(342d) 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.