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584
some catalytic species. If there was no interaction between the support and the catalyst, MoS2 formed from pure MOO, should have the same catalytic effect as the Mo(1V) species in the catalyst. However, the Mo(V1) species which interacts with the alumina is sulfided in a different way than the MoSz formed from pure MOO, and may not have the same catalytic effect. The data at 450 OC, Table I, indicated that the major reactions were cleavage of carbon-sulfur bonds and hydrogenation. As expected from the previous studies, the reaction sequence was the conversion of thiophene to butenes and butenes to butane. Conversion did not occur on the catalyst in its oxide form. Hydrogen is known to absorb on sulfur species (Wright et al., 1980). The paucity of sulfur in the oxide catalyst may have been responsible for the paucity of reaction with the oxidic catalyst. We have not established the hydrogen adsorption characteristics of either the sulfide anions or the nonstoichiometric amorphous sulfur. The major findings may be summarized as follows. Under reaction conditions at atmospheric pressure, the catalyst in its working state contains nonstoichiometric amorphous sulfur in addition to the stoichiometric sulfide anions. Only some of the molybdenum cations are fully sulfided. A substantial fraction of the molybdenum remains as an oxide probably associated with the alumina. The molybdenum oxide in a Mo03-Co0-A1203 HDS catalyst responds much differently to sulfiding by H2S/H2 than does pure MOO,. The composition of the catalyst after reaction with thiophene for periods up to 2 h is much the same as after sulfiding, but before exposure to the reaction mixture. This would indicate that the sulfiding step transforms the catalyst into its working state. The use of XPS as a semiquantitative probe in catalysis research has also been shown in this study. Registry No. COO,1307-96-6; Moo3, 1313-27-5.
Aoshima, A.; Wise, H. J. Catal. 1074,5 0 , 190. Apecetche, M. A.; Houalla, M.; Delmon, 8. Surf. Int. Anal. 1081, 3 , 90. Bancroff, G. M.; Gupta, R. P.; Hardln, A. H.; Ternan, M. Anal. Chem. 1979, 51, 2102. Breysse, M.; Bennet, B. A.; Chadwlck, D. J . Catal. 1981, 77, 430. Brlnen, J. S.; Armstrong, W. D. J. Catal. 1978,5 4 , 57. Carlson, T. A. “Photoelectron and Auger Spectroscopy”; Plenum Press: New York, 1974. Chang, C. H.; Chan, S. S. J. Catal. 1081, 72, 139. Cheng, C. P. Ph.D. Thesis, University of Delaware, 1981. deBeer, V. H. J.; Schuit, G. C. A. Ann. N.Y. Acad. Scl. 1076, 272, 61. Decierck-Grlmee, R. K.; Canesson, P.; Friedman, R. M.; Fripiat, J. J. J. Phys. Chem. 1978,82, 885. Dufresne, P.; Grimblot, J.; Bonnelle, J. P. Bull. SOC.Chlm. 1980,89. Delgass, W. N.; Hughes, T. R.; Fadley, C. S. Catal. Rev. 1970,4 , 179. Frensen, T.; Mars, P.; Gellings, P. T. J. Colloid Interface Sci. 1970,70, 97. Hercules, S. H.; Hercules, D. M. “Surface Characterization by Electron Spectroscopy for Chemical Analysis (ESCA)”; Kane, P. F.; Larrabee, G. B.; Ed.; Plenum Press: New York, 1974; pp 307-335. Jagannathan, K.; Srlnlvasin, A.; Rao, C. N. R. J. Catal. 1081, 69, 418. Jepsen, J. S.; Rase, H. F. Ind. Chem. Prod. Res. Dev. 1981, 2 0 , 467. Lipsch, J. M. J. G.; Schuit, G. C. A. J. Catal. 1969, 15, 179. Massoth, F. E.; Klbby, C. I.J. Catal. 1977,47, 300. Massoth, F. E. J. Catal. 1977,50, 190. Massoth, F. E.; Chung, K. S.; Ramachandran, R. Fuel Proc. Techno/. 1979, 2 , 57. Maksimov, Y. V.; Suzdalev, I. P.; Gd’danskil, V. I.; Krylov, D. V.; Margolis, L. Y.; Nechltailo, A. E. Chem. Phys. Lett. 1975,3 4 , 172. Medema, J.; van Stam, C.; de Beer, V. H. J.; Konlngs, A. J. A,; Konlngsburger, D. C. J. Catal. 1078,53, 386. Mitchell, P. C. H.; Triflro, F. J. Catal. 1974,33, 350. Okamoto, Y.; Smimdtawa, T.; Imanaka, T.; Teranishi, S.J . Cats/. 1970,57, 153. Okamoto, Y.; Imanaka, T.; Teranishl, S. J . Catal. 1080, 65, 448. Pollack, S. S.; Makovsky, L. E.; Brown, F. R. J . Catal. 1979,5 9 , 452. Powell, C. J.; Larson, P. E. Appl. Surf. Sci. 1978, 7 , 186. Richardson, J. T. Ind. Eng. Chem. Fundam. 1964,3 , 154. Siegbahn. K. Science 1982, 217, 111. Silbernagel, B. G. J. Magn. Magn. Mater. lS83,31-34, 885. Stevens, G. C.; Edmonds, T. J. Catal. 1975, 37, 544. Ternan, M. Can. J. Chem. Eng. 1083, 6 1 , 133. Wagner, C. D. Anal. Chem. 1977,49, 1282. Wagner, C. D.; Rlggs, W. M.; Davis, L. E.; Mulden, J. F.; Muilenberg,; G. E., Ed. “Handbook of X-ray Photoelectron Spectroscopy”; Perkln-Elmer Corp.; physical Electronlcs Division, Eden Praire, MN, 1979. Wentrcek, P. R.; Wise, H. J. Catal. 1978,45, 349. Zingg, D. S.; Makovsky, L. E.; Tischer, R. E.; Brown, F. R.; Hercules, D. M. J. Phys . Chem . 1980,84, 2898.
Received for review April 21, 1983 Revised manuscript received February 22, 1984 Accepted April 12, 1984
L i t e r a t u r e Cited Ahuja, S. P.; Derrlen. M. L.; LePage, J. F. Ind. Eng. Chem. Prod. Res. Dev. 1970,9 ,272.
Kinetics of NO Reduction with NH, on “Chemical Mixed” and Impregnated V2O5=TiO2Catalysts W. C. Wong and Ken Nobe’ Department of Chemical Engineering, University of California, Los Angeles,
Los Angeles. California
90024
The reduction of NO with NH, in the presence of 0, on TiO, (anatase)- and TiO, (rutile)-supported V,05 catalysts prepared by the “chemical mixing” technique and the impregnation method was studied in a flow reactor below 350 OC. An impregnated V,O,-AI,O, catalyst was also studied for comparison. The kinetic data, corrected for pore diffusion effects, were correlated with a power law rate expression. The intrinsic rates of the NO-NH,-0, reaction are first order with respect to NO and zero order with respect to NH, for all catalysts studied. O2 reaction orders are 0.25 and 0.5 for “chemical mixed” and impregnated catalysts, respectively. The intrinsic activity of the V205-Ti02 catalysts prepared by the “chemical mixing” technique is greater than that prepared by the impregnation method. TiO, (anatase)-supported catalysts are shown to be substantially more active than the Al,O,-supportd catalyst. The Ti02-supportedcatalysts with the rutile structure have the lowest acthrities due mainly to their low surface areas.
Research in the synthesis of organometallic chemicals has provided a wide variety of compounds which can be used as materials in the preparation of catalysts. The particular organometallics of interest in this study are the 0196-4321/84/1223-0564$01.50/0
alkoxides. There is a considerable literature on the synthesis of alkoxides (Bradley et al., 1978). These organometallic intermediates have been used to prepare highpurity submicron ceramics with electronic properties. For 0
1984 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 585
example, Mazdiyasni and his co-workers (1969, 1971) prepared BaTiO, by hydrolyzing a stoichiometric mixture of barium diisopropoxide and titanium tetra-tert-amyl oxide. The particle size of the final dried powder ranged from 50 to 100 A with a purity of 99.98+%. Superior quality electronic grade ceramic materials can be prepared by this method because of thorough mixing of the product components in liquid solution, precise control of adding small amounts of selected dopants, and the relative absence of ionic impurities. These are characteristics which are also highly desirable for catalysts. Thus,this method of preparing mixed oxides for electronic applications has interesting possibilities for catalytic chemistry applications. We have recently reported the development of a “chemicalmixing” technique for the preparation of a V2O5 catalyst (Pearson et al., 1983). V205catalysts are noted for their activity in hydrocarbon and SO2oxidation processes. The specific catalyst prepared in the previous study was V205-Ti02*(anatase), and ita activity for the complete reduction of NO to N2 with NH3 in the presence of excess O2 was examined. The preliminary results obtained showed that it was effective catalyst for the reaction. The “chemical mixing” technique can be applied to prepare a wide range of known industrial catalysts (e.g., metals, metal oxides, mixed metal oxides, etc., on a variety SiO,, Zr02,Ti02,etc.). of common supports such as A1203, Furthermore, the technique is not limited to liquid alkoxides but is applicable to an almost unlimited number of solid and liquid organometallic complexes. This paper is the follow-up to the communication published recently (Pearson et al., 1983). In the present paper the kinetics of the NO-NH3-O2 reaction on “chemical mixed” V205-Ti02* (anatase), “chemical mixed” V205Ti02* (rutile),impregnated V205-TiO, of both the anatase and rutile structures, and V205-A1203catalysts are compared. The intrinsic kinetic parameters of the NO-NH3-02 reaction on the six catalysts have been determined and are presented. This reaction is important in the control of NO, emissions from stationary sources (e.g., power plants) by selective catalytic reduction methods. Previous results from our laboratory showed that V205-A1203is an effective and durable catalyst for the reduction of NO with NH, in excess O2 (Bauerle et al., 1975; Wu and Nobe, 1977) even in the presence of substantial amounts of SO, (Bauerle et al., 1978). Recently, Inomata et al. (1982) reported that V205-Ti02is also active for the NO-NH3-02 reaction, and at V205contents of 5 mol % and lower, it is more active than V205-A1203.Their catalysts were prepared by the impregnation method.
Experimental Section A schematic of the experimental setup is shown in Figure 1. Liquid-Carbonics gases were used: 99.999% pure N2, 99.0% NO, 99.995% NH,, 99.5% OF A molecular sieve (Type 4A) trap was used to remove water, and a column containing copper turnings maintained at 400 OC was used to remove oxygen. N2was used as the carrier gas, and the gas flow rate through the reactor was maintained at 300 L/h (25 “C, 1 atm). The catalyst was loaded into an upflow Pyrex reactor (2.2 cm i.d.) and was supported by a plug of glass wool on top of a section of 5 mm diameter glass beads. Five chromel-alumel thermocouples were inserted into the thermocouple wells to monitor the temperature of the catalyst bed. The bed reactor section was wrapped with heating tapes for temperature control. The entire reactor was encased in 2.5 cm thick thermobestos block insulation.
Figure 1. Experimental apparatus: (1) molecular sieve (type 4A); (2) Vycor column filled with copper turnings at 400 “C; (3) soap bubble buret; (4) Drierite; (5) needle valve; (6) rotameter;(7) shut-off valve; (8) gas mixer; (9) glass wool; (10)catalyst bed; (11) thermocouple wells; (12) glass beads; (13) asbestos insulation; (14) pressure regulator; (15) thermocouples; (16) temperature recorder; (17) gas chromatograph; (18) recorder; (19) thermo-electron NO-NO, analyzer; (20) recorder; (21) pump; (22) ozone scrubber; (23) gas surge tank.
A Perkin-Elmer Model 900 gas chromatograph was used for analysis of N20. The NO, NO2, and NH3 analyses were carried out with a Thermo-Electron Chemiluminescent NO-NO,NO, series 10 analyzer. It is equipped with a stainless steel converter, which was maintained at 750 “C, and a molybdenum converter was used to convert NO2and NH, to NO. In this work, the N 2 0 formation was negligible for temperatures below 350 “C, and NO2 formation was not observed. The procedure for the preparation of “chemical mixed” V205-Ti02 catalyst consists of forming tetrakis tertiary amyloxides of titanium and vanadium separately, followed by controlled hydrolysis of an appropriate mixture of the alkoxides. The details of the preparation of alkoxides are described by Pearson et al. (1983). The oxide mixture precipitate is then filtered and dried overnight at 110 OC. The dried oxides are pressed into pellets X in.). A 5 wt 9‘0 V205-Ti02catalyst was prepared this way and calcined at 400 “C in flowing air for 4 h to obtain the anatase structure of Ti02. Another “chemical mixed” catalyst (10 wt % V205-Ti02)was calcined at 750 “C for 1h in flowing air to obtain the rutile structure of the Ti02 Three V205catalysts were prepared by impregnation of Ti02. Ti02 was precipitated by hydrolyzing TiC14. The precipitate was filtered and dried overnight at 110 OC and pressed into pellets (1/8 X 1/8 in.). The support was calcined at 400 “C for 4 h. The impregnation method involved soaking the pellets in an aqueous solution of NH4V03and oxalic acid. Two catalysts, 5 wt % and 10 wt % V205-Ti02 (anatase) were prepared and calcined at 400 “C for 4 h. Another 10 wt % V205-Ti02 (rutile) catalyst impregnated on Ti02 was further calcined at 750 “C for 4 h. A 10 wt % V205-A1203catalyst was prepared by the impregnation method to compare its activity with the in.) Ti02-supported catalysts. The A1203pellets (1/8 X were Filtrol grade 86 alumina. Table I gives pertinent physical properties for the 6 catalysts. Results and Discussion Kinetic Analysis. Some typical differential reactor kinetic data for the 6 catalysts are presented in Table 11. Averaged apparent or global reaction rates are determined from such data. Preliminary calculations showed that external heat and mass transfer effects are negligible at
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and then correlated with the power law rate equation
Table I. Physical Properties of Catalysts sp surf. area, cat.' 5% VzOs-Ti02* (A) 5% V20s-Ti02 (A) 10% V205-Ti02 (A) 10% V205-A1203 10% V205-Ti02* (R) 10% V205-Ti02 (R)
m2/a 77 34 26 200 2 1
pellet density, g/cm3 1.48 1.58 1.57 1.08 1.96 2.01
pore vol, cm3/g 0.34 0.32 0.32 0.46 0.26 0.25
mean pore radius,
A
88 180 250 46 2600 5000
'Asterisk (*) indicates that the catalyst was prepared by 'chemical mixing" technique. (A) Anatase structure for TiO> (R) Rutile structure for TiO?
the high total gas flow rate of 300 L (NTP)/h maintained. Internal heat transfer effects can also be neglected because of the low reactant concentrations. However, strong internal mass transfer reffects were indicated for the A1203-supportedand the Ti02 (anatase)-supported catalysts. The intrinsic reaction rates were determined from the reduction of NO with NH3 in the presence of O2 4N0 4NH3 O2 = 4N2 6H20
+
+
+
where
R = Ae-E/RT The inlet concentrations of NO and NH3 were varied, as shown in Table I1 in order to determine the reaction orders n and m for NO and NH,, respectively. The O2concentration was maintained in excess at 2.2% (a typical concentration in power plant emissions) for these measurementa. Calculations to correct for pore diffusion effects were performed in order to obtain the dependence of the intrinsic reaction rates on the concentration of the reactants. The calculation procedures were similar to those followed by others (e.g., see Smith, 1981). Some typical values for effectiveness factors (Q) and intrinsic reaction rates (P) for the 6 catalysts at constant temperature conditions and various concentrations of NO and NHs but constant excess concentration of O2 (2.2%)are given in Table 11. It has been well established that catalytic reduction of NO with NH, is enhanced by the presence of O2 (e.g, see
Table 11. Typical Differential Reactor Kinetic Data for Determination of NO and NHI Reaction Orders (Po+= 0,022 atm) S.V., pNO PNH8 i x io3, cat. temp, O C L/(g of cat. h) lo6, atm. lo6, atm. n mol/(a of cat. h) 52.7 449 0.22 2.56 5% Vz05-TiOz* (A) 204 1030 863 0.23 4.43 939 1260 0.23 6.28 870 1770 0.23 9.05 835 943 0.22 2.80 149 185 75.0 1090 0.24 2.65 268 1000 0.22 2.76 487 1010 0.20 3.60 960 435 0.15 3.09 202 75.2 1050 5% V205-Ti02 (A) 4.04 1010 0.20 958 1530 0.20 6.08 863 2070 0.23 6.10 818 872 0.19 3.76 199 75.0 504 0.19 909 3.85 797 3.94 0.19 1290 885 871 3.78 0.19 1740 450 0.46 0.67 1030 177 75.0 10% V2O5-TiO2 (A) 980 0.94 0.69 965 1570 1.37 0.64 995 1900 2.13 0.65 1110 932 1.33 0.37 450 177 75.0 1.29 932 0.38 903 0.43 1210 913 0.95 911 0.41 1.03 1450 0.18 0.84 1042 396 196 75.0 0.20 1.35 974 834 2.42 1345 0.19 920 1943 0.19 3.45 908 947 0.17 2.04 518 204 75.0 2.23 950 0.17 845 0.16 2.40 1360 953 0.16 2.42 2090 955 0.18 504 0.85 991 204 42.9 10% V2O5-Tiz* (R) 0.32 1060 0.88 918 0.48 1600 880 0.88 0.52 865 0.89 2080 967 0.29 204 42.9 586 0.88 0.31 984 929 0.87 0.32 1440 0.85 857 838 0.30 1800 0.86 430 0.07 10% Vz05-Ti02 (R) 204 21.4 1090 0.93 0.11 0.94 909 995 1390 0.17 915 0.95 1700 0.21 906 0.95 0.13 21.3 0.94 948 204 460 0.12 0.94 955 880 0.11 957 1350 0.95 0.13 1790 0.94 968
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 4, 1984 567
Il.
v2 o5 - h o t
n
59.
0
59. V2 O5 -Ti02
A 0
109.
V
109. V 2 O 5 - T l O t ( R )
A
I09.V205-T~02 ( R I
v2
Table IV. Typical Integral Reactor Kinetic Data for Determination of Intrinsic Arrhenius Parameters (Pol= 0.022 atm)
Os - 1 1 0 ~
10% V 2 O 5 - A I 2 O 3
cat. (wt, 9)
5%
4.
temp,
"C
P N O x
P N H s X
IO6;atm 106,atm
XNO XNHs
k"
0.15 0.14
3.147
143
1090
975
204 260 304 157
1090 1090 1090 1050
975 975 975 lo00
0.29 0.45 0.57 0.17
0.22 15.90 0.32 47.88 0.43 105.6 0.11 5.20
193 252 299 93
1110 1100 1110 1010
1000 1010 1010 lo00
0.32 0.56 0.69 0.11
0.23 16.39 0.44 79.29 0.55 224.2 0.15 0.82
154 188 216 154
1010 1010 1010 1020
lo00 lo00 lo00 1050
0.41 0.62 0.78 0.14
0.37 0.57 0.63 0.20
216 271 348 166
1020 1020 1020 1040
1050 1050 1050 945
0.30 0.69 0.88 0.10
0.32 20.04 0.61 90.79 0.79 516.1 0.09 0.567
221 271 329 163
1040 1040 1040 1075
945 945 945 1040
0.21 0.33 0.46 0.08
0.21 0.31 0.48 0.08
204 254 321 354
1075 1075 1075 1075
1040 1040 1040 1040
0.11 0.11 0.20 0.17 0.34 0.32 0.38
3 -
e1-0'; 8 6 -
34.
3 .
2 -
-'.01 8 6 5 -
4r
6.63 23.06 58.05 2.21
I 2
3
4 S 6 7 8 9 ,c3
PARTIAL
2
PRESSURE
3
OF
4 5
s 7 eslo-z
2
3
OXYGEN ( o m 1
Figure 2. Oxygen reaction order plot: PNo= 0.001 atm; Pms = 0.001 atm: T = 204 O C . Table 111. Calculated Intrinsic Kinetic Parameters cat. n m P A" 5% VzOs-TiO2* (A) 1 0 0.25 9.2 X 1.94 X lo7 5% VzOS-TiOz (A) 1 0 0.5 10% V2OS-TiO2(A) 1 0 0.5 3.5 X lo' 8.52 X 10" 10% Vz06-Alz03 1 0 0.5 10% V2Os-TiOZ*(R) 1 0 0.25 1.08 X l@ 1.08 X lo3 10% V205-TiOz (R) 1 0 0.4
le
"Units: mol/(g of cat. h atom"+"'+q).
Eb
10.40 12.90 12.90 14.80 6.54 6.54
kcal/g-mol.
Bauerle et al., 1975, and references therein). In the present study the dependence of the NO reduction rate on oxygen was determined by varying O2concentrations at constant temperature and constant concentrations of NO (1000 ppm) and NH3 (1000 ppm). Reaction order plots for O2 (intrinsic reaction rate vs. O2 concentration) are shown in Figure 2. Based on the above calculations, the NO, NH3, and O3 reaction orders that provided the best correlation with the intrinsic reaction rate expression, eq 1, are given in Table 111. For all 6 catalysts, intrinsic reactions rates are first order with respect to NO and zero order with respect to NH,. For both anatase and rutile structures of the "chemical mixed" Ti02* supports, the reaction rate was one-fourth order with respect to 02. The other catalysts prepared by the impregnation method were one-half order with respect to O2 (the impregnated rutile Ti02-supported catalyst was actually four-tenths order). The intrinsic Arrhenius parameters ( A and E) were determined from integral reactor kinetic data. An iterative calculation procedure was used to correct for pore diffusion effects and to determine the intrinsic rate constant for each bed temperature. Table IV presents some typical results. The A and E values for each catalyst determined from these intrinsic rate constants are given in Table 111. Table I11 shows that the * chemical mixed" 5% V205Ti02* (anatase) catalyst has a lower activation energy for the reaction than the impregnated 5 and 10% V205-TiO, (anatase) catalysts. The latter catalysts have the same activation energy, but 10% V205has an Arrhenius prefactor about twice the value of the 5% V205 catalyst, as expected. The alumina catalyst has an activation energy
1.268 2.337 4.674 0.66 1.17 2.07 4.25 5.85
Gunits: mol/(g of cat. h atmn+"'+q).
about 2 kcal larger than the impregnated Ti02 (anatase) catalysts. The "chemical mixed" Ti02* (rutile) and the impregnated Ti02 (rutile) catalysts have identical Arrhenius prefadars and activation energies. The latter value is about one-half that of the impregnated Ti02 (anatase) catalysts. Previous work indicates that improved efficacy of catalysts supported on Ti02compared to A1203, for example, can be ascribed to interaction of the active metal and the Ti02 support (e.g., Cole et al., 1976; Tauster et al., 1978; Roozeboom et al., 1980; Inomata et al., 1981). As shown in Table 111, the intrinsic activation energies decrease in the following order: 10% V205-A1203(14.80 kcal) > 10% V205-Ti02(anatase) (12.90 kcal) = 5% V205-Ti02 (anatase) (12.90 kcal) > 5% V205-Ti02*(anatase) (10.40 kcal) > 10% V205-Ti02 (rutile) (6.54 kcal) = 10% V205-Ti02* (rutile) (6.54 kcal). I t is surmised that the decrease in activation energy may be the result of increasing interaction between V205and Ti02. Thus, the more intimate mixing during preparation of "chemical mixed" 5% ' V205-Ti02* (anatase) would provide a more significant interaction than impregnated V205-Ti02 (anatase) catalysts. The low activation energies of the two rutile catalysts indicate the strongest interaction, possibly compound formation between V205and the rutile form of Ti02 (Cole et al., 1976). Comparison of Catalytic Activity. Figure 3 shows a semilog plot of the intrinsic reaction rates vs. 1/T. This plot provides a relative measure of the catalytic activities for the 6 catalysts. The "chemical mixed" 5% V205-Ti02*
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Table V. Comparison of Intrinsic Catalytic Activity" conversion of NO (xNn) 100 "C 125 "C 150 "C 175 "C 5% VzO5-TiO2* (A) 0.28 0.54 0.82 0.97 5% VzO6-TiO2 (A) 0.09 0.24 0.51 0.81 10% VzO6-TiO2 (A) 0.15 0.39 0.72 0.95 10% V205-A1203 0.03 0.10 0.28 0.58 0.11 0.18 0.26 10% VzO6-TiO2* (R) 0.07 10% VzO5-TiO2 (R) 0.04 0.07 0.11 0.16 unsupported V z 0 2 0.03 0.09 0.20 0.38 "Reaction conditions: PONO = 1000 X IO* atm; poNH3 = 1000 X lo* atm; PO, = 0.022 atm; Nz balance, total gas flow rate (NTP) = 300 L/h; cat. w t = 14 g. *Surface area = 5.4 m2/g (Inomata et al., 1980).
i
16
I8
20
i x lo3
2 2
2 9
2 6
28
1 K-'I
Figure 3. Intrinsic reaction rate vs. 1/T: PN0= 0.001 atm; PmI= 0.001 atm; Po2 = 0.022 atm.
(anatase) catalyst has been found to be the most active with the catalytic activities of the 6 catalysts decreasing in the following order: 5% V205-Ti02* (anatase) > 10% V205-Ti02 (anatase) > 5% VzO5-TiO2(anatase) > 10% VZO5-Al2O3> 10% Vz05-Ti02* (rutile) > 10% V205-Ti02 (rutile). It is clear that the TiOz (anatase)-supported VZO5 catalysts are more active than the A1203-supportedV2O5 catalyst even though the surface area of the latter is much larger than the former. The conversion of NO on the 6 supported catalysts between 100 and 175 "C are compared in Table V. Conversion calculations are based on the intrinsic kinetic parameters given in Table 111. Also included in Table V are results based on the work of Ionomata et al. (1980) for unsupported V2OP The results of NO conversion provide another measure of catalytic activity. Thus, in the temperature range examined, Table V shows that the effectiveness of the TiO, (anatase)-supported catalysts in reducing NO is far greater than the other catalysts. The rutile structure Ti0,-supported catalysts have the lowest activity, mainly because of their low surface areas. The same reason holds for the unsupported V205. The A1203-supportedcatalyst (200 m2/g) has almost two orders of magnitude larger surface area than the unsupported V205(5.4 m2/g),yet their activities are virtually equivalent up to 150 "C. As Table V shows, the "chemical mixed" and the impregnated V205-Ti02 (anatase) catalysts are far more effective in reducing NO with NH3 in excess oxygen than V205-A1203.Recently, Inomata et al. (1982) reported that for VzOj catalysts of 5 mol % (approximately 10 w t %)
and lower, Ti0,-supported catalysts are more active than A1203-supported catalysts. Their results are consistent with those given in Figure 3 and Table V. Similar results have also been reported by Shikada et al. (1981) for a V,O5-TiO2 catalyst. Figure 3 and Table V clearly show that "chemical mixed" 5% V2O5-TiO2*(anatase) is the most active catalyst examined with impregnated 10% V205-Ti02 (anatase) next but with a significantly lower activity. It seems reasonable, as discussed above, that the higher activity of the former may be attributed to the intimate mixing, and consequently, stronger interaction of the catalyst components (V2O5 and TiO& This preliminary study shows that catalysts of high activity can be prepared by the "chemical mixing'! technique. Natural extensions of this initial work would be to introduce promoters, poisons, alloying elements, and other selected additives by this technique. The preparation of such homogeneous catalyst systems may lead to interesting results not obtained heretofore. Registry NO.NO, 10102-43-9; NHB, 7664-41-7; V,05,1314-62-1; TiO,, 13463-67-7.
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Received for review March 12, 1983 Revised manuscript received August 7, 1984 Accepted September 4, 1984
