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Smooth ultrahigh vacuum-cleaned PtRu alloys used as model catalysts for methanol oxidation, present a continuous loss of activity under potentiostatic conditions. After a potential step, e.g., from 50 to 500 mV vs. reference hydrogen electrode, chronoamperometric curves first show a steep decrease over 5-10 min followed by a slower decrease over several hours. The latter decay exhibits a time Ϫ1 behavior for different catalyst compositions, with higher slopes for the catalysts with less activity. The first current decay can be repeatedly observed by restarting the experiment after setting the potential back to the initial value, thus indicating a certain degree of reversibility. The slow decrease in activity, however, cannot be recovered by this means. But stepping the potential to higher anodic values, e.g., up to 1.2 V, the original surface activity can be obtained again. Optimized porous PtRu surfaces, on the other hand, do not show any comparable decrease after a respective potential step. The possible origin of the different behavior of smooth and porous surfaces is discussed.

The pressureless sintering to near theoretical density of highpurity titanium diboride powder, produced in a plasma arc from halogen vapor reactants, is described. Physical and mechanical property measurements between 25" and 1400°C are reported for materials densified at T= 1800" to 2275°C. Titanium diboride exhibits several unusual and interesting properties, which include high electronic conductivity, a large positive strength-vs-temperature coefficient, and an intrinsic resistance to high-temperature slow crack growth (negative M.Property trends are explained in terms of microstructural development and residual grain-boundary stresses.

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