Poisons and Active Components

In certain instances of poisoning, especially in the case of base metal catalysts, the deactivation can be simply explained by the formation of new bulk solid phases between the base metal and the poison. Examples are the formation of lead vanadates (14) in vanadia catalysts, or of sulfates in copper-chromite and other base metal catalysts (81). These catalyti-cally inactive phases are identifiable by X-ray diffraction. Often, the conditions under which deactivation occurs coincide with the conditions of stability of these inert phases. Thus, a base metal catalyst, deactivated as a sulfate, can be reactivated by bringing it to conditions where the sulfate becomes thermodynamically unstable (45). In noble metal catalysts the interaction is assumed to be, in general, confined to the surface, although bulk interactions have also been postulated. 

An attempt was made in our laboratory (97) to examine the specificity of the association of noble metal and lead, using the carefully controlled exhaust produced by the pulse-flame generator (30). A microscope slide was covered by a 100-A-layer of alumina. Two separate strips of Pt (or Pd), 50 or 300 A thick, were sputtered on top of the alumina layer. A constant temperature in the 350°-700°C range was kept during the experiment. 

The surface of the exposed specimens was examined by the electron microprobe. As the electron-microprobe traces of a few of the experiments indicate (Fig. 23), an enhanced Pb affinity for platinum seems to be, indeed, the case. The lower trace (A) shows that there is much more lead retained on the areas covered by Pd, while the middle trace (B) indicates the same for the areas covered by Pt. This phenomenon recurs at each of the four interfaces between the metal and A1203 segments of the slide. In the experiment, the slides were exposed to approximately 72 hr of treatment (which is equivalent to 2000-2500 miles of simulated vehicle operation), and the fuel contained 0.5 g Pb/gal. In case of the traces (A) and (B), there was no scavenger in the fuel. 

The most plausible interpretation of the traces in Fig. 23 is as follows the volatile species transporting lead decomposes on the noble metal, and the lead diffuses copiously through cracks in the A1203 film into the glass slide underneath. 

Additional evidence was obtained from Auger surface analysis of the Pt-sapphire specimens. Peak-to-peak heights of the lead line at 94 eV were taken from six spots on the Pt-covered area and from six spots on the adjoining sapphire area after a 24-hr exposure to pulsator exhaust. There was, after correction of the 94-eV line for backscattered electrons, more lead on the Pt-covered area in five out of six comparisons. On the average there was a 50% higher lead content on the Pt-covered areas. At lower lead exposures the differences are difficult to discern, since the accumulation of the ubiquitous particulate lead is not expected to vary from one kind of surface to another. Such behavior is essentially in accord with the findings of Williams and Baron (95). At present, however, we still must consider the preferential association of lead with Pt or Pd only as eminently plausible but in need of further, still more direct confirmation. 

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The future work on the poisoning of automotive catalysts will have to deal, primarily, with the specific interaction of particular poisons with the active components. The present trend toward more complex catalytic systems, containing several active components, will make the task still more difficult. One could foresee the use of modem, more sophisticated methods of surface analysis for studying the interactions between poison and active components. 

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