Gas Phase Routes to Solid Catalyst Materials

Peter C. Stair, PhD
Northwestern University

Seminar Abstract: The traditional route to solid catalyst materials involves solution phase deposition or liquid-surface reactions. Examples include impregnation, deposition-precipitation, and solution phase grafting of molecular precursors. Atomic Layer Deposition (ALD) is a gas-solid deposition methodology having enormous potential for the synthesis of advanced heterogeneous catalysts with control of composition and structure at the atomic scale. The ability of ALD to produce conformal oxide coatings on porous, high-surface area materials can provide completely new types of catalyst supports. At the same time ALD can achieve highly uniform catalytically active metal and oxide phases with (sub-) nanometer dimensions. This lecture will provide examples from the laboratories at Northwestern University and Argonne National Laboratory of ALD used to synthesize oxide supports, catalytic oxide overlayers, single-site catalysts, metal nanoparticles, and new porous structures. These materials have been characterized by SEM, XRF, ICP, UV-Vis absorption
spectroscopy, Raman spectroscopy and evaluated for a variety of catalytic reactions including oxidative and non-oxidative alkane dehydrogenation, combustion, and selective hydrogenation. A particular focus has been the synthesis of supported metal nanoparticles with the development of a procedure in which the metal and support materials are grown sequentially in each ALD cycle. This method makes possible the synthesis of exceptionally small, ca. 0.5 nm, and uniform metal nanoparticles. Using additional ALD support layers at the conclusion of the growth, a process we call overcoating, the metal particles can be stabilized against sintering and leaching while still remaining active under harsh conditions in both gas and liquid phase reactions. Through proper annealing procedures the overcoating oxide develops porosity, thus ALD becomes a method for introducing and controlling pore structures. With appropriate choice of precursors single-site catalysts have been prepared that afford non-traditional entry into the catalytic cycle. With templated ALD it is possible to prepare surface cavities, which we call nanobowls. These structures exhibit size selectivity, where large molecules are excluded from interacting with the catalyst material. The lecture will conclude with a perspective on what may be possible with additional advances in ALD methodology and synthesis strategies.