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Catalysis Communications 8 (2007) 1244–1248

Ag/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogel catalysts forbenzene combustion: Relationships between operating

synthesis variables and catalytic activity

Stephanie Lambert a,*, Caroline Cellier b, Eric M. Gaigneaux b,Jean-Paul Pirard a, Benoıt Heinrichs a

a Laboratoire de Genie Chimique, B6a, Universite de Liege, B-4000 Liege, Belgiumb Unite de Catalyse et Chimie des Materiaux Divises, Universite Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium

Received 13 June 2006; received in revised form 14 November 2006; accepted 15 November 2006Available online 19 November 2006

Abstract

The aim of this work is to explore the applicability of the sol–gel method for the preparation of Ag/SiO2, Cu/SiO2 and Pd/SiO2 cat-alysts and to see whether such a method can yield silver, copper and palladium species stabilized by the carrier in the case of benzeneoxidation. So Ag/SiO2, Cu/SiO2 and Pd/SiO2 xerogel catalysts were synthesized by cogelation of tetraethoxysilane (TEOS) and chelatesof Ag, Cu and Pd with 3-(2-aminoethylamino)-propyltrimethoxysilane (EDAS). The resulting catalysts are composed of completelyaccessible metallic crystallites with a diameter of about 3 nm located inside silica particles.� 2006 Elsevier B.V. All rights reserved.

Keywords: Sol–gel process; Metallic nanoparticles; Benzene combusion

1. Introduction

The reaction studied in this work is oxidation of benzeneover Ag/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogel cata-lysts. In fact, volatile organic compounds (VOCs) are rec-ognized as major contributors to air pollution, eitherdirectly through their toxic or malodorous nature, or indi-rectly as ozone precursors and smog precursors [1]. Whenthere is no interest in recovering VOCs, they are usuallydestroyed by deep oxidation. Catalytic incineration ofVOCs is preferred over thermal incineration for severalreasons. A thermal incinerator operates at higher tempera-tures (700–1200 �C) than a catalytic unit (200–500 �C), andthermal incineration often has to be supplemented withfuel to maintain a flame in a dilute VOC waste stream (lessthan 1000 ppm). Moreover, thermal incineration couldproduce toxic substances and NOx [2]. Therefore, funda-

1566-7367/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2006.11.018

* Corresponding author. Tel.: +32 4366 4771; fax: +32 4366 3545.E-mail address: stephanie.lambert@ulg.ac.be (S. Lambert).

mental knowledge of catalytic oxidation mechanisms canbe valuable to develop effective methods to control airpollution.

The total oxidation of trimethylamine and/or n-hexanehas been examined previously on c-MnO2 and Au/TiO2

catalysts [3,4]. In the former case, a coupling between cat-alytic results and extensive characterization of the catalystssuggested a redox Mars and van Krevelen mechanism inwhich the nature of the VOC dictates its conversion levelthrough its ability to reduce the catalyst and thus acceleratethe incorporation rate of gaseous oxygen.

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 sinter already at relatively low tempera-tures, these ones should be strongly anchored on a supportmaterial which itself is thermally stable and keeps a highspecific surface area up to high temperatures. The aim ofthis work is to explore the applicability of the sol–gelmethod for the preparation of Ag/SiO2, Cu/SiO2 and

S. Lambert et al. / Catalysis Communications 8 (2007) 1244–1248 1245

Pd/SiO2 catalysts and to see whether such a method canyield silver, copper and palladium species stabilized bythe carrier in the case of benzene oxidation. Characteriza-tion of the catalyst structure by several physical and chem-ical techniques, including nitrogen adsorption–desorptionisotherms, mercury porosimetry measurements, X-ray dif-fraction, transmission electron microscopy and chemisorp-tion measurements, has been used to correlate themicrostructure of Ag/SiO2, Cu/SiO2 and Pd/SiO2 catalystswith their catalytic performance.

2. Experimental

Five Ag/SiO2, four Cu/SiO2 and one Pd/SiO2 catalystswere synthesized in ethanol from corresponding metal ace-tate (OAc), TEOS (Si(OC2H5)4), EDAS (NH2–CH2–CH2–NH–(CH2)3–Si(OCH3)3), and a solution of aqueous 0.18 NNH3. Furthermore, three monometallic catalysts with atheoretical metal loading of 1.50 wt.% of Ag, Cu and Pdwere made in ethanol from corresponding metal acetylacet-onate (acac), TEOS, EDAS, and a solution of aqueous0.18 N NH3. Metal precursor and EDAS were mixedtogether in half the total volume of ethanol (the molarratios EDAS/Ag(OAc) = 2, EDAS/Ag(acac) = 2, EDAS/Pd(OAc)2 = 2, EDAS/Pd(acac)2 = 2, EDAS/Cu(OAc)2 =4 and EDAS/Cu(acac)2 = 4 [5,6]). The slurry was stirredat room temperature until a colourless solution wasobtained for silver, a clear blue solution for copper, anda clear yellow solution for palladium (about half an hour).After addition of TEOS, a solution of aqueous 0.18 N NH3

in the other half of the total volume of ethanol was addedto the mixture under vigorous stirring. The hydrolysisratio, that is the molar ratio H = H2O/(TEOS + 3/4EDAS) (factor 3/4 is due to the fact EDAS contains onlythree hydrolysable groups in relation to TEOS which con-tains four hydrolysable groups), and the dilution ratio, thatis the molar ratio R = ethanol/(TEOS + EDAS) were keptconstant at the values of 5 and 10, respectively, for all sam-ples. The vessel was then closed and heated up to 70 �C for3 days (gelation and aging [7]). The wet gels were dried

Table 1Synthesis operating variables of Ag/SiO2, Cu/SiO2 and Pd/SiO2 catalysts

Sample Metal powder (mmol) nEDAS (mmol) nTEOS (mmol) nH

Ag0.40 0.442 0.85 188 940Ag1.00 1.073 2.13 186 940Ag1.50 1.608 3.21 185 940Ag2.60 2.705 5.39 183 930Ag3.65 3.832 7.63 181 930

Cu1.00 1.757 7.20 181 940Cu1.50 2.716 10.86 178 930Cu2.15 3.814 15.31 173 920Cu4.50 8.036 32.08 156 900

Pd1.50 1.632 3.50 186 940Ag1.50acac 1.628 3.23 186 940Cu1.50acac 2.742 10.91 178 930Pd1.50acac 1.569 3.16 186 940

under vacuum according to the following procedure: theflasks were opened and put into a drying oven at 60 �C,and the pressure was slowly decreased (to prevent gel burst-ing) to the minimum value of 1200 Pa after 90 h. The dry-ing oven was then heated at 150 �C for 72 h. The resultingsamples are xerogels [7]. Synthesis operating variables ofAg/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogel catalystsare shown in Table 1. For each catalyst, Ag, Cu or Pddenote the name of metal followed by the theoreticalweight percentage of metal in the sample. For Ag, Cuand Pd-based samples synthesized from correspondingmetal acetylacetonate, the name of each sample is followedby the abbreviation ‘‘acac’’. The conditions of calcinationfor Ag/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogel cata-lysts were as follows: the sample was heated up to 400 �Cat a rate of 120 �C/h under flowing air (0.022 mmol s�1);this temperature was maintained for 12 h in air(0.11 mmol s�1). The catalysts were then reduced. So Ag/SiO2 and Pd/SiO2 cogelled xerogel catalysts were heatedto 350 �C at a rate of 350 �C/h under flowing H2

(0.25 mmol s�1) and maintained at this temperature for3 h (same flow). The Cu/SiO2 cogelled xerogel catalystswere heated to 400 �C at a rate of 350 �C/h under flowingH2 (0.25 mmol s�1) and maintained at 400 �C for 3 h (sameflow).

The size of metallic particles was examined by transmis-sion electron microscopy (TEM) with a Philips CM100microscope. All 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 60 �C and60 nm slices were then cut up with a Reichert–Jung Ultra-cut E microtome. Finally, these slices were put on a coppergrid. X-ray diffraction (XRD) was also used to determinemetal particles sizes. Patterns were obtained with hand-pressed samples mounted on a Siemens D5000 goniometerusing the Cu Ka line (Ni filter). Metal dispersion in Pd/SiO2, Ag/SiO2 and Cu/SiO2 catalysts was determined fromCO chemisorption at 30 �C on a Fisons Sorptomatic 1990device and O2 chemisorption at 150 �C for Ag and at 30 �Cfor Cu on a Micrometrics device, respectively. Before mea-

2O (mmol) nC2H5OH (mmol) Gel time (min) Metal loading ±0.05%

1890 170 0.401890 51 1.001890 50 1.551890 50 2.601890 48 3.65

1890 69 1.001890 54 1.501890 49 2.151890 40 4.50

1890 66 1.551890 18 1.801890 20 1.501890 24 1.90

1246 S. Lambert et al. / Catalysis Communications 8 (2007) 1244–1248

surements, the calcined sample was reduced in situ in flow-ing H2 (0.003 mmol s�1) at 350 �C for 3 h for palladiumand silver, and at 400 �C for 4 h for copper. After, this sam-ple was outgassed under vacuum at 340 �C for 16 h for pal-ladium and silver, and at 200 �C for 20 h for copper. Adouble adsorption method was used: (i) first adsorptionisotherm was measured, which includes both physisorptionand chemisorption; (ii) after a 2 h outgassing at 30 �C forCO or at 150 �C (Ag) or 30 �C (Cu) for O2, a second iso-therm was measured which includes physisorption only.Both isotherms are determined in the pressure range of10�8 to 2 · 10�1 kPa. The difference between first and sec-ond isotherms gave the CO or O2 chemisorption isotherm[8–11].

For benzene oxidation over Ag/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogel catalysts, 0.007 mmol s�1 of C6H6/N2 mixture (2550 ppm of benzene in nitrogen),0.01 mmol s�1 of oxygen and 0.052 mmol s�1 of nitrogenwere used. 0.1 g of catalyst in the form of pellets with adiameter between 0.2 and 0.315 mm was diluted in 7 cm3

of glass beads with the same diameter, previously checkedto be inactive. The reactants and products of a possibleincomplete combustion were separated by gas chromatog-raphy and then quantified using a flame ionisation detector(FID). The conversion for all metallic cogelled xerogel cat-alysts has been measured at atmospheric pressure and atvarious temperature levels from 100 �C to 360 �C by 20 �C.

3. Results and discussion

From nitrogen adsorption–desorption isotherms andmercury porosimetry measurements, it was demonstratedthat all Ag/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogelcatalysts contain micropores (width < 2 nm), mesopores

Table 2Metal particle size and dispersion of Ag/SiO2, Cu/SiO2 and Pd/SiO2 catalysts

Catalyst TEM

dTEM1 (nm) rTEM1 (nm) dTEM2 (nm)

Ag0.40 4.0 0.5 –a

Ag1.00 3.6 0.3 –a

Ag1.50 3.3 0.3 13Ag2.60 3.2 0.3 20Ag3.65 2.9 0.2 30

Cu1.00 3.2 0.4 –a

Cu1.50 3.1 0.4 8Cu2.15 2.8 0.2 15Cu4.50 2.4 0.2 30

Pd1.50 2.6 0.3 18Ag1.50acac 3.3 0.4 –a

Cu1.50acac 3.0 0.3 –a

Pd1.50acac 2.7 0.2 11

dTEM1, dTEM2: mean diameters of small and large metal particles, respectiveldTEM1 and dTEM2, respectively; D: metal dispersion measured by chemisorption;particles estimated from X-ray line broadening.

a Not observed.b Not measurable.

(2 nm < width < 50 nm) and macropores (width > 50 nm)[6]. For all metallic catalysts, the total pore volume variesbetween 2 and 7 cm3/g, and the specific surface area variesbetween 200 and 600 m2/g [5,6]. The high values of thetotal pore volume and specific surface area show thatcogelled xerogel catalysts are very porous, but do not provethe accessibility of silver, copper and palladium.

All samples were examined by TEM, by chemisorptionof CO (Pd catalysts) or O2 (Ag and Cu catalysts) and byXRD to obtain metallic particle size. Results are presentedin Table 2. TEM analysis indicates that the metal is dis-persed in tiny particles below ca. 1 wt.%. Beyond a valuebetween 1 and 1.5 wt.%, all samples exhibit metal particlesdistributed in two families of different sizes: small crystal-lites between 2.4 and 4 nm, dTEM1, and large crystallitesbetween 10 and 30 nm, dTEM2 (Fig. 1). In the case of Pd/SiO2 and Pd–Ag/SiO2 xerogel catalysts prepared in thesame way, it has been shown form 3-D electron tomogra-phy [12] and rotating TEM [13] that the small metal crys-tallites are located inside silica particles exhibiting a sizebetween 10 and 60 nm. The mean size of metal particleshas been also derived from metal dispersion, D, measuredby CO chemisorption for Pd/SiO2 catalysts and O2 chemi-sorption for Ag/SiO2 and Cu/SiO2 catalysts [5]. A meandiameter, dchem, of Pd, Ag and Cu crystallites has beenderived from dispersion values assuming those crystallitesto be roughly spherical in shape. Values of dchem are givenin Table 2. In each series, it is observed that D decreases,and so dchem increases, when the metal loading increases.Moreover, at comparable metal loadings, slightly higherdispersions are reached with Pd catalysts in comparisonwith Ag and Cu catalysts. Furthermore, metallic xerogelcatalysts synthesized from metal acetylacetonate precursorspresent higher dispersions than the same catalysts synthe-

CO or O2 chemisorption XRD

rTEM2 (nm) dchem (nm) D (%) dXRD (nm)

–a 4.2 28 –b

–a 4.2 28 –b

6 5.8 20 75 7.4 16 108 9.8 12 14

–a 3.8 28 –b

2 5.2 20 126 6.4 16 189 8.6 12 24

4 4.6 24 11–a 4.2 28 –b

–a 3.8 28 –b

1 3.4 32 4

y measured by TEM; rTEM1, rTEM2: standard deviations associated withdchem: mean diameter derived from dispersion D; dXRD: mean size of metal

1

10

100

1000

0 2 4 6

metal loading (wt.%)

spec

ific

act

ivit

y (m

mol

s-1

kg-1

met

al)

Fig. 2. Specific activity at T = 260 �C as a function of metal loading forthe series Ag (j) and for the series Cu ð�Þ.

Fig. 1. TEM micrographs of Ag3.65 sample (250,000· and 350,000·).

S. Lambert et al. / Catalysis Communications 8 (2007) 1244–1248 1247

sized from metal acetate precursors. X-ray diffraction is thethird experimental technique that has been used to evaluatethe size of metal crystallites. A mean size, dXRD, has beencalculated from Scherrer’s formula based on line broaden-ing analysis (Table 2) [11].

A comparison between dXRD, dTEM1 and dTEM2 in Table2 shows that XRD and TEM results are in agreement sincedXRD values are between dTEM1 and dTEM2 values. A veryimportant concern about cogelled catalysts is the accessibil-ity of the active centers. Because the metal (palladium, sil-ver or copper) is located inside silica particles, there is arisk that it may not be accessible. The values of the totalpore volume of all catalysts [5] show that the ability of dry-ing under vacuum to retain porosity is high, but do notprove the accessibility of palladium, silver or copper. Nev-ertheless, it is observed in Table 2 that dTEM1 @ dchem forsamples with one metal particles series (metal loading<1.5 wt.%) and that dTEM1 6 dchem 6 dTEM2 for sampleswith two metal particles series (metal loadingP1.5 wt.%). So in each case, metal particle sizes obtainedby CO and O2 chemisorption measurements are in agree-ment with those determined by TEM. The convergence ofTEM and chemisorption measurements proves that metalparticles located inside silica particles are completelyaccessible.

A comparison between dXRD and dchem indicates thatthe latter is systematically lower than the former. A firstglance this could be surprising since XRD does not givea particle size but rather a crystalline domain size. In con-sequence, such a result indicates that metal particles areprobably mostly constituted of a single crystal. Moreover,XRD gives a volume weighted size and thus is more sensi-tive to large particles than chemisorption which gives a sur-face weighted size [11]. That is the reason why sizes fromXRD are higher than size from chemisorption in the pres-ent case of a particle size distribution.

It has been shown in a previous paper [14] that the veryparticular structure of those catalysts allows to avoid diffu-

sional limitations. Indeed, in Pd/SiO2, Ag/SiO2 and Cu/SiO2 xerogel catalysts, in order to reach active sites, reac-tants must first diffuse through large pores located betweenaggregates of SiO2 particles and then through smaller poresbetween those elementary particles inside the aggregates.Finally, they diffuse through micropores inside the silicaparticles. It was shown that there is no problem of masstransfer at each of the three levels. So there are no pore dif-fusion limitations and the observed rate is equal to theintrinsic rate of the chemical reaction.

Over Ag/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogelcatalysts used in this study, benzene is oxidized in H2Oand CO2 only. As indicated in Table 2, the dispersion inAg/SiO2 and Cu/SiO2 xerogel catalysts decreases whenmetal loading increases. In consequence, as it is observedin Fig. 2, the specific activity decreases when metal loadingincreases. So the catalytic tests of benzene oxidation withAg/SiO2 and Cu/SiO2 catalysts confirm again the evolutionof metal dispersion with metal loading. In Fig. 3, it isobserved that benzene conversion is a function of the nat-ure of the metal: Pd/SiO2 > Ag/SiO2 > Cu/SiO2. Suchresults are in agreement with literature data. Indeed, inthe case of metal catalysts on Fe2O3, Scire et al. [15]observed that the activity towards the oxidation of volatileorganic compounds (VOC) has been found to be in theorder: Au/Fe2O3 > Ag/Fe2O3 > Cu/Fe2O3 > Fe2O3 in therange of temperature investigated (100–400 �C). Papaefthi-miou et al. [16] showed that at 180 �C for benzene oxida-tion, Pd/Al2O3 and Pt/Al2O3 catalysts are more activethan catalysts synthesized with metal from group IB.Finally, in Fig. 3, it is observed that metallic xerogel cata-lysts synthesized from metal acetylacetonate precursorspresent higher conversions than the same catalysts synthe-sized from metal acetate precursors. Indeed, in Table 2,metallic xerogel catalysts synthesized from metal acetyl-acetonate precursors present higher dispersions than thesame catalysts synthesized from metal acetate precursors.So from results in Fig. 3 and Table 2, it is concluded that

0

25

50

75

100

100 140 180 220 260 300 340Temperature (˚C)

Ben

zene

con

vers

ion

(%)

Fig. 3. Benzene conversion as a function of temperature, nature of themetal and nature of metal precursor. Ag1.50 (j), Ag1.50acac (�), Cu1.50ð�Þ, Cu1.50acac (�), Pd1.50 (m) and Pd1.50acac (D).

1248 S. Lambert et al. / Catalysis Communications 8 (2007) 1244–1248

Ag/SiO2, Cu/SiO2 and Pd/SiO2 activity increases withmetal dispersion and therefore, depends of operating syn-thesis variables.

4. Conclusions

The sol–gel method applied to Ag/SiO2, Cu/SiO2 andPd/SiO2 catalysts allows to stabilize silver, copper and pal-ladium species by the silica network. Although metallicparticles are located in SiO2 particles, their complete acces-sibility, via the microporosity, has been shown for benzeneoxidation. In fact, the greater intrinsic kinetic rates areobserved for cogelled xerogel catalysts with lower metalloading and higher dispersion. Benzene conversion is alsoa function of the nature of the metal: Pd/SiO2 > Ag/SiO2 > Cu/SiO2, with a complete benzene conversion intoH2O and CO2 for Pd/SiO2 cogelled xerogel catalysts from200 �C.

Acknowledgements

The authors thank the Centre d’Enseignement et deRecherche des Macromolecules, C.E.R.M., from the Uni-

versity of Liege for TEM analysis and the Laboratoire deChimie Inorganique Structurale from the University ofLiege for XRD analysis. S. Lambert is grateful to the Bel-gian Fonds National de la Recherche Scientifique(F.N.R.S.) for a postdoctoral researcher position. Theauthors also thank the Belgian Fonds de la RechercheFondamentale Collective, the Ministere de la RegionWallonne, the Belgian Ministere de la CommunauteFrancaise and the Fonds de Bay for their financialsupport.

References

[1] J.A. Horsley, Catalytica Environmental Report E4, CatalyticaStudies Division, Mountain View, CA, 1993.

[2] P.O. Larsson, A. Andersson, L.R. Wallenberg, B. Swensson, J. Catal.163 (1996) 279.

[3] C. Cellier, V. Ruaux, C. Lahousse, P. Grange, E.M. Gaigneaux,Catal. Today 117 (2006) 350.

[4] C. Cellier, S. Lambert, E.M. Gaigneaux, C. Poleunis, V. Ruaux, P.Eloy, C. Lahousse, P. Bertrand, J.-P. Pirard, P. Grange, Appl. Catal.B., in press.

[5] S. Lambert, C. Cellier, P. Grange, J.-P. Pirard, B. Heinrichs, J. Catal.221 (2004) 335.

[6] S. Lambert, C. Alie, J.-P. Pirard, B. Heinrichs, J. Non-Cryst. Solids342 (2004) 70.

[7] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics andChemistry of Sol–Gel Processing, Academic Press, San Diego, 1990.

[8] J.J.F. Scholten, J.A. Konvalinka, F.W. Beekman, J. Catal. 28 (1973)209.

[9] J.H. Sinfelt, Bimetallic Catalysts – Discoveries, Concepts, andApplications, Wiley, New York, 1983.

[10] E. Iglesia, M. Boudart, J. Catal. 81 (1983) 204.[11] G. Bergeret, P. Gallezot, in: G. Ertl, H. Knozinger, J. Weitkamp

(Eds.), Handbook of Heterogeneous Catalysis, Wiley-VCH, Wein-heim, 1997, p. 439.

[12] C.J. Gommes, K. de Jong, J.-P. Pirard, S. Blacher, Langmuir 21(2005) 12378.

[13] B. Heinrichs, J.W. Geus, S. Lambert, J.P. Pirard, J. Catal. 241 (2006)229.

[14] B. Heinrichs, J.-P. Schoebrechts, J.-P. Pirard, AICHE J. 47 (2001)1866.

[15] S. Scire, S. Minico, C. Crisafulli, S. Galvagno, Catal. Commun. 2(2001) 229.

[16] P. Papaefthimiou, T. Ioannides, X.E. Verykios, Appl. Catal. B: Env.13 (1997) 175.