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Journal of Non-Crystalline Solids 352 (2006) 2751–2762

Methods for the preparation of bimetallic xerogel catalysts designedfor chlorinated wastes processing

Stephanie Lambert *, Fabrice Ferauche, Benoıt Heinrichs, Natalia Tcherkassova,Jean-Paul Pirard, Christelle Alie

Laboratoire de Genie Chimique, B6a, Universite de Liege, B-4000 Liege, Belgium

Received 8 July 2005; received in revised form 15 March 2006Available online 6 June 2006

Abstract

The aim of this work is to simplify and generalize the synthesis procedure of bimetallic supported catalysts by sol–gel process. For Pd–Ag/SiO2 co-gelled xerogels catalysts a number of synthesis procedures were compared: use of one or two specific alkoxides able to form achelate with palladium and/or silver cations, reagent mixing in one or two steps, use of industrial grade chemicals instead of laboratorygrade chemicals. The catalysts obtained are quite similar: same metal dispersion, same tailored morphology, same localization andaccessibility of Pd–Ag alloy nanoparticles inside microporous silica, same activity and selectivity for hydrodechlorination of 1,2-dichlo-roethane into ethylene. For catalyst production at large scale the synthesis can be achieved in one step with 3-(2-aminoethyl)aminopro-pyltrimethoxysilane of industrial grade as chelating alkoxide, tetraethylorthosilicate (TEOS) of industrial grade and ethanol denaturedwith diethyl phthalate.� 2006 Elsevier B.V. All rights reserved.

PACS: 81.20.Fw; 82.33.Ln; 61.43.Gt

Keywords: Catalysis; Nuclear and chemical wastes; Silica; Sol–gels (xerogels)

1. Introduction

For more than fifty years it has been well known that thecombination of atoms of a Group VIII metal and a GroupIB metal as well as the combination of two different GroupVIII metals present selectivity effects in catalysis [1]. A highsurface area is generally necessary for a bimetallic catalystto be of practical interest. An effective approach to gener-ate high surface area is the dispersion of the metal on a car-rier. The bimetallic species are generally prepared either byco-impregnation of a carrier with an aqueous solution ofmetal compounds, by co-precipitation method or bysequential precipitation method [1].

0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.03.055

* Corresponding author. Tel.: +32 4 366 4771; fax: +32 4 366 3545.E-mail address: stephanie.lambert@ulg.ac.be (S. Lambert).

More recently monometallic particles finely dispersed ona mineral support were obtained by realising the supportsynthesis by sol–gel process in a solution containing theactive metal species [2–4]. A particularly interesting sol–gel method by co-gelation has been developed to dispersemetal species in a silica matrix [5,6]. This method consistsin using alkoxides of the type (RO)3Si–X–A in which afunctional organic group A able to form a chelate withmetal cation is linked to the hydrolysable silyl group(RO)3Si– via an inert and hydrolytically stable spacer X.The co-condensation of such molecules with a network-forming reagent such as tetraethylorthosilicate, TEOS,results in materials in which the metal is dispersed or evenanchored to the silica matrix.

This sol–gel method by co-gelation seems particularlysuitable to achieve an intimate mixing of two metals inorder to prepare in a single step synthesis bimetallic clusters

2752 S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762

dispersed on a carrier [7–15]. Several choices exist for thesynthesis procedure and for the nature of the metal precur-sors, network-forming reagent and alkoxides able to form achelate. It is necessary to define the simplest and best strat-egy to generate bimetallic particles as well dispersed as pos-sible with a support texture adequate for easy reactantdiffusion and active site accessibility. In order to expandapplications it is essential to adapt the co-gelation methodfor preparing bimetallic xerogels catalysts to facilitate thescaling up of the synthesis procedure. Does each metalrequire its specific alkoxide of the type (RO)3Si–X–A? Dothe two metals need separate mixing with the modified alk-oxide(s) before gelation or can they directly be mixedtogether? What is the influence of the chemicals purity?To answer all these questions a catalyst well known andwell characterized has been chosen: Pd–Ag/SiO2 xerogelcatalysts prepared by co-gelation and active for selectivehydrodechlorination of 1,2-dichloroethane into ethylene[9]. In the latter study, the palladium precursor is palla-dium acetylacetonate (Pd(acac)2) and (RO)3Si–X–A = 3-(2-aminoethyl)aminopropyltrimethoxysilane (EDAS) isused to complex palladium. The silver precursor is silveracetate and (RO)3Si–X–A = 3-(aminopropyl)triethoxysi-lane (AS) is used to complex silver. The study of the disper-sion, localization and accessibility of Pd–Ag crystallites aswell as the study of the texture show that the co-gelationmethod allows obtaining, after drying, calcination andreduction, low-density xerogels catalysts containing smallPd–Ag alloy particles (2–4 nm) that seem to be locatedinside microporous silica particles (10–20 nm) and thatare easily accessible [16].

The two main purposes of this study are to simplify thesynthesis procedure of bimetallic Pd–Ag/SiO2 co-gelledxerogel catalysts described in [9] and the replacement oflaboratory grade reagents by industrial grade reagents soas to investigate how the co-gelation method to preparebimetallic catalysts could be up-scaled to a profitableindustrial process.

Table 1Synthesis operating variables of Pd–Ag/SiO2 co-gelled xerogel catalysts

Catalyst Pd(acac)2

(mmol/l)Ag(OAc)(mmol/l)

EDAS (mmol/l)mixture X

EDAS (mmol/l)mixture Y

EA2LAB33 9.70 4.85 19.44 –EA2LAB50 9.84 9.66 19.60 –EA2LAB67 9.87 19.60 19.85 –

E2LAB33 9.56 4.85 19.16 9.42E2LAB50 9.62 9.56 19.29 19.03E2LAB67 9.86 19.46 19.61 38.71

E1LAB33 9.57 4.75 28.65 –E1LAB50 9.63 9.53 38.39 –E1LAB67 9.81 19.40 58.32 –

E1IND33 9.70 4.80 28.90 –E1IND50 9.70 9.60 38.70 –E1IND67 9.90 19.50 58.90 –

2. Experimental

2.1. Sample designation

Twelve samples containing various amounts of palla-dium and silver were prepared and their synthesis operat-ing variables are presented in Table 1. For each sample,E denote the use of EDAS (NH2–CH2–CH2–NH–(CH2)3–Si(OCH3)3) and A denote the use of AS (NH2–(CH2)3–Si(OC2H5)3) in Pd–Ag/SiO2 co-gelled xerogelcatalysts. Furthermore, two different synthesis methodsare used in this work and they correspond to either asynthesis method in one step, distinguishable in Table 1by the number 1 immediately after the letter E, or a synthe-sis method in two steps, distinguishable in Table 1 by thenumber 2 immediately after the letters E and A. Finally,LAB or IND denote the laboratory or industrial grade ofreagents, followed by the weight percentage of silver withregard to the total weight of metals in the sample.

2.2. Catalyst preparation

The first series of catalysts (EA2LAB33, EA2LAB50,EA2LAB67) was synthesized from reagents with a labora-tory grade according to a synthesis method in two steps asproposed by Heinrichs et al. [9]: for mixture X, a definedamount of EDAS was added to a suspension of insolublepalladium acetylacetonate powder (Pd(CH3COCH@C-(O–)CH3)2, Pd(acac)2) in a quarter of the total volume ofethanol; for mixture Z, a defined amount of AS was addedto a suspension of insoluble silver acetate powder (Ag(CH3-CO(O–)), Ag(OAc)) in another quarter of the total volumeof ethanol. These actions represent the first step of this syn-thesis method. Mixtures X and Z were stirred at ambienttemperature in closed vessels until clear yellow and color-less solutions were obtained for Pd and Ag respectively(about half an hour) after which mixtures X and Z weremixed together. These actions represent the second step

AS (mmol/l)mixture Z

TEOS(mmol/l)

H2O(mmol/l)

NH3

(mmol/l)C2H5OH(mmol/l)

Gel time(min)

9.95 1092 5565 17.64 11138 1519.31 1084 5272 16.70 11224 1539.56 1059 5509 17.46 11164 13

– 1077 5497 17.83 11165 33– 1071 5484 17.78 11165 26– 1045 5458 17.70 11165 24

– 1077 5497 17.83 11165 23– 1071 5484 17.78 11165 20– 1041 5458 17.70 11165 20

– 1087 5543 17.97 11157 34– 1077 5532 17.94 11160 34– 1058 5510 17.87 11167 34

S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762 2753

of this synthesis method. TEOS (tetraethoxysilane,Si(OC2H5)4) was then added. Finally, a solution containingaqueous 0.18 N NH3 in the remaining ethanol, was slowlyadded under vigorous stirring.

The second series of catalysts (E2LAB33, E2LAB50,E2LAB67) is identical to the first, except that in these syn-theses, Ag(OAc) was mixed with EDAS instead AS in aquarter of the total ethanol volume (Mixture Y).

The third series of catalysts (E1LAB33, E1LAB50,E1LAB67) is identical to the second, except that in thesesyntheses, Pd(acac)2 and Ag(OAc) were mixed togetherwith EDAS in half of the total ethanol volume (synthesismethod in one step).

Series 1: EA2LAB

Laboratory grade reagents

Pd+EDAS Ag+AS

Series 2: E2LAB

Laboratory grade reagents

Pd+EDAS Ag+EDAS

Series 3: E1LAB

Laboratory grade reagents

Pd+Ag+EDAS

Series 4: E1IND

Industrial grade reagents

Pd+Ag+EDAS

Fig. 1. Evolution scheme of synthesis method of Pd–Ag/SiO2 catalysts.

The fourth series of catalysts (E1IND33, E1IND50,E1IND67) is identical to the third, except that in these syn-theses, the grade of reagents is industrial. The industrialEDAS used is known as Dynasilan DAMO (ABCR,1760-24-3) and the industrial TEOS used is the productnamed Dynasil (ABCR, 78-10-4). The denaturing agentof the industrial ethanol used was diethyl phthalate(0.5%). It was necessary to choose a denatured alcohol witha very low content in water because water would prema-turely induce the hydrolysis of the alkoxides.

In Fig. 1, the successive modifications of the synthesismethod of Pd–Ag/SiO2 catalysts co-gelled xerogel catalystsare represented.

For all samples, the hydrolysis ratio, that is the molarratio H = [H2O]/([TEOS] + 3/4[EDAS] + 3/4[AS]), andthe dilution ratio, that is the molar ratio R = [ethanol]/([TEOS] + [EDAS] + [AS]) were kept constant at 5 and10, respectively, as proposed by Heinrichs et al. [9]. Themolar ratios EDAS/Pd(acac)2, EDAS/Ag(OAc) and AS/Ag(OAc) were chosen to have a numerical value of 2 asexplained in [9,17]. For all samples, after the addition ofthe last reagent, the vessel was then tightly closed andheated up to 70 �C for 3 days (gelling and aging [18]).The gel time of all samples are presented in Table 1 andis defined as the time elapsed between the introduction ofthe last reactive component to the solution and gelationat 70 �C. It is measured at the moment when the liquidno longer flows when the flask is tipped at an angle of 45�.

After the stay at 70 �C for 3 days, the wet gels were driedunder vacuum according to the following procedure: theflasks were opened and put into a drying oven at 80 �C,and the pressure was slowly decreased to the minimumvalue of 1200 Pa after 90 h to prevent gel bursting. The dry-ing oven was then heated at 150 �C for 72 h. The resultingsamples are xerogels [18].

The conditions of calcination and subsequent reductionfor all samples were as follows. In the calcination step, thesample was heated up to 400 �C at a rate of 120 �C/h underflowing air (0.02 mmol s�1); this temperature was main-tained for 12 h in air (0.1 mmol s�1). In the reduction step,the catalyst was heated to 350 �C at a rate of 350 �C/hunder flowing H2 (0.23 mmol s�1) and maintained at thistemperature for 3 h (same flow).

2.3. Catalyst characterization

Apparent densities [19] were measured by helium pyc-nometry on a Micromeritics AccuPyc 1330. Nitrogenadsorption–desorption isotherms were measured at 77 Kon a Fisons Sorptomatic 1990 after outgassing for 24 h atambient temperature. After a 2-h outgassing at ambienttemperature, mercury porosimetry measurements were per-formed with sample monoliths using a manual porosimeterfrom 0.01 to 0.1 MPa and a Carlo Erba Porosimeter 2000from 0.1 to 200 MPa.

The sizes of SiO2 particles and metal particles wereexamined by transmission electron microscopy (TEM).

2754 S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762

TEM analyses were performed on a Philips CM100 micro-scope. All the samples were impregnated with an epoxyresin (EPON 812) to which an amine was added to serveas a hardener. Hardening went on for 72 h at the tempera-ture of 60 �C, and then 60-nm slices were cut up with aReichert-Jung Ultracut E microtome. Finally, these sliceswere put on a copper grid.

X-ray diffraction (XRD) was used to determine metalparticles’ sizes. Furthermore, the bulk composition ofmetal particles was measured from XRD patterns by usingVegard’s law [20]. Patterns were obtained with hand-pressed samples mounted on a Siemens D5000 goniometerusing the Cu-Ka line (Ni filter).

The composition of the metallic particles was also exam-ined by scanning transmission electron microscopy coupledwith energy dispersive X-ray spectroscopy (STEM-EDX).STEM-EDX analyses were performed on a Siemens Elmi-skop 102 transmission electron microscope. Samples wereprepared by means of impregnation of xerogel catalystswith an epoxy resin (Europox 710) to which an aminewas added to serve as a hardener. Hardening went on for48 h, after which a 60-nm slice was cut up with a ReichertSupernova ultramicrotome.

Palladium dispersion was determined from CO chemi-sorption at 30 �C on a Fisons Sorptomatic 1990 device.Before measurements, the calcined sample was reducedin situ in flowing H2 (0.003 mmol s�1) at 350 �C for 3 hfor all Pd–Ag/SiO2 catalysts. Each sample was then out-gassed under vacuum at 340 �C for 16 h for all Pd–Ag/SiO2 catalysts. A double adsorption method was used asdescribed by Heinrichs et al. [21]. Palladium dispersionwas also determined from H2 chemisorption at 30 �C ona Fisons Sorptomatic 1990 device. This technique wasgiven up because a spillover phenomenon appeared overPd–Ag/SiO2 co-gelled xerogel catalysts, which overesti-mated the palladium dispersion [22].

2.4. Catalytic experiments

All Pd–Ag/SiO2 co-gelled xerogel catalysts were testedfor 1,2-dichloroethane hydrodechlorination, which wasconducted in a stainless steel tubular reactor (internal diam-eter: 8 mm) at a pressure of 0.3 MPa. The reactor wasplaced in a convection oven. A constant flow of each reac-tant was maintained by a Gilson piston pump for CH2Cl–CH2Cl and Brooks mass flow controllers for H2 and He.The effluent was analyzed by gas chromatography (Ther-moFinnigan with FID) using a Porapak Q5 packed column.

Prior to each experiment, all Pd–Ag/SiO2 co-gelled xero-gel catalysts were reduced in situ at atmospheric pressure inflowing H2 (0.023 mmol s�1) while being heated to 350 �Cat a rate of 350 �C/h and were maintained at this tempera-ture for 3 h. After reduction, all Pd–Ag/SiO2 co-gelledxerogel catalyst were cooled in flowing H2 to the desiredinitial reaction temperature of 200 �C.

For each catalytic experiment, 0.11 g of catalyst pellets,sieved between 250 and 500 lm, were tested. The total flow

of the reactant mixture was 0.45 mmol s�1 and consisted ofCH2Cl–CH2Cl (0.011 mmol s�1), H2 (0.023 mmol s�1), andHe (0.42 mmol s�1). The temperature was successively keptat 200, 250, 300, 350 and back to 300 �C to check a possibledeactivation of the catalyst with time. The effluent was ana-lyzed every 15 min.

3. Results

3.1. Metal loading

The actual metal loading in co-gelled catalysts is generallyhigher than theoretical loading because the actual catalystmass after gelling, aging, vacuum drying, calcination andreduction steps, is lower than theoretical mass. This isexplained by the fact that some TEOS and/or EDAS and/or AS often remain unreacted and are volatilized during dry-ing [23]. This theoretical mass (mth) is calculated from Eq. (1)

mth ¼ nPdðMMPdÞ þ nAgðMMAgÞþ ðnTEOS þ nEDAS þ nASÞðMMSiO2

Þ; ð1Þ

where nPd is the amount of palladium in the gel (mmol);MMPd is the palladium atomic weight, 106.42 g mol�1;nAg is the amount of silver in the gel (mmol); MMAg is thesilver atomic weight, 107.868 g mol�1; nTEOS, nEDAS andnAS are respectively the amount of TEOS, EDAS and ASin the gel (mmol); MMSiO2

is the molecular weight of SiO2,60.085 g mol�1. In this equation, it is assumed that allTEOS, EDAS and AS molecules are converted into SiO2.

Results in Table 2 clearly show that for all series, theweight loss decreases when the silver loading increases inall Pd–Ag/SiO2 co-gelled xerogel catalysts. The atomicratio Ag/(Pd + Ag) is also presented in Table 2. For Pd–Ag/SiO2 co-gelled xerogel catalysts, the atomic ratio Ag/(Pd + Ag) is equal to the weight ratio, because Pd andAg have nearly identical atomic weights.

3.2. Catalytic experiments

It has been shown in a previous study [16] that the veryparticular structure of bimetallic co-gelled xerogel catalystsallows avoiding diffusional limitations. Indeed, in Pd–Ag/SiO2 co-gelled xerogel catalysts, in order to reach activesites, reactants must first diffuse through large poreslocated between aggregates of SiO2 particles and thenthrough smaller pores between those elementary particlesinside the aggregates. Finally, they diffuse through microp-ores inside the silica particles. It was shown that there is noproblem of mass transfer at each of the three levels. TheWeisz modulus, which compares the observed reaction rateto the diffusion rate, has a value much smaller than 1 atthree discrete levels (macroscopic pellet, aggregate of silicaparticles, elementary silica particle containing an activemetal crystallite). So there are no pore diffusion limitationsand the observed rate r is equal to the intrinsic rate of thechemical reaction.

Table 2Pd and Ag loadings in Pd–Ag/SiO2 co-gelled xerogel catalysts

Catalyst Theoretical metal loading Weight lossa (wt%) Actual metal loading

Pd (wt%) Ag (wt%) Ag/(Pd + Ag) (at.%) Pd (wt%) Ag (wt%) Ag/(Pd + Ag) (at.%)

EA2LAB33 1.5 0.75 33 32 2.2 1.1 33EA2LAB50 1.5 1.5 50 35 2.3 2.2 49EA2LAB67 1.5 3.0 67 21 1.9 3.7 66

E2LAB33 1.5 0.75 33 28 2.1 1.1 34E2LAB50 1.5 1.5 50 20 1.9 1.9 50E2LAB67 1.5 3.0 67 4 1.6 3.2 66

E1LAB33 1.5 0.75 33 22 1.9 1.0 33E1LAB50 1.5 1.5 50 20 1.9 1.9 50E1LAB67 1.5 3.0 67 6 1.6 3.2 66

E1IND33 1.5 0.75 33 32 2.2 1.1 33E1IND50 1.5 1.5 50 24 1.9 1.9 50E1IND67 1.5 3.0 67 10 1.7 3.3 67

a Weight loss = 100 (mth � ma)/mth, where mth is the theoretical mass and ma is the actual catalyst mass measured after vacuum drying, calcination andreduction steps.

0

20

40

60

80

100

0 5 10 15 20

Time (h)

Con

vers

ion,

sel

ecti

viti

es

(mol

%)

150

200

250

300

350

400T

emperature (˚C

)

(a)

0

20

40

60

80

100

0 5 10 15 20Time (h)

Con

vers

ion,

sel

ecti

viti

es

(mol

%)

150

200

250

300

350

400

Tem

perature (˚C)

(b)

Fig. 2. 1,2-Dichloroethane hydrodechlorination of samples (a) E1IND33,and (b) E1IND67. (d) ClCH2–CH2Cl conversion, (·) C2H4 selectivity, (h)C2H6 selectivity, (—) Temperature.

S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762 2755

In Fig. 2, conversion as well as C2H6 and C2H4 selectiv-ities are shown as a function of time and temperature oversamples E1IND33 and E1IND67, as examples. At the startof each catalytic test, small amounts of C2H5Cl are alsoobserved with all Pd–Ag/SiO2 co-gelled xerogel catalysts,

but after 2 h, C2H5Cl selectivity is equal to 0 mol%.Although for each temperature, conversion does not pres-ent exactly constant value over time, mean value of 1,2-dichloroethane conversion is given at each temperature inTable 3 for all Pd–Ag/SiO2 co-gelled xerogel catalysts. Fur-thermore, in Fig. 2, the examination of conversion curvesshows that a deactivation, which is faster when the temper-ature increases from 200 to 350 �C, is observed with allsamples. Nevertheless, this deactivation becomes slowerwhen the Ag loading is increased and is scarcely distin-guishable with samples EA2LAB67, E2LAB67, E1LAB67and E1IND67. Finally, all bimetallic catalysts clearly showthat C2H4 selectivity increases, not only with temperature,but also with time. This increase corresponds to an induc-tion period during which an eventual rearrangement of thePd–Ag alloy surface probably occurs. After this inductionperiod, ethylene selectivity depends only to temperature.For all further discussion in this paper, the induction per-iod will not be considered. In Table 3, only C2H4 selectivityafter 12 h of the start of the catalytic test will be presented.

In Table 3, it is observed that all samples are active for1,2-dichloroethane hydrodechlorination between 200 and350 �C and that C2H4 selectivity depends on the silver con-tent in bimetallic catalysts. So for all Pd–Ag/SiO2 co-gelledxerogel catalysts of each series, increasing silver content inbimetallic catalysts results in an increase in ethylene selec-tivity, and for samples EA2LAB50, EA2LAB67, for sam-ple E1LAB67, and for sample E1IND67, this selectivityreaches 100% in the conditions of the catalytic test. In allseries, conversion of 1,2-dichloroethane decreases at eachtemperature when the silver loading is increased.

Although it has been demonstrated in a previous study[24] that C2H4 and HCl influence kinetic results andalthough these both products are not present at the reactorinlet, the consumption rate of 1,2-dichloroethane, r, is cal-culated in first approximation from chromatographic mea-surements of C2H6 and C2H4 concentrations in the reactoreffluent and from the differential reactor equation when

Table 3Kinetic data for 1,2-dichloroethane hydrodechlorination in Pd–Ag/SiO2 co-gelled xerogel catalysts

Catalyst Conversion (mol%) ±8% C2H4 selectivity after12 h (mol%)

r after 12 h ðmmol kg�1cat s�1Þ ±8%

200 �C 250 �C 300 �C 350 �C 300 �C

EA2LAB33 2 4 16 32 12 60 ± 2 14EA2LAB50 2 2 8 22 8 100 10EA2LAB67 –a 2 6 18 6 100 6

E2LAB33 4 10 26 50 18 60 ± 2 20E2LAB50 2 4 10 32 10 88 ± 1 12E2LAB67 –a 2 10 30 10 98 ± 1 10

E1LAB33 2 8 24 46 18 64 ± 2 20E1LAB50 2 6 18 42 16 90 ± 1 18E1LAB67 –a 2 10 26 10 100 10

E1IND33 2 4 16 32 14 58 ± 2 16E1IND50 2 2 10 28 10 98 ± 1 12E1IND67 –a 2 8 26 8 100 8

r: Consumption rate after 12 h at 300 �C of 1,2-dichloroethane reported to catalyst mass.a Not observed.

35 40 45 50 552θ (degree)

Inte

nsit

y (a

.u.)

Ag(111)

Pd(111)

Ag(200)

Pd(200)

(a)

(b)

(c)

Fig. 3. X-ray diffraction patterns of samples (a) E1LAB33, (b) E1LAB50,and (c) E1LAB67.

35 40 45 50 552θ (degree)

Inte

nsit

y (a

.u.)

Ag(111)

Pd(111)

Ag(200)

Pd(200)

(a)

(b)

(c)

Fig. 4. X-ray diffraction patterns of samples (a) E1IND33, (b) E1IND50and (c) E1IND67.

2756 S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762

1,2-dichloroethane conversions are <20%. This equation iswritten as follows:

r ¼ F A þ F E

WðF A0 and F E0 ¼ 0Þ; ð2Þ

where r is the consumption rate ðmmol kg�1cat s�1Þ, FA is the

molar flowrate of ethane at the reactor outlet (mmol s�1),FA0 is the molar flowrate of ethane at the reactor inlet(mmol s�1), FE is the molar flowrate of ethylene at the reac-tor outlet (mmol s�1), FE0 is the molar flowrate of ethyleneat the reactor inlet (mmol s�1) and W is the catalyst massinside the reactor (kgcat). For all Pd–Ag/SiO2 co-gelledxerogel catalysts, r has been calculated from Eq. (2) at300 �C after 12 h because 1,2-dichloroethane conversionsare <20%. These results are presented in Table 3. It is ob-served that these results are very similar for the four series,namely that conversion and consequently r increase withtemperature and that conversion and consequently r de-crease when the silver loading increases in Pd–Ag/SiO2

co-gelled xerogel catalysts.

3.3. Size and localization of metal particles, bulk and

surface compositions of Pd–Ag alloy particles

3.3.1. X-ray diffraction (XRD)

X-ray diffraction is used to evaluate the size of metalcrystallites. That technique also allows determining thepresence and the bulk composition of alloy crystallites. Ina previous work, Heinrichs et al. [21] used this techniqueto evaluate the size and the bulk composition of alloy crys-tallites for the samples EA2LAB33, EA2LAB50 andEA2LAB67. In this study, the results for series 2, 3 and 4are very similar to these one of the samples EA2LAB33,EA2LAB50 and EA2LAB67. So as examples, Fig. 3 showsthe diffractograms obtained for samples E1LAB33,E1LAB50, E1LAB67 and Fig. 4 shows the diffractogramsobtained for samples E1IND33, E1IND50, E1IND67.Between the (111) Bragg lines of either Pd and Ag for sam-

ples E1LAB33, E1LAB50, E1LAB67, and samplesE1IND33, E1IND50, E1IND67, all these samples exhibit

Table 4Sizes of metal particles, bulk and surface compositions of Pd–Ag alloy particles

Catalyst TEM XRD Chemisorption xPds (at.%)

dTEM1

(nm)r TEM1

(nm)dTEM2

(nm)rTEM2

(nm)ds

(nm)DPd–Ag

(%)dXRD1

(nm)dXRD2

(nm)[Ag/(Pd + Ag)]XRD

(at.%)DPd (%)

EA2LAB33 2.4 0.7 9.3 0.3 2.8 41 1.9 >30 31 20 ± 2 30 ± 5EA2LAB50 3.0 0.7 6.8 0.8 3.3 35 2.1 >30 43 9 ± 1 15 ± 3EA2LAB67 3.0 1.2 9.8 1.8 3.8 30 2.0 18 46 7 ± 1 12 ± 2

E2LAB33 2.3 0.4 5.8 0.4 2.4 47 1.9 –b 28 22 ± 2 35 ± 5E2LAB50 2.2 0.3 8.1 1.1 2.3 49 2.0 11.7 39 18 ± 2 20 ± 4E2LAB67 1.9 0.2 9.6 0.8 2.1 54 1.9 11.4 46 12 ± 1 12 ± 2

E1LAB33 2.3 0.3 –a –a 2.4 47 2.1 –a 29 24 ± 2 35 ± 5E1LAB50 2.2 0.4 5.4 0.5 2.2 52 2.3 –b 43 22 ± 2 24 ± 4E1LAB67 2.0 0.2 8.2 0.7 2.2 52 2.3 9.1 47 12 ± 1 12 ± 2

E1IND33 2.5 0.5 8.8 0.7 2.7 42 2.6 9.2 30 22 ± 2 35 ± 5E1IND50 2.5 0.3 6.3 1.2 2.6 43 2.4 9.2 36 14 ± 1 20 ± 4E1IND67 2.6 0.3 5.7 1.1 2.8 41 2.6 10.4 40 8 ± 1 12 ± 2

dTEM1, dTEM2: Mean diameters of small and large metal particles, respectively, measured by TEM; rTEM1, rTEM2: standard deviations associated withdTEM1 and dTEM2, respectively; ds, DPd–Ag: mean surface diameter and overall metal dispersion of small metal particles estimated from TEM; dXRD1 anddXRD2: mean sizes of small and large metal particles estimated from X-ray line broadening; [Ag/(Pd + Ag)]XRD: atomic ratio or bulk compositiondetermined from XRD; DPd: palladium dispersion measured by CO chemisorption; xPds

: fraction of Pd atoms present at the surface of Pd–Ag alloyparticles estimated from the combination of CO chemisorption, XRD and TEM results.

a Not observed.b Not measurable.

Fig. 5. TEM micrograph of sample E1IND67 (500000·).

S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762 2757

a broad peak, which demonstrates the presence of a solidsolution. Furthermore, for sample E1LAB67 (Fig. 3(c)),and samples E1IND33, E1IND50, E1IND67 (Fig. 4), thepresence of some unalloyed pure silver particles is observedby the presence of two very small peaks that is characteris-tic of pure silver. Finally, the composition of the solid solu-tion was calculated from the unit cell parametercorresponding to the broad peak between the (111) Bragglines of Pd and Ag (Vegard’s law) [20]. Results in Table 4indicate for the four series that as the silver loadingincreases, the atomic ratio or bulk composition of alloyparticles, [Ag/(Pd + Ag)]XRD, deviates more and morefrom the atomic ratio calculated from mass balances (Table2). This observation is the result that a few large silver par-ticles are present in Pd–Ag/SiO2 co-gelled xerogel catalystswhen the silver loading increases.

With the assumption that all the bimetallic particles ofone sample have the same Pd–Ag composition, a meansize, dXRD, has been calculated from Scherrer’s formulabased on line broadening analysis (Table 4) [21]. In allthe samples, bimetallic particles are found to be finely dis-persed (dXRD1), whereas pure silver particles present in sev-eral samples are much larger (dXRD2).

3.3.2. Transmission electron microscopy (TEM)

The mean diameter of small crystallites, dTEM1, is thearithmetic mean of 50 diameters of small particles mea-sured on TEM micrographs, an example of which is givenin Fig. 5 for sample E1IND67. Values are presented inTable 4. When large crystallites are present, their numberis much smaller than for small crystallites and their meandiameter, dTEM2, is then estimated from an average of

two to twenty crystallites depending on the sample. Theresults obtained from the examination of TEM micro-graphs of all Pd–Ag/SiO2 co-gelled xerogel catalysts aresimilar. So all samples exhibit metal particles distributedin two families of different sizes: small crystallites between1.9 and 3.0 nm and some large crystallites between 5.4 and9.8 nm (Table 4). So from XRD measurements and TEMobservations, it is concluded that Pd–Ag alloy particleswould be small crystallites (dXRD1, dTEM1, Table 4) andpure silver particles present in several samples would belarge crystallites (dXRD2, dTEM2, Table 4).

From TEM measurements, it is possible to calculate theoverall metal dispersion, DPd–Ag, that is, the ratio betweenthe number of metal atoms at the surface of the Pd–Ag

2758 S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762

alloy particles and the total number of metal atoms in thoseparticles. Let us immediately specify that we consider hereonly palladium and silver atoms, which are present in alloyparticles in all Pd–Ag/SiO2 co-gelled xerogel catalysts. DPd–

Ag is given by Eq. (3) [25]

DPd–Ag ¼6ðvm=amÞ

ds

ð3Þ

with

ds ¼P

nid3iP

nid2i

; ð4Þ

where vm is the mean volume occupied by a metal atom inthe bulk of the alloy (nm3), am is the mean surface areaoccupied by a surface metal atom (nm2), ds is the mean sur-face diameter of metal particles (nm), di is the metal parti-cles diameter (nm) and ni is the number of metal particlesof a given diameter di. For palladium and silver, the valuesof vm are 0.01470 and 0.01706 nm3, respectively, and thevalues of am are 0.0793 and 0.0875 nm2 respectively [25].For each bimetallic catalyst, the mean surface diameter,ds, is calculated from the diameters di of the same 50 metalparticles, already used to calculate the arithmetic meandiameter, dTEM1, and measured on TEM micrographs. Val-ues of ds are presented in Table 4. From these values, theoverall metal dispersion, DPd–Ag, of alloy crystallites is cal-culated by means of Eq. (3) and given in Table 4. The val-ues taken for vm and am are weighted means calculated byusing the atomic ratio of each metal in the alloy particlesderived from XRD (explained above) as weight factors.

Concerning localization of metal crystallites (Fig. 5), itappears that all Pd–Ag/SiO2 co-gelled xerogel catalystsare composed of elementary silica particles arranged inpearls or aggregates [26–28], and although TEM gives onlya 2D view, it seems that small metal particles are locatedinside silica particles, whereas large metal are located attheir surface. This silica matrix structure and this metalparticles’ localization were also observed in previous stud-ies in the case of monometallic Pd/SiO2, Ag/SiO2, Cu/SiO2

and bimetallic Pd–Ag/SiO2 and Pd–Cu/SiO2 co-gelledaerogel and xerogel catalysts [9,15,17,29].

3.3.3. Chemisorption measurements

CO chemisorption on monometallic palladium catalystsis a well-known phenomenon widely used to measure palla-dium dispersion [22]. According to the literature, pure silverdoes not chemisorb CO [22]. Since, in monometallic Pd/SiO2

and Ag/SiO2 co-gelled xerogel catalysts, CO chemisorptionoccurs on palladium, but not on silver, it will be assumedthat on the surface of bimetallic particles in Pd–Ag/SiO2

co-gelled xerogel catalysts, CO chemisorption occurs on pal-ladium atoms only. This hypothesis is supported for Pd–Ag/SiO2 catalysts by Soma-Noto and Sachtler’s work [30], inwhich no IR absorption band that would correspond to apossible bond between carbon monoxide and silver isdetected, either on pure Ag or on Pd–Ag alloys.

In all Pd–Ag/SiO2 co-gelled xerogel catalysts, the silverbulk composition, [Ag/(Pd + Ag)]XRD (Table 4), is higherthan 25 at.%. In a previous work [30], it was demonstratedfrom IR spectra of CO adsorbed on Pd–Ag alloy particlesof various bulk compositions, that the introduction of silverin palladium strongly reduces the presence of multicenterCO in relation to linear CO, which becomes almost the onlyspecies beyond 25 at.% of silver in the bulk of the alloy. Soin this work, it can then be admitted that CO is chemisorbedon palladium in the linear form only. As a consequence, thechemisorption mean stoicheometry, XCO = 1, will be usedfor the calculation of palladium dispersion, DPd, in Pd–Ag/SiO2 co-gelled xerogel catalysts (Table 4).

In each series, it is observed that DPd decreases when thesilver loading increases, indicating an enrichment in silverfor Pd–Ag alloy particles’ surface according to the litera-ture [21,22,30]. So the bimetallic surfaces of Pd–Ag/SiO2

co-gelled xerogel catalysts of each series are more and moreenriched with Ag to the detriment of Pd when the silverloading increases, as will be explained below.

3.3.4. Surface composition of Pd–Ag alloy particlesBefore to explain the calculation of the surface compo-

sition of Pd–Ag alloy particles in bimetallic co-gelled xero-gel catalysts, several hypotheses must be put forward. First,it is considered here only palladium and silver atoms, whichare present in alloy particles in all bimetallic samples. Silveratoms present in pure Ag particles are not taken intoaccount in the developments below. Moreover, it isassumed that palladium is present only in the form of aPd–Ag alloy. Furthermore, it is assumed that on the sur-face of bimetallic particles in Pd–Ag/SiO2 co-gelled xerogelcatalysts, CO chemisorption occurs on palladium atomsonly. This hypothesis is supported for Pd–Ag/SiO2 cata-lysts by Soma-Noto and Sachtler’s work [30]. Finally, forthe determination of the bulk composition of alloy parti-cles by XRD, it is assumed that all the bimetallic particlesof one sample have the same Pd–Ag bulk composition.This hypothesis is also supported by the fact that whenthe mean size of bimetallic particles, dXRD1, is calculatedby XRD from Scherrer’s formula based on line broadeninganalysis (Table 4) [20], the size of bimetallic particles,dXRD1, presents similar values than dTEM1, that is the meandiameter of small metal particles measured by TEM. So thevery wide peaks between the (111) Bragg lines of either Pdand Ag in Figs. 3 and 4 are due only to the small size of thebimetallic particles and no to a distribution of composition.

Pd–Ag alloy particles’ surface composition is by defini-tion the fraction xPds of palladium atoms present at the sur-face of Pd–Ag alloy particles,

xPds ¼nPds

nPds þ nAgs

; ð5Þ

where nPds is the number of Pd atoms at the surface of Pd–Ag alloy particles and nAgs

is the number of Ag atoms at thesurface of Pd–Ag alloy particles. Eq. (5) of xPds can bedeveloped as

0.01

0.1

1

10

0.1 1 10 100 1000

Pore size (nm)

Cum

ulat

ive

volu

me

(cm

³/g)

(a)

0.01

0.1

1

10

0.1 1 10 100 1000

Pore size (nm)

Cum

ulat

ive

volu

me

(cm

³/g)

(b)

Fig. 6. Pore size distributions of (a) samples EA2LAB33 (h), EA2LAB50(e), EA2LAB67 (n), and samples E2LAB33 (j), E2LAB50 (�),E2LAB67 (m), of (b) samples E1LAB33, E1LAB50, E1LAB67 (h), andsamples E1IND33, E1IND50, E1IND67 (j).

S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762 2759

xPds ¼nPds

nPd

nPd

nPd þ nAg

nPd þ nAg

nPds þ nAgs

¼ DPdxPd

1

DPd–Ag

¼ DPdð1� ½Ag=ðPdþAgÞ�XRDÞ1

DPd–Ag

; ð6Þ

where nPd is the total number of Pd atoms in Pd–Ag alloyparticles and nAg is the total number of Ag atoms in Pd–Agalloy particles [21]. In Eq. (6), the first factor, nPds=nPd, isthe palladium dispersion, DPd, that is, the ratio betweenthe number of surface Pd atoms and the total number ofPd atoms in the catalyst. It is determined by CO chemi-sorption measurements as explained above (Table 4). Thesecond factor, nPd/(nPd + nAg), is the fraction xPd of Pdatoms in the Pd–Ag alloy particles. This fraction corre-sponds to the bulk composition of alloy particles, deter-mined by XRD. So xPd = 1 � [Ag/(Pd + Ag)]XRD (Table4). The third factor, (nPd þ nAgÞ=ðnPds þ nAgs

), is the inverseof the overall metal dispersion, DPd–Ag, of the alloy parti-cles with no distinction between palladium and silveratoms. So DPd–Ag is the ratio between the number of metalatoms at the surface of Pd–Ag alloy particles and the totalnumber of metal atoms in those particles. DPd–Ag has alsobeen measured from TEM experiments as explained above,and the values are given in Table 4.

Values obtained for the surface composition, xPds , arepresented in Table 4. A very marked impoverishment inpalladium and hence a very marked enrichment in silverfor Pd–Ag alloy particles’ surface compared to their bulkis observed for each series of Pd–Ag/SiO2 co-gelled xerogelcatalysts.

3.4. Textural properties

The catalysts apparent density, qapp, which is the densityincluding closed pores according to IUPAC [19], is veryclose to the true density, which is the density excludingpores according to IUPAC, of dried alkoxy-derived silica

Table 5Textural properties of Pd–Ag/SiO2 co-gelled xerogel catalysts

Catalyst qapp (g/cm3)± 0.01

VHg (cm3/g)± 0.05

Vcum<7.5 nm (cm3/g)± 0.01

EA2LAB33 2.14 4.30 0.07EA2LAB50 2.19 4.70 0.06EA2LAB67 2.22 6.10 0.04

E2LAB33 2.15 2.80 0.08E2LAB50 2.18 2.45 0.08E2LAB67 2.21 2.05 0.09

E1LAB33 2.13 3.20 0.05E1LAB50 2.16 3.40 0.06E1LAB67 2.20 2.60 0.05

E1IND33 2.17 3.70 0.09E1IND50 2.20 3.55 0.08E1IND67 2.24 3.35 0.07

qapp: Apparent density measured by helium pycnometry; VHg: specific pore volpores of diameter between 2 and 7.5 nm determined by Broekhoff-de-Boer theorVcum<7.5 nm and Vm; SBET: specific surface area obtained by BET method; dSiO

gels, that is 2.2 g cm�3 [31] (Table 5). Nevertheless, in eachseries, qapp increases with metal loading because metal den-sity is greater than silica density.

Vm (cm3/g)± 0.01

Vv (cm3/g)± 0.1

SBET (m2/g)± 5

dSiO2ðnmÞ

�1

0.13 4.5 380 150.11 4.9 290 180.12 6.3 320 18

0.11 3.0 435 140.11 2.6 425 120.12 2.3 445 10

0.10 3.4 395 140.10 3.6 395 130.11 2.8 400 11

0.15 3.9 400 170.17 3.8 395 150.16 3.6 395 11

ume measured by mercury porosimetry; Vcum<7.5 nm: cumulative volume ofy; Vm: microporous volume; Vv: pore volume obtained by addition of VHg,

2: silica particle diameter measured by TEM.

2760 S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762

In Table 5, the total pore volume, Vv, that is, the porevolume obtained by addition [9,26] of pore volume mea-sured by mercury porosimetry, VHg, cumulative volumeof mesopores of widths between 2 and 7.5 nm measuredby nitrogen adsorption–desorption, Vcum<7.5 nm, and vol-ume of micropores, Vm, does not present strong variationsin each series except to in samples EA2LAB33,EA2LAB50, EA2LAB67, in which Vv increases as the sil-ver content and thus the AS content increases in those sam-ples. Furthermore, in all series, the specific surface areaobtained by the BET method, SBET, does not displaystrong variations with the silver loading (Table 5).

Fig. 6 shows the evolution of the cumulative volume dis-tributions over the entire pore size range for all Pd–Ag/SiO2 co-gelled xerogel catalysts. These curves wereobtained by applying a combination of various methodsto their respective validity domains and by adding the por-ous volume distributions corresponding to each domain[9,26]. All bimetallic catalysts are characterized by a steepvolume increase around 0.8 nm followed by a plateau. Inthe range of meso- and macropores, one observes that allsamples exhibit a broad distribution. In Fig. 6(b), the dis-tribution curves of all Pd–Ag/SiO2 co-gelled xerogel cata-lysts synthesized in one step from laboratory andindustrial EDAS, are very similar.

Texture and morphology of Pd–Ag/SiO2 co-gelled xero-gel catalysts have also been examined by TEM and the sizeof SiO2 elementary particles, dSiO2

, has been evaluated(Fig. 5). Sizes given in Table 5 represent the arithmeticmean on 50 particles. It is observed that dSiO2

increasesslightly as the silver loading and thus the amount of ASin the catalysts increases for samples EA2LAB33,EA2LAB50 and EA2LAB67 (series 1), whereas dSiO2

decreases slightly as the silver loading increases for series2, 3 and 4 prepared with EDAS only (Table 5).

4. Discussion

The purposes of this study are to simplify the synthesisprocedure of bimetallic Pd–Ag/SiO2 co-gelled xerogel cata-lysts described in [9] and to investigate how the co-gelationmethod for the preparation of bimetallic Pd–Ag/SiO2 cat-alysts could be scaled up to expand applications. Thus, inthis work, it is shown that the successive modifications ofthe synthesis method of Pd–Ag/SiO2 co-gelled xerogel cat-alysts, as represented in Fig. 1, do not influence catalyticand physico-chemical properties of bimetallic catalysts.So by comparison of the four series, it is observed thatall the results are very similar for the surface compositionof Pd–Ag alloy particles, the catalytic properties and thetextural properties.

4.1. Textural properties

In Table 5, the total pore volume, Vv, the specific surfacearea obtained by BET method, SBET, and the size of SiO2

elementary particles, dSiO2, do not present strong variations

in each series except to in samples EA2LAB33,EA2LAB50, EA2LAB67, in which Vv increases as the sil-ver content and thus the AS content increases in those sam-ples [15,29].

4.2. Surface composition of Pd–Ag alloy

For all samples, the metal crystallites with a size from1.9 to 3.0 nm (dTEM1, Table 4) seem to be located insidethe silica particles (Fig. 5). The latter ones exhibit a mono-disperse micropore distribution centered on a pore width ofabout 0.8 nm (Fig. 6). So this characteristic width of themicropores is smaller than the diameter of the metal parti-cles. A few larger metal particles (dTEM2) are also observedat the surface of the SiO2 particles in all Pd–Ag/SiO2 co-gelled xerogel catalysts except for sample E1LAB33. Theformation of a Pd–Ag alloy is demonstrated by XRD,but this technique also shows the presence of some largerpure silver crystallites (Figs. 3 and 4). Moreover, theatomic ratio or bulk composition determined from XRDdiffractograms, [Ag/(Pd + Ag)]XRD, is smaller than theatomic ratio calculated from mass balances (Tables 2 and4). So the large particles detected by TEM and XRD wouldcorrespond, according to XRD diffractograms, to pure sil-ver. All these results lead to the conclusion that the smallmetal crystallites located inside silica particles would bePd–Ag alloy crystallites in all Pd–Ag/SiO2 co-gelled xero-gel catalysts, whereas large metal particles situated outsidethe silica matrix would consist of pure silver. The conclu-sion arising from the comparison of the TEM and XRDresults was confirmed by an additional STEM-EDX ofsample EA2LAB67. Due to their sufficient size, the focus-ing of the electron beam on individual large particles waspossible and showed that they are composed of pure silver.On the contrary, it was not possible to focus the beam onindividual small nanoparticles. Only results of groups ofsmall nanoparticles could be obtained. Despite the veryweak intensity of the signal compared to the backgroundnoise, X-rays emitted by such groups were characteristicof Pd and Ag in sample EA2LAB67.

The surface enrichment with silver for Pd–Ag alloy par-ticles presented in Table 4 for each series is in agreementwith the theoretical prediction according to which, at ther-modynamic equilibrium, alloys forming a solid solution(completely miscible metals) exhibit, when under vacuum,a surface enriched with the metal having the lowest surfaceenergy [22]. In the case of the Pd–Ag alloy, a surfaceenrichment 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 to be expected and is confirmed by severalauthors [21,32]. Values of the surface composition, xPds ,for all Pd–Ag/SiO2 co-gelled xerogel catalysts, presentedin Table 4 are very similar and are in agreement with resultscalculated from Auger and IR spectroscopies for variousPd–Ag alloys [30].

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50

x Pds (%at.)

TO

F (

s-1)

Fig. 7. TOF as a function of the fraction of palladium atoms present atthe surface of Pd–Ag alloy particles, xPds , at T = 300 �C after 12 h forsamples EA2LAB33, EA2LAB50, EA2LAB67 (·), for samples E2LAB33,E2LAB50, E2LAB67 (e), for samples E1LAB33, E1LAB50, E1LAB67(n), and for samples E1IND33, E1IND50, E1IND67 (s).

S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762 2761

4.3. Hydrodechlorination of 1,2-dichloroethane over

Pd–Ag/SiO2 co-gelled xerogel catalysts

In Table 3, it is observed for all the series that ethyleneselectivity increases with silver content, and for bimetallicsamples EA2LAB50, EA2LAB67, for sample E1LAB67and for sample E1IND67, this selectivity reaches 100% inthe conditions of the catalytic test. Furthermore, at eachtemperature (200, 250, 300, 350 and 300 �C), the conver-sion decreases when the silver loading is increased (Table3). So the evolution of catalytic properties (increasing eth-ylene selectivity with silver content) is seen both for cata-lysts made from laboratory grade reagents (series 1, 2and 3) or industrial grade reagents (series 4). Therefore,the mechanism of 1,2-dichloroethane hydrodechlorination,studied in detail by Heinrichs et al. over the sampleEA2LAB67 [24], can be considered as similar over all othercatalysts containing 1.5 wt% of Pd and 3.0 wt% of Ag fromseries 2, 3 and 4 (samples E2LAB67, E1LAB67 andE1IND67). This mechanism is based on a sequence of ele-mentary steps involving chlorination of the silver surfaceby 1,2-dichloroethane followed by a hydrodechlorinationof that surface by hydrogen adsorbed on palladium. Usedalone, silver deactivates rapidly due to coverage by chlorineatoms. Thanks to the activation of hydrogen by dissocia-tive chemisorption, palladium present in the alloy supplieshydrogen atoms for the regeneration of metallic silver. Thepresence of hydrogen adsorbed on Pd also causes undesiredethylene hydrogenation leading to a loss of olefinselectivity.

From the surface composition of Pd–Ag alloy particlesand the catalytic properties of all Pd–Ag/SiO2 co-gelledxerogel catalysts, which are very similar, it is possible tocalculate the turnover frequency (TOF), that is, the numberof 1,2-dichloroethane molecules consumed per active sur-face metal and per second. It goes without saying thatTOF must be similar for all series. So TOF is calculatedat 300 �C after 12 h from the beginning of the catalytic test.As explained by Lambert et al. in [15], TOF, in all Pd–Ag/SiO2 co-gelled xerogel catalysts, must be calculated as afunction of active surface palladium. By assuming that allpalladium atoms present in the catalyst are contained insmall alloy metal particles, TOF is calculated as follows:

TOF ¼ rð%PdÞ 10�6ðMMPdÞ

1

DPd

; ð7Þ

where r is the specific consumption rate of 1,2-dichloroeth-ane at 300 �C after 12 h (mmol kg�1

cat s�1) (Table 3); %Pd isthe actual palladium loading calculated from mass balance(Table 2); MMPd is the palladium atomic weight,106.42 g mol�1; DPd is the palladium dispersion in smallPd–Ag alloy particles located inside silica particles calcu-lated from CO chemisorption measurements (Table 4). Pal-ladium dispersion, DPd, of fresh Pd–Ag/SiO2 co-gelledxerogel catalysts is used to calculate TOF at 300 �C. In-deed, in a previous study [17], it was checked for 1,2-dichlo-

roethane hydrodechlorination over Pd/SiO2 co-gelledxerogel catalysts that metal dispersion preserves a similarvalue during the catalytic test.

In Fig. 7, TOF is shown as a function of the fraction ofpalladium atoms present at the surface of Pd–Ag alloy par-ticles, xPds (Table 4), for all Pd–Ag/SiO2 co-gelled xerogelcatalysts at the temperature of 300 �C (after 12 h fromthe beginning of the catalytic test). For all Pd–Ag/SiO2

co-gelled xerogel catalysts, within experimental error, itappears that TOF does not vary with the synthesis methodand the purity of reagents (laboratory or industrial grade).Furthermore, it seems that TOF does not significantly varywith the fraction of palladium atoms present at the surfaceof Pd–Ag alloy particles, xPds . Thus, a single horizontal linerepresents the mean value (TOF = 0.48 s�1) calculatedfrom TOF values for all Pd–Ag samples. This mean valueis quasi-identical to this one in [15], wherein the TOF meanvalue for Pd–Ag/SiO2 co-gelled xerogel catalysts is equal to0.46 s�1.

5. Conclusions

The first purpose of this study was to simplify the syn-thesis procedure of Pd–Ag/SiO2 co-gelled xerogel catalysts,namely first to replace 3-(aminopropyl)triethoxysilane (AS)by 3-(2-aminoethyl)aminopropyltrimethoxysilane (EDAS)to complex silver (series 2) and secondly to mix in the samevessel Pd(acac)2, Ag(OAc) and EDAS in half of the totalethanol volume (series 3). In both series, highly dispersedPd–Ag/SiO2 co-gelled xerogel catalysts were obtained.These catalysts were active and selective for 1,2-dichloro-ethane hydrodechlorination: increasing silver content inbimetallic catalysts results in an increase in ethyleneselectivity.

The second purpose of this work was to extrapolate thesynthesis of Pd–Ag/SiO2 co-gelled xerogel catalysts with

2762 S. Lambert et al. / Journal of Non-Crystalline Solids 352 (2006) 2751–2762

tailored morphology to semi-industrial scale. In this way, itwas necessary to move from laboratory grade reagents toindustrial grade reagents. The evolution of the texturalproperties, the localization and accessibility of Pd–Ag alloyparticles located inside microporous silica particles, thecomposition of these alloy particles and their catalyticactivity and selectivities in the case of 1,2-dichloroethanehydrodechlorination, of series made from laboratory gradereagents or industrial grade reagents are parallel. As thechemicals purity seems to have no influence on texturalproperties and catalytic performances, no further investiga-tion has been done to identify the impurities present in theindustrial grade chemicals.

So the use of industrial grade reagents such as DynasilanDAMO (industrial EDAS), Dynasil (industrial TEOS) andindustrial ethanol denaturated with diethyl phthalate,allowed obtaining highly dispersed Pd–Ag/SiO2 co-gelledxerogel catalysts, these catalysts being very active andselective for selective hydrodechlorination of 1,2-dichloro-ethane into ethylene. This work concerning the industrial-ization process of co-gelled xerogel catalysts isencouraging for the follow of the study concerning theextrapolating of the synthesis of metallic co-gelled catalyststo an industrial scale.

Acknowledgments

The authors thank the Centre d’Enseignement et deRecherche des Macromolecules, C.E.R.M., from the Uni-versity of Liege for TEM analysis, the Laboratoire de Chi-mie Inorganique Structurale from the University of Liegefor XRD analysis, and the Solvay Company for STEM-EDX measurements. C. Alie and S. Lambert are gratefulto the Belgian Fonds National de la Recherche Scientifiquefor a position of Postdoctoral Researcher. The authors alsothank the Belgian Fonds de la Recherche FondamentaleCollective (FRFC convention no. 2.4523.01), the Ministerede la Region Wallonne – Direction Generale des Technol-ogies, de la Recherche et de l’Energie – (Projet Gredecat:convention no. 14573, and Programme de formation a larecherche scientifique et technologique), the Belgian Min-istere de la Communaute Francaise – Direction de laRecherche Scientifique – (ARC no. 00/05-265) and theFonds de Bay for their financial support.

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