Events observed and evidence for crystallite migration in [formula omitted] catalysts

Abstract

The sintering behavior of model Pt Al 2O 3 catalysts when heated in hydrogen, and alternately in H 2 and O 2 was investigated at different temperatures (between 300 and 800 °C) by means of transmission electron microscopy. At both relatively low (500 °C) and high temperatures (750 °C), shortdistance migration (usually 1–3 particle diameters) and coalescence of particles was observed to contribute considerably toward sintering. At 500 °C and following two or three cycles of alternate heating in H 2 and O 2, significant migration of particles (up to 8-nm particles migrating over 25 nm) were observed on exposure of the oxidized particles to H 2 and/or of the reduced particles to O 2. Sintering is fast and pronounced in the initial 4 to 6 h of heating of a fresh sample; then it becomes slow. However, further continuous heating for extended periods of 12 h or more causes additional significant sintering by particle migration and coalescence among other mechanisms. This suggests that investigations based on only 1 or 2 h of heating, as often reported in the literature, yield incomplete information. The present results indicate that at both 500 and 750 °C, sintering by ripening (apparently only of a localized kind) also occurs. The present results are evidence of the occurrence of a variety of phenomena such as short-distance migration and coalescence of particles, migration toward or away from another particle, decrease in size and/or disappearance of larger particles near unaffected smaller particles, decrease in size and/or disappearance of both small and larger particles, decrease in size and/or disappearance of small particles near larger particles (ripening), decrease in size and subsequent migration of particles or vice versa, collision without coalescence, and collision-coalescence-separation into two particles again. The particles seem to feel the presence of other nearby particles via long-range interparticle forces that induce the migration of a particle toward another or emission of atoms toward a nearby particle. The two major mechanisms of sintering of supported metal crystallites appear to be (i) short-distance, direction-selective (in contrast to random) migration of particles followed by (a) collision and coalescence or (b) direct transfer of atoms between the two approaching particles, or (ii) localized ripening (direct ripening) between a few stationary, adjacent particles.