(63a) Atomic Layer Deposition for the Synthesis of Nanostructured Catalysts

Atomic layer deposition (ALD) is a thin film growth technique that uses cycles of alternating, self-limiting chemical reactions between gaseous precursors and a solid surface to deposit material in an atomic layer-by-layer fashion. Saturation of the individual ALD surface reactions and gaseous diffusion of the chemical precursors ensure that all exposed surfaces of a template become uniformly coated. These attributes, coupled with a broad palate of available materials, make ALD a versatile technique for synthesizing catalysts. Conceptually, ALD can be used to build a catalyst as a series of discrete layers which perform specific functions such as tuning the porosity, serving as the catalyst support, providing catalytic activity, and imparting thermal stability. This capability allows the physical structure and porosity of a template to be decoupled from the surface chemistry and catalytic properties. Consequently, the same catalyst can be synthesized simultaneously on a variety of templates such as a planar single crystal to facilitate detailed characterization, or a high surface area powder to boost conversion for catalytic testing.

In addition to depositing continuous layers, ALD also enables the synthesis of nanoparticles. Using ALD, the nanoparticle size can be modulated by adjusting the number of cycles performed, and the number of nanoparticles can be controlled by pretreating the surface to change the concentration of surface functional groups on which the ALD initiates. By alternating between different ALD surface chemistries, mixed-metal nanoparticles (e.g. Pt-Pd) can be formed. Furthermore, the layer-by-layer nature of ALD enables specific nanostructures for these mixed-metal nanoparticles such as core/shell (Pd/Pt or Pt/Pd) and well-mixed alloy. Finally, ultrathin ALD metal oxide layers applied after the metal nanoparticle ALD can stabilize the nanoparticles to inhibit sintering at elevated temperatures. Quite remarkably, these metal oxide “overcoats” grow selectively on specific sites on the noble metals and on the support between the nanoparticles such that the catalytic activity is maintained or even enhanced.