Structure and catalytic activity of palladium–platinum aggregates obtained by laser vaporisation of bulk alloys

Journal of Alloys and Compounds 328 (2001) 50–56

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Invited lecture

Structure and catalytic activity of palladium–platinum aggregates obtained by laser vaporisation of bulk alloys Albert Renouprez*, Jean Luc Rousset, Anne Marie Cadrot, Yvonne Soldo, Lorenzo Stievano Institut de Recherches sur la Catalyse, CNRS, 2 Avenue A. Einstein, Villeurbanne F69626 Cedex, France Received 12 June 2000; received in revised form 17 July 2000; accepted 22 December 2000

Abstract The European emission legislation concerning the pollution abatement of the exhaust gas from light cars imposes a reduction of the concentration of aromatics in gasoline, down to 2%. The hydrogenation of these hydrocarbons can be performed on Pd or Pt catalysts. However, these catalysts are poisoned by traces of sulphur — 100–300 ppm — remaining in the fuels. It is claimed in the literature that bimetallic Pd–Pt catalysts are less affected by this poisoning than the pure metals. To verify this point, both Pd–Pt and pure metal aggregates were synthesised by laser vaporisation of bulk alloys and deposited on alumina. Analytical microscopy and EXAFS have shown that these clusters have a very narrow composition distribution and form alloys. The study of their activity in the hydrogenation of Tetralin, a model molecule, in the presence of variable amounts of H 2 S, has shown that Pt is more active than Pd at low sulphur concentration, whereas Pd becomes more active for the highest H 2 S content. Contrary to what is claimed in the literature, no synergetic effect has been found found by alloying these two metals; actually their activity is the simple additivity of that of the two metals.  2001 Elsevier Science B.V. All rights reserved. Keywords: Production of aggregates by laser vaporisation; Hydrogenation of aromatics; Thioresistance of Pd–Pt catalysts

1. Introduction In both Europe and the US the legislation now imposes a strong reduction of the aromatic hydrocarbons contained in car gasoline and diesel fuels, because they are carcinogenic [1]. Also, their presence lowers the fuel quality, specially the cetane index. To eliminate these mono- or poly-cyclic aromatics, a catalytic conversion under 4–10 MPa of hydrogen, on transition metals, is generally performed. The main difficulty is the presence of sulphur in the feedstocks so that, even if hydro-desulphurization treatments have achieved great progress, its concentration is still of the order of 200–500 ppm. Thus a thiotolerance of the catalyst is requested, because even at this low concentration, sulphur rapidly deactivates the catalysts. Many patents and articles claim that palladium–platinumsupported bimetallic catalysts are more active in the presence of sulphur than the pure metals, in the hydrogenation of aromatic hydrocarbons [2], especially for a

Pd / Pt ratio of the order of 4 / 1 [3]. Many reasons are invoked to explain this effect; for some authors, Pd has an electronic effect on Pt inducing a charge transfer from this metal, inhibiting H 2 S adsorption. For others, this synergy is induced by the supports. Actually, most of these catalysts are prepared on acidic supports such as dealuminated Y–zeolites [3] or silica–alumina and the presence of electrodeficient Pt atoms would be the consequence of a support effect. However, even catalysts prepared for academic studies, are complex systems with generally not well defined composition and structure. With the goal of studying in detail this so-called synergetic effect between the two elements, well defined model catalysts were prepared by using soft landing deposition on a low acidity alumina support, of platinum–palladium aggregates, obtained by laser vaporisation of bulk alloys [4].

2. Production of the bimetallic clusters *Corresponding author. Tel.: 133-4-7244-5300; fax: 133-4-72445399. E-mail address: [email protected] (A. Renouprez).

A Nd-YAG laser is focused onto a metallic rod, driven in a slow screw motion. Short helium bursts, synchronised with the laser pulses, cool the plasma and a supersonic

0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01346-9

A. Renouprez et al. / Journal of Alloys and Compounds 328 (2001) 50 – 56

expansion occurs at the exit of the source, inducing the formation of a beam of clusters, containing neutral and ionized species. The masses of the charged clusters are analysed by a time of flight spectrometer. These clusters are sent in a vacuum chamber and deposited either on amorphous carbon to perform an easy characterisation by electron microscopy or on a high surface alumina for catalytic studies. For catalytic activity studies, the clusters, consisting of neutral low-energy aggregates, were deposited onto galumina powder (Condea, Puralox SCFa-240, specific surface area5240 m 2 / g). The typical deposition rates, monitored by a quartz microbalance, were of 5 nm / cm 2 per min. In order to homogeneously deposit the clusters on the powder surface, a device has been designed which stirs the powder in front of the cluster beam. Four rods were used with the Pd 100 , Pd 65 Pt 35 , Pd 17 Pt 83 and Pt 100 compositions.

3. Structure and composition of the clusters

3.1. Electron microscopy studies Electron microscopy experiments were performed with a Jeol 2010 instrument equipped with a microanalysis device. The histogram of the diameters of these particles deposited on an amorphous carbon corresponding to a 0.2 nm equivalent thickness, is represented in Fig. 1. The mean diameters are very close for the two compositions, 2.4 and 2.5 nm. It was also verified that the nature of the support and the thickness of the deposits (up to 1 nm), have a negligible influence on the size of these clusters. The main point, which justifies the choice of this method of production, is the expected uniformity of the cluster compositions. To determine to what extent this goal has been achieved, EDX studies were undertaken by

Fig. 1. Diameter distribution histograms corresponding to the Pd 65 Pt 35 (hatched symbols) and Pd 17 Pt 83 (clear symbols) clusters.

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measuring the intensity of the Pd and Pt La emissions of individual particles. Thanks to the field emission gun of this microscope, the investigated area can be as small as 1 nm 2 . In this way, individual particles can be analysed. As shown in Fig. 2, the distributions of composition of the bimetallic clusters measured by EDX are narrow, close to the composition of the initial alloy rod.

3.2. EXAFS experiments Thus, EDX has demonstrated that bimetallic particles with a uniform composition have been obtained; but one does not know if alloying of these two elements does occur. To verify this point, EXAFS experiments have been performed at the ESRF on the ‘CRG Interface’ beamline. Because only the Si(111) monochromator reflection was available, the maximum energy limit was 20 keV and the experiments at he Pd K edge impossible. The spectra were thus only recorded at the Pt L III edge, above 11 563 eV. For this purpose, 0.4-nm and 2.4 equivalent thickness layers of clusters were deposited on a light material, 2-mm thick Suprasil disks. With this type of discontinuous film, the only possible detection is the fluorescence mode. It was achieved by a 30 elements Ge device, from Camberra. Pt 4-mm thick and Pd 95 Pt 5 40-mm foils were used as references, to extract back-scattering amplitudes and phase shifts corresponding to the Pt–Pt and Pt–Pd pairs. A modelling of the first co-ordination shell was thus performed for the two compositions. As shown in Fig. 3, because of the low metal amount in ˚ 21 . The the beam, the spectra are noisy above 12 A modelling shown in Fig. 4 was thus performed only ˚ 21 . It can be seen in Table 1 that for between 3 and 11.5 A the Pd 65 Pt 35 aggregates, the total co-ordination number, 10.5, does correspond to 4–5-nm particles. It is observed that for the 2.4-nm thick deposit, the Pt–Pd and Pt–Pt ˚ distances are identical, 2.74 A. Conversely, in the case of the 0.4-nm thick layer, the

Fig. 2. Distribution of the Pd concentration in the aggregates measured by analytical microscopy for the clusters with (a) the Pd 17 Pt 83 and (b) the Pd 65 Pt 35 nominal compositions.

A. Renouprez et al. / Journal of Alloys and Compounds 328 (2001) 50 – 56

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Fig. 3. EXAFS spectrum corresponding to a 0.4-nm thick layer of Pd 65 Pt 35 aggregates.

Fig. 4. Modelling of the EXAFS spectrum corresponding to a 2.4-nm thick layer of Pt 65 Pd 35 clusters.

˚ This phenomPt–Pd distances are shortened by 0.04 A. enon of radial contraction has indeed already been observed by LEED on (100) surfaces of Pd–Ni alloys [5]. In that case, the distance between the first and second planes is shortened by 2.7%. Considering now the NPt – Pd /Ntotal values reported in Table 1, it appears that they are in all cases lower than expected from the stoichiometry. This too large number of Pt atoms in the first coordination shell of platinum is the

indication of a partial segregation of the two elements inside the particles.

3.3. Surface composition of the clusters EXAFS has shown that alloying between the two elements has occurred but also that some heterogeneity in the distribution of the two types of atoms inside the particles is present. Actually, this last point has been

Table 1 Results of the fits of the first coordination shells Sample

NPt – Pt

R Pt – Pt

Pd 65 Pt 35 , 2.4 nm Pd 65 Pt 35 , 0.4 nm Pd 17 Pt 83 , 2.4 nm

5.23 6.32 9.08

2.74 2.75 2.75

˚ ( A)

NPt – Pd

R Pt – Pd

5.2 4.42 0.58

2.74 2.70 2.73

˚ ( A)

2

NPt – Pd /Ntotal

˚ ) s 2 (A 23 310

0.498 0.41 0.06

8.2 9 7.34

A. Renouprez et al. / Journal of Alloys and Compounds 328 (2001) 50 – 56

clearly demonstrated in a previous paper [4], using low energy ion scattering. Indeed, this method is a probe of the composition of the outmost layer and provided suitable calibration samples are available, it leads to the composition of the ultimate surface. A typical LEIS spectrum obtained with the Pd 17 Pt 83 sample is shown in Fig. 5, together with its successive modifications in the course of the analysis. The Pd and Pt signals are observed at 865 and 920 eV, respectively. In the first spectrum the ratio of the magnitude of the two peaks leads to the actual composition of the surface plane. This quantitative analysis is based on a calibration using (111) Pd and Pt single-crystal faces. For longer periods of analysis, ion etching occurs and the Pd signal is progressively decreased, indicating that this element is preferentially located at the surface of the particles. The variations of Pd surface concentration with the time of analysis are shown in Fig. 6. The first measurements, performed during 1 min, lead, respectively, to surface atomic concentrations of 38 and 86%, whereas the mean values are 17 and 65%. After 30 min of ion etching, Pd has been removed and the probed layer is Pt enriched. The surface composition of bimetallic particles can be predicted using a tight-binding approach with a model based on a concept called ‘equivalent medium approximation’. As described in detail in Ref. [6], it is necessary to use a Monte Carlo procedure to obtain the optimised configuration of particles of different size, assuming that all of them have a cubo-octahedral shape, a reasonable assumption considering the electron microscopy observations. The results of these calculations can be compared in Table 2 with the LEIS measurements. One can note the marked influence of the cluster size on the surface composition, at low Pd nominal concentration. Indeed the surface enrichment is more pronounced for the large particles. This can indeed be ascribed to the limited supply of segregating element inside the small particles.

Fig. 5. Modification of the LEIS spectra of the Pd 17 Pt 83 sample after (a) 75, (b) 165, (c) 210, (d) 660 and (e) 1785 s sputtering time. Note the presence at 820 eV, after 600 s of sputtering, of the peak belonging to the Cu support grid.

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We have therefore shown that very well-defined bimetallic clusters have been obtained by this method, with two completely different situations with respect to the surface composition. In one case, the surface is fully covered with Pd atoms, with Pt atoms first neighbours, in the underlying layer. In the second case, the surface is composed of an equal amount of the two elements. The four catalysts can thus be tested in the conversion of aromatics to determine if any synergy can be observed in their thioresistance.

4. Catalytic activity As mentioned in Section 1, these bimetallic catalysts are industrially employed in the hydrogenation of aromatics, due to their higher resistance to sulphur poisoning compared to the two pure metals. For this reason, the catalytic activity of the bimetallic supported clusters was compared to that of the pure metals in the hydrogenation of tetralin (1,2,3,4-tetrahydronaphthalene), in the presence of sulphur, introduced as H 2 S. The experiments were performed in the gas phase at 573 K, in a fixed-bed catalytic microreactor, with a total pressure of H 2 of 4.50 MPa and a partial pressure of tetralin of 6.0 kPa. In order to investigate the influence of H 2 S on the catalytic properties, the H 2 S concentration was varied between 0 and 500 ppm. The products of hydrogenation were analysed by an in-line gas chromatograph equipped with a flame-ionisation detector. Cis and trans decaline were always obtained together with small amounts of naphthalene, whereas no detectable amounts of isomerisation or cracking products are formed. Samples of Pd /Al 2 O 3 , Pd 17 Pt 83 /Al 2 O 3 , Pd 65 Pt 35 /Al 2 O 3 and Pt /Al 2 O 3 were heated up to 573 K in H 2 , and only once the reaction temperature had been reached, the reaction gas containing up to 500 ppm of H 2 S was introduced. At each different concentration of H 2 S, all samples reach their steady-state activity almost immediately, without showing any considerable deactivation even after more than 120 h of continuous use. The reaction rate is generally expressed as a ‘turnover number’, i.e. the number of reactant molecules transformed per unit of time per surface metal atom. In the present case, electron microscopy has shown that all the catalysts have a very similar dispersion; the rate can thus be just expressed as a function of the metal concentration. One should however mention that these clusters are less homogeneously distributed at the surface of the alumina than on amorphous carbon: a sintering of the particles occurs in the course of the activation treatment performed at 3008C. It was also verified by measuring the cluster sizes after the reaction that no further sintering occurs during it. The catalytic activity results are reported in Table 3, whereas the catalytic behaviour of the different samples

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Fig. 6. Variation of the palladium surface concentration measured by LEIS as a function of the etching time.

with the variation of the sulphur concentration is shown in Fig. 7. The activation energy obtained from the Arrhenius plot was found to be approximately 15 kcal / mol for all samples, i.e. high enough to exclude the presence of a diffusion regime. The first important result of this catalytic test shown in Fig. 7, is that platinum is more active than palladium, at low sulphur concentration. Inversely, for the highest H 2 S partial pressure, Pd becomes slightly more active. The second point is that no synergy effect can be evidenced on Table 2 Comparison between the measured and predicted surface compositions Sample

Pd 65 Pt 35 Pd 17 Pt 83

Pd at.% LEIS

87 38

Predicted Pd surface concentration as a function of the number of atoms in the clusters 201

586

1289

2406

93 27

95.5 36

97.2 44

98.6 52

the thioresistance of the Pd–Pt alloys, whose activities always lie between those of the pure metals. This is a consequence of the observed pseudo-order of reaction with respect to H 2 S, which is comparable on the pure supported metals and on the alloys. The Pd /Al 2 O 3 sample, which is supposed to be the least active in absence of sulphur, is also the sample that shows the best thioresistance properties, since its deactivation slope is the lowest (Table 3). On the contrary, the Pt / Al 2 O 3 sample is the most active at low sulphur concentrations, whereas its activity decades faster on increasing the sulphur concentration. Of the two alloy samples, Pd 65 Pt 35 /Al 2 O 3 has a reactivity and thioresistance very similar to that of pure supported palladium. This result agrees well with the strong segregation of palladium to the surface. In fact, as shown above, with a nominal palladium concentration of 65 at.%, the palladium surface concentration is nearly 100%. In this case, therefore, the bimetallic catalyst behaves as a monometallic supported palladium catalyst,

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Table 3 Comparison of the catalytic properties of the Pd–Pt /Al 2 O 3 catalysts Sample Metal loading (%) Activation energy (kcal / mol) Reaction rate with 50 ppm of H 2 S (mol s 21 mol 21 (metal ) ) Reaction rate with 500 ppm of H 2 S (mol s 21 mol 21 (metal ) ) Deactivation slope

Pd /Al 2 O 3 0.038 16

Pd 65 Pt 35 /Al 2 O 3 0.032

Pd 17 Pt 83 /Al 2 O 3 0.080

17

17

Pt /Al 2 O 3 0.048 16

3.00310 23

2.40310 23

3.22310 23

6.71310 23

1.29310 23

1.07310 23

7.48310 24

1.03310 23

20.37

20.34

with negligible influence of the underlying Pt-rich layer. For the Pd 17 Pt 83 /Al 2 O 3 sample, instead, the reactivity and thioresistance are the average of those of the two pure metals, taking into account a measured surface palladium concentration of about 40%.

5. Discussion and conclusions Since the thioresistance of this system is practically the average between those of the two pure metals, weighed for the surface concentration of the two metals, our catalytic results indicate that no synergy effect is observed. These results are in contradiction with those of Yasuda and co-workers [7,8], who found, for Pd–Pt metal particles supported on zeolites, a maximum in the thioresistance at a Pd concentration of about 80 at.%. A similar effect was found more recently by Fujikawa et al. [9], who compared the hydrogenation of light cycle oils on Pd–Pt supported on Al 2 O 3 –SiO 2 and pure Al 2 O 3. They did observe that for the same Pd / Pt ratio, the Al 2 O 3 –SiO 2 -supported catalyst is the most sulphur resistant, with a maximum for a 0.7Pd /(Pd1Pt) weight ratio. This effect of a synergy for a well-defined stoichiometry

Fig. 7. Hydrogenation rate of Tetralin as a function of the pressure of H 2 S on the pure metals and on the bimetallic catalysts.

20.63

20.80

of the noble metals is difficult to understand, since, as shown above, for particles of diameters of 2–3 nm, the composition of the surface is almost only composed of pure Pd in the 60–100% Pd nominal concentration range. The situation can be different when the support is for example a Y–zeolite. In that case, the metal particles encaged in the large cavities are smaller than 1 nm (60– 65% dispersion). A mass balance then occurs, which limits the concentration of the segregating element at the surface. The poor behaviour of pure Pt catalysts in the presence of sulphur is attributed to the agglomeration of the metal particles under the effect of H 2 S, as shown by Chang and co-workers [10,11], using EXAFS. But the beneficial role played by Pd, claimed by many authors, is not easy to explain. It has to be considered, however, that Yasuda and co-workers studied the reaction in the liquid phase with sulphur introduced as dibenzothiophene. The results reported in the present paper, on the contrary, are obtained in the gas phase, using H 2 S as the sulphur carrier. This can be a first possibility to explain the differences in the observed thioresistance. Another important effect in the reactivity of these systems, in the presence of sulphur, can be the influence of the support. In particular, its acidity can play a major role in the catalytic activity. In fact, the use of supports such as zeolites seems to have a considerable effect on the electronic structure of the metal particles; which in turn can change the strength of the M–S bond and therefore influence the thioresistance (M5Pd, Pt) [7]. No clear proof of a modification of the electronic structure of Pt under the influence of Pd has been given up to now. Our attempts to detect it, by studying the Pd 4d and Pt 5d levels by XPS, were unsuccessful. One can thus present an alternative hypothesis, based on the adsorption– desorption equilibrium between H 2 S and H 2 at 573 K. At this temperature, in the presence of strong acidic supports, the surface concentration of sulphur would be lowered, so that Pt would not undergo any structural modifications. As a conclusion, we have shown that alloying Pd to Pt cannot explain the thioresistance of the Pd–Pt bimetallic catalysts. None of the synergetic effects between the two

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metals claimed in the literature was observed with these well-defined catalysts in the hydrogenation of Tetralin. An effect of the support is therefore undoubtedly at the origin of the thioresistance observed by many authors.

[5] [6] [7]

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