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Experimental Microkinetic Approach of De-NO by NH on VO/WO/ TiO Catalysts: Part I- Individual Heats of Adsorption of Adsorbed NH Species on a Sulfate-free TiO Support Using Adsorption Isobars 2

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François Giraud, Christophe Geantet, Nolven Guilhaume, Sébastien Gros, Lynda Porcheron, Mohamed Kanniche, and Daniel Bianchi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Jun 2014 Downloaded from http://pubs.acs.org on June 26, 2014

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The Journal of Physical Chemistry

1

Experimental Microkinetic Approach of De-NOx by NH3 on V2O5/WO3/TiO2 Catalysts: Part I- Individual Heats of Adsorption of Adsorbed NH3 Species on a Sulfate-free TiO2 Support using Adsorption Isobars

AUTHOR NAMES François Giraud,1 Chistophe Geantet,1 Nolven Guilhaume,1 Sébastien Gros,2 Lynda Porcheron,2 Mohamed Kanniche,2 and Daniel Bianchi1*

AUTHOR ADDRESS 1

Institut de recherche sur la catalyse et l’environnement de Lyon (IRCELYON), UMR 5256

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

2

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

78401, Chatou-France. KEYWORDS TiO2, ammonia, NH3-SCR, heat of adsorption, AEIR, TPAE.

ACS Paragon Plus Environment


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2 ABSTRACT: The present study is dedicated to the development of experimental procedures allowing the measurement of the individual heats of adsorption of adsorbed NH3 species on a sulfate-free TiO2 solid (P25 from Degussa). This solid has been selected because it is frequently used as support of V2O5- or/and WO3- TiO2 model catalysts for the understanding of the surface processes implicated in the Selective Catalytic Reduction of NOx by NH3 (NH3-SCR) Two original analytical procedures denoted adsorption equilibrium infrared spectroscopy (AEIR) and temperature programmed adsorption equilibrium (TPAE) (developed in previous works) were applied. These methods are based on (a) the experimental measurement of the change in the adsorption equilibrium coverage of the individual adsorbed species in isobar conditions and (b) the comparison of the experimental data to an adsorption model. It is shown that in the ranges of the ammonia partial pressures and reaction temperatures of the NH3-SCR process, only two adsorbed NH3 species on Lewis sites (Ti+δ) are detected on the solid dehydrated at 673 K. These species noted NH3ads-L1 and NH3ads-L2 are differentiated by their δs NH3 IR bands at 300 K: 1149 cm-1 and 1228 cm-1 respectively, whereas their δas IR bands are at the same position: 1596 cm-1. The AEIR and TPAE methods indicate that the heats of adsorption of the NH3ads-L1 and NH3ads-L2 species (noted EL1(θ) and EL2(θ) accuracy ± 5 kJ/mol) vary linearly with their respective coverages θ from EL1(1)= 56 kJ/mol to EL1(0)= 105 kJ/mol and from EL2(1)=105 kJ/mol to EL2(0)= 160 kJ/mol. These values are compared to (a) isosteric heat of adsorption provided by the Clausius-Clapeyron method and (b) literature data using temperature programmed desorption, microcalorimetry and DFT calculations. Forthcoming articles show that the simplicity of the analytical procedures allows studying the impact of the presence of sulfate and VOx/WOy depositions over TiO2 on the nature and heats of adsorption of adsorbed NH3 species. TEXT:


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3 1. Introduction The abatement of NOx (NO+NO2) in the exhaust gases emitted from thermal power plants and others stationary sources is currently performed by transformation into N2 using NH3 as a reducing agent on solid catalysts (noted NH3-Selective Catalytic Reduction: NH3SCR).1 Numerous compositions of catalysts have been investigated for this reaction in particular

metal-exchanged

zeolites.2,3

However,

stationary

sources

mainly

use

V2O5/WO3/TiO2 catalysts explaining periodic review articles dedicated to the state of the art on these NH3-SCR catalysts.4-7 It is known that the structure/composition of those catalysts is more complex than the notation x wt% V2O5/ y wt% WO3/ TiO2 with x and y in the ranges of 0.7-2 and 9-13 for commercial catalysts.8,9 For instance,4-7 at vanadium loadings of interest for the NH3-SCR, mono- and poly- meric VxOy species are present on the TiO2 surface without V2O5 particles and the support TiO2 is preferred in its anatase form and it is significantly sulfated. The high performances of the NH3-SCR on TiO2 based catalysts explain that this process has been applied for the abatement of the NOx emitted by duty car.10 Even if the NH3-SCR technology on TiO2-based catalysts, is widely applied worldwide, the reaction mechanism and the kinetic parameters controlling the catalytic activity are still debated.4-7 However, there is a consensus on the fact that the reactant NH3 is strongly adsorbed,4-7 and numerous studies on MOx (M= Ti, V and W) metal oxides (pure and supported) have evidenced different parameters of the NH3 adsorption such as: (a) the nature of the adsorption sites (Lewis and/or Brønsted acidic sites), (b) the structure of the adsorbed species,11-13 (c) the surface concentration of the adsorption sites14-15 and (d) the average heat of adsorption of NH3 using microcalorimetry16-18 and temperature programmed desorption methods.19-20 The debate is more open on the elementary steps involving NOx and on the pivotal adsorbed NHx intermediates species.5



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4 The present study is a contribution to the mastering of the kinetic parameters controlling the catalytic activity of x% V2O5/ y% WO3/ TiO2 catalysts by an experimental microkinetic approach (denoted EMA) of NH3-SCR as performed previously for others catalytic reactions such as (a) the CO/O2 reaction on Pt/Al2O3,21,22 (b) the catalytic oxidation of Diesel soot by ceria23,24 and (c) the photocatalytic oxidation of isopropyl alcohol on TiO2 .25-26 The final aim of the present EMA is the understanding of how chemical poisons associated to the coal combustion affect the kinetic parameters of the surface elementary steps that control the reaction rate. The EMA of a catalytic reaction is a branch of the microkinetic,27 its aims are the measurements (using adapted experimental procedures) of the kinetic and thermodynamic parameters of the surface elementary steps that control the rate of the catalytic reaction on a conventional solid catalyst. The first stage of the EMA is to propose a plausible kinetic model of the reaction (from either literature data or a formal kinetic approach of the reaction) and then, experimental procedures are developed to study each elementary step. Different kinetic models of the NH3-SCR have been proposed in the literature. However, as discussed by Busca et al.5 few of them take into account accepted experimental facts and/or basic scientific concepts such as (a) the stoichiometry of the NH3SCR reaction: 4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O, (b) the production of N2 via a coupling reaction between NH3 and NO, (c) the role of oxygen as re-oxidation agent of the sites that have been reduced by the others reactants and (d) the respect of the equilibrium ion charges in the formalism of the kinetic model. Two main detailed mechanisms of the NH3-SCR on TiO2 based catalysts emerge.5 They differ by the nature of (a) the adsorption sites of NH3 (either Lewis or Brønsted acidic sites) and (b) the adsorbed intermediate species after adsorption of NH3 : either neutral -NH2 via the adsorption of NH3 on Lewis site according to a mechanism referred as “amidenitrosamide” and proposed by Ramis et al.28 or NH4+ ions via the adsorption of NH3 on


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5 Brønsted sites as proposed by Topsøe et al.29,30 The EMA approach has been developed considering that the two kinetic models are evenly plausible and experiments have been performed to find arguments supporting or not each of the proposals. The present study which is a part of a larger work, concerns the first step of the EMA: it is dedicated to the characterization of the NH3 adsorption on a TiO2 support of models x% V2O5/ y% WO3/ TiO2 catalysts. There are two main aims: (a) the identification of the different NH3 species formed on the surface of the solids (i.e. adsorption on Lewis and Brønsted sites) and the quantification of their amounts: this constitue a revisit of literature data. and (b) the measurement of the individual heats of adsorption (at different coverages) of the different adsorbed species present in experimental conditions (adsorption pressure and température) of the NH3-SCR, by adapting two original analytical methods developed previously for the study of adsorbed CO species on supported metal particles (denoted AEIR31-33 and TPAE34). This is the original contribution of the present work: literature data concern the average heat of adsorption of NH3. Moreover, the two methods allow using experimental conditions (i.e. PNH3, T, presence of H2O and/or O2) mimicking those of the NH3-SCR process and they present different advantages as compared to classical methods such as Temperature Programmed Desorption, isosteric procedures and microcalorimetry.31-34 The present Part I is dedicated to the development of the AEIR and TPAE methods for the measurement of the individual heat of adsorption of adsorbed NH3 species on a sulfatefree TiO2 solid. These procedures allow us showing, in forthcoming articles, how the presences of sulfate groups and/or the deposition of VxOy or/and WxOy species, at loadings of interest for the NH3-SCR, modify the natures and heats of adsorption of the adsorbed NH3 species. 2. Experimental.



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6 2.1 TiO2 solid, pretreatments and structure/texture characterizations The sulfate free TiO2 solid: P25 from Degussa (≈ 55 m2/g) is a mixture of anatase (≈ 85%) and rutile (≈15%) structures with traces of SiO2 (0.2 wt%).11,35,36 It has been selected because (a) it is often used as support of SCR model catalysts and (b) the characterization of the NH3 adsorption on its surface has been performed in different studies (i.e. nature of the adsorbed species as shown in the Results and Discussion section). This allows us showing the original contributions of the present study on the characterization of the adsorption of NH3 on TiO2 surfaces. We have verified using conventional analytical methods that the P25 sample was conform to literature data concerning the BET area and the crystallographic structures. Before NH3 adsorption, the solid was treated on the different analytical systems as follows: O2 (713 K, 10 min)→ He (713 K, 5 min)→ He (300 K). The same sample of solid was used to perform a series of experiments and it was pretreated as above before each experiment. 2.2 IR Cell in Transmission mode The experiments have been performed using a Nicolet-6700 FTIR spectrometer equipped with a small internal volume (≈ 2 cm3) homemade stainless steel IR cell reactor in transmission mode using CaF2 windows previously described in more detail.37 Briefly, it allowed in-situ treatments at atmospheric pressure 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 selected using different valves and purified by different traps in particular cool traps for H2O impurities. The molar fraction of NH3 at the outlet of the IR cell was followed using a second FTIR spectrometer (Nicolet protege) with a conventional gas IR cell. After a pretreatment procedure the time required to cool down the sample from 713 K to 300 K was ≈ 1 hour. Low IR transmissions do not allow accurate quantitative exploitation of the IR bands of the adsorbed species. In the present study quantitative exploitations were performed for IR


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7 transmission higher than ≈ 7% after adsorption of NH3. This comment concerns particularly the wavenumber range below 1200 cm-1 that reveals the presence of different adsorbed NH3 species on Lewis sites via their δs IR bands (see below). In this wavenumber range a low IR transmission comes from (a) the decrease in the IR transmission of the CaF2 windows and of the TiO2 solids that is dependent on its temperature and (b) the intensity of the IR bands of the adsorbed species. The contribution of TiO2 is depending on the weight of the pellet: light pellets favor the IR transmission but decreases the amount of adsorbed species imposing a compromise in the design of the experiments. For IR transmission lower than 7% (this may concerns a fraction of an IR band) semi quantitative exploitation are performed. For instance if two species provide overlapped IR bands with one of them having IR transmission > 7% a decomposition can be performed to quantify this species with a reasonable accuracy whereas the second species is not quantified. 2.3 Volumetric measurements using experiments in the transient regime. To support the quantitative exploitation of the IR spectra, the adsorption of NH3 has been studied using an analytical system for transient experiments as described previously.38 Briefly, the composition (molar fractions Xi) of the gas mixture at the outlet of a quartz micro reactor containing the pretreated catalyst (weight range of 0.2-0.5 g, deposited on quartz wool) was determined, by using a quadrupole mass spectrometer (Inficon, Transpector CPM), during either switches between regulated gas flows (1 atm, flow rate in the range of 100-200 cm3/min) at a constant temperature or an increase in the temperature at a constant flow rate. The temperature is recorded via a stainless steel K type thermocoax (Φ= 0.25 mm) inserted in the sample of catalyst. All the stainless steal tubes, before and after the microreactor until the source of the mass spectrometer, were heated to 373 K. This allowed the quantification of H2O production during experiments. After a pretreatment procedure the time required to cool down the sample from 713 K to 300 K was ≈ 4 min. The molar fractions provided the rates of



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8 either formation of a product j: Rj= Xj MF/mC or consumption of a reactant i: Ri= (Xin-Xi) MF/mC where MF and mC were the total molar flow rate and the weight of catalyst respectively and Xin was the molar fraction at the inlet of the reactor. For instance, the adsorption/desorption of NH3 at Ta= 300 K on the TiO2-P25 solid was studied according to the following switches: He→ 0.5% NH3/0.5% Ar/He (Ar was used as tracer) providing the total amount of strongly and weakly adsorbed NH3 species followed by 0.5% NH3/0.5% Ar/He → He providing the amount of reversibly adsorbed species, that was confirmed by a new switch He→ 0.5% NH3/0.5% Ar/He. After the adsorption equilibrium at 300 K, the temperature was increased progressively in the presence of the partial pressure of NH3 to perform a TPAE experiment.34 Blank experiments with the empty reactor (with thermocouple and quartz wool) using the switch He → 0.5% NH3/0.5% Ar/He indicated the absence of NH3 adsorption/decomposition at 300≤T≤ 723 K (i.e. absence of a significant delay between Ar and NH3 at 300 K). The experiments have been designed to prevent the impact of mass transfer limitations as described previously,34 and it has been verified that in experimental conditions of interest for the TPAE, the modifications of particle size fractions, sample sizes and gas flow rates have no impact on the measurements.

2.4 AEIR and TPAE methods for the measurement of the heats of adsorption. AEIR method The experimental procedure of the adsorption equilibrium IR spectroscopy method has been described in detail in previous studies.31-33 However, it is briefly summarized to facilitate the presentation considering NH3 adsorption on TiO2. After pretreatment of the solid, an x% NH3/He mixture (x< 3) was absorbed at 300 K (adsorption pressure of NH3: Pa= x 103 Pa). After the adsorption equilibrium was reached, evolutions of IR bands of adsorbed species were followed during a progressive increase in the adsorption temperature Ta in the presence


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9 of NH3. This provided the evolution of the coverage θX(Ta) of each adsorbed NH3 species X with Ta according to:39,40 θX(Ta) = AX(Ta)/AX(M)

(1)

where AX(Ta) and Ax(M) were the area of an IR band characteristic of the Xads species at Ta and at saturation of the sites respectively. The saturation was ascertained by verifying that the IR band area was unmodified by increasing either Pa at 300 K or Ta at a fixed Pa. The heats of adsorption of the Xads NH3 species at different coverages were determined comparing the experimental curve θX(Ta) to a theoretical curve which was often the generalized expression of the Temkin’s model (linear decrease of the heat of adsorption with the increase in the coverage):

θ=

1 + KX (0) .Pa RTa ln( ) ΔEX 1 + KX (1) .Pa

(2)

where KX(0) (EX(0)) and KX(1) (EX(1)) were the adsorption coefficients (heats of adsorption) at θ= 0 and θ= 1 of the Xads NH3 species respectively, ΔEX= EX(0)-EX(1) and R was the ideal gas constant. The EX(0) and EX(1) values were obtained considering that the Xads species was localized and that the adsorption coefficients KX(θ) was provided by the statistical thermodynamics: KX (θ ) =

h3 (2 π

1 m )3 / 2

( k T a )5 / 2

exp( EX d

EX (θ ) (θ ) − EX a (θ ) ) = A(T ) exp( ) R Ta R Ta

(3)

Where h and k were Planck’s and Boltzmann’s constants, m was the mass of the molecule and EXd(θ) and EXa(θ) were the activation energies of desorption and adsorption respectively (EX(θ)= EXd(θ)-EXa(θ) was the heat of adsorption equal to EXd(θ) for non-activated NH3 adsorption). The EX(0) and EX(1) values were those leading to theoretical curves θXth= f(Ta) (from Eqs. (2) and (3)) consistent with the experimental data. It must be noted, as discussed previously,31 that EX(1) fixed the Ta value that initiated the decrease in the coverage from 1 in



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10 isobar conditions whereas EX(0) was related to the slope of the linear section of the isobar θXth= f(Ta). This means that the heats of adsorptions at low and high coverages can be obtained from a limited section of the experimental isobar including θX(Ta)= 1. The accuracy on the heat of adsorption measurements (considering differences between experimental and theoretical curves) was of ≈ ±5 kJ/mol. As discussed in a previous work for the adsorption of CO,34 Eq. 3 is obtained via approximations onthe partition functions of rotation and vibration of the molecule in the gas phase : qgr and qgv, respectively and of the adsorbed molecule qa. The justification of these approximations for the adsorption of NH3 is discussed below. TPAE method. The temperature programmed adsorption equilibrium method was similar to the AEIR procedure except that the coverages of the adsorbed species were determined by a volumetric method.34 After the adsorption equilibrium of the NH3ads species at 300 K using an x% NH3/x% Ar/He mixture, the adsorption temperature was increased progressively (≈20 K/min). The increase in the NH3 partial pressure due to the net NH3 desorption rate: Ri(t) in µmol/(g×s) was followed by the MS system in parallel to the adsorption temperature Ta. The experimental net desorption rate provided the evolution of the coverage of the adsorbed species according to: ta

∫ R i (t ) dt

θ X (T a ) =

0

Qs

(4)

where ta (in s) was the time to obtain the adsorption temperature Ta starting for Ta= 300 K at ta=0 and Qs (µmol/g) was the amount of NH3 at saturation of the site. This experimental curve θX= f(Ta) at a quasi-constant adsorption pressure can be (a) compared to that obtained using the AEIR method: an agreement justifies the validity of Eq. (1) and (b) exploited using Eqs (2) and (3) for the determination of the heats of adsorption of the adsorbed species. There are


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11 similarities between TPAE and TPD procedures,34 however, the presence of the inlet NH3 pressure during the TPAE ensures that the coverage decreases progressively whereas the adsorption equilibrium is maintained. This situation modifies and facilitates the design of the experiments (control of the readsorption) and their exploitation as compared to a TPD procedure.34 However, TPAE imposes a careful design of the experiments (weight of the sample and choice of the heating rate) to prevent the following situations: (a) a large net NH3 desorption rate (high amount of adsorbed species in the reactor and/or high heating rate) may prevent assuming that Pa remains quasi constant (this condition is fulfilled if the increase in the Pa is < ≈30 % of the inlet partial pressure)34 and (b) a low net NH3 desorption rate (small amount of adsorbed species in the reactor and/or low heating rate) cannot be quantified with an acceptable accuracy using the MS system.

3. Results and Discussion. The AEIR and TPAE methods have been used to determine the individual heats of adsorption of the adsorbed NH3 species formed of TiO2-P25 in experimental conditions of interest for the NH3-SCR. 3.1 Nature of the adsorbed NH3 species at 300 K on TiO2-P25. TiO2-P25 has been studied by different authors considering the adsorption of NH3 and this allows us showing the original contributions of the present study to the characterization of the adsorbed NH3 species on TiO2 based catalysts. FTIR characterizations of the pretreated TiO2-P25 The inset in Fig. 1 shows the evolution of the IR bands of the OH groups on P25 during the cooling stage (≈ 1 h) in helium, after the pretreatment of the solid at 713 K. It can be observed that the IR bands at 3704, 3670 and 3630 cm-1 for T= 713 K (spectrum a) increase progressively with the decrease in T associated with the appearance of a new IR band



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12 at 3690 cm-1 for T= 498 K followed at 423 K by a very small IR band at 1620 cm-1 (result not shown). The IR bands observed at 300 K: 3720, 3690, 3672 and 3640 (inset Fig. 1, spectrum d) and their relative intensities are similar to those described on P25 at 300 K by different authors such as : 3735, 3715 3662 and 3640 cm-1,11 and 3736, 3717, 3688, 3672, 3642 cm-1.41 These IR bands are ascribed to different OH groups,11,41 whereas some authors assign the IR band at 3720 cm-1 to Si-OH groups due to the presence of SiO2 impurities.11,42,43 DFT calculations44,45 have shown that the different IR bands can be assigned to TiX–μ1OH and TiX– μ1OH2 species on different planes of TiO2 due to dissociated and undissociated water respectively (the notations describe the various local ionic coordinations: X= V or VI for the coordination of Ti whereas μ1 indicates that either OH or H2O are monocoordinated to a surface atom). At 300 K, the intensity of the IR band at 1620 cm-1 due to a low amount of undissociated water,43,46 represents 8% of the IR band of the OH group at 3720 cm-1 (result not shown). The increase in the IR bands of the OH groups during the cooling stage is probably due to the adsorption of tiny H2O traces present in the helium flow (i.e. desorption from the stainless steal tubes after the cool traps): this is consistent with comments of Deiana et al.41 considering the FTIR study of undissociated/dissociated H2O adsorption after pretreatments of P25 in the 300-873 K range. It must be noted that the IR transmission of the IR Cell/P25 pellet system decreases with the increase in the temperature in the 1500-1000 cm1

range: for instance at 1500 cm-1, it is of 74 % and 62 % at 300 K and 713 K respectively

whereas for the same temperatures there are of 42% and 12% at 1200 cm-1 and of 22 % and 1 % at 1100 cm-1 respectively. This creates difficulties in the quantitative exploitation of the IR bands of the adsorbed NH3 species in the wavenumber range below ≈1200 cm-1 during the increase in the temperature (see below) FTIR characterizations of adsorbed NH3 species at 300 K on TiO2-P25


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13 Parts A and B in Fig.1 show the evolutions of the IR spectra during adsorption of 0.1% NH3/He at 300 K: the IR bands of the OH groups (Fig. 1A) decrease progressively with time on stream whereas different IR bands increase at 3396, 3348, 3245, 3190, 3145 cm-1 and 1596, 1142 with shoulders at 1680, 1477 and 1215 cm-1. A steady state is obtained after an adsorption duration of ta= 96 s. It must be noted that (a) this duration is controlled by the apparent rate of NH3 adsorption which is fixed by the inlet NH3 molar flow rate and (b) at the adsorption equilibrium for PNH3= 0.1 kPa, the IR bands of the OH groups are still detected with small intensities (Fig. 1A, spectrum e). The IR bands in Fig. 1 are consistent with different literature data on TiO2-P2511,35,47,48 (the similarity is remarkable with two of them)47,48 and there is an agreement in their assignments:11,47,48 (a) the strong IR band at 1142 cm-1 and the shoulder at 1215 cm-1 are due to the symmetric deformation (δs NH3) of two adsorbed NH3 species on Lewis sites coordinatively unsaturated (TiV or TiIV) having different acidic strength: to facilitate the presentation they are denoted NH3ads-L1 and NH3ads-L2 for the IR bands at 1142 cm-1 and 1215 cm-1 respectively (the notation NH3ads-L is used without distinction of the two adsorbed species on the Lewis sites), (b) the IR band at 1596 cm-1 (Fig. 1B) is due to the overlap of the asymmetric deformation δas of the two NH3ads-L1 and NH3ads-L2 species and (c) the shoulder at 1680 cm-1 and the broad IR band at 1477 cm-1 (that is detected as two IR bands in some experiments at 1424 and 1497 cm-1) are ascribed to the antisymmetric and symmetric deformation of NH4+ species.11,48 The ratio of the intensities of the IR bands at 1596 and 1477 is similar to that of Hadjiivanov et al.48 but it is smaller than that of Busca et al.11,35 who used an higher adsorption pressure (Pa= 50 torr): this indicates that NH4+ is weakly adsorbed and its coverage increases at 300 K with Pa. This is consistent with the fact that Busca et al.11 have observed the total disappearance of the IR band of the NH4+ species after desorption at room temperature.The IR bands at 3396, 3348, 3245, 3190 and 3145 cm-1 in Fig. 1A are similar to those observed at 300 K and PNH3= 50 torr by Rami et



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14 al.35: 3393, 3348, 3255, 3195 and 3155 cm-1 respectively. They correspond to asymmetric and symmetric ν NH3 and the first overtone of asymmetric deformation band δas NH3 of the two NH3ads-L species.35 3.2 Experimental procedure of the AEIR method. Saturation of the Lewis adsorption sites. Figure 2A shows the evolution of the intensities of the IR bands of the adsorbed NH3 species during the increase in the adsorption pressure Pa at the adsorption temperature Ta=300 K according to the following switches: 0.1% NH3/He (spectrum a)→ 0.5% NH3/He (spectrum b)→ 1 % NH3/He (spectrum c). The IR band at 1596 cm-1 is not modified by the increase in Pa (same observation for the IR bands < 1300 cm-1, not shown). This indicates that the IR bands area of the NH3ads-L1 and NH3ads-L2 species at 300 K, for Pa= 0.1 kPa, can be used as AX(M) value in Eq. 1. The IR bands of the NH4+ species increase with the increase in Pa (Fig. 2A) (whereas the IR bands of the remaining OH groups decreases slightly, not shown) indicating weakly adsorbed species in agreement with literature data.11,35 This is consistent with the fact that a switch 0.1% NH3/He → He for td= 260 s (Fig. 2A spectrum d) leads to the decrease in the IR bands of the NH4+ species in agreement with literature data11,36 and a more limited decrease of that of the NH3ads-L species. It must be noted that the saturation of the Lewis sites at 300 K for the PNH3= 0.1 kPa, prevents the presence of detectable amount of physisorbed like NH3 species (H bonded) formed for high adsorption pressures,11, 48 that is a favorable situation for the AEIR method. Reproducibility of the IR data. The AEIR method imposes that the evolutions of the IR bands of adsorbed species during the increase in the adsorption temperature Ta in isobar condition must be only due to the modification of the adsorption equilibrium. This is ascertained by comparing the evolutions of IR bands during different cycles: heating to Tf and then cooling to 300 K, in the


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15 presence of NH3 (denoted H(Tf)/C(300 K) cycle where Tf is the highest adsorption temperature). For instance, Fig. 2B compares the intensities of the IR bands of the adsorbed NH3 species at 300 K for Pa= 0.1 kPa after (a) the pretreatment procedure (spectrum a), and (b) a first (spectrum b) and a second (spectrum c) H(673 K)/C(300K) cycle. It can be observed that the IR band of the NH4+ species (1477 cm-1) decreases after the first cycle associated to a slight increase in the IR bands of the NH3ads-L species (1596, 1147 and 1215 cm-1) whereas a second cycle has no impact (spectrum c). The modifications of the IR bands after the first cycle is probably due, in part, to the fact that the presence of strongly adsorbed NH3 species prevents the readsorption of tiny traces of H2O on the P25 surface (competitive chemisorption) decreasing the number of Brønsted sites. The heats of adsorption of the NH3ads-L species are measured after a first H(673 K)/T(300K) cycle leading to reproducible IR spectra. Adsorption of NH3 in the temperature range of 300-673 K. After a first H(673K)/C(300 K) cycle, Fig. 3 shows the evolutions of the IR bands in the wavenumber range of 1100-2000 cm-1 characteristic of the NH3ads-L and NH4+ species during the increase in Ta in the presence of 0.1% NH3/He (Pa= 0.1 kPa): (a) the common δas IR band of the two NH3ads-L species at 1596 cm-1 decreases associated to a slight shift to higher wavenumbers: 1606 cm-1 at 673 K (Fig. 3e), (b) in parallel, the IR band at 1142 cm-1 (δs NH3 of NH3ads-L1) decreases without any shift for Ta ≤ 373 K (Fig. 3b) and then with a progressive shift 1167 cm-1 and 1199 cm-1 at Ta= 473 K (Fig. 3c) and 673 K (Fig. 3e) respectively, (c) the intensity of the shoulder at 1215 cm-1 (NH3ads-L2) remains roughly unmodified up to 473 K (Fig. 3c) and then decreases and shifts slightly to higher wavenumbers (1229 cm-1 at 673 K) and (d) the broad IR bands of the NH4+ species decrease sharply with the increase in Ta and they are not detected at 423 K (Fig. 3c) in agreement with literature data.9,36 Clearly NH4+ species adsorbed on P25 at 300 K can not be present in the



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16 temperature range of NH3-SCR process. This explains that its heat of adsorption has not been studied. It must be noted that, in the 3800-3000 cm-1 range, the IR bands of the NH3 species decrease progressively with the increase in Ta in isobar condition (Pa= 0.1 kPa) and they are still detected at 3372, 3248 and 3156 cm-1 for Ta= 673 K (inset of Fig. 3, spectrum b). The IR bands of the OH groups increase with Ta due to the removal of the NH4+ species (result not shown) at low temperature and then to the decrease in the NH3ads-L species. At Ta= 673 K, their intensities are lower than that after the pretreatment procedure (inset Fig. 3). This can be due to either interactions between the OH groups and the remaining fraction of the NH3ads-L species or as proposed by auhtors,49,50 to the fact that the dehydroxylation of a catalyst already calcined at high temperatures is favored by the presence of adsorbed ammonia species which promote the proton migration. This last interpretation is consistent with the TPAE data (see below). The fraction of NH3ads-L species remaining at 673 K (inset Fig. 3) is eliminated after a pretreatment at 713 K in O2 leading to the spectrum of the fresh TiO2 solid (inset Fig. 1). 3.3 Individual heats of adsorption of the NH3ads-L species using the δs IR bands. In Fig. 3, the two IR bands at 1215 and 1142 cm-1, after adsorption at 300 K, are candidate for the measurement of the individual heats of adsorption of the NH3ads-L1 and NH3ads-L2 species using the AEIR method. However, there are two difficulties (a) due to the overlap of the δs IR bands their decomposition must be performed at each adsorption temperature and (b) the IR transmission below ≈1180 cm-1 decrease to values < 7% for Ta> 400 K leading to inaccuracies in the quantification of NH3ads-L1 species. For instance, the inset of Fig. 4 shows the decomposition of the δs IR bands after adsorption of NH3 at 300 K corresponding to the saturation of the two Lewis sites. This indicates that the contributions of NH3ads-L1 (1142 cm-1) and NH3ads-L2 (1215 cm-1) species to the total IR band area are of x1 ≈ 0.7 and x2 ≈ 0.3 respectively. Assuming identical IR absorption coefficients these values correspond to the


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17 proportion of the two NH3ads-L species at full coverage of the Lewis sites. According to DFT calculations,44,45 x1 and x2 are probably related to the TiIV and TiV sites on different planes. Finally, after decomposition of the two δs IR bands at each temperature, symbols and S in Fig. 4 provide (from Eq. 1, the two AM are measured after decompoiston at Ta=300 K) the experimental evolutions of the coverages of the NH3ads-L1 and NH3ads-L2 species respectively for Pa= 0.1 kPa. For NH3ads-L1, the calculations are are limited to Ta in the 300400 K range due to the decrease in the IR transmission for Ta > 400 K. The individual heats of adsorption of the two NH3ads-L species are obtained by determining the values that must be used in Eqs (2) and (3) to obtain theoretical curves θth= f(Ta) consistent with the experimental data. Curve a in Fig, 4 which overlaps symbols is obtained using EL1(1)= 56 kJ/mol and EL1(0)= 112 kJ/mol for the NH3ads-L1 species. Curve b in Fig. 4 which overlaps symbols S is obtained using EL2(1)= 104 kJ/mol and EL2(0)= 160 kJ/mol for the NH3ads-L2 species. These values show clearly that the NH3ads-L2 species is more strongly adsorbed than the NH3ads-L1 species (whatever their respective coverages) and that it is the dominant adsorbed species in the Pa and Ta ranges of interest of the NH3-SCR-process. The difficulties (decomposition of the IR bands and low IR transmission at high temperatures for the NH3ads-L1 species) encountered in the measurement of the individual heats of adsorption of the NH3ads-L species by using their δs IR bands are significantly increased on sulfated and V2O5 and/or WO3 supported TiO2 solids (forthcoming studies) and this prevents using the δs IR bands for the AEIR method. This leads us developing the AEIR method using the common δas IR bands of the NH3ads-L1 and NH3ads-L2 at 1596 cm-1 (Fig. 3) which is in a wavenumber range of high IR transmission for the different TiO2 based solids. 3.4 Individual heats of adsorption of the NH3ads-L species using the δas IR band The area of the IR band at 1596 cm-1 does not (a) increase at Ta= 300 K for Pa in the 0.1-1 kPa range (Fig. 2A) and (b) decrease at Pa= 0.1 kPa for Ta in the range of 300-313 K



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18 (result not shown). This indicates the full coverage of the Tiδ+ adsorption sites of the NH3ads-L1 and NH3ads-L2 species for Pa= 0.1 kPa and Ta= 300 K. The area of the IR band at 1596 cm-1 can be used in Eq. 1 to follow the evolution of the average coverage θA (Ta) of the two NH3ads-L species as shown by symbols z in Fig. 4 (AM in Eq. 1 is measured at Ta=300 K). Assuming that the two NH3ads-L species have similar IR absorption coefficients and taking into account that they are at full coverage of their sites for Pa= 0.1 kPa and Ta= 300 K, then AM= A1M + A2M where A1M and A2M are the contribution of NH3ads-L1 and NH3ads-L2 respectively to the area of the IR band at 1596 cm-1. At an adsorption temperature Ta> 300 K the average coverage θA (Ta) is given by:

θ A (Ta ) ==

A(Ta ) θ L1 = AM

A1M + θ L 2 AM

A2 M

= x1 θL1(Ta) + x2 θL2(Ta)

(5)

where θL1(Ta) and θL2(Ta) are the theoretical coverages of NH3ads-L1 and NH3ads-L2 respectively provided by Eqs. (2)-(3) and x1=A1M/AM and x2=A2M/AM are their contributions (in fraction) to the δas IR band at 300 K respectively. Curve c in Fig. 4, which overlaps symbols z has been obtained using in Eq. 5: x1= 0.73 and x2= 0.27, EL1(1)=56 kJ/mol, EL1(0)=105 kJ/mol, EL2(1)= 105 kJ/Mol, EL2(0)= 160 kJ/mol. It can be noticed that the individual heats of adsorption of the NH3ads-L1 and NH3ads-L2 species are consistent with those obtained using the

δs IR bands (curves a and b in Fig. 4). Moreover the proportions of the two species are consistent with the decomposition of the δs IR bands at 300 K (inset Fig. 4). This shows that the common δas IR band of the two NH3ads-L species, can be used to determine by the AEIR method both the proportions and heats of adsorption of the adsorbed NH3 species on Lewis sites. This procedure is particularly useful for solids with low IR transmission below 1300 cm1

such as TiO2 surface modified by sulfate, VxOy and WxOy depositions.

3.5 Heats of adsorption of the adsorbed NH3 species on TiO2-P25 using the TPAE method.


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19 The AEIR method is based on the assumption that there is a linear relationship between the intensities of the IR bands of the adsorbed species and their amounts on the surface (Eq. 1). The experimental ascertainment of this relationship has been made in few cases for the adsorption of CO on solids leading to the formation of a single adsorbed species.39,40.This cannot be performed if two or more adsorbed species are simultaneously present on the surface, as for the NH3 adsorption on P25. To validate Eq. 1, another method consists comparing the experimental evolution, during the increase in Ta in isobar condition, of the adsorption equilibrium coverage of NH3 using a volumetric method via the MS system (TPAE method,)34 to the average coverage of the adsorbed species via FTIR spectroscopy (AEIR method) using the δas IR band. Amount of adsorbed NH3 species formed on the P25 solid. Part A in Fig. 5 shows the evolution of the molar fractions of the gases during the switch He→ 0.5% NH3/0.5% Ar/He on the pretreated TiO2-P25 solid. The amount of adsorbed NH3 species is obtained from the difference between the Ar and NH3 signal: 332 µmol of NH3/g. This represents: ≈0.36 1019 NH3 species/m2 which can be compared to the approximate theoretical amount of sites on a solid surface: ≈ 1019 NH3 sites/m2. The switch 0.5% NH3/0.5% Ar/He → He in Fig. 5B, shows that a fraction of this amount is reversibly adsorbed: 96 µmol/g which corresponds to the desorption of weakly adsorbed species NH4+ species11,36 and of a small fraction of the NH3ads-L1 species (Fig. 2A). After the readsorption of NH3 in Fig. 5C, Fig. 5D shows the evolution of the molar fractions during the increase in Ta in the presence of 0.5% NH3/0.5% Ar/He (TPAE method). It can be observed that there is a first NH3 peak at low temperatures (Tm= 315 K) due to the decrease in the coverage of the weakly adsorbed NH4+ species followed by a broad asymmetric peak ending at ≈730 K, with a maximum at Tm ≈ 400 K, ascribed to the decrease in the adsorption equilibrium coverage of the two NH3ads-L species. A small H2O production: 17 µmol/g at Tm= 715 K, is observed



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20 without N2 production (similarly to NH3-TPD experiments on TiO2)49 indicating that the oxidation of the NH3ads-L species by the P25 solid is not operant. This H2O peak is consistent with the higher deshydration of the TiO2 surface in the presence of adsorbed NH3 species (inset Fig. 3) which are considered as favorable to the proton mobility.49,50 Note that on the M.S system, the fast cooling stage (i.e. ≈ 4 min in Fig. 4E) after the pretreatment procedure associated to the larger amount of solid (as compared to the IR cell) limits significantly the impact of the readsorption of tiny traces of H2O on the surface properties of the solid sample. The broad NH3 peak during the TPAE (Fig. 5 part D) is consistent with NH3-TPD experiments on TiO2 solids performed by different groups14,19 who observed a single broad NH3 peak in the range of 300-≈800 K associated with a peak of H2O at low temperature (Tm= 420 K)20 ascribed to H2O impurities (this peak is not observed in the present study and it is probably dependent on the level of H2O impurities for each experimental setup). Went et al.14 detect N2, NO and N2O productions at high temperatures not observed in the present study. Srnak et al.19 indicate a small shoulder at 400 K on a broad TPD NH3 peak at high temperature that seems correspond to the first NH3 peak at TM= 315 K during the TPAE procedure (Fig. 5D). The detection of this peak (due to weakly adsorbed species) during TPD experiments is dependent on the duration of the desorption before the increase in the desorption temperature. In Fig. 5D, the total amount of NH3 desorbed during the TPAE is QT= 328 µmol/g. The comparison with the total amount of adsorbed species at 300 K: 332 µmol/g is due to the fact that low apparent desorption rates (at low coverage of the surface for high temperatures) cannot be quantified with accuracy because the outlet and inlet molar fractions of NH3 in the reactor are too close. The decomposition of the first NH3 peak at 315 K during the TPAE as shown in Fig. 5D, allows us estimating the amount of NH4+ species: 25 µmol/g consistent with the low intensity of the IR band at 1474 cm-1 (Figs. 1 and 2) indicating that the amount


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21 of NH3ads-L species is ≈ (332-25)= 307 µmol/g (used as Qs in Eq. 4). Figure 5E shows the NH3 consumption due to its readsorption during the cooling stage to 300 K. The new TPAE in Fig. 5F shows (a) the good reproducibility of the NH3 peak as compared to Fig. 5D (total amount: 323 µmol NH3/g, first ≈14 µmol NH3/g) and (b) the absence of H2O production confirming that the H2O peak in Fig. 5D is due to the dehydratation of the solid in the presence of NH3 (the desorbed OH groups in Fig. 5D are regenerated neither by tiny H2O traces nor by the NH3 adsorption at low temperatures) Individual heats of adsorption of the adsorbed NH3 species on TiO2-P25 using TPAE. Using Eq. 4, the NH3 production, due to the decrease in the coverage of the NH3ads-L species during the TPAE (Fig. 5D) (after decomposition of the NH3 peak at 315 K ascribed to NH4+ species), leads to the evolution of their average coverage during the increase in Ta as shown by symbols z in Fig. 6. Considering the uncertainty in the measurement of low apparent desorption rate, we have limited the calculations to Ta= 673 K. The experimental curve in Fig. 6 is very similar to that provided by symbols z in Fig. 4 obtained using the evolution of the δas IR band at 1596 cm-1. Note that the highest increase in Pa during the TPAE (Fig. 5D) is 30% of the inlet partial pressure of NH3: this justifies the assumption of a quasiconstant adsorption pressure.34 Curve a in Fig. 6, for Pa= 0.5 kPa, is obtained similarly to curve c in Fig. 4 using Eqs. 2, 3 and 5 by considering two adsorbed NH3 species (noted X1 and X2 because TPAE does not allow characterizing the nature of the adsorption sites) with the following parameters: x1= 0.73, x2= 0.27, EX1(1)= 58 kJ/mol, EX1(0)= 104 kJ/mol, EX2(1)= 105 kJ/mol and EX2(0)= 150 kJ/mol. These values are consistent with those determined using the AEIR method considering that X1 is NH3ads-L1 (IR band at 1142 cm-1) and X2 is NH3ads-L2 (IR band at 1215 cm-1). This indicated that the amounts of NH3ads-L1 and NH3ads-L2 species: are (307 × 0.73)≈ 224 µmol/g and (307 × 0.27)≈ 83 µmol/g respectively. The comparison



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22 between TPAE and AEIR procedures validates the assumption of the linear relationship between the IR band area of the adsorbed species and their amounts on the surface (Eq. 1). 3.6 Isosteric heat of adsorption and adsorption model. The use of the δas IR band of the NH3ads-L species for the measurement of their individual heat of adsorption according to Eqs 1-3 and 5 is particularly useful because sulfated and VOx/WOy containing TiO2 solids have a very low IR transmission below 1300 cm-1 preventing using the δs IR bands of the NH3ads-L species for quantitative applications. Considering this situation we have used the δas IR band of the NH3ads-L species to compare the individual heats of adsorption of the two NH3ads-L species from the AEIR and TPAE methods to the isosteric heat of adsorption (noted Qiso) from the Clausius-Clapeyron Equation:51 Q ⎛ ∂lnPa ⎞ ⎜⎜ ⎟⎟ = iso R ⎝ ∂ (1 / T a ) ⎠θ

(6)

The isosteric heat of adsorption involves assumptions neither on the adsorption model nor on the adsorption coefficient: an agreement between the heats of adsorption obtained by the two procedures (Clausius-Clapeyron equation and AEIR) justifies the different assumptions involved in the AEIR method. Inset of Fig. 7 compares (on the same P25 pellet), the intensities of the δas IR band of the NH3ads-L species for two adsorption pressures: Pa= 0.1 kPa and 0.01 kPa, at two adsorption temperatures: Ta= 300 K (spectra a and b) and 673 K (spectra can d). The ratio of the two IR bands at 300 K (spectra a and b, in Fig. 7) provides θL= 0.89 for Pa= 0.01 kPa considering θL= 1 for Pa=0.1 kPa. Symbols and z in Fig. 7 give the evolutions of θL for the two NH3 adsorption pressures. The slopes of ln Pa=f(1/Ta) at different θL values provide Qiso(θL)=f(θL) as shown by symbols in Fig 8 (it must be noted that experimental uncertainties have a strong impact of Qiso: for instance ± 1 K leads to ≈Qiso(θL) ± 4 kJ/mol). It can be see that Qiso(0.89)= 59 kJ/mol is consistent with EL1(0.9)= 60 kJ/mol due to the fact that high average coverages θL are dominated by the NH3ads-L1 species:


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23 this validates the adsorption model and the expression of the adsorption coefficient (Eq. 2-3). At low θL there are significant differences with the EL2 values obtained from the AEIR method: i.e. Qiso(0.1)= 118 kJ/mol and EL2(0.1)= 154 kJ/mol. This is due to the fact that θL is obtained from AM= (AML1+AML2) and that at 300 K the adsorption of NH3 on P25 is dominated by the NH3ads-L1 species. The isosteric heat of adsorption at low coverage can be calculated considering that at Ta ≥ 600 K for Pa= 0.1 kPa, the intensity of the δas IR band is mainly due to the NH3ads-L2 species as indicated by curves a and b in Fig. 4. Curve b in Fig. 4 indicates that the coverage of this species is θL2=0.75 at 600 K for Pa= 0.1 kPa. Considering that the IR band area of the δas IR band at 600 K corresponds to this value, then the evolution of the coverage θL2 for Pa= 0.1 kPa and 0.01 kPa at Ta> 600 K can be obtained using the δas IR band as shown by symbols and { in Fig. 7. According to Eq. 6, these data provide the isosteric heat of adsorption of the NH3ads-L2 species: Qiso(θL2), as shown by symbols z in Fig. 8. For instance Qiso(0.37) = 143 kJ/mol whereas the AEIR method provides EL2=140 kJ/mol at θL2=0.37. This good agreement between Qiso(θL2) and EL2 validates the different assumptions and approximations associated to the AEIR and TPAE methods. Finally curves a and b in Fig.7 are obtained from Eqs. 2-3 and 5 using (a) Pa= 0.1 kPa and 0.01 kPa respectively, (b) the proportions x1=0.73 and x2=0.27 and (c) EL1(1)= 56 kJ/mol, EL1(0)= 105 kJ/mol, EL2(1)= 105 kJ/mol, EL2(0)= 155 kJ/mol. The different values are consistent with those used to obtain curve c in Fig. 4. The good agreement between experimental data and theoretical model for two adsorption pressures of NH3 reinforces the interest of the AEIR procedure. This is also confirmed by the comparison, in Fig. 7, of the experimental data for the NH3ads-L2 species (symbols and { from the δas IR band) and curves c and d for the two adsorption pressures using EL2(1)= 105 kJ/mol, EL2(0)= 155 kJ/mol. The net advantages of the AEIR method as compared to the Clausius-Clapeyron procedure are that (a) the experiments are more simple (a single isobar) and (b) it provides an mathematical expression of the adsorption coefficient for



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24 the adsorbed NH3 species which is a required step in the development of an EMA of the NH3SCR reaction on TiO2 based catalysts. The good agreement between the heats of adsorption of the NH3ads-L species obtained from the AEIR and Clausius-Clapeyron methods validates the mathematical expression of the adsorption coefficient provided by Eq. 3. This can also be justified by the evaluation of the impacts of the assumptions involved in this equation (see supporting information).

3.7 Comparison of the heats of adsorption of the NH3ads-L species with literature data. One of the advantages of the AEIR method is that it allows measuring the individual heats of adsorption of well identified adsorbed species present simultaneously on the surface of a solid. For the adsorption of NH3 on P25, we have focused the study on the adsorbed species which are present in the experimental conditions of the NH3-SCR (PNH3< 0.5 kPa and T > 523 K) and identified as adsorbed on Lewis sites (TiIV, TiV) (for these experimental condition NH4+ species are not detected). This must be taken into account for the comparison with literature data using temperatures lower than 523 K in experimental conditions allowing the formation of NH4+.

Comparison with experimental data on dispersed TiO2. The experimental measurements of the heats of adsorption (equal to the activation energy of desorption for non-activated chemisorption) of the adsorbed NH3 species on TiO2 solids are mainly performed using TPD methods and microcalorimetry. After adsorption of NH3 at low temperatures (≤ 300 K) on powdered TiO2 solids, TPD leads to a broad NH3 peak in the range of 300-800 K14,19,20,52,53 which is mainly used to measure the total amount of strongly adsorbed NH3 species and very rarely to measure the activation energy of desorption. This is due to the fact that readsorption of NH3 during the TPD experiments prevents using the classical mathematical formalism of TPD.54 This has been particularly quantified by Gorte

et al.55,56 who proposed different criteria allowing one to evaluate, using catalyst powders, the


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25 impact of different physical (i.e. diffusion) and chemical (i.e. readsorption) processes on the TPD data. These criteria reveal how difficult it is to design experiments preventing the readsorption for adsorbed species during TPD.34,57 One of the solution is to modify the classical TPD procedure by limiting the decrease in the coverage of the adsorbed species (quasi constant coverage assumption) and so the rate of desorption according to a procedure denoted Interrupted TPD58 as used in a previous work for the adsorption of NH3 on Al2O3.59 However, this procedure is time consuming and requires an analytical system able detecting traces of desorbing gases. The difficulties associated to TPD procedures were one of the driving forces for the development of the TPAE procedure.34 However, Srnak et al.19 have exploited NH3 TPD spectra (in the range of 300-673 K) on sulfate-free TiO2 (Toho Titanium, extra pure) using a mathematical formalism similar to that described by Gorte et al.55,56 involving (a) the impact of the NH3 readsorption and (b) the resolution of three differential equations describing the different material balances for ammonia in the nonsteady state TPD cell. This allows the authors providing the adsorption enthalpy of the adsorbed NH3 species. They consider that the broad NH3 peak observed in the 300-673 K range (at this temperature the peak is not ended) is due to three adsorbed species in proportion 0.2, 0.5 and 0.3 with the following heats of adsorption: 75 kJ/mol, 91kJ/mol and 102 kJ/mol.19 Their lowest heat of adsorption is higher than 58 kJ/mol found in the present study because the purge in an inert gas at 300 K, before the temperature ramp of the TPD procedure, removes weakly adsorbed species from the surface in particular a small fraction of the NH3ads-L species (Fig. 2A spectrum d). The other values are consistent with those determined in the present study for the sulfate-free TiO2-P25. However, as the broad TPD peak is not ended at the highest temperature (673 K), the authors,19 cannot quantify heats of adsorption higher than 102 kJ/mol, whereas we obtained 160 kJ/mol at low coverages for NH3ads-L2 species. Curve b in Fig. 6 provides the evolution of the coverage of the TiO2 surface by NH3 species in isobar



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26 condition: Pa= 0.5 kPa, considering that three adsorbed species are present on the surface (Langmuir model) with the heats of adsorption and proportions of Srnak et al.19 It can be seen that, if curve b is included in the temperature range determined in the present study, there are significant under estimations of the coverage at temperatures of interest for the SCR process (Ta >≈ 550 K) that are unacceptable for the development of a EMA of the NH3-SCR process. Microcalorimetry has been used for the characterization of the differential heat of adsorption of NH3 on TiO2. The comparison with the present study must take into account (a) the fact that these measurements provide an average of the heats of adsorption of the different adsorbed species and (b) according to the experimental conditions (particularly at low temperatures for strongly adsorbed species) the adsorption equilibrium is not reached in the whole solid sample.60 Sun et al.61 have measured the differential heat of adsorption of NH3 at 423 K on pure anatase TiO2 (141 m2/g). The heat of adsorption of 187 kJ/mol at coverage ≈ 0 decreases to ≈160 kJ/mol after adsorption of ≈10 µmol NH3/g. This amount corresponds to a coverage of 0.025 considering the total amount of NH3 adsorption: 393 µmol/g (see Table 2 in Ref. (61)). For higher coverage the heat of adsorption decreases roughly linearly with the increase in the coverage to ≈ 25 kJ/mol at high coverage. The initial heat of adsorption: 187 kJ/mol is probably associated to a very small amount of strongly adsorbed species that are not detected in the present study. However, 160 kJ/mol at θ= 0.025 is consistent with EL2(0) (Table 1). Moreover the linear decrease in the heat of adsorption in a large coverage range is consistent with the conclusions of the present study (Temkin model). The heat of adsorption at full coverage: 25 kJ/mol,61 is lower than that of the NH3ads-L1 species: EL1(1)= 56 kJ/mol because microcalorimetry measurements involve different adsorbed species in particular the weakly adsorbed NH4+ species and physorbed like species according to the highest adsorption pressure. Demartin-Chomel et al.18 have performed similar studies for the adsorption of NH3 at 353 K on a commercial TiO2 (GP350, 220 m2/g from Millenium Inorganic Chemicals).


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27 They observe an initial heat of adsorption of 150 kJ/mol that is consistent with EL2(0)=160 kJ/mol. in the present study. For higher coverages the heat of adsorption decreases slightly, at the difference of the present study and of Sun et al.61 This is probably due to the low adsorption temperature,18 which does not allow obtaining the adsorption equilibrium in the sample.60

Comparison with DFT calculations. Wanbayor and Ruangpornvisuti62 have evaluated the adsorption parameters of di, tri and polyatomic gases on anatase TiO2 (001) and (101). For NH3, they determine that it is adsorbed via the N atom on Ti Lewis sites with heats of adsorption of 125 kJ/mol and 164 kJ/mol on (001) and (101) faces respectively (there is no indication of the coverage). These values are consistent with the heats of adsorption at low coverage of the two NH3ads-L species. Onal et al63 have performed DFT calculations for the adsorption and dissociation of H2O and NH3 on the same anatase TiO2 surfaces modeled as a Ti2O9H10 cluster. They determine that NH3 is bonded to Ti4+ site via nitrogen atom according to a non-activated adsorption process with heat of adsorption of 107 kJ/mol and 129 kJ/mol on partially relaxed and fixed TiO2 (101), respectively, and 111 kJ/mol and 99 kJ/mol on partially relaxed and fixed TiO2 (001), respectively.63 The calculations of the heat of adsorption of ammonia adsorbed via H bonding are significantly lower < 60 kJ/mol.63 In an early study, Markowits et al.64 have determined using ab initio Hartree-Fock crystalline orbital program CRYSTAL the heats of adsorption of NH3 on TiO2 at two coverages: ≈ 134 kJ/mol and 183 kJ/mol at coverage of 1 to and 1/2 respectively. These values are significantly different of those found in the present study but they indicate an increase in the heat of adsorption with the decrease in the coverage that is consistent with the Temkin model. Erdogan et al.59 have determined the heat of adsorption of NH3 at θ= 0.25 on TiO2 (001): 104 kJ/mol, consistent with the values found in the present study. The situation is similar with the study of Pan et al.65 on anatase (001) TiO2 nanotubes,



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Page 28 of 55

28 who determined 95 kJ/mol and 110 kJ/mol for the heats of adsorption of NH3 for TiO2 nanotubes arrays of 12 and 18 respectively. Guo et al.66 have shown by DFT calculations, that the heat of adsorption of NH3 on Ti sites of TiO2-B (100) decreases linearly from 105 to 75 kJ/mol with the increase in the coverage in the range of 1/6-1. This relationship is consistent with the conclusion of the present study even if the values are slightly different. Mc Grill and Idriss67 have performed similar calculations on the Ti sites of the (011) surface of rutile TiO2: at coverage 0.5 and 1 they determine heats of adsorption of 100 and 58 kJ/mol that are consistent with the values of the NH3ads-L1 species particulary at high coverage.

Comparison with surface sciences data. We limit the comparison to a recent study of Kim et al.68 who refer to different works on this approach. The authors use a TPD procedure in UHV conditions to measure the activation energy of desorption of NH3 on the Ti sites of the rutile TiO2(110)-1 × 1 surface, as a function of the coverage. The point of interest is that the authors show that the heat of adsorption increases linearly : from 55 to 120 kJ/mol, with the decrease in the coverage in the range of 1- ≈0.15 (the value at coverage 1 is consistent with the EL1= 56 kJ/mol) whereas in the 0.15-0 range, there is a second linear relationship with a different slope. This suggests two adsorbed NH3 species with different heats of adsorption as observed in the present study. Moreover the heat of adsorption at full coverage is in good aggrement with that obtained from the AEIR method for NH3ads-L1 species which is the dominant species at low temperature.. The above comparisons show that literature data on the experimental and theoretical heats of adsorption of adsorbed NH3 species on sulfate free TiO2 powders are in the range of those determined, using either the AEIR or the TPAE method, for the NH3ads-L1 and NH3ads-L2 species. However, the present procedure allows us differentiating the adsorbed species NH3 species on Lewis sites leading to their individual heats of adsorption at different coverages. Moreover, the experimental conditions are well defined (adsorption equilibrium) that is not


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29 the situation of all TPD and microcalorimetry studies. The measurement of the individual heat of adsorption of two NH3ads-L species which can be simultaneously present on the TiO2 surface has not been previously performed at this level of description of adsorbed species and constitutes a key step of the experimental microkinetic approach of the NH3-SCR process. In particular this allows (a) to take into account the individual role of each adsorbed NH3 species for the understanding of the surface processes of the NH3-SCR: for instance the coverage of the NH3ads-L1 species decreases significantly in the experimental conditions of the NH3-SCR process whereas that of the NH3ads-L2 species remains very high and (b) to use the mathematical expression of the adsorption coefficient and the kinetic model provided by Eqs. 2-3 favoring the kinetic modeling of the catalytic activity associated to the EMA for TiO2 based catalysts.

4- Conclusion

The present study has been dedicated to the measurement of individual heats of adsorption of the adsorbed species formed on a sulfate-free TiO2-P25 from Degussa (used in numerous studies, as support of model V2O3/WO3/TiO2 NH3-SCR catalysts) by adapting two original experimental procedures (denoted AEIR and TPAE) developed in previous work for the adsorption of CO on supported metal particles and pure metal oxides. The two methods which use adsorption temperature (300-673 K) and partial pressure of NH3 (< 0.5 kPa) representative of the NH3-SCR process show that the TiO2 solid leads mainly to two adsorbed species on Lewis sites (denoted NH3ads-L1 and NH3ads-L2) and a small amount of NH4+ species on Brønsted sites. Heats of adsorption of NH4+ species are significantly lower than those of NH3ads-L species: they are not detected at temperature and pressure of the NH3-SCR process. The proportions and the individual heats of adsorption of the NH3ads-L1 and NH3ads-L2 species have been determined using either their characteristic δs IR band at 1142 cm-1 and 1215 cm-1



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Page 30 of 55

30 (at 300 K) respectively or their common δas IR band at 1596 cm-1. It has been shown that the heats of adsorption of the two species vary linearly with the coverage of their sites as indicated in Table 1. It must be noted that the low stability of NH4+ species explains that they are not present on the surface at high temperatures in the absence of H2O in the gas phase (present study). However, the significant partial pressure of H2O during NH3-SCR may favor the conversion of the NH3ads-L species to NH4+ species. This supposes that the heat of adsorption of H2O is not much lower than those of the NH3ads-L species, allowing the simultaneous presence of NH3 and H2O adsorbed species on the surface. This is clearly conceivable taking into account that the coverage of NH3ads-L1 species is low under experimental conditions representative of the NH3-SCR process, releasing some sites for the formation of H2Oads species. Forthcoming articles, using the procedures describe in the present work will be dedicated to the impacts on the nature of the adsorbed species and their individual heat of adsorption of (a) the presence of superficial SO4 groups (sulfated TiO2), (b) the deposition VOx or/and WOy on sulfate free and sulfated TiO2 and (c) the co adsorption of NH3 and H2O.


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31 FIGURES:

A

3720 3690 3672 0.02

3245

0.04 3348

3690

3704

3640

3145

3672

e 3640

Absorbance

a

d

B

a

g

3630 3670 3800 3700 3600 3500

1596

1215

0.04

e 3660 3700

1142

3190

3396

3720 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


a 3500 3300 3100 Wavenumber (cm-1)

1680 1800

1477

a

1600 1400 1200 Wavenumber (cm-1)

Figure 1: Adsorption of 0.1% NH3/He at 300 K on TiO2-P25 pretreated at 713 K in helium as

a function of adsorption duration ta: (a-g) 0, 9, 18, 30, 40, 50, 96 s. Inset: Evolution of the IR bands of the OH groups during the cooling stage in helium after pretreatment: (a-d) T= 713, 498, 423 and 300 in K.



32

0.11

1596

A

Absorbance 1477

1680

b,c

0.04

d

0.02

1142 a

B

a,b,c

Absorbance

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

Page 32 of 55

1215

1596 a 1477

a

d

b,c 1900

1700

1500

Wavenumber (cm-1)

1300

2000

1800 1600 1400 1200 Wavenumber (cm-1)

Figure 2: Interaction of NH3 with TiO2-P25. Part A: Impact of the adsorption pressure of

NH3 at 300 K on the IR bands of the adsorbed NH3 species: (a-c) PNH3= 0.1, 0.5 and 1 kPa, (d) in helium after desorption during 260 s. Part B: Impact of the number of adsorption (673 K)/ cooling to 300 K cycles using 0.1% NH3/He, on the intensities of the IR bands of the adsorbed NH3 species (a) after the pretreatment procedure, (b) and (c) after the first and the second cycle respectively.


Page 33 of 55

33

3704 3671

1142

Absorbance

0.02

Absorbance

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


a 3372

b

3248 3156

a 3700 3300 Wavenumber (cm-1)

1596

1167

1188 1215 1199 e 1229 1477

0.04 1606 2000

1800

1600 1400 1200 -1 Wavenumber (cm )

Figure 3: Impact of the adsorption temperature Ta on the IR bands of the adsorbed NH3

species on TiO2-P25 using 0.1% NH3/He: (a-e) Ta= 300, 373, 473, 573 and 673 K. Inset: comparison of the IR band of the OH groups before (spectrum a) and after (spectrum b) adsorption of 0.1% NH3/He at 673 K.



34

1

Absorbance

Coverage of the NH3 adsorbed species

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

Page 34 of 55

0.8 0.6

NH3ads-L1 NH3ads-L2

1400 1300 1200 1100

Wavenumber (cm-1)

0.4 b

0.2

a c

0

300

400

500 600 700 Temperature (K)

800

900

Figure 4: Heats of adsorption of the adsorbed NH3 species on TiO2-P25 using the AEIR

method. □ and S coverages of the NH3ads-L2 and NH3ads-L1 adsorbed species using their δs IR bands respectively; full lines: (a) and (b) theoretical coverages of the NH3ads-L1 and NH3ads-L2 species considering the Temkin model using EL1(1)= 56 kJ/mol, EL1(0)= 102 kJ/mol, EL2(1)= 102 kJ/mol and EL2(0)=160 kJ/mol; z average coverage of the NH3ads-L1 and NH3ads-L2 species using δas IR band at 1596 cm-1; (c) theoretical evolution of the average coverage of the NH3ads-L species considering molar fractions x1=0.73 and x2=0.27 for the NH3ads-L1 and NH3adsL2

species respectively using of EL1(1)= 56 kJ/mol, EL1(0)= 105 kJ/mol, EL2(1)= 105 kJ/mol

and EL2(0)=160 kJ/mol. Inset: Decomposition of the δs IR bands of the NH3ads-L2 and NH3adsL1

species at 300 K.


Page 35 of 55

35

A|B|C|

Molar fractions of the gases

1

D

Tm=315 K

Ar×160

Tm= 400 K

| E |

F

773

NH3 ×160

0.8

673

0.6

573

T

0.4

473

0.2

373 Tm= 715 K

0

0

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


20

40

60

H 2O ×500

80 Time (min)

100

120

273

140

Figure 5 : Adsorption (Part A)/ desorption (Part B)/readsorption (Part C) at 300 K using 0.5%

NH3/0.5% Ar /He on TiO2-P25 followed by TPAE (Part D), readsorption during cooling in NH3 (Part E) and a second TPAE (Part F) (see the text for more details).



36


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 36 of 55

1

0.8 0.6

0.4

0.2 b 0

300

a

400 500 600 700 800 Adsorption temperature (K)

900

Figure 6 : Experimental evolution of the average coverage of the NH3ads-L species on TiO2-

P25 during the TPAE procedure (symbols z) for Pa= 0.5 kPa and theoretical coverage (curve a) using the Temkin model considering the presence of two adsorbed NH3 species (X1 and X2) with the following parameters: X1, molar fraction x1= 0.73, heats of adsoption EX1(1)= 58 kJ/mol and EX1(0)= 104 kJ/mol; X2, molar fraction x2= 0.27, heats of adsorption EX2(1)= 105 kJ/mol and EX1(0)= 150 kJ/mol. (see the text for more detail). Curve b evolution of the theoretical coverage of the adsorbed NH3 species with Ta for Pa= 0.1 kPa according to activation energies of desorption of Ref (19).


Page 37 of 55

37

1596 a b

1

0.8

c

d

Absorbance


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


0.02

0.6

0.4

d

a

b

1604 c

1700 1600 1500 Wavenumber (cm-1)

0.2 0

300

400

500


900

Figure 7: Comparison of the isosteric heat of adsorption (Clausius-Clapeyron equation) and

AEIR methods: and z average coverage of the NH3ads-L species from the δas IR band for Pa= 0.1 kPa and 0.01 kPa respectively; and { coverage of the NH3ads-L2 species at high temperatures from the δas IR band (see the text for more detail); (a) and (b) theoretical evolution of the average coverage of the NH3ads-L species for Pa= 0.1 kPa and 0.01 kPa respectively considering: molar fractions x1=0.73 and x2=0.27 for the NH3ads-L1 and NH3adsL2

species respectively and heat of adsorption of EL1(1)= 56 kJ/mol, EL1(0)= 105kJ/mol,

EL2(1)= 105 kJ/mol, EL2(0)= 155 kJ/mol; (c) and (d) theoretical evolution of the coverage of the NH3ads-L2 species considering EL2(1)= 105 kJ/mol and EL2(0)= 155 kJ/mo for Pa = 0.1 kPa and Pa=0.01 kPa respectively. Inset: Comparison of the δas IR band at Ta= 300 K (spectra a and b) and 673 K (spectra c and d) for Pa= 0.1 kPa and Pa= 0.01 kPa respectively.



38

Isosteric heat of adsorption of NH3ads species (kJ/mol)

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

Page 38 of 55

160 140 120 100 80 60 40

0

0.2 0.4 0.6 0.8 Coverage θL () and θL2(z)

1

Figure 8: () Isosteric heat of adsorption of the NH3ads-L species on TiO2-P25 using the δas IR

band (inset Fig. 7) as a function of the average coverage θL; (z) Isosteric heat of adsorption of the NH3ads-L2 species using the δas IR band at high temperatures (see the text for more details).


Page 39 of 55

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39 TABLES:

Adsorbed species

E(0) kJ/mol

E(1) kJ/mol

Proportion

AEIR using the δs IR bands NH3ads-L1

112

56

≈0.7

NH3ads-L2

160

104

≈0.3

AEIR using the common δas IR band NH3ads-L1

105

56

0.73

NH3ads-L2

160

105

0.27

Table 1 : Heats of adsorption (accuracy ± 5 kJ/mol) at low and high coverages and

proportions of the NH3ads-L1 and NH3ads-L2 adsorbed species formed on two Lewis sites of TiO2-P25 according to the AEIR procedure using their characterisitic δs and δas IR bands.



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 40 of 55

40 AUTHOR INFORMATION Corresponding Author

*: To whom correspondence should be addressed. Tel:0033472431419 E-mail: [email protected]

ACKNOWLEDGMENT.

D.B thanks the “Institut de Chimie de Lyon” for the purchase of the mass spectrometer in the framework of the “Contrat de Projets Etat-Région” Rhône-Alpes (2007-2013).

SUPPORTING INFORMATION AVAILABLE: Evaluation of the impact of the approximations involved in the expression of the adsorption coefficient on the heats of adsorption of the NH3 species. This information is available free of charge via the Internet at http://pubs.acs.org


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41 REFERENCES (1) Forzatti, P. Environmental Catalysis for Stationary Applications. Catal. Today 2000, 62, 51-65. (2) Li, J.; Chang, H.; Ma, L.; Hao, J.; Yang, R. T. Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Metal Oxide and Zeolite Catalysts—A Review.

Catal.Today 2011, 175, 147-156. (3) Brandenberger, S.; Kröcher, O.; Tissler, A.; Althoff, R. The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts. Catal.

Rev.: Sci. Eng. 2008, 50, 492-531. (4) Bond, G. C.; Tahir, S. F. Vanadium Oxide Monolayer Catalysts Preparation, Characterization and Catalytic Activity. Appl. Catal. 1991, 71, 1-31. (5) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and Mechanistic Aspects of the Selective Catalytic Reduction of NOx by Ammonia over Oxide Catalysts: A review. Appl.

Catal., B 1998, 18, 1-36. (6) Calatayud, M.; Mguig, B.; Minot, C. Modeling Catalytic Reduction of NO by Ammonia over V2O5. Surf. Sci. Rep. 2004, 55, 169-236. (7) Centi, G.; Perathoner, S. The Role of Ammonia Adspecies on the Pathways of Catalytic Transformation at Mixed Metal Oxide Surfaces. Catal. Rev.: Sci. Eng. 1998, 40, 175-208. (8)

Shang, X.; Li, J.; Yu, X.; Chen, J.; He, C. Effective Regeneration of Thermally

Deactivated Commercial V-W-Ti Catalysts. Front. Chem. Sci. Eng. 2012, 6, 38-46. (9) Nova, I.; dall’Acqua, L.; Lietti, L.; Giamello, E.; Forzatti, P. Study of Thermal Deactivation of a De-NOx Commercial Catalyst. Appl. Catal., B. 2001, 35, 31-42. (10) Liu, Z.; Woo, S. I. Recent Advances in Catalytic DeNOx Science and Technology. Catal.

Rev.: Sci. Eng. 2006, 48, 43-89. (11) Busca, G.; Saussey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V. FTIR Characterization of Surface Acidity of Different Titanium Dioxide Anatase Preparation. Appl. Catal. 1985, 14, 245-260. (12) Rajadhyaksha, R. A.; Knozinger, H. Ammonia Adsorption on Vanadia Supported on Titania-Silica Catalyst: An Infrared Spectroscopic Investigation. Appl. Catal. 1989, 51, 81-92. (13) Dines, T. J.; Rochester, C. H.; Ward, A. M. Infrared and Raman Study of the Adsorption of NH3, Pyridine, NO and NO, on Anatase. J. Chem. Soc., Faraday Trans. 1991, 87, 643-651.



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 55

42 (14) Went, G. T.; Leu, L.; Lombardo, S. J.; Bell, A. T. Raman Spectroscopy and Thermal Desorption of NH3 Adsorbed on TiO2 (Anatase)-Supported V2O5. J. Phys. Chem. 1992, 96, 2235-2241. (15) Jung, S. M.; Grange, P. Evidence of Correlation Between Electronic Density and Surface Acidity of Sulfated TiO2. Catal. Lett. 2001, 1-2, 27-30. (16) Guimon, C.; Gervasini, A.; Auroux, A. XPS Study of the Adsorption of SO2 and NH3 over Supported Tin Dioxide Catalysts Used in de-NOx Catalytic Reaction. J. Phys. Chem. B 2001, 105, 10316-10325.

(17) Zou, H.; Li, M.; Shen, J.; Auroux, A. Surface Acidity of Supported Vanadia Catalysts. J.

Therm. Anal. Calorim. 2003, 72, 209-221. (18) Desmartin-Chomel, A.; Flores, J. L.; Bourane, A.; Clacens, J. M.; Figueras, F.; Delahay, G.; Giroir Fendler, A.; Lehaut-Burnouf, C. Calorimetric and FTIR Study of the Acid Properties of Sulfated Titanias. J. Phys. Chem. B 2006, 110, 858-863. (19) Srnak, T. Z.; Dumesic, J. A.; Clausen, B. S.; Tornqvist, E.; Topsøe, N.Y. TemperatureProgrammed Desorption/Reaction and in Situ Spectroscopic Studies of Vanadia/Titania for Catalytic Reduction of Nitric Oxide. J. Catal. 1992, 135, 246-262. (20) Lietti, L; Forzatti, P. Temperature Programmed Desorption/reaction of ammonia over V2O5/TiO2 De-NOxing Catalysts. J. Catal. 1994, 147, 241-249. (21) Bourane, A.; Bianchi, D. Oxidation of CO on a Pt/Al2O3 Catalyst: From the Surface Elementary Steps to Light-Off Tests : I. Kinetic Study of the Oxidation of the Linear CO Species. J. Catal. 2001, 202, 34-44. (22) Bourane, A.; Bianchi, D. Heats of Adsorption of the Linear CO Species on Pt/Al2O3 using Infrared Spectroscopy: Impact of the Pt Dispersion. J. Catal. 2003, 218, 447-452. (23) Bianchi, D.; Ristori, A.; Jean, E.; Vonard, R. Catalytic Oxidation of a Diesel Soot Formed in the Presence of a Cerium Additive. III. Microkinetic-Assisted Method for the Improvement of the Ignition Temperature. Energy Fuels 2005, 19, 1453-1461. (24) Bassou, B.; Guillaume, N.; Lombaert, K.; Mirodatos, C.; Bianchi, D. Experimental Microkinetic Approach of the Catalytic Oxidation of Diesel Soot by Ceria Using Temperature-Programmed Experiments. Part 1: Impact and Evolution of the Ceria/Soot Contacts during Soot Oxidation Energy Fuels 2010, 24, 4766-4680. (25) Arsac, F.; Bianchi, D.; Chovelon, J. M.; Ferronato, C.; Herrmann, J. M. Experimental Microkinetic Approach of the Photocatalytic Oxidation of Isopropyl Alcohol on TiO2. Part 1. Surface Elementary Steps Involving Gaseous and Adsorbed C3HxO Species. J. Phys. Chem. A 2006, 110, 4202-4212.


Page 43 of 55

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


43 (26) Arsac, F.; Bianchi, D.; Chovelon, J. M.; Ferronato, C.; Herrmann, J. M. Experimental Microkinetic Approach of the Photocatalytic Oxidation of Isopropyl Alcohol on TiO2. Part 2. From the Surface Elementary Steps to the Rates of Oxidation of the C3HxO Species. J. Phys.

Chem. A 2006, 110, 4213-4222. (27) Dumesic, J. A.; Rudd, D. F.; Aparicio, L. M. ; Rekoske, J. E.; Treviño, A. A. The

Microkinetics of Heterogeneous Catalysis; ACS Professional Reference Book, Am. Chem. Soc.: Washington, DC., 1993. (28) Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. Fourier Transform-Infrared Study of the Adsorption and Coadsorption of Nitric Oxide, Nitrogen Dioxide and Ammonia onVanadiaTitania and Mechanism of Selective Catalytic Reduction. Appl. Catal. 1990, 64, 259-278. (29) Topsøe, N. Y. Mechanism of the Selective Catalytic Reduction of Nitric Oxide by Ammonia Elucidated by in Situ On-Line Fourier Transform Infrared Spectroscopy. Science 1994, 265, 1217-1219.

(30) Topsøe, N. Y.; Topsøe, H.; Dumesic, J. H. Vanadia/Titania Catalysts for Selective Catalytic Reduction of Nitric Oxide by Ammonia: 1. Combined Temperature Programmed in situ FTIR and On-Line Mass Spectroscopy Studies. J. Catal. 1995, 151, 226-240. (31) Dulaurent, O.; Bianchi, D. Adsorption Isobars for CO on a Pt/Al2O3 Catalyst at High Temperatures using FTIR Spectroscopy: Isosteric Heat of Adsorption and Adsorption Model. Appl. Catal., A 2000, 196, 271-280. (32) Derrouiche, S.; Gravejat, P.; Bianchi, D. Heats of Adsorption of Linear CO Species Adsorbed on the Au° and Ti+δ Sites of a 1% Au/TiO2 Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium. J. Am. Chem. Soc. 2004, 126, 13010-13015. (33) Derrouiche, S.; Bianchi, D. Heats of Adsorption of the Linear and Bridged CO species on a Ni/Al2O3 Catalyst by Using the AEIR Method. Appl. Catal., A 2006, 313, 208-217. (34) Derrouiche, S.; Bianchi, D.

Heats of Adsorption Using Temperature Programmed

Adsorption Equilibrium: Application to the Adsorption of CO on Cu/Al2O3 and H2 on Pt/Al2O3. Langmuir 2004, 20, 4489-4497. (35) Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Fourier Transform Infrared Study of the Adsorption and Coadsorption of Nitric Oxide, Nitrogen Dioxide and Ammonia on TiO2 Anatase. Appl. Catal. 1990, 64, 243-257. (36) Gallardo Amoresa, J. M.; Sanchez Escribanoa, V.; Ramis, G.; Busca, G. An FT-IR Study of Ammonia Adsorption and Oxidation over Anatase-supported Metal Oxides. Appl. Catal.,

B. 1997, 13, 45-58.



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 55

44 (37) Chafik, T.; Dulaurent, O.; Gass, J. L.; Bianchi, D. Heat of Adsorption of Carbon Monoxide on a Pt/Rh/CeO2/Al2O3 Three-Way Catalyst Using in-Situ Infrared Spectroscopy at High Temperatures. J. Catal. 1998, 179, 503-514. (38) Bourane, A.; Bianchi, D. Oxidation of CO on a Pt/Al2O3 Catalyst: From the Surface Elementary Steps to Lighting-Off Tests II. Kinetic Study of the Oxidation of Adsorbed CO Species Using Mass Spectroscopy. J. Catal. 2002, 209, 114-125. (39) Bourane, A.; Dulaurent, O.; Bianchi. D. Comparison of the Coverage of the Linear CO Species on Pt/Al2O3 Measured under Adsorption Equilibrium Conditions by Using FTIR and Mass Spectroscopy. J. Catal. 2000, 195, 406-411. (40) Zeradine, S.; Bourane, A.; Bianchi, D. Comparison of the Coverage of the Linear CO Species on Cu/Al2O3 Measured under Adsorption Equilibrium Conditions by Using FTIR and Mass Spectroscopy. J. Phys. Chem. B 2001, 105, 7254-7257. (41) Deiana, C.; Fois, E.; Coluccia, S.; Martra, G. Surface Structure of TiO2 P25 Nanoparticles: Infrared Study of Hydroxy Groups on Coordinative Defect Sites. J. Phys.

Chem. C 2010, 114, 21531-21538. (42) Griffiths D. M.; Rochester, C. H. Infrared Study of the Adsorption of Water on to the Surface of Rutile. J. Chem. Soc., Faraday Trans. 1977, 73, 1510-1529. (43) Tanaka K.; White, J. M. Characterizatlon of Specles Adsorbed on Oxidized and Reduced Anatase. J. Phys. Chem. 1982, 86, 4708-4714. (44) Arrouvel, C.; Digne, M.; Breysse, M.; Toulhoat, H.; Raybaud, P. Effects of Morphology on Surface Hydroxyl Concentration: a DFT Comparison of Anatase–TiO2 and γ-Alumina Catalytic Supports. J. Catal. 2004, 222, 152-166. (45) Dzwigaj, S.; Arrouvel, C.; Breysse, M.; Geantet, C.; Inoue, S.; Toulhoat, H.; Raybaud, P. DFT Makes the Morphologies of Anatase-TiO2 Nanoparticles Visible to IR Spectroscopy.

J. Catal. 2005, 236, 245-250. (46) Parfitt. G. D.; Ramsbotham, J.; Rochester, C. H. Infra-red Study of Ammonia Adsorption on Rutile Surfaces. J. Chem. Soc., Faraday Trans. 1971, 67, 841-847. (47) Guglielminotti, E.; Boccuzzi, F. Infrared Spectroscopic Study of Ammonia and Nitric Oxide Adsorption and Reactivity on a Ru/TiO2 Catalyst: Effect of Oxo-Reducing Treatments.

J. Chem. Soc., Faraday Trans. 1991, 87, 337-343. (48) Hadjiivanov, K.; Klissurski, D.; Busca, G.; Lorenzelli, V. Benzene-Ammonia Coadsorption on TiO2 (Anatase). J. Chem. Soc., Faraday Trans. 1991, 87, 175-178.


Page 45 of 55

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


45 (49) Nova, I.; Lietti, L.; Casagrande, L.; Dall'Acqua, L.; Giamello, E.; Forzatti, P. Characterization and Reactivity of TiO2-Supported MoO3 De-NOx SCR Catalysts. Appl.

Catal., B 1998, 17, 245-258. (50) Lietti, L.; Nova, I.: Ramis, G.; Dall’Acqua, L.; Busca, G.; Giamello, E.; Forzatti, P.; Bregani, F. Characterization and Reactivity of V2O5–MoO3/TiO2 De-NOx SCR Catalysts. J.

Catal. 1999, 187, 419-435. (51) Tompkins, F. C. Chemisorption of Gases on Metals; Acad Press (London) LTD: NewYork, 1978. (52) Choo, S. T.; Lee, Y. G.; Nam I. S.; Ham, S. W.; Lee, J. B. Characteristics of V2O5 Supported on Sulfated TiO2 for Selective Catalytic Reduction of NO by NH3. Appl. Catal., A 2000, 200, 177-188.

(53) Turco, M.; Bagnasco, G.; Russo, G.; Ciambelh, P; Patrono, P.; Massucci, M. A.; Vecchio, S. NH3 TPD Study and Thermal Behaviour of Vanadium-Exchanged Titanium Phosphates as Catalysis: Reduction of NO with NH3. J. Therm. Anal. 1996, 47, 215-225. (54) Lemaitre, J. C. Characterization of Heterogeneous Catalysts; Marcel Dekker: New York, 1984. (55) Gorte, R. J. Design Parameters for Temperature Programmed Desorption from Porous Catalysts. J. Catal. 1982, 75, 164-174. (56) Demmin, R. A.; Gorte, R. J. Design Parameters for Temperature-Programmed Desorption from a Packed Bed. J. Catal. 1984, 90, 32-39. (57) Efsthathiou, A.; Bennett, C. O. Enthalpy and Entropy of H2 Adsorption on Rh/Al2O3 Measured by Temperature-Programmed Desorption. J. Catal. 1990, 124, 116-126. (58) Gaillard, F.; Amdat, M.; Joly, J. P.; Perrard, A. An Intermittent Temperatureprogrammed Desorption Method for Studying Kinetics of Desorption from Heterogeneous Surfaces. Appl. Surf. Sci. 2004, 238, 91-96. (59) Joly, J. P.; Khalfallah, M.; Bianchi, D.; Pajonk, G. M. Acidity of a Microporous Amorphous Alumina Measured by Intermittent Temperature-Programmed Desorption of Ammonia. Appl. Catal., A. 1993, 98, 61-70. (60) Babitz, S. M.; Williams, B. A.; Kuehne, M. A.; Kung, H. H.; Miller, J. T. Surface Equilibration in Adsorption Microcalorimetry of Bases on H-USY. Thermochim. Acta 1998,

312, 17-25. (61) Sun, Q.; Fu, Y.; Liu, J.; Auroux, A.; Shen, J. Structural, Acidic and Redox Properties of V2O5-TiO2-SO42- Catalysts. Appl. Catal., A 2008, 334, 26-34.



46 (62) Wanbayor, R.; Ruangpornvisuti, V. Adsorption of di-, tri- and Polyatomic Gases on the Anatase TiO2 (0 0 1) and (1 0 1) Surfaces and their Adsorption Abilities. J. Mol. Struct.:

THEOCHEM 2010, 952, 103-108. (63) Onal, I.; Soyer, S.; Senkan, S. Adsorption of Water and Ammonia on TiO2-Anatase Cluster Models. Surf. Sci. 2006, 600, 2457-2469. (64) Markovits, A.; Ahdjoudj, J.; Minot, C. A. Theoretical Analysis of NH3 Adsorption on TiO2. Surf. Sci. 1996, 365, 649-661. (65) Pan, X.; Cai, Q. X.; Chen, W. L.; Zhuang, G. L.; Li, X. N.; Wang, J. G. A DFT Study of Gas Molecules Adsorption on the Anatase (001) Nanotube Arrays. Comput. Mater. Sci. 2013,

67, 174-181. (66) Guo, X. J.; Liu, W.; Fang, W.; Cai, L.; Zhu, Y.; Lu, L.; Lu. X. DFT Study of CoverageDepended Adsorption of NH3 on TiO2-B (100) Surface. Phys. Chem. Chem. Phys. 2012, 14, 16618-16625. (67) McGill, P. R.; Idriss, H. Ab Initio Study of Surface Acid-Base Reactions. The Case of Molecular and Dissociative Adsorption of Ammonia on the (011) Surface of Rutile TiO2.

Langmuir 2008, 24, 97-104. (68) Kim, B.; Li, Z.; Kay, B. D.; Dohnalek, Z.; Kim, Y. K. The Effect of Oxygen Vacancies on the Binding Interactions of NH3 with Rutile TiO2(110)-1 × 1. Phys. Chem. Chem. Phys. 2012, 14, 15060-15065.

SN Synopsis TOC

Heats of adsorption of NH3 species on TiO2-P25 NH3ads-L1 EL1(1)=56 kJ/mol EL1(0)=112 kJ/mol

AEIR method 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 55

PNH3= 0.1 kPa Ta = 300 K 473 K 0.04 2000

673K 1800

NH3ads-L1 + NH3ads-L2

NH3ads-L2 EL2(1) =104 kJ/mol EL2(0) =140 kJ/mol NH4+

1600 1400 Wavenumber (cm-1) ACS Paragon Plus Environment

1200

Page 47 of 55

A

3720

3245

3690

3704

g Absorbance

e 3640

1142

3640

3145

3672

B

0.02

3190

3396

3720

a

3348

d

3672 3690

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


3630 3670

a

3800 3700 3600 3500

1596

1215

0.04 e 3660 3700

a 3500 3300 3100 Wavenumber (cm-1)

1680 1800

Figure 1


1477

a

1600 1400 1200 -1 Wavenumber (cm )


1596

A

a,b,c

0.04

d

0.02

1142 a

B

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 48 of 55

1477

1680

1215

1596 a 1477

a

d

b,c 1900

1700

Wavenumber

1500

(cm-1)

1300

2000

1800 1600 1400 1200 Wavenumber (cm-1)

Figure 2 ACS Paragon Plus Environment

b,c

Page 49 of 55

3704 3671

1142

Absorbance

0.02

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


a 3372

b

3248 3156

1167

1188

a 3700 3300 Wavenumber (cm-1)

1215

1596

1199 e 1229 1477

0.04 1606 2000

1800

1600 1400 1200 Wavenumber (cm-1) Figure 3


1

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47



0.8

Page 50 of 55

NH3ads-L1 NH3ads-L2

0.6 1400 1300 1200 1100

Wavenumber (cm-1)

0.4 b

0.2

a c

0 300

400



800

900

Page 51 of 55

A|B|C| Molar fractions of the gases

1

D

Tm=315 K

Ar×160

Tm= 400 K

| E |

F 773

NH3 ×160

0.8

673

0.6

573

T

0.4

473

0.2

373 Tm= 715 K

0

H2O ×500

0

20

40

60

80 Time (min)


273

100

120

140

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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47



Page 52 of 55

1

0.8 0.6

0.4

0.2 0

300

400

b

a


800


900

1596 a b

1 c

0.8

d

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47



Page 53 of 55

0.02

0.6 d

a

0.4

b

1604 c

1700 1600 1500 Wavenumber (cm-1)

0.2 0 300

400

500


Figure 7


900

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

Isosteric heat of adsorption of NH3ads species (kJ/mol)


Page 54 of 55

160 140 120 100 80 60 40

0

0.2 0.4 0.6 0.8 Coverage θL () and θL2(z) Figure 8


1

Page 55 of 55

Heats of adsorption of NH3 species on TiO2-P25 NH3ads-L1 EL1(1)=56 kJ/mol EL1(0)=112 kJ/mol

AEIR method 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


PNH3= 0.1 kPa Ta= 300 K 473 K 0.04 2000

673K 1800

NH3ads-L1 + NH3ads-L2

NH3ads-L2 EL2(1) =104 kJ/mol EL2(0) =140 kJ/mol NH4+

1600 1400 Wavenumber (cm-1)

Graphical abstract


1200

WO3

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