J. Phys. Chem. 1984,88, 2135-2141 of the wave function for the zero vibrational level corresponds to the shaded area, which gradually changes from 'Lb to 'La character as Ao is varied through solvent effects. This leads to the continuous ~2) andi of kf (~ ~~4) andi~should~~also lead ~ variation of p ( to a strong solvent effect on the dipole moment of the fluorescent state, formally describable as an abnormally high polarizability. Electrooptical emission measurements to verify this prediction are
2735
under way.
Acknowledgment. Support by the Deutsche Forschungsgethe Bundesministerium fur Forschung und Technologie ~ meinschaft, ~ (Project 05 286LI p/% and the Fonds der C h f ~ ~ i s c h eIndustrie n is gratefully acknowledged. Registry No. EICEE, 78357-15-0; EIN, 89937-22-4.
Catalytic Reactions on Well-Characterized Vanadium Oxide Catalysts. 1. Oxidation of Carbon Monoxide Kenji Mori,+Akira Miyamoto,* and Yuichi Murakami Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa- ku, Nagoya 464, Japan, and Kinu- ura Research Department, JGC Company, Sunosaki-cho, Handa, Aichi 475, Japan (Received: July 27, 1983)
The oxidation of carbon monoxide over unsupported and supported vanadium oxide catalysts was investigated from the standpoint of structure sensitivity. The activity of unsupported VzOscatalyst changed greatly with the treatment of the catalyst: The fusion of unsupported V,05 markedly decreased the turnover frequency, while the reduction-oxidation treatment of the fused catalyst increased it. The turnover frequency of V20S/Ti02with low V20s content was much smaller than that of the V2O5/TiO2 with high V205 content or the unsupported V2Os. Such a retarding effect of the TiOz support on the activity of the oxidation of carbon monoxide is in contrast to the known promoting effect of TiOz for the oxidations of various hydrocarbons. From these results coupled with the characterization of the catalysts, it was concluded that the oxidation of carbon monoxide on vanadium oxide catalysts is a structure-sensitive reaction and that the activity of surface defects such as steps, kinks, and vacancies is much higher than that of the surface V=O species in the smooth (010) face of V20,.
Introduction Many investigators have already clarified that the surface V=O species on vanadium oxide catalysts acts as the active site for various reactions including the oxidation of hydrocarbons and the reaction of NO with NH3 in the presence of Oz.1-8 Although much research has been done for the oxidation of CO on vanadium oxide catalyst^,^*^-'^ the active site for the reaction has been a subject of controversy. Hirota et aL2 used I8Oas a tracer in studying the mechanism of the oxidation of CO and found that carbon dioxide formed by the oxidation of CO with '*02 had a lower I8O content than it should have had if the reaction were going by way of an interaction between adsorbed oxygen from the gas phase and adsorbed carbon monoxide. From the result they suggested that a certain amount of lattice oxygen in the catalyst participates in the reaction. More recently, Kera supported the validity of this conclusion by using single crystals of V2O5." On the other hand, Marshneva et al." drew a conclusion that an associative mechanism makes the main contribution to the oxidation of CO on V,Os catalyst rather than the oxidation-reduction mechanism, and Kon' et a1.I2 suggested the participation of adsorbed oxygen in this reaction. The effects of various supports on the activity of V205 for the oxidation of C O have also been investigated. Goldwasser and Trimm showed that coordination effects induced by the support affect both the adsorption of reactants and the catalytic activity.I3 Roozeboom et al.14 found that the oxidation of C O on V z 0 5 supported on y-A1203,ZrOZ, Ti02, and C e 0 2 proceeds by the reduction-oxidation mechanism. Recently, it has been emphasized that investigation of structure sensitivity in a catalytic reaction provides valuable information about the nature of active site^.'^-^^ Although Roozeboom et al.14 have found that the activity of unsupported V2O5 for the JGC Co. *Address correspondence to this author at the Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan.
0022-3654/84/2088-2735$01.50/0
oxidation of CO changes with treatment of the catalyst, further details of the structure sensitivity have not been studied for this reaction. We have previously established the rectangular pulse technique which allows the determination of the number of surface V=O species of vanadium oxide catalyst and have characterized the structures of V20S/Ti02catalysts.22-zs This would make it
(1) Tarama, K.; Teranishi, S.; Yoshida, S . ; Tamura, N. Proc. Int. Congr. Catal., 3rd, 1964 1965, 282. (2) Hirota, K.; Kera, Y.; Teratani, S . J. Phys. Chem. 1968, 72, 3133. (3) Cole, D. J.; Cullis, C. F.; Hucknall, D.J. J. Chem. SOC.,Faraday Trans. 1 1976, 72, 2185. (4) Ahmoto, M.; Usami, M.; Echigoya, E. Bull. Chem. SOC.Jpn. 1978, 51, 2195. (5) Andersson, A. J . Solid State Chem. 1982, 42, 263. (6) Inomata, M.; Miyamoto, A.; Murakami, Y . J. Chem. SOC.,Chem. Commun. 1980, 233. (7) Inomata, M.; Miyamoto, A,; Murakami, Y . J. Catal. 1980, 62, 140. (8) Inomata, M.; Miyamoto, A.; Ui, T.; Kobayashi, K.; Murakami, Y. Ind. Eng. Chem. Prod. Res. Deu. 1982, 21, 424. (9) Hughes, M. F.; Hill, G . R. J. Phys. Chem. 1955, 59, 388. (10) Kera, Y. Bull. Chem. Soc. Jpn. 1979, 52, 888. (1 1) Marshneva, V. I.; Boreskov, G. K.; Sokolovskii, V. D. Kinef. Katal. 1972, 13, 1209. (12) Kon', M. Ya.; Shvets, V. A,; Kazanskii, V. G.Kine?. Katal. 1972, 13, 735. ( 1 3) Goldwasser, M. R.; Trimm, D. L. Ind. Eng. Chem. Prod. Res. Deu. 1979, 18, 27. (14) Roozeboom, F.; van Dillen, A. J.; Geus, J. W.; Gellings, P. J. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 304. (15) Boudart, M. Proc. In?. Congr. Catal., bth, I976 1977, 1. (16) Hardeveld, R. V.; Hartog, F. Adu. Caial. 1972, 22, 7 5 . (17) Somorjai, G. A. Ado. Cafal.1977, 26, 1. (18) Volta, J. C.; Forissier, M.; Theobald, T.; Pham, T. P. Faraday Discuss. Chem. SOC.1981, 72, 225. (19) Tatibouet, J. M.; Germain, J. E. J. Catul. 1982, 72, 375. (20) Tatibouet, J. M.; Germain, J. E.; Volta, J. C. J . Catal. 1983, 82, 240. (21) Ziolkowski, J.; Janas, J. J. Catal. 1983, 81, 298.
0 1984 American Chemical Society
2736 The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 possible to investigate the structure sensitivity in this reaction in terms of the turnover frequency. It should also be emphasized that the relationship between the structure of the metal oxide on support and the activity of the catalyst has barely been clarified in terms of the turnover frequency because of the lack of a well-established method to determine the number of active sites on supported metal oxide catalysts. The purpose of this study was (i) to investigate the structure sensitivity and effects of Ti02 supports on the CO oxidation over V2Os catalyst and (ii) to discuss the active sites for this reaction from these results. Experimental Section Catalysts. A V2O5-U catalyst was prepared by the thermal decomposition of ammonium metavanadate at 773 K in flowing 02.A V20j-F 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 V20j-R0 catalyst 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 5 times. In the case of supported vanadium oxide catalysts, two kinds of Ti02, denoted by Ti02(a) and Ti02(r), were used as supports; they consisted of anatase and rutile, respecti~ely.2~ Ti02(a) and Ti02(r) were prepared by hydrolysis of Ti(S0,J2 and TiC14, respectively, followed by calcination in air at 873 K for TiO,(a) and in O2at 773 K for Ti02(r). Respective BET surface areas of TiO,(a) and TiOz(r) were 48.2 and 16.8 m2 g-l. Vanadium oxides supported on Ti02, denoted by V205/Ti02(a)and V205/Ti02(r), were prepared by impregnation of Ti02(a) or Ti02(r) support with an oxalic acid solution of ammonium metavanadate followed by calcination at 773 K in a stream of 02. A V,05/Ti02(a)-R0 (2 mol % V205) catalyst was prepared from the V,05/Ti0,(a) (2 mol % V205) catalyst by the reductionoxidation treatment, that is, the same procedure as that for the preparation of the V205-ROcatalyst from the V205-Fcatalyst. Catalytic Activity Measurement. Kinetic studies were carried out by using the continuous flow reaction technique under the following conditions: total pressure = 1 atm, temperature = 613-745 K, partial pressure of CO (Pco) = 0.01-0.16 atm, partial pressure of O2(Po2)= 0 . 7 5 2 atm, and W/F= 3.54 X 102-1.56 X lo4g s mol-". Nitrogen was used as a balance gas. Conversion of CO was kept below 20% by controlling the amount of catalyst used. The analysis of the reaction gas was done by gas chromatography. Particular care was paid to eliminating the heat of reaction and therefore to the control of reactor temperature (f1 K). The glass reactor was heated to the reaction temperature by using a fluidized bed, and the catalyst was diluted with cu-A1203. Characterizations of Catalysts. X-ray diffraction diagrams of the catalysts were obtained with a Rigaku G F 2035 X-ray diffractometer with Cu target. UV-visible reflectance spectra of the catalysts were observed in the 250-800-nm range with a Jasco UNIDEC-505 spectrophotometer. X-ray photoelectron spectra (XPS) of the catalysts were measured on a Shimadzu ESCA 750 electron spectrometer. The number of surface V=O species on thecatalysts was determined by using the rectangular pulse technique described previously.22-2sThe BET surface area of the catalysts was determined by using a conventional flow-type apparatus with N2 as an adsorbate. IR spectra of the catalysts under steady-state conditions at various concentrations of O2were observed by using a Jasco EDR-3 1 emissionless IR diffuse reflectance spectrometer with KBr as a diluent.26 Electron mi(22) Miyamoto, A.; Yamazaki, Y.; Inomata, M.; Murakami, Y. J . Phys. Chem. 1981,85, 2366. (23) Inomata, M.; Miyamoto, A.; Murakami, Y.J. Phys. Chem. 1981,85, 2372. (24) Murakami, Y . ; Inomata, M.; Miyamoto, A,; Mori, K. Proc. I n t . Congr. Catal., 1981, 1344. (25) Inomata, M.; Mori, K.; Miyamoto, A,; Ui, T.; Murakami, Y. J . Phys. Chem. 1983,87,154. (26) Niwa, M.; Hattori, T.; Takahashi, M.; Shirai, K.; Watanabe, M.; Murakami, Y . Anal. Chem. 1979, 51, 46.
Mori et al. TABLE I: Physical Characteristics of the V205-U,V,Os-F, and V2OS-ROCatalyst"
JV
mol S(oio)/ SBET/ Cv=o/
catalyst g-l) V205-U 22 V2O5-F 4 V205-R0 4
(m2 g-l) (m2 g-') 2.7 5.4 0.49 0.8 0.49 0.8
S(oio)/
(pmol m-2) SnFr N 4.1 0.5 504 5 .O 0.61 2750 5.0 0.61 2750
L is the number of surface V=O species. S(olo) is the specific surface area of the (010) face of VzO5. SBET is the BET (Brunauer-Emmett-Teller) surface area. C Vis the ~ density of surface V=O species. N is the number of V205 layers.
Figure 1. Micrograph of the V205-Ucatalyst.
crographs were observed with a Hitachi H-700H electron microscope, which was operated at 200 kV. Results Characterization of Catalysts. Table I shows results of the number of surface V=O species, L, the BET surface area, SBET, the specific surface area of the (010) face of V2O5, S(olo), the density of surface V=O species, Cv4, and the number of V205 layers, N , for the unsupported V,O5-U, V,05-F, and V205-RO was calculated from L together with the catalysts. Here, S(olo) surface density of the V=O species in the (010) face of V20j crystal, 4.872 nmT2,and Cv4 is defined as L/SBET. If the catalyst forms uniform layers of V,05, the number of V2O5 layers for the unsupported V205catalyst is calculated from L as follows: 23,25 N = 2/(LMv,o,)
(1)
where Mv,o, is the molecular weight of V2O5. The results of N are also indicated in Table I. As shown in Table I, the values of L and SBET for the V,Oj-U were much larger than those for the V,05-F or V205-RO. However, the density of surfaces V=O species, Cv-o, or the fraction of the (010) face, S(oio)/SBET, did not change significantly with the kind of catalyst. Figures 1-3 show examples of scanning electron micrographs of the VzOj-U, V205-F, and V,05-R0 catalysts, respectively. As shown, the particle size of the Vz05-U was much smaller than that of the
Catalytic Reactions on Vanadium Oxide Catalysts
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2737
TABLE II: Rate ( R ) and Turnover Frequency (TF) for the Oxidation of CO on Unsupported V205 and V205/Ti02 Catalysts' v205 Rb/(pmol catalyst content/mol % SBET/(m2g-l) L/(pmol g-l) N g-1 s-1) V,O,/TiO2(a) 2 45.2 120 1-2 9.9 5 10 25 50
V205/TiO2(r)
1 2 5 10 25 50
v205-u
100
Vz05-F VZOS-RO V2O5/TiOZ(a)-RO
100 100 2
26.3 22.8 10.3 7.4 16.8 18.4 15.9 14.8 17.2 12.7 5.4 0.8 0.8 46.0
184 135 60 31 71 109 118 105 91 60 22 4 4 134
2-3 5-8 30-40 50-60 1-2 1-2 3-6 5-10 20-30 60-70
1-2
9.7 22.8 28.4 15.8 2.4 10.2 14.8 23.2 18.9 16.7 9.7 0.06 0.11 42.1
TF/ks-' 82 53 169 473 510 31 94 126 22 1 208 279 439 15 27 314
O S B E T is the BET (Brunauer-Emmett-Teller) surface area. L is the number of surface V=O species. N is the number of V205layers on Ti02 support. bReaction conditions: temperature = 673 K; Pco = 0.045 atm; Po, = 0.752 atm.
Figure 2. Micrograph of the V,Os-F catalyst.
Figure 3. Micrograph of the V,OS-RO catalyst.
V205-F or Vz05-R0 catalyst, in accordance with the results of the number of V2O5 layers. Furthermore, the surface of the V2O5-F was much smoother than that of the V205-U or V205-RO, indicating the change of the surface of unsupported V205 by the fusion or reduction-oxidation treatment. All of the X-ray diffraction peaks of the VzOs-U, V2Os-F, and V 2 0 5 - R 0were assigned to the V2Os phase, while no peaks of other compounds such as V6OI3and VOz were observed. Figure 4 shows UV-visible reflectance spectra of the VZO5-U, V2O5-F, and V 2 0 5 - R 0catalysts. Here, the absorption in the wavelength region below 500-600 nm has been assigned to charge-transfer transition from 02-to V5+.25,27As shown, the spectrum did not change greatly with the kind of catalyst, indicating the absence of significant changes in the electronic state of the catalysts. The absence of absorption bands for the one-electron d-d transition in the 600-
800-nm region indicates that the number of V4+ions is small for these catalysts. No noticeable difference was observed in the XPS spectra of the V205-U, V205-F,and Vz05-R0catalysts; the vanadium was in its highest oxidation state (V5+) and no peaks of impurities were observed except for the peak of CISat 284.6 eV. Structures of V205/Ti02(a)and V205/Ti02(r)have been investigated by using the rectangular pulse technique coupled with various physicochemical measurements including X-ray diffraction, IR, ESR, UV-visible, and in situ IR spectra of absorbed amm ~ n i a Results . ~ ~ of the BET surface area, the number of surface V=O species, and the number of V205layers on the support, N , thus determined are shown in Table 11. Here, N was determined experimentally by analyzing the tailing behaviors of N2produced by the reaction of the NO and NH3 mixture with the preoxidized cataly~t.~~-~~ Oxidation of CO on Unsupported V205Catalysts. Figure 5 shows results of the steady-state rate (R)of CO oxidation on the V2O5-U and VzO5-F catalysts at various partial pressures of O2
(27) Gritskov, A. M.; Shvets, V. A,; Kazanskii, V. B. Kine?. Kutaf. 1973, 14, 1062.
Mori et al.
2738 The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 100
,'.----------
I
~'OS-U
J W U
I
01
200
,
3QO
I
400
600
500
WAVE LENGTH /
700
8
NM
Figure 4. UV-visible reflectance spectra of the V205-U, V2O5-F, and
V205-R0catalysts.
0
0,Ol 0,OZ
PCQ
'ip,
0,03 0,04 0,05 / ATM
Figure 6. Rate of the CO oxidation at various partial pressures of CO: (a) V205-Ucatalyst, (b) V205-Fcatalyst. Reaction conditions: temperature = 743 K, Pol = 0.752 atm.
,
,(a) ,
,
0
- - - - -?O
(C)
p02 /
ATM
Figure 5. Rate and the amount of V=O in the catalysts in the steady state of the CO oxidation at various partial pressures of 02:(a) V2O5-U catalyst; reaction conditions: temperature = 658 K, Pco = 0.045 atm. (b) V205-Ucatalyst; reaction conditions: temperature = 743 K, Pco = 0.045 atm. (c) V205-Fcatalyst; reaction conditions: temperature = 743 K, Pco = 0.045 atm. Open circles: reaction rate. Closed circles: the
amount of V=O.
(Po,). As for the V205-Ucatalyst at 658 K, the rate increased with increasing Po2 to 0.2 atm, while it was almost constant above 0.2 atm. The rate for the V205-Uat 743 K also increased with increasing Po2 to ca. 0.3 atm and attained almost a constant value above this partial pressure. As for the reaction on the V20s-F at 743 K, the rate increased steeply with Po, to 0.05 atm, increased gradually to 0.2 atm, and attained a constant value above 0.2 atm. Figure 6 shows results of the steady-state rate at various partial
pressures of CO (Pco). As shown, the rate increased monotonically with increasing Pco up to 0.045 atm for both the V20s-U and V205-Fcatalysts. Figure 7 shows IR spectra of the V2OS-U and V20,-F catalysts in the steady-state CO oxidation at various partial pressures of 02.As shown in Figure 7a, the V205-U catalyst exhibited an absorption at 1020 cm-' which is assigned to the stretching vibration of V=O species.1*7~8~28-30 The relative amount of the V=O species in the catalyst was calculated from the absorption peak at 1020 cm-' by using the Kubelka-Munk equation and the results are shown in Figure 5a. As for the V205-Ucatalyst at 658 K, the amount of the V=O species did not change with Po,, while the reaction rate changed greatly. As shown in Figure 5b, also the amount of the V=O species in the V205-Ucatalyst at 743 K did not change with Po2 except for the reaction in the absence of 02.Figure 7b shows IR spectra of the V20,-F catalyst in the steady-state CO oxidation at various partial pressures of 02.As shown, the IR spectrum changed considerably with Po2. The relative amount of the V=O species determined from the spectra changed with Po, similarly to the reaction rate (Figure 5c), a behavior much different from that for the V205-U catalyst. Figure 8 depicts the change of the reaction rate on the V20,-U and V,O,-F catalysts after the stoppage of gaseous 0, supply. For the V205-Ucatalyst, the rate markedly decreased immediately, and no detectable amount of C 0 2was produced after the stoppage of O2supply. For the reaction on the V2O5-F catalyst, on the other hand, the reaction took place even after the stoppage of 0 2 supply. Table I1 shows the rate and turnover frequency (TF) on the V20s-U,V205-F,and V 2 0 5 - R 0catalysts for the CO oxidation when an excess of oxygen was present, where the rate of the reaction was zeroth order with respect to PO, and where the catalyst in its highest oxidation state, Le., V5+. Here, the turnover frequency is defined as the rate ( R ) divided by the number of surface V=O species, L. This was done in order to normalize (28) Tarama, K.; Yashida, S.; Ishida, S.; Kakioka, H . Bull. Chem. SOC. Jpn. 1969, 41, 2840. (29) Kera, Y.; Teratani, S.; Hirota, K . Bull. Chem. SOC.Jpn. 1967, 40, 2458. (30) Miyamoto, A,; Yamazaki, Y.; Murakami, Y. Nippon Kuguku Kuishi 1977, 619.
Catalytic Reactions on Vanadium Oxide Catalysts
The Journal of Physical Chemistry, Vol. 88. No. 13, 1984 2739
0
20
40
60
80 100
0
20
40
EO
80 100
TINE / Y I N TIPIE / M ! N Figure 8. Changes in the rate of the CO oxidation after the stoppage of 0,supply: (a) V205-Ucatalyst, (b) V2O5-Fcatalyst. Reaction conditions: temperature = 745 K, Pc0 = 0.16 atm, Po2 = 0.496-0 atm.
h '
h 0 . 4 9 5 )
1020 J
a
I
1200
*
I
1000
o
'
800
WAVE NUMBER /
600
CM-'
shows only results for the reaction at 673 K, a similar relationship was also found to hold for the reaction between 673 and 743 K; the apparent activation energies for these catalysts were 20-23 kcal mol-'. Oxidation of CO on V205/Ti02(a) and V2O5/TiOZ(r)Catalysts. Neither Ti02(a) nor Ti02(r) support was active for the oxidation of CO under the present experimental conditions. Table I1 shows the rate and turnover frequency for the oxidation of C O on the V205/Ti02(a)and V205/Ti02(r)catalysts when excess oxygen was present, where the reaction rate was zeroth order with respect to Po,, and where the catalyst was confirmed to be in the V5+state. Here, the turnover frequency is defined as the rate of CO oxidation divided by the number of surface V=O species. This was done in order to eliminate a trivial effect of Ti02supports on the activity, Le., the increase in the surface area of V2O5 by spreading the V205 on the support. As shown in Table 11, when the content of V2O5 in the V205/Ti02(a) was low (e.g., 2 or 5 mol % V2O5), the turnover frequency was 5-7 times smaller than that of the unsupported V2O5-U catalyst. Similarly, the turnover frequency for the V205/Ti02(r)decreased markedly with decreasing content of V2O5 in the catalyst. Furthermore, the turnover frequency for the VzOS/TiO2(a)-RO(2 mol % V205)was about 4 times larger than that for the V,05/Ti02(a) (2 mol % V2O5). This means that the reduction-oxidation treatment of the V205/Ti02(a)( 2 mol % V205) markedly increased the turnover frequency for the oxidation of CO. Although Table I1 shows only results for the reaction at 673 K, a similar relationship was also found to hold for the reaction between 673 and 745 K; the apparent activation energies for the supported V2O5 catalysts were 20-23 kcal mol-'.
Discussion Structure Sensitivity of the Reaction. As described above, the
>
.
1200
'
a
1000
'
.
800
'
6013
WAVE NUMBER f C M - ~ Figure 7. Infrared spectra of the V20s-U and V2OS-Fcatalysts in the steady state of the reaction at various partial pressures of 02:(a) V,03-U catalyst; reaction conditions: temperature = 658 K, Pco = 0.045 atm. (b) V205-Fcatalyst; reaction conditions: temperature = 743 K, PCO = 0.045 atm. The numbers in parentheses represent the partial pressures of 0,.
the difference in the surface area among the catalysts. As shown did in Table I, the density of surface V = O species or S(olo)/SBET not change significantly with the kind of unsupported V2O5 catalysts. Therefore, the relationship for the activities of the V205-U,V,OS-F, and V 2 0 5 - R 0catalysts is not affected by the measure to represent the surface area of the catalyst. As shown in Table 11, the turnover frequency for the V2O5-U catalyst was ca.30 times larger than that for the V205-F catalyst. Furthermore, the turnover frequency for the V205-ROcatalyst was almost twice as large as that for the V20S-F catalyst.31 Although Table I1
V2O5-F catalyst was prepared by fusing the V205-U catalyst, while the V 2 0 5 - R 0catalyst was prepared by the reduction-oxidation treatment of the VzO5-F. As shown in Table 11, the turnover frequency for the CO oxidation changed markedly with this difference in the preparation method of the unsupported V205 catalysts; the fusion of the V2O5-U catalyst markedly decreased the turnover frequency and the reduction-oxidation treatment of the V205-Fincreased the turnover frequency.31Since no impurities were detected in the XPS spectra of the catalysts (except for CIS), this means that the oxidation of C O on the V205catalyst is a structure-sensitive reaction. Unchanged electronic states as revealed by UV-visible and XPS support the validity of this conclusion. Although various reactions on metal catalysts have been classified into the structure-sensitive reactions,I5-I7 a structure(31) The error in determining the number of surface V=O species was within k 10%. Turnover frequency for the structure-insensitivereactionsuch as the ethylene oxidation-on V205-Fcatalyst was almost equal to that on V 2 0 ~ - R catalyst 0 (within &lo%); confer, for example, Table I in: Mori, K.; Miyamoto, A.; Murakami, Y. J . Phys. Chem., following article in this issue. Consequently, the experiments were accurate enough to discuss the structure sensitivity from the data of turnover frequencies on V205-Fand V,05-R0 (Table 11).
2740
The Journal of Physical Chemistry, Vol, 88, No. 13, 1984
TABLE 111: Values of kcoa and kOP for the V20s-U and V305-F Catalysts at 743 K
catalyst kCob kcOc kCOaV ko,b ko; koT V?OvU 55.1 52.1 53.6 40.0 44.3 42.2 Vi0i-F 1.58 1.78 1.69 5.81 5.32 5.57 aUnits: s-l atm-I. bValue derived from TF'vs. Po;'. derived from TF' vs. Pc0-'.
Mori et al.
0
kCOBVI
koT
1.27 0.30 CValue
sensitive reaction on metal oxide catalyst has barely been established.'s-21 The oxidation of CO on vanadium oxide catalysts provides another example of the structure-sensitive reaction on metal oxide catalysts. Effect of T i 4 Support on the Reaction. It is generally accepted that a Ti02 support promotes the oxidation of hydrocarbons over vanadium oxide ~ a t a l y s t s , 3 ! ~and $ ~ this ~ - ~effect ~ has been quantitated for the oxidation of benzene in terms of the turnover frequency; the turnover frequency for the benzene oxidation on V205/Ti02(a)catalysts has increased significantly with decreasing content of VZO5 in the catalyst. As shown in Table 11, on the other hand, the turnover frequency for the CO oxidation on Vz05/ Ti02(a) or V205/TiOZ(r)catalysts decreased significantly with decreasing V2O5 content in the catalyst. Such a retarding effect by the Ti02 support on the activity of VZOS for the CO oxidation is in contrast to the known promoting effect of T i 0 2 for the oxidations of h y d r o c a r b o n ~ , 3 -suggesting ~ ~ ~ ~ - ~ ~that active sites for the CO oxidation are different from those for the oxidation of hydrocarbons. Kinetics of the Reaction. As shown in Figures 5 and 6, kinetic behaviors in the CO oxidation on the V205-U are different from those on the V2O5-F. In order to quantitate this difference, the results are analyzed according to the reduction-oxidation mechanism: 2,9JO,14
co + (0)2%
CO2 + ( )
(2)
2%
'/202 + ( 1 (0) (3) where (0)and ( ) represent oxidized and reduced sites, respectively. Among possible rate equations for reactions 2 and 3, eq 4 was found to be the simplest equation to satisfy experimental
T F ' = kco-'Pco-'
+ ko['Po[l
(4)
relationships between the turnover frequency and Po, (or Pco), where kco and ko, stand for the rate constants of reactions 2 and 3 (s-' atm-'), respectively. As shown for example in Figure 9, the observed turnover frequencies for the reaction on the VzO5-U and VzO5-F catalysts were well represented by eq 4, and the rate constants, kco and ko,, were determined from the relationships and are shown in Table 111. As indicated, the rate constants derived from the relationship between T F ' and P0;l agree with those derived from the relationship between TF' and Pc0-'.This indicates that the rates of CO oxidation on the VzOs-U and VzO5-F catalysts are formulated by eq 4. As shown in Table 111, the rate constants, kcOavand koT, for the V2O5-U are much larger than those for the VZOS-F. Furthermore, the ratio of the rate constants, k c o a v / k o ~for , the V205-U is significantly different from that for the V2O5-F. Since the rate constants are a sensitive reflection of the nature of active sites, these results indicate that the active sites for the V2O5-U are much different from those for the V2OS-F. Active Sites for the CO Oxidation. As for the V2O5-F catalyst, the rate and the amount of V=O species changed similarly with Po (Figure 5c). As shown in Figures 1-3, the surface of the V2b5-F was much smoother than that of the Vz05-Uor V,05-R0. Since the V=O species is located on the (010) face of the VZOS crystal, these results suggest that the active site for the V2O5-F is the surface V = O species in the smooth (010) face. The gradual (32) Vejux, A.; Courtine, P. J . Solid State Chem. 1978, 23, 93. (33) Gond, G. C.; SBrkPny, A. J.; Parfitt, G. D. J . Catal. 1979, 57, 176. (34) Bond, G. C.;Briickrnan, K. Faraday Discuss. Chem. SOC.1981,72, 225.
Figure 9. Relations of TF' vs. Po;' and TF' vs. Pco-': (a) V,05-U catalyst, (b) V205-Fcatalyst.
decrease of the reaction rate after the stoppage of gaseous O2 supply supports the validity of this conclusion. This is because the subsurface V=O species can reproduce the surface V=O species even if the surface V=O species is consumed by the reaction with various reactants.2~7~8~22-24~30 As shown in Table I, S(olo)/SeEr for V20S-Fis 0.61. This indicates that various faces of VZO5 are exposed to the surface in addition to the (010) face. Since the active site for the CO oxidation on VzO5-F is located on the (010) face of V2OS,other faces of V20s-especially low Miller index faces responsible for the smooth ~ u r f a c e ~ ~ - a r e considered to be less active than the (010) face. From the results of the turnover frequency (Table 11), the amount of V=O species in the catalysts (Figure sa), and the kinetics of the reaction (Table I11 and Figure 8), the surface V = O species in the smooth (010) face is not the active site for the CO oxidation on the V2O5-U catalyst. Fusion of a solid would generally lead to a smooth surface with a decreased number of surface defects. According to Figures 1-3, the surface of the VZ05-Uwas more rugged than that of the V205-F. Furthermore, no impurity peaks were observed in the XPS spectrum of the Vz05-U, V205-F, or V2OS-RO. It is therefore considered that the active sites for the CO oxidation on the V205-U are located at the surface defects ~~~~
(35) E.g.: Davis, S. M.; Somorjai, G. A. "The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis"; King, D. A,, Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1982; Vol. 4, Chapter 7.
J. Phys. Chem. 1984,88, 2741-2744 (e.g., steps, kinks, or v a ~ a n c i e s ) . Since ~ ~ severe redox treatment of a solid with few surface defects would tend to increase their number, the turnover frequency for the V205-RObeing higher than that for the V2O5-F supports the validity of this conclusion.37 Reasonfor the Unusual Supporting Effects of Ti02. According to Vejux and C o ~ r t i n ethere , ~ ~ is a remarkable fit of the crystallographic patterns between the (010) face of VzO5 and the TiO, surface. On the basis of the characterizations of V20S/Ti02 catalysts by using the rectangular pulse technique coupled with various physicochemical measurements, we have previously found the validity of their c o n c l ~ s i o n .At ~ ~low contents of VzO5, the catalyst grows epitaxially on the T i 0 2 support to expose its (010) face, whereas various faces are exposed on the unsupported V20s-Ucatalyst. It is therefore considered that a smooth VZO5 surface with few defects is formed for the VzOS/Ti02(a)and V205/TiOz(r)catalysts having a low concentration of VzOs and that the number of surface defects increases with increasing content of V205. Since the activity of the surface defects is much higher than that of the surface V=O species in the smooth (010) face, this explains the significant reduction in the turnover fre-
2741
quency for the V20S/Ti02(a)and V205/Ti02(r)compared with that for the unsupported V205-U catalyst and the increase of the turnover frequency with increasing content of V20, in the V205/Ti02(a)and V205/Ti02(r).38The marked increase in the turnover frequency for the V205/TiOz(a) (2 mol % V2O5) after redox treatment (Table 11)39also supports the validity of this conclusion, because the severe redox treatment of VzOs increases the number of surface defects. In conclusion, the structure sensitivity and the unusual supporting effect of TiOz in the CO oxidation over vanadium oxide catalysts were reasonably understood in terms of the active sites for the reaction. Reaction mechanisms on these sites will be discussed on the basis of additional experimental results including those of ‘*O-tracer experiments.
Acknowledgment. We thank Mr. Masatoshi Yamada (Kinuura Research Department, JGC Co.) for taking the electron micrographs of the catalysts. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan (no. 57470055). Registry No. CO, 630-08-0; V,Os, 1314-62-1; TiOz, 13463-67-7.
(36) Since smooth and rugged surfaces would form low Miller index faces and high Miller index faces, respecti~ely,’~ the observed changed in the turnover frequency with V205-U,VZOS-F, and V z 0 5 - R 0could be understood in terms of the differences of the crystal faces exposed. This is not in conflict with the present conclusion that surface defects provide active sites for the C O oxidation, because high Miller index faces usually form surface defects such as steps or kinks.3s Confer also ref 39. (37) The decrease in the amount of the V=O species for the reaction at 743 K in the absence of 0,(Figure 5b) is ascribable to the reaction on the V = O species in the smooth (010) face, since the latter reaction can proceed at a measurable rate at such a high temperature as 743 K.
(38) The results of characterizations of Vz05/TiOz(a)and Vz05/Ti0,(r) catalysts indicate that the formation of mixed oxide between V,05 and TiOz is 11egligible.2~Thus, the retarding effect of TiOZis not due to the formation of the mixed oxide. (39) Since the number of V20s layers for Vz05/Ti0,(a) (2 mol % V,05) or V,O,/TiO,(a)-RO (2 mol % V205)is 1-2 (Table II), this result suggests that surface roughness in such thin Vz05layers is enough to form the active site for the CO oxidation. Surface defects such as steps, kinks, or vacancies seem more appropriate than high Miller index faces to describe the roughness in the thin V205layers.
Catalytic Reactions on Well-Characterized Vanadium Oxide Catalysts. 2. Oxidation of Ethylene Kenji Mori,t Akira Miyamoto,* and Yuichi Murakami Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa- ku, Nagoya 464, Japan, and Kim-ura Research Department, JGC Company, Sunosaki-cho Handa, Aichi 475, Japan (Received: October 31, 1983)
Catalytic activities of unsupported and supported vanadium oxides for ethylene oxidation were investigated in relation to the catalyst structures. From results of reaction rates and steady-state catalyst structures at various 0, concentrations, the active oxygen species was found to be the surface V=O species. The specific activity of the surface V=O species, or the turnover frequency, on unsupported Vz05catalyst did not change with the treatment of the catalyst: Neither fusion nor reduction-oxidation treatment affected the turnover frequency. This indicates that ethylene oxidation on vanadium oxide catalyst is a structure-insensitive reaction. The turnover frequency for Vz05/Ti02(anatase) was considerably higher than that for V205/Ti02(rutile) or unsupported V2O5 at any V205content. This indicates the promoting effect of Ti02 (anatase) support on the activity of vanadium oxide and provides experimental evidence for the validity of previous inference in solid-state chemistry.
Introduction Supported metal oxide catalysts exhibit interesting catalysis depending on the kind of 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 number of active sites on supported metal oxide catalysts. As for the supported vanadium oxide catalysts, we have previously established JGC Co.
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at the Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan.
0022-3654/84/2088-2741$01.50/0
the rectangular pulse technique which allows the determination of the number of surface V=O species and the number of V,05 layers on support.’ Furthermore, the structures of V205/T10, catalysts have been determined by using various physicochemical measurements together with the rectangular pulse technique.2 The V2O5/TiO2catalyst is active and selective for the oxidations (1) (a) A. Miyamoto, Y . Yamazaki, M. Inomata, and Y . Murakami, J. Phys. Chem., 85, 2366 (1981); (b) M. Inomata, A. Miyamoto, and Y . Murakami, Ibid., 85, 2372 (1981). (2) Y. Murakami, M. Inomata, A. Miyamoto, and K. Mori, Proc. Int. Congr. Catal., 7th, 1980, 1344 (1981); (b) M. Inomata, K. Mori, A. Miyamoto, T. Ui, and Y. Murakami, J . Phys. Chem., 87, 754 (1983); Y . Murakami, M. Inomata, K. Mori, T. Ui, K. Suzuki, A. Miyamoto, and T. Hattori, Prep. Catal. 3, Proc. Int. Symp., 3rd, 1982, 531 (1983).
0 1984 American Chemical Society
