UV-Vis diffuse reflectance and FT-IR spectroscopic investigation
of V
2O
5/SiO
2and MoO
3/SiO
2catalysts for partial oxidation
of methane to formaldehyde (*)
G. MARTRA(1)(**), P. VITTONE(1), S. COLUCCIA(1), F. ARENA(2) and A. PARMALIANA(2) (1) Dipartimento di Chimica, IFM dell’Università di Torino
via P. Giura 7, I-10125 Torino, Italy
(2) Dipartimento di Chimica Industriale dell’ Università di Messina
Salita Sperone 31, I-98186, S. Lucia (Messina), Italy
(ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — The structure of the supported phase of differently loaded V2O5/SiO2 and MoO3/SiO2 catalysts and the interaction of the surface species with the carrier were studied by DR UV-Vis and FT-IR spectroscopy. In the case of the V2O5/SiO2 systems, an evolution from isolated to clusterized tetrahedral (Td) V5 1 containing species was observed passing from low to high loading, whereas complex polymeric structures containing Mo6 1 ions in octahedral coordination were found to be overwhelmingly present in MoO3 catalysts at any loading. The isolated V5 1( Td ) species are suggested to be responsible for the high activity of low-loaded V2O5/SiO2 samples, while the poor catalytic performances of MoO3/SiO2 indicate that polymolybdate species are almost inactive in the partial oxidation of methane to formaldehyde.
PACS 82.65 – Surface and interface chemistry.
PACS 78.40 – Absorption and reflection spectra: visible and ultraviolet. PACS 82.65.Jv – Heterogeneous catalysis at surfaces.
PACS 01.30.Cc – Conference proceedings.
1. – Introduction
The great amount of research interest focused during the past decade on the catalytic methane partial oxidation (MPO) to formaldehyde evidenced that the SiO2-based catalysts are very effective, and considerable efforts have been addressed
to ascertain the working mechanism of such oxide systems and to identify the nature of their active sites [1, 2].
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
(**) E-mail: martraHch.unito.it
On this account, the unique suitability of pure SiO2 surfaces in catalysing the title
reaction has been previously pointed out [3, 4], assessing also the influence of the preparation methods, Na loading and thermal and mechanical pretreatments on their reactivity [3]. In addition, it has been found that the functionality of the very active “precipitated” SiO2towards the formation of HCHO is significantly promoted by V2O5,
while it is depressed by MoO3[5, 6]. Namely, a monotonic decrease in the activity with
increasing metal oxide loading has been found for MoO3, while a significant enhancing
of the original activity of the bare SiO2has been noted for V2O5loadings in the range
0.2–5.3 wt% [5].
Furthermore, literature data [7, 8] indicate that the structure and the dispersion of the supported phase strongly depend on the V2O5 or MoO3 loading, suggesting that
some structure-activity relationship could be hypothesized to explain the observed catalytic behaviour of the supported oxide catalysts.
To this aim, the results of a spectroscopic characterization study of V2O5/SiO2 and
MoO3/SiO2 catalysts previously tested in the partial oxidation of methane to
formaldehyde [5, 6] are reported in this paper, focusing on the structure of the supported phase at different oxides loading and on the interaction between the surface species and the carrier.
2. – Experimental
MoO3/SiO2 and V2O5/SiO2 catalysts were prepared according to the procedure
described elsewhere [5, 6] using a “precipitated” SiO2 sample (Akzo product, Grade Si
4-5P; S.S.A.BET4 400 m2/g ) as support.
For both types of catalysts, a series of samples containing different loadings of the supported oxides were obtained. The loading amounts, as determined by atomic absorption spectrometry, along with the BET surface area, the calculated number of V5 1 or Mo6 1 cations per nm2 and the notation used to identify the investigated
catalysts are summarised in table I.
To perform the diffuse reflectance UV-Vis measurements, fine powders of the samples were put into a cell with an optical quartz wall, while for the IR measurements the powders were pressed to obtain self-supporting pellets, and then placed into a cell equipped with KBr windows.
In both cases the cells were permanently connected to conventional vacuum lines (residual pressure 4131026Torr; 1 Torr 4133.3 Pa) allowing all thermal treatments
to be carried out in situ. Before recording the spectra, the samples were: a) outgassed
TABLEI.
Sample Loading (wt%) (a) BET ( m2
Og ) (a) Mn 1Onm2 V2S V5S V10S M2S M4S M7S 2.0 ( V2O5) 5.3 ( V2O5) 10.3 ( V2O5) 2.0 ( MoO3) 4.1 ( MoO3) 7.2 ( MoO3) 257 231 200 311 187 120 0.26 ( V5 1) 0.76 ( V5 1) 1.80 ( V5 1) 0.21 ( Mo6 1) 0.68 ( Mo6 1) 1.90 ( Mo6 1) (a) From refs. [5, 6].
at 600 7C; b) oxidized for 1 hour in 100 Torr O2(high purity gas, Matheson) at the same
temperatures; c) cooled at room temperature under oxygen and then pumped off for 1 hour. Following this procedure a high dehydration of the samples without reduction of the transition metal ions was obtained.
Diffuse reflectance UV-Vis spectra were obtained using a Perkin-Elmer Lambda 19 spectrophotometer equipped with an integrating sphere and using as reference BaSO4
powder put in the same type of cell employed for the samples. A band at 210–-215 nm was always present in our spectra. It was an artifact due to a defect in the integrating sphere and was eliminated from the figures reported in this paper.
Infrared measurements were carried out by a Bruker IFS 48 spectrometer, using 4 cm21resolution and 256 accumulating scans.
3. – Results and discussion
3.1. V2O5/SiO2catalysts. – In fig. 1 the DR UV-Vis spectra of the V2O5/SiO2samples
are shown. At low loading (curve a), a broad and asymmentric band is present in the 200–350 nm range, exhibiting a well-defined maximum at 250 nm and an evident shoulder at A300 nm. As the V2O5 loading increases, the 200–350 nm absorption
becomes more intense (curves b, c), still showing the main components at 250 and 300 nm, buth with different relative intensities. In fact, for the V5S sample two absorptions at 250 and 300 nm of similar intensity are present (curve b), whereas the spectrum of the V10S sample is dominated by a maximun at 300 nm (curve c), the component at 250 nm now appearing as a shoulder (curve c).
According to the literature [8-10], both spectral components can be assigned to charge transfer transitions from oxygen ligands to V5 1 ions, their position depending
on the structure of the vanadium-containing species and on the number of ligands
Fig. 1. – Diffuse reflectance spectra of the V2O5/SiO2 catalysts with increasing V2O5 loadings: a) V2S (2.0 wt%), b) V5S (5.3 wt%) and c) V10S (10.3 wt%).
surrounding the metal ion. In particular, the component at 250 nm is assignable to isolated monomeric species with V5 1 ions in tetrahedral (Td) coordination [8-10] (scheme 1), while that at 290–300 nm is due to clusterized oligomeric species, still containing V5 1 (Td) ions, forming chains or bidimensional patches on the silica
surface [8, 10] (scheme 2).
Furthemore, it must be noticed that in this last case, besides the main absorption in the 200–350 nm range, also a weak but not negligible band centred at 465 nm is present.
On this basis, the observed evolution of the relative intensity of the components at 250 and 300 nm passing from the V2S to the V10S sample essentially evidences that a progressive increase of the amount of the oligomeric structure with respect to the isolated terahedral species occurs by increasing the V2O5loading.
Furthermore, the presence in the spectrum of the V10S sample of a weak band at 465 nm, characteristic of crystalline V2O5, indicates that at such high loading even
small amounts of tridimensional structure containing V5 1 ions in octahedral
coordination are formed on the surface [9, 10].
Previous studies [5, 6] showed that the catalytic behaviour in the MPO reaction of these V2O5/SiO2 samples depends on the loading, and then, as different loaded
catalysts contain isolated and clusterized structures in a different ratio, it could be suggested that these two species exhibit different catalytic activity.
In particular, as catalytic tests indicated that V2S and V5S samples are highly active in the MPO reaction [5, 6], it seems reasonable to suppose that isolated tetrahedral species, which in these cases respresent a consistent fraction of the supported phase, are efficient catalytic centres, whereas the lower activity observed at higher V2O5 content, when clusterized tetrahedral species prevail, suggests that such
more complex structures are catalytically less active.
3.2. MoO3/SiO2 catalysts. – The results of similar spectroscopic measurements
carried out on the MoO3/SiO2catalysts are reported in fig. 2.
For all samples a complex absorption in the 200–400 nm range is observed, its overall intensity increasing as the MoO3loading increases.
All spectra exhibit as main features a maximum at 235 nm and a shoulder at 285 nm, this last showing a wide tail zeroing at A380 nm. Such components appear broad and poorly resolved in the case of the Mo2S sample (curve a), but become progressively narrower and better defined at higher loading (curves b, c).
As for the V2O5/SiO2 systems, the observed absorptions are assignable to
ligand-metal charge transfer transitions [6-11]. The relationships between UV spectral features and molybdenum symmetry has been widely debated in the literature [11-16], some difficulties for a straightforward assignement deriving from significant overlaps
Fig. 2. – Diffuse reflectance spectra of the MoO3/SiO2 catalysts with increasing MoO3loadings: a) Mo2S (2.0 wt%), b) Mo4S (4.1 wt%) and c) Mo7S (7.2 wt%).
of the components attributed to Mo6 1 ions in tetrahedral (Td) coordination and to
Mo6 1ions in octahedral (Oh) coordination, and on the dependence of the band position
not only on the local symmetry but also on the molybdenum environment (e.g., structure and dimension of surface aggregates, interaction with the support).
However, on the basis of well established data literature, it seems reasonable to conclude that: a) isolated Mo6 1( Td ) species absorb at 220–225 and 265–270 nm [11, 12]; b) polymolybdate-like structure containing Mo6 1(Oh) ions exhibit bands at 230–235 nm
and at 280–295 nm [13-16]; c) Mo6 1( Oh ) ions in crystalline structure such as MoO 3are
characterized by absorptions in the 300–400 nm range [15, 16].
Following this type of assignment, the spectral pattern of polymolybdate species can be recognized as the dominant features (maximum at 235 nm and shoulder at 285–290 nm) of all the spectra reported in fig. 2. No defined components due to isolated Mo6 1( Td ) species can be observed in the spectra, but the particular broadness of the
spectral features of the Mo2S sample suggests that non-resolved absorptions at 220 and 265 nm could be also present in this case (fig. 2a).
However, the observed predominance of the 235 and 285 nm components for all samples indicates that polymolybdate species are overwhelming in all cases, whereas isolated Mo6 1( Td ) structures, if present, must correspond to a minor fraction of the
supported phase even at low loading.
Such isolated structures should be typical of highly dispersed systems [17, 18], but it can be concluded that by the adopted preparation method [5, 6] they are not produced even in the most diluted samples; noticeably, in the spectrum of the Mo2S sample a weak absorption in the 300–380 nm is observed, indicating that also MoO3
microparticles, characteristic of poorly dispersed systems [15, 16] are present at the lowest loading.
Fig. 3. – FTIR spectra of the bare SiO2(a) and of the Mo2S (b) and Mo4S (c) samples. The spectra are reported in absorbance, having subtracted, as background, the spectra of the samples outgassed at room temperature, where no “strained bridges” are present [19-21].
The overwhelming presence of agglomerated structures could be related to the low catalytic activity exhibited in the MPO reaction by all the MoO3/SiO2systems studied,
which result even less active than the bare carrier [5, 6]. Indeed, they have to be almost inactive towards the MPO reaction, as their presence on the surface causes a decreasing of the intrinsic activity of the support.
A plausible explanation of such poisoning effect was provided by FTIR studies of these systems. The obtained results are reported in fig. 3, where the 930–870 cm21
range of the infrared spectra of the bare SiO2and of the Mo2S and Mo4S samples are
shown.
In this range absorption bands characteristic of surface “strained” siloxane bridges [19-21] are observed after dehydroxylation at high temperature. These structures are edge-shared silicate tetrahedral rings produced by condensation of two adjacent hydroxyls during dehydroxylation at temperature above 500 7C [15], and in a previous study were suggested to contribute to the silica surface activity in the MPO reaction [21] promoting CH4dissociation by the following mechanism:
As reported in fig. 3, the intensity of the bands due to these “strained bridges” is high on the bare silica (curve a) and progressively decreases as the amount of MoO3
This behaviour clearly indicates that the formation of the “strained bridges” is hindered by the supported species, as a consequence of the fact that the -OH surface groups that are the precursors of such structures are consumed in the anchoring process of the supported species. Consequently, if these last are almost inactive, as is the case of polymolybdate species, the resulting systems exhibit a lower catalytic activity than the bare carrier.
A similar effect on the population of strained Si-O-Si structures, not shown for the sake of brevity, was also observed in the case of V2O5/SiO2catalysts. However, in this
case, such bridges are subsituted by isolated tetrahedral vanadium species, which are even more active, and then an overall increase of the catalytic activity occurs.
4. – Conclusions
DR UV-Vis spectroscopic data evidence that, a for similar loading, V2O5/SiO2 and
MoO3/SiO2 catalysts significantly differ in dispersion and structure of the supported
phase.
In the case of V2O5/SiO2systems, an evolution from isolated to clusterized V5 1(Td)
containing species are observed passing from low to high loading, whereas complex polymeric species, containing Mo6 1 ions in octahedral coordination, are found to be
overwhelmingly present in MoO3/SiO2at any loading.
On the basis of the catalytic performances of both types of systems, some suggestions about the relationship between the structure and the activity in the MPO reaction of the supported species can also be derived: a) isolated V5 1(Td) species are
efficient promoters of the MPO reaction, while clustered structures are less active; b) polymolybdate-like species exhibit a poor activity and, as the presence of supported species hinders the formation of the “intrinsic” active sites of SiO2, their presence
causes a decrease of the catalytic performance of these systems in comparison with the bare carrier.
* * *
Financial support of this study by CNR and MURST is gratefully acknowledged.
R E F E R E N C E S
[1] PITCHAIR. and KLIERK., Catal. Rev.-Sci. Eng., 28 (1986) 13. [2] BROWNM. J. and PARKYNSN. D., Catal. Today, 8 (1991) 305.
[3] PARMALIANAA., FRUSTERIF., MICELID., MEZZAPICAA., SCURRELM. S. and GIORDANON.,
Appl. Catal., 78 (1991) 47.
[4] PARMALIANAA., FRUSTERIF., MEZZAPICAA., SCURRELM. S. and GIORDANON., J. Chem. Soc.,
Chem. Commun. (1993) 751.
[5] PARMALIANAA., FRUSTERIF., MEZZAPICAA., MICELID., SCURRELM. S. and GIORDANON., J.
Catal., 143 (1993) 262.
[6] MICELID., ARENAF., PARMALIANAA., SCHURRELM. S. and SOKOLOVSKIIV., Catal. Lett., 18 (1993) 283.
[7] LIUT., FORISSIERM., COUDURIERG. and VE´DRINEJ. C., J. Chem. Soc., Faraday Trans. 1, 85 (1989) 1607.
[8] SCHRAML-MARTHM., WOKAUMA., POHLM. and KRAUSSH.-L., J. Chem. Soc. Faraday Trans.,
87 (1991) 2635.
[9] HANKEW. and BIENERTR., Z. Anorg. Allg. Chem., 414 (1975) 109. [10] CENTIG., PERATHONERS. and TRIFIRO´ F., J. Phys. Chem., 96 (1992) 2617. [11] ASHLEYJ. H. and MITCHELLP. C. H., J. Chem. Soc. A (1968) 2821.
[12] GIORDANON., BARTJ. C. J., VAGHIA., CASTELLANA. and MARTINOTTIG., J. Catal., 36 (1975) 81.
[13] PRALIAUDH., J. Less-Common Met., 54 (1977) 387.
[14] JEZIOROWSKIJ. and KNO¨ZINGERH., J. Phys. Chem., 83 (1979) 1166.
[15] FOURNIERJ., LOUISC., CHEM., CHAQUINP. and MASURED., J. Catal., 119 (1989) 400. [16] WILLIAMSC. C., EKERDITJ. G., JEHNGJ.-M., HARDCASTLEF. D., TUREKA. M. and WACHSI.
E., J. Phys. Chem., 95 (1991) 8781.
[17] IWASAWAY. and OGASAWARAS., J. Chem. Soc., Faraday Trans. 1, 75 (1979) 1465. [18] LOUISC., TATIBOUETJ. M. and CHEM., J. Catal., 109 (1988) 354.
[19] LYGINV. I., Kinet. Catal., 35 (1994) 480.
[20] BUNKERB. C., HAALANDD. M., WARDK. J., MICHALSKET. A., SMITHW. L., BINKLEYJ. C., MELIUSC. F and BALFEC. A., Surf. Sci., 210 (1989) 406.
[21] VIKULOVK., MARTRAG., COLUCCIAS., MICELID., ARENAF., PARMALIANAA. and PAUKSHTIS E. A., Catal. Lett., 37 (1996) 235.
