Precursor Effect on the Molecular Structure, Reactivity, and Stability of

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Precursor Effect on the Molecular Structure, Reactivity, and Stability of Alumina-Supported Vanadia A. E. Lewandowska,*,† M. A. Banãres,*,† D. F. Khabibulin,‡ and O. B. Lapina‡ Catalytic Spectroscopy Laboratory, Institute of Catalysis and Petroleumchemistry CSIC, E-28049-Madrid Spain, and BoreskoV Institute of Catalysis SB RAS, Prosp. Akad. LaVrentieVa 5, 630090 NoVosibirsk, Russia ReceiVed: June 19, 2009; ReVised Manuscript ReceiVed: October 2, 2009

This work studies the influence of specific vanadium precursors on alumina-supported vanadium oxide catalysts. Vanadyl sulfate, ammonium metavanadate, and vanadyl acetylacetonate were used as precursors. All catalysts have the same surface vanadium coverage (4 V atoms/nm2). The structural features of surface vanadium oxide species were studied by 51V-NMR, UV-vis, and Raman spectroscopy. The vanadium precursor determines the strength of interaction between supported VOx species and alumina. The reducibility of vanadium ions was estimated by conventional TPR/TPO cycles. The structural and electronic changes during reduction/oxidation cycles were studied by in situ Raman and UV-vis spectroscopy (TPR/TPO-Raman, TPR/ TPO-UV-vis). Surface sulfate species reduce concomitantly to surface vanadium oxide species. 1. Introduction Vanadium-based solid materials have been widely studied in catalytic oxidation reactions both in the gas1-5 and in the liquid6-9 phase. Depending on their application, conventional supported vanadium oxide systems10-12 or vanadium oxide supported on the mesoporous matrices3,13,14 are selected. The structure and performance of supported vanadium oxide catalysts depend strongly on the specific support, vanadium loading, preparation method, and precursor. There is a wide range of the preparation methods available in the synthesis of vanadiumcontaining materials; among these, gas techniques, such as moleculardesigneddispersion(MDD),15,16 vapor-phasegrafting,17-19 or chemical vapor deposition (CVD)20-22 have aroused increasing interest in the last years. However, conventional impregnation methods are preferred for industrial catalyst applications. Liquid-phase techniques are most commonly applied to prepare a catalyst; among them, the aqueous impregnation with ammonium metavanadate is a particularly common one.23-27 Addition of oxalic acid into the aqueous NH4VO3 solution improves precursor solubility via the formation of an oxalate, which decomposes easily in air flow. Ammonium and oxalate ions leave no traces on the catalyst surface after calcination at 673 K. The use of an acidic solution to the impregnation mixture, even weak oxalic acid, may lead to a partial dissolution of the support,28 which promotes the formation of undesired phases, such as AlVO4. Nonaqueous alcoholic solutions, for example methanol29-32 or isopropanol,4,33 are used with moisture and air-sensitive vanadium precursors, like vanadium triisopropoxide (VO(O-Pri)3) or vanadyl acetylacetonate.3,13 This method is limited by its moisture sensitivity and need of glovebox equipment for impregnation and drying steps. Vanadyl sulfate is widely used in the synthesis and modification of mesoporous molecular sieves.6-8,14,34 This vanadium compound possesses high solubility and stability in air and moisture. Despite its convenient properties, this type of vana* To whom correspondence should be addressed. E-mail: banares@ icp.csic.es (M.A.B.), [email protected] (A.E.L.). † Institute of Catalysis and Petroleumchemistry CSIC. ‡ Boreskov Institute of Catalysis SB RAS.

dium precursor is not commonly used to impregnate oxide supports, mainly because of the presence of sulfur residues upon calcination. The sulfate ion can form the tridentate SO42- surface species under dehydrated conditions.2,35 The tridentate sulphates present on alumina support transform into protonated bidentate surface species in the presence of humidity.35-37 This process leads to the formation of S-O-H groups, which increase the Brønsted acidity of the support2,36 thus affecting the catalytic properties.38 This work investigates the influence of the vanadium oxide precursor on the structure of surface vanadium oxide and the strength of its interaction with an alumina support. Combined Raman and UV-vis in situ spectroscopic studies during reduction/reoxidation cycles provide an insight on the relevance of vanadium oxide precursors on the structural-reactive features of supported vanadium oxides. 2. Experimental Section Preparation. The alumina support was γ-Al2O3 (kindly supplied by Sasol Puralox SCCa-5/200, SBET ) 193 m2/g). The vanadium oxide was dispersed on alumina using the wet impregnation method39 with different precursors. In method 1, an aqueous solution of VOSO4 (Aldrich, 99.99%) was stirred at 323 K for 50 min. Then, γ-Al2O3 was added. In method 2, an aqueous solution of NH4VO3 (Sigma, 99.99%) was mixed with oxalic acid (Panreac, 99.5%) to facilitate its dissolution. In method 3, an ethanolic solution of vanadyl acetylacetonate (VO(C5H7O2)2, Aldrich, 99.99%) was used as vanadium precursor. After impregnation, the aqueous solution was evaporated in a rotatory evaporator at 338 K (method 1 and 2) or at 313 K for the ethanolic solution (method 3). In all methods, the resulting solid was dried at 388 K for 20 h and then calcined at 673 K for 4 h in air. The heating rate was 5 K min-1. A summary of preparation methods is presented in Table 1. The amount of vanadium was determined so that total coverage of vanadium would be 4 atoms/nm2 of alumina support vanadia monolayer coverage on alumina is reached at ca. 7 vanadium atoms/nm2.40 The general nomenclature is 4 VX, where X indicates the specific vanadium precursor: S”for VOSO4, M for NH4VO3 and A for VO(acac)2, respectively.

10.1021/jp9057884 CCC: $40.75  2009 American Chemical Society Published on Web 11/09/2009

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TABLE 1: Summary of Preparation Method preparation method

series

precursor

dissolvent

1 2

VS VM

3

VA

VOSO4 (Aldrich, 99.99%) NH4VO3 (Sigma, 99.99%) Oxalic acid (Panreac, 99.5%) VO(C5H7O2)2, (Aldrich, 99.99%)

rotatory evaporator

drying

calcination

water water

383K 383K

388K/20h 388K/20h

673K/4h 673K/4h

ethanol

313K

388K/20h

673K/4h

TABLE 2: List of the Studied Catalysts V Content catalyst 4 VS 4 VM 4 VA

Bulk Atomic Ratio

BET (m2/g)

nominal coverage (V atoms/nm2)

(V atoms/nm2)

V (wt. %)

V/Al

S/Al

175 178 185

4 4 4

3.9 4.4 3.0

10.4 11.8 7.9

0.10 0.12 0.08

0.03

TXRF. The vanadium oxide loading was determined by X-ray fluorescence recorded on a TXRF 8030c FEI spectrometer using excitation W-LB (thin) and a tube current of 47 mA. 51 V MAS NMR. Solid-state NMR experiments performed using Bruker MSL-400 (9.4 T) spectrometer at resonance frequencies 105.20 MHz for 51V. Bruker 4.0 and 2.5 mm MAS probes were used for acquisition of static and 15-35 kHz MAS spectra. The single pulse sequence with rf - pulse duration of 1 µs (less than π/10) and recycling time from 0.1 to 5 s was used. The chemical shift values were referenced to external reference VOCl3. 51V MAS NMR spectra were performed for hydrated samples. Simulations of 51V static and MAS NMR spectra were performed taking into account second-order quadruple correction using general purposes simulation program NMR1 described earlier.41 The relative content of different vanadium species were determined from integral intensities of the spinning sidebands of the central transition. TPR/TPO. Temperature-programmed reduction and temperature-programmed oxidation (TPR/TPO) experiments were performed in a fixed-bed quartz reactor fitted to a Micromeritics TPD/TPR 2900 analyzer. A thermal conductivity detector measured hydrogen and oxygen consumption. The reduction of the samples was carried out using H2/Ar (10 vol %) as reducing agent (flow rate ) 40 cm3min-1). Thirty milligrams of the sample was placed in the reactor and was activated in a flow of synthetic air at 673 K at a rate of 5 K min-1. Subsequently, the sample was cooled to room temperature in synthetic air, and then it was heated at a rate of 10 K min-1 to 1023 K in a flow of reducing mixture. After the reduction experiment (H2-TPR) the catalyst was cooled to room temperature in He. O2/He mixture was used as an oxidizing agent (flow rate ) 75 cm3min-1) in the temperature-programmed oxidation (TPO). The oxidation of the materials was performed heating linearly from 313 to 1023 K at a rate of 10 K min-1. The reoxidized sample was purged in helium at room temperature, and a second TPR process was performed on it (following the same procedure). The maximum reduction temperature was 1223 K. The water produced during the reduction was condensed in a cold trap immersed in a mixture of isopropanol and liquid nitrogen. Bulk V2O5 (Fluka, 99%) and gamma-alumina TPR profiles were run as references to the TPR profiles. In Situ UV-vis. UV-vis spectra were run with AVANTES equipment connected to a fiber optic reflectance probe. The AvaSpec-2048TEC spectrometer was equipped with a CCD detector (refrigerated to 243 K) and a source of UV and visible light (AvaLight-DHS). UV-vis spectra were recorded with integration time of 6 ms using 50 accumulations every 120 s. A Halon white reference standard was used as a white reference. 300 mg of the sample was placed in home-developed operando

quartz reactor (custom-made by HELLMA),42,43 fitted to an adhoc oven (custom-made by PID Eng. & Tech.). This reactor was positioned by XYZ-goniometer positioning stages (Melles Griot). The position of the fiber optic was perpendicular to the window of the quartz operando reactor. Prior to the reduction experiment, the catalysts were dehydrated and calcined clean in 20 vol % O2/He (Air Liquid) at 673 K at a rate of 10 K min-1. The sample was then cooled to 323 K in O2/He mixture, and purged in helium during 30 min. For the TPR, the feed was switched to 10 vol % H2/Ar (Air Liquid) and the temperature was ramped from 323 to 823 K at 10 K · min-1 and hold 30 min at 823 K. After reduction the sample was cooled to 423 K in a flow of reducing mixture and then purged in He at 323 K. To run the TPO on the reduced sample, the feed was switched to 20 vol % O2/He (Air Liquid) and ramped from 323 to 823 K at 10 K · min-1 and held 30 min at 823 K; then it was cooled to 323 K. The TPR/TPO treatments were recorded with a Hiden HPR20 mass spectrometer. V2O5 (Fluka, 99%) was used as a bulk oxide reference for UV-vis absorption and estimation of edge energy (Eg) value. In Situ Raman. Raman spectra were run with a singlemonochromator Renishaw System-1000 microscope Raman equipped with a cooled CCD detector (200 K) and holographic Edge filter. The powder samples were excited with the 514 nm Ar+ line; spectral resolution was ca. 3 cm-1 and spectrum acquisition consisted of 20 accumulations of 10s. The spectra were obtained under reductive and oxidative conditions in an in situ hot stage (Linkam TS-1500). Prior to the reduction experiment, the catalysts were dehydrated in synthetic air flow at 673 K at a rate of 5 K · min-1. After the sample was cooled to room temperature in synthetic air, and the reduction of the materials was carried out from 303 to 1023 K in a flow of 1 vol % H2 in Ar (SEO-L’Air Liquid) at 10 K min-1. After the reduction, the sample was cooled to room temperature under the reducing mixture. It was then heated at 10 K min-1 to 1023 K under a flow of synthetic air. The oxidized catalyst was cooled to room temperature in an oxidizing mixture. 3. Results Table 2 summarizes some characterization data of aluminasupported vanadia catalysts. BET surface area of alumina slightly decreases after impregnation for all samples (Table 2). The vanadia surface density determined by X-ray fluorescence of 4 VS (3.9 atoms/nm2) and 4 VM (4.4 atoms/nm2) are close to the nominal value. The catalyst prepared via vanadyl acetylacetonate precursor, 4 VA, contains a rather lower vanadium loading. Sulfur traces are apparent for the catalyst prepared via vanadyl sulfate.

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Figure 1. 51V MAS NMR spectra of 4 VOx/Al2O3 samples prepared from: vanadyl sulfate (4 VS), ammonium metavanadate (4 VM), and vanadyl acetylacetonate (4 VA).

Figure 2. The percentage values of different vanadium surface species; tetrahedral VO4 weakly bonded (red), VO4 strongly bonded (blue), V10O286- (green), and V2O5 (orange). 51

V MAS NMR analyzes structural features of the surface vanadium oxide species (Figure 1). NMR signals at -420 and -500 ppm indicate the decavanadate polyanions (V10O286-) in all samples. The presence of this species is due to the hydrated conditions used during 51V MAS NMR analyses. NMR signals at -570 and -620 ppm evidence the presence of VO4 species in all catalysts. The chemical shift δ (51V) at -570 ppm corresponds to VO4 species weakly bonded to the surface via one or two bonds.28,44,45 The 51V MAS NMR signal at -620 ppm is assigned to VO4 tetrahedral species strongly bonded to the alumina surface via two or three bonds. 4 VA catalyst also exhibits some tiny amounts of crystalline V2O5 phase (-610 ppm). The structure of surface vanadium oxide species observed after calcination is the same for all catalysts, in line with previous works.28,44,45 The precursor affects the extent of vanadia interaction with alumina support. Figure 2 shows the distribution of vanadium species (tetrahedral VO4 weakly and strongly bonded to support, V10O286- and V2O5) present on alumina surface based on the 51V MAS NMR measurement. The population of decavanadate polyanions in the hydrated samples increases as: 4 VA < 4 VM < 4 VS (Figure 2, green bar). The precursor affects the population distribution of weakly and

Lewandowska et al. strongly bonded tetrahedral VO4 species. Their populations are balanced for 4 VS material. Weakly bonded tetrahedral VO4 species dominate in 4 VM catalyst and are slightly more abundant than decavanadate polyanions. 4 VA catalyst prepared under nonaqueous solution shows the highest population of strongly bonded tetrahedral VO4 species. Apparently, the population of decavanadate species can be an indicator of weaker interaction between VOx species and Al2O3. Figure 3 illustrates UV-vis spectra of fresh dehydrated, reduced and dehydrated reoxidized vanadium catalysts. Table 3 summarizes the edge energy (Eg) values. Reference bulk V2O5 oxide absorbs at 245, 335, and 480 nm, with an edge energy of 2.18 eV. Regardless to experimental conditions, all spectra show UV-vis bands near 256 and 334 nm attributed to ligand-tometal charge transfer (LMCT) transitions of V5+.4,46,47 These bands originate from VO4 species in tetrahedral coordination.47,48 4 VM material exhibits an additional band near 380 nm (a and c in part B of Figure 3), which may correspond to an LMCT transition of V5+ in octahedral coordination (VO6).47,48 Reduced catalysts exhibit still bands near 256 and 334 nm attributed to an LMCT transition of V5+ (b in part A, b in part B, and b in part C of Figure 3). The UV-vis band at 423 nm may correspond to V2O3-like species (b in part A of Figure 3). Reduced 4 VS shows shoulders at 672 and 757 nm, which would suggest the presence of nonstoichiometric V7O13- and V6O13like species. The edge energy values of hydrated samples increase as 4 VS (2.29 eV) < 4 VM (2.39 eV) < 4 VA (2.71 eV); this evolution is probably related to an increase of VO6 units in comparison with VO4 units. The edge energy of polymerized VO4 units possesses a higher value than polymerized VO6 units.47 4 VS edge energy changes significantly upon dehydration (from 2.29 to 2.48 eV); this evidence displays drastic changes in the coordination sphere of the VOx species. Indeed, dehydrated 4 VS does not exhibit UV-vis bands at 380 nm. 4 VM edge energy is less sensitive to hydration/ dehydration. The Eg values of 4 VA hardly change upon hydration/dehydration, which suggests a stronger interaction of VOx species with the alumina surface for this sample.46 The VOx species of 4 VA mostly show tetrahedral coordination independently on the conditions. The reduction and subsequent reoxidation process significantly increases the polymerization degree of vanadium oxide species in 4 VA catalyst. Its edge energy drops from 2.71 to 2.57 eV (Table 3). The Eg value of 4 VS remains essentially constant (2.49 eV vs 2.48 eV) upon reduction and reoxidation cycle. It suggests negligible changes in the vanadium oxide species upon reduction/reoxidation. Figure 4 shows Raman spectra of fresh hydrated, fresh dehydrated, and dehydrated reoxidized vanadium catalysts, and the spectrum of nano-particles of bulk V2O5 formed during catalyst preparation (a’ in part C of Figure 4). Fresh hydrated samples exhibit essentially decavanadate polyanions (V10O286-) bands (a in part A, a in part B, and a in part C of Figure 4). Raman bands corresponding to decavanadate ion are 990, 945, 840, 700, 604, 592, 365, 323, 256, 191, 188 cm-1.49-51 Hydrated catalysts also exhibit Raman bands near 980, 830-810, 580-550, 500, 300-290 cm-1 aside from decavanadate ions. They probably derive from polymeric tetrahedral vanadates.52 Fresh dehydrated catalysts exhibit Raman bands near 1023, 1000 (assigned to the VdO bond in polymerized and isolated surface vanadia,29,53,54 respectively), and near 900, 795, 770, 615, 540, 485 (associated to V-O-V modes29,30,39,54-56), and near 350 and 250 cm-1 (bending modes of V-O and V-O-V bonds, b in part A, b in part B, and b in part C of Figure 4).



Figure 3. UV-vis spectra of 4 VS (A), 4 VM (B), and 4 VA (C) catalysts recorded at 323 K under: fresh dehydrated (a, green), reduced (b, red), and dehydrated reoxidized after TPR (c, blue) conditions.

TABLE 3: Summarized Edge Energy Values Eg, eV catalyst 4 VS 4 VM 4 VA

hydrated

dehydrated

after TPR-TPO

2.29 2.39 2.71

2.48 2.45 2.71

2.49 2.41 2.57

Reduction/reoxidation cycles have significant effects on the supported vanadia phase (c in part A, c in part B, and c in part C of Figure 4); the Raman bands near 1023 and 770 cm-1 blue shifts and become more intense. In addition, Raman bands near 875, 795 and especially 540 cm-1 change their relative intensities. A new strong band becomes apparent for all samples at ca. 895 cm-1. 4 VM and 4 VA do not exhibit the stretching mode of the terminal VdO bond of isolated VOx species near 1000 cm-1. Temperature-programmed reduction profiles illustrate the influence of vanadium precursor on reducibility of alumina supported vanadium catalysts. Figure 5 shows the TPR profiles of alumina-supported vanadia oxide (solid lines). The TPR profiles of 2S/Al2O3 material and bulk V2O5 (dash line) are added as a references. A reduction profile of alumina supported sulfur exhibits a reduction peak at 924 K with a shoulder at 906 K (part A of Figure 5). This maximum is related to reduction of SOx species to H2S.58 The V2O5 reduction profile is dominated by three peaks at 945, 979, and 1064 K (part B of Figure 5). The observed multiple reduction maxima correspond to a progressive reduction through intermediate compounds: V2O5 f V6O13 f V2O4 f V2O3.18,23-25,59,60 It should be noted that the reduction profiles for bulk V2O5 reported in the literature differ in the temperature and the number of the reduction peaks.23,25 Increasingly important diffusion limitations in bulk V2O5 retard the reduction temperature in comparison with alumina-supported vanadia.23 Bulk vanadia reduction is strongly affected by the preparation method, crystal size, vanadia loading and level of impurities. Figure 5 illustrates reduction profiles of supported vanadium catalysts. The first (I) TPR run corresponds to the fresh calcined sample (Figure 5, green line). The catalyst is purged and reoxidized after the TPR; the reoxidized sample is reduced in a second (II) TPR run (Figure 5, pink line). Table 4 summarizes

the H2-TPR data. The H2-TPR profile of 4 VS(I) exhibits one sharp maximum at 747 K (part A of Figure 5). The sharp and rather narrow shape of the reduction maximum (fwhm 35 K, Table 4) could indicate uniform distribution of surface vanadium oxide species, which should be narrower than that in 4 VM and 4 VA samples. This is consistent with vanadium species’ population distributions determined by 51V NMR (Figure 2). For the second TPR, the onset reduction temperature shifts to a lower value at 450 K (Table 4). The TPR profile of 4 VS(II) reveals an asymmetric peak near 760 K and a tailing to higher temperatures (ca. 882 and 1100 K). The reduction maximum shape broadening (fwhm 101 K) and Tmax shift to a higher temperature suggest changes in the surface vanadium oxide species or in the strength of the vanadium oxide-alumina interaction. Reduction of fresh 4 VM(I) exhibits a broad asymmetric peak near 743 K (part B of Figure 5). Its broad reduction peak (fwhm 115 K) suggests the coexistence of different surface vanadium oxide species.11,23-25,53 The second TPR of 4 VM (4 VM(II)) exhibits a similar broad asymmetric peak near 746 K (fwhm 99 K). The similar reduction profiles of 4 VM during first and second reduction suggest that the structure of supported vanadium oxide species is essentially the same. The TPR profile of fresh 4 VA (4 VA(I)) exhibits a rather sharp asymmetric peak at 744 K (fwhm 68 K) (part C of Figure 5). Similarly to 4 VS, the reduction profile upon reoxidation (fwhm 120 K) shifts to higher temperature (Tmax, 754 K), which suggest a broader distribution of structures. The formal average oxidation states of vanadium upon reduction were estimated as described elsewhere,10,18,61 that is, we assume that the of V5+ to V4+ corresponds to a consumption of 0.5 H2/V. H2 consumption values, represented as H/V atomic ratio, close to 2 indicates that the V5+ species reduce to V3+.4,11 The formal vanadium average oxidation states (AOS) are listed in Table 5. The highest hydrogen consumption is observed for the alumina supported vanadyl sulfate (4 VS), which reaches an AOS below +3; 4 VA and 4 VM reach formal AOS near +4; the latter is consistent with literature.3,10,22 Similar formal AOS values in both TPR experiments for all samples suggest a reversible regeneration upon reduction. The sharp reduction profile of fresh 4 VS and the surprisingly low AOS value deserve additional insight. The first TPR profile

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Figure 4. Raman spectra of fresh hydrated (a, blue; a’, light blue - 10x minimized, spectrum recorded at other point), fresh dehydrated (b, red) and dehydrated reoxidized (c, green) alumina-supported oxides at room temperature: (A) 4 VS, (B) 4 VM, (C) 4 VA.

Figure 5. H2-TPR profiles of A - 4 VS catalyst (VOSO4), B - 4 VM material (NH4VO3), and C - 4 VA catalyst (VO(acac)2). The profiles represent (I) the first reduction and (II) the second TPR. The profile mark by dash line on A described the reduction of 2S/Al2O3 and on B reduction of V2O5.

TABLE 4: H2-TPR Results for the Catalysts

4 4 4 4 4 4 4

VS(I) VS(I)a VS(II) VM(I) VM(II) VA(I) VA(II)

Total H2 consumption (µmol/gcat)

H2/V (mol/mol)

AOS of V

H/V (atomic ratio)

To,red [K]

Tmax [K]

fwhm [K]

1469 1469 1092 674 765 632 703

1.18 0.93 0.88 0.48 0.54 0.67 0.74

+2.8 +3.1 +3.2 +4.0 +3.9 +3.7 +3.5

2.37 1.85 1.76 0.96 1.09 1.34 1.49

515 515 450 430 490 455 450

747 747 762 743 746 744 754

35 35 101 115 99 68 120

Calculation based on total amount of (V + S) atoms/nm2l (I) The first reduction cycle, (II) the second reduction cycle, and AOS - formal average oxidation state of V after reduction. a

of 4 VS is particularly sharp and it may be indicative of a very narrow distribution of vanadium oxide species, this may be due to some additional feature. TXRF analyses confirm the presence of sulfur species in 4 VS catalyst (vanadyl sulfate precursor). SOx species reduce to H2S above 900 K (part A of Figure 5, dashed line). However, its reduction peak is not evident for 4 VS (part A of Figure 5, solid lines). On line mass spectrometry during TPR was used to further understand this feature (Figure

6). The mass fragmentation at m/z ) 32 is characteristic of H2S and O2, which is the main fragment for both species.62 The m/z ) 32 fragment is not observed during 4 VM and 4 VA TPR, but it is during that of 4 VS, which also exhibits a significantly higher hydrogen consumption (Table 4). This is consistent with a concomitant reduction of both surface sulfate and surface vanadium oxide species. It accounts for the significantly lower AOS value estimated for vanadium during the first TPR run.



Figure 6. H2-TPR-MS results of A - 4 VS catalyst (VOSO4), B - 4 VM material (NH4VO3), and C - 4 VA catalyst (VO(acac)2).

Figure 7. UV-vis spectra of 4 VM catalyst during temperature-programmed reduction (A) and reoxidation (B).

AOS values assume that only vanadium species reduce. Sulfate species reduce in the same temperature range as supported vanadia, and thus it would not be evident to H2-TPR profiles. Figure 7 illustrates the TPR/TPO-UV-vis experiments of 4 VM catalyst. The changes observed during TPR/TPO cycle are the same for all catalysts (Figures S1 and S2 of the Supporting Information). The LMCT bands’ intensities gradually decrease with increasing reduction temperature. The absorption band near 330 nm remains after reduction and is assigned to the LMCT transition of V5+ in tetrahedral coordination due to incomplete reduction. VO6 units reduce easier than VO4 units for 4 VM material. The absorption band at ∼380 nm, related to the VO6 species, significantly decreases during reduction (Figure 7). The V3+/V4+ cations d-d electronic transitions are located above 335 nm.4 It is difficult to distinguish these bands because of their broadness and weakness. However, an increasing of absorption background above 500 nm suggests the reduction of V5+ and formation V3+/V4+ cations (part A of Figure 7). The onset temperature of this phenomenon depends on the catalysts. These changes are reversible upon reoxidation (part B of Figure 7). Temperature-programmed Raman analyses (TP-Raman) study the evolution of vanadium oxide species during reduction and reoxidation. Figures 8-10 show the structural changes of alumina supported vanadia catalysts during redox cycle. The V-O-V bridging Raman bands decrease quicker than those

of terminal VdO bonds during catalyst reduction (Figure 8-10). Thus, polymeric species would reduce faster than surface isolated ones. Raman band at ∼1013 cm-1 assigned to the stretching mode of terminal VdO bond disappears above 923 K for 4 VS and 4 VA samples (part A of Figure 8 and part A of Figure 10) and at 823 K for 4 VM (part A of Figure 9). TPR-Raman was run up to 1023 K (spectra not shown due to blackbody radiation). All catalysts exhibit a Raman band near 910 cm-1 after reduction (parts B Figures 8-10). 4 VS and 4 VM also exhibit weak features near 990 and 1000 cm-1, respectively. Reduced 4 VA exhibits significant fluorescence, which decreases during reoxidation treatment (part B of Figure 10). Reduced alumina-supported vanadium oxide catalysts exhibit similar trends during reoxidation. Polymeric vanadium surface oxide species are restored quicker than isolated tetrahedral ones. Two Raman bands at 1020 and 875 cm-1 and a shoulder near 780 cm-1 become apparent at 1023 K during reoxidation conditions for all materials (parts B of Figures 8-10). 4. Discussion About Structure of Hydrated Catalysts. The calcination process removes most of the organic and inorganic components of the supported vanadium oxide precursor, except surface sulfate species. The vanadium precursor does not determine the

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Figure 8. Selected Raman spectra of the 4 VS catalyst during the TPR-Raman of fresh sample (A) and during the TPO-Raman of the reduced sample (B).

Figure 9. TPR/TPO selected Raman spectra of the 4 VM material during (A) reduction of fresh sample and (B) oxidation of the reduced sample.

structures of calcined vanadium oxide species28,44,45,52 but affects their relative populations because it determines different degrees of interaction with the support. These catalysts exhibit different sensitivity to hydration. 4 VS material prepared from VOSO4 contains both vanadium oxide species and sulfate species. The amount of sulfate species is much lower than surface VOx species (Table 2). The sulfate species adsorbed on the surface do not coordinate with surface VOx species because bulk VOSO4 phase is not formed. Raman spectra confirm the absence of crystalline VOSO4 phase (Figure 4). Interaction between surface vanadium oxide and surface sulfate species cannot be excluded. It is reasonable to assume that surface sulfate species would titrate first the most basic hydroxyls, whereas surface vanadate species titrate both basic and neutral support hydroxyls. Therefore, surface vanadia and surface sulfate species compete for surface hydroxyl sites. Surface sulphates increase Brønsted acidity.2,36 The acidic impurities decrease the aqueous pH at PZC of alumina,52 increasing the formation of octahedral surface vanadium oxide species (V10O286-) under humid conditions. The

weakly and strongly bonded tetrahedral VO4 species populations are near equal in 4 VS (Figure 2). Hydrated 4 VA catalyst exhibits a high level of strongly and weakly bonded tetrahedral VO4 species. It seems to result from dominant role of the pH of precursor solution and its composition. The presence of decavanadate polyanions increases as 4 VA < 4 VM < 4 VS and the edge energy values decreases in the same order 4 VA (2.71 eV) < 4 VM (2.39 eV) < 4 VS (2.29 eV). The differences between hydrated 4 VS, 4 VM, and 4 VA samples detected by 51 V NMR (Figures 1 and 2) are consistent with edge energy values (Table 3). The VO6 species dominate in 4 VS and 4 VM hydrated samples. 4 VA mostly consists of VO4 species in contrast with 4 VS and 4 VM. About Structures of Dehydrated Species. The differences in the relative amounts of vanadium oxide species are less evident for dehydrated samples than for hydrated ones, and the pH value of the PZC of alumina does not control the surface VOx species under dehydrated conditions. The edge energy values decreases as 4 VA (2.71 eV) < 4 VS (2.48 eV) < 4 VM



Figure 10. Selected Raman spectra of the 4 VAl catalyst during the TPR-Raman of fresh sample (A) and during the TPO-Raman of the reduced sample (B).

(2.45 eV) (Table 3). SOx species appears to affect the facility of VO6/VO4 species formation during hydration of 4 VS catalyst because the edge energy value of hydrated (2.29 eV)/dehydrated (2.48 eV) samples differs significantly. The relative amounts of surface vanadium oxide species at 4 VA seem to be insensitive on hydrated/dehydrated conditions. It could result from a lower vanadium loading (Table 2). The surface VOx species is a function of surface VOx density, specific support, and impurities (e.g., sulfate species). According to the Raman measurements, isolated VOx species exist at low vanadium oxide loadings ( 4 VA > 4 VM (Table 3). Polymeric V-O-V species reduce first, as confirmed by in situ Raman studies during reduction. Additionally, VO6 species in octahedral coordination reduced easier than VO4 species as confirmed by in situ UV-vis studies. Ruitenbeek et. al31 observe by XANES that V3+ ions are not stable at the surface of alumina support and they migrate to Al3+ octahedral positions after reduction. Their experiment shows the formation of 4-fold coordinated (VO4) to 6-fold coordinated (VO6) species during reduction.31 Magg et. al67 actually described the partial incorporation of vanadia (V3+) particles into the surface alumina film in a model catalyst. These experiments underline the strong interaction between vanadia and alumina upon reduction. Our Raman data show that reduced V/Al2O3 exhibit bands at 900 and 1000 cm-1. DFT calculation of the VOx/γ-Al2O3 cluster model predicts V-O-Al vibrational frequencies in the 925-955 cm-1 range,66 and Raman bands near 900-950 cm-1 have been assigned to V-O-Al bonds in supported model catalyst on alumina thin film66-68 and in model VOx/δ-Al2O3catalysts.54,66 Thus, the Raman band near 900 cm-1 probably corresponds to V3+-O-Al3+ or V4+-O-Al3+ bonds of surface-reduced vanadia species. 5. Conclusions The different precursors for alumina-supported vanadia do not determine the nature of surface vanadium oxide species but do determine the degree of interaction with alumina, which appears to be weaker for 4 VS. In this sample, surface sulfate species coexist with surface vanadia. During reduction, both surface vanadia and surface sulfate species reduce simulta-

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neously, resulting in a single reduction peak, sharper than typical reduction profiles for alumina-supported vanadia. As a precursor, vanadyl sulfate is interesting because the catalyst exhibits high reactivity and stability; sulfate species can be removed from the alumina surface by reduction treatment. Reduction/reoxidation cycles affect the strength of interaction of vanadia with alumina. Upon reduction, surface vanadia species interact strongly with alumina, and the Raman mode near 900-1000 cm-1 suggests the formation of strong V-O-Al bonds. Vanadyl sulfate allows the preparation of materials with higher dispersion of vanadium oxide species than the commonly used ammonium metavanadate. Acknowledgment. The NATO ESP.NR. NRCLG 981857 grant support, CTQ2008-02461/PPQ Spanish Ministerio de Educacioń y Ciencia project and the RFBR grant 07-03-00695a are acknowledged. A.E.L. acknowledges the Spanish Ministerio de Educacioń y Ciencia for a “Juan de la Cierva” postdoctoral position. SASOL, Inorganic Specialty Chemicals, is acknowledged for providing alumina support. Supporting Information Available: UV-vis spectra of 4VS and 4VA materials. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Banãres, M. A. Catal. Today 1999, 51, 319. (2) Dunn, J. P.; Stenger Jr, H. G.; Wachs, I. E. Catal. Today 1999, 51, 301. (3) Concepcioń, P.; Navarro, M. T.; Blasco, T.; Lopez Nieto, J. M.; Panzacchi, B.; Rey, F. Catal. Today 2004, 96, 179. (4) Gao, X.; Banãres, M. A.; Wachs, I. E. J. Catal. 1999, 188, 325. (5) Banãres, M. A.; Cardoso, J. H.; Agullo´-Rueda, F.; Correa-Bueno, J. M.; Fierro, J. L. G. Catal. Lett. 2000, 64, 191. (6) Ziolek, M.; Lewandowska, A.; Renn, M.; Nowak, I. Stud. Surf. Sci. Catal. 2004, 154, 2610. (7) Ziolek, M. Lewandowska, A. Kilos, B. Nowak, I. Book of Extended Abstracts - 4th World Congress on Oxidation Catalysis, Vol. II(2001) 79 (8) Mal, N. K.; Kumar, P.; Fujiwara, M.; Kuraoka, K. Stud. Surf. Sci. Catal. 2002, 142B, 1307. (9) Trukhan, N. N.; Derevyankin, A. Y.; Shmakov, A. N.; Paukshtis, E. A.; Kholdeeva, O. A.; Romannikov, V. N. Microporous Mesoporous Mater. 2001, 45, 603. (10) Harlin, M. E.; Niemi, V. M.; Krause, A. O. I. J. Catal. 2000, 195, 67. (11) Routary, K.; Reddy, K. R. S. K.; Deo, G. Appl. Catal. A 2004, 265, 103. (12) Arena, F.; Frusteri, F.; Parmaliana, A. Appl. Catal. A 1999, 176, 189. (13) Solsona, B.; Blasco, T.; Lopez Nieto, J. M.; Penã, M. L.; Rey, F.; Vidal-Moya, A. J. Catal. 2001, 203, 443. (14) Santamarıá-Gonza´lez, J.; Luque-Zambrana, J.; Me´rida-Robles, J.; Maireles-Torres, P.; Rodrı´guez-Castelloń, E.; Jimeńez-Lo´pez, A. Catal. Lett. 2000, 68, 67. (15) Baltes, M.; Collart, O.; van Der Voort, P.; Vansant, E. F. Langmuir 1999, 15, 5841. (16) Baltes, M.; van Der Voort, P.; Collart, O.; Vansant, E. F. J. Porous Mater. 1998, 5, 317. (17) Haber, J.; Kozlowska, A.; Kozłowski, R. J. Catal. 1986, 102, 52. (18) Koranne, M. M.; Goodwin, J. G.; Marcelin, G. J. Catal. 1994, 148, 369. (19) Bond, G.; Tahir, S. Appl. Catal., A 1991, 71, 1. (20) Inumaru, K.; Misono, M.; Okuhara, T. Appl. Catal., A 1997, 149, 133. (21) Inumaru, K.; Okuhara, T.; Misono, M. J. Phys. Chem. 1991, 95, 4826. (22) Kera¨nen, J.; Auroux, A.; Ek, S.; Niinisto¨, L. Appl. Catal., A 2002, 228, 213. (23) Reddy, E. P.; Varma, R. S. J. Catal. 2004, 221, 93. (24) Ferreira, R. S. G.; Oliveira, P. G. P.; Noronha, F. B. Appl. Catal., B 2001, 29, 275. (25) Shiju, N. R.; Anilkumar, M.; Mirajkar, S. P.; Gopinath, C. S.; Rao, B. S.; Satyanarayana, C. V. J. Catal. 2005, 230, 484.

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