Dispersed NbOx Catalytic Phases in Silica Matrixes: Influence of

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J. Phys. Chem. C 2008, 112, 14064–14074

Dispersed NbOx Catalytic Phases in Silica Matrixes: Influence of Niobium Concentration and Preparative Route Paolo Carniti,*,† Antonella Gervasini,*,†,‡ and Matteo Marzo† Dipartimento di Chimica Fisica ed Elettrochimica (CFE), and Centro di Eccellenza CIMAINA, UniVersita` degli Studi di Milano, Via Camillo Golgi 19, 20133 Milano, Italy ReceiVed: April 11, 2008; ReVised Manuscript ReceiVed: June 9, 2008

Niobium oxide catalytic phase was dispersed in/over silica host structures by using different Nb sources (ammonium niobium oxalate complex and niobium pentaethoxide) and methodologies (coprecipitation, sol-gel, and impregnation). Three series of completely amorphous silica-niobia catalysts were obtained with 5, 15, 30, 45, and 60 mass % Nb2O5 by coprecipitation (aqueous route, from ammonium niobium oxalate) and sol-gel (organic route, from niobium pentaethoxide) and with 5, 10, and 20 mass % Nb2O5 by impregnation on a finite silica, from ammonium niobium oxalate. The surface and bulk catalyst properties of all the series of samples were studied by several physicochemical techniques [N2 adsorption/desorption, thermal gravimetric analysis (TGA), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflection spectra (UV-vis DRS), temperature-programmed reduction (TPR)], and the acid properties of the surfaces were investigated by base (2-phenylethylamine, PEA) titration in liquid (cyclohexane). Evidence from both XPS (3d Nb bands and O 1s band) and UV-vis DRS (ligand-to-metal charge-transfer transitions (LMCT) from O2- to Nb5+ and edge energies) measurements showed that the Nb dispersion increased with Nb dilution in the host silica structures. The number of coordinated oxygens around the Nb center was 6, for the highest diluted samples, and converged to 2.5, indicating the progressive formation of Nb2O5 nanodomains. As the Nb2O5 concentration decreased in the catalyst composition, the surface area values expanded and Nb dispersion increased, owing to Nb-O-Si linkage formation. The synthesis by sol-gel permitted accommodating a higher surface concentration of niobia than by the coprecipitation route. 1. Introduction High-acidity and water-tolerant properties are the most interesting features of Nb2O5-containing catalytic materials.1-4 The recognition of the relations between acidity of such oxide phase and its catalytic properties was disclosed early and reviewed by Tanabe et al.5 and more recently by Okuhara6 and Busca.7 Bulk niobia or Nb2O5-containing catalysts (mixed oxides of niobia, niobia supported on various oxides, metal oxides supported on niobia, niobia as an additive, surface-modified niobia, mesoporous molecular sieves, and Nb-doped mesoporous silica)8 find application for some fine chemical acid-catalyzed processes,9-11 among these the fructose dehydration reaction,12,13 and for selective oxidation, hydrogenation and dehydrogenation, hydrocarbon conversion, photocatalysis, and polymerization processes.14-18 The control of the local structure of niobium species in the catalysts containing niobia is a key factor to govern their catalytic performances. Concerning the supported niobia catalysts, many investigations were done by selecting suitable support materials,19,20 niobium precursors,21 and preparation methods,20c,22 to control the formation of the surface niobia species. Different niobia species have been recognized on the different supports by in situ Raman and X-ray absorption nearedge spectroscopy (XANES) studies:19,23 highly dispersed monomeric and oligomeric fourfold-coordinated NbO4 species, * Corresponding authors. E-mail: [email protected] (P.C.); antonella. [email protected] (A.G.). † Dipartimento di Chimica Fisica ed Elettrochimica. ‡ Centro di Eccellenza CIMAINA.

fivefold-coordinated NbO5 species, and sixfold-coordinated polymeric NbO6 species. The relative distribution of these niobia species is a function of the Nb loading and of the nature of the oxide support. More recently, niobium has been incorporated in mesoporous siliceous matrixes, such as MCM-41 and MCM-48.3,14,24 The obtained samples possess only the Nb5+ cations incorporated into the matrixes as isolated NbO4 species; consequently, they display unique physicochemical and oxidation properties. On the other hand, the control of the nature, dispersion, acidity, and catalytic activity of the Nb centers inserted on amorphous silicas is really more difficult and challenging because of the low number and reactivity of the different silica surface hydroxyls.8d,20a Silica-supported niobia catalysts have been mainly developed and investigated as acid and redox catalysts.8d,20c,23c The presence of dispersed Nb species in/on a high surface area and chemically inert host matrix, as silica, leads to surface expansions through Si-O-Nb linkages, compared with bulk niobia.25 The relationships between the structure, acidity, and dispersion of niobium oxide in silica matrixes is not well-established, also concerning the influence of the preparation method and Nb loading. In this study, we present a systematic investigation on the dispersion and relative surface properties of niobia in/on silica structures, which have been prepared by different methods and contained Nb concentrations in a wide interval. The detailed knowledge and comparative viewing of the surface properties of the different series samples, in particular for the structure of the dispersed Nb species, will be useful for establishing sound correlations with the catalytic activities measured in the fructose

10.1021/jp803140x CCC: $40.75  2008 American Chemical Society Published on Web 08/14/2008

Dispersed NbOx Phases in Silica Matrixes

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TABLE 1: Presentation of the Three Si-Nb Series Samples and Their Main Properties Nb2O5 content

Nb2O5 concentration

sample

/mass %

/mol %

Nb/Si

Si-Nb/5aq Si-Nb/15aq Si-Nb/30aq Si-Nb/45aq Si-Nb/60aq Si-Nb/5org Si-Nb/15org Si-Nb/30org Si-Nb/45org Si-Nb/60org Si-Nb/5wi Si-Nb/10wi Si-Nb/20wi

5 15 30 45 60 5 15 30 45 60 5.08 9.15 16.61

1.2 3.8 8.8 15.6 25.3 1.2 3.8 8.8 15.6 25.3 1.2 2.2 4.3

0.024 0.080 0.194 0.370 0.678 0.024 0.080 0.194 0.370 0.678

a

/atomNb · nm-2

SBET/m2 · g-1

Vp/cm3 · g-1

rp,av/nma

acid sites/mequiv · g-1 b

1.19 1.77 4.09

274 200 165 141 93 317 434 448 262 134 193 234 184

0.59 0.43 0.36 0.29 0.41 1.00 0.93 0.42 0.17 0.10 0.65 0.65 0.64

2.14 1.80 1.75 1.83 1.79 4.51 3.72 1.85 1.64 1.71 4.16 4.14 4.88

0.44 0.35 0.27 0.21 0.18 0.34 0.52 0.50 0.41 0.10 0.28 0.29 0.27

Main pore size evaluated from BJH pore distribution. b Determined under PEA equilibrium concentration of 5× 10-5 M.

Figure 1. Thermogravimetric profiles of selected dried samples at low and high Nb content obtained by coprecipitation (left side) (A) Si-Nb/5aq, (B) Si-Nb/60aq, and by impregnation (right side) (C) Si-Nb/5wi, (D) Si-Nb/20wi.

dehydration reaction running in water and highly protic solvents, pursuing our experimental activity in this field.13 2. Experimental Section 2.1. Material Preparation. Ammonium niobium oxalate complex (NH4[NbO(C2O4)2(H2O)] · (H2O)m, ANBO), kindly furnished from Companhia Brasileira de Metalurgia e Mineraçaõ (CBMM), and niobium(V) ethoxide (Nb(OCH2CH3)5, NBE, 99.95%), purchased from Aldrich, were used as Nb sources. Tetraethyl orthosilicate (Si(OC2H5)4, TEOS, g98%), purchased from Fluka, was used for the silica synthesis. A first series of silica-niobia samples was prepared in water by coprecipitation (aqueous route synthesis) with 5, 15, 30, 45, and 60 mass % of Nb2O5 (Si-Nb/xaq, with x representing the Nb2O5 mass %). To prepare 1 g of sample, aqueous solution of ANBO (from 0.11 to 1.4 g) was gently dropped into TEOS (from 3.3 to 1.4 g), previously hydrolyzed with hydrochloric acid (0.05 M) at room temperature (r.t.) for ca. 60 min. The

HCl/TEOS molar ratio was kept constant to 4, in any case. Then, ammonia solution (28 mass %) was added dropwise to the obtained limpid solution, until complete precipitation. The unripe solid was aged at r.t. for 24 h, dried under vacuum at 40 °C for 2 h, and eventually calcined at 550 °C for 8 h. A second series of silica-niobia samples was prepared by sol-gel (organic route synthesis) with 5, 15, 30, 45, and 60 mass % of Nb2O5 (Si-Nb/xorg, with x representing the Nb2O5 mass %), adopting the synthetic procedure described in ref 26. To prepare 1 g of sample, NBE (from 0.12 to 1.4 g, handled under flowing nitrogen) was added under vigorous stirring to an ethanolic (EtOH) solution of TEOS (from 3.3 to 1.4 g) hydrolyzed with concentrated hydrochloric acid (0.1 M) at r.t. for ca. 60 min. The [TEOS + EtOH]/HCl and EtOH/TEOS volume ratios were kept equal to 58 and 3.8, respectively. Clear solutions were obtained, in every case. Then, the addition of a defined amount of tetrapropylammonium hydroxide ((C3H7)4NOH, TPAOH, 20 mass % in water) led to gelation. For a

14066 J. Phys. Chem. C, Vol. 112, No. 36, 2008

Figure 2. XRD patterns of three selected samples for each of the three Si-Nb series (panel A, coprecipitation; panel B, sol-gel; panel C, impregnation).

complete and fast gelation, the TPAOH/(Nb + Si) molar ratio was kept equal to 0.30. The unripe solid was aged at r.t. for 24 h, dried under vacuum at 40 °C for 2 h, and eventually calcined at 550 °C for 8 h. A third series of silica-niobia samples was prepared by wet impregnation of a finite silica with ANBO, as Nb source. Silica (370 m2 · g-1, 0.8 cm3 · g-1, and ca. 10 nm of average pore size) was prepared as described elsewhere.27 The adequate amount of ANBO (from 0.57 to 2.28 g) was dissolved in water and added to the slurry containing the silica support (5 g), obtaining a total volume of ca. 40 mL. After 16 h of contact, water was evaporated and the solid was dried (120 °C for 16 h) and calcined at 550 °C for 8 h. The final samples contained 5, 10, and 20 mass % of Nb2O5 (Si-Nb/xwi, with x representing the Nb2O5 mass %). 2.2. Characterization Techniques. Surface areas were determined from N2 adsorption and desorption isotherms at liquid nitrogen temperature by using the BET method. The N2 isotherms were collected in an automatic analyzer (Sorptomatic 1900 instrument). Prior to measurement, the sample (ca. 0.1-0.3 g) crushed and sieved as 45-60 mesh particles was introduced in the sample holder and thermally activated at 350 °C for 16 h under vacuum. The pore volume was determined from the total amount of N2 adsorbed at P/Po ) 0.99 (cm3 · g-1 (STP)) converted into liquid volume, cm3 · g-1, using the N2 density in

Carniti et al. the normal liquid state (F ) 0.8081 g · cm-3). Pore size distribution was determined from the desorption branch of the isotherm by the Barret-Joyner-Halenda (BJH)28 method. Scanning electron micrographs (SEM) were collected by a JEOL JSM-5500LV coupled with energy-dispersive X-ray spectroscopy (EDS) working with an accelerating voltage of 20 kV. X-ray diffraction (XRD) patterns of the powder samples were recorded using a Philips PW3020 diffractometer, using a Cu KR radiation (λ ) 1.5418 Å) and fixed power source (40 kV). The samples were scanned at a rate of 1 deg · min-1 (2θ) over a range of 5-80°, which comprises the characteristic diffraction peaks of niobium oxide. The sample surfaces were qualitatively and quantitatively studied by X-ray photoelectron spectroscopy (XPS) by a Kratos Analytical AXIS ULTRA DLD spectrophotometer (Al KR monochromatized exciting radiation, 1486.6 eV). All binding energy (BE) measurements were corrected for charging effects with reference to the C 1s peak of the adventitious carbon (284.6 eV). UV and visible light diffuse reflection spectra (UV-vis DRS) were recorded on a double-beam Perkin-Elmer Lambda 35 spectrophotometer equipped with a DR accessory (RSA-PE-20 model, 50 mm) coated with Spectralon as reference material. Spectra were measured in reflectance mode in the 1100-190 nm range. Thermal gravimetric analysis (TGA) was conducted on the dried samples in a TG analyzer from Perkin-Elmer (TGA7), with a scan of 10 °C · min-1, from 25 to 800 °C under flowing air. For better evidencing the thermal events, differential thermogravimetric curves (DTGA) were calculated from the parent TGA profiles. Temperature-programmed reduction (TPR) analysis was carried out on the calcined samples (sieved as 45-60 mesh particles), activated at 350 °C for 1 h under flowing air (45 mL · min-1), in a Thermo Fisher TPD/R/O-1100 instrument, equipped with a quartz reactor with a porous septum (i.d., 8 mm), and a filter of soda lime for trapping acid gases and water. The measurements were carried out with the following conditions: temperature range, from 40 to 920 °C at constant heating rate of 8 °C · min-1; reducing gas, H2/Ar mixture (5.03% v/v, Sapio, Italy) flowing at 15 mL · min-1. Experiment conditions forTPRrunswerechosenaccordingtotheliteraturerecommendations,29-31 in order to achieve good resolution of the component peaks (sample mass comprised between 0.050 and 0.15 g, corresponding to 150-200 µmol of Nb2O5). The H2 consumption was detected by a thermal conductivity detector (TCD), and peak areas were calibrated with pure H2 (>99.99999%, from Sapio, Italy) injections and high-purity CuO wires. The acidity experiments were performed in solvent (cyclohexane) using a high-performance liquid chromatography (HPLC) apparatus, comprising a pump (Waters, model 510) and a monochromatic UV detector (Waters, model 2487, λ ) 254 nm), coupled to a personal computer for the collection, storage, and processing of the data. The apparatus, already described for the determination of the solid acidity in liquids of various proticity and polarity by a pulse technique,32,33 has been modified to work as a recirculating method.34 The titration of the acid sites was carried out at fixed temperature of 17 °C by the 2-phenylethylamine (PEA) probe. The sample (ca. 0.050 g, crushed and sieved as 80-200 mesh particles) was first activated at 350 °C for 16 h in flowing air in a stainless steel tube (i.d. 2 mm, length 12 cm); the tube was successively thermostatted and mounted in place of the chromatographic column for the


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Figure 3. N2 adsorption/desorption isotherms (left side) and BJH pore distribution (right side) of two selected samples at low and high Nb content for each of the three Si-Nb series.

Figure 4. Trends of surface area vs bulk molar Nb2O5 content for all the samples of the three Si-Nb series.

analysis. Successive dosed amounts of PEA solution (50 µL, 0.15 M in cyclohexane) were injected into the line in which cyclohexane continuously circulated. For each injection, the PEA solution recirculated onto the sample until adsorption equilibrium was achieved. The collected data allowed us to draw adsorption isotherms. Assuming a 1:1 stoichiometry for the PEA adsorption on the acid site, the total number of acid sites per catalyst mass was found (mequiv · g-1) with a percentage error of 7-8%.

3. Results and Discussion The three different series of silica-niobia (Si-Nb) samples were prepared starting from two different Nb precursors (ANBO and NBE) and employing three different preparation routes (coprecipitation, sol-gel, and impregnation) to understand the effect of dilution of the silica matrix on the properties of the niobium oxide phase. For each series of samples, a large interval of Nb concentrations was taken into account (from 5 to 60 mass % of Nb2O5, corresponding to 1-25 mol %, for the samples prepared by coprecipitation and sol-gel, and from 5 to 17 mass % of Nb2O5, corresponding to 1-4 mol %, for the impregnated samples) to control the limit of the diluting ability of the host silica, also in relation with the preparative route and Nb precursor used (Table 1). The synthetic procedure for the preparation of the Si-Nb/ xaq and Si-Nb/xorg samples proceeded in a similar way. In both the preparations, the Si font was TEOS which was hydrolyzed in acid-aqueous or acid-ethanolic solutions, for the Si-Nb/ xaq and Si-Nb/xorg samples, respectively. The Nb sources, ANBO for the Si-Nb/xaq samples and NBE for the Si-Nb/xaq samples, were added to the relevant aqueous or ethanolic TEOS solutions, then the pH was increased by ammonia or TPAOH addition, respectively, causing the precipitation or gelation of the unripe silica-niobia mass. In these two series of samples,


Carniti et al.

Figure 5. SEM micrographs of the surfaces of each of the three Nb-Si series magnified at 500×: (A1) Si-Nb/15aq, (A2) Si-Nb/15org, and (A3) Si-Nb/20wi; (B1-B3) atomic maps of Nb (yellow) and Si (blue) of the same regions observed in the images in panels A1-A3.

TABLE 2: Quantitative Results of the XPS Experiments on Three Si-Nb Series Samples Nb (V) 3d BE/eV

O 1sBE/eV

sample

Nb/Si

Nb2O5/mol %

3d5/2

3d3/2

∆3d/eV

fwhm Nb 3d5/2/eV

Si-Nb/5aq Si-Nb/15aq Si-Nb/30aq Si-Nb/45aq Si-Nb/60aq Si-Nb/5org Si-Nb/15org Si-Nb/30org Si-Nb/45org Si-Nb/60org Si-Nb/5wi Si-Nb/10wi Si-Nb/20wi

0.012 0.044 0.101 0.151 0.379 0.014 0.057 0.154 0.289 0.531 0.014 0.025 0.029

0.57 2.15 4.82 7.03 15.93 0.67 2.77 7.13 12.63 20.97 0.70 1.24 1.42

208.2 207.9 207.9 207.8 207.8 207.9 207.8 207.5 207.6 207.4 207.5 207.3 207.4

210.5 210.4 210.6 210.5 210.6 210.6 210.6 210.2 210.3 210.2 210.1 210.0 210.1

2.22 2.50 2.73 2.79 2.76 2.68 2.76 2.76 2.75 2.78 2.59 2.67 2.75

3.41 3.08 2.59 1.76 1.49 2.50 2.12 1.78 1.60 1.55 2.51 2.12 1.65

a

band

O-Nb peak

O-Si peak

530.8 (11.5)a 530.7 (25.9) 530.5 (3.4) 530.9 (7.6) 530.7 (14.7) 530.7 (23.2) 530.6 (31.4)

532.7 (88.5)b 532.4 (74.1) 533.0 (96.6) 533.0 (92.4) 532.6 (85.3) 532.6 (76.8) 532.3 (68.6)

532.6 532.3 532.6

532.9 533.0

Percent contribution of O-Nb. b Percent contribution of O-Si.

the Nb phase should be homogeneously distributed at both the surface and the mass of the samples. On the contrary, the Si-Nb/xwi samples were prepared by deposition of the Nb precursor (ANBO) over silica (surface area, 370 m2 · g-1; pore volume, 0.8 mL · g-1; average pore size, 10 nm);27 therefore, the Nb phase should only be present at the outer shell of the silica particles. For a closer comparison of the surface properties among the three series of Si-Nb samples, the interval of loading of Nb on the Si-Nb/xwi samples was more limited than in the other samples (see Table 1). To complete the formation of the

SiO2-Nb2O5 oxide phases, the drying and calcination steps were performed. The calcination temperature was chosen based on the results obtained from the thermogravimetric analysis performed on the dried materials. It can be deduced that it was necessary to heat the samples up to 550 °C to remove all of the organic matter and to complete the oxide phases. Figure 1 shows the thermogravimetric profiles of two selected samples obtained by coprecipitation and by impregnation at low and high niobia content, prepared from the same Nb source (ANBO). The thermograms show, centered at about 100 °C, losses of mass


Figure 6. Surface Nb/Si ratio vs bulk Nb/Si ratio for the three Si-Nb series samples.

Figure 7. X-ray photoelectron spectra of the O 1s region for all the samples prepared by sol-gel: (A) Si-Nb/5org, (B) Si-Nb/15org, (C) Si-Nb/30org, (D) Si-Nb/45org, (E) Si-Nb/60org. Deconvolution of the (E) spectrum is shown in the inset.

related to physical desorption of water; they were heavier as higher the silica content in the sample (compare part A of Figure 1 with part B and part C with part D). The main DTGA peaks, associated with the most intense losses of mass and centered around 200-240 °C, could be attributed to the decomposition of ANBO. The mass loss regularly increased with the Nb content in the sample, for both the Si-Nb/xaq and Si-Nb/xwi series samples, as expected. It can be noticed a light shift at higher temperature of the decomposition peak (Tmax) of ANBO with the amount of Nb in the sample (e.g., 188, 198, 207, 236, and 244 °C for Si-Nb/5aq, Si-Nb/15aq, Si-Nb/30aq, Si-Nb/45aq, and Si-Nb/60aq, respectively), as shown in Figure 1 for representative examples. The Tmax values of the Si-Nb/xwi samples were always higher than those of the Si-Nb/xaq series samples. This behavior could suggest the presence of different degrees of aggregation of the NbOx centers between the two sample series. An analogous clear interpretation of the thermogravimetric curves for the Si-Nb/xorg samples could not be done, because the presence of TPAOH in the dried samples, whose thermal decomposition dimmed that of NBE, the used precursor of the niobia phase. All the calcined samples, also those at very high niobia content, are completely amorphous, as Figure 2 displays. The XRD patterns show very broad and low intensity bands around 15-25° 2θ, typical of the presence of unstructured silica. It seems that no phase separation occurred and any defined XRD reflexes from Nb2O5 or other niobium oxide phase did not occur in the Si-Nb samples. When samples contained a very high Nb amount and likely Nb aggregation could occur, the formed niobium oxide phase did not arrange in a well-ordered way. Direct, by SEM-EDS analysis, and indirect, by N2 adsorption and desorption isotherms, investigations were carried out to

J. Phys. Chem. C, Vol. 112, No. 36, 2008 14069 study the textural properties of the samples. The main textural properties of all the samples are summarized in Table 1. Interesting differences emerged among the three Si-Nb series from the collected isotherms of N2 adsorption-desorption and relevant pore size distributions, comparatively depicted in Figure 3. For each of the three Si-Nb series, two chosen samples containing different amounts of niobia are depicted, as examples. The samples prepared by impregnation did not show any noticeable difference, neither for the shape of the N2 isotherms nor for the pore size distribution. The N2 isotherms and pore distribution curves of the Si-Nb/xwi samples are typical of mesoporous siliceous materials. On the other hand, the samples prepared by aqueous route and, in particular, those prepared by organic route (Si-Nb/xaq and Si-Nb/xorg) showed significant differences of the morphologic features with the variation of the Nb concentration. At low Nb content, they exhibited higher adsorption and higher porosity than the high Nb content samples and the pore size became narrower with the increase of the Nb content in the samples. The sample pore size distribution evolved from a broad pore size distribution, typical of the amorphous silica materials, to that typical of niobium oxide.22d,35 For the Si-Nb/xaq series samples, the specific BET surface areas, the pore volumes, and the pore size maximum regularly decrease with Nb addition (Figure 4). This behavior is in agreement with the decrease of the Si-O-Nb linkages with the niobia increase into the silica matrix. A different trend emerged for the Si-Nb/ xorg series samples. For niobia concentration up to 9 mol %, the surface area increased and then abruptly decreased for higher niobia content (Figure 4). Thus, it can be inferred that silica can accommodate niobium at high dispersion, promoting the Si-O-Nb linkages, up to a certain Nb concentration; when this threshold is overcome, niobium agglomeration occurs with dramatic change of the textural properties of the Si-Nb samples. The porosity (pore volumes and pore size maximum) of the Si-Nb/xorg samples regularly decreased with the niobia increase as already observed for the Si-Nb/xaq samples. The surface area and porosity values of the Si-Nb/xorg samples were higher than those of Si-Nb/xaq. Scanning electron microscopy attests the images of the calcined materials Si-Nb/15aq, Si-Nb/15org, and Si-Nb/5wi (Figure 5). The morphology is similar for the different types of materials containing low or high niobia, independent of the preparation method; irregular polyhedral shaped particles of various sizes could be observed. The regions of the three samples observed by SEM were analyzed by EDS to study the Nb distribution on the surfaces. The Nb atomic maps for the two representative samples at similar nominal Nb concentration prepared by coprecipitation and sol-gel (Figure 5B1-B3) revealed good homogeneous distribution of Nb on silica, the Nb phase appearing more dense on Si-Nb/15org than on Si-Nb/ 15aq. Well-visible uncovered silica regions could be observed on Si-Nb/20wi with islands of aggregated niobia, heterogeneously spread onto the silica matrix. This proves the expected inability of the impregnation method to homogeneously disperse oxidic phases on oxide supports. The qualitative and quantitative study on the niobia presence at the surfaces is of great importance because of the catalytic function of the materials. XPS results for all the Si-Nb samples are reported in Table 2 and Figures 6-8. The values of Nb 3d binding energy and the peak shapes for all the samples are characteristic for niobium with a formal charge of +5 in an oxide surrounding, similar to Nb2O5 oxide.25,36 From quantitative XPS results, the surface Nb2O5 molar concentrations could be determined from the surface Nb/Si ratios (Table 2), directly


Carniti et al.

Figure 8. X-ray photoelectron spectra of the Nb 3d5/2 and Nb 3d3/2 regions for the three Si-Nb series samples.

Figure 9. UV-vis DRS spectra of one selected Si-Nb sample for each series and bulk niobia.

Figure 10. Edge energies as a function of niobia molar concentration for all the Si-Nb samples.

obtained from the Nb 3d and Si 2p bands, respectively, and compared with the relevant molar Nb2O5 concentrations from the composition data (Table 1). As expected, the niobia presence on the surfaces was always lower than the total amount introduced into the samples, in particular when the highly concentrated niobia samples were concerned. The direct comparison of the surface versus bulk Nb/Si ratios for all the samples is shown in Figure 6; regularly increasing linear trends were observed for the Si-Nb/xaq and Si-Nb/xorg series samples, each

point of the latter series laying over that of the first series. This confirms the high ability of the sol-gel method to accommodate host phases, as niobia, at the surface of the guest silica matrix. On the other hand, the Si-Nb samples prepared by impregnation did not show a regular increasing trend of their surface Nb/Si ratios with the Nb enrichment in the composition. In this case, the impregnated Nb phase could penetrate into the large porosities of the silica support where the surface analytical technique could not detect it.


Figure 11. TPR experiments: H2 consumption vs analysis time (analysis temperature is indicated in the upper axis) of three selected samples for each of the three Si-Nb series (panel A, coprecipitation; panel B, sol-gel; panel C, impregnation).

Figure 12. TPR experiments: signal vs analysis time (analysis temperature is indicated in the upper axis) of one selected sample (at the highest Nb2O5 loading) for each of the three Si-Nb series.

The copresence at the surface of silica and niobia was indicated from the O 1s band that became even more asymmetric as the surface niobia increased. Figure 7 shows the O 1s XPS peak for the samples of the Si-Nb/xorg series with an example of band deconvolution into the two peak components (533 and 531 eV for the O-Si and O-Nb, respectively) for the most niobia-concentrated sample. The O 1s peak of the O-Nb

J. Phys. Chem. C, Vol. 112, No. 36, 2008 14071 component increased with the niobia concentration (Table 2, ninth column), as expected. From the quantitative XPS data (Nb and O-Nb atomic percentages, Table 2), it can be determined that, on average, the oxygen coordination around each Nb center was 6, for the lowest concentrated sample (Si-Nb/5org), and decreased to 2.5 (typical of Nb2O5) for the highest concentrated samples (Si-Nb/45org and Si-Nb/60org). This result points out that the Nb dispersion goes parallel with the Nb dilution into the silica matrix. On this point, it is of interest to observe that the values of BE of the Nb 3d5/2 component are increasing (from 207.3 to 208.2 eV) with decreasing Nb loading (Table 2). In the literature,36 the shift toward higher values than that typical of Nb2O5 species (207.3 eV) has been attributed to atomic dispersion of niobium on the support and to the change in the Nb coordination by formation of Nb-O-Si linkages. Moreover, the Nb dilution produces a progressive Nb 3d peak broadening (Figure 8) and it results in an increase in the values of the full width at half-peak maximum (fwhm) of Nb 3d5/2 and a reduction in the Nb 3d valley between the 5/2 and 3/2 components (Table 2), in agreement with what already observed by Dragone et al.25 All the effects are clearly observed for the Si-Nb/xaq and Si-Nb/xorg samples (∆3d), and accordingly, the BE values for Nb 3d5/2 are higher for the most diluted samples. Electronic properties of the dispersed Nb phase in the Si-Nb samples were investigated by UV-vis DRS spectroscopy. The DRS spectra of representative Si-Nb samples for each series and pure niobia (from CBMM, Brazil) in the UV region are shown in Figure 9. The spectra are dominated by intense bands corresponding to ligand-to-metal charge-transfer transitions (LMCT from O2- to Nb5+), without the possibility to clearly discriminate among the presence of tetrahedral (230 nm) or fiveand six-coordinated Nb species (≈250 nm).37 Moreover, only for the samples prepared by impregnation and for the high Nbloaded samples (Si-Nb/45aq, Si-Nb/60aq, Si-Nb/45org, Si-Nb/ 45org), absorption at 320 nm could be observed, corresponding to niobia in small nanodomains.37 The bands are seen to increase in intensity and to shift to higher wavelength as the niobium content increases. The corresponding edge energies (Eg) reported as a function of the niobia concentration are shown in Figure 10. The absorption edges are seen to decrease from ca. 4.0 to 3.3 eV for the Si-Nb/xaq and Si-Nb/xorg samples as the niobium loading increases from 1 to 25 mol %, and they are comprised between 3.7 and 3.4 eV for the impregnated samples (from 1 to 4.5 Nb mol %). The higher values are slightly lower to that reported by Gao et al.3 for isolated NbO4 species in MCM-41supported niobium, suggesting that these samples (Si-Nb/xaq and Si-Nb/xorg with niobia content up to 9 mol %) are not dominated by polymerized NbOx units and high Nb-dispersion species are formed. On the opposite, the lower Eg values are close to that for bulk niobia. In this case too, the poor Nb dispersion in the impregnated samples and in the highest Nbloaded samples emerges. The reducing properties (TPR) of the samples containing the dispersed Nb(V) phases were studied, even if the quite complete irreducible character of the Nb-O-Nb bonds are well-known.9 Integrated hydrogen uptakes were well-detected only for the highest loaded samples of the Si-Nb/xaq and Si-Nb/xorg series and the three impregnated samples, Si-Nb/xwi. For these samples, the Nb(V) phase was reduced in various steps that regularly increased their intensity as the niobium content did (Figure 11C). The hydrogen consumed accounts for reduction of ca. 50% of Nb(V) to Nb(IV), in any case. On the contrary for the other two series samples, any reduction could not be observed for the lowest Nb-containing samples (Si-Nb/5aq(org)


Carniti et al.

Figure 13. Equilibrium isotherms of PEA adsorption at 17 °C in cyclohexane on the Si-Nb series samples, compared at similar Nb content.

and Si-Nb/15aq(org)), slight reductions were observed for the medium Nb-containing samples (Si-Nb/30aq(org)), and higher reductions were observed for the highest Nb-containing samples, without appreciable increase of hydrogen uptake with the niobium content (Si-Nb/45aq(org) and Si-Nb/60aq(org)) (Figure 11, parts A and B). In this case from the hydrogen uptake, a reduction of Nb(V) to Nb(IV) not higher than 20% could be calculated, in any case. The presence of a Nb threshold for a clearly detected reduction could go parallel with the high dispersion of the NbOx species in the samples silent to TPR and with well-developed niobia nanodomains in the highest Nbconcentrated samples. As concerns the TPR profile shape, illdefined maxima of low intensity at ca. 400, 500, and 800 °C and well-defined maxima at temperatures of ca. 920 °C or higher could be observed for the Si-Nb/xaq and Si-Nb/xorg series samples (Figure 12). Two better defined maxima were observed for the Si-Nb/xwi samples at ca. 500 and 920 °C (Figure 12) that account for the prevalent formation of Nb2O5 nanodomains, in agreement with the TPR profile of bulk niobia.38,39 The surface acid properties are of prominent interest in view of the catalytic functionality of the samples. A change of the acidity of an oxide surface when mixed with other oxide components is a well-known phenomenon, already presented and discussed in the literature. Tanabe and co-workers5,40 ascribe this effect to chargeunbalanced localized bonds formed in the mixed composition, Connell and Dumesic41 consider a balance of the formal charge of the guest cation by surface coordination of lattice oxygen anions,

thus leading to cus species (coordinatively unsaturated cations) acting as Lewis acid sites, and Gervasini and co-workers42 introduce the ISE effect (specific effect of ions) to rationalize the evolution of the acidity of modified oxide formulations by introduction of guest metal ions. In the present case, the almost exclusive Brønsted acidity of silica surface turned toward Lewis acidity when dispersed Nb centers were introduced in its structure. Then, both types of Lewis and Brønsted acid sites are expected to coexist on the Si-Nb surfaces of the samples; the relative proportion of the nature of acid sites depends on the silica to niobia ratio and preparation procedure. A decrease in the abundance of Lewis acid sites with increasing Nb loading and a progressive increase in the abundance of Brønsted acid sites have been reported in the literature for different systems (Nb/TiO2,4,43 Nb/ZrO2,44 and Nb/Al2O345). Increasing the Nb concentration in/on silica, not only the nature but the total amount of acid sites is expected to vary, too. The acidic properties of the Nb-containing surfaces were chemically probed by titration with a strong base probe (PEA) in the apolar cyclohexane solvent. In Figure 13, the equilibrium isotherms of PEA adsorption on the Si-Nb samples are shown, bringing together the isotherms of the samples of the different series with similar Nb concentration. The determination of the number of acid sites on each surface (expressed as mequiv · g-1) was made comparing the amount of PEA adsorbed under very low equilibrium PEA concentration (5 × 10-5 M); the values are listed in Table 1. A progressive decrease of the abundance of the acid sites of the Si-Nb/xaq series samples with Nb

Dispersed NbOx Phases in Silica Matrixes addition could be noticed (about 50% of acid sites were lost comparing Si-Nb/5aq and Si-Nb/60aq), whereas a trend passing through a maximum (in correspondence of Si-Nb/30org, ca. 9 mol % of Nb2O5) characterized the Si-Nb/xorg series samples. The evolution of the amounts of acid sites as a function of the Nb content for the samples of the Si-Nb/xaq and Si-Nb/xorg series was very similar to that already observed in Figure 4. Any clear increasing or decreasing trend of the amount of acid sites with Nb concentration on the Si-Nb/xwi series samples was not found (mean value of sites of 0.34 mequiv · g-1), accounting for the poor Nb dispersion on the silica support. Other important information could be derived from the adsorption isotherms of Figure 13. All the isotherms showed a Langmuirian shape with a more or less pronounced knee at defined equilibrium PEA concentration. The two parameters in the Langmuir equation represent the amount of adsorbed species at the monolayer and the adsorption constant (or Langmuir constant, b). As higher the bond between the surface site and the adsorbed species is, as higher the Langmuir constant is observed. By confining the numerical interpretation of our isotherms to the first part (at very low PEA equilibrium concentration), the b values could be calculated; these values account for the average strength of interaction between the stronger acid sites with the PEA probe. For the two Si-Nb/xaq and Si-Nb/xorg catalyst series, the same trend of the increasing b values with increasing Nb loading was obtained (b values from 50 000 to 140 000 M-1, from the lowest to highest Nbconcentrated sample of the Si-Nb/xaq series, and from 50 000 to 100 000 M-1, from the lowest to highest Nb-concentrated sample of the Si-Nb/xorg). This means that the average acid strength of the surfaces is increasing with the surface enrichment of niobium. 4. Concluding Remarks The present study makes it clear that the control of the local structure of niobium species may be governed by the concentration of the host niobia phase into the guest matrix, with the possibility to modulate the surface and acidic properties of the obtained materials and to influence their catalytic properties. The one-step synthesis of Si-Nb oxides led to materials with improved characteristics compared with conventional materials obtained by niobia deposition on finite silica support. Niobium oxide phases were dispersed in/over silica host structures in wide concentration intervals using different Nb sources and methodologies. The surface and acidic properties of the samples were governed by the local structure of the Nb species. The modulation of the surface properties of the samples could be achieved by varying the Nb concentration. Acknowledgment. This work has been partially supported by the Italian Research Ministry (FIRST project 2007). References and Notes (1) Ziolek, M. Catal. Today 2003, 78, 47. (2) Tanabe, K.; Okazaki, S. Appl. Catal., A 1995, 133, 191. (3) Gao, X.; Wachs, I. E.; Wong, M. S.; Ying, J. Y. J. Catal. 2001, 203, 18. (4) Onfroy, T.; Manoilova, O. V.; Bukallah, S. B.; Hercules, D. M.; Clet, G.; Houlla, M. Appl. Catal., A 2007, 316, 184. (5) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases, their Catalytic Properties; Delmon, B., Yates, J. T., Eds.; Studies in Surface Science and Catalysis, Vol. 51; Kodansha: Tokyo, Elsevier: Amsterdam, 1989. (6) Okuhara, T. Chem. ReV. 2002, 102, 3641. (7) Busca, G. Chem. ReV. 2007, 107, 5366. (8) (a) Jehng, J. M.; Turek, A. M.; Wachs, I. E. Appl. Catal., A 1992, 83, 179. (b) Jehng, J. M.; Wachs, I. E. Catal. Today 1993, 16, 417. (c)

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JP803140X

Dispersed NbOx Catalytic Phases in Silica Matrixes: Influence of

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