J. Phys. Chem. 1 9 ~ 387, , 761-768
cm-' (Figure 2), diffuse hyperfine structure in the ESR spectra (Figure 3), and strong absorption in the 500-600nm region in the UV-visible spectra (Figure 4) for these catalysts. As shown in Figure 9, various crystal faces of V205are exposed on the surface of V2O5/TiOz(a)(25 and 50 mol % V205). This agrees with the results shown in Figure 6 that S(OIO)/SBETdecreases with increasing V205 content of more than 10 mol %. This change in s(O1O)/sBET is reasonable, because the effect of the interaction at the V205-Ti02interfacek7 on the surface structure of V205is considered to decrease with the number of V205 layers. 4. V205/TiOz(a) Monolayer Catalyst. In accordance with the experimental number of V205layers shown in Table I, monolayer of V205spreads over the TiOz surface in the V205/Ti02(a)monolayer catalyst (Figure 9). As shown in Figure 8, both 1610- and 1420-cm-' bands appear in the IR spectrum of NH3 adsorbed on the VzO5/TiO2(a) monolayer catalyst. This means that both TiOz and V205 are exposed on the catalyst surface. The strong peak at
761
1420 cm-' further indicates that the monolayer of V205 covers a considerable part of the whole catalyst surface. This can also be seen in the results of S(olo) and SBmshown in Table I. According to Yoshida et al.,1° the treatment of a supported vanadium oxide catalyst with an ammoniacal solution dissolves the isolated, massive vanadium oxide into the solution, while the residual vanadium oxide is regarded to interact chemically with the support. A considerable amount of surface V=O species present on the V205/ Ti02(a)monolayer catalyst (Table I) therefore indicates the intimate interaction between V205and Ti02. This agrees with the conclusion of previous investigations on the V2O5/TiOZ~atalyst."~ Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan (No. 57470055). Registry No. Vz05, 1314-62-1;TiOz, 13463-67-7.
Structures of Supported Vanadium Oxide Catalysts. 2. V,O,/AI,O, Makoto Inomata,+ Kenjl
MorIlt Aklra
Mlyamoto," and Yulchl Murakaml
Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan (Received: February 23, 7982; In Final Form: September 9, 1982)
Structures of vanadium oxide supported by AZO3have been investigated by using the rectangular pulse technique coupled with various physicochemical measurements including X-ray diffraction and IR, ESR, and UV-visible spectroscopy. In situ IR spectra of adsorbed ammonia and benzaldehyde have also been measured to characterize the catalysts. It has been found that the structure of VzO5/Al2O3 differs significantly from that of VzO5/TiOZ. At lower Vz05content (1and 2 mol % V205),the Vz05loaded on A1203barely forms surface V=O species but does form inactive vanadium ions, most of these ions acting as Bronsted acid sites. With a further increase in the Vz05content to 25 mol 70, small particles of V205grow on the A1203support, which leads to the gradual increase in the number of layers of Vz05lamellae on the support and in the fraction of the (010) face of VZOb As for the Vz05/A1203catalyst (25 mol % Vz05), 2-4 layers of V205 lamellae almost cover the surface of Alz03 and about half of the catalyst surface is occupied with the (010) face of Vz05. When the V205content is 35 mol % or more, Vz05completely covers the Alz03surface. However, the fraction of the (010) face of V205 on the whole catalyst surface does not exceed the value of the unsupported V205 catalyst (SOYO),which is in contrast to the behavior for the VzO5/TiO2catalyst. This indicates that various crystal faces of Vz05are exposed to the surface of Vz05/A1203 because of the lack of any crystallographic fit at the Vz05-A1203 interface.
Introduction We have previously investigated the structure of V205/Ti02catalysts by using the rectangular pulse technique coupled with a variety of physicochemical measurements.l It has been found that the V=O species, or the (010) face of V205,is selectively exposed on the surface of VzO5/TiO2. This behavior is independent of the kind of TiOz supports, i.e., anatase, rutile, and mixture of anatase with rutile. According to the V205-Ti02 interface model proposed by Vejux and Courtine,2 the V205*upport interaction is expected to be greatly affected by the kind of supports. Therefore, it seems interesting to investigate the effect of the kind of supports on the structure of V205. In this study, we investigated the structure of vz05/&03 catalysts and discussed it in comparison with the structure of V2O5/TiOZcatalysts. The V205/A1203catalyst has been 'Present Address: Kinu-ura Research Department, JGC Co., Sunosaki-cho, Handa, Aichi 475, Japan. 0022-365418312087-0761$01.50/0
used for the oxidation of hydrocarbons and recently for the reduction of NO with NH3.3-9 Although valuable information has been obtained on the structure of V205/A1203catalysts by using various physicochemical mea~urements,4~J"-'~ further investigations are necessary (1) M. Inomata, K. Mori, A. Miyamoto, T. Ui, and Y. Murakami, J. Phys. Chem., preceding article in this issue. (2) A. Vejux and P. Courtine, J. Solid State Chem., 23, 93 (1978). (3) J. K. Dixon and J. E. Longfield, Catalysis, 7, 281 (1960). (4) F. Roozeboom, A. J. van Dillen, 3. W. Gem, and P. J. Gellings,Ind. Eng. Chem. Prod. Res. Deu., 20, 304 (1981). (5) M. Ya. Kon, V. A. Shveta, and V. B. Kazanskii, Kinet. Katal., 14, 403 (1973). (6) D. V. Spiridonova, E. A. Fokina, K. N. Spiridonov, and 0. V. Krvlov. Kinet. Katal.. 18. 1495 (1977). 17) G. L. Bauerle, S C.'Wu, &d K. Nobe, Ind. Eng. Chem. Prod. Res. Deu., 14, 268 (1975). (8) G . L. Bauerle, S. C. Wu, and K. Nobe, Ind. Eng. Chem. Prod. Res. Deu., 17, 117 (1978). (9) M. Inomata, A. Miyamoto, T. Ui, K. Kobayashi, and Y. Murakami, Ind. Eng. Chem. Prod. Res. Deu., 21, 424 (1982).
0 1983 American Chemical Society
Inomata et al.
The Journal of Physical Chemistty, Vol. 87, No. 5, 1983
762
\ y 5 / 4 I2 c;
:f
(4OL RATIO)
(DE'OEE)
:c
;c
'2
I
I
I
j;
60
70
1 0
2
:
98
5
:
?5
13
'
PP
so
I
P
PP
P
P
PP
P
P
I
I
:I:: Li
c
:
10s
I
1
I
I
P I?
I
IIIII I
P I
r
IIIII
P
I
I
I
Figure 1. X-ray diffraction diagrams of V2O5/AI2O3with various V2O5 content: (-) V205, (4) AI,O,.
in order to precisely determine the structure of VZO5/Al2O3 catalysts. Experimental Section An A.120support 3 was obtained commercially (Sumitomo 7-A1203), and its BET surface area was 230 m2 g-l. V205/A1203catalysts were prepared by impregnation of the A1203support with an oxalic acid solution of ammonium metavanadate followed by calcination at 773 K in a stream of O2 for 3 h. V205/A1203(25 mol % V205) treated with an ammoniacal solution was prepared in a manner similar to that of Yoshida et al.;13 this catalyst is hereafter referred to as a V205/A1203monolayer catalyst. Characterizations of the V205/A1203catalyst were carried out by using the rectangular pulse technique coupled with various physicochemical measurements including X-ray diffraction and IR, ESR, and UV-visible spectroscopy, which have been described previously.' Infrared spectra of NH, adsorbed on the catalyst were measured in the same manner as described previously.' In addition, infrared spectra of benzaldehyde adsorbed on the catalyst were recorded in situ on a Jasco IR-G spectrometer as follows: A disk of the catalyst was heated in situ under vacuum for 1 h at 673 K followed by the adsorption of benzaldehyde at 573 K for 5 min and subsequent evacuation at 573 K for 15 min. After the temperature decreased to a room temperature, the infrared spectrum was measured. Results X-ray Diffraction. Figure 1 shows X-ray diffraction diagrams of V205/A1203catalysts with various V205 content. As shown, the A1203support consisted of 7-A1203 (10)K. Tarama, S.Yoshida, S. Ishida, and H. Kakioka, Bull. Chem. SOC. Jpn., 41,2840 (1969). (11)H.Takahashi, M. Shiotani, H. Kobayashi, and J. Sohma, J. Catal., 14, 134 (1969). (12)M. Akimoto, M. Usami, and E. Echigoya, Jpn., _ . Bull. Chem. SOC. 51,'2195 (1978). (13)S.Yoshida, T. Iguchi, S. Ishida, and K. Tarama, Bull. Chem. SOC. Jpn., 45,376 (1972). '
-
1203
i m
'WAVE hUI'BE?
330
/ cY1
Flgure 2. Infrared spectra of V,O,/AI,O, with various V,05 content. The number in parentheses represents the V,05 content of the catalyst.
and all of the diffraction peaks of the V205/A1203catalyst were assigned to either V205or 7-A1203:when the content of V205was 25 mol % or lower, only 7-A1203peaks were observed. When the V205content was 35 mol % or higher, the V205phase appeared in addition to the 7-Al2O3phase, and the Vz05peaks became stronger with increasing V2O5 content. No peaks assignable to a compound between V2O5 and A1203were observed. IR Spectra of the Catalysts. Figure 2 shows infrared spectra of V205/A1203catalysts with various V205content. Although the spectra did not exhibit definite peaks, a shoulder around 1020 cm-l was observed for catalysts with a V205content of 5 mol % or higher. This is assigned to the V5+=0 stretching vibration.14J5 ESR Spectra of the Catalysts. Figure 3 shows the ESR spectra of v205/&03 catalysts with various V205content, which can be assigned to the V4+ ion.l@12 Hyperfine structures due to electron spin-nuclear spin coupling were observed in the spectra; the parameters are determined below. As the V205content increased, the signal intensity increased, while the hyperfine structure became diffuse gradually. The amount of V4+ ions as determined by double-integration of the ESR signal was at most a few percent of vanadium ions supported. Figure 3b shows an ESR spectrum of V205/A1203(10 mol % V205)which has been reduced by treatment with a mixture of NO (1000 ppm) and NH3 (1000 ppm) at 573 K for 80 h.gJ5 The intensity for reduced V205/A1203(10 mol % V2O5) was considerably stronger than that for untreated VzO5/Al2O3 (10 mol % V205),and the hyperfine structure for the reduced catalyst was more diffuse than that for the untreated catalyst. According to Hecht and Johnstone,16ESR parameters for the V4+ion with an axial symmetry can be determined (14)L. D. Frederickson and D. M. Hansen, A d . Chem., 35, 818 (1963). (15)M.Inomata, A. Miyamoto, and Y. Murakami, J. Catal., 62,140 (1980). (16)H.G.Hecht and T. S. Johnstone, J. Chem. Phys., 46,23(1967).
The Journal of Physical Chemistry, Vol. 87, No. 5, 1983 763
Structure of V205/Ai203Catalysts
by eq 1 and 2,17 where HI,and H, are parallel and perHI,= 2Ho/qll- (All/gllS)mI (for 8 = 0) (1)
H,
= 2H0/g, - (A,/g,P)m,
(for 8 = r / 2 ) ( 2 )
pendicular principal components, respectively, of extremum points of ESR absorption peaks; /3 is the Bohr magneton; mI is the nuclear magnetic quantum number of the V4+ion; g,,and g , are the parallel and perpendicular principal components, respectively, of the g tensor; A,,and A , are parallel and perpendicular principal components, respectively, of the hyperfine coupling tensor. Similar to the method employed in previous investigations,",l2HI,and H, were determined as shown, for example, in Figure 4a, and these are plotted against mI in Figure 4b. From the observed straight lines coupled with eq 1 and 2 , values of the ESR parameters were determined and summarized in Table I. These values are close to those of the V4+ion formed separately on the A1203support.'lYl2 UV-Visible Reflectance Spectra of the Catalysts. Figure 5 shows the optical reflectance spectra of VZO5/ A1203catalysts with various V205content. As shown, the spectra of catalysts with low V205content (1or 2 mol % Vz05)were different from those of catalysts with a VzO5 content of 5 mol % or higher. The red edge of the absorption was ca. 500 nm for V205/A1203(1 or 2 mol % Vz05)while it was ca. 600 nm for V205/A1203( 5 , 10, 25, and 50 mol % V205) and unsupported V205. These absorption bands have been assigned to charge-transfer transitions from 02-to V5+.18719The absence of absorption bands for one-electron d-d transitions in the 600-800-nm region indicates that the amount of V4+ions is small for all catalysts shown in Figure 5. The Number of Surface V=O Species and the Number of Layers of V205Lamellae. Figure 6 shows examples of concentration profiles of N2 produced by the reaction of the rectangular pulse of the NO and NH, mixture with the V205/A1203catalyst. Using the method described in the previous papers,mwe determined the amount of the initial sharp N2 signal from the concentration profile of N2 The amount of the initial sharp N2 signal was constant and independent of the reaction temperature. From the constant value, the number of surface V=O species on the catalyst, L , was determined, and the results are shown in Table 11. The specific area of the (010) face of V205,S(olo), can be calculated from L by dividing by the surface density of V=O on the (010) face of V205,4.872 nm-2.20 This is also shown in Table I1 with the results of the BET specific surface area of the catalyst, SBm.Figure 7 shows the ratio of S(olo) to SBm,which indicates the fraction of the (010) face of V205on the whole area of the catalyst surface. When the V205content was 0, S(OlO)/&T was equal to 0, indicating the surface of an uncovered A1203 support. When the V2O5 content was 1or 2 mol % , S(olo)/SBET was negligibly small. This means that the loaded V2O5 barely forms surface V=O species. As the V205content increased 5-25 mol % , S(OIO)/SBETincreased abruptly and attained almost a constant value above 25 mol % V2OP However, the constant value did not exceed the value of the unsupported V206catalyst (50%),where various crystal faces of VzO5 are exposed in addition to the (010) face.m Figure 8 shows the dispersion of Vz05, D, which is defined as the (17) The form of the spin Hamiltonian for this system is given by eq 1 in ref 11. ~.~ (18) C. K. Jorgensen, "Adsorption Spectra and Chemical Bonding in Complexes", Pergamon Press, Oxford, 1962. (19) A. M. Gritakov, V. A. Shveta, and V. B. Kazanskii,Kinet. Katal., 14, 1062 (1973). (20) A. Miyamoto, Y. Yamazaki, M. Inomata, and Y. Murakami, J. Phys. Chem., 85,2366 (1981); Chem. Lett., 1355 (1978). ~
~~~
~~
:
b:
/G\
Figure 3. ESR spectra of (a) V205/A120, with various V,05 content and (b) V2O5/AI2O3(10 mol % V2OS)reduced by the treatment with a mixture of NO (1000 ppm) and NH3 (1000 ppm) at 573 K for 80 h. The number in parentheses represents the V206content of the catabst.
ratio of the number of surface V=O species to the number of V205in the catalyst. The dispersion was very small for catalysts with 1-5 mol % V205,while it increased with
764
Inomata et al.
The Journal of Physical Chemistty, Vol. 87, No. 5, 1983
TABLE I: SDin Hamiltonian Parameters o f the Spectrum o f V4+ for V,O./Al,O, with Various V:O. Content
2 5 10 25
177 182 179 174
65 64 70 65
1.946 1.944 1.937 1.944
102 103 106 101
1.987 1.985 1.991 1.990
3455 3450 3465 3450
1.973 1.971 1.973 1.975
3385 3380 3370 3396
TABLE 11: Number o f Surfuce V=O Species ( L ) ,S(,,,), and Number of Layers o f V,O, Lamellae (N)for V,O,/Al,O, with Various V,O, Content
L,
mol
mol g-l
S(,,,); m 2 g-
1 2 5 10 25 35 50 monolayer
a 3 77 355 405 365 249 20
0.4 9.4 43.6 49.7 44.8 30.5 2.4
% v,o,
N
SBET; m zg-
exptl
cdcd
216.1 221.3 219.1 167.6 114.1 101.1 65.8 174.2
1-2 1-2 1-3 2-4 3-7 5-15 1
(62.5) (6.2) 2.6 5.1 7.4 14.3
(-)
Trace.
AJELEITi
INY
Flgure 5. UV-visible reflectance spectra of V20,/AIz03 with various V205content. The number In parentheses represents the V205content of the catalyst.
I
m
I
=
7/2
I
I
5/2
-3/2
Q+ :
:
3= 3=X/2
a-o
1
l
l
-5/2
-7/2
b
f
Flgure 4. (a) AssQnment of each peak !n the spectrum of V4+ formed in V,O,/AI,O, (10 mol % V20,). (b) Plot of the magnetlc field of the observed peaks against the nuclear quantum number m , .
further increasing of the V205 content up to 10 mol %, attained its maximum at 10 mol % V2O5, and then decreased to a very small value for the unsupported V2O5 catalyst
.
By comparing the experimental concentration profiles of N2 (Figure 6) with theoretical ones,21 the number of layers of V205lamellae, N , was determined and the results are shown in Table 11. If all of the loaded V205 forms uniform layers of V205 on A1203,N is given by the reciprocal of D. The number of layers thus calculated is also indicated in Table 11. The calculated number of layers is larger than the experimental one, while the difference decreased with increasing Vz05content in the For the catalyst with a V205content of 1-10 mol 7'0,the number of Vz05layers on A1203remained 1 , 2 , or 3. The number of V205layers increased gradually with further increase in the V205 content, being 2-4 layers for V205/ A 1 2 0 3 (25 mol % V2O5) and 5-15 layers for V2O5/AlZO3 (50 mol % V205). Table I1 also shows the values of L, S(olo), and N for the V205/A1203monolayer catalyst. As shown, the number of layers was 1, meaning a monolayer of V205 was on the AI2O3support. Infrared Spectra of Ammonia and Benzaldehyde Adsorbed on Catalysts. Figure 9 shows the infrared spectra of NH, adsorbed on V205/A1203catalysts with various V205content. The IR spectrum of NH, adsorbed on the NZO3support exhibited only absorption bands at 1230 and 1610 cm-l assignable to coordinately held NH3.23924In the IR spectra of NH3 adsorbed on V205/A1203catalysts with a V205content of 10 mol % or lower, bands of coordinately held NH3 and that of NH4+(1420 or 1450 cm-') were both observed. As for the VzO5/Al2O3catalyst (1 or 2 mol % (21) M. Inomata, A. Miyamoto, and Y. Murakami, J. Phys. Chem., 86, 2372 (1981). (22) The dieagreementbetween experimental and calculatedvalues for V106/A1203(1,2,or 5 mol % Vz06)is due to the formation of inactive vanadium ions. (23) (a) L. H. Little, "InfraredSpectra of Adsorbed Species",Academic Press, New York, 1966,Chapter 7. (b) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry",Marcel Dekker, New York, 1967, Chapter 5. (c) A. V. Kiselev and V. I. Lygin, "Infrared Spectra of Surface Compounds". Wiley, New York, 1975, Chapter 8. (24) H. Knbinger, Adu. Catal., 25, 184 (1976).
The Journal of Physical Chemistry, Vol. 87, No.
Structure of V,05/AI,03 Catalysts
0
4C
20 TWE
60 /
20
0
80
40
60
80
5, 1983 765
100
V23j CONTENT / MOLX
SEC
Flgure 7. Fraction of the (010) face of V,05 over the whole catalyst surface, Le., S(o,o)/Sm: (soli line) V2O5/AI2O3,(dotted line) V205/li02.
1,C
c*s
w Y
z
0
t
a Y w w
D
0
20
40 TIME
60 /
80
SEC
20
40
80
60
V p 0 j CCNTENT
/ MOLW
Figure 8. Dispersion of V,05 for V,05/AI,03: (dotted line) V,05/Ti0,.
D
20
40 TIME
60 /
100
(soli line) V2O5/AI2O3,
80
SEC
Flgwe 6. Concentration profiles of N, produced by the reaction of the rectangular pulse of NO and NH, with V,05/A1,03 at various temperatures. The number in parentheses represents the V205 content of the catalyst.
Vz05),the band of the adsorbed NH4+appeared at 1450 cm-l, while it shifted to 1420 cm-' for catalysts with a Vz05 content of 5 mol % or higher. As can be seen in Figure 9, the intensity of the bands of coordinately held NH, relative to that of NH4+decreased with increasing Vz05 content up to 10 mol %. When the V205content was 25 mol % , the bands of coordinately held NH3 disappeared, and only the band of NH4+was observed for catalysts with a Vz05 content of 25 mol % or higher. In the infrared spectrum of NH3 adsorbed on the V205/A1203monolayer catalyst, the absorption bands of coordinately held NH3 appeared and the band of NH4+was observed at 1450 cm-l.
1-159
1703
1503
1700
300 'IAVE NUP'BE'I
1500
1300
/ CM-'
Flgure 9. Infrared spectra of NH, adsorbed on the V,O,/AI,O, catalysts: (broken line) background, (solld line) I R spectra of NH, adsorbed on the catalyst. (A) Al,03, (B) 2 mol % V,05, (C) 5 mol % V,05, (D) 10 mol % V,05, (E) 25 mol % V,O,, (F) monolayer.
This is significantly different from the spectrum for the V205/A1203(25 mol % V205)from which the monolayer
766
The Journal of Physical Chemistty, Vol. 87, No.
5, 1983
Inomata et at.
35 1
125:
L
4
c
r
r
L
n
I
...........
1-2-
1E'^ 'IAL'E
IiU"SEF
1 41111
V205/A1,03catalyst was prepared by treatment with an ammoniacal solution. Figure 10 shows the infrared spectra of benzaldehyde adsorbed on Vz05/A1203catalysts. Absorption peaks appeared at 1312, 1435, 1455, 1500, 1550, and 1600 cm-' which are assigned to an adsorbed benzoate ion.25926As shown, the spectra for V205/A1203 (10 and 25 mol % ) exhibited the bands of the adsorbed benzoate ion, while no absorption bands were observed in the spectrum for Vz05/A1203(35 mol % Vz05). The absence of the bands of the benzoate ion was also confirmed for unsupported V205catalyst.
Discussion Structures of V205/A1203 Catalysts. On the basis of the above-mentioned results, structures of V205/A1203catalysts with various Vz05content were determined as shown in Figure 11. Here, bold lines refer to the (010) face of V205exposed on the catalyst surface, while small closed circles represent inactive vanadium ions (V5+ or V4+)27 interacting strongly with the A1203support. A detailed structure of the inactive vanadium ion is shown in Figure 12; the ion acts as a Bronsted acid site. The proposed structures are in accordance with the above-mentioned experimental results and conclusions of previous investig a t i o n ~ l , as ~ Jfollows: ~~~ 1. Vz05/A1203 ( 1 and 2 mol % V205).According to the proposed structures of Vz05/A1203(1and 2 mol % V2O5), the loaded V205 barely forms surface V=O species but does form inactive vanadium ions. This agrees with the negligibly small value of L, S(OIO)/SBET, or D for the cata(25) M. Niwa, H. Ando, and Y. Murakami, J. Catal., 49, 92 (1977). (26) A. E. T. Kuiper, J. Medema, and J. J. G. M. Van Bokhoven, J . Catal., 29, 40 (1973). (27) The inactive vanadium ion is neither reduced by the mixture of NO and NH3 nor oxidized by 02.According to our preliminary experiments, this ion is inactive for various oxidation reactions of hydrocarbons.
. .n. -
Figure 11. Structures of Vz05/AIz03catalysts with various V,O, content and V2O5/AlZO3monolayer catalyst: (bold line) the (010) face of Vz05exposed on the catalyst surface, (small closed circle) inactive , , content vanadium ion. The number in parentheses represents the VO of the catalyst.
/ c
Figure 10. Infrared spectra of benzaldehyde on the Vz05/AIzOB catalysts: (broken line) background, (solld line) IR spectra of benzaldehyde adsorbed on the catalyst. The number in parentheses represents the V,05 content of the catalyst.
monolayer
. . n. . . . n.
LCYE?S
OH AI-0
I
-V-0
-AI
Figure 12. Structure of the inactive vanadium ion.
lysts (Table 11, and Figures 7 and 8). The absence of the Vz05peaks in the X-ray diffraction diagrams (Figure 1) and the absence of the peak of the V=O stretching vibration at 1020 cm-' in the IR spectra of the catalysts (Figure 2) are consistent with the formation of the inactive vanadium ion. As shown in Figure 5, the red edge of the absorption in the UV-visible spectra of V205/A1203(1 and 2 mol % V205) (ca. 500 nm) is different from that of unsupported V205 (ca. 600 nm). Since the absorption in this wavelength region is assigned to a charge-transfer transition from 0,- to V5+,the difference in the red edge means that the coordination of oxygens around the V5+ ion in V,05/A1203(1and 2 mol % V205)is significantly different from that in unsupported Vz05. This agrees with the proposed structure because the coordination of oxygens around an inactive vanadium ion (Figure 12) is much different from that around V5+ in the V205crystal. The hyperfine structure in the ESR spectra of V205/A1203(1 and 2 mol % V205) (Figure 3) indicates that the V4+ ion is dispersed on the A1203support. This agrees with the proposed catalyst structures in which inactive vanadium ions are highly dispersed. It should be noted that the number of V4+ ions measured by ESR was much smaller than the number of vanadium ions supported. This means that the inactive vanadium ion is mainly composed of the V5+ ion. The absence of absorption bands for one-electron d-d transitions in the 600-800-nmregion in the UV-visible spectra (Figure 4) supports the validity of this conclusion. The bands of coordinately held NH3 at 1230 and 1610 cm-' in the IR spectrum of NH3 adsorbed on the A1203 support (Figure 9A) indicate the presence of Lewis acid sites on the A1203support. This agrees with the conclusions of previous investigation^.^^^^^ In the IR spectrum of NH, adsorbed on V205/A1203(2 mol % V205) (Figure 9B),the band of NH4+(1450 cm-') appears in addition to (28) J. B. Peri, J. Phys. Chem., 69, 220 (1965). (29) E. P. Parry, J. Catal., 2, 371 (1962).
Structure of V,O,/AI,O,
Catalysts
the bands of coordinately held NH3. This means that the inactive vanadium ion plays the role of a Bronsted acid site. It should be noted that the wavenumber of the NH4+ band for V205/A1203(2 mol % V205) (1450 cm-') is different from that for unsupported V205 (1420 cm-l). Since the loaded V205mainly forms inactive vanadium ions in V205/A1203 (2 mol % V205),the band at 1450 cm-' is assignable to NH, adsorbed on the inactive vanadium ion, while the band at 1420 cm-l has been assigned to NH3 adsorbed on the Bronsted acid site on layers of V205lamellae. (5, 10, and 25 mol % V2O5). In the 2. V205/A1203 proposed structures of V205/A1203(5,10, and 25 mol % V205)(Figure l l ) , small particles of V205grow on the A 1 2 0 3 support. This agrees with the marked increase in S(olo)/ SBmwith increasing V205content to 25 mol % (Figure 7). In accordance with the results shown in Table 11, the number of Vz05layers in the proposed structure is 1-2, 1-3, and 2-4 for catalysts with 5,10, and 25 mol % V2O5, respectively. Such small V205 particles in the catalysts explain the absence of the V205 peaks in the X-ray diffraction diagrams (Figure 1). According to Takahashi et al.,ll the agglomeration of vanadium ions leads to a singlet broad peak of V4+in the ESR spectrum. The hyperfine structure remaining in the spectrum of reduced V205/ A1203(10 mol % V205)therefore indicates that the V205 particles in V205/A1203(10 mol % V205)are not very large, which supports the validity of the proposed structure. According to the proposed structure, layers of V2O5 lamellae are formed on the A1203support in catalysts with a V205content of 5 mol % or higher. This is in accordance with the red edge in the W-visible spectra of the catalysts (ca. 600 nm), because layers of V205 are relevant to the red edge at ca. 600 nm in the spectra. The shoulder around 1020 cm-'-assignable to the V=O stretching vibrationin the IR spectra of the catalyst with a V2O5 content of 5 mol % or higher is also compatible with the presence of V205layers in the proposed structure. As shown in Figure 9, V205/A1203(5 and 10 mol % V205) catalysts exhibit both bands of coordinately held NH, (1230 and 1610 cm-') and NH4+(1420 or 1450 cm-'). As the V205content increases to 25 mol 90,the intensity of the former bands decreases while that of the latter band increases. This means that the A1203surface is gradually covered by V205with increasing V205content to 25 mol %, supporting the validity of the proposed structure. As shown in Figure 9C, V205/A1203(5 mol % V2O5) exhibits two kinds of NH4+bands at 1420 and 1450 cm-l, which are respectively assigned to NH, adsorbed on layers of V2O5 lamellae and NH, adsorbed on inactive vanadium ion. This means that the V205particle and the inactive vanadium ion are both exposed on the surface of V205/A1203 (5 mol % Vz05),in accordance with the proposed structure. On the other hand, the NH4+band appears only at 1420 cm-l for V205/A1203(10 and 25 mol % V205). This indicates that the V205particles are exposed on the catalyst surface more than the inactive vanadium ion in these catalysts, in agreement with the proposed structure [Figures ll(10) and 11(25)]. The absence of bands of coordinately held NH3 at 1230 and 1610 cm-I in the IR spectrum for V205/A1205(25 mol % V205) indicates that the A1203surface is almost covered by V205. According to Niwa et al.125the absorption bands in the IR spectra of benzaldehyde adsorbed on V205/A1203 surface, are brought about by the benzoate ion on the A1203 while the benzoate ion adsorbed on V205is unstable. This coincides with the result obtained in this study, because unsupported Vz05did not exhibit bands of the adsorbed
The Journal of Physical Chemistry, Vol. 87,
No. 5, 1983 767
benzoate ion. The presence of the bands of the benzoate ion for V205/A1203(10 and 25 mol % V2O5) (Figure 10) therefore indicates that the A1203 surface is exposed on the catalyst surface. These conclusionsprove the validity of the proposed structure. Although the bands of coordinately held NH, do not appear in the spectrum of NH3 adsorbed on V2O5/AI2O3(25 mol % V2O5), this may be due to the extinction coefficient of coordinately held NH, being smaller than that of NH4+. 3. V205/A1203 (35and 50 mol % V205).In the proposed structures of V205/A1203(35 and 50 mol % V205),layers of V205lamellae completely cover the A1203surface; their number is 3-7 in V205/A1203(35 mol % VZO5) and 5-15 in VzO5/Al2O3(50 mol % V205). These structures agree with the number of V205 layers (Table 11),peaks of V2O5 in the X-ray diffraction diagrams (Figure l),diffuse hyperfine structure in ESR spectra (Figure 3), strong absorption in the 500-600-nm region in UV-visible spectra (Figure 5), and the absence of the IR bands of the benzoate ion [Figure 10(35)] for these catalysts. According to the proposed structures of these catalysts, the (010) face of V205is not selectively exposed on the catalyst surface, but other faces of V205 are considerably exposed. This agrees with the results of s @ 1 o ) / s B for ~ the catalysts, which does not exceed the value for unsupported V205(50%) (Figure 7). 4. V205/A1203 Monolayer Catalyst. The number of V2O5 layers in the proposed structure of the V205/A1203 monolayer catalyst is 1, in accordance with the result shown in Table 11. The small coverage of the (010) face of V205 on the whole catalyst surface (Figure 11)is consistent with the small value of L or S(olo) for the monolayer catalyst (Table 11). As shown in Figure 9, NH, adsorbed on the V205/A1203monolayer catalyst exhibits bands of coordinately held NH3 at 1230 and 1610 cm-l. This indicates that the A1203surface is considerably exposed on the catalyst surface, in accordance with the proposed structure of the catalyst. Comparison of the Structure of V205/A1203 with That of V205/Ti02.By comparing the structure of V205/A1203 catalysts shown in Figures 11 and 1 2 with that of V205/ Ti02 catalysts (Figure 9 in ref l), we can note following points: 1. When the content of V205is 1or 2 mol 90,inactive vanadium ions are mainly formed on the V205/A1203 catalyst, while the (010) face of V205spreads over the Ti02 surface of the V205/Ti02catalyst. In other words, the loaded V2O5 effectively forms surface V = O species on the V205/Ti02catalyst. 2. The (010) face of V2O5 is selectively exposed on the surface of the V205/Ti02catalyst; namely, the fraction of the (010) face of V205 over the whole catalyst surface becomes as high as 90% for V205/Ti02(5 or 10 mol % V205).lp21 On the other hand, various crystal faces of V205 are exposed on the surface of V205/A1203in addition to the (010) face. This supports the validity of the conclusion proposed in previous investigation^,',^*^^^^^ since a crystallographic fit between the (010) faces of the V205 and Ti02 surfaces would cause the selective exposure of the (010) face of V205on the surface of VzO5/TiO2,while such an effect cannot be expected for the V205-A1203interface. 3. The number of surface V=O species on the V205/ A1203monolayer catalyst is much smaller than that on the V205/Ti02monolayer catalyst, while that on V205/A1203 (30) G. C. Bond, A. J. S i r k h y , and G. D. Parfitt, J. Catal., 57, 476 (1979). (31)D.J. Cole, C. F. Cullis, and D. J. Hucknall, J. Chem. Soc., Faraday Trans. 1, 72,2185 (1976).
768
J. Phys. Chem. 7903, 87, 768-775
(25 mol % V2O5) is considerably larger than that on V205/Ti02(10 mol % Vz05).32 According to Yoshida et al.,I3 the treatment of a supported vanadium oxide catalyst with an ammoniacal solution dissolves the isolated, massive vanadium oxide into the solution, while the residual vanadium oxide is regarded to interact chemically with the support. This means that the number of surface V=O species on the monolayer catalyst gives information about the strength of V205-supportinteraction: If the interaction is not strong enough, the supported vanadium oxide dissolves into the ammoniacal solution and the surface V=O (32) The Vz05/Alz03and V2O5/TiO2monolayer catalysts were prepared from VZ05/A1203(25 mol % Vz05) and VZO5/TiO2(10 mol % VZO,), respectively.
species is barely formed on the monolayer catalyst. The relationships for the number of surface V=O species in the monolayer catalyst therefore indicate the presence of an intimate interaction at the V205-Ti02interface and its absence at the V205-A1203interface. This also supports the validity of the conclusion proposed in previous investigation~.~~~~~~~~~ In conclusion, the structure of supported vanadium oxide catalyst is greatly changed with the kind of support. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan (No. 57470055). Registry No. Vz05,1314-62-1; A1,0,, 1344-28-1; NH3, 766441-7; benzaldehyde, 100-52-7.
Effective Surfaces of Semiconductor Catalysts for Light-Induced Heterogeneous Reactions Evaluated by Slmultaneous Photodeposition of both Oxidation and Reduction Products Tetsuhlko Kobayashl, Yoshlkunl Tanlguchl, Hlroshl Yoneyama,
and Hldeo Tamura
Deparfment of Applied Chemistty, Faculty of Engineering, Osaka University, Yamadaska, Suite, Osaka 565, Japan (Received: May 3, 7982; In Final Form: September 8, 7982)
Illumination of the front surface of a well-etched Ti02single crystal with nonillumination of the back in aqueous solutions containing both ruthenium(II1)chloride and chloroplatinic acid caused simultaneous photodeposition of ruthenium dioxide mainly onto the illuminated surface and of platinum onto the dark back surface. In the case where there were flaws in the surface, they served as selective reduction sites for the platinum deposition irrespective of whether the surface was illuminated or not. Similar selectivity of the semiconductor surfaces for oxidation and reduction processes of heterogeneous reactions was observed for the simultaneous photodeposition of lead dioxide and palladium onto Ti02single crystals and of polypyrrole and silver onto n-Gap, n-CdS, and n-Si single crystals. The simultaneous photodeposition continued to occur until the semiconductor surfaces were completely covered with reaction products, indicating that all the surface sites serve as effective sites for either oxidation or reduction processes of light-induced heterogeneous reactions.
Introduction The function of semiconductor photocatalysis in lightinduced heterogeneous reactions is to provide both positive holes having a high oxidizing power and electrons having a high reducing power a t specific reaction sites of the photocata1yst.l Concerning the reaction sites of n-type semiconductors, illuminated parts of the photocatalyst surfaces serve as oxidation sites, while dark parts serve as reduction sites, as judged from the photodeposition of c ~ p p e r . ~Such ? ~ selectivity of reaction sites is supported by electrochemical analysis of several heterogeneous reactions on Ti02;4,5in these cases, the rates of the heterogeneous reactions roughly coincided with those derived from the intersection point of dark cathodic and photoanodic current-potential curves, as typically demon(1) A. J. Bard, Science, 207, 139 (1980); J. Phys. Chem., 86, 172 (1982), and references cited therein. (2) M. S.Wrighton, P. T. Wolczanski, and A. B. Ellis, J . Solid State Chem., 22, 17 (1977). (3) H. Reiche, W. W. Dunn, and A. J. Bard, J . Phys. Chem., 83, 2248
strated by the photobleaching reaction of methylene blue on Ti02.* However, such selectivity is not always observable. For photodeposition of palladium and platinum onto Ti02 single crystals: for example, a fraction of the deposited metals was recovered from the illuminated surface. Furthermore, it was reported that the photodecomposition of water on metal-free SrTi03single crystals7 occurred on the illuminated surface alone; the dark back surface of the semiconductor was inactive in this case. It is thus judged to be important to investigate effective surfaces of semiconductor photocatalysts in detail. For this purpose, it is desirable to choose reaction systems in which both the oxidation and the reduction products are simultaneously deposited on effective surfaces for the respective processes. Observations and analysis of the deposited substances on the two-dimensional surface planes give direct information in detail on effective surfaces for individual processes of heterogeneous reactions. With this in mind, we searched for suitable reaction systems. Included in this paper are studies of the si-
I1 ~ 979\ -._,. -
(4) H. Yoneyama, Y. Toyoguchi, and H. Tamura, J. Phys. Chem., 76, 3460 (1972). (5) F. Mollers, T. J. Tolle, and R. Memming, J. Electrochem. SOC.,121, 1160 (1974).
(6)H. Yoneyama, N. Nishimura, and H. Tamura, J. Phys. Chem., 85, 268 (1981). (7) F. T. Wagner and G. A. Somorjai, J . Am. Chem. SOC., 102, 5492 (1980).
0022-365418312087-0768$01.50/0 0 1983 American Chemical Society
