535

Adsorption and Catalytic Properties of Co/ZSM-5 Zeolite Catalysts for COOxidation

Ludmila P. Oleksenko1*, Vitaly K. Yatsimirsky1, German M. Telbiz2 and Larisa V.Lutsenko1 (1) Department of Chemistry, Kiev Shevchenko National University, Vladimirskaya Str., 64, 01033 Kiev-33,

Ukraine. (2) Institute of Physical Chemistry, Ukrainian National Academy of Sciences, Nauki Avenue, 28, 03039 Kiev-39,

Ukraine.

(Received 20 January 2004; revised form accepted 28 April 2004)

ABSTRACT: Studies were made of the adsorption and acidic properties, thecoordination state of the Co ion in Co2+, Co3+-containing ZSM-5 zeolite systemsand the catalytic activity of these zeolites in CO oxidation. A correlation wasestablished between the catalytic activity of these Co-containing zeolites and thenumber of Brönsted acid sites on the surface and in the system bulk as obtainedby different methods. The content of various forms of Co ion coordination statesand the ratio of Brönsted to Lewis acidic sites at the surface of the Co-containingzeolites defined their activity in CO oxidation.

INTRODUCTION

One of the factors determining the physicochemical properties of zeolite surfaces in adsorptionand catalytic processes is the presence of acid sites (Breck 1974; Jacobs 1977). Clarification ofthe mechanism of pyridine adsorption on metal-containing zeolites provides an example of theconsiderable interest in using such materials in various catalytic and adsorption processes (Datkaet al. 1996; Buzzoni et al. 1996; Paukshtis 2000). In addition, modification of the zeolite surfaceby metal ions can influence the distribution of cations and allow the use of various zeolite systemscontaining transition metal ions, where the active component may be present as atoms, isolatedions or clusters of metal ions.

It is well known that Co3O4 is one of the most active catalysts of CO oxidation among oxidesystems (Golodetz 1977). For this reason, it was of interest to investigate the influence of the methodof modifying a ZSM-5 zeolite surface with cobalt ions and the acidity of the resulting Co-containingzeolites on their adsorption properties and catalytic activity in CO oxidation.

EXPERIMENTAL

The present work on Co-containing systems involved the high silica content zeolite ZSM-5. Theinitial zeolite, obtained as HZSM-5, was first compressed into tablet form (P = 150 N/m2), granu-lated and the 0.5–1.0-mm fraction used in the experiments. This fraction was then used tosynthesize Co-containing zeolite catalysts via the impregnation method and metal cation exchange.

*Author to whom all correspondence should be addressed. E-mail: olexludmil@ukr.net; pavlisvv@phyche.freenet.kiev.ua

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Thus, the modified zeolite 1.2% Co2+/ZSM-5 was prepared via impregnation and ion exchangeusing Co(NO3)2 solution, while the 0.42% Co3+/ZSM-5 zeolite system was obtained using[Co(NH3)6]Cl3 solution and allowing the mixture to stand at room temperature overnight. Themixed-valence Co-containing zeolites 0.83% Co2+−0.42% Co3+/ZSM-5 and 1% Co2+−0.42%Co3+/ZSM-5 were obtained via ion exchange by introducing Co2+ ions into 0.42% Co3+/ZSM-5employing different concentrations of cobalt nitrate solution. All the prepared samples werewashed with distilled water and dried at 60–80°C. The metal content in the samples prepared bycation exchange was determined by atomic absorption spectroscopy.

A Specord M-40 spectrometer was employed to measure the diffuse reflectance spectra of thesamples over the spectral range 12 000–30 000 cm−1, while the corresponding IR spectra of KBrwafers of the Co-containing zeolites were recorded over the range 400–4000 cm−1 using a SpecordIR-75 spectrometer.

Pyridine (Py) was used as a spectral probe for examining the numbers of Brönsted (B) andLewis (L) sites in studies of the adsorption and acidic properties of the prepared zeolites. Thus,the IR spectra of pyridine adsorbed in situ on catalysts were recorded over the spectral range1300–1800 cm−1. Before such measurements, all samples were degassed at room temperature at apressure P = 1.33 Pa and 450°C over a period of 30 min. The pyridine was adsorbed onto the sam-ples at 150°C by exposure to the vapour for 30 min. After adsorption, the samples were exposedto low pressures in order to remove any physically adsorbed pyridine from their surfaces.

The catalytic activities of the Co-containing zeolites in CO oxidation were investigated employ-ing a flowing gas reactor with chromatographic control of the composition of the reaction mixture.Gas separation (O2, CO, CO2) was conducted using a column filled with active carbon to whichNiSO4 had been applied. Catalytic measurements were carried out over the temperature range20–500°C at atmospheric pressure employing a reaction gas mixture containing excess oxygen(0.5% CO + 20% O2). The gas mixture flow was maintained at 0.1 l/min and the mass of catalystused in each experiment was 0.25 g. Comparison of the catalytic activity was made at the tem-perature where complete conversion of CO was achieved (T100%). All the Co/ZSM-5 zeolites wereused as catalysts after treatment in a hydrogen gas flow at 300–350°C over a period of 3 h.

RESULTS AND DISCUSSION

Modification of zeolites by metal cations and thermal treatment of their surfaces may lead tochanges in their structures and their acidic properties, and at the same time influence their adsorp-tion and catalytic properties. In the present studies, investigations of the structures of theCo-containing zeolites were carried out by IR spectroscopy. The frequencies of the adsorptionbands for the IR spectra of HZSM-5 and of Co/ZSM-5 systems after treatment in a hydrogen floware presented in Table 1.

As it evident from the table, the structure of HZSM-5 remained unchanged after modificationof the zeolite matrix with cobalt ions and following treatment in hydrogen flow. This view is sup-ported by the presence of absorption bands in the IR spectra corresponding to the asymmetricstretching vibrations of the Si–O–Si and Si–O–Al bonds between tetrahedra (1084–1100 cm−1)and deformation vibrations T–O inside tetrahedra (427–449 cm−1) characterized by very largeintensities (Rabo 1980). The existence of an absorption band shift corresponding to the vibrationfrequency of double five-membered rings (538 cm−1) for the 1.2% Co/ZSM-5 zeolite system sup-ported the location of the Co2+ cation in positions near the centres of double rings or insidehexagonal prisms. The preservation of the crystalline structure of Co-containing zeolites after

536 Ludmila P. Oleksenko et al./Adsorption Science & Technology Vol. 22 No. 7 2004

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modification and thermal treatment in hydrogen flow was supported by the unchanged intensityof the structure-sensitive absorption bands corresponding to the vibrations of double five-membered rings at 538–560 cm−1.

The presence of the absorption band at 538–560 cm−1 in the IR spectra of Co/ZSM-5 systemswas caused by the hydrated state of the cations in the zeolites, the electrostatic field of the cationcausing the dissociation of water molecules and consequent cation hydrolysis. Thus, hydroxylgroups were formed in zeolites containing polyvalent ions; this is reflected by the existence ofregions corresponding to stretching (3400–3700 cm−1) and deformation (1610–1630 cm–1) vibra-tions for the −OH groups in the IR spectra (Table 1). The absorption bands in the regioncorresponding to −OH group stretching vibrations may be associated with structural hydroxylgroups, adsorbed water and hydroxyl groups reacting directly with cations. These simultaneouslyprovided protonic Brönsted sites on the zeolite surface.

Earlier research (Oleksenko et al. 2001) of the acidic properties of Co-containing zeolites viathe thermo-programmed desorption of ammonia (TPD) showed that, in comparison to the initialHZSM-5 zeolite, after heat treatment in a hydrogen flow the ratio of weakly associated andstrongly associated adsorption contributions decreased in Co2+/ZSM-5 systems.

Since the active sites of zeolite systems are able to contain cobalt ions in different oxidationstates (Oleksenko et al. 2002), investigations of Py adsorption and catalytic activity in CO oxida-tion for univalent 0.42% Co3+/ZSM-5 and 1.2% Co2+/ZSM-5 and for mixed-valence 0.83%Co2+−0.42% Co3+/ZSM-5 and 1% Co2+−0.42% Co3+/ZSM-5 zeolites were carried out after theirtreatment in a hydrogen flow.

Data relating to investigations of the acidic properties of Co-containing zeolites prepared viaion exchange and formed during catalytic reactions are presented in Figure 1. After Py adsorptiononto Co3+/ZSM-5 zeolite (Figure 1, spectrum 1), absorption bands were observed in the IR spec-trum at 1440 and 1450 cm–1 corresponding to the coordination bond of Py with the metal ions. Incontrast, the spectrum of pyridine adsorbed onto the Co2+−Co3+/ZSM-5 system after its use forcatalysis (Figure 1, spectrum 2) exhibited bands at 1610 cm–1 corresponding to bonding betweenPy and L-sites. Two bands at 1440, 1460 cm–1 and a shoulder at 1470 cm–1 that are clearly sepa-rate demonstrate the existence of various types of L-sites, i.e. those formed with Al3+ (1440 cm–1),

Adsorption/Catalytic Properties of Co/ZSM-5 Zeolite Catalysts for CO Oxidation 537

TABLE 1. IR Vibration Frequencies (cm−1) for ZSM-5 Zeolite Systems

Vibrations HZSM-5 1.2% Co2+−ZSM-5 0.83% Co2+−0.42% Co3+−ZSM-5 1.2% Co2+−ZSM-5

InternalAsym. stretching 1088 1100 1084 1084; 1088Sym. stretching 665 sh 635 sh 659 sh 665 shDeformation T–O 449 427 437 444

ExternalFive-membered ring 542 560 546 538Sym. stretching 788 814 766 780Asym. stretching 1220 1240 1198 1210

δ(OH) 1629 1627 1613 1613ν(OH) 3450 3480 3480 3493M–OH 3624 3664 3616 3600

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Co2+ (1460 cm–1) and Co3+ (1470 cm–1). The small intensity of the adsorption bands at 1640 and1545 cm–1 demonstrates the extensive substitution of hydroxyl protons by cobalt ions and theremoval of part of the −OH groups during thermal treatment and catalytic reaction.

A comparison of the intensities of all the absorption bands corresponding to Lewis acid sitesshowed that the number of such sites on the surface of the mixed-valence Co-containing zeolitewas greater than that for Co3+/ZSM-5. This could account for the considerably higher activity of Co-containing systems containing cobalt ions with different oxidation numbers, i.e. Co2+−Co3+/ZSM-5,relative to the behaviour of the zeolite containing only Co3+ ions. Thus, the temperatures for totalCO conversion with the mixed-valence 0.83% Co2+−0.42% Co3+/ZSM-5 and 1% Co2+−0.42%Co3+/ZSM-5 zeolites were 345oC and 250oC, respectively in the second catalysis cycle comparedwith the much higher temperature of 450oC necessary to achieve 20% CO conversion with 0.42%Co3+/ZSM-5.

Since the nature and quantity of acid sites and localization of the cobalt ions on the surface and inthe framework of Co2+ zeolites as obtained by different methods can lead to a variety of adsorptionand catalytic properties, the IR spectra of adsorbed pyridine on all these various samples weremeasured.

The IR spectrum of pyridine adsorption onto the ion-exchanged Co2+/ZSM-5 system beforecatalysis [Figure 2(a), spectrum 1] exhibited absorption bands at 1640, 1620, 1545, 1490 and

538 Ludmila P. Oleksenko et al./Adsorption Science & Technology Vol. 22 No. 7 2004

1440 1460

1480

16101630

16401610

1450

1545

1300 1400 1500 1600 1700 1800

Wavenumber, ν (cm−1)

Spectrum 2

Spectrum 1

Figure 1. IR spectra of adsorbed pyridine on cobalt-containing zeolites: spectrum 1, Co3+/ZSM-5; spectrum 2,Co2+−Co3+/ZSM-5.

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1440 cm–1, respectively. The bands at 1640 and 1545 cm–1 corresponded to bonding between Py anda proton and may be used to determine the corresponding Brönsted acidity, while the bands at 1620and 1440 cm–1 may be related to the coordination of Py with a metal (Co, Al) and is indicative of Lewisacidity. After adsorption of Py onto exchanged Co2+/ZSM-5 after its use in catalysis [Figure 2(a), spec-trum 2], the presence of bands at 1542, 1438, 1450 and 1625 cm–1 characterizing proton acid sites andbands at 1438, 1450 and 1625 cm–1 characteristic of Py associated with metal cations were noted inthe IR spectrum. This suggested an increase in the ratio L/B of Lewis (L) to Brönsted (B) acid sitesin the sample after catalytic reaction relative to the situation before such use.

The IR spectrum of adsorbed Py on impregnated 1.2% Co2+/ZSM-5 [Figure 2(b)] showed theexistence of intense absorption bands related to both L and B sites. It should be noted that thissample exhibited a rather greater number of proton acid sites compared to the corresponding sit-uation with the ion-exchanged material, thereby demonstrating that the cobalt cation was mainly

Adsorption/Catalytic Properties of Co/ZSM-5 Zeolite Catalysts for CO Oxidation 539

1440

1630

1490

1545

1438

1450

14901590

1625

1640

1542

1300

(a) (b)

1400 1500 1600 1700 1800

Spectrum 2

Spectrum 1

1440

1486

1610

1630

1540

Spectrum 1

Spectrum 2

1518

1453

14331580

16101645

1300 1400 1500 1600 1700 1800

Wavenumber, ν (cm−1) Wavenumber, ν (cm−1)

Figure 2. IR spectra of adsorbed pyridine on 1.2% Co2+/ZSM-5 prepared by (a) ion exchange or (b) impregnation. In bothcases, spectrum 1 corresponds to the situation before catalysis while spectrum 2 was obtained after the catalyst had beenemployed in a catalytic reaction.

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localized on the external surface of the zeolite. The value of the L/B ratio for the impregnated sam-ple changed from 13.2 to 16.2 after its use in a catalytic reaction. The temperature for total COconversion employing the 1.2% Co2+/ZSM-5 system obtained by impregnation was 115 to 85oClower than that for the system obtained via ion exchange (Figure 3).

Studies of the coordination state of the Co ions in metal-containing zeolites, i.e. Co2+/ZSM-5,by diffuse reflectance spectroscopy showed that in the zeolite obtained by impregnation theCo2+ ions surrounded by O2− ions exhibited Td and Oh coordination states. The diffuse reflectancespectrum for the impregnated catalyst 1.2% Co/ZSM-5 was characteristic of Co(II) compoundswith a low-spin electron configuration exhibiting octahedral coordination. The spectra of 1.2%Co/ZSM-5 systems after their formation in a hydrogen flow and subsequent to their use in cat-alytic reactions changed generically although the positions of the absorption bands corresponding

to , and remained fixed. It should be noted that in the spectra of the most active

Co-containing zeolites the absorption bands corresponding to and were practically of

equal intensity. The reduced catalytic activity of the ion-exchanged Co/ZSM-5 relative to thatobtained by impregnation could have been caused by a smaller quantity of active centres presentat its surface. According to the diffuse reflectance spectral results, the cobalt present as existed in octahedral coordination.

CONCLUSIONS

The results obtained indicated that during the catalytic oxidation of CO the properties of thehydroxyl groups and the number of Lewis sites changed in Co-containing catalysts synthesized bydifferent methods. Thus, for the ion-exchanged sample, a smaller number of Co cations associated

CoOh

2 +

CoOh

3 +CoTd

2 +

CoTd

2 +CoOh

3 +CoOh

2 +

540 Ludmila P. Oleksenko et al./Adsorption Science & Technology Vol. 22 No. 7 2004

100

Temperature (°C)

80

60

40

50 100 150 200 250

4′ 4

3′

2′

2 1

1′

3

20

0

Xco

(%

)

Figure 3. The dependence of the degree of CO oxidation (XCO) on the reaction temperature for a gas mixture consisting of0.5% CO + 20% O2 + 79.5% He in the presence of Co-containing zeolite catalysts obtained by different methods. Thus, for1.2% Co/ZSM-5 (obtained by ion exchange): curves 1,1′ correspond to the first catalytic cycle while curves 2,2′ correspondto the second catalytic cycle. Similarly, for 1.2% Co/ZSM-5 (obtained by impregnation): curves 3,3′ correspond to the firstcatalytic cycle while curves 4,4′ correspond to the second catalytic cycle.

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with ZSM-5 crystals on the external surface of the catalyst took part in the reaction, with the cat-alytic activity being mainly confined to those situated in the channel structure. However, with thesample obtained by impregnation, the metal cations occurred mainly on the external surface of theZSM-5 crystals with their content in the pores being minimal. A proportion of the acid sites on theexternal surface of the crystals was blocked by metal ions, whereas ZSM-5 crystals located in thechannel structure were free from such ions.

The presence of ‘weak’ H-acid sites on the surface of zeolite crystals capable of being removedduring catalytic reaction can lead to a relatively greater number of active sites on the surface anda correspondingly higher activity for impregnated catalysts relative to the behaviour of the corre-sponding ion-exchanged materials. The hydroxyl group cover of the ion-exchanged samplechanged to a lesser degree during catalysis. In addition, the location of the metal cations in thezeolite structure and their coordination state on the surface and in the zeolite framework plays arole in the overall process. Diffuse reflectance spectra of the 1.2% Co/ZSM sample demonstratedthe importance of the existence of and Co2+ ions in tetrahedral and octahedral coordinationstates at the surface of the zeolite system as well as a definite ratio for their content.

REFERENCES

Breck, D.W. (1974) Zeolite Molecular Sieves, Wiley, New York.Buzzoni, R., Bordiga, S., Ricchiardi, G., Lamberti, G. and Zecchina, A. (1996) Langmuir 12, 930.Datka, J., Sulikowski, D. and Gil, B. (1996) J. Phys. Chem. 100, 11 242.Datka, J., Marschmeyer, S., Neubauer, T., Meusinger, J., Papp, H., Schutze, F.-W. and Szpyt, J. (1996)

J. Phys. Chem. 100, 14 451.Golodetz, G.I. (1977) Heterogeneous Catalytic Reactions with Participation of Molecular Oxygen, Naukova

dumka, Kiev.Jacobs, P.A. (1977) Carboniogenic Activity of Zeolites, Elsevier, Amsterdam.Oleksenko, L.P. and Lutsenko, L.V. (2001) Bull. Donetsk Univ., Ser. A 2, 225.Oleksenko, L.P., Yatsimirsky, V.K. and Lutsenko, L.V. (2002) Adsorp. Sci. Technol. 20, 371.Paukshtis, E.A. (2000) J. Mol. Catal. A 158, 37.Rabo, J. (1980) Chemistry of Zeolites and Catalysis on Zeolites, Mir, Moscow.

CoOh

3 +

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