SiO2 cogelled xerogel catalysts

Applied Catalysis A: General 270 (2004) 201–208

Determination of surface composition of alloy nanoparticles and relationships with catalytic activity in Pd–Cu/SiO2 cogelled xerogel catalysts Stéphanie Lambert a , Benoˆıt Heinrichs a,∗ , Alain Brasseur a , André Rulmont b , Jean-Paul Pirard a b

a Laboratoire de Génie Chimique, B6a, Université de Liège, B-4000 Liège, Belgium Laboratoire de Chimie Inorganique Structurale, B6b, Université de Liège, B-4000 Liège, Belgium

Received in revised form 4 May 2004; accepted 5 May 2004

Abstract The combination of results from carbon monoxide chemisorption, X-ray diffraction, and transmission electron microscopy allowed calculating the surface composition of the palladium–copper nanoparticles in Pd–Cu/SiO2 cogelled xerogel catalysts. Values obtained indicate a very pronounced surface enrichment with copper. Surface compositions obtained with this method, which combines three different experimental techniques, are in agreement with the literature data previously obtained for surface segregation in Pd–Cu/SiO2 catalysts by other techniques as low energy ion scattering and X-ray photoelectron spectroscopy. While 1,2-dichloroethane hydrodechlorination over pure palladium mainly produces ethane, increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity, to reach 100% in ethylene selectivity for the sample containing 1.4 wt.% of palladium and 3.0 wt.% of copper. © 2004 Elsevier B.V. All rights reserved. Keywords: Sol–gel process; Pd–Cu/SiO2 catalysts; Pd–Cu alloy nanoparticles; Surface composition; CO chemisorption; TEM; XRD; Hydrodechlorination

1. Introduction Noble metals catalysts (Group VIII), and particularly palladium, are very active for the hydrodechlorination reaction [1–3]. In the case of 1,2-dichloroethane hydrodechlorination, the noble metal participates in a catalytic cycle, in which the reactant is dechlorinated by chlorination of the metal surface, which is then itself dechlorinated by reduction with hydrogen. Because of the high reactivity of hydrogen on noble metals, the dechlorinated organics, C2 H4 in the present case, is immediately converted into the fully hydrogenated product, C2 H6 [2–5], which is much less useful from an industrial point of view. However, several authors demonstrated the ability of bimetallic catalysts, composed of alloys such as Pd–Ag [6], Pt–Cu [7,8], Pd–Cu [9,10], to convert chlorinated alkanes selectively into less or not chlorinated alkenes. That selectivity change in the par∗ Corresponding author. Tel.: +32-4-366-35-05; fax: +32-4-366-35-45. E-mail address: [email protected] (B. Heinrichs).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.05.005

ticular case of hydrodechlorination shows once again that the addition of a second metal in a monometallic catalyst significantly alters its activity and selectivity [1]. To understand the mechanism of selective hydrodechlorination of 1,2-dichloroethane into ethylene on a supported alloy and to calculate a turnover frequency (TOF), that is, the number of 1,2-dichloroethane molecules consumed per active surface metal and per second, it is very important to know its actual surface composition. Indeed, the latter can strongly deviate from the bulk composition [11,12]. In the case of the palladium–silver alloy, this composition difference between surface and bulk has been shown by Kuijers and Ponec from results calculated from Auger electron and infrared spectroscopies [13] and by Heinrichs et al. from CO chemisorption, X-ray diffraction, and transmission electron microscopy [14]. The surface enrichment with silver for Pd–Ag alloy particles is in agreement with the theoretical prediction according to which, at thermodynamic equilibrium, alloys forming a solid solution (completely miscible metals) exhibit under vacuum a surface enriched with the metal having the lowest surface energy [11,15,16]. Indeed, a

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surface enrichment with silver, whose energy is much lower (1.26 J m−2 at 0 K) than that of palladium (2.09 J m−2 at 0 K), is then expected. For Pd–Cu alloy, although the difference between the surface energies at 0 K of palladium and copper (1.84 J m−2 ) is lower [11], a theoretical study of the surface segregation in Pd–Cu single crystals preferentially locates Cu atoms on the edges of the small crystals and on the (1 0 0) open faces, rather than on the (1 1 1) faces [17–19]. This result is also confirmed by studies of O2 and C2 H4 adsorption on Cu modified Pd/SiO2 catalysts, for which a very marked enrichment in copper on the surface of the Pd–Cu alloy particles has been observed when copper loading increases [20]. A high activity of a supported catalyst often calls for a large active surface area and, thus, for small particles, i.e. a high dispersion of the active phase. Because small metal particles tend to already sinter at relatively low temperatures, these generally are applied into a support material which itself is thermally stable and maintains a high specific surface area up to high temperatures [21]. In this way, Schubert and co-workers have developed an interesting method to disperse metal particles in a silica matrix [22,23]. Heinrichs et al. [6] and Lambert et al. [4,5,24] used this cogelation method for the preparation of Pd/SiO2 , Ag/SiO2 , Cu/SiO2 and Pd–Ag/SiO2 catalysts. All these authors used alkoxides of the type (RO)3 Si–X–A in which a functional organic group A, able to form a chelate with a cation of a metal such as palladium, silver, copper, etc., is linked to the hydrolysable silyl group (RO)3 Si– via an inert and hydrolytically stable spacer X. The co-condensation of such molecules with a network-forming reagent such as TEOS, Si(OC2 H5 )4 , results in materials in which the metal is anchored to the SiO2 matrix. So this method can allow obtaining a mean diameter of metal particles of about 2 nm [4,5] whereas metal catalysts prepared by a classical method as impregnation present a mean diameter of metal particles of about 5–30 nm [25]. The best metal dispersion values obtained in the case of Pd/SiO2 , Ag/SiO2 , Cu/SiO2 and Pd–Ag/SiO2 cogelled xerogel catalysts come from the structure of cogelled catalysts: metallic crystallites with a diameter of about 2–3 nm are located inside silica particles exhibiting a monodisperse microporous distribution centered on a pore size of about 0.8 nm [4–6,24]. Because metallic crystallites are larger than the micropores of the silica particles in which they are located, the metallic crystallites in cogelled catalysts are trapped and are then unable to migrate outside silica particles. In consequence, those catalysts are sinter-proof during treatments at high temperatures. In metallic impregnated samples, metal particles are not trapped inside silica matrix. Therefore, metal particles are very mobile during treatments at high temperatures and sintering occurs [25]. The objective of the present paper is to validate the experimental method proposed by Heinrichs et al. [14], combining carbon monoxide chemisorption, X-ray diffraction and transmission electron microscopy, for determining the surface composition of Pd–Cu alloy particles in Pd–Cu/SiO2

cogelled xerogel catalysts and compare our results with those published in the literature and obtained with other methods. The second objective is to establish relationships between catalytic activity and the surface composition of alloy nanoparticles for 1,2-dichloroethane hydrodechlorination in these Pd–Cu/SiO2 cogelled xerogel catalysts.

2. Experimental The three bimetallic catalysts Pd–Cu/SiO2 studied in this paper are xerogels prepared in alcohol by a one-step sol– gel procedure which consists in the cogelation of the silica precursor, tetraethoxysilane (TEOS), with 3-(2-aminoethyl) aminopropyl-trimethoxysilane (EDAS) forming chelates with palladium and copper ions. In these syntheses, palladium acetylacetonate powder (Pd(CH3 COCH=C(O–)CH3 )2 , Pd(acac)2 ) and copper acetate powder (Cu(CH3 CO(O–))2 , Cu(OAc)2 ) were mixed together with EDAS in half of the total ethanol volume. The slurry was stirred at room temperature until a clear blue solution was obtained for the Pd–Cu mixture (about half an hour). After addition of TEOS, a 0.54N NH3 aqueous solution in the remaining half of the total ethanol volume was added to the mixture under vigorous stirring. The vessel was then tightly closed and heated up to 80 ◦ C for 3 days (gelling and aging [26]). For all samples, the volume of the final solution was 155 ml. The hydrolysis ratio, H = [H2 O]/([TEOS] + 3/4[EDAS]), and the dilution ratio, R = [ethanol]/([TEOS] + [EDAS]) were kept constant at values of 5 and 10 respectively for all samples. The molar ratios EDAS/Pd(acac)2 was chosen equal to 2, and the molar ratio EDAS/Cu(OAc)2 was chosen equal to 4 as in the case of monometallic Pd/SiO2 and Cu/SiO2 cogelled xerogel catalysts studied in [4]. The resulting alcogels were dried under vacuum at 423 K, calcined in air at 673 K, and finally reduced in hydrogen at 623 K. Synthesis operating variables of Pd–Cu/SiO2 cogelled xerogel catalysts are presented in Table 1. Samples are denoted Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67) (numbers in parentheses refer to the weight composition in the sample). The surface composition of Pd–Cu bimetallic particles in Pd–Cu/SiO2 cogelled xerogel catalysts is determined through a combination of various characterization results obtained by transmission electron microscopy (TEM), X-ray diffraction (XRD), and carbon monoxide chemisorption. Details of technical equipments and experimental procedures are given in [4,14]. Samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33– 67) were tested for 1,2-dichloroethane hydrodechlorination, which was conducted in a stainless steel tubular reactor (10 mm i.d.) at a pressure of 0.3 MPa. The reactor was placed in a convection oven. A constant flow of each reactant was maintained by a Gilson piston pump for CH2 Cl–CH2 Cl and Brooks mass flow controllers for H2 and He. The effluent was analyzed by gas chromatography (ThermoFinnigan with FID) using a Porapak Q5 packed column. Prior to

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Table 1 Synthesis operating variables of Pd–Cu/SiO2 cogelled xerogel catalysts Sample

Pd(acac)2 (mmol)

Cu(OAc)2 (mmol)

EDAS (mmol)

TEOS (mmol)

H2 O (mmol)

C2 H5 OH (mmol)

Gel timea (min)

Pd–Cu(67–33) Pd–Cu(50–50) Pd–Cu(33–67)

1.494 1.500 1.521

1.246 2.505 5.064

7.95 13.24 23.60

164 158 148

848 841 828

1715 1715 1715

6 4 10

a Gel time is defined as the time elapsed between the introduction of the last reactive component to the solution and gelation at 80 ◦ C, and it is measured at the moment when the liquid no longer flows when the flask is tipped at an angle of 45◦ .

each experiment, Pd–Cu/SiO2 cogelled xerogel catalysts were reduced in situ at atmospheric pressure in flowing H2 (0.023 mmol s−1 ) while being heated to 623 K at a rate of 623 K/h and were maintained at this temperature for 3 h. After reduction, Pd–Cu/SiO2 cogelled xerogel catalyst were cooled in flowing H2 to the desired initial reaction temperature of 473 K. For each catalytic experiment, 0.11 g of catalyst pellets, sieved between 250 and 500 ␮m, were tested. The total flow of the reactant mixture was 0.45 mmol s−1 and consisted of CH2 Cl–CH2 Cl (0.011 mmol s−1 ), H2 (0.023 mmol s−1 ), and He (0.42 mmol s−1 ). The temperature was successively kept at 473, 523, 573, 623 and 573 K. The effluent was analyzed every 15 min.

3. Results 3.1. Bulk and surface compositions, size and localization of metal particles The particles surface composition is defined by the following molar fraction xPds , nPds (1) xPds = nPds + nCus where nPds is the number of Pd atoms lying on the surface of the Pd–Cu alloy particles, and nCus is the corresponding number of Cu atoms. xPds can be developed as follows [14]: xPds =

nPds nPd nPd + nCu 1 =DPd xPd nPd nPd + nCu nPds + nCus DPd–Cu

(2)

where nPd and nCu refer to the total number of atoms in the Pd–Cu alloy particles. The first factor, nPds /nPd , is the palladium dispersion, DPd , that is, the ratio between the number of surface Pd atoms and the total number of Pd atoms in the catalyst. DPd is determined from CO chemisorption measurements. The second factor, nPd /(nPd +nCu ), is the fraction xPd of Pd atoms in the Pd–Cu alloy particles. This fraction corresponds to the bulk composition of alloy particles, which are determined from XRD diffractograms. The third factor, (nPd + nCu )/(nPds + nCus ), is the inverse of the overall metal dispersion, DPd–Cu , of the alloy particles with no distinction between palladium and copper in samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67). DPd–Cu is the ratio between the number of metal atoms at the surface of Pd–Cu

alloy particles and the total number of metal atoms in those particles and can be calculated from TEM experiments. 3.1.1. Calculation of the palladium dispersion DPd from CO chemisorption To study the surface of Pd–Cu bimetallic particles in samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67), it is essential to know if CO is chemisorbed on both palladium and copper, or only selectively on one of these metals. CO chemisorption on monometallic palladium catalysts is a well-known phenomenon widely used to measure palladium dispersion [11,27,28]. The possible presence of strong interactions between CO and copper has been checked experimentally through the measurement of CO adsorption isotherms on a pure copper sample: chemisorption is nonexistent on this sample, which is in agreement with the literature data [7,27,29]. Since, in monometallic Pd/SiO2 and Cu/SiO2 cogelled xerogel catalysts, CO chemisorption occurs on palladium, but not on copper, we expect the same behavior on the surface of bimetallic particles in Pd–Cu/SiO2 cogelled xerogel catalysts. This hypothesis is supported by Renouprez’s work, in which a strong decrease is observed for the volume of adsorbed CO when the Cu content increases, the uptake being reduced by 50% for a Cu concentration of 17 at.% [18]. For 1,2-dichloroethane hydrodechlorination over Pt–Cu/SiO2 catalysts, the addition of CO into the CH2 Cl–CH2 Cl + H2 reaction mixture at 200 ◦ C to block Pt sites only, resulted in an improvement in the ethylene selectivity of the bimetallic catalysts at the expense of ethane. These observations were consistent with the idea that with Pt–Cu catalysts, ethylene forms on Cu sites, which were not blocked by carbon monoxide [8]. CO chemisorption isotherms determined for samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67) by the double adsorption method [4,5,14] are presented in Fig. 1. As chemisorption occurs on palladium only and that the Pd total concentration (Table 2) differs from one bimetallic catalyst to another, the amounts of chemisorbed CO are given in mmol gPd −1 . One observes that the amount of chemisorbed CO with respect to the weight of Pd decreases when copper loading increase. To be able to calculate palladium dispersion, DPd , for samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67), the chemisorption mean stoichiometry, XPd–CO , that is the mean number of Pd atoms on which one CO molecule is adsorbed, must then be determined. It is well known that carbon monoxide can adsorb on palladium

Adsorbed CO (mmol gPd-1)

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3

Pd (111)

Cu (111)

Pd (200)

Cu (200)

(c)

2

(b)

1

(a)

0 0

5

10

15

35

Pressure (kPa) Fig. 1. CO chemisorption isotherms of samples Pd–Cu(67–33) (䊐); Pd–Cu(50–50) (); Pd–Cu(33–67) (䊊).

40

45

50

55

Fig. 2. X-ray diffraction patterns of (a) Pd–Cu(67–33); (b) Pd–Cu(50–50); (c) Pd–Cu(33–67).

between the (2 0 0) Bragg lines of Pd and Cu is present, as well as one peak characteristic of unalloyed pure copper. The composition of the solid solution was calculated from the unit cell parameter corresponding to the broad peak between the (1 1 1) Bragg lines of Pd and Cu by using the Végard’s law [14,31]. Results are presented in Table 2.

in various configurations (linear CO and/or multicenter CO) and that the chemisorption mean stoichiometry, XPd–CO , depends on palladium dispersion [11,27,30]. From IR spectra of CO adsorbed on Pd–Cu alloy particles of various bulk compositions, the introduction of copper in palladium strongly reduces the presence of multicenter CO with respect to linear CO, which becomes almost the only species beyond 40 at.% of copper in the bulk of the alloy [18]. In samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67), the copper bulk composition, 1 − xPd , in bimetallic particles are higher than 40 at.% (Table 2). It can thus be admitted that CO is chemisorbed on palladium in the linear form only. As a consequence, the chemisorption mean stoichiometry XPd–CO = 1 will be used for the calculation of palladium dispersion, DPd , in Pd–Cu/SiO2 cogelled xerogel catalysts (Table 2). It is observed that DPd decreases when copper loading increases, indicating an enrichment in copper at Pd–Cu alloy particles surface as already reported in [11,27].

3.1.3. Calculation of the overall dispersion DPd–Cu of alloy particles from TEM A TEM micrograph is presented in Fig. 3 for sample Pd–Cu(33–67) as an example. TEM analysis indicates that the three bimetallic samples exhibit small metal crystallites between 2.5 and 4 nm, which correspond to the broad XRD peak lying between the (1 1 1) Bragg lines of pure Pd and Cu. Nevertheless, for sample Pd–Cu(33–67), we observed large crystallites between 10 and 20 nm, whose the presence can be correlated with the narrow XRD peak, characteristic of unalloyed pure copper (Fig. 2). All these results lead to the conclusion that the small metal particles would be Pd–Cu alloy crystallites, whereas large metal particles would consist of pure copper [18]. Concerning the localization of metallic crystallites, it appears that cogelled catalysts are composed of silica particles arranged in strings or aggregates, and although TEM gives only a 2D view, it seems that small metal particles are located inside silica particles, whereas large metal particles are located at their surface

3.1.2. Calculation of the Pd atoms fraction xPd in the bulk of alloy particles from XRD Fig. 2 shows the patterns obtained for samples Pd–Cu(67– 33), Pd–Cu(50–50) and Pd–Cu(33–67). Between the (1 1 1) Bragg lines of Pd and Cu, all these samples exhibit a broad peak, which demonstrates the presence of a solid solution. For sample Pd–Cu(33–67) (Fig. 2c), a second broad peak Table 2 Surface composition of alloy particles Sample

Pd–Cu(67–33) Pd–Cu(50–50) Pd–Cu(33–67)

Metal loading Pd (wt.%)

Cu (wt.%)

Chemisorption, DPd (%)

1.5 1.5 1.4

0.8 1.5 3.0

22 16 10

XRD, xPd (at.%)

TEM ds (nm)

DPd–Cu (%)

51 45 32

2.7 3.4 4.0

40 32 26

xPds (at.%)

xCus (at.%)

28 22 12

72 78 88

DPd : palladium dispersion measured by CO chemisorption; xPd : atomic ratio or bulk composition determined from XRD; ds , DPd–Cu : mean surface diameter and overall metal dispersion of small metal particles estimated from TEM; xPds : fraction of Pd atoms present at the surface of Pd–Cu alloy particles estimated from the combination of CO chemisorption, XRD and TEM results; xCus : fraction of Cu atoms present at the surface of Pd–Cu alloy particles estimated from the combination of CO chemisorption, XRD and TEM results.

205

100

425

80

375

60

325

40

275

20

225

0

175 0

5

10

15

20

Time (h)

(a)

DPd–Cu =

6(vm /am ) ds

(3)

with


ni d 3 ds =
i2 ni di

425

80

375

60

325

40

275

20

225

0

175 0

(b)

Temperature (˚C)

(Fig. 3). In previous studies [4,5,24], it was demonstrated that the localization of metal inside the silica particle was induced by a nucleation process initiated at the EDAS ligand site complexed by the metal. Because of its very reactive methoxy groups, EDAS reacts first giving rise to hydrolysis and condensation reactions, and forms the silica nuclei on which TEOS condenses in a later stage to form larger silica particles; a core–shell configuration is thus obtained. The overall metal dispersion, DPd–Cu , is given by Eq. (3) [27]:

Conversion, selectivities (mol %)

100

Fig. 3. TEM micrograph of sample Pd–Cu(33–67) (500,000×).

Temperature (˚C)

Conversion, selectivities (mol %)

S. Lambert et al. / Applied Catalysis A: General 270 (2004) 201–208

5

10

15

20

Time (h)

Fig. 4. 1,2-Dichloroethane hydrodechlorination: (a) Pd–Cu(67–33); (b) Pd–Cu(50–50). (䊉) ClCH2 –CH2 Cl conversion; (×) C2 H4 selectivity; (䊐) C2 H6 selectivity; () C2 H5 Cl selectivity; (—) temperature.

(4)

where vm is the mean volume occupied by a metal atom in the bulk of the alloy (nm3 ), am the mean surface area occupied by a surface metal atom (nm2 ), ds the mean surface diameter of metal particles (nm), di the metal particles diameter (nm) and ni the number of metal particles of a given diameter di . For palladium and copper, the values of vm are 0.01470 and 0.01183 nm3 , respectively, and the values of am are 0.0793 and 0.0685 nm2 , respectively [27]. For each bimetallic catalyst, the mean surface diameter ds is calculated from the diameters di of 50 small metal particles measured on TEM micrographs [4,5]. Values of ds are presented in Table 2. From these values, the overall metal dispersion DPd–Cu of metal crystallites is calculated by means of relation [3] and given in Table 2. The values taken for vm and am are weighted means calculated by using the atomic ratio of each metal in the alloy particles derived from XRD as weight factors. For the calculation of xPds in Pd–Cu/SiO2 cogelled xerogel catalysts, only palladium and copper atoms, which are present in alloy particles are considered. So copper atoms present in pure Cu particles for samples Pd–Cu(33–67) are

not taken into account in the developments above. Moreover, it is assumed that palladium is present only in the form of a Pd–Cu alloy in samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67). Values obtained for the surface composition, xPds and xCus = 1 − xPds , are shown in Table 2. It is observed on the particles surface a significant palladium concentration decrease coupled with the corresponding copper enrichment. 3.2. Catalytic experiments In Figs. 4 and 5, conversion as well as C2 H6 , C2 H4 and C2 H5 Cl selectivities are shown as a function of time and temperature over samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67). It is observed that all samples were active for 1,2-dichloroethane hydrodechlorination between 473 and 623 K. Increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity, and for sample Pd–Cu(33–67), this selectivity reaches 100% in the conditions of the catalytic test. Conversion of 1,2-dichloroethane decreases at each temperature when the copper loading is increased.

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425 DCE conversion Ethylene selectivity Ethane selectivity EC selectivity Temperature

80

375

60

325

40

275

20

225

0

Temperature (˚C)

Conversion, selectivities (mol %)

100

175 0

10

20

30

40

50

60

70

Time (h) Fig. 5. 1,2-Dichloroethane hydrodechlorination over sample Pd–Cu(33–67). (䊉) ClCH2 –CH2 Cl conversion; (×) C2 H4 selectivity; (䊐) C2 H6 selectivity; () C2 H5 Cl selectivity; (—) temperature.

In Figs. 4 and 5, the examination of conversion curves shows that a deactivation, which is faster when the temperature increases from 473 to 623 K, is observed with all samples. Nevertheless, this deactivation becomes slower when the Cu loading is increased and is scarcely distinguishable with samples Pd–Cu(50–50) and Pd–Cu(33–67). For sample Pd–Cu(33–67), the catalytic cycle is repeated three times to show its stability over time (Fig. 5). During the three catalytic cycles, at each level of temperature, 1,2-dichloroethane conversion preserves the same value and the selectivity towards C2 H4 is equal to 100%.

subsequently reduced. It was concluded that the surface of the bimetallic particles, with a mean particle size of 3–4 nm, was enriched with copper [32]. Catalysts made by coimpregnation of KL-zeolithe with Cu and Pd nitrates were characterized by XANES and IR spectroscopy, and the formation of substitutionally disordered alloys was observed [33]. For Pd–Cu/SiO2 cogelled xerogel catalysts, the results presented in Table 2 and Fig. 6 demonstrate that a segregation of copper at the surface of the Pd–Cu alloy occurs. These experimental results are completely corroborated by works

4. Discussion The surface enrichment with copper for Pd–Cu alloy particles given in Table 2 is in agreement with the theoretical prediction according to which, at thermodynamic equilibrium, alloys forming a solid solution (completely miscible metals) exhibit under vacuum a surface enriched with the metal having the lowest surface energy [11,15,16]. Indeed, surface energy of copper (1.84 J m−2 at 0 K) is lower than that of palladium (2.09 J m−2 at 0 K). Furthermore, a theoretical study of the surface segregation in Pd–Cu single crystals preferentially locates Cu atoms on the edges of the small crystals and on the (1 0 0) open faces, rather than on the (1 1 1) faces [17–19]. Finally, the alloying of Cu and Pd has been also reported in several studies. In studies of O2 and C2 H4 adsorption on Pd/SiO2 catalysts modified by Cu, a very marked enrichment in copper for Pd–Cu alloy particles’ surface compared to their bulk is observed when copper loading increases [20]. In a combined XPS, IR absorption, and catalytic reaction study, an alloy was formed when CuPd(OAc)4 was chemisorbed on dehydrated ␥-alumina and

Surface composition x Pds (at. %)

100 90 80 70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

90

100

Bulk composition x Pd (at. %) Fig. 6. Surface composition as a function of bulk composition for Pd–Cu alloys: (䊏) surface composition obtained in this study with samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67); () surface composition determined by low energy ion scattering [17]; (䊉) surface composition determined by X-ray photoelectron spectroscopy [34].

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of Renouprez et al. [17,18]. In their studies, low energy ion scattering (LEIS) experiments have been performed on Pd–Cu/SiO2 catalysts with bulk atomic ratios of 31, 46 and 83 at.% of palladium. The LEIS measurements performed on each sample within the first minute reflect the actual surface composition, before any ionic erosion has occurred. Fig. 6 shows that the fraction of Pd atoms lying on the surface of the Pd–Cu alloy particles, xPds , is equal to about 11, 25 and 64 at.%, respectively [17]. Moreover, Venezia et al. determined the most likely structure of Pd–Cu catalysts supported on pumice by a combination of a surface technique, as XPS, and a bulk technique, as XRD [34]. These results are also in complete agreement with the data obtained from the combination of CO chemisorption, XRD, and TEM in this study (Fig. 6). In Figs. 4 and 5, it is observed for bimetallic samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67) that ethylene selectivity increases with copper content, and for bimetallic sample Pd–Cu(33–67), this selectivity reaches 100% in the conditions of the catalytic test. As explained by several authors [1,35,36], in the case of the hydrogenolysis of hydrocarbons over bimetallic catalysts, this selectivity effect could be explained by the hypothesis that the production of C2 H6 requires a surface site consisting of an array of adjacent active metal atoms which is larger than that required for C2 H4 production. Diluting active Pd atoms in a Pd–Cu alloy with increasing of inert Cu (xPds of sample Pd–Cu(67–33) = 28 at.%, xPds of sample Pd–Cu(50–50) = 22 at.%, xPds of sample Pd–Cu(33–67) = 12 at.%), would then favor C2 H4 rather than C2 H6 . The mechanism of 1,2-dichloroethane hydrodechlorination has been studied in detail by Heinrichs et al. over a 1.9%Pd–3.7%Ag/SiO2 cogelled xerogel catalyst with a surface composition, xPds = 10 at.% [14,37]. This mechanism is based on the sequence of elementary steps, which suggests a process of chlorination of the silver surface by 1,2-dichloroethane followed by a hydrodechlorination of that surface by hydrogen adsorbed on palladium. Used alone, silver deactivates rapidly due to its covering by chlorine atoms. Thanks to its activation power of hydrogen by dissociative chemisorption, palladium present in the alloy supplies hydrogen atoms for the regeneration of the chlorinated silver surface into metallic silver. The presence of hydrogen adsorbed on Pd also causes undesired ethylene hydrogenation leading to a loss of olefin selectivity. The same mechanism can be suggested for Pd–Cu/SiO2 cogelled xerogel catalysts, that is, chlorination of the copper surface by 1,2-dichloroethane followed by its dechlorination. Indeed, samples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67) present a very marked impoverishment in palladium for Pd–Cu alloy particles’ surface (Table 2). Furthermore, pure copper samples presents a very low activity at each temperature for 1,2-dichloroethane hydrodechlorination [4]. Surface Cl could not be removed easily due to a lack of surface hydrogen. Palladium could therefore be needed to provide an abundant source of dissociated hydrogen, to re-

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duce surface CuCl species and form HCl. According to the study of Fung and Sinfelt concerning the hydrogenolysis of methyl chloride CH3 Cl on metals [38], metals from group Ib such as Ag and Cu, are able to form a metal-chlorine bond, as demonstrated by the existence of stable chlorides. Furthermore, Vadlamannati et al. suggested the same mechanism for 1,2-dichloroethane hydrodechlorination over Pt–Cu/C catalysts [7]. Finally, for 1,2-dichloroethane hydrodechlorination over Pt–Cu/SiO2 catalysts, the addition of CO into the CH2 Cl–CH2 Cl + H2 reaction mixture at 200 ◦ C to block Pt sites only results in an improvement in the ethylene selectivity of the bimetallic catalysts at the expense of ethane. These observations were consistent with the idea that with Pt–Cu catalysts, ethylene forms on Cu sites that were not blocked by carbon monoxide [8].

5. Conclusions The combination of results from carbon monoxide chemisorption, X-ray diffraction, and transmission electron microscopy allowed calculating the surface composition of the palladium–copper particles in Pd–Cu/SiO2 cogelled xerogel catalysts. Values obtained indicate a very pronounced surface enrichment with copper. The concentration increase of copper at the particle surface results from the fact that the surface energy of copper is lower than the surface energy of palladium. Furthermore, the surface enrichment with Cu could also result from a preferential localization of copper atoms on low coordination sites. Surface compositions obtained with this method, which combines three different experimental techniques, are in agreement with the literature data previously obtained for surface segregation in Pd–Cu/SiO2 catalysts by other techniques such as low energy ion scattering and X-ray photoelectron spectroscopy. While 1,2-dichloroethane hydrodechlorination over pure palladium mainly produces ethane, increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity. Used alone, copper deactivates rapidly due to its covering by chlorine atoms. Thanks to its activation power of hydrogen by dissociative chemisorption, palladium present in the Pd–Cu alloy supplies hydrogen atoms for the regeneration of the chlorinated copper surfaces into metallic copper.

Acknowledgements The authors thank the Centre d’Enseignement et de Recherche des Macromolécules, C.E.R.M., from the University of Liège for TEM analysis. S.L. is grateful to the Belgian Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture, F.R.I.A., for a Ph.D. grant. The authors also thank the Belgian Fonds National de la Recherche Scientifique, the Fonds de Bay, the Fonds de la Recherche Fondamentale et Collective, the Ministère de la Région Wallonne and the Ministère de la Communauté

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Française (Action de Recherche Concertée No. 00-05-265) for their financial support.

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