Catalyst hysteresis

The objective of this work was to provide a technique for measuring catalytic reaction kinetics over oxides in a manner unaffected by hysteresis effects. Hysteresis is commonly introduced by changes in the stoichiometry of the catalyst in response to the reaction conditions (JL). We wanted to measure the reaction kinetics in a time sufficiently short that the catalyst stoichiometry would not have changed between the beginning and the end of a series of measurements. To this end, it was necessary to substantially decrease the time on stream per data point, and the use of a pulse technique was therefore attractive. 

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

An example of such hysteresis is given in Figure 2 for the oxidation of CO over S MnFeOg and I MnFeOg. Elimination of this hysteresis would enable us to collect a set of data descriptive of a constant state of the catalyst, and hence analyzable in terms of a true reaction order and activation energy. 

In this work we attempt to measure kinetics data in a time short compared with the response time of the catalyst stoichiometry. An alternative is to measure kinetics in a true steady state, i.e., to increase the line-out time at each reactor condition until hysteresis is eliminated. The resulting apparent reaction orders and activation energies would be appropriate for an industrial mathematical model of reactor behavior. 

A square concentration pulse flow technique has been developed to study the kinetics of catalytic reactions over catalysts which change their stoichiometry in response to the reaction conditions. The technique makes it possible to obtain hysteresis-free kinetics data while greatly reducing the time during which the catalyst is exposed to the reaction mixture. 

Oxide Catalysts - Under appropriate conditions a bulk metal will form an oxide overlayer and this layer will be responsible for governing the catalytic behaviour. In a similar manner a bulk oxide can undergo reduction with a formation of a metallic surface layer. Such behaviour can be responsible for rate oscillations or hysteresis in reaction rate and is important when considering the catalytic behaviour of bulk oxides. 

Synthesis gas production. Alqahtany et al.92 have studied synthesis gas production from methane over an iron/iron oxide electrode-catalyst. Although the study was essentially devoted to fuel cell operation, for purposes of comparison some potentiometric work was performed at 950°C. It was found that under reaction conditions Fe, FeO or Fe304 could be the stable catalyst phase. Hysteresis in the rates of methane conversion were observed with much greater rates over a pre-reduced surface than over a pre-oxidised surface possibly due to the formation of an oxide. 

In between these tangencies, the curves R and L have three intersections, so the system has multiple stationary states (Fig. 7.3(b)). We see the characteristic S-shaped curve, with a hysteresis loop, similar to that observed with cubic autocatalysis in the absence of catalyst decay ( 4.2). 

The synthesis and characterization of the structural defects within aluminosilicate mesoporous materials were provided. We further discussed the fascinating adsorption-desorption hysteresis behaviors and the influencing factors in the formation of the structural defects. However, mesoporous MCM-41 can act as catalyst support for many catalytic reactions, especially involve bulk oiganic molecules, due to its large surface area and pore size. The ability to synthetically control the connectivity of the mesoporous materials may have important applications in catalysis. 

We should note that this article by Ya.B. apparently remained little noticed in its time. In any case, we are unaware of any reference to it in the works of other authors. This is explained by the fact that its ideas were far ahead of their time. Only in recent years, due to the wide application of physical methods in studies of adsorption and catalysis, have the changes in the surface (and volume) structure of a solid body during adsorption and catalysis been proved. Critical phenomena have been discovered, phenomena of hysteresis and auto-oscillation related to the slowness of restructuring processes in a solid body compared to processes on its surface. Relaxation times of processes in adsorbents and catalysts and comparison with chemical process times on a surface were considered in papers by O. V. Krylov in 1981 and 1982 [1] (see references at end of Introduction). 

Fleisch et al. (1984) measured the catalyst surface area and pore volume changes that occurred after severe deactivation of a 100- to 150-A pore catalyst. The results of these measurements are shown in Table XXVIII for various positions in the reactor bed. Catalyst surface area and pore volume are substantially reduced in the top of the bed due to the concentrated buildup of metals in this region. The pore volume distribution of Fig. 44 reveals the selective loss of the larger pores and an actual increase in smaller (<50-A) pores due to the buildup of deposits and constriction of the larger pores. Fleisch et al. (1984) also observed an increase in the hysteresis loop of the nitrogen adsorption-desorption isotherms between fresh and spent catalysts, which reflects the constrictions caused by pore... 

Fig. 1. Hysteresis loop for H2 oxidation on a single catalyst particle in a packed bed of inactive pellets. Linear gas flow rate w = 1.7 cm/sec. 0 H2 percentage increasing, percentage decreasing (75). (Reprinted with permission from Advances in Chemistry Series. Copyright by the American Chemical Society.)...

Taylor et al. [90], when studying the oxidation of CO over Ir (110), also found hysteresis on decreasing and increasing the temperature. This hysteresis is ascribed primarily to the non-linear kinetic dependences of the conversions of surface substances [90]. However, in our opinion, this hysteresis is also most likely to be "false since temperature variations of catalyst were sufficiently high ( 1.25Ks ) and the steady state could hardly be achieved. 

Let us now take normal conditions. For CO oxidation over Pt catalysts, sufficiently complete studies have been carried out to identify the dependence of the reaction rate on the CO concentration VF(Cco) and temperature JF(T). For these dependences we can observe specific critical effects. They are the clockwise hysteresis for VF(Cco) and counter-clockwise hysteresis for W(T). It is these characteristics that we have obtained for the kinetic model of the adsorption mechanism. 

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