J . Phys. Chem. 1985, 89, 4265-4269
4265
Catalytic Reactions on Well-Characterized Vanadium Oxide Catalysts. 4. Oxidation of
Kenji Mori? Akira Miyamoto,* and Yuichi Murakami Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo- Cho, Chikusa- ku, Nagoya 464, Japan, and Kinuura Research Department, JGC Corporation, Sunosaki-cho, Handa, Aichi 475, Japan (Received: November 28, 1984) The activity and selectivity in the oxidation of butane (a saturated hydrocarbon) on unsupported and supported V205catalysts were investigated in connection with the catalyst structure. It was found that the reaction rate at various concentrations of O2 was proportional to the amount of V5+=0 species in the catalyst, indicating that the surface V=O plays the active oxygen species for the reaction. The specific activity of surface V=O species on unsupported V2O5 changed greatly with the surface structure of the catalyst: The fusion of V2OS markedly decreased the specific activity, while the severe redox treatment of the fused catalyst increased the activity. This means that the butane oxidation on V205catalyst is a structure-sensitive reaction. The specific activity of the surface V=O species on the V205/Ti02(anatase) and V205/Ti02(rutile) catalysts was smaller than that of the unsupported V205,indicating the retarding effect of the support on the activity. This is in contrast to the known promoting effect of TiO, support on the activity of V205. The selectivity to CO or C 0 2 was also found to be sensitive to the surface structure of V205:A rough V205surface is favorable for the formation of CO while a smooth surface leads to the selective formation of C 0 2 .
Introduction Supported metal oxides can exhibit interesting catalytic properties depending on the nature of the support and on the composition of the catalysts.’ However, the activity and selectivity on the supported metal oxide catalyst have not been well clarified in terms of the structure of the metal oxide on support. This seems to be due to the lack of a well-established method to determine the structure of the supported metal oxide, especially the number of active sites. As for the supported vanadium oxide catalysts, we have previously established the rectangular pulse technique which allows the determination of the number of surface V=O species and the number of V205layers on support.2 Furthermore, the structures of V205/Ti02and V205/A1203catalysts have been determined by using various physicochemical measurements together with the rectangular pulse t e c h n i q ~ e . ~By investigating the oxidation of CO, H2 (simple diatomic molecules), ethene (simplest olefin), and benzene (simplest aromatic) on the wellcharacterized vanadium oxide we have found the activity of the catalyst to change greatly with the kind of reactant molecules. A rough V2O5 surface is effective for the oxidation of C 0 , 4 while the activity for the H2 oxidation is determined only by the number of surface V=O species and independent of the surface structure, kind of support, or V205 c0ntent.j Although the activity for the oxidation of ethene or benzene is mainly determined by the number of surface V = O species, the specific activity of the surface V=O species is increased by the Ti02 (anatase) Before discussing detailed reasons for the structure-activityfselectivity correlations on vanadium oxide catalysts, it is indispensable to establish the correlation for various types of reactant m ~ l e c u l e s . Since ~ ~ ~ both ethene and benzene have unsaturated T bond(s), the catalytic behavior of a saturated hydrocarbon seems very interesting. The purpose of this study is then to reveal the activity and selectivity in the oxidation of butane on the well-characterized vanadium oxide catalysts. According to our preliminary experiments, neither methane, ethane, nor propane was oxidized on vanadium oxide catalysts below 773 K, and butane is the simplest saturated hydrocarbon available for kinetic experiments. It should also be noted that this reaction has received much attention in relation to the industrial production of maleic anhydride.*O Experimental Section Catalysts. A V205-Ucatalyst was prepared by the thermal decomposition of NH4V03in a stream of O2at 773 K. A V205-F ‘JGC Corp. *Address correspondenceto this author at the Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606, Japan. ~
catalyst was prepared by fusing the V2O5-U catalyst at 1073 K for 18 h in air, followed by gradual cooling to room temperature. A V205-ROcatalyst was prepared from the V2O5-F catalyst by the reduction-xidation treatment, that is, the reduction in flowing H2 at 673 K for 1 h followed by the reoxidation in flowing O2 (20%) at 673 K for 1 h, and this redox cycle was repeated five times. The number of surface V=O species ( L ) on the catalyst has been determined by using the rectangular pulse technique,, and the results are shown in Table I together with the results of The number of V 2 0 5 layers (N) the BET surface area (SBET). for the catalysts was calculated from L by
N = 2/[LM(V*O,)l
(1)
where M(V205)is the molecular weight of V205. According to the results of x-ray diffraction, UV-visible spectra, IR spectra, X-ray photoelectron spectra, and scanning electron micrographs of the catalyst^,^.^ electronic properties of the catalysts do not ( 1 ) (a) Hucknall, D. J. ‘Selective Oxidation of Hydrocarbons”; Academic Press: New York, 1974. (b) Wainwright, M. S.; Foster, N. R. Coral. Rev. 1979, 19,211. (c) Dadyburjor, D. B.; Jewur, S. S.; Ruckenstein, E. Cafal. Reu. 1979, 19, 293. (d) BielaAski, A,; Haber, J. Catal. Reu. 1979, 19, 1 . (e) Cullis, C . F.; Hucknall, D. J. “Catalysis”; The Chemical Society: London, 1982; Vol. 5 , p 273. (f) Haber, J. Proc. Int. Congr. Catal., 8th 1984 1985, 1-85 and references therein. ( 2 ) (a) Miyamoto, A,; Yamazaki, Y.; Inomata, M.; Murakami, Y. J. Phys. Chem. 1981, 85, 2366. (b) Inomata, M.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1981, 85, 2372. ( 3 ) (a) Inomata, M.; Mori, K. Miyamoto, A,; Ui, T.; Murakami, Y. J . Phys. Chem. 1983, 87, 754. (b) Inomata, M.; Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1983, 87, 761. (c) Murakami, Y . ;Inomata, M.; Mori, K.; Ui, T.; Suzuki, K.; Miyamoto, A.; Hattori, T. ‘Preparation of Catalysts 111”;Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; p 5 3 1 . ( 4 ) Mori, K.; Miyamoto, A,; Murakami, Y. J. Phys. Chem. 1984,88,2735. ( 5 ) Mori, K.; Miura, M.; Miyamoto, A,; Murakami, Y. J. Phys. Chem. 1984,88, 5232. ( 6 ) Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1984,88,2741. ( 7 ) Mori, K.; Inomata, M.; Miyamoto, A.; Murakami, Y. J . Chem. SOC., Faraday Trans. 1 1984, 80, 2655. ( 8 ) (a) Bond, G . C.; Briickman, K. Faraday Discuss. Chem. SOC.1981, 72, 235. (b) Cole, D. J.; Cullis, C. F.; Hucknall, D. J. J . Chem. SOC.,Faraday Trans. I 1976, 72, 2185. (c) Srivastava, R. D.; Stiles, A. B. J . Catal. 1982, 77, 192. (d) van Hengstum, A. J.; van Ommen, J. G.; Bosch, H.; Gellings, P. J. Proc. Int. Congr. Catal., 8th, 1984 1985, IV-297. (e) Koziolowski, R.; Pettifier, R. F.; Thomas, J. M. J . Phys. Chem. 1983,87, 5175. (0 Ono, T . ; Nakagawa, Y.; Miyata, H.; Kubokawa, Y . Bull. Chem. Soc. Jpn. 1984,57, 1205. (g) Wachs, I. E.; Chan, S.S.; Chersich, C. C.; Saleh, Y.“Catalysis on the Energy Scene”; Kaliaguine, S.,Mahay, A,, Eds.; Elsevier: Amsterdam, 1984; p 275. ( 9 ) Miyamoto, A.; Mori, K.; Inomata, M.; Murakami, Y. Proc. Inr. Congr. Catal., 8 f h , 1984 1985, IV-285 and references therein. (10) (a) Matsuura, I. Proc. Inr. Congr. C a r d . , 8th, 1984 1985, IV-473. (b) Ai, M. Proc. Int. Congr. Catal., 8th, 1984, 1985, V-475 and references therein.
0022-365418512089-4265$01.50/0 0 1985 American Chemical Society
4266 The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Mori et al.
TABLE I: Physical and Catalytic Properties of Unsupported V2O5 Catalysts"**
catalyst V20s-U V,Os-F VzOyRO
'SBET/ m2 g-l
Llrmol g-'
5.4 0.8 0.8
22 4 4
LISBETI pmol g-' 4.1 5 .O 5 .O
s-I
R/~mol g-l s-' 0.60 0.0060 0.024
RISBETI pmol m-2 s-' 0.1 1 0.0075 0.030
TF/ ks? 27 1.4 6.0
TF(CO)/ ks-l 16 0.15 2.2
WCW/ ks-l
S(CO)/%
S(C02)/%
11 1.25 3.8
58 11 37
42 89 63
SBET. the BET (Brunauer-Emmett-Teller) surface area; L, the number of surface V=O species; R, reaction rate; TF, turnover frequency; TF(CO), turnover frequency for the formation of CO; TF(C02), turnover frequency for the formation of CO,; S(CO), selectivity to CO; S(C02), selectivity to COz. bReaction conditions: temperature, 738 K; PB = 0.035 atm; Po = 0.381 atm. TABLE II: Physical and Catalytic Properties of V205/Ti02(a) and V205/Ti02(r) Catalysts'Bb
catalyst V,0S/Ti02(a)
V20s/TiOz(r)
v205-u
mol %
SBET/
V,Os
m2 g-'
1 2 5 10 25 50 1 2 5 10 25 50
47 45 26 23 10 7.4 17 18 16 15 17 13 5.4
Llpmol g-l 56 120 184 135 60 31 77 109 118 105 91 60 22
N 1-2 1-2 2-3 5-8 30-40 50-60 1-2 1-2 3-6 5-10 20-30 60-70 504
MLI% 32 67 280 600 2800 6400 91 160 460 930 1700 3700
Rlpmol g-' s-' 0.42 1.02 1.91 3.78 1.62 0.71 0.61 1.42 2.63 2.71 2.14 1.81 0.60
TF/
ks-' 7.5 8.5 10.4 28.0 27.0 22.9 7.9 13.0 22.3 25.8 27.4 30.2 27.3
TF(CO)/ ks-' 0.2 0.1 0.6 13.2 15.4 13.2 0.2 1.o 9.6 14.7 16.2 17.8 15.8
TF(CO,)/
ks-'
S(CO)/% 2 1 6 47 57 58 2 8 43 57 59 59 58
7.3 8.4 9.8 14.8 11.6 9.6 7.7 12.0 12.7 11.1 11.2 12.4 11.5
S(COJ/% 98 99 94 53 43 42 98 92 57 43 41 41 42
ON,number of V2Os layers on support; ML, percent theoretical monolayer of V 2 0 s calculated by eq 2; as for the definition of SBET, L, R, TF, TF(CO), TF(CO2), S(CO), and S(C02), see Table I. bReaction conditions: temperature, 738 K; PB = 0.035 atm; Po = 0.381 atm. change from each other while the surface of V20,-U or V 2 0 S - R 0 is rougher than that of V2OS-F. In the case of supported vanadium oxide catalysts, two kinds of T i 0 2were used as supports; they consisted of anatase and rutile, denoted by Ti02(a) and Ti02(r), respectively. Ti02(a) and Ti02(r) were prepared by hydrolysis of Ti(S0,)2 and TiCl.,, respectively, followed by calcination in air at 873 K for Ti02(a) and in O2 at 773 K for Ti02(r). The respective BET surface areas of Ti02(a) and TiO,(r) were 48 and 17 m2/g. Vanadium oxides supported on Ti02, denoted by V205/Ti02(a)and V205/Ti02(r), were prepared by impregnation of the carrier with an oxalic acid solution of NH,V03 followed by calcination at 773 K in a stream of O2 for 3 h. The number of surface V=O species ( L ) , the number of V2Os layers on support (N), and the BET surface area of the supported catalysts have been determir~ed,~ and the results are shown in Table 11. ML is the percent theoretical monolayer of V20s,11which is calculated from the V205content (x) and S, by
where N A is Avogadro's number, a(V205) is the area occupied by a V205unit (20.6 A2), and M(Ti02) is the molecular weight of Ti02. Unless otherwise stated, the particle size of the catalyst was in the range 28-48 mesh. Catalytic Activity Measurements. Kinetic studies were carried out by using the continuous flow reaction technique under the following conditions: total pressure = 1 atm (1 atm = 101.3 P a ) , temperature = 693-763 K, partial pressure of O2 (Po) = 0 . 4 5 atm, and partial pressure of butane (P,)= 0.035 atm. Nitrogen was used as a balance gas. C O and C 0 2were identified as reaction products, and they were analyzed by using gas chromatography. Particular care was paid to eliminating the heat of reaction and therefore to the control of reactor temperature ( f l K). The glass reactor was heated to the reaction temperature by using a fluidized bed of sand, and the catalyst was diluted with a-A1203. Characterization of Catalysts. A catalyst in the steady-state reaction was rapidly cooled down to room temperature to measure the steady-state catalyst structure. IR spectra of the catalyst was (1 1) Roozeboom, F.; Fransen, T.; Mars, P.; Gellings, P. J. Z . Anorg. Allg. Chem. 1979, 25, 449.
0.5,
0-
1
0.1
0.3
0.2 Po /
0.4
c.5
ATM
Figure 1. Effect of Po on R (a), R ( C 0 ) (0),and R ( C 0 2 ) ( 0 ) for the V20s-Uat 725 K. PB = 0.035 atm and W/F = 2058 g s mo1-I.
observed on a Jasco EDR-3 1 emissionless IR diffuse reflectance spectrometer with KBr as a diluent.I2 Desorption of oxygen from the catalyst surface was investigated by using a temperatureprogrammed desorption (TPD) apparatus similr to that used by Amenomiya et al.13
Results Effects of 4 Partial Pressure on Reaction Rates and Catalyst Structures. In the oxidation of butane on vanadium oxide catalysts, the following reactions were found to take place: C4Hlo + (9/2)O2 ---* 4CO 5H2O (3) C4Hlo + (13/2)0,
-
+
4CO2
+ 5H2O
(4)
From the stoichiometries of eq 3-4, the rate of formation of each product [ R ( C O ) or R ( C 0 2 ) ]is defined as the rate of butane converted to the product. The total reaction rate ( R ) and selectivities to C O [S(CO)] and C 0 2 [S(C02)] are given by R = R(C0)
+ R(C02)
(5)
S(C0) = R(CO)/R
(6)
S(C02) = R(CO,)/R
(7)
(12) Niwa, M.; Hattori, T.; Takahashi, M.; Shirai, K.; Watanabe, M.; Murakami, Y . Anal. Chem. 1979, 51, 46. (13) Cvetanovic, R. J.; Arnenorniya, Y . Adu. Catal. 1967, 17, 103.
-
Oxidation of Butane on VZOS Catalysts
L
50
2 40
.
c
I
-""I . cn
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4261
1
r Y
w 2
(0.05)
=: 3 3 . 2u . 3 1
*
0 3
3 2 ' j
04
125
, ATfl 10%
Figure 2. Effect of Po on S ( C 0 ) (0)and S(C02)(0)for the V205-U at 725 K. PB = 0.035 atm.
I
-
1200
1000
800
WAVENUMBER / 3
21 TlrlE /
40
60
80
MIN
Figure 3. Change in R ( O ) , R ( C 0 ) (0) and R ( C 0 2 ) ( 0 ) after the at 725 K. Po = 0.381 atm at stoppage of O2gas supply for the V205-U t < 0, and Po = 0 atm at t > 0.
Figure 1 shows the effects of Po on R , R(CO), and R ( C 0 2 ) for the V205-Ucatalyst at 725 K. The rates increased gradually with increasing Po to 0.23 atm, while they were almost constant at Po above 0.23 atm. In spite of the change of the reaction rates with Po, S(C0) and S ( C 0 2 ) were independent of Po as shown in Figure 2. According to the results of change in the reaction rates after the interruption of O2gas supply (Figure 3), the rates decreased gradually with time and were negligible 8 h after the reaction began. It was also confirmed that both the reaction rate and infrared spectrum of the catalyst attained a steady state under this condition. Figure 4 shows IR spectra of the V2O5-U catalyst in the steady-state reaction at various partial pressures of O2 The catalyst in the steady-state reaction above 0.23 atm of PO gave absorption bands at 1020 and 825 cm-I which are assigned to the stretching vibration of V=O species and the coupled vibration between V = O and V-0-V, re~pectively.'~*'~ The absorption at 1020 and 825 cm-' gradually decreased as Po decreased below 0.23 atm, and a new absorption band at 910 cm-I was observed for the catalyst in the steady-state reaction below 0.10 atm of Po. This band is assigned to the lattice vibration of V204.16 In order to relate quantitatively the change in the amount of V=O species with Po, the absorbance at 1020 cm-' was calculated from the spectra in Figure 4, and the results are shown in Figure 5. As shown, the amount of V = O species increases almost linearly with Po to 0.23 atm and it was constant above this value of Po. Activity and Selectivity in Butane Oxidation under Excess Oxygen Conditions. Table I shows the results of R, R/SBET, S(CO), and S ( C 0 2 )for unsupported vanadium oxide catalysts under excess oxygen conditions where the reaction rate was zeroth order with respect to Po and where the catalyst was confirmed to be in the highest oxidation state, i.e. Vs+. It should be poted that the specific activity (R/SBET)for V205-U is much Iqrger than that for V205-F and that for V 2 0 S - R 0is larger than that for (14) Inomata, M.; Miyamoto, A.; Murakami, Y. J . Catal. 1980, 62, 140. (15) Tarama, K.;Yoshida, S.;Ishida, S.;Kakioka, H.Bull. Chem. SOC. Jpn. 1969, 41, 2840. (16) Frederickson, L. D.; Hansen, D. M. Anal. Chem. 1963, 35, 818.
600
CM-'
Figure 4. Infrared spectra of the V2OS-U in the steady state of butane oxidation at various partial pressures of 0,. Reaction temperature = 725 K and PB = 0.035 atm. The number in parentheses represents the partial pressure of 02.
0
0.1
0.2 /
3.3
0,4
05
ATH
Figure 5. Amount of V = O in the V205-U in the steady state of butane oxidation at various partial pressures of 0,.
V2OS-F. The selectivity also changes greatly with the kind of unsupported V20S catalysts: S(C0) for VZO5-U is much larger than S(C0) for VZOS-F, and S ( C 0 ) for V 2 0 5 - R 0is larger than S ( C 0 ) for V2O5-F. The V205-F catalyst produces CO, almost selectively. Table I1 shows the results of R , S(CO), and S ( C 0 2 ) for V20S/Ti02(a) and V205/Ti02(r) catalysts with various V2O5 contents. TiO,(a) or Ti02(r) alone had a negligible activity for the butane oxidation. The reaction rate (R) for VzOS/Ti02(a) increased markedly with an increase in V205 content from 0 to 10 mol %, passed a maximum at 10 mol % V205, and then decreased to the value of V205-U with further increase in V205 content. The R for V205/Ti02(r)exhibited similar behavior to that for V20s/Ti02(a)with respect to the change in V205 content. The selectivity to CO [S(CO)]was low for V,O,/TiO,(a) containing 1, 2, or 5 mol % V2O5, while it attained values as high as 50% for catalysts with a V2O5 content above 10 mol %. S ( C 0 ) for V205/Ti02(r)with low V205content (1 or 2 mol %) was low, while the catalyst with V205content higher than 5 mol % produced a considerable amount of CO in addition to CO,. Although Tables I and I1 show results for the reaction at 738 K, sjmilar relationships were found to at any temperature examined (693-763 K): The apparent activation energy for R was 21-25 kcal/mol for the unsupported Vzo5catalysts and 19-20 kcal/mol for V205/Ti02(a)
4268
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Mori et al.
and V20S/Ti02(r)catalysts. The selectivity to CO did not change significantly with reaction temperature.
Discussion Active Oxygen Species f o r the Butane Oxidation. As shown in Figure 1, the rate of butane oxidation increased with increasing Po to 0.23 atm for V2Os-U and attained a constant value above this value of Po. The steady-state amount of the V s + = O species in the catalyst changed with Po similarly to the reaction rate (Figure 5 ) . This suggests that the reaction proceeds by the reduction-oxidation mechanism (or Mars-van Krevelen mechanism)” and that the active oxygen species for the butane oxidation is surface V=O species. Adsorbed oxygen species are not responsible for this reaction, because no adsorbed oxygen species, were detected on the catalysts by either such as Of, 0-,or 03-, ESR or TPD measurement. Moreover, even if the surface of the VIOS-U were initially covered completely with adsorbed oxygen species, these species should be consumed by the reaction with butane within 10 s of stopping the supply of 0, and the rate should decrease abruptly, contrary to the experimental result shown in Figure 3. The observed relationship between reaction rate and Po (Figure 1) can be explained in terms of the reduction-xidation mechanism as follows: In the absence of O2 (Po = 0 atm), the catalyst is in the reduced state and no surface V=O species is present to oxidize butane.I8 As Po increases, the reoxidation of the reduced catalyst by 0, proceeds to give the higher oxidation state of the catalyst and to increase the number of surface V=O species. This means that the reaction rate increases with increasing Po. In the presence of excess oxygen (Po > 0.23 atm), the catalyst is in the highest oxidation state, i.e. V2OS. Therefore, the increase in Po does not lead to further increase in the oxidation state or the number of surface V=O species and the reaction rate does not increase with Po under excess oxygen conditions. According to the results of our preliminary experiments on the reaction of butane with 180over 2 the V20s-Ucatalyst, oxygens of V2Os (i.e., I6O)were preferentially incorporated into the carbon oxides. This is consistent with the conclusion that the surface V = O is the active oxygen species for the reaction. Structure Sensitivity of the Reaction. The surface of VIOS-F has been found to be significantly different from that of V20s-U or V205-R0.4The surface of V2OS-F is much smoother than that of V2OS-U or V20S-RO. Since the surface V 4 species has been found to be the active oxygen species for the butane oxidation, the turnover frequency (TF) for this reaction can be defined by
TF = R / L
(8)
Values of T F at 738 K were calculated from the results of R and L (Tables I and 11) for various catalysts, and the results are also shown in these tables. It is evident from Table I that T F changes significantly with the kind of unsupported catalysts; TF for V20s-U is much larger than TF for V,OS-F, and TF for V20S-ROis larger ( 1 7 ) Mars, P.; van Krevelen, D. W. Chem. Eng. Sci. 1951, 3, 41. (18) The Mars-van Krevelen mechanism is formulated by following two reactions:
hydrocarbon
+ oxidized catalyst
-
oxidation products
reduced catalyst + oxygen
-
+ reduced catalyst (1)
oxidized catalyst
(11)
The oxidized catalyst can oxidize the hydrocarbon to oxidation products, while the reduced catalyst cannot oxidize the hydrocarbon. In the absence of O2 (Po= 0 atm), the catalyst is in the reduced state, and therefore the reaction does not proceed under the conditions. Although the active oxygen species in the oxidized catalyst has not been specified in the original Mars-van Krevelen mechanism, the present investigation has revealed that the V=O species is the active oxygen species for the butane oxidation. In other words, all of the oxygens in vanadium oxide are not the active oxygen species, but only a vanadium oxide with V = O species is active for the reaction. Although the catalyst in the steady-state reaction in the absence of O2does not oxidize butane (Figure l), this does not mean a lack of activity of the fresh V205which is prepared by the thermal decomposition of NH4V0, in O2at 773 K. Judging from the result in Figure 3 and infrared spectrum of the fresh V205,the catalyst is expected to show initial activity even in the absence of O2but the activity would decline with time, a behavior similar to that in Figure 3.
I-/
-:
1
r-
2
1 -
I_
1ci
;‘,I
Figure 6. Relationship between the turnover frequency (TF) for butane oxidation and the number of V 2 0 , layers on support: (0). V20,/Ti02(a); (O), V20s/Ti02(r); (Oh V 2 0 5 - U .
than T F for V2OS-F. These behaviors are similr to those in the C O oxidation4 and indicate that the butane oxidation on V2Os catalysts is a structure-sensitive reaction. Fusion of a solid would generally lead to a smooth surface with a decreased number of surface defects (e.g. steps, kinks, or vacancies), while severe redox treatment of a solid with few surface defects would tend to increase no impurity peaks were observed their n ~ r n b e r .Furthermore, ~ in the XPS spectrum of the V20s-U,VZOS-F, or V20S-RO. Since the surface V=O species has been shown to be the active oxygen species, the surface V=O species at the surface defects are considered to be much more active than that in the smooth (010) plane. Activity of Supported Catalysts. In general, the activity of supported catalyst is determined by two factors: (i) the number of active sites and (ii) the specific activity of the active site, that is, the turnover frequency. The separation of these two factors is indispensable for a detailed understanding of the role of support in a given reaction. As shown in Table 11, the rate ( R ) for V20S/Ti02(a)and V20s/Ti02(r) catalysts is much larger than that for V2Os-U, indicating the promoting effect of TiO, support. The number of surface V=O species ( L ) for V20S/Ti02catalysts (5 mol % V2OS) is 5-8 times larger than that for the unsupported V,Os-U, while the maximum increase in turnover frequency (TF) by TiO, support is only 11%. This means that the promoting effect of TiO, is mainly to increase the number of surface V=O species but the specific activity of the surface V=O species is barely increased by the support. Figure 6 shows the relationship between the turnover frequency (TF) and the number of V2Os layers (N) for V2O5/TiO2(a)and V20s/Ti02(r). The turnover frequency for V2O5/TiO2 decreases monotonically with the decreasing number of V2OS layers on Ti02. This indicates that the retarding effect of TiO, on the specific activity of the surface V = O species becomes more significant with decreasing number of VIOS layers. According to Vejux and Courtine,” there is a remarkable fit of the crystallographic patterns between the (010) face of V2Os and the TiO, surface. It is therefore considered that a smooth V20s surface with few defects is formed for the V20S/Ti02 catalysts having a low concentration of V2OS and that the number of surface defects increases with increasing content of V,Os. Since the activity of the surface V=O at the surface defect is much higher than that in the smooth (010) face, this explains the significant reduction in the turnover frequency for V205/Ti02 compared with that for V20s-U and the increase of the turnover frequency with increasing content of V,05 in V20s/Ti02. In other words, the retarding effect of Ti0, support on T F is due to the formation of a smooth V2Os surface for V2OS/TiO2. Selectivity in Butane Oxidation. Figure 7 shows relationships between the selectivity to CO [S(CO)] and the conversion of butane, which were obtained from the results under excess oxygen conditions at various temperatures. As shown, S ( C 0 ) is independent of the conversion for any catalyst. This indicates that consecutive oxidation of C O to C 0 2 was negligible under the present experimental conditions. In other words, the difference (19) Vejux, A.; Courtine, P. J . Solid State Chern. 1978, 23, 93.
J . Phys. Chem. 1985, 89, 4269-4272
n
. -
-40c
3 Lo
0
-"
0
4 - A L A -
-
20 .
in the selectivity among catalysts is brought about by the difference in the catalyst structure but not by the consecutive oxidation of co to coz. As shown in Figure 5, the oxidation state of the VzOs-U catalyst changes greatly with Po. Under excess oxygen conditions, the catalyst is kept in the highest oxidation state, Le. Vs+, while it is reduced as Po decreases. As shown in Figure 2, S ( C 0 ) does not change with Po. This means that the selectivity in butane oxidation is independent of the oxidation state of the catalyst at least under the present experimental conditions where conversion of butane is below 20% and consecutive oxidation of C O to COz is negligible and where the catalyst is in its steady state at a given level of Po. As shown in Table I, the selectivity changes greatly with the kind of unsupported V20s: The V205-Fis effective for the formation of COz, while the Vz05-Uand VZOs-ROproduct both CO and COz. As discussed above, the number of surface defects on
Effect of Cation Polarizing Power on Ai,Br,Nuclear Quadrupole Resonance
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the VzOS-Fis considered to be smaller than that on the Vz05-U and V205-R0. The result of the selectivity thus suggests that the surface V = O species at the surface defect produces both C O and C 0 2 , while that in the smooth (010) face leads to the selective formation of COz. As shown in Table 11, the selectivity also changes greatly with the kind of VzOs/TiO2 catalysts: VZO5/TiOzwith low Vz05 content forms COzselectively, while that with high Vz05content produces both C O and C 0 2 . Since a smooth VzOs surface is expected to be formed for VzOS/TiO2with low Vz05content, the result can also be explained by the above-mentioned idea that the surface V=O species at the surface defect produces both C O and COz, while that in the smooth (010) face leads to the selective formation of COz.zo In conclusion, catalytic behaviors of unsupported and supported Vz05for butane oxidation are explained in terms of the roughness of catalyst surface: Surface V=O at the surface defect exhibits high activity for the reaction to form both CO and COz, while that in the smooth (010) face is less active and produces COz selectively.
Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (No. 59470097) and for Encouragement of Young Scientists (No. 59750655) from the Ministry of Education, Science, and Culture, Japan. Registry No. CH,(CH2)2CH3,106-97-8; V 2 0 5 , 13 14-62-1; rutile, 1317-80-2;anatase, 1317-70-0. (20) Although control of the roughness of the V 2 0 5 surface is the main factor of the catalytic behavior of V205/Ti02catalysts, the following points should also be noted as for the effect of the TiO, support from the results of turnover frequencies for the formation of CO [TF(CO) = R(CO)/L] and CO, [TF(C02)= R(C02)/L]: As shown in Table I, TF(C0) for V205-Fis ca. 100 times smaller than TF(C0) for V205-U, and TF(C0,) for V2O5-F is ca. 10 times smaller than TF(C02)for V,O,-U. TF(C0) for V205/TiOzwith low V2O5 content is also ca. 100 times smaller than TF(C0) for V~OS-U,a difference similar to that between V205-Fand V2O5-U. On the other hand, TF(C02)for V205/Ti02with low V,05 content is comparable to TF(C02) for V205-U,a behavior different from that for V,05-F and V205-U. This suggests that Ti02 support plays some role in the oxidation of butane to CO, on V205/Ti02with thin V,05 layers.
Anion Studied by means of ''Br and *,AI
Koji Yamada* and Tsutomu Okuda Department of Chemistry, Faculty of Science, Hiroshima University, Naka- ku, Hiroshima 730, Japan (Received: January 7 , 1985; In Final Form: May 13, 1985)
Bromine-81 and aluminum-27 nuclear quadrupole resonance (NQR) data have been obtained for a series of MAI2Br7compounds (M = NH4, (CH,),N, and (C2H5)4N).For the cation change from (C2HJ4N+to NH4+,the bridging 8'Br NQR frequency increases about 10 MHz, which is about 13% of that in the (C2H5),NAI2Br7compound and too large to arise from only crystal field effects. This large frequency shift is suggestive of the polarization of the anion induced by neighboring cations. That is, the negative charges of the anion are more localized at the terminal positions than at the bridging position with increasing polarizing power of the cation. The nuclear quadrupole coupling constants of 27Alincrease with increasing cation radius. This finding also supports the anion polarization model.
Introduction In the Friedel-Crafts reaction AlzX7- (X = C1 or Br) anions have an important role to stabilize organic cations in a proper solvent.' Many salts containing A12Br7- anions have been confirmed in the solid state for alkali metal2 and alkylammonium (1) Mirda, D.; Rapp, D.; Kramer, G. M. J . Org. Chem. 1979,44, 2619. ( 2 ) Cronenberg, V. C. T. H. M.;Van Spronsen, J. W. Z . Anorg. Allg. Chem. 1967, 354, 103.
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cations and studied by vibration3 and NQR spectra." According to X-ray analysis the AlZBr7- anion consists of two A1Br4 tetrahedra sharing one Br atom with a bridging bond angle close to regular tetrahedral angle.7s8 (3) Manteghetti, A.; Potier, A. Spectrochim. Acta, Parr A 1982, 38A. 141. (4) Merryman, D. J.; Edwards, P. A.; Corbett, J. D.; McCarley, R. Inorg. Chem. 1974, 13, 1471. ( 5 ) Deeg, T.; Weiss, Al. Ber. Bunsenges. Phys. Chem. 1975, 79, 497. (6) Yamada, K. J . Sci. Hiroshima Uniu., Ser. A : Phys. Chem. 1977,41, 71.
0 1985 American Chemical Society
