State of dispersion of platinum in alumina-supported catalysts

Abstract

A study of the effects of various heat treatments on the state of dispersion of platinum in alumina-supported catalysts has shown that during heat treatments in hydrogen and air the hydrogen-chemisorption capacity (expressed as the H Pt ratio) of the catalysts decreases. The mechanism by which this occurs, however, is not the same in the two cases. During heat treatment in an oxygen-containing atmosphere above 500 °C agglomeration of platinum takes place. This results in a bimodal size distribution of Pt crystallites: Besides well-dispersed platinum (crystallites below 3 nm), a fraction of particles of about 10–15 nm is formed. There are practically no particles of intermediate size. A carefully controlled oxidation at low temperature (below 500 °C) is not capable of redispersing the “big” crystallites. During heat treatment in hydrogen hardly any agglomeration of platinum takes place. However, a certain amount of platinum becomes “inaccessible” and causes the observed decrease in the H Pt ratio. The actual mechanism by which this occurs still needs further investigation. Oxidation at a carefully controlled, low temperature (below about 500 °C) restores the original contribution to the H Pt value of the platinum fraction which has become “inaccessible” as a result of heat treatment in a hydrogen atmosphere. These results suggest that it is generally incorrect to derive the average platinum crystallite size solely from H 2-chemisorption data: the existence of a direct link between “decrease in H Pt ratio” and “sintering” is often highly debatable. For this reason the conclusions of various published studies are suspect.