Effect of number and properties of specific sites on alumina surfaces for Pt-Al2O3 catalysts

In this work, how the number and properties of specific sites on alumina surfaces affect the specific interaction between Pt and alumina was investigated by using X-ray diffraction, ethanol temperature programmed desorption, diffuse reflectance infrared Fourier transform spectroscopy, H-2 chemisorption, scanning transmission electron microscopy and benzene hydrogenation reaction. Here, we chose two sets of model aluminas having different number of sites with the identical properties and different properties of sites with the same number based on ethanol TPD. The H-2 chemisorption results for the model aluminas show that H/Pt are all similar for low Pt loadings, but significantly different for high Pt loadings. For 1 wt% Pt/Al2O3, the number of specific sites on all the aluminas was sufficient to disperse all the Pt, leading to only highly dispersed Pt clusters (similar to 1 nm). However, at 10 wt% Pt/Al2O3, the number of Pt atoms is greater than that of the specific sites on the alumina surface, resulting in a bimodal distribution of large agglomerated Pt (> 10 nm) and highly dispersed Pt clusters (< 3 nm) revealed by XRD and TEM. Overall, the results clearly demonstrated that Pt shows higher dispersion with increasing number of sites and interaction strength, because the Pt atoms can interact with specific sites on alumina in greater numbers and more strongly. However, these Pt dispersion changes do not represent the gradual change in Pt cluster sizes, but the relative population change of small (< 3 nm) and large agglomerated Pt clusters (> 10 nm) under bimodal distribution. The number of large agglomerated Pt clusters decreased with increasing number of sites and interaction strength. This fundamental understanding provides an important perspective for designing Al2O3-based supported catalysts.