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.
Bibliographical noteFunding Information:
We gratefully acknowledge financial support from the National Science Foundation (Grant DMR-1334928). The Clean Catalysis Facility of the Northwestern University Center for Catalysis and Surface Science is supported by a grant from the DOE (DE-SC0001329). The CleanCat Core facility acknowledges funding from the U.S. Department of Energy (Grant DE-FG02-03ER15457) used for the purchase of the Altamira AMI-200. DFT calculations were made possible by the NERSC computational resources of the U.S. Department of Energy (DOE). This work made use of the J.B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (Grant DMR-1121262) at the Materials Research Center of Northwestern University. This work made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (Grant NSF NNCI-1542205); the MRSEC program (Grant NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois through the IIN. Metal analysis was performed at the Northwestern University Quantitative Bioelement Imaging Center. C.L.W. would also like to thank Stephanie Kwon for SBA-15 synthesis, David Chen for his assistance in MOF characterization, Neil Schweitzer for helpful discussion, and Benjamin Schweitzer for his assistance with DFT calculations.
© 2017 American Chemical Society.