Madon-Boudart method

To verify the absence of any heat transfer effects, the Madon-Boudart method can also be utilized. If plots of In vs. In L are obtained at two or more temperatures, and the slopes are unity at each temperature, then thermal gradients, as well as concentration gradients and other artifacts such as poisoning, can also be ruled out [63]. If the reaction order is known, the criterion in equation 4.73 can also be used to check for isothermal operation. 

In real situations, the question arises frequently as to whether or not a marked influence of heat and mass transfer on the observable reaction rate may be expected under certain reaction conditions. Quite often then, one has to deal with reactions obeying complex kinetics where either none or only a very cumbersome analytical solution is possible based upon the methods described in Section 6.2.3. For such cases a number of useful diagnostic criteria have been developed in the past, either derived from asymptotic solutions of the governing differential equations or from perturbation methods [86], Most of these criteria have been explained in a detailed review by Mears [76]. More recent surveys of diagnostic transport criteria have been given by Butt [12] and by Madon and Boudart [74],... 

However, this criterion also has its limitations, as it is not always easy or possible at all to prepare catalyst samples with different concentration of the active material without simultaneously changing some other important properties of the catalyst, for example its diffusional characteristics. Details about the application of the Koros-Nowak criterion, as well as some methods how to prepare the required catalyst samples, can be found in the original paper by Koros and Nowak [64] or in a more recent article by Madon and Boudart [74]. 

Madon and Boudart propose a simple experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions [R. J. Madon and M. Boudart, Ind. Eng. Chem. Fundam., 21 (1982) 438]. The experiment involves making rate measurements on catalysts in which the concentration of active material has been purposely changed. In the absence of artifacts from transport limitations, the reaction rate is directly proportional to the concentration of active material. In other words, the intrinsic turnover frequency should be independent of the concentration of active material in a catalyst. One way of varying the concentration of active material in a catalyst pellet is to mix inert particles together with active catalyst particles and then pelletize the mixture. Of course, the diffusional characteristics of the inert particles must be the same as the catalyst particles, and the initial particles in the mixture must be much smaller than the final pellet size. If the diluted catalyst pellets contain 50 percent inert powder, then the observed reaction rate should be 50 percent of the rate observed over the undiluted pellets. An intriguing aspect of this experiment is that measurement of the number of active catalytic sites is not involved with this test. However, care should be exercised when the dilution method is used with catalysts having a bimodal pore size distribution. Internal diffusion in the micropores may be important for both the diluted and undiluted catalysts. 

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