(452a) Comparison of Strong Electrostatic Adsorption and Dry Impregnation of Pt into Alumina Bead Supports

Strong electrostatic adsorption (SEA) is an established and useful method for synthesis of ultra-small monometallic and bimetallic nanoparticles on oxide and carbon supports. However, studies of SEA have been conducted with powdered and not formed supports, so the influence of imbibition and diffusion of protons and metal precursors on the metal distribution in the support structure is unknown.

To study the effect of mass transfer of protons and metal precursors as a function of the alumina support pore distribution, six types of formed alumina were selected with different surface areas and microporous or mesoporous pore size distributions. These were impregnated with acidic solutions with and without Pt hexachloride metal precursors to measure the transients in proton and metal uptake equilibria. XRF was used to characterize the degree of penetration of Pt into the spheres as a function of time.

The time required to achieve uniform metal profiles was remarkably long – requiring days of contact for 3 mm alumina spheres. As expected, mesoporous aluminas equilibrated much quicker than microporous aluminas, but still required many hours for uniform metal distributions.

To study the effect of the dry impregnation (DI) method on Pt catalyst particle size and distribution in the support structure, DI was also applied to the set of alumina beads. The concentration profiles showed that uniform distributions of Pt were immediately obtained. However, XRD measurements showed larger particle size from DI compared to SEA as result of stronger adsorption of metal catalyst to the alumina surface in SEA. Thus there is a tradeoff between 1) short contact time, which can be used with DI to give uniform metal profiles with lower metal dispersion, or eggshell metal profiles with SEA with lower metal loadings and high metal dispersion, or 2) high contact time for high metal dispersion with uniform metal profiles.