Thermal Deactivation versus Poisoning

It is simple to demonstrate that a catalyst operated in the absence of poisons (54, 55) still can show significant activity loss, albeit to a much smaller degree than in their presence. This deactivation process is induced by thermal effects. A separation of chemical and thermal deactivation requires considerable efforts. 

If one could disregard the complicated influence of poisons on mass transfer processes, it would be possible to state in a first approximation that catalyst activity for a selected reaction is a monotonic function of the surface area occupied by the active component. The problem that arises is the measurement of the catalytic surface area in the presence of a support material. In the case of Pt such a measurement is relatively simple, done by hydrogen chemisorption (56, 57) or titration (55), although even in this case there are uncertainties associated with surface stoichiometry (59, 60). These problems become more complicated when Pd, or other noble metals are incorporated at the same time, and still more so, when the catalysts have been contaminated (61). 

Examination of automotive catalysts by various chemisorption techniques has shown that a loss in noble metal surface area caused by higher temperatures correlates monotonically with various activity indices (62, 63). Moreover, Dalla Betta and co-workers (64) were able to separate the additional effect of poisons on the surface of the precious metal by painstaking attention to detail. They developed techniques for accurately measuring the crystallite-size distribution in used automotive catalysts by 

Such a sharp drop in surface area of the noble metals does not result in a corresponding activity decrease. As measured by various empirical criteria, such as conversion at a certain temperature, it is found that activity loss is initially not nearly as steep as the indicated loss in site accessibility. The reason is that such measurements are usually carried out under conditions of mass transport control, when the vast majority of the active surface is not utilized in the catalytic process. However, once the active surface has dropped below a certain value, catalytic activity diminishes rapidly (66). These results emphasize that to begin with, a huge reserve of activity is required if the statutory service life of 50,000 miles is to be achieved. How large this reserve has to be is determined to a large extent by the poison levels. 

Actual surface area (CO adsorption, O) and expected surface area if chemical contaminants were not present (electron microscopy by tissue grinding, ) vs. observed catalyst activity. [From DalTa Betta et al. (64).] (Reprinted with permission of the American Chemical Society.) 

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Typical experimental results are presented in Figures 1 and 2 in Table T, relevant experimental operating conditions are presented. The exit conversion versus time-on stream data are presented for an isothermally deactivating bed The discontinuity in conversion data occurs where the poison-contaiiting feed was stopped for thermal activation of the bed for the following cycle. The initial conversion level at time zero in... 

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