React. Kinet. Catal. Lett., Vol. 10, No. 3,281-285 (1979) E N E R G Y D I S S I P A T I O N AND S E L E C T I V I T Y IN T H E C A T A L Y T I C O X I D A T I O N O F A M M O N I A A. Wojnowski and J. Barcicki Department of ChemicalTechnology, Institute of Chemistry Maria Curie-SklodowskaUniversity, 20-031 Lublin, Nowotki 12, Poland ReceivedApril 12, 1978 Accepted December 28, 1978 A qualitative interpretation of selectivity changes in ammonia oxidation, based on considerations of energy dissipation processes is presented. IIpHsO/IHTC~I KaqeCTBeHHa~ HHTepIIpeTaII,H~I H3MeHeHH~ C~HCKTHBHOCTH B OKHCYlCHHHabiMHalr OCHOBaHHa,$[Ha IIpI4H~THH B ytleT llpOl.teCCOB p a c e e l m a H l ~ 3HeprHH.

INTRODUCTION High exothermielty of ammonia oxidation reactions (316.7, 236.1 and 226.4 kl per moI NH3 for N2. N20 and NO production, respectively) suggests the necessity of dealing with the problem of heat dissipation (generation) in this system.

EXPERIMENTAL The total heat generation was examined by measuring the overheating of the working catalyst relative to the gas and calculating the heat release on this b a s i s / 1 , 2 / . The experiments were performed mainly on pure oxides of metals.

RESULTS AND DISCUSSION In all the cases it was found that with increasing catalyst temperature, there ooo cttrred a maximum of heat generation, while the relative curves, i , e , qt/qmax = f~t) always had very similar shapes (Fig. 1). The phenomenon eould not result from catalyst 281

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deactivation, since with the lowering of the temperature in the region discussed there again occurred an increase in the quantity of generated heat. Moreover, a general diagram (Fig. 2) common for metal and metal oxides, constructed on the basis of the detailed literature d a t a / 3 - 8 / s h o w s the phenomenon too and thus the curve of heat generation seems to be a limit for the generation of total heat under the given concentration and flow conditions. We should search for physical (which suggests the independence of the phenomenon of the type of catalyst) rather than chemical factors, which cause a systematic fall in the quantity of generated heat with an increase in the catalyst temperature. To begin the interpretation we assume that the elementary steps in ammonia oxidation do not differ from those in atom and small radical recombinations because of the high exothermicity and small number of bonds in the species participating in 282

WOJNOWSKI,BARCICKI:ENERGY DISSIPATION

total heat generation ~q=qNZ+qN20+qNO ' .... h e a t g e n e r a t i o n in individual reaction .......... q u a n t i t y of product ,n (moll AHw~o 8HA, qN0=nNo'AHNo)qNZO=nNIO"AHN0 J qN-=nNZ AHN0 :D

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both reaction types. It is well known that in the course of a recombination reaction the rate-controlling step is the dissipation of energy from the bonds created during the r e a c t i o n / 9 / , but even in the presence of solids some limits in energy dissipation exist/10, i l / . As a result we can present the following argumentation. In the course of an exothermic reaction, the catalyst is heated by the energy from the bonds formed, which leads to the conclusion that there may exist a surface temperature whose value could result from the establishment of an atomic scale thermal equilibrium between the bonds of an intermediate and those of the solid surface. After this surface state has been reached (due Cog. to independent electrical or reaction heating only), energy transfer from the bonds created would be impossible because of the lack of differences in the "oscillatory temperatures". This would completely 283

WOJNOWSKI, BARCICKI:ENERGY DISSIPATION inhibit the exothermic reactions, which corresponds to the intersection of the heat generation curve with the temperature axis in Fig. 2. This irdaibition may be influenced by other factors, typical for gas, solid energy transfer (see for example p, 459 in Ref. /12/). At lower catalyst temperatures one may in turn expect larger "oscillatory temperature" differences, in view of increasing heat generation up to the temperature of catalyst extinction. On the basis of the data presented above, one may offer a hypothesis about the phenomenon which may be called "energetic competition in bond stabilization" occurring on the surface of the catalyst, Due to this competition, under the conditions limiting energy flow (solid state energy transport properties may interact he~e, e . g . heat conduction by the bulk phase), there are formed only those bonds (hence also molecules) whose stabilization requires dissipation of a smaller amount of energy. With increasing catalyst temperature and greater difficulties in energy dissipation this leads to a change in the selectivity of ammonia oxidation which passes from the product of the most exothermic reaction (N2), through that of lower exothermicity (N20) to the least exothermic reaction (to NO) and finally, to the inhibition of the latter (the behavior of the reaction in the latter region seems to be analogous to the maximum observed in CO oxidation/13/, thus the whole ammonia oxidation system represents a more general (see Fig, 9.) case of heat generation inhibition), As a consequence of this inhibition, there appears unreacted ammonia at high temperatures and/or an endothermic NH3 decomposition leading to oscillatory p h e n o m e n a / 1 4 / . Thus the frequently reported presence of nitrogen in this region may be a result either of the reaction of nitrogen oxides with unreacted ammonia, or of endothermic ammonia decomposition. How the above competition influences the structural changes of surface species (intermediate forms) in the region of main selectivity changes it is difficult to answer at present.

284

WOJNOWSKI, BARCICKh ENERGY DISSIPATION REFERENCES

1. A. Wojnowski, J. Chmieh React. Kinet. Catal. Lett., 4, 443 (1976). 2. J. Barcicki. A. Wojnowski, J. ChmieI: Environ. Protec~ Engng., 3, 103 (I977). 3. Y.M. Fogel, et al.: Kinet. Katal., _5, 496 (1964). 4. C.W. Nutt, S. Kapur: Nature, 220, 697 (1968); 224, 169 (1969). 5. J. Zawadzki: Disc. Faraday Soc., 8, 140 (1950). 6. N. Giordano, et al. : Chim. Ind., 4.55, 15 (1963). q. J.E. Germain, R. Perez: Bull. Soe. Chim. Ft., 2042 (1972). 8. N.I. Ilchenko, et al.: Kinet. Katal., 16._., 1455 (1975); 17.._, 378 (1976). 9. G.M. Panchenkov, V.P. Lebedev: Chemical Kinetics and Catalysis. Moscow 1976. 10. B.J. Wood, et al.: J. Phys. Chem., 67.._, 1462 (1963). 11, Ch. SteinbrUchel, L.D. Schmidt: Surf. Sci., 40, 693 (1973). 12. L.D. Schmidt: The Physical Basis for Heterogeneous Catalysis, Ed. by E. Drauglts, IL.I. Jaffee. New York (1975). 13. J.S. Close, J.M. White: J. Catal., 36, 185 (1975). 14. J. Barcicki, A. Wojnowski: React. Kinet. Catal. Lett., 9, 59 (1978).

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