Synthesis of Oxide Nanocavities for Size-Selective Catalysis

Enzymes have been the inspiration for catalyst design and are known for their reactant and product selectivity. For reactions not able to be carried out in zeolites or other microporous solids, translating this size- or shape-selectivity onto heterogeneous catalysts is a challenge. In this study, we present first results on a route to incorporating size selectivity in an arbitrary oxide catalyst. This approach involves templated atomic layer deposition (ALD) on the surface of an existing catalyst, where bulky molecules such as p-tertbutylcalix[4]arene or adamantanecarboxylic acid are immobilized on the oxide catalyst surface and are used as templating agents. ALD's highly conformal coverage and angstrom-level control over film thickness is ideal for this application. After typically depositing less than 1.5 nm of an inert barrier oxide, the subsequent removal of the molecular template generates nanoscale cavities in the ALD layers, which we call nanocavities. Unlike previously reported templated ALD where generated features are hundreds of nanometers to microns in dimension, this templating process generates features that are 1-2 nm or less, the range where size- and shape- selective catalysis can be carried out. The catalysts are characterized throughout the synthesis process using DRUV-vis, TGA, and N₂ physisorption. The existence of the nanocavities is demonstrated by QCM studies during synthesis, SAXS and TEM. Size selectivity of the catalysts is demonstrated through selective photooxidation of benzyl alcohol, 1-hexanol and 2-adamantanol over what is normally a non-selective titania photocatalyst. Results showed that 2-adamantanol accesses relatively little of the titania surface when nanocavities deeper than ~0.5 nm are in place. In contrast, benzyl alcohol and 1-hexanol are still relatively reactive with these nanocavities, allowing for selective reaction of mixtures. This selective photocatalytic oxidation of alcohols demonstrates how nanocavities can discriminate size and opens up possibilities in designing multifunctional, all-oxide catalysts with properties that begin to mimic enzymes, or bulk oxide catalysts with zeolite-like selectivity.