DOI: 10.1002/cctc.201200562

Iron-Functionalized Silica Nanoparticles as a HighlyEfficient Adsorbent and Catalyst for Toluene Oxidation inthe Gas PhaseMargarita Popova,[b] Alenka Ristic,[a] Karoly Lazar,[c] Darja Maucec,[a] Mihaela Vassileva,[b] andNatasa Novak Tusar*[b, d]

Introduction

Volatile organic compounds (VOCs) are the main class of airpollutants emitted by various industrial processes.[1–12] The listof pollutants includes more than 300 compounds, such as oxy-genates, aromatic hydrocarbons, and halogen hydrocarbons.The catalytic oxidation has been considered as the most ap-propriate method for VOC removal from main industrial emis-sions, such as the petroleum industry. In addition, many localsources such as printing and painting processes emit low-con-centration VOCs. In this case, the adsorption process is themost promising control technology. The advanced VOC remov-al process is composed of an adsorption unit and a catalytic in-cinerator.[13, 14] Many efforts have been made to design catalystswith good activity and selectivity.[1–12] Transition-metal oxidesare an alternative to the currently used noble metal-containingcatalysts because of their resistance to halogens, low cost, andhigh catalytic activity and selectivity.[1–4, 8] Their catalytic proper-

ties were described by using the Mars–van Krevelen mecha-nism.[9] The nature of the support is also an important factorbecause the surface area and functionality determine thenature and dispersion of metal oxide particles and thereforetheir catalytic behavior.[1, 8] Over the last decades, mesoporoussilica supports have been of interest as catalyst supports be-cause of their uniform mesoporous channel structure and highspecific surface area.[15–26] Various metal ions (Fe, Ti, V, Cr, Cu,etc.) have been introduced into the silica matrix to obtainmodified mesoporous materials with tunable catalytic proper-ties.[21–23] The applied method of modification strongly influen-ces the nature (location, dispersion, and oxidation state) of theloaded metal species.[24–28] Their introduction in the host matrixcould be realized during the silica synthesis process as well asby various postsynthesis techniques (e.g. , impregnation andgrafting).[24–28] Adsorption and catalytic oxidation of aromaticVOCs are of importance because they are emitted from diversesources, for example, from the petrochemical industry, printing,pressing, automobile exhaust, and traffic-related processes.The total oxidation of toluene as a model aromatic VOC hasbeen studied with different metals (Pt, Cu, Fe, V, Co, Cr) on dif-ferent types of supports (Al2O3, CeO2, TiO2 activatedcarbon).[2, 3, 7, 8, 20, 29–31]

We show that iron-functionalized silica nanoparticles with in-terparticle porosity (disordered mesoporous KIL-2 materialfunctionalized by iron) act as highly efficient adsorbents andcatalysts for toluene elimination. Spectroscopic techniquesdemonstrate that the superior activity of the catalyst can be at-tributed uniquely to the formation of stable Fe3 + ions in the

Catalytic oxidation is one of the most important industrially ap-plicable processes for the decomposition of volatile organiccompounds (VOCs) in polluted air. The advanced VOC removalprocess is composed of an adsorption unit and a catalytic in-cinerator. Many efforts have been made to design a combinedadsorption–catalytic unit with optimal activity and selectivity.We demonstrate that iron-functionalized silica nanoparticleswith interparticle mesoporosity (FeKIL-2) act as highly efficientadsorbents and catalysts with optimal Fe/Si molar ratios of0.01 in toluene oxidation as model VOCs in the gas phase. Byusing UV/Vis, FTIR, and Mçssbauer spectroscopic techniques,

we prove that the enhanced activity of the catalyst is attribut-ed to iron incorporated into the silica matrix, which dependson the iron content. The iron content with Fe/Si�0.01 leads tothe formation of stable Fe3+ ions in the silica matrix, which en-sures easier oxygen release from the catalyst (Fe3 +/Fe2 + redoxcycles). The increase in the iron content with Fe/Si>0.01 leadsto the formation of oligonuclear iron complexes. The materialthus introduces a promising, environmentally friendly, cost-ef-fective, and highly efficient catalyst with combined adsorptionand catalytic properties for the removal of low-concentrationVOC from polluted air.

[a] Dr. A. Ristic, D. MaucecNational Institute of Chemistry1000 Ljubljana (Slovenia)

[b] Dr. M. Popova, M. Vassileva, Prof. Dr. N. Novak TusarInstitute of Organic Chemistry with Centre of PhytochemistryBulgarian Academy of Sciences1113 Sofia (Bulgaria)

[c] Dr. K. LazarCentre for Energy ResearchInstitute of IsotopesHungarian Academy of Sciences1525 Budapest (Hungary)

[d] Prof. Dr. N. Novak TusarUniversity of NovaGorica5000 Nova Gorica (Slovenia)

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silica matrix, which ensures easier oxygen release from the cat-alyst (Fe3+/Fe2+redox cycles).

Results and Discussion

Physicochemical characterization of the samples

We have prepared the heterogeneous catalysts through the in-corporation of iron into the silicate nanoparticles of the disor-dered mesoporous KIL-2 structure with interparticle porosity,which have recently been developed in our laboratory.[31–33]

Heterogeneous iron catalysts with various iron concentrations(Fe/Si ratios ranging from 0.005 to 0.05) were preparedthrough direct two-step solvothermal synthesis (see details inExperimental Section). The XRD patterns of FeKIL-2 samples atlow angles indicate their disordered mesoporous nature (notshown). At high angles, they do not show detectable diffrac-tion peaks that could be assigned to iron oxide phases(Figure 1); thus, either such oxides are not present within

FeKIL-2 samples or their crystallites are very small with typicaldimensions of 5 nm or less and thus are not detectable byusing XRD. High-angle XRD patterns (Figure 1) show only onediffraction peak at 2 q= 44.39o corresponding to the sampleholder and one broad peak at2 q�23o corresponding to glass-like amorphous silicate nanopar-ticles.[34] SEM images of theFeKIL-2 catalysts are shown inFigure 2.

Nitrogen adsorption isothermsfor FeKIL-2 materials are shownin Figure 3, whereas structuralparameters determined on thebasis of these isotherms arelisted in Table 1. The differenceof isotherms types can be seenclearly in Figure 3. 005FeKIL-2

and 01FeKIL-2 samples show adsorption isotherms typical forKIL-2 silica,[31–33] that is, with a relatively narrow type IV hystere-sis loop.[35] The 005FeKIL-2 sample shows the adsorption iso-therm with a capillary condensation step at relative pressuresof approximately 0.88, which indicates the presence of large

Figure 1. XRD patterns of 005FeKIL-2 (Fe/Si = 0.005), 01FeKIL-2 (Fe/Si = 0.01),02FeKIL-2 (Fe/Si = 0.02), and 05FeKIL-2 (Fe/Si = 0.05): a) at low angles in the2 q range from 0.5 to 58 (not shown) and b) at high angles in the 2 q rangefrom 5 to 608.

Figure 2. SEM of a) 005FeKIL-2 (Fe/Si = 0.005), b) 01FeKIL-2 (Fe/Si = 0.01),c) 02FeKIL-2 (Fe/Si = 0.02), and d) 05FeKIL-2 (Fe/Si = 0.05). Scalebars = 200 nm.

Figure 3. Nitrogen sorption isotherms with pore size distribution curves ofFeKIL-2 samples. PSD = pore size distribution.

Table 1. Physicochemical properties of the studied samples.[a]

Sample Fe/Si Fe[wt %]

SBET

[m2 g�1]wBJH

[nm]Vme

[cm3 g�1]Vt

[cm3 g�1]Vmi

[cm3 g�1]Sme

[m2 g�1]Sex

[m2 g�1]

KIL-2 – – 545 19.9 1.392 1.480 0.054 370 38005FeKIL-2 0.005 0.47 472 27.0 1.149 1.292 0.080 246 3701FeKIL-2 0.01 0.92 556 21.6 1.326 1.459 0.077 332 3902FeKIL-2 0.02 2.91 213 – 0.546 0.976 – 143 7305FeKIL-2 0.05 3.84 366 – 0.368 0.640 0.069 136 70

[a] SBET = BET surface area; Sex = external surface area, Sme = mesoporous surface area, Vme = primary mesoporevolume, and Vmi = micropore volume, each evaluated by using the as plot method; Vt = total pore volume eval-uated from adsorption isotherm at the relative pressure of 0.98; and wBJH = mesopore diameters at the maxi-mum of the BJH pore size distribution.

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mesopores. The sample 01FeKIL-2 undergoes a capillary con-densation step at a lower relative pressure of approximately0.82, which evidences smaller mesopores in this sample com-pared to 005FeKIL-2. Thus, the increase in the iron amount re-sults in an increase in the pore volume (from 1.292 to1.459 cm3 g�1) and a decrease in the pore diameter.[36] Notably,higher amounts of the incorporated iron lead to a markedchange in the shape of the nitrogen adsorption isotherms; forexample, the nitrogen isotherms of 02FeKIL-2 and 05FeKIL-2samples change to a type II isotherm, which also confirms thepresence of macroporosity.[37] A capillary condensation step of02FeKIL-2 and 05FeKIL-2 samples occurs at a very high relativepressure (p/p0�0.9), which is an evidence of an interparticularporosity.[38]

BET specific surface areas of 005FeKIL-2 and 01FeKIL-2 mate-rials increase with a higher amount of iron, whereas the specif-ic surface areas of 02FeKIL-2 and 05FeKIL-2 materials decreasesignificantly. The pore size distribution of different mesoporousmaterials has been determined by using the BJH model widelyused for this type of samples.[39, 40] Although this model oftenunderestimates pore sizes, it is appropriate for comparativepurposes.[41] The pore size distribution determined from ad-sorption isotherms shows one distinguished maximum for005FeKIL-2 and 01FeKIL-2 samples. 02FeKIL-2 and 05FeKIL-2samples show very broad pore size distributions with no ap-parent maximum because of the technique limits of the nitro-gen adsorption–desorption apparatus.[38]

Adsorption and catalytic performance

The toluene adsorption data of the studied materials are pre-sented in Figure 4. All samples show close total adsorption ca-pacity (29.3–34.9 mg toluene/100 mg sample) but differenttimes (t= 5–25 min) to achieve the total capacity. The disor-dered silica structure of KIL-2 materials facilitates the access tothe adsorption sites compared with ordered mesoporous sili-cates.[42] The earlier breakthrough of toluene (Figure 4) for the

05FeKIL-2 sample could be explained by the higher amount ofiron slowing the toluene adsorption.

All catalysts were tested in total toluene oxidation, and theresults are shown in Figure 5. CO2 is the only registeredcarbon-containing product in all cases and H2O. Of all the

modified materials, 01FeKIL-2 possesses the best catalytic ac-tivity and stability. The catalytic activity increases with the in-crease in the iron content (up to 0.9 wt %); a further increasein the iron content (02FeKIL-2 and 05FeKIL-2) leads to lowercatalytic activity. Of the modified materials, 005FeKIL-2 and01FeKIL-2 possess the best catalytic stability at 380 8C (Fig-ure 5 b); the conversion remains approximately constant over3 h. The catalysts maintain their catalytic activity in three reac-tion cycles. A well-defined trend of catalytic activity decrease isobserved for 02FeKIL-2. The stable activity of 05FeKIL-2 couldbe related to the low activity of this sample.

The formation of Si�O�Fe species results in easier oxygenrelease, which is an essential step in the oxidation reaction ac-cording to the Mars–van Krevelen mechanism.[43–45] The mecha-nism of oxidation assumes the adsorption of the VOC moleculeon the catalyst surface and its oxidation with lattice oxygenfollowing the oxidation of the reduced catalysts. The surfaceproperties (acidic and basic) of the catalyst affect the sorptionof organic molecules and therefore its conversion in the oxida-tion reaction. The activity in the total oxidation of toluene isconnected with the interaction of aromatic electrons withacidic centers of the catalyst, which increases the possibility ofthe electrophilic attack of adsorbed oxygen and the combus-tion of toluene molecules. Iron ions could act as Lewis acidsites, which facilitates the interaction of toluene moleculeswith the support and thus increases the catalytic activity. Thereduction properties of the samples can be used for the inter-pretation of their catalytic performance as well (see the nextsection).

Figure 4. Breakthrough curves for toluene adsorption on the initial silicatenanoparticles (denoted as KIL-2) and iron-containing samples with Fe/Simolar ratios 0.005 (denoted as 005FeKIL-2), 0.01 (denoted as 01FeKIL-2), 0.02(denoted as 02FeKIL-2), and 0.05 (denoted as 05FeKIL-2). Operating condi-tions: Tads = 25 8C, atmospheric pressure, Ptoluene = 0.9 kPa.

Figure 5. a) Catalytic activity versus temperature and b) catalytic activityversus time on stream at 380 8C on the iron-containing silicate nanoparticleswith Fe/Si molar ratios 0.005 (denoted as 005FeKIL-2), 0.01 (denoted as01FeKIL-2), 0.02 (denoted as 02FeKIL-2), and 0.05 (denoted as 05FeKIL-2).Operating conditions for part a: T = 300–480 8C, atmospheric pressure,Ptoluene = 0.9 kPa, weight hourly space velocity = 1.2 h�1. TOF = turnoverfrequency.

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Local environment of iron

The information on the local environment of iron is obtainedthrough FTIR, UV/Vis, Mçssbauer, and temperature-pro-grammed reduction (TPR) experiments.

A comparison of the FTIR spectra of parent and iron-func-tionalized materials is shown in Figure 6. The asymmetricstretching vibrations of Si�O�Si from the silica matrix appear

at approximately 1090 cm�1. The slight shift of this band tolower wavenumbers is observed for the iron-containing materi-als. The band at 960 cm�1 probably arises from both Si�O�Hand Si�O�Ti stretching vibrations. The simultaneous shift ofthe bands at 1090 and 960 cm�1 to the lower wavenumbers isusually ascribed to the presence of tetrahedrally coordinatedmetal ions in the silica matrix.[19] The negligible changes in thespectrum of 02FeKIL-2 and 05FeKIL-2 in comparison with005FeKIL-2 and 01FeKIL-2 silicas are indicative of the lowerdegree of iron incorporation into the silica matrix of thesesamples.

Diffuse reflectance UV/Vis spectroscopy was used to investi-gate the nature of Fe3 + ions in FeKIL-2 samples (Figure 7). TheUV/Vis spectra of iron-containing zeolites are characterized byintense Fe3 +!O charge transfer bands, the position of whichprovided information on the coordination and degree of ag-glomeration.[46] Thus, isolated Fe3 + ions give rise to bandsbelow 300 nm, whereas signals of small oligonuclear FexOy

clusters within the pores appear between 300 and 450 nm andFe2O3 nanoparticles at the external surface of the crystal above450 nm.[47, 48] Two charge transfer transitions were observed formore or less distorted isolated Fe3 + sites; for tetrahedral Fe3 +

ions in the framework positions of iron silicalite-1, they are ob-served at 214 and 240 nm, whereas for isolated Fe3 + ions inthe octahedral symmetry, they are observed between 250 and300 nm.[49] Similar observations were found in disordered mes-oporous materials.[19, 33] The analysis of UV/Vis spectra of005FeKIL-2 and 01FeKIL-2 samples reveals peaks at 245 and290 nm. These bands are assigned to the charge transfer be-tween the iron and oxygen atoms of Fe�O�Si in the network,which indicates the presence of tetrahedrally and octahedrallycoordinated Fe3 + species, respectively.[47, 49] The UV/Vis spectraof 02FeKIL-2 and 05FeKIL-2 samples are different from those ofthe other two spectra. The analysis of both spectra revealpeaks at 250 and 355 nm. The first peak corresponds to isolat-ed Fe3+ species, whereas the latter can be assigned to the oli-gonuclear iron complex. An additional peak at 213 nm attribut-able to the presence of the isolated tetrahedral Fe3+ ions isalso present in the spectrum of 02FeKIL-2. No contributiongreater than 450 nm is observed, which excludes the occur-rence of iron oxide particles in all samples.[48]

The TPR thermogravimetric (TG) profiles (Figure 8) show dif-ferent reduction behaviors of the modified materials in termsof dependence of iron content. 02FeKIL-2 and 05FeKIL-2 sam-ples are characterized with two reduction peaks at low temper-

atures in the intervals 250–380 8C and 480–600 8C, whereas005FeKIL-2 and 01FeKIL-2 samples possess a single reductionpeak at high temperatures (480–600 8C). The reduction peak atlow temperatures is attributed to the presence of oligonucleariron clusters, which are formed mainly in the samples withhigh iron content (02FeKIL-2 and 05FeKIL-2). The reduction ofiron ions incorporated into the silica matrix of 005FeKIL-2 and01FeKIL-2 is observed at high temperatures.[42] This assumptionis also supported by the FTIR and UV/Vis data of the samples,and the presence of tetrahedrally coordinated Fe3 + ions canbe observed in the latter case. The TPR data of 02FeKIL-2 and05FeKIL-2 samples show a peak at a low temperature and

Figure 6. FTIR spectra of the initial and modified samples.

Figure 7. UV/Vis spectra of the modified samples.

Figure 8. TPR-TG profiles of the iron-modified samples. The samples were re-duced in a 50 vol % H2 in Ar (flow rate = 100 cm3 min�1) to a temperature of600oC (heating rate = 5 8C min�1), which was maintained for 1 h. DTG = differ-ential thermal gravimetry.

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lower reduction ability; thus, the redox cycles in these samplesare more difficult compared with those in 005FeKIL-2 and01FeKIL-2 samples (Table 2).

To obtain specific information on the local environment ofiron, in situ Mçssbauer spectra were recorded for 01FeKIL-2,02FeKIL-2, and 05FeKIL-2 samples under different conditions(after calcination and subsequent treatments at 640–650 K, i.e. ,evacuation and reduction in hydrogen, respectively). The spec-tra are shown in Figure 9, and the data obtained from the de-composition of spectra are listed in Table 3.

The signal of the tetrahedrally coordinated isolated Fe3 +

ions can be easily identified in the spectra. These ions demon-strate a characteristic Fe3+ doublet with large quadrupole split-ting (quadrupole splitting = 1.8–2.0 mm s�1 and isomer shift =

0.26–0.30 mm s�1) in the spectra recorded after evacuation.[50]

Notably, the proportion of this component decreases with theincrease in the iron content; the corresponding relative intensi-ty value is 67 % for 01FeKIL-2, whereas it is only 44 % in05FeKIL-2 in the first series of evacuation. The other character-istic observation in the analysis of spectra is that a largeamount of iron participates in the reversible Fe3+ÐFe2 + pro-cess. In microporous ferrisilicates, this process is not prevailing;in their rigid crystalline structure, the Fe3 + to Fe2+ reduction isnot allowed for the framework-substituted tetrahedral Fe3 +

ions, or if the Fe3 + to Fe2+ reduction proceeds, then the re-duced Fe2 + irons are expelled from the tetrahedral frameworksite to the extra-framework position.[48]

In contrast, in mesoporous materials, the reversibleFe3 +ÐFe2+ reduction proceeds on isolated iron ions becausethe structure is more flexible and the crystallinity is less strict.In comparison to the formation of silanolic Si�OH groups, Fe2 +

�OH groups form during the reduction of Fe3 +�O bonds.[50, 51]

This reversibility plays a role during the oxidation reaction bytransferring of oxygen atoms.[19] Furthermore, the Fe3 +!Fe2 +

reduction also proceeds on the mentioned polyferrate Fe�O�Fe�O� chains through the simple extraction of oxygen. How-ever, this latter process is not reversible when the hydrogentreatment is reverted to the repeated evacuation. The Fe2 +

component formed upon the extraction of oxygen starts toform oligomeric separate FexOy species, which does not partici-pate in further reversible Fe3+ÐFe2+ process, that is, in trans-ferring oxygen. The proportion of this “immobilized” iron in ag-glomerates increases with the iron content, as demonstratedby the increase in the remaining Fe2+ part after the hydrogentreatment and the subsequent second evacuation. Thus, theMçssbauer studies demonstrate the concentration dependenceof the proportions of tetrahedrally coordinated single Fe3 +

ions, and they also prove that a large proportion of tetrahe-drally coordinated iron ions can participate in the reversibleFe3 +ÐFe2+ redox process. Similar in situ spectra were ob-tained on a related analogous mesoporous Fe-TUD-1 (Fe/Si =0.02) sample.[52]

Hypothesis

The results from physicochemical studies of the local iron envi-ronment indicate that iron is incorporated into the frameworkfor materials with an Fe/Si ratio up to 0.01. If the iron concen-tration exceeds this value, iron deposits in the form of an oli-gonuclear iron complex. The iron ions incorporated into thesilica matrix can participate in Fe3 +/Fe2 + redox cycles moreeasily, which is an essential step in the oxidation process ac-cording to the Mars–van Krevelen mechanism. We assumethat, in the case of samples with lower iron content (005FeKIL-2 and 01FeKIL-2), iron is incorporated predominantly as stableFe3 + ions into the silica structure, whereas the oligonucleariron complexes formed in samples with higher iron content(02FeKIL-2 and 05FeKIL-2) are more prone to the agglomera-tion process. The TPR-TG experiments (Figure 8) support ouridea and there is no significant change in the reduction abilityfor 01FeKIL-2 during the second TPR-TG experiment(Figure 10). The same experiment is performed with 05FeKIL-2.The second TPR-TG experiment with 05FeKIl-2 shows a shift ofthe peak at low temperature to a higher temperature of414 8C. This effect can be explained with some difficulties inthe Fe3+!Fe2+ transition related to the presence of an oligo-nuclear iron complex.

Table 2. Redox properties of iron-modified KIL-2 materials.

Samples Weight loss Theoretical weight loss230–410 8C[wt %]

410–600 8C[wt %]

Fe3+!Fe2 +

[wt %]Fe2 +!Fe0

[wt %]Fe3+!Fe0

[wt %]

005FeKIL-2 0 0.07 0.07 0.13 0.2001FeKIL-2 0.02 0.13 0.13 0.26 0.3902FeKIL-2 0.06 0.25 0.42 0.83 1.2505FeKIL-2 0.18 0.33 0.55 1.10 1.65

Figure 9. In situ Mçssbauer spectra recorded for 01FeKIL-2, 02FeKIL-2, and05FeKIL-2 samples a) as received, b) after evacuation at 360 8C, c) after reduc-tion in H2 at 350 8C, and d) after 2nd evacuation at 360 8C.

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Conclusions

Iron-containing silicate nanoparticles with interparti-cle mesoporosity (pore diameter 2–50 nm) are highlyactive adsorbents and catalysts for toluene removalfrom polluted air. The iron content influences thenature of the metal species in the silica matrix. Thepresence of Fe3+ ions in the tetrahedral position isobserved for all modified materials. Oligonuclear ironcomplexes are present in FeKIL-2 samples with anFe/Si molar ratio greater than 0.1. All the iron-func-tionalized samples show high adsorption capacity.The highest catalytic activity is observed for the01FeKIL-2 sample with an Fe/Si molar ratio of 0.01.The predominate incorporation of isolated Fe3 + ionsinto the silica matrix is observed for 01FeKIL-2, whichfavors the fast Fe3 +!Fe2 + transition, essential for theoxidation reaction. Iron incorporated into silica nano-particles with interparticle mesoporosity (disorderedmesoporous KIL-2) was more efficient than iron incor-porated into ordered mesoporous silicates MCM-41and SBA-15.[42]

Experimental Section

Synthesis

Iron with Fe/Si molar ratios ranging from 0.005 to 0.05was incorporated into a new inorganic silicate matrixfrom the KIL family (KIL-2) through a two-step solvother-mal synthesis. KIL-2 is a mesoporous silicate formedfrom silicate nanoparticles (10–20 nm in size) with inter-particle porosity.[27] We denoted the potential catalysts005FeKIL-2 (Fe/Si = 0.005, 0.270 g), 01FeKIL-2 (Fe/Si =0.01, 0551 g), 02FeKIL-2 (Fe/Si = 0.02, 1.10 g), and05FeKIL-2 (Fe/Si = 0.05, 2.75 g). In the first step, iron(III)chloride hexahydrate (FeCl3·6 H2O, Aldrich; 005FeKIL-2:0.270 g, 01FeKIL-2: 0551 g, 02FeKIL-2: 1.10 g, and05FeKIL-2: 2.75 g), tetraethyl orthosilicate (98 %, Acros;42.52 g), and triethanolamine (99 %, Fluka; 15.07 g) were

stirred for 30 min. Finally, demineralized water was added to theabove mixture, followed by the addition of tetraethylammoniumhydroxide (20 %, Acros; 14.73 g). The solution was mixed witha magnetic stirrer to obtain a homogeneous gel. The final gel witha molar composition of tetraethyl orthosilicate/0.5 triethanolamine/(0.005–0.05) iron/0.1 tetraethylammonium hydroxide/11 water wasaged overnight at RT and then dried in an oven at 50 8C for 24 h.In the second step, the gel was solvothermally treated in ethanolin Teflon-lined stainless steel autoclaves at 150 8C for 48 h. Thetemplate was removed through calcination at 500 8C for 10 h(ramp rate = 1 8C min�1) in the flow of air. This also proved the ther-mal stability of the product. The hydrothermal stability of the prod-ucts was verified by keeping them in boiling water for 2 h.

Characterization

Basic characterization

The basic structural characterization of prepared catalysts was per-formed by using XRD, SEM, and N2 sorption measurements. XRDdiffractograms were recorded on a PANalytical X’Pert PRO (HTK)

Table 3. Comparison of characteristic data extracted from in situ Mçssbauer spectrafor FeKIL-2 samples.[a]

Samples Treatment Component IS[mm s�1]

QS[mm s�1]

FWHM[mm s�1]

RI[%]

01FeKIL-2 as received Fe3 + 0.24 0.73 0.50 20Fe3 + 0.35 1.19 0.84 80

evacuation at 350 8C Fe3 + 0.29 1.08 0.75 23Fe3 + 0.26 1.78 0.76 67Fe2 + 1.03 1.96 0.67 11

H2 at 360 8C Fe3 + 0.37 0.54 0.90 45Fe3 + 0.84 2.02 0.35 123Fe2 + 1.13 2.17 0.57 42

2nd evacuation at 350 8C Fe3 + 0.35 1.11 0.75 59Fe3 + 0.30 1.89 0.51 21Fe2 + 0.87 2.47 1.00 20

02FeKIL-2 as received Fe3 + 0.32 0.74 0.43 30Fe3 + 0.36 1.21 0.65 70

evacuation at 350 8C Fe3 + 0.33 1.21 0.67 37Fe3 + 0.28 1.91 0.67 57Fe2 + 1.09 2.28 0.49 6

H2 at 360 8C Fe3 + 0.37 0.73 1.00 32Fe3 + 0.92 1.95 0.52 31Fe3 + 1.21 2.04 0.51 37

2nd evacuation at 350 8C Fe3 + 0.33 1.15 0.62 53Fe3 + 0.30 1.77 0.52 30Fe2 + 1.06 2.11 0.70 17

05FeKIL-2 as received Fe3 + 0.34 0.77 0.42 40Fe3 + 0.36 1.27 0.58 60

evacuation at 350 8C Fe3 + 0.35 1.15 0.62 37Fe3 + 0.29 1.83 0.59 44

H2 at 360 8C Fe2 + 1.08 2.01 0.72 19Fe3 + 0.42 0.50 0.82 15Fe3 + 0.91 1.92 0.51 39Fe3 + 1.18 2.06 0.53 46

2nd evacuation at 350 8C Fe3 + 0.34 1.05 0.59 41Fe3 + 0.30 1.65 0.55 32Fe2 + 1.05 2.07 0.73 27

[a] IS = isomer shift, related to metallic iron; FWHM = full width at half maximum;QS = quadrupole splitting; RI = relative intensity.

Figure 10. Second TPR-TG profiles of the iron-modified samples. After thefirst reduction (Figure 8), the samples were reoxidized in a flow of air toa temperature of 500 8C (heating rate = 10 8C min�1), which was maintainedfor 1 h, and then the samples were reduced again in a 50 vol % H2 in Ar(flow rate = 100 cm3 min�1) to a temperature of 600 8C (heating rate = 5 8C min�1), which was maintained for 1 h.

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high-resolution diffractometer with CuKa1 radiation (1.5406 �) inthe 2 q range from 0.5 to 58 (100 s per step of 0.0398) and from 5to 608 (100 s per step of 0.0168) for the samples and from 108 to908 (100 s per step of 0.0168) for the sample holder equipped witha fully opened X’Celerator detector. Morphology and surface prop-erties of the samples were observed by using SEM on a ZeissSupra 3VP microscope. Elemental analysis of all samples was per-formed by using the energy-dispersive X-ray method with INCAEnergy system attached to a Zeiss Supra 3VP microscope. Nitrogenphysisorption measurements were performed at 77 K with a Micro-meritics ASAP 2020 volumetric adsorption analyzer. Before the ad-sorption analysis, the samples were outgassed under vacuum at473 K for 2 h in the port of the adsorption analyzer. The BET specif-ic surface area was calculated from the adsorption data in the rela-tive pressure range 0.06–0.165.[53] The total pore volume was esti-mated on the basis of the amount adsorbed at a relative pressureof 0.98.[36] The mesopore volume Vme and the external surface areaSex were determined by using the as plot method from the adsorp-tion data in the range of the standard reduced adsorption from 2.5to 3.0.[54, 55] The same method was applied to show the presence ofmicroporosity in these materials. The micropore volume Vmi was es-timated from the as interval 0.75–1.00.[55] In the as plot calculations,a macroporous silica material LiChrospher Si-1000 (SBET =22.1 m2 g�1) was used as a reference adsorbent.[56] The pore sizedistributions were calculated from nitrogen adsorption data byusing an algorithm based on the ideas of Barrett, Joyner, and Ha-lenda.[40] The mesopore diameters were determined as the maxi-mum on the pore size distribution for given samples.

Local environment of iron

The local environment of iron was investigated by using diffuse re-flectance UV/Vis, FTIR, and Mçssbauer spectroscopies. The diffusereflectance UV/Vis spectra of the samples in the UV/Vis regionwere recorded on a Lambda 40P (Perkin–Elmer) UV/Vis spectropho-tometer equipped with an integrating sphere. Spectra were decon-voluted to the Gaussian bands by using the Microcal Origin 8.1software). The FTIR spectrum was recorded from KBr pellets(99 wt % of KBr) on a Brucker Vector 22. In situ Mçssbauer spectrawere recorded for samples treated in a special measuring cell.[49]

Pellets were pressed from the 400–800 mg amounts of samplesand were evacuated at approximately 0.1 Pa and treated subse-quently in hydrogen (both treatments at 640 K). The 57Co/Rh(1 GBq) source was used for the measurements. Isomer shift valuesare related to a-iron. The obtained spectra were decomposed toLorentzian-shaped components. The accuracy of the positional pa-rameters is approximately �0.03 mm s�1. The TPR-TG investigationswere performed in a Setaram TG92 instrument. Typically, 40 mg ofthe sample was placed in a microbalance crucible and heated ina 50 vol % H2 in Ar (flow rate = 100 cm3 min�1) to a temperature of600 8C (heating rate = 5 8C min�1), which was maintained for 1 h.Before the TPR-TG experiments, the samples were treated in situ ina flow of air to a temperature of 500 8C (heating rate = 10 8C min�1),which was maintained for 1 h. After the first reduction, the sampleswere reoxidized in a flow of air (heating rate = 10 8C min�1) toa temperature of 500 8C, which was maintained for 1 h, and thenthe samples were heated in a 50 vol % H2 in Ar (flow rate =100 cm3 min�1) to a temperature of 600 8C (heating rate =5 8C min�1), which was maintained for 1 h.

Adsorption

The system for toluene adsorption assessment consisted of a fixed-bed flow reactor coupled with a GC system. The samples weretreated at 400 8C for 1 h before the adsorption experiment thatused a N2 flow of 60 mL min�1. The toluene adsorption was per-formed at 25 8C in N2 flow (flow rate = 30 mL min�1). By using thistechnique, the breakthrough curves, which represented the con-centration of toluene in the outlet gas versus time, were obtained.From the numerical integration of these curves, an adsorption ca-pacity of 880 ppm toluene was determined.

Catalysis

Before the catalytic test, the samples were pretreated in air at450 8C for 1 h. Toluene oxidation was studied at atmospheric pres-sure by using a fixed-bed flow reactor, air as the carrier gas, andthe catalyst (30 mg; prepared as agglomerates with 0.2–0.8 mmparticle size) diluted with glass beads (60 mg) of the same diame-ter previously checked to be inactive. The air stream was passedthrough a saturator filled with toluene and equilibrated at 0 8C(ptoluene = 0.9 kPa). The activity was determined in the temperatureinterval of 350–450 8C at a weight hourly space velocity of 1.2 h�1.The online analysis of the reaction products was performed byusing the GC apparatus equipped with a flame ionization detectorand a thermal conductivity detector on a 25 m PLOT Q capillarycolumn (25 m � 0.53 mm � 20 mm) with the polystyrene–divinylben-zene stationary phase (Chrompack). The turnover frequency wascalculated as the number of toluene molecules converted per ironatom per second. The samples were treated in air at 500 8C for 1 hafter every catalytic cycle.

Acknowledgements

The financial support from the Bulgarian Scientific Fund (Bulga-ria-Slovenia bilateral project HTC 02/205), the Slovenian ResearchAgency (research program P1-0021, research project Z1-9144,Slovenia-Hungary bilateral project BI-HU/11-12-010, Slovenia-Bul-garia bilateral project BI-BG/11-12-009), and the EN-FIST Centreof Excellence is acknowledged. The authors thank Dr. M. Dimitrovfor technical support.

Keywords: heterogeneous catalysis · interparticlemesoporosity · iron–silica · nanoparticles · oxidation

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Received: August 16, 2012

Published online on January 4, 2013

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