Applied Catalysis B: Environmental 50 (2004) 127–140

Improvement of metal dispersion in Pd/SiO2 cogelled xerogelcatalysts for 1,2-dichloroethane hydrodechlorination

Stéphanie Lambert, Jean-François Polard, Jean-Paul Pirard, Benoı̂t Heinrichs∗

Laboratoire de Génie Chimique, B6a, Université de Liège, B-4000 Liège, Belgium

Received 18 August 2003; received in revised form 12 January 2004; accepted 12 January 2004

Abstract

The study of the influence of synthesis operating variables (nature and concentration of complexing silane, palladium percentage, temper-atures of gelling, aging and vacuum drying of xerogels, molar ratio between the complexing silane and palladium, molar concentration ofammonia solution, and use of tetramethylammonium hydroxide as base instead of NH3) allows improving metal dispersion in Pd/SiO2 co-gelled xerogel catalysts. The use of 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) or 3-(2-aminoethylamino)propyltriethoxysilane(EDAES) to complex palladium in an ethanolic solution containing tetraethoxysilane (TEOS) and an ammonia solution of 0.54 mol/l al-lows obtaining a Pd/SiO2 xerogel catalyst with a mean metal particle diameter of 2.4 nm located inside silica particles. Indeed, complexesPd(EDA(E)S)xn+ induce a nucleation mechanism because of their higher reactivity compared to the network-reagent (TEOS). Although metalparticles are located inside the silica particles, their complete accessibility, via the micropore network, has been shown. 1,2-dichloroethanehydrodechlorination over Pd/SiO2 catalysts mainly produces ethane and the specific hydrodechlorination rate per gram of palladium increasesproportionally with palladium dispersion. Hydrodechlorination over Pd/SiO2 cogelled xerogel catalysts is a structure insensitive reaction withregard to the ensemble size concept.© 2004 Elsevier B.V. All rights reserved.

Keywords: Cogelled xerogel catalysts; Tailored morphology; Nucleation; Metal dispersion; Hydrodechlorination

1. Introduction

There is an increasing demand for technology that willconvert chlorocarbons as by-products of industrial processesinto more useful or environmentally benign products. Forexample, hydrodechlorination of chlorinated organics is aparticularly attractive alternative compared with incinerationof wastes from the chlorine industry from both economicand environmental points of view[1]. The hydrodechlori-nation reaction often consists of the carbon-chlorine bondhydrogenolysis,≡C–Cl + H2 → ≡C–H + HCl, in whichhydrogen atoms are substituted for chlorine atoms. Noblemetal catalysts (Group VIII) like Pd, Pt,. . . are very ac-tive for the hydrodechlorination reaction[2–9]. In the caseof 1,2-dichloroethane hydrodechlorination, the noble metalparticipates in a catalytic cycle in which the reactant isdechlorinated by chlorination of the metal surface, which is

∗ Corresponding author. Tel.:+32-4-366-35-05; fax:+32-4-366-35-45.E-mail address: b.heinrichs@ulg.ac.be (B. Heinrichs).

then itself dechlorinated by reduction with hydrogen. Be-cause of the high reactivity of hydrogen on noble metals, thedechlorinated organics, C2H4 in the present case, is imme-diately converted into the fully hydrogenated product, C2H6[3–9].

A high activity of a supported catalyst often calls for alarge active surface area and, thus, for small particles, i.e.a high dispersion of the active phase. Because small metalparticles tend to already sinter at relatively low temperatures,these generally are applied into a support material whichitself is thermally stable and maintains a high specific surfacearea up to high temperatures[10].

Schubert et al. have developed an interesting method todisperse metal particles in a silica matrix[11,12]. Heinrichset al. [5,13] and Lambert et al.[14] used this cogelationmethod for the preparation of Pd/SiO2, Ag/SiO2, Cu/SiO2and Pd-Ag/SiO2 catalysts. All these authors used alkoxidesof the 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 stable

0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apcatb.2004.01.015

128 S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140

spacer 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.

The study of textural properties, metal dispersion and cat-alytic activity measurements show the complete accessibil-ity of nanometer-sized metallic particles located inside silicaparticles[14]. Nevertheless, beyond a value of metal loadingwhich is >1.0 wt.% in all Pd/SiO2, Ag/SiO2 and Cu/SiO2catalysts, samples exhibit metal particles distributed in twofamilies of different sizes: small crystallites between 2 and4 nm and large crystallites between 10 and 30 nm[14]. Soseveral hypotheses could explain the metal dispersion in twofamilies of different sizes in cogelled xerogel catalysts: (i)large metal particles could result from the sintering of smallmetal particles, which are not sufficiantly trapped inside ele-mentary silica particles; (ii) the insufficient coating of metalparticles could be a consequence that some TEOS sometimesremains unreacted and is volatilized during vacuum drying.

The main purpose of the present work is to improve pal-ladium dispersion in cogelled xerogel catalysts in such away as to increase their activity for 1,2-dichloroethane hy-drodechlorination. This study consists in canceling weightlosses in cogelled catalysts by mastering the influences ofsynthesis operating variables: the nature and concentrationof complexing silane, the metal percentage, the molar ratiobetween the complexing silane and metal, the temperaturesof gelling, aging and drying, and the nature and molar con-centration of basic aqueous solution. The nucleation phe-nomenon by the metal complex in the formation of silicaparticles is also examined.

2. Experimental

2.1. Catalyst preparation

2.1.1. Gel synthesisTable 1shows synthesis operating variables of Pd/SiO2

xerogel catalysts. These catalysts were made in ethanol(Merck 1.00983.2500, purity: 99.9%) from TEOS=Si(OC2H5)4 (Merck 8.00658.1000, purity: 98%), EDAES=NH2–CH2–CH2–NH–(CH2)3–Si(OC2H5)3 (Dow CorningZ-6021, purity: 86%) (series 1) or EDAS= NH2–CH2–CH2–NH–(CH2)3–Si(OCH3)3 (Aldrich 10,488-4, pu-rity: 97%) (series 2–6), and a solution of aqueous NH3(series 1–5) or a solution of aqueous TMAH (tetram-ethylammonium hydroxide, (CH3)4NOH) (series 6). Themetal precursor is a palladium acetylacetonate powder= Pd(CH3C(O)–C=C(O)CH3)2 (Aldrich 28,278-2, purity:97%). Pd(acac)2 and EDAS or EDAES were mixed togetherin the half of the total volume of ethanol (the molar ratioEDA(E)S/Pd are given inTable 1). The slurry was stirred atroom temperature until a clear yellow solution was obtainedfor palladium (color characteristic of palladium complex).After addition of TEOS, a solution of aqueous NH3 or a

solution of aqueous TMAH in the other half of the totalvolume of ethanol was added to the mixture under vigor-ous stirring. The hydrolysis ratio, that is, the molar ratioH = [H2O]/([TEOS] + 3/4[EDAS or EDAES]) (factor 3/4is due to the fact that EDAS or EDAES contains only threehydrolysable groups in relation to TEOS, which containsfour hydrolysable groups), was kept constant at the value of5 for all samples. The dilution operating variable, that is themolar ratioR = [ethanol]/([TEOS]+ [EDAS or EDEAS])was kept constant at the value of 10 for all samples. Thevessel was then closed and heated up for gelling and ag-ing [15] to the chosen temperatureTd for 3 days. In theTables, Pd/SiO2 catalysts are presented in six series: series1 (EDAES) and 2 (EDAS), variation of palladium loading;series 3, variation of temperatures of gelling, aging and vac-uum drying; series 4, variation of molar ratio EDA(E)S/Pd;series 5 (NH3) and 6 (TMAH), variation of molar con-centration of basic aqueous solution. In the figures, whenseries 1 and 2 are presented, each sample is named by thetheoretical palladium loading; when series 5 and 6 are pre-sented, each sample is named by the corresponding base(NH for NH3 or TH for TMAH) followed by its molarconcentration.

2.1.2. DryingThe wet gels were dried under vacuum according to the

following procedure: the flasks were opened and put intoa drying oven at the chosen temperatureTd (Table 1), andthe pressure was slowly decreased to the minimum value of1200 Pa after 90 h to prevent gel bursting. The drying ovenwas then heated at 150◦C for 72 h, except for samples 1dand 2d, which were heated at 150◦C for 144 h. The resultingsamples are xerogels[15].

2.1.3. CalcinationThe conditions of calcination for all Pd/SiO2 samples

were as follows: the sample was heated up to 400◦C at a rateof 120◦C/h under flowing air (0.02 mmol s−1) (Alphagaz,purity: 98%); this temperature was maintained for 12 h inair (0.1 mmol s−1).

2.1.4. ReductionThe Pd/SiO2 catalysts were heated to 350◦C at a rate of

350◦C/h under flowing H2 (0.23 mmol s−1) (Alphagaz, pu-rity: 99%) and maintained at this temperature for 3 h (sameflow).

2.2. Catalyst characterization

Apparent densities[16] were measured by helium py-cnometry on a Micromeritics AccuPyc 1330. Nitrogenadsorption–desorption isotherms were measured at 77 Kon a Fisons Sorptomatic 1990 after outgassing for 24 hat ambient temperature. After a 2 h outgassing at ambienttemperature, mercury porosimetry measurements were per-formed with sample monoliths using a manual porosimeter

S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140 129

Table 1Synthesis operating variables, theoretical and actual metal loading of Pd/SiO2 xerogel catalysts

Sample Td (◦C) Basicityofaqueoussolution(mol l−1)

E/Pd nPd

(mmol)nE

(mmol)nTEOS

(mmol)nH2O

(mmol)nC2H5OH

(mmol)Gel time(min)

Theoreticalmetal loading(wt.%)

Weight loss±1% (wt.%)

Actual metalloading±0.1% (wt.%)

1a 60 0.18 2 0.488 0.989 84.6 430 857 28 1.00 16 1.21b 60 0.18 2 0.738 1.474 84.3 426 857 26 1.50 22 1.91c 60 0.18 2 1.016 2.080 83.8 422 857 25 2.15 28 2.81d 60 0.18 2 2.187 4.365 81.5 420 857 25 4.50 40 7.2

2a 60 0.18 2 1.074 2.154 186 940 1886 30 1.00 10 1.12b 60 0.18 2 1.569 3.162 185 938 1886 26 1.50 24 1.92c 60 0.18 2 2.289 4.588 184 934 1886 26 2.15 32 3.02d 60 0.18 2 4.811 9.623 179 930 1886 24 4.50 42 7.4

3a 25 0.18 2 1.621 3.254 185 938 1886 90 1.50 40 2.53b = 2b 60 0.18 2 1.569 3.162 185 938 1886 26 1.50 24 1.93c 80 0.18 2 1.617 3.208 185 938 1886 17 1.50 15 1.7

4a = 3c 80 0.18 2 1.617 3.208 185 938 1886 17 1.50 15 1.74b 80 0.18 4 0.735 2.956 82.8 425 857 14 1.50 6 1.64c 80 0.18 6 0.735 4.423 81.5 422 857 12 1.50 0 1.54d 80 0.18 8 0.735 5.889 79.7 420 857 10 1.50 0 1.5

5a 80 0 2 0.747 1.467 84.2 426 857 30 1.50 58 3.75b 80 0.09 2 1.621 3.254 185 938 1886 18 1.50 30 2.15c = 3c 80 0.18 2 1.617 3.254 185 938 1886 17 1.50 15 1.75d 80 0.36 2 1.620 3.254 185 938 1886 15 1.50 0 1.55e 80 0.54 2 1.622 3.254 185 938 1886 13 1.50 0 1.55f 80 2 2 1.635 3.254 185 938 1886 11 1.50 0 1.5

6a 80 0.001 2 1.630 3.254 185 938 1886 25 1.50 56 3.46b 80 0.005 2 0.738 1.466 84.2 426 857 20 1.50 52 3.16c 80 0.05 2 0.733 1.466 84.2 426 857 15 1.50 50 3.06d 80 0.5 2 0.735 1.466 84.2 426 857 8 1.50 32 2.26e 80 1 2 0.739 1.466 84.2 426 857 6 1.50 18 1.86f 80 2 2 0.730 1.466 84.2 426 857 2 1.50 0 1.5

Td: temperatures of gelling, aging and vacuum drying; E/Pd: molar ratio between the mole number of EDAS or EDAES and the mole number ofPd(acac)2; nPd: mole number of Pd(acac)2; nE: mole number of EDAS or EDAES; weight loss= 100 × (mth − ma)/mth, wheremth is the theoreticalmass andma the actual catalyst mass measured after drying, calcination and reduction.

from 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 were ex-amined by transmission electron microscopy (TEM). TEManalyses were performed on a Philips CM100 microscope.All the samples were impregnated with an epoxy resin(EPON 812) to which an amine was added to serve asa 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.

Metal dispersion in Pd/SiO2 xerogel catalysts was deter-mined from CO chemisorption (Alphagaz, purity: 99%) at30◦C on a Fisons Sorptomatic 1990 device. Before measure-ments, the calcined sample was reduced in situ in flowingH2 (0.003 mmol s−1) (Alphagaz, purity: 99%) at 350◦C for3 h. Afterwards, this sample was outgassed under vacuum at340◦C for 16 h. A double adsorption method was used: (i)first adsorption isotherm was measured, which includes bothphysisorption and chemisorption; (ii) after a 2 h outgassing

at 30◦C, a second isotherm was measured, which includesphysisorption only. Both isotherms were determined in thepressure range of 10−8 to 2× 10−1 kPa. The difference be-tween the first and second isotherms gave the CO chemisorp-tion isotherm. The latter theoretically exhibited a horizon-tal linear region corresponding to the complete coverage ofmetallic sites by a monolayer of adsorbate. However the lin-ear region of experimental chemisorption isotherms oftenexhibited a slope and the monolayer uptake was obtained byback extrapolation of the linear region to zero pressure[17].

2.3. Catalytic experiments

The four Pd/SiO2 catalysts of series 2 and one sampleof series 1, 4 and 5 (samples 1b, 4d and 5e) were testedfor 1,2-dichloroethane hydrodechlorination, which was con-ducted in a stainless steel tubular reactor (internal diameter:10 mm) at a pressure of 0.3 MPa. The reactor was placed in aconvection oven. A constant flow of each reactant was main-tained by a Gilson piston pump for ClCH2–CH2Cl (Aldrich

130 S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140

31,992-9, purity: 99.5%) and Brooks mass flow controllersfor H2 (Alphagaz, purity: 99%) and He (Alphagaz, purity:99%). The effluent was analyzed by gas chromatography(ThermoFinnigan with FID) using a Porapak Q5 packedcolumn. Prior to each experiment, the Pd/SiO2 catalystswere reduced in situ at atmospheric pressure in flowing H2(0.023 mmol s−1) while being heated to 350◦C at a rate of350◦C/h and were maintained at this temperature for 3 h. Af-ter reduction, the Pd/SiO2 catalysts were cooled in H2 to thedesired initial reaction temperature of 300◦C. The total flowof the reactant mixture was 0.45 mmol s−1 and consisted ofClCH2–CH2Cl (0.011 mmol s−1), H2 (0.023 mmol s−1), andHe (0.42 mmol s−1). The temperature was successively keptat 300◦C (10 h to allow activity stabilization after fast ini-tial deactivation), 350◦C (2 h), 300◦C (2 h), 250◦C (2 h),and 300◦C (2 h). The effluent was analyzed every 15 minand eight analyses were made at each temperature (40 forthe first level). For each catalytic experiment, the mass ofcatalyst pellets, sieved between 250 and 500�m, was cal-culated so that 1,2-dichloroethane conversion was approx-imately equal to 10% at 300◦C. Reaction rate values ob-tained in the present catalytic study were not falsified bydiffusional limitations. Thus, those values reflect the intrin-sic chemical kinetics[18].

3. Results

3.1. Synthesis

Synthesis operating variables of cogelled samples aregiven inTable 1.

Gel time is defined as the time elapsed between the intro-duction of the last reactive component to the solution andgelation at the chosen temperature, and it is measured at themoment when the liquid no longer flows when the flask istipped at an angle of 45◦. The comparison between all thesamples indicates that the gel time decreases in each serieswhen either palladium loading and therefore the amountsof EDAES and EDAS (series 1 and 2), or the temperaturesof gelling, aging and vacuum drying (series 3), or the mo-lar ratio EDA(E)S/Pd (series 4), or the basicity of aqueoussolution (series 5 and 6) increase.

The actual palladium loading in cogelled catalysts is gen-erally higher than theoretical loading because the actual cat-alyst mass after gelling, aging, drying, calcination and reduc-tion steps, is lower than theoretical mass. In fact, some TEOSand/or EDAS often remain unreacted and are volatilized dur-ing drying. This theoretical mass (mth) is calculated fromEq. (1):

mth = nmMm + (nTEOS+ nE)MSiO2 (1)

wherenm is the amount of metal in the gel (mmol);Mm thepalladium atomic weight;nTEOS andnE are respectively theamount of TEOS and EDAES or EDAS in the gel (mmol);MSiO2 the molecular weight of SiO2, 60.085 g mol−1. In

this equation, it is assumed that all TEOS and EDAES orEDAS molecules are converted into SiO2. Results inTable 1clearly show that the weight loss increases with EDAES andEDAS concentrations (series 1 and 2). In other series, theweight loss decreases and sometimes down to 0% when thetemperatures of gelling, aging and vacuum drying (series 3),the molar ratio EDA(E)S/Pd (series 4), and the basicity ofaqueous solution (series 5 and 6) increase.

3.2. Texture of catalysts

The results of textural properties are presented inTable 2.The catalyst apparent density,ρapp, which is the density

including closed pores according to IUPAC[16], is veryclose to the true density, which is the density excluding poresaccording to IUPAC, of dried alkoxy-derived silica gels, thatis 2.2 g cm−3 [19].

Submitted to an increasing mercury pressure, all thesamples exhibit two successive behaviors[20]. The seriesEDAES–TEOS are presented inFig. 1 as an example.

Table 2Sample textural properties

Sample ρapp

(g cm−3)Pt

(MPa)Vv

(cm3/g)SBET

(m2 g−1)dSiO2 (nm)

1a 2.058 40 3.2 340 19.5± 2.01b 2.112 70 2.8 420 15.0± 1.41c 2.152 95 2.5 500 13.8± 1.51d 2.226 125 2.0 540 7.9± 1.3

2a 2.092 35 3.2 365 19.5± 1.32b 2.192 75 2.8 375 17.9± 1.22c 2.220 100 2.5 495 12.9± 1.32d 2.256 120 2.0 570 9.8± 1.1

3a 2.132 90 2.8 370 15.2± 1.13b = 2b 2.192 75 2.8 375 17.9± 1.23c 2.134 60 3.0 355 19.2± 1.8

4a = 3c 2.134 60 3.0 355 19.2± 1.84b 2.164 65 3.7 350 16.4± 1.64c 2.104 65 4.0 340 15.6± 1.54d 2.126 75 4.0 340 13.8± 1.6

5a 2.164 120 2.1 500 10.9± 1.45b 2.152 75 2.2 475 13.0± 1.85c = 3c 2.134 60 3.0 355 19.2± 1.85d 2.106 55 3.5 345 20.4± 2.35e 2.070 50 3.5 335 22.9± 2.05f 2.030 35 4.7 225 29.7± 2.6

6a 2.250 80 3.4 450 13.4± 1.76b 2.208 70 3.4 420 16.2± 2.06c 2.166 50 4.4 400 21.4± 4.36d 2.130 1.5 4.6 390 38.2± 6.06e 2.112 0.5 5.2 340 >60a

6f 2.084 0.5 5.9 300 >60a

ρapp: apparent density measured by helium pycnometry;Pt: pressure ofchange of mechanism during mercury porosimetry;Vv: total cumulativespecific pore volume;SBET: specific surface area obtained by BET method;dSiO2: silica particle diameter measured by TEM.

a It is impossible to measure silica particle size because 60 nm sliceswere cut up from samples for TEM observations. Then if the silicaparticles have a size >60 nm, the Ultracut microtome cut in silica particles.

S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140 131

Fig. 1. Mercury porosimetry curves (volume variation as a function of mercury pressure) of the series EDAES–TEOS (series 1): samples Pd1.00% (�),Pd1.50% (�), Pd2.15% (�) and Pd4.50% (�).

At low pressure, the sample collapses under the isostaticpressure, and above a pressure of transition (Pt), which ischaracteristic of the material composition and microstruc-ture, mercury can enter the network of small pores notdestroyed during the compression at low pressure. Thisappears in the pressurization curve by a sudden change ofslope atPt on the curve of pressure increase. The curve ofpressure decrease can also be divided in two distinct partsseparated by an abrupt change of slope. At high pressure,there is a large volume variation reversibility with a hys-teresis, whereas at low pressure, there is only a very limitedvolume variation indicating the mainly irreversible natureof the crushing phenomenon that occurred during pressur-ization. Two models can then be used in order to calculatethe pore size distribution from mercury porosimetry: themercury intrusion in small pores abovePt described byWashburn’s model[21] and the collapse of larger poresbelow Pt described by Pirard’s model[20]. In Table 2, Ptincreases as either the metal loading and as a consequencethe amount of EDAES and EDAS in the catalyst (series 1and 2) or the molar ratio EDA(E)S/Pd (series 4) increase,whereasPt decreases as either the temperatures of gelling,aging and vacuum drying,Td (series 3), or the basicity ofaqueous solution (series 5 and 6) increase.

The nitrogen adsorption analysis reveals the presence oftwo types of isotherms. The�-type nitrogen adsorption–desorption isotherm is characterized by: (i) at low rela-tive pressure, a sharp increase of the adsorbed volume isfollowed by a plateau that corresponds to type I isothermaccording to BDDT classification[21], which is character-istic of microporous adsorbents; (ii) at high pressure, theadsorbed volume increases quickly like in type II isotherm,which is characteristic of macroporous adsorbents. Thevolume adsorbed betweenp/p0 = 0.95 and 1 is large, indi-cating that the sample contains pores of large dimensions;(iii) all samples exhibit a narrow adsorption–desorption hys-teresis loop forp/p0 values close to 1 and this hysteresis is

characteristic of capillary condensation in large mesopores.The�-type nitrogen adsorption–desorption isotherm has thefollowing characteristics: a nitrogen adsorption–desorptiontype IV isotherm according to the BDDT classification[21]with a broad hysteresis. This observation could be explainedby the absence of macropores and very large mesoporesand therefore an increase of the specific surface area,SBET.

The �-type nitrogen adsorption–desorption isotherm ischaracteristic at low palladium loading with either EDASor EDAES as additive (Fig. 2a, samples Pd1. 00% and Pd1.50%) and at high basicity of aqueous solution (Fig. 2band c, samples NH0. 54, NH2, TH1 and TH2). The�-typeis characteristic at high palladium loading (Fig. 2a, sam-ple Pd4. 50%) and at low basicity of aqueous solution(Fig. 2(b and c), samples NH0, NH0. 09, TH0. 001 andTH0. 005). For intermediate values of palladium loading(Fig. 2a, sample Pd2. 15%) and of basicity of aqueous so-lution (Fig. 2b and c, samples NH0. 18, NH0. 36, TH0. 05and TH0. 5), Pd/SiO2 xerogel catalysts nitrogen isothermsevolve from the�-type at low palladium loading and at highbasicity of aqueous solution to the�-type at high palladiumloading and at low basicity of aqueous solution. Nitrogenadsorption–desorption isotherms of the Pd/SiO2 catalystssynthesized from EDAES (series 1) are not presented in thisstudy because the evolution from�-type to�-type is similarto that of the Pd/SiO2 catalysts synthesized from EDAS (se-ries 2). InTable 2, the specific surface area,SBET, increaseswith the palladium content in series 1 and 2 and decreases asthe basicity of aqueous solution increases in series 5 and 6.

Nitrogen adsorption–desorption isotherms of series 3 and4 are not presented in this work because the�-type isothermsremain at similar values when either the temperatures ofgelling, aging and vacuum drying,Td, or the molar ratioEDA(E)S/Pd vary. InTable 2, the specific surface area,SBET,is quasi constant when the temperatures of gelling, aging andvacuum drying,Td, (series 3) or the molar ratio EDA(E)S/Pd(series 4) increases.

132 S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140

Fig. 2. Nitrogen adsorption–desorption isotherms of (a) the seriesEDAS–TEOS with the variation of palladium loading (series 2): samplesPd1. 00% (�), Pd1. 50% (�), Pd2. 15% (�) and Pd4. 50% (�), of (b)the series EDAS–TEOS with the variation of NH3 solution basicity (se-ries 5): samples NH0 (�), NH0. 09 (�), NH0. 18 (�), NH0. 54 (�),and NH2 (×), and of (c) the series EDAS–TEOS with the variation ofTMAH solution basicity (series 6): samples TH0. 001 (�), TH0. 005(�), TH0. 05 (�), TH1 (�) and TH2 (×).

Fig. 3 shows the evolution of the cumulative volume dis-tributions over the entire pore size range as a function ofeither palladium loading or basicity of aqueous solution forPd/SiO2 catalysts. These curves were obtained by applyinga combination of various methods to their respective valid-

Fig. 3. Pore size distributions as a function of (a) palladium loadingand therefore the amount of EDAES (full symbols) and EDAS (emptysymbols) (series 1 and 2): samples Pd1.00% (� and �), Pd1.50% (�and �), Pd2.15% (� and ), Pd4.50% (� and �); (b) the basicity ofNH3 solution (series 5): samples NH0 (�), NH0.18 (�), NH0.54 (�)and NH2 (�), and of (c) the basicity of TMAH solution (series 6):samples TH0.005 (�), TH0.05 (�), TH1 (�) and TH2 (�).

ity domains and by adding the porous volume distributionscorresponding to each domain as proposed by Alié et al.[20]. All Pd/SiO2 catalysts are characterized by a steep vol-ume increase around 0.8 nm followed by a plateau (Fig. 3).In the range of meso- and macropores, one observes that allsamples exhibit a broad distribution. The distribution shiftstowards the smaller pores when palladium loading, and asa consequence, the content in EDAES or EDAS increase(Fig. 3a). The distribution shifts towards the larger pores

S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140 133

when the basicity of the aqueous NH3 (Fig. 3b) or TMAH(Fig. 3c) solution increases.

The total pore volume,Vv, that is, the pore volume ob-tained by addition of pore volume measured by mercuryporosimetry and cumulative volume of micropores andpores of width between 2 and 7.5 nm measured by N2adsorption–desorption, decreases as the palladium contentincreases in series 1 and 2. The total pore volume,Vv,increases as the basicity of aqueous solution increases inseries 5 and 6 (Table 2).

Cumulative volume distributions of the series 3 and 4are not presented in this work because these curves remainsimilar values over the entire pore size range when the tem-peratures of gelling, aging and vacuum drying,Td, vary andover the micro- and mesopore size range when the molarratio EDA(E)S/Pd varies. Indeed, the total pore volume,Vv, slightly increases with the molar ratio EDA(E)S/TEOS(Table 2).

Texture and morphology of Pd/SiO2 cogelled catalystshave also been examined by TEM and the size of SiO2 ele-mentary particles,dSiO2, has been evaluated. Sizes given inTable 2represent the arithmetic mean on fifty particles. Itis observed thatdSiO2 decreases as either the metal loadingand as a consequence the amount of EDAES and EDAS inthe catalyst (series 1 and 2) or the molar ratio EDA(E)S/Pd(series 4) increase, whereasdSiO2 increases as either thetemperatures of gelling, aging and vacuum drying,Td (se-ries 3), or the basicity of aqueous solution (series 5 and 6)increase. Furthermore, it has been shown[20] that the rela-tion between the pressure of transition,Pt, and the size ofSiO2 particles,dSiO2, is given by the relationshipdSiO2 ÷1/P0.75

t in the case of the buckling of the brittle filamentsof mineral oxide under an axial compressive stress. In allthe series, whendSiO2 increases,Pt decreases and inversely(Table 2).

3.3. Dispersion and localization of metal

Table 3gives palladium particle size determined by TEMand CO chemisorption measurements.

TEM analysis indicates that the most samples exhibitmetal particles distributed in two families of different sizes:small crystallites between 1.9 and 2.8 nm and large crystal-lites between 10 and 33 nm (Fig. 4). For all catalysts, themean diameter of small crystallites,dTEM1, is the arithmeticmean of fifty diameters of small metal particles measuredon TEM micrographs. When large crystallites are present,their number is much smaller than for small crystallites andtheir mean diameter,dTEM2, is then estimated from an aver-age of three to twenty crystallites. In the series 1, 2 and 4,dTEM1 seems to decrease, whereasdTEM2 seems to increasewhen palladium loading and the molar ratio EDA(E)S/Pdincrease. Concerning localization of metallic crystallites, itappears that cogelled catalysts are composed of silica par-ticles arranged in strings or aggregates, and although TEMgives only a 2D view, it seems that small metal particles are

Table 3Metal average particle size and dispersion

Sample TEM COchemisorption

n1/n2

dTEM1

(nm)σTEM1

(nm)dTEM2

(nm)σTEM2

(nm)dchem

(nm)D (%)

1a 2.5 0.2 –a –a 2.6 44 –1b 2.5 0.3 11 1 3.3 34 1901c 2.2 0.2 14 2 4.0 28 2201d 1.9 0.3 33 5 5.6 20 2230

2a 2.7 0.2 –a –a 2.6 44 –2b 2.7 0.2 11 1 3.0 38 4402c 2.3 0.3 18 4 4.6 24 3602d 2.1 0.3 24 7 7.0 16 450

3a 2.8 0.3 30 5 4.6 24 16203b = 2b 2.7 0.2 11 1 3.0 38 4403c 2.7 0.4 10 2 3.0 38 320

4a = 3c 2.7 0.4 10 2 3.0 38 3204b 2.6 0.4 10 2 3.0 38 2604c 2.1 0.2 15 4 3.3 34 5004d 2.0 0.2 30 6 6.2 18 1280

5a 2.8 0.3 22 5 4.0 28 9305b 2.8 0.4 11 3 3.1 36 4105c = 3c 2.7 0.4 10 2 3.0 38 3205d 2.5 0.3 7 1 2.6 42 3405e 2.5 0.2 –a –a 2.4 46 –5f 2.7 0.3 5 1 2.8 40 80

6a 2.3 0.2 –a –a 2.4 46 –6b 2.6 0.3 –a –a 2.4 46 –6c 2.6 0.3 –a –a 2.6 44 –6d 3–40 4.6 24 –6e 3–40 4.6 24 –6f 3–40 6.2 18 –

dTEM1, dTEM2: mean diameter of small and large palladium particles re-spectively measured by TEM;σTEM1, σTEM2: standard deviations associ-ated withdTEM1 and dTEM2, respectively;D: metal dispersion measuredby chemisorption;dchem: mean diameter derived from dispersionD; n1/n2:ratio between the number of metal particles located inside (n1) and out-side (n2) the silica particles.

a Not observed.

located inside silica particles, whereas large metal particlesare located at their surface (Fig. 4).

A mean diameter,dchem, of palladium particles has beenalso derived from metal dispersion,D, measured by COchemisorption for Pd/SiO2 catalysts as proposed by Lam-bert et al.[14]. Values ofdchem are given inTable 3. In allseries except series 3 and 5, it is observed thatD decreases,and sodchem increases, when either palladium loading (se-ries 1 and 2), or the molar ratio EDA(E)S/Pd (series 4), orthe basicity of TMAH solution (series 6) increase. Never-theless, in series 3 and 5, it is observed thatD increasesin a first time with the temperatures of gelling, aging andvacuum drying and the basicity of NH3 solution and sotends to become less or non-dependent on these synthesisoperating variables. One has to note the agreement betweenTEM and chemisorption for samples containing, accord-ing to TEM analysis, one single family of particle size.In the case of samples containing two families of particle

134 S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140

Fig. 4. TEM micrograph of sample Pd1.50% from the series EDAS-TEOS (series 2) (500,000×).

sizes, values ofdchem are between values ofdTEM1 anddTEM2.

From Bergeret and Gallezot[17], the mean surface diam-eter, ds = ∑

nid3i /

∑nid

2i , which defines the fraction of

particles giving the major contribution to the total surfacearea and thus to the catalytic properties, can be comparedwith the mean diameter calculated from surface area mea-surements using gas adsorption methods (chemisorptionmethods). So ifn1 and n2 are the number of palladiumparticles located, respectively, inside and outside the sil-ica particles,dTEM1 and dTEM2 are the mean diameter ofmetallic particles measured by TEM and located respec-tively inside and outside the silica particles (nm) anddchemis the mean diameter of metallic particles measured by COchemisorption (nm) (Table 3), it can be written

dchem= n1d31 + n2d

32

n1d21 + n2d

22

(2)

Finally,

n1

n2= d3

2 − dchemd22

dchemd21 − d3

1

(3)

The ration1/n2 is given inTable 3, but these calculationsare not very precise. WhenD ≥ 44%, the ration1/n2 ≈0.For 42%≥ D ≥ 28%, n1/n2 values vary between 190 and930. WhenD < 28%, the ration1/n2 ≥1000 except for sam-ples 2c and 2d because it seems thatdTEM2 values are un-derestimated owing to the very small number of large metalparticles. So when the metal dispersion,D, decreases, thenumber of palladium crystallites situated outside the silicaparticles seems to decrease, but their mean size strongly in-creases. Moreover, this conclusion has been checked fromTEM measurements.

3.4. Catalytic experiments

In Fig. 5, conversion as well as C2H6, C2H4 and C2H5Clselectivities are given as a function of time and temperatureover sample 1b, for example. All Pd/SiO2 catalysts mainlyproduce ethane, C2H6, with a selectivity between 70 and90% with conversions between 10 and 20%. Two secondaryproducts are observed: ethyl chloride, C2H5Cl, and ethy-lene, C2H4. In each case, a temperature increase from 250to 350◦C is beneficial to C2H4 selectivity and detrimentalto C2H6 and C2H5Cl selectivities. The examination of con-version curves shows that a deactivation, which is faster atthe beginning of the catalytic test and when the temperatureincreases, is observed with all samples.

The consumption rate of 1,2-dichloroethaner is calculatedfrom chromatographic measurements of C2H6, C2H5Cl andC2H4 concentrations in the reactor effluent and from thedifferential reactor equation that is written as follows:

r = FA + FCl + FE

W(FA0, FCl0 andFE0 = 0) (4)

wherer is the consumption rate (mmol kg−1Pd s−1), FA the

molar flowrate of ethane at the reactor outlet (mmol s−1),FA0 the molar flowrate of ethane at the reactor inlet(mmol s−1), FCl the molar flowrate of ethyl chloride at thereactor outlet (mmol s−1), FCl0 the molar flowrate of ethylchloride at the reactor inlet (mmol s−1), FE is the molarflowrate of ethylene at the reactor outlet (mmol s−1), FE0 themolar flowrate of ethylene at the reactor inlet (mmol s−1)andW the palladium mass inside the reactor (kgPd).

For samples 1b, 2(a–d), 4d and 5e,r has been calculatedfrom Eq. (4)at the temperatures of 250 and 300◦C and theseresults are presented as a function of palladium dispersionin Fig. 6. It is observed thatr increases with palladiumdispersion.

S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140 135

Fig. 5. Conversion of 1,2-dichloroethane (�) and selectivity of ethane (�), ethylene (�) and ethyl chloride (�) as a function of time for samplePd1.00% from the series EDAES–TEOS (series 1).

Fig. 6. Consumption rate of 1,2-dichloroethane atT = 250◦C (�) andT = 300◦C (�) as a function of palladium dispersion.

4. Discussion

4.1. Nucleation mechanism

TEM micrographs obtained for Pd/SiO2 cogelled xerogelcatalysts show metal particles inside microporous silica par-ticles. In previous studies[13,14,20], it was demonstratedthat this localization of the metal inside the silica matrix wasinduced by a nucleation process initiated by the ligand ofthe metal, EDAS and thus by the complex M(EDAS)xn+.In this work, to establish the nucleation mechanism by thecomplexing silane (EDAES or EDAS) and thus by the com-plex M(EDA(E)S)xn+, where M is palladium, the relationbetween SiO2 particle volume and TEOS and EDA(E)S con-centrations can be expressed by[22].

d3SiO2

= C1[TEOS]+ [EDA(E)S]

(1 − ε)[EDA(E)S]z(5)

wheredSiO2 is the diameter of SiO2 particles (nm), [TEOS]and [EDA(E)S] the concentrations of TEOS, EDAES orEDAS (mol l−1), ε the void fraction of the silica particles

andC1 a constant. The exponentz is experimentally deter-mined from the relation

Ns = C2[EDA(E)S]z (6)

whereNs is the number of SiO2 particles in one liter of xe-rogel (particle l−1) (Table 4). Ns was calculated by dividingthe volume of silica per liter by the volume of matter ofone silica particle:Ns = 6M/[πρappd

3SiO2

(1 − ε)] whereM

is the mass of silica obtained from one liter gel (g l−1) andρapp the apparent density measured by helium pycnometry(g cm−3). For series 1, 2 and 4, the values of the exponentzare respectively equal to 1.54, 1.29 and 1.20. These valuesare similar and can be compared with the values ofz calcu-lated by Lambert et al. in the case of Pd/SiO2 and Ag/SiO2xerogel catalysts and xerogels without metal[22].

The curvesd3SiO2

= f {([TEOS] + [EDA(E)S])/(1 −ε)[EDA(E)S]z} for the series 1, 2 and 4 are shown inFig. 7. Itis observed that there is effectively growth of silica particleswhen the ratio ([TEOS]+ [EDA(E)S])/(1− ε)[EDA(E)S]z

increases, i.e. as [EDA(E)S] decreases. The linearity

136 S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140

Table 4Calculation of number of silica particles per literNs, number of EDA(E)S molecules and number of metal atoms per silica particle

Sample ε Ns (particles l−1) [EDA(E)S] (mol l−1) EDA(E)S/particle Metal/particle dmetal (nm)

1a 0.20 1.10× 1019 0.013 699 350 2.11b 0.25 2.09× 1019 0.019 549 274 2.01c 0.28 3.20× 1019 0.027 508 254 1.91d 0.32 1.10× 1020 0.057 310 155 1.6

2a 0.21 1.14× 1019 0.013 685 342 2.12b 0.25 1.84× 1019 0.019 621 310 2.02c 0.27 3.16× 1019 0.027 515 258 1.92d 0.31 7.70× 1019 0.057 445 222 1.8

4a 0.15 8.36× 1018 0.019 1369 684 2.74b 0.15 1.78× 1019 0.038 1297 648 2.64c 0.15 2.87× 1019 0.057 1195 598 2.64d 0.16 4.11× 1019 0.076 1120 560 2.5

ε: void fraction of silica particle;Ns: number of silica particles per liter; [EDA(E)S]: concentration of EDAES or EDAS; EDA(E)S/particle: number ofEDA(E)S molecules per silica particle; metal/particle: number of metal atoms per silica particle;dmetal: palladium particle diameter calculated from thenumber of metal atoms per silica particle.

betweend3SiO2

and the ratio ([TEOS]+ [EDA(E)S])/(1 −ε)[EDA(E)S]z confirms the hypothesis of nucleation byEDA(E)S, since more EDA(E)S decreasesd3

SiO2, due to

more nucleation sites. The samples from series 3, 5 and 6 arenot presented inFig. 7because the concentrations of TEOSand EDAS remain similar in all samples in these series.

In Fig. 7, the nucleation phenomenon is demonstratedwhen palladium is complexed either by EDAES (series 1) orby EDAS (series 2 and 4). Indeed, both Pd(EDA(E)S)22+complexes present a similar structure and therefore hy-drolyzed Pd(EDA(E)S)22+ complexes can act as a nucle-ation agent leading to silica particles with a hydrolyzedPd(EDA(E)S)22+ core and a shell principally made ofhydrolyzed TEOS. Nevertheless, in a previous study[23],Alié et al. demonstrated that in xerogels without metal, thenucleation mechanism by the additive (EDAS or EDAES)depends only on the difference of reactivity between addi-tive and TEOS and when both EDAES and TEOS containethoxy groups, the nucleation mechanism by EDAES does

Fig. 7. Nucleation of the silica particles for the series 1 (EDAES) and 2 (EDAS) where theoretical palladium loading varies from 1.0 to 4.5 wt.%: EDAES(�), EDAS (�), and for the series 4 where the molar ratio EDAS/Pd varies from 2 to 8 ().

not take place. Nevertheless, when palladium is introducedin cogelled xerogels synthesized from either EDAES orEDAS, the Pd(EDA(E)S)22+ complexes are much more re-active than TEOS and the nucleation mechanism is checked.

From the number of silica particlesNs and the concen-tration of EDA(E)S transformed in number of EDA(E)Smolecules per liter by multiplying by Avogadro’s number, itis also possible to determine an average number of EDA(E)Smolecules per silica particle, which is also the number ofEDA(E)S molecules per nucleus. As can be seen inTable 4,the number of EDA(E)S molecules per particle decreases asthe palladium loading (series 1 and 2) and the molar ratioEDA(E)S/Pd (series 4) increase. For Pd/SiO2 xerogel cat-alysts, it is also possible to determine an average numberof metal atoms per silica particle by dividing the numberof EDA(E)S molecules per silica particle by 2 because inan aqueous solution, palladium forms with two ethylene-diamine molecules (NH2–CH2–CH2–NH2) a plane-squarecomplex[24] and it is assumed that in an ethanolic solution,

S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140 137

this stoechiometric ratio remains the same value betweenpalladium and ethylenediamine. From these average num-bers of metal atoms per silica particle, an average diameterof metal particles located in one silica particle,dmetal, canbe calculated as follows:

dmetal =(

6(metal/particle)vmetal

π

)1/3

(7)

where metal/particle is the number of metal atoms per sil-ica particle,vmetal the mean volume occupied by one metalatom in the bulk metal particle (nm3). For palladium, thevalue of vmetal are 0.01470 nm3 [12]. The values ofdmetalare given inTable 4. It is observed thatdmetal decreaseswhen the metal loading (series 1 and 2) and the molar ratioEDA(E)S/Pd (series 4) increase. Furthermore,dmetal, dTEM1anddchem for samples with only one family of small crys-tallites (Table 3) present similar values. So it is concludedthat the three methods used in this study for the determina-tion of metal particle sizes by TEM, CO chemisorption andby mass balance present similar results.

4.2. Completion of hydrolysis and condensationreactions

In all cogelled xerogel catalysts[14], the actual metalloadings were often higher than theoretical loadings be-cause the actual catalyst mass after drying, calcination andreduction steps was lower than theoretical mass. In fact,some organosilanes sometimes remained unreacted and werevolatilized during drying. Although hydrolysis and conden-sation reactions of alkoxides can proceed thermally withoutinvolving catalysts, their use especially in organosilanes isoften necessary. Numerous catalysts have been employed:generally compounds exhibiting acid or base character, butalso neutral salts and alkoxides[15]. In this work, three basesare used: EDAS, NH3 and TMAH. Furthermore, when co-gelled catalysts are dried by a supercritical drying, a longstay of gels in autoclave allowed a complete condensationreaction from TEOS in silica[13]. So the increase of thetemperatures of gelling, aging and vacuum drying of gels,Td, allows canceling weight losses in catalysts as in the caseof supercritical drying.

In Table 1, the weight losses decrease whenTd increasesfrom 25 to 80◦C (series 3) and when the amount of base in-creases in gels (series 4–6). Indeed, hydrolysis and conden-sation reaction rates are improved and all TEOS moleculesare transformed into silica. Therefore, TEOS is not morevolatilized during vacuum drying.

In Table 2, silica particle size,dSiO2, increases withTd(series 3) and with the amounts of NH3 and TMAH (series 5and 6): it can be explained by a more complete condensationreaction from TEOS in silica. The fact thatdSiO2 increasesallows explaining the reduction in shrinkage during vacuumdrying. Indeed, the pores existing between large silica par-ticles or aggregates are themselves large. Due to the factthat the capillary pressure is inversely proportional to the

pore radius[25], larger pores induce a reduction of shrink-age during drying. So, the total pore volume,Vv, increasesslightly with Td (series 3) and strongly with the amountsof NH3 and TMAH in gels (series 5 and 6)[26]. Further-more, the specific surface area,SBET, in series 3, 5 and 6 de-creases whenTd and the basicity of gels increase, which is inagreement with growing silica particle sizes[15]. Neverthe-less, when the molar ratio EDAS/Pd increases and thereforethe amount of EDAS in gels (series 4), silica particle size,dSiO2, decreases, but the total pore volume,Vv, increasesslightly and the specific surface area,SBET decreases veryslightly (Table 2). These observations seem completely incontradiction with the explanation above. In fact, EDAS hasa double function: on the one hand, it is a nucleant agentas demonstrated in point (I) of this discussion. So whenthe amount of EDAS increases in gels,dSiO2 decreases dueto more nucleation sites; on the other hand, by the pres-ence of amines in EDAS molecule, this one exhibits a basecharacter. So when the amount of EDAS increases in gels,Vv increases andSBET decreases. Nevertheless, these textu-ral properties variations are slighter than in those series inwhich either the agent nucleant role or the base character isdominating.

From TEM and CO chemisorption measurements(Table 3), metal dispersion increases withTd (series 3) andthe amount of NH3 in gels (series 5) because thanks to amore complete condensation reaction of TEOS in silica,metal particles are more trapped inside silica particles andthen are unable to migrate and to sinter during calcinationand reduction steps. The best palladium dispersion value,D, is obtained for sample 5e, in which the molarity ofNH3 solution is equal to 0.54 mol l−1. Nevertheless, if theamount of NH3 is still increased in gels, palladium disper-sion tends to become less or non-dependent on the amountof NH3. Indeed, NH3 can play a twofold role. On the hand,NH3 catalyzes hydrolysis and condensation reactions andtherefore TEOS conversion increases with the amount ofNH3 in the gel. On the other hand, NH3 can complex metallike Pd. The amount of complexes Pd(NH3)x

n+ are nottrapped inside silica matrix and then are able to migrate andto sinter during calcination and reduction steps. And so themetal dispersion decreases. For this reason, another organicbase that does not complex palladium, TMAH, is used inthis study.

When the molar concentration of TMAH solution variesbetween 0.001 and 0.05 mol l−1, samples 6(a–c) presentpalladium dispersion values of about 45% and therefore amean diameter of metal particles of about 2.5 nm (Table 3).So these Pd/SiO2 catalysts are highly nanometer-sized dis-persed. Nevertheless, the weight losses of samples 6(a–c)are ≥50%. When the amount of TMAH in gels is in-creased to cancel the weight losses (samples 6(d–f)), pal-ladium dispersion very strongly decreases and a very largemetal particles distribution is observed. These observationscould be explained by Hofmann elimination[27]: TMAHthermally decomposes to yield methanol (CH3OH) and

138 S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140

trimethylamine ((CH3)3N) as follows

((CH3)4N)+OH− !−→(CH3)3N + CH3OH (8)

If some amount of TMAH could be decomposed in trimethy-lamine and methanol during aging and vacuum drying ofgels, and as trimethylamine exhibits a lower base characterthan TMAH, it would be necessary therefore to have a greatamount of trimethylamine to cancel the weight losses in xe-rogel catalysts. It could be for this reason that the weightlosses are equal 0% for sample 6f for which the molarity ofTMAH solution is equal 2 mol l−1. In samples 6(a–c), pal-ladium dispersion values are about of 45% because TMAHand/or trimethylamine would not complex palladium. Thisone could be therefore complexed by EDAS and trappedin silica matrix. For samples 6(d–f), it is always unknownwhy palladium dispersion strongly decreases. So the use ofTMAH as an organic base which does not complex pal-ladium does not make Pd/SiO2 xerogel catalysts synthesiseasier.

For Pd/SiO2 cogelled xerogel catalysts, palladium dis-persion values,D, reach 46% and therefore a mean diam-eter of metal particles of about 2.5 nm (Table 3) whereasPd/SiO2 catalysts prepared by a classical method as impreg-nation present a mean diameter of palladium particles ofabout 7–30 nm[13]. The best palladium dispersion valuesobtained in the case of Pd/SiO2 cogelled xerogel catalystscome from the structure of cogelled catalysts: palladiumcrystallites with a diameter of about 2–3 nm are located in-side silica particles exhibiting a monodisperse microuporousdistribution centered on a pore size of about 0.8 nm. Becausepalladium crystallites are larger than the micropores of thesilica particles in which they are located, the palladium crys-tallites in cogelled catalysts are trapped and are then unableto migrate outside silica particles. In consequence, those cat-alysts are sinter-proof during treatments at high tempera-tures. In Pd/SiO2 impregnated samples, metal particles arenot trapped inside silica matrix. Therefore, metal particles

Fig. 8. TOF as a function of palladium dispersion for hydrodechlorination atT = 250◦C (�) andT = 300◦C (�).

are very mobile during treatments at high temperatures andsintering occurs[13].

4.3. Catalytic activity

A high activity of a metal catalyst often calls for a largeactive surface area and, thus, for small particles, i.e. a highdispersion of the active phase. Furthermore, a very impor-tant concern about cogelled catalysts is the accessibility ofthe active centers. Because palladium is located inside sil-ica particles, there is a risk that it may not be accessible.The values ofVv in Table 2show that the ability of dryingunder vacuum to retain porosity is high, but do not provethe accessibility of palladium. Nevertheless, it is observedin Table 3that dTEM1 ∼= dchem for samples with one metalparticles size and thatdTEM1 ≤ dchem≤ dTEM2 for sam-ples with two metal particles sizes. So in each case, metalparticles sizes obtained by CO chemisorption measurementsare related to TEM measurements. The convergence of TEMand chemisorption measurements proves that metal particleslocated inside silica particles are completely accessible.

In their study of 1,2-dichloroethane hydrodechlorinationover Rh/SiO2 catalysts at 92–280◦C, Bozelli et al.[4] foundthat C2H6 and HCl are the major products of the reaction.Ethyl chloride CH3–CH2Cl is a minor product. These au-thors proposed a reaction scheme where ethyl chloride isan intermediate product, which is produced from the reac-tion of dichloroethane with hydrogen. Ethyl chloride canbe further hydrodechlorinated to ethane. Results obtainedin this study (Fig. 5) and in previous studies[5,14] withPd/SiO2 cogelled catalysts show that this mechanism canapply for palladium, which mainly produces C2H6 and HClas well as small amounts of CH3–CH2Cl at 250–350◦C.In Fig. 5, the selectivity in C2H4 increases from 250 to350◦C, whereas the selectivity in CH3–CH2Cl decreasesfrom 250 to 350◦C. In one hand, this phenomenon couldsuggest that ethylene production over Pd/SiO2 cogelled xe-rogel catalysts may occur according to the following reac-

S. Lambert et al. / Applied Catalysis B: Environmental 50 (2004) 127–140 139

tion: CH3 − CH2Cl → C2H4 + HCl as observed on nickeland iron films at higher temperatures (284 and 263◦C, re-spectively)[28]. On the other hand, the presence of ethy-lene could result from the breaking of two C–Cl bondsin 1,2-dichloroethane molecules. A high temperature couldtherefore provide sufficient energy for adsorbed C2H4 to re-turn to the gas phase.

The specific consumption rate of 1,2-dichloroethane,r,is proportional to palladium dispersion (Fig. 6). So thecatalytic tests of 1,2-dichloroethane hydrodechlorinationover Pd/SiO2 catalysts confirm the evolution of metal dis-persion with synthesis operating variables, as explainedin Section 4.2. The turnover frequency (TOF), that is, thenumber of molecules consumed per surface metal atom andper second, for 1,2-dichloroethane hydrodechlorination ispresented inFig. 8 as a function of metal dispersion. Al-though a light deactivation is observed at 350◦C in Fig. 5,metal dispersion of fresh Pd/SiO2 catalysts are used to cal-culate TOF at 250 and 300◦C because CO chemisorptionmeasurements for tested sample 1c give a value of 24%for metal dispersion instead of 28% for fresh sample 1c. Itis observed inFig. 8 that TOF for 1,2-dichloroethane hy-drodechlorination over Pd/SiO2 catalysts remains constantwith metal dispersion atT = 250 and 300◦C. Although thestructure sensitivity of C–Cl hydrogenolysis with the en-semble size concept has been pointed out by several authorsin the case of chlorinated alkanes and chlorinated aromat-ics over Pd/Al2O3 and Pd/C catalysts[3,29,30], it seemsthat 1,2-dichloroethane hydrodechlorination over Pd/SiO2catalysts is a structure insensitive reaction[14].

5. Conclusions

The main purpose of the present work was to improvethe metal dispersion in Pd/SiO2 cogelled xerogels in sucha way as to increase their activity for 1,2-dichloroethanehydrodechlorination by the study of the influences ofsynthesis operating variables (nature and concentrationof complexing silane, metal percentage, temperatures ofgelling, aging and drying under vacuum of xerogels,molar ratio between the complexing silane and palla-dium, and nature and concentration of basic aqueoussolution). This purpose is reached because the use of3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) or3-(2-aminoethylamino)propyltriethoxysilane (EDAES) tocomplex palladium in an ethanolic solution containingtetraethoxysilane (TEOS) and an ammonia solution of0.54 mol l−1 allows obtaining a Pd/SiO2 xerogel catalystwith a mean metal particle diameter of 2.5 nm located in-side silica particles. Indeed, complexes Pd(EDA(E)S)xn+induce a nucleation mechanism because of their higherreactivity compared to the network-reagent (TEOS).

This study has provided several answers such as: (i) theuse of EDAES or EDAS as complexing silane allows obtain-ing Pd/SiO2 catalysts with very similar textural properties,

metal dispersion and catalytic activity; (ii) xerogel catalystswith a metal loading >2 wt.% present a too low metal dis-persion to present an interest in catalysis; (iii) the use of amolar ratio between the complexing silane and palladiumgreater than the stoechiometric ratio very strongly decreasespalladium dispersion because silica particle size decreasesdue to more nucleation sites.

Although metallic particles are located inside the silicaparticles, their complete accessibility, via the micropore net-work, has been shown. 1,2-dichloroethane hydrodechlori-nation over Pd/SiO2 catalysts mainly produces ethane andthe hydrodechlorination activity is proportional to palladiumdispersion. The improvement of palladium dispersion from16 to 46% in Pd/SiO2 cogelled xerogel catalysts allows toincrease very strongly the hydrodechlorination activity. Hy-drodechlorination over Pd/SiO2 xerogel cogelled catalysts isa structure insensitive reaction with regard to the ensemblesize concept.

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 tothe Belgian 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 deRecherche Fondamentale et Collective, the Ministère dela Région Wallonne and the Ministère de la CommunautéFrançaise (Action de Recherche Concertée no. 00-05-265)for their financial support.

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