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Applied Catalysis A: General 239 (2003) 8794

Surface structure and catalytic behavior of silica-supported coppercatalysts prepared by impregnation and solgel methods

Zhenl Wang, Qingsheng Liu, Jianfeng Yu, Tonghao Wu, Guojia Wang

Department of Chemistry, Jilin University, Changchun 130023, PR China

Received 6 February 2002; received in revised form 24 June 2002; accepted 24 June 2002

Abstract

Supported copper on silica catalysts prepared by solgel and impregnation methods were studied in this paper. The surfacestructures of these catalysts were characterized by various techniques, including BET, XRD, FTIR, XPS, TPR, and ESR. Theresults showed that the distribution of copper species was in different ways in the catalysts prepared by the two methods.Cu(II) species highly dispersed in the silica matrix for the solgel catalyst, while copper oxide clusters are dominant in theCuO-SiO2 sample prepared by the impregnation method. The catalysts were then used for dehydrogenation of 2-butanol.Obvious differences of catalytic behavior were observed for the catalysts prepared by the two methods. High selectivity(>90%) toward dehydrogenation and high 2-butanol conversion was observed for the impregnated catalyst; however, forthe CuO-SiO2 solgel catalyst, very low dehydrogenation selectivity and 2-butanol conversion were obtained. The surfacestructures of catalysts were closely related to the preparation methods, and the catalytic behaviors were affected subsequently.The copper oxide clusters, which may be reduced to Cu0, are responsible for the dehydrogenation reaction. The highlydispersed Cu(II) ions were inactive for catalyze 2-butanol dehydrogenation. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Cu-SiO2; Solgel; Impregnation; 2-Butanol; Dehydrogenation

1. Introduction

Supported copper catalysts have been attracted con-siderable attention due to recent practical uses in pro-moting methanol steam reforming [1], dehydrogena-

tion [2], and ester hydrogenolysis [3]. It is importantto study the preparation method and the nature of theinteraction between the support material and the cat-alytic species in gaining a deeper understanding ofcopper-containing catalysts.

To achieve high catalytic activity, it is neces-sary to disperse fine particles in well-defined pores

Corresponding author. Tel.: +86-431-8922331x3214;fax: +86-431-8949334.

E-mail address: [email protected] (G. Wang).

in a support. By conventional methods for catalystpreparation such as impregnation, however, inho-mogeneous agglomeration of active species at grainboundary of support occurs especially at higher con-tent, and large-sized particles result [4]. Although

the ion-exchange method stabilizes copper specieson silica and leads to the better dispersion of copperon silica than the wet impregnation method does [5],this ion-exchange method can not be widely usedbecause of the low loading due to the limit terminalOH groups on the surface of silica. For stabilizingof the active phase in supported copper catalyst, thesolgel technique offers some advantages. With thispreparation method, copper species are effectivelyincorporated into ionic oxide network [6,7] and themethod may led to more stable catalysts than those

0926-860X/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0926-86 0X(02 )0042 1-0

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88 Z. Wang et al./ Applied Catalysis A: General 239 (2003) 8794

prepared by conventional methods. Because synthesisparameters are available, solgel processing offersversatility in final material properties not available

by other catalyst synthesis methods [810]. How-ever, lower activities were usually observed due toencapsulation in some solgel catalysts [11].

It has been suggested that more than one form ofcopper species exist on the surface of catalysts. De-pending on the different preparation processes andexperimental conditions, various species would playdifferent roles in catalytic reactions [1215]. Guer-reiro et al. [14] reported the presence of low-interactedCu(II) species, highly dispersed and surface-interactedspecies in silica-supported copper catalyst prepared byionic exchange. Sakata et al. [16] reported that iso-lated and aggregated copper(II) ions co-existed on theCu-Pd-H3PO4/SiO2 catalyst. The isolated Cu(II) ionscoordination with PO43 was found to be reversiblewith alternate reductionoxidation treatments and wasresponsible for catalytically active sites in the directoxidation of benzene. Marchi et al. [17] reported thattwo CuO phases are present on the CuO-SiO2 catalystwith different properties.

In the present work, silica-supported copper cat-alysts were prepared by solgel technique and con-ventional impregnation method, and the two types

were calcined at the same temperature. The sur-face structures of the catalysts were studied byBET, XPS, XRD, FTIR, and ESR techniques. The2-butanol-dehydrogenation was chosen as a testing re-action to investigate the catalytic behaviors of the cat-alysts. The main aim is to understand the relationshipsamong the preparation methods, the surface structureand the catalytic performances of these catalysts.

2. Experimental

2.1. Catalyst preparation

The CuO-SiO2 samples were prepared by thesolgel techniques from Si(OC2H5)4 (TEOS) andCu(NO3)23H2O precursors with CuO content of5.0 wt.%. This method has been successfully used forpreparing Me-SiO2 (Me = ZnO, Fe2O2) nanocom-posites [18,19]. TEOS was mixed with ethanol andan aqueous solution of Cu(NO3)3H2O. The molarratio TEOS:C2H5OH:H2O was 1:3.85:10.2. After 1 h

stirring, the clear sol was then poured into a Teflonbeaker and allowed to gel in air. The gel was dried in anoven by slowly raising the temperature to 100 C over

a week. The sample was then powdered and heated to500 C, in steps of 100 C maintaining the temperatureat each step for 30 min. Such a sample was denotedas SG. The CuO-SiO2 catalyst containing 5 wt.% ofCuO was synthesized by the incipient-wetness im-pregnation method. The SiO2 was impregnated withan aqueous solution of Cu(NO3)23H2O. The slurryformed was dried at 70 C for 18 h, and then calcinedin the same process as used for SG catalyst. Theprepared sample is denoted as IM.

2.2. Catalyst characterization

BET specific surface areas of the samples were mea-sured on a Micromeritics ASAP 2010 system underliquid-N2 temperature using N2 as the adsorbate. TheXRD analyses were recorded on a D/Max-rA diffrac-tometer. The scans were taken in a 2= 2080 rangewith step size of 0.05 using Cu K radiation at 50 kVand 150 mA. The chemical states of the copper parti-cles were checked by a VG ESCA LAB MK II XPSsystem, using a standard Al K (1486.6eV) source.Charging effects were corrected by adjusting the C 1speak to 284.6 eV. ESR measurements were made atroom temperature on a Bruker ER 200D-SRC spec-trometer operated at X-Band frequencies, 9.8GHzKlystron frequencies, and 100 KHz magnetic fieldmodulations. DPPH was used as a field marker (g =2.0036). FTIR measurements were performed on aNicolet-impact 410 spectrometer (KBr tablet). Spec-tra were recorded in the range of 4000400cm1.Temperature programmed reduction (TPR) was car-ried out in a flow reactor system. Fifty milligramcatalyst was placed in a quartz reactor, the reduction

gas was 5% H2Ar mixture with a total flow rate of30cm3/min. The heating rate was 10 C/min with therange 100800 C and using a thermal conductivitydetector measured the hydrogen consumption. Themixture was dried in a molecular-sieves trap.

2.3. Catalyst testing

The catalytic conversion of 2-butanol was carriedout in a fixed bed microreactor operated at atmospheric

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Z. Wang et al. / Applied Catalysis A: General 239 (2003) 8794 89

pressure. The typical run was performed on 0.3 g ofcatalyst sieved to 4060 mesh and pretreated in situfor 0.5 h by a flow of pure argon at 673 K. 2-Butanol

was directly introduced into the reactor with a pulse-less pump (made in Beijing, China) without any car-rier gas. The GHSV was maintained at 1600h1 atthe reaction temperature. The reaction mixture was an-alyzed quantitatively by on-line gas chromatographyequipped with a flame detector and an integrator (Shi-madzu C-R6A Chromatopac).

3. Results and discussion

3.1. Surface properties

BET surface data of SG and IM catalysts are sum-marized in Table 1, together with the colors of thefresh catalysts. The specific surface area of SG cat-alyst (425.1m2/g) is higher than that of IM catalyst(365.4cm2/g), suggesting that higher specific surfaceareas can be obtained by the solgel method. The IMsamples show a clear decrease in specific surface areasand average pore diameters relative to their supports.This is expected because the pores in each support arepartially filled by CuOparticles. The IM sample is dark

Fig. 1. X-ray diffraction patterns of (a) fresh SG catalyst; (b) used SG catalyst; (c) fresh IM catalyst and (d) used IM catalyst.

Table 1BET surface characterization of SG and IM catalysts

Catalysts BET surface

areas (m

2

/g)

Average pore

diameters ()

Colors

SG 425.1 21.8 Clear blueIM 365.4 21.2 Dark graySiO2 435.2 24.8 White

gray in color, which suggests that CuO phase existsin the sample. However, the SG catalyst is clear blue,characteristic of the presence of cupric ions Cu2+ in-corporated into silica [20], indicating that Cu cationsare dispersed in silica gel matrix in atomic level andthat no separate CuO phase was formed during the

calcination [4].Fig. 1 shows the XRD patterns of both calcinated(fresh) and reduced (used) SG and IM catalysts. TheSG catalysts, both fresh and used, do not show thesignal of either metallic copper or of copper oxidebut do show a broad peak attributed to amorphoussilica (Fig. 1a and b), This indicates that the copperspecies particles, which are amorphous or very small,are highly dispersed within the matrix. In the caseof IM catalysts, the typical pattern of CuO is ob-served in the fresh catalyst (Fig. 1c) and three peaks,centered at 43.4, 50.3, and 74.1, attributed to metal

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Fig. 2. FTIR spectra of (a) SG catalyst and (b) IM catalyst.

copper crystallite appear in the used one (Fig. 1d).The results indicate that Cu species would be betterstabilized in the silica support by this solgel methodthan by impregnation method during the calcination.

The typical FTIR absorbance spectra of SG and IM

catalysts are shown in Fig. 2. No obvious differenceis observed between SG and IM catalysts. The spec-tra have a broad absorption band at approximately3460cm1, which is assigned to the OH stretchingvibration of water, ethanol and silanols. Similarly, theband near 1630cm1 is associated with the bend-ing mode of OH groups of adsorbed water [21].Bands at approximately 1200 and 1100cm1 corre-spond to stretching vibrations of SiO bonds; thatat 964.2 cm1 is due to the presence of non-bondedoxygen SiO [22]. The bands at approximately 800

and 575cm

1

are also attributed to bending vibra-tions of SiO bonds, and that at 464.7 cm1 belongsto bending vibrations of OSiO bonds [23]. Thevibration of CuO bonds that appear at 575, 500, and460cm1 [24] cannot be observed due to the presenceof a broad band at 467.4cm1 from the silica support.

In order to obtain more information about the Cuspecies in the samples, the chemical compositions ofthe catalysts were determined by X-ray photoelectronspectroscopy (Figs. 3 and 4). For SG catalysts, theCu 2p3/2 peak is at 933.7 eV with a shoulder on the

high BE side, which is characteristic of Cu2+ species[25,26] in a fresh sample (Fig. 3a), and the inten-sity of the satellites shows no obvious decrease afterthe reaction (Fig. 3b). The color of the sample is stillclear blue. The results suggest that the Cu(II) species

does not change during the reaction. In the case offresh IM catalyst, the position of the Cu 2p3/2 peak

Fig. 3. The XPS spectra of Cu 2p3/2 core level of (a) fresh SGcatalyst; (b) used SG catalyst; (c) fresh IM catalyst and (d) usedIM catalyst.

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Fig. 4. The XPS spectra of Cu LLM lines of (a) fresh IM catalystand (b) used IM catalyst.

is at 934.2eV (Fig. 3c) and the Cu LMM peak is at569.4eV (Fig. 4a), consistent with CuO [27,28]. TheCu2+ satellites are clearly visible in the Cu 2p XPSspectrum. After reaction, the Cu2+ satellites nearlydisappeared, the Cu 2p3/2 peak shifted to 932.5eV(Fig. 3d), and the Cu LMM shifted to 568.2 eV (Fig.4b), consistent with the formation of copper metal

[27,28].

Fig. 5. ESR profiles of (a) fresh SG catalyst; (b) used SG catalyst and (c) IM catalyst.

Electron spin resonance (ESR) is an absolutely nec-essary method to investigate the Cu(II) species in thecatalysts. The room temperature ESR spectra of SG

and IM catalysts are reported in Fig. 5.A strong axially symmetric signal (g = 2.07) isobtained for fresh SG catalyst (Fig. 5a). The axiallysymmetrical signals, which are generally attributed tohighly dispersed or isolated Cu(II) species interactingwith the support, have been reported in the ESR stud-ies [15,2931]. Lack of hyperfine structure may bedue to clusters located on the support. The ESR spec-trum of the used SG catalyst is shown in Fig. 5b. TheESR signal is narrower than that of the fresh catalystand exhibits a typical hyperfine axially symmetricalstructure. The parameters (gxx = gyy = g = 2.07and gzz = g = 2.36, A = 140G) are close to theones reported for highly dispersed Cu(II) coordinatedwith lattice oxygen of the support [3234]. The phe-nomena suggest that highly dispersed Cu(II) speciesprevail in the SG catalyst and that clusters are alsopresent. After reaction, the clusters disappear andthe highly dispersed Cu(II) species are still present.For IM catalysts, the ESR result showed a differencecase compared with that of SG sample. Fig. 5c showsthat the axial symmetric becomes broader and itsintensity decreases with respect to SG catalyst. The

broad isotropic signal can probably be attributed to

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dipolar and spinspin exchange interactions amongCu(II) ions of clusters, which are located close to oneanother. The result indicates that copper oxide clusters

are the main form in the IM catalyst.Temperature-programmed reduction, containingsome information about the surface structure of thecatalysts, has been extensively applied to the char-acterization of reducible catalysts including metaland metal oxide system. It is especially suitable forstudying highly dispersed systems with low loading.

The species of copper in the catalysts are stronglyaffected by the support, loading, preparation meth-ods, calcination temperature, and experimental condi-tions. Depending on different factors, the TPR showsone, two, or three peaks in a wide temperature range(230800 C).

Dow et al. [35] investigated the TPR of supportedCuO in hydrogen. For copper oxide supported on-Al2O3, two TPR peaks can be observed. The peakat low temperature region is attributed to the reductionof a highly dispersed copper species and the peak athigh temperature region is ascribed to the reductionof bulk CuO. The highly dispersed copper speciesinclude isolated Cu2+ ions that strongly interact withthe support and the isolated Cu2+ that weakly interactwith the support (the cupric ions have close contact

Fig. 6. TPR spectra of (a) SG catalyst and (b) IM catalyst.

with each other), and small two-dimensional clustersor three-dimensional ones. The peak at higher temper-ature is attributed to large three-dimensional clusters.

Here, three TPR peaks are observed in IM catalyst(Fig. 6b). The peak centered at 356 C would be as-cribed to the reduction of larger CuO clusters. The twolower reduced centered at 287 and 245 C, should bedue to the reduction of highly dispersed Cu(II) speciesand small CuO clusters, respectively. From the inten-sities of peaks, it can be confirmed that the main cop-per species present on the IM catalyst are larger CuOclusters. For SG catalyst, only two peaks in the lowtemperature region are observed (Fig. 6a) comparedto IM catalyst in the TPR profile. The results suggestthat no larger CuO particles but only highly dispersedCu(II) species and small clusters exist in SG catalyst,which is in agreement with XRD results.

3.2. Catalytic activity

The 2-butanol dehydrogenation has been widelyused for correlating selectivity with physicochemicalproperties of the catalysts. In the present paper, thecatalytic activities for 2-butanol dehydrogenation onSG and IM catalysts as a function of reaction temper-ature are shown in Table 2.

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Table 2Catalytic activities for 2-butanol dehydrogenation on SG and IMcatalysts

Catalysts Reactiontemperature(C)

Selectivity (%) Conversion of2-butanol (%)MEK Butene

SG 230 0290 15.6 84.4 1.5350 15.7 84.3 4.1

IM 220 99.8 0.2 67.4280 97.8 1.9 93.4350 92.1 6.2 99.0

Reaction conditions: reaction time is 20 min; GHSV = 1600h1,no carrier gas; catalysts: 0.30 g. Before reaction each catalyst ispretreated by argon for 30min at 400 C. MEK = methyl-ethyl-ketone.

One may observe from the table that the highselectivity to methyl-ethyl-ketone (MEK, the dehy-drogenation product) is obtained at all the reactiontemperatures for IM catalyst. The selectivity decreasesfrom 99.8 to 92.1% and the 2-butanol conversionincreases from 67.4 to 99.0% with reaction temper-ature increase. For SG sample, very low conversionof 2-butanol is observed. Only 4.1% 2-butanol con-verted to products even if the reaction temperature

increased up to 350 C. At the same time, relativelylow selectivity to MEK is observed (about 16%) overall the range of reaction temperature. From the abovecharacterization of catalysts, we conclude that thedifferences of selectivity and conversion would not berelated to the SBET (both SG and IM have the similarsurface data). The state and distribution of Cu speciesin the catalysts are the key factors to the reaction.

Several relevant investigations have dealt with theimportance of the different oxidation states of cop-per in the dehydrogenation reactions. However, there

is not yet a general agreement between the authors.Most of the studies suggest dehydrogenation mech-anisms involving both Cu2+, Cu+, and Cu0 species[17,3638]. Marchi et al. [17] reported that the CuOphase with a low reduction temperature, a small parti-cle size and a poor crystallinity is the one responsiblefor the appearance of the finely dispersed Cu0, whichis the active phase for 2-propanol dehydrogenationon a Cu-SiO2 catalyst.

In this paper, no Cu+ species is detected in the freshor used SG and IM catalysts. For SG catalysts, the

main form of Cu species is highly dispersed Cu(II)ions, and it is stable during the reaction. The smallamount of clusters detected in fresh SG catalyst, how-

ever, disappears after reaction. Low selectivity to de-hydrogenation and 2-butanol conversion are observedon this catalyst. For IM catalyst, the main form is CuOcluster, including small or larger clusters, which arereduced to Cu0 during the reaction (Fig. 1). At thesame time, high selectivity to dehydrogenation and2-butanol conversion are obtained. So it is easy toconclude that the ability of dehydrogenation is relatedto the CuO clusters, which may be reduced to Cu0

during dehydrogenation reaction, and Cu0 is the mainreactive site for dehydrogenation of 2-butanol. Theresults suggest that the highly dispersed Cu(II) ionswere not the active phase for dehydrogenation. Thisphenomenon is probably attributed to isolated Cu(II)enveloped within the SiO2 matrix or strongly inter-acting with the support, which might find access dif-ficult or reduced by 2-butanol molecules. This view-point is in agreement with that of Guerreiro et al. [14],who reported that highly dispersed copper (isolated Cuspecies) is unable to catalyze the dehydrogenation ofmethanol.

4. Conclusion

High selectivity to dehydrogenation and high con-version of 2-butanol were observed on IM catalyst andlow selectivity to MEK and low 2-butanol conversionwere observed on the SG sample. Based on the resultsof XRD, XPS, ESR, and TPR, the different distribu-tion of cupric species over SG and IM catalyst havebeen observed: highly dispersed Cu(II) species are themain form in the former but clusters are predominantin the latter. The surface structure of the catalysts is

determined by the preparation method. The activitiesof dehydrogenation of the SG and IM catalysts werestrongly dependent on their surface structures. TheCuO clusters that may be reduced to metallic copperare responsible for the dehydrogenation reaction. Butthe highly dispersed copper(II) species that cannot bereduced were inactive to catalyze the dehydrogena-tion reaction. The results presented here providedinsight into how the surface structures of catalystswere determined by the preparation methods and howthe catalytic behaviors were consequently affected.

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