Periodic operation effects

Fig. 5. Periodic operation effect on Pt/Al203 catalyst in 3 -02 reaction.

Fig. 6. Periodic operation effect on Pd/Al203 catalyst in CO-Oz reaction. Oscillation periods 0s( ), 1 s(3), 5s( ), 10 s(O) and 20 s(0). Periodic operation effect on Pd/Al203 catalyst in C0-02 reaction.

PERIODIC OPERATION EFFECTS ON AUTOMOTIVE NOBLE METAL CATALYSTS — REACTION ANALYSIS OF BINARY GAS SYSTEMS —... 

Catalytic activities and periodic operation effects in various binary gas systems (C0-02, NO-CO, N0-H2, C3H6-02, and C3Hg-02) over Pt, Pd, and Rh/a-Al203 were compared. In all reaction systems, periodic operation effects were found to some extent. That is, the conversion improved in the cycling feed compared to the static one. The periodic operation effects occurred most noticeably for catalysts having lower catalytic activity as a result of the difference of adsorption capability between the two reactants. 

We observed that the reaction using noble metal catalysts, particularly Pt and Pd, under cycling conditions was superior to that under static conditions, and that catalyst perfomance depends on the cycling period and feedstream composition. These phenomena may be used to improve the activity of three-way catalysts by selection of suitable periodic conditions. Further, it is very important to clarify the mechanism of the periodic operation effects over various noble metal catalysts. Our object was to investigate the performance of conversions over noble metal catalysts under periodic conditions. 

This paper introduces (1) the detailed behaviour of the N0-H2 reaction under both static and dynamic conditions over three kinds of noble metal catalysts (Pt, Pd, and Rh/a-Al 2O3), and (2) the relationships between catalytic activity and periodic operation effects obtained from the various binary gas systems over the same catalysts. Finally, in order to explore the role of periodic operation, the concept of self-poisoning of reactants contained in the binary gas systems is postulated, allowing reasonable interpretation of our results obtained under the cycling conditions. 

Figure 4 illustrates the effect of cycling period on NO conversion at several temperatures over a Rh catalyst. At lower temperatures (below 300°C), the periodic operation effect was most noticeable. 

Die negative order, that is NO inhibition, in N0-H2 reactions over noble metal catalysts is essentially concerned with the periodic operation effect. The periodic operation effect is perhaps due to the surface state of the Rh catalyst. That is, the catalyst surface under static conditions is easily oxidized by NO, so that NO chemisorbs to a smaller degree and reaction rates are suppressed. However, under the optimum cycling feeds, catalyst surface is sufficiently reduced and suitable surface compositions of reactants are maintained in order for reaction to proceed. As a result, the reaction rate reaches the maximum value. The periodic operation effect can be interpreted in terms of the strong affinity between Rh and oxygen and this concept can be extended to interpret the behaviour observed in the C0-02 (Ref. 8) and CO-NO (Ref. 9) systems. 

Catalytic activities and periodic operation effects in various binary gas systems over noble metal catalysts... 

In order to clarify the mechanism of the periodic operation effect over noble metal catalysts, we studied several binary gas systems, which occur in automotive exhaust gas CO-NO, C0-02, C3H6-02, C3H8-02, and N0-H2. 

Under dynamic conditions, periodic operation effects were observed in every binary gas system. The most efficient, catalysts under periodic condition were ... 

Comparing the results under static and dynamic conditions, it is clearly found that periodic operation effects occurred most noticeably for catalyst of lower catalytic activity and at lower reaction temperatures. These facts indicate that if catalysts have a poor catalytic activity or they are reacting at a lower temperature, their activities can be improved by periodic operation. 

In all cases where periodic operation effects were noticed, the product of two partial reaction orders, m x n, became negative. From these results, it seems that one of the reactants self-poisons a reaction occurring over noble metal catalysts and that the potential for self-poisoning corresponds to the degree of periodic operation effect. 

The periodic operation effect depends on temperature and cycling period and each catalyst behaves differently. To improve catalytic activity, it is important that the optimum cycling period for each temperature is used. 

The detailed behaviour of noble metal catalysts in NO reduction with H2 was investigated and the order of catalytic activity was found to be Pt > Pd > Rh. Periodic operation effects were found that is, the conversion of NO is improved by the cycling feedstream compared to the static one. This was most noticeable with Rh catalyst. 

In binary gas systems, periodic operation effects are observed according to the catalyst used and the reaction temperature. 

Periodic operation effects occurred most noticeably for catalysts having lower activity or when catalysts were operating at lower temperatures. 

From the results of the partial reaction order, either reactant contained in binary gas systems can self-poison the surface of the catalyst and suppress the reaction. Ihe potential for self-poisoning corresponds with the degree of the periodic operation effect. 

From these findings, we consider that periodic operation effects arise from a difference of adsorption capability between the two reactants on the catalyst surface, that is, the self-poisoning reactant is the one more strongly adsorbed on the catalyst surface. Accordingly, the catalyst surface under static conditions is almost covered by the stronger adspecies, and expected reactions are suppressed. Conversely, under optimum cycling conditions, these adspecies are eliminated and surface compositions are suitable for reaction to take place. Under these circumstances, the reaction rate reaches the maximum value. 

Periodic operation effects can be applied to improve the reactivity of... 

Periodic Operation Effects on Automotive Noble Metal Catalysts. Reaction Analysis of Binary Gas Systems",... 

Shinjoh, H., H. Muraki and Y. Fujitani, 1987, Periodic operation effects on automotive noble metal catalysts Reaction analysis of binary gas systems, in Catalysis and Automotive Pollution Control, eds A. Crucg and A. Frennet (Elsevier, Amsterdam) pp. 187-197. 

The behaviour of TWC s under continuous operation has been extensively studied. Due to the step-like response of the oxygen sensors the gas composition oscillates with a frequency of about 1 Hz around the stoichiometric set point. Therefore, most studies focus on the behaviour of catalytic converters under oscillating exhaust gas composition. In particular, the contribution of ceria to the dynamic behaviour of automotive catalysts under transient air/fuel conditions [3, 4, 5, 6, 7, 8] has been investigated. Binary gas mixtures have been applied to clarify the mechanisms of the periodic operation effects over different noble metal catalysts [9,10,11,12]. Muraki et al. used simulated exhaust gas to examine the performance of noble metals on a-Al Oj [9]. 

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