XPS Study of the Adsorption of SO2 and NH3 over Supported Tin

10316

J. Phys. Chem. B 2001, 105, 10316-10325

XPS Study of the Adsorption of SO2 and NH3 over Supported Tin Dioxide Catalysts Used in de-NOx Catalytic Reaction C. Guimon,† A. Gervasini,‡ and A. Auroux*,§ UMR(CNRS)5624, UniVersite´ de Pau et des Pays de l’Adour, 2 aVenue du Pre´ sident Angot, 64053 Pau Cedex 9, France, Dipartimento di Chimica Fisica ed Elettrochimica, UniVersita` degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy, and Institut de Recherches sur la Catalyse, CNRS, 2 AVenue Einstein, 69626 Villeurbanne Cedex, France ReceiVed: March 8, 2001; In Final Form: August 7, 2001

Alumina and titania supported tin dioxide catalysts presenting various Sn loadings were prepared by impregnation. The acidity and basicity of the samples were determined by adsorption of ammonia and sulfur dioxide, respectively, using adsorption microcalorimetry and X-ray photoelectron spectroscopy. The surface reactivity of the samples and the chemical composition of the adsorbed species were determined as well as the heats of adsorption of the probe molecules. The catalysts were tested in the selective catalytic reduction of NO by C2H4. Both calorimetry and XPS experiments have shown that the SnO2/TiO2 series of samples was markedly more acidic than the SnO2/Al2O3 series, as the N/(Ti+Sn) molar ratios were noticeably higher than the corresponding N(Al+Sn) ratios. The number of acidic sites seemed to increase with the tin content when tin dioxide was well dispersed on the support. They are of Lewis type. It was shown that sulfur dioxide adsorption led to the formation of three types of species: SO2, sulfites, and sulfates. The basicity of the SnO2/Al2O3 series of samples was weaker than that of the alumina support and passed through a minimum around 12 wt % Sn. On the contrary, as the acidity, the basicity of the SnTi series did not seem dependent on the Sn concentration. This can be correlated to the bad dispersion of SnO2 on TiO2. In the NO reduction by C2H4 reaction, the turnover frequency mainly depends on the Sn dispersion, and the Sn centers are very active even at low amounts.

1. Introduction The surface properties of many oxides can conveniently be described in terms of surface acidic and basic sites, and even reducing and oxidizing sites may occur. Acidic and basic sites may exist in many different configurations which will lead to a distribution of their strength. Acid-base pair sites may play a substantial role1 conjointly with redox sites. This is the case for tin dioxide based catalysts. Tin dioxide is a n-type semiconductor widely used in sensors2 and CO2 lasers but also in oxidation catalysis and more recently in automotive exhaust.3-9 In this paper, we report the sorption capacities, the differential heats of adsorption, and the nature of the adsorbed species when SO2 and NH3 probe molecules are in contact with alumina or titania supported tin dioxide catalysts presenting various Sn loadings. For this purpose, besides the techniques allowing investigation of the structure (X-ray diffraction, NMR, and BET), we have used X-ray photoelectron spectroscopy which allows studies both of the superficial chemical composition and of the surface reactivity by using the adsorption of probe molecules. The molecular and acid surface properties can be correlated with the core-level binding energy (BE) shifts of elements constituting the acid center and an adsorbed phase.10 To complete this study, we have also used adsorption microcalorimetry and adsorption isotherms of the same acid* To whom correspondence should be addressed. E-mail: auroux@ catalyse.univ-lyon1.fr. † Universite ´ de Pau et des Pays de l’Adour. ‡ Universita ` degli Studi di Milano. § Institut de Recherches sur la Catalyse.

base probe molecules to determine the strength and number of acid-base sites. Heat-flow microcalorimetry is one of the best methods known for measuring accurately the differential heats of adsorption and also of characterizing a catalyst by the energy distribution of its surface sites. It allows us to determine the heat evolved during adsorption of appropriate probe molecules from the gas phase. The various thermodynamic parameters derived from the isotherms of adsorption and calorimetric curves, the differential heat, and the integral heat of adsorption of the adsorbate were determined as a function of the coverage. As said above, two probe molecules, ammonia and sulfur dioxide, were used in this study. The adsorption of ammonia gives both qualitative and quantitative information about surface acidity.11,12 A correlation exists between the BE and charge on the nitrogen atom of the base and between the observed BE shifts of the N1s band and the strength of acid-base interaction. Moreover quantitative assessment of specific surface species involved in the acidbase interactions can be performed. Ammonia can be considered as a strong Lewis base molecule able to titrate the total acidity of solids. Interactions should be quite strong because ammonia is a fairly hard base able to interact even with weak sites with an appreciable heat evolution. Ammonia can also act as a reactive gas in the selective catalytic reduction of nitrogen oxides. The adsorption of ammonia leads to a variety of chemically distinct species and different types of sites are responsible for the formation of these surface species.1 However, ammonia in some cases is a too reactive probe and can give rise to reactive adsorption.

10.1021/jp0108869 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001

XPS Study of the Adsorption of SO2 and NH3

J. Phys. Chem. B, Vol. 105, No. 42, 2001 10317

TABLE 1: Physicochemical Characterization of the Alumina- and Titania-Supported Tin Dioxide Catalysts

sample

Sn surface area coverage (wt %) (m2 g-1) atom Sn nm-2

γ-Al2O3 SnAl-0.2 SnAl-2 SnAl-8 SnAl-12 SnAl-24 SnO2

0.24 2.1 8.3 11.5 23.7 78.8

112 118 113 115 102 94 58

TiO2 SnTi-5 SnTi-12 SnTi-21

5.0 12.3 21.3

108 91 83 71

Sn/Me Me)Al,Ti (XPS) (molar ratios)

0.109 0.95 3.76 5.21 10.74

nd 0.02 0.09 0.14 0.55

2.26 5.57 9.65

0.14 0.14 0.14

The choice of sulfur dioxide as probe molecule is not denied of interest. First it can be considered as an acidic probe molecule and for this purpose can help in determining the number of basic sites of the solid catalyst under study. Second, sulfur dioxide is formed from both the oxidation of sulfur contained in fossil fuels and in the industrial processes that treat and produce sulfur-containing compounds13 and has a significant impact because of the associated sulfur oxide (SOx) emissions. So the adsorption of SO2 on tin dioxide based catalysts can make a valuable contribution in the study of pollutant gas sensors and environmental catalysts such as SnO2.14 In addition, some catalytic data on the activity of these series of catalysts in the selective reduction of NO by hydrocarbon (SCR) relevant to the discussion of the properties of the surfaces will be also given here, and details on the catalytic activity will be reported elsewhere.15 2. Experimental Section Commercial nonporous γ-alumina (oxid C, from Degussa) and porous titania (DT51, anatase, from Rhoˆne-Poulenc) were used as the starting materials. The supports were impregnated with aqueous solutions of tin tetrachloride (SnCl4, 5 H2O, from Aldrich). The impregnates were dried at 393 K overnight and then calcined at 773 K for 5h under dioxygen flow. The composition and the physicochemical properties of the catalysts are summarized in Table 1. The catalysts are labeled as SnAl-x and SnTi-x where x represents the Sn loading in wt %. The surface area was calculated by the BET method from the N2 adsorption-desorption isotherms recorded at 77 K. The chemical composition was determined by inductive coupled plasma emission spectroscopy. NH3 and SO2 were analytical grade of purity >99.9 wt % and supplied by Air Liquide. Before any use they were purified by successive freeze-thaw cycles. 2.1. XPS Studies. XPS analyses were performed at room temperature with an SSI 301 spectrometer using monochromatic and focused (spot diameter ) 600 µm, 100 W) Al KR radiation (1486.6 eV) under a residual pressure of 5 × 10-8 Pa. Charge effects were compensated by the use of a flood gun (5 eV). The hemispherical analyzer functioned at a constant pass energy of 50 eV for high-resolution spectra and at 150 eV for quantitative analyses. The experimental bands were fitted to theoretical bands (80% Gaussian, 20% Lorentzian) with a leastsquares algorithm using a nonlinear baseline. Quantitative analyses were performed using the appropriate Scofield factors. Reference binding energies (BE) were Al2p (74.4 eV, Al2O3) and Ti2p3/2 (458.8 eV, TiO2) in agreement with the literature.

The value of binding energy for Sn (II, IV) is 486.7 ( 0.2 eV. As pointed out by Lau and Wertheim16 one cannot distinguish between Sn(II) and Sn(IV) in XPS because their binding energy difference is too small. Because of the approximations (in particular the choice of Scofield values as intensity factors) done in the calculations of the XPS compositions, we must only consider the relative evolution of the ratios [Sn/M]. After pretreatment at 673 K under helium, the samples were exposed to SO2 or NH3 at 353 K and then submitted to 1 h of desorption at 353 or 673 K. The XPS analyses were performed on samples after desorption without any contact with the atmosphere. 2.2. Calorimetric and Volumetric Measurements. The heats of adsorption of NH3 and SO2 were measured in a heat flow microcalorimeter of the Tian-Calvet type, C80 from Setaram, linked to a glass volumetric line to permit the introduction of successive small doses of gases. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge (Datametrics). The adsorption temperature was maintained at 353 K in order to limit physisorption interactions between the probe molecules and the samples surface. Successive doses were sent onto the sample until a final equilibrium pressure of 133 Pa was obtained. The samples were then evacuated for 1 h at the same temperature, and a second adsorption was performed. For each probe molecule, the primary and secondary adsorption isotherms were collected at the same temperature, to calculate, by subtraction, the irreversible chemisorbed amount (Virr). 2.3. Catalytic Experiments of NO Reduction with C2H4 (SCR). Catalytic activity was measured in a flow apparatus at atmospheric pressure already detailed in refs 3 and 17. The apparatus included a feeding section where streams of He, 2% NO in He, 2% C2H4 in He, and pure O2 (from Sapio, Italy) were regulated by four independent mass flow controller meters (Bronkhorst, Hi-Tec.). After mixing, the gas mixture entered in a quartz reactor (i.d. 5 mm) positioned upright in an electrical heater. The catalyst (0.1-0.5 g) was put between two plugs of quartz wool and thermally activated in air at 673 K for 2 h followed by 2 h of He flowing before the experiments. Experiments were done at space velocities (GHSV) in the 10 000 and 50 000 h-1 range and in the 150-500 °C domain. The feed consisted of 0.5% of both NO and C2H4 and 9% O2, with balance He. The reactor outflow was analyzed by gas chromatography (Chrompack, CP-9000 equipped with TCD detector) equipped with a 60/80 Carboxen-1000 column (Supelchem). Activity was calculated controlling the NO conversion to N2 and N2O and C2H4 conversion to CO and CO2. On the basis of N2, the specific integral rate of NO conversion (rNO) was calculated and expressed as molN2 gcat-1 s-1. 3. Results and Discussion The tin loadings are reported in Table 1, expressed in weight percent. The naming of the samples according to the tin loadings as well as the values of surface areas are summarized in the same Table 1. The surface coverage calculated in atom of Sn per square nanometer of catalysts is also given. As can be seen, the BET surface areas did not change appreciably upon loading of tin dioxide on alumina but were more affected on titania, especially at high loading. For the highly loaded sample on titania (SnTi-21) the mesoporosity of the support could be responsible for the decrease in surface area by pore plugging. The Sn/Me molar ratios (Me ) Al or Ti) as determined by XPS are given in column 5 of Table 1. The low Sn loading

10318 J. Phys. Chem. B, Vol. 105, No. 42, 2001

Guimon et al.

TABLE 2: Calorimetric and Volumetric Data of NH3 and SO2 Adsorption NH3

SO2 number of sitesd

sample

Qinit kJ mol-1

VTa

Virrb

γ-Al2O3 SnAl-0.2 SnAl-2 SnAl-8 SnAl-12 SnAl-24 SnO2 TiO2 SnTi-5 SnTi-12 SnTi-21

199 127 155 171 158 164 180 204 172 180 150

263 286 323 338 337 265 221 358 330 302 256

161 173 215 236 229 181 171 235 227 199 170

number of sitesd

Qintc

Q> 150 kJ mol-1

150 > Q> 120

120 > Q> 100

100 > Q> 80

Qinit/ kJ mol-1

VTa

Virrb

28 27.1 32.8 36.8 33.7 28.2 29.0 41.0 35.5 34.0 27.7

27 0 0 11.5 19 40 114 32 30 37 0

54 36 145 150 92 76 18 156 119 114 114

73 110 23 23 67 33 9 59 60 51 50

33 44 42 76 34 32 9 39 39 39 21

176 170 160 161 128 166 180 143 147 149 160

227 165 153 125 125 115 101 165 83 89 78

193 131 122 86 102 90 86 98 44 43 43

Qintc

Q> 150 kJ mol-1

150 > Q> 120

120 > Q> 100

100 > Q> 80

29.1 19.9 14.9 13.5 12.2 12.8 14.5 18.4 8.2 9.1 8.5

104 50 35 9.5 0 6 65 0 0 0 7

45 45 43 49 43 66 13 61 19 20 19

16 18 14 20 38 7.5 7 44 19 18 14

11 12 12 17.5 10 7.5 3.5 41 25 20 17

a Total amount of gas (in µmol g-1) adsorbed under an equilibrium pressure of 0.2 Torr (26.6 Pa). b Number of strong sites corresponding to the irreversibly adsorbed gas amount, calculated by subtracting the primary and secondary isotherms (in µmol g-1). c Integral heat corresponding to the total heat evolved after adsorption of V (in J g-1). d Number of sites (in µmol g-1) which adsorb NH3 or SO2 with a given evolved heat.

samples on alumina are well dispersed, whereas the presence of SnO2 islands on the titania surface and on the highly loaded alumina samples has been established. 3.1. Calorimetric Measurements. The results of adsorption calorimetry experiments performed with ammonia and sulfur dioxide on the various samples and supports are given in Table 2. The probe chemisorbed amounts (Virr) determined by the difference between the total adsorbed amount under an equilibrium pressure of 0.2 Torr (VT) and the physisorbed amount (determined at the same pressure by readsorption after pumping) are given together with the integral heats of adsorption (total heat evolved under 0.2 Torr of equilibrium pressure). We can consider that Virr measures the amount of strongly adsorbed probe molecule on the surface at 353 K. Table 2 gives also the number of sites which adsorb ammonia or sulfur dioxide with an evolved heat chosen in a given interval. On varying the concentration of tin dioxide deposited on γ-Al2O3, the amount of chemisorbed ammonia (Virr) passed through a maximum at a tin concentration in the vicinity of 8 wt % (around the monolayer). The total number of acid sited (VT) as well as the corresponding integral heat (Qint) followed the same trend. Indeed, a coverage of about 8 wt % of Sn gives rise to the highest acidity in terms of number and strength of sites. The initial heats of adsorption (at coverage close to zero) are more subjected to experimental errors and also influenced by traces of impurities or defects in the sample. The addition of tin dioxide on titanium oxide creates a slight decrease in the initial heats of NH3 adsorption and in the number and strength of acid sites with increasing loading, which can be attributed mainly to the decrease in surface area of the samples. In fact, the calorimetric data (VT, Virr, Qint) recalculated per m2 (instead per g) show that the number and strength of acid sites remain nearly constant and these results can be attributed to a bad dispersion of SnO2 on the titania surface with increasing loading. This poor dispersion is confirmed by a constant Sn/Ti molar ratio of 0.14 for the samples as measured by XPS. The calorimetric data concerning the adsorption of sulfur dioxide on tin dioxide samples on alumina are also reported in Table 2. The initial heat of SO2 adsorption on γ-Al2O3 was approximately 175 kJ mol-1. The addition of SnO2 decreased the initial heat of SO2 adsorption from 170 kJ mol-1 for 0.2 wt % Sn to ≈130 kJ mol-1 for 12 wt % Sn. Moreover the coverage with SO2 decreased from about 227 µmol g-1 for γ-Al2O3 to

115 µmol g-1 upon increasing the SnO2 loading. The amounts of SO2 irreversibly adsorbed were found to decrease from about 190 µmol g-1 to less than 90 µmol g-1. The minimum uptake is reached for the sample close to the monolayer (≈8 wt % Sn) which is certainly the loaded sample with the best dispersion as confirmed by XRD. Tin dioxide supported on titanium oxide also creates a drastic decrease in the number of basic sites compared to the bare support but the loading has little influence. Indeed, the bad dispersion of tin dioxide at the surface of titania may explain these results. 3.2. XPS Measurements. XPS spectroscopy is specifically a surface technique, as the depth of analysis is well below 10 nm. It mostly gives quantitative information, obtained from the intensities of the bands. The dispersion of tin dioxide on the two supports, alumina, and titania can be estimated from the Sn/Me molar ratio given in Table 1. As can be seen the dispersion is much better on alumina than on titania with increasing loading. In adsorption studies, the acidic surface reacts with the incoming base to form an acid-base pair which can readily be identified and measured by XPS. The adsorption of NH3 followed by evacuation of the extra base sets the stage for quantitative measurements. The concentration of surface acid sites is directly related to the concentration of adsorbed base. The amount of elements constituting the surface acid sites and the amount of adsorbed ammonia are determined from the intensity ratio of the specific XPS peaks. Peak intensity is obtained by careful curve fit followed by integration of the peaks using a mix of Gaussian (80%) and Lorentzian (20%) functions. In particular, the ratio between the intensity of the main band associated to the adsorbed molecule (here the S2p band for SO2 or the N1s band for NH3) and that of the bands associated to elements of the catalysts (here Al2p for Al2O3, Ti2p for TiO2, and Sn3d5/2 for SnO2) makes it possible to monitor the evolution of adsorption as a function of various parameters such as activation temperature, adsorption or desorption temperature, and chemical composition of the solid. This ratio is representative of the number of sites on which the probe molecule adsorbed. More qualitative information can also be obtained: indeed, measurements of the binding energy BE associated to sulfur



(S2p3/2) or nitrogen (N1s) make it possible to determine the adsorption type and the resulting species. Ammonia reacts with Lewis acid centers by donation of the free electron pair on nitrogen which forms a dative bond with a Lewis acid on the surface, whereas reaction with a Bro¨nsted acid site involves a proton transfer from the surface to form NH4+. The observed core-level shifts in the N1s line are greater when ammonia is protonated than when interacting with a Lewis acid.10 When the ammonia nitrogen gains a net positive charge by electron pair donation to a Lewis acid site or proton attachment from a Bro¨nsted site, the acid site itself loses the charge and, in theory, the BE shifts are in opposite direction (to lower BEs) than those of the N1s emission of the base. In fact, these shifts are not frequently observed because of the charge delocalization on the oxide skeleton. The BE value of the main element of the probe molecule depends on its atomic environment. For nitrogen (NH3 adsorption) N1s, the following species can be formed: i. NH2- species associated to BE ∼399 eV18-20 ii. NH3-Me species associated to BE ∼400.5 eV21 iii. NH4+ species associated to BE ∼402.5 eV21 Similarly for sulfur (SO2 adsorption) there are classically four different environments, associated to different S2p3/2 binding energies: i. sulfide associated to BE ∼162 eV22 ii. sulfur dioxide associated to E ∼166 eV23

Figure 1. N1s spectra, after NH3 adsorption and desorption at 353 K on the SnAl samples.

iii. sulfite associated to BE ∼167.5 eV23-25

TABLE 3: Components of the N1s Band after NH3 Adsorption and Desorption at 353 K on the SnAl Samples (Tactiv ) 673 K) NH2-

iv. sulfate associated to BE ∼169.2 eV26

3.2.1. XPS Study of the Surface Acidity of the SnAl and SnTi Series. All samples were pretreated under helium at 673 K during 8 h, except for bare TiO2 which was also subjected to a pretreatment at 973 K. Ammonia adsorption was performed at 353 K, followed by a desorption during 1 h under helium gas flow at either 353 or 673 K. XPS analyses were performed on the samples after desorption without exposing the samples to air. The molar ratio N/(Me+Sn) with Me ) Al or Ti is related to the amount of NH3 adsorbed at the surface of the sample. As the experiments are performed under 10-9 mbar, the probe molecules adsorbed on the weakest sites do not remain on the surface and cannot be observed by XPS.

Lewis

samples

N/(Me+Al) (atomic ratio)

BE N1s (eV)

peak area %

BE N1s (eV)

peak area %

γ-Al2O3 SnAl-2 SnAl-8 SnAl-12 SnAl-24 SnO2

0.006 0.010 0.014 0.017 0.017 0.051

399.2 399.2 399.2 399.1

40 40 30 20

400.7 400.9 400.7 400.8 400.3 400.2

60 60 70 80 100 100

NH3 Adsorption on the SnAl Series. Similarly to Al2O3, the SnAl-2, -8 and -12 samples present basic and acidic Lewis sites (Table 3). The former are associated to the N1s band around 399.2 eV, which can be attributed to NH2-. Indeed, the formation of NH2- over basic sites has been reported in FTIR studies of MgO18,19 and ZnO.20 The intensity of this band decreases as the coverage of alumina by SnO2 increases (Figure 1). The N1s spectra of SnAl-2 and SnAl-24 are not represented on this figure because they are identical to those of bulk Al2O3 and SnO2, respectively. The acidic sites are associated to the N1s band around 400.7 eV, which can be attributed to the formation of NH3-Me species corresponding to a Lewis acidic site. The values of the binding energies and the proportions of the various nitrogen species present on the SnAl series samples after desorption of ammonia at 353 and 623 K are listed in Tables 3 and 4, respectively. The relative contribution of basic sites becomes smaller and smaller as the tin oxide content


Guimon et al.

TABLE 4: Components of the N1s Band, after NH3 Adsorption at 353 K and Desorption at 673 K on the SnAl Samples (Tactiv ) 673 K) NH2-

Lewis

samples

N/(Me+Al) (at)

BE N1s (eV)

peak area %

BE N1s (eV)

peak area %

γ-Al2O3 SnAl-2 SnAl-8 SnAl-12 SnAl-24 SnO2

0.003 0.005 0.009 0.015 0.014 0.034

399.2 399.4 399.3 398.7

50 45 30 25

400.8 400.8 400.8 400.4 400.4 400.1

50 55 70 75 100 100

TABLE 5: Components of the N1s Band, after NH3 Desorption at 673 K on Bulk TiO2 Activated at 353 and 973 K Lewis TiO2

Bro¨nsted

Tactivation

N/(Me+Al) (at)

BE N1s (eV)

peak area %

BE N1s (eV)

peak area %

353 K 973 K

0.063 0.038

400.1 400.2

60 85

402.3 402.3

40 15

increases. However, ammonia adsorbed on the Lewis acidic sites is slightly less resistant to desorption at 673 K than the species formed on the basic sites (Tables 3 and 4). Because of the low amounts of NH3 adsorbed by Al2O3 and by the samples with low Sn contents, it is difficult to build arguments upon the quantitative data. Nevertheless, it seems that acidity tends to increase (both in number and strength) with tin content. For SnAl12, and for SnAl-24 as well, the amount desorbed between 353 and 673 K is much less than for the other samples. Indeed, the proportion of strong acid sites (still covered at 673 K; Table 4) with respect to the total number of surface Lewis acid sites (covered at 353 K; Table 3) is equal to 42%, 46%, 64%, 83%, 82%, and 67% for Al2O3, SnAl-2, SnAl-8, SnAl-12, SnAl-24, and SnO2, respectively. NH3 Adsorption on the SnTi Series. The TiO2 support presents Lewis (N1s around 400.1 eV) and Bro¨nsted (around 402.3 eV) acidic sites, with proportions 60%/40% for the sample activated for 8 h at 673 K under helium. The concentration of acidic sites decreases markedly when the sample is activated at 973 K (Table 5). The proportion of Bro¨nsted sites is also remarkably decreased (it drops from 40% to 15%) because of the dehydroxylation of the surface (Figure 2). This dehydroxylation can be monitored by observing the O1s band, where the intensity of the component associated to OH groups (around 531.5 eV27-29) decreases after activation at 973 K (Figure 3). The values of the binding energies and the proportions of the various nitrogen species present on the SnTi series after adsorption and desorption at 353 K are reported in Table 6. Unlike TiO2 which presents acidic sites of both Bro¨nsted and Lewis types, bulk SnO2 only possesses Lewis sites. The ratio between the amounts of Lewis and Bro¨nsted acidic sites increases slightly for SnTi-12 and SnTi-21 (30% of Bro¨nsted sites) in comparison with TiO2 and SnTi-5 (40%; Figure 4). This relatively small difference shows that the surface of TiO2 is only partially covered by SnO2. This is in agreement with the fact that the molar Sn/Ti ratio remains constant (0.14) independently of the tin content, indicating a very bad dispersion and the formation of SnO2 clusters or aggregates. Quantitatively, the concentration of surface acidic sites decreases slightly when the tin content increases. This trend is attenuated for the strongest sites as can be seen in Table 7 when ammonia is adsorbed and desorbed at higher temperature (673 K). The influence of the desorption temperature can be more easily evidenced in Figure 6 which represents the respective molar

Figure 2. N1s spectra, after NH3 adsorption and desorption at 353 K on TiO2 (pretreated at 673 and 973 K).

Figure 3. O1s spectra of bulk TiO2 pretreated at 673 and 973 K under helium.

TABLE 6: Components of the N1s Band, after NH3 Adsorption and Desorption at 353 K on the SnTi Series (Activated at 673 K) Lewis

Bro¨nsted

samples

N/(Me+Al) (at)

BE N1s (eV)

peak area %

BE N1s (eV)

peak area %

TiO2 SnTi-5 SnTi-12 SnTi-21 SnO2

0.063 0.058 0.046 0.041 0.051

400.1 400.2 400.2 400.1

60 60 70 70

402.3 402.3 402.3 402.1 400.2

40 40 30 30 100

ratios N/(Sn+Ti) after desorption of NH3 at 353 and 673 K, respectively. 3.2.2. XPS Study of the Surface Basicity of the SnAl and SnTi Series. The identification of the nature of the sulfur-containing species on tin oxide based gas sensors has motivated surface studies by infrared spectroscopy or by X-ray photoelectron spectroscopy. The XPS spectrum characteristic of the 2p electrons of the sulfur atom (S2p) in sulfur dioxide adsorbed at the surface of an oxide can give information on the nature of the complexes formed on the sites, because the binding energy (BE) associated to S2p3/2 depends on the environment of the sulfur atom. In the case of sulfur dioxide adsorption on bulk tin oxide, the various species observed in the literature are SO32-



Figure 5. Molar ratios N/(Sn + Ti) after desorption of NH3 over SnTi catalysts at 353 and 673 K.

Figure 4. N1s spectra, after NH3 adsorption and desorption at 353 K on the SnTi samples.

TABLE 7: Components of the N1s Band after NH3 Adsorption at 353 K and Desorption at 673 K on the SnTi Series (Activated at 673 K) Lewis

Bro¨nsted

samples

N/(Me+Al) (at)

BE N1s (eV)

peak area %

BE N1s (eV)

peak area %

TiO2 SnTi-5 SnTi-12 SnTi-21 SnO2

0.047 0.040 0.037 0.034 0.035

400.0 400.2 400.1 400.2

55 60 70 75

402.2 402.4 402.2 402.2 400.1

45 40 30 25 100

(BE ) 168.4 ( 0.1 eV), SO42- (BE ) 169.9 ( 0.1 eV), and S2- (BE ) 162.4 ( 0.1 eV).30,31 Berger30,31 has reported the predominance of sulfate species after treatment of SnO2 by SO2 between 573 and 973 K. Adsorption of SO2 on γ-Al2O3 and SnO2. We have studied the SO2 adsorption sites of bulk tin oxide and γ-alumina as a function of the activation temperature and the desorption temperature. The S2p binding energy is evaluated from the BE of Ti2p3/2 of TiO2 at 458.8 eV and BE of Al2p of Al2O3 at 74.4 eV. For each type of sulfur species, there is a doublet consisting of S2p3/2 and S2p1/2 (spin-orbit coupling) with the intensity ratio 2:1. The S2p peaks have been analyzed assuming a width of 1.8 eV for the theoretical components. The values of the binding energies and the proportions of the various types of sulfur species present over the bulk SnO2 and Al2O3 samples after activation at various temperatures and adsorption of sulfur dioxide at 353 K are reported in Table 8 and Figure 6, respectively.

Figure 6. Influence of the activation temperature on the respective amount of SOx species (associated to the BE S2p3/2) on γ-Al2O3 after SO2 adsorption and desorption at 353 K.

TABLE 8: Components of the S2p3/2 Band for Bulk SnO2 Activated at Various Temperatures after SO2 Adsorption and Desorption at 353 K SO42-

SnO2

SO32-

Tactivation/ (K)

S/Sn (at)

BE S2p3/2 (eV)

peak area %

BE S2p3/2 (eV)

peak area %

423 673 973

0.017 0.023 0.032

169.0 168.9 169.0

25 80 60

167.5

75 20 40

167.5

When the activation temperature increases, the S/Sn ratio increases drastically, whereas the amount adsorbed on alumina remains almost constant. The value of the S2p binding energy over bulk SnO2 is 169.0 eV for sulfate and 167.5 eV for sulfite species. It can be observed that an increase of the activation temperature (from 673 to 973 K), i.e., of the dehydroxylation of the surface, leads to improved adsorption of SO2, mostly in the form of sulfite species. Over alumina, the width of the S2p peak makes it necessary to take into account another type of adsorbed species, associated to a S2p binding energy of 166.1 ( 0.1 eV (Figure 7). This value can be attributed to sulfur bonded to an Al atom, corresponding to SO2 adsorbed over an anionic vacancy, characteristic of tri-coordinated aluminum. The concentration of this type of adsorbate over alumina tends to increase with activation temperature, which corresponds to the creation of Lewis-type acidic sites because of the dehydroxylation of the alumina surface.32 The XPS spectra after SO2 adsorption on alumina yield the following values for the S2p binding energies: 167.5 ( 0.2 eV for sulfur surrounded by three oxygens (sulfite type) and 169.3


Guimon et al.

Figure 8. Influence of the desorption temperature on the respective amount of SOx species (associated to the BE S2p3/2) on γ-Al2O3 (activated at 673 K) after SO2 adsorption at 353 K.

Figure 9. Relative contributions of the SOx species after adsorption and desorption at 353 K on the SnAl series.

TABLE 9: Components of the S2p3/2 Band for Various SO2 Desorption Temperatures on SnO2 (Activated at 673 K) after SO2 Adsorption at 353 K SO42-

SnO2

Figure 7. S2p spectra, after adsorption at 353 K and desorption at 353 or 673 K of SO2 on tin dioxide and alumina pretreated at 673 K.

( 0.1 eV for sulfur surrounded by four oxygens (sulfate type). These species are formed by reaction of SO2 with one or two surface oxygen atoms or hydroxyl groups. Therefore, they correspond to adsorption on basic sites, and their presence characterizes surface basicity. In particular, the presence of these three types of sulfur species after activation at various temperatures demonstrates the amphoteric character of alumina. To study the strength of SO2 adsorption sites on the two bulk oxides, the desorption has been performed at various temperatures. The amounts and proportions of the various types of sulfur species detected after desorption over alumina and tin dioxide are reported in Figure 8 and Table 9, respectively.

Tdesorption (K)

S/Sn (at)

BE S2p (eV)

353 673

0.023 0.025

169.0 168.9

3/2

SO32-

peak area % 80 100

BE S2p (eV) 167.5

3/2

peak area % 20

The SO2 species adsorbed on Lewis acid sites of alumina, although a minority, remain present up to 423 K. Beyond this temperature, these species disappear, which shows the relative weakness of the acid sites, as already observed with NH3 adsorption. The S/Al ratio decreases sharply as the desorption temperature increases. However the ratio of concentrations SO4-2/SO3-2 increases with temperature, as sulfate species are much harder to desorb. After desorption at 673 K, only sulfate species remain present at the surface of SnO2 (Figure 7, Table 9). Still, the S/Sn ratio remains very close to that measured after desorption at 353 K. This seems to indicate the occurrence of a reaction of oxidation of sulfites to sulfates. This is in agreement with the marked oxidizing character of SnO2, especially compared to Al2O3. Berger31 had previously reported the predominance of sulfate species over SnO2 between 573 and 973 K. Adsorption of SO2 on the SnAl Series. When alumina is covered by SnO2, the amount of SO2 adsorbed tends to decrease (Figure 9). The evolution of the adsorbed amount as a function of tin dioxide content presents a minimum for SnAl-8, i.e., approximately when alumina is covered by a monolayer of SnO2. The low tin content sample (SnAl-2) is the only one to display adsorption of SO2 on Lewis acidic sites (associated to a measured S2p binding energy of 165.8 eV). Therefore it seems



Figure 10. Molar ratios S/(Sn + Al) determined from XPS data after desorption of SO2 over SnAl catalysts at 353 and 673 K.

Figure 12. S2p spectra, after SO2 adsorption and desorption at 353 K on TiO2 pretreated at 673 or 973 K.

TABLE 10: Components of the S2p3/2 Band after Adsorption of SO2 and Desorption at 353 K on the SnTi Series Activated at 673 K (and 973 K for the Pure Oxides) SO32samples desorption 353 K Figure 11. Relative contributions of the SOx species after adsorption at 353 K and desorption at 673 K on the SnAl series.

that, beyond a certain concentration, SnO2 tends to cover these acidic sites, which become inaccessible to SO2. Sulfur dioxide is preferentially adsorbed as sulfite species over all samples, especially in the case of sample SnAl-12 (Figure 9) which corresponds to the theoretical monolayer coverage of Al2O3 by SnO2. Beyond this coverage, the characteristics of adsorption tend to those of SnO2. After desorption at 673 K, the amounts of SO2 adsorbed are considerably decreased (Figure 10), except on bulk SnO2. This indicates that most basic sites are relatively weak. As in the case of the support, SO2 adsorbed as sulfite is more easily desorbed than the sulfate species (Figure 11). A minimum is observed for samples SnAl-8 and SnAl-12, with equal contributions of sulfates and sulfites for the former. In agreement with the calorimetric data, the samples with tin content close to the monolayer coverage are those which display the weakest basicity. Adsorption of SO2 on the SnTi Series. The species observed after SO2 adsorption at 353 K on TiO2 pretreated at 673 K are predominantly SO4-2. This can be explained by the oxidizing character of this support, and may also be related to its surface state which, as shown by the NH3 adsorption data, presents a large number of hydroxyl groups. Titania differs from Al2O3 and SnO2 in the sense that the activation temperature affects markedly the number of basic sites, which drops by a factor of 2.5 between 673 and 973 K (Table 10). Dehydroxylation plays an important role at 973 K, as the proportion of sulfites changes from 15 to 80% (Figure 12). This evolution is more evident than in the case of SnO2, for which this proportion varied from 20 to 40% only. This behavior may be related to the different reducibilities of the two oxides, with SnO2 being more reducible and therefore more oxidizing than TiO2.

TiO2 (act. 673 K) TiO2 (act. 973 K) SnTi-5 (act.673 K) SnTi-12 (act.673 K) SnTi-21 (act.673 K) SnO2 (act. 673 K) SnO2 (act. 973 K)

SO42-

peak peak S/(Me+Ti) BE S2p3/2 area BE S2p3/2 area (at. ratio) % % (eV) (eV) 0.080 0.032 0.059 0.058 0.058 0.023 0.032

167.1 167.2 167.3 167.4 167.3 167.5 167.5

15 80 10 10 25 20 40

168.8 168.9 168.8 169.0 168.9 169.0 169.0

85 20 90 90 75 80 60

TABLE 11: Components of the S2p3/2 Band after Adsorption of SO2 at 353 K and Desorption at 673 K on the SnTi Series Activated at 673 K (and 973 K for TiO2)a samples desorption 673 K

S/(Me+Ti) (atomic ratio)

TiO2 (act. 673 K) TiO2 (act. 973 K) SnTi-5 (act. 673 K) SnTi-12 (act. 673 K) SnTi-21 (act. 673 K) SnO2 (act. 673 K)

0.066 0.014 0.058 0.057 0.058 0.025

SO42BE peak area (eV) % S2p3/2

168.9 168.3 168.9 169.0 168.9 168.9

100 100 100 100 100 100

a For T desorption ) 973 K on TiO2 (activated at 673 K), S/Ti ) 0.049 with 100% sulfate.

After adsorption and desorption of SO2 at 673 K on TiO2 pretreated at 673 K (Table 11), there are no sulfite species remaining, whereas the totality of the SO4-2 species remain present (the S(SO42-)/Ti ratio remains constant between 353 and 673 K). After adsorption and desorption of SO2 at 673 K on TiO2 pretreated at 973 K (Table 12), about two-thirds of the SO3-2 species have been desorbed, whereas one-third has been oxidized to SO4-2. Figure 13 represents the molar ratios S/(Sn + Ti) after SO2 adsorption over SnTi catalysts at 353 K and desorption at 353 and 673 K respectively. In agreement with the calorimetric measurements, the basicity of the SnTi series of samples appears to remain roughly constant, regardless of the Sn loading and also of the desorption temperature, except for the support. 3.3. NO Reduction with C2H4. Irrespective of the amount of Sn content, all samples had stable activities toward the


Figure 13. Molar ratios S/(Sn + Ti) after desorption of SO2 over SnTi catalysts at 353 and 673 K.

selective reduction of NO by C2H4, used as reducing species, in the 423-773 K temperature domain. The activity of the supports (Al2O3 and TiO2) was very less pronounced than those of the relevant catalysts in terms of yields of N2 formation and of C2H4 conversion. On the other hand, the role of the support can be considered very limited on the more Sn-concentrated samples. Only on SnAl-1, NO conversion to N2 did not reach a well definite maximum (TN2,max) as a function of temperature, whereas on all of the other Sn catalysts a typical volcano curve was observed for the N2 curve as a function of temperature. On more concentrated samples, NO conversion reached TN2,max at a lower temperature (673 K rather than 748 K). SCR activity was higher for SnAl-1 and then decreased with increasing the Sn loading without significative differences among the samples with Sn loading greater than 5 wt % (>2 Sn atoms nm-2) (SnAl-1 . SnAl-2 > SnAl-8 ≈ SnAl-12 ≈ SnAl-24). On TiO2 support too, not important differences of SCR activity were observed between the SnTi-5 and SnTi-21 samples, the latter being less active. C2H4 conversion very light increased in the 423-623 K temperature domain and, from 623 K, abruptly rose to 90100% at 773 K. In this case, the more concentrated samples attained the maximum (TC2H4,max) at lower temperature. Comparison of the catalytic activities showed higher rNO values at any temperature tested on the less concentrated sample (SnAl-1), and they lightly decreased with Sn content. The decreasing of SCR activity with increasing Sn content points out to active centers containing Sn both well dispersed on the supports (oxo-tin species) and present as large aggregates. However, the maintenance of sustained activity for high Sn coverage points out to well structured crystallites of SnO2, as already suggested in the literature.4,5 On the basis of the experimental data obtained, specific integral reaction rates could be calculated for all of the Sn catalysts. Reporting rNO in a Arrhenius-like plot, groups of lines were obtained with increasing absolute values of the slopes from the lowest to the highest concentrated samples. This dependence from the Sn content suggests that a different molecular complex and mechanism are involved over the differently loaded Sn catalysts. Moreover, from the values of rNO, turnover frequencies (N) can be calculated and expressed as molN2 molSn-1 s-1, once the amount of Sn per unit mass of catalyst is known. Figures14 and 15 show the linear trends obtained by plotting ln N as a function of inversion of temperature over the Sn catalysts supported on Al2O3 and TiO2, respectively. All the trends are satisfactorily linear in the whole temperature domain investigated, indicating the little volcano shape of the curves of N2

Guimon et al.

Figure 14. ln N (turnover frequency) as a function of inversion temperature over SnAl catalysts.

Figure 15. ln N (turnover frequency) as a function of inversion temperature over SnTi catalysts.

formation vs reaction temperature. The lines are well apart the one from the other. The highest N values are associated with the lowest concentrated catalysts (SnAl-1 and SnTi-5 for the series supported on Al2O3 and TiO2, respectively). On SnAl-1, that was loaded with the lowest Sn amount (0.11 atoms nm-2), all of the Sn centers can be assumed active sites, in a first approach, and the N values associated should therefore be regarded as a limit. Obviously, not all Sn deposited on the support could be active. On the series of Sn-Al2O3 catalysts, it is worth noticing that the increasing of Sn by a factor of 10 (SnAl-1 and SnAl-2 with 0.11 and 0.95 Sn atoms/nm2, respectively) leads to N values decreased by a factor of about 10. Moreover, for SnAl-12 and SnAl-24 (5.21 and 10.7 Sn atoms/nm2, respectively), the increasing of Sn by a factor of 2 leads to the decreasing of the N values by a factor of 2. This observation points out that small Sn centers are surely more active than large ones but formation of structured crystallites of SnO2 led to an active Sn phase. Moreover, it seems confirmed the above assumption that increasing Sn content, the SCR mechanism changes probably due to the variation of the structure of active sites constituted of crystallites containing Sn, of different nuclearity. Analogous evidences can be obtained for TiO2 support too. In this case, the variation of Sn by a factor of about 5 (SnTi-5 and SnTi-21, 2.26 and 9.65 Sn atoms/nm2) when the N values change by a factor of about 8 suggests an important role played by the support. Comparing catalysts at similar Sn content, i.e., SnAl-8 and SnTi-5 or SnAl-24 and SnTi-21, one can obtain differences of the N values around 10, with the catalysts on TiO2 being more active than those on Al2O3 (see Figures 14 and 15). From this behavior, it is well evident the

XPS Study of the Adsorption of SO2 and NH3 role of support in the stabilization of Sn centers more active toward SCR.

J. Phys. Chem. B, Vol. 105, No. 42, 2001 10325 the tin dioxide amount. The support is responsible for the quality of the dispersion, and Sn centers are very active even at low amounts.

4. Conclusions The SnO2/TiO2 series of samples is markedly more acidic than the SnO2/Al2O3 series, as the N/(Ti + Sn) molar ratios are noticeably higher than the corresponding N/(Al + Sn) ratios. This is due to the much more important acidity of TiO2 compared to that of Al2O3 (0.074 vs 0.006 after adsorption and desorption at 353 K), whether desorption occurs at 353 or 673 K. The acidic sites created by the addition of tin are of Lewis type. The number of acidic sites seems to increase with the tin content when tin dioxide is well dispersed on the support. This is the case when alumina is used as support, up to 12% Sn content. Moreover, the newly generated sites are stronger than those of alumina (Figure 2). As discussed previously, in the case of TiO2-supported catalysts, the tin loading affected neither the number nor the strength of the adsorption sites, because of the bad dispersion of the active phase (Figure 5). Despite the decrease in the number of Bro¨nsted acid sites observed by XPS after deposition of tin dioxide over TiO2, the global acidity of these solids is not affected. It has been shown that sulfur dioxide adsorption leads to the formation of three types of species: SO2, sulfites, and sulfates. These species reflect the presence of Lewis basic sites, and also of Lewis acidic sites in the case of Al2O3 (SO2‚‚‚Men+). Over SnO2 and TiO2, desorption at high temperature favors the formation of sulfate species. However, the basicity of the SnAl series of samples is weaker than that of the alumina support and seems to pass through a minimum (around SnAl12). On the contrary, the basicity of the SnTi series appears to be independent of the Sn concentration, just like the acidity. This can be correlated to the bad dispersion of SnO2 on TiO2, as evidenced by the XPS data. The S/(Sn + Al) molar ratios after desorption of SO2 at 353 K and the calorimetric data (Virr and number of sites with Q > 100 kJ mol-1) determined at the same temperature vary in the same following order: Al2O3 > SnAl-2 > SnAl-12 > SnAl-24 > SnAl-8. The S/(Sn + Al) molar ratios after desorption of SO2 at 673 K vary in the same order as the number of strong sites which give rise to a heat of adsorption >150 kJ mol-1, i.e., Al2O3 > SnAl-2 > SnAl-8 > SnAl-24 > SnAl-12. The comparison among tin dioxide catalysts with different Sn content and on different supports, active in NO reduction by C2H4, suggests that the turnover frequency mainly depends on the Sn dispersion, but the structure of the Sn crystallites and the support are also important variables. The acidity is more related to the uncovered support than to

Acknowledgment. We thank B. Gergely for assisting in catalysts preparation. References and Notes (1) Kno¨zinger, H. AdV. Catal. 1976, 25, 185. (2) Jimenez, V. M.; Mejias, J. A.; Espinos, J. P.; Gonzalez-Elipe, A. R. Surf. Sci. 1996, 366, 545. (3) Auroux, A.; Sprinceana, D.; Gervasini, A. J. Catal. 2000, 195, 140. (4) Park, P. W.; Kung, H. H.; Kim, D.-W.; Kung, M. C. J. Catal. 1999, 184, 440. (5) Kung, M. C.; Park, P. W.; Kim, D. W.; Kung, H. H. Top. Catal. 1999, 181, 1. (6) Miyadera, T.; Yoshida, K. Chem. Lett. 1993, 1483. (7) Teraoka, Y.; Harada, T.; Iwasai, T.; Ikeda, T.; Kagawa, S. Chem. Lett. 1993, 773. (8) Tabata, M.; Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T. Catal. Lett. 1994, 25, 55. (9) Kung, M. C.; Kung, H. H. Top. Catal. 2000, 10, 21. (10) Johansson, M.; Klier, K. Top. Catal. 1997, 4, 99. (11) Gergely, B.; Redey, A.; Guimon, C.; Gervasini, A.; Auroux, A. J. Therm. Anal. Cal. 1999, 56, 1233. (12) Auroux, A.; Sprinceana, D.; Gervasini, A. Stud. Surf. Sci. Catal. 2000, 130, 1433. (13) Dunn, J. P.; Strenger, H. G.; Wachs, I. E. J. Catal. 1999, 181, 233. (14) Gergely, B.; Guimon, C.; Gervasini, A.; Auroux, A. Surf. Interface Anal. 2000, 30, 61. (15) Gervasini, A.; Auroux, A.; et al. Manuscript in preparation. (16) Lau, C. I.; Wertheim, G. K. Vac. Sci. Technol. 1978, 15, 622. (17) Gervasini, A.; Carniti, P.; Ragaini, V. Appl. Catal. B: EnViron. 1999, 22, 201. (18) Coluccia, S.; Garrone, E.; Borrello, E. J. Chem. Soc. Faraday Trans. I 1983, 79, 607. (19) Coluccia, S.; Lavagnino, S.; Marchese, L. J. Chem Soc. Faraday Trans. I 1987, 83, 477. (20) Belokopytov, Y. V.; Kholyavenko, K. M.; Gerei, S. V. J. Catal. 1979, 60, 1. (21) Guimon, C.; Zouiten, A.; Bore´ave, A.; Pfister-Guillouzo, G.; Schulz, P.; Fitoussi, F.; Quet, C. J. Chem. Soc., Faraday Trans. 1994, 90, 3461. (22) Ng, K. T.; Hercules, D. M. J. Phys. Chem. 1976, 80, 2095. (23) Datta, A.; Cavell, R. G.; Tower, R. W.; George, Z. M. J. Phys. Chem. 1985, 89, 443. (24) Waqif, M.; Saad, A. M.; Bensitel, M.; Bachelier, J.; Saur, O.; Lavalley, J. C. J. Chem. Soc., Faraday Trans 1992, 88, 2931. (25) Nam, S. W.; Gavalas, G. R. Appl. Catal. 1989, 55, 193. (26) Lindberg, B. J.; Hamrin, K.; Johansson, G.; Gelius, U.; Fahlman, A.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 1, 277. (27) Kim, K. S.; Baitinger, W. E.; Amy, J. W.; Winograd, N. J. Elect. Spectrosc. Relat. Phenom. 1974, 5, 351. (28) McIntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208. (29) Munuera, G.; Gonzalez-Elipe, A. R.; Espinos, J. P.; Navio, A. Mater. Sci. Forum 1988, 25-26, 467. (30) Berger, F.; Touyeras, F.; Chambaudet, A.; Klein, D.; Planade, R.; Tiret, C. Analusis 1995, 23, 80. (31) Berger, F.; Beche, E.; Berjoan, R.; Klein, D.; Chambaudet, A. Appl. Surf. Sci. 1996, 93, 9. (32) Karge, H. G.; Dala Lana, I. F. J. Phys. Chem. 1984, 88, 1538.

XPS Study of the Adsorption of SO2 and NH3 over Supported Tin

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