γ-Al2O3

γ-Al2O3

Catalysis Communications 2 (2001) 369±374 www.elsevier.com/locate/catcom SO2 Reactions over c-Al2O3; Pt=c-Al2O3; Sn=c-Al2O3 and Pt±Sn/c-Al2O3 Griseld...

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Catalysis Communications 2 (2001) 369±374 www.elsevier.com/locate/catcom

SO2 Reactions over c-Al2O3; Pt=c-Al2O3; Sn=c-Al2O3 and Pt±Sn/c-Al2O3 Griselda Corro *, Angeles Velasco, Ram on Montiel Laboratorio de Catalisis Ambiental, Instituto de Ciencias, Benem erita Universidad Aut onoma de Puebla, 14 Sur 6303, Puebla, Puebla 72570, Mexico Received 20 June 2001; received in revised form 29 November 2001; accepted 29 November 2001

Abstract Platinum and Platinum±tin bimetallic catalysts supported on alumina were prepared by co-impregnation of both metallic precursors on the support and used as catalysts for the oxidation of SO2 . Platinum dispersion was determined by means of H2 ±O2 titration. Tin addition (1 and 2 wt%) only slightly decreased the exposed platinum atoms suggesting that tin is mainly over the support. At temperatures lower than 300 °C, SO2 did not react with oxygen. Nevertheless, when the temperature was increased, the SO2 oxidation began. The ignition temperatures for SO2 oxidation (taken at 50% conversion) were 345 °C for 1% Pt/Al2 O3 and 520 °C for 1% Pt±2% Sn=Al2 O3 . The strong displacement on activity suggests that tin plays an important role as inhibitor of the SO2 oxidation reaction. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: SO2 adsorption; SO2 oxidation; Platinum/alumina catalyst; Platinum±tin/alumina

1. Introduction Exhaust SO2 is a signi®cant poison for noble metal catalysts contained on the catalytic automotive converter. These poisoning e€ects will become increasingly serious as emission standards tighten. SO2 in the exhaust is also a problem in controlling diesel engine particulate emission. Moreover, in the presence of an oxidation catalyst, SO2 will be oxidized to sulfur trioxide (SO3 ),

*

Corresponding author. Fax: +52-2-2-29-55-51. E-mail address: [email protected] (G. Corro).

which can form sulfates by reacting with water and other compounds in the exhaust and be adsorbed on the soot [1,2]. In spite of the important concentration of SO2 in the gas exhaust, specially in diesel automotive trucks, only small e€orts have been done to develop catalysts which diminish SO2 oxidation on platinum converter catalysts working under oxidizing conditions. With this aim, in the present study, temperature programmed sulfur dioxide adsorption and sulfur dioxide oxidation were investigated over c-Al2 O3 , Pt/c-Al2 O3 , Sn/c-Al2 O3 and Pt±Sn/c-Al2 O3 catalysts in order to develop catalysts with better tolerance for sulfur poisoning.

1566-7367/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 7 3 6 7 ( 0 1 ) 0 0 0 6 2 - 0

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Table 1 Catalyst characterization Catalyst

wt% Pt

wt% Sn

wt% Cl

Pt dispersion

1% Pt/c-Al2 O3 1% Pt±1% Sn/c-Al2 O3 1% Pt±2% Sn/c-Al2 O3 1% Sn/c-Al2 O3 c-Al2 O3

1.02 1.0 1.0 0 0

0 1.2 2.2 1.1 0

1.2 1.2 1 1.2 1.1

0.35 0.28 0.22 ± ±

2. Experimental 2.1. Preparation of the catalysts The support used was a c-Al2 O3 Merck with a grain size of 0.063±0.200 mm (70±230 mesh ASTM). Before use, the support was calcined for 6 h at 600 °C in air. Pt, Sn and Pt±Sn catalysts supported on alumina were prepared by impregnation using SnCl4  5H2 O and H2 PtCl6  6H2 O as the precursors for the metals in an acid medium (0.1 M HCl). After drying at 120 °C overnight, the catalysts were calcined in ¯owing air for 6 h at 500 °C The catalysts were ®nally activated by reduction in pure hydrogen for 8 h at 500 °C. A reference alumina support was prepared in the same way using only diluted hydrochloric acid. Table 1 shows the characteristics of the obtained catalysts. 2.2. Metal accessibility measurements Platinum accessibility measurements were obtained by the H2 ±O2 titration method at room temperature in a static volumetric apparatus. Isotherms were obtained in the 0±50 Torr range. Extrapolation to the origin value was used to calculate the number of Pt exposed atoms and dispersion (Table 1). The metal contents of the catalysts were determined by energy disperse X-ray spectrometer interfaced to a scanning electron microscope. 2.3. Activity measurements Temperature programmed SO2 adsorption and temperature programmed SO2 oxidation reactions

were carried out in a quartz tubular down ¯ow reactor (i.d. 7 mm), and using a program temperature rate of 10 °C min 1 from 25 to 700 °C. The SO2 adsorption measurements were obtained using a feed volume ¯ow rate of 100 cm3 min 1 consisting of 30 ppm of SO2 and balance N2 , whereas the oxidation activity was carried out with a feed ¯ow composition consisting of 30 ppm of SO2 , 10 vol% of O2 and balance N2 . The catalyst load in the reactor was of 200 mg for both experiments. The concentrations of SO2 and O2 consumed, were detected every minute and recorded during the experiments using a KM 9106 Quintox Gas Analyzer. Temperature programmed SO2 adsorption and SO2 oxidation reaction curves were obtained following the evolution of SO2 as a function of temperature. 3. Results and discussion 3.1. SO2 temperature programmed adsorption In Fig. 1, the SO2 temperature programmed adsorption is illustrated for c-Al2 O3 , 1% Pt/cAl2 O3 , 1% Sn/c-Al2 O3 and Pt±Sn/c-Al2 O3 catalysts. SO2 adsorption began at 25 °C and the maximum SO2 uptake was found around 150 °C. At higher temperatures, SO2 adsorption diminishes and no additional uptake was observed up to 500 °C. The similarity of the adsorption pro®les for alumina, platinum and bimetallic platinum±tin catalysts, in spite of its metallic composition, suggests that SO2 is adsorbed mainly on the support. However, small amounts of SO2 on the metallic particles cannot be discarded [3].

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Fig. 1. Temperature programmed %SO2 adsorption pro®le on catalysts. : c-Al2 O3 ; r: 1% Pt/c-Al2 O3 ; ±: 1% Sn/-cAl2 O3 ; j: 1% Pt± 1% Sn/c-Al2 O3 . Feed: 35 ppm SO2 and balance N2 .

Fig. 2. %SO2 uptake on temperature programmed SO2 ‡ O2 reaction. : c-Al2 O3 ; ±: 1% Sn/c-Al2 O3 . Feed: 35 ppm SO2 , 10% O2 and balance N2 .

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3.2. SO2 temperature programmed oxidation The SO2 oxidation pro®les for the c-Al2 O3 support and the 1% Sn/c-Al2 O3 reference samples are shown in Fig. 2. It can be seen in both alumina and tin±alumina catalysts a SO2 adsorption phenomena with a maximum uptake at around 150 °C. SO2 adsorption diminishes as temperature increases up to 300 °C. At higher temperatures,a second SO2 uptake began at 400 °C on Al2 O3 and at 500 °C on 1% Sn=Al2 O3 . On the other hand, the evolution of SO2 uptakes as a function of temperature is shown in Fig. 3 for platinum containing catalysts. In this ®gure, an initial SO2 adsorption with a very similar behavior as for alumina and tin±alumina samples can be observed. Nevertheless, on 1% Pt/c-Al2 O3 , 1% Pt±1% Sn/c-Al2 O3 and 1% Pt±2% Sn/c-Al2 O3 samples, the SO2 oxidation initiates at around 200, 350 and 450 °C, respectively. Using infrared spectroscopy to investigate SO2 adsorption on c-Al2 O3 Chang [4] identi®ed at least ®ve di€erent adsorption SO2 sites on Al2 O3 . This investigation showed that under oxidizing conditions, SO2 can be adsorbed and then oxidized to

form SO3 on at least one type of alumina oxygen lattice atoms. At temperatures lower than 400 °C no oxidation reaction was observed, however, at higher temperature SO2 reacts with oxygen to form SO3 , SO2…g† ‡ 12O2…g† $ SO3…g†

…1†

Moreover, in oxidizing environments, at around 400 °C, sulfur can also be stored through the oxidation of SO2 to form SO3 that reacts with c-Al2 O3 components to form sulfates, 3SO3…g† ‡ Al2 O3…s† $ Al2 …SO4 †3…s†

…2†

In Fig. 2, the low temperature SO2 uptake can be assigned to SO2 adsorbed on the support. Thus, high temperature SO2 uptake corresponds to SO2 oxidation in agreement with the interpretation given by Chang [4]. Note that we do not take into account a probable chlorine e€ect in SO2 reactivity, since its content is practically constant on the support and on the supported catalysts (Table 1). 3.2.1. SO2 ‡ O2 reaction over 1% Sn/c-Al2 O3 . The e€ect of temperature on the SO2 ‡ O2 reaction over 1% Sn/c-Al2 O3 is shown in Fig. 2. As

Fig. 3. %SO2 uptake on temperature programmed SO2 ‡ O2 reaction. r: 1% Pt/c-Al2 O3 ; j: 1% Pt±1% Sn/c-Al2 O3 ; N: 1% Pt±2% Sn/cAl2 O3 ). Feed: 35 ppm SO2 , 10% O2 and balance N2 .

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in the case of c-Al2 O3 , the ®rst low temperature uptake occurred around 200 °C. However, at higher temperature, the oxidation reaction was drastically diminished, a second signal appearing at temperatures higher than 500 °C. This result is explained by the massive deposition of SnO on cAl2 O3 as it has been reported [5,6]. The SnO deposition diminishes SO2 oxidation sites on c-Al2 O3 but they do not a€ect the low temperature SO2 adsorption sites. Sn…SO4 †2 is probably formed from the oxidation of SO2 at temperatures higher than 500 °C [7].

These facts lead to conclude that Sn induces a strong e€ect on Pt behavior for SO2 oxidation and according to our knowledge, these results were not reported previously. SO2 adsorption on metallic Pt [3,8,9] and on Pt supported on SiO2 [10] have been reported as SO2 dissociate adsorption over the metal according to the following reactions:

3.2.2. SO2 ‡ O2 reaction over Pt±Sn/c-Al2 O3 Fig. 3 shows the SO2 signals recorded from SO2 ‡ O2 reaction over Pt±Sn/c-Al2 O3 bimetallic catalysts including the 1% Pt/c-Al2 O3 catalyst. In this ®gure it can be seen that the ®rst low temperature SO2 adsorption occurred at the same temperature over the bimetallic catalysts as on 1% Pt/c-Al2 O3 . Oxygen on the feed does not modify the SO2 adsorption over the catalysts at low temperature. The second SO2 uptake began at 200 °C on 1% Pt/c-Al2 O3 and at higher temperature on 1% Pt±1% Sn/c-Al2 O3 (300 °C) and on 1% Pt±2% Sn/c-Al2 O3 (400 °C). Thus, on the bimetallic catalysts an important diminution of platinum oxidation activity is induced by tin. Moreover, Table 2 gives the values obtained for the ignition temperature de®ned as the temperature required to obtain a 50% conversion of SO2 (high temperature uptake). In 1% Pt/c-Al2 O3 this temperature is 95 and 170 °C higher than that of 1% Pt±1% Sn/c-Al2 O3 and 1% Pt±2% Sn/c-Al2 O3 , respectively.

The results in Table 2 show that the activity for the oxidation reaction diminishes when tin is added to platinum catalysts. Thus tin plays an essential role on the oxidation of SO2 . Tin may modify the SO2 reactivity following two hypothesis: · Tin acts as a dilution agent of the Pt atoms ensembles required for the SO2 adsorption. This dilution e€ect of tin on platinum ensembles has been reported previously [11,12]. · SO2 dissociate adsorption is diminished due to the modi®cation of the electron density on platinum induced by the tin oxide molecules surrounding the platinum particles (egg shell model), that might lower SO2 adsorption strength [13,14]. The modi®cation of the oxidation state of noble metals by SnO2 has been reported for the Pd=SnO2 system in which the stabilization of PdO strongly depends on its interaction with the SnO2 support [15]. In general, it has been reported that SnO2 induces electron de®ciency on platinum leading to Pt…d‡† particles which may lead to lower electronic interactions with the anti-bonding orbital of SO2 molecule, thus lowering its dissociate adsorption.

Table 2 Catalytic activity of the catalysts in SO2 ‡ O2 reaction

…SO2 †g $ …SO2 †ad

…3†

…SO2 †ad $ …SO†ad ‡ …O†g

…4†

…SO†ad $ …S†ad ‡ …O†g

…5†

Catalyst

Temperature, °C for 50% SO2 conversion. Second uptake

1% Pt/cAl2 O3 1% Pt±1% Sn/c-Al2 O3 1% Pt±2% Sn/c-Al2 O3 1% Sn/cAl2 O3 c-Al2 O3

350

4. Conclusions

445

The results obtained from the quantitative analysis of SO2 temperature programmed adsorption and SO2 ‡ O2 temperature programmed reaction over c-Al2 O3 , 1% Pt/c-Al2 O3 , 1% Sn/cAl2 O3 and bimetallic Pt±Sn/c-Al2 O3 catalysts, lead us to the following conclusions:

520 656 622

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In the absence of oxygen: 1. Over the catalysts studied in this work, SO2 adsorption occurs at low temperature (200 °C) with or without oxygen contained in the reactor ¯ow. However, this adsorbed SO2 must be mainly adsorbed on the support. With oxygen present: 2. At low temperature, oxygen does not interfere with SO2 adsorption. 3. SO2 oxidation initiates at temperatures higher than 300 °C. 4. The SO2 reacting with O2 at high temperatures over 1% Pt/c-Al2 O3 , is higher than over Pt±Sn/ c-Al2 O3 . Thus, on the bimetallic Pt±Sn catalysts, lower amounts of sulfur species are generated. Acknowledgements The authors wish to thank SIZA-CONACYT and DEGUSSA CATALYATS S.A. de C.V. for their valuable support (Project 980602005).

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