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The nature of activity enhancement for propane oxidation over supported Pt catalysts exposed to sulphur dioxide
Applied Catalysis B: Environmental 19 (1998) 199±207
The nature of activity enhancement for propane oxidation over supported Pt catalysts exposed to sulphur dioxide R. Burch*, E. Halpin, M. Hayes, K. Ruth, J.A. Sullivan Catalysis Research Centre, Chemistry Department, The University of Reading, Whiteknights, Reading RG6 6AD, UK Received 19 April 1998; received in revised form 12 July 1998; accepted 12 July 1998
Abstract The nature of the active sites, the role of the support, and the mechanism by which hydrocarbons are activated over supported Pt catalysts have been investigated for the combustion of propane in the presence and absence of SO2. A strong enhancement in the activity for propane oxidation has been con®rmed either when SO2 is introduced with the propane or with a pre-sulphated alumina-supported catalyst. No equivalent effects were found with silica-supported catalysts. Fluorination of the alumina support also leads to an increase in activity. The addition of pulses of SO2 into the propane-containing gas stream produces a very large, but short-lived, increase in activity in addition to a more gradual and progressive activity enhancement. Reasons for these different effects are discussed. Attempts to correlate the permanent enhancement in activity with the total acidity of the support were unsuccessful. It appears that the increase in activity is due to a more subtle effect and a model is presented in which the possible role of perimeter sites at the metal±support interface is emphasised. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Pt Catalysts; C3H8 combustion; SO2; Al2O3; Interface; Acidity
1. Introduction The activation of hydrocarbons is of interest both because of the desire to functionalise inexpensive feedstocks and also because catalytic combustion offers a possible means to generate power without creating excessive amounts of nitrogen oxides. Platinum catalysts are well known to be the most active for the combustion of hydrocarbons containing more than one carbon atom whereas palladium is the most active catalyst for methane combustion. For propane combustion over an Al2O3-supported Pt catalyst, Hubbard et al. [1±3] have reported a *Corresponding author.
substantial reaction rate increase after having exposed the catalyst to an oxygen-rich mixture containing SO2 at 5008C. Earlier work had shown a similar promotional effect when a small amount of SO2 (20 ppm) was present in the feed gas [4]. Results presented by Lambert and co-workers [5], however, show that adsorption of propane on a Pt single crystal surface is greatly enhanced if SO2 and O2 are co-adsorbed prior to the introduction of the propane. In the absence of SO2 the sticking probability for propane was almost zero. These workers concluded that the promoting effect of SO2 can arise from a direct interaction on the metal surface and suggest that ``support mediated effects are not necessary for SO2 promotion of Pt catalysed propane combustion''. This interpretation is
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not consistent with the observation of Hubbard et al. [2] that propane combustion over Al2O3-supported Pt is promoted by high temperature pre-treatment of the catalyst in an oxygen rich stream of SO2 whereas the reaction over SiO2-supported Pt is unaffected. In addition,earlyworkcarriedoutbyYaoetal.[4]demonstrated that the extent of the promotional effect of SO2 was directly related to the Pt dispersion on the support. The purpose of the present work was to re-assess these two con¯icting mechanisms by investigating the effect of different treatments with SO2-containing gas streams on the activities for propane combustion of supported Pt catalysts. In order to further assess the importance of the overall acidity of the support, catalysts were also exposed to a high temperature treatment with ¯uoromethane since it is known that ¯uorination of alumina has the effect of greatly increasing the Lewis acidity [6,7]. 2. Experimental The supports used for the Pt-containing catalysts were g-Al2O3 (Akzo Chemie, CK300, 190 m2 gÿ1) and silica (Grace 432, 290 m2 gÿ1). Before use the silica was washed with 2 M nitric acid at 70±808C for 2 h to remove any metal impurities. Both supports were crushed and sieved to 250±600 mm and dried at 3008C before impregnation. The catalysts were prepared by dry impregnation of the support with the appropriate amount of aqueous dinitrodiammine (DNDA) Pt(II) (Johnson Matthey). The materials thus formed were dried at 1208C before being calcined in air at 5008C for 4 h. The catalyst tests were performed in a continuous ¯ow tubular quartz reactor (inner diameter 5 mm), placed in a programmable furnace. The catalyst temperature was controlled by a thermocouple mounted internally. Product analysis was performed by gas chromatography using a Perkin-Elmer 8410 gas chromatograph equipped with a Porapak QS column and an FID detector. Methanation of carbon oxides was carried out prior to detection. The catalyst sample (50 mg) was exposed to a gas ¯ow consisting of 4000 ppm C3H8 (72 cm3 minÿ1 1% C3H8/He), 4% O2 (36 cm3 minÿ1 air) and balanced with N2 (all gases from BOC Special Gases) to yield a total ¯ow of 180 cm3 minÿ1. Activity measurements were made as
the samples were heated in the range 150±4508C. The temperature of the sample was varied randomly to minimise systematic errors and held at each temperature until the catalyst was seen to have reached a steady state. The time required for this to occur varied enormously with the catalyst, its pre-treatment and the extent of local heating associated with the reaction exotherm. Sulphation of the catalyst sample was achieved by exposing the catalyst for 2 h at 5008C to a 36:1 O2:SO2 gas stream which comprised of 22 cm3 minÿ1 500 ppm SO2 in N2 (BOC Special Gases) and 2 cm3 minÿ1 air. Fluorination of Al2O3-supported Pt catalysts was done by treating a 50 mg sample in a ¯ow of CHF3 and air (29:1 air:CHF3, total ¯ow 60 cm3 minÿ1) at 5008C for 1 min. This treatment was suf®cient to expose the catalyst to a quantity of ¯uorine ®ve times greater than the amount of SO2 which had been introduced during a 2 h sulphation. The orders of reaction with respect to [C3H8] and [O2] were calculated from data obtained from rates of reaction observed when the reactor was known to be operating under differential conditions. Initially, at 2408C with a constant total gas ¯ow of 180 cm3 minÿ1, [C3H8] was varied between 1000 and 6000 ppm while [O2] was held constant at 4%. In subsequent experiments, [C3H8] was held constant at 4000 ppm and [O2] was varied between 2% and 6%. In the case of the Al2O3-supported Pt catalyst, which had been exposed to a high temperature SO2 pretreatment, differential behaviour was only observed at lower temperature because of the much higher activity. 1758C was selected. To analyse the in¯uence of sulphation on samples of different metal dispersion, a part of the sample prepared as described above was subsequently heat treated at 8008C for 30 min. The metal dispersion was determined by H2-chemisorption as follows. The sample was ®rst held under vacuum at 2508C for 30 min and then reduced for 30 min under a H2 atmosphere of 20 kPa pressure and a temperature of 3008C. In the case of sulphated samples, the reduction temperature was decreased to 1008C to reduce the Pt, but not the sulphur. Finally, the sample was evacuated at 3008C for 1 h prior to measuring the H2-adsorption isotherm at room temperature. To determine the acidity of the catalysts, temperature-programmed ammonia desorption analysis was
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used. For this, 50 mg samples were pretreated in an O2/He ¯ow at 5508C for 1 h before NH3 (1% NH3 in N2) was passed over the catalyst at 1508C for 1 h. The sample was then purged at the same temperature for 30 min and the temperature then lowered to 1008C prior to the temperature-programmed desorption into a ¯ow of He. Desorption of NH3, N2, NO and N2O were monitored by quadrupole mass spectrometry as a function of temperature over the range 100±5508C (heating rate of 208C minÿ1). 3. Results 3.1. Propane combustion reactions Fig. 1 shows the characteristic activity versus temperature curves for a fresh sample of a 1 wt% Pt/Al2O3 catalyst and for the same sample after exposure to a 36:1 O2:SO2 stream at 5008C for 2 h. It can be quite clearly seen that the sulphation procedure leads to a dramatic enhancement in the catalytic activity for Fig. 2. Arrhenius plots for the combustion of C3H8 over 1% Pt/ Al2O3: (*) untreated sample; () sulphated sample.
Fig. 1. Effect of SO2 pre-treatment on propane combustion activity as a function of temperature over 1% Pt/Al2O3.
propane combustion. The characteristic temperature at which 50% conversion is observed (T50) is over 1008C lower for the sulphated sample (1858C) than for the fresh sample (>3008C), and at 2508C the conversion to CO2 is 15 times greater. The sulphated sample also exhibits a much sharper light-off pro®le. This is re¯ected in the Arrhenius plots (shown in Fig. 2). which have been derived from the data presented in Fig. 1 by assuming that under conditions of excess oxygen propane combustion over Pt is ®rst order with respect to [C3H8] and zero order with respect to [O2] [8]. The apparent activation energy of 78 kJ molÿ1 over the fresh catalyst is consistent with the value reported by Otto et al. [8] but is in stark contrast to the value of 129 kJ molÿ1 for the sulphated sample. In compensation, the pre-exponential factor associated with the sulphated catalyst is over seven orders of magnitude greater than that for the fresh sample. It should be noted that sulphation of the bare alumina support resulted in no activity for propane combustion under these reaction conditions and in the temperature range studied.
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Measurements on the equivalent SiO2-supported Pt catalyst revealed that exposure of this system to the same high temperature sulphation treatment resulted in no discernible increase in catalytic activity for propane combustion although the fresh SiO2-supported Pt catalyst was found to be somewhat more active than the fresh Al2O3-supported system. This has been previously attributed to substantial deactivation of the alumina-supported system resulting from a strong interaction between platinum and g-Al2O3 [2] and we would concur with this interpretation. In order to gain additional information on the extent and reversibility of the SO2/O2 promotional effect for propane combustion over Al2O3-supported Pt, samples were brought to steady state in 20:1 O2:C3H8 at 3008C before introduction of a 30 s pulse of a gas stream additionally containing 100 ppm SO2. A sample of the reactor ef¯uent was analysed immediately after the SO2 had been switched off and at short intervals thereafter. It can be quite clearly seen from Fig. 3 that there is an immediate increase in propane conversion upon introduction of SO2. However, even 2 min after the end of the SO2 pulse much of the enhanced activity had already been lost, though it appears that decay to a more active steady state occurs. After eight pulses (at 20 min intervals) it can be seen that the catalyst is signi®cantly more active for propane combustion than an equivalent system which had not been exposed to SO2. These results suggest that there is more than one effect of sulphation on the catalyst surface. There is a transient effect which causes a temporary sharp increase in activity. And there is a more long-lasting effect which may be associated with accumulation of a sulphate species on the alumina support [9,10].
Fig. 3. The effect of a series of 30 s pulses of a 30:1 O2:SO2 mixture on propane conversion on 1% Pt/Al2O3 at 3008C.
The in¯uence of sulphation was compared for two samples of different metal dispersion and the results are shown in Table 1. Comparison of the light-off temperatures of the two catalysts, before and after sulphation, shows a strong promotional effect. This effect is much larger for the sample having a high metal dispersion. For the sake of completeness, we report the apparent dispersions for the sulphated sample. However, the latter must be treated with caution because (a) the results are within the experimental
Table 1 Influence of sulphation on Pt/g-Al2O3 catalysts having different metal dispersions Pre-treatment in air
5008C for 4 min
5008C for 4 min and 8008C for 30 min
Apparent dispersion before sulphation Apparent dispersion after sulphation
60% 1%
5% 0.4%
Light-off temperature Before sulphation After sulphation
2958C 1598C
2328C 1528C
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Fig. 4. Dependence of the rate of propane combustion on the propane concentration for untreated (&) and sulphated (^) 1% Pt/Al2O3.
error of the system, (b) the pre-reduction temperature (1008C) may not have been suf®ciently high to reduce Oads on the Pt surface and (c) this pre-reduction may have led to the reduction of sulphate to H2S which could certainly poison Pt sites. What we can see is that the surface area for H2 adsorption is much decreased by the sintering and that sulphation seems to have a negative effect on chemisorption. In an attempt to further explore the effect of sulphation we determined the kinetic orders of reac-
tion with respect to C3H8 and O2. Fig. 4 shows that the propane combustion reaction over Al2O3-supported Pt remains ®rst order with respect to [C3H8] even after exposure to an oxygen-rich SO2 stream at 5008C. The order of reaction with respect to [O2], however, is substantially more negative over the sulphated catalyst (Fig. 5). Fig. 6 shows that high temperature ¯uorination of Al2O3-supported Pt does cause some promotion of the propane combustion reaction with T50 for the ¯uori-
Fig. 5. Dependence of the rate of propane combustion on the oxygen concentration for untreated (*) and sulphated (~) 1% Pt/Al2O3.
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Fig. 6. Effect of CHF3 and SO2 pre-treatment on propane combustion as a function of temperature for 1% Pt/Al2O3: () untreated sample; () fluorided sample; ( ) fluorided and sulphated sample.
nated catalyst being about 408C lower than for the fresh catalyst. Comparison with Fig. 1 shows, however, that the effect is much less extreme than that found after the sulphation treatment. Fig. 6 also shows that exposure of the ¯uorinated catalyst to a subsequent sulphation treatment results in further promotion of the reaction, but again comparison with Fig. 1 shows sulphation of the fresh sample to be the most effective treatment for the promotion of propane combustion activity. Arrhenius plots derived from the activity versus temperature data presented in Fig. 6 reveal that the apparent activation energy for propane combustion over ¯uorinated Al2O3-supported Pt (65 kJ molÿ1) is comparable to the value calculated for the same reaction over the fresh sample (79 kJ molÿ1). The temperature-programmed ammonia desorption experiments shown in Fig. 7 (see also Table 2) indicate that desorption of NH3 occurs over broadly the same temperature range in all cases. This indicates that the acidic sites are all of comparable strength but there is a continuous increase in the amount of ammonia adsorbed as the time of sulphation was increased.
Fig. 7. NH3 temperature-programmed desorption profiles from 1% Pt Al2O3 sulphated for various times: (&) 0; (*) 5; () 10; (}) 15; (&) 30; (!) 60; (r) 120 min. Table 2 Amounts of N-containing products desorbed in ammonia temperature-programmed desorption experiments Sulphation time (min) 0 5 10 15 30 60 120
NH3a
Total N-containing products detecteda,b
142 132 106 113 164 160 296
142 132 155 152 208 258 399
a
Amount desorbed in units of mmol gÿ1 cat . Contains mainly NH3 with significant amounts of N2 and trace amounts of nitrogen oxides over the heavily sulphated samples.
b
4. Discussion The results of propane combustion over Al2O3supported Pt presented in Fig. 1 are consistent with literature reports which suggest that high temperature treatment of the catalyst with SO2 results in a very strong promotion of the catalytic activity for propane combustion [1±4]. Figs. 1 and 2 show that the
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sulphated sample behaves very differently from the fresh sample and exhibits a very much higher apparent activation energy although this is balanced by a preexponential factor which is seven orders of magnitude larger. This result suggests the development of a new site on the sulphated catalysts which can facilitate C± H bond activation in propane. Interestingly the effect is neither observed over SiO2-supported Pt nor over a physical mixture of Pt/SiO2 and g-Al2O3. It is well known that the oxidation of SO2 to SO3 is catalysed by Pt at temperatures greater than 2008C [11±13]. SO3 can react with the Al2O3 support to form Al2(SO4)3 and this reaction is further catalysed by the presence of Pt [13]. SiO2-supported sulphate type species have been reported [17] but it would seem unlikely that they would be formed or persist under the conditions of this study. A silicon analogue of Al2(SO4) has not been reported [14]. Our results are consistent with the work of Hubbard et al. [2] but are not in line with the assertion of Lambert and co-workers [5] that the SO2 promotion of Pt catalysed propane combustion is a metal-only phenomenon. In Fig. 3 both a transient and a permanent promotion effect can be observed. The transient effect could be due to rapid oxidation of SO2 to SO3 over oxygencovered Pt at 3008C which causes the removal of surface oxygen thus facilitating adsorption of propane and results in the sharp increase in the propane conversion. Note the negative dependency on oxygen partial pressure shown in Fig. 5. Another possible explanation for the observed sharp increase and decrease in the activity might be related to the contribution of SOx species at the metal±support interface. SO3 is formed from SO2 and Oads on the metal particle from where it diffuses rapidly to the metal±support interface. This results in the formation of the new active site (see later) which is far more active for the combustion reaction. This is then followed by the slower diffusion of this SOx species from the metal±support interface to more stable adsorption sites on the Al2O3, the destruction of the newly formed active sites and hence a decrease in the activity of the catalyst. The creation of a new, and increasingly more active steady state, is consistent with the build-up of a sulphate deposit at the Pt±support interface. If the active site for propane combustion over sulphated Al2O3-supported Pt lies at the metal±support interface then one would expect the extent of the promotional
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effect to be directly related to the interfacial area and so be related to Pt dispersion. This is indeed the case as shown by the limited results presented in Table 1, demonstrating that a decreased metal dispersion results in a lower promotion effect. Kinetics experiments carried out to calculate the orders of the propane combustion reaction with respect to [C3H8] and [O2] over the fresh and the pre-sulphated samples (Figs. 4 and 5, respectively) show that the reaction remains ®rst order with respect to [C3H8] even after high temperature treatment with SO2. The sulphation treatment, however, causes the order of reaction with respect to [O2] to become more negative (cf. ÿ0.7 for the fresh catalyst and ÿ1.5 for the sulphated sample), correlating well with oxygen disruption at the active site for propane combustion over the sulphated sample which is thought to be a dÿ couple (see model presented later). Ptd ÿ SO2ÿ 4 This type of site is consistent with the proposal of Bondareva et al. [9] based on IR spectral evidence derived from a study of the role of SO2 in the oxidation of CO over Al2O3-supported Pt. It is well known that high temperature sulphation of alumina has the effect of greatly increasing the support acidity [1,2,7]. However, Hubbard et al. [1,2] report no correlation between the support acidity as measured by Hammett indicators and the catalytic activity of the corresponding Pt catalysts for propane combustion. In contrast, Niwa et al. [16] report that the speci®c activity of supported Pt catalysts for the oxidation of methane increases with the acidity of the support in the order Pt/SiO2
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Fig. 8. Comparison of the amount of ammonia adsorbed (&) and rate constant for propane combustion at 2008C (&) as a function of the time of sulphation.
The ammonia temperature-programmed desorption experiments shown in Fig. 7 (see also Table 2) can be taken as an approximate guide to the total acidity of the catalyst. Fig. 8 shows the relationship between the total amount of ammonia adsorbed and the ®rst order rate constant for propane combustion as a function of the time of sulphation. A temperature of 2008C was selected where the conversion of propane is always less than 8%. The acidity, as measured by NH3 TPD, increases linearly with time of sulphation. However, the catalytic activity exhibits a more complex behaviour. Initially the activity rises with increasing acidity but after a sulphation time of 20 min it plateaus out. However after a sulphation time of 90 min the activity rises sharply before again reaching a plateau. This pattern of behaviour can be explained as follows. Exposure of the Pt±Al2O3 catalyst to an SO2O2 mixture at 5008C results in the formation of SO3 on platinum. However, as the results on the Pt±SiO2 show, the SO3 is not stable on the Pt and it will migrate to the Al2O3 surface. The Al2O3 is heterogeneous so there will be sites where the SO3 is trapped strongly and other sites where it is trapped less strongly. We envisage that at short sulphation times all the SO3 can migrate from the Pt particles to isolated sites on the Al2O3. However, as the Al2O3 surface gradually saturates any additional SO3 will have to be accommodated at less favourable sites. One such site could be at
the interface between the Pt particle and the support. Thus we propose initially that sulphation of isolated sites on the support causes large changes in the acidity of the Al2O3 but this is only re¯ected in small changes in the catalytic activity. Eventually surface sites at the perimeter of the Pt particles become sulphated and a large increase in catalytic activity is observed. The complex behaviour of the catalytic activity shown in Fig. 8 can be explained on this basis if we accept that sulphation of isolated Al2O3 sites has little or no promoting effect on the activity for propane combustion whereas perimeter sites are strongly promoted by SO3. Thus we propose a model involving the build-up of an electronegative sulphate SO2ÿ 4 deposit which lies in close proximity to the edge of a Pt particle (Fig. 9). This would withdraw electron density from the Pt particle thereby conferring on Pt atoms at the perimeter has a partial positive charge. A catalytic site consisting of adjacent cationic and anionic moieties could then develop and this would facilitate the initial (heterolytic) C±H bond activation at the basic anionic (sulphate) site in a manner similar to that proposed by Choudhary and Rane [15] for methane activation over oxide catalysts. This model, in which the overall acidity of the catalyst is not the key factor, is consistent with the results reported by Hubbard et al. on Al2O3 supported catalysts but the proposed role of
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Fig. 9. Perimeter interface model to interpret the promoting effect of sulphation on the propane combustion reaction on alumina-supported Pt catalysts.
perimeter sites does not support the view of Wilson et al. derived from single crystal studies. 5. Conclusions This study has demonstrated the role of different surface oxygen species in the activation of hydrocarbons for combustion. The effect of sulphation on the activity of supported Pt catalysts for propane combustion is con®rmed to be support-speci®c and so cannot be solely due to SO2 promotion of Pt [5]. Equally, it does not seem that the overall, or total acidity, of the support is an accurate measure of the enhancement in activity. As an alternative it is proposed that perimeter sites at the Pt/support interface may be important in the activation of the ®rst C±H bond in propane. Polarisation of atoms at these perimeter sites by sulphation of the support could explain the enhanced activity observed on Al2O3-supported catalysts Acknowledgements We are pleased to acknowledge ®nancial support for this research from the EU (EV5V-CT94-0535) and the EPSRC (grants GR/K16371 and GR/K70403) and
would like to thank Mr D.J. Crittle for carrying out some of the experiments. References [1] C.P. Hubbard, K. Otto, H.S. Gandhi, K.Y.S. Ng, Catal. Lett. 30 (1995) 41. [2] C.P. Hubbard, K. Otto, H.S. Gandhi, K.Y.S. Ng, J. Catal. 144 (1993) 484. [3] H.S. Gandhi, M. Shelef, Appl. Catal. 77 (1991) 175. [4] H.C. Yao, H.K. Stepien, H.S. Gandhi, J. Catal. 67 (1981) 231. [5] K. Wilson, C. Hardacre, R.M. Lambert, J. Phys. Chem. 99(38) (1995) 13755. [6] A.K. Ghosh, R.A. Kydd, Catal. Rev.-Sci. Eng. 27 (1985) 539. [7] P. Berteau, M.-A. Kellens, B. Delmon, J. Chem. Soc., Faraday Trans. 87 (1991) 1425. [8] K. Otto, J.M. Andino, C.L. Parks, J. Catal. 131 (1991) 243. [9] N.K. Bondareva, V.I. Panchnishnyi, V.A. Matyshak, Kinet. Katal. (English Translation) 33 (1992) 1113. [10] C.C. Chang, J. Catal. 53 (1978) 374. [11] G.C. Bond, Catalysis by Metals, Academic Press, London, 1962. [12] G.I. Godolets, Stud. Surf. Sci. Catal. 15 (1983) 365. [13] D.D. Beck, M.H. Krueger, D.R. Monroe, SAE Paper no. 910844 (1991). [14] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed. [15] V.R. Choudhary, V.H. Rane, J. Catal. 130 (1991) 411. [16] M. Niwa, K. Awano, Y. Murakami, Appl. Catal. 7 (1983) 317. [17] B.A. Morrow, R.A. McFarlane, M. Lion, J.C. Lavalley, J. Catal. 107 (1987) 232.
The nature of activity enhancement for propane oxidation over supported Pt catalysts exposed to sulphur dioxide R. Burch*, E. Halpin, M. Hayes, K. Ruth, J.A. Sullivan Catalysis Research Centre, Chemistry Department, The University of Reading, Whiteknights, Reading RG6 6AD, UK Received 19 April 1998; received in revised form 12 July 1998; accepted 12 July 1998
Abstract The nature of the active sites, the role of the support, and the mechanism by which hydrocarbons are activated over supported Pt catalysts have been investigated for the combustion of propane in the presence and absence of SO2. A strong enhancement in the activity for propane oxidation has been con®rmed either when SO2 is introduced with the propane or with a pre-sulphated alumina-supported catalyst. No equivalent effects were found with silica-supported catalysts. Fluorination of the alumina support also leads to an increase in activity. The addition of pulses of SO2 into the propane-containing gas stream produces a very large, but short-lived, increase in activity in addition to a more gradual and progressive activity enhancement. Reasons for these different effects are discussed. Attempts to correlate the permanent enhancement in activity with the total acidity of the support were unsuccessful. It appears that the increase in activity is due to a more subtle effect and a model is presented in which the possible role of perimeter sites at the metal±support interface is emphasised. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Pt Catalysts; C3H8 combustion; SO2; Al2O3; Interface; Acidity
1. Introduction The activation of hydrocarbons is of interest both because of the desire to functionalise inexpensive feedstocks and also because catalytic combustion offers a possible means to generate power without creating excessive amounts of nitrogen oxides. Platinum catalysts are well known to be the most active for the combustion of hydrocarbons containing more than one carbon atom whereas palladium is the most active catalyst for methane combustion. For propane combustion over an Al2O3-supported Pt catalyst, Hubbard et al. [1±3] have reported a *Corresponding author.
substantial reaction rate increase after having exposed the catalyst to an oxygen-rich mixture containing SO2 at 5008C. Earlier work had shown a similar promotional effect when a small amount of SO2 (20 ppm) was present in the feed gas [4]. Results presented by Lambert and co-workers [5], however, show that adsorption of propane on a Pt single crystal surface is greatly enhanced if SO2 and O2 are co-adsorbed prior to the introduction of the propane. In the absence of SO2 the sticking probability for propane was almost zero. These workers concluded that the promoting effect of SO2 can arise from a direct interaction on the metal surface and suggest that ``support mediated effects are not necessary for SO2 promotion of Pt catalysed propane combustion''. This interpretation is
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not consistent with the observation of Hubbard et al. [2] that propane combustion over Al2O3-supported Pt is promoted by high temperature pre-treatment of the catalyst in an oxygen rich stream of SO2 whereas the reaction over SiO2-supported Pt is unaffected. In addition,earlyworkcarriedoutbyYaoetal.[4]demonstrated that the extent of the promotional effect of SO2 was directly related to the Pt dispersion on the support. The purpose of the present work was to re-assess these two con¯icting mechanisms by investigating the effect of different treatments with SO2-containing gas streams on the activities for propane combustion of supported Pt catalysts. In order to further assess the importance of the overall acidity of the support, catalysts were also exposed to a high temperature treatment with ¯uoromethane since it is known that ¯uorination of alumina has the effect of greatly increasing the Lewis acidity [6,7]. 2. Experimental The supports used for the Pt-containing catalysts were g-Al2O3 (Akzo Chemie, CK300, 190 m2 gÿ1) and silica (Grace 432, 290 m2 gÿ1). Before use the silica was washed with 2 M nitric acid at 70±808C for 2 h to remove any metal impurities. Both supports were crushed and sieved to 250±600 mm and dried at 3008C before impregnation. The catalysts were prepared by dry impregnation of the support with the appropriate amount of aqueous dinitrodiammine (DNDA) Pt(II) (Johnson Matthey). The materials thus formed were dried at 1208C before being calcined in air at 5008C for 4 h. The catalyst tests were performed in a continuous ¯ow tubular quartz reactor (inner diameter 5 mm), placed in a programmable furnace. The catalyst temperature was controlled by a thermocouple mounted internally. Product analysis was performed by gas chromatography using a Perkin-Elmer 8410 gas chromatograph equipped with a Porapak QS column and an FID detector. Methanation of carbon oxides was carried out prior to detection. The catalyst sample (50 mg) was exposed to a gas ¯ow consisting of 4000 ppm C3H8 (72 cm3 minÿ1 1% C3H8/He), 4% O2 (36 cm3 minÿ1 air) and balanced with N2 (all gases from BOC Special Gases) to yield a total ¯ow of 180 cm3 minÿ1. Activity measurements were made as
the samples were heated in the range 150±4508C. The temperature of the sample was varied randomly to minimise systematic errors and held at each temperature until the catalyst was seen to have reached a steady state. The time required for this to occur varied enormously with the catalyst, its pre-treatment and the extent of local heating associated with the reaction exotherm. Sulphation of the catalyst sample was achieved by exposing the catalyst for 2 h at 5008C to a 36:1 O2:SO2 gas stream which comprised of 22 cm3 minÿ1 500 ppm SO2 in N2 (BOC Special Gases) and 2 cm3 minÿ1 air. Fluorination of Al2O3-supported Pt catalysts was done by treating a 50 mg sample in a ¯ow of CHF3 and air (29:1 air:CHF3, total ¯ow 60 cm3 minÿ1) at 5008C for 1 min. This treatment was suf®cient to expose the catalyst to a quantity of ¯uorine ®ve times greater than the amount of SO2 which had been introduced during a 2 h sulphation. The orders of reaction with respect to [C3H8] and [O2] were calculated from data obtained from rates of reaction observed when the reactor was known to be operating under differential conditions. Initially, at 2408C with a constant total gas ¯ow of 180 cm3 minÿ1, [C3H8] was varied between 1000 and 6000 ppm while [O2] was held constant at 4%. In subsequent experiments, [C3H8] was held constant at 4000 ppm and [O2] was varied between 2% and 6%. In the case of the Al2O3-supported Pt catalyst, which had been exposed to a high temperature SO2 pretreatment, differential behaviour was only observed at lower temperature because of the much higher activity. 1758C was selected. To analyse the in¯uence of sulphation on samples of different metal dispersion, a part of the sample prepared as described above was subsequently heat treated at 8008C for 30 min. The metal dispersion was determined by H2-chemisorption as follows. The sample was ®rst held under vacuum at 2508C for 30 min and then reduced for 30 min under a H2 atmosphere of 20 kPa pressure and a temperature of 3008C. In the case of sulphated samples, the reduction temperature was decreased to 1008C to reduce the Pt, but not the sulphur. Finally, the sample was evacuated at 3008C for 1 h prior to measuring the H2-adsorption isotherm at room temperature. To determine the acidity of the catalysts, temperature-programmed ammonia desorption analysis was
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used. For this, 50 mg samples were pretreated in an O2/He ¯ow at 5508C for 1 h before NH3 (1% NH3 in N2) was passed over the catalyst at 1508C for 1 h. The sample was then purged at the same temperature for 30 min and the temperature then lowered to 1008C prior to the temperature-programmed desorption into a ¯ow of He. Desorption of NH3, N2, NO and N2O were monitored by quadrupole mass spectrometry as a function of temperature over the range 100±5508C (heating rate of 208C minÿ1). 3. Results 3.1. Propane combustion reactions Fig. 1 shows the characteristic activity versus temperature curves for a fresh sample of a 1 wt% Pt/Al2O3 catalyst and for the same sample after exposure to a 36:1 O2:SO2 stream at 5008C for 2 h. It can be quite clearly seen that the sulphation procedure leads to a dramatic enhancement in the catalytic activity for Fig. 2. Arrhenius plots for the combustion of C3H8 over 1% Pt/ Al2O3: (*) untreated sample; () sulphated sample.
Fig. 1. Effect of SO2 pre-treatment on propane combustion activity as a function of temperature over 1% Pt/Al2O3.
propane combustion. The characteristic temperature at which 50% conversion is observed (T50) is over 1008C lower for the sulphated sample (1858C) than for the fresh sample (>3008C), and at 2508C the conversion to CO2 is 15 times greater. The sulphated sample also exhibits a much sharper light-off pro®le. This is re¯ected in the Arrhenius plots (shown in Fig. 2). which have been derived from the data presented in Fig. 1 by assuming that under conditions of excess oxygen propane combustion over Pt is ®rst order with respect to [C3H8] and zero order with respect to [O2] [8]. The apparent activation energy of 78 kJ molÿ1 over the fresh catalyst is consistent with the value reported by Otto et al. [8] but is in stark contrast to the value of 129 kJ molÿ1 for the sulphated sample. In compensation, the pre-exponential factor associated with the sulphated catalyst is over seven orders of magnitude greater than that for the fresh sample. It should be noted that sulphation of the bare alumina support resulted in no activity for propane combustion under these reaction conditions and in the temperature range studied.
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Measurements on the equivalent SiO2-supported Pt catalyst revealed that exposure of this system to the same high temperature sulphation treatment resulted in no discernible increase in catalytic activity for propane combustion although the fresh SiO2-supported Pt catalyst was found to be somewhat more active than the fresh Al2O3-supported system. This has been previously attributed to substantial deactivation of the alumina-supported system resulting from a strong interaction between platinum and g-Al2O3 [2] and we would concur with this interpretation. In order to gain additional information on the extent and reversibility of the SO2/O2 promotional effect for propane combustion over Al2O3-supported Pt, samples were brought to steady state in 20:1 O2:C3H8 at 3008C before introduction of a 30 s pulse of a gas stream additionally containing 100 ppm SO2. A sample of the reactor ef¯uent was analysed immediately after the SO2 had been switched off and at short intervals thereafter. It can be quite clearly seen from Fig. 3 that there is an immediate increase in propane conversion upon introduction of SO2. However, even 2 min after the end of the SO2 pulse much of the enhanced activity had already been lost, though it appears that decay to a more active steady state occurs. After eight pulses (at 20 min intervals) it can be seen that the catalyst is signi®cantly more active for propane combustion than an equivalent system which had not been exposed to SO2. These results suggest that there is more than one effect of sulphation on the catalyst surface. There is a transient effect which causes a temporary sharp increase in activity. And there is a more long-lasting effect which may be associated with accumulation of a sulphate species on the alumina support [9,10].
Fig. 3. The effect of a series of 30 s pulses of a 30:1 O2:SO2 mixture on propane conversion on 1% Pt/Al2O3 at 3008C.
The in¯uence of sulphation was compared for two samples of different metal dispersion and the results are shown in Table 1. Comparison of the light-off temperatures of the two catalysts, before and after sulphation, shows a strong promotional effect. This effect is much larger for the sample having a high metal dispersion. For the sake of completeness, we report the apparent dispersions for the sulphated sample. However, the latter must be treated with caution because (a) the results are within the experimental
Table 1 Influence of sulphation on Pt/g-Al2O3 catalysts having different metal dispersions Pre-treatment in air
5008C for 4 min
5008C for 4 min and 8008C for 30 min
Apparent dispersion before sulphation Apparent dispersion after sulphation
60% 1%
5% 0.4%
Light-off temperature Before sulphation After sulphation
2958C 1598C
2328C 1528C
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Fig. 4. Dependence of the rate of propane combustion on the propane concentration for untreated (&) and sulphated (^) 1% Pt/Al2O3.
error of the system, (b) the pre-reduction temperature (1008C) may not have been suf®ciently high to reduce Oads on the Pt surface and (c) this pre-reduction may have led to the reduction of sulphate to H2S which could certainly poison Pt sites. What we can see is that the surface area for H2 adsorption is much decreased by the sintering and that sulphation seems to have a negative effect on chemisorption. In an attempt to further explore the effect of sulphation we determined the kinetic orders of reac-
tion with respect to C3H8 and O2. Fig. 4 shows that the propane combustion reaction over Al2O3-supported Pt remains ®rst order with respect to [C3H8] even after exposure to an oxygen-rich SO2 stream at 5008C. The order of reaction with respect to [O2], however, is substantially more negative over the sulphated catalyst (Fig. 5). Fig. 6 shows that high temperature ¯uorination of Al2O3-supported Pt does cause some promotion of the propane combustion reaction with T50 for the ¯uori-
Fig. 5. Dependence of the rate of propane combustion on the oxygen concentration for untreated (*) and sulphated (~) 1% Pt/Al2O3.
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Fig. 6. Effect of CHF3 and SO2 pre-treatment on propane combustion as a function of temperature for 1% Pt/Al2O3: () untreated sample; () fluorided sample; ( ) fluorided and sulphated sample.
nated catalyst being about 408C lower than for the fresh catalyst. Comparison with Fig. 1 shows, however, that the effect is much less extreme than that found after the sulphation treatment. Fig. 6 also shows that exposure of the ¯uorinated catalyst to a subsequent sulphation treatment results in further promotion of the reaction, but again comparison with Fig. 1 shows sulphation of the fresh sample to be the most effective treatment for the promotion of propane combustion activity. Arrhenius plots derived from the activity versus temperature data presented in Fig. 6 reveal that the apparent activation energy for propane combustion over ¯uorinated Al2O3-supported Pt (65 kJ molÿ1) is comparable to the value calculated for the same reaction over the fresh sample (79 kJ molÿ1). The temperature-programmed ammonia desorption experiments shown in Fig. 7 (see also Table 2) indicate that desorption of NH3 occurs over broadly the same temperature range in all cases. This indicates that the acidic sites are all of comparable strength but there is a continuous increase in the amount of ammonia adsorbed as the time of sulphation was increased.
Fig. 7. NH3 temperature-programmed desorption profiles from 1% Pt Al2O3 sulphated for various times: (&) 0; (*) 5; () 10; (}) 15; (&) 30; (!) 60; (r) 120 min. Table 2 Amounts of N-containing products desorbed in ammonia temperature-programmed desorption experiments Sulphation time (min) 0 5 10 15 30 60 120
NH3a
Total N-containing products detecteda,b
142 132 106 113 164 160 296
142 132 155 152 208 258 399
a
Amount desorbed in units of mmol gÿ1 cat . Contains mainly NH3 with significant amounts of N2 and trace amounts of nitrogen oxides over the heavily sulphated samples.
b
4. Discussion The results of propane combustion over Al2O3supported Pt presented in Fig. 1 are consistent with literature reports which suggest that high temperature treatment of the catalyst with SO2 results in a very strong promotion of the catalytic activity for propane combustion [1±4]. Figs. 1 and 2 show that the
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sulphated sample behaves very differently from the fresh sample and exhibits a very much higher apparent activation energy although this is balanced by a preexponential factor which is seven orders of magnitude larger. This result suggests the development of a new site on the sulphated catalysts which can facilitate C± H bond activation in propane. Interestingly the effect is neither observed over SiO2-supported Pt nor over a physical mixture of Pt/SiO2 and g-Al2O3. It is well known that the oxidation of SO2 to SO3 is catalysed by Pt at temperatures greater than 2008C [11±13]. SO3 can react with the Al2O3 support to form Al2(SO4)3 and this reaction is further catalysed by the presence of Pt [13]. SiO2-supported sulphate type species have been reported [17] but it would seem unlikely that they would be formed or persist under the conditions of this study. A silicon analogue of Al2(SO4) has not been reported [14]. Our results are consistent with the work of Hubbard et al. [2] but are not in line with the assertion of Lambert and co-workers [5] that the SO2 promotion of Pt catalysed propane combustion is a metal-only phenomenon. In Fig. 3 both a transient and a permanent promotion effect can be observed. The transient effect could be due to rapid oxidation of SO2 to SO3 over oxygencovered Pt at 3008C which causes the removal of surface oxygen thus facilitating adsorption of propane and results in the sharp increase in the propane conversion. Note the negative dependency on oxygen partial pressure shown in Fig. 5. Another possible explanation for the observed sharp increase and decrease in the activity might be related to the contribution of SOx species at the metal±support interface. SO3 is formed from SO2 and Oads on the metal particle from where it diffuses rapidly to the metal±support interface. This results in the formation of the new active site (see later) which is far more active for the combustion reaction. This is then followed by the slower diffusion of this SOx species from the metal±support interface to more stable adsorption sites on the Al2O3, the destruction of the newly formed active sites and hence a decrease in the activity of the catalyst. The creation of a new, and increasingly more active steady state, is consistent with the build-up of a sulphate deposit at the Pt±support interface. If the active site for propane combustion over sulphated Al2O3-supported Pt lies at the metal±support interface then one would expect the extent of the promotional
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effect to be directly related to the interfacial area and so be related to Pt dispersion. This is indeed the case as shown by the limited results presented in Table 1, demonstrating that a decreased metal dispersion results in a lower promotion effect. Kinetics experiments carried out to calculate the orders of the propane combustion reaction with respect to [C3H8] and [O2] over the fresh and the pre-sulphated samples (Figs. 4 and 5, respectively) show that the reaction remains ®rst order with respect to [C3H8] even after high temperature treatment with SO2. The sulphation treatment, however, causes the order of reaction with respect to [O2] to become more negative (cf. ÿ0.7 for the fresh catalyst and ÿ1.5 for the sulphated sample), correlating well with oxygen disruption at the active site for propane combustion over the sulphated sample which is thought to be a dÿ couple (see model presented later). Ptd ÿ SO2ÿ 4 This type of site is consistent with the proposal of Bondareva et al. [9] based on IR spectral evidence derived from a study of the role of SO2 in the oxidation of CO over Al2O3-supported Pt. It is well known that high temperature sulphation of alumina has the effect of greatly increasing the support acidity [1,2,7]. However, Hubbard et al. [1,2] report no correlation between the support acidity as measured by Hammett indicators and the catalytic activity of the corresponding Pt catalysts for propane combustion. In contrast, Niwa et al. [16] report that the speci®c activity of supported Pt catalysts for the oxidation of methane increases with the acidity of the support in the order Pt/SiO2
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Fig. 8. Comparison of the amount of ammonia adsorbed (&) and rate constant for propane combustion at 2008C (&) as a function of the time of sulphation.
The ammonia temperature-programmed desorption experiments shown in Fig. 7 (see also Table 2) can be taken as an approximate guide to the total acidity of the catalyst. Fig. 8 shows the relationship between the total amount of ammonia adsorbed and the ®rst order rate constant for propane combustion as a function of the time of sulphation. A temperature of 2008C was selected where the conversion of propane is always less than 8%. The acidity, as measured by NH3 TPD, increases linearly with time of sulphation. However, the catalytic activity exhibits a more complex behaviour. Initially the activity rises with increasing acidity but after a sulphation time of 20 min it plateaus out. However after a sulphation time of 90 min the activity rises sharply before again reaching a plateau. This pattern of behaviour can be explained as follows. Exposure of the Pt±Al2O3 catalyst to an SO2O2 mixture at 5008C results in the formation of SO3 on platinum. However, as the results on the Pt±SiO2 show, the SO3 is not stable on the Pt and it will migrate to the Al2O3 surface. The Al2O3 is heterogeneous so there will be sites where the SO3 is trapped strongly and other sites where it is trapped less strongly. We envisage that at short sulphation times all the SO3 can migrate from the Pt particles to isolated sites on the Al2O3. However, as the Al2O3 surface gradually saturates any additional SO3 will have to be accommodated at less favourable sites. One such site could be at
the interface between the Pt particle and the support. Thus we propose initially that sulphation of isolated sites on the support causes large changes in the acidity of the Al2O3 but this is only re¯ected in small changes in the catalytic activity. Eventually surface sites at the perimeter of the Pt particles become sulphated and a large increase in catalytic activity is observed. The complex behaviour of the catalytic activity shown in Fig. 8 can be explained on this basis if we accept that sulphation of isolated Al2O3 sites has little or no promoting effect on the activity for propane combustion whereas perimeter sites are strongly promoted by SO3. Thus we propose a model involving the build-up of an electronegative sulphate SO2ÿ 4 deposit which lies in close proximity to the edge of a Pt particle (Fig. 9). This would withdraw electron density from the Pt particle thereby conferring on Pt atoms at the perimeter has a partial positive charge. A catalytic site consisting of adjacent cationic and anionic moieties could then develop and this would facilitate the initial (heterolytic) C±H bond activation at the basic anionic (sulphate) site in a manner similar to that proposed by Choudhary and Rane [15] for methane activation over oxide catalysts. This model, in which the overall acidity of the catalyst is not the key factor, is consistent with the results reported by Hubbard et al. on Al2O3 supported catalysts but the proposed role of
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Fig. 9. Perimeter interface model to interpret the promoting effect of sulphation on the propane combustion reaction on alumina-supported Pt catalysts.
perimeter sites does not support the view of Wilson et al. derived from single crystal studies. 5. Conclusions This study has demonstrated the role of different surface oxygen species in the activation of hydrocarbons for combustion. The effect of sulphation on the activity of supported Pt catalysts for propane combustion is con®rmed to be support-speci®c and so cannot be solely due to SO2 promotion of Pt [5]. Equally, it does not seem that the overall, or total acidity, of the support is an accurate measure of the enhancement in activity. As an alternative it is proposed that perimeter sites at the Pt/support interface may be important in the activation of the ®rst C±H bond in propane. Polarisation of atoms at these perimeter sites by sulphation of the support could explain the enhanced activity observed on Al2O3-supported catalysts Acknowledgements We are pleased to acknowledge ®nancial support for this research from the EU (EV5V-CT94-0535) and the EPSRC (grants GR/K16371 and GR/K70403) and
would like to thank Mr D.J. Crittle for carrying out some of the experiments. References [1] C.P. Hubbard, K. Otto, H.S. Gandhi, K.Y.S. Ng, Catal. Lett. 30 (1995) 41. [2] C.P. Hubbard, K. Otto, H.S. Gandhi, K.Y.S. Ng, J. Catal. 144 (1993) 484. [3] H.S. Gandhi, M. Shelef, Appl. Catal. 77 (1991) 175. [4] H.C. Yao, H.K. Stepien, H.S. Gandhi, J. Catal. 67 (1981) 231. [5] K. Wilson, C. Hardacre, R.M. Lambert, J. Phys. Chem. 99(38) (1995) 13755. [6] A.K. Ghosh, R.A. Kydd, Catal. Rev.-Sci. Eng. 27 (1985) 539. [7] P. Berteau, M.-A. Kellens, B. Delmon, J. Chem. Soc., Faraday Trans. 87 (1991) 1425. [8] K. Otto, J.M. Andino, C.L. Parks, J. Catal. 131 (1991) 243. [9] N.K. Bondareva, V.I. Panchnishnyi, V.A. Matyshak, Kinet. Katal. (English Translation) 33 (1992) 1113. [10] C.C. Chang, J. Catal. 53 (1978) 374. [11] G.C. Bond, Catalysis by Metals, Academic Press, London, 1962. [12] G.I. Godolets, Stud. Surf. Sci. Catal. 15 (1983) 365. [13] D.D. Beck, M.H. Krueger, D.R. Monroe, SAE Paper no. 910844 (1991). [14] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed. [15] V.R. Choudhary, V.H. Rane, J. Catal. 130 (1991) 411. [16] M. Niwa, K. Awano, Y. Murakami, Appl. Catal. 7 (1983) 317. [17] B.A. Morrow, R.A. McFarlane, M. Lion, J.C. Lavalley, J. Catal. 107 (1987) 232.