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Experimental Microkinetic Approach of De-NOx by NH3 on V2O5/WO3/TiO2 Catalysts. 4. Individual Heats of Adsorption of Adsorbed H2O Species on Sulfate-Free and Sulfated TiO2 Supports. Daniel Bianchi, François Giraud, Julien Couble, Christophe Geantet, Nolven Guilhaume, Eric Puzenat, Sébastien Gros, Lynda Porcheron, and Mohamed Kanniche J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 17 Jun 2015 Downloaded from http://pubs.acs.org on June 18, 2015

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The Journal of Physical Chemistry

1

Experimental Microkinetic Approach of De-NOx by NH3 on V2O5/WO3/TiO2 Catalysts. 4. Individual Heats of Adsorption of Adsorbed H2O Species on Sulfate-Free and Sulfated TiO2 Supports.

AUTHOR NAMES François Giraud,1,2 Julien Couble, Christophe Geantet,1 Nolven Guilhaume,1 Eric Puzenat,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 NH3-SCR, TiO2, sulfated TiO2, water, FTIR, heat of adsorption,

ACS Paragon Plus Environment


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2 ABSTRACT: The present study is a part of an experimental microkinetic approach of the removal of NOx from coal-fired power plants by reduction with NH3 on V2O5/WO3/TiO2 catalysts (NH3-SCR). It is dedicated to the characterization of the heats of adsorption of molecularly adsorbed H2Oads species formed on sulfate free and sulfated TiO2 supports. Water which is always present during the NH3-SCR, may either be in competition or/and react (formation of NH4+) with the adsorbed NH3 species controlling the coverage of the adsorbed intermediate species of the reaction. Mainly, an original experimental procedure named adsorption equilibrium infra-red spectroscopy (AEIR) previously used for the adsorption of NH3 species on the same solids is adapted for the adsorption of H2O. At Ta= 300 K and for PH2O≤ 1 kPa, three main H2Oads species are formed (associated with a minor amount of dissociated H2O species) on the two TiO2 solids. The species are identified by the position of their IR bands in the 3750-3000 cm-1 range. Considering the decreasing order of stability, they are (a) coordinated to strong (L2) and weak (L1) Lewis sites and denoted H2Oads-L2 and H2OadsL1

respectively and (b) hydrogen bonded to the H2Oads-L species and on O2-/OH sites of the

solids (denoted H2Owads). The three species have a common well defined δH2O IR band at a position in the range 1640-1610 cm-1 according to the total coverage of the surface. According to the AEIR method, the evolution of the intensity of this IR band during the increase in the adsorption temperature Ta in isobaric condition, provides the evolution of the average coverage of the three species and then to their individual heats of adsorption as a function of their coverage. It is shown that there are no significant differences on the two TiO2 solids. In particular, the heat of adsorption of the H2Oads-L2 species varies from ~114 to 61 kJ/mol at low and high coverages respectively, indicating that it can be present in the experimental conditions of the NH3-SCR. In a forthcoming article, the competitive chemisorptions and reaction between adsorbed H2O and NH3 species are studied and modeled on the TiO2 supports and model and commercial V2O5/WO3/TiO2 catalysts.


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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 by NH31 (named NH3-selective catalytic reduction: NH3-SCR) considering the experimental conditions of flue gas denitrification in coal-fired power plants (T> 573 K). Stationary sources use mainly x% V2O5/y% WO3/sulfated-TiO2 (weight %) catalysts with x and y in the ranges 0.7-2 and 9-13 respectively.2,3 The aims of EMA are (a) the identification of the surface elementary steps controlling the rate of the reaction in particular the nature of the pivotal adsorbed NH3 species allowing the reduction of NOx and (b) the measurement of the kinetic/thermodynamic parameters of interest. Previous works have been dedicated to the characterization of the adsorbed NH3 species on different solids involved in the catalyst preparation, from sulfate free and sulfated TiO2 supports to V2O5 and/or WO3 supported on TiO2.4-6 The originality of these studies is the measurement of the individual heats of adsorption of (a) two NH3ads-L formed on acidic Lewis sites of different strength whatever the solid composition and (b) a strongly adsorbed NH4+ species on the WO3 containing solids. This has been obtained by adapting an analytical method (named AEIR) developed in previous works for the adsorption of CO on supported metal particles. This method is based on the quantitative use of the evolutions of the IR bands of an adsorbed species during the increase in the adsorption temperature Ta in isobaric conditions.7-10 Considering the high partial pressure of H2O coming from the coal combustion, it can be suggested that the coverage of the NH3ads-L and NH4+ species can be modified due to (a) a competitive chemisorption between NH3 and H2O species and (b) surface elementary steps leading to total/partial transformation of NH3ads-L into NH4+ species. These modifications may have either negative or positive impacts on the rate of the NOx reduction according to the nature of the pivotal adsorbed intermediate species. To clarify



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4 these points before developing the EMA by studying the reactivity of the NH3ads–L and NH4+ species with NO, the present study is dedicated to the measurement of the heats of adsorption of H2O species formed on sulfate free and sulfated TiO2 supports to quantify the competitive chemisorption NH3/H2O on the different solids involved in NH3-SCR catalysts. The adsorption of H2O on these solids leads to more complex IR spectra than those due to the adsorption of NH3. This results from the fact that acidic and basic sites of the solids form different coordinated, hydrogen bonded (abbreviation H-bonded) and dissociated H2O species which may adsorb new H2O species by H-bond.11-17 This imposes an adaptation of the AEIR method to obtain the individual heats of adsorption of the H2O species, particularly those present during the NH3-SCR which may modify the coverage of the intermediate adsorbed NH3 species of the reaction. These original data will allow us in a forthcoming article to study and to model the competitive chemisorption/reaction of H2O and NH3 adsorbed species on TiO2 based solids particularly on model and commercial V2O5/WO3/TiO2 catalysts in the adsorption temperature Ta and pressure Pa ranges of the NH3-SCR. Moreover, the individual heats of adsorption of H2O species on TiO2 solids, can be of interests for other fields of heterogeneous catalysis because TiO2 is used either as catalyst (i.e. the photocatalytic oxidation of organic compounds (Refs 18, 19 and references therein) and water splitting)20 or support (for metal supported particles) in catalytic reactions involving H2O as either product or reactant. 2. Experimental. Solids, characterizations and pretreatments In the present study, the sulfate free and sulfated TiO2 solids are P25 from Degussa (55 m2/g) and DT51 (80 m2/g) from Millenium Inorganic Chemical respectively which have been (a) used as supports of NH3-SCR catalysts and (b) characterized considering their


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5 properties for NH3 adsorption.4-6 TiO2-P25 has been selected because it is used in numerous literature data dedicated to the characterization of adsorbed species particularly H2O. Before H2O 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 successive experiments and it was pretreated as above before each experiment. IR Cell in Transmission mode The FTIR characterizations have been performed using a Nicolet-6700 FTIR spectrometer equipped with a home made small internal volume (≈ 2 cm3) stainless steel IR cell reactor in transmission mode using CaF2 windows and described in more detail previously.4,21 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 at atmospheric pressure selected using different valves and purified by different traps in particular cool traps for H2O impurities (i.e. 77 K for helium).4-6 The thermal inertia of the IR cell led to a slow cooling rate: ≈ 1 h from 713 K to ∼300 K leading to the adsorption of tiny traces of H2O on the solids after the pretreatment procedure (see below).4-6 Volumetric measurements using mass spectrometry To support the exploitation of the IR spectra, the amounts of adsorbed H2O species have been quantified with an analytical system for transient experiments using a mass spectrometer providing the molar fractions of the gas mixture at the outlet of a quartz microreactor as described previously.4-6,22 The low thermal inertia of the microreactor which allowed a fast cooling from 713 K to 300 K (≈ 4 min) and the amount of solids (200-300 mg) limited the impact of H2O traces on the measurements. This system provided the rates of either consumption (i.e. adsorption) or formation (i.e. desorption) of a gas with time on



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6 stream: R(t) (see the Supporting Information for more details). The M.S setup allowed us measuring (a) QTads(Ta, Pa) the total amount of adsorbed H2O species at the adsorption equilibrium at Ta and Pa using x% H2O/y% Ar/He gas mixtures (Ar is used as a tracer providing the mixing curve of the reactor) and (b) Qdes(Ta, Pa), the amount of reversible species by desorption in helium at Ta. The difference Qsads (Ta, Pa)= QTads(Ta, Pa) - Qdes(Ta, Pa) provided the amount of strongly adsorbed H2O species at Ta. H2O adsorption on the TiO2 solids After the pretreatment of the solid at 713 K followed by cooling in helium, the adsorption of H2O was studied at 300 K according to the switch He  x% H2O/He (FTIR system) or x% H2O/1% Ar/He (M.S system). The H2O contents of the gas mixture were fixed by mixing two gas flow rates (a) helium passing through a vaporizator (containing liquid H2O at 303 K)/condensor (at Tc< 300 K) system and (b) either helium (FTIR) or a y% Ar/He mixture (S.M). This allowed modifying the partial pressure of H2O using Tc and/or the two gas flow rates. Isothermal desorption at 300 K and TPD procedure were performed to facilitate the identification of the different adsorbed H2O species. Heats of adsorption of the adsorbed H2O species using the AEIR and TPAE methods The procedures for the measurement of the individual heats of adsorption of adsorbed H2O species using the AEIR7-10 and TPAE23 methods were similar to those described in Parts 1-34-6 for the adsorbed NH3 species. For a Pa value, after the adsorption equilibrium of H2O on the pretreated solid at 300 K, Ta was increased progressively and the evolution of the coverage of the adsorbed species was followed by the decrease in either the intensities of the IR bands or the net H2O desorption rate with the FTIR and M.S system respectively. The experimental curves provided the heats of adsorption of the different species by comparison with an adsorption model (see Supporting Information eqs ES1-ES4).


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7 3. Results and Discussion. The first steps of the AEIR method are (a) the identification of the adsorbed species formed at 300 K and (b) the selection of the IR bands to follow the coverage of the different species with the increase in Ta in isobaric conditions. FTIR characterizations of the adsorption of H2O at 300 K on TiO2 solids (anatase and rutile) followed by a progressive desorption at different temperatures (TPD) have been largely described in the literature.11-17 However, the assignment of the different IR bands is not straightforward and some of them are still debated as discussed by Deiana et al,16 even with additional material from NMR spectroscopy, surface sciences and theoretical calculations.24-30.In spite of this situation, there is an agreement on the counting of the different undissociated adsorbed H2O species (abbreviation H2Oads in the present study) which can be formed on a TiO2 surface according to Ta and Pa. For instance, Soria et al.17 list the following H2Oads species in the increasing order of stability : (a) multilayer of water molecule arrangements (MAs) constituted by weakly adsorbed and highly mobile species (formed at low adsorption temperature) which desorb at room temperature facilitating the observation of the remaining less mobile species, (b) H2O species in a second layer H-bonded to strongly chemisorbed H2O species and desorbing at ∼350 K, (c) H-bonded species to bridging O2- anions desorbing in the same temperature range (d) H-bonded species to hydroxyl groups and (e) strongly adsorbed H2O species coordinated to Ti4+ cations forming the first layer of very restricted mobility and desorbing at T > ∼500 K.17,24 This description of H2O adsorption on dispersed TiO2 solids is in line with the conclusions of Henderson30 considering TiO2 (110) facet. Moreover, this author has shown that on some TiO2 faces and defect sites, in parallel to the undissociated adsorption, dissociative H2O chemisorption forms small amounts of strongly adsorbed hydroxyl groups.29,30 Taking into account these literature data the present study has two main aims:



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8 (a) in the temperature range 300-650 K, the identification by use of FTIR and M.S spectroscopy of the adsorbed H2O species formed on the P25 and DT51 TiO2 solids and the justification of the choice of the IR bands used for the AEIR method. This part of the study constitutes mainly a revisit of literature data. However, the performances of the present IR cell in transmission mode allows us obtaining new experimental data leading to reconsider some previous conclusions, such as the state of the TiO2 surface before adsorption. and (b) the measurement of the individual heats of adsorption (at different coverages) of the adsorbed H2O species present in experimental conditions (Pa and Ta) of the NH3-SCR by adapting the AEIR and TPAE methods.4-6 This constitutes the main original contribution of the present work. The experiments and procedures developed in line with these two aims are provided in detail for the adsorption of H2O on TiO2-P25. Then, they are applied to TiO2-DT51.

3.1 Adsorption of H2O on TiO2-P25 using the FTIR and M.S systems. Comments on the interpretations of the IR spectra of adsorbed H2O species In the mid-IR range (4000-1100 cm-1) allowed by CaF2 windows, the adsorbed H2O species on TiO2 and other metal oxides can be identified using their IR bands in two wavenumber ranges: 1700-1600 and 3800-3000 cm-1.11-17 They correspond to the fundamental (a) deformation (ν2 or δH2O) and (b) stretching (ν2 and ν3 or νsym, and νasym respectively) vibrations of H2O species respectively. Moreover, the H-bonds formed during the adsorption contribute to an intense and broad IR band below 3600 cm-1 whereas the OH groups of the solids provide IR bands above 3600 cm-1. Often, the IR bands of adsorbed species present similarities with the liquid/solid phases favoring the identification of the IR bands of the adsorbed species particularly for H2O. In the gas phase, the fundamental ν1, ν2 and ν3 vibrations of H2O are situated at 3652, 1595, 3756 cm-1 respectively and the overtone 2 ν2 is


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9 detected at 3151 cm-1.31 In the liquid/solid phases the fundamental ν1 and ν3 IR bands are (a) shifted to lower wavenumbers, (b) significantly increased and (c) strongly overlapped with the IR bands due to the four H-bonds of each H2O molecules. This leads to a broad IR band with a maximum at ∼3400 cm-1 including the 2 ν2 overtone.32-34 The approximate positions of the ν1 and ν3 vibrations: ∼3530 and ∼3690 cm-1 respectively, have been determined from their combinations IR bands in the near infrared.33 The δH2O vibration in the liquid/solid phases provides a well defined IR band shifted to higher wavenumbers as compared to the gas phase ∼ 1645 cm-1.32-34 The comparison between adsorbed and condensed H2O phases must take into account recent experimental and theoretical studies dedicated to ice clusters. They have shown that H2O molecules at the surface of the clusters have a coordination number lower than four leading to dangling H (not involved in H-Bond, abbreviations d-H)36,37 which are characterized by well defined IR bands such as 3720 and 3698 cm-1 for d-H of H2O molecules with 2 and 3 H-bonds respectively.37 The adsorption of gases such as N2 on the clusters shifts the d-H IR band by ∼ 20 cm-1 to lower wavenumbers,36 showing the high dependence of the IR band to the d-H environment. Moreover, it is considered that core and subsurface H2O molecules do not contribute significantly to the intensity of the sharp and structured deformation δH2O IR band (∼ 1650 cm-1) of ice clusters which is mainly due to the surface H2O molecules.36 Similarly to small ice clusters, a sharp IR band at 3690 cm-1 and a broad IR band below 3600 cm-1 in the spectra of H2O species adsorbed (at room temperature and for Pa= 18 torr) on TiO2-P25 and Al2O3 have been ascribed by Kakeuchi et al.15 to d-H and Hbond IR bands respectively of polymeric H2O chains. It must be noted that the positions of the d-H IR bands,36-37 are in the same wavenumber range than the OH groups of the metal oxides and the ν1 and ν3 IR bands of isolated H2O species,11-17 leading to difficulties in the interpretation of the IR spectra of adsorbed H2O species.



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10 After adsorption of H2O on TiO2, different authors report the presence of a sharp IR band at ∼ 3420 cm-1 overlapping the broad IR band of the H-bond.11,13,14,16,17 Deiana et al.16 discuss its assignment considering different arguments of the literature (such as due to the rutile TiO2 phase) and they show that there is no definitive conclusion. Comparison with the theoretical IR spectra of (H2O)n ice clusters, may provide new arguments.36 It is shown that the IR spectra change significantly with the size of the cluster. For n= 20 (all the H2O molecules are at the surface) well defined IR bands are present in the range 3600-3000 cm-1 whereas for n≥ 48 the IR spectra are dominated by the characteristic broad H-bond IR band at 3400 cm-1. The adsorption of H2O on metal oxides such as TiO2 may lead to adsorbed species with a structure similar to that at the surface of small ice clusters providing the well defined IR band at ~3400 cm-1 overlapping the broad H-bond IR band due to other adsorbed species. The interpretation of the IR spectra after adsorption of H2O on TiO2 must also take into account the fact that this solid favors long range interactions between adsorbed species leading to the modification of their characteristic IR bands. For instance, on TiO2-DT51, we have shown that a small amount of NH3 species adsorbed on Lewis sites disturbs the totality of the IR band of the SO4 group (the position is shifted to lower wavenumbers and the intensity is decreased).5 It is shown below that the situation is similar with adsorbed H2O species. The interpretations of the IR spectra during the adsorption of H2O on the present TiO2 solid are made on the basis of these different comments taking into account previous literature data. Impact of the H2O traces on the IR spectra of the pretreated TiO2-P25. It is accepted that outgassing pretreatments of TiO2 solids at T > 600 K is associated to the dehydration/ dehydroxylation of the surface creating different acidic (Lewis: Tiδ+ and Brønsted: OH) and basic (O2-) sites able to adsorb H2O at 300 K.11-13, 16,17,28 Figure 1 shows the evolution of the IR bands of TiO2-P25 during the cooling stage in helium after the


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11 pretreatment procedure. At 673 K (spectrum a) two main IR bands are observed at 3702 (denoted B1) and 3669 cm-1 (denoted B2) with a small shoulder at 3630 cm-1 whereas there is no δH2O IR band (inset Figure 1). These IR bands must correspond to two isolated OH groups coming from either the solid preparation or the dissociation of tiny traces of H2O which can not be prevented.4-6 These two OH groups can be either situated on different TiO2 faces in line with recent DFT calculations27 such as 3760 and 3728 cm-1 on TiO2 (001) and (110) respectively or/and formed on different sites leading to terminal (on Ti4+ sites) and bridging (on O2- sites) hydroxyls for the IR bands at 3730 and 3670 cm-1 respectively.38 Cooling from 673 to 473 K leads to an increase in the IR bands with a shift of B1 to 3712 cm-1 (Figure 1, spectrim c) which can be interpreted as the increase in the coverage of OH groups due to the adsorption equilibrium under a very low H2O pressure. These modifications continue in the 473-423 K range associated with the appearance of (a) two new IR bands at 3690 cm-1 (denoted B3) and 1622 cm-1 which is the δH2O IR band of an undissociated H2O species. It must be noted that the formation of H2Oads species at T= 473 K in the presence of H2O traces indicates a high heat of adsorption as expected for an adsorption on strong Lewis sites. For T< 423 K (Figure 1 spectra e and f) all the IR bands increase whereas at Ta= 300 K, a small and broad IR band is clearly detected below 3600 cm-1 (denoted B4). At 300 K the IR bands at 3747, 3720 (B1), 3690 (B3), 3672 (B2) and 3642 cm1

(Figure 1f) and their relative intensities are similar to those observed by different authors on

TiO2-P25,12,14,16 such as 3736, 3717, 3688, 3672 and 3642 cm-1 with a small shoulder at 3659 cm-1 (not observed in the present study).16 These IR bands are conventionally ascribed to different OH groups on the TiO2 surface,12,16 and some authors consider that B1 is due to SiOH groups of the small amount of SiO2 impurities in TiO2-P25.12 Figure 1 shows clearly that this assignment must be partially reconsidered due to the adsorption of H2O traces during the cooling stage in helium. This is consistent with comments of Deiana et al.16 evoking H2O



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12 adsorption during the cooling stage of TiO2-P25 pretreated in vacuum at high temperatures. The B3 IR band which increases simultaneously with the δH2O IR at 1622 cm-1 must be ascribed to a strongly adsorbed H2Oads on Tiδ+ Lewis sites noted H2Oads-L. According to our previous comments on the interpretation of the IR spectra of adsorbed H2O species, two interpretations of the B3 IR band can be proposed. It is either (a) the ν3 vibration of an isolated H2O species having its ν1 IR band (at lower wavenumbers) overlapped with the IR of the isolated OH group at 3672 cm-1 (this is consistent with DFT calculations,27 which indicate for instance that the stretching vibration of adsorbed H2O species not involved in H-bonds, should absorb at 3665 and 3646 cm-1 on TiO2 (101)) or/and (b) the d-H IR band of an H2Oads-L species having its second H involved in a H-bond with O2-/OH sites at proximity of the Tiδ+ sites. This may explain the small and broad IR band below 3600 cm-1 detected at 300 K indicating the presence of H-bonds. Note that the two interpretations may represent two situations encountered by the H2Oads-L species according to the different arrangement of the Tiδ+ and O2-/OH sites. This is in line with Henderson considering the mechanism of H2O dissociation.29 The author notes that water dissociation is observed on TiO2 (100) and not on TiO2 (110) surfaces.29 This is interpreted by a longer distance between acidic (five-coordinate Ti4+ binding H2O) and basic (bridging two-coordinate O2-) sites on TiO2(110) than on TiO2(100) surface preventing the formation of H-bond precursor of the H2O dissociation.29 The author indicates that the TiO2(110) surface can be active for water dissociation when structural defects such as steps or kinks are created (i.e. by electron-beam irradiation). It can be expected that the two situations described by Henderson are simultaneously present on a dispersed TiO2 solid allowing or not H-bond between with O2-/OH sites and H2Oads-L species leading to d-H and ν3, ν1 IR bands respectively. Finally, Figure 1 shows that the IR spectrum of the pretreated TiO2-P25 at 300 K, before H2O adsorption, is partially due to the adsorption of tiny traces of H2O during the


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13 cooling stage. However, this spectrum does not correspond to a homogeneous sample composition because H2O adsorption is controlled by an apparent rate (in the present study the inlet molar flow rate of the H2O traces) and proceeds according to a breakthrough curve. During the cooling stage, in addition to the amounts of each type of adsorption site, three main parameters have an impact on the IR spectra at each temperature (a) Pa (depending on the amount of H2O traces) which fixes the H2O molar flow rate and the adsorption equilibrium, (b) Ta which fixes the formation of the different adsorbed species according to their heats of adsorption and (c) the cooling rate which fixes the duration of the adsorption at each temperature. These parameters are dependent on the analytical setup and this explains the differences in literature data considering the number and intensity of the IR bands in the 3750-3600 cm-1 range.12,14,16 In the present study the amount of H2O impurities in helium is very low (the H2O traces are due to desorption from the tubes between the liquid nitrogen trap and the IR cell) whereas there is a long cooling stage (∼1 h) due to the thermal inertia of the IR cell. Adsorption of H2O at 300 K on the pretreated TiO2-P25 using FTIR Figure 2 shows the evolutions of the IR spectra at 300 K with time on stream ta after introduction of 0.25% H2O/He (200 cm3/min) on pretreated TiO2-P25 (inset A gives more details in the 3780-3660 cm-1 range). The modifications appear as the continuation of those observed during the cooling stage (Figure 1) with a strongly higher apparent rate of adsorption. Inset B indicates the progressive increase in the δH2O IR band of the adsorbed H2O species, which shifts from 1620 to 1635 cm-1 at the adsorption equilibrium for ta≥ 130 s (Figure 2, spectra f and g). In the range 3750-3600 cm-1, the introduction of H2O leads to the progressive disappearance of the IR band at 3720 cm-1 (not detected for ta> 80 s), indicating that this OH group is involved in H-bond with adsorbed H2O species in agreement with the strong increase in the broad IR band below 3600 cm-1. In parallel the IR bands at 3692 (B3)



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14 and 3673 cm-1 (B2) increase without any shift (breakthrough curve). It must be noted the disappearance of B1 and the increase of B3 and B2 at ta≈80 s whereas the steady state for the B4 and δH2O IR bands is obtained for ta ≥130 s, indicating that two types of adsorbed species must be considered. Inset A in Figure 2 shows that there is an isosbestic point at 3705 cm-1 for the decrease in B1 and increase in B2, suggesting that the disappearance of the OH group and the formation of H2O species characterized by the B3 IR band are linked. The overlap of the ν1 IR band of the H2Oads-L with that (B2) of the isolated OH group does not allow us determining if this group is modified during the adsorption of H2O. However, Finnie et al.38 have shown on TiO2 that the terminal OH (3730 cm-1) is involved in H-bonds with H2Oads species and not the bridging OH (3670 cm-1). For ta> 80 s, the B2 IR band at 3673 cm-1 becomes broader and shifts to 3666 cm-1 at the adsorption equilibrium, indicating interactions between strongly (on Lewis sites H2Oads-L) and weakly (i.e. H-bonded to O2- sites and/or H2Oads-L) adsorded species. After the adsorption equilibrium with Pa= 0.25 kPa, the increase in Pa to 1 kPa (see spectrum a in Figure 4) leads at the adsorption equilibrium to the increase in (a) the broad IR bands below 3600 cm-1 which overlaps the IR band 3666 cm-1 and (b) the δH2O IR band indicating that the amount of weakly adsorbed H2O species increases. Spectrum a in Figure 4 is similar to that observed at 300 K on TiO2-P25 by Takeuchi et al.15 after adsorption equilibrium with PH2O=18 torr who ascribe the IR band at 3690 cm-1 to a d-H at the end of polymeric chain of H2O. To improve the identification of the adsorbed H2O species at the adsorption equilibrium at 300 K, their amount must be known, for instance to determine if the formation of several multilayer of H2O must be take into account. Amount of adsorbed H2O species at 300 K using the M.S system On the M.S system, a fast (∼4 min) cooling to 300 K after the pretreatment procedure associated with a large amount of solid limits the impact of H2O traces on the adsorption


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15 properties of TiO2. Figure 3A gives the evolutions of the molar fractions of the gases during the switch He (200 cm3/min)  0.5% H2O/0.5% Ar/ He (200 cm3/min) at 300 K on pretreated TiO2-P25. The difference between the Ar and H2O molar fractions provides the total amount of weakly and strongly adsorbed H2O species; QTads(300 K, 0.5 kPa)= 562 µmol/g. Note that (a) the molar fraction of H2O is equal to 0 during several minutes indicating that the H2O molar flow rate controls the adsorption and (b) the surface concentration of adsorbed H2O species: 6.8 H2O/nm2 is consistent with the amount determined by Finnie et al.38 on a TiO2 film: 5 H2O/nm2 using hydrated air. Figure 3B shows that a large fraction of the adsorbed species desorb at 300 K during the switch 0.5% H2O/0.5% Ar/He (200 cm3/min)  He (100 cm3/min) (the lower helium flow rate favors the accuracy of the measurements): Qdes(300 K, 0.5 kPa) = 251 µmol/g which are readsorbed in Figure 3C (266 µmol/g) during the switch He (100 cm3/min) 0.5% H2O/0.5% Ar/He (200 cm3/min). This fraction corresponds to the weakly adsorbed H2O species (named H2Owads): Qwads = (300 K, 0.5 kPa)≈ 266 µmol/g, indicating that the amount of strongly adsorbed species is Qsads = (562-266) = 296 µmol/g (3.6 H2O/nm2). This amount is similar to the total amount of NH3 species adsorbed at saturation of two types of Lewis sites (L1 and L2 in the increasing order of acidic strength): 332 µmol/g,4 suggesting that the same sites are involved in the adsorption of the strongly adsorbed H2O species. The difference between the two quantities can be due to the desorption at 300 K of a small fraction of chemisorbed H2O species on the L1 sites which is consistent with the slow H2O desorption at the end of Part B in Figure 3. Considering the cross-sectional area of an adsorbed H2O molecule: σ= 0.105 nm2, obtained from liquid density of H2O at 298 K assuming spherical shape and hexagonal close packing:39-41 the H2Owads on TiO2-P25 (BET area= 55 m2/g) represent 0.3 monolayer (abbreviation ML) for Pa= 0.5 kPa and Ta= 300 K (total coverage with the strongly adsorbed species: 0.65 ML) which is significantly lower than a monolayer. However, it has been suggested that different interactions dependent on the



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16 nature of the solid lead to a more open packing in the adsorbed state increasing σ up to twice the theoretical value.39,40 Considering a factor of 2 on the σ value, the coverage of the surface by H2Owads species remains lower than the monolayer. It must be considered that all the IR bands in Figure 2 are not relevant of several multilayers (such as MAs in Ref. 17). At 300 K for PH2O= 0.5 kPa, (a) the strongly adsorbed species (296 µmol/g) seems formed on the Lewis sites (H2Oads-L species) without and with d-H according to the arrangement of the Ti4+ and O2/OH sites and (b) the H2Owads species (∼266 µmol/g, desorbing at 300 K) are H-bonded either on O2-/OH sites or/and on the H2Oads-L species forming a second layer. Similarly to ice clusters the H-bonded species may have d-H providing IR band in the 3750-3600 cm-1 range.15 Isothermal and temperature programmed desorption of the adsorbed H2O species Temperature programmed desorption of adsorbed H2O species is a conventional procedure to obtain supplementary data for the identification of the adsorbed species using FTIR.11,14-16 Figure 4A shows the evolution of the spectra during the switch 1% H2O/He → He at 300 K after the adsorption equilibrium. The spectra recorded after different isothermal desorption duration td have been shifted according to the Y axis to facilitate the presentation (Figure S1 of the supporting information provides another representation facilitating the comparison of the modification of the intensities of the IR bands with td). The introduction of He leads to the immediate (spectrum b) decrease in the broad B4 IR band below 3600 cm-1 and then to the B3 IR band at 3696 cm-1 which shifts to lower wavenumbers. For td= 32 s (spectrum c) this allows the detection of IR bands at 3663 (B2) and 3630 cm-1 (new IR band denoted B5) whereas B3 is detected at 3692 cm-1. Moreover a narrow IR band is detected at 3418 cm-1 (new IR band B6) overlapping the broad IR band below 3600 cm-1. This indicates that the progressive desorption of H-bonded H2Owads modifies the interactions with the strongly adsorbed species. These evolutions continue with the increase in td and for td= 212 s


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17 (a) the B3 is detected as a shoulder at 3689 cm-1 of the B2 which increases and shifts to 3667 cm-1, (b) B5 and B6 are not modified. For td ≈ 11 min, B2 splits in two IR bands with maximum at 3672 and 3650 cm-1. At quasi steady state (very slow decrease in the IR bands) for td≈ 1 h, IR bands are detected at 3679 (B2), 3653 (B7), 3630 (B5 is slightly decreased as compared to td=11 min) and 3418 (B6) cm-1 which overlaps the remaining broad B4 IR band. Inset of Figure 4A shows that the δH2O IR band decreases progressively with td associated to a shift from 1637 to 1622 cm-1 for td≈ 1h confirming that the modification of the IR bands in the 3750-3000 cm-1 range are due to the desorption of H2Owads species. The data in Figure 4A show that H2Owads are linked to the B3 (at 3696 cm-1) and B4 (below 3600 cm-1) IR bands. Takeuchi et al.,15 have ascribed an IR band at 3690 cm-1 after adsorption of H2O (18 torr) on TiO2-P25 and Al2O3 to a d-H at the end of weakly adsorbed polymeric H2O chains. However, the amount H2Owads lower than a ML does not support the view that numerous polymeric chains are formed (there is no quantitative data in Ref. 15). We suggest that B3 is a d-H IR band of isolated H-bonded species. Moreover, (a) during adsorption, this IR band seems associated to the disappearance on the IR band at 3702 cm-1 ascribed to an OH group of TiO2 considering the isosbestic point at 3705 cm-1 (inset A of Figure 2) and (b) the disappearance of the B3 by desorption at 300 K does not lead to the appearance of the OH IR band. This suggests that B3 (3696 cm-1) is the d-H IR band of a H2Owads species formed on strongly adsorbed H2O species (H2Oads-L) having an H-bond with the OH group giving the IR band at 3702 cm-1. Indeed this IR band reappears progressively at high temperatures during the TPD procedure (Figure 4B). Others H2Owads species H-bonded with O2-/OH sites of the surface may contribute to the broad IR band below 3600 cm-1. After one hour of desorption at 300 K, the increase in Td (TPD procedure, Figure 4B) leads to the decrease in the B5 IR band (Figure 4B) without any shift associated with the decrease in the B4 and δH2O IR bands (inset Figure 4B). At Td= 473 K the remaining B5 IR



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18 band is detected at a small shoulder of the B7 IR band. Note that the decrease in B5 is associated with a slight increase in the B2 and B7 IR bands which can be due to long range interactions between the different strongly adsorbed species. In the 3750-3600 cm-1 range the decrease in B5 is not associated to the decrease of a second IR band indicating that B5 is a dH IR band of a strongly adsorbed species. Considering the presence of the two L1 and L2 Lewis sites on the TiO2-P25,4 B5 is ascribed the adsorption of H2O on the L1 sites (denoted H2Oads-L1). The second hydrogen form H-bond with O2-/OH sites explaining that the broad B4 IR band decreases in parallel to the B5 and δH2O IR bands. Finally for Td> 473 K, the decrease in the remaining different IR bands are due to the desorption of the strongly adsorbed H2O species on the L2 sites denoted H2Oads-L2 with and without d-H according to the arrangement of the L2 and O2-/OH sites. The IR band at 3411 cm-1, which overlaps the remaining broad B4 IR band, is associated to this species. The vibrations of this species presents some similarities with those of small ice clusters.36 Note that the progressive decrease in the amount of H2OadsL2

leads to the parallel increase in the intensity of the IR band of the OH group at 3702 cm-1

(Figure 4B) and at Td=673 K the spectrum is similar to that of the pretreated solid. Finally, the data in Figures 1-4 lead to the following identification of the adsorbed species after adsorption of H2O at Ta = 300 K for Pa = 1 kPa (a) H2Owads species (desorbing at 300 K), in an amount lower than a monolayer, H-bonded on O2-/OH sites and H2Oads-L species (these species contribute to the broad B4 IR Band below 3600 cm-1 whereas that adsorbed on the H2Oads-L species has a d-H IR band at 3692 cm-1), (b) a more strongly adsorbed species of the L1 Lewis site (H2Oads-L1) having d-H due to a H-bond with O2-/OH sites of the surface and (c) a strongly adsorbed H2O species on the L2 Lewis sites (H2Oads-L2) with and without d-H according to the arrangement of the L2 and O2-/OH sites. This species interacts with the OH group characterized by the IR band at 3702 cm-1. At 673 K in helium all the H2Oads species are desorbed and only two isolated OH groups are present maybe associated to the H2O


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19 dissociation of H2O traces. The IR spectra of the adsorbed species during adsorption equilibrium at different temperatures confirm these assignments.

Adsorption of H2O on TiO2-P25 at different adsorption temperatures using FTIR The dotted line spectra in Figure 5A shows the evolution of the IR bands of adsorbed H2O species during the first increase in Ta in the presence of 0.25% H2O/He. There are qualitative similarities with the TPD experiments of Figure 4B. In the 300-360 K range, the broad B4 IR band in the 3600-2800 cm-1 range decreases significantly associated with those of the IR bands at 3693 cm-1 (not detected for Ta > 363 K) and 1635 cm-1 (inset Figure 5A) which shifts to lower wavenumbers (1622 cm-1 at 398 K). This indicates that the amount of the H2Owads species decreases to ∼0 at Ta ≈360 K due to its low heat of adsorption in agreement with its disappearance by desorption at 300 K (Figure 4A). This suppresses the interactions between this H2Owads and H2Oads-L species and the IR spectra at Ta = 363 K for PH2O = 0.25 kPa is similar to that after 1 h of desorption at 300 K, with three main IR bands at 3673, 3655 and 3630 cm-1. Moreover, the strong decrease in the B4 IR band allows the detection at Ta ∼ 400 K of the B6 IR band at 3412 cm-1 associated with a small IR band at 3270 cm-1 ascribed to the overtone 2 δH2O of strongly adsorbed H2Oads-L species. At Ta = 495 K, the IR band at 3630 cm-1 is no more detected and the IR spectra is similar to that observed in Figure 4B for Td ≈ 423 K. For Ta > 495 K, the remaining IR bands decrease progressively with the increase in Ta however, those at 3673 and 3655 cm-1 are more overlapped with the increase in Ta than during TPD. The δH2O IR band decreases progressively with the increase in Ta in the 300-620 K range until its disappearance (insets of Figure 5A-B5). At Ta = 673 K the IR spectrum in the presence of 0.25% H2O/He (Figure 5B spectrum l) presents a main IR band at 3662 cm-1 (B2) with a shoulder at 3702 cm-1 (B1). The intensity of these IR bands and the B2/B1 ratio are higher than those after the pretreatment in helium (Figure 5B spectrum



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20 m). This indicates that there is an adsorption equilibrium for the dissociated H2O species controlled by the adsorption pressure Pa. It must be noted that for Ta> 495 K, the broad B4 IR band below 3600 cm-1 is still detected, indicating that a fraction of the H2Oads-L2 species are involved in H-bonds with O2-/OH sites which is consistent with the concerted mechanism of the H2O dissociation proposed by Henderson.29 Repeatability of the experimental AEIR data for the adsorption of H2O on TiO2-P25 Previous works on different gas/solid systems have shown that during the first increase in Ta different surface processes might contribute to the evolution of the IR spectra of the adsorbed species such as surface reconstruction for the adsorption of CO on supported metal particules42,43 and modification of the ratio between NH3ads-L and NH4+ species on TiO2 and V2O5 or WO3 supported on TiO2.4-6 These processes are often irreversible (without a new pretreatment) and the modifications of the IR spectra during a second increase in Ta are only due to the adsorption equilibrium. This has been verified for the adsorption of H2O on TiO2P25 by performing heating/cooling cycles denoted H(TaM)/C(300 K) where TaM is the highest adsorption temperature with PH2O = 0.25 kPa. The IR spectra are identical for the two heating stage (a) in the 4000-1100 cm-1 range for Ta ≥ 495 K (Figure 5B, same comment for a third heating stage, result not shown) and (b) in the position and intensity of the δH2O IR band whatever Ta (insets Figure 5A and B). For Ta< 498 K, the main differences between the IR spectra recorded during the first (dooted line spectra) and second (full line spectra) heating stage are (Figure 5A) (a) the IR bands at 3412 and 3270 cm-1 associated with the H2Oads-L2 species are detected at lower Ta for the second heating and (b) the IR band at 3630 cm-1 (H2Oads-L1 species) is more clearly detected at low temperatures (Figure 5A spectra a-e) and it disappears for Ta > 495 K. This seems indicate that the interactions between adsorbed H2O species are dependent on how the adsorption equilibrium is obtained, either by adsorption at Ta = 300 K or by the decrease of Ta from 673 K. The explanation of these differences is not


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21 the purpose of the present study. However, considering these observations and even if the δH20 IR band is not modified by the number of H/C cycles, the AEIR method has been applied after a first H/C cycle in H2O. Choice of the IR bands for the application of the AEIR method In agreement with literature data,17 the FTIR and MS data have shown that the adsorption of H2O at 300 K on TiO2-P25 for Pa ≤ 1 kPa leads mainly to three adsorbed undissociated species named in the decreasing order of stability H2Oads-L2, H2Oads-L1 (L2 and L2 two Lewis sites) and H2Owads (which represent different H-bonded species on O2-/OH sites of TiO2 and on the H2Oads-L species) in amount lower than a ML. These species have been identified by their characteristic IR bands in the 3750-3600 range in particular their d-H IR bands (this concerns H2Owads species H-bonded to H2Oads-L, the main part of the H2Oads-L1 species and a fraction of the H2Oads-L2). Moreover, it has been shown that there are different interactions between H2Owads and H2Oads-L modifying the positions and intensities of their IR bands in the 3750-3000 cm-1 range according to their respective coverages. The different H2Oads species are not distinguishable by their δH2O IR bands: they all contribute to the intensity of a well defined IR band at a position which varies with the total coverage of the surface from 1635 cm-1 at 300 K in the presence of H2Owads to ∼ 1620 cm-1 at Ta > 495 K with the H2Oads-L2 species. The AEIR method is based on the quantitative use of the evolutions of the area of IR bands characteristic of each adsorbed species with Ta in isobaric condition. In the 3750-2800 cm-1 range, if the IR bands of H2Oads species allow their identification, their strong overlaps associated with the modification of their positions and intensities, due to the different interactions, impose complex decompositions with large uncertainties preventing their use for the AEIR method. This is not the case for the δH2O IR, band due to the contribution of the different H2Oads species, which is well defined whatever Ta and Pa facilitating its



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22 quantification. Moreover, the selection of this IR band leads to a situation similar to that encountered with the common δas IR band at 1596 cm-1 of the NH3ads-L1 and NH3ads-L2 species and we have shown how a development of the AEIR method allows us obtaining the individual heats of adsorption of the two species. This new procedure has been developed using the δH2O IR band and considering the contributions of the three H2Owads, H2Oads-L1 and H2Oads-L2 species. This development concerns particularly the following point: for the NH3 adsorption, it has been shown that the area of the δas IR band of the NH3ads-L species at 300 K is not modified for Pa in the 0.1-1 kPa range indicating the saturation of the Lewis sites. This permits to know with accuracy the value of AM providing the evolution of the average coverage of the NH3ads-L species as a function of Ta and Pa using eq. ES1 of the supporting information (see also Ref 4). This is not the situation for the δH2O IR band because the amount of H2Owads species increase at 300 K in the adsorption pressure range allowed in the FTIR system (Pa ≤ 2 kPa). An adaptation of the AEIR procedure has been used to overcome this situation.

3.2 Heats of adsorption of the three H2Oads species on TiO2-P25 using the AEIR method Area of the δH2O IR band at saturation of the surface at 300 K. Taking into account that the saturation of the H2Owads species can not be obtained for Pa ≤ 2 kPa, the estimation of AM used in eq. ES1 has been obtained according to the following procedure. After the adsorption equilibrium at 300 K using Pa = 2 kPa (ascertained by a constant intensity of the δH2O IR band with the adsorption duration ta), Pa is decreased step by step to 1, 0.8, 0.5 and 0.2 kPa until obtaining of new adsorption equilibrium at each adsorption pressure as shown in Figure 6. The estimation of AM for the δH2O IR band is obtained assuming that the average coverage of the H2Oads species follows the Langmuir model:


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23

θ=

(T ) P K A(300 K , Pa ) H 2O a a = A 1 + K (T ) P M H 2O a a

(1)

where KH2O(Ta) is an averaged adsorption coefficient for the H2Oads species. According to eq. 1, the plot of [1/(A(300 K, Pa)] = f(1/Pa) must be a straight line providing AM as observed in the inset of Figure 6 for Pa in the range 0.2-1 kPa, indicating that the average coverage of the adsorbed H2O species for Pa = 0.25 kPa and Ta = 300 K is ∼ 0.6. Note that A(300 K, 2 kPa) is not consistent with eq. 1 because the Langmuir model is an approximation only valid in narrow adsorption pressure ranges (it is shown below that the H2Oads species follows the Temkin model). Moreover for 2 kPa the amount of adsorbed species is close to 1 ML. This means that the AM value includes different approximations and their impacts on the heats of adsorption of the H2Oads species are discussed below. It must be noted that the quantitative exploitation of the δH2O IR band is based on the fact that there is a linear relationship between its area and the total amount of H2Oads species on the TiO2-P25 surface, as shown in previous studies for the δas IR band of adsorbed NH3ads-L species.4-6 This is justified below considering measurements performed with the M.S system. However, this linear relationship has also been verified by (a) Finnie et al.38 on an anatase film desorbed 10 min at 723 K and (b) Wang et al.41 on a boehmite by plotting the area of the δH2O IR band versus the amount of H2Oads species measured by gravimetric methods. M.S verification of the Langmuir approximation for narrow adsorption pressure range The impact of the adsorption pressure on the amount of adsorbed H2O species has been quantified using the M.S system as follows: after the adsorption equilibrium for Pa = 0.5 kPa (Figure 3 Part C), the switch 0.5% H2O/0.5% Ar/He (200 cm3/min)  0.23% H2O/0.23% Ar/He (130 cm3/min) indicates (Figure 3 Part D) the net desorption of 85 µmol/g of H2O mainly due to weakly adsorbed H2O species leading to QTads(300 K, 0.23 kPa) = 477 µmol/g and Qwads (300 K, 0.23 kPa)≈ 181 µmol/g. Similar experiments have been performed for Pa =



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24 0.17 kPa leading to QTads(300 K, 0.17 kPa) = 450 µmol/g and Qwads(300 K, 0.17 kPa) ≈ 154 µmol/g. These values can be compared to the Langmuir model using θ = Qwads (300 K, Pa)/QMx in eq. 1. The plot of [1/Qwads(300 K, Pa)] = f(1/Pa) provides a straight line (see Figure S2 of the Supporting Information) in agreement with the Langmuir adsorption model leading to QMw = 417 µmol/g at the saturation of the weakly adsorbed species. This represents 0.48 ML (σH2O = 0.105 nm2) confirming the absence of multilayer at 300 K. It must be noted that the relationship [1/QTads(300 K, Pa)] = f(1/Pa) provides also a straight line leading to QMT = 645 µmol/g for the average of the strongly and weakly H2Oads species (0.74 ML). This is due to the fact that whatever the complexity of an adsorption process (in the present case the formation of different weakly and strongly adsorbed H2Oads species), the Langmuir model can be applied in narrow Ta and Pa ranges. This explains that this model is often used in the kinetic formalism of catalytic reactions. Heats of adsorption of the adsorbed H2Oads species Symbols  in Figure 7 give the evolutions of the average coverage of the H2Oads species with the increase in Ta for Pa = 0.25 kPa using the area of the δH2O IR band (insets of Figure 5A-5B). It is not possible to fit the experimental data considering a single H2Oads species (eqs. ES2-ES3). Curve a is obtained using eqs ES2-ES4 assuming two adsorbed H2Oads species having different heats of adsorption (named H2Oads1 and H2Oads2 in the increasing order of stability) with the following parameters: Eads1(1) = 45 kJ/mol, Eads1(0) = 61 kJ/mol, Eads2(1) = 61 kJ/mol, Eads2(0) = 110 kJ/mol, x1 = 0.7 and x2 = 0.3. Curves b and c provide the evolution of the coverage of the two species using the heats of adsorption from curve a. The H2Oads1 species corresponds to the average of the H2Owads and H2Oads-L1 species. This explains that, compared to the latent heat of liquefaction of H2O ∼43.8 kJ/mol at 298 K,44,45 the heats of adsorption of H2Oads1 is (a) slightly higher at full coverage (as expected for the first layer of physisorbed species) which is dominated by the H2Owads species and (b)


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25 significantly higher at low coverage due to the contribution of the H2Oads-L1 species. The H2Oads2 species corresponds to the H2Oads-L2 species formed on the L2 Lewis sites. The impacts of the approximations linked to the estimation of AM on the heats of adsorption of the H2Oads-L2 (which is that of interest for the competitive H2O/NH3 chemisorption in the experimental conditions of the NH3-SCR) are limited. For instance, (a) decreasing AM to obtain an average coverage of the H2Oads species of 1 at 300 K for PH2O = 0.25 kPa leads to the following values: Eads1(1) = 58 kJ/mol, Eads2(0) = 59 kJ/mol, EL2(1) = 61 kJ/mol, EL2(0) = 110 kJ/mol, x1= 0.5 and x2= 0.5 and (b) increasing in AM to obtain an average coverage of 0.5 at 300 K, leads to the following values: Eads1(1) = 40 kJ/mol, Eads1(0) = 61 kJ/mol, EL2(1) = 61 kJ/mol, EL2(0) = 110 kJ/mol, x1 = 0.75 and x2 = 0.25. This shows that the approximations involved in the AM value modify the heats of adsorption of the H2Oads1 species (average of H2Owads and H2Oads-L1) but have no impact on the heats of adsorption of H2Oads-L2. Taking into account that at 300 K the δH2O IR band is due to the contribution of three H2Oads species: H2Owads, H2Oads-L1 and H2Oads-L2, eq. ES4 coming from the measurement of the heats of adsorption of the NH3ads-L1 and NH3ads-L2 species can be modified as follows:

θ A (Ta ) =

A(Ta ) = xwads θwads(Ta)+ xL1 θL1(Ta) + xL2 θL2(Ta) AM

(2)

where xwads xL1 and xL2 are the contribution of the H2Owads, H2Oads-L1 and H2Oads-L3 species respectively to the δH2O IR band area at 300 K and θwads, θL1, θL2 their coverage accordin to eqs. ES3-ES3. Equation 2 provides curve d in Figure 7, which is overlapped with curve a, with the following parameters: xwads = 0.42, Ewads(0) = 55 kJ/mol, Ewads(1) = 44 kJ/mol, xL1 = 0.28, EL1(0) = 61 kJ/mol, EL1(1) = 55 kJ/mol, xL2 = 0.3, EL2(0) = 110 kJ/mol, EL2(1) = 61 kJ/mol. Note that calculations using eq. 2 have no impact of the heats of adsorption of the H2Oads-L2 species which is that of interest in the NH3-SCR. Curves e, f, g in Figure 7 give the evolution (es. ES2-ES3) of the coverage of the H2Owads, H2Oads-L1 and H2Oads-L2 with Ta for Pa



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26 = 0.25 kPa according the parameters used in eq. 2 to obtain curve d. Transient experiments using the M.S system provide additional materials supporting the exploitation of the IR spectra in particular the approximations linked to the value of AM. TPAE method. After the adsorption of 0.5% H2O/0.5% Ar/He (gas flow rate 200 cm3/min) the adsorption temperature Ta is increased (∼ 5 K/min), leading to the decrease in the adsorption equilibrium coverage of the adsorbed species according to a net desorption rate as shown by inset A of Figure 7. This allows determining the evolution of the average coverage of the TiO2 surface (including undissociated and dissociated H2O species) using eq. ES5 of the Supporting Information as shown by symbols  in inset B of Figure 7. Curve a in inset B of Figure 7 is obtained considering two adsorbed species with the following parameters: E1(1) = 45 kJ/mol, E1(0) = 63 kJ/mol, E2(1) = 67 kJ/mol, E2(0) = 98 kJ/mol, x1 = 0.64 and x2 = 0.36. These values are consistent with those used to obtain curve a in Figure 7 using the δH2O IR band. The slightly lower value of E2(0), as compared to EL2(0) = 110 kJ/mol, results from the fact that at high temperatures, the accuracy of the TPAE method is limited because of the small difference between the inlet and oulet H2O molar fractions. This is due to the low heating rate imposed by the heats of adsorption of the H2Owads species (which have a high net desorption rate) to limit the increase in Pa to accept the assumption that Pa remains quasi constant during TPAE as considered in the mathematical formalism of the method.23 This inaccuracy reduces the amount of H2Oads-L2 species at low coverages and decreases E2(0) as compared for instance to the AEIR method. However, the fact that the heats of adsorption of the AEIR (using an IR band characterizing molecularly adsorbed H2Oads species) and TPAE methods (which does not allow the distinction between H2Oads and dissociated adsorbed H2O species) are consistent (a) confirms that the H2Oads species dominates the H2O adsorption (the amount of dissociated H2O species is very limited on the TiO2-P25 (see below)) and (b)


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27 validates the procedure of quantification of the δH2O IR band according to eq. ES1 in line with literature data.38, 41 Temperature Programmed desorption of adsorbed H2O species The inset of Figure 3 gives the TPD spectra of the adsorbed H2O species obtained as follows; after the adsorption equilibrium at 300 K using 0.17% H2O/0.17% Ar/He, a desorption in helium is performed during 55 min (a longer desorption duration than in Figure 3B allowing us to consider that the H2Owads species have been removed from the surface) then the temperature is increased (23 K/min). The total amount of H2O production during the TPD (inset Figure 3) is 276 µmol/g. It can be observed that the molar fraction of H2O increases as soon as Td increases leading to a first peak at TM1 = 368 K that can be ascribed to the H2Oads-L1 species. Then a second peak, overlapping the first peak, is observed at TM2 = 427 K that can be ascribed to the H2Oads-L2. There is a third H2O production leading to a small and broad shoulder at TM3 = 523 K overlapping the second peak indicating the presence of a strongly adsorbed species which can be tentatively ascribed to dissociated H2O in line with spectra f and g in Figure 4B. Indeed simple calculations based on modeling of TPD spectra with readsorption23 (which can not be prevented in the experimental conditions of the inset of Figure 3)46,47 show that the H2Oads-L2 species (assuming non activated chemisorption) is totally desorbed at Td ≥ 620 K whereas inset of Figure 3 indicates a H2O desorption until 700 K. An approximate decomposition of the second and third peaks indicates that the amount of dissociated H2O species is ≈ 20 µmol/g of catalyst leading to an amount of H2Oads of 276-20 ≈ 256 µmol/g which is consistent with the amount of strongly adsorbed species estimated from the experiments in Figure 3A and 3B: 296 µmol/g. The difference arises from the longer purge in helium at 300 K before TPD which leads to desorption of a fraction of the H2Oads-L1 species. The low amount of dissociated H2O species on TiO2-P25 is consistent with the FTIR



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28 results (Figure 1) and the conclusions of Henderson indicating that H2O dissociation is limited to specific TiO2 faces and defect sites.28,29 3.3 Heat of adsorption of H2Oads-L2 on TiO2-P25 using the Clausius-Clapeyron Equation Curves c and e in Figure 7 indicate that the coverages of the H2Oads-L1 and H2Oads-L2 species are of 0.8 and ∼0 respectively at Ta = 400 K for Pa = 0.25 kPa. This allows us studying the heats of adsorption of the H2Oads-L2 which is that of interest in relationship with the NH3SCR from the Clausius-Clapeyron equation (see eq (6) in Ref. 4) using the change with Ta of the δH2O IR band for two isobars. Taking into account the impacts of the experimental uncertainties on the Clausius-Clapeyron method, the two adsorption pressures must be significantly different such as Pa= 1000 and 35 Pa in the present study. Calculations similar to curves c and e in Figure 7 indicate that Ta must be equal to 360 and 412 K for Pa = 35 and 1000 Pa to obtain simultaneously a coverage of 0.8 for H2Oads-L2 and ∼0 for H2Oads-L1. The experiments have been performed according to the following procedure after the pretreatment of TiO2-P25 and a first H(673 K)/C(300 K) cycle with 1% H2O/He, a second heating is performed using 1% H2O/He. Then, after cooling in H2O, a third heating stage is realized using 0.035% H2O/He. Figure 8 compares the IR spectra recorded at Ta = 443 K and 523 K with Pa = 1 kPa (spectra a and b) and Pa = 0.035 kPa (spectra c and d). Considering from curve c in Figure 7 that the coverage of the H2Oads-L2 species is 0.8 at Ta = 412 K and Pa = 1000 kPa then symbols

 and  in inset A of Figure 8 gives the evolution with Ta of the experimental coverage of the H2Oads-L2 species for Pa = 35 and 1000 Pa. These data provide the change of the isosteric heats of adsoption of the H2Oads-L2 species with its coverage (in the range 0.8-0.1) from the Clausius-Clapeyron equation as shown in inset B of Figure 8 which confirms the linear relationship between the heats of adsorption and the coverage of the H2Oads-L2 species as considered in the Temkin model. Moreover, the isosteric heats of adsorption are higher by ∼ 5


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29 kJ/mol than those of the AEIR method confirming the validity of the different approximations associated to this method. Finally, curves a and b in the inset A of Figure 8, which overlap the experimental data, are obtained from eqs. ES2-S3 considering a single adsorbed species (H2Oads-L2) with (a) Pa = 1000 and 35 Pa and (b) heats of adsorption of 59 and 114 kJ/mol at high and low coverages. These values are consistent with those obtained by curves a and d in Figure 7. It must be remembered that the advantage of the AEIR procedure as compared to the Clausius-Clapeyron equation is that it provides the mathematical expression of the adsorption coefficient and the relationship between the coverage and the experimental parameters Ta, Pa (i.e. Temkin model) which are crucial for the development of the EMA of the NH3-SCR.

3.4 Adsorption of H2O on TiO2-DT51 and Heats of adsorption of H2Oads species Adsorption of H2O on the pretreated TiO2-DT51 solid Similarly to TiO2-P25, traces of H2O are adsorbed on DT51 during the cooling stage from 673 K, as shown by inset A of Figure 9, which compares the IR bands in the range 37503000 cm-1 at 673 K and 300 K after cooling in He. A 673 K, only OH groups are detected with a main IR band at 3659 cm-1 and two shoulders at 3687 and 3621 cm-1 associated with a strong IR band at 1373 cm-1 due to S=O bond of sulfate groups.5 During cooling the main IR band increases and shifts to 3666 cm-1 at 300 K whereas IR bands at 3390 cm-1 overlapping a broad IR band below 3600 cm-1 and 1618 cm-1 (δH2O not shown) appear and increase for T< 473 K indicating the formation of strongly adsorbed H2Oads species on Lewis sites as observed on TiO2-P25 (Figure 1). However on this solid the IR band at 3390 cm-1 was not observed under helium at 300 K. The adsorption of 0.25% H2O/He at 300 K leads at the adsorption equilibrium to (a) a shift of the S=O IR band to lower wavenumbers (below 1300 cm-1) associated to broadening (Figure 9 spectrum a) and (b) different IR bands similar to those observed on TiO2-P25. There are (a) a strong IR band at 1631 cm-1 due to the δH2O vibration



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30 of different H2Oads species and (b) an IR band at 3690 cm-1 associated with a broad IR band below 3600 cm-1 due to weakly adsorbed H2O species (Inset B of Figure 9). Note that this IR band is identical to that of TiO2-P25 (Figure5A) and Al2O3,15 indicating that the nature of the support has no significant impact. This is consistent with its assignment to a d-H IR band of H2Owads species H bonded (broad IR band below 3600 cm-1) to H2Oads-L species. Figure S3 of the supporting information shows that during the adsorption of H2O at 300 K, the S=O IR band of solid at 1373 cm-1 disappears progressively with time on stream without any shift confirming that the adsorption is limited by the H2O molecular flow rate and proceeds according to a breakthrough curve. Figure 9 gives the evolution of the IR spectra in the range 1800-1250 cm-1 with the increase in Ta using 0.25% H2O/He. Similarly to TiO2-P25, the δH2O IR band of the H2Oads species decreases and shifts progressively from 1631 cm-1 at 300 K to 1619 cm-1 at 423 K and it is still detected with a very low intensity at 673 K (Figure 9i). The decrease in the amount of H2O species leads in parallel to the increase in the S=O IR band which becomes sharper and shifts to higher wavenumbers (Figure 9). At 673 K, it is detected at 1364 cm-1 (Figure 9i) in the presence of H2O whereas after the pretreatment procedure in helium at this temperature it is situated at 1370 cm-1 (Figure 9j) indicating that a small amount of H2Oads species disturbs the all sulfate groups as observed with the NH3ads-L2 species,4 due to long range interactions. In the 3800-2800 cm-1 range (inset B Figure 9) the increase in Ta leads to the decrease in (a) the IR band at 3690 cm-1 which disappears at 373 K (confirming that it is due to a H2Owads species H bonded to the H2Oads-L species) and (b) the broad IR band below 3600 cm-1. This allows the detection of two strongly overlapped IR bands at 3666 and 3648 cm-1 which are differently modified by the increase in Ta as shown in more details Figure 10. The decrease in the IR band 3648 cm-1 for Ta > 300 K, leads to the increase in the IR band at 3666 cm-1 as observed on TiO2-P25 (Figure 4B, the IR band at 3630 cm-1 decreases whereas those at 3676


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31 and 3653 cm-1 increase). This leads us considering that the IR bands at 3648 and 3667 cm-1 (which overlaps the IR band due to the OH groups of DT51) are due H2Oads-L1 and H2Oads-L2 species respectively adsorbed on the L1 and L2 Lewis sites of TiO2-DT51 revealed by the NH3 adsorption.5 Figure 10 shows that for Ta> 473 K only the H2Oads-L2 species is present on the surface and that similarly to TiO2-P25 it is associated with two well defined IR bands at 3390 and 3275 cm-1 (overtone 2 δH2O) overlapping the broad IR band of the H-bonds. Note that if the positions of the overtone are similar on TiO2 -P25 and -DT51 those of the second IR band differ significantly 3412 (Figure 5) and 3390 cm-1 on P25 and DT51 respectively. This reveals the impact of differences in the surface composition (presence of sulfate groups on DT51) and the Ti+δ/O2-/OH site arrangements. Spectra i and j of the inset of Figure 9 recorded at 673 K in the presence and in the absence of H2O show that similarly to TiO2-P25, the IR bands of the OH groups are higher by a factor ∼2.5 in the presence of H2O indicating that there is adsorption equilibrium for dissociated adsorbed H2O species. Moreover, similarly to TiO2P25 (Figure 5), H(673 K)/C(300K) cycles in the presence of 0.25% H2O/He have no impact on the IR band of the H2Oads at 1631 cm-1 whereas the IR band at 3390 cm-1 is detected at lower temperatures as compared to the first increase in Ta (result not shown). Similarly to TiO2-P25 the overlaps of the IR bands in the range 3700-3000 cm-1 prevent their use for the AEIR method. The evolution of the IR band at 1631 cm-1 in Figure 9 has been selected to determine the heats of adsorption of H2Oads-L species. The total and reversible amount of H2O adsorbed at 300 K on DT51 using the M.S system (results not shown) has been measured using 0.2% H2O/0.2% Ar/He: QTads(300 K, 0.2 kPa) = 864 µmol/g with Qwads(300 K, 0.2 kPa) = 340 µmol/g. The difference in the amounts of strongly adsorbed H2O species at 300 K on TiO2 -DT51 and -P25 (ratio of 524/296 = 1.8) is mainly due to the difference in the BET area (ratio 80/50∼1.6). Heats of adsorption of H2Oads species on TiO2-DT51



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32 Similarly to TiO2-P25 an estimation of AM on TiO2-DT51 has been obtained according to the procedure described in the Figure 6 in the 0.1-1 kPa Pa range and Ta = 300 K. After a first H(673 K)/C(300 K) cycle, symbols  and  in Figure 11 give the evolutions of the average coverage of the H2Oads species during the heating and cooling stage of the second cycle respectively showing the good repeatability of the experiments. Curve a which overlaps the experimental data is obtained considering the presence of two adsorbed H2Oads species (named ads1 and ads2) using eqs ES2-ES4 with the following paramteres Eads1(1) = 45 kJ/mol, Eads1(0) = 61 kJ/mol, Eads2(1) = 61 kJ/mol, Eads2(0) = 114 kJ/mol, x1 = 0.65 and x2 = 0.35. The heats of adsorption of the two H2Oads species are similar to those determined on TiO2-P25 whereas their proportions are slightly different. The H2Oads2 species corresponds to the H2OadsL2

which is formed on the L2 Lewis sites whereas H2Oads1 represents the average of the

H2Owads and H2Oads-L1 species. Considering that three H2Oads species are present on the surface then eq. 2 and eqs ES2-ES3 provides a curve overlapped with the experimental data (inset Figure 11) with the following parameters: xwads = 0.35, Ewads(0) = 50 kJ/mol, Ewads(1) = 45 kJ/mol, xL1 =0.30, EL1(0) = 60 kJ/mol, EL1(1) = 54 kJ/mol, xL2 = 0.35, EL2(0) = 114 kJ/mol, EL2(1) = 61 kJ/mol. This confirms that the heats of adsorption of the three H2Oads species are not significantly different on TiO2 -P25 and -DT51 as observed previously for the NH3ads-L1 and NH3ads-L2 species on the two solids. However, the heat of adsorption of the H2Oads-L2 species is higher by ∼ 4 kJ/mol on DT51 than on –P25 explaining that the δH2O IR band is observed with a small intensity at 673 K on TiO2-DT51. 3.5 Comparison of the individual heat of adsorption of the adsorbed H2O species. Table 1 summarizes the heats of adsorption of the three undissociated H2Oads species on the TiO2 -P25 and -DT51 supports and their contribution to the δH2O IR band at 300 K. The heats of adsorption in Table 1 can be compared to experimental and theoretical literature. Comparison with experimental data on dispersed TiO2


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33 They are mainly obtained by TPD and microcalorimetry methods. The comparison with the present study must take into account that the AEIR method provides the heats of adsorption of identified H2Oads species whereas neither TPD methods nor microcalorimetry allow a net identification of the nature of the adsorbed species. Using TPD and according to the experimental conditions the differentiation between the two types of Lewis sites can be difficult (see inset Figure 3). Moreover, microcalorimetry provides an average of the heats of adsorption the different adsorbed species and may include the heat involved in reactions parallel to to adsorption (i.e. the surface reconstruction). Finally, it has been shown that using microcalorimetry, the adsorption equilibrium is not reached in the whole solid sample (particularly at low temperatures for strongly adsorbed species) for the measurement of the differential heats of adsorption.48 For TPD experiments with TiO2 powders, we have considered literature data using a mathematical formalism including free readsorption because this process can rarely be prevented in these experimental conditions.46,47 For TPD performed in the experimental conditions of surface sciences, we assume that the activation energy of desorption is equal to the heats of adsorption (non activated chemisorption). Egashira et al.49 have used TPD procedures with a thermal conductivity detector to study the adsorption of H2O at 300 K on home made anatase and rutile TiO2 solids pretreated 1 h at 873 K in helium. They observe three overlapped TPD peaks which provide activation energy of desorption of 36, 64 and 104 kJ/mol consistent with the heats of adsorption of the H2Owads, H2Oads-L1 and H2Oads-L2 in Table 1 taking into account that the authors have not considered the impact of the coverage. The authors tentatively ascribe the different peaks in the increasing order of stability to physisorption, H2Oads species on surface oxygen ions through hydrogen bonds and chemisorption on Ti4+ ions through coordination boond49 whereas we consider that the two more strongly adsorbed species are formed on different Ti+δ Lewis sites in agreement with the adsorption of NH3 on the two TiO2 solids.4,5 Note that the authors mention that a small



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34 fraction of the more strongly adsorbed species is involved in the dissociative chemisorption of H2O49 in agreement with the present study. Srnak et al.50 have studied by TPD in vacuum the adsorbed H2O species formed on a TiO2 deposit (sputtering of TiO2 from Toho Titanium) on a tungsten foil support (200-250 nm layer of TiO2) after adsorption in the range 120-150 K. Two peaks are observed leading to activation energies of desorption of 46 kJ/mol and 75 kJ/mol. The first peak is consistent with the H2Owads species and the second appears as the average of the heats of adsorption of the two H2Oads-L species as a function of their coverage. Lechenko et al.51 have studied the differential heat of adsorption of H2O at 298 K by microcalorimetry, on a series of anatase (three samples) and rutile (two samples) TiO2 solids of different BET surface area. The solids are desorbed during 3 days at 373 K before adsorption. Simple calculations using the values of Table 1 indicate that the two H2Oads-L species are fully desorbed for the pretreatment procedure of Ref. (51). Whatever the TiO2 solids, the authors show that the differential heat of adsorption decreases with the increase in the H2O coverage from 150 kJ/mol at ∼0 H2O/nm2 to a value slightly lower that the heat of H2O condensation at high coverages (7 H2O/nm2).51 However, for a small increase in the coverage (from 0 to ∼0.5 and ∼0.2 H2O/nm2 on rutile and anatase TiO2 respectively), the heat of adsorption decreases strongly from 150 kJ/mol to ∼120 kJ/mol which is ascribed to a small amount of dissociative H2O chemisorption. The variation of the heat of adsorption between 120 to ∼40 kJ/mol,51 for molecularly adsorbed H2O species is consistent with the values of Table 1. Harju et al.52 have performed a similar study using plasma sprayed TiO2 (Amperit1 782.1) pretreated or not 12 h in air at 1273 K. Before H2O adsorption at 300 K, two drying temperatures have been used 383 K and 673 K. On the solids pretreated at 1273 K and dried at 673 K the heat of adsorption decreases with the amount of H2O adsorption from ∼125 to ∼45 kJ/mol at ∼0.5 and 5 H2O/nm2 respectively whereas the others solids provide similar profiles with a lower heats of adsorption at low coverages (∼ 85 kJ/mol).


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35 Comparison with surface sciences data They are numerous surface sciences studies (particularly using TPD methods) dedicated to the adsorption of H2O on well defined TiO2 surfaces. Hugenschmidt et al53 have studied the adsorption of H2O on TiO2 (110) with different concentrations of defects created by annealing in the temperature range 900-1000 K. On the perfect TiO2 surface the adsorption of H2O at 100 K gives two peaks with maximum at (a) 160 K ascribed to multi layers of physisorbed H2O and (b) in the range 250-350 K according to the coverage of the surface ascribed to molecularly adsorbed H2O species on Ti4+. There is a tail in the second peak for T > 430 K ascribed to a small amount of dissociated H2O species. On defect surfaces this tail is more pronounced providing a broad peak at 500 K with an amount representing 10% of the second peak. Using a first order kinetic model the authors found that the activation energy of desorption Ed of the species producing the second peak varies linearly with the coverage θ according to Ed(θ) = (71.1 - 8.8 θ) kJ/mol with a preexponential factor of 1012 s-1 (a preexponential value of kT/h≈1013 s-1 as considered in the formalism of eq. ES3 increases the value by ∼10 kJ/mol). This linear relationship is consistent with the Temkin model whereas the heats of adsorption are in the range of those of the H2Oads-L2 species. Brinkley et al.54 have performed a similar study on three well-defined surface configurations of TiO2 (110) (1×1, 1×2, and Ar-sputtered) using TPD and modulated molecular beam scattering. The three surfaces provide similar data except at very low coverage (between 10-2 and 10-4 monolayer) for the sputtered surface. Three TPD peaks are observed at 160, 200 and 300 K. The amount of H2O in the two firsts peaks exceed the monolayer whereas that in the third peak varies with H2O exposure without exceeding the monolayer : 5.2 1014 H2O/cm2, which corresponds to the amount of five fold-coordinated titanium sites on the surface.54 The temperature at the maximum of this third peak varies with the coverage and this has been used to obtain the variation of the activation energy of desorption Ed with the coverage according to a first



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36 kinetic order (modulated molecular beam experiments indicate that H2O is molecularly adsorbed) : Ed(θ) = (80- 35 θ) kJ/mol. This is consistent with (a) the Temkin model used in the present study and the values of H2Oads-L species and (b) the large variation of Ed with the coverage as observed in the present study for the H2Oads-L2 species in contrast with Hugenschmidt et al.53 This dependence of Ed on θ is ascribed both to the heterogeneity of the sites and repulsive interactions.54 On the sputtered surface there is a fourth very small peak in the range 400-600 K ascribed to a small amount of dissociated H2O species which is consistent with the conclusion of the present study considering FTIR and M.S data. Comparison with DFT calculations The Grätzel group55,56 has calculated, by first-principles molecular dynamics, the heat of adsorption of adsorbed H2O on TiO2 anatase (101), (100), (010) and (001) at different coverages. On (101), (100) and (010) surfaces (which are dominant for anatase TiO2), the H2O adsorption is undissociated. Its heats of adsorption at full coverage is 69 kJ/mol on the (101), increasing slightly to 72 kJ/mol at a coverage of 0.5. On TiO2 (110), Gonikowski and Gillan57 have determined a heat of adsorption at full coverage of 79 kJ/mol for the H2Oads species on five fold coordinated Ti cations whereas on the same surface Stefanovich and Truaong58 have obtained 125 kJ/mol at low coverages. These values are consistent with those of the H2Oads-L2 species in Table 1. On the anatase (001) minority surface for a coverage of 0.5, the adsorption of H2O can be both molecular and dissociative with heats of adsorption of 89 and 139 kJ/mol respectively.56 Using the MSINDO method, Homann et al.59 have studied the adsorption of H2O on anatase (100) surface which is modeled by different clusters (TiO2)n(H2O)m (n = 33–132, m = 17–48). They show that the heat of adsorption of H2Oads species changes slightly with the cluster size in the range of 94-101 kJ/mol. Onal et al.60 have performed a similar study using DFT calculations on Ti2O9H10: the heat of H2Oads are 121 and 105 kJ/mol on fixed and relaxed cluster respectively. Aschauer et al.61 have performed DFT


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37 calculations for the adsorption of H2O on the TiO2 (101) surface considering the presence of different defects. They show that (a) at low coverage the adsorption of H2O is mainly molecular, with a heat of adsorption varying according to the type of defects in the range ∼100-64 kJ/mol which are consistent with the present study and (b) the dissociated state of H2O is energetically not favorable. However, the barrier for dissociation is small enough to allow for a small population of dissociated water molecules which is consistent with the present study.61 The above comparisons show that the heats of adsorption of H2Oads species on the sulfate-free and sulfated TiO2 solids provided by the AEIR methods are consistent with values obtained by different authors using experimental and theoretical procedures. However, considering surface sciences and theoretical calculations and taking into account the EMA approach of the NH3-SCR, the AEIR method appears as more accurate because it provides the individual heats of adsorption of three identified H2Oads species H2Owads, H2Oads-L1 and H2OadsL2 at

different coverages on dispersed TiO2 solids used in De-NOx applications. Moreover, the

AEIR method uses adsorption equilibrium conditions which prevent different difficulties linked to others experimental methods such as TPD and microcalorimetry used for dispersed solids. The strongly chemisorbed H2Oads-L2 species, formed on the strongest Lewis sites, can be present in the experimental conditions of the NH3-SCR and it can therefore interact therefore with the adsorbed NH3 species (i.e. competitive chemisorption). It is remarkable, as noted previously for the adsorption of NH3 on TiO2 -P25 and -DT51 that the composition of the surface (without and with sulfate groups) has no significant impact on the heats of adsorption of the H2Oads species.

4- Conclusion



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38 The present study has been dedicated to the measurement of individual heats of adsorption of H2Oads species, by use of an original experimental method denoted AEIR, on two sulfate-free (-P25) and sulfated (-DT51) TiO2 solids which are used as supports of V2O5/WO3/TiO2 NH3-SCR catalysts. At Ta= 300 K, on the two solids, it has been shown that three molecularly adsorbed species are formed identified by their IR bands in the range 38003000 cm-1 (stretching, d-H and H-bond IR bands) whereas their deformation IR bands δH2O are overlapped providing a well defined IR band at a position in the range 1635-1620 cm-1 according to the total coverage of the surface. Two of these species are adsorbed on Lewis sites of different acidic strength which are denoted H2Oads-L1 and H2Oads-L2 in the increasing order of stability. The third species (denoted H2Owads) of low stability represents different H bonded H2O molecules on (a) O2- /OH sites and (b) H2Oads-L species. These H2Oads species are associated with a small amount of dissociated species having the highest heat of adsorption. In the experimental conditions of the present study, the total amount of these different species is lower than a monolayer. Due to different difficulties linked to the quantification of the IR bands in the range 3800-3600 cm-1, the δH2O IR band has been selected for the AEIR method. In line with previous studies dedicated to the measurement of the heats of adsorption of NH3 species, it has been shown how the AEIR method can be developed to obtain the individual heats of adsorption of the three H2Oads species as a function of their coverage from the change of the area of the δH2O IR band with the increase in Ta in isobaric conditions. For instance for the TiO2-P25 the heats of adsorption of the adsorbed species vary linearly with their coverage as follows: Ewads(0) = 50 kJ/mol to Ewads(1)= 45 kJ/mol for the H2Owads, EL1(0) = 60 kJ/mol to EL1(1) = 54 kJ/mol for the H2Oads-L2 , and EL2(0 )= 110 kJ/mol to EL2(1) = 61 kJ/mol for the H2Oads-L2. It has been shown that the presence of sulfate groups (TiO2-DT51) has no significant impact on the heats of adsorption of the three molecularly except EL2(0) which is increased by 4 kJ/mol . These values lead to the conclusion that only the H2Oads-L2 species can


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39 be present in the experimental conditions (partial pressure and temperature) of the NH3-SCR associated with a competitive chemisorption and/or reaction (formation of NH4+ species) with the adsorbed NH3ads species. These two processes may have either negative or positive impacts on the catalytic activity according to the pivotal intermediate species of the NH3SCR: NH3ads-L or NH4+ respectively. Considering the microkinetic approach of the NH3-SCR reaction this imposes (a) to study by experimental procedure the competitive chemisorption between the H2O and NH3 species and the transformation of NH3ads-L species to NH4+ in the presence of H2O and (b) to model the data taking into account that the heats of adsorption of the NH3ads-L2 and H2Oads-L2 species vary linearly with the coverage of L2 sites. This is presented in a forthcoming article, dedicated to the heats of adsorption of H2O on model and commercial V2O5/WO3/TiO2 catalysts according to the AEIR procedure developed in the present study and the competitive chemisorption and reaction between H2Oads-L2 and NH3ads-L2 adsorbed species in the experimental conditions of De-NOx by NH3-SCR.



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

40


Page 40 of 73

Page 41 of 73

41

FIGURES:

B1 3720

B3

δH2O

3690 B2 3672

f

1620

f

3712

f

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


d

3702 0.002

3642

0.02

a

a,b,c 3669

3630

1800

1700

1600

f

1500

B4

a 3800

3700

3600

3500

3400

Wavenumber (cm-1) Figure 1 : Evolution of the IR bands of TiO2-P25 pretreated at 673 K during the cooling stage to 300 K with the indication of the temperature and the duration from time 0 of the cooling process.(a)-(f) T = 673 (0 s), 573 (292 s), 473 (742 s), 423 (1086 s), 373 (1613 s) and 300 (3600 s) K



42

δH2O B4

0.1 3692

0.04 3190

1626

e

f, g

3720 a

B

a e

1635

f,g

3354

A

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 42 of 73

a

1620

3705

3780 3720

3660

1800 1700 1600 1500

B2 B3 3666 3692

B1 3720

a 3673

f,g 3800

a 3600

3400

3200

Wavenumber (cm-1)

3000

2800

Figure 2: Evolution of the IR bands on TiO2-P25 pretreated in helium at 673 K during the adsorption of H2O at Ta = 300 K using 0.25% H2O/He: (a)-(g) ta = 0, 27, 46, 66, 93, 130 and 393 s. Inset A : Details of the range 3780-3660 cm-1. Inset B : Evolution of the deformation IR band δH2O.


Page 43 of 73

43

1 A

|

B

3

Molar fractions

Ar ×100

0.4

C

1 368 K

|

200 cm3/min

100 cm /min

0.8

0.6

| 3

200 cm /min

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


D 130 cm3/min

427 K H2O × 200

0.5

523 K 0 400 600 Temperature (K)

H2O ×100

0.2

0 0

20

40

Time (min)

60

80

100

Figure 3: Adsorption (Part A)/ desorption (Part B)/readsorption (Part C) at Ta = 300 K using 0.5% H2O/0.5% Ar /He on TiO2-P25 followed by a decrease in the adsorption pressure to 0.23% H2O/0.23% Ar/He (Part D). Inset: Temperature programmed desorption after adsorption of H2O at 300 K followed by 1h in helium at 300 K (see the text for more details).



44

0.1

A

a

B6

B3/B2 B7 3653 3676

B

B5 3630

1622 c c

3630

3418

B6

a

B5

f

3418

a

0.02

3663 3667

B3 3696

3630

3692

0.04 B7

3689

Absorbance

3600 3400 3200 3000 2800

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 44 of 73

a

1800 1600

B1

a

3702

3672 3679

3671

B2

g

3653a

B4

g f

1637

0.04

B3/B2 f

1622

0.02

3669

a 3411 g

f f

1800 1700 1600 1500

3740

3700 3660 Wavenumber (cm-1)

3620

3800

3700

3600

3500

Wavenumber (cm-1)

3400

Figure 4: Evolutions of the IR bands on TiO2-P25 after adsorption of 1% H2O/He at 300 K during isothermal desorption at 300 K (Part A) and temperature programmed desorption (Part B). Part A: (a) 1% H2O/He at 300 K and (b)-(f) td = 10, 32, 212, 652 and 3672 s in helium at 300 K. Part B: temperature programmed desorption in helium: (a)-(g) Td = 300, 353, 413, 443, 473, 513 and 673 K


Page 45 of 73

45

1635

a 3655

0.04

g

b c

0.1

Absorbance

3673 3693 3630

h

3662

0.04

3270

1800 1700 16001500

g

0.01 g

B6 3412

1619

3673

a

1619

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


i k l

3702

l

1800

g

1700

1600

3412

d 3270

e

3669

m

f 3702

3655 3702

3700

g 3500 3300 Wavenumber (cm-1)

3700

3500 Wavenumber (cm-1) 3300

Figure 5: Evolution of the IR band of the adsorbed H2O species on TiO2-P25 during the increase in Ta for 0.25% H2O/He. Part A: (a)-(g) Ta = 300, 320, 340, 360, 395, 420 and 495 K (dotted lines first increase in Ta, full lines second increase after a H/C cycle in H2O). Part B: (g)-(l) Ta = 495, 520, 570, 595, 620 and 673 K and (m) in helium at 673 K.



46

1640 0.025

0.1

1/A

a

0.02

0.015

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 46 of 73

1800

0.01 0

0.002

0.004

1/Pa

e

0.006

1632

1700

1600

Wavenumber (cm-1)

1500

1400

Figure 6: Impact of Pa at 300 K on the δH2O IR band of H2Oads species on TiO2-P25. (a)-(e) Pa = 2, 1, 0.8, 0.5 and 0.2 kPa. Inset: Estimation of AM at 300 K for the δH2O IR band using the Langmuir model in the Pa range 2-0.2 kPa.


Page 47 of 73

c, g a, d

0.8 f 0.6

1

a

0.4 0.2

0.6 0.4 0.2 0 200

300 400 500 Temperature (K)

e 0

200

B

0.8

Molar fractions

Coverage of the H2Oads species

1

Coverage of the H2 Oads species

47

1

314 K

A

0.5

523

273

0

b 400

773

H2 O × 100

0

300

600 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 48 49 50 51 52 53 54 55 56 57 58 59 60


500 600 Temperature (K)

1000 Time (s)

700

2000

800

Figure 7: Heats of adsorption of the H2Oads species on TiO2-P25 using the AEIR method:  average coverage of the H2Oads species during the second increase in Ta for 0.25% H2O/He, (a) theoretical curve from eqs. ES2-ES4 assuming the presence of two H2Oads species with the following parameters; H2Oads1, 45 and 61 kJ/mol at coverage 1 and 0 respectively with x1 = 0.7 and for H2Oads-L1, 61 and 110 kJ/mol at coverage 1 and 0 respectively with x2 = 0.3; b) and c) theoretical evolutions of the coverages of H2Oads1 and H2Oads2 species respectively; (d) average coverage considering the presence of the three H2Owads, H2Oads-L1 and H2Oads-L2 species; (e), (f), (g) theoretical evolutions of the coverages of H2Owads, H2Oads-L1 and H2Oads-L2 species respectively (see the text for more details and Table 1 for the parameters) Inset A: Evolution of the molar fraction of H2O during TPAE experiment using the M.S system with 0.5% H2O/0.5% Ar/He. Inset B:  evolution of the average coverage of the H2O species during TPAE, a) theoretical curve from eqs. ES2-ES4 assuming the presence of two H2Oads species with the following parameters; for H2Oads1, 45 and 66 kJ/mol at coverage 1 and 0 respectively with x1 = 0.64 and for H2Oads2, 67 and 98 kJ/mol at coverage 1 and 0 respectively with x2 = 0.36 (see the text for more details).



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 48 of 73

48

Figure 8: Heats of adsorption of the H2Oads-L2 species on TiO2-P25 using the ClausiusClapeyron equation. (a) and (b): δH2O IR band for PH2O =1 kPa at Ta = 443 and 523 K respectively, (c) and (d): δH2O IR band for PH2O=0.035 kPa at Ta = 443 and 523 K respectively. Inset A:  and  Evolution of the coverage of the H2Oads-L2 species with Ta for Pa = 0.035 and 1000 kPa respectively; (a) and (b) theoretical curves from eqs. ES2-ES4 with EL2(1) = 59 and EL2(0) = 114 kJ/mol for Pa = 0.035 and 1 kPa respectively. Inset B: Heats of adsorption of the H2Oads-L2 species at different coverages using the Clausius-Clapeyron equation.


Page 49 of 73

49 3666

a

0.2

0.02

A 3692

3636 3659 3390

1370 1364

3667 3648 3690

b

3621

a

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


3700

3600

a

a

3687

j 3500

3800 3600 3400

1354

1335

j

1631 1619

0.2

1800

1318

j

1700

a 1600

1500

Wavenumber (cm-1)

1400

1300

Figure 9: Evolution of the IR bands of the H2Oads species on the pretreated TiO2-DT51 at the adsorption equilibrium using 0.25% H2O/He during the increase in Ta: (a)-(i) Ta = 303, 323, 358, 423, 473, 523, 573, 623 and 673 K in H2O/He; (j) in helium at 673 K. Inset A: Evolution of the IR bands of TiO2-DT51 during the cooling stage in helium after the pretreatment procedure: (a) and (b) at 673 and 300 K respectively.



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 50 of 73

50

Figure 10: Details of the evolutions of the IR bands of the H2Oads species in the 3800- 3300 cm-1 range on the pretreated TiO2-DT51 at the adsorption equilibrium using 0.25% H2O/He during the increase in Ta: (a)-(f) Ta = 363, 448, 523, 573, 633 and 673 K.


Page 51 of 73


51

1

e

0.8


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


c,f

0.6

1

a

0.8 0.6 0.4 0.2 0 200


800

0.4

0.2 a

d

b

0 200

300


600

700

800

Figure 11: Heats of adsorption of the H2Oads species on the sulfated TiO2-DT51 support using the AEIR method:  and  average coverage of the H2Oads species during the increase and decrease in Ta for the second cycle in 0.25% H2O/He, (a) theoretical curve from eqs. ES2-ES4 assuming the presence of two H2O species H2Oads1 and H2Oads2 with the following parameters; for H2Oads1, 45 and 59 kJ/mol at coverage 1 and 0 respectively with x1 = 0.65 and for H2Oads2 61 and 113 kJ/mol at coverage 1 and 0 respectively with x2 = 0.35; b) and c) theoretical evolutions of the coverages of H2Oads1 and H2Oads2 species respectively, (d), (e) and (f) theoretical evolutions of the coverages of H2Owads, H2Oads-L1 and H2Oads-L2 species respectively considering the presence of three adsorbed H2Oads (see the text for more details). Inset: average coverage of the H2Oads species during the increase in Ta for the second cycle in 0.25% H2O/He, (a) theoretical curve considering the presence of three H2O species H2Owads, H2Oads-L1 and H2Oads-L2 (see the parameters in Table 1).



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 52 of 73

52 TABLE . Solid

H2Owads*

Heat of adsorption kJ/mol TiO2- P25

Ewads(1) Ewads(0)

TiO2- DT51

H2Oads-L1*

H2Oads-L2*

EL1(1)

EL1(0)

EL2(1)

EL2(0)

Fraction of δH2O at 300 K xwads xL1 xL2

44

55

55

61

61

110

0.42

0.28

0.3

45

50

54

60

61

114

0.35

0.3

0.35

Table 1: Heats of adsorption at different coverages Ex(θ) of the three molecularly adsorbed H2Oads species formed on TiO2 solids and their contribution of the δH2O IR band at 300 K using the AEIR method.*: H2Owads hydrogen bonded to different sites, H2Oads-L1 and H2OadsL2

coordinated to two Lewis sites of different acidic strength.


Page 53 of 73

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53 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 TiO2-DT51 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). Thanks are due to King Abdullah University of Science and Technology, (KAUST, Saudi Arabia) for the financial support (award No. UK-C0017) of J. C. postdoctoral grant in the framework of CADENCED project.

SUPPORTING INFORMATION AVAILABLE: Details on the calculations using the M.S system, the equations used for the AEIR and TPAE methods, the IR spectra during temperature programmed desorption of adsorbed H2O species, the Langmuir model using M.S data, the IR spectra during the adsorption of H2O at 300 K on DT51.This information is available free of charge via the Internet at http://pubs.acs.org



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Page 59 of 73

59

SN SYNOPSIS TOC

Heats of adsorption of H2 O species on TiO2 -P25 using the AEIR method

E(θ) kJ/mol

1

PH2O= 0.1 kPa

H2Oads-L2 E(0)= 110 E(1)= 61

0.8

H2Oads-L1 E(0)= 61

0.6

E(1)= 55

0.4

H2Owads E(0)= 55 E(1)= 44

0.2

Ta (K)

Absorbance

Coverage of the H2 Oads 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


303 323 363 423 498 525 573 623

0.04 1800

0 200

300

400

1700

1600

Wavenumber (cm-1)



700

1500

800


B1 3720

B3

δH2O

3690 B2 3672

f

1620

f

3712

f 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

Page 60 of 73

d

3702 0.002

3642

0.02

a

a,b,c 3669

3630

1800

1700

1600

1500

f

B4

a 3800

3700

3600

Wavenumber (cm-1) Figure 1 ACS Paragon Plus Environment

3500

3400

Page 61 of 73

δH2O B4

0.1 3692

0.04 3190

1626

e

f, g

3720 a

B

a e

1635

f,g

3354

A

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


a 1620

3705

3780 3720

3660

1800 1700 1600 1500

B2 B3 3666 3692

B1 3720

a 3673

f,g 3800

a 3600

3400

3200

Wavenumber

(cm-1)

Figure 2 ACS Paragon Plus Environment

3000

2800


1 A

|

B

200 cm3/min

Molar fractions

Ar ×100

0.6

0.4

|

100 cm3/min

0.8 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

Page 62 of 73

1 368 K

C

|

200 cm3/min

D 130 cm3/min

427 K H2O × 200

0.5

523 K 0 400 600 Temperature (K)

H2O ×100

0.2

0 0

20

40

Time (min)

60


80

100

Page 63 of 73

0.1

B6

A

a

B3/B2 B7

3653 3676

B

B5 3630

1622 c c

3630

3418

a

0.02 3418

f

B6

a

B5

3600 3400 3200 3000 2800

3663 3667

B3 3696

3630

3692

0.04 B7

3689

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


f

B1

3702

3672 3679

a

1800 1600

a

3671

B2

g

3653a

B4

g

1637

0.04

B3/B2

3669

0.02 f

1622

a 3411 g

f f

1800 1700 1600 1500

3740

3700 3660 Wavenumber (cm-1)

3620 Figure 4

3800


3700

3600

3500

Wavenumber (cm-1)

3400


1635

a 3655

0.04

g

b c

0.1

Absorbance

3673 3693 3630

h

3662

0.04

3270

1800 1700 16001500

g

0.01 g

B6 3412

1619

3673

a

1619

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

Page 64 of 73

i k l

3702

l

1800

g

1700

1600

3412

d 3270

e

3669

m

f 3702

3655 3702

3700

g 3500 3300 -1 Wavenumber (cm ) Figure 5

3700


3500

Wavenumber (cm-1)

3300

Page 65 of 73

1640 0.025

0.1

1/A

a

0.02

0.015

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


1800

0.01 0

0.002

0.004

1/Pa

e

0.006

1632

1700

1600

Wavenumber

(cm-1)

1500


1400

c, g a, d

0.8 f 0.6

1

0.6 0.4 0.2 0 200

0.2 e 200

B

0.8

0.4

0

a

300 400 500 Temperature (K) Molar fractions


1

Page 66 of 73

1

314 K

400

0.5

523

0

273

0



773

A

H2O × 100

b 300

600

1000 Time (s)

700

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



2000

800

Page 67 of 73

Heat of adsorption kJ/mol

Coverage H2Oads-L2

1623

140

1

a

0.8 0.6

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


A

120

b

100

0.4 0.2 0 300

B

400

500

600

Temperature K

a b

700

80 0

c 1617

d

0.02 1800

0.2 0.4 0.6 0.8 1 Coverage H2Oads-L2

1700

1600

Wavenumber

(cm-1)

Figure 8


1500


3666

a

0.2

0.02

A 3636 3659 3390

3692

1370 1364

3667 3648 3690

a

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

Page 68 of 73

b

3621

j 3700

3600

a

a

3687

3500

1354

1335

3800 3600 3400

j

1631 1619

0.2

1800

1318

j

1700

a 1600

1500

Wavenumber

(cm-1)

1400


Figure 9

1300

Page 69 of 73

a

3666

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


3275

3648

3390

0.1

3621 f 3800

3700

3600

3500

Wavenumber


3400

(cm-1)

3300

1

e

0.8


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



c,f

0.6

1

Page 70 of 73

a

0.8 0.6 0.4 0.2 0 200


800

0.4

0.2 a

d

b

0 200

300



Figure 11

600

700

800

Page 71 of 73

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


a 3679

3650 3630

3696

b 3702

3671

d

c

3669

e 3720

3660

3600

Wavenumber (cm-1)

Figure S1 ACS Paragon Plus Environment

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

1/ Q(300 K, Pa) in (µmol/g)-1


Page 72 of 73

0.008

0.006

0.004

0.002

0

0

0.002

0.004

0.006

1/Pa in Pa-1 Figure S2


0.008

Page 73 of 73

1373

Absorbance

a

e

0.1

3690

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


3665

e

a

e

3600 3400 3200 3000 2800 Wavenumber (cm-1)

e

1630 1618

a 0.1

1800

a 1700

1600

1500

1400

Wavenumber (cm-1)

Figure S3


1300

1200

WO3

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