Journal of Colloid and Interface Science, March 1982 [antikvár]

Wetting Phenomena during Alternating Heating in O2 and H2 of Supported Metal Crystallites E. RUCKENSTEIN' and J. J. CHEN Department of Chemical Engineering, State University of New York, Buffalo, New York 14260 Received February 16, 1981; accepted June 1, 1981 Specimens of Pd crystallites (20 to 200 A in size) supported on thin alumina films have been heated alternately in O2 and H2. The changes in size, shape, and position of each crystallite after each heat treatment have been followed by examining the same areas of the specimens with a transmission electron microscope. During heating in O2, most of the crystallites extended by spreading on the alumina surface and became irregular in shape. Pits and cavities developed on many of the crystallites. During heating in Hj, most of the crystallites which extended during the previous heating in O2 contracted to a more circular form. Some large crystallites, however, grew in size. Some small crystallites disappeared during heating in O2 or H2. The extension and recon-traction are explained in terms of wetting. Pit formation is explained as either a surface tension gradient driven phenomenon enhanced by a spreading-produced stress, and/or as a crack propagation phenomenon induced by the spreading-generated tensile stress and enhanced by the oxidation of the freshly exposed atoms at the tip of the crack. INTRODUCTION The behavior of AI2O3 supported Pd crystallites during heating in hydrogen or oxygen has been investigated and reported recently by the authors (1,2). Severe sintering of the Pd crystallites was found to occur during heating in H2 above 650°C. During heating in O2, the formation of pits on the crystallites, the coalescence of these pits into cavities, the extension by spreading of the oxidized crystalHtes on the alumina surface and fragmentation induced by spreading have been observed. The scope of the present paper is to examine the behavior of Pd crystallites (20 to 200 A in initial size) supported on alumina, under heat treatment in alternating oxidizing and reducing atmospheres. These experiments have been carried out because the alternation between the two extreme conditions can better emphasize the phenomena which occur in each of them and ' To whom correspondence should be addressed. the effect of the chemical atmosphere. Palladium crystallites supported on thin alumina films have been heated alternately in 1 atm O2 at 750°C and 1 atm H2 at 550 or 600°C, for various time intervals. A JEOL lOOU transmission electron microscope was used to examine the same areas of the specimens after each treatment in order to follow the changes in size, shape, and position of each crystallite. A variety of events has been identified during the experiments. Most Pd crystallites expanded by spreading on the alumina surface during heating in O2 and recontracted during heating in H2. Some large crystallites grew in size during heating in H2. During heating in either O2 or H2, some small crystallites disappeared. Pits formed on the crystallites during heating in O2 and disappeared during heating in H2. While extensive literature is available concerning the sintering of supported metal crystallites and its mechanisms [some references are (3-17)], no evidence seems to be available on the spreading of the crystal- Joumal of Colloid and Interface Science, Vol. 86, No. 1, March 1982 0021-9797/82/030001-ll$02,00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.