J. Phys. Chem. 1981, 85,2366-2372
2366
TABLE I: Aggregation Numbers in 0.1 M SDS Solutions g
(lit.) 0.1 M SDS in water 0.1 M SDS in 0.2 M Na,SO, solution g o . 2 M salt /gno salt a
g
g (qT(calcd))’ (TIT = 1.0)
62,23 6324 1202
76 153
61 89
l e g 4
2.01
1.46
See text.
highest pyrene concentration used. Since the limiting quantum yields of excimer and monomer are virtually identi~al,’~ the acceptor fluorescence intensity is proportional to the area under the entire corrected pyrene fluorescence spectrum, irrespective of the extent of excimer formation. Since the maximum occupation number is low, eq 6 applies. The plots of I D / I A vs. 1/cA are the straight lines as predicted. The values of p ~ were g found from the slopes. In order to find the aggregation number g, it is necessary to know qT as a function of aggregation number. This information can be obtained from the Monte-Carlo method described above provided the size and shape of the core can be related to the aggregation number. The minimum micelle size of SDS micelles is generally thought to correspond to a core of radius 18 A.6 On addition of electrolyte micelles are likely to be cylinders capped with hemispherical ends of this radius. If one takes the molar volume of the hydrocarbon chain as that of dodecane, the size and shape of the core are easily calculated for any aggregation number. In fact we find that the dependence of pT on g is very similar for other micellar shapes, at least in the range of aggregation numbers 60-200. Aggregation numbers obtained in this way, together with literature values, are shown in Table I. In contrast to many methods of determining aggregation numbers, e.g., osmosis, con-
ventional and quasielastic light scattering, the energy transfer method does not require intermicellar interaction to be taken into account. Such interactions could be the reason for discrepancies between quasi-elastic light-scattering results.26 Our results are somewhat higher than those found by other methods, but the ratio of aggregation numbers in 0.2 M Na2S04to that without added electrolyte is the same. In the last column of Table I, we show aggregation numbers calculated by assuming pT = 1. This might be applicable if donor and acceptor are confined to the central part of the core. The agreement with literature values is better for SDS without added salt, but the effect is too small to be significant. It is clear however that g lies in the region of 60-80. On the other hand, the aggregation number in 0.2 M Na2S04is too low if pT is 1and the ratio of g to the aggregation number without salt is in poor agreement with the literature. It seems therefore that the fluorescent species are distributed throughout the micelle core and that pT has to be calculated for enlarged micelles by the Monte-Carlo method. The aggregation number in 0.2 M Na2S04is -150. The main source of error in the method is probably the quantum yields. These can be determined by the method described to ca. &lo%. The aggregation number, which is proportional to the ratio of two quantum yields, would have an error of ca. &15%. Supplementary Material Available: Discussion of correction for trivial reabsorption and of energy transfer in micelles with several acceptors (4 pages). Ordering information is available on any current masthead page. ~~~~~
~
(25)(a) N.A. Mazer, G. B. Benedek, and M. C. Carey, J.Phys. Chern., 80,1075(1976); (b) A. Fbhde and E. Sackmann, J. Colloid Interface Sci., 70,494 (1979).
Determination of the Number of V=O Species on the Surface of Vanadium Oxide Catalysts. 1. Unsupported V205and V205/Ti02Treated with an Ammoniacal Solution Aklra Mlyamoto,* Yutaka Yamarakl, Makoto Inomata, and Yulchl Murakami Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan (Received: January 6, 198 1; In Final Form: April 16, 198 1)
-
The number of V=O species on the surface of Vz06has been determined by using the rectangular pulse technique to detect the concentration profile of N2 produced by the following reaction, NO + NHS+ V=O N2 + H20 + V-OH. The obtained number of V=O species on the surface of an unsupported V206has been consistent with the BET surface area of the catalyst. The application of this method to a V206/TiOztreated with an ammoniacal solution has shown the validity of the previous inference that the treatment of a supported vanadium oxide catalyst with an ammoniacal solution dissolves the isolated, massive vanadium oxide into the solution, while the residual oxide is regarded to interact chemically with the carrier. Furthermore, the simulation of both surface reaction and diffusion processes in V205has proved the validity and soundness of the proposed method. Introduction As far as supported metal catalysts are concerned, methods to determine the number of active sites or the surface area of the active component have been established,lP2and various reactions on the catalysts have been
classified as structure-sensitive and structure-insensitive reactions on the basis of the turnover frequencies of the reactions.13 Although supported metal oxide catalysts (or multicomponent metal oxide catalysts) exhibit interesting catalyses depending on the kind of support (or additive)
(1) (a) A. D. 0. Cinneide and K. A. Clarke, Catal. Reu., 7,213(1973); (b) J. R. Anderson, “Structure of Metallic Catalysts”, Academic Press, New York, 1975,Chapters 4 and 5; (c) R.V. Hardeveld and F. Hartog, Adu. Catal., 22, 75 (1972).
(2)R. J. Farrauto, AIChE Symp. Ser., 70,9 (1974). (3) (a) 0. M. Poltorak, V. S. Boronin, and A. N. Mitrofanova, Proc. Int. Congr. Catal., 4th, 1968, 276 (1971); (b) M. Boudart, Adu. Catal., 20, 153 (1969); (c) M.Boudart, Proc. Int. Congr. Catal. 6th, 1976, 1 (1977).
0022-365418112085-2366$01.2510 0 1981 American Chemical Society
V-0
Species on the Surface of V,O, Catalysts
and on the composition of the cataly~ts?~ the relationship between the structure of the metal oxide on support and the activity of the catalyst has not been well clarified. 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 (or multicomponent metal oxide catalysts).236,7 Vanadium oxide has been widely used as a main catalyst in the oxidations of hydrocarbons and SOZa4From the environmental point of view, the catalyst has recently recieved attention as one of the best catalysts for the reduction of nitric oxide with ammonia.8~~In many studies concerning the mechanism of the catalytic reactions on vanadium oxide, the V=O species have been considered to play a significant role as the active sites for the react i o n ~ . ~ JTherefore, ~ it is highly desirable to know the number of V=O species on the surface of a vanadium oxide catalyst in order to evaluate the specific activity of the catalyst and to demonstrate the role of the additive or support of the vanadium oxide catalyst in terms of the turnover frequency. A method to determine the number of V=O species on the surface of the catalyst, however, has not been established. Although the infrared absorption peak near 1020 cm-I gives important information about the number of V=O s p e c i e ~ , the ~ J ~peak can hardly distinguish between surface and bulk V=O species. ESR spectra, magnetic susceptibility, electrical conductivity, X-ray diffraction, thermal analysis, and reduction by chemicals-such as CO, H2, and benzene-also provide important knowledge of the nature and the structure of vanadium oxide on s u p p ~ r t S . ~However, ~J~ the informa(4)(a) D. J. Hucknall, “Selective Oxidation of Hydrocarbons”, Academic Press, New York, 1974; (b) J. K. Dixon and J. E. Longfield, Catalysis, 7, 281 (1960); (c) D. B. Dabydurjor, S. S. Jewur, and E. Ruckenstein, Catal. Rev., 19,293 (1979); (d) M. S. Wainwright and N. R. Foster, ibid., 19,211 (1979); (e) A. Bielahski and J. Haber, ibid., 19, 1 (1979);(fj R.Higgins and P. Hayden, “Catalysis”, Vol. 1, The Chemical Society, London, 1977,Chapter 5, p 168. (5)(a) G. W. Keulks, L. D. Krenzke, and T. N. Notermann, Adu. Catal., 27, 265 (1978); (b) F. E. Massoth, ibid., 27, 265 (1978). !6) (a) H. C. Yao and M. Shelef, “The Catalytic Chemistry of Nitrogen Oxldes”,R. L. Klimisch and J. G. Larson, Eds., Plenum Press, New York, 1975,p 45; (b) B. S. Parakh and S.W. Weller, J . Catal., 47,100 (1977); (c) D. Pope, D. S. Walker, L. Whalley, and R. L. Moss, ibid., 31, 335 (1973);(d) C. R. F. Lund, J. J. Schorfheide, and J. A. Dumesic, ibid., 57, 105 (1979); (e) M. R. Goldwasser and D. L. Trimm, Znd. Eng. Chem. Prod. Res. Deu., 18,27 (1979). (7)(a) W.N. Delgsss, G. L. Haller, R. Kellerman, and J. H. Lunsford, Eds., “Spectroscopy in Heterogeneous Catalysis”, Academic Press, New York, 1979,Chapter 8; (b) P. Meriaudeau and J. C. Vedrine, Nouu. J . Chim., 2, 133 (1978). (8)(a) G. L. Bauerle, S. C. Wu, and K. Nobe, Ind. Eng. Chem. Prod. Res. Deu., 14,268(1975);(b) ibid., 17,117(1978);(c) N. Todo, M. Kurita, H. Hagiwara, H. Ueno, and T.Sato, Preprinta of Papers for the JapanU.S.A.Seminar on Catalytic NO, Reactions, 3-1 (1975). (9)(a) M. Inomata, A. Miyamoto, and Y. Murakami, J . Catal., 62,140 (1980); (b) M. Inomata, A. Miyamoto, and Y. Murakami, Chem. Lett., 799 (1978); (c) A. Miyamoto, Y. Yamazaki, and Y. Murakami, Nippon Kagaku Kaishi, 619 (1977). (10)(a) K. Tarama, S. Teranishi, S. Yoshida, and N. Tamura, Proc. Znt. Congr. Catal., 3rd, 1964,282(1965); (b) K. Tarama, S. Yoshida, S. Ishida, and H. Kakioka, Bull. Chem. SOC.Jpn., 41,2840 (1969); (c) K. Hirota, Y. Kera, and S. Teratani, J. Phys. Chem., 72,3133 (1968); (d) D.J. Cole, C. F. Cullis, and D. J. Hucknall, J. Chern. SOC.,Faraday Trans. 1,72,2185 (1976); (e) G. C. Bond, A. J. Stirkhy, and G. D. Parfitt, J . Catal., 57,176(1979);(fj M. Akimoto, M. Usami, and E. Echigoya, Bull. Chem. SOC.Jpn., 51,2195(1978); (9) S.K. Bhattacharyya, K. Janakiam, and N. D. Ganguly, J. Catal., 8, 128 (1967); (h) D. C. Agarwal, P. C. Nigam, and R. D. Srivastava, ibid., 55, 1 (1978); (i) M.Nakamura, K. Kawai, and Y. Fujiwara, ibid., 34,345 (1974). (11)(a) Y. Kera, S. Teratani, and K. Hirota, Bull. Chem. SOC. Jpn., 40,2458 (1967); (b) H. Takahashi, M. Shiotani, H. Kobayashi, and J. Sohma, J . Catal., 14,134 (1969); (c) V. A. Fenin, V. A. Shveta, and V. B. Kazanskii, Kinet. Katal., 16,1046 (1975); (d) S.Yoshida, T. Iguchi, 5. Ishida, and K. Tarama, Bull. Chem. SOC.Jpn., 45,376 (1972);(e) S. Yoshida, T. Murakami, and K. Tarama, Bull. Inst. Chern. Res., Kyoto Uniu., 51,195 (1973);(0 K. Tarama, S. Teranishi, and S. Yoshida, ibid., 46,185 (1968); (g) D. J. Cole, C. F. Cullis, and D. J. Hucknall, J . Chem. SOC.,Faraday Trans.1, 72,2744 (1976).
The Journal of Physical Chemistty, Vol. 85, No. 16, 1981 2367
aq .^
Flgure 1. Apparatus for the rectangular pulse technique: (1) titanium metal spon e column; (2) molecular sieve trap; (3) pressure regulator; (4) 200-cm syringes for the preparation of a NO and NH, mixture; (5) buffer tube; (6) six-way valve for sampling; (7) TCD at the Inlet of reactor; (8) TCD at the outlet of reactor; (9) capillary; (IO)thermocouple; (1 1) electric furnace; (12) reactor; (13) liquid-nitrogen trap; (14) six-way valve for the treatment with 0,.
f
tion obtained by these methods contains both surface and bulk contributions, and complete separation of surface and bulk information seems difficult. We have previously shown that the V=O species on a V205catalyst serves as the active site for the reaction of NO with NH3 (the NO-NH3 reaction) in the presence of O2 and that this reaction proceeds according to the following steps: NO NH3 V=O N2 + H2O + V-OH (1)
+
2V-OH
+
+
gaseous O2 or bulk V = O
2V=O
+ H20
(2)
According to this scheme, first NO and NH3 react at the surface V=O site to form N2, H20, and V-OH species (reaction 1). Then, the reoxidation of the V-OH species by gaseous O2or bulk V = O follows to form V=O and H20 (reaction 2). Consider the reactions when the NO and NH3 mixture is introduced onto vanadium oxide catalyst in the absence of gaseous oxygen. In the initial stage of the reaction, N2 will be produced according to reaction 1due to the surface V=O. Then, N2formation due to the V=O reproduced by the diffusion of bulk V=O to the surface (reaction 2’) will follow. Consequently, if we can separate
bulk V=O
2V-OH 2V=O + H2O (2’) the initial reaction caused by the surface V=O from the secondary process caused by the reoxidation of the surface, we can expect to determine the number of V=O species on the surface of vanadium oxide catalyst. In the present investigation, unsupported V205and V205/Ti02treated with an ammoniacal solution were used as samples in order to prove the validity and soundness of the method proposed. Experimental Section Catalysts and Reagents. An unsupported V2O5 catalyst was prepared by thermal decomposition of ammonium metavanadate at 773 K for 3 h in a stream of oxygen gas. The BET surface area of the catalyst was 5.4 m2 g-l. A VzO5/TiO2catalyst treated with an ammoniacal solution was prepared as follows. Ti02 composed of anatase with a small amount of rutile was prepared by calcination of Ti02 (Nippon Aerosil) in O2 at 773 K. V205/Ti02(25 mol % V205)was prepared by impregnating the Ti02 support with an oxalic acid solution of ammonium metavanadate followed by calcination at 773 K in a stream of 02.The treatment of the catalyst with an ammoniacal solution was made in a manner similar to that used by Yoshida et al.’ld The BET surface area of the catalyst was 41.3 m2 8’. A carrier gas (helium) was purified by using a molecular sieve
2368
The Journal of Physical Chemistry, Vol. 85, No. 16, 1981
Miyamoto et al.
I
Flgure 3. Model of the reaction and diffusion of V=O species in V205 with Nlayers. C, is the concentration of V=O species in the ith layer.
I b l
605 K 573
b
493
3
40
20 -1WE
f
60
SEC
Flgure 2. TCD responses at the inlet and outlet of the reactor: (a) Inlet rectangular pulse of the mixture of NO and NH3. (b) Concentration profiles of N, produced by the reaction of rectangular pulse of the NO and NH, mixture with the unsupported V,05 at various temperatures. Experiments were conducted under the standard condition.
trap and a titanium metal sponge column heated above 1073 K. Commercial NO (97.3% purity; impurities were 2.0% of N2 and 0.7% of N20), NH3 (99.9% purity), and O2 (99.8% purity) was used without further purification. In order to examine the effect of NO purity, we also carried out experiments using NO gas of more than 99.5% purity. Apparatus and Procedure. The rectangular pulse apparatus employed in the present study is shown in Figure 1. A mixture of NO and NH3 with a given composition was prepared and stored in two syringes (4) and made to flow slowly through a sampling loop in a six-way valve (6). The six-way valve was turned so that the mixture in the sampling loop could flow through a stainless-steel capillary (9) to the main stream of the carrier gas and then be carried to the reactor (12). The pulse width was varied by changing the time at which the six-way valve was returned. The shapes of the pulses at the inlet and the outlet of the reactor were recorded by thermal conductivity detectors (7 and 8, respectively). The composition of the NO and NH3 mixture in the pulse was determined by means of gas chromatography. The elution profile of the product (Nz) a t the outlet of the catalyst bed was obtained by passing the reaction products through a liquid-nitrogencooled trap (13), where components other than Nz, i.e., NO, NH3, N20, and H20, were completely trapped, but the shape of N2 was not deformed. Between the pulses of the NO and NH3 mixture, the catalyst was treated with an O2 gas stream at 773 K in order to hold the oxidation state of the catalyst before the measurement in the highest oxidation state, Le., V205,and to remove any adsorbates on the catalyst. Although the number of V = O species was measured under various experimental conditions, most experiments were carried out under the following standard condition: weights of samples = 0.1 g for the unsupported V205and 0.02 g for V205/Ti02;flow rate of the carrier gas = 150 cm3min-l; initial concentration of NO = 4.86 X mol ~ m - initial ~ ; concentration of NH3 = 9.25 X lo-' mol cm-,; pulse width = 60 s; reaction temperature = 382-610 K. By using the apparatus shown in Figure 1,we obtained a rectangular pulse with a definite pulse width and composition, as shown in Figure 2a. Model and Mathematical Analysis In order to determine the number of surface V=O species on a catalyst, one should separate the amount of
N2 due to the surface V=O from the amount of N2 due to the reoxidation of the surface. Since the rates of reactions 1 and 2' are considered to change with various experimental variables, a definite method should be established for determining the number of surface V=O species from the measured concentration profiles of N2 This is the reason that mathematical analysis was made in the present study. Taking into account the lamellar structure of V2O5 and the fact that the V=O species is located on the (010) face of V205crystal,12 we assume an N-layer model of V205 shown in Figure 3, where the consumption of V=O due to the NO-NH, reaction takes place on the first layer. The reproduction of the V=O species thus consumed is made at the expense of the V=O species in the second layer, which is then supplied by the V=O species in the third layer. The sequence is followed up to the Nth layer. Since the rate of the NO-NH3 reaction is first order with respect to both the concentration of NO in the gas phase (CNo) and that of V=O in the first layer (CJ, and zeroth order with respect to the concentration of NH3 ( C N H ) , 9 the processes mentioned above can be represented h y the following simultaneous differential equations: dCi/dt = -kCiCNO + Di(C2 - CJ (3) dC,/dt = D(C3 - C2) - Di(C2 - CJ dC,/dt = D(Ci+l - 2Ci
+ Ci-1)
(i = 3, 4, ...,N
(4) - 1)
(5)
dCN/dt = -D(CN - CN-1)
(6)
where the diffusivity between the first and second layers (Dl) is delineated from that belbw the second layer (D). This is because D1 contains the effect of the rate of the reoxidation of the surface (reaction 2') and should be distinguished from the simple diffusivity in the bulk of the solid (D). The rates of the formation of N2 (u) and of the reproduction of the surface V=O (u)can be given by the following equations: U = kC1CNo (7)
u = Dl(CZ - C,) (8) Since the sample is fully oxidized before the measurement, the initial condition becomes as follows: ci =
C"
(i = 1, 2, ..., N)
(9)
at t = 0. Transforming eq 3-9 into the dimensionless forms, we get the following equations: dXl/dT = -X1Y + A(X2 - Xi) (10) dXz/dT = AB(X3 - X,) - A(X2 - Xi)
(11)
(12)A.Bystrom, K.A. Wilhelmi, and D.Brotzen, Acta Chem. Scand., 4, 1119 (1950).
V=O
The Journal of Physical Chemistty, Vol. 85, No. 16, 1981 2369
Species on the Surface of V,05 Catalysts
dXi/dT = AB(Xi+l - 2Xi
+ Xi-1)
(i = 3, 4, ..., N - 1)
T
(12)
dXN/dT = -AB(XN - X N - ~ )
(13)
v = v/(kCNO"C")= x 1 Y
(14)
u = U/(kCNo°Co)= A(X2 - xi) (at T = 0; i = 1, 2, ..., N) Xi = 1
(15) (16)
where
x, = C;/C"
(i = 1, 2, ..., N)
(17) 0.7
Y = CNO/~NO"
(18)
T = kC,o"t
(19)
A = oi/(kCNo")
(20)
B = D/D1
(21)
In order to approximate better the experimental rectangular pulse shown in Figure 2, the dimensionless concentration of NO (Y) was given by the trapezium shown in Figure 4. Solutions of eq 10-16 were calculated for various values of parameters by the Runge-Kutta-Gill method. Numerical calculations were made with a FACOM 230-75 computer, Computer Center, Nagoya University. Results of Mathematical Analysis Judging from the BET surface area of the unsupported V,O, catalyst, we consider the number of layers (N) of this catalyst to be as large as several hundred. Since we are interested here in both unsupported Vz05and VzO5/TiOZ treated with an ammoniacal solution, calculations were made for the two following cases: N = 1, and N is large. The behaviors of the intermediate layers will be discussed in the following paper, where the number of surface V=O species and the structure of Vz05on support will be described precisely. First of all, we calculated the relationship between V and T for the case where N = 1. As shown in Figure 5, as T increases, V increase abruptly at first and then decreases monotonically to zero. When N = 1, the reoxidation of the surface by the bulk V = O cannot take place; therefore, the relationship shown in Figure 5 is independent of both parameters, A and B, and U is zero at any T. In accordance with eq 9, the integration of V with T becomes 1, meaning that all of the surface V = O species are consumed by reaction 1. Figure 6 shows calculated results of V (solid line) and U (dotted Iine) as functions of T with A as a parameter, when N is 50. Since the calculated results for N = 50 were confirmed to be almost the same as those for N = 90 or N = 30, these are considered to represent the behaviors of the reaction and diffusion in V205catalyst with several hundred layers of V205lamellae. As can be seen from Figure 6, when A = 0, the relation between V and T for N = 50 is the same as that for N = 1, and U is zero irrespective of the value of T. When A = 0.05, the tailing part of V appears at T > 2, although the shape of the initial peak of V at T I2 does not vary from that for A = 0. Corresponding to the tailing part in V, U is no longer zero for A = 0.05, which shows that the reoxidation of the surface occurs at T L 2. When A is increased further, that is, as the reoxidation of the surface becomes faster, the values of V and U at the tailing part increase further, although the initial part of V does not change significantly. At T > 6, U is almost the same as V, meaning that the
0,6 0 5
-
04 -
=. 0.3 0 2
01
'
-
Figure 5. Calculated relationship between Vand T for N 1.0; To = a.
= 1.
a=
0.7
0'6 0.5 0.4
-1
3 jr
0.3 A
0.2
=
0.3
0,2
0.1
0,l
n nr.
0
10
20
30
Flgure 6. Calculated values of V (solid line) and U (dotted line) as functions of Tat various A . a = 1.0; To = m; B = 100; N = 50.
consumption of the surface V=O by reaction 1proceeds as fast as the reoxidation of the surface by the bulk V=O (reaction 2'). Figure 7 shows calculated values of V and U as functions of T with B as a parameter. As shown in Figure 7, when B becomes larger, that is, when the diffusion of the bulk V=O becomes faster than the reoxidation of the surface, V values at the tailing part stop decreasing with T. Correspondingly,U values at the tailing part stop decreasing at T > 4. Although the shape of the tailing part changes with T depending on the values of A and B , the shape of the initial part does not change significantly. In other words, the shape of the initial part of V does not change markedly with the parameter, A or B, unless the rate of the reoxidation of the surface is much faster than the reaction rate on the surface. This is the theoretical
2370
No. 16, 1981
The Journal of Physical Chemistiy, Vol. 85,
Miyamoto et al.
'3,6h 0.5
, 1
,
I. 0
I I
30
20
10
0 450
30
5>3
b13
6jC
T E M P ~ R A T ~ /Q FY
Figure 9, Amount of the initial peak (IN) and the total amount of NP (IN 4- TL) for the measurements at various temperatures. Experiments were carried out under the standard condition.
Figure 7. Calculated values of V (solid line) and U (dotted line) as functions of Tat various 6. a = 1.0; To = a;A = 0.2; N = 50.
I
I
I
I
40
20 T~~~
/
I
I
60
SEC
Figure 8. Way of separation of the concentration profile of N, into the initial part (IN) and the tailing part (TL).
reason that we can determine the number of surface V=O species from the relation between V and T, especially from the initial peak of V.
Experimental Results Figure 2b shows examples of the concentration profiles of Nz produced by the reaction of the NO and NH3 mixture with the unsupported V205catalyst at various temperatures. At 447 K, only the initial N2peak was observed, whereas, at higher temperatures such as 493,573, and 605 K, the tailing of the Nz peak appeared; the concentration of Nz at the tailing part became greater with increasing temperature. Taking into account the results of the theoretical analysis, the initial Nz peak (IN) was separated from the tailing part (TL) in the manner shown in Figure 8. As shown in Figure 9, total amount of Nz (IN + TL) increased monotonically with increasing temperature, whereas the amount of N2 corresponding to the initial part (IN) was constant at temperatures higher than 553 K. The amount of the initial N2 (IN) at the constant region was mol 8-l. This value was confirmed to (2.2 f 0.1) X be constant irrespective of the change in the experimental variables such as the weight of the V205 catalyst, the carrier gas flow rate, the concentrations of NO and NH3 in pulse, and the pulse width. Furthermore, the difference in the purity of NO, 97.3% or 99.5%, did not affect the amount of the initial N2. Figure 10 shows examples of the concentration profiles of Nz for the Vz05/Ti02treated with the ammoniacal solution. Comparing the results with those of the unsup-
0
23
GO
43 TIME /
30
SEC
Figure 10. Concentration profiles of N2 produced by the reaction of the NO and NH, mixture with V205/Ti02treated with the ammoniacal solution at various temperatures. Experiments were carried out under the standard condition.
ported VzO5 catalyst shown in Figure 2b, one notes that the tailing part of N2 for the VZO5/TiOzis markedly smaller than that for the unsupported VZO+ It should be emphasized that the concentration of Nzat the tailing part for the VzO5/TiOZwas very low even at temperatures as high as 610 K. At this temperature, the concentration of N2 at the tailing part for the unsupported V2O5 was considerably high, as shown in Figure 2b. By repeated introduction of the NO and NH3 mixture to the catalyst, furthermore, the small tailing part for the Vz05/Ti02 above 574 K was confirmed to be due not to the reoxidation of the surface by the bulk V=O but to the reaction of NO and NH3 catalyzed by V-OH species of V4+species produced by reaction 1. It can therefore be said that the tailing of N2 due to the reoxidation of the surface does not occur at any temperature, when a VzO5/TiO2catalyst treated with an ammoniacal solution is used as a sample. Although the amount of the initial Nz for the V205/Ti02 decreased with decreasing temperature below 573 K, it was constant at temperatures higher than 573 K. The amount of the initial N2 above 573 K for the V20,/Ti02 was 9.3 x mol g-l, which was 4.2 times more than the amount for the unsupported V205.
Discussion Comparison of the Theory with Experiments. As can be seen from the comparison of Figure 2b with Figure 6, the experimental concentration profiles of N2 for the unsupported V2O5 catalyst are well simulated by the calcu-
V=O
Species on the Surface of V,05 Catalysts
lated ones. Both experimental and calculated profiles of N2 show the initial peak of N2 followed by the tailing part. As shown in Figure 2b, the concentration of N2 at the tailing part increased with increasing temperature. According to the calculated results shown in Figure 6, this is due to the larger value of A at the higher temperature; in other words, this is due to the faster reoxidation of the surface by the bulk V=O. The calculated profiles of N2, i.e., V , at various B values shown in Figure 7 indicate that the value of V at the tailing part for small B decreases monotonically with T, while that for large B (100 in Figure 7) does not depend on T. Since the experimental profile of N2 at the tailing part did not change with time at any temperature of 493, 573, and 605 K (Figure 2b), the experimental B value is considered to be very large. B is defined as the ratio of the diffusivity of V=O between the first and second layers (Dl) to the bulk diffusivity (D). The large B value then suggests that D1 does not refer to a simple diffusion, which can be understood as follows: In order to diffuse the V=O in the second layer to the first layer, the oxygen should reoxidize the surface according to reaction 2'; therefore, the process involved in D1 is different from the simple diffusion of bulk V=O. According to Yoshida et al.lld 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 carrier. Thus, if the assignment of the peak described in Figure 8 and the theoretical treatments mentioned above are valid, the tailing (TL) due to the reoxidation of the surface by the bulk V=O species should disappear when a V205/Ti02catalyst is treated with an ammoniacal solution. In other words, the concentration profiles of N2 for the V205/Ti02treated with the ammoniacal solution should be similar to the profile for N = 1shown in Figure 5. In fact, as shown in Figure 10, the results for the V205/Ti02treated with the ammoniacal solution are in accord with the prediction; namely, only the initial part (IN) was observed at any temperatures shown in Figure 10, and the amount of the tailing part was very small. Furthermore, as mentioned above, the small tailing part of the V205/Ti02above 574 K was confirmed to be due not to the reoxidation of the surface by the bulk V=O but to the reaction of NO and NH3 catalyzed by the V-OH or V4+ species. Consequently, the concentration profiles of N2were well simulated by the calculated profiles shown in Figures 5 and 6 for both the unsupported V2O5 and V205/Ti02treated with the ammoniacal solution. Validity and Soundness of the Method Proposed. Although the above-mentioned agreement between the experimental profiles of N2 and the calculated ones provides important evidence for the validity of the proposed method, more evidence can be obtained from the following four parameters: 1. The number of surface V=O species on the unsupported V205 catalyst. According to Bystrom et a1.,12the V=O species is located on the (010) face of V205crystal. Thus, if it is tentatively assumed that the (010) face is selectively exposed to the surface of the unsupported V205 catalyst, the number of surface V=O species coupled with the lattice parameter of V2O5 gives the specific surface area of the (010) face of V205to be 2.7 m2 g-l, which is one-half of the BET surface area of the unsupported V205catalyst, i.e., 5.4 m2 g-l. Since the powder of the unsupported Vz05 may actually expose various crystal faces in addition to the (010) face, it can be said that the agreement between both data is satisfactory.
The Journal of Physical Chemistty, Vol. 85, No. 16, 1981 2371
2. Effect of experimental variables. Since the number of surface V=O species on the unsupported V206 or VzO5/TiO2catalyst is a quantity which depends only on the structure of the catalyst, it should not change with the experimental variables such as the reaction temperature or the weight of the catalyst. The constancy of the amount of the initial Nz above 553 K for the unsupported V205 (Figure 9) satisfies the condition. Futhermore, the amount of the initial Nz for the VzO5/TiO2treated with the ammoniacal solution was constant at temperatures above 573 K. Although the amount of the initial N2 for the unsupported V206 (Figure 9) decreased with decreasing temperature below 553 K (573 K for the V&/Ti02), this may be due to unreacted V=O species remaining on the surface because of the slower reaction rate at lower temperatures. As mentioned above, it was also confirmed that the amount of the initial N2 did not vary with the carrier gas flow rate, the weight of the catalyst, the concentration of NO or NH3 in pulse, or the pulse width. The desorption of the V=O species from the catalyst surface in the atmosphere of the carrier gas did not occur and therefore did not affect the measurements, provided that the catalyst was cooled to the reaction temperature in a stream of O2 after treatment with O2 at 773 K. Moreover, the reoxidation of the surface by Oz as an impurity in the carrier gas (30 ppm at maximum) was proved not to affect the measurements of the number of V=O species on the surface. 3. Reactions involved in the measurements. The present method for the determination of the surface V=O is based on the assumption that the reaction of an NO and NH3 mixture with an oxidized vanadium oxide catalyst proceeds according to reaction 1. Although the validity of reaction 1 has been supported by many experimental results described previo~sly,~ the results of 15Ntracer experiments coupled with the pulse reaction technique should be mentioned as additional evidence. According to reaction 1,if a mixture of 15N0and 14NH3is introduced onto the V205 catalyst, 15N14Nshould be selectively produced. This was confirmed e~perimenta1ly.l~ As mentioned above, the reaction of NO and NH3 catalyzed by V-OH or V4+ may bring about experimental incompletion by increasing the concentration of Nz at the tailing part at high temperatures. This effect does not seem significant since, as shown in Figure 10, the tailing of N2 due to the reaction of NO and NH3 catalyzed by V-OH or V4+is small even at 610 K, and the contribution of this effect can easily be evaluated by the repeated introduction of the rectangular pulses of the NO and NH3 mixture without the oxidation treatment of the catalyst between the pulses. 4. Surface oxygen species other than V=O. No ESR spectra assignable to 02-, 0-, or 03-was observed on the unsupported V205 and V205/Ti02treated with the ammoniacal solution. Furthermore, temperature-programmed desorption experiments for the V205catalyst indicated no desorption peak from room temperature to 773 K. These data are reasonable, since such oxygen species as 02-, 0-, and 03have been detected only for prereduced vanadium oxide catalysts14 and, in the present study, the catalysts were fully oxidized at 773 K in a stream of O2 before the measurement. On the other hand, infrared spectra of the unsupported V206catalyst treated with the NO and NH, mixture showed that the amount of V=O species in the (13) A. Miyamoto, K. Kobayashi, M. Inomata, and Y. Murakami, unpublished data. (14) (a)V. B.Kazansky, Proc. Int. Congr. Catal., 6th, 1976,50 (1977); (b) S.Yoshida, T. Matsuzaki, K. Kashiwazaki, K. Mori, and K. Tarama, Bull. Chem. SOC.Jpn., 47,1564 (1974); ( c ) J. H.Lunsford, Catal. Reu., 8, 135 (1972).
2372
J. Phys. Chem. 1981, 85,2372-2377
catalyst decreases until all of the V=O species are consumed by reactions 1 and Yq9 Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan (No. 455310).
Nomenclature A dimensionless diffusivity of V=O between the first and second layers a parameter which determines the shape of Y , dimensionless B ratio of diffusivity of V=O between the first and second layers to bulk diffusivity, dimensionless concentration of V=O in the ith layer, mol cm2 Ci concentration of NO in the gas phase, mol cmw3 CNO CNOat the plateau of the rectangular pulse, mol CNO” cm-3 C N ~ B concentration of NH3 in the gas phase, mol cm-3
C” D D~ k N T U
U
V
X, Y
initial concentrationof V=O in a layer of V206,mol cm-2 diffusion coefficient of V=O below the second layer, s-l diffusion coefficient of V=O between the first and second layers, si-l reaction rate constant, cm3 mol-’ s-l number of layers, dimensionless time, s dimensionless time rate of the reproduction of V=O in the first layer, mol cm-2 s-l dimensionless rate of the reproduction of V=O rate of the formation of N2, mol cm-2 s-l dimensionless rate of the formation of N2 dimensionless concentration of V=O in the ith layer dimensionless concentration of NO
Determination of the Number of V=O Species on the Surface of Vanadium Oxide Catalysts. 2. V20,/Ti02 Catalysts Makoto Inomata, Akira Mlyamoto,* and Yuichi Murakami Depadment of Synthetic Chemistty, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan (Received: January 6, 1981; In Final Form: April 16, 1981)
-
By using the rectangular pulse technique coupled with the reaction NO + NH3 + V=O Nz + H20 + V-OH, we have determined the number of surface V=O species or the specific area of the (010) face of Vz05 for V2O5/TiO2catalysts with various contents of VzO5. It has been found that the V=O species or the (010) face of V2O5 is selectively exposed to the surface of VzO5/TiOZcatalysts. As the content of V2O5 in the catalyst increases to 5 mol %, the surface of TiOz support is gradually covered by the (010) face of V205. When the content of V206is 10 mol %, the maximum fraction of the (010) face of Vz05on the catalyst surface (90%) is attained, indicating that almost all of the surface of the V205/TiOzcatalyst is covered by the (010) face of Vz05. When the content of Vz05 increases further, the fraction of the (010) face decreases to reach the value of the unsupported VzO5 (50%)where various crystal faces of V2O5 are exposed in addition to the (010) face. It has been pointed out that these results agree with previous studies of VzO5/TiO2catalysts and prove the validity of these studies quatitatively. In addition to the number of surface V=O species, the number of V2O6 lamellae on a Ti02support has been determined by the comparison of experimental concentration profiles of N2with simulated ones. The number of layers thus determined has agreed with the number of layers calculated from the content of Vz05 and the specific area of the (010) face of VzOp On the basis of the above-mentioned data, the applicabilityof the proposed method to supported Vz06and V205-containingmulticomponent catalysts has been concluded.
Introduction It has been shown in the preceding study1 that the rectangular pulse technique coupled with reaction 1is an
NO
+ NH, + V=O
-
N2
+ H2O + V-OH
(1)
effective way to determine the number of surface V=O species on an unsupported V205catalyst. In order to investigate the effects of supports or additives on the catalysis of metal oxides in terms of the turnover frequency, one should determine accurately the number of active sites on supported metal oxide catalysts or multicomponent catalysts. In this regard, however, little work has been done, though some attempts have been made to determine the ratio of the amount of an adsorbed gas to the estimated (1) A. Miyamoto, Y. Yamazaki, M. Inomata, and Y. Murakami, J. Phys. Chem., preceding paper in this issue; Chern. Lett. 1355 (1978).
amount of metal ions on the surface of unsupported metal
oxide^.^-^
Vanadium oxide has been used as a main catalyst for the selective oxidation of hydrocarbons; V205/Ti02catalysts exhibit the best selectivity and activity.&* Intimate interaction between V205and Ti02has been suggested to (2) R. J. Farrauto, AIChE Symp. Ser. 70, 9 (1974). (3) M. R. Goldwasser and D. L. Trimm, Ind. Eng. Chem. Prod. Res. Deu., 18, 27 (1979). (4) H. C. Yao and M. Shelef, “The Catalytic Chemistry of Nitrogen Oxides”, R. L. Klimisch and J. G. Larson, Eds., Plenum Press, New York, 1975. (5) C. R. F. Lund, J. J. Schorfheide, and J. A. Dumesic, J . Catal., 57, 105 (1979). (6)D. J. Hucknall, “Selective Oxidation of Hydrocarbons”,Academic Press, London, 1974. (7) R. Higgins and P. Hayden, “Catalysis”, Vol. 1, The Chemical Society, London, 1977, Chapter 5, p 168. (8) G. C. Bond, A. J. SBrkiny, and G. D. Parfitt, J. Catal. 57, 476 (1979).
0 1981 American Chemical Society
