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 V206 is 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
V,0S/Ti02 Catalysts
explain the promoting effect of Ti02. When V2O5 is supported on Ti02 (anatase), a simultaneousreduction of V205 and transformation of anatase into rutile has been observed."1° It has been suggested that these reactions are topotactic reactions activated by the remarkable fit of the crystallographic patterns in contact at the VzO5-TiO2 (anatase) interface.'O For the unsupported V205catalyst, the fraction of the (010) face in the total surface area of the catalyst has been found to be 50%.' This means that various crystal faces of V2O5 are exposed to the surface of the unsupported V205in addition to the (010) face where the V=O species is located. If the (010) face of V205 intimately interacts with the surface of Ti02-for VzO5/ TiOz catalysts-the surface of Ti02should be well covered by V205and the (010) face of V205should be selectively exposed to the surface. Consequently, V205/Ti02catalysts with various contents of V2O5 are appropriate samples to examine the applicability of the proposed method to supported V,05 catalysts and V205-containingmulticomponent catalysts. When the rectangular pulse technique' is used, information of the structure of Vz05 on support can also be obtained in addition to the number of surface V=O species. For the unsupported V205, which is composed of almost 500 layers of V205lamellae, significant tailing of N,-due to the reoxidation of the surface by bulk oxygen-has been found to follow the initial sharp peak of Nz due to the surface V=O. For the Vz05/Ti02treated with an ammoniacal solution which is considered to be a monolayer catalyst, such a tailing does not occur. When the content of V205in supported V205catalyst increases, the number of layers of V2O5 lamellae on support is considered to increase. The shape of the outlet peak of Nz then should reflect the change in the number of layers due to the content of VzO5. This was also examined in the present study. Experimental Section TiOz composed of anatase with a small amount of rutile was prepared by calcination of Ti02obtained from Nippon Aerosil in Oz at 773 K. Vanadium oxide catalysts supported on the carrier were prepared by impregnation of the carrier with an oxalic acid solution of ammonium metavanadate followed by calcination at 773 K in the stream of OF Apparatus and procedure of the rectangular pulse technique were described in the preceding paper.' Infrared spectra and ESR spectra of the catalysts before and after the reaction with the mixture of NO and NH3 were observed by using a JASCO IRA-3S spectrometer with a KBr disk method, and at the X band employimg a JEOL ME 1X spectrometer, respectively. Mathematical Analysis Model and Method of Analysis. Using a model similar to the one described in the preceding paper,l we conducted a theoretical analysis for a uniform N-layer structure of V2O5 on support (Figure 1). Since calculations have already been made for the cases N = l and N = 50,l detailed analysis was carried out for the intermediate number of layers which might be significant for supported V205 catalysts. For the unsupported V205catalyst,' the ratio of diffusivity of the V=O between the first and second layers to bulk diffusivity, B, has been determined to be 100 (cf. Nomenclature). This value was also adopted in this study. (9) D.J. Cole, C. F. Cullis, and D.J. Hucknall, J. Chern. Soc., Faraday Trans. 1 , 72, 2185 (1976). (10) A. Vejux and P.Courtine, J. Solid S t a t e Chern., 23, 93 (1978).
The Journal of Physical Chemistty, Vol. 85,No. 16, 1981
2373
~EACTION
c1
c2
c3 c4
4-1
; ; 1
\ 4
DIFCUSIOV
f
DIFFUSION
\
.1
t
\
;
DIFFUSION
f
~1FFusioN
4
4 / / / / / / I /
I I 11/11
SUPPORT
Flgure 1. Model of the reaction and diffusion of the V=O species in supported V 2 0 catalyst. C, = concentration of the V=O in the ith layer, mol cm-9.
Results of Mathematical Analysis. Results of calculations are shown in Figures 2 and 3 for N = 2 , 3 , 4 , 5 , and 10. As shown in Figure 2, when the reoxidation of the surface by bulk oxygen does not take place, that is, when A = 0, the shape of V is identical with that for N = 1, irrespective of N. Although the tailing of V due to the reoxidation of the surface at T > 2 increases with increasing A value, the shape of the initial sharp peak due to the surface V=O at T i 2 does not change significantly when A is smaller than 0.2 (Figure 2 and 3A). This provides a theoretical basis for the measurement of the number of surface V=O species on supported Vz05 catalysts using the rectangular pulse technique coupled with reaction 1. As the value of A further increases, it becomes more difficult to distinguish the initial sharp N2 peak from the tailing N2. However, it should be noted that the tailing part at such large A values as 0.5 and 1.0 reflects the difference in the number of layers. Figure 3B shows the calculated results of V at A = 0.5. When N = 1,the tailing does not take place at all because of the lack of bulk oxygen. When N I2, the tailing appears and it becomes greater with an increase in the number of layers. This suggests the possibility of determining the number of layers of lamellar structure of V205 on support by using the rectangular pulse technique coupled with reaction 1. Experimental Results Concentration Profile of N2 and the Amount of t h e Initial Sharp N2. Figure 4 shows examples of experimental concentration profiles of N2produced by reaction 1for the Vz05/Ti02catalysts with various contents of VzO5. For Vz05/Ti02(2 mol %), the tailing of Nz was small at any temperature, indicated in Figure 4A. Even at such a high temperature as 620 K, the tailing in the concentration profile of N2 was not significant. Furthermore, the repeated introduction of the mixture of NO and NH3 without the oxidation treatment with O2 revealed that the small tailing is not due to the reoxidation of the surface by the bulk V=O, but to the reaction of NO and NH, catalyzed by a V-OH or V4+ species. The profiles of N2 for V205/Ti02(10 mol %), especially those measured at 610 K (Figure 4B), were markedly different from those for catalysts with different contents of V205 In the profiles of Nz for V205/Ti02(25 mol %), significant tailing appeared as shown in Figure 4C. When V205/Ti02(50 mol %) was used, the tailing of Nz became greater and the profiles of N2 were almost the same as those for the unsupported Vz05catalyst.' Consequently, the concentration profiles of Nz produced by reaction 1 changed markedly with the content of V205in the catalyst and the reaction temperature. Using the method described in the preceding paper,' we separated the amount of the initial sharp N2 from that of the tailing N,. The amount of the initial sharp N2 for
2374
The Journal of Physical Chemistry, Vol. 85, No. 16, 1981
Inomata et ai.
0.7
0,7
0.6
0,6
0,5 0,4
0.4 >
2
0.3
0.3
0,2
3.2
0.1
0.1
0
10
20
0
30
10
20
30
1
>
0
20
10
30
0
10
20
30
T
0.7
0,6 0.5
0.4 0.3
0
10
20
30
Flgure 2. Simulated profiles of N, ( V ) for various values of A : (A) N = 2; (B) N = 3; (C)N = 4; (D) N = 5; (E) N = 10. B = 100; a = 1.0; To = m (cf. Nomenclature).
various catalysts is plotted against the reaction temperature, and is shown in Figure 5. As can be seen, the amount of the initial sharp N2 was independent of the reaction temperature at any content of V2O5 in the catalyst. Although the amount of the initial N2for V205/Ti02(50 mol ?%)at 532 K was smaller than that at higher temperatures, this may be due to the unreacted V=O species remaining on the catalyst at low temperature.' Number of Surface V=O Species on V 2 0 5 / T i 0with 2 Various Contents of V20,. According to the results of the theoretical analysis, the number of surface V=O species on the catlaysts with various contents of V205can be ob-
tained from the amount of the initial sharp N2 shown in Figure 5, and the results of the number of surface V=O species are shown in Figure 6. Since the V=O species is located on the (010) face of V2O5 crystal and the surface density of the V=O in the (010) face is known to be 4.872 nm-2,11the specific area of the (010) face of V2O5, S(olo), can be calculated from the measured number of surface V=O species. This is also plotted in Figure 6 with the BET specific surface area of the catalyst, SBm,against the (11)A.Bystrbm, K.A. Wilhelmi, and 0.Brotzen, Acta Chem. Scand., 4, 1119 (1950).
The Journal of Physical Chemistry, Vol. 85,No. 16, 1981 2375
V205/Ti02Catalysts
:d
A
A
=
11
0,2
0.4
I \\
I
TIME /
0
10
20
SEC
30
T
TIME /
SEC
>
C
0
10
20
I
30
0
20
Figure 3. Simulated profiles of N2(V) for the various numbers of layers (N): (A) A = 0.2; (B) A = 0.5 (cf. Nomenclature).
content of V20b Figure 7 shows the ratio of S(olo) to SBET which indicates the fraction of the (010) face of V206in the surface of the catalysts. When the content of V205is 0, S(OlO)/SBET equals 0, indicating the surface of an uncovered Ti02support. As the content of V2O5 increased up to 5 mol % (Figure 7), the fraction of the (010) face of V205 increased linearly. This means that the surface of Ti02 is gradually covered by V2O5 with increasing V2O5 content. When the content of V2O5 was 10 mol %, the maximum fraction of the (010) face of V2O5 (90%) was attained, and here the specific area of the (010) face of V205 (26.4 m2 g-l) was -10 times that of the unsupported V205 (2.7 m2 g-l). When the content of Vz06 increased further, the fraction of the (010) face decreased to the value of the unsupported V2O5 (50% at 100 mol % of V2O5 content) where various crystal faces of V205 aree considered to be exposed in addition to the (010) face. In other words, when the content of V206is 10 mol %, V206covers the surface of TiOz to selectively expose the (010) face to the surface. IR and ESR Spectra of Catalysts Before and After the Measurement. Infrared spectra of V205/Ti02catalysts before the introduction of the mixture of NO and NH3 indicated an absorption peak at 1020 cm-l assignable to the V=O stretching vibration,12-14for example, as shown (12) K. Tarama, S. Teraniehi, S. Yoshida, and N. Tamura, Proc. Int. Congr. Catal., 3rd, 1964, 202 (1965). (13) A. Miyamoto, Y. Yamazaki, and Y. Murakami, Nippon Kugaku Kaishi,619 (1977).
o
60
40 TIME /
zn
40 TIME
SO
SEC
60
$0
SEC
Figure 4. Concentration profiles of N2 produced by the reaction of the rectangular pulse of the mixture of NO and NH3 with V2O5/TiO2catalysts at various temperatures: (A) V205/Ti02(2 mol %); (B) V2O6/TiO2 (10 mol %); (C) V206/Ti02(25 mol %); (D) V206/Ti02(50 mol %). Pulse width = 80 s except data measured at 596 and 610 K for V205/Ti02(10 mol % ) (120 s).
in Figure 8. After the repeated introduction of the rectangular pulses of the mixture, the peak at 1020 cm-' disappeared but the spectrum of V20413J4was observed. In conformity with the change in the infrared spectra, the quantity of the V4+ion measured with the ESR method was markedly increased by the reaction of the catalysts with the mixture of NO and NH3. Furthermore, the total amount of N2 produced by the reaction of the mixture of NO and NH3 with the catalysts was approximately equal (14) M. Inomata, A. Miyamoto, and Murakami, J. Catul., 62, 140 (1980); Chem. Lett., 799 (1978).
2376
The Journal of Physical Chemistry, Vol. 85,No. 10, 1981
Inomata et ai.
1
1200
1
1
1000 H A V E HUMBER
L 550 EO0 E4 0
520
l
I
I
SO0 / CM-1
Flgure 8. Infrared spectra of V2O5/T1O2(25 mol %) before and after the repeated introduction of the rectangular pulses of the mixture of NO and NH3 at 603 K: (a) before the reaction; (b) after the reaction.
TEMPERATURE / K
Flgure 5. Amount of the initial sharp N2 measured at various temperatures: (0)V205/Ti02(2 mol %); (0) V2O5/T1O2(10 mol %); (0) V205/Ti02(25 mol %); (A)V205/TI02(50 mol %).
TABLE I: Number of Layers of V,O, on TiO, Support for V,O,/TiO, with Various Contents of V,O,
-____
content of V,O,, mol % 1 2 5
NO
,
10 25 35 50
30
looc
no. of layers -exptlQ calcdb 1 or 2 1 or 2 2 or 3 4 or 5 10 > 20 > 20 > 50
-
1.8
1.7 3.0 5.2 12.4 24.5 44.1 503.6
Determined by comparison of experimental profiles of N, with theoretical ones shown in Figure 2. Calculated from eq 2. Unsupported V,O,. 20
0
40
80
60
V205 CONTENT
/
100
MOL^
Figure 6. Number of surface V=O species (closed circle), area of the (010) face of V205 (S(olo,,closed circle), and the BET surface area (Sm, open circle) for V205/Ti02catalysts with various contents of v2°5.
1
0
20
43 !'235 C O ~ T E N T
60
30
109
profiles of N2 a t the tailing part. Furthermore, the experimental profiles of N2 shown in Figure 4 indicate that the profiles differ significantly depending on the content of V2O5 in the catalysts. These data suggest the possibility of determining the number of layers of V205by comparison of experimental profiles of N2with theoretical ones. Note the data for V205/Ti02(2 mol % ) shown in Figure 4A. Comparing the data with the simulated results shown in Figure 2, especially those in Figure 2A, one can consider this catalyst to be composed of one or two layers of V205 on Ti02 support. Similarly, the V205/Ti02(10 mol %) is considered to be composed of four or five layers of V205 on Ti02 support. The number of layers of V205for the other contents of V205was similarly estimated by comparison of experimental profiles of N2with theoretical ones, and the results are indicated in Table I. If the V205in V205/Ti02catalysts forms uniform layers of V205on Ti02, the number of layers, N , can be calculated on the basis of the content of V2O5 in the catalyst, x , and the specific area of the (010) face of VZO5, S(olo), as follows:
VOLI
Figure 7. Fraction of the (010) face of V2O5 in the surface of the V205/Ti02Catalysts.
to the amount of oxygen necessary for the change from V2O4 to VZO5.
Discussion Number of Layers of V205on Ti02Support. By using the rectangular pulse technique coupled with reaction 1, one can measure the number of surface V=O species or the specific area of the (010) face of V205 on the V205/Ti02 catalysts, as shown in Figure 6. According to the theoretical analysis mentioned above, the number of layers of V205on support can be determined from the concentration
N=
1.236 X lo5 X 181.8~ + 79.9(100 -X) S(o10)
(2)
The number of layers thus calculated is also indicated in Table I. If one takes into account that the method of preparation makes the system heterogeneous, and the contribution of the V4+ion-which is incorporated into the carrier and cannot be oxidized to the V=O species by the treatment with O2 at 773 K15-the agreement between the experimental and calculated number of layers seems satisfactory. In other words, the variation of the concentra(15) M. Akimoto, M. Usami, and E.Echigoya, Bull. Chem. SOC.Jpn., 51, 2195 (1978).
V205/TI02Catalysts
tion profile of N2 with the content of V2O5 in the catalyst has been well understood in terms of the number of layers of V205 lamellae on Ti02 support. The concentration profiles of N2 at various temperatures (shown in Figure 4) can be explained by assuming that the rate of surface reoxidation relative to the surface reaction rate, that is, A , becomes greater with increasing temperature. This assumption has been shown to be valid for the unsupported V2O5 catalyst. Selective Exposure of the V=O Species on the Surface of V 2 0 5 / T i 0 2Catalysts. As shown in Figure 7, the fraction of the (010) face of V2O5 for the V2O5 catalysts is higher than that of the unsupported V2O5 catalyst except V205/Ti02(1mol 5%) and V205/Ti02(2 mol 5%). Noting the data of S(olo) shown in Figure 6, for the latter catalysts, the surface of Ti02 is considered not to be well covered by V205because of the small content of V205. It can then be said that the V=O species, or the (010) face of VZO5, is selectively exposed to the surface of the V205/Ti02 catalysts. This suggests the existence of an intimate interaction between V205and Ti02 in V205/Ti02catalysts. When V205is supported on Ti02 (anatase), a simultaneous reduction of V205 and transformation of anatase into rutile has been observed."1° Although the reactions occur at much higher temperatures than 773 K-at which V2O5/TiO2catalysts employed in this study were treated with 02-the observations clearly indicate the intimate interaction between VzO5 and Ti02 (anatase). According to Vejux et al.,1° the reactions have been interpreted in terms of the remarkable fit of crystallographic patterns in contact at the V205-Ti02 (anatase) interface. The data shown in Figure 7 are then in accordance with the previous studies of V205/Ti02catalysts"1° and furthermore quantitatively prove the validity of these studies. Applicability of the Method to the Determination of the Number of Surface V=O Species on Supported V205 Catalysts. Since no other method has been proposed to determine the number of surface V=O species on supported V2O5 catalysts, the method employed in this study cannot be calibrated with other methods. However, evidence for the validity of the method in the following four categories assures the applicability of the method to supported V205catalysts: 1. Constancy of the amount of the initial sharp N2. Since the number of surface V=O species on a supported V205 catalyst is a quantity which depends only on the structure of Vz05 on support, it should not depend on experimental variables such as temperature, carrier gas flow rate, or weight of sample. As shown in Figure 5, the amount of the initial n2 did not depend on reaction temperature at any content of V205 Also, carrier gas flow rate, weight of sample, pulse width, or the concentration of NO or NH3 in pulse did not affect the amount of the initial sharp N2. 2. Reasonable change in the number of surface V=O species with the content of V205 As mentioned above, the measured number of surface V=O species shown in Figure 6, or the fraction of the (010) face of V2O5 shown in Figure
The Journal of Physical Chernktty, Vol. 85, No. 16, 1981 2377
7, exhibits reasonable change with the content of V205 Furthermore, the selective exposure of the V=O species on the Vz05/Ti02catalysts agrees with the previous investigations of Vz05/Ti02catalysts.&'O 3. Oxygen species responsible for the reaction with the mixture of NO and NH3. As mentioned in the preceding paper,' such oxygen species as 02-,0-, or 03-cannot play a role in the reaction of catalysts with the mixture of NO and NH3, since the catalysts were fully oxidized at 773 K in the stream of O2before the measurement. In agreement with this, no ESR spectra assignable to 02-,0-,or Of were observed on the catalysts employed in the present study. On the other hand, the infrared spectrum of the catalyst after the reaction shown in Figure 8 indicates that the V=O species in the V205/Ti02catalyst is all consumed by the reaction with the NO and NH3 mixture. Furthermore, the total amount of N2 produced by repeated introduction of the NO and NH3 mixture into the catalyst was approximately equal to the amount of oxygen necessary for the change from V2O4 to V205 These data indicate that the V=O species is the sole oxygen species responsible for the reaction with the mixture of NO and NH3. 4. The number of layers of V205on TiO> As shown in Table I, the experimental numbers of layers of V206were approximately equal to the calculated ones. In addition to the above-mentioned evidence, pulse experiments on the reaction of the V2O5/TiO2catalyst with the 15N0and 14NH3mixture indicated that 15N14Nis selectively produced as a reaction product.16 This supports the validity of reaction 1. In conclusion, the number of surface V=O species and the number of layers of V2O5 on Ti02support have been satisfactorily determined by using the rectangular pulse technique coupled with reaction 1. This assures the applicability of the proposed method to variously supported V205catalysts and V205-containing multicomponent catalysts.
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 the V=O between the first and second layers parameter which determines the shape of the inlet U rectangular pulse (cf. Figure 4 in ref 1) B Ratio of diffusivity of the V=O between the first and second layers to bulk diffusivity, dimensionless N number of layers of V2O5 lamellae, dimensionless BET specific surface area, m2 g-' specific area of the (010) face of Vz05, m2 g-l S(Ol0) T dimensionless time dimensionless pulse width (cf. Figure 4 in ref 1) TO V dimensionless rate of the formation of Nz As for the definitions of A , B, T , and V , confer ref 1. (16) A. Miyamoto, K. Kobayashi, M. Inomata, and Y. Murakami, unpublished data.
