(196g) Polymer-Nanocrystal Hybrid Catalysts Demonstrate Control of Transition State and Product Diffusion

Polymer-Nanocrystal Hybrid
Catalysts Demonstrate Control of Transition State and Product Diffusion

Andrew Riscoe , Cody Wrasman
, Andrew Herzing , Adam Hoffman , Aditya Menon , Alexey Boubnov , Maria Vargas
, Simon Bare, Matteo Cargnello

Nearly all industrial chemical transformations involve a
catalyst.  The most effective catalysts deliver accelerated rates and complete
selectivity towards desired reaction products.  Enzymes are examples of near
perfect catalysts which use specifically tailored binding pockets with
complementary shape and functionality to their specific substrate so that desired
reaction transition states have maximum stability and undesired product
pathways have highly energetically unfavorable transition states.  Most of
these binding pockets are on the order of the size of a small molecule on the
order of 5-10 Å. Surrounding
these active sites are pathways shaped/ functionalized to promote transport of
desired species and stop unwanted transport.  In comparison, most
industrial heterogeneous catalysts are composed of metallic surfaces where reactants
and intermediates bind and catalytic rates and selectivities are determined by
relative binding strengths of intermediates and transition states.  The
transport of species to these exposed surfaces is uncontrolled as well, mostly
governed by macroscopic factors of reactor construction.

In this work we aim to export some of the control used in
enzymes to heterogeneous catalyst’s extended metal surfaces, namely a confined
binding pocket at the catalytically active site and control over a transport of
species.  We use microporous Porous Organic Frameworks (POFs) to encapsulate
colloidally synthesized transition metal nanocrystals.  We introduce POF with
additional functionality, demonstrate that these POFs are thermally stable in heterogeneous
catalysis conditions above 250C, maintain their microporous characteristics when
included in hybrid metal-POF catalysts, and are completely encapsulating metal nanocrystals.
In a probe reaction of CO oxidation with POF-Palladium hybrid catalysts, we
note that the apparent transition state of the reaction is altered when the
chemistry of the encapsulating POF layer is altered which we attribute to the
presence of altered functionality at the active binding pocket.  We also
observe the appearance of oscillations in reaction rates under specific
conditions with specific POF overlayers.  We correlate the adsorption properties
of the POFs with the presence of the oscillations in a surface diffusion
model.   We support our conclusions with catalytic measurements, TEM, electron
tomography, XPS, XAS and adsorption measurements.