For catalysts containing supported metal nanoparticles, particle sizes and distribution of sizes are often critical to obtain optimized performance; this is referred to as structure-sensitivity. The concentrations of exposed facets or corner and edge atoms vary as particle sizes change. In fact, the smallest particles consist primarily of corners and edges, while low index basal planes such as (111) and (100) surfaces predominate as particles become larger. Many catalysts, such as those prepared by dry impregnation, give wide size distributions and random spatial separations, making structure-sensitive trends difficult to identify. However, the strong electrostatic adsorption (SEA) method has been shown to produce ultra-small particles with narrow size distributions and uniform densities on support surfaces. By using electroless deposition (ED) to deposit additional metal atoms over existing particles made by SEA, sizes of the monometallic particles can be systematically increased by changing the amount of Pd that has been deposited. For our studies, 1.88 wt% Pd/SiO2 (Aerosil 150) was prepared by SEA with &lt;1.5 nm particles estimated from XRD line broadening (and 3.1 nm from chemisorption). An ED bath for Pd was developed using [Pd(NH3)4]2+ as the Pd precursor, ethylenediamine (EN) as stabilizing (chelating) agent, and dimethylamine borane (DMAB) as reducing agent, all at pH = 6. The [Pd(NH3)4]2+ and DMAB solutions were added separately at 25°C using two motor-driven syringe pumps for fixed periods of time. In this manner the ED bath remained stable and did not undergo thermal reduction and the rate of Pd deposition could be controlled to give more uniform Pd particle sizes. Characterization data, including STEM, XRD, and selective chemisorption will be presented to support particle size arguments.

Precision control of supported palladium nanoparticle sizes via electroless deposition

ACS Fall 2021

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