Nanodispersed Fe Oxide Supported Catalysts with Tuned Properties

Mar 4, 2008 - Studi di Milano, Via C. Golgi 19, 20133 Milano, Italy, Dipartimento di ... Nazionale delle Ricerche (CNR-ISTM), Via Golgi 19, I-20133 Mi...
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J. Phys. Chem. C 2008, 112, 4635-4642

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Nanodispersed Fe Oxide Supported Catalysts with Tuned Properties Antonella Gervasini,*,† Claudia Messi,‡ Alessandro Ponti,§ Simone Cenedese,| and Nicoletta Ravasio| Dipartimento di Chimica Fisica ed Elettrochimica (CFE) & Centro di Eccellenza CIMAINA, UniVersita` degli Studi di Milano, Via C. Golgi 19, 20133 Milano, Italy, Dipartimento di Chimica Fisica ed Elettrochimica (CFE), UniVersita` degli Studi di Milano, Via C. Golgi 19, 20133 Milano, Italy, Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle Ricerche (CNR-ISTM), Via Golgi 19, I-20133 Milano, Italy, UdR Milano UniVersita` , INSTM, Via Giusti 9, 50121 Firenze, Italy, and Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle Ricerche (CNR-ISTM), Via Golgi 19, I-20133 Milano, Italy ReceiVed: NoVember 9, 2007; In Final Form: January 7, 2008

Catalysts comprising amorphous Fe2O3 nanoparticles dispersed on silica and silica-zirconia are presented. A molecular Fe precursor was grafted on the oxide support surface by an equilibrium-adsorption method. Afterward, calcination of the material produced amorphous Fe2O3 nanoparticles on the oxide support surface. The different nature of the oxide support (SixZr1-xO2, 0.715 < x < 1) imparted tunable electronic (studied by UV-vis diffuse reflectance spectroscopy), dimensional (studied by electron paramagnetic resonance spectroscopy, EPR), and redox (studied by thermal programmed reduction, TPR) properties to the samples, with following down toward their catalytic properties. The different support surfaces were covered by the iron oxide phase (from 5 to 10%), constituted of nanosized particles in a narrow size interval (between 2 and 9 nm, studied by EPR line shape and transmission electron microscopy, TEM, analyses). Homogeneous surface distribution was achieved in any case (studied by scanning electron micrographs coupled with energy dispersive X-ray spectroscopy, SEM-EDS). The precise arrangement and nuclearity of the iron oxide nanoparticles on the surface depended on the support nature. Besides, from Fe2O3 nanoaggregates (3d and 2d nanoparticles), the presence of isolated Fe3+ ions in strong interaction with the oxide support was revealed, the amount increasing with the zirconia content in the support.

Introduction In the last decades the development of nanotechnology in heterogeneous catalysis led to improved design and control of catalysts and catalytic processes.1-2 One of the main key points for a successful catalytic process is the design of a catalyst with perfect selectivity and acceptable activity in order to obtain satisfactory productivity, absence of waste products, reduction of energy consumption, and easier separation and purification processes. With this goal in mind, different families of catalytic materials have been developed at the nanoscale, metal oxides being the most widely studied compounds. In particular, many benefits came from the design and development of supported metal oxides in which the active oxide aggregates are dispersed at the nanometric scale over various supports.3 The nature and properties of the support can effectively modify not only the geometric and electronic properties of the supported phase but also its chemical and catalytic properties. The so-called metalsupport interaction is a well-known phenomenon4,5 that can be exploited to regulate the properties of the supported phase. Unsupported amorphous Fe oxide in the form of nanoparticles has recently found much success in different fields of applied science.6 In catalysis, supported Fe oxide phases have revealed promising activity in various reactions such as the decomposition and reduction of nitrous oxide and nitric oxide, the partial * To whom correspondence should be addressed. Phone: +02 50314254. Fax: +02 50314300. E-mail: [email protected]. † Universita ` degli Studi di Milano, Dpt. CFE and CIMAINA. ‡ Universita ` degli Studi di Milano, Dpt. CFE. § CNR-ISTM, Milano, and INSTM, Firenze. | CNR-ISTM, Milano.

oxidation of hydrocarbons, and several acid transformations of organic substrates (e.g., Friedel-Crafts reactions, isomerizations, etc.).7-11 For most catalytic applications, high dispersion of the iron centers enhances the activity-selectivity pattern of the catalyst. To obtain such high Fe dispersion, conventional methods exploit the ion exchange properties of crystalline substrates (zeolites, MCM, SBA, etc.),12-17 while the formation of isolated, or highly dispersed, metal centers on amorphous oxide supports is less straightforward because the low ion exchange capacity of the latter ones. Moreover, the properties of supported Fe oxides largely depend on the support nature and characteristics and on the route chosen for their synthesis.12,17-19 In this investigation, we chose as supports for the Fe oxide phase silica (S), with its eminently neutral surface constituted of silanol and siloxane groups, and zirconia-modified silica (SZ series samples), with well-developed acid properties. Zirconia was introduced into the host silica structure in different proportions (SixZr1-xO2, 0.715 < x < 1) by a sol-gel method assuring high homogeneity of the synthesized supports. On the surface of each of these supports, a Fe molecular complex was grafted, yielding, after calcination, the final oxide materials (Fe concentration between 4 and 7% mass/mass). The Fe oxide properties were dependent on the support nature, as emerged from the suite of techniques employed for this study. Experimental Section Material Preparation. Oxide supports were prepared by a modification of the sol-gel procedure of Serrano et al.,20 and

10.1021/jp710742g CCC: $40.75 © 2008 American Chemical Society Published on Web 03/04/2008

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TABLE 1: Properties of Fe Catalysts compositiona (mass %) sample

SiO2

ZrO2

Fe2O3

Zr/Si (mol)

Fe concentration (atoms nm-2)

Fe coverageb (%)

SAc (m2 g-1)

ravd (nm)

Fe/S Fe/SZ-5 Fe/SZ-15 Fe/SZ-30 Fe/SZ-45

92.7 89.4 80.2 65.6 52.6

4.7 14.2 28.1 43.1

7.3 5.9 5.6 6.4 4.3

0.026 0.086 0.21 0.40

1.5 1.9 1.3 1.0 0.8

11.0 13.6 9.3 7.4 5.4

239.2 237.4 231.5 298.1 237.5

2.8 2.0 1.8 1.8 1.8

a SZ-5, SZ-15, SZ-30, and SZ-45 supports contained 5, 15, 30, and 45 mass % of ZrO2. b Theoretical support coverage by Fe2O3 ordered in a monolayer.18 c Surface area calculated by BET equation from N2-adsorption measurements. d Average pore radius calculated by BJH equation.

the iron precursor was deposited by a previously described procedure based on an equilibrium-adsorption method.21-22 For the synthesis of silica (S), an ethanol solution of TEOS (tetraethylortosilicate) was added dropwise under vigorous stirring to an aqueous solution of TPAOH (tetrapropylammonium hydroxide) (TPAOH/Si ) 0.09 and EtOH/H2O ) 2) to form the gel. For the synthesis of the SixZr1-xO2 samples (hereafter denoted as SZ-X, where X denotes the percentage mass content of zirconia), a 1-propanol (n-PrOH) solution of Zr(OC3H7)4 (zirconium tetrapropoxide) was added dropwise to a solution of TEOS, TPAOH, and hydrochloric acid (Zr/n-PrOH ) 0.04, HCl/Si ) 4, TPAOH/(Si+Zr) ) 0.09, and Zr/Si ) 0.026, 0.086, 0.21, and 0.40 for the synthesis of SZ-5, SZ-15, SZ-30, and SZ-45, respectively) to form the gel. The gels were aged for 18 h at room temperature, dried at 120 °C overnight, and finally calcined at 550 °C for 8 h. Fe(III) acetylacetonate (water/propanol solution, 0.5 L, 0.03 M) was used as a precursor of the Fe phase. It was adsorbed on the supports suspended in a water/propanol solution (1/1 v/v, 1 g in 0.12 L) at the temperature of 0 °C and pH 10, maintained by ammonia solution. After equilibration under vigorous stirring at room temperature for 24 h, dark orange solids were recovered by filtration, dried at 120 °C overnight, and finally calcined at 500 °C for 4 h. The obtained samples were labeled as Fe/S or Fe/SZ-X. The iron content of the samples was measured by ion-exchange chromatographic analysis after solid dissolution in acid mixtures and reaction in a microwave digestor at 220 °C. The composition of each sample, expressed as mass precentage of Fe2O3, SiO2, and ZrO2, is reported in Table 1. Characterization Techniques and Methods. Surface area (Brunauer-Emmett-Teller, BET) and porosity were determined by conventional N2 adsorption/desorption at -196 °C using a Carlo Erba Sorptomatic 1900 instrument. Prior to the analysis, the calcined supports and Fe samples (0.2-0.3 g) were outgassed at 350 °C for 16 h. Pore volume distribution was calculated from the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) model equation. Scanning electron micrographs (SEM) were obtained by a JEOL JSM-5500LV coupled with energy dispersive X-ray spectroscopic (EDS) analyzer working at 20 keV to obtain quantitative information on the distribution of Fe and Zr atoms. The samples were analyzed under a moderate vacuum after gold coating. Transmission electron micrographs (TEM) were obtained by a Zeiss EFTEM LEO 912AB microscope. Prior to the analysis, the samples were grounded, ultrasonically dispersed in ethanol, and deposited over a copper grid. X-ray diffraction (XRD) of the powder samples was carried out by a Philips PW1710 vertical goniometer diffractometer using Ni-filtered Cu KR1 radiation (λ ) 1.54178 Å). The chamber rotated around the sample at 1° (2θ) min-1 from 5 to 80° (2θ). UV-vis diffuse reflectance spectroscopy (UV-DRS) measurements were performed on fine powders of the Fe samples

put into a cell with optical quartz walls by a Perkin-Elmer Lambda 35 instrument equipped with an integrating sphere and Spectralon as reference material. Spectra were measured in reflectance mode. X-ray photoelectron spectroscopy (XPS) analyses of the supports and catalysts were carried out by a Kratos Analytical AXIS ULTRA DLD spectrophotometer, with Al KR monochromatised exciting radiation (1486.6 eV). A pass energy of 160 eV or 40 eV for the acquisition of the general (0-1100 eV) or high-resolution (C 1s, O 1s, Si 2p, Fe 2p, Zr 3d) spectra was used, respectively. The residual pressure in the analysis chamber was around 10-9 mbar. All binding energy (BE) measurements were corrected for charging effects with reference to the C 1s peak of the adventitious carbon (284.6 eV). The electron paramagnetic resonance (EPR) spectra were measured by a Bruker Elexsys E560 X-band spectrometer. Typical recording conditions were microwave frequency of ca. 9.4 GHz, microwave power of 5 mW (16 dB), magnetic field sweep range of 8000 G (2048 points), modulation frequency of 100 kHz, modulation amplitude of 4 G, and sweep time of 168 s. Spectra were taken at room temperature and at -196 °C (cold finger). The magnetic field was measured with a Bruker ER036TM teslameter; microwave frequency was measured by a Hewlett-Packard HP 5340A frequency counter. The dependence of the EPR line shape on temperature and on nanoparticle size has been theoretically modeled21,22 (see Supporting Information for details). TPR experiments were performed on the Fe samples using a TPD/R/O-1100 instrument from Thermo Electron Corporation. The sample mass used (ca. 0.07 g sieved between 45 and 60 mesh) corresponded to about 30 µmol of Fe2O3 for all the analyses. The samples were initially pretreated in air flow (45 mL min-1) at 350 °C for 1 h. After cooling to 50 °C, the H2/Ar (5.03% v/v) reducing mixture flowed through the sample at 15 mL min-1, while the temperature increased from 50 to 900 °C at a constant rate of 8 °C min-1. The H2 consumption was detected by a thermal conductivity detector. Peak areas were calibrated with pure H2 injections (Sapio, Italy; 6.0 purity) and with thermal reduction of high-purity CuO wires. Results and Discussion All synthesized SixZr1-xO2 oxides were amorphous, high surface area materials. The surface area of the oxides was in the 300-450 m2 g-1 range with porosity about 0.85 cm3‚g-1 and pore size about 8 nm. A clear trend of the surface area vs the zirconia content could not be observed. The introduction of low or high zirconia amounts in the host silica structure did not remarkably alter the external morphology of the final mixed oxides. SEM-EDS spectroscopy measurements indicated that the amount of zirconia in the first surface layers (probing depth of about 10 nm for ceramic materials) was very similar to that introduced in the host silica structure during synthesis (e.g., measured amount of ZrO2 by EDS of 29.88 ( 1.7 mass % for

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Figure 1. Surface molar Zr/Si ratios measured by XPS compared with the analogous bulk ratios from the amounts used in the preparation of the SixZr1-xO2, supports (circle) and Fe catalysts (diamonds).

Figure 2. Deconvolution of the O 1s XPS band of the S and SZ series supports: SiO2, 533.13 ( 0.06 eV; ZrO2, 530.98 ( 0.31 eV. The percent surface oxygen concentration of SiO2 and ZrO2 is indicated in the inset.

the SZ-30 sample), except for the SZ-45 sample that showed lower zirconia content. XPS surface analysis confirmed the presence of Zr4+ (Zr 3d at BE ) 183.03 ( 0.33 eV) besides Si4+ (Si 2p at BE ) 103.60 eV). From the quantitative analysis, the surface Zr/Si molar ratios could be determined and compared with the analogous bulk ratios (obtained from the amounts of Zr and Si used during synthesis), as shown in Figure 1. Regular surface enrichment in zirconium (increase of the Zr/Si surface ratios) with the increasing zirconium content in the samples was observed up to a Zr/Si synthesis ratio (bulk Zr/Si) of 0.2 (SZ-30). A further zirconium increase in the oxide support (SZ-45) led to a decrease of the surface Zr/Si ratio, indicating an incipient segregation phenomenon. This behavior was expected due to the different density of zirconia compared with silica, which likely led to an incipient zirconia collapse (sintering) during the calcination step. Because of the presence of Zr on all the sample surfaces, two different oxygen types were observed by XPS: the predominant one accounting for the silica (533 eV) and the second one for the zirconia (531 eV) presence. Figure 2 shows the deconvolution of the XPS band of the oxygen for all the samples with quantitative evaluation of the two peak components. From the deconvolution data the surface Zr content was determined as

Figure 3. Thermogravimetric curves (colored line) and first derivative of the curves (black line) of two selected dried Fe catalysts compared with the thermal profile of Fe(III) acetylacetonate (Fe(acac)3), mechanically mixed with sand (5% w/w).

1, 5, 15, and 21 mol % for SZ-5, SZ-15, SZ-30, and SZ-45, respectively. The observed trend regularly followed the increasing amounts of zirconia introduced in the silica structure. Therefore, the SixZr1-xO2 supports had the desired surface zirconia enrichment that guaranteed the development of the acid centers acting as anchorage for the supported iron phase. The number of acid centers on the support surface was determined by PEA (2-phenyl-ethyl-amine) thermal desorption after complete surface saturation with the PEA base probe.23-27 A regular increase of the amount of acid centers was observed from S to SZ-45 (0.431, 0.451, 0.943, 0.985, and 1.352 µmol g-1 for S, SZ-5, SZ-15, SZ-30, and SZ-45, respectively).28 It is known29 that the low reactivity of the hydroxyl groups of oxide surfaces may induce a low dispersion of the supported phases. To achieve high dispersion in grafting the iron phase to the oxide supports, we used Fe(III) acetylacetonate as precursor and we deposited it on the support surface by an equilibrium-adsorption method. The chosen calcination temperature (500 °C) guaranteed the complete decomposition of iron acetylacetonate and formation of the oxide iron phase, as proved by the thermogravimetric experiments. The thermograms (TGA analysis) and relevant first-derivative curves (DTGA) of the dried Fe samples reported in Figure 3 show the decomposition of the supported iron complex, compared with the unsupported Fe(III) acetylacetonate complex, as a function of temperature. Much higher temperatures (>500 °C) were neces-

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Figure 4. SEM-EDS analysis of the Fe/S (A) and Fe/SZ-30 (B) surfaces: A1 and B1, EDS spectra; A2 and B2; micrographs magnificated at 500×; A3 and B3, atomic maps of Fe (red), Si (yellow), and Zr (green) of the same regions observed in A2 and B2.

Figure 5. TEM micrographs of sample Fe/S at different magnification. Left, 25 000×; right, 80 000×.

sary for the complete decomposition of the supported complex compared with that of unsupported Fe(III) acetylacetonate (around 200 °C), as expected due to chemical interaction of the iron complex with specific surface sites of the support. The observed shape and peak position of the DTGA curves were similar for the different samples. At higher zirconia content in the support, more broadened DTGA profiles were observed, due to the decomposition of the iron complex in a more heterogeneous environment. The amount of iron deposited on the different supports was comprised in a small range in any case (Table 1), suggesting that the deposition method directed the arrangement of the precursor of the iron oxide phase over the different supports. The calculated surface coverage of the supports by Fe2O3 ordered in a monolayer30 decreased with the amount of zirconia in the support (Table 1), except for the Fe/S sample, which

Figure 6. XP spectrum of the Fe 2p region of Fe/SZ-45 (the sample has been chosen as a representative example of XP spectra of the Fe catalysts).

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Figure 7. UV-DRS spectra of calcined Fe catalysts (A). Examples of curve deconvolution for selected samples (a-d) (maximum absorption wavelengths and percentage area of the different species are indicated for each sample).

contained more iron than Fe/SZ-5. On the surface of each catalyst there are large uncovered regions of supporting oxide besides the supported iron phase. All the samples maintained amorphous characteristics (Supporting Information provides XRD spectra of the Fe samples). The surface morphology of the catalyst was investigated by SEM-EDS analysis (Figure 4). Images typical of amorphous phases with piles of irregular polyhedral-shaped grains with various dimensions were observed in any case. Over several grains, much smaller particles could be observed, likely due to the supported iron phase. The observed regions were mapped to study the atomic distribution of the components (Fe, Si, and Zr). The iron phase appeared homogeneously distributed over

all the support surfaces (see the A3 and B3 images of Figure 4, as examples). When the support contained zirconia (see B3 image of Figure 4, as example), more agglomerated iron phase could be individuated. On selected samples, TEM micrographs were also collected, as reported, for example, for Fe/S in Figure 5. Fe/S displays a grainy structure even at the scale of the tens of nanometers. Furthermore, well-dispersed iron oxide nanoparticles several nanometers long can be seen on the grain surface. The deposited iron phase did not significantly alter the Zr/Si surface ratio of the supports. The XPS-measured Zr/Si surface molar ratios for the supports and catalysts are very similar to the corresponding Zr/Si bulk ratios (see Figure 1). This

4640 J. Phys. Chem. C, Vol. 112, No. 12, 2008 observation suggested that the adopted equilibrium-adsorption method led to homogeneous deposition of small iron particles over the different supports without special affinity for the silica or zirconia phase. After calcination, Fe3+ was the only iron species detected in all the samples by XPS. Figure 6 depicts the Fe 2p core level of a selected sample (Fe/SZ-45), showing spin-orbit splitting of the Fe 2p3/2 ground state at 711.3 eV and the Fe 2p1/2 excited state at 724.8 eV. On all the other catalyst surfaces, very similar positions of the Fe 2p level were measured (Fe 2p3/2 ) 711.3 ( 0.32 and Fe 2p1/2 ) 724.8 ( 0.69). The peak position and energy difference between Fe 2p3/2 and Fe 2p1/2 (13.5 eV) is typical of the Fe3+ state in Fe2O3.31-34 The dispersion and nature of the supported iron species could also be inferred from the UV-DRS investigation. The literature suggests that 3d Fe2O3 nanoparticles show a band around 500 nm, low nuclearity (2d) FeOx species show a band between 300 and 500 nm, while a band below 300 nm is typical of isolated Fe3+ centers in an oxidic environment. These species are characterized by increasing interaction strength with support, respectively.10,32,35,36 Figure 7 shows the UV-DRS spectra of all the studied samples (Figure 7A) and the deconvolution analysis of the spectra for representative samples (parts a-d of Figure 7). In any case, the experimental curves could be deconvoluted into three subcurves with maxima around 500, 300, and 200 nm (see the exact values in the inset of each picture of parts a-d of Figure 7), whose position depended on the zirconia content in the support. A clear blue shift of the experimental spectra could be observed with increasing zirconia content. The spectra were dominated by the high-intensity charge-transfer bands at high frequency (curves at 200-230 nm), attributed to isolated Fe3+ species, indicating the obtained high dispersion of the iron phase in all the samples. The percent area of the bands at 310-370 nm (2d FeOx species) was 35,1. 37.2, 41.2, and 47.5% for Fe/S, Fe/SZ-15, Fe/SZ-30, and Fe/ SZ-45, respectively. Hence, the concentration of the species associated with this band increased with zirconia content in the support. On the opposite, the percent area of the bands at 480500 nm (3d Fe2O3 nanoparticles) decreased with increasing the zirconia content in the support (17.0, 7.1, 5.8, and 2.5% for Fe/S, Fe/SZ-15, Fe/ SZ-30, and Fe/SZ-45, respectively). Interesting information about the nature and size of the Fe oxide particles came from EPR investigation. (For the sake of clarity, we report on the detailed results for two representative samples: Fe/S and Fe/SZ-30.) The EPR spectra of Fe/S and Fe/SZ-30 recorded at room temperature and at -196 °C are shown in Figure 8. Each spectrum is the superposition of the signals from two different types of Fe(III) ions. The signal at g ) 4.3 (B0 = 1600 G) is typical of isolated, oxygen-coordinated Fe(III) ions in a distorted tetrahedral environment.37 Its intensity, larger at lower temperature as dictated by Curie law, is larger in the zirconia-containing sample Fe/SZ-30. EPR spectra are however dominated by another signal with a complex shape, exhibiting a narrow component centered at g ) 2.0 (B0 = 3360 G) and a broader component shifted to lower field. The latter becomes more intense at the expense of the narrow component and dramatically broadens when the temperature is lowered. This behavior is associated with the presence of superparamagnetic iron oxide nanoparticles.38,39 We fitted the experimental spectra to a theoretical spectral shape model to obtain information about the size of the particles. The used model has been successfully applied to the simulation of EPR spectra of Fe oxide particles in various matrices.40,41 To increase the accuracy and confidence of the best-fit

Gervasini et al.

Figure 8. EPR spectroscopy of Fe/S and Fe/SZ-30. (a-d) Experimental (black) and best-fit (red) spectra: (a) Fe/S at RT; (b) Fe/SZ-30 at RT; (c) Fe/S at -196 °C; (d) Fe/SZ-30 at -196 °C. (e-f) Fe oxide particle diameter distribution from spectral fitting: (e) log-normal distribution for Fe/S; (g) discrete distribution for Fe/SZ-30, including error bars.

parameters, we simultaneously fitted both the room temperature and -196 °C spectra to a single parameter set. The results are presented in parts a-d of Figure 8 (of course, the g ) 4.3 peak could not be reproduced by the model). The spectra of Fe/S could be well reproduced assuming that the nanoparticle diameter is distributed following the usual log-normal distribution42 (Figure 8e) with median 〈d〉 ) (2.90 ( 0.08) nm and deviation σd ) (0.40 ( 0.02). The particles turned out to approximately be oblate ellipsoids with demagnetizing factor difference N||-N⊥ ) 0.23 ( 0.02, which translates43 into an ellipsoid axis ratio of about 0.55. Unexpectedly, the spectra of Fe/SZ-30 could not be reproduced with a log-normal distribution. Since other choices would be arbitrary, we employed a discrete, histogramlike diameter distribution ranging from 0 to 10 nm with a 0.5-nm step. Then, a good fit to the experimental data was obtained (parts b and d of Figure 8). In this case, N||-N⊥ ) 0.20 ( 0.02 is a little smaller, hinting at slightly more spherical nanoparticles on the surface of Fe/SZ-30. The diameter distribution is clearly bimodal, as also observed in hexagonal mesoporous silicate catalysts5 showing two well-distinct peaks in the ranges of 1.5-3.5 nm and 6.5-8 nm (Figure 8f). The smaller particles, similar in size to those observed on Fe/S, account for (46 ( 4)% of the nanoparticles, whereas the larger ones, probably induced by the presence of zirconia, constitutes the (54 ( 10)% of the Fe oxide particles. In addition to the interaction with the support matrix, the nuclearity and size of the Fe oxides particle are expected to affect the redox properties of such phases. We chose to probe the reducing properties of the Fe samples by TPR experiments. Figure 9a collects the reduction profiles of the calcined samples. From a qualitative point of view, two main reduction peaks with well-defined maxima at ca. 400 °C (Tmax,1) and 800-900 °C

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Figure 9. TPR profiles of calcined Fe catalysts collected in the 50-900 °C interval (A). Examples of curve deconvolution of the first TPR peak (in the 140-650 °C interval corresponding to 1000-5000 s of analysis time) for selected samples (B). Temperatures of maximum rate of reduction and percentage areas are indicated for each sample.

(Tmax,2) dominated the spectra in all cases. The two well-defined TPR peaks suggests a two-step reduction of hematite to metallic iron through wu¨stite (FeO) and/or magnetite (Fe3O4) intermediates, in agreement with the evidence reported in the literature.32,44-45 However, the quantitative calculation of the hydrogen consumption indicated that these simple reduction paths could not hold.46 Beside the wu¨stite formation from hematite reduction (Fe2O3 f FeO), a part of hematite could be directly reduced to metallic iron (Fe2O3 f Fe). Indeed, deconvolution analysis of the low-temperature reduction peaks (Tmax,1) (Figure 9b) clearly showed in any case the contribution of two subcurves (Tmax,1a located at ca. 320 °C and Tmax,1b at ca. 420 °C). The relative proportion of the areas of the two curves changed with the zirconia content. The reduction curve at higher temperature (Tmax,1b) was prevalent in all samples, and it decreased with decreasing zirconia content in the support, while the opposite trend was observed for the low-temperature curve (Tmax,1a). The direct Fe2O3 f Fe reduction seemed favored when a strong interaction of iron with zirconia is involved.

Moreover, by increasing the zirconia content in the support, the peaks at high temperature (Tmax,2), due to the reduction of wu¨stite to metallic iron, shifted to higher temperatures, accounting for a strong metal-support interaction of the iron species. The functional character of the studied samples is related to their acid properties, which determine the catalytic properties. The nature of the acid centers of the samples is not unique.47 The surface comprise Lewis species (dispersed Fe(III) centers), Brønsted centers (hydroxyl groups), and M-[ ] vacancies in the uncovered SixZr1-xO2 surface.48 This complex acid site distribution affected the sample catalytic properties. A detailed study on activity, selectivity, and productivity in the isomerization of R-pinene oxide to the corresponding campholenic aldehyde,49 chosen as test reaction, was carried out.50 The most significant results are listed in Table 2. All Fe catalysts proved to be very active even in very mild conditions, and the obtained product distribution was in agreement with the eminently Lewis acid character of these materials. Best selectivity to campholenic aldehyde was observed over Fe/S (reaction path catalyzed by

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TABLE 2: Catalytic Performances of the Fe Catalysts in the Reaction of Isomerization of r-Pinene Oxide to Campholenic Aldehyde50 catalysta

tb (min)

selectivity (%) to campholenic aldehyde

productivityc (mmol‚gcat-1‚h-1)

Fe/S Fe/SZ5 Fe/SZ15 Fe/SZ30 Fe/SZ45

35 5 5 5 5

72 65 61 56 57

8 53 50 44 45

a Catalyst mass, 100 mg; substrate mass (R-pinene oxide), 100 mg; solvent, toluene; reaction temperature, 25 °C. b Time required to obtain complete conversion of substrate. c Amount of campholenic aldehyde produced at total conversion of substrate.

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