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Single crystal α-iron whiskers have been used to study early stages in the oxidation of iron at high temperatures. Well defined oxide patterns are observed on clean iron surfaces and there are characteristic differences between the patterns on {100} and {110} faces. These differences tend to be eliminated if a natural oxide film is present before the high temperature oxidation is performed. Unusual mosaic oxide patterns which form on clean {100} iron surfaces are shown.

The activity of single crystal lanthanum lead manganite (La0.7Pbo.3Mn03) was compared to Pt/Al203, Co304, and CuO under conditions important to automotive emission control. In simulated automotive exhaust ambients, the observed order of activity of fresh catalyts toward both CO and C3H6 oxidation was found in general to be: Pt/Al203 > Co304 > La0.7Pbo.3Mn03 > CuO. Pt/Al203 remained the most active of the materials throughout tests In which the susceptibility of catalysts to various modes of deactivation, namely, high temperature exposure, S02 poisoning and reaction with -203, was measured. The results for the single crystal manganite catalyst showed it to be susceptible to thermal deactivation but resistant to S02 poisoning. Manganite was also found to react extensively with • 203 at 1000°C suggesting potential difficulties with supported catalysts.Basic differences were found between the properties of single crystal and polycrystalline forms of La0.7Pbo.3Mn03. They showed different oxidation kinetics and the specific catalytic activity of the single crystals was found to be roughly 100 times higher than the polycrystalline materials. The data suggest that the enhanced activity might be due to the presence of small quantities of Pt. In addition to single crystal LaojPbo.sMnOs and Pt/ AI2O3, two base metal catalysts, C03O4 and CuO, were included in the study to broaden the scope by which the activity of manganite catalysts could be assessed. The activity of polycrystalline Lao^Pbo.sMnOs materials was also investigated. Experimental SectionMaterials. A total of four manganite samples were investigated. These included two samples of crushed Lao.7Pbo.3Mn03 single crystals (37-250 µ ) which were grown from a lead borate flux in a Pt crucible and subsequently etched 3-5 min in hot (80°C) 20% nitric acid (Voorhoeve et al., 1973). The two samples were prepared at different laboratories. The first (sample A) was obtained from J. P. Remeika (Bell Telephone Laboratories), and the second (sample B) was prepared by the authors. The remaining samples were polycrystalline. Sample C was prepared from coprecipitated hydroxides which were calcined in A1203 crucibles for 24 hr at 600°C and etched 5 min in hot (50°C) 15% nitric acid (Voorhoeve et al., 1973). Testing was conducted on particles in the 149-250-µ range. Samples A-C were found by X-ray diffraction analysis to be single phase material with well defined perovskite structures.