J. Phys. Chem. 1991,95,4826-4832
4826
tween the colloids can be estimated. In doing so we find with decreasing separation, with only one exception, an increasing attraction down to a separation of 1 A. The depth of this attraction ranges from 4- to 8kBT. At shorter separations a repulsion ap-
pears. The exception is found for Zdl = -80 in a 0.1 M salt solution where the initial long-ranged potential is repulsive. Most likely these effective potentials, acting between colloids in a dispersion, would be strong enough to cause aggregation.
Elementary Surface Reactions in the Preparation of Vanadium Oxide Overiayers on Silica by Chemical Vapor Deposition Kei Inumaru, Toshio Okubara,* and Makoto Misono Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan (Received: September 28, 1990)
Elementary processes of the formation of vanadium oxide overlayers by the adsorption4ecomposition of VO(OC&), vapor on high-surface-area Si02were studied by IR (infrared) spectroscopy,TPD (temperature-programmed decomposition), and the stoichiometryof the surface reactions. In this study the adsorption4ecomposition process will be called a CVD (chemical vapor deposition) cycle. The structure of the vanadium oxide prepared by repeated CVD cycles was characterized by XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy). After the introduction of VO(OC2H5)3onto the Si02 at 423 K, the 1R peak due to the surface Si-OH groups disappeared, and the number of ethanol molecules evolved agreed with that of the surface Si-OH, indicating that all the Si-OH groups reacted with VO(OC2H&. On the basis of the stoichiometry of the gas-phase products and the vanadium atoms, it was confirmed that two surface species, Si-O-VO(OC2H5)2 (1) and (Si-0)2-VO(OC2HS) (2) were formed, the fraction of species 2 being 0.72482 and 0.57-0.59 for Si02pretreated at 523 and 773 K, respectively. IR and TPD revealed that these species decomposed upon heat treatment to form vanadium oxide through the formation of V-OH and ethylene. The XPS peak ratios of V to Si as well as the XRD data showed that repeated CVD cycles gave highly dispersed vanadium oxides on SiOz as compared with that prepared by an impregnation method, especially at high loading levels.
Introduction Recently, thin films of metal oxides dispersed on oxide supports have attracted much attention as catalysts. By the formation of oxide thin film, not only an increase in the surface area of the oxide overlayer but also the generation of active sites having novel functions is expected.' Vanadium oxide is interesting in this respect, as the catalytic activity of supported vanadium oxide has been reported to be sensitive to its microstructure. Mori et al. classified various oxidation reactions into structure-sensitive and -insensitive reactions on the basis of the activity per one V = O on the surface.2 Oyama et al. observed that the selectivity of ethane oxidation over V20s/Si02 depended strongly on the vanadium loadings, while that of ethanol oxidation was insen~itive.~ According to Wachs et al., V20s monolayers supported on Ti02 showed a high selectivity for oxidation of o-xylene.' Therefore, control of the microstructure may be necessary if the catalytic performance of supported vanadium oxide is to be improved. Chemical vapor deposition (CVD) is a useful method for the preparation of highly dispersed oxide Here, we call the process of deposition using reaction between surface sites such as OH groups and vapors of metal compounds a CVD methodeg Bond et al. found that a V205/Ti02 catalyst prepared by the reaction between VOC13 vapor and surface OH groups showed a high selectivity in the oxidation of o-xylene to phthalic anhydride.5 We reported previously that a V2Os overlayer on S i 0 2 obtained from VO(OC2H5)3 vapor was highly dispersed.'O Besides these examples, there are several reports on CVD preparations of solid acid catalysts," and on AES" and analyses of oxide overlayers. In order to control the structure of V205overlayers formed by CVD, the elucidation of its elementary reactions during CVD is necessary. Kijenski et al. studied the reaction between VO(0C4H& and the surface OH group of oxides and observed that the numbers of V atoms deposited were nearly equal to those of the surface OH groups in the cases of A1203and Si02.13 How* T o whom correspondence should be addressed.
ever, elementary reactions during the adsorption and deposition are still obscure. In the present study, we have tried to elucidate the elementary surface reactions in the CVD process using VO(OC2H5)3 and the structure of vanadium oxide overlayers on SiOz by using XPS, XRD, IR, and TPD. Experimental Section Materials. Si02(Aerosil 200; 203 m2 g-I) was calcined at 773 K for 5 h in air and was stored at room temperature. The density of OH groups on the surface of the S i 0 2 was measured by two methods. The first was a titration method using sodium naphthalene as described in the 1iterat~re.l~The second was a H, D exchange between Si-OH and (CD3)&O, which is similar to a method described by Larson et aLI5 After S O z (about 0.3 g) (1) Bond, G. C.; Flamerz, 1989, 87, 65.
S.;Shukri, R. Faraday Discuss. Chem. Soc.
(2) (a) Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1984,88, 2735. (b) Mori, K.; Miyamoto, A.; Murakami, Y. J . Phys. Chem. 1985,89, 4265. (3) Oyama, T. S.;Somorjai, G. A. J . Phys. Chem. 1990, 94, 5022. (4) Wachs, I. E.; Saleh, R. Y.; Chan, S.S.; Chersich, C. C. Appl. Caral. 1985, 15, 339. (5) (a) Bond, G. C.; Brcckman, K. Faraday Discuss. Chem. Soc. 1981, 72, 235. (b) Bond, G. C.; Kbnig, P. J . Caral. 1982, 77, 309. (6) Niwa, M.; Hibino, T.; Murata, H.; Katada, N.; Murakami, Y. J . Chem. Soc., Chem. Commun. 1989, 289. (7) Imizu, Y.; Tada, A. Chem. Lett. 1989, 1793. (8) Sato, S.;Urabe, K.; Izumi. Y. J . Catal. 1986, 102, 99. (9) Niwa, M.; Kato, M.; Hattori, T.; Murakami, Y. J . Phys. Chcm. 1986, 90, 6233. (IO) Inumaru, K.; Okuhara, T.; Misono, M. Chem. Lerr. 1990, 1207. (11) Ohhara. T.; White, J. M. Appl. Surf, Sci. 1987, 29, 223. (12) Jin, T.; Okuhara, T.; White, J. M. J . Chem. Soc., Chem. Commun. 1987, 1248. (13) Kijenski, J.; Baiker, A.; Glinski, M.; Dollenmeier, P.; Wokaun, A. J . Caral. 1986, 101, 1. (14) Kijenski, J.; Hombek, R.; Malinowski, S. J . Carol. 1977, 50, 186. (15) Larson, J. G.;Hall, W. K. J . Phys. Chem. 1965, 69, 3080.
0022-365419 1 12095-4826SO2.5010 0 1991 American Chemical Society
CVD Preparation of V2O5 Overlayer on Silica
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4821
e n
9
. I v hI
$
6
8
4
.9 U-d
Figure 1. Illustration of high-vacuum system: (a) microbalance; (b) mass spectrometer; (c) sample basket; (d) VO(OC2H5),in Pyrex tube;
it/
(e) ionization gauge; (f) stop cock; (g) Pirani gauge; (h) to turbomolecular pump.
was evacuated at 523 or 773 K in a closed-circulation system, it was exposed to 30 Torr of (CD3)2C0 at 423 K. The isotopic composition of the acetone was measured by a mass spectrometer (Inficon Quadrex-100). The number of surface OH groups on the Si02was calculated, assuming that the equilibrium fraction of D in acetone is equal to that in the hydroxy groups on the SOz. VO(OC2H5)3 (Tri Chemical Laboratory; 99.9999% on a metal base) was purified by freeze-pumpthaw cycles and was stored in a glass tube covered by Al foil to prevent decomposition by The vapor pressure Of V0(0C2H5)3 by a Pirani gauge was 7 X Torr at 298 K. The details of the CVD preparation will be described below. Conventional V205/Si02catalysts were prepared by incipient wetness impregnation of Si02with an aqueous oxalic acid solution containing NH4V03 (pH was about 2). The solution used was 1.7 cm3/g of Si02,where the concentration of the solution was in the range from 0.23 to 1.56 mol.dm-). The sample was dried a t 363 K overnight and calcined at 623 K for 1 h in air. The loading amounts of vanadium were varied from 3.4 to 19.1 wt % as V205. These catalysts are denoted as V,O5/SiO2(Imp). Apparatus and Procedure of CVD. As illustrated in Figure 1, the high-vacuum system used here is equipped with a microbalance (an improved Shimadzu TG-30) and a mass spectrometer. This system can be evacuated to Torr by using a turbomolecular pump. Si02(20-50 mg) in the quartz basket on the microbalance was preevacuated a t 523 or 773 K for 1 h and then cooled to the desired temperature. After the pressure of the system reached less than lo-6 Torr, VO(OC2H5)3 (7 X IO-, Torr) was introduced, keeping the sample at 423 K (in some cases, at 298 K). The weight change by the adsorption (or reaction) was monitored with the microbalance. After the sample weight became constant, it was heated in a vacuum a t the rate of 10 K m i d up to 623 or 723 K and was kept at the temperature for 1 h. The spectra of desorbing gases were recorded with the mass spectrometer. This adsorption4e”position process is called a CVD cycle. The first run of the CVD cycle is denoted as 1st-CVD cycle. The sample was cooled down to 423 K after the 1st-CVD cycle, and then the CVD cycle was repeated (2nd-CVD cycle, 3rd-CVD cycle, etc.). The catalysts prepared by CVD cycles are designated as V2OS/SiO2(CVD-623or -723), where the figure in parentheses is the decomposition temperature. Some of them were calcined a t 623 K in air for 1 h; these are shown as, e.g., V205/Si02(CVD-723)-calcined. In order to determine the amounts of gaseous products during the CVD process, CVD was performed in a closed-circulation system (200 cm3) equipped with an on-line GC (TCD; Shimadzu 8A). After the Si02(about 0.3 g) was evacuated a t 523 or 773 K for 1 h, VO(OC2H5)3was introduced at 423 K, together with He (IO Torr) as a carrier gas. The gases evolved were collected in a trap at liquid nitrogen temperature and were analyzed after the vaporization. Then the sample was heated up to 723 K at the rate of IO K min-I. The gas-phase molecules produced during the thermal decomposition were also collected and were analyzed. Measurements of IR and XPS. A self-supporting disk of Si02 (20 mg, 2 cm in diameter) was evacuated at 773 K for 1 h in an IR ce11,16 and it was exposed to 7 X Torr of VO(OC2H5),
l‘
0
10
20
30
40
50
60
Time / min Figure 2. Weight changes of SiO, during the 1st-CVD cycle using VO(OC,H,),: (a) 298 K adsorption: (b) 423 K adsorption; (i) evacuation; (ii) heating to 723 K (iii) after the heating. The horizontal broken line shows the weight corresponding to the surface monolayer of VO(0C2H5)3.
at 423 K for 3 h. After evacuation at 423 K, the IR spectra were recorded at 298 K with a JIR-10 FT-IR spectrometer (JEOL). Then the sample was heated stepwise to 723 K and was kept for 0.5 h for each temperature. The IR spectra were taken at 298 -, K.
For the XPS measurement, self-supporting disks (about 10 mg, 1 cm diameter) were used, and the spectra were recorded with
a JPS-90SX spectrometer (JEOL). The pressure in the chamber was kept under IO” Torr. The binding energy of V was estimated in reference tc: that of Si2p3/2,1/2of Si02 (103.8 eV), which was determined on the basis of that of Au (4f7/,, 84.0 eV) evaporated on Si02. Other Measurements. X-ray diffraction patterns were obtained with a Rotaflex (Rigaku Denki Co. Ltd.) using a Cu Ka source. The content of vanadium was measured by ICP (Nippon Jarrell-Ash Co. Ltd., ICAP-575), where the samples were dissolved in an aqueous solution of HF. Surface areas were determined by the BET method using N2 adsorption a t 77 K after the pretreatment a t 573 K. Results Gravimetric Measurement during CVD. Figure 2 shows the weight changes observed during the adsorption and thermal decomposition in the case of SiO, preevacuated at 773 K. The weight changes are normalized by the surface area of the original Si02. When VO(OC2H5)3vapor was introduced at 298 K, the weight increased rapidly and became constant in about 10 min. At 423 K, the sample weight became constant after 30 min. The final increases in the weight were 8.1 X 10-4 and 2.8 X l0-l g m-2 at 298 and 423 K, respectively. During the evacuation at 298 K, a great decrease in the weight was observed for the 298 K adsorption sample. On the other hand, the decrease during the evacuation at 423 K was only 4% for the 423 K adsorption sample. The amounts that remained after the decomposition at 723 K were 1.8 X IO4 and 1.6 X lo4 g m-2 for the 298 and 423 K adsorption, respectively. When Si02was pretreated at 523 K, the weight increase on adsorption at 423 K was 4.4 X 10-4g m-,, which is about 1.5 times that for SiOl preevacuated at 773 K. The weight of vanadium oxide that remained after the decomposition was also 1.5 times greater. Figure 3 shows the amounts of vanadium oxide loaded on Si02 as a function of the number of CVD cycles. The time taken for each CVD cycle was about 3 h. The loaded amounts of V20J measured by the microbalance during CVD were in agreement (16) Mizuno, K.; Ikeda, M.; Imokawa, T.; Take, J.; Yoneda, Y. Bull. Chem. SOC.Jpn. 1976, 49, 1788.
4823 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
Inumaru et al.
r 3745
0
1
2
3
4
5
6
7
-
8
numberofCVD cydm
Figure 3. Changes of the loading amount of vanadium oxide as a function of run number of CVD cycles: 0 , measured by microbalance; 0 , measured by ICP. VO(OC2HI)3was introduced at 423 K to SO2pretreated at 773 K. The decomposition temperature was 723 K.
r 3145
3900
3800
3700
9600
3500
wave ?umber / cm-1 spectra of 16.8 wt % V20JSi02(CVD-723): (a) preevacuated at 773 K; (b) evacuated at 723 K after the 7th-CVD (c) calcined in air (600 Torr) at 623 K for 1 h; (d) treat4 with 20 Torr of H 2 0 at 623 K for 1 h and evacuated at 623 K. Figure 5. IR
Wave number / cm-1 Figure 4. Changes in the IR spectra in the OH region of SO2 during the 1st-CVD: (a) precvacuated at 773 K; (b) after introduction of VO(OC2H& at 423 K; (c) evacuated at 523 K; (d) 623 K; (e) 723 K; (f) V201 evacuated at 573 K. with those by ICP. For the 1st-CVD cycle, 3.6 wt % of V205was loaded. The amount of vanadium oxide loaded in the 2nd-CVD cycle was about half that of the 1st-CVD cycle. After 2nd-CVD, the loaded amount in each CVD was nearly equal to that of the 2nd-CVD. On repeating CVD cycles seven times, a sample having 16.9 wt % of V205was obtained. Changes in IR Spectra during 0. The IR spectra in the OH stretching region changed after the 1st-CVD cycle as given in Figure 4. For S O 2 evacuated at 773 K, a sharp peak due to Si-OH groups appeared at 3745 cm-I (Figure 4a), together with a broad band centered at about 3660 cm-*. The sharp peak disappeared completely after the introduction of VO(OC2HS)3 at 423 K (Figure 4b), while the broad peak was not influenced. As will be described later, this broad band is assigned to Si-OH in the Si02bulk. When this sample was heated at 523 K in a vacuum, a new peak was observed at 3660 cm-' (Figure 4c). At the same time, a part of the Si-OH peak (3745 cm-') reappeared. The intensities of the two peaks increased as the temperature was raised to 623 K (Figure 4d). A further increase to 723 K decreased the peak at 3660 cm-', but the Si-OH peak continued to increase. The intensity of the Si-OH peak restored at 723 K was about 80% of the one of the fresh S O 2 (Figure 4e). Upon repeated CVD cycles, the IR spectra changed similarly to those in Figure 4b-e, although the intensities of the Si-OH peak that reappeared as a result of the heating were only 65 and 50% of the fresh Si02ones after the 2nd-CVD and the 3rd-CVD cycles, respectively. The bulk V 2 0 5 gave a sharp peak at 3671 cm-I (Figure 40. This wavenumber is close to the peak at 3660 cm-I in Figure 4c-e. The 1R spectra in the OH stretching region after the 7th-CVD are given in Figure 5. The intensity of the Si-OH peak decreased to only 20% of that of the fresh SiOz. The Si-OH peak was not affected by the calcination in air or the treatment with H 2 0 (20 Torr) at 623 K.
I
C
I
Temperature/ K Compounds observed during temperature programmed decomposition of VO(OC2H,), on SO2during CVD cycles: (a) 1st-CVD (b) 2nd-CVD; (c) 3rd-CVD (-) C2H,; (---) C2H,0H; (-) CH,CHO (-) H20. VO(OC2HS),was adsorbed at 423 K on SO2pretreated at Figure 6.
173 K. Tempemture-Prognmmed Decomposition of VO(OC2H5)3on SiO,. Results of the thermal decomposition of VO(OC2HS),on SiO, during the CVD cycles are given in Figure 6, where the relative desorption rates were calculated from the signal intensity and relative sensitivity of the mass spectrometer. For the 1st-CVD cycle, C2H,0H appeared from about 470 K and C2H4 at about 490 K (Figure 6a). A peak of H 2 0 was observed at a temperature similar to that for C2H4. It should be noted that the amounts of C,H50H and CH3CH0 desorbed were appreciable in the 2ndand 3rd-CVD, while these amounts were small in the 1st-CVD. The amounts of C2HSOHand CHICHO increased as the number of CVD cycles increased (Figure 6b-c). Cas-pknss Roductsduring CM)Recesses. The density of OH groups and the number of V atoms after the 1st-CVD cycle are listed in Table I. The densities of OH groups determined by the
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4829
CVD Preparation of VzOs Overlayer on Silica TABLE I: A w t Of G~-Ph.seP d W t in tbt lst-CVD C V C ~
amount of amount of gas phase productd/molecules Vr/atoms lit? N(C2H50H)' N(others)c N(TD)' total nm-2 titrn 7.17 2.52 4.2 4.34 2.55 523 4.2 0.28 1.6 1.8 1.20 773 1.7 1.91 0.13 1.61 3.65 "Pretreatment temperature of SOl. bDensity of OH groups, nm-'. cReference 14. dAs C2 compounds. uProducts when VO(OC2H,)3 was adsorbed at 423 K: N(C2H50H),the amount of ethanol; Mothers), the total amount of CH,CHO, (C2HJ)20and C2H,. /Products during thermal decomposition up to 723 K. ZMeasured by ICP. pretreat.O temp/K
Si-OHb/nm-2 H,D-exch 3.7
1.o
z
0.8
H
t
0.6
H
0.4
I
0.2
n 0
I
I
I
10
20
30
VzO, loadings / wt% F'tgure 7. XPS intensity ratio ( f v / f s )as a function of the loading amount
of VzO!: 0 , V2O1/SiO2(CVD-623); 0, V20~/Si02(CVD-723);m, V2OI/S~OZ(CVD-623)-calcined; 0 , V20,/SiO2(CVD723)-calcined; A, V205/Si02(Imp). VO(OC2H5),was adsorbed at 423 K on Si02 pretreated at 773 K. two different methods and those in the literature" were nearly q u a l . When the vapor of VO(OC2HS)3was introduced at 423 K, C2HSOH appeared in the gas phase. The amounts of the gaseous products are also given in Table I. It is noted that the amounts of C2HSOHevolved during the adsorption are very close to those of the OH groups. It was confirmed that a negligibly small amount of C2HSOHwas adsorbed on S O 2under the same conditions. If the amounts (based on C2) of the gaseous products formed in the adsorption steps and those in the thermal decomposition steps are added for each cycle, they are approximately 3 times the amounts of the deposited V atoms that were determined by ICP (Table 1). This shows that all the ethoxy groups were decomposed into gaseous products. In the 2nd-CVD cycle, the amount of C2HSOHevolved during the adsorption was about half the amount for the 1st-CVD cycle. Instead of C2HsOH, C H 3 C H 0 increased. The total amount of the products (based on C,) was 2.4 times that of V, suggesting that some carbon species remained on the surface in the 2nd-CVD. X-ray Photoelectron Spectra and XRD Patterm of V205/Si02. The relative intensities of the XPS peak of V2p312 to that of Si2p are compared in Figure 7 for the samples prepared by CVD and by impregnation as a function of the amount of V deposited. The ratio Iv/Isi, did not increase much for V20s/Si02(Imp) as V content increased and was 0.23 at the 19.1 wt 7% loading. On the other hand, it continued to increase to higher levels of V loading for V2OS/SiO2(CVD-623,-723), and reached 1.1 at 24.7 wt %. Although the difference in the decomposition temperature in the CVD cycles did not affect the ratio before the calcination, it had a remarkable effect after calcination. The Iv/Isiratio decreased greatly by the calcination of V20S/Si02(CVD-623), while the decrease was not significant in the case of V20J/Si02(CVD-723). XRD patterns of V20s/Si02and bulk V20s are shown in Figure 8. XRD peaks due to VzOs were observed for V20s/Si02(Imp) containing 5.8 wt 3'% or above of V20s. The XRD patterns were similar for V205/Si02(Imp)and V2O5, while the intensities were smaller for the former. For V2OS/SiO2(CVD-723) (3.6-16.9 wt %), no peak due to V,Os was detected (Le., Figure 80, except that weak peaks due to V20, were found only for a highly loaded (17) Fripiat, J. J.; Uytterhaven, J. J. Phys. Chem. 1962, 66, 800.
i
-3fi h
Oh-
-
j ,
I
I
I
,
I
10 15 20 25 30 35 40 45 50 55
20 / deg Figure 8. XRD patterns of V20,/Si02: (a) 3.4 wt 4% V20JSi02(Imp); (b) 5.8 wt 96 V205/Si02(Imp);(c) 8.9 wt 96 V20S/Si02(Imp); (d) 18.4 wt % V20,/Si02(Imp); (e) 7.9 wt 96 (CVD-723)-calcined; ( f ) 16.9 wt 5% V205/Si02(CVD-723);(g) 16.9 wt % ' V2O5/SiO2(CVD723)-calcind, (h) 17.0 wt % V205/Si02(CVD-623);(i) 17.0 wt 96 V20,/Si02(CVD623)-calcined; (j)bulk V2O5. VO(OC2H5), was adsorbed at 423 K on SiO, pretreated at 773 K.
sample after calcination (16.9 wt % V2OS/SiOz(CVD-723)-caIcined (Figure 8g)). It should be noted that the XRD pattern of this sample was quite different from the bulk V20s; the (101). (200), and (301) peaks were the main peaks for 16.9 wt % V20s/Si02(CVD-723)-calcined, and the (010) peak, which was the main peak of the bulk V20s, was negligibly small. In the case of the V20S/Si02(CVD-623),broad peaks due to V,Os crystallites appeared for the 17.0 wt 96 sample before calcination (Figure 8h). Calcination of the sample made the peaks sharper and more intense (Figure 89. Discussion Elementary Surface Reactions in 1st-CVD Cycle. As shown by the horizontal broken line in Figure 2, the weight corresponding to the surface monolayer of VO(OC2H5)3is 7 X 10-4 g m-z. This was estimated from the surface area of SiO, and the molecular cross section of VO(OC2HS)3,0.48 nm2. The molecular cross section was calculated from the density of the liquid.'* The amounts of VO(OC#S)~ adsorbed at 298 and 423 K were nearly equal to and about one-third of a monolayer, respectively. In spite of this difference, the amounts deposited as vanadium oxide after (18) Bradley, D. C.; Mchrotra, R. C.; Gaur, 0. P. Metol Alkoxtdes; Academic Press: New York, 1978; p 109.
4830 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
1
2
Inumaru et al. TABLE II: Fraction of Swcies 2 at the 1st-CVD n pretreat. temp W / ( 1 + 2)) of SiO,/K eQ2 eQ4 eQ2 eQ4 523 773
\
YSi.
1.74 1.58
1.82 1.57
0.74 0.58
0.82 0.57
\ O\ 7 Si-0 ;V=O
-\sp /
Figure 9. Suggested surface vanadium species formed on Si02.
decomposition were nearly equal in both cases. This indicates that a considerable part of the VO(OC2HS), was physically adsorbed at 298 K and was readily desorbed during the thermal decomposition. The amounts of vanadium oxide deposited in the 1st-CVD cycle corresponded to about 1/8 the monolayer over S O 2 , where the (010) layer of V205 crystallites is assumed for the calculation of monolayer. The elementary surface reaction in the 1st-CVD consists of two parts: (1) the reaction of VO(OC2H5), with the surface Si-OH to form surface species and (2) the decomposition of the surface species to vanadium oxide on SiOz. As shown in Table I, the OH densities obtained by the two different methods agreed with the values in the literature.” Therefore, the densities determined here may be reliable. The number of C2HSOHmolecules produced by the reaction of VO(OC2H5), with the surface of SiO, at 423 K (Table I) was close to that of the surface OH. This result demonstrates that all surface OH groups were involved in the reaction, and one OH group produced one molecule of C2H50H in the gas phase, forming one V-0-Si bond on the surface. The IR data in Figure 4 support the Occurrence of the above reaction. The Si-OH band (3745 cm-I, Figure 4a) disappeared on the introduction of VO(OC2HS),. As for the broad Si-OH peak, Davydov et al. reported that this band did not change by contact with D 2 0 (gas) at rmm temperature, while the sharp OH band at 3750 cm-I changed rapidly to Si-OD.19 The broad band is probably due to the OH groups in the Si02bulk, which is inert for the reaction with VO(OC2HS), as shown in Figure 4b. As described above, one surface OH group can react with one ethoxy group of VO(OC2H5)3.So, one molecule of VO(OC,HS), possibly reacts with either one, two, or three OH group(s) as shown in eq 1, where n is the number of OH group(s) with which one VO(OC2HS), molecule reacts ( n = 1-3). VO(OC2H5), + nSi-OH (Si0-)nVO(OC2H5)3-n+ nC2H50H ( n = 1-3) (1)
-
Plausible surface species formed by the reaction of OH groups with VO(OC2H5), are shown in Figure 9. The species 1,2, and 3 may be formed when one molecule of VO(OC2H5)3reacts with one, two, and three OH group(s), respectively. Since one OH group produces one CzHSOHby the reaction, n can be derived from eq 2, where N(CzH50H) is the number of CzH50Hmolen = N(C2H50H)/N(V) (2) cules produced when VO(OCzH5)3was in contact with SiOz and N(V) is the number of V atoms loaded onto the SiO,. n can also be calculated another way. Equation 3 also holds as long as no carbon-containing species remained on the surface during the 1st-CVD cycle, where N(TD) is the number of gasphase molecules (C, base) formed during thermal decomposition. N(C2HSOH) + N(TD) = 3N(V) (3) As shown in Table I, eq 3 is valid for the 1st-CVD. Then, n can be expressed by eq 4. n = 3N(C2H50H)/(N(C2H50H)+ N(TD)) (4) Therefore, n can be calculated from either eq 2 or eq 4, using the different data obtained by GC and ICP. The values of n (19) Davydov, V. Ya.; Kiselev, A. V.;Kiselev, S.A.; Polotnyuk. V. 0.-V. J . Colloid Interface Sei. 1980, 74, 378.
(100)
( 1 11)
00
@OH Figure 10. Surface structures of cristobalite.
si
obtained are listed in Table 11. The two equations gave similar values: n’s are 1.7-1.8 and 1.6 for SiO, pretreated at 523 and 773 K, respectively. Now, the structure of the surface species formed from VO(0C,H5)3 and their fractions will be discussed. The surface structure of cristobalite has often been used as a model of the surface of amorphous Peri et a1.20and Sindorf et aL2’ discussed the behavior of dehydration-hydration using the (100) and (1 11) surfaces of cristobalite. Hockey adopted the (1 11) face including some kinds of defects as his model of Accordingly, it may be reasonable to use the (100) and (1 1 1) faces of cristobalite as models of the Si02surface. Figure 10 illustrates top views of the (1 11) face and the partially dehydrated (100) face of cristobalite.20The distance between the two nearest OH groups on the (1 11) face is estimated to be 5.06 A. The distance between oxygen atoms of the 0-V-O bonds of VO(OC2HS)j is about 2.6 A on the basis of data for VO(OCH3)3.D Thus, one VO(OC2H5), molecule can react with one OH group to form a Si-0-V but cannot react with two bonds at the same time. In the case of the (100) face, the shortest distance between OH groups is 2.53 A, and the distance between the rows of OH groups is 5.06 A. Depending on the location of the OH groups, one VO(OC2H5), molecule can react with either one or two OH group(s) on the (100) face, but the reaction with three OH groups is excluded for steric reasons. Therefore, among the three structures in Figure 9 , l and 2 are the possible candidates for the species formed from VO(OC2H5)3on SiO,. If the fraction of 2 is 9 (=2/(1+ 2)), the value of n is calculated to be 2 9 + (1 - 9 ) = 9 1. The values of CP calculated from n are summarized in Table 11. It is reasonable that the value of 9 is greater for the S i 0 2 with the higher density of surface OH groups, since the pair sites of OH must increase with the increase in the OH density. Elementary Reactions during the Thermal Decomposition Step of CVD. Ethylene was the main product in the thermal decomposition (Figure 6). A new peak that appeared at 3660 cm-I after the 5 2 3 K evacuation (Figure 4c) is assigned to V-OH, since its peak position is close to that of the OH group on V2O5 (Figure 4f). These results demonstrate that the ethoxy groups of the surface species (1 or 2) decomposed at the initial stage as in eq
+
5.
>V(=O)--OC,HS
-
>V(-O)--OH
+ C2H4
(5)
The changes in the peak intensities of V-OH and Si-OH upon heating (Figure 4) suggest that Si-OH is regenerated by the (20) Peri, J. B.;Hensley, A. L.,Jr. J . Phys. Chem. 1968, 72,2926. (21) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 1487. (22) Hockey, J. A. Chem. Ind. (London) 1965. 57. (23) Caughlan, C. N.; Smith, H. M.; Watenpaugh, K.Inorg. Chem. 1966, 5, 2131.
CVD Preparation of V2OSOverlayer on Silica
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4831 L,.... e -----A1 - 8 -d,
:
(010)layers
i
Si02 Figure 13. Model for calculation of Iv/lsi. Figure 11. Suggested scheme of Si-OH regeneration with the formation of V O S i and V O V bond.
Figure 12. Scheme of Si-OH regeneration with the aggregation of vanadium oxide.
reaction of V-OH with Si-0-Si or Si-0-V, as shown schematically in Figure 1 1. Another possibility for the regeneration of Si-OH is the aggregation of the vanadium oxide through migration on the surface, breaking some of the V 4 - S i bonds. A probable scheme for this process is given in Figure 12. If the migration of the vanadium species actually took place, decomposition at higher temperatures would have formed greater aggregates. As will be described later, however, the experimental facts show that V205/Si02(CVD)was more highly dispersed when it was decomposed at the higher temperature (Figure 7, CVD-723 series). Therefore, the second possibility may be excluded. As for the surface vanadium species, Went and OyamaU have claimed recently from Raman spectroscopy that the vanadium species held by three V-0-Si links ( V = = O ( U S i ) , ) is present for V205/Si02at low loadings. Although there is no firm evidence, the proposed species H O - V ( = O ) ( U S i ) 2 in Figure 11 may change to the V ( = O ) ( U S i ) 3 through the surface restructure of Si02 caused by, e.g., calcination. In the case of the 2nd-CVD, two kinds of surface sites are available for the reaction of VO(OC2HS)3: (i) the Si-OH regenerated as described above and (ii) the sites on the surface of the vanadium oxide originally loaded. The IR peak of V-OH (3660 cm-I) as well as that of Si-OH (3745 cm-I) disappeared when VO(OC2H5)3was added to the sample giving spectrum d in Figure 4, indicating that V-OH is also active for the reaction with VO(OC2H5),. Lewis acid sites on V205/Si02(CVD-723) or coordinatively unsaturated sites of V which were indicated by the IR of the adsorbed pyridine are also probable active sites. As shown in Figure 3, the amount of vanadium oxide deposited at each CVD cycle after the 2nd-CVD did not change, although the number of the Si-OH decreased. Therefore, not only Si-OH but also vanadium oxide is an active site for the deposition. It is to be noted that, among the two kinds of reactive sites, the Si-OH takes part in lateral growth of the vanadium oxide overlayers along the surface of SO2, and the sites on vanadium oxide thicken the overlayers. Therefore, the relative amounts and reactivities of the two kinds of sites are the key factors determining the structure of the overlayer. If the decomposition is performed at 623 K, at which temperature only a little Si-OH was regenerated (Figure 4d), the vanadium oxide deposited becomes more important as an active site. Contrary to this, since the decomposition at 723 K regenerated a large number of Si-OH sites with very few V+H, Si-0-V bonds are mainly formed by CVD cycles. This is the reason for the formation of highly dispersed vanadium oxide by high-temperature decomposition as will be discussed in the next section. The Structure of Vanadium Oxide on Si02. The structure of supported metal oxides has often been studied using XPS. Bond observed that the ratio of the XPS peak intensities (Iv/ITi) et (24) Went, G. T.; Oyama, S.T.; Bell, A. T.J . Phys. Chem. 1990,944240.
of V20S/Ti02became constant when the content of vanadium oxide exceeded about 5 wt %. They inferred that the V2O5 crystallites grew perpendicularly to the surface like towers, mering only 5-1295 of the first monolayer formed on Ti02. The differences in the Iv/Isiratios among the three catalysts, V2OS/SiO2(Imp),V205/Si02(CVD-623),and V205/Si02(CVD723) are shown in Figure 7. The low Iv/Isifor V20S/Si02(Imp) indicates the formation of aggregates of vanadium oxide. The high Iv/Isifor both V205/Si02(CVD)implies highly dispersed overlayers (thin films or fine particles). The slight decreases of Zv/Isi after the calcination for V205/Si02(CVD-723) indicate the high stability of dispersed vanadium oxide. Contrary to this, the considerable decreases for V20s/Si02(CVD-623) after calcination are due to the aggregation of vanadium oxide. If the decomposition temperature is high, the formation of V U S i bonds is predominant over V-0-V formation in each CVD cycle, as discussed above, and the vanadium oxide overlayer is stabilized by the V U S i bond. The results of the XPS are supported by the XRD measurements. The obvious XRD peaks of V205/Si02(Imp) (Figure 8b-d) mean that V2OSon V20S/Si02(Imp)is present as threedimensional crystallites similar to bulk V2O5. The size of the vanadium oxide crystallites on 18.4 wt % V205/Si02(Imp)was estimated to be 340 and 210 A from the peak widths of the (101) and (010) faces, Roozeboom et al. reported, from Raman spectroscopy studies, that microcrystallitesof vanadium oxide were formed on S i 0 2 when V2O5 was supported by the impregnation method?' while two-dimensional vanadium oxides were produced on A1203and Ti02. The absence of the diffraction peak for V205/Si02(CVD-723) revealed the high dispersion of vanadium oxide. Since the XRD peaks of 16.9 wt % V2OS/SiO2(CVD-723)were very small even after the calcination (Figure 8g), it is probable that most of the vanadium oxide on this sample was still highly dispersed. It is notable that the (010) peak, which is the main peak of the bulk V2OS,was absent for the 16.9 wt % V2OS/SiO2(CVD-723)-calcined, suggesting that the thickness of the crystallites is less than about 30 A. Thus, the crystallites are very thin in the [OIO] direction, having a width of about 120 A as estimated from the (101) peak. Finally, let us speculate about the average thickness of the vanadium oxide on the basis of the XPS data. Here, the overlayers are assumed to have uniform (010) layers of V205, the coverage being B (Figure 13). If the inelastic mean free path (IMFP) of photoelectrons from Si through the V2OSoverlayers is Xsi,v, the number of photoelectrons is reduced by a factor of exp(-Nt/kv) by passing through V205layers having a thickness of Nt, where N is the number of (010) layers and t is the thickness of one (010) layer of V2O5. If Xv,v is the IMFP of photoelectrons from V in , ~ ~IMFP of photoelectrons from Si a V2OSoverlayer and X S ~ the in bulk Si02, the XPS intensity ratio, Iv/Isi, is expressed by eq 6, where uv and usi are the ionization cross sections for V and -I(V) - - avAv,vdv~(1 - exp(-Nt/ AV,V)) (6) I(Si) usiXsUidsi(1 - 0 + B exp(-Nz/Xsi,v)) Si,28respectively, dVand dsi are the numbers of V and Si atoms (25) Bond, G.C.; Zurita, J. P.; Flamerz, S.Appl. Card. 1986, 27, 353. (26) Schemer, P. Gdttinger Nuchrichten 1918, 2, 98. (27) Roozeboom, F.; Mittelmcijer-Hazeleger, M.C.; Moulijn, J. A.; Medema, J.; de Beer, V. H. J.; Gellings, P. J. J. Phys. Chem. 1980, 84, 2783. (28) Scofield, J. H.J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.
4832
J . Phys. Chem. 1991, 95.4832-4837
V2 05 loadings I wt% Figure 14. Calculated XPS intensity ratio (fv/fsi) as a function of the amount of V2OS.The parameter in this figure is the number of (010)
and the specific surface area of the support, respectively. The calculated Iv/Isiis also given in Figure 14 for various values of N. The parameters used in this calculation are given in Table 111. The values of dv, t , and dsi are derived from the crystallographic data of V20529and cristobalite,m and Xsi~i,and Xsi,v are calculated according to the literat~re.~'When N varies from 10 to 2, Iv/Isiincreases by a factor of about 3. The ratios Iv/Isiexperimentally obtained for V2OS/SiO2(CVD-723)-calcined are about 3 times higher than those for V205/Si02(Imp) at high loadings. Therefore, the average thickness of the V2O5 overlayers of V20S/Si02(CVD-723) are estimated to be less than 20% that ' of V205/Si02(Imp). If the thickness of V2O5 on 18.4 wt % V205/Si02(Imp) is assumed to be 210 A, that of 16.9 wt 96 V20S/Si02(CVD-723)dcinedis less than 40 A. This estimate is compatible with the XRD results.
layers accumulated (N). TABLE IIk h8-m U d in Qlculrti~Of Iv/Za IMFP of V photoelectron in V205 XV,V 1.0 nm IMFP of Si 2p photoelectron in V205 hsi,v 1.5 nm IMFP of Si 2p photoelectron in S O z ,Isisi 1.9 nm number of Si atoms in unit volume of SiOz dsi 21.8 number of V atoms in unit volume of Vz05 dv 22.3 r 0.437 nm thickness of ideal V205(010) monolayer A 1.47 X weight of ideal Vz05(010) monolayer
Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. We greatly appreciate partial support by the Asahi Glass Foundation and by the Foundation for the Promotion of Material Science and Technology of Japan. g m-2
in unit volumes of V2O5 and SiO,, respectively. B can be expressed by eq 7,where w, A, and S are weight ratio of V2OSloaded and e = W/(SAN) (7) Si02, the weight of an ideal V2OS(010) monolayer of unit area,
Rdstry NO. VO(OCzH5)3, 1686-22-2;VIOS,13 14-62-1.
(29)Bachmann, H.G.;Ahmcd, F.R.; Barnes, W. H. 2.Krisrallogr. 1961, 115, 110. (30) Wyckoff, R. W. G. Crysral Srrucrures, 2nd ed.; lishers: New York, 1963; Vol. 1, p 318.
(31)
Interscience Pub
Penn, D. R. J. Electron Specrrosc. Relar. Phenom.
1976, 9, 29.
Uttrasonlc Absorption Studies of Surfactant Exchange between Micelles and Bulk Phase In Aqueous Micellar Solutions of Nonionic Surfactants with Short Alkyl Chains. 1. 1,2-HexanedIol and 1,2,3-0ctanetriol M. Frindi, B. Micbels, Laboratoire de SpectromOtrie et d'lmagerie Ultrasonores, 4, rue Blaise Pascal, 67000 Strasbourg, France
and R. Zana* Institut Charles Sadron (CRM-EAHPJ, CNRS- ULP Strasbourg, 6, rue Boussingault, 67083 Strasbourg CPdex, France (Received: December 17, 1990) 1,2-Hexanediol (1,2-HD) and 1,2,3-octanetriol (1,2,3-OT) are known to self-associate in a manner very similar to that of conventional surfactants to give rise to micellelike aggregates (Hajii, S.;et al. J . Phys. Chem. 1989,93,4819). This paper reports on fluorescenceprobing, with pyrene as a probe, of these aggregates and on a study of the kinetics of monomer exchange between aggregates and bulk phase by means of the ultrasonic relaxation method in the frequency range 0.5-100 MHz. Thus the polarity sensed by pyrene solubilized in the aggregates is lower than for conventional nonionic surfactants of the C,E, type. The critical micellization concentrations determined by fluorescence probing are in agreement with the reported values. The ultrasonic relaxation amplitude A and frequencyfi have been found to vary with concentration as expected from the expressions derived for conventional surfactants on the basis of the Aniansson and Wall treatment for the kinetics of surfactant exchange. The full use of these expressions permitted us to obtain the values of the rate constants k+ and kfor the incorporation of a surfactant monomer into, and the dissociation of a monomer from, a micelle, respectively, as well as the standard deviation characterizing the distribution of micelle aggregation numbers (polydispersity) and the volume change upon incorporation. These results are discussed and compared to those obtained for conventional surfactants.
Introduction
systems are now rather well-known. In particular it is widely
*Towhom correspondence should be addressed. 0022-3654/91/2095-4832602.50/0
( 2 ) degiorgio, V. Physics of Amphiphiles: Micelles Vesicles and Micre emulsions; Degiorgio, V., Corti, M.,Eds.; North Holland: Amsterdam, 1985, and references therein.
0 1991 American Chemical Society
