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TiO2 catalysts
JOURNAL OF
MOLECULAR CATALYSIS ELS EV 1ER
Journal of Molecular Catalysis 88 (1994) 311-324
1H and 5 1 V high-resolution solid state nuclear magnetic resonance studies of supported V205/TiO2 catalysts L.G. Pinaeva*, O.B. Lapina, V.M. Mastikhin, A.V. Nosov, B.S Balzhinimaev Bore ~kov In ~tm~te of Catalysts. Novoslbirsk 630090. Russian Federatton
( Received April 3, 1993, accepted September 30, 1993 )
Abstract tH solid state NMR data show the existence of several types of hydroxyl groups on T102 surface, depending on the surface impurities. VOC13 selectively interacts with these groups. According to 5tV NMR data, the structure of the surface vanadium complexes, independently of the preparation method, is determined by the type of surface hydroxyls. On clean TiO2 surface at low vanadium concentration two types of surface complexes with vanadium in the distorted tetrahedral environment of oxygen atoms are formed, one of them containing OH groups in the coordination sphere At high V content associated species, with V in octahedral coordination, are formed. nuclear magnenc resonance studies; solid state; titania: vanadia Key words: JH-NMR spectroscopy: nuclear magnenc resonance: mania: vanadla, 5W-NMR spectroscopy
1. Introduction Supported V20~/TiO~_ catalysts are widely used in industry for NOx reduction by NH3 [ 1 ] and tbr selective oxidation of hydrocarbons [2]. An important information on local environment of vanadium atoms in supported vanadium catalysts was obtained recently by a number of physical methods. The structure of vanadium complexes with two terminal and two bridged oxygen atoms on TiO2 surface has been proposed [3]. Vanadium atoms in tetrahedral oxygen coordination on anatase were observed with laser Raman spectroscopy
[4]. So-called monolayer catalysts obtained by the grafting of V O C I 3 from gaseous phase tc TiO2 and subsequent hydrolysis are nowadays widely used as the model VzO~/TiO2 cata* Corresponding author, fax ( + 7-383)2355766. 0304-5102/94/$07 O0 © 1994 EIse',ier Science B.V. All rights reserved S S D I 0 3 0 4 - 5 I 021 93 ) E0277-N
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lysts of partial oxidation of hydrocarbons. According to ESCA measurements, it was proposed that vanadium in these catalysts forms a film of 7-10 ,~ thickness with a V/Ti ratio of about 1 [5]. An excess of vanadium gives V205 globules. This model is consistent with published data [6]. So, these catalysts can be effectively used for studying surface interactions, resulting in the formation of the active complexes. It is obvious that the structure of the active component of supported catalysts depends on the interaction of vanadium compounds used for catalyst preparation with the surface hydroxyl groups. Infrared spectroscopic studies [ 7-13] have revealed the dependence of the type of OH groups and their concentration in anatase on the method of preparation. Recently, LH NMR became an effective method for the studies of surface OH groups [ 14-16], while 51V NMR is a powerful tool for characterization of the local vanadium environment [ 17-19]. In this work we have used both these methods to study the interaction of surface OH groups of anatase with VOCI3 as well as for identification of surface vanadium complexes.
2. Experimental Four types of TiOz obtained by different preparations were used as a support. Sample l was prepared by precipitation from TIC14 with ammonia with subsequent washing, drying and calcination at 773 K during 2 h: sample 2 was obtained by pyrolysis of TiCI4 in 02: sample 3 was prepared by hydrolysis of titanyl sulfate, sample 4 was obtained by Ti(iBuO) 4 hydrolysis with Na2CO3. In Table 1 the main characteristics of TiO, samples ( SBET, impurities) are presented. Supported vanadium oxide catalysts were prepared by a grafting technique described previously [20]. TiO2 (anatase) was initially heated in a He flow at 523-773 K for 2 h and then treated by VOCI~ in the gaseous phase at 293 K. The excess of Table 1 Properties of anatase samples and their ~H chemical shifts Sample
Preparation method
S~LT, m~-g ~
Main mapuntles
Chemical shift 6/ppm ( + 0 3 ppm)
TIOz- 1"
precipitation of TICI4 with NH~
55
0.1% AI. 0. 1% CI
2 9, 6.4
TiO,-2 b
pyrolysis of TIC[4 m 02
31
3 7% Sl, 0 4% CI, 0 3% Fe
1.9, 3 2
TIO2-3
hydrolysis of tltanyl sulfate
289
3 9% S, 0 1% Fe
3.9:7 6" 11 6
TIO2-4
hydrolysis of TI( l-BuO)4 with NazCO~
35
2% Na, 0.1% Fe
2.9; 7 4
"Other impurities are less than 5 × 10 3%. b Sample contained an admixture of futile
L. G Pmae va et al / Journal of Molecular Catalysts 88 ~1994 ) 311-324
313
removed by heating in a He flow at 673 K. To increase the vanadium coverage, several supporting cycles were used. After hydroxylation of the surface by moist air at 340 K during 2 h with subsequent removal of physically adsorbed water by heating in a He flow up to 410 K, another treatment with VOCI3 at 293 K was performed. Depending on the number of supporting cycles and the temperature of the TiO, pretreatment in He, the vanadium content in the samples varied from 0.85 up to 6.15 wt.%. Then the catalysts were calcined at 723 K during 2 h. Other samples were prepared by the impregnation of these supports with VOSO4 solution. They contained about l wt.% of V. The ~H NMR M A S spectra were recorded at 300.066 MHz using a Bruker CXP-300 spectrometer. The spectral range was 50 kHz, the pulse duration 5 /xs and the interval between pulses 5 s. The accumulation number was 103. Before the measurements the samples were placed in a quartz reactor and then dehydrated in a flow of He at different temperatures (from 523 to 773 K) during 1 h. After this procedure they were sealed off in the special glass tubes of 12 mm length and 7 mm outer diameter under the He atmosphere. To diminish the signal from the traces of water on the outer walls of the sample tubes, the latter were heated in the flow of hot air and then placed in the quartz rotor for the ~H NMR M A S measurements. The rotation frequency was about 3 kHz. The chemical shifts (6) were measured with respect to external tetramethylsilane ( T M S ) . The 5~V NMR spectra were recorded at 105.2 MHz at Bruker MS L-400 spectrometer in a frequency range of 250 kHz, the pulse width was 2 / z s and the interval between pulses was I s. The accumulation number ranged from 103 to 3 × 105. The 5IV NMR MAS spectra were measured at 78.86 MHz using a Bruker CXP-300 spectrometer in a frequency range of 150 kHz, the pulse width was 3 # s and interval between pulses was from 0.1 to I s. The number of scans was from 103 to 10 4 . After the measurement of the 5 ~V NMR spectra, the samples were treated in a He flow at 723 K during l h and sealed in the special quartz ampoules (40 mm length, 10 mm outer diameter), which were then opened in an argon glove box and placed in plastic rotors for M A S measurements, isolated from the atmosphere with paraffin. The rotation frequency was about 4 kHz. The chemical shifts in 5IV NMR spectra were measured with respect to VOCI3 as an external reference. VOC13 was
3. Results 3.1. I H N M R M A S s p e c t r a
The ~H NMR MAS spectra of anatase samples dehydroxylated in a He flow during 1 h at different temperatures are presented in Fig. 1. They comprise two main peaks each consisting of three overlapping lines with 6 = 1.5, 2.3, 3.5 ppm (peak A) and ¢5= 5.6:6.8 and 7.6 ppm (peak B) (Fig. 1, spectrum 1 ). The ratio of peak intensities depends on the dehydroxylation temperature. As the temperature increases from 523 to 773 K, the total intensity of the signal decreases due to the decrease of the surface area; the peak B decreasing more than peak A (Fig. 1, spectrum 2). Evacuation of the samples at 773 K and P = 10 Torr during 2 h results in complete disappearance of peak B from the spectrum (Fig. 1, spectrum 3). The total concentration of the surface hydroxyl groups calculated from the
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L.G. Pinaeva et al. / Journal o f Molecular Catalysis 88 t 1994 ~ 311-324
2.9
l
6.8
2.9
$.8 6,6
•
2O
t0
0
-10
i
~,ppm
Ftg 1. ~H NMR MAS spectra of anatase TiO2-1, dehydroxylated at various temperatures: I I ~ 523 K in the He flow; (2) 773 K m the He flow, (3) 773 K m the vacuum of l0 Ton- t4) after supporting of 3.7,% vanadmm (monolayer coverage ) at 623 K in He flow The spectra are scaled to equivalent height of the most intense peak, which is the same for all the samples. A narrow line at 1.0 ppm m these and other spectra ~s due to the traces of water on the outer surface of the samples and rotor. The side bands caused by the sample rotation are marked w~th asterisks ( * ).
c o m p a r i s o n w i t h the signal o f the r e f e r e n c e s a m p l e was d e c r e a s e d f r o m 4.2 n m - 2 (at 523 K) t o 2 . 3 n m 2(at773K). W h e n v a n a d i u m w a s s u p p o r t e d on the surface o f the TiO~-I s a m p l e in the f o r m o f VOCI3, the intensity o f p e a k B d e c r e a s e d . At m o n o l a y e r c o v e r a g e o f the surface (5.1 V a t o m s n m - 2 ) [ 12] this peak a l m o s t c o m p l e t e l y d i s a p p e a r s f r o m the s p e c t r u m (Fig. l, spectrum 4).
The IH N M R s p e c t r a o f TiO2 s a m p l e s o f o t h e r preparations are p r e s e n t e d in Fig. 2. In
L.G. Pmaeva et al. / Journal of Molecular Catalysis 88 (1994) 3 l l - 3 2 4
315
St
11
B
t,9 3.2
7,6
2 2.9
I
I
I
I
20
t0
0
-10
I
8,ppm
Fig 2 ~HNMR MAS spectraof the samples after dehydrationin the He flowat 623 K: ( 1) TIO2-2(prepared by pyrolysis of TiCI4in O_.)" (2) TIOz-3(prepared by hydrolysisof titanyl sulphate); (3) TIO2-4(prepared by hydrolysisof Tl( i-BuO)4 with Na2CO3) the spectrum of sample TIO2-2 that contains the admixture of silicon as an impurity (about 3.7 wt.%), the intensity of peak B is low compared with that of peak A (Fig. 2, spectrum 1). Peak A overlaps with the narrow intense line ( 6 = 1.9 ppm) that is typical for Si-OH groups [ 14,15 ]. A new line ( 6 = 11.6 ppm) appears as a shoulder of the most intense peak B in the spectrum of TIO2-3 prepared by hydrolysis of titanyl sulfate (Fig. 2, spectrum 2). Spectrum of TIO2-4 sample contains the peak A ( 6 = 2.9 ppm) and the peak B ( 6 = 7.4 ppm) of low intensity (Fig. 2, spectrum 3). The ~H NMR MAS spectra of the TIO2-2, TIO2-3 and TIO2-4 samples after interaction with VOCI3 are presented in Fig. 3 together with the spectrum of V205. For the TIO2-3 sample the decrease of the intensity of the low-field peaks ( 6 = 7.6 and 6 = 11.6 ppm) and the shift of peak A into the region of 6-- 2.7 ppm (Fig. 3, spectrum 3) are observed, while the line in the spectrum of TIO2-2 characterized by 6 = 1.9 ppm completely disappears ( Fig.
316
L.G. Pmaeva et al / Jour~lal of Molecular Catalww~"88 (1994) 311-324 2.9
7.6
11.
3 2.6 1.3
I
I
30
20
I
I
|
10
0
-10
i
,~ ,ppm
Fig 3 'H NMR MAS spectra of TIO~samples after s,pportmg VOCI~ (about 5 V atoms nm -'): (I) T]O,-4, {2 ~TtO,-2- ( 3 ) TIO2-3:and (4 ~ V20~ spectrum. 3, spectrum 2). The intensity of TIO2-4 spectrum does not change, but the lines are broadened substantially (Fig. 3, spectrum 1 ). The spectrum of V205 (SBE-V= 1 m z. g ' ) is also presented in Fig. 3 (spectrum 4). It contains the single line (~i= 2.9 ppm) from V - O H groups. The line in the region of 6 = 1.3 ppm observed in some spectra is, undoubtedly caused by the water vapour on the outer walls of the sample tubes or the rotor. 3.2. 51V N M R s p e c t r a
Fig. 4 shows the 5IV NMR spectra of the samples with various V concentrations that were prepared by supporting VOCI~ on TiO2 with subsequent hydration and removal of the excess of water in a He flow at 523 K. The spectra of all samples are composed of several lines with their intensity, depending on total V content. Line I ( peak at - 440 ppm) prevails
L G. Pmaeva et al. / Jou¢~al of Molecular Catalysis 88 (l 994) 311-324
317
-300 {iin, iVl
/
-NO IIIn_eIt)
-11170
II
t _ . . .
. . . .
I
0
. . . .
*
. . . .
i
~
.
.
.
.
.
.
.
.
.
I
.
.
-t000
.
.
.
.
.
.
.
n
.
.
-1600
.
.
.
.
.
.
,~ ,plan
Fig. 4, 5~V NMR spectra of V/TIO2-1 samples as a function of vanadium surface concentration, (nm -2) ( I ) 5. l; (2) 5.6, (3) 8.9; (4) 10.7. Samples were obtained by repeated procedure of VOCI3 supporting from the gas phase on the samples treated in He flow at 723 K during 2 h with the subsequent hydroxylation and treating in He flow at 623 K.
at lowest V content. Line II (peak at - 560 ppm) appears at about 5 V atoms per nm 2. The line with the axial anisotropy of the chemical shift tensor, line I I l ( 6 . = - 320, 6 n = - 1370 ppm) and line IV ( 6 ± = - 300, 6, = - 1270 ppm) are typical for the samples with increased V content. Line IV prevails at the highest V concentration. The use of magic angle spinning narrows the lines I, I I and IV, but does not change considerably the width of line III. Fig. 5 illustrates the effect of magic angle spinning for the sample V/TiO2-1. Analysis of the 5,V NMR M A S spectra obtained at different rotation frequencies has allowed more precise measurement of the spectral parameters. Line I has 6,~o= - 4 4 0 ppm, 3 6 = 6 . - 6 , = 500 ppm; for line I I 6,so = - 6 0 0 ppm, A 6 = 300 ppm; line I I I has ,z16= 1000 ppm; for line IV 6,so = - 6 1 0 ppm; ,,:16= 900 ppm. Effect of water adsorption on TiO2-1 sample having about one monolayer of vanadium atoms on its surface is illustrated in Fig. 6. Water adsorption results in the transformation of the spectra from those typical for tetrahedral complexes (Fig. 6, spectrum 1 ) to that from V atoms in a distorted octahedral coordination ( Fig. 6, spectrum 2). The evacuation of this sample at 673 K restores the initial spectrum. Fig. 7A shows the 5 ~V N M R spectra obtained for samples prepared by a single supporting cycle of VOCI 3 on TiO2-1, TIO2-2, TIO2-3 and TIO2-4. All of these spectra are composed of different lines. For TiO2-1 the spectrum is composed mainly of lines I I and I, while for
318
L.G. Pmaeva et al. / Journal of Molecular Catalysis 88 (1994) 311-324 44O
I
i
I
O
-600
-100(I
I 6 ,ppm
Fig. 5. Effect of magic angle spinning' ' on-5~V NMR spectra of V/TIO,-_1sample ( V content 5.1 nm - 2, hydroxylated and treated m He flow at 523 K): ( 1) without spinning; (2) MAS spectrum. .,loo
i
i
I
o
400
.1000
I
~ ,ppm
Fig. 6 5~V NMR spectra of TiOz- 1 sample with 5.1 nm 2 V atoms ( I ) treated m He flow at 623 K; (2) after H20 adsorption on the sample 1. TIO2-3 ( c o n t a i n i n g sulfate anions) line I I I prevails in the spectrum. For TIO2-2 ( containing an admixture o f silicon) a line with parameters 6~_ = - 5 0 0 , 6al = - 1000 ppm, which are different f r o m those for lines I, II, I I I and IV, predominate in the spectrum. A relatively narrow line with 6lSO= - 535 ppm and A u = 20 kHz has been o b s e r v e d in the spectrum o f the catalyst prepared from TIO2-4. Fig. 7B illustrates the 51V N M R spectra o f the samples prepared by impregnation o f different TiO2 supports with the solution o f vanadyl sulfate (2 V atoms per n m 2) f o l l o w e d
L.G. Pmaeva et al. / Journal of Molecular Catalysis 88 (1994) 311-324
A
B
I
0
I
I
-1000
I
5 ,pin
I
0
319
.-636
I
I
-'1000
I
,~ , p p m
Fig. 7.5Iv NMR spectra of the samples prepared by different methods on different TiO2 supports: ( 1) TIO2-1; (2) TIO2-3; (3) TIO2-2; (4) TIO2-4; (A) VOCI3 grafted to TiO2 from He flow at 623 K hydrolyzed and treated in He flow at 623 K; ( B ) prepared by impregnation from aqueous solution of VOSO4 followed by drying at 383 K and calcination at 723 K.
by drying at 383 K and calcination at 773 K. As can be seen from the comparison of Figs. 7A and 7B, the spectra of the samples prepared by two different methods are similar and composed of the same lines.
4. Discussion According to the literature data [ 11,21], the TiO2 surface contains two main types of hydroxyl groups localized on the terminal and bridged oxygen atoms with a formal charge of - 1 / 3 and + 1/3 on the protons, respectively. Existence of two types of hydroxyls follows also from IR data [7-13,22]. Two main peaks, A and B, at 6 = 2.9 (A) and 6.4 ppm (B), respectively, (Fig. 1) agree with this model. Downfield peak (B) can be attributed to the positively charged, more acidic hydrogen atoms localized on the bridged oxygens and forming the weak hydrogen bonds with adjacent oxygens. The highfield peak (A) at 6 = 2.9 ppm can be ascribed to the "more basic" hydrogen atoms bonded to the terminal oxygens. Such an assignment is confirmed additionally by the fact that the peak B is easily removed during dehydroxylation while much higher temperatures are necessary to remove peak A from the spectrum ( Fig. 1). A decrease of the concentration of the OH groups during surface dehydroxylation of TiO2 from a value of 9.34 nm -2 (at 300 K) to about 3.2 nm -2 (at 710 K) has also been
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L G. Pmaeva et al. /Journal of Molecular Catalysts 88 (1994) 311-324
observed [ 16]. Unresolved lines in each peak are most probably due to the contribution of OH groups localized on cleavage planes, other than (001). The absence of peak B in the spectrum of the TIO2-2 sample can be explained by formation of S i - O H groups ( 8 ~ 1.9 ppm) on the surface of this sample, containing a large amount of silicon impurity. Second, it is necessary to take into account that the given TiO2 sample is a mixture of rutile and anatase ( see Table 1 ). Rutile, being the more thermodynamically stable and densely packed TiO, modification, is expected to locate on the surface [6]. Indeed, two types of hydroxyl groups were observed in the IR spectra of anatase, and only one for rutile [22]. For the TIO2-4 sample the most obvious explanation of the absence of peak B is the replacement of acidic protons with Na ions from Na2CO3 used for hydrolysis of the titanium isobutoxide. The line at 8 = 11.6 ppm m the spectrum of TiO,-3 containing a large quantity of sulfur most probably is due to S - O H groups on the surface of this sample. According to published data [ 9 ], S - O H groups can exist o n T i O 2 surface up to an evacuation temperature of 773 K. The interaction of VOCI~ with surface OH groups proceeds due to reaction VOCI3 + n T i - O H ~ (TiO),,VOCI3_,, + n H C I
( ! ~
(1)
which results in a decrease of the number of OH groups on the T i O 2 surface. For different values of n the surface V species bound with TiO 2 by one, two or three oxygen atoms can be formed. The ~H NMR data show that V O C I 3 interacts only with OH groups of B type. No interaction with the more basic OH groups of A type with VOCI, has been observed for any of the four types of TiO2 samples studied. This does not support the supposition on the VOCI3 interaction with basic OH groups made previously [6]. The uncertainty in determination of the concentration of surface OH groups from IH NMR spectra prevents finding the exact value of n in reaction ( 1 ). The comparison of the decrease of the intensity of signal from OH groups with the number of V atoms supported on TiO~-1 surface shows that supporting of 9.7 × 1019 g ~ V atoms decreases the OH group quantity by ( 8 . 4 + 2 ) × 10 ~') g J. This means that the averaged stoichiometry of reaction ( 1 ) is between 1 and 2, and suggests the tormation of different types of surface V species including those containing CI atoms in their coordination sphere. The formation of several surface V species tbllows also from 51V NMR spectra. The lines in the 5~V NMR spectra have been assigned on the basis of previously published reports [ 17-19]. As has been shown there, the anisotropy of chemical shift tensor is the most sensitive parameter for characterization of the local environment of the V nuclei. For the tetrahedral coordination in the most cases 8~ 4:62 4: 63, / i 6 = ] 63 -- 81 ] < 600 ppm, while for the distorted octahedral coordination the axial anlsotropy (81 = 82 4: 8~) is typical with anisotropy parameter _18> 600 ppm. Therefore, lines I and II belong to tetrahedral and lines I I I and IV, to distorted octahedral V atoms. The line IV can be attributed to crystalline V205, since its parameters coincide with those of unsupported V205. At the same time, the lines I, II and I I l have parameters different from those for any crystalline V compounds and can be ascribed, therefore, to V complexes bound to support. Adsorption of water results in transformation of the tetrahedral coordination of surface complexes ~lines I and II) to
L.G. Pinaeva et al. / Joun~al of Molecular Catalysts 88 (1994) 311-324
321
the octahedral one. This follows from the appearance of the axially anisotropic line after H20 adsorption (Fig. 6). Line I I I can be attributed to octahedral V atoms in the highly disordered surface complexes. The latter follows from the inefficiency of the magic angle spinning, which cannot decrease the width of the inhomogeneously broadened lines. On the contrary, the lines I and II belong to more regular surface species. Additional information about the possible structure of surface complexes could be obtained from 6,so values of surface species. The dependence of 6,.,o for many chlorinesubstituted compounds of the VOCln(OR)3_, type, where R is an organic or inorganic radical, has been reported [ 23 ]. For VOCI2OR compounds 6,so were about - 300 ppm; for VOCI(OR)2 they were in the range - 4 0 0 to - 5 1 0 ppm and for VO(OR)3, in the range - 4 5 0 to - 6 6 0 ppm. Using these data, the lines from VOCI3 supported on TiO2 before hydration can be identified. For these samples the anisotropic spectra with 611 in the range 0 to - 2 0 0 ppm and 6 . in the range - 3 0 0 to - 4 0 0 ppm are observed (6,so varies from - 290 to - 450 ppm). This means that before hydration the supported samples contain two types of complexes bound with the TiO2 surface by one or two bonds: VOCI3-OTi and V O C I - ( O T i ) 2- The hydration would result in the formation of the species of the VO (OH) n(OTi) 3 ,, type, which are supposed to undergo further rearrangement on thermal treatment. At the moment, the detailed structure of surface complexes is not established. However, the complexes I and lI can be attributed to tetrahedral V species with an almost regular structure, probably having OH group in their coordination sphere and bonded with TiO2 surface by one or two bonds. Octahedral complexes I I I detected at higher V concentration most probably are formed due to association of the complexes I and II, which results in rearrangement of their environment. V205 is formed at largest V content as it follows from 611 and 6~_ values observed for line IV. The impurities, contained on support drastically influence the structure of supported complexes. The presence of sodium (TIO2-4) results in appearance of slightly anisotropic line with 6.,o = - 535 ppm, .:16~ 150 ppm. These parameters coincide with those of sodium orthovanadate Na3VO4, indicating its formation in TiO2 sample containing Na impurities. As has been expected, H~O adsorption does not influence the spectrum of this compound. The interaction of supported V with Na, and not with OH groups in TIO2-4 follows also from the ~H NMR spectra, since the intensity of ~H NMR spectra does not change after supporting of the V complexes. The Si-OH groups in TIO2-2 interact with supporting VOC13, as it follows from appearance of a new line in the 5~V spectrum with parameters, typical for the V/SiO2 system (6± = - 5 0 0 ppm, 61L= --1000 ppm, (Fig. 7 A ) ) . This line transforms to a line with 61 = - 300 ppm, 611= - 1200 ppm upon H20 adsorption. The change of the spectrum on water adsorption shows that this line belongs to surface species, while the 6± and 61i values indicate the tetrahedral environment of V nuclei before H20 adsorption. The decrease of intensity of the peak from silanol groups observed in ]H spectra confirms the interaction of V complexes with surface Si-OH groups. Thus, ]H and 5~V data allows one to attribute this line to tetrahedral species bonded with support surface via V - O - S i bond. The presence of SO4 anions on the TiO2 surface results in formation of octahedral surface complexes, most probably, having sulfate anions in their coordination sphere. On the surface of the most clean anatase sample (TiO2-1 ) the complexes of the L II and I I I type are present.
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Comparison of the data presented in Fig. 7A and 7B show that for the same support the structure of surface complexes is determined mainly by the structure of support surface. On the same support different preparation methods result in formation of very similar surface structures provided the treatment temperature is high enough (673-773 K). The smaller line width in the spectra of samples prepared from TIO2-4 and the higher percentage of line II for catalyst prepared from TiO2-1 might be caused by the different treatment temperatures of the supports. (The spectra in Fig. 7A correspond to the samples calcined at 623 K, while the spectra in Fig. 7B correspond to the samples calcinated at 773 K.) This resulted in the formation of Na3WO4 for TIO2-4 and an increase of the concentration of complexes I I I for TiO2-1. Therefore, one can conclude that the main factor that determines the structure and composition of supported vanadium, is the nature of the support surface (i.e., the concentration and the type of OH groups and impurities). The treatment temperature and V concentration of the samples also influences the structure of surface complexes. Recently the 5~V NMR data on the vanadium oxide species formed on anatase and rutile for ambient atmospheric conditions have been reported [ 24]. The authors reported about the marked influence of surface contaminations on the structure of the surface V complexes. This is in agreement with the results obtained in this work. Here we show that formation of the surface species takes place as a result of their interaction with surface OH groups or with contaminations of the support. It is also shown here, that on a clean TiO2 surface at low V content the formation of two tetrahedral sites takes place, while at increased V content octahedral vanadium is also present on the catalyst surface. The tetrahedral coordination is readily transformed to an octahedral one on water adsorption. Thus, the data presented in this paper provide important information on the types of V species on the TiO2 surface.
5. Conclusions
From the studies of the V-Ti samples obtained by different ways on different T i O 2 supports, the following conclusions can be made: (i) Supported V species selectively interact with surface OH groups. (ii) The types of the surface vanadium complexes depend mainly on the surface chemistry of the support and do not depend significantly on the preparation method, provided the treatment temperature is high enough (600-700 K). On a clean TiO~ surface and on the surface containing S O l - ions, V O C I 3 interacts with the more acidic OH groups. On T i O 2 containing silicon or sodium a t o m s V O C I 3 interacts with SiOH groups or Na ions, respectively. (iii) Several types of V complexes are formed on TiO2 surface. Vanadium in the surface complexes at low vanadium concentration has a distorted tetrahedral environment from oxygen atoms (complexes I and |I). Complex I most probably has OH group in its coordination sphere. The increased vanadium content results in formation of associated complexes with V in octahedral environment (complex III).
L.G. Pmaeva et al /Journal of Molecular Catalysis 88 (1994) 311-324
323
6. Acknowledgement We thank V.Ph. Lyakhova for the preparation of impregnated with vanadyl sulfate TiO2 samples.
7. References [ 1] [21 [3] [4]
F Cavani, F. Tnfiro, Catal. Today, 4 (1989) 253. G.C. Bond, K. Brukman, Faraday Discuss. Chem. Soc., 72 ( 1981 ) 235. R. Kozlovskt, R.F. Pettifer, J.M Thomas, J. Phys. Chem, 87 (1983) 5176. K. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijn, J. Medema, V H.J. de Beer, P.J. Gelhngs, J Phys. Chem., 84 (1980) 2783. [ 5] L.G. Pinaeva, A.A. Rar, A.V. Kalinkin, V.I. Zaikovsku, P.P. lvanov, B.S. Balzhimmaev, React. Kmet. Catal. Lett., 41 ~ 1990) 375. [6] G C. Bond, J.P. Zurida, S. Flammerz, P.J. Gelhngs, H. Bosch. J.G. van Ommen, B J Kip, Appl. Catal., 22 (1986) 361 [7] G. Busca, H. Saussey, O. Saur, J.C. Lavalley, V Lorenzelli, Appl. Catal., 14 (1985) 245. [8] K.I Hadjiivanov, A.A. Davydov, D.G. Klissurskl, Kinet. Katal., 29 (1988) 161. [9] K.I. Had.liivanov, A.A. Davydov, Kmet. Katal, 29 (1988) 460 [ 10] K.I. Hadjuvanov, D.G. Klissurskl, A.A Davydov, J. Catal., 116 (1989) 498. [ 11 ] M. Primer, P. Plchat, M.-V. Mathleu, J. Phys. Chem, 76 ( 1971 ) 1216 [ 12] K. Tanaka, J.M. White, J. Phys. Chem., 86 (1982) 4708. [ 13] N E. Tretjakov, V.M Fdimonov, l~net. Katal, 13 (1972) 815 [ 14] H. Pfelfer, D. Freude, M. Hunger, Zeolites, 5 (1985) 274. [ 15] V.M. Mastikhin, I L. Mudrakovsky, A.V. Nosov, Progr. NMR Spectrosc., 23 ( 1991 ) 259. [ 161 M.A. Enriquez, C. Doremleux-Morim J. Frissard, J. Sohd State Chem., 40 ( 1981 ) 233. [ 17] V.M. Mastikhin, O.B, Lapina, V.N. Krasilmkov, A A. Ivakin, React Kinet. Catal. Lett., 24 (1984) 189. [ 18] O.B. Lapina, V.M. Mastlkhln, A.A. Shubm, V.N. Krasdmkov, K.I. Zamaraev, Prog NMR Spectrosc, 24 (1992) 457. [ 19] H. Eckert, I.E. Wachs, J. Phys. Chem, 93 (1989) 6796. [20] G.C. Bond, P Konig, J. Catal., 77 (1982) 309. [21 ] H.P. Boehm, Adv. Catal., 16 f 1966) 179. [22] D.J C. Jates, J. Phys Chem., 65 ( 1961 ) 184 [23] D. Rehder, Bull. Magn. Reson., 4 (1982) 33 [24] H. Eckert, G. Deo, I E. Wachs, A.M. Hirt, Colloids Surf., 45 (1990) 347.
MOLECULAR CATALYSIS ELS EV 1ER
Journal of Molecular Catalysis 88 (1994) 311-324
1H and 5 1 V high-resolution solid state nuclear magnetic resonance studies of supported V205/TiO2 catalysts L.G. Pinaeva*, O.B. Lapina, V.M. Mastikhin, A.V. Nosov, B.S Balzhinimaev Bore ~kov In ~tm~te of Catalysts. Novoslbirsk 630090. Russian Federatton
( Received April 3, 1993, accepted September 30, 1993 )
Abstract tH solid state NMR data show the existence of several types of hydroxyl groups on T102 surface, depending on the surface impurities. VOC13 selectively interacts with these groups. According to 5tV NMR data, the structure of the surface vanadium complexes, independently of the preparation method, is determined by the type of surface hydroxyls. On clean TiO2 surface at low vanadium concentration two types of surface complexes with vanadium in the distorted tetrahedral environment of oxygen atoms are formed, one of them containing OH groups in the coordination sphere At high V content associated species, with V in octahedral coordination, are formed. nuclear magnenc resonance studies; solid state; titania: vanadia Key words: JH-NMR spectroscopy: nuclear magnenc resonance: mania: vanadla, 5W-NMR spectroscopy
1. Introduction Supported V20~/TiO~_ catalysts are widely used in industry for NOx reduction by NH3 [ 1 ] and tbr selective oxidation of hydrocarbons [2]. An important information on local environment of vanadium atoms in supported vanadium catalysts was obtained recently by a number of physical methods. The structure of vanadium complexes with two terminal and two bridged oxygen atoms on TiO2 surface has been proposed [3]. Vanadium atoms in tetrahedral oxygen coordination on anatase were observed with laser Raman spectroscopy
[4]. So-called monolayer catalysts obtained by the grafting of V O C I 3 from gaseous phase tc TiO2 and subsequent hydrolysis are nowadays widely used as the model VzO~/TiO2 cata* Corresponding author, fax ( + 7-383)2355766. 0304-5102/94/$07 O0 © 1994 EIse',ier Science B.V. All rights reserved S S D I 0 3 0 4 - 5 I 021 93 ) E0277-N
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lysts of partial oxidation of hydrocarbons. According to ESCA measurements, it was proposed that vanadium in these catalysts forms a film of 7-10 ,~ thickness with a V/Ti ratio of about 1 [5]. An excess of vanadium gives V205 globules. This model is consistent with published data [6]. So, these catalysts can be effectively used for studying surface interactions, resulting in the formation of the active complexes. It is obvious that the structure of the active component of supported catalysts depends on the interaction of vanadium compounds used for catalyst preparation with the surface hydroxyl groups. Infrared spectroscopic studies [ 7-13] have revealed the dependence of the type of OH groups and their concentration in anatase on the method of preparation. Recently, LH NMR became an effective method for the studies of surface OH groups [ 14-16], while 51V NMR is a powerful tool for characterization of the local vanadium environment [ 17-19]. In this work we have used both these methods to study the interaction of surface OH groups of anatase with VOCI3 as well as for identification of surface vanadium complexes.
2. Experimental Four types of TiOz obtained by different preparations were used as a support. Sample l was prepared by precipitation from TIC14 with ammonia with subsequent washing, drying and calcination at 773 K during 2 h: sample 2 was obtained by pyrolysis of TiCI4 in 02: sample 3 was prepared by hydrolysis of titanyl sulfate, sample 4 was obtained by Ti(iBuO) 4 hydrolysis with Na2CO3. In Table 1 the main characteristics of TiO, samples ( SBET, impurities) are presented. Supported vanadium oxide catalysts were prepared by a grafting technique described previously [20]. TiO2 (anatase) was initially heated in a He flow at 523-773 K for 2 h and then treated by VOCI~ in the gaseous phase at 293 K. The excess of Table 1 Properties of anatase samples and their ~H chemical shifts Sample
Preparation method
S~LT, m~-g ~
Main mapuntles
Chemical shift 6/ppm ( + 0 3 ppm)
TIOz- 1"
precipitation of TICI4 with NH~
55
0.1% AI. 0. 1% CI
2 9, 6.4
TiO,-2 b
pyrolysis of TIC[4 m 02
31
3 7% Sl, 0 4% CI, 0 3% Fe
1.9, 3 2
TIO2-3
hydrolysis of tltanyl sulfate
289
3 9% S, 0 1% Fe
3.9:7 6" 11 6
TIO2-4
hydrolysis of TI( l-BuO)4 with NazCO~
35
2% Na, 0.1% Fe
2.9; 7 4
"Other impurities are less than 5 × 10 3%. b Sample contained an admixture of futile
L. G Pmae va et al / Journal of Molecular Catalysts 88 ~1994 ) 311-324
313
removed by heating in a He flow at 673 K. To increase the vanadium coverage, several supporting cycles were used. After hydroxylation of the surface by moist air at 340 K during 2 h with subsequent removal of physically adsorbed water by heating in a He flow up to 410 K, another treatment with VOCI3 at 293 K was performed. Depending on the number of supporting cycles and the temperature of the TiO, pretreatment in He, the vanadium content in the samples varied from 0.85 up to 6.15 wt.%. Then the catalysts were calcined at 723 K during 2 h. Other samples were prepared by the impregnation of these supports with VOSO4 solution. They contained about l wt.% of V. The ~H NMR M A S spectra were recorded at 300.066 MHz using a Bruker CXP-300 spectrometer. The spectral range was 50 kHz, the pulse duration 5 /xs and the interval between pulses 5 s. The accumulation number was 103. Before the measurements the samples were placed in a quartz reactor and then dehydrated in a flow of He at different temperatures (from 523 to 773 K) during 1 h. After this procedure they were sealed off in the special glass tubes of 12 mm length and 7 mm outer diameter under the He atmosphere. To diminish the signal from the traces of water on the outer walls of the sample tubes, the latter were heated in the flow of hot air and then placed in the quartz rotor for the ~H NMR M A S measurements. The rotation frequency was about 3 kHz. The chemical shifts (6) were measured with respect to external tetramethylsilane ( T M S ) . The 5~V NMR spectra were recorded at 105.2 MHz at Bruker MS L-400 spectrometer in a frequency range of 250 kHz, the pulse width was 2 / z s and the interval between pulses was I s. The accumulation number ranged from 103 to 3 × 105. The 5IV NMR MAS spectra were measured at 78.86 MHz using a Bruker CXP-300 spectrometer in a frequency range of 150 kHz, the pulse width was 3 # s and interval between pulses was from 0.1 to I s. The number of scans was from 103 to 10 4 . After the measurement of the 5 ~V NMR spectra, the samples were treated in a He flow at 723 K during l h and sealed in the special quartz ampoules (40 mm length, 10 mm outer diameter), which were then opened in an argon glove box and placed in plastic rotors for M A S measurements, isolated from the atmosphere with paraffin. The rotation frequency was about 4 kHz. The chemical shifts in 5IV NMR spectra were measured with respect to VOCI3 as an external reference. VOC13 was
3. Results 3.1. I H N M R M A S s p e c t r a
The ~H NMR MAS spectra of anatase samples dehydroxylated in a He flow during 1 h at different temperatures are presented in Fig. 1. They comprise two main peaks each consisting of three overlapping lines with 6 = 1.5, 2.3, 3.5 ppm (peak A) and ¢5= 5.6:6.8 and 7.6 ppm (peak B) (Fig. 1, spectrum 1 ). The ratio of peak intensities depends on the dehydroxylation temperature. As the temperature increases from 523 to 773 K, the total intensity of the signal decreases due to the decrease of the surface area; the peak B decreasing more than peak A (Fig. 1, spectrum 2). Evacuation of the samples at 773 K and P = 10 Torr during 2 h results in complete disappearance of peak B from the spectrum (Fig. 1, spectrum 3). The total concentration of the surface hydroxyl groups calculated from the
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L.G. Pinaeva et al. / Journal o f Molecular Catalysis 88 t 1994 ~ 311-324
2.9
l
6.8
2.9
$.8 6,6
•
2O
t0
0
-10
i
~,ppm
Ftg 1. ~H NMR MAS spectra of anatase TiO2-1, dehydroxylated at various temperatures: I I ~ 523 K in the He flow; (2) 773 K m the He flow, (3) 773 K m the vacuum of l0 Ton- t4) after supporting of 3.7,% vanadmm (monolayer coverage ) at 623 K in He flow The spectra are scaled to equivalent height of the most intense peak, which is the same for all the samples. A narrow line at 1.0 ppm m these and other spectra ~s due to the traces of water on the outer surface of the samples and rotor. The side bands caused by the sample rotation are marked w~th asterisks ( * ).
c o m p a r i s o n w i t h the signal o f the r e f e r e n c e s a m p l e was d e c r e a s e d f r o m 4.2 n m - 2 (at 523 K) t o 2 . 3 n m 2(at773K). W h e n v a n a d i u m w a s s u p p o r t e d on the surface o f the TiO~-I s a m p l e in the f o r m o f VOCI3, the intensity o f p e a k B d e c r e a s e d . At m o n o l a y e r c o v e r a g e o f the surface (5.1 V a t o m s n m - 2 ) [ 12] this peak a l m o s t c o m p l e t e l y d i s a p p e a r s f r o m the s p e c t r u m (Fig. l, spectrum 4).
The IH N M R s p e c t r a o f TiO2 s a m p l e s o f o t h e r preparations are p r e s e n t e d in Fig. 2. In
L.G. Pmaeva et al. / Journal of Molecular Catalysis 88 (1994) 3 l l - 3 2 4
315
St
11
B
t,9 3.2
7,6
2 2.9
I
I
I
I
20
t0
0
-10
I
8,ppm
Fig 2 ~HNMR MAS spectraof the samples after dehydrationin the He flowat 623 K: ( 1) TIO2-2(prepared by pyrolysis of TiCI4in O_.)" (2) TIOz-3(prepared by hydrolysisof titanyl sulphate); (3) TIO2-4(prepared by hydrolysisof Tl( i-BuO)4 with Na2CO3) the spectrum of sample TIO2-2 that contains the admixture of silicon as an impurity (about 3.7 wt.%), the intensity of peak B is low compared with that of peak A (Fig. 2, spectrum 1). Peak A overlaps with the narrow intense line ( 6 = 1.9 ppm) that is typical for Si-OH groups [ 14,15 ]. A new line ( 6 = 11.6 ppm) appears as a shoulder of the most intense peak B in the spectrum of TIO2-3 prepared by hydrolysis of titanyl sulfate (Fig. 2, spectrum 2). Spectrum of TIO2-4 sample contains the peak A ( 6 = 2.9 ppm) and the peak B ( 6 = 7.4 ppm) of low intensity (Fig. 2, spectrum 3). The ~H NMR MAS spectra of the TIO2-2, TIO2-3 and TIO2-4 samples after interaction with VOCI3 are presented in Fig. 3 together with the spectrum of V205. For the TIO2-3 sample the decrease of the intensity of the low-field peaks ( 6 = 7.6 and 6 = 11.6 ppm) and the shift of peak A into the region of 6-- 2.7 ppm (Fig. 3, spectrum 3) are observed, while the line in the spectrum of TIO2-2 characterized by 6 = 1.9 ppm completely disappears ( Fig.
316
L.G. Pmaeva et al / Jour~lal of Molecular Catalww~"88 (1994) 311-324 2.9
7.6
11.
3 2.6 1.3
I
I
30
20
I
I
|
10
0
-10
i
,~ ,ppm
Fig 3 'H NMR MAS spectra of TIO~samples after s,pportmg VOCI~ (about 5 V atoms nm -'): (I) T]O,-4, {2 ~TtO,-2- ( 3 ) TIO2-3:and (4 ~ V20~ spectrum. 3, spectrum 2). The intensity of TIO2-4 spectrum does not change, but the lines are broadened substantially (Fig. 3, spectrum 1 ). The spectrum of V205 (SBE-V= 1 m z. g ' ) is also presented in Fig. 3 (spectrum 4). It contains the single line (~i= 2.9 ppm) from V - O H groups. The line in the region of 6 = 1.3 ppm observed in some spectra is, undoubtedly caused by the water vapour on the outer walls of the sample tubes or the rotor. 3.2. 51V N M R s p e c t r a
Fig. 4 shows the 5IV NMR spectra of the samples with various V concentrations that were prepared by supporting VOCI~ on TiO2 with subsequent hydration and removal of the excess of water in a He flow at 523 K. The spectra of all samples are composed of several lines with their intensity, depending on total V content. Line I ( peak at - 440 ppm) prevails
L G. Pmaeva et al. / Jou¢~al of Molecular Catalysis 88 (l 994) 311-324
317
-300 {iin, iVl
/
-NO IIIn_eIt)
-11170
II
t _ . . .
. . . .
I
0
. . . .
*
. . . .
i
~
.
.
.
.
.
.
.
.
.
I
.
.
-t000
.
.
.
.
.
.
.
n
.
.
-1600
.
.
.
.
.
.
,~ ,plan
Fig. 4, 5~V NMR spectra of V/TIO2-1 samples as a function of vanadium surface concentration, (nm -2) ( I ) 5. l; (2) 5.6, (3) 8.9; (4) 10.7. Samples were obtained by repeated procedure of VOCI3 supporting from the gas phase on the samples treated in He flow at 723 K during 2 h with the subsequent hydroxylation and treating in He flow at 623 K.
at lowest V content. Line II (peak at - 560 ppm) appears at about 5 V atoms per nm 2. The line with the axial anisotropy of the chemical shift tensor, line I I l ( 6 . = - 320, 6 n = - 1370 ppm) and line IV ( 6 ± = - 300, 6, = - 1270 ppm) are typical for the samples with increased V content. Line IV prevails at the highest V concentration. The use of magic angle spinning narrows the lines I, I I and IV, but does not change considerably the width of line III. Fig. 5 illustrates the effect of magic angle spinning for the sample V/TiO2-1. Analysis of the 5,V NMR M A S spectra obtained at different rotation frequencies has allowed more precise measurement of the spectral parameters. Line I has 6,~o= - 4 4 0 ppm, 3 6 = 6 . - 6 , = 500 ppm; for line I I 6,so = - 6 0 0 ppm, A 6 = 300 ppm; line I I I has ,z16= 1000 ppm; for line IV 6,so = - 6 1 0 ppm; ,,:16= 900 ppm. Effect of water adsorption on TiO2-1 sample having about one monolayer of vanadium atoms on its surface is illustrated in Fig. 6. Water adsorption results in the transformation of the spectra from those typical for tetrahedral complexes (Fig. 6, spectrum 1 ) to that from V atoms in a distorted octahedral coordination ( Fig. 6, spectrum 2). The evacuation of this sample at 673 K restores the initial spectrum. Fig. 7A shows the 5 ~V N M R spectra obtained for samples prepared by a single supporting cycle of VOCI 3 on TiO2-1, TIO2-2, TIO2-3 and TIO2-4. All of these spectra are composed of different lines. For TiO2-1 the spectrum is composed mainly of lines I I and I, while for
318
L.G. Pmaeva et al. / Journal of Molecular Catalysis 88 (1994) 311-324 44O
I
i
I
O
-600
-100(I
I 6 ,ppm
Fig. 5. Effect of magic angle spinning' ' on-5~V NMR spectra of V/TIO,-_1sample ( V content 5.1 nm - 2, hydroxylated and treated m He flow at 523 K): ( 1) without spinning; (2) MAS spectrum. .,loo
i
i
I
o
400
.1000
I
~ ,ppm
Fig. 6 5~V NMR spectra of TiOz- 1 sample with 5.1 nm 2 V atoms ( I ) treated m He flow at 623 K; (2) after H20 adsorption on the sample 1. TIO2-3 ( c o n t a i n i n g sulfate anions) line I I I prevails in the spectrum. For TIO2-2 ( containing an admixture o f silicon) a line with parameters 6~_ = - 5 0 0 , 6al = - 1000 ppm, which are different f r o m those for lines I, II, I I I and IV, predominate in the spectrum. A relatively narrow line with 6lSO= - 535 ppm and A u = 20 kHz has been o b s e r v e d in the spectrum o f the catalyst prepared from TIO2-4. Fig. 7B illustrates the 51V N M R spectra o f the samples prepared by impregnation o f different TiO2 supports with the solution o f vanadyl sulfate (2 V atoms per n m 2) f o l l o w e d
L.G. Pmaeva et al. / Journal of Molecular Catalysis 88 (1994) 311-324
A
B
I
0
I
I
-1000
I
5 ,pin
I
0
319
.-636
I
I
-'1000
I
,~ , p p m
Fig. 7.5Iv NMR spectra of the samples prepared by different methods on different TiO2 supports: ( 1) TIO2-1; (2) TIO2-3; (3) TIO2-2; (4) TIO2-4; (A) VOCI3 grafted to TiO2 from He flow at 623 K hydrolyzed and treated in He flow at 623 K; ( B ) prepared by impregnation from aqueous solution of VOSO4 followed by drying at 383 K and calcination at 723 K.
by drying at 383 K and calcination at 773 K. As can be seen from the comparison of Figs. 7A and 7B, the spectra of the samples prepared by two different methods are similar and composed of the same lines.
4. Discussion According to the literature data [ 11,21], the TiO2 surface contains two main types of hydroxyl groups localized on the terminal and bridged oxygen atoms with a formal charge of - 1 / 3 and + 1/3 on the protons, respectively. Existence of two types of hydroxyls follows also from IR data [7-13,22]. Two main peaks, A and B, at 6 = 2.9 (A) and 6.4 ppm (B), respectively, (Fig. 1) agree with this model. Downfield peak (B) can be attributed to the positively charged, more acidic hydrogen atoms localized on the bridged oxygens and forming the weak hydrogen bonds with adjacent oxygens. The highfield peak (A) at 6 = 2.9 ppm can be ascribed to the "more basic" hydrogen atoms bonded to the terminal oxygens. Such an assignment is confirmed additionally by the fact that the peak B is easily removed during dehydroxylation while much higher temperatures are necessary to remove peak A from the spectrum ( Fig. 1). A decrease of the concentration of the OH groups during surface dehydroxylation of TiO2 from a value of 9.34 nm -2 (at 300 K) to about 3.2 nm -2 (at 710 K) has also been
320
L G. Pmaeva et al. /Journal of Molecular Catalysts 88 (1994) 311-324
observed [ 16]. Unresolved lines in each peak are most probably due to the contribution of OH groups localized on cleavage planes, other than (001). The absence of peak B in the spectrum of the TIO2-2 sample can be explained by formation of S i - O H groups ( 8 ~ 1.9 ppm) on the surface of this sample, containing a large amount of silicon impurity. Second, it is necessary to take into account that the given TiO2 sample is a mixture of rutile and anatase ( see Table 1 ). Rutile, being the more thermodynamically stable and densely packed TiO, modification, is expected to locate on the surface [6]. Indeed, two types of hydroxyl groups were observed in the IR spectra of anatase, and only one for rutile [22]. For the TIO2-4 sample the most obvious explanation of the absence of peak B is the replacement of acidic protons with Na ions from Na2CO3 used for hydrolysis of the titanium isobutoxide. The line at 8 = 11.6 ppm m the spectrum of TiO,-3 containing a large quantity of sulfur most probably is due to S - O H groups on the surface of this sample. According to published data [ 9 ], S - O H groups can exist o n T i O 2 surface up to an evacuation temperature of 773 K. The interaction of VOCI~ with surface OH groups proceeds due to reaction VOCI3 + n T i - O H ~ (TiO),,VOCI3_,, + n H C I
( ! ~
(1)
which results in a decrease of the number of OH groups on the T i O 2 surface. For different values of n the surface V species bound with TiO 2 by one, two or three oxygen atoms can be formed. The ~H NMR data show that V O C I 3 interacts only with OH groups of B type. No interaction with the more basic OH groups of A type with VOCI, has been observed for any of the four types of TiO2 samples studied. This does not support the supposition on the VOCI3 interaction with basic OH groups made previously [6]. The uncertainty in determination of the concentration of surface OH groups from IH NMR spectra prevents finding the exact value of n in reaction ( 1 ). The comparison of the decrease of the intensity of signal from OH groups with the number of V atoms supported on TiO~-1 surface shows that supporting of 9.7 × 1019 g ~ V atoms decreases the OH group quantity by ( 8 . 4 + 2 ) × 10 ~') g J. This means that the averaged stoichiometry of reaction ( 1 ) is between 1 and 2, and suggests the tormation of different types of surface V species including those containing CI atoms in their coordination sphere. The formation of several surface V species tbllows also from 51V NMR spectra. The lines in the 5~V NMR spectra have been assigned on the basis of previously published reports [ 17-19]. As has been shown there, the anisotropy of chemical shift tensor is the most sensitive parameter for characterization of the local environment of the V nuclei. For the tetrahedral coordination in the most cases 8~ 4:62 4: 63, / i 6 = ] 63 -- 81 ] < 600 ppm, while for the distorted octahedral coordination the axial anlsotropy (81 = 82 4: 8~) is typical with anisotropy parameter _18> 600 ppm. Therefore, lines I and II belong to tetrahedral and lines I I I and IV, to distorted octahedral V atoms. The line IV can be attributed to crystalline V205, since its parameters coincide with those of unsupported V205. At the same time, the lines I, II and I I l have parameters different from those for any crystalline V compounds and can be ascribed, therefore, to V complexes bound to support. Adsorption of water results in transformation of the tetrahedral coordination of surface complexes ~lines I and II) to
L.G. Pinaeva et al. / Joun~al of Molecular Catalysts 88 (1994) 311-324
321
the octahedral one. This follows from the appearance of the axially anisotropic line after H20 adsorption (Fig. 6). Line I I I can be attributed to octahedral V atoms in the highly disordered surface complexes. The latter follows from the inefficiency of the magic angle spinning, which cannot decrease the width of the inhomogeneously broadened lines. On the contrary, the lines I and II belong to more regular surface species. Additional information about the possible structure of surface complexes could be obtained from 6,so values of surface species. The dependence of 6,.,o for many chlorinesubstituted compounds of the VOCln(OR)3_, type, where R is an organic or inorganic radical, has been reported [ 23 ]. For VOCI2OR compounds 6,so were about - 300 ppm; for VOCI(OR)2 they were in the range - 4 0 0 to - 5 1 0 ppm and for VO(OR)3, in the range - 4 5 0 to - 6 6 0 ppm. Using these data, the lines from VOCI3 supported on TiO2 before hydration can be identified. For these samples the anisotropic spectra with 611 in the range 0 to - 2 0 0 ppm and 6 . in the range - 3 0 0 to - 4 0 0 ppm are observed (6,so varies from - 290 to - 450 ppm). This means that before hydration the supported samples contain two types of complexes bound with the TiO2 surface by one or two bonds: VOCI3-OTi and V O C I - ( O T i ) 2- The hydration would result in the formation of the species of the VO (OH) n(OTi) 3 ,, type, which are supposed to undergo further rearrangement on thermal treatment. At the moment, the detailed structure of surface complexes is not established. However, the complexes I and lI can be attributed to tetrahedral V species with an almost regular structure, probably having OH group in their coordination sphere and bonded with TiO2 surface by one or two bonds. Octahedral complexes I I I detected at higher V concentration most probably are formed due to association of the complexes I and II, which results in rearrangement of their environment. V205 is formed at largest V content as it follows from 611 and 6~_ values observed for line IV. The impurities, contained on support drastically influence the structure of supported complexes. The presence of sodium (TIO2-4) results in appearance of slightly anisotropic line with 6.,o = - 535 ppm, .:16~ 150 ppm. These parameters coincide with those of sodium orthovanadate Na3VO4, indicating its formation in TiO2 sample containing Na impurities. As has been expected, H~O adsorption does not influence the spectrum of this compound. The interaction of supported V with Na, and not with OH groups in TIO2-4 follows also from the ~H NMR spectra, since the intensity of ~H NMR spectra does not change after supporting of the V complexes. The Si-OH groups in TIO2-2 interact with supporting VOC13, as it follows from appearance of a new line in the 5~V spectrum with parameters, typical for the V/SiO2 system (6± = - 5 0 0 ppm, 61L= --1000 ppm, (Fig. 7 A ) ) . This line transforms to a line with 61 = - 300 ppm, 611= - 1200 ppm upon H20 adsorption. The change of the spectrum on water adsorption shows that this line belongs to surface species, while the 6± and 61i values indicate the tetrahedral environment of V nuclei before H20 adsorption. The decrease of intensity of the peak from silanol groups observed in ]H spectra confirms the interaction of V complexes with surface Si-OH groups. Thus, ]H and 5~V data allows one to attribute this line to tetrahedral species bonded with support surface via V - O - S i bond. The presence of SO4 anions on the TiO2 surface results in formation of octahedral surface complexes, most probably, having sulfate anions in their coordination sphere. On the surface of the most clean anatase sample (TiO2-1 ) the complexes of the L II and I I I type are present.
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Comparison of the data presented in Fig. 7A and 7B show that for the same support the structure of surface complexes is determined mainly by the structure of support surface. On the same support different preparation methods result in formation of very similar surface structures provided the treatment temperature is high enough (673-773 K). The smaller line width in the spectra of samples prepared from TIO2-4 and the higher percentage of line II for catalyst prepared from TiO2-1 might be caused by the different treatment temperatures of the supports. (The spectra in Fig. 7A correspond to the samples calcined at 623 K, while the spectra in Fig. 7B correspond to the samples calcinated at 773 K.) This resulted in the formation of Na3WO4 for TIO2-4 and an increase of the concentration of complexes I I I for TiO2-1. Therefore, one can conclude that the main factor that determines the structure and composition of supported vanadium, is the nature of the support surface (i.e., the concentration and the type of OH groups and impurities). The treatment temperature and V concentration of the samples also influences the structure of surface complexes. Recently the 5~V NMR data on the vanadium oxide species formed on anatase and rutile for ambient atmospheric conditions have been reported [ 24]. The authors reported about the marked influence of surface contaminations on the structure of the surface V complexes. This is in agreement with the results obtained in this work. Here we show that formation of the surface species takes place as a result of their interaction with surface OH groups or with contaminations of the support. It is also shown here, that on a clean TiO2 surface at low V content the formation of two tetrahedral sites takes place, while at increased V content octahedral vanadium is also present on the catalyst surface. The tetrahedral coordination is readily transformed to an octahedral one on water adsorption. Thus, the data presented in this paper provide important information on the types of V species on the TiO2 surface.
5. Conclusions
From the studies of the V-Ti samples obtained by different ways on different T i O 2 supports, the following conclusions can be made: (i) Supported V species selectively interact with surface OH groups. (ii) The types of the surface vanadium complexes depend mainly on the surface chemistry of the support and do not depend significantly on the preparation method, provided the treatment temperature is high enough (600-700 K). On a clean TiO~ surface and on the surface containing S O l - ions, V O C I 3 interacts with the more acidic OH groups. On T i O 2 containing silicon or sodium a t o m s V O C I 3 interacts with SiOH groups or Na ions, respectively. (iii) Several types of V complexes are formed on TiO2 surface. Vanadium in the surface complexes at low vanadium concentration has a distorted tetrahedral environment from oxygen atoms (complexes I and |I). Complex I most probably has OH group in its coordination sphere. The increased vanadium content results in formation of associated complexes with V in octahedral environment (complex III).
L.G. Pmaeva et al /Journal of Molecular Catalysis 88 (1994) 311-324
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6. Acknowledgement We thank V.Ph. Lyakhova for the preparation of impregnated with vanadyl sulfate TiO2 samples.
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