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

Determination of surface composition of alloy nanoparticlesand relationships with catalytic activity in Pd–Cu/SiO2

cogelled xerogel catalysts

Stéphanie Lamberta, Benoıt Heinrichsa,∗, Alain Brasseura,André Rulmontb, Jean-Paul Pirarda

a Laboratoire de Génie Chimique, B6a, Université de Liège, B-4000 Liège, Belgiumb 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 calculat-ing the surface composition of the palladium–copper nanoparticles in Pd–Cu/SiO2 cogelled xerogel catalysts. Values obtained indicate a verypronounced surface enrichment with copper. Surface compositions obtained with this method, which combines three different experimentaltechniques, are in agreement with the literature data previously obtained for surface segregation in Pd–Cu/SiO2 catalysts by other techniquesas low energy ion scattering and X-ray photoelectron spectroscopy. While 1,2-dichloroethane hydrodechlorination over pure palladium mainlyproduces ethane, increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity, to reach 100% in ethyleneselectivity 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 particularlypalladium, are very active for the hydrodechlorination reac-tion [1–3]. In the case of 1,2-dichloroethane hydrodechlo-rination, the noble metal participates in a catalytic cycle,in which the reactant is dechlorinated by chlorination ofthe metal surface, which is then itself dechlorinated byreduction with hydrogen. Because of the high reactivityof hydrogen on noble metals, the dechlorinated organics,C2H4 in the present case, is immediately converted into thefully hydrogenated product, C2H6 [2–5], which is muchless useful from an industrial point of view. However, sev-eral 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 ornot 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: b.heinrichs@ulg.ac.be (B. Heinrichs).

ticular case of hydrodechlorination shows once again thatthe addition of a second metal in a monometallic catalystsignificantly alters its activity and selectivity[1].

To understand the mechanism of selective hydrodechlo-rination of 1,2-dichloroethane into ethylene on a supportedalloy and to calculate a turnover frequency (TOF), that is,the number of 1,2-dichloroethane molecules consumed peractive surface metal and per second, it is very important toknow its actual surface composition. Indeed, the latter canstrongly deviate from the bulk composition[11,12]. In thecase of the palladium–silver alloy, this composition differ-ence between surface and bulk has been shown by Kuijersand Ponec from results calculated from Auger electron andinfrared spectroscopies[13] and by Heinrichs et al. fromCO chemisorption, X-ray diffraction, and transmission elec-tron microscopy[14]. The surface enrichment with silver forPd–Ag alloy particles is in agreement with the theoreticalprediction according to which, at thermodynamic equilib-rium, alloys forming a solid solution (completely misciblemetals) exhibit under vacuum a surface enriched with themetal having the lowest surface energy[11,15,16]. Indeed, a

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

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

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 at0 K), is then expected.

For Pd–Cu alloy, although the difference between the sur-face energies at 0 K of palladium and copper (1.84 J m−2)is lower [11], a theoretical study of the surface segregationin Pd–Cu single crystals preferentially locates Cu atoms onthe 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 alsoconfirmed by studies of O2 and C2H4 adsorption on Cu mod-ified Pd/SiO2 catalysts, for which a very marked enrichmentin copper on the surface of the Pd–Cu alloy particles hasbeen observed when copper loading increases[20].

A high activity of a supported catalyst often calls for alarge active surface area and, thus, for small particles, i.e. ahigh dispersion of the active phase. Because small metal par-ticles tend to already sinter at relatively low temperatures,these generally are applied into a support material which it-self is thermally stable and maintains a high specific surfacearea up to high temperatures[21]. In this way, Schubert andco-workers have developed an interesting method to dispersemetal particles in a silica matrix[22,23]. Heinrichs et al.[6] and Lambert et al.[4,5,24]used this cogelation methodfor the preparation of Pd/SiO2, Ag/SiO2, Cu/SiO2 andPd–Ag/SiO2 catalysts. All these authors used alkoxides ofthe type (RO)3Si–X–A in which a functional organic groupA, able to form a chelate with a cation of a metal such aspalladium, silver, copper, etc., is linked to the hydrolysablesilyl group (RO)3Si– via an inert and hydrolytically stablespacer X. The co-condensation of such molecules with anetwork-forming reagent such as TEOS, Si(OC2H5)4, re-sults in materials in which the metal is anchored to the SiO2matrix. So this method can allow obtaining a mean diam-eter of metal particles of about 2 nm[4,5] whereas metalcatalysts prepared by a classical method as impregnationpresent a mean diameter of metal particles of about 5–30 nm[25]. The best metal dispersion values obtained in the caseof Pd/SiO2, Ag/SiO2, Cu/SiO2 and Pd–Ag/SiO2 cogelledxerogel catalysts come from the structure of cogelled cata-lysts: metallic crystallites with a diameter of about 2–3 nmare located inside silica particles exhibiting a monodispersemicroporous distribution centered on a pore size of about0.8 nm [4–6,24]. Because metallic crystallites are largerthan the micropores of the silica particles in which theyare located, the metallic crystallites in cogelled catalystsare trapped and are then unable to migrate outside silicaparticles. In consequence, those catalysts are sinter-proofduring treatments at high temperatures. In metallic impreg-nated samples, metal particles are not trapped inside silicamatrix. Therefore, metal particles are very mobile duringtreatments at high temperatures and sintering occurs[25].

The objective of the present paper is to validate the exper-imental method proposed by Heinrichs et al.[14], combin-ing carbon monoxide chemisorption, X-ray diffraction andtransmission electron microscopy, for determining the sur-face composition of Pd–Cu alloy particles in Pd–Cu/SiO2

cogelled xerogel catalysts and compare our results with thosepublished in the literature and obtained with other meth-ods. The second objective is to establish relationships be-tween catalytic activity and the surface composition of alloynanoparticles for 1,2-dichloroethane hydrodechlorination inthese Pd–Cu/SiO2 cogelled xerogel catalysts.

2. Experimental

The three bimetallic catalysts Pd–Cu/SiO2 studied in thispaper are xerogels prepared in alcohol by a one-step sol–gel procedure which consists in the cogelation of the silicaprecursor, tetraethoxysilane (TEOS), with 3-(2-aminoethyl)aminopropyl-trimethoxysilane (EDAS) forming chelateswith palladium and copper ions. In these syntheses, palla-dium acetylacetonate powder (Pd(CH3COCH=C(O–)CH3)2,Pd(acac)2) and copper acetate powder (Cu(CH3CO(O–))2,Cu(OAc)2) were mixed together with EDAS in half of thetotal ethanol volume. The slurry was stirred at room temper-ature until a clear blue solution was obtained for the Pd–Cumixture (about half an hour). After addition of TEOS, a0.54N NH3 aqueous solution in the remaining half of the to-tal ethanol volume was added to the mixture under vigorousstirring. The vessel was then tightly closed and heated up to80◦C for 3 days (gelling and aging[26]). For all samples,the volume of the final solution was 155 ml. The hydrolysisratio,H = [H2O]/([TEOS]+3/4[EDAS]), and the dilutionratio, R = [ethanol]/([TEOS] + [EDAS]) were kept con-stant at values of 5 and 10 respectively for all samples. Themolar ratios EDAS/Pd(acac)2 was chosen equal to 2, andthe molar ratio EDAS/Cu(OAc)2 was chosen equal to 4 asin the case of monometallic Pd/SiO2 and Cu/SiO2 cogelledxerogel catalysts studied in[4]. The resulting alcogels weredried under vacuum at 423 K, calcined in air at 673 K, andfinally reduced in hydrogen at 623 K. Synthesis operatingvariables of Pd–Cu/SiO2 cogelled xerogel catalysts arepresented inTable 1. Samples are denoted Pd–Cu(67–33),Pd–Cu(50–50) and Pd–Cu(33–67) (numbers in parenthesesrefer to the weight composition in the sample).

The surface composition of Pd–Cu bimetallic particlesin Pd–Cu/SiO2 cogelled xerogel catalysts is determinedthrough a combination of various characterization resultsobtained by transmission electron microscopy (TEM),X-ray diffraction (XRD), and carbon monoxide chemisorp-tion. Details of technical equipments and experimentalprocedures 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 placedin a convection oven. A constant flow of each reactant wasmaintained by a Gilson piston pump for CH2Cl–CH2Cl andBrooks mass flow controllers for H2 and He. The effluentwas analyzed by gas chromatography (ThermoFinniganwith FID) using a Porapak Q5 packed column. Prior to

S. Lambert et al. / Applied Catalysis A: General 270 (2004) 201–208 203

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

Sample Pd(acac)2 (mmol) Cu(OAc)2 (mmol) EDAS (mmol) TEOS (mmol) H2O (mmol) C2H5OH (mmol) Gel timea (min)

Pd–Cu(67–33) 1.494 1.246 7.95 164 848 1715 6Pd–Cu(50–50) 1.500 2.505 13.24 158 841 1715 4Pd–Cu(33–67) 1.521 5.064 23.60 148 828 1715 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 ismeasured 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 catalystswere reduced in situ at atmospheric pressure in flowing H2(0.023 mmol s−1) while being heated to 623 K at a rate of623 K/h and were maintained at this temperature for 3 h.After reduction, Pd–Cu/SiO2 cogelled xerogel catalyst werecooled in flowing H2 to the desired initial reaction temper-ature of 473 K. For each catalytic experiment, 0.11 g of cat-alyst 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 CH2Cl–CH2Cl (0.011 mmol s−1), H2(0.023 mmol s−1), and He (0.42 mmol s−1). The tempera-ture 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 localizationof metal particles

The particles surface composition is defined by the fol-lowing molar fractionxPds,

xPds = nPds

nPds + nCus

(1)

wherenPds is the number of Pd atoms lying on the surfaceof the Pd–Cu alloy particles, andnCus is the correspondingnumber of Cu atoms.

xPds can be developed as follows[14]:

xPds=nPds

nPd

nPd

nPd + nCu

nPd + nCu

nPds + nCus

=DPdxPd1

DPd–Cu(2)

wherenPd andnCu refer to the total number of atoms in thePd–Cu alloy particles. The first factor,nPds/nPd, is the palla-dium dispersion,DPd, that is, the ratio between the numberof surface Pd atoms and the total number of Pd atoms in thecatalyst.DPd is determined from CO chemisorption mea-surements. The second factor,nPd/(nPd+nCu), is the fractionxPd of Pd atoms in the Pd–Cu alloy particles. This fractioncorresponds to the bulk composition of alloy particles, whichare determined from XRD diffractograms. The third factor,(nPd+nCu)/(nPds +nCus), is the inverse of the overall metaldispersion,DPd–Cu, of the alloy particles with no distinctionbetween palladium and copper in samples Pd–Cu(67–33),Pd–Cu(50–50) and Pd–Cu(33–67).DPd–Cu is the ratio be-tween the number of metal atoms at the surface of Pd–Cu

alloy particles and the total number of metal atoms in thoseparticles and can be calculated from TEM experiments.

3.1.1. Calculation of the palladium dispersion DPd fromCO chemisorption

To study the surface of Pd–Cu bimetallic particles insamples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67),it is essential to know if CO is chemisorbed on both palla-dium and copper, or only selectively on one of these metals.CO chemisorption on monometallic palladium catalysts isa well-known phenomenon widely used to measure palla-dium dispersion[11,27,28]. The possible presence of stronginteractions between CO and copper has been checkedexperimentally through the measurement of CO adsorp-tion isotherms on a pure copper sample: chemisorption isnonexistent on this sample, which is in agreement with theliterature data[7,27,29]. Since, in monometallic Pd/SiO2and Cu/SiO2 cogelled xerogel catalysts, CO chemisorp-tion occurs on palladium, but not on copper, we expectthe same behavior on the surface of bimetallic particles inPd–Cu/SiO2 cogelled xerogel catalysts. This hypothesis issupported by Renouprez’s work, in which a strong decreaseis observed for the volume of adsorbed CO when the Cucontent increases, the uptake being reduced by 50% for aCu concentration of 17 at.%[18]. For 1,2-dichloroethanehydrodechlorination over Pt–Cu/SiO2 catalysts, the addi-tion of CO into the CH2Cl–CH2Cl + H2 reaction mixtureat 200◦C to block Pt sites only, resulted in an improvementin the ethylene selectivity of the bimetallic catalysts at theexpense of ethane. These observations were consistent withthe idea that with Pt–Cu catalysts, ethylene forms on Cusites, which were not blocked by carbon monoxide[8].

CO chemisorption isotherms determined for samplesPd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67) by thedouble adsorption method[4,5,14] are presented inFig. 1.As chemisorption occurs on palladium only and that the Pdtotal concentration (Table 2) differs from one bimetallic cat-alyst to another, the amounts of chemisorbed CO are given inmmol gPd

−1. One observes that the amount of chemisorbedCO with respect to the weight of Pd decreases when copperloading increase. To be able to calculate palladium dis-persion, 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 oneCO molecule is adsorbed, must then be determined. It iswell known that carbon monoxide can adsorb on palladium

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

0

1

2

3

0 5 10 15

Pressure (kPa)

Ads

orbe

d C

O (

mm

ol g

Pd-1

)

Fig. 1. CO chemisorption isotherms of samples Pd–Cu(67–33) (�);Pd–Cu(50–50) (�); Pd–Cu(33–67) (�).

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 spec-tra of CO adsorbed on Pd–Cu alloy particles of variousbulk compositions, the introduction of copper in palladiumstrongly reduces the presence of multicenter CO with re-spect to linear CO, which becomes almost the only speciesbeyond 40 at.% of copper in the bulk of the alloy[18]. Insamples Pd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67),the copper bulk composition, 1−xPd, in bimetallic particlesare higher than 40 at.% (Table 2). It can thus be admittedthat CO is chemisorbed on palladium in the linear form only.As a consequence, the chemisorption mean stoichiometryXPd–CO = 1 will be used for the calculation of palladiumdispersion,DPd, in Pd–Cu/SiO2 cogelled xerogel catalysts(Table 2). It is observed thatDPd decreases when copperloading increases, indicating an enrichment in copper atPd–Cu alloy particles surface as already reported in[11,27].

3.1.2. Calculation of the Pd atoms fraction xPd in the bulkof alloy particles from XRD

Fig. 2shows 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 broadpeak, which demonstrates the presence of a solid solution.For sample Pd–Cu(33–67) (Fig. 2c), a second broad peak

Table 2Surface composition of alloy particles

Sample Metal loading Chemisorption,DPd (%)

XRD,xPd (at.%)

TEM xPds (at.%) xCus (at.%)

Pd (wt.%) Cu (wt.%) ds (nm) DPd–Cu (%)

Pd–Cu(67–33) 1.5 0.8 22 51 2.7 40 28 72Pd–Cu(50–50) 1.5 1.5 16 45 3.4 32 22 78Pd–Cu(33–67) 1.4 3.0 10 32 4.0 26 12 88

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

35 40 45 50 55

(a)

(b)

(c)

Pd(111)

Cu(111)

Pd(200)

Cu(200)

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, aswell as one peak characteristic of unalloyed pure copper. Thecomposition of the solid solution was calculated from theunit cell parameter corresponding to the broad peak betweenthe (1 1 1) Bragg lines of Pd and Cu by using the Végard’slaw [14,31]. Results are presented inTable 2.

3.1.3. Calculation of the overall dispersion DPd–Cu ofalloy particles from TEM

A TEM micrograph is presented inFig. 3 for samplePd–Cu(33–67) as an example. TEM analysis indicates thatthe three bimetallic samples exhibit small metal crystallitesbetween 2.5 and 4 nm, which correspond to the broad XRDpeak lying between the (1 1 1) Bragg lines of pure Pd andCu. Nevertheless, for sample Pd–Cu(33–67), we observedlarge crystallites between 10 and 20 nm, whose the pres-ence can be correlated with the narrow XRD peak, charac-teristic of unalloyed pure copper (Fig. 2). All these resultslead to the conclusion that the small metal particles wouldbe Pd–Cu alloy crystallites, whereas large metal particleswould consist of pure copper[18]. Concerning the localiza-tion of metallic crystallites, it appears that cogelled catalystsare composed of silica particles arranged in strings or ag-gregates, and although TEM gives only a 2D view, it seemsthat small metal particles are located inside silica particles,whereas large metal particles are located at their surface

S. Lambert et al. / Applied Catalysis A: General 270 (2004) 201–208 205

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

(Fig. 3). In previous studies[4,5,24], it was demonstratedthat the localization of metal inside the silica particle wasinduced by a nucleation process initiated at the EDAS ligandsite complexed by the metal. Because of its very reactivemethoxy groups, EDAS reacts first giving rise to hydrolysisand condensation reactions, and forms the silica nuclei onwhich TEOS condenses in a later stage to form larger silicaparticles; a core–shell configuration is thus obtained.

The overall metal dispersion,DPd–Cu, is given byEq. (3)[27]:

DPd–Cu = 6(vm/am)

ds(3)

with

ds =∑

nid3i∑

nid2i

(4)

wherevm is the mean volume occupied by a metal atom inthe bulk of the alloy (nm3), am the mean surface area oc-cupied by a surface metal atom (nm2), ds the mean surfacediameter of metal particles (nm),di the metal particles di-ameter (nm) andni the number of metal particles of a givendiameterdi . For palladium and copper, the values ofvm are0.01470 and 0.01183 nm3, respectively, and the values ofam are 0.0793 and 0.0685 nm2, respectively[27]. For eachbimetallic catalyst, the mean surface diameterds is calcu-lated from the diametersdi of 50 small metal particles mea-sured on TEM micrographs[4,5]. Values ofds are presentedin Table 2. From these values, the overall metal dispersionDPd–Cu of metal crystallites is calculated by means of rela-tion [3] and given inTable 2. The values taken forvm andam are weighted means calculated by using the atomic ratioof each metal in the alloy particles derived from XRD asweight factors.

For the calculation ofxPds in Pd–Cu/SiO2 cogelled xero-gel catalysts, only palladium and copper atoms, which arepresent in alloy particles are considered. So copper atomspresent in pure Cu particles for samples Pd–Cu(33–67) are

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Fig. 4. 1,2-Dichloroethane hydrodechlorination: (a) Pd–Cu(67–33); (b)Pd–Cu(50–50). (�) ClCH2–CH2Cl conversion; (×) C2H4 selectivity; (�)C2H6 selectivity; (�) C2H5Cl selectivity; (—) temperature.

not taken into account in the developments above. More-over, it is assumed that palladium is present only in the formof a Pd–Cu alloy in samples Pd–Cu(67–33), Pd–Cu(50–50)and Pd–Cu(33–67). Values obtained for the surface compo-sition, xPds andxCus = 1 − xPds, are shown inTable 2. Itis observed on the particles surface a significant palladiumconcentration decrease coupled with the corresponding cop-per enrichment.

3.2. Catalytic experiments

In Figs. 4 and 5, conversion as well as C2H6, C2H4 andC2H5Cl selectivities are shown as a function of time and tem-perature over samples Pd–Cu(67–33), Pd–Cu(50–50) andPd–Cu(33–67). It is observed that all samples were activefor 1,2-dichloroethane hydrodechlorination between 473 and623 K. Increasing copper content in bimetallic catalysts re-sults in an increase in ethylene selectivity, and for samplePd–Cu(33–67), this selectivity reaches 100% in the condi-tions of the catalytic test. Conversion of 1,2-dichloroethanedecreases at each temperature when the copper loading isincreased.

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

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DCE conversionEthylene selectivityEthane selectivityEC selectivityTemperature

Fig. 5. 1,2-Dichloroethane hydrodechlorination over sample Pd–Cu(33–67). (�) ClCH2–CH2Cl conversion; (×) C2H4 selectivity; (�) C2H6 selectivity;(�) C2H5Cl selectivity; (—) temperature.

In Figs. 4 and 5, the examination of conversion curvesshows that a deactivation, which is faster when the tem-perature increases from 473 to 623 K, is observed with allsamples. Nevertheless, this deactivation becomes slowerwhen the Cu loading is increased and is scarcely distin-guishable with samples Pd–Cu(50–50) and Pd–Cu(33–67).For sample Pd–Cu(33–67), the catalytic cycle is repeatedthree times to show its stability over time (Fig. 5). Dur-ing the three catalytic cycles, at each level of temperature,1,2-dichloroethane conversion preserves the same value andthe selectivity towards C2H4 is equal to 100%.

4. Discussion

The surface enrichment with copper for Pd–Cu alloy par-ticles given inTable 2is in agreement with the theoreticalprediction according to which, at thermodynamic equilib-rium, alloys forming a solid solution (completely misciblemetals) exhibit under vacuum a surface enriched with themetal having the lowest surface energy[11,15,16]. Indeed,surface energy of copper (1.84 J m−2 at 0 K) is lower thanthat of palladium (2.09 J m−2 at 0 K). Furthermore, a the-oretical study of the surface segregation in Pd–Cu singlecrystals preferentially locates Cu atoms on the edges of thesmall crystals and on the (1 0 0) open faces, rather than onthe (1 1 1) faces[17–19]. Finally, the alloying of Cu and Pdhas been also reported in several studies. In studies of O2and C2H4 adsorption on Pd/SiO2 catalysts modified by Cu, avery marked enrichment in copper for Pd–Cu alloy particles’surface compared to their bulk is observed when copperloading increases[20]. In a combined XPS, IR absorption,and catalytic reaction study, an alloy was formed whenCuPd(OAc)4 was chemisorbed on dehydrated�-alumina and

subsequently reduced. It was concluded that the surface ofthe bimetallic particles, with a mean particle size of 3–4 nm,was enriched with copper[32]. Catalysts made by coimpreg-nation of KL-zeolithe with Cu and Pd nitrates were charac-terized by XANES and IR spectroscopy, and the formationof substitutionally disordered alloys was observed[33].

For Pd–Cu/SiO2 cogelled xerogel catalysts, the resultspresented inTable 2andFig. 6 demonstrate that a segrega-tion of copper at the surface of the Pd–Cu alloy occurs. Theseexperimental results are completely corroborated by works

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Bulk composition x Pd (at. %)

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ds (

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Fig. 6. Surface composition as a function of bulk composition for Pd–Cualloys: (�) surface composition obtained in this study with samplesPd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67); (�) surface composi-tion determined by low energy ion scattering[17]; (�) surface composi-tion determined by X-ray photoelectron spectroscopy[34].

S. Lambert et al. / Applied Catalysis A: General 270 (2004) 201–208 207

of Renouprez et al.[17,18]. In their studies, low energyion scattering (LEIS) experiments have been performed onPd–Cu/SiO2 catalysts with bulk atomic ratios of 31, 46 and83 at.% of palladium. The LEIS measurements performedon each sample within the first minute reflect the actual sur-face composition, before any ionic erosion has occurred.Fig. 6 shows that the fraction of Pd atoms lying on the sur-face 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 sup-ported on pumice by a combination of a surface technique,as XPS, and a bulk technique, as XRD[34]. These resultsare also in complete agreement with the data obtained fromthe combination of CO chemisorption, XRD, and TEM inthis study (Fig. 6).

In Figs. 4 and 5, it is observed for bimetallic samplesPd–Cu(67–33), Pd–Cu(50–50) and Pd–Cu(33–67) thatethylene selectivity increases with copper content, and forbimetallic sample Pd–Cu(33–67), this selectivity reaches100% in the conditions of the catalytic test. As explained byseveral authors[1,35,36], in the case of the hydrogenolysisof hydrocarbons over bimetallic catalysts, this selectivityeffect could be explained by the hypothesis that the produc-tion of C2H6 requires a surface site consisting of an arrayof adjacent active metal atoms which is larger than thatrequired for C2H4 production. Diluting active Pd atoms ina Pd–Cu alloy with increasing of inert Cu (xPds of samplePd–Cu(67–33) = 28 at.%,xPds of sample Pd–Cu(50–50) =22 at.%,xPds of sample Pd–Cu(33–67) = 12 at.%), wouldthen favor C2H4 rather than C2H6.

The mechanism of 1,2-dichloroethane hydrodechlorina-tion has been studied in detail by Heinrichs et al. overa 1.9%Pd–3.7%Ag/SiO2 cogelled xerogel catalyst with asurface composition,xPds = 10 at.%[14,37]. This mecha-nism is based on the sequence of elementary steps, whichsuggests a process of chlorination of the silver surface by1,2-dichloroethane followed by a hydrodechlorination ofthat surface by hydrogen adsorbed on palladium. Used alone,silver deactivates rapidly due to its covering by chlorineatoms. Thanks to its activation power of hydrogen by dis-sociative chemisorption, palladium present in the alloy sup-plies hydrogen atoms for the regeneration of the chlorinatedsilver surface into metallic silver. The presence of hydrogenadsorbed on Pd also causes undesired ethylene hydrogena-tion leading to a loss of olefin selectivity.

The same mechanism can be suggested for Pd–Cu/SiO2cogelled xerogel catalysts, that is, chlorination of the cop-per surface by 1,2-dichloroethane followed by its dechlori-nation. Indeed, samples Pd–Cu(67–33), Pd–Cu(50–50) andPd–Cu(33–67) present a very marked impoverishment inpalladium for Pd–Cu alloy particles’ surface (Table 2). Fur-thermore, pure copper samples presents a very low activityat each temperature for 1,2-dichloroethane hydrodechlorina-tion [4]. Surface Cl could not be removed easily due to a lackof surface hydrogen. Palladium could therefore be needed toprovide an abundant source of dissociated hydrogen, to re-

duce surface CuCl species and form HCl. According to thestudy of Fung and Sinfelt concerning the hydrogenolysis ofmethyl chloride CH3Cl on metals[38], metals from group Ibsuch as Ag and Cu, are able to form a metal-chlorine bond, asdemonstrated by the existence of stable chlorides. Further-more, Vadlamannati et al. suggested the same mechanism for1,2-dichloroethane hydrodechlorination over Pt–Cu/C cat-alysts[7]. Finally, for 1,2-dichloroethane hydrodechlorina-tion over Pt–Cu/SiO2 catalysts, the addition of CO into theCH2Cl–CH2Cl + H2 reaction mixture at 200◦C to block Ptsites only results in an improvement in the ethylene selec-tivity of the bimetallic catalysts at the expense of ethane.These observations were consistent with the idea that withPt–Cu catalysts, ethylene forms on Cu sites that were notblocked by carbon monoxide[8].

5. Conclusions

The combination of results from carbon monoxidechemisorption, X-ray diffraction, and transmission electronmicroscopy allowed calculating the surface composition ofthe palladium–copper particles in Pd–Cu/SiO2 cogelled xe-rogel catalysts. Values obtained indicate a very pronouncedsurface enrichment with copper. The concentration increaseof copper at the particle surface results from the fact thatthe surface energy of copper is lower than the surface en-ergy of palladium. Furthermore, the surface enrichmentwith Cu could also result from a preferential localizationof copper atoms on low coordination sites. Surface com-positions obtained with this method, which combines threedifferent experimental techniques, are in agreement with theliterature data previously obtained for surface segregationin Pd–Cu/SiO2 catalysts by other techniques such as lowenergy ion scattering and X-ray photoelectron spectroscopy.

While 1,2-dichloroethane hydrodechlorination over purepalladium mainly produces ethane, increasing copper con-tent in bimetallic catalysts results in an increase in ethyleneselectivity. Used alone, copper deactivates rapidly due to itscovering by chlorine atoms. Thanks to its activation power ofhydrogen by dissociative chemisorption, palladium presentin the Pd–Cu alloy supplies hydrogen atoms for the regener-ation of the chlorinated copper surfaces into metallic copper.

Acknowledgements

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

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

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

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