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Carbon-supported binary PtRu electrocatalysts were prepared by coimpregnation using ethanolic solutions of Pt(NH 3 ) 2 (NO 2 ) 2 as the Pt source, various Ru sources [RuCl 3 , Ru 3 (CO) 12 , and RuNO(NO 3 ) x ], and carbon black by thermal decomposition under reducing conditions, and their structure, morphology, and electrocatalytic properties were investigated. X-ray diffraction analysis and high resolution scanning electron microscopy indicated that the use of Cl-free Ru sources, i.e., Ru 3 (CO) 12 or RuNO(NO 3 ) x , afforded highly dispersed and uniform PtRu nanoparticles. Surface area measurements conducted by electro-oxidation of preadsorbed carbon monoxide indicated that the use of Ru 3 (CO) 12 as the Ru source yielded high surface area catalysts. In terms of the surfacearea specific current density (current density normalized by the specific surface area of PtRu metal obtained from preadsorbed CO electro-oxidation measurements), the electrocatalytic activity of Pt(NH 3 ) 2 (NO 2 ) 2 -Ru 3 (CO) 12 and Pt(NH 3 ) 2 (NO 2 ) 2 -RuNO(NO 3 ) x were equal. PtRu/C electrocatalysts prepared from ethanolic solutions of Pt(NH 3 ) 2 (NO 2 ) 2 -Ru 3 (CO) 12 resulted in high mass-specific activity toward methanol oxidation, with mass-specific current density as high as 159 mA mg Ϫ1 Pt at 500 mV. The efficiency of PtRu/C electrodes is discussed based on the significance of the use of Cl-free Ru sources.

Steady-state quadrupole shape oscillations of air bubbles trapped in water were excited by amplitude modulation of the acoustic radiation pressure used for levitation. This method of exciting controlled shape oscillations may make possible noncontact dynamical measurements of the rheological properties of bubbles. Bubble sizes ranged from 1.6- to 12-mm diameter corresponding to observed quadrupole mode frequenices of 190 to 17 Hz. Small-amplitude oscillations were detected by interference of scattered laser light. Some larger amplitude oscillations were detected by the unaided eye or with a television camera. The structure of the acoustic field in the levitator needed for the levitation of large bubbles is discussed. In the absence of modulation the levitated bubbles had an oblate shape.

In the case of radiography of a cylindrically symmetric object, the Abel transform is useful for describing the tomographic measurement operator. The inverse of this operator is unbounded, so regularization is required for the computation of satisfactory inversions. We introduce the use of the total variation seminorm for this purpose, and prove the existence and uniqueness of solutions of the corresponding variational problem. We illustrate the effectiveness of the total-variation regularization with an example and comparison with the unregularized inverse and the H 1 regularized inverse.