Activated Carbon Supported Palladium Nanoparticle Catalysts Synthesized by Atomic Layer Deposition: Genesis and Evolution of Nanoparticles and Tuning the Particle Size

We report a novel method of synthesizing uniform Pd nanoparticle catalysts with tunable particle size on activated carbon (AC) supports. Pd is deposited on AC supports by atomic layer deposition (ALD) using Pd hexafluoroacetylacetonate and HCOH as the precursors. The ALD surface reaction becomes saturated with long precursor exposures. The saturated Pd loading is roughly linear to the number of ALD cycle. The increased number of ALD cycles leads to growth in particle density and particle size. Modifying the surface chemical state of the AC support provides an alternative method of controlling the size of Pd nanoparticles. Acid treatment and high temperature calcination are applied to the AC to adjust the type and concentration of surface functional groups. In order to investigate the genesis and evolution of Pd nanoparticles, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and X-ray diffraction characterizations are carried out after each step of Pd ALD on the AC supports undergoing different treatments. The characterization results reveal that the formation of Pd nanoparticles on AC supports goes through two different phases: tiny Pd nanoparticles are first generated during the metal precursor exposure; the following HCOH reduction could cause further growth or agglomeration of Pd nanoparticles. The surface functional groups play different roles at the different phases of Pd ALD: they promote dispersion of Pd species during the metal precursor exposure; on the other hand, instability of -COOH in the reducing atmosphere could facilitate agglomeration of metal nanoparticles. It is concluded that on AC supports nanoparticles in a specific size range can be synthesized through proper support treatment combined with controlling the number of Pd ALD cycle. The activities of the ALD Pd/AC catalysts with different Pd particle sizes are evaluated in hydrogenation of nitrobenzene. The turnover frequencies of the medium (4-5 nm) and large (>6 nm) Pd nanoparticles are comparable and are nearly 2x higher than that of the smaller (<3 nm) Pd nanoparticles. The most probable reason for the depressed activity of the smaller Pd nanoparticles is the steric restrictions resulting from the porous structure of the AC support.