Langmuir 1986, 2, 568-577
568
Chemical and Spectroscopic Study of the Nature of a Vanadium Oxide Monolayer Supported on a High-Surface-Area Ti02Anatase Guido Busca,*+ Gabriele Centi,I Leonard0 Marchetti,? and Ferruccio Trifirox Istituto Chimico, Facoltci di Ingegneria, Universitci di Bologna, 40136 Bologna, Italy, and Istituto di Technologie Chimiche Speciali, Facoltci di Chimica Industriale, Universitci d i Bologna, 40136 Bologna, Italy Received February 11, 1986. I n Final Form: M a y 7, 1986 Vanadium titanium oxides with a vanadium coverage up to 1.6 of the theoretical monolayer (bidimensional sheet of vanadium oxide on the Ti02 surface), prepared by wet impregnation, grafting, and exchange techniques, are studied by X-ray diffraction, electron spin resonance, infrared, UV-vis-near-IR diffuse reflectance, and chemical analysis of the valence state and of the total amount of the vanadium present, before and after washing with a basic solution. Results indicate that three vanadium oxide surface structures can be formed. At the lower coverages, vanadium reacts with surface OH groups of TiOz, forming strongly interacting and stabilized VIv ions, part of which are coordinativelyunsaturated. This species is present also at higher coverages together with a second vanadium oxide species, interpreted as a bidimensional cluster of Vv ions weakly interacting with the TiOz surface. The vanadium oxide in excess of a monolayer is present as multilayer crystalline Vz05. The nature of the vanadium oxide monolayer is discussed in relation to the presence of these species and to IR analysis of the first overtone of v(V=O), which suggests the presence of V ions with (i) a single oxo group and (ii) a higher frequency of v(V=O) as compared to the bulk VZOb The shift to lower frequencies found by other authors is probably related to the presence of adsorbed water.
Introduction In the recent years increasing attention has been directed toward the study of the modifications induced on the reactivity properties of transition-metal oxides when they are supported on an oxide carrier. There is much evidence showing that the properties of a thin layer of a transition-metal oxide interacting with the support are strongly modified as compared to the properties of the same bulk 0 ~ i d e . l This ~ ~ effect is an illustration of how it is possible to change, at a molecular level, the reactivity properties of a transition-metal ion and it has been shown for several supported oxide catalysts such as M003/A1203,4 W03/A1203,5and, particularly, Vz05/Ti02.6 For the last system, effective for a wide range of catalytic reactions such as the selective oxidation or ammoxidation of aromatic compounds7or NO, reduction with ammonia: there is a general agreement that the optimal catalytic performance is shown by a catalyst where the amount of vanadium present corresponds to that necessary to form a bidimensional layer (called monolayer) of vanadium oxide on the T i 0 2surface and that for greater amounts of vanadium, the presence of crystalline V205,which is substantially inactive in most of the reactions previously cited, can be detected. However, the specific nature of this vanadium oxide monolayer is a controversial matter. Wachs et al.6 indicate that the vanadium oxide monolayer is the active phase in o-xylene oxidation; the vanadia species coordinated to the T i 0 2 support would be Vv and are characterized by a broader v(V=O) stretching band in the Raman spectrum, shifted to lower frequencies with respect to that of bulk Vz05. In studies of the specific reaction of V0Cl3 with titania hydroxy groups, Bond and Bruiickmanngalso have pointed out the peculiar catalytic behavior of the vanadium oxide monolayer and indicated that the monolayer has the same density as a two-dimensional layer of Vz05. Naka*To whom all correspondence should be addressed. Istituto Chimico.
* Istituto di Technologie Chimiche Speciali. 0743-7463/86/2402-0568$01.50/0
gawa et al.IOalso found a shift from 1020 (bulk V205)to 980 cm-l of the V=O bond stretching frequency in the infrared spectra for the vanadium oxide monolayer species on titanium oxide. According to these authors, the vanadium on the surface is present as amorphous V205at low vanadium coverage and amorphous and crystalline V2O5 at high surface vanadium coverage. However, they suggested that in the two cases the concentration of V=O bonds does not change, only their reactivity is modified. In contrast, Miyamoto et aL1* discussed principally the activity and selectivity of the vanadium-titanium catalysts in terms of selective exposure of the (010) plane of Vz05 deriving from the spreading of the vanadium oxide on the Ti02surface. This idea is in agreement with that of Vejux and Courtine12who suggest a close epitactic match between the (010)planes of V205and the predominant (001)growth planes of the TiOz anatase substratum; this situation would result in a high surface density of exposed V=O groups. Accordingly, Gasior and Machej13evidenced the role of the (1) Haber, J. Pure Appl. Chem. 1984, 56, 1663. (2) Volta, J. C.; Portefaix, J. L. Appl. Catal. 1985, 18, 1. (3) Courtine, P. In Solid State Chemistry in Catalysis; Grasselli, R. K., Brazdil, J. F., Eds.; American Chemical Society: Washington, DC, 1985; p 37. (4) Stencel, J. M.; Makovsky, L. E.; Sarkus, T. A.; de Vries, J.; Thomas, R.; Moulijn, J. A. J. Catal. 1984, 90, 314. (5) Soled, S.; Murrel, L. L.; Wachs, I. E.; Mc Vicker, G. B.; Sherman, L. G.; Chan, S. S.; Dispenziere, N. C.; Baker, R. T. K. In Solid State Chemistry in Catalysis; Grasselli, R. K., Brazdil, J. F., Eds.; American Chemical Society: Washington, DC, 1985; p 165. (6) Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Chersich, C. Appl. Catal. 1985, 15, 339. Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Chersich, C. CHEMTECH 1985 (Dec), 756. (7) Wang, W. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Deu. 1984,23, 564. (8) Inomata, M.; Miyamoto, A.; Vi, T.; Kobayashi, K.; Murakami, Y. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 424. (9) Bond, G. C.; Briickmann, K. Faraday Discuss. Chem. SOC.1981, No. 72, 235. (10) Nakagawa, Y.; Ono, T.; Miyata, H.; Kubokawa, Y. J . Chem. SOC., Faraday Trans. 1 1983, 79, 2929. (11) Inomata, M.; Mori, K.; Miyamoto, A.; Vi, T.; Murakami, Y. J. Phys. Chem. 1983,87, 754. Miyamoto, A.; Mori, K.; Inomata, M.; Murakami, Y. Proc. Int. Congr. Catal., sth, 1984 1984, 4, 285. (12) Vejux, A.; Courtine, P. J . Solid State Chem. 1978, 23, 93. (13) Gasior, M.; Machej, T. J . Catal. 1983, 83, 472.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 5, 1986 569
Study of a Vanadium Oxide Monolayer
(010) plane of bulk Vz05suggesting that the active centers for selective oxidation of o-xylene are located on this plane. Kozlowski et al.14 and Haber15 suggest a different picture of the surface vanadium oxide structure, involving a new species being formed at the interface presenting two terminal V=O double bonds and two bridging V-0-Ti bonds; such a structure looks very similar to that proposed on other supported oxide catalysts such as Mo03/A12034 and wo3/&03.5 This new species would more easily give up oxygen in a selective oxidation yielding a V”’ species; no VIv species are taken into account in this model,15in agreement with Wach’s opinion.6 However, the presence of VIv species on the surface of a V monolayer was suggested on the basis of visible diffuse reflectance spectrOecopyl6and chemical and ESR analyses.l7Ja This agrees with the stabilization of the valence four state of vanadium by interaction with the TiOz surface, found by Hermann et al.19 Summarizing all these literature data, three types of hypotheses have been advanced to explain the superior catalytic performances of the vanadium monolayer species. The titanium oxide support (i) induces a preferential exposure of V=O double bonds, (ii) modifies the strength of V=O double bonds, and (iii) induces an unusual vanadium oxide surface structure. In order to try to clarify what types of vanadium oxide species form in the interaction of vanadium oxide with TiOz, near monolayer vanadium oxide on a pure highsurface-area anatase TiOz support was prepared by different techniques. The support was chosen in order to have as high a concentration of the surface vanadium oxide species as possible on a highly pure support, as a model system of the active phases present in real catalysts. Both physicochemical and chemical techniques of analysis were utilized for their characterization. Experimental Section Preparation o f Samples. Supported vanadium-titanium samples were all prepared from the same pure rutile-free anatase powder (CLDD 1764/2 from Tioxide Ltd.), having 117 m2/g surface area. Wet impregnated samples (VTIO1, VTI05, VTIOS, and VTI16) were prepared by impregnation of TiOz with a hot (373 K) solution of NH4V03 (Merck) in double-distilled water, drying a t 420 K and dcinating in air for 3 h at 720 K. The amount of ammonium vanadate used corresponds to that necessary to have a final composition in the calcinated samples of about 1%, 5%, 9%, and 16% wt of V205,respectively. “Grafted” sample^^^'^ (VTG) were prepared by specific reaction of VOC13 in hot (373 K) anhydrous benzene with surface hydroxyl groups of Ti02 (previouslyhydrated by reaction at 420 K with double-distilled water); the amount of V0Cl3 (Aldrich) used corresponds to that necessary to have 16% wt of Vz05 in the final sample in the case of complete reaction with TiOP After they were filtered and dried, the samples were hydrolized by the addition of a few drops of double-distilled water to the hot solid. Finally the samples were calcined a t 720 K. “Exchanged” samples (VTE) were prepared by the exchange of Ti02with a VOC12.2H20(Carlo Erba) solution is isopropyl alcohol, followed by hydrolysis and calcination as in the case of the VTG samples. Experimental Techniques. Chemical analysis: The V-Ti sample, 0.5 g, was moistened with 50 cm3 of saturated NaOH (14) Kozlowski, R.; Pettifer, R. F.; Thomas,J. M. J. Phys. Chem. 1983, 87, 5176. (15) Haber, J. Proc. Znt. Congr. Catal., 8th, 1984 1984,1, 85. (16) Busca, G.; Marchetti, L.; Centi, G.; TrifirB, F. J. Chem. SOC., Faraday Trans. 1 1985,81, 1003. (17) Gasior, M.; Gasior, I.; Grzybowska, B. Appl. Catal. 1984,10,87. Rusiecka, M.; Grzybowska, B.; Gasior, M. Appl. Catal. 1984, 10, 101. (18) Meriaudeau, P.; Vedrine, J. N o w . J. Chim. 1978, 2, 133. (19) Herrmann, J.-M.; Vergnon, P.; Teichner, S.J. Bull. SOC.Chim. Fr. 1976, 7-8, 1056.
Table I. Characterization of the V-Ti Oxides by Chemical Analysis and Surface Area Measurements
catalyst VTIOl VTI05 VTIO9 VTI16 VTG VTE Vz05coverage, wt % surface area, m2/g fraction of theoretical monolayer total Vz05, % extracted in a basic medium V,05 coverage after extraction, wt % fraction of theoretical monolayer after extraction
1.3 95 0.09
5.2 79 0.42
9.6 63 0.94
16.4 57 1.62
8.3 66 0.79
2.7 91 0.20
0
38
64
79
55
0
1.3
3.2
3.4
3.4
3.4
3.7
0.09
0.26
0.33
0.33
0.35
0.20
solution and then filtered after 10 min under agitation. The amount of vanadium was then determined separately in the filtered solution and in the residual sample which was dissolved in an appropriate concentration of boiling H2S04aqueous solution. The total amounts and valence states of vanadium in the two solutions were determined by titration according to a previously reported method.20 Standard tests were performed to verify the stability of the valence state of vanadium upon dissolution of the sample. A Cary 14 spectrophotometer with a diffuse reflectance attachment, a Varian E4 X-band first-derivative ESR spectrometer, and a Nicolet MX 1Fourier-transform IR spectrophotometer were used, all connected with conventional gas manipulation and evacuation ramps and measurement cells similar to those reported in literature.21r22 Reflectance spectra were digitalized and converted in the Kubelka-Munk function22[F(R’,)], which is proportional to the absorption coefficient,using MgO as the reference standard. All spectra were recorded in the presence of O2to avoid, by quenching, possible effects of fluorescence which artificially increase the reflectance value.22 X-ray diffraction patterns were recorded with a Philips diffractometer using Ni-filtered Cu K a radiation. Surface areas were determined by the BET method with nitrogen adsorption using a Carlo Erba Sorptomatic instrument.
Results Surface Area. Table I reports the surface areas of the samples after calcination at 720 K. From the initial value of 117 m2/g for the pure TiOz support, a decrease in the surface area, almost proportional to the amount of vanadium present, is observed in the V-Ti oxides. By use of these data to calculate the surface area per gram of TiOz support and from the assumption that, according to Bond et al.9 and Roozeboom et al.,23the theoretical monolayer to be the amount of VOz.5units necessary to have complete coverage of the TiOz surface, the fraction of a theoretical monolayer present in our catalysts after calcination was calculated (Table I). Chemical Analyses. The chemical analyses of the amount of vanadium present in the V-Ti oxides are reported in Table I for the various samples together with the amount of vanadium remaining on the catalyst after extraction with a saturated aqueous solution of NaOH. All the extracted vanadium is present as Vv, as determined (20) Centi, G.; Fornasari, G.; Trifirb, F. J. Catal. 1984, 89, 44. (21) Busca, G.; Saussey, H.; Saur, 0.; Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985,14, 245. (22) Stone, F. S. In Surface Properties and Catalysis by non-Metals; Bonnelle, J. P., Delmon, B., Derouane, E., Eds.; Reidel Dordrecht, 1983; p 237. Peri, J. B. In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 5, p 171. Schoohlydt, R. A. In Characterization of Heterogeneous Catalysts; Delannay, F., Ed.; Dekker: New York, 1984; p 125. Vedrine, J. C. In Characterization of Heterogeneous Catalysts; Delannay, F., Ed.; Dekker: New York, 1984; p 161. (23) Roozeboom, F.; Mittelmejer-Hazeleger, C.; Moulijn, A.; Medema, J.; de Beer, V. H. J.; Gellings, P. J. J. Phys. Chem. 1980, 84, 2783.
570 Langmuir, Vol. 2, No. 5, 1986
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Table 11. Valence State of Vanadium (As Determined by Chemical Analysis) in the V-Ti Oxides after Different Treatments catalyst VTIOl VTI05 VTIOS VTIOS VTIOS VTG VTG VTI16 treatment calcination in air at 720 K X X X X X X X X X X extraction in a basic medium X X torr) evacuation at 720 K 0
0
0
0"
66 34
68 32
21 79
62" 38"
10 55
0 18
35
82
11
0"
60 29
61" 39"
Percentage of the total amount remaining after extraction.
v
Ti02tanatase)
I
"'2'5
I
Figure 2. EPR spectra of samples calcined a t 720 K in air: (A) pure TiOz, (B) VTIO1, (C) VTIOS, (D) VTI16, and (E) Vz05.All spectra are recorded in vacuo a t 150 K. 65
55
45
35
25
29
15
Figure 1. XRD patterns of VTIOS (a) and VTI16 (b) samples calcined in air at 720 K.
by titration; in contrast, the vanadium remaining on the catalyst is mainly present as VIv (Table 11). The evacuation treatment utilized in EPR and in some IR measurements (evacuation at loT5torr at 720 K) also modifies the valence state of vanadium, inducing the reduction of Vv with the formation of VIv and a small amount of VI1' (Table 11). X-ray Diffraction (XRD). The XRD patterns of the VTIOS and VTI16 samples after calcination at 720 K are compared in Figure 1. In the case of the VTIOS sample, only the lines of Ti02 anatase (ASTM 4-0477) are present and there is no evidence of the presence of a rutile T i 0 2 phase (ASTM 4-05551). In the VTI16 sample together with the reflections of anatase, new lines appear at d = 4.38, 3.40, and 2.88 A which may be attributed to the presence of crystalline Vz05 (ASTM 9-387). Electron Spin Resonance (ESR). Reported in Figure 2 are the ESR spectra, recorded at 150 K, of the pure TiO, support, of samples VTIO1, VTIOS, and VTI16, and of pure crystalline V205. All samples were treated in vacuo torr) for 3 h at 720 K in order to obtain complete desorption of water. Under these conditions two signals occur for TiO,; the sharp single feature at g = 2.003 is assigned to localized conduction band electrons, while the much broader one at g = 1.947 is due to Ti3+ions24produced by partial reduction during heating in vacuo. The latter signal (24) Servicka,E.; Schlierkamp, M. W.; Schindler, R. N. Z . Naturforsh., A 1981,36A,226. Gravelle, P.C.; Juillet, F.; Meriaudeau, P.; Teichner, S. J. Faraday Discuss. Chem. SOC.1971,No. 52, 140.
I
mi
4 / I
--
40
A
* . g; 1.922 t tg;, 1.920
A;I 192 G
,
A',, 166 G '
Figure 3. EPR spectra of the VTIOl sample after different torr) treatments: (A) spectrum in air, (B) after evacuation at 470 K, after (C)evacuation at 720 K, and (D) in the presence of 10 torr of water. Spectra recorded a t 150 K.
is no longer present in sample VTIO1, where an 8-foldhyperfine signal due to 51V'v species is observed. This signal is again observed for samples VTIOS and VTI16 but is progressively superimposed by another signal centered at g = 1.970 which lacks hyperfine splitting and is rather broad (called signal C). Signal C also is observed on pure
Langmuir, Vol. 2, No. 5, 1986 571
Study of a Vanadium Oxide Monolayer
A)
I
1
'g,l. 920
Figure 5. EPR spectra of (A)sample VTI16 after washing with a basic solution and (B) sample VTI16 before (B.a)and after (B.b) reduction with butadiene at 670 K. All spectra are recorded at 150 K.
Figure 4. EPR spectra at 150 K of samples VTG (A)and VTE (B). V205under the same conditions and may be assigned to magnetically interacting VN species in a slightly reduced V2O5 phase. Reported in Figure 3 are the ESR spectra of sample VTIOl after different treatments. The hyperfine signal is already observable at room temperature and is sensibly strengthened by cooling down to 150 K. A further increase in its intensity is obtained by evacuation at 520 and 720 K. Under these conditions, two sets of parallel hyperfine components are clearly observable, which have a clearly distinguishable splitting due to a different parallel coupling constant. Perpendicular components are also apparently multiple, but they cannot be clearly distinguished. Exposure to water vapor causes the disappearance of the set of parallel hyperfine components characterized by the higher value of A,, (Figure 3D) (this signal is called A). The spectra of samples VTG and VTE after evacuation at 720 K for 3 h are reported in Figure 4. Analogous to that observed in Figure 1for sample VTIO9, the spectrum of V?'G derives from the overloading of a signal with hyperfine structure to a broad signal centered at ( g ) = 1.970 (signal C ) . In contrast, only the signal with hyperfine structure is present in sample VTE. The signals with hyperfine structure are substantially identical and similar to those observed on VTIOl after evacuation at 720 K and following exposure to water vapor. In both cases, only one single set of parallel components is observed, related to the smaller parallel hyperfine coupling component (this signal is called B). The ESR parameters, as calculated from this spectrum are as follows (signal B): gl = 1.991, A1 = 70 G; g2 = 1.984, A2 = 84 G; g3 = 1.920, A3 = 166 G . For the surface sensitive signal A, which disappears with water adsorption, only the parallel parameters may be evaluated from these spectra. However, in other samples
obtained by a different preparation procedure,25the same signal is more clearly observable and also allows the identification of the perpendicular parameters: g, = 1.991, A , = 72 G ; g,, = 1.922, A,, = 192 G. The spectrum of sample VTI16 after it has been washed with a basic solution is reported in Figure 5. In the same figure, the spectrum after a reduction treatment with butadiene at 750 K is also reported. The washing treatment causes a considerable decrease in the intensity of the broad signal C, signals A and B now being more intense and predominant. The reducing treatment causes a greater decrease in the intensity of the ESR signal; after reduction, signal B is still observable, whereas signal C completely disappears. Fourier Transform Infrared Spectra (FT-IR). 1. KBr Disk Technique. Reported in Figure 6 are the Ft-IR spectra in air of calcined vanadium-titanium oxide samples after subtraction of the contribution of the support. The VTIOl spectrum shows a weak and relatively broad absorption near 980 cm-l. This band further increases in intensity with increasing vanadium concentration in the catalyst, but weak shoulders appear at 940 and 890 cm-'. Instead, a further band at 1020 cm-' is predominant on VTI16, together with those already cited. The band at 1020 cm-', rather weak, also is observed on VTG; however, in this sample the shoulders at 940 and 890 cm-' are not present. In both cases (samples VTI16 and VTG), the band at 1020 cm-l disappears after a washing procedure with a basic solution, but the band a t 980 cm-' is still observable after such a treatment. It is worth noting that in crystalline Vz05, a strong band centered at 1019 cm-' and a very weak shoulder at 985 cm-' are present in the 1100-850-cm-' range, both assigned to v(V=O) stretching modes.26 The band at 980 cm-l present in ( 2 5 ) Busca, G.; Cavani, F.; Trifir6, F., manuscript in preparation.
Busca et al.
572 Langmuir, Vol. 2, No. 5, 1986
2100
2000
v,cm.l
1900
Figure 7. FT-IR spectra (pressed disk technique) in the 2v(V==O) region of V-Ti oxides: (a) VTI05 evacuated at 670 K, (b) VTIOS evacuated at room temperature, (c) VTIOS in vacuo, (d) VTI16 in vacuo, and (e) Vz05 in vacuo. 10120
L -l a 1 4
1100
950
E--
.1
~
~
(26) Fabbri, G.; Baraldi, P. Anal. Chem. 1972, 44, 1325. (27) Busca, G.; Lavalley, J. C. Spectrochim. Acta, Part A 1986, 42A,
443.
So, both these features are to be attributed to surface vanadium species. It is worthwhile to note that in the case of crystalline Vz05,three u(V=O) overtones are observable (Figure 7, spectrum e) at 2025, 1990, and 1975 cm-' corresponding to the poorly resolved fundamentals observed at 1020 (strong) and 985 (shoulder) cm-' in KBr.26 In the case of vanadium titanium oxide with low vanadium coverage, therefore, the observation of only one single overtone band allows the exclusion of its assignment to bulk V205; moreover, the presence of only one single overtone mode for u(V=O) indicates the presence of a single V 4 bond. No evidence was found for the presence of surface dioxovanadium groups suggested by Haber15in the low vanadium coverage samples. In such species, in fact, two V=O fundamentals are expected (u,(V=O) and u,(V=O)); separated by about 10-20 cm-1,28that would produce a resolved triplet or, at least, a broad overtone band. With increased vanadium coverage, the maximum of the first u(V=O) overtone falls at similar frequencies (2400 cm-') but a weak shoulder with a maximum centered around 2020 cm-' is present at lower frequencies in the VTI16 sample. This shoulder, according to the spectrum of crystalline V205(spectrum e), may be interpreted as due to the presence of V205 together with the vanadium monolayer, in agreement with IR data (KBr technique) and with XRD data. The FT-IR spectra in the OH stretching region for the VTI05 sample are reported in Figure 8 for different evacuation treatments. After evacuation at room temperature, a sharp band centered at 3650 cm-' is superimposed on a very broad absorption band in the 30003600-cm-' region. The broad absorption progressively disappears upon evacuation, revealing the single band at 3650 cm-l. According to our previous discussion concerning different pure TiOz supportsz1and vanadium titanium oxide samples,16this band was assigned to free OH groups bonded to V (V-OH). However, after evacuation at higher temperature (670 K), corresponding to that use for the other spectroscopic measurements, the v(OH) band at 3650 cm-' disappears, indiating that under these conditions (28) Ahlborn, E.; Diemann, E.; Mtiller, A. 2.Anorg. Allg. Chem. 1972, 394, 1. Satyanarayana, D. N. Bull. Chem. SOC.Jpn. 1964,37, 1736.
Langmuir, Vol. 2, No. 5, 1986 573
Study of a Vanadium Oxide Monolayer
'-
I
I
.
3
0
15
25
I
v.103 c m "
I
I
I
4000
,
,
,
,
I
,
3500 v,cm-'
3000
Figure 8. FT-IR spectra (pressed disk technique) in the v(OH) region of the sample VTI05 after different evacuation treatments: (a) evacuation at room temperature, (b) evacuation torr) at 470 K, and (c) evacuation at 670 K.
I
1700
1600 y,cm-'
1500
I
Figure 9. FT-IR spectra (pressed disk technique) in vacuo of the VT105 (-) and VTIOS (- - -) samples. neither surface-bonded water molecules nor free OH'S are present on the vanadium titanium oxide catalyst surface. Shown in Figure 9 are the IR spectra of evacuated VTIO9 and VTI05 samples in the 1450-1650-cm-' region. In addition to the peak at 1610 cm-l related to the ammonia formed during preparation (decomposition of NH4V03),which remains adsorbed on the catalyst surface even after evacuation, a relatively broad absorption is present in the 1500-1600-cm-' range. This weak absorption increases in intensity with increasing vanadium content in the catalyst and it is not present in the samples with low amounts of vanadium (VTIO1 and VTE). It is worth noting that in pure Vz05a broad absorption centered at 820 cm-' is present.26 This band is attributed to a v,(VOV) stretching mode of V-0-V bonds, but the presence of a weak bond between the vanadium ions and the titanium surface probably results in a shift of the stretching frequency of this bond to lower frequencies. The direct observation of this band in V-Ti oxides is prevented by the strong absorption of the Ti02 support in this region. However, on the basis of these considerations, the broad absorption in the 1500-1600-cm~1 region may be tentatively attributed to the first overtone of the V-0-V bond interacting with the titania support on the samples with medium coverage. Diffuse Reflectance (DR) Spectra. Reported in Figure 10 are the DR spectra in the 35 000-10 OOO-cm-' region of sample VTI16 after different treatments and of pure TiO,. In addition to the strong absorption in the ultraviolet region (27 000-35 000 cm-l) attributed to an
Figure 10. DR spectra in the UV-vis-near-IR region of the sample VTI16 after different treatments (spectra 1-4) and of pure Ti02 (spectrum 5): (1) sample after calcination in air at 720 K, (2) sample after evacuation at 10-4 torr for 3 h at 750 K, (3) sample after evacuation at 1r2torr for '/z h at 720 K, (4) sample calcined in air after washing with a basic solution, and (5) pure TiOz support. electron transition from the valence band to the upper band of the TiOZa and present also in pure TiOz (spectrum 5), a broad absorption in the 20 000-28 000-cm-' region and a weaker absorption in the 11000-18 000-cm-' region are present in the VTI16 sample calcined in air a t 720 K (spectrum 1); thus, these absorptions may be attributed to the electronic transitions of vanadium compounds. In general, the electronic spectra of vanadium ions3*in the examined region are characterized for the vanadium(V) ions by the LCT transition (related to charge transfer between vanadium and terminal oxygen3') falling in the 20 000-25 000-cm-' region for octahedral coordination; for lower coordinations such as tetrahedral vanadium(V), the LCT transition is expected at higher frequencies (30 00035000-cm-' region). The LCT transition for vandium(IV) ions falls at higher frequencies (35OOCMO 000-cm-l region), whereas the d-d transitions of octahedral V02+ ions fall in the region examined: the b,(d,,) e(dxy,dyz) near 13000 cm-' and the b,(dx ) bl(dxz-,,z)near 16000 cm-'. The d-d transition a t higier frequencies (b2(dxy) al(dx2)) is generally maskedN by CT transitions. The intensities of these d-d transitions are generally about 10 times lower than those of the CT transitions. On the basis of the previous discussion, the absorption in the VTI16 sample after calcination in air (Figure 10, spectrum 1)may be attributed to LCT transitions of octahedral vanadium(V) ions (20000-25000-~m-~ region) and to d-d transitions of octahedral V02+ ions (1100018000-cm-' region) partially covered by the broad CT bands. The evacuation treatments at 720 K under various conditions (spectrum 3, lo-, torr, '/, h; spectrum 2, lo4 torr, 3 h) strongly modify the electronic spectrum of the VTI16 sample. A strong increase in the 10000-20000-~m~~ region is found together with a modification of the absorption in the 20 000-28 000-cm-' region. According to data from the literature30 indicating that octahedral V"' ions show two absorption bands near 25 000 (more intense) and 18000 cm-l, corresponding to 3T1,(F) 3T1,(P)and to 3T1g 3T2 transitions, respectively, and according to that previody reported for the bands of VIv, increased absorption between 13000 and 20 000 cm-' in the spectra of sample VTI16 after evacuation treatments may indicate
-
-
-
-
-
(29) Iwaki, T.; Miura, M. Bull. Chem. SOC.Jpn. 1971,44, 1754. (30) Hush. N. S.: Hobbs,R. J. M. Prom. Znora. Chem. 1968.10.259. Kidg,'E. F.; Good,'M. L. &'pectrochim.-Acta, Part A 1973, 29A; 707. Selbin, J. Chem. Rev. 1965, 65, 153. (31) So, H.; Pope, M. T. Znorg. Chem. 1972,11,1441. Iwamoto, M.;
Furukawa, H.; Mataukami, K.; Takenaka, T.; Kagawa, S. J . Am. Chem. SOC.1983,105, 3719.
Busca et al.
574 Langmuir, Vol. 2, No. 5, 1986
I
0'
2 5 \
25
\
"\\
20
I
v .to3cm.' Figure 11. DR spectra in the vis region of V-Ti oxides calcined in air at 720 K after subtraction of the TiOz contribution to the spectrum: (1)VTIO1, (2) VTE, (3) VTI05, (4) VTG, (5)VTIOS, (6) VTI16, and (7)mechanical mixture of 16% w t V,05 and TiOz.
the appearance of reduced vanadium or the formation of nonstoichiometric oxide layers, in agreement with chemical analyses (Table 11). After the washing procedure with a basic solution (spectrum 41, a strong decrease of the absorption in the 20 000-28 000-cm-' region (vanadium(V) ions) is found with a shift of the maximum (after subtraction of the TiOz contribution) from 23 000 (spectrum 1)to 25000 cm-l (spectrum 4). In contrast, no substantial change is found in the absorption at lower frequencies related to crystal field transitions of V02+ ions. These results agree with the chemical analysis data (Table 11). Reported in Figure 11 are the DR spectra of calcined vanadium titanium oxide samples after substraction of the contribution due to TiOz and, for comparison, the spectrum of a mechanical mixture of 16% wt V206and TiOz (spectrum 7). This procedure of subtraction of the support absorption, based on the assumption that the absorption edge in V-Ti02 is unperturbed as compared to that in pure Ti02, may be taken as correct because we are concerned only with supported catalysts; the anatase conduction band is expected to be unperturbed. As previously discussed, the absorption in this region is related to LCT bands of octahedral Vv. Increasing the amount of V in the sample causes a change in the absorption maximum. For the lower vanadium coverages, where ESR analysis shows only noninteracting vanadium ions, a single maximum around 25000 cm-' is found. With further increases in the amount of vanadium, the maximum of the absorption is shifted to lower frequencies (around 23000 cm-'), but it is still present as a shoulder around 25000 cm-'. It is worthwhile to note that washing the samples with a basic solution drastically decreases the absorption at 23 000 cm-', and only a weaker absorption centered at 25 000 cm-l remains. In addition to these two absorptions, in the VTI16 sample and less intense in the VTG sample, a further shoulder around 21 O00 cm-l is observed. It is worth noting that in crystalline V205(spectrum 7), the LCT band is found at 21000 cm-' and that in these two samples (VTI16 and VTG), IR and/or XRD analyses show the presence of crystalline V206. Discussion ESR Spectra. Vanadium(IV), both in the form of vanadyl ions (V02+in a square-pyramidal or octahedral coordination sphere) and as V4+ ions in octahedral and tetrahedral environments, is generally detectable3*by ESR even at room temperature. Cooling causes an increase of
the signal intensity and a sharpening of the hyperfine components due to a decreased effect of the relaxation processes that cause line broadening. In the cases where tumbling motion averages the anisotropic effect at room temperature, cooling may also cause the appearance of the anisotropy of the signal. Only in a few cases, related to the relatively rare tetrahedral have very effective relaxation processes been reported which prevent the observation of vanadium(1V) at temperatures similar to those actually obtained in our apparatus (down to 100 K). Our experimental conditions, therefore, appear to be reliable at least for the qualitative analysis of V", while an accurate quantitative evaluation is prevented by the different effects of relaxation phenomena involving the different species. However, the main problem is whether or not the ESR analysis may be representative of the surface situation of vanadium. Chemical analysis data show that VIv is indeed the predominant valence state present in the part of the vanadium which is not extracted by the washing procedure. Furthermore, the evacuation pretreatment used for ESR and some IR experiments causes a strong further lowering of the content of Vv with formation of VIv and also small amounts of V"'. This is confirmed by the DR spectra where a drastic change occurs which may be interpreted as due to the reduction of Vv and the formation of VIv and VIr1. The reduction can be reversed by oxidation and the reduction-oxidation cycle can be made several times without substantial changes in the species present, suggesting that this type of treatment modifies the valence state of vanadium but not the nature and coordination of the species present, in agreement also with DR analysis. These data, therefore, suggest that although only vanadium(1V) species could be detected, the ESR analysis may be successfully applied to the analysis of the surface structure of vanadium in V-Ti oxide catalysts. Three different types of VIv species (called A, B, and C) were identified from ESR analysis. The value of the isotropic hyperfine coupling constant in signals A and B indicate a relevant asymmetry of the coordination sphere of vanadium(IV), similar to that of vanadyl (V02+)complexes. Signal A, apparently associated with axial symmetry, shows a large dipolar parallel hyperfine coupling constant, indicating a probable high ionicity of the in-plane V-0 bonds, as well as an appreciable delocalization of the electron on the vanadylic oxygen. Signal A, which is much weaker than signal B, disappears on adsorption of water, while signal B is unperturbed. We may suppose that the species responsible for signal A is transformed in another form, undistinguishable from that responsible for signal B, even if its transformation into a species undetectable is such conditions cannot be excluded. According to our hypothesis, the perturbation of the ESR signal consists in a lowering of the symmetry (from axial to orthorombic) and a lowering of the values of A,,as well as a small lowering of (g). Such perturbation appears too strong to be justified by simple hydrogen bonding, but it is similar to that undergone by the ESR signal of VO(acac) when the 6th coordination is saturated.33 This supports the identification of species A as a coordinatively unsaturated surface vanadyl (V02+) ion. Signal B is not sensibly perturbed by water adsorption and may be due to coordinatively saturated vanadyls. The (32)Siegel, I. Phys. Rev. 1964,134,A 193. Ballhausen, C.J.; Gray, H. B.Znorg. Chem. 1962,1, 111. Selbin, J. Chem. Rev. 1965,65, 153. (33)Bramman, P.F.;Lund, T.; Raynor, J. B.; Willis, C. S. J. Chem. SOC.,Dalton 1976,45.Guzy, C.M.;Raynor, J. B.; Symons, M. C. R.J. Chem. SOC.A 1969,2791.
Study of a Vanadium Oxide Monolayer
Langmuir, Vol. 2, No. 5, 1986 575
coordination sphere of vanadium may be completed both by surface anions of the support as well as by adsorbed water, in a similar way to that proposed for vanadia-ilica catalysts on the basis of ENDOR results.34 Signal C is clearly due to near-lying vanadium centers (also supposed to be in the form of vanadyl) that interact with each other magnetically. Due to the lack of hyperfine splitting, little information may be obtained from the spectrum; however, it is worth noting that this species is related to the part of vanadium present on the surface which is soluble in a basic medium, as shown by the variation in the ESR spectra after the washing procedure, as well as by the absence of such a signal in the VTIOl and VTE samples. Surface Structure of V-Ti Oxides. The presence of vanadium on the surface of Ti02induces a decrease in the surface area almost proportional to the amount of vanadium present in the catalyst. Using these data to calculate the surface area per gram of Ti02support and assumin823 the theoretical monolayer to be the amount of V02,5units necessary to have complete coverage of the Ti02 surface (corresponding to 0.1466% wt of V205per m2 of support), it is possible to calculate the fraction of theoretical monolayer present in our catalysts after calcination; the results obtained show a coverage from 0.09 to 1.6 times higher than the monolayer (Table I). In this range of vanadium coverage, different surface structures of vanadium are found. X-ray diffraction analysis shows that crystalline V205 is present only when the amount of vanadium exceeds the amount necessary for the theoretical monolayer. In agreement, a new band appears at 1020 cm-' in the IR spectrum (KBr disk technique), characteristic of the more intense stretching mode of V = O bonds in crystalline V203 Furthermore, the presence of a weak shoulder in the DR spectra and in the IR spectra (pressed-disk technique) is also in agreement with the presence of bulk V205. For lower vanadium coverages, the presence of V205 is not found with XRD analysis nor by the other spectroscopic techniques used. Up to the theoretical monolayer amount, the vanadium is thus stabilized by interaction with the T i 0 2 surface in a form not detectable as bulk V205. It should be noted that the utilization of a high-surface-area TiOz support, as compared to the 10-20 m2/g of surface area usually used in the preparation of V-Ti catalysts: enhances the resolution and detectability of the types of vanadium oxide present. Nevertheless, no indication of the presence of bulk V205in the vanadium coverage region below the vanadium oxide monolayer was found. This constitutes further evidence of the spreading effect of the Ti02 surface on the vanadium oxide as previously suggested.6-'0 However, different species of vanadium can be found in the vanadium oxide monolayer region. Washing with a basic solution partially dissolves the vanadium present on the surface and increasing percentages are dissolved by increasing the amount of vanadium deposited on the Ti02. However, it is worth noting that the fraction of theoretical monolayer after extraction is almost constant about 0.33 (Table I). In the catalysts with lower amounts of vanadium deposited (VTIO1 and VTE), (i) no vanadium is extracted in the basic medium and (ii) ESR spectroscopy shows only the presence of magnetically isolated vanadium species. Furthermore, in the case of the catalysts containing amounts of vanadium higher than 3% wt V205,after the washing procedure, signal C in the ESR spectra (lacking
hyperfine splitting and assigned to magnetically interacting VIv species) disappears, whereas the hyperfine signals A and B are still observable. Chemical analysis of the valence state of vanadium after extraction shows that it is largely present as VIv, whereas the dissolved vanadium is almost all V". DR spectra indicate that (i) in the VTIOl samples, the LCT band of V(V) is centered around 25000 cm-', whereas in the samples with higher vanadium coverages the maximum is shifted toward lower frequencies (around 23 000 cm-l), even though a shoulder at 25 OOO cm-l is still observable, and (ii) washing the sample in a basic medium causes a considerable decrease in the absorption in this region and only a weaker band with a maximum centered at 25000 cm-' remains. All these data suggest that different vanadium oxide species are present on the surface of V-Ti oxides at different coverages. At low coverages isolated species of vanadium (mainly as VIv) are present; as coverage increases, V,O, clusters appear which contain near-lying vanadium ions; at coverages higher than the vanadium oxide monolayer the formation of crystalline V205begins. a. Low Coverage. In this region the ESR analysis of VTIOl shows the presence of two species with hyperfine signals (signals A and B). As noted above, the two signals may be assigned to two V02+ species, the first lacking coordination while the second is coordinatively saturated. It is worth noting that these signals are present not only in the catalysts prepared by wet impregnation but also in those prepared by ionic exchange (VTE) and by grafting at low vanadium coverages.16 In these two cases, the preparation involves a specific reaction between V0C12 (VTE) or VOCl, (VTG) with the hydroxyl groups present on the surface of Ti02. It is thus reasonable to conclude that those species showing hyperfine structures and thus attributable to isolated vanadium ions may derive from the specific interaction of vanadium with the OH surface groups of titania and that such vanadium oxide species are still present even when the surface coverage is nearly complete. This conclusion is suggested by (i) the ESR signal of VTIO9 showing a hyperfine signal superimposed on the broad signal C and (ii) by the ESR after the washing tests. In agreement, we have previously observed for this first vanadium oxide species (i) insolubility in a basic medium, (ii) a strong stabilization of valence four of vanadium, and (iii) a higher frequency for the LCT band. All these indications are consistent with the proposal that specific reaction between vanadium ions and the OH surface groups of Ti02 gives rise to a strong interaction between V and Ti02. However, it should be noted that the relative amount of this first species is probably related to the amount of OH groups present on the surface; this value depends on the specific nature of the Ti02 (impurities, type of preparation, and treatments). In a previous study we found16 that on Ti02 with about 5% of sulfate ions, the number of OH groups present is very low and that the complete reaction of VOC13with all surface OH groups occurs for a vanadium coverage corresponding to about 0.04 of the theoretical monolayer. Furthermore, recently it has been pointed out by Morishige et that in the hydrated surface of a very pure Ti02anatase the population of hydroxy groups is smaller than that of strongly adsorbed molecular water, which covers a large fraction of the Ti02surface. According to these authorss this effect is only slightly influenced by the crystal structure, i.e., by the type of exposed crystalline planes. This observation is in contrast to previous assumptions that the surface of
(34)Narayana, M.;Narasimhan, C. S.; Kevan, L. J. Catal. 1983,79, 237.
(35) Moriahige, K.; Kanno, F.; Ogawara, S.; Sasaki, S. J. Phys. Chem. 1985,89,4404.
576 Langmuir, Vol. 2, No. 5 , 1986
anatase mainly consists of a (001) crystal plane on which water molecules dissociate to give hydroxy groups.15 This indicates that also in our case the amount of hydroxy groups present on the TiOz surface and available for the specific reaction with vanadium is smaller than the amount calculated on the basis of crystallographic considerations of selective exposition of a completely hydroxylated (001) crystal plane. This amount (about 10 OH nm-2 for a (001) plane) would give rise to complete reaction of all vanadium ions of the theoretical monolayer (about 9.7 V nm-2) with OH surface groups. The value of 0.33 of the theoretical monolayer that we found, seems therefore more in agreement with the number of OH free groups present on TiOz anatase as deduced on the basis of the determinations of Morishige et Finally, the model of Morishige et al.35 also is more in agreement with the observation (Table I) that changing the method of impregnation (wet or anhydrous-VTI and VTG) does not cause great variations in the fraction of vanadium oxide still present after extraction. b. Coverages Near the Theoretical Monolayer. In this region, a second vanadium oxide species is present. EPR signal C indicates that the VIv species are magnetically interacting, while IR analysis shows the presence of a weak absorption in the 1500-1600-~m~~ region which may be tentatively attributed to the first overtone of the V-0-V bond-stretching mode. However, no evidence was found of bulk Vz05or other vanadium oxides and CO adsorption (followed by IR spectroscopy36)indicates that titanium ions are not exposed on the surface; thus, the formation of bidimensional vanadium oxide clusters is deduced. This vanadium oxide species may be dissolved in a basic medium, suggesting a weakened interaction with the titania support as compared to previous isolated vanadium ions. However, the spreading effect observed suggests that an oxide-oxide interface interaction occurs. In fact, an enhancement of surface free energy when a bulk multilayer oxide is converted to a bidimensional sheet is expected. Thus, the spreading effect is justified by the reduction of the total free energy by such a second driving force. In agreement, the DR spectra may be interpreted as a decrease of the interaction of V with the TiOz surface and chemical analysis shows that almost all of the vanadium is present as V" even if it may be partly reduced by the evacuation treatment. The weak interfacial interaction would be sufficient to cause a considerable change in the properties of the V=O bond, such as stretching frequency (IR analysis) and the LCT band (DR analysis), as compared to bulk crystalline VZO,. c. Coverages Higher Than the Theoretical Monolayer. In this region, all techniques also showed the presence of bulk crystalline V205,which seems substantially no different than unsupported Vz05. XRD anslysis also gave no evidence of a particularly exposed crystalline plane nor of a quasi-amorphous structure (very small particle dimensions). This means that the Vz05particles are multilayer. On the other hand, they are evident even a t coverages only 0.6 higher than the monolayer so that the entire process cannot be interpreted merely as a consecutive formation of a second layer on the first (mono)layer, since in this case the V205particles would not be detectable by XRD. This is a further indication that in the absence of specific interaction with the Ti02surface, the driving force is only the reduction of free surface energy and thus the formation of multilayer crystalline bulk Vz05 particles. (36) Busca, G. Langgmuir, in press.
Busca et al.
Nature of the Vanadium Oxide Monolayer. In accordance with other authors,l0Jl we found that with the KBr disk technique, the stretching frequency in the vanadium monolayer falls at 980 cm-l. When crystalline V,05 is present, a futher band appears centered at 1020 cm-'. On the basis of this shift to a lower frequency on the v(V==O)IR band of vanadium oxide monolayer species as compared to the same band in crystalline Vz05,several authorsg-'l have attributed the enhanced catalytic properties of the vanadium oxide monolayer to weakening of the V=O bond produced by the interaction with titania. We may mention that surface vibrations are frequently observed at higher frequencies than the corresponding ones for crystals or solutions, due to electrostatic or interaction effects. In agreement, the IR analysis with the pressedpowder disk technique shows that after evacuation, the u(V=O) fundamental band falls at 1035 cm-I and the 2u(V=O) overtone band at 2045 cm-l; after adsorption of water, both the v(V=O) fundamental and its overtone shift to lower frequencies. This indicates that the observed shift to lower frequencies of the V=O stretching band in monolayer species is not due to the interaction with titanium oxide but to the perturbing effect of adsorbed water. A further indication given by the analysis of the v(V=O) first overtone band is that it is single for monolayer vanadium oxide. Dioxovanadium species are generally characterized by two u(V=O) bands separated by 10-20 cm-l, due to coupling. This would generate resolved triplet or, at least, a broad overtone band. The observation of only one single sharp overtone thus seems to contradict the model proposed by some author^'^,'^ involving the presence of a surface >V(==O)2species on the TiOz in order to explain the superior catalytic performances of the vanadium oxide monolayer. Furthermore, the LCT band of this surface dioxo species considered to be in tetrahedral coordination would be expected at higher frequencies than that observed by us, and we prefer to attribute this band to LCT transitions of distorded octahedral Vv ions, Also DR analysis in the visible region does not support the hypothesis of formation of dioxovanadium species, although it is not possible to exclude their presence together with the vanadium oxide species we observed and tentatively assigned to (i) isolated vanadium ions strongly interacting with TiOz and (ii) bidimensional vanadium oxide clusters weakly interacting with Ti02. Conclusions The present results indicate the formation of three different vanadium oxide structures as the vanadium oxide content is increased on catalysts supported on anatase. They are (i) isolated VIv ions, part of which are coordinatively unsaturated, produced by reaction with the surface hydroxy groups of the support and strongly bonded to it; (ii) bidimensional clusters of vanadium oxide, containing mainly Vv after calcination, but reducible even under mild conditions to Vlv and to some extent also to VIn (such species weakly interact with the support surface); and (iii) V205, appearing when coverage is only slightly higher than that corresponding to the monolayer and present as a bulk multilayer structure. It has been shown that catalysts containing an amount of vanadium oxide near that required to form a monolayer are the most active and selective in several catalytic reactions.6-'0 In such catalysts, isolated vanadyls that are able to act as adsorption sites are observed together with vanadium oxide clusters, which constitute centers (or multicenters) with oxidizing properties. Accordingly, the properties of TiOz-supported monolayer catalysts are considerably different from those of pure vanadium oxides.
Langmuir 1986,2, 577-582 Spectroscopic techniques, in particular DR and FT-IR, indicate the peculiar spectral features of these phases. However, a care€ulanalysis of the measurement conditions indicates that they are different from those reported in other studies.615 Both v(V=O) and LCT bands are shifted to higher frequencies as compared with those in unsupported Vz05, suggesting a possible strengthening of the V = O bond. Apparent weakening of this bond6J0J1seems to be due mainly to the presence of adsorbed water during measurements. Moreover, no evidence is found of the presence of surface dioxovanadium groups, which have been proposed as the characterizing surface phase present in active V-Ti c a t a l y s t ~ , ' ~analogous J~ to those proposed on other systems such as Mo03/A1203and W03/A1203.4v5 The present results support the idea of the formation of a particular vanadium oxide structure on the support surface, characterized by peculiar spectroscopic as well as catalytic behaviors. According to our model, such a surface
577
phase is formed by two different driving forces: (i) specific reaction with OH surface groups (leading to strongly bonded isolated V02+)and (ii) a spreading effect, due to interfacial oxide-oxide interaction (leading to reducible vanadium oxide clusters, more weakly bonded to the support). According to the similarity of our system with other oxide-supported oxide the present model may be proposed as a generalized way to obtain special sirface structures and to modify drastically the spectroscopic, reactivity, and catalytic properties of a transition-metal oxide.
Acknowledgment. Support by Minister0 Pubblica Istruzione to the research group on Structure and Reactivity of Surfaces is gratefully acknowledged. Registry No. VzO5,1314-62-1;TiO,, 13463-67-7;V, 7440-62-2; vanadium oxide, 11099-11-9.
FT-IR Study of the Surface Chemistry of Anatase-Supported Vanadium Oxide Monolayer Catalysts Guido Busca Istituto Chimico, Facoltci di Ingegneria, Universitci di Bologna, 40136 Bologna, Italy Received February 11, 1986. I n Final Form: M a y 7, 1986 The surface chemistry of anatase-supported vanadium oxide catalysts whose loading corresponds to that needed to complete the geometric monolayer has been studied by FT-IR spectroscopy of adsorbed probe molecules. The suppression of the linear chemisorption of CO indicates that the support surface is no longer exposed on the supported catalysts. Adsorption of ammonia, pyridine, and acetonitrile indicates that very strong Lewis acid sites and medium-strong Brcansted acid sites are present on the vanadium oxide monolayer, identified as coordinativelyunsaturated V02+ions and VOH groups, respectively. Acidic OH groups are also active in the adsorption of propylene to produce isopropoxy groups and of alcohols to produce alkoxy groups that are further easily oxidized to the corresponding carbonylic compounds. The stability of adsorbed benzaldehyde and the weak adsorption of COz seem to indicate that basic and nucleophilic surface anions are very weak.
Introduction Vanadium titanium oxides exhibit particularly good performances as catalysts for hydrocarbon-selective oxidations and ammoxidations.'s2 Even if, depending on the V/Ti atomic ratio and the preparation procedure, several different phases are frequently detectable in such catalysts, a particularly good activity seems to be developed by vanadium oxide monolayers on anatase-exposed face^.^!^ However, the origin of the .activation of Vanadium oxide when supported on TiOs is still unclear, possibly also due to the lack of a systematic study of the surface chemistry and structure of such catalysts. In previous studies we have performed a characterization of the surface structure of anatase-supported vanadium oxide catalysts, whose loading corresponds to that of a complete geometric m ~ n o l a y e rand , ~ of the surface chem(1) Cullis, C. F.; Hucknall, D. J. In Catalysis; The Royal Society of Chemistry: London, 1982;Vol. 5, p 273. (2)Wainwright, M. S.;Foster, N. R. Catal. Reu. 1979,19,211. (3)Bond, G.C.; Briickmann, K. Discuss. Faraday SOC.1982,72,235. (4)Wachs, I. E.;Chan, S. S.; Chersich, C. C.; Saleh, R. J. In Catalysis i n the Energy Scene; Kaliaguine, S., Mahay, A., Eds.; Elsevier: Amsterdam, 1984;p 275.
0743-7463/86/2402-0577$01.50/0
istry of pure anatase supports.6 As a further development of such research, we report here the results of a study of the surface chemistry of near-monolayer anatase-supported vanadium oxide catalysts, performed using the technique of the IR spectroscopy of adsorbed probe molecules.
Experimental Section Samples used were VTIO9 and VTI16, whose preparation and ~ vanadium characterization have been reported p r e v i ~ u s l y .The oxide loading, expressed as the fraction of the theoretical geometric monolayer, is 0.94 and 1.62, respectively. They were passed into self-supporting disks and usually activated in the IR cell, by evacuation at 673 K, before adsorption experiments. IR spectra were recorded by a Nicolet MX 1 Fourier transform instrument, connected with conventional gas-manipulationjevacuation greaseless ramps and IR cell (NaC1 windows).
Results As reported previously) the IR spectra of VTI samples show, with respect to that of the pure TiOz support, some (5)Busca, G.;Centi, G.; Marchetti, L.; TrifirB, F. Langmuir, preceding paper in this issue. ( 6 ) Busca, G.; Saussey, H.; Saw, 0.; Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985,14,245.
0 1986 American Chemical Society
