Precursor Nuclearity Effects in Supported Vanadium Oxides Prepared

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Precursor Nuclearity Effects in Supported Vanadium Oxides Prepared by Organometallic Grafting Staci L. Wegener,† Hacksung Kim,†,‡ Tobin J. Marks,*,† and Peter C. Stair*,†,‡ †

Department of Chemistry and Center for Catalysis and Surface Science, Northwestern University, Evanston, Illinois 60208, United States, and ‡Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

ABSTRACT Despite widespread importance in catalysis, the active and selective sites of supported vanadium oxide (VOx) catalysts are not well understood. Such catalysts are of great current interest because of their industrial significance and potential for selective oxidation processes.1-4 However, the fact that the nature of the active and selective sites is ambiguous hinders molecular level understanding of catalytic reactions and the development of new catalysts. Furthermore, complete structural elucidation requires isolation and characterization of specific vanadium oxide surface species, the preparation of which presents a significant synthetic challenge. In this study, we utilize the structural uniformity inherent in organometallic precursors for the preparation of supported vanadium oxide catalysts. The resulting catalysts are characterized by UV-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray absorption spectroscopy (XAS), UVRaman spectroscopy, and H2-temperature programmed reduction (H2-TPR). Significant structural and reactivity differences are observed in catalysts prepared from different organometallic precursors, indicating that the chemical nature of surface vanadia can be influenced by the nuclearity of the precursor used for grafting. SECTION Surfaces, Interfaces, Catalysis

S

tructure determinations for vanadium oxide surface species have traditionally relied on a variety of spectroscopic techniques, including solid-state NMR spectroscopy,5-9 X-ray absorption spectroscopy (XAS),10-15 UV-vis diffuse reflectance spectroscopy (UV-vis DRS),16-20 and Raman spectroscopy.21-32 It is generally agreed that there are three main classes of VOx surface species present on alumina supports: monovanadate, polyvanadate, and crystalline V2O5. Monomeric species predominate at low vanadium coverages and are generally assumed to consist of a vanadium center coordinated to four oxygen atoms.1-4 Recent work from this laboratory has shown that at low vanadium loadings, a mixture of monomers having differing mono-, bi-, or tridentate surface coordination is formed (Figure 1).33 When the vanadium loading is increased, polymeric or cluster species begin to form, the exact structures of which are unknown, until the entire support is eventually covered by a vanadium oxide monolayer.29,31,32 Surfaces with vanadium loadings that exceed monolayer coverage are dominated by V2O5 crystallites.29 The loading dependence of vanadium surface structures makes it difficult to isolate individual VOx species for study. At high loadings below monolayer coverage, a mixture of monomers, dimers, and polymeric VOx surface species is present.29 These structures contribute differently to the various spectroscopic signatures, rendering quantitative structural determination a challenging task.30,31,34 Further complicating the analysis

r 2011 American Chemical Society

Figure 1. Monomeric structures present on θ-alumina supports.33

of reported spectroscopic data is the multitude of preparation techniques used for the synthesis of supported vanadia.9,35-37 The inhomogeneity inherent to traditional catalyst preparative techniques including incipient wetness impregnation and chemical vapor deposition cannot reliably isolate specific surface components. Preparation of well-defined, “single-site” supported metal oxide catalysts therefore presents a significant synthetic challenge. By utilizing the structural uniformity inherent in molecular organometallic precursors, we aim to synthesize well-defined, variable nuclearity VOx surface structures by organometallic grafting. The synthesis of uniform surface structures should provide better structural definition of monomeric, dimeric, Received Date: November 2, 2010 Accepted Date: January 6, 2011 Published on Web Date: January 10, 2011

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DOI: 10.1021/jz101490p |J. Phys. Chem. Lett. 2011, 2, 170–175

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metal charge transfer (O2p f V3d - O2p).42,43 The position of this edge is influenced by the vanadium coordination and can provide information about the degree of vanadia polymerization.17-20 For the supported catalysts V1, V2, and V4, the edge energies are at 3.9 to 4.0 eV (Table 1), which fall in the isolated VO4 region (>2.8 eV), indicating monomeric vanadia as the dominant surface species. Using the formula developed by Gao et al., which relates the degree of polymerization of supported vanadium oxides to the edge energy position,18 there is no evidence of polymeric V-O-V type chains in any of the samples. Regardless of the precursor used for deposition, the resulting vanadium sites have similar UV-vis-deduced coordination geometries with an average vanadium 5þ oxidation state for V1, V2, and V4. X-ray absorption spectra of the V1, V2, and V4 supported catalysts were also measured to provide detailed metrical information about the vanadium coordination geometry, structural environment, and oxidation state.11,13-15,44-46 The normalized X-ray absorption spectra of the supported catalysts show a distinct preedge feature, a broad XANES region, and a flat featureless EXAFS region (Figure S2). The preedge peak height is 0.69 for each of the supported samples, indicating that the structure of the vanadium oxide species is a distorted tetrahedron.46 In agreement with DRS data, the edge position of the XANES region indicates that the samples have vanadium in the 5þ formal oxidation state.14,15 The Fourier transform magnitude of the chi function shows little difference in features between the catalysts prepared from the different precursors. There is no distinguishable higher shell feature near 3 Å, signifying an absence of detectable V-O-V

and polymeric supported vanadium oxide species and aid in developing a more detailed mechanistic understanding of the role of oxide surface centers in catalytic reactions. In this contribution, we explore the scope of the organometallic grafting technique using organovanadium precursors having preselected nuclearity. To elucidate vanadium surface structure, the catalysts38 are characterized by UV-vis DRS spectroscopy, X-ray absorption spectroscopy, UV-Raman spectroscopy, and H2-TPR. It will be seen that both the structural and chemical properties of surface vanadium oxides can be significantly controlled via the organometallic precursor used for deposition. The organometallic precursors VO(Mes)3, [(Mes)3V]2(μ-O), and [(η-C5Me5)V]4(μ-O6) were synthesized and purified as described in the relevant refs 39 and 40. Under strictly anaerobic/anhydrous conditions, these precursors were dissolved in a hydrocarbon solvent and reacted with the θ-Al2O3 support (Johnson Matthey). The surface reactions proceed via ligand protonolysis, as verified by in situ 1H NMR spectroscopy (Scheme 1). The resulting supported catalysts were then collected, washed, dried, and calcined prior to characterization. Vanadium loading was quantified by ICP-AE spectroscopy. Physical properties of the as-prepared VOx/Al2O3 samples are summarized in Table 1 below. UV-vis DRS spectroscopy is commonly used to provide information about the local coordination environment of supported metal oxide catalysts. A charge transfer band at ∼4.5 to 5.6 eV, characteristic of vanadium in the 5þ oxidation state,41 is observed in all of the present samples (Figure S1). Typical d-d transitions indicating vanadium species in lower oxidation states41 are not observed. There is a sharp rise in absorption between 3 and 4 eV that results from ligand-to-

Scheme 1. Organometallic Grafting Reactions of Precursors VO(Mes)3, [(Mes)3V]2(μ-O), and [(η-C5Me5)V]4(μ-O6) on θ-Al2O3a

a Surface species present immediately after ligand protonolysis were not isolated or structurally analyzed prior to oxygen treatment. The supported vanadia catalysts V1, V2, and V4 are characterized after calcination in oxygen.


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Table 1. Summary of Characterization Data for Samples Prepared by Organovanadium Grafting sample

organometallic precursor

loading (atoms V/nm2)

edge energy (eV)

H2-TPR Tmax (°C)

VdO stretching mode (cm-1)

surface coordination

V1 V2

VO(Mes)3 [(Mes)3V]2(μ-O)

0.21 0.22

4.0 4.0

488 432

1023 1021, 1035

tridentate tridentate, bidentate

V4

[(η-C5Me5)V]4(μ-O6)

0.23

3.9

496

1007, 1022

molecular, tridentate

structures is expected to result in V-OH type surface bonds when grafted to the alumina support (V2, V4, Scheme 1). Consistent with this picture, the spectral difference in VdO bands for the three adsorbates match well with band assignments for the molecular (1007 cm-1), bidentate (1035 cm-1), and tridentate (1022 cm-1) structures (Figure 1) computed theoretically and observed experimentally by Kim and coworkers.33,50 Using these assignments, the tridentate structure is predominant in V1, a mixture of bidentate and tridentate structures in V2, and a mixture of tridentate and molecular structures in V4. Note that the number of OH ligands coordinated to vanadium in these structures correlates with the number of μ-oxo bonds in the precursor. The broad Raman band at 800-1000 cm-1 is difficult to assign because of the absence of distinctive features. However, in addition to the central 900 cm-1 component of the band observed in V1, V2, and V4, there is a second component centered at 948 cm-1 present in V2. This contribution is associated with the symmetric V-O stretching vibrations in the bidentate structure.50 Evidently, the tridentate, bidentate, and molecular structures of monomeric surface vanadium oxide, all of which have vanadium in the 5þ oxidation state and the VO4 coordination, are distinguishable by Raman spectroscopy but indistinguishable by UV-vis DRS and EXAFS measurements. Note that UV-Raman spectroscopic differences in VOx structures are observed even after calcination at temperatures well above the bulk V2O5 Tammann temperature. This raises questions as to the applicability of bulk oxide Tammann temperatures for understanding the mobilities of supported metal oxide species. The Tammann expression, TTam ≈ 0.5Tbulk,melt, where Tbulk,melt is the bulk melting point and TTam is the

linkages in any of the samples, and no significant structural differences between V1, V2, and V4 are apparent.47 UV-Raman spectroscopy is a powerful tool for characterizing solid catalysts, providing detailed structural information as well as the unique ability to probe specific components of heterogeneous mixtures.21,30 For typical dehydrated vanadia catalysts, there are two main features in the UV-Raman spectra: a terminal VdO stretching mode with a distinctive band in the 1000-1030 cm-1 range and a broad 800-1000 cm-1 feature, which is a combination of coupled V-O-Al and V-O-V modes.48 The ν(VdO) and ν(V-O-Al or V-O-V) bands of V1, V2, and V4 present distinct spectroscopic signatures (Figure 2). Although the spectral changes are subtle, they are reproducible and were observed in three sample sets prepared at different times. The ν(VdO) frequency shifts reflecting differences in VdO and V-O bond lengths are influenced by a combination of the V-coordination number, V-O environment, and differences in V-O-Al or V-O-H coordination. Similar differences in VdO stretching frequencies have previously been associated with increased vanadium loadings and were attributed to distortions associated with polymeric surface VO4 species.17,29,49 Recently, changes in ν(VdO) for samples with low vanadium loadings were assigned to monomeric surface geometries having different V-O-H linkages.33,50 Because the vanadium loading of each sample in the present study is identical and the vanadium coordination is determined to be monomeric by UV-vis DRS and EXAFS, the observed shifts in ν(VdO) indicate the presence of structurally distinct monomeric surface species (Figure 1), a direct consequence of the organometallic precursor structure. The presence of μ-oxo bonds in the precursor

Figure 2. UV-Raman spectra of supported vanadia samples. (A) Full spectra. The sharp peak at 1554 cm-1 is a gas phase O2 band inherent to the sample measurement conditions. (B) Expanded spectra in the V-O region.


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reveal distinct structural differences between the supported catalysts prepared from different precursors. There is a correlation between the organovanadium precursor structure and the resulting vanadia surface structure. From H2-TPR, it is also clear that there are chemical differences between the different monomeric vanadium structures because V2 (bidentate) begins reduction at a lower temperature than V1 (tridentate) and V4 (tridentate þ molecular). It is therefore apparent that the structural and chemical reactivity differences of supported VOx catalysts are non-negligibly influenced by the molecular precursor used for deposition. We believe that this work is the first of its kind to show that the chemical nature of surface vanadia can be controlled by the nuclearity of the precursor used for grafting. Analogous structural and chemical behavior has previously only been observed as a function of vanadium loading.

SUPPORTING INFORMATION AVAILABLE Supplementary

Figure 3. H2-TPR profiles of the supported samples.

figures and description of catalyst preparation, UV-visible diffuse reflectance spectroscopy, X-ray absorption spectroscopy, UV-Raman spectroscopy, and H2 temperature- programmed reduction. This material is available free of charge via the Internet at http://pubs.acs.org.

Tammann temperature, is applicable to bulk metal oxide surfaces and supported oxides at high loadings.51 Note that the Tammann temperature of the alumina support (890 °C) is far higher than that of bulk V2O5 (209 °C) and higher than the calcination temperature.51 Moreover, isolated molecular vanadia surface species should have properties far different from those of bulk V2O5 and likely have significantly different Tammann temperatures. H2-TPR was also used to probe for chemical differences in the supported vanadium oxides (Figure 3). A single reduction peak is observed in each of the samples with no significant changes in peak shape/position observed over multiple redox cycles for individual samples, indicating thermally stable surface structures. Importantly, high-temperature reduction peaks characteristic of V2O5 are not observed.3,52,53 V4 exhibits a TPR profile similar to that of V1 with a slightly higher maximum uptake temperature. The TPR profile for V4 is consistent with surface decomposition of the V4 precursor, [(η-C5Me5)V]4(μ-O6), to form monomeric fragments, as expected for the known chemical reactivity of [(η-C5Me5)V]4(μO6).40,54 Note that there are differences in the reduction temperature maximum (Tmax) between the V1 catalyst prepared with VO(Mes)3 and the other two materials. Similar shifts in reduction temperature have been observed in multiple sample sets prepared at various times. The 56 °C lower Tmax for V2 reflects the combination of V-O-support bonds present in the assigned V2 bidentate structure, which renders reduction more facile than that for the V-O-support bonds in the tridentate and molecular species of V1 and V4, respectively. Detailed mechanistic studies will be required to elucidate completely how the differing populations of tridentate, bidentate, and molecular structures that comprise the V1, V2, and V4 samples lead to the observed TPR profiles. The present results underscore the limitations of UV-vis DRS and XAS for detecting subtle chemical and structural changes in vanadia species on alumina surfaces. Both of these methods conclude that V1, V2, and V4 give rise to similar monomeric-type vanadium surface species, regardless of the organometallic precursor used for grafting. However, the ν(VdO) band position differences and the additional 948 cm-1 component of the 800-1000 cm-1 band in the UV-Raman spectrum of V2


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: (T.J.M.) [email protected]; (P.C.S.) [email protected].

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Science, under contracts DE-AC02-06CH11357 and DE-FG02-03-ER15457.

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