In Situ Raman and FTIR Spectroscopy of Molybdenum(VI) Oxide

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In Situ Raman and FTIR Spectroscopy of Molybdenum(VI) Oxide Supported on Titania Combined with 18O/16O Exchange: Molecular Structure, Vibrational Properties, and Vibrational Isotope Effects George Tsilomelekis and Soghomon Boghosian* Department of Chemical Engineering, University of Patras and Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation of Research and Technology - Hellas (FORTH/ICH-HT), Patras, Greece ABSTRACT: Molybdenum oxide deposited on anatase with Mo surface densities in the range 1.8-17.0 Mo/nm2 was studied by in situ vibrational (Raman and FTIR) spectroscopies and 18O/16O isotopic exchange experiments combined with in situ Raman spectra at 450 °C. The vibrational isotope effects and the combined interpretation of the observed Raman fundamental, IR overtone, as well as calculated zero-order band wavenumbers and characteristics suggest a mono-oxo configuration for the deposited molybdena phase at low (up to 3 Mo/nm2) as well as at high (4-6 Mo/ nm2) coverage, irrespective of the extent of association (polymerization). The Raman band due to ModO stretching is observed at 994 cm-1 for all samples with surface densities up to 6 Mo/nm2. Isolated mono-oxo monomolybdates in C3v OdMo(-O-Ti)3 configuration predominate at low loadings, while the presence of mono-oxo polymolybdates is evidenced at high surface coverage. A “next-nearest-neighbor 18O/16O substitution” vibrational isotope effect is observed, resulting in red shifts (6-7 cm-1) of the Mod16O Raman band wavenumber. All observations are addressed and discussed, and consistent band assignments and interpretations are made. A mechanism accounting for the 18O/16O exchange process is proposed and discussed at the molecular level.

1. INTRODUCTION Supported molybdena catalysts have attracted significant interest during the last decades due to their catalytic performance in a number of processes.1-7 Sound proposals for the presence of the same outermost layer in bulk and supported catalysts have been reported.8 The structural configuration of the dispersed oxo molybdenum species is strongly influenced by the choice of a specific support through a dependence on the pH at the point of charge (PZC), characteristic for each support, and following the speciation of molybdate in solution.9 The local structure of the dispersed (MoOx)n phase in oxide supports at dehydrated conditions has been studied extensively, mainly by in situ Raman spectroscopy,10 and for surface Mo concentrations below the monolayer coverage evolves (with increasing coverage) from isolated monomeric molybdates (MoOx) to larger polymolybdates possessing Mo-O-Mo bridges.5,6,11 At coverages exceeding the monolayer, crystalline MoO3 or mixed metal oxide crystals (e.g., Al2(MoO4)3,6 Zr(MoO4)25,12) are formed. Questions like, e.g., whether the surface monomolybdate (abundant at low Mo surface densities) occurs in mono-oxo tetrahedral OdMo(-O-M)3, dioxo tetracoordinated (Od)2Mo(-O-M)2, or tetragonal pyramidal OdMo(-O-M)4 (M = support metal atom) configuration are still open and have been the subject of recent elegant theoretical (DFT) investigations for molybdena supported on silica,13 γ-alumina,14 and titania.15 Figure 1 depicts the schematic molecular models for the above structural configurations of isolated surface molybdates. In particular, the DFT r 2010 American Chemical Society

calculations on the structure of isolated (monomeric) Mo oxide in dehydrated conditions have shown that di-oxo (Od)2Mo(-OSi)2 species are favored on model supported MoO3/SiO213 that the mono-oxo tetragonal pyramidal OdMo(-O-Al)4 configuration is dominant on the most exposed (110) facet of γ-alumina,14 while the most stable configuration on the majority (101) crystal termination of titania(anatase) is the mono-oxo tetrahedral OdMo(-O-Ti)3 arrangement.15 Moreover, a comprehensive experimental investigation on group 5-7 transition metal oxides supported on SiO2 including in situ spectroscopic work16 and Raman studies assisted by 18 O/16O isotope exchange measurements17 points undoubdtedly to an exclusive occurrence of dehydrated dispersed metal oxide species in isolated (monomeric) form on silica. For the case of the MoO3/ SiO2 system, dioxo (Od)2Mo(-O-Si)2 species prevail with small amounts of OdMo(-O-Si)4 also evidenced,16,17 thereby confirming the pertinent DFT results.13 However, the dispersion of supported metal oxide layers on SiO2 has its own special character, being very low because of the low reactivity and greater acidic nature of the SiO2 surface hydroxyls. To the contrary, more complex molecular structures are present in “non-SiO2” supports (e.g., Al2O3, ZrO2, TiO2) where the dispersion is much higher and isolated metal oxide species coexist with polymeric domains.10 With direct relevance to the present work, in situ Raman and Mo LIII-edge XANES Received: October 15, 2010 Revised: December 6, 2010 Published: December 30, 2010 2146

dx.doi.org/10.1021/jp1098987 | J. Phys. Chem. C 2011, 115, 2146–2154

The Journal of Physical Chemistry C

Figure 1. Schematic structural models for monomolybdates deposited on oxide supports: (a) mono-oxo tetrahedral unit (C3v); (b) di-oxo tetracoordinated unit (C2v); and (c) mono-oxo tetragonal pyramidal unit (C4v).

characterization of MoO3/TiO2 catalysts points to the occurrence of primarily isolated and tetrahedral species at low coverage and primarily associated (polymeric) and octahedral species at high coverage.9 In situ Raman spectroscopy combined with in situ IR spectroscopy can shed light for the differentiation of mono-oxo and poly-oxo deposited species.18-20 It is generally adopted that the occurrence of a single ModO stretching vibration over 1000 cm-1 is supportive of a proposal for a mono-oxo Mo oxide species;14 such a proposal which is based on the vibrational properties (ModO modes) of gaseous oxyhalides21 is strengthened and confirmed when the wavenumbers of the observed Raman and IR fundamentals coincide. A di-oxo species possesses symmetric vs (strong in Raman and weak in IR) and antisymmetric vas (weak in Raman and strong in IR) modes, the splitting of which lies in the 10-30 cm-1 range.21 Interestingly, the IR overtone region for a di-oxo unit would exhibit multiple features due to overtones and combination bands of the vs and vas modes. The DFT calculations for the isolated molybdena on γ-alumina predict a ∼1015 cm-1 band for the dominant mono-oxo OdMo(-O-Al)4 unit and a 980/960 cm-1 vs/vas pair for the much less abundant dioxo species.14 The corresponding DFT results for isolated Mo oxide species on anatase predict a 1004 cm-1 band for the dominant mono-oxo OdMo(-O-Ti)3 unit and a 986/ 974 cm-1 vs/vas pair for the much less stable dioxo (Od)2Mo(-O-Ti)2 species.15 In situ vibrational (Raman and IR) spectroscopies combined with 18O isotopic labeling studies19,20 can lead to differentiation between mono-oxo MdO, di-oxo M(dO)2, or tri-oxo M(dO)3 configurations, and the relevant systematics are well-known and have been elegantly summarized.22 Very briefly (with relevance to combined in situ Raman and 18O isotopic labeling23), the MdO band of a mono-oxo species will undergo simple splitting (bands due to Md16O and Md18O), whereas complex (multiple) splitting will be exhibited for the MdO bands of a poly-oxo unit due to the occurrence of partially 18O/16O-substituted moieties.21 However, growing concern appears on why three or four Raman vibrational bands have not been observed for dispersed metal oxides with evidenced (or suspected) poly-oxo configuration.17 DFT results on model MoO3/SiO2 suggested that the vibrational properties of 18 O/16O partially exchanged surface metal oxides are not as simple as previously thought and described above and that the observed isotope effects are based on the total mass of the metal oxide complex.13 Research aiming at the understanding of structural and vibrational properties of supported surface transition metal oxides is of topical character.13-17,24 Despite the extensive investigation of supported molybdena materials, there is still a lack of general understanding of the factors controlling the dispersion and local MoOx coordination/structure of supported molybdena and their

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dependence on the nature of supports and surface coverage.24 The motivation of the present article, reporting on our pertinent endeavors on molybdena catalysts supported on titania(anatase), is 3-fold: (i) to differentiate for the first time between mono-oxo and di-oxo configurations for the deposited Mo(VI)-oxo species on titania using in situ Raman and FTIR spectroscopies combined with 18O/16O isotopic substitution; (ii) to examine the site/phase multiplicity at low (1.8 Mo/nm2) and high (∼6 Mo/nm2) Mo surface densities below monolayer coverage; and (iii) to exploit the vibrational isotope effects to infer structural properties for the surface Mo oxides and to study the reducibility of the various O sites (ModO, Mo-O-Mo, and Mo-O-support). The new insights generated from this comprehensive study allow the understanding of the vibrational properties of deposited Mo(VI)-oxo species on TiO2 and, for the first time, the experimental determination of the number of ModO bonds.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. The catalysts were prepared by wet impregnation of TiO2-anatase (Alfa, 127 m2/g) using aqueous solutions (pH = ∼4) of (NH4)6Mo7O24 3 4H2O (Alfa). The solutions, of which the concentrations were adjusted to correspond to the desired MoO3 wt %, were subjected to rotation at 45 °C for 1 h and then to rotary evaporation at 45 °C under reduced pressure for 1 h for removing the solvent. They were then dried overnight at 100 °C and calcined in air in a muffle furnace at 480 °C for 4 h. Surface areas were measured by N2 adsorption using a Micromeritics Gemini II 2370 analyzer and standard multipoint BET analysis methods. Samples were evacuated for 2 h before N2 adsorption measurements. Powder X-ray diffraction patterns were obtained at room temperature using a Philips PW 1830 diffractometer and Cu KR radiation. The catalyst characteristics are summarized in Table 1. The samples are denoted by xMoTi, x being the wt % of MoO3 loading. 2.2. In Situ Raman Spectroscopy and 18O/16O Exchange. A homemade Raman cell was used for recording the in situ Raman spectra of the studied catalysts (Figure 2) consisting of a double-walled quartz-glass transparent tube furnace mounted on a xyz plate allowing it to be positioned on the optical table. The inner furnace tube (23 mm o.d., 20 mm i.d., and 10 cm long) is kanthal wire-wound for heating the cell. The cell has a gas inlet and a gas outlet as well as a thermocouple sheath possessing a sample holder at its tip. Approximately 180 mg of each catalyst was pressed into a wafer and mounted on the holder that could be vertically adjusted in the in situ cell. The 488.0 nm line of a Spectra Physics Stabilite 2017 Arþ laser was used for exciting the Raman spectra. The incident light (adjusted to 40 mW) was slightly defocused with a cylindrical lens to reduce sample irradiance. The scattered light was collected at 90° (horizontal scattering plane), analyzed with a 0.85 Spex 1403 double spectrometer, and detected by a -20 °C cooled RCA PMT interfaced with Labspec data acquisition software. In situ Raman spectra under oxidizing conditions were recorded at 450 °C under flowing 16O2 (99.999%) after treatment for 1 h at a 15 cm3/min flow rate. Afterwards, each catalyst was subjected to reducing conditions under flowing 4.5% H2/N2 for 15 min at a 50 cm3/ min flow rate and reoxidized with 2% 18O2/He (Messer) for 15 min at a 10 cm3/min flow rate. Intermittent in situ Raman spectra under flowing 2% 18O2/He were then recorded. The reduction and reoxidation step durations were crucial, to ensure that reduction and 2147

dx.doi.org/10.1021/jp1098987 |J. Phys. Chem. C 2011, 115, 2146–2154


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Table 1. Properties of the Catalysts

a

catalyst

MoO3 (wt %)a

calcination

surface area (m2/g)

surface density ns (Mo/nm2)

3MoTi

3

480 °C/4 h

68.1

1.8

TiO2 (anatase)

6MoTi

6

480 °C/4 h

80.7

3.1

TiO2 (anatase)

9MoTi

9.1

480 °C/4 h

82.5

4.6

TiO2 (anatase)

5.9

TiO2 (anatase)

crystalline phases

15MoTi

15

480 °C/4 h

106.0

21MoTi

21.1

480 °C/4 h

107.9

8.2

TiO2 (anatase), MoO3

35MoTi

35

480 °C/4 h

86.3

17.0

TiO2 (anatase), MoO3

Values based on chemical analysis by atomic absorption spectrometry.

Figure 2. Homemade in situ Raman cell.

reoxidation (18O/16O exchange) take place to a satisfactory extent without waste of the expensive 18O2. The reduction/reoxidation protocol was established by “simulating” the process using 2% 16O2/ He as the reoxidant. This simulation showed that a duration of 15 min for each step was capable of attaining 90% of the steady state in each cycle at 450 °C. 2.3. In Situ FTIR Spectroscopy. In situ Fourier transform infrared (FTIR) experiments were carried out under flowing 16 O2 at 450 °C using a Nicolet 740 FTIR spectrometer equipped with a diffuse reflectance (DRIFT) cell (Spectra Tech), an MCTB detector, and a KBr beam splitter. The oxygen flow rate through the DRIFT cell was 30 cm3/min.

3. RESULTS AND DISCUSSION 3.1. In Situ Vibrational (Raman and FTIR) Spectra of Oxidized Catalysts under Flowing 16O2(g) . 3.1.1. In Situ Raman Spectra. Figures 3 and 4 show the in situ Raman spectra

obtained for the studied MoO3/TiO2 catalysts under flowing 16 O2 at 450 °C and include the corresponding in situ Raman spectra of TiO2 (anatase) (spectra Figures 3a and 4a) and crystalline MoO3 (spectrum Figure 4d) for comparison. The spectra in Figure 3, pertaining to samples of which the coverage does not exceed the monolayer (spectra 3b-3e of samples with

ns up to 5.9 Mo/nm2), are normalized with respect to the strong v1(Eg) TiO2 (anatase) band observed at 633 cm-1. This is done to illustrate the increase in the relative intensity of bands due to deposited MoOx species as compared to the main TiO2 band intensity with increasing coverage. A well-defined band (in the ModO stretching region) is seen at 994 cm-1 which is at fixed position irrespective of loading (as marked by the dashed line in Figure 3). The spectra in Figure 3 show that for ns up to 5.9 Mo/nm2 molybdena species occur exclusively in the form of dispersed amorphous metal oxides, in conformity with the reported ∼5 Mo/nm2 molybdate saturation capacity on several metal oxide supports.9,12 A weak and broad band at ∼925 cm-1 due to Mo-O-Mo functionalities9 becomes visible with increasing loading (see spectra 3d and 3e of samples with ns of 4.6 and 5.9 Mo/nm2) thereby witnessing a low presence of associated (polymeric) molybdates with increasing loading. The assignment of the broad weak ∼925 cm-1 band to Mo-O-Mo functionalities is in agreement with the general consensus in catalysis literature.9,10,19,20,23,25-36 Contrary to the case of MoO3/ZrO2 and MoO3/Al2O3 catalysts,5,6,37 the ∼925 cm-1 MoO-Mo band does not dominate at monolayer loading, probably indicating a low extent of surface association between MoOx units on TiO2. Bands observed below ∼800 cm-1 in Figure 3 are due to the weak, 2v4(B1g)(∼792 cm-1), and the very strong, v1(Eg)(633 cm-1), v2(B1g)(510 cm-1), and v4(B1g)(393 cm-1), TiO2 2148



Figure 3. In situ Raman spectra of oxidized MoO3/TiO2 catalysts (surface densities in the range 1.8-5.9 Mo/nm2) under flowing 16O2 at 450 °C: (a) TiO2; (b) 3MoTi; (c) 6MoTi; (d) 9MoTi; (3) 15MoTi. Laser wavelength, λ0 = 488.0 nm; laser power, w = 40 mW; time constant, τ = 0.5 s; spectral slit width, ssw = 7 cm-1.

(anatase) support bands. The spectra in Figure 3 are in agreement with the ones reported earlier by Wachs et al.9 The asterisk in spectrum Figure 3e of the sample with ns of 5.9 Mo/nm2 marks a weak band at ∼820 cm-1 superimposed on the ∼792 cm-1 2v4(B1g)TiO2 band, which is due to traces of crystalline MoO3 (not detected by XRD). The spectra shown in Figure 4b and 4c obtained for samples with ns of 8.2 and 17.0 Mo/nm2 (by far exceeding the molybdate saturation capacity) are dominated by bands due to bulk crystalline MoO3, of which the in situ Raman spectrum is shown for comparison in Figure 4d. 3.1.2. In Situ FTIR Spectra and Anharmonicity. Figure 5 shows the in situ FTIR spectra of the studied catalysts (spectra Figure 5b-5g) in the oxidized state under flowing 16O2 at 450 °C and includes the in situ FTIR spectra of the TiO2 support and bulk crystalline MoO3 under the same conditions for comparison. Unfortunately, only the overtone region can be exploited. The FTIR spectra in Figure 5b-5e, pertaining to samples with ns ranging between 1.8 and 5.9 Mo/nm2 (i.e., below monolayer) show one single band in the ModO overtone region at 1965 cm-1 (marked by a dashed line in Figure 5) at fixed position, irrespective of loading. The intensity of this band increases with increasing loading in accordance with a correspondingly increasing presence of deposited MoOx species. For coverages exceeding the molybdate saturation capacity (i.e., for ns > 6 Mo/nm2), the FTIR spectra in Figure 5f and 5g exhibit a band at 1945 cm-1 together with a shoulder at 1865 cm-1 (best seen in the spectrum in Figure 5g) that are due to bulk crystalline MoO3, of which the spectrum is displayed in Figure 5h. Observing one single band in the IR overtone region of spectra Figure. 5b-5e (with ns ranging between 1.8 and 5.9 Mo/nm2) is indicative of a mono-oxo configuration for the dispersed molybdena phase (see pertinent discussion in the Introduction).

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Figure 4. In situ Raman spectra of oxidized MoO3/TiO2 catalysts (surface densities in the range 8.2-17.0 Mo/nm2) under flowing 16O2 at 450 °C: (a) TiO2; (b) 21MoTi; (c) 35MoTi; (d) MoO3. Recording parameters: see Figure 3 caption.

However, the observed 1965 cm-1 ModO IR overtones are not the same as would be predicted by doubling the corresponding observed Raman fundamentals (i.e., 2 994 = 1988 cm-1). This is ascribed to anharmonicity, which (in the first place) is the reason for the modification of the vibrational selection rule and for allowing the overtone vibrational transition. By exploiting the wavenumbers of the observed Raman vMo=O fundamentals (vR,ModO,1r0) and observed IR overtones (vIR,ModO,2r0) and by adopting a diatomic approximation, we have calculated the zero-order wavenumbers (corrected for anharmonicity), ωModO, as well as the corresponding anharmonic constants by using the equations Fundamental : vModO;1r0 ¼ ωModO ð1 - 2xModO Þ

ð1Þ

First overtone : vModO;2r0 ¼ 2ωModO ð1 - 3xModO Þ

ð2Þ

that relate the observed values of the fundamentals and first overtones with the zero-order wavenumber and the anharmonicity constant,21 and the results are: ωModO = 1017 cm-1 and xModO = 0.011. The ∼0.01 value for the anharmonic constant is very reasonabe, thereby qualifying the proposal for mono-oxo configuration. 3.1.3. Vibrational Spectra and Structural Implications. The structure of dispersed molybdena on TiO2 (anatase) is far from being unambiguously resolved.10 The combined application of Raman and FTIR spectroscopy can provide complementary information on the molecular structure of the dispersed MoOx phase as described in the Introduction.19 It is generally adopted as a “rule of thumb” that ModO stretching vibrations above 1000 cm-1 for molybdena deposited on oxide carriers are supportive of the proposal for mono-oxo species, based on the vibrational properties (ModO modes) of gaseous oxyhalides.21 However, the prediction 2149



Figure 5. In situ FTIR spectra of oxidized MoO3/TiO2 catalysts under flowing 16O2 at 450 °C.

of the exact location of ModO vibrations for supported MoOx from data pertaining to gaseous oxyhalides is not straightforward since the halide ligands affect the wavenumber of the oxo functionalities.16 Thus, the vibration of a halide-free oxo complex would serve as a more accurate reference for supported MoOx (e.g., 1006 cm-1 reported for Keggin H3SiM12O40 clusters that contain mono-oxo ModO units16). In the present work, for all MoO3/TiO2 samples with coverages up to monolayer, i.e., with ns in the range 1.8-5.9 Mo/nm2, the observed Raman fundamental, vR,1r0, and IR first overtone, vIR,2r0, constitute (together with the very reasonable anharmonicity constant of ∼0.01) a set of values which is consistent with “coinciding” Raman and IR fundamentals at a zero-order wavenumber of 1017 cm-1 (vide ante). One should always remember that the observation of the overtone per se precludes the coincidence of the observed IR first overtone with the doubled observed Raman fundamental. Thus, the above results point to mono-oxo configurations for the surface MoOx species on TiO2 for coverages up to approximate monolayer. Furthermore, this proposal complements very well the DFT results for isolated oxomolybdenum species on TiO2 suggesting a mono-oxo tetrahedral OdMo(-O-Ti)3 arrangement as the most stable configuration on the majority (101) crystal termination of anatase.15 The same computational investigation predicts a 1004 cm-1 band for the ModO stretching of the proposedly dominant OdMo(-O-Ti)3 species,15 which is consistent with the experimentaly determined zero-order wavenumber, ωModO, of 1017 cm-1. Notably, the same DFT work for the much less stable di-oxo species on the minor (001) crystal termination of anatase predicts a vs/vas pair at 986/974 cm-1, far below the experimentaly determined zero-order wavenumber of 1017 cm-1. Therefore, the proposal for mono-oxo configuration for MoOx on TiO2 is further justified. In addition, the DFT proposal15 for tetracoordination of the isolated deposited oxomolybdenum unit is in agreement with

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the Mo LIII-edge XANES-based proposal9 for tetracoordination of MoOx deposited on anatase at low surface coverages. The in situ Raman spectra of the samples with high coverages (i.e., spectra in Figure 3d and 3e of samples with ns of 4.6 and 5.9 Mo/nm2) are indicative of a progressive association of surface molybdates and formation of polymeric units exhibiting a weak and broad band due to Mo-O-Mo functionalities at ∼925 cm-1. The intensity of this band relative to the intensity of the 994 cm-1 band due to ModO terminal stretching is low, indicating probably a low extent of polymerization for the molybdena units on titania. However, Mo LIII-edge XANES work on molybdena deposited on anatase at high surface coverages provided evidence for primarily polymerized species with five-fold coordination for the Mo atom.9 In addition, concern on the possibility for laser wavelength dependence of the relative intensities of bands due to the deposited species led to the discovery that Raman bands due to out-of-plane symmetric stretching vibrations of bridging oxygen units (Mo-O-Mo) appear much stronger with UV excitation compared to the corresponding band intensity when visible laser excitation was used.38 Therefore, one should be rather careful when suggesting a low extent of association (polymerization) of MoOx units on anatase, based solely on the low relative intensity of the ∼925 cm-1 Mo-O-Mo band, as compared to the intensity of the 994 cm-1 ModO band. 3.2. In Situ Raman Spectra and 18O/16O Isotope Exchange . 3.2.1. Introductory Remarks and Raman Spectra. The theory of vibrational isotope effects in polyatomic molecules is quite elaborate and has been well documented for a long time.39 The situation becomes much simpler if the diatomic approximation can be adopted, like, e.g., in the case of a mono-oxo MdO configuration of a dispersed metal oxide phase. On the basis of equal interatomic M-O distances between the Md16O and Md18O counterparts, it can easily be shown that the isotopic ratio, defined as the vMd16O/vMd18O ratio, is equal to21 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 þ 16 m m M O vMd16O isotopic ratio : ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð3Þ vMd18O 1 1 þ mM m18O where mi are the respective atomic masses. For the case of a Mo-O diatomic oscillator, the isotopic ratio is equal to 1.0513. Figure 6 shows sequential snapshots of intermittent in situ Raman spectra obtained at 450 °C for the low-loaded 3MoTi (1.8 Mo/nm2) sample after successive reduction/oxidation cycles (with 4.5% H2/N2 and 2% 18O2/He, respectively) as indicated by each spectrum. Figure 7 illustrates the respective series of spectra obtained for the high-loaded 15MoTi (5.9 Mo/nm2) sample. The following observations are made: (i) The bands due to Mod16O (observed at 994 cm-1 for both 3MoTi (Figure 6) and 15MoTi (Figure 7)) progressively lose intensity, and new bands appear (observed at 944 cm-1 for both 3MoTi (Figure 6) and 15MoTi (Figure 7)) which can be assigned to Mod18O, remain at fixed position (944 cm-1), and progressively gain intensity as a function of reduction/oxidation cycles, indicating a progress in 18O/16O exchange; (ii) Only one new band appears for each sample in the Mod18O stretching region; (iii) Almost 90% of the progress achieved after 30 reduction/ oxidation cycles in 18O/16O exchange is reached after 15-20 reduction/oxidation cycles under the applied conditions; and 2150



Figure 6. In situ sequential Raman spectra obtained for the 3MoTi sample at 450 °C under flowing 2% 18O2/He after reduction (H2)/ oxidation (18O2) cycles as indicated by each spectrum. Laser wavelength, λ0 = 488.0 nm; laser power, w = 40 mW; time constant, τ = 2.5 s; spectral slit width, ssw = 3.5 cm-1.

(iv) The position of the Mod16O band is gradually red-shifted from 994 to 988 cm-1 for the low-loaded 3MoTi (Figure 6) and to 987 cm-1 for the high-loaded 15MoTi (Figure 7) after the completion of the performed reduction/oxidation cycles for 18O/16O exchange. Furthermore, it is of importance to note that the red shift of the Mod16O band is precedent to the appearance of the Mod18O band (see spectra Figures 6b, 6c, 7b, and 7c). All pertinent information on band wavenumbers as well as the theoretically expected band positions for the Mod18O modes (calculated using the isotopic ratio 1.0513) are listed in Table 2. It is noteworthy that separate series of more than 50 sequential reduction/oxidation cycles with intermittent recording of Raman spectra were initially run for both samples with a lower resolution (spectral slit width) of 7 cm-1 (not shown, for brevity). The spectra in Figures 6 and 7 are recorded with a higher resolution of 3.5 cm-1 so that the gradual red shift of the Mod16O band becomes unambiguous. 3.2.2. Vibrational Isotope Effects, Interpretations, and Structural Implications. The evidence arising from all experimental observations made in the present work is consistent with a monooxo configuration for the dispersed molybdena phase on TiO2 both at low (1.8 Mo/nm2) and at high coverage (5.9 Mo/nm2). The same conclusion has very recently been reached for MoO3/Al2O3 and MoO3/ZrO2 catalysts.37 In situ Raman spectra combined with 18 O/16O exchange has been earlier used to propose a mono-oxo configuration for the molybdena phase on a high-loaded (approximate monolayer) MoO3/ZrO2 catalyst,23 and the strategy applied in that work is adopted also in the present study to a large extent. The present proposal for the mono-oxo configuration which is based on the indications provided from the study of the vibrational isotope effects builds on the evidence found in the combined in situ

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Raman and FTIR spectra (vide ante). It is also in agreement with the computational results based on DFT calculations.15 Below, we discuss the various spectral observations, structural implications, and interpretations at the molecular level. The first indication of the 18O/16O isotope exchange pointing to mono-oxo configuration comes from Figures 6 and 7, showing that the isotopic substitution follows the scenario of two bands being present in the ModO stretching region during the sequential 18O/16O isotope exchange, namely, at 994 cm-1 for Mod16O (before the initiation of the isotope exchange) and at 944 cm-1 for Mod18O. The second indication comes from the corroboration of the plausible hypothesis for the existence of a diatomic unit containing terminal oxygen (ModO) which enables a theoretical prediction of the isotopic effect on the vibrational wavenumber based on the isotopic ratio 1.0513. This is best seen in Table 2, where the observed Mod18O wavenumbers (944 cm-1) are in excellent agreement with those theoretically predicted (945 cm-1). However, it is worth mentioning that there is one single possibility for a dioxo unit to exhibit one single band in the Raman spectrum due to degeneration effects, if the angle within the OdModO bond is 90° and the force constants of the independent Mo-O bonds are the same.19,23,40 However, it is extremely unlikely (given also the molybdena site multiplicity) that such a coincidence could occur in low as well as high loadings. Finally, we address the red shift observed for the Mod16O Raman band, i.e., the difference between the initial (before isotopic exchange) and the final (after completion of the performed reduction/oxidation cycles resulting in 18O/16O exchange) position of the Mod16O band (see Figures 6 and 7 and Table 2). It is furthermore of importance to underline that the red shift observed for the Mod16O Raman band occurs gradually in the beginning (i.e., during the first few cycles) of the sequential reduction/oxidation cycle series of the 18O/16O exchange process. Moreover (best seen in Figure 7 where the signal-to-noise ratio is higher), the red shift in the Mod16O band wavenumber precedes the appearance of the Mod18O band and is completed before the emerging Mod18O band starts gaining intensity (see spectra Figure 7b-7d). In addition, it should be pointed out that when performing reduction/oxidation cycles using 2% 16O2/He as the oxidizing agent there is no effect on the position of the Mod16O band. Therefore, the observed red shift is a result of a perturbation caused by the 18O/16O exchange, explained as follows. We ascribe the gradual red shift in the Mod16O wavenumber to a gradual perturbation in bond length (elongation) and bond strength (weakening) of the Mod16O bond caused by the 18O/16O exchange of O sites which are next-nearest-neighbors to the terminal 16O of the Mod16O bond. Such oxygen sites can belong primarily to Mo-18O-Ti anchoring sites (because of the evidenced predominance of OdMo(-O-Ti)3 species among the deposited molybdena phase) or (to a smaller extent at high surface coverage) to Mo-18O-Mo0 units bridging to another Mo atom (denoted Mo0 ) in associated (polymeric) units. The effective nuclear charge of the 18O is lower due to the higher number of neutrons, thereby resulting in a slightly lower electronegativity for the 18O atom in Mo-18O-Ti and/or Mo-18O-Mo0 sites (should an 18O/16O exchange had taken place within these units). In turn, a slightly higher basicity (electron-donating ability) results for the 18O atom, and thereby a slight strengthening of the Mo-18O(-Ti) and/or Mo-18O(-Mo0 ) bonds takes place. Such an effect will cause a slight 2151



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Figure 7. In situ sequential Raman spectra obtained for the 15MoTi sample at 450 °C under flowing 2% 18O2/He after reduction (H2)/oxidation (18O2) cycles as indicated by each spectrum. Recording parameters: see Figure 6 caption.

an extent that corresponds to the extent of 18O/16O exchange of the O sites which are next-nearest-neighbors to the terminal 16 O of the Mod 16 O bond, i.e., following the sequence

Table 2. Vibrational Isotope Effects for MoO3/TiO2 Catalystsa Mod18O

catalyst sample

Mod16O b

(Mo/nm2)

(Raman)

(Raman)

(calculated) d

(Raman)

3MoTi (1.8)

994

944

945

988

15MoTi (5.9)

994

944

945

987

18

Mod16O c

16

18

weakening of the Mod16O terminal bond and a slight red shift of its wavenumber, by that means explaining the observed shifts in the respective Mod16O band wavenumbers (Figures 6 and 7 and Table 2). The effect proceeds gradually to

O=16 O

16

OdMoð- 18 O- TiÞð- 16 O- TiÞ2 sf

16

OdMoð- 18 O- TiÞ2 ð- 16 O- TiÞ sf

18

a

Observed and theoretically calculated Raman band wavenumbers (Figures 6 and 7). b Mod16O band position before 18O/16O exchange [16OdMo(-16O-Ti)3]. c Final position of the Mod16O band after 30 reduction/oxidation cycles of 18O/16O exchange [16OdMo(-18OTi)3]. d Based on the isotopic ratio: 1.0513.

O=16 O

OdMoð- 16 O- TiÞ3 sf

16

OdMoð- 18 O- TiÞ3

O=16 O

ð4Þ

At this point, it is of direct relevance to state that according to DFT calculations on the reducibility of titania-supported molybdena under dehydrated conditions the monomeric oxomolybdenum species is less reducible by hydrogen than the anatase support.41 According to the calculations, the most favorable “mechanism” for H2 adsorption (which is the first step of the reduction process) is a dissociative one resulting in H atom 2152



Figure 8. In situ Raman spectra obtained for the 15MoTi sample at 450 °C under flowing 16O2 (bottom) and under flowing 2% 18O2/He after reduction (H2)/oxidation (18O2) cycles as indicated by each spectrum (b-c). Recording parameters: see Figure 6 caption.

adsorption on unsaturated surface O atoms of Ti-O-Ti sites.41 Thus, hydrogen tends to reduce preferentially the TiO2 surface rather than the ModO site. This is in full agreement with the experimental observations described in the present section. According to the proposed scenario, 16O atoms of surface Ti-O-Ti sites are the first ones to be easier exchanged by 18 O as suggested also from the DFT calculations; subsequently, the mobility of the deposited molybdena phase enables a surface diffusion/spillover mechanism by which 16O atoms belonging to Mo-16O-Ti sites are exchanged by surface 18O atoms according to the sequence described in eq 4; the 18O/16O exchange of the oxygen site of the Mod16O bond occurs less easily (later), as seen in Figures 6 and 7 and as proposed based on the pertinent DFT calculations. Figure 8 shows the in situ Raman spectra obtained at 450 °C for the 15MoTi sample in the 300-1050 cm-1 range under flowing 16O2 and after successive reduction/oxidation cycles of 18 O/16O exchange under flowing 2% 18O2/He as indicated by each spectrum. This is done to cover the spectral range of the main Raman bands due to the anatase support. The isotope effect (appearance of bands due to Mod18O) and the newly reported effect of the “next-nearest-neighbor 18O/16O substitution” which results in a red shift of the Mod16O band are clearly observed, while all bands due to the TiO2-anatase support undergo a slight red shift, thereby indicating that indeed under the applied conditions certain O sites of the support have undergone isotopic substitution.

4. CONCLUSIONS The molecular structure and configuration of the molybdena phase deposited on TiO2-anatase was studied by in situ vibrational (Raman and FTIR) spectroscopies. In addition, in situ Raman spectroscopy was applied in combination with 18O/16O isotope exchange studies. The vibrational isotope effects and the combined interpretation of observed Raman fundamental and IR overtone band wavenumbers and characteristics point to a mono-oxo configuration for the deposited molybdena phase at

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low as well as at high surface coverage with a Mod16O stretching observed at 994 cm-1 irrespective of polymerization degree (i.e., for isolated monomolybdates and associated/polymeric molybdates). Deposited molybdena occurs predominantly in the form of monooxo monomolybdates in distorted tetrahedral (C3v) OdMo(-O-Ti)3 configuration at low coverage (up to ∼3 Mo/nm2), while a certain degree of association (polymerization) leading to formation of mono-oxo polymolybdates is evidenced at higher loadings (4-6 Mo/nm2). A “next-nearest-neighbor 18O/16O substitution” vibrational isotope effect is observed which results in a red shift (6-7 cm-1) of the Mod16O band. This effect takes place upon substitution of oxygen atoms which are next-nearest-neighbors to the terminal 16O, i.e., oxygen atoms of Mo-O-Ti or (to a less extent) of Mo-O-Mo sites. Such an isotopic substitution takes place selectively before the substitution of the terminal 16O of the ModO site. According to the proposed mechanism, unsaturated surface oxygen atoms of Ti-O-Ti sites are substituted by 18 O much easier, and this step is followed by a surface diffusion/ spillover of the molybdena overlayer resulting in 18O/16O exchange of Mo-16O-Ti sites.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support was provided by the Research Committee of the University of Patras (C. Caratheodory program C.583). The authors are grateful to P. Panagiotopoulou (University of Patras) for the FTIR analysis of the studied samples. ’ REFERENCES (1) Handzlik, J. J. Phys. Chem. B 2005, 109, 20794. (2) Jehng, J. M.; Hu, H. C.; Gao, X. T.; Wachs, I. E. Catal. Today 1996, 28, 335. (3) Liu, H. C.; Iglesia, E. J. Catal. 2002, 208, 1. (4) Chen, K. D.; Bell, A. T.; Iglesia, E. J. Catal. 2002, 209, 35. (5) Christodoulakis, A.; Boghosian, S. J. Catal. 2008, 260, 178. (6) Christodoulakis, A.; Heracleous, E.; Lemonidou, A. A.; Boghosian, S. J. Catal. 2006, 242, 16. (7) Nova, I.; Lietti, L.; Casagrande, L.; Dall'Acqua, L.; Giamello, E.; Forzatti, P. Appl. Catal., B 1998, 17, 245. (8) Burcham, L. J.; Briand, L. E.; Wachs, I. E. Langmuir 2001, 17, 6175. (9) Hu, H.; Wachs, I. E.; Bare, S. R. J. Phys. Chem. 1995, 99, 10897. (10) Mestl, G.; Srinivasan, T. K. K. Catal. Rev. Sci. Eng. 1998, 40, 451. (11) Tsilomelekis, G.; Christodoulakis, A.; Boghosian, S. Catal. Today 2007, 127, 139. (12) Xie, S.; Chen, K.; Bell, A. T.; Iglesia, E. J. Phys. Chem. B 2000, 104, 10059. (13) Chempath, S.; Zhang, Y.; Bell, A. T. J. Phys. Chem. C 2007, 111, 1291. (14) Handzlik, J.; Sautet, P. J. Phys. Chem. C 2008, 112, 14456. (15) Hamraoui, K.; Cristol, S.; Payen, E.; Paul, J.-F. J. Phys. Chem. C 2007, 111, 3963. (16) Lee, E. L.; Wachs, I. E. J. Phys. Chem. C 2007, 111, 14410. (17) Lee, E. L.; Wachs, I. E. J. Phys. Chem. C 2008, 112, 6487. (18) Wachs, I. E. Catal. Today 1996, 27, 437. (19) Busca, G. J. Raman Spectrosc. 2002, 33, 348. (20) Banares, M. A.; Wachs, I. E. J. Raman Spectrosc. 2002, 33, 359. (21) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 6th ed.; Wiley-Interscience: New York, 2009. 2153



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In Situ Raman and FTIR Spectroscopy of Molybdenum(VI) Oxide

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