the sites via the heats of adsorption. As commented in Part I,8 the measurement of the individual heats of adsorption of identified adsorbed NH3 species on TiO2 based solids has been rarely made (literature data concern mainly average heats of adsorption of NH3). This has been performed8 for the NH3 adsorbed species on a sulfate free TiO2 solids (P25 from Degussa) present in the experimental conditions (adsorption temperature Ta and pressure Pa) of interest for the NH3SCR, by adapting two experimental procedures denoted adsorption equilibrium infra-red spectroscopy (AEIR)9-12 and temperature programmed adsorption equilibrium (TPAE)13



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4 developed in previous works. Two main adsorbed NH3 species on coordinatively unsaturated Lewis sites (TiV or TiIV) are detected in these experimental conditions on the dehydrated P25 solid. These species denoted NH3ads-L1 and NH3ads-L2 species have been (a) identified by their symmetric deformation (δs NH3) IR bands at 1149 cm-1 and 1228 cm-1 for Ta= 300 K and Pa= 0.1 kPa respectively (their asymmetric deformation δas IR bands were overlapped at 1596 cm1

) and (b) quantified by a volumetric method: 224 µmol/g and 83 µmol/g respectively.8 Their

heats of adsorption EL1(θ) and EL2(θ) which linearly vary with their coverage θ, are significantly different. They are (a) EL1(1)= 56 kJ/mol and EL1(0)= 105 kJ/mol for the NH3adsL1

and (b) EL2(1)= 105 kJ/mol and EL2(0)= 160 kJ/mol for NH3ads-L2 species.8

On the

dehydrated TiO2-P25 solid, a small amount of NH4+ species is detected at low adsorption temperatures (< 500 K) with small IR bands at 1680 and 1477 cm-1 (at 300 K) due to antisymmetric and symmetric deformation.8 However, the coverage of the NH4+ species decreases to a very low value for Pa and Ta used in the NH3-SCR reaction, indicating weakly adsorbed species in agreement with literature data (Ref 8 and references therein). Using the methods developed in Part I,8 the present study is dedicated to the impacts of the presence on the TiO2 surface of well dispersed (a) sulfate (commercial catalysts use sulfated TiO2 solids) and/or (b) VxOy species (at loadings 1 species) is discussed below considering the Raman spectra of the solids and literature data. Before NH3 adsorption, the solids were treated on the different analytical systems as follows: O2 (713 K, 10 min)→ He (713 K, 5 min)→ He (300 K). The same sample of solid was used to perform a series of experiments and it was pretreated as above before each experiment. 2.2 Analytical methods Different analytical methods have been used to characterize the prepared solids allowing a comparison with literature data such as: (a) elementary analysis using ICP-AES to ascertain the V loadings and to measure the presence of others components such as S, Si, (b) N2 physisorption at 77 K for the measurement of the BET area, (c) XRD for the identification of the structures of the solids and (d) Raman and IR spectroscopies to study the dispersion of the VxOy species on the TiO2 supports (see Supporting information for more details). IR Cell in Transmission mode



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6 The FTIR characterizations that constitute the main part of the present study, have been performed using a Nicolet-6700 FTIR spectrometer equipped with a small internal volume (≈ 2 cm3) stainless steel IR cell in transmission mode using CaF2 windows and described in more detail previously.8,27 Briefly, it allowed in-situ treatments of a compressed disk of solid (Φ= 1.8 cm, m≈ 40-80 mg), in the temperature range of 293-800 K with a controlled gas flow rate in the range of 150-2000 cm3 min-1 at atmospheric pressure selected using different valves and purified by different traps in particular cold traps for H2O impurities (77 K using helium and 203 K using ammonia containing gas mixtures). The thermal inertia of the IR cell led to a low cooling rate after a pretreatment procedure: 1 h to cool down from 673 K to 300 K leading to the readsorption on the solids of tiny H2O traces. Volumetric measurements using mass spectrometry To support the exploitation of the IR spectra, the amounts of adsorbed NH3 species on the different TiO2 solids have been quantified with an analytical system for transient experiments using a quartz micro reactor and a mass spectrometer as described previously,8,28 providing the rates of eitheir formation of a product or consumption of a reactant with time on stream: R(t) (see Supporting information for more details). The low thermal inertia of the microreactor which allowed a fast cooling from 713 K to 300 K (≈ 4 min) and the high amount of solids limited the impact of H2O traces on the surface properties of the solids.

2.3 Heats of adsorption of the adsorbed NH3 species using the AEIR and TPAE methods The experimental procedure of the adsorption equilibrium IR spectroscopy method9-12 and its modification for the measurement of the heats of adsorption of adsorbed NH3 species have been described in detail in Part I.8 Briefly (see Supporting information for more details: Equations ES1-ES4), this method allowed the measurement of the individual heats of adsorption of each adsorbed species present simultaneously on a catalyst surface from the


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7 evolution of the IR band area of each adsorbed Xads species with the increase in Ta in isobaric conditions. This provided the evolution of the coverage θX(Ta) (Eq. ES1) which was compared to an adsorption model often the Temkin model (Eq. ES2) assuming localized adsorbed species (Eq. ES3). The adjustement of the theoretical curve to the experimental data provided Ex(1) and Ex(0) the heats of adsorption at high and low coverages of the Xads species respectively. In Part I,8 the AEIR method has been applied to measure the individual heats of adsorption of two adsorbed NH3 species on Lewis sites present simultaneously on the surface: denoted NH3ads-L1 and NH3ads-L2 respectively using their characteristic δs IR bands at 1145 and 1225 cm-1 (at Ta= 300 K) respectively. Moreover, it has been shown that the individual heats of adsorption of each adsorbed NH3 species can be obtained using their common δas IR band at 1596 cm-1 (at 300 K).8 This IR band provided the average experimental coverage θex(Ta) of the two NH3ads-L species which was compared to the average theoretical coverage θth (Ta) (Eq ES4). This last procedure was particularly usefull for the V and S containing solids due to either a low IR transmission (< 7%) or/and the strong overlap of several IR bands in the wavenumber range 523 K (Fig. 3). This shows that, similarly to P25, the NH4+ species are more weakly adsorbed than the NH3ads-L species on DT51: in particular they are not present in the temperatures and NH3 partial pressures ranges of interest for the NH3-SCR: this explains that their heats of adsorption have not been measured (superficial SO4 groups do not increase significantly the stability of NH4+) Considering the low IR transmission and the overlap of the different IR bands below 1300 cm-1, the δs IR bands of the NH3ads-L species have not be used for the AEIR method. The individual heats of adsorption of the NH3ads-L species have been obtained, according to the procedure detailed in Part I (supporting Information) using the evolutions of the δas IR band.8 3.2 Heats of adsorption of NH3ads-L species on TiO2-DT51 using the AEIR method. Using the IR spectra in Fig. 3, symbols z and in Fig. 4 displays the evolutions of the average experimental coverage of the NH3ads-L species (Eq. ES1, IR band at 1604 cm-1) with Ta for Pa= 0.1 kPa during the heating stages of the second (Fig. 3) and third (not shown) H(673 K)/C(300 K) cycles respectively. The overlap of the two experimental curves shows the good repeatability of the data. Their profiles are similar to those on P25,8 indicating that the δas IR band is due to the contribution of two main adsorbed NH3ads-L species with different heats of adsorption. Curve a in Fig. 4, which overlaps the experimental data, gives the theoretical evolution of their average coverage (Eqs. ES2-ES4) using the following values: x1= 0.65, x2= 0.35, EL1(1)= 56 kJ/mol, EL1(0)= 102 kJ/mol, EL2(1)= 110 kJ/mol and EL2(0)= 140 kJ/mol. These values are reported in Table 1 for comparison of the different solids: they are similar to those obtained on P25 (Table 1 line 3) showing that the presence of sulfate groups has a limited impact on these parameters except E2(0) that is decreased by 20 kJ/mol. Curves b and c in Fig. 4 show the theoretical evolution of the coverage of the NH3ads-L1 and NH3-adsL2 species considering the heats of adsorption determined from curve a. They indicate



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12 that, at T> 500 K (NH3-SCR reaction), NH3ads-L2 species is present at high coverages whereas the main part of the sites adsorbing the NH3ads-L1 species are free for the activation of other molecules. The limited difference in the nature and heat of adsorption of the NH3ads-L species on P25 and DT51 may suggest that similar adsorption sites are involved on the two solids. However, the interaction between the NH3ads-L species and the sulfate groups is clearly detected by the position and intensity of the S=O IR band which strongly depend on the coverage of the two NH3ads-L species (Fig. 4), in agreement with literature data.15 In particular, the inset A of Fig. 4 shows that, at Ta= 673 K and Pa= 0.1 kPa, the small amount of NH3ads-L species remaining adsorbed disturbs the S=O vibration observed at 1375 and 1354 cm-1 before (spectrum a) and after (spectrum b) NH3 adsorption respectively. The NH3ads species must be totally desorbed by using the pretreatment procedure to fully recover the S=O IR band at 1375 cm-1 (confirming that the sulfate groups are not consumed during the increase in Ta). This impact of a low amount of adsorbed NH3 species on the totality of the sulfate groups indicates a long range interaction between the two species, via the TiO2 support.33 This is consistent with literature data indicating a similar red shift of the S=O IR band after the chemisorption of pyridine on the Lewis sites of sulfated TiO2 and ZrO2-TiO2.34,35 The authors suggest that pyridine is bound to Lewis-acidic metal cations and that sulfate groups enhance their acidity. The chemisorbed pyridine can backdonate electrons through the metal cation to the sulfate group, changing the character of the highly covalent S=O bond to more ionic.34,35 This change induces the shift of the S=O band toward lower frequencies. A similar process can be considered for the present NH3 species with long rang perturbations. It must be noted that the heats of adsorption indicate that only the NH3ads-L2 species is present at 673 K (compare curve b and c in Fig. 4). This is consistent with the presence of only one IR band at 1216 cm-1 at Ta= 673 K (in inset A of Fig. 4) which is the δs IR band of the NH3ads-L2 species. Inset B of Fig. 4 compares the IR spectra at 673 K in the 3700-3100 cm-1 range, after the pretreatment


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13 procedure (spectrum a) and after adsorption of NH3 (spectrum b). It can be observed a decrease in the intensity of the IR band of the OH group at 3648 cm-1 without any shift. Similarly to the adsorption of NH3 on P25, this can be tentatively ascribed according to literature data (Ref 8 and references therein) to the fact that the adsorbed NH3ads-L species favor the mobility of protons on the surface allowing a higher dehydratation of the solid as compared to the pretreatment procedure. 3.3 Heats of adsorption of adsorbed NH3 species on TiO2-DT51 using the TPAE method Experimental data of the TPAE procedure Part A in Fig. 5 shows the evolution of the molar fraction of the gases during the switch He→ 0.5% NH3/0.5% Ar/He at 300 K on the pretreated DT51 solid: the total amount of NH3 adsorption (difference between Ar and NH3 curves) is 498 µmol of NH3/g (≈ 0,37*1019 NH3 species/m2 a value not significantly different than on P25:8 ≈ 0.36 1019 NH3 species/m2). The switch: 0.5% NH3/0.5% Ar/He → He (Fig. 5B) leads to the desorption 180 µmol/g due to weakly adsorbed species (NH4+ and a fraction of the NH3ads-L1 species) which are readsorbed in Part C of Fig. 5. The increase in Ta in the presence of 0.5% NH3/0.5% Ar/He (TPAE procedure) leads to a first NH3 peak (Fig. 5D) with a maximum at Tm= 323 K mainly due to the weakly adsorbed NH4+ species: 97 µmol/g (after decomposition as shown Fig. 5D) followed by a broad asymmetric peak due to the decrease in the coverage of the NH3ads-L species: the total amount of NH3 desorbed in Fig. 5D is 440 µmol/g. This value is lower than the amount of NH3 adsorbed at 300 K: 498-440= 58 µmol/g because (a) a small fraction of NH3ads-L2 species remains adsorbed at the highest temperature of the TPAE: 716 K (see curve c in Fig. 4) and (b) there is a NH3 consumption at high temperatures leading to a negative NH3 peak: 56 µmol/g at 707 K associated to the production of N2: 53 µmol N2/g and H2O: 170 µmol H2O/g. The ratio H2O/N2 ≈ 3, (similarly to P25, the excess of H2O may



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14 indicate that adsorbed NH3 species favor the dehydratation of the solid) suggests that NH3ads-L is oxidized by oxygen species of DT51 according to: 2 NH3ads-L + 3 Oads → N2 + 3 H2O

(1)

Where Oads represents surface oxygen species able to react with the strongly adsorbed NH3ads-L2 species. Assuming that the difference between the total amounts of NH3 adsorbed and desorbed is due to NH3 oxidation then the total amount of NH3 consumption is 58+56= 114 µmol/g leading to a ratio N2/NH3 ≈ 1/2 consistent with Eq. (1). After the cooling step in NH3 (Fig. 5E) associated to the readsorption of NH3, there is no NH3 oxidation during the second TPAE in Fig. 5F indicating that the solid has a limited capacity to provide Oads species (no deep reduction). The total amount of NH3 desorbed during the TPAE is 428 µmol of NH3/g with 43 µmol NH3/g in the first peak at 323 K (NH4+). The decrease in the amount of NH4+ after a first TPAE is consistent with the decrease in the IR band at 1454 cm-1 after a first H(673 K)/C(300 K) cycle (inset Fig. 2). Grange et al.36 have performed H2-TPR on sulfated TiO2 solids and observed three peaks at T≥ 783 K ascribed to the reduction of the SO4 groups. This suggests that the Oads species in Eq. (5) come from this superficial species (sulfate free TiO2-P25 was inactive for NH3 oxidation at T≤715 K).8 Considering the S content of DT51: 0.56%, the amount of Oads associated to SO4 group is 700 µmol O/g indicating that NH3 oxidation involves ≈1/4 of these oxygen which are not associated to the S=O bond because the IR band at 1375 cm-1 is detected at 673 K in the presence of NH3 (inset A Fig. 4). All the amounts of NH3, N2, H2O associated to the transient experiments are reported in Table 2. Individual heats of adsorption of the adsorbed NH3 species from the TPAE procedure. The heats of adsorption of the two NH3ads-L species can be obtained using the second TPAE in Fig. 5 after removing of the contribution of the peak at 323 K due to the NH4+ species. Symbols S in Fig. 4, obtained using Eq. ES5 provides the evolution of the average coverage of the NH3ads-L species with Ta at the quasi constant adsorption pressure Pa= 0.5 kPa


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15 (highest increase 30% of the inlet adsorption pressure). Note that (a) the calculations are limited to Ta= 650 K due to difficulties in the accurate quantification of the NH3 production at low apparent desorption rate (small differences between the inlet and outlet molar fractions) and (b) QM in Eq. ES5 is equal to the total amount of NH3 adsorbed at 300 K minus the amount in the first peak of the TPAE. Curve d in Fig. 4 which overlaps symbols S is obtained considering in Eqs. ES2-ES4: Pa= 0.5 kPa, x1= 0.64, x2= 0.36, EL1(1)= 54 kJ/mol, EL1(0)= 98 kJ/mol, EL2(1)= 102 kJ/mol and EL2(0)= 140 kJ/mol which are consistent with the values determined using the AEIR procedure validating Eq. ES1. Considering the x1 and x2 values, the amounts NH3ads-L1 and NH3ads-L2 species on DT51 are of ≈ 237 ((428-57)×0.64) and ≈ 133 µmol NH3/g respectively. It must be noted that the slight reduction of the solid during the first increase in Ta does not modify significantly the heat of adsorption of the NH3ads-L species: for instance, using the AEIR procedure and limiting the calculation of the average coverage of the NH3ads-L species during the first heating to the onset temperature of H2O and NH3 productions (Fig. 5): 590 K, then the heats of adsorption and the proportion of the NH3ads-L species are not significantly modified (result not shown). Similarly to DT51, the AEIR and TPAE procedures have been applied to study the impact of the V deposition on the nature and heats of adsorption of the adsorbed NH3 species formed on TiO2 P25 and DT51. 3.4 Characterization of the VxOy dispersion on P25 and DT51. Before adsorption of NH3, the dispersion of the VxOy species on the two supports has been studied by Raman and FTIR spectroscopies for comparison with literature data. Raman and FTIR spectroscopyof the VxOy species. The Raman and IR spectra of pretreated V containing solids were consistent with literature data (see Supporting information for more details, Fig. 1S). Raman spectra indicate the absence of V2O5 particules (characterized by a band at 995 cm-1)37,38 and the presence of well dispersed VxOy species on P25 and DT51 (Raman bands in the range 1024-1029 cm-1



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16 ascribed to the terminal ν(V=O) stretching vibrations of vanadate species)38-43 with different small x values.44-45 This has been confirmed (Fig. 2S) considering the overtone IR bands of the VxOy species showing an IR band at 2045 cm-1 at low V loading and a second overlapped band at ≈2050 cm-1 at high V loadings consistent with literature data.46-50 IR bands of the OH groups of the V2O5 containing catalysts. The dispersion of VxOy species on the TiO2 supports has been confirmed by studying the impacts of the V loading on the IR bands of the OH groups (see Suporting information for more details). At low V loading, the OH groups of the supports were partially disturbed whereas they are totally disturbed at high loading (as expected considering that the V precursor react with the OH groups)20 leading to a single strong IR band at 3660 cm-1 on P25 and 3651 on DT51 ascribed to V-OH surface hydroxyl groups.47,51-53 IR band of sulfate species after V deposition on DT51. The position of the S=O IR band is not strong modified by the VxOy depositions (see Supporting information) indicating that the SO4 groups remain of covalent nature. The Raman and FTIR spectroscopies lead to the conclusion that the TiO2 surfaces of P25 and DT51 are strongly modified by the deposition of 1.5% and 2.25% V2O5 respectively which forms well dispersed VxOy species with small x values. Their impacts on the heats of adsorption of the adsorbed NH3 species have been studied by the AEIR and TPAE methods. 3.5 FTIR study of the adsorption of NH3 at 300 K on V2O5/TiO2-P25. After a first H(673 K)/C(300 K) adsorption cycle in 0.1% NH3/He (the impact of this cycle on the adsorbed NH3 species is shown below), Fig. 6 compares the IR spectra at adsorption equilibrium at 300 K on P25, 0.5% V2O5/P25 and 1.5% V2O5/P25. The negative IR band at 1360 cm-1 after V deposition (spectra b and c) is due to adsorbed sulfate species coming from the oxalate impurities (0.08% of SO4 from Alpha Aesar) used for the dissolution of the V precursor (the intensity of this IR band is ≈ 2% of that on DT51, Fig. 1). Lietti et al.54


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17 mention the presence of a similar IR band at 1378 cm-1 ascribed to sulfate impurities on a V containing TiO2 solid prepared via the oxalate method. Similarly to DT51 (Figs. 2-3), the adsorption of NH3 leads to a shift of this small IR band towards lower wavenumbers. The IR bands in Fig. 6 reveal the modifications of the adsorbed NH3 species with the V loading. According to literature data,8 the IR bands on P25 (spectrum a) are ascribed as follows: 1142 and 1215 cm-1 are the δs IR bands of two species adsorbed on Lewis sites (denoted NH3ads-L1 and NH3ads-L2 respectively) having their δas IR band overlapped at 1596 cm-1. The shoulders at 1680 and 1477 cm-1 are due to a small amount of NH4+ species.8 The deposition of 0.5% V2O5 (Fig. 6b) has limited impacts: (a) the intensity of the IR bands of the NH4+ increases and its main IR band is detected at 1445 cm-1 indicating that dispersed VxOy species favor the amount of Brønsted sites in agreement with literature data52,53 (the ratio between of the IR band area at 1445 cm-1 (NH4+) and 1596 cm-1 (δs NH3L(ads) species is denoted AB/AL ratio), (b) the shoulder at 1215 cm-1 on P25 is detected at 1222 cm-1 on 0.5% V2O5/P25, (c) a new shoulder is detected at 1180 cm-1 indicating that a third NH3ads-L species is formed in the presence of VxOy groups. These data show that NH3 adsorption on 0.5% V2O5/P25 involves both the TiO2 support and the VxOy species in agreement with literature data.49,52,55 The increase in the V loading (spectrum c) confirms the modifications of the adsorbed NH3 species as compared to P25: (a) the shoulder observed at 1180 cm-1 in Fig. 6b is detected as a main IR band at 1185 cm-1 with a small shoulder at ≈ 1222 cm-1 and (b) the AB/AL ratio is significantly increased as compared to spectrum b mainly due to the increase in the IR band of the NH4+ species. This is consistent with the observations of Topsøe et al.49,52,55 for x% V2O5/TiO2 (x= 2 and 6) and it indicates that V deposition favors the amount of Brønsted sites. The δas IR band of the adsorbed NH3ads-L species is only slightly shifted: 1600 cm-1 as compared to P25: 1596 cm-1. This position is similar to that observed by Ramis et al.56 on a 5% V2O5/P25 after adsorption of NH3 at 300 K: 1605 cm-1 with an AB/AL ratio higher56 than in Fig. 6c. This can be due to



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18 differences (a) in the V loading and/or (b) the H2O impurities (see below). The absorbance mode used in Fig. 6 improves the display of the IR spectra, however, similarly to DT51, the IR transmission of the solid/IR cell system is < 7% below 1230 cm-1, therefore preventing using the δs IR bands of the NH3ads-L species for the AEIR method. In the 3500-3800 cm-1 range (Fig. 6) the negative IR bands are due to the perturbation of the OH groups of the solids by adsorption of NH3. As compared to P25 (the assignment of the IR bands according to literature data has been detailed in Part I)8 the V deposition has limited impacts: (a) new IR bands are observed for the 1.5% V2O5 loading (Fig. 6c) at 3042 and 2755 cm-1 ascribed to the stretching vibrations of NH4+ species,48, 52 and (b) the IR bands at 3345 cm-1 on P25 and 0.5%V2O5/P25 increases and shifts to 3356 cm-1 on 1.5%nV2O5/P25 (Fig. 8c) as observed on a 5% V2O5/P25 solid.56 It is shown below that these modifications are strongly related to the presence of NH4+ species (they disappear with the NH4+ IR bands). Comparing the IR spectra of NH3 adsorbed species on P25 based solids leads to the following conclusions in agreement with literature data: (a) IR spectra are not strongly different suggesting similar adsorbed NH3 species on Lewis and Brønsted sites,56,57 and (b) the V deposition on TiO2 favors the formation of NH4+ species as compared to pure P25.46, 49,52,55 The two δs IR bands at 1185 and 1222 cm-1 on 1.5% V2O5/TiO2 are ascribed to two species NH3 speceis on Lewis sites: NH3ads-L1 and NH3ads-L2 respectively, involving VxOy groups. Moreover, considering that (a) the adsorption of NH3 on 0.5% V2O5/TiO2 involves the adsorption sites of both the TiO2 support and the VxOy species and (b) the compositions of VxOy species are similar for the two V loading as considered by Freund et al.44,45 (the average value of x in the VxOy species may increases with loading), we present mainly the experimental data on 1.5% V2O5/P25 (this V loading disturbs the whole P25 surface). 3.6 Heats of adsorption of the NH3ads-L species on 1.5% V2O5/P25. AEIR experimental procedure for Pa= 0.1 kPa.


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19 After a first H(673 K)/C(300 K) cycle in 0.1% NH3/H, Fig. 7A shows the evolutions of the IR bands of the adsorbed NH3 species during the increase in Ta from 300 K to 673 K for Pa= 0.1 kPa: (a) the IR band at 1185 cm-1 (NH3ads-L1) at 300 K decreases associated to a shift to higher wavenumbers: 1198 cm-1 at Ta= 473 K (this shift leads to a strong overlap with the IR band of the NH3ads-L2 species detected as a shoulder at 1222 cm-1 at 300 K), (b) the common δs NH3 IR bands decreases progressively and slightly shifts to higher wavenumbers (1600 cm-1 and 1604 cm-1 at Ta= 300 and 673 K respectively) and (c) the IR band at 1445 cm-1 strongly decreases for Ta ≤ 473 K (Fig. 7A) and it is no more detected at Ta= 523 K (this indicates that the V deposition favors the amount of NH4+ species (Fig. 6) as compared to P25 without increasing their stability). This is consistent with literature data,46 based on outgassing procedures at different temperatures, which indicate that NH4+ species formed on V containing species are significantly less stable (absent at ≈ 520 K) than the NH3ads-L species (still present at 620 K). The IR bands of the NH3ads-L species in the 3400-2600 cm-1 range decrease with the increase in Ta (results not shown) (a) the IR band at 3356 cm-1 decreases and disappears simultaneously with the NH4+ IR bands (3042 and 2755 cm-1) and (b) the other IR bands are still detected at Ta= 673 K for Pa= 0.1 kPa as shown in Fig. 7B which indicates that the three P25 based solids present the same IR bands at 3361, 3262 and 3166 cm-1 corresponding to the more strongly adsorbed NH3ads-L2 species. The involvement of the VxOy species in the adsorption of NH3 is revealed by the evolution of their IR bands with Ta for Pa= 0.1 kPa (Fig. 7C): the two overtones at 2050 and 2045 cm-1 on the pretreated 1.5% V2O5/P25 (Supporting information Fig. 2S) are detected as broad IR bands ≈ 1962 ad 1916 cm-1 after adsorption of NH3 at 300 K (Fig. 7C spectrum a) consistent with FTIR,48,52,55 and Raman58 literature data on similar solids. The decrease in the coverage of the NH3 species during the increase in Ta (Fig. 7A) for Pa= 0.1 kPa leads to a progressive shift of the overtones which become sharper: for instance at Ta= 473 K, they are



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20 detected at 2018 and 1985 cm-1 (Fig. 7C-b) whereas at Ta= 673 K, there are strongly overlapped giving a single IR bands at 2030 cm-1 (Fig. 7C-c). Similar evolutions have been previously observed on 6% V2O5/TiO2.52 It must be noted that at Ta= 673 K, the remaining fraction of the NH3ads-L2 species disturbs the totality of the VxOy groups: the overtones are detected at 2040 cm-1 (Fig. 7C-d) and 2030 cm-1 (Fig. 7C-c) before and after adsorption at 673 K respectively. This suggests long rang interactions between NH3ads-L and VxOy as observed on DT51 for the sulfate groups. The difference in the position of the main overtone IR band on the pretreated solid as a function of the temperature: 2050 cm-1 (Fig. 7C-e) and 2040 cm-1 (Fig. 7C-d) at 300 K and 673 K is consistent with the observation of Topsøe et al.52 on 6% V2O5/TiO2: 2044 cm-1 and 2036 cm-1 at 300 and 625 K respectively. Heat of adsorption of the adsorbed NH3 species on the 1.5% V2O5/TiO2-P25. Similarly to P25 and DT51, inset A of Fig. 8 shows that the first H(673K)/C(300 K) cycle in 0.1% NH3/He decreases the IR bands of the NH4+ (i.e. 1450 cm-1) and increases that of the NH3ads-L species (following cycles have no impact). After a first cycle, inset B in Fig. 8 shows that the increase from 303 K to 313 K has no impact on the intensity of the δas NH3 IR band (1600 cm-1) which decreases slightly at Ta= 323 K whereas the IR band of the NH4+ species (1445 cm-1) decreases with the increase in Ta from 300 K. This indicates that (a) the heat of adsorption of NH4+ species is lower than that of the NH3L-ads species and (b) the area of the δas IR band at 300 K for Pa= 0.1 kPa can be used as AX(M) value (Eq. ES1). After a first H(673 K)/C(300 K) cycle, symbols z in Fig. 8 give the experimental evolution with Ta, of the average coverage of the NH3ads-L species (Eq. ES1) on 1.5% V2O5/P25 at Pa= 0.1 kPa, using the change in the δas NH3 IR band at 1600 cm-1 (Fig. 7A). Eqs. ES2-ES4 provide curve a in Fig. 8 using: x1= 0.5, x2= 0.5, EL1(1)= 58 and EL1(0)= 94 kJ/mol, EL2(1)= 99 and EL2(0)= 146 kJ/mol. These data are similar to those observed on the P25 and DT51 supports (Table 1). Curves b and c in Fig. 8 give the theoretical evolutions of the


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21 coverage of the NH3ads-L1 and NH3ads-L2 species respectively according to Eqs. ES2-ES3 and considering the heats of adsorption determined from curve a. It must be noted that Topsøe et al.55 have evaluated the average coverage of adsorbed NH3ads-L species using the δas IR band on V2O5/TiO2 catalysts for different Ta and Pa values: (a) at Ta= 525 K under 500 ppm NH3/8% O2/Ar (Pa= 0.05 kPa) the coverage was 0.3 on a 6% V2O5/TiO2 catalyst and (b) at the same temperature, using 500 ppm NH3/500 ppm NO/ 8% O2/Ar the coverage was 0.6 on a 2% V2O5/TiO2 catalyst (see Table 1 in Ref (55)). For Pa= 0.05 kPa, the heats of adsorption obtained from curve a in Fig. 8 lead to an average coverage of 0.43 at 525 K that is reasonably consistent with Topsøe et al.55 considering the differences in the experimental procedures. According to curve b in Fig. 8 there is no more NH3ads-L1 at Ta= 600 K for Pa= 0.1 kPa. This allows us considering that, at Ta> 600 K, the intensity of the δas NH3 IR band is only due to the NH3ads-L2 species. Assuming that the IR band area at Ta= 600 K corresponds to the theoretical coverage provided by curve c, then the evolution of the δas IR band for Ta≥ 600 K gives the experimental evolution of the coverage of the NH3L2-ads species as shown by symbols in Fig. 8 which are consistent with the theoretical curve c. Transient experiments using the M.S system on 1.5% V2O5/P25 and TPAE method. Transient experiments similar to those described for DT51 (Fig. 5) have been performed on the pretreated 1.5% V2O5/P25 solid using 0.5% NH3/0.5% Ar/He and except quantitative differences (i.e. amount of adsorbed NH3 species/g of catalyst) there are strong similarities with DT51 (see Supporting information, Fig 3S). Table 2 summarizes the amounts of NH3 implicated in the different stages of the experiment and shows that V deposition (a) increases slightly the amount of NH3 adsorption at 300 K (column 2) however this concerns mainly the weakly adsorbed species that desorbs at 300 K (column 3) and (b) allows the NH3 oxidation at high temperatures (not observed on P25) with the formation of N2 and H2O (Eq. 1). Similarly to DT51, the experimental evolution of the average coverage of the NH3ads-L



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22 species (Eq. ES5) has been obtained (results not shown) from the second TPAE (after removal of the contribution of the first NH3 peak due to NH4+ species) leading to heats of adsorption and proportion values similar to those used to obtain curve a in Fig. 8. 3.7 Heat of adsorption of the NH3ads-L species on the V2O5/P25 solids in the presence of O2. One of the advantages of the AEIR method is that it allows measuring the heats of adsorption of adsorbed species in the presence of coadsorbed species including the possibility of catalytic reactions provided that they do not disturb the adsorption equilibrium which is a conventional assumption in numerous kinetic models at low conversions.12 The TPAE experiments (see Supporting information, Fig. 3S) reveal that NH3ads-L species are oxidized at high temperatures by oxygen species of 1.5% V2O5/TiO2 during the first heating in the presence of NH3. This may lead to the view that the measurement of the heats of adsorption of the NH3ads-L species by the AEIR method performed after a first H(673 K)/C(300 K) cycle are relevant to a reduced surface and not of the surface state of a catalyst during the NH3-SCR process which take place in excess O2. To clarify this point, the AEIR procedure has been applied using a 0.1% NH3/5% O2/He mixture. After a first H(673K)/C(300K) cycle in 0.1% NH3/5% O2/He, Fig. 9 gives the evolution of the IR bands of the adsorbed NH3 species during the increase in Ta for 1.5% V2O5/P25. There are significant difference neither in the position of the IR bands nor in the ratios on their intensities as compared to 0.1% NH3/He (Fig. 7A) indicating that the nature of the adsorbed species (spectrum a Fig. 9) at 300 K are no modified in the presence of O2: two NH3ads-L species with δs IR bands at 1185 and 1222 cm-1 and δas at 1600 cm-1 and the NH4+ species with the IR band at 1445 cm-1. Moreover the evolution of the IR band with Ta is qualitatively similar in the absence (Fig. 7) and in the presence (Fig. 9) of O2. Symbols { in Fig. 8 give the evolution of the experimental average coverage of NH3ads-L species in the presence of 0.1% NH3/5% O2/He using the IR band at 1600 cm-1: they are overlapped with symbols z obtained using 0.1% NH3/He except at Ta> 630 K where symbols


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23 { indicate lower coverage values due to the fact that the catalytic NH3 oxidation disturbs the adsorption equilibrium. At Ta< 630 K the adsorption equilibrium is not disturbed by the reaction as considered in numerous kinetic models of catalytic reactions. 3.8 Adsorption of NH3 at 300 K on V2O5/TiO2-DT51 After a first H(673 K)/C(300 K) cycle in 0.1% NH3/He, Fig. 10A compares, at the adsorption equilibrium, the IR bands observed at 300 K in the 1700-1200 cm-1 range on DT51 based solids with different V loadings. Whatever the solids, the position of the δas IR band of the NH3ads-L species is 1602 cm-1 whereas its intensity is slightly different depending on the V2O5 loading: 2.25% > 0.7% > 0. The reverse order is observed for the IR band of NH4+ species at 1454 cm-1. The main difference between the three DT51-based solids is the profile of the IR bands below 1300 cm-1: the absorbance of the 2.25% V2O5/DT51 solid is lower than the two other solids which are similar. This might indicate that the shift of the S=O IR band to lower wavenumbers is stronger for the 2.25% V2O5 loading. Similarly to the P25 support, the deposition of 0.7% V2O5 on DT51 does not strongly modifiy the IR spectrum of DT51 explaining that we present mainly the adsorption of NH3 on 2.25% V2O5/DT51 for the study of the impact of VxOy species on the heats of adsorption of adsorbed NH3 species. Adsorption of NH3 on 2.25% V2O5/DT51 in the 300-673 K Ta range After a first H(673 K)/C(300 K) cycle, the adsorption equilibrium at 300 K using 0.1% NH3/He leads to spectrum a in Fig. 11 which indicates (a) the formation of NH4+ (i.e. IR bands at 1454, 1675, 2786 and 3042 cm-1) and NH3ads-L (i.e. IR bands at 1602 cm-1 and > 3150 cm-1), (b) the strong shift of the S=O IR band to lower wavenumbers revealing that the sulfate groups are disturbed by NH3 adsorption (as for DT51 this shift is dependent on the NH3ads-L coverage) and (c) the strong decrease in the IR band of the V-OH species (detected as a negative IR band at 3660 cm-1 (Fig. 11). Spectra a and b in the inset of Fig. 11 show that the NH3 adsorption at 300 K significantly modifies the IR bands of the VxOy species: they shift



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24 by from 2050 cm-1 to 1934 cm-1 indicating that there are involved in the adsorption process. As compared to 1.5% V2O5/P25 (Fig. 7C), the overtone IR bands are sharper in Fig.11. The increase in Ta for 0.1% NH3/He on 2.25% V2O5/DT51 (Fig. 11) leads to the decrease in (a) the IR bands of NH4+ species (i.e. 1454 cm-1) which disappears at Ta≈ 503 K (Fig. 11) and (b) the δas IR band of the NH3ads-L species that is still detected at 703 K. This indicates that NH4+ species are (a) more weakly adsorbed than NH3ads-L species and (b) not present in significant amount at the temperatures of the SCR in the absence of H2O. Inset in Fig. 11 shows that the decrease in the amount of the NH3ads-L species leads to the shift of the IR band of the VxOy species to higher wavenumbers: 2027 cm-1 at 703 K (spectrum c) which is lower than after the pretreatment procedure (2038 cm-1 not shown) due to the remaining fraction of adsorbed NH3ads-L species. A significant difference between the DT51-based solids concerns the impact of NH3 adsorption of the S=O IR band. Spectra a-c in Fig. 10B recorded at 673 K after the pretreatment procedure on DT51, 0.7% V2O5/DT51 and 2.25% V2O5/DT51 respectively reveal slight difference in the position of the S=O IR bands. Spectra d-f recorded under 0.1% NH3/He at 673 K indicate a similar shift of the S=O IR band to lower wavenumbers as compared to the bare pretreated surfaces. However, the intensity of the S=O IR band on 2.25%V2O5/DT51 (Fig. 10B-f) is significantly lower than on the other solids indicating differences in the interaction between NH3ads-L and sulfate groups. Heat of adsorption of the NH3ads-L species on 2.25% V2O5/DT51 using the AEIR method Symbols z in Fig. 12 give the experimental evolution of the average coverage of the NH3ads-L species from Eq. ES1 using the δas IR band at ≈1600 cm-1 in Fig. 11. Similarly to the other solids it has been verified that adsorption 0.1% NH3/He at 300 K leads to the saturation of the Lewis sites. Curve a in Fig. 12, which overlaps symbols z has been obtained using in Eqs. ES2-ES4: x1= 0.6, x2= 0.4, EL1(1)= 55 kJ/mol, EL1(0)= 98 kJ/mol, EL2(1)= 102 kJ/mol, EL2(0)= 150 kJ/mol. These values are similar to those measured on the others solids (Table 1).


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25 Curves b and c in Fig. 12 provide the theoretical evolutions of the coverage of NH3ads-L1 and NH3ads-L2 species respectively according to Eqs ES2-ES3 considering the heats of adsorption determined from curve a. According to curve b, the coverage of the NH3ads-L1 species is 0 at Ta= 600 K. This allows us considering that, at Ta> 600 K, the intensity of the δas NH3 IR band is only due to NH3ads-L2 species. Assuming that the IR band area at Ta= 600 K corresponds to the theoretical coverage provided by curve c then the evolution of the δas NH3 IR band for Ta≥ 600 K gives the experimental evolution of the coverage of the NH3ads-L2 species represented by symbols in Fig. 12 which are consistent with curve c. Transient experiments using the M.S system on 2.25% V2O5/DT51 and TPAE method. Experiments similar to Fig. 5 on DT51 have been performed on the pretreated 2.25% V2O5/DT51 solid using 0.5% NH3/0.5% Ar/He (see Supporting information, Fig. 4S). The amount of NH3 involved in the different steps of the experiment are indicated Table 2. The main difference with the DT51 support is the higher capacity of the solid for NH3 oxidation (Eq. 1) at T> 515 K during the first TPAE. Moreover, small H2O and N2 productions are detected during the second TPAE indicating that the solid has a higher capacity to provide Oads species for NH3 oxidation than DT51. Using the second TPAE the evolution of the average coverage of the NH3ads-L species have been obtained (symbols S in Fig. 12) using the procedure described for DT51 (Fig. 4). Curve d in Fig. 12 has been obtained using Eqs ES2ES4 with the following values: Pa= 0.5 kPa, x1= 0.68, x2= 0.32, EL1(0)= 98 kJ/mol, EL1(1)= 56 kJ/mol, EL2(0)= 145 kJ/mol, EL2(1)= 102 kJ/mol. These values are consistent with those used to obtain curve a in Fig. 12 using the δas IR band. 3.9 On the modeling of the NH3ads-L coverage using the Temkin Model The Temkin model derives from the Langmuir model assuming a distribution of the adsorption sites based on a linear relationship between the heats of adsorption and the coverage. The mathematical development does not provide information on the origin of this



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26 heterogeneity that can be either intrinsic to the solid surface (depending on the preparation method) or induced by adsorption. For the adsorption of linear CO species on supported metal particles the heterogeneity is mainly induced due to interactions between adsorbed molecules (i.e. coupling at high coverage) that lead for instance to the shift of IR bands to higher wavenumbers with the increase in the coverage.9-12 This type of interaction seems improbable for the NH3ads-L species on the present solids considering their surface concentrations. However, an induced heterogeneity can be associated to processes similar to those involved in the modifications of the S=O and V=O IR bands revealing long range interactions via the TiO2 support: a small amount of NH3ads-L species (i.e. at 673 K) disturbs all sulfate or/and VxOy groups. These interactions are probably relevant of the observations of the Dielbold groups.59 They have shown that TiO2 anatase has specific properties as compared to TiO2 rutile considering the mobility of surface O vacancies (Vo’s) created at the surface of the solid. They show that surface Vo’s are less stable than bulk Vo’s explaining their migration to subsurface sites. Moreover they suggest that the subsurface defects have to be reckoned with when considering the surface chemistry of TiO2 anatase and this has been considered for adsorption of formic acid60 and H2O61 on TiO2 anatase. 3.10 Comparisons of heats of adsorption of NH3ads-L species with literature data. Table 1, summarizing the heats of adsorption of the two NH3ads-L species formed on the different solids using the AEIR method (TPAE method provides similar data considering the respective experimental uncertainties), shows clearly that the differences in the surface composition of the solids have limited impacts on (a) the numbers of the adsorbed NH3ads-L species: two species, (b) their heats of adsorption at different coverage and (c) their respective proportions. Finally it appears that sulfate and VxOy species (a) maintain the presence of strongly adsorbed NH3ads-L species (even if the nature of the sites can be different) allowing their presence at a significant coverage under experimental conditions: Pa and Ta, relevant of


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27 the NH3-SCR process and (b) provide reactive surface oxygen species for ammonia oxidation at high temperatures which can be involved in the NH3-SCR reaction at lower temperatures. The values of the heats of adsorption in Table 1 can be compared to experimental literature data taking into account that they report essentially microcalorimetry and TPD methods which provide the average heat of adsorption of NH3 whereas the AEIR procedure allows measuring the individual heats of adsorption of identified adsorbed species present simultaneously on the surfaces. Moreover, for microcalorimetry (a) the measurement may involve other surface processes (i.e. reactions)15 and (b) the homogeneity of the solid sample, considering the adsorption equilibrium, is strongly dependent on the experimental conditions (i.e. Ta) particularly for strongly adsorbed species.62,63 Impact of sulfate groups on the heat of adsorption of NH3ads-L species. Guimon et al.18 have measured the heat of adsorption of NH3 on DT51 using microcalorimetry: successive small doses of NH3 are sent on the sample at 353 K increasing progressively the adsorption equilibrium pressure up to Pa= 133 Pa. For each dose, the authors measure the amount of NH3 adsorbed and the heat released during the adsorption. They find a total amount of strongly adsorbed NH3 species of 235 µmol/g which is consistent with the present experimental data: 278 µmol/g obtained considering (a) the amount of strongly adsorbed NH3ads-L species at 300 K (Table 2) and (b) the coverage: 0.8, calculated from the adsorption equilibrium conditions of the authors18 and the heats of adsorption of the present study (Table 1) (at 353 K the amount of NH4+ is significantly decreased, Fig. 3). Note that the authors18 quantify the amount of strongly adsorbed species using the two isotherms method, which is strongly dependent on the desorption duration between the two isotherms, particularly when the activation energy of desorption increases with the decrease in the coverage (i.e. the NH3ads-L species). The authors18 find an initial heat of NH3 adsorption of 204 kJ/mol significantly higher than 140 kJ/mol at low coverage of the NH3ads-L2 species (Table 1).



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28 This might be due to either the presence of a small amount of sites with a strong heat of adsorption (not detected in the present study) or the contribution of exothermic surface processes in parallel to NH3 adsorption (i.e. involving H2O impurities). At full coverage, the authors18 classify the total amount of adsorbed species into four groups according to a range of heats of adsorption: > 150 kJ/mol (32 µmol/g), between 150 and 120 kJ/mol (156 µmol/g), between 120 and 100 kJ/mol (59 µmol/g) and between 100 kJ/mol and 80 kJ/mol (39 µmol/g). This arbitrary decomposition of the sites does not allow a clear comparison with the present AEIR method which provides the individual heats of adsorption of two NH3ads-L species, however they are in the range of those determined in the present study. In a previous work, Spinceana et al.64 have also studied the heat of adsorption of NH3 on DT51 at 423 K using microcalorimetry: they find (see Fig. 2 in Ref. (64) that it varies roughly linearly from ≈160 kJ/mol to 50 kJ/mol between low and high coverages respectively. This is consistent with the heats of adsorption of the NH3ads-L1 at high coverage and NH3ads-L2 at low coverage determined in the present study (Table 1). The difference between the microcalorimetric measurement on DT51 in Refs (18) and (64) is probably related to the difference in the adsorption temperature.62,63 Using microcalorimetry, Desmartin-Chomel et al.15 have compared the initial heat of adsorption of NH3 at 383 K on a sulfate free TiO2 (GP350 from Millenium Organic Chemical): 150 kJ/mol (this values is consistent with that found on P25: 160 kJ/mol)8 and sulfated TiO2 solids. On the these solids the introduction of the two first small increments of NH3 are associated to very low heat releases ascribed to the overlap of different processes: i.e. adsorption (exothermic) and dissociation of NH3 (endothermic) on a small numbers of sites.15 Then the heat of adsorption of NH3 is of ≈ 150 kJ/mol decreasing to 50 kJ/mol. at high coverage, showing that the sulfate groups have a limited impact as observed in the present study. Finally, the authors15 show that, at the difference of pure TiO2, sulfated surfaces have a small number of sites able to dissociate NH3. These sites are


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29 deactivated during the NH3 adsorption allowing the measurement of the heats of adsorption of the adsorbed NH3 species.15 The good agreement between the individual heats of adsorption of the two NH3ads-L species on DT51 and the measurement by microcalorimetry,15,64 is probably due to the adsorption temperature used by the different authors which limits the amount of NH4+ species and favors the homogeneity of the solid sample. Jin et al.34 propose that sulfate groups increase the acidic strength of surface sites on metal oxides based on the shift of the IR band of S=O group (i.e; Fig. 2, 3) to lower wavenumbers during the adsorption at 423 K of basic molecules such as pyridine and ammonia: the larger the shift (i.e. on sulfated ZrO2 and TiO2) the stronger is the acidity (small shifts are observed on sulfated Al2O3 and SnO2 and no shift on SiO2 and Bi2O3).34 The authors provide an explanation to this shift by suggesting a change in the electronic structure of the sulfate groups due to NH3 adsorption: the covalent S=O bond before NH3 adsorption has a strong tendency to lose its double-bond character (or to decrease the bond order) due to an electronic transfer from the adsorbed species. However, according to the conclusion of the present study and of Desmartin-Chomel et al.15 these interactions between SO4 groups and NH3ads-L species have a very limited impact on the heats of adsorption of the NH3ads-L species. This is probably due to the fact that the relationship between the shift in the IR band of the S=O band and the heat of adsorption of the NH3 species is not straightforward. In particular Jin et al.34 provide an explanation of the electronic transfer based on a short range interaction (the SO4 group is adjacent to the cation sites adsorbing NH3) whereas it seems that long range interaction via the TiO2 support59-61 are implicated in the shift of the IR band of the S=O bond (small amounts of NH3ads-L species at 673 K disturb the totality of the sulfate groups, Fig. 4). Impact of VxOy species on the heat of adsorption of NH3ads-L species. For these solids it should be mentioned that several authors considered that NH3 adsorption is dominated by Brønsted sites whereas the AEIR method is applied only to



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30 NH3ads-L species because it has been shown that NH4+ species are weakly adsorbed whatever the composition of the solids and not present with a significant coverage for Ta and Pa values of interest for the NH3-SCR reaction. The modifications of the V=O overtone IR band by NH3 adsorption (i.e. Fig. 7) is conventionally ascribed to the participation of the VxOy groups in the adsorption. However, Table 1 shows that the number and heats of adsorption of the NH3ads-L species are not significantly modified by the presence of VxOy species. These conclusions are in line with the study of Zhao et al.65 on the impact of V2O5 deposition (preparation method as in present study, loading in the 5-25% range) on TiO2 (G5 From Millenium, anatase, 209 m2/g) in the presence and in the absence of sulfate species (the solids have been sulfated after V2O5 deposition) on the heat of adsorption of NH3 measured by microcalorimetry. The authors65 show that in the absence of sulfate group and whatever the V loading, the heat of adsorption of the NH3 decreases linearly as a function of the coverage as observed in the present study. For V loading < 25%, it decreases from 150 kJ/mol to 50 kJ/mol at low et high coverage respectively65 in good agreement with the present study considering that the NH3ads-L1 and NH3ads-L2 species are dominant at high and low coverages of the surface respectively. Moreover, at very low coverages (two first NH3 doses), the authors65 find a decrease from ≈ 206 to ≈150 kJ/mol that can be due to the presence of a very small amount of sites with high heats of adsorption not detected in the present study. The sulfatation of the VxOy/TiO2 solids65 has significant impact neither on the linear relationship between heat of adsorption of NH3 and coverage nor on their values.65 Moreover, the authors shows that the V loading and the presence of sulfate have a limited impact on the proportion of adsorbed species having an heat of adsorption in arbitrary ranges: 60 kJ/mol 600 K; S experimental average coverages of the NH3ads-L according to TPAE method using Pa= 0.5 kPa (see the text for more details); d) theoretical average coverage of the NH3ads-L1 species using in Eqs. 2-5 Pa= 0.5 kPa, x1= 0.68, x2= 0.32, EL1(0)= 98 kJ/mol, EL1(1)= 56 kJ/mol, EL2(0)= 145 kJ/mol, EL2(1)= 102 kJ/mol.



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 46 of 69

46 TABLES:

Solids

NH3ads-L1

NH3ads-L2

EL1(1) kJ/mol

EL1(0) kJ/mol

x1

EL2(1) kJ/mol

EL2(0) kJ/mol

x2

TiO2-P25

56

112

0.7

104

160

0.3

TiO2-DT51

56

102

0.65

110

140

0.35

1.5% V2O5/P25

58

94

0.5

99

146

0.5

2.25% V2O5/DT51

55

98

0.6

102

150

0.4

Table 1: Heats of adsorption: EX(θ) and proportions at 300 K: xi of the two NH3ads-L species present on the different TiO2 based solids at the adsorption equilibrium for Ta and Pa of interest for the NH3-SCR.


Page 47 of 69

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47

Solids

TiO2-P25*

Ads. 300 K

Des. 300 K

µmol/g

µmol/g

NH3ads

NH3rev

332

96

TPAE-1 µmol/g NH3des

N2P

H2OP

TPAE-2 µmol/g NH3c

NH3des

328

323

P1= 25

P1= 18 Tm=679 K

0

17

0

N2P

H2OP

NH3c

0

0

0

trac.

trac.

0

4

32

≈4

9

75

10

Tm=684 K

440

428 §

TiO2-DT51

1.5% V2O5/P25

2.25% V2O5/DT51

498

357

481

180

134

174

(-58)

P1= 97

53

170

56

(-70) § P1= 43

TM=716 K

Tm=715 K

280

307

(-77)

§

P1= 17

31

128

10

(-50) § P1= 12

TM=685 K

Tm=679 K

334

374

(-147)

§

P1= 34

86

373

31

Tm=694 K

(-107) § P1= 25 Tm=691 K

*: see Ref (8) §

: Difference with the amount of adsorbed species at 300 K.

P1: amount of NH3 in the first peak of the TPAE. TM: highest temperature of the TPAE procedure.

Table 2: Amounts of NH3, N2 and H2O involved in the different steps of the transient experiments on the different TiO2 based solids using the M.S system (see Fig. 5 for DT51).



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Page 48 of 69

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

ACKNOWLEDGMENT We thank Mrs Eva Diaz-Marti from Millenium Inorganic Chemicals, a Cristal Global company, for providing the DT51 TiO2 sample. D.B thanks the “Institut de Chimie de Lyon” for the purchase of the mass spectrometer in the framework of the “Contrat de Projets EtatRégion” Rhône-Alpes (2007-2013).

SUPPORTING INFORMATION AVAILABLE: Catalyst preparations, analytival methods using FTIR and M.S, equations used in the AEIR and TPAE methods, Raman and FTIR spectrocopies characterizations of the pretreated solids, transient experiments with 0.5% NH3/0.5% Ar/He on 1.5% V2O5/P25 and 2.25% V2O5/DT51 using M.S. This information is available free of charge via the Internet at http://pubs.acs.org


Page 49 of 69

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15

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54

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55 (73) Soyer, S.; Uzun, A.; Senkan, S.; Onal, I. A Quantum Chemical Study of Nitric Oxide Reduction by Ammonia (SCR reaction) on V2O5 Catalyst Surface. Catal. Today 2006, 118, 268-278.



56 SN Synopsis TOC

Heats of adsorption of NH3 species on 2.25 wt%V2O5/TiO2-DT51

AEIR method

NH3ads-L1

PNH3= 0.1 kPa 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 56 of 69

T a= 300 K 423 K

+

EL1(1)= 55 kJ/mol EL1(0)= 98 kJ/mol

NH3ads-L2 EL2(1)= 102 kJ/mol EL2(0)= 150 kJ/mol NH4+

568 K 0.04 673 K 1700

1600 Wavenumber (cm-1)


1500

Page 57 of 69

d

c d b

1610

0.5 a

0.5% Transmission %

Transmission %

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


d

a 3648

d 3658

a

3700 3600 3500

Wavenumber (cm-1)

1700

1375

1500 1300 Wavenumber (cm-1)

Figure 1 ACS Paragon Plus Environment

1100


a 5% f

1668

f 1600

Transmission %

Transmission %

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

Page 58 of 69

b,c b,c a 1668 a a b-c 1600 1454

a

1454

5%

1800

1800

1258

1600 1400 1200 Wavenumber (cm-1)

1600

1212

1375

a



f 1138 1200

Page 59 of 69

1600

0.1

a 0.02

1353 1338

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


1215

i 1668

i

1322 1604

a 1740 1700 1660 1620 1580 1540

a

1600 i 1600

1800

a

1454

i 1400 -1 Wavenumber (cm )


1200

1

Page 60 of 69

1374

b

0.8 0.6

0.4 0.2

d

3648

a

Wavenumbers

300

a

1600 1400 1200 Wavenumbers

b 0.02

b

1604

3127

B

1354 1216

A

3368 3271

3700 3500 3300 3100

0

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

Coverage of the NH3 adsorbed species


a

c

400 500 600 700 800 Adsorption temperature (K) Figure 4 ACS Paragon Plus Environment

900

Page 61 of 69

1 A

| B |C|

D

| E |

773

F

Molar fractions of the gases

Tm=707 K

0.8

0.6

Tm=323 K

673

T

Ar×100

573

NH3 ×100

473

H2O ×1000

373

0.4

0.2

Tm=423 K Tm=323 K

NH3 ×100

Temperature K

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


N2 ×1000

0 0

273

20

40

60 Time (min)


80

100

120


0.1

0.1

3248

3345

1185 1198

3356 3388

1142

1596

3190

1600 3042

Absorbance

1180

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

Page 62 of 69

2755

c

3152

b

3145

a 3397

1360 1445

1675

c b

1215 1222

3800

3400

3000

Wavenumber (cm-1)

2600

a 1800

1600

1400

Wavenumber (cm-1)

Figure 6


1200

Page 63 of 69

C

1185

2050

e

2040

A Absorbance

1198

a

0.1

2018

B c 1445 1665 1604

1800 1600 1400 1200 Wavenumber (cm-1)

2030

0.004

1222

1600

Absorbance

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


1985 a 1916 1962

2100 2000 1900 Wavenumber (cm-1) 3361 3262 3166

c

0.02

a 3600 3200 Wavenumber (cm-1)

Figure 7



B Absorbance

1 c b

0.8

1600 c

a,b

0.04 1450 a c

0.6 1800 1600 1400 Wavenumber (cm-1)

A b

0.4

0.2

0

Absorbance

Coverage of the NH3 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

Page 64 of 69

0.04

1600 1450

a

1700 1500 Wavenumber (cm-1)

300

400

a 500

600

700

Adsorption temperature (K)


800

900

Page 65 of 69

1185 a 1201 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


1209 1222

1600

1215 1220 e 1445

1800

1604 1600 1400 1200 Wavenumber (cm-1) Figure 9



A

B a

1371 1368

a 1376

b

c Absorbance

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

Page 66 of 69

0.2 c

1602

1356 1352

0.2

a 1454

e

d 1352

a f

c 1500

1700 1500 1300 Wavenumber (cm-1)

1400

1300

Wavenumber (cm-1)

Figure 10


1200

Page 67 of 69

0.04 3353

3251

1602 2050

3151

0.04

a 3042 2786

h

Absorbance

a

Absorbance

3307

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


2 10-3

2027

c

a

b

2100 2000 1900 Wavenumber (cm-1)

1454

3363

h

h 1675

a 3660 3800 3400 3000 2600 Wavenumber (cm-1) Figure 11

1934

a

1606 h

1700 1600 1500 Wavenumber (cm-1)



1 Coverage of the NH3 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

Page 68 of 69

c 0.8

b

d

0.6 0.4

0.2

0

a 300

400

500

600

700

Adsorption Temperature (K)


800

900

Page 69 of 69

Heats of adsorption of NH3 species on 2.25 wt%V2O5/TiO2-DT51

AEIR method

NH3ads-L1

PNH3= 0.1 kPa 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


+

Ta= 300 K

EL1(1)= 55 kJ/mol EL1(0)= 98 kJ/mol

NH3ads-L2 EL2(1)= 102 kJ/mol EL2(0)= 150 kJ/mol

423 K

NH4+

568 K 0.04 673 K 1700


Graphical abstract


1500

Experimental Microkinetic Approach of De-NO - American

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