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Characterization and reactivity of Pd–Pt bimetallic supported catalysts obtained by laser vaporization of bulk alloy
Applied Surface Science 164 Ž2000. 163–168 www.elsevier.nlrlocaterapsusc
Characterization and reactivity of Pd–Pt bimetallic supported catalysts obtained by laser vaporization of bulk alloy J.L. Rousset a,) , F.J. Cadete Santos Aires a , F. Bornette a , M. Cattenot a , M. Pellarin b, L. Stievano a , A.J. Renouprez a a
b
Institut de Recherches sur la Catalyse, CNRS 2 aÕenue A. Einstein, F69626, Villeurbanne Cedex, France Laboratoire de spectrometrie uniÕersite´ Claude Bernard Lyon I, 43 Bd du 11 noÕ. 1918, 69622 Villeurbanne, ´ ionique et moleculaire, ´ Cedex, France
Abstract Bimetallic Pd–Pt clusters produced by laser vaporization of bulk alloy have been deposited on high surface alumina. Energy dispersive X-ray ŽEDX. analysis and transmission electron microscopy ŽTEM. show that they have a perfectly well-defined stoichiometry and a narrow range of size. Therefore, they constitute ideal systems to investigate alloying effects towards reactivity. Pd–Pt alloys are already known for their applications in the hydrogenation of unsaturated hydrocarbons, especially aromatics, because this system is highly resistant to sulfur and nitrogen poisoning. In this context, the catalytic properties of this system have been investigated in the hydrogenation of tetralin in the presence of hydrogen sulfide. Preliminary results show that this model catalyst is more sulfur-resistant than each of the pure supported metals prepared by chemical methods. q 2000 Elsevier Science B.V. All rights reserved. PACS: 82.65.J; 61.16.B Keywords: Laser vaporisation; Well-defined bimetallic catalysts; TEM; Reactivity
1. Introduction Metallic clusters constitute an exciting field of research, and over at least the past 20 years, a great deal of effort has been devoted to them. A fact, which is very attractive in this field, is certainly the transition of their properties when going from the bulk to small clusters. Since these differences are expected to affect the catalytic behaviour, and considering that the supported-metal catalysts used in practice always consist of small particles, these parti) Corresponding author. Tel.: q33-4-72-44-54-34; fax: q33-472-44-53-99. E-mail address: [email protected] ŽJ.L. Rousset..
cle size effects are of great interest to those who investigate chemisorptive or catalytic properties. Moreover, in the field of heterogeneous catalysis, a promising class of catalysts is constituted by bimetallic particles, which are found to exhibit superior properties, compared to pure metals, in terms of activity, selectivity, stability and resistance to poisoning w1,2x. The effects of alloying on the improvement of the catalytic behaviour may be explained by an ensemble andror a ligand effect w1,2x. It is somewhat unusual to associate two noble metals like Pd and Pt. Actually, bimetallic catalysts based on the association of these two elements can show either a simple additivity of the reactivity of each component, as observed in the dehydrogenation
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 3 3 6 - 6
164
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
of cyclohexane w3x or an enhancement of activity, compared to that of the pure metals, as for example, in the isomerization and hydrocracking of alkanes w4x and in the hydrogenation of aromatics w5x. Pd and Pt are generally both active in hydrogenation reactions, but the former is generally preferred because of its lower cost and its larger selectivity for partial hydrogenation. On the other hand, Pd–Pt bimetallic catalysts are known to be more resistant than monometallic ones to sulfur poisoning, an element present in fuels w6x. Most of the bimetallic catalysts prepared by chemical methods suffer from a lack of compositional homogeneity and, in some cases, one cannot even be sure that alloys are really formed. To overcome this difficulty, the laser vaporization of bulk alloys, which ensures a perfect homogeneity of composition of the particles, was proposed w7,8x. Up to now, we have used a pulsed vaporization source, which permitted us to deposit only low quantities of clusters on flat supports. By the use of a modified source, we have been able to deposit bimetallic Pd–Pt clusters on high-area-surface oxides. In this paper, we report on the production and characterization of this catalysts and on their reactivity towards hydrogenation of tetralin.
Žpositively and negatively. ionized clusters. After removal by electrostatic deflection of the ionized clusters, the neutral low-energy clusters are deposited onto the substrates. The incoming flux of clusters is monitored by a quartz microbalance. In these conditions, typical deposition rates are 0.5 nm cmy2 miny1 w10x. These rates are quite acceptable to deposit thin films on low specific area samples Žflat surfaces.; however, they are very low when deposits are performed on high specific area samples Žpowders.. Since our goal is to obtain bimetallic particles supported on high specific surfaces Žtypically 300 m2rg., the cluster source has been modified. In the ‘‘new’’ source, the Nd:YAG laser is still pulsed, while there is a continuous flux of inert gas Žnow a mixture of He and Ar.. This yields higher deposition rates of about 5 nm cmy2 miny1 . The catalytic supports used in this study are powders of g-alumina ŽCondea, Puralox SCFa-240. and non-porous silica ŽDegussa, Aerosil 380. with a specific surface of 240 and 380 m2rg, respectively. In order to deposit the clusters on these samples, a device has been made which stirs the powder in front of the cluster beam. We expect this device to favor homogeneous cluster deposition on most powder grains. After deposition, the samples are air transferred and characterized andror used in catalytic reactions.
2. Experimental set-up
2.2. Transmission electron microscopy (TEM) and energy dispersiÕe X-ray spectroscopy (EDX-S)
2.1. Low-energy cluster beam deposition The laser vaporization technique w9x has been used to obtain a low-energy cluster beam of bimetallic particles from a rod alloy obtained by melting of palladium Ž99.99% purity. and platinum Ž99.95% purity.. In the classical set-up w10x, a pulsed Nd:YAG laser is focussed on a rod of alloy and a plasma is formed Žthe rod is animated by a slow helical movement in order to allow the laser beam to be focussed on different fresh regions.. The introduction of a synchronized pulse of an inert gas ŽHe. leads to nucleation and growth of metallic clusters. The mean size of the free clusters may be qualitatively monitored by adjusting the inert gas pressure and the time of residence in the nucleation cell. The cluster beam is obtained by differential pumping extraction through a skimmer. This beam is formed by neutral and
Morphologic characterization and compositional analysis of the supported clusters has been performed, respectively, by TEM and high spatial resolution EDX-S in the TEM. In order to obtain suitable samples for TEM characterization, the as-obtained powders are dispersed in ethanol by ultrasonication. A drop of the solution is then deposited onto a thin holey-carbon film supported on a copper microscopy grid Ž200 mesh, 3.05 mm. and left to dry. The powder grains, containing the alloy particles, are then well separated and supported on the thin holey-carbon film and may thus be characterized by TEM. The experiments were performed in a JEOL JEM 2010-F transmission electron microscope operating at 200 kV and equipped with a field emission gun ŽFEG., a high resolution UHR pole piece Žpoint
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
resolution: 0.196 nm. and a Pentafet-Link ISIS EDX spectrometer from Oxford Instruments. The size of the clusters and verification of homogeneous distribution over the supports have been obtained by TEM imaging. Intense enough condensed probes Ž0.5–2.4 nm. can be obtained with the FEG, which allow EDX-S analysis of very reduced volumes and, thus, of individual nanometer-size particles. This was performed to verify the chemical homogeneity of the deposited clusters. Large scale Ž) 20 nm. EDX-S analysis of more or less large collections of particles was also performed by decondensing the electron probe to the required size.
165
the catalyst, the H 2 S concentration was varied between 0 and 500 ppm. These experimental conditions were chosen in order to minimise the dehydrogenation of decalin to naphthalene, and to obtain a conversion lower than 15%, at which a differential model for the determination of the specific rates of reaction can be applied. The products of the hydrogenation were analysed by an in-line gas chromatograph with a flame-ionisation detector. Cis and trans decalins were the only products obtained, together with small amounts of naphthalene. No detectable amounts of isomerisation or cracking products were formed.
2.3. Catalytic actiÕity measurements 3. Results and discussion The catalytic properties of the Pd–Ptralumina catalyst specimen have been tested in the hydrogenation of 1,2,3,4-tetrahydronaphtalene Žtetralin.. The experiments were performed in the gas phase at 573 K in a fixed-bed catalytic microreactor, with a pressure of H 2 of 4.50 MPa and a partial pressure of tetralin of 6.0 kPa, which was kept constant by using a gas phase saturator system. In order to investigate the influence of H 2 S on the catalytic properties of
There are only few results on the nucleation of deposited clusters w11x. In our case, deposits have been made on high-surface-area silica or alumina powders. The mean size and standard deviation have been found nearly identical, whatever the support, indicating that clusters diffusion and, hence, coalescence, must be very weak. Consequently, for suitable deposits, the supported particles are well separated
Fig. 1. TEM images of the Pd–Pt clusters deposited on g-alumina Ža. and silica Žb.. Scale bars are 20 nm.
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J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
and homogeneously distributed on the substrate. This is shown in Fig. 1, which represents TEM images of silica and alumina supported catalysts. The size histograms, corresponding to 460 and 439 analyzed particles, respectively, for silica and alumina systems are shown in Fig. 2. The mean cluster size Žstandard deviation. are about 3.5 nm Ž1 nm. for Pd–Ptrsilica and 3.7 nm Ž1.2 nm. for Pd–Ptralumina. EDX analysis has been used to study the composition of individual particles by reducing the probe area down to 1 nm2 . Consequently, for both systems, the composition homogeneity has been checked and found equal to that of the vaporized rod. Fig. 3 shows the composition corresponding to the analysis of supported particles for Pd–Ptralumina catalyst. The chemical analysis was carried out by inductively coupled plasma ŽICP. spectrometry ŽSpectroflame, ICP, D model.. We found 0.04 and 0.08 wt.% of metal, respectively, for alumina and silica supports. This metal loading, which is somewhat lower than the usual metal content in supported catalysts, is more than enough to perform catalytic tests. Bimetallic catalysts, based on the Pd–Pt system, are industrially employed in the hydrogenation of aromatics, due to their higher resistance to sulphur poisoning compared to the two pure metals w6x. For
Fig. 2. Size distribution histograms of Pd–Pt clusters deposited on g-alumina Žblack line. and silica Žgrey line..
Fig. 3. Palladium concentration of a collection of alumina-supported Pd–Pt clusters. Triangles and circles correspond, respectively, to large area and individual particles analysis.
this reason, the catalytic activity of the Pd–Ptr alumina specimen was tested in the hydrogenation of tetralin in the presence of sulphur, which was introduced as H 2 S. The catalyst sample, used without any previous activation treatment, was heated up to 673 K in the reaction gas, containing 500 ppm of H 2 S, and reached its steady state activity immediately at the beginning of the catalytic test, which has been found equal to 0.5 = 10y8 mol sy1 gy1 . The sample did not show any considerable deactivation, even after more than 120 h of continuous use. It is interesting to note that, after prolonged use, TEM shows ŽFig. 4. that the majority of the Pd–Pt crystallites have a mean size of 5 nm, which is slightly larger than that of the fresh catalyst. Since the catalytic properties of the Pd–Ptralumina catalyst do not vary considerably along the catalytic test, it is reasonable to assume that the observed increase of particle size occurs during the initial period of the hydrogenation reaction, and that later on, the structure of the catalyst does not further change.
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
167
The thio-resistance of the Pd–Ptralumina catalyst can be evaluated by measuring the reaction rate at different concentrations of H 2 S ŽFig. 5.. The resulting pseudo-order of reaction with respect to H 2 S is y0.34. This result can be compared to that of a sample of Ptralumina prepared by impregnation of Al 2 O 3 , with an aqueous solution of H 2 PtCl 6 , having a Pt loading of 0.3% and a higher dispersion Žaverage particle size of 1.5 nm as made, 2.5 nm after catalytic test.. The reaction rate of the PtrAl 2 O 3 sample under the same experimental conditions used for the Pd–PtrAl 2 O 3 catalyst and with a H 2 S concentration of 500 ppm is of 3.0 = 10y8 mol sy1 gy1 , which, once corrected for the metal loading, is comparable to that of Pd–Ptralumina, in spite of the
Fig. 5. Reaction rate of the Pd–PtrAlumina catalyst in the presence of H 2 S. The solid line represents the best fit data curve. The equation of this fit is displayed in the inset.
higher dispersion of the PtrAl 2 O 3 sample. Moreover, the resulting pseudo-order of reaction of PtrAl 2 O 3 with respect to H 2 S is y1.0 w12x, showing that the activity of the bimetallic systems, on increasing the concentration of the H 2 S, decreases more slowly than the activity of supported pure platinum.
4. Conclusion
Fig. 4. TEM image of the Pd–Ptralumina catalyst after reaction. Scale bar is 50 nm.
The laser vaporization technique permits to synthesize bimetallic supported clusters with a rather narrow size distribution and remarkably well-defined composition. The slightly modified source enables us to obtain metal loadings on high-specific-area oxides, large enough to perform standard catalytic test. The possibility of obtaining well-tailored supported particles will be used in the near future to study in details the resistance to sulfur poisoning, as a function of the composition. In order to do this, we plan to synthesize, by the method described in this paper, supported catalysts of pure Pd and Pt, as well as other Pd–Pt alloys.
168
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
Acknowledgements The authors gratefully thank F. Bourgain for technical assistance in the realization of the powder mixing device.
References w1x J.H. Sinfelt, J. Catal. 29 Ž1973. 308. w2x J.H. Sinfelt, G.H. Via, F.W. Lytle, J. Chem. Phys. 72 Ž1980. 4832. w3x M.E. Ruiz-Vizcaya, O. Novaro, J.M. Ferreira, R. Gomez, J. Catal. 51 Ž1978. 108.
w4x E. Blomsma, J.A. Martens, P.A. Jacobs, J. Catal. 165 Ž1997. 241. w5x G. Carturan, G. Cocco, J. Mol. Catal. 26 Ž1984. 375. w6x T.B. Lin, C.A. Jan, J.R. Chang, Ind. Eng. Chem. Res. 34 Ž1995. 160. w7x J.L. Rousset, A.M. Cadrot, L. Lianos and A.J. Renouprez, Eur. Phys. J.D, Žin press.. w8x J.L. Rousset, A.J. Renouprez, A.M. Cadrot, Phys. Rev. B 58 Ž1998. 2150. w9x R.E. Smalley, Laser Chem. 2 Ž1983. 167–184. w10x J.L. Rousset, A.M. Cadrot, F.J. Cadete Santos Aires, A. Renouprez, P. Melinon, A. Perez, M. Pellarin, J.L. Vialle, M. ´ Broyer, J. Chem. Phys. 102 Ž1995. 8574–8575. w11x G. Fuchs, P. Melinon, F. Santos Aires, M. Treilleux, B. ´ Cabaud, A. Hoareau, Phys. Rev. B 44 Ž1991. 3926. w12x S. Pessayre, 1999. Private communication.
Characterization and reactivity of Pd–Pt bimetallic supported catalysts obtained by laser vaporization of bulk alloy J.L. Rousset a,) , F.J. Cadete Santos Aires a , F. Bornette a , M. Cattenot a , M. Pellarin b, L. Stievano a , A.J. Renouprez a a
b
Institut de Recherches sur la Catalyse, CNRS 2 aÕenue A. Einstein, F69626, Villeurbanne Cedex, France Laboratoire de spectrometrie uniÕersite´ Claude Bernard Lyon I, 43 Bd du 11 noÕ. 1918, 69622 Villeurbanne, ´ ionique et moleculaire, ´ Cedex, France
Abstract Bimetallic Pd–Pt clusters produced by laser vaporization of bulk alloy have been deposited on high surface alumina. Energy dispersive X-ray ŽEDX. analysis and transmission electron microscopy ŽTEM. show that they have a perfectly well-defined stoichiometry and a narrow range of size. Therefore, they constitute ideal systems to investigate alloying effects towards reactivity. Pd–Pt alloys are already known for their applications in the hydrogenation of unsaturated hydrocarbons, especially aromatics, because this system is highly resistant to sulfur and nitrogen poisoning. In this context, the catalytic properties of this system have been investigated in the hydrogenation of tetralin in the presence of hydrogen sulfide. Preliminary results show that this model catalyst is more sulfur-resistant than each of the pure supported metals prepared by chemical methods. q 2000 Elsevier Science B.V. All rights reserved. PACS: 82.65.J; 61.16.B Keywords: Laser vaporisation; Well-defined bimetallic catalysts; TEM; Reactivity
1. Introduction Metallic clusters constitute an exciting field of research, and over at least the past 20 years, a great deal of effort has been devoted to them. A fact, which is very attractive in this field, is certainly the transition of their properties when going from the bulk to small clusters. Since these differences are expected to affect the catalytic behaviour, and considering that the supported-metal catalysts used in practice always consist of small particles, these parti) Corresponding author. Tel.: q33-4-72-44-54-34; fax: q33-472-44-53-99. E-mail address: [email protected] ŽJ.L. Rousset..
cle size effects are of great interest to those who investigate chemisorptive or catalytic properties. Moreover, in the field of heterogeneous catalysis, a promising class of catalysts is constituted by bimetallic particles, which are found to exhibit superior properties, compared to pure metals, in terms of activity, selectivity, stability and resistance to poisoning w1,2x. The effects of alloying on the improvement of the catalytic behaviour may be explained by an ensemble andror a ligand effect w1,2x. It is somewhat unusual to associate two noble metals like Pd and Pt. Actually, bimetallic catalysts based on the association of these two elements can show either a simple additivity of the reactivity of each component, as observed in the dehydrogenation
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 3 3 6 - 6
164
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
of cyclohexane w3x or an enhancement of activity, compared to that of the pure metals, as for example, in the isomerization and hydrocracking of alkanes w4x and in the hydrogenation of aromatics w5x. Pd and Pt are generally both active in hydrogenation reactions, but the former is generally preferred because of its lower cost and its larger selectivity for partial hydrogenation. On the other hand, Pd–Pt bimetallic catalysts are known to be more resistant than monometallic ones to sulfur poisoning, an element present in fuels w6x. Most of the bimetallic catalysts prepared by chemical methods suffer from a lack of compositional homogeneity and, in some cases, one cannot even be sure that alloys are really formed. To overcome this difficulty, the laser vaporization of bulk alloys, which ensures a perfect homogeneity of composition of the particles, was proposed w7,8x. Up to now, we have used a pulsed vaporization source, which permitted us to deposit only low quantities of clusters on flat supports. By the use of a modified source, we have been able to deposit bimetallic Pd–Pt clusters on high-area-surface oxides. In this paper, we report on the production and characterization of this catalysts and on their reactivity towards hydrogenation of tetralin.
Žpositively and negatively. ionized clusters. After removal by electrostatic deflection of the ionized clusters, the neutral low-energy clusters are deposited onto the substrates. The incoming flux of clusters is monitored by a quartz microbalance. In these conditions, typical deposition rates are 0.5 nm cmy2 miny1 w10x. These rates are quite acceptable to deposit thin films on low specific area samples Žflat surfaces.; however, they are very low when deposits are performed on high specific area samples Žpowders.. Since our goal is to obtain bimetallic particles supported on high specific surfaces Žtypically 300 m2rg., the cluster source has been modified. In the ‘‘new’’ source, the Nd:YAG laser is still pulsed, while there is a continuous flux of inert gas Žnow a mixture of He and Ar.. This yields higher deposition rates of about 5 nm cmy2 miny1 . The catalytic supports used in this study are powders of g-alumina ŽCondea, Puralox SCFa-240. and non-porous silica ŽDegussa, Aerosil 380. with a specific surface of 240 and 380 m2rg, respectively. In order to deposit the clusters on these samples, a device has been made which stirs the powder in front of the cluster beam. We expect this device to favor homogeneous cluster deposition on most powder grains. After deposition, the samples are air transferred and characterized andror used in catalytic reactions.
2. Experimental set-up
2.2. Transmission electron microscopy (TEM) and energy dispersiÕe X-ray spectroscopy (EDX-S)
2.1. Low-energy cluster beam deposition The laser vaporization technique w9x has been used to obtain a low-energy cluster beam of bimetallic particles from a rod alloy obtained by melting of palladium Ž99.99% purity. and platinum Ž99.95% purity.. In the classical set-up w10x, a pulsed Nd:YAG laser is focussed on a rod of alloy and a plasma is formed Žthe rod is animated by a slow helical movement in order to allow the laser beam to be focussed on different fresh regions.. The introduction of a synchronized pulse of an inert gas ŽHe. leads to nucleation and growth of metallic clusters. The mean size of the free clusters may be qualitatively monitored by adjusting the inert gas pressure and the time of residence in the nucleation cell. The cluster beam is obtained by differential pumping extraction through a skimmer. This beam is formed by neutral and
Morphologic characterization and compositional analysis of the supported clusters has been performed, respectively, by TEM and high spatial resolution EDX-S in the TEM. In order to obtain suitable samples for TEM characterization, the as-obtained powders are dispersed in ethanol by ultrasonication. A drop of the solution is then deposited onto a thin holey-carbon film supported on a copper microscopy grid Ž200 mesh, 3.05 mm. and left to dry. The powder grains, containing the alloy particles, are then well separated and supported on the thin holey-carbon film and may thus be characterized by TEM. The experiments were performed in a JEOL JEM 2010-F transmission electron microscope operating at 200 kV and equipped with a field emission gun ŽFEG., a high resolution UHR pole piece Žpoint
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
resolution: 0.196 nm. and a Pentafet-Link ISIS EDX spectrometer from Oxford Instruments. The size of the clusters and verification of homogeneous distribution over the supports have been obtained by TEM imaging. Intense enough condensed probes Ž0.5–2.4 nm. can be obtained with the FEG, which allow EDX-S analysis of very reduced volumes and, thus, of individual nanometer-size particles. This was performed to verify the chemical homogeneity of the deposited clusters. Large scale Ž) 20 nm. EDX-S analysis of more or less large collections of particles was also performed by decondensing the electron probe to the required size.
165
the catalyst, the H 2 S concentration was varied between 0 and 500 ppm. These experimental conditions were chosen in order to minimise the dehydrogenation of decalin to naphthalene, and to obtain a conversion lower than 15%, at which a differential model for the determination of the specific rates of reaction can be applied. The products of the hydrogenation were analysed by an in-line gas chromatograph with a flame-ionisation detector. Cis and trans decalins were the only products obtained, together with small amounts of naphthalene. No detectable amounts of isomerisation or cracking products were formed.
2.3. Catalytic actiÕity measurements 3. Results and discussion The catalytic properties of the Pd–Ptralumina catalyst specimen have been tested in the hydrogenation of 1,2,3,4-tetrahydronaphtalene Žtetralin.. The experiments were performed in the gas phase at 573 K in a fixed-bed catalytic microreactor, with a pressure of H 2 of 4.50 MPa and a partial pressure of tetralin of 6.0 kPa, which was kept constant by using a gas phase saturator system. In order to investigate the influence of H 2 S on the catalytic properties of
There are only few results on the nucleation of deposited clusters w11x. In our case, deposits have been made on high-surface-area silica or alumina powders. The mean size and standard deviation have been found nearly identical, whatever the support, indicating that clusters diffusion and, hence, coalescence, must be very weak. Consequently, for suitable deposits, the supported particles are well separated
Fig. 1. TEM images of the Pd–Pt clusters deposited on g-alumina Ža. and silica Žb.. Scale bars are 20 nm.
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J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
and homogeneously distributed on the substrate. This is shown in Fig. 1, which represents TEM images of silica and alumina supported catalysts. The size histograms, corresponding to 460 and 439 analyzed particles, respectively, for silica and alumina systems are shown in Fig. 2. The mean cluster size Žstandard deviation. are about 3.5 nm Ž1 nm. for Pd–Ptrsilica and 3.7 nm Ž1.2 nm. for Pd–Ptralumina. EDX analysis has been used to study the composition of individual particles by reducing the probe area down to 1 nm2 . Consequently, for both systems, the composition homogeneity has been checked and found equal to that of the vaporized rod. Fig. 3 shows the composition corresponding to the analysis of supported particles for Pd–Ptralumina catalyst. The chemical analysis was carried out by inductively coupled plasma ŽICP. spectrometry ŽSpectroflame, ICP, D model.. We found 0.04 and 0.08 wt.% of metal, respectively, for alumina and silica supports. This metal loading, which is somewhat lower than the usual metal content in supported catalysts, is more than enough to perform catalytic tests. Bimetallic catalysts, based on the Pd–Pt system, are industrially employed in the hydrogenation of aromatics, due to their higher resistance to sulphur poisoning compared to the two pure metals w6x. For
Fig. 2. Size distribution histograms of Pd–Pt clusters deposited on g-alumina Žblack line. and silica Žgrey line..
Fig. 3. Palladium concentration of a collection of alumina-supported Pd–Pt clusters. Triangles and circles correspond, respectively, to large area and individual particles analysis.
this reason, the catalytic activity of the Pd–Ptr alumina specimen was tested in the hydrogenation of tetralin in the presence of sulphur, which was introduced as H 2 S. The catalyst sample, used without any previous activation treatment, was heated up to 673 K in the reaction gas, containing 500 ppm of H 2 S, and reached its steady state activity immediately at the beginning of the catalytic test, which has been found equal to 0.5 = 10y8 mol sy1 gy1 . The sample did not show any considerable deactivation, even after more than 120 h of continuous use. It is interesting to note that, after prolonged use, TEM shows ŽFig. 4. that the majority of the Pd–Pt crystallites have a mean size of 5 nm, which is slightly larger than that of the fresh catalyst. Since the catalytic properties of the Pd–Ptralumina catalyst do not vary considerably along the catalytic test, it is reasonable to assume that the observed increase of particle size occurs during the initial period of the hydrogenation reaction, and that later on, the structure of the catalyst does not further change.
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
167
The thio-resistance of the Pd–Ptralumina catalyst can be evaluated by measuring the reaction rate at different concentrations of H 2 S ŽFig. 5.. The resulting pseudo-order of reaction with respect to H 2 S is y0.34. This result can be compared to that of a sample of Ptralumina prepared by impregnation of Al 2 O 3 , with an aqueous solution of H 2 PtCl 6 , having a Pt loading of 0.3% and a higher dispersion Žaverage particle size of 1.5 nm as made, 2.5 nm after catalytic test.. The reaction rate of the PtrAl 2 O 3 sample under the same experimental conditions used for the Pd–PtrAl 2 O 3 catalyst and with a H 2 S concentration of 500 ppm is of 3.0 = 10y8 mol sy1 gy1 , which, once corrected for the metal loading, is comparable to that of Pd–Ptralumina, in spite of the
Fig. 5. Reaction rate of the Pd–PtrAlumina catalyst in the presence of H 2 S. The solid line represents the best fit data curve. The equation of this fit is displayed in the inset.
higher dispersion of the PtrAl 2 O 3 sample. Moreover, the resulting pseudo-order of reaction of PtrAl 2 O 3 with respect to H 2 S is y1.0 w12x, showing that the activity of the bimetallic systems, on increasing the concentration of the H 2 S, decreases more slowly than the activity of supported pure platinum.
4. Conclusion
Fig. 4. TEM image of the Pd–Ptralumina catalyst after reaction. Scale bar is 50 nm.
The laser vaporization technique permits to synthesize bimetallic supported clusters with a rather narrow size distribution and remarkably well-defined composition. The slightly modified source enables us to obtain metal loadings on high-specific-area oxides, large enough to perform standard catalytic test. The possibility of obtaining well-tailored supported particles will be used in the near future to study in details the resistance to sulfur poisoning, as a function of the composition. In order to do this, we plan to synthesize, by the method described in this paper, supported catalysts of pure Pd and Pt, as well as other Pd–Pt alloys.
168
J.L. Rousset et al.r Applied Surface Science 164 (2000) 163–168
Acknowledgements The authors gratefully thank F. Bourgain for technical assistance in the realization of the powder mixing device.
References w1x J.H. Sinfelt, J. Catal. 29 Ž1973. 308. w2x J.H. Sinfelt, G.H. Via, F.W. Lytle, J. Chem. Phys. 72 Ž1980. 4832. w3x M.E. Ruiz-Vizcaya, O. Novaro, J.M. Ferreira, R. Gomez, J. Catal. 51 Ž1978. 108.
w4x E. Blomsma, J.A. Martens, P.A. Jacobs, J. Catal. 165 Ž1997. 241. w5x G. Carturan, G. Cocco, J. Mol. Catal. 26 Ž1984. 375. w6x T.B. Lin, C.A. Jan, J.R. Chang, Ind. Eng. Chem. Res. 34 Ž1995. 160. w7x J.L. Rousset, A.M. Cadrot, L. Lianos and A.J. Renouprez, Eur. Phys. J.D, Žin press.. w8x J.L. Rousset, A.J. Renouprez, A.M. Cadrot, Phys. Rev. B 58 Ž1998. 2150. w9x R.E. Smalley, Laser Chem. 2 Ž1983. 167–184. w10x J.L. Rousset, A.M. Cadrot, F.J. Cadete Santos Aires, A. Renouprez, P. Melinon, A. Perez, M. Pellarin, J.L. Vialle, M. ´ Broyer, J. Chem. Phys. 102 Ž1995. 8574–8575. w11x G. Fuchs, P. Melinon, F. Santos Aires, M. Treilleux, B. ´ Cabaud, A. Hoareau, Phys. Rev. B 44 Ž1991. 3926. w12x S. Pessayre, 1999. Private communication.