Ind. Eng. Chem. Prcd. Res. D ~ V .~
52
NO conversion was measured by introducing NH3 into the gas mixture for a short period of time, 10-20 min. It was found that the catalyst lost activity to some extent, larger extent at the lower temperature. The loss of activity could be ascribed to the SO3 adsorption of the active sites of catalyst. The loss of activity was found to be temporary, since the catalytic activity was recovered quickly when SO3 was removed from the gas mixture. Taking into account the poisoning by the SO3 adsorption and the ABS condensation, the calculated activity was in good agreement with the experimental value. An extensive study on the SO3adsorption has been in progress in our laboratory and a quantitative treatment will be reported in the future. Appendix Reaction Rate and Pore Volume. Based on the Thiele model for a catalytic reaction which is controlled by reactant diffusion in micropore, the reaction rate R in one pore of radius r and length 2L is given by
R(r) = C o 2 a r m tanh(h) h =
Ldm
(AI)
(A21
where co is the reactant concentration to which the rate is linearly proportional, k is the intrinsic rate constant per unit area, and D is the diffusion coefficient. In the case of NO-NH3 reaction on the catalyst used in the present study, h is in a range of lo2-lo3, and tanh (h) = 1. Equation A1 is reduced to
R(r) = C 0 2 7 r r m (-43) The mean free path of NO is calculated to be about 1300 A at 350 “C and normal pressure. The transfer of NO in micropores of the catalyst is controlled by the Knudsen diffusion, since the mean pore radius is about 200 A.
D = 9.7 x
1 0 3 ~ m a r ~
22 1 , , ~ s
where T is temperature in K and M is the molecular weight. From eq A3 and A4,
R(r) 0: r2 (A5) On the other hand, pore volume V of radius r is given by V(r)= 2L.7rr2
(A6) Under the assumption that the length of pore L is independent of r, we derive from eq A5 and A6
R(r) a V(r)
(A7) Then, the total rate of reaction on one catalyst particle will be
Literature Cited Bartok, W.; Crawford, A. R.; Cunningham, A. R.; Hail, H. J.; Manny, E. J.; Skopp, A. “Systems Study of Nitrogen Oxldes Control Methods for Stationary Sources”, National Technical Information Services, Springfield. VA, Government Report PB-192-789, 1969. Ikeda, T.; Koyata, K. Karyoku Genshkyoku Hatsuden 1978, 29, 59. Imnari, M.; Watanabe, Y.; Matsuda, S.; Nakajlma, F. Preprints of Contrlbuted Papers, 7th International Congress on Catalysis, 5 9 , Tokyo, Japan, 1980. Kasaoka, S.;Sasaoka, E.; Yamanaka, T. “ y o Kyokaishi 1977, 56, 834. Kato, A.; Matsuda, S.;Kamo, T.; Nakajlma, F.; Kuroda, H.; Nab, T. J . phvs. Chem., 1981, 85. 1710. Kuroda, H.; Nakajima, F. “Some Experlences of NO, Removal In Pilot Plants and Utility Boilers”; presented at 2nd EPRI NO, Control Technology Seminar, Denver. CO, 1978. Matsuda, S.; Takeuchi, M.; HlsMnwna, T.; Nakajlma, F.;Narlta, T.; Watanabe, Y.; Imanari, M. J . Air Pollut. ControlAssoc. 1978, 28,350 (presented at National Meetlng of APCA, Portland. 1976). Nakajima, F.; Hishinuma, T.; Kumura, T.; Arikawa, Y.; Narita, T. “Catalytic Reduction of NO, In Stack Gases”; presented at ACSlCSJ Chemical Congress, Honolulu, HI, 1979. Stuli, D. R.; Porphet, H. “JANAF Thermochemical Tables”, 2nd ed.; Natbnai Bureau of Standards, Washington, DC, 1971. Wheeler, A. “Cetalysis”; P. H. Emmett, Ed., ReinhoM Publishing Corp., New York, 1955; Voi. 11, Chapter 2, p 133.
Received for review M a r c h 31, 1981 Revised manuscript received A u g u s t 31, 1981 Accepted S e p t e m b e r 17, 1981
(A4)
Effect of Sulfur Poisoning on the Hydrogenolysis Activity of Pt in Pt-AIzOa CataIysts P. Govlnd Menon, Guy B. Marln,‘ and Gilbert F. Fromenl’ Laboratorlum voor Petrochemische Techniek, Rijksuniversitek Gent, Krijgshan 27 1, 9000 Gent, Eelglum
Using the hydrogenolysis of n-pentane at 300-380 O C as an indicator reaction, the S poisoning of Pt-AI,03 catalysts (0.3-2.0% Pt) was studied by the gas chromatographic pulse titration technique. The hydrogenolysis activii was reduced to zero only when the atomic ratio of S per exposed Pt atom, SIR,, was about unity. The breakthrough of H,S from the catalyst bed can also be used to indicate the end point in the S titration and hence to estimate the Pt dispersion on the catalyst surface. A fresh catalyst and one containing only irreversibly held S have practically the game activity and selectivity for reforming reactions of n-hexane at 423 OC and 10 bar pressure. The suppression of hydrogenolysis and enhancement of isomerization and aromatization can be achieved only by reversibly adsorbed S on the Pt over and above that held irreversibly. Here also, the hydrogenolysis is effectively suppressed only when all surface Pt atoms are covered by S.
Introduction Poisoning of the activity of supported platinum catalysts by traces of S is well known (cf. Smith et al, 1971). SoDepartment
of C h e m i c a l Engineering, Stanford U n i v e r s i t y ,
Stanford, CA 94305. 0196-4321/82/1221-0052$01.25/0
morjai (1972) suggests that S may catalyze the recrystalization of (111)crystal planes of Pt characterized by sixfold rotational symmetry to (100) planes with fourfold rotational symmetry, and that sensitivity to poisoning by S indicates that a particular reaction is structure-sensitive and not facile, according to Boudart’s (1969) classification. 0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 53
Blakely and Somorjai (1976) propose that a structuresensitive reaction like hydrogenolysis of hydrocarbons can take place only at the coordinatively most unsaturated "kink" sites in the steps separating terraces of Pt atoms in a Pt crystallite. If this is true, then the very first amounts of the S poison should be retained by the Pt atoms at the kink sites, to be followed only later by those at the steps and the terraces. Menon and Prasad (1977) have reported that on exposing commercial 0.6% Pt-AlzO3 reforming catalyst to thiophene at 500 "C, all surface Pt atoms are sulfded irreversibly (as seen from no measurable H2 chemisorption on the catalyst at 25 "C), but the activities at 315 "C for dehydrogenation of cyclohexane, and at 500 "C for dehydro-isomerization of methylcyclopentane and dehydrocyclization of n-hexane, determined in a pulse reactor, are affected only marginally. The suppression of hydrogenolysisand associated enhancement of aromatization on the catalyst at atmospheric pressure and at 500 "C are achieved by the S held on the Pt reversibly over and above that held irreversibly. Gonzalez-Tejuca et al. (1977, 1978), using the S poisoning titration technique, have found that the number of active centers for the catalytic hydrogenation of olefins and equilibration of Hz-D2 mixtures is almost equal to that determined by H2 chemisorption technique. Fischer and Keleman (1978), from high-vacuum studies on a Pt (100) plane, have identified three different S poisoning mechanisms: (a) when the surface is covered with one S atom per two surface Pt atoms, it is chemically inert; (b) at lower S/Pt coverages, the strong chemical bond to S modifies the chemical properties of the Pt surface and weakens its interaction with adsorbates; and (c) when the S coverage is one S atom per four surface Pt atoms, a regular S overlayer is established on which reactant molecules can adsorb without undergoing further reactions, however. In the present work, the pulse-poisoning titration technique has been applied to determine the amount of S required to completely suppress the hydrogenolysis activity of Pt in Pt-AlzO3 catalysts at 300-380 "C. Investigations have also been carried out on (a) the role of reversibly held S, over and above that held irreversibly, on Pt in suppressing the hydrogenolysis of n-hexane in a continuousflow integral reactor at 423 "C and 10 bar pressure, and (b) the quantity of S retained by the catalyst with different S concentrations in the 'feed.
Experimental Section The low-pressure experimenta were carried out in a pulse microreactor containing 0.5 g of catalyst at 300 "C, over which 0.1-pL or higher doses of thiophene were injected, followed 5 min later by 2-pL doses of n-pentane. Hydrogen was used as the carrier gas during both types of injections. The injection series were continued until the hydrogenolysis activity of the catalyst was reduced to zero. During thiophene injections, the exit from the reactor was made to bypass the GC column and end up in a fine jet just below a wet lead acetate paper. In this way the breakthrough point of HzS (from the reaction of thiophene and H2over the Pt catalyst) could be detected during the sulfur titration of the Pt surface. This simple lead acetate paper test is sensitive to 500 OC) in H2 also attenuates the hydrogenolysis, presumably due to a self-inhibition by a stronger chemisorption of H2 (Menon and Froment, 1979) at the catalytic sites of Pt responsible for C-C bond fission. The similarity of the results obtained on pretreating the catalyst by the above three different methods is somewhat analogous to a generalizing hypothesis just forwarded by Biloen et al. (1980). These authors propose that the beneficial effects on catalyst selectivity and stability, brought about by widely different catalyst modifiers (Re, Au, Sn, S, C of coke, etc.) to Pt catalysts, are largely due to one common cause, namely, the division of the Pt surface into very small ensembles of 1-3 Pt atoms. Acknowledgment This work was undertaken thanks to a "Center of Excellence" Grant awarded by the Belgian Ministry of Scientific Affairs within the framework of the "Action Concertie Interuniversitaire Catalyse". One of us (G.B.M.) is grateful to the Belgian "Instituut tot Aanmoediging vmr
het Wetenschappelijk Onderzoek in Nijverheid en Landbouw" for a grant over the period 1977-79. We thank Dr. W. F. de Vleesschauwer of Texaco Research Center, Gent, for analysis of the sulfur content in our catalyst samples.
Literature Cited Benson, J. E.; Bodart, M. J . Catel. 1965, 4 , 704. Biloen, P.; Helle, J. N.; Verbeek, K.; Dautzenberg, F. M.; Sachtk, W. M. H. J . Catal. 1980. 63, 112. Blakely, D. W.; Somorjal, 0.A. J . Catal. 1978, 42, 181. Boudart, M. A&. Catel. 1969, 20, 153. De Pauw, R. P.; Froment. G. F. Chem. fng.Sc/. 1975, 30, 789. Flscher, T. E.; Keleman, S. R. J . Catal. 1978, 53, 24. Freei, J. J . Catel. 1972, 25, 139. Gonzales-Tejuca, L.; Aka, K.; Namba, S; Turkevich, J. J . phys. Chem. 1977, 87,1399. Gonzales-Tejuca, L.; Turkevich, J. J . Chem. Soc.,Faraday Trans. 7 1978, 7 4 , 1064. Menon. P. G.: Sieders. J.; Streefkerk, F. J.; Van Keulen, G. J. M. J . Catal. 1 9 7 3 ~ 2 9 ,isa. Menon, P. G.; Rased, J. Roc. Int. Congr. Catel. 6th 1977, 1601. Menon, P. G.: De Pauw, R. P.; Froment, G. F. Ind. Eng. Chem. Prod. Res. D e v . 1979, 78, 110. Menon. P. G.; Froment, 0. F. J . Catal. 1979, 59, 138. Smith, R. L.; Naro, P. A.; SHvesby, A. J. J . Catel. 1971, 20, 359. c Somorjai, G. A. J . Catel. 1972, 27, 453. Thomas, L. C.; Chamberlln, G. J. "Cobrlmetric Chemical Analytical Methods", published by The Tintometer LM.. Salisbury, England, and distributed by John Wlley 8 Sons, London, 1974, p 198. Truesdale, E. C. Ind. Eng. Chem.. Anal. Ed. 1930, 2, 299.
Received for review April 27, 1981 Accepted September 24, 1981
Complete Conversion of NO over the Composite Catalyst Supported on Active Carbon Tomoyukl Inul, Toshlro Otowa, and Yoshlnobu Takegaml +
Lbpafiment of Hydrocarbon Chemlstiy, Faculty of Englneerlng, Kyoto Unherslty, YoshMa, Sakyo-ku, Kyofo 606 Japan
Complete conversion of low concentrations of NO (such as 0.6%) was achieved by a composite catalyst supported on active carbon without additional reductive gases. Several weight percent of Ni or Co was supported as the catalyst substrate, and small amounts of a rare earth oxMe and a platinum metal were used as co-catalyst components. The reaction of NO and active carbon occurred above 300 OC,and the complete conversion of NO was achieved at about 400 O C at a high hourly space velocity. In particular, above 440 O C , NO was converted quantitattvely Into N, and COP NO conversion was retarded by the coexistence of O,, but the NO was still rapidly converted beiow 420 O C in succession to the complete O2 conversion. The morphological observations of the change of active carbon during the reaction support the pitting mechanism similar to the catalytic oxidation of graphite. The composite catalyst also exerted high activity In the surface reactions of NO with H, and CO.
reactions with reducing agents such as H2, NH3,or CO are also thermodynamically more favorable than the direct decomposition of NO into N2 and 02.Therefore, NO has usually been made to react with some gaseous reducing agent such as hydrogen, ammonia, or methane (Adlhart et al., 1971; Katzer, 1975). However, this method is troublesome to apply for small sources or moving vehicles. In this paper, a novel and simple method is presented to convert NO,, irrespective of the coexistence of 02,into innoxious N2 and C 0 2 (reactions I1 and 111)without any 2N0 C N2 + COZ (11)
Introduction Many studies have been conducted recently to remove detrimental NO in exhaust gases from various internal combustion engines and furnaces by means of catalytic conversion (Dwyer, 1972; Shelef, 1975). The most simple method to remove NO is believed to be the direct decomposition of NO into N2 and O2 (reaction I). Some reduced 2N0 N2 + 0 2 .(I) transition metal catalysts have high activities for NO decomposition but are deactivated rapidly owing to the combination of metals with the oxygen released by the decomposition of NO. In contrast, oxide catalysts such as perovskite are stable against oxidative reaction conditions but have low activities for NO decomposition (Moser, 1975; Voohoeve, 1977). Indirect decomposition of NO via +
0196-4321/82/1221-0056$01.25/0
+ 2N20 + C
--
2Nz + COZ
(111)
additional gaseous reducing agents. These reactions were achieved at considerably low temperatures by a catalytic 0
1982 American Chemical Society
