Siting and redox properties of iron in porous ferrisilicates ∗)

K. Lazar

Institute of Isotopes, Hung. Acad. Sci., P.O.B. 77, H-1525, Budapest, Hungary

G. Pal-Borbely, A. Szegedi

Institute of Surface Chemistry and Catalysis, CRC, Hung Acad. Sci., P.O.B. 17, H-1525,

Budapest, Hungary

P. Fejes

Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B.

ter 1, 6720-Szeged, Hungary

F. Martinez

Environmental and Chemical Engineering Group, Rey Juan Carlos University, 28933

Mostoles, Spain

Received 13 June 2006

Siting of iron and its ability to take part in the Fe3+↔ Fe2+ redox process are

correlated in certain micro- and mesoporous ferrisilicates as deduced from changes in co-ordination and oxidation states of iron followed from in situ Mossbauer spectroscopy. Thethree-dimensional crystalline character stabilizes the Fe3+ state in microporous frame-work sites, whereas the mentioned reversible redox process proceeds more easily on extra-framework sites in microporous systems, as well as on iron located in the pore walls ofthe mesoporous ferrisilicates. Correlations with catalytic properties are also mentioned incertain instances.

PACS : 76.80.+yKey words: Mossbauer spectroscopy

1 Introduction

Properties of porous silicates can be modified by inserting iron into the struc-ture. The effects may be different, depending on the original structure, whether itwas micro- or mesoporous. Microporous structures are strictly crystalline (in threedimensions – with characteristic pore sizes of about 0.5 nm). Iron may be insertedeither to the centre of the primary tetrahedral [SiO4/2] building units, replacingfour-valent silicon with three-valent iron. This is the so-called framework (FW) sit-ing. When Fe3+ is introduced to FW sites, a charge deficiency is generated (sincethe charges of the four oxygens are only partially compensated by Fe3+). Thiscompensating charge may be provided by Na+ (used in the synthesis), or by pro-tonic hydrogen (resulting in the formation of Bronsted acidity). Iron species may

∗) Presented at International Colloquium “Mossbauer Spectroscopy in Materials Science”,Kocovce, Slovak Republic, June 11–15, 2006.

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K. Lazar et al.

also supply the compensating charge, e.g., FeO+. These compensating charges arelocalized in the pores, in the so called extra-framework (EFW) positions [1].

The structure of mesoporous silicate materials is less defined. They only exhibitXRD reflections at low angles (2Θ < 5◦), which correspond to lattice parametersof 3–5 nm. The walls of their pores and channels are partly amorphous and maycontain Si-OH silanolic groups [2]. Iron can be also introduced into these materialsduring the synthesis.

The iron-modified porous silicates may be applied—among others—in the fieldof catalytic processes. Insertion of iron, in particular, opens an additional pathfor redox processes. Namely, Fe3+

↔ Fe2+ process may proceed, even reversibly,accompanied, e.g., by the electron or oxygen transfer as fundamental steps of redoxcatalytic processes.

For studying the porous ferrisilicates, in situ Mossbauer spectroscopy is a suit-able tool, being particularly sensitive to the coordination and oxidation states ofiron. Mossbauer data collected from numerous studies performed on ferric- andferrous silicate minerals [3], can be preferably used for determining the actual statesof iron in the porous ferrisilicates.

In the present study, various examples are presented to illustrate the interplay ofthe structure and the Fe3+

↔ Fe2+ process, in particular, how the latter depends onthe structure and on the actual siting of iron. In certain instances the applicationsin catalytic processes are also discussed.

2 Experimental

Microporous samples were prepared by hydrothermal syntheses, using the ap-propriate templates to synhesize the different ferrisilicate frameworks [4]. Namelyethanolamine, pyrrolidine, and tetrapropylammonium perchlorate were used to syn-thesize Fe-MCM-22, Fe-FER, and Fe-MFI, respectively. Synthesis of Fe-LTA wasperformed by an amorphisation and recrystallization [5]. Fe-MCM-41 mesoporoussamples were prepared by two methods. The sample of the first type was preparedby regular hydrothermal process using sodium silicate and cetyltrimethylammo-nium (C16) bromide, as template. The second Fe-MCM-41 was prepared by usingtetraethylorthosilicate (TEOS) as a silica source, the same C16 template as pre-viously, in partially ethanolic media [6]. Fe-SBA was prepared by using TEOS,Pluronic P123 triblock copolymer, also in partially ethanolic media [7].

Mossbauer spectra were recorded in most cases in series, one after another in asequence of various treatments. Most of the spectra were recorded at 300K (someexceptions are mentioned in the text). The isomer shift values are related to metalliciron. Spectra were collected in a constant acceleration mode. For decomposition,Lorentzian line shape was assumed for the separate lines. The accuracy of positionaldata is about ±0.03mm/s.

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Siting and redox properties of iron in porous ferrisilicates

3 Results and discussion

3.1 Framework ions in microporous systems (Fe-FER and Fe-MCM-22)

Fe-MCM-22 and Fe-FER were synthesized containing Fe3+ mostly in frameworksubstituted siting (Si/Fe = 20 and 16, respectively). Fig. 1 shows the spectrarecorded after the hydrothermal synthesis, after template removal with calcination,and after a subsequent evacuation.

as synth

calc720 K

-4 -2 0 2 4

evac650 K

Fe-FERFe-MCM-22

Velocity, mm/s

-4 -2 0 2 4

Fig. 1. 300 K Mossbauer spectra of MCM-22 and Fe-FER samples: after synthesis (con-taining the template used for synthesis – top), after removal of the template by calcination

(middle) and after evacuation (bottom).

Almost all the iron atoms are situated in all cases at the tetrahedral positions,although the spectra are apparently different. In the first instance, the templatecations provide smeared, less distorted charge distribution around the FW iron,with the charge compensation of the [FeO4/2]

− unit being achieved on an extendedregion. Thus, the observed quadrupole splitting (∆EQ) is small. Calcination re-moves the template, the chemisorbed water is, however, left behind, which is stillable to smear the charges, the compensation is provided by the hydrated H3O

+

hydroxonium ions (0.8 < ∆EQ < 1.0mm/s). Upon removing water by evacua-tion, a bare proton supplies the compensating charge in the third case – mostprobably linked to a single oxygen in a Bronsted acidic form, O-Si-O-Fe(OH)–Si.In this case, the symmetry is strongly lowered and a large splitting is detected(1.8 < ∆EQ < 2.0mm/s).

A minor Fe2+ component appears also upon evacuation. It is formed most prob-ably from the EFW iron upon dehydration: 2 Fe3+

EFW(OH)2 → 2Fe2+EFW(OH) + H2O

+ 12O2. It is also worth mentioning that the FW/EFW distinction using the ap-

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K. Lazar et al.

pearance of the large ∆EQ for the FW Fe3+ does not depend on the particularmicroporous structure; MCM-22 and FER are rather different ones, and they ex-hibit spectra of similar shapes. In MFI, etc., the same component with large ∆EQ

can be detected upon evacuation, as well.

3.2 Exchange and migration of EFW ions reflected in the changes of ∆EQ

value of the FW ions

The change in the ∆EQ of the FW Fe3+ ions can be interpreted as reflecting theexchange and migration of EFW ions, e.g., in a series of sequential treatments onan Fe-MCM-22 (Si/Fe = 20, and the sample is partly in Na+ form, Si/Na = 35).

1) calc. 5) 2nd calc. 670 K

2) vac / 640 K 6) 3rd evac 640 K

3) H2 / 620 K 7) 2nd H

2

620 K

-4 -2 0 2 4

4) 2nd evac 640 K

1st ser. MCM-22 2nd ser.

-4 -2 0 2 4

Velocity, mm/s

8) 4th evac 640 K

Fig. 2. Sequences of 300 K in situ spectra recorded stepwise on Fe-MCM-22 sample inthe series of treatments indicated.

The first treatment, evacuation, does not result in a dominant appearance of thehigh ∆EQ component. Another dominant component appears instead, exhibitingonly a modest ∆EQ (0.86mm/s). This can be attributed to the FW ions charge com-pensated with the bulky Na+ cations and EFW FeO+ species. Upon reduction inhydrogen, EFW FeO+ is reduced probably to Fe(OH)+, and partial Na+

→ H3O+

exchange may also take place on the charge compensating sites of the [FeO4/2]−

units. In the second calcination, a further removal of Na+ takes place (formingprobably Na2O), and upon the second evacuation, the H+-compensated FW com-ponent of 29% appears, exhibiting the characteristic large ∆EQ (2.13mm/s at thepresent instance). The system is more or less stabilized: upon reduction and re-peated evacuation, it still contains the high ∆EQ FW iron in displaying a spectralcontribution of 20%. The assignment of the tetrahedral siting is also supported bya low δ value (0.24mm/s).

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Siting and redox properties of iron in porous ferrisilicates

The presented series also attest that single FW ions are able to preserve theirFe3+ state, whereas on the EFW ions, the Fe3+

→ Fe2+ reduction process mayproceed under the same conditions. (For further details see Table 1.)

Table 1. Mossbauer data derived from the spectra of (Na+, H3O+) Fe-MCM-22 shown in

Fig. 2.

First series Repeated calcination

Treatment Comp.δ ∆EQ Ar δ ∆EQ Ar

(mm/s) (mm/s) (%) (mm/s) (mm/s) (%)

after Fe3+ 0.32 0.82a 50a 0.33 1.32 63calcination Fe3+ — — 50a 0.30 0.74 37

Fe3+ — — — 0.24 2.13 29

evac/Fe3+ 0.26 1.61 10 0.30 1.39 49

640KFe3+ 0.31 0.86 71 0.30 0.82 21Fe2+ 0.79 2.16 7 — — —Fe2+ 1.17 2.29 11 — — —

Fe3+ 0.33 1.20 2 0.28 1.51 8H2/ Fe3+ 0.31 0.56 48 0.36 0.74 39

620K Fe2+ 0.98 1.93 38 0.90 2.00 32Fe2+ 1.13 2.45 11 1.24 2.04 22

Fe3+ 0.26 1.60 7 0.24 1.96 202nd evac/ Fe3+ 0.41 0.66 56 0.41 1.02 45

640K Fe2+ 0.96 1.94 29 — — —Fe2+ 1.09 2.60 8 1.07 1.89 35

adoublet with two lines of the same intensity with different line widths allowed

3.3 Reversible Fe3+↔ Fe2+ processes with extra-framework ions

A new synthesis method was applied to construct a LTA structure, and 57Fe wasalso added to the synthesis mixture to incorporate iron [5]. The LTA structure israther compact, thus the incorporation was only partial. In the course of a seriesof in situ treatments, the changes of the coordination and oxidation states of ironions were followed. The obtained spectra can be basically interpreted as mostlycharacterising the behavior of EFW ions (Fig. 3).

Specifically, upon evacuating the calcined sample, a large part is reduced toFe2+, thus the presence of large proportion of EFW iron is suggested (2nd stage).The small splitting of one of the Fe2+ components (δ = 0.85mm/s, ∆EQ =0.45mm/s) is characteristic; it can probably be assigned to a distorted, half-sidedcoordination – to Fe2+ attached to the wall of the wider pores. Upon reduction at570K in hydrogen, almost all the iron is converted to Fe2+ state (3rd stage). Fe2+

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K. Lazar et al.

6) air, 300 K

3) H2,

570 K

4) N2O,

470 K

5) H2,

570 K

-4 -2 0 2 4

2) evac. 650 K

-4 -2 0 2 4

Velocity, mm/s

1) calc, 750 K

Fig. 3. Series of 300 K Mossbauer spectra obtained on Fe-LTA exposed to various treat-ments.

can be almost fully converted to Fe3+ by oxidation in N2O atmosphere at 470K(4th stage). The Fe2+

→ Fe3+ process can be afterwards reverted by a repeatedhydrogen treatment: the Fe2+ component with a small splitting (δ = 0.70mm/s,∆EQ = 0.53mm/s) is dominant in the spectrum recorded after treatment at 540Kin hydrogen. The last spectrum of the series was recorded after storing the reducedsample in open air at room temperature for 7 h. Partial Fe2+

→ Fe3+ oxidation hadtaken place and the Fe2+ with the small ∆EQ is converted to a component exhibit-ing large splitting (δ = 1.16mm/s, ∆EQ = 2.11mm/s) reflecting most probablythe sorption of moisture collected from the air during the storage.

This series illustrates the fact that Fe3+↔ Fe2+ changes are almost fully re-

versible for the EFW ions, and the changes in the coordination of Fe2+ ions areapparently reflected in the changes of the respective δ and most expressively, in the∆EQ values.

3.4 Stable (and independent) FW and EFW combination: active in N2Ocatalytic decomposition

Various microporous ferrisilicates were found to exhibit excellent catalytic activityand stability in selective oxidation, N2O decomposition, etc. Immense literaturediscusses the process, e.g. [8,9], including Mossbauer studies as well [10–12]. Someauthors attribute the activity to dinuclear pairs of iron stabilized in the framework,whereas others interpret the activity as exerted by single EFW ions.

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Siting and redox properties of iron in porous ferrisilicates

In our recent study, a sample was studied in which FW 57Fe3+ is combinedwith EFW Fe2+. In the first stage, enriched 57Fe is introduced to FW sites of MFI(Si/Fe = Si/Al = 200), then in the second stage, Fe2+ of normal isotope compo-sition was introduced to EFW sites from methanolic solution of FeSO4 resultingin an average Si/Fe = 50 ratio in the sample, providing an FW :EFW distributionof approximately 1 : 3 [13]. Under the same conditions, a temperature-programmedreaction was also performed – the highest catalytic activity was obtained at 620K.Various stages of the synthesis and spectra, recorded during and after N2O decom-position are presented in Fig. 4.

77 Kas synth.300 K+ FeCl

2

(exch.)

77 Kevac. 650 K

620 K N2O

620 K

-12 -9 -6 -3 0 3 6 9 12Velocity, mm/s

300 Kevac. 650 K

-12 -9 -6 -3 0 3 6 9 12

300 Kevac.650 K

57Fe3+ (FW) + nFe2+(exch - EFW)57Fe3+ (FW)

Fig. 4. Mossbauer spectra obtained with a composed system containing iron in bothFW and EFW sites in MFI host. Left: 57Fe (FW): measured at 77K (top and middle)and measured at 300 K (bottom). Right: the 57Fe FW combined with EFW Fe. After ionexchange on the previous 57Fe FW sample (top), under reaction conditions in catalyticdecomposition of N2O (recorded in N2O at 620 K for 6 h – middle), after a subsequent

evacuation (bottom).

Spectra on the left side of Fig. 4 exhibit paramagnetic relaxation. Single, sepa-rated ions may exhibit this feature (the temperature dependence is also illustrated).Addition of EFW Fe2+ in a significant excess compared to the amount of FW 57Fewith impregnation from methanolic solution hardly affects the paramagnetic be-havior, the quadrupole splitting of EFW Fe2+ is simply added to the paramagnetic57Fe FW background on the spectrum recorded after impregnation and evacuation.The spectrum recorded during the very reaction of the catalytic N2O decompo-sition at 620K still exhibits the presence of Fe2+ component. Thus, it is clearlydemonstrated that Fe2+ is stabilized in the EFW sites in strongly oxidizing condi-

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tion, in N2O at 620K (c.f. the respective spectrum recorded on the EFW ions ofLTA in Fig. 3, where the Fe2+

→ Fe3+ oxidation was complete in N2O atmospherealready at 470K). The FW Fe3+ + EFW Fe2+ system is stable; after keeping thesample under reaction conditions at 620K for 6 h, the original spectrum can beagain observed (the last spectrum of the series).

This four examples presented above illustrate that the distinction between FWand EFW sitings of iron is appropriate in the case of microporous systems. The Fe3+

state is more stable in FW sites, it withstands Fe3+→ Fe2+ reduction better than

EFW iron. In the EFW sites, the Fe3+↔ Fe2+ process may proceed reversibly. On

the other side, however, EFW Fe2+ can be also stabilised in oxidizing conditions.The following two examples will illustrate certain differences in the behavior of ironwhen comparing the microporous hosts with the mesoporous ones.

3.5 Iron siting in MCM-41: dependence on the synthesis and pore wall thick-ness

Two different synthesis methods were used for preparing Fe-MCM-41 and the prop-erties of the obtained samples were compared. In the first case, hydrothermal pro-cedure was applied using hydrous sodium silicate as a silica source. In the secondcase, the silica source was TEOS—the synthesis was faster—upon hydrolysing theTEOS [6]. The characteristic data of the two samples are presented in Table 2.The most characteristic spectra of the two samples are shown in Fig. 5 and thecorresponding data are listed in Table 3.

Table 2. Comparison of characteristic data of Fe-MCM-41 samples [6].

Synthesis method Si/Fed100

aa0

b ΦBJHc

Vpd

Wde WTH1

f WTH2g

(nm) (nm) (nm) (cm3/g) (nm) (nm) (nm)

Hydrohermal 140 3.38 3.90 2.43 0.79 3.27 1.47 0.63

TEOS/ethanol 20 3.47 4.01 2.31 0.62 3.20 1.70 0.81a lattice distance, determined from d100 reflexionsb unit cell constantc average pore diameter, determined from sorption isotherms by the Barrett-Joyner-Halenda

methodd pore volume

e diameter of cylindrical pores: Wd = 1.2d100

√

2.2Vp

1+2.2Vp

f wall thickness, approximated as a0 − ΦBJHg wall thickness, approximated as a0 − Wd

The difference in the synthesis methods results in different structures, leading,among others, to different wall thicknesses. The starting structure formed by hy-drothermal synthesis probably contains silanolic –OH groups in a larger extent,thus 30% of iron can be reduced to Fe2+ during removal of water by evacuation asthe corresponding spectrum shows. Simultaneously, the largest quadrupole splittingcomponent ∆EQ of Fe3+ is only 1.68mm/s. In contrast, evacuation of the sample

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Siting and redox properties of iron in porous ferrisilicates

calc. 750 K

evac. 670 K

-4 -2 0 2 4

H2 620 K

-4 -2 0 2 4

Fe-MCM-41 (TEOS / methanol)Fe-MCM-41 (hydrotherm.)

Velocity, mm/s

Fig. 5. 300 K in situ Mossbauer spectra of Fe-MCM-41 samples prepared with differentpore wall thicknesses.

Table 3. Mossbauer date derived from spectra of Fe-MCM-41 synthesized by hydrothermaland by TEOS/ethanolic route.

Sample Hydrothermal synth. TEOS/ethanol synth.

Treatment Comp.δ ∆EQ Ar δ ∆EQ Ar

(mm/s) (mm/s) (%) (mm/s) (mm/s) (%)

calcin.Fe3+ 0.35 0.68 40 0.34 0.79 38Fe3+ 0.37 1.13 60 0.34 1.30 62

Fe3+ — — — 0.28 2.02 39evac/ Fe3+ 0.28 1.68 44 0.32 1.42 39670K Fe3+ 0.34 1.04 27 0.35 0.90 22

Fe2+ 1.07 2.03 30 — — —

Fe3+ — — — 0.17 1.72 20

H2/Fe3+ 0.12 0.57 5 0.25 1.06 39

620KFe2+ 0.59 0.83 32 — — —Fe2+ 0.86 1.10 33 0.91 1.53 32Fe2+ 0.87 1.70 30 0.97 2.05 9

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K. Lazar et al.

synthesized in ethanolic media does not result in detectable Fe3+→ Fe2+ reduc-

tion, and the largest ∆EQ component for Fe3+ is 2.02mm/s. These features can beinterpreted as attesting that the structure of pore wall in the first sample is mostlyamorphous, whereas in the second sample, it can be partly crystalline, i.e., regionsmay exist in which the elementary building unit is the same as for the microporouszeolite analogues, namely the iron-centered [FeO4/2]

− tetrahedron. This interpre-tation is supported by the results of the hydrogen treatment at 620K. In the firstsample, the Fe3+

→ Fe2+ reduction is almost complete (except 5% of Fe3+ only),whereas in the sample synthesized from TEOS, the dominant oxidation state is thepreserved Fe3+ (59%) after the treatment.

Thus, for the mesoporous structures, the FW vs. EFW distinction is probablyless meaningful, since those coordination and oxidation states may occur insidethe pore walls which were characteristic exclusively only for EFW sites in themicroporous structures.

It should be mentioned that both MCM-41 samples exhibit catalytic activity.The first sample was evaluated in CO oxidation [14], whereas the second one inalkylation of toluene with ethylbenzene [15].

3.6 Composite mesoporous Fe-SBA-15 + Fe2O3 (hematite) system

SBA-15 also designates a mesoporous structure. The difference to the previousMCM-41 is the way of synthesis, which is carried out in a more acidic media usingtriblock copolymer templates. The acidic media is also advantageous for an incor-poration of iron, since the hydrolysis of iron is retarded and iron exists mostly inthe form of monomeric hydrated Fe3+ ions. Three samples were prepared. Firstly,the block copolymer was dissolved, and then iron was added. Thereafter, tetraethy-lorthosilicate was hydrolysed. DS-1 sample was aged in the original acidic media(pH < 1). Sample DS-2 was aged in less acidic conditions (pH = 3.5), whereassample DS-3 was aged under neutral conditions (pH = 7). The three samples haddifferent iron contents (1.2, 16 and 22 wt. %, respectively.) High-resolution TEMshowed the ordered mesoporous structure for DS-1, whereas in DS-2 and DS-3,dark spots of iron oxide of 50–100nm in size were also embedded into the regularmesoporous network. The catalytic activity of the samples was also evaluated in awet total oxidation of phenol with hydrogen peroxide [7].

The Mossbauer spectra of these samples are presented in Fig. 6. The spectraof synthesized samples correspond with the description of synthesis and results ofTEM studies. In the DS-1 sample, all the iron is incorporated to the SBA-matrixin an ionic dispersion. To prove the ionic dispersion, further in situ spectra wererecorded on DS-1 sample (Fig. 6, right side). These spectra are apparently similar tothe spectra obtained for the mesoporous MCM-41 of thicker walls (shown in Fig. 5).Thus, the structures and sitings of iron are probably similar in the DS-1 and in thelatter particular MCM-41. Samples DS-2 and DS-3 contain large oxide particles – asalso reflected by the presence of sextets of hematite in the corresponding spectra.It is, however, important to notice that both the spectra of DS-2 and DS-3 stillcontain a small doublet with the same parameters as found in DS-1, i.e., beside the

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Siting and redox properties of iron in porous ferrisilicates

Velocity, mm/s

DS-1DS-1, DS-2, DS-3

77 K

DS-1 calc.770 K

300 K

DS-2 evac.650 K

-12 -9 -6 -3 0 3 6 9 12

300 K

DS-3

-4 -2 0 2 4

H2

620 K

Fig. 6. Mossbauer spectra of various Fe-SBA samples. Left: 1.2 wt. % (top), 16 wt.%,(middle) and 22wt. % (bottom). Right: 300 K in situ spectra of the sample with 1.2 wt. %iron, after calcination (top), after evacuation (middle) and after subsequent treatment in

hydrogen (bottom).

large oxide particles, a small part of iron may exist in the ionic dispersion in themesoporous host. The catalytic activity of the three samples in oxidation of phenolwas measured (Fig. 7). Surprisingly, all the three samples exhibit similar activities,regardless to the iron content.

Thus, the interpretation is obvious: the catalytic activity originates from theseparated Fe3+ ions dispersed in the pore walls of the mesoporous host, and thelarge hematite particles do not practically contribute to the catalytic performance.

4 Conclusions

The correspondence between the siting of iron and the ability to take part inthe Fe3+

↔ Fe2+ redox process is illustrated in certain micro- and mesoporousferrisilicates as investigated by means of in situ Mossbauer spectroscopy. In mi-croporous 3D crystalline systems, the framework/extra-framework sitings can beclearly distinguished. Fe3+ is more stable in the former sites, whereas the valencychange is easier at the latter ones. In the mesoporous systems the pore walls arepartly amorphous – the FW/EFW distinction is less distinct. The stability of Fe3+

strongly depends on the structure and thickness of the pore walls. Examples arealso presented for catalytic applications: e.g., the stability of EFW Fe2+ in micro-

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K. Lazar et al.

0 20 40 60 80 100

0

20

40

60

80

100

Blank

DS-1, DS-3

DS-2

TO

C C

onv

ersi

on (%

)

Time (min)

Fig. 7. Catalytic activity of the three various Fe-SBA-15 samples in the total oxidationof phenol.

porous MFI is demonstrated in N2O decomposition, and the activity of separatedFe3+ ions in mesoporous Fe-SBA-15 on oxidation of phenol is also evidenced.

The financial support of the National Science Research Fund, Grant No. OTKA

T 46970 is gratefully acknowledged.

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