Synthesis of Oxide ‘Nanobowls' and ‘Armor-Coated' Active Sites by Templated ALD: a New Paradigm In Heterogeneous Catalyst Synthesis

One of the most important goals in heterogeneous catalyst development is to increase selectivity and stability. Zeolites have long been used to impart selectivity based on size discrimination, and they are efficient at maintaining catalyst sites dispersed and active. Here, we utilize atomic layer deposition (ALD), which has been used for years in microelectronics fabrication but only recently applied to heterogeneous catalysts synthesis, to grow thin oxide layers perforated with cavities over an existing oxide catalyst. We call these nanoscale cavities (<2 nm deep, <2 nm in diameter) ‘nanobowls'. Their presence introduces a size-sieving feature to an existing catalyst surface, and they can ‘armor-coat' individual active sites on a surface. In this study, the nanobowls are generated by using a molecular template immobilized on the catalyst surface before ALD. Bulky template molecules such as p-tertbutylcalix[4]arene and adamantanecarboxylic acid are ideal for this purpose. They are deposited on oxide supports and a wall of oxide is grown around the immobilized template molecule by ALD. Template removal generates the nanobowl. For the case of ‘armor-coated' catalyst sites, the template is both the metal-containing catalyst precursor and the structure directing agent. Nanobowl synthesis is monitored by DRUV-vis, TGA and N₂ physisorption, while the existence of the nanobowls is elucidated by in-situ QCM studies, small angle x-ray scattering (SAXS) and transmission electron microscopy. Size selectivity of the catalysts is demonstrated through selective photooxidation of benzyl alcohol, 1-hexanol and 2-adamantanol over a reference, though traditionally non-selective titania photocatalyst. For ALD films more than a few tenths of a nanometer thick, bulky secondary alcohols like 2-adamantanol are able to access fewer active sites on the titania surface than do terminal alcohols like benzyl alcohol and 1-hexanol. Thus, the relative rate of alcohol oxidation, or conversion after a set time, is a strong function of the nanobowl depth. This selective photocatalytic oxidation of alcohols demonstrates how nanobowls can discriminate size on an arbitrary catalyst surface and introduces a new paradigm in heterogeneous catalyst synthesis.