Elucidating the Nanoparticle–Metal Organic Framework Interface of Pt@ZIF‑8 Catalysts

Composites of metal nanoparticles
encapsulated in metal–organic
frameworks (NP@MOFs) have emerged as heterogeneous catalysts for regioselective
reactions. While numerous NP@MOF composite combinations have been
synthesized, characterization of the nanoparticle–MOF interface
and the encapsulated nanoparticle surface have yet to be determined.
In this work, Pt@ZIF-8 synthesized by the controlled encapsulation
method was chosen as a representative NP@MOF, and in situ characterization
methods coupled with density functional theory (DFT) calculations
were used to probe the nanoparticle surface. CO adsorption diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed
that Pt@ZIF-8 exhibits red-shifted linear- and bridge-bound CO peaks
and a linear peak associated with cationic Pt. DFT calculations and 1H NMR suggest that these sites arise from the binding and
electronic donation of the MOF linker, 2-methylimidazole, to the Pt
surface. DRIFTS under argon reveals that linker fragments may be present
on the Pt nanoparticle surface, suggesting a reaction between the
nanoparticle and the MOF linker during controlled encapsulation synthesis.
Finally, CO oxidation reveals via DRIFTS that the red-shifted linear
CO and bridging CO sites are active sites, while the cationic Pt is
not. Overall, these results show that Pt@ZIF-8 contains unique Pt
surface sites and indicate that the nanoparticle–MOF interface
contains a heterogeneous mixture of framework 2-methylimidazole, free-standing
2-methylimidazole, and linker fragments. These findings expose the
complex nature of the nanoparticle surface in NP@MOF composites and
demonstrate the importance of characterizing their surface to understand
their catalytic behavior.