Small angle X-ray scattering investigation of platinum metal dispersions on alumina catalysts

Abstract

The metal particle size diameters and particle size distributions of platinum supported on alumina catalysts have been investigated using small angle X-ray scattering techniques. The interfering background scattering from the micropores of the alumina support was eliminated by adsorption of small concentrations of CH2I2 and C4H9I, organic liquids of electron density similar to that of the alumina. Although CH2I2 is twice as effective as C4H9I in reducing the pore scattering from the η-alumina support, care must be exercised to prevent an excess of organic material which will cause unwanted scattering. The primary parameters derived from the scattering of X-rays by the platinum crystallites are RG, the Guinier radius, and RP, the Porod radius. These radii are expressed as the ratio of moments of a distribution which is related to σ2 and μ, the variance and geometric mean, respectively, of a log-normal particle size distribution function. Using this simple procedure, we find that both the mean particle diameter and the particle size distribution of an aged 0.62 wt % Pt on alumina catalyst have shifted to larger sizes. In the experimental intensity curves of a 0.35 wt % Pt catalyst and that of the fresh 0.62 wt % Pt catalyst, shoulders are observed that are more or less pronounced. This behavior indicates interparticle scattering effects, suggesting the presence of small clusters of platinum particles approximately 200 Å in diameter before any agglomeration has ocurred.