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.封装于金属有机框架 (metal–organic frameworks, MOFs) 内的金属纳米粒子复合材料（NP@MOFs）已作为区域选择性反应的多相催化剂崭露头角。尽管已合成诸多NP@MOF复合体系，但纳米粒子-金属有机框架界面与封装纳米粒子表面的表征仍有待完成。本研究选用通过可控封装法合成的Pt@ZIF-8作为代表性NP@MOF体系，并结合原位表征方法与密度泛函理论 (density functional theory, DFT) 计算，对纳米粒子表面进行探究。CO吸附漫反射傅里叶变换红外光谱 (CO adsorption diffuse reflectance infrared Fourier transform spectroscopy, DRIFTS) 结果显示，Pt@ZIF-8表现出红移的线性吸附与桥连吸附CO特征峰，以及与阳离子Pt相关的线性吸附峰。密度泛函理论计算与核磁共振氢谱 (proton nuclear magnetic resonance, ¹H NMR) 结果表明，这些位点源于金属有机框架配体2-甲基咪唑与Pt表面的结合及电子捐赠作用。氩气氛围下的DRIFTS测试显示，Pt纳米粒子表面可能存在配体碎片，这表明在可控封装合成过程中，纳米粒子与金属有机框架配体发生了反应。最后，通过DRIFTS开展的CO氧化实验表明，红移的线性吸附CO与桥连吸附CO位点为活性位点，而阳离子Pt位点则不具备催化活性。综上，本研究结果证实Pt@ZIF-8存在独特的Pt表面位点，并表明纳米粒子-金属有机框架界面存在多组分混合体系，包括框架结合的2-甲基咪唑、游离态2-甲基咪唑以及配体碎片。这些研究结果揭示了NP@MOF复合材料中纳米粒子表面的复杂特性，并证实了对其表面进行表征以理解催化行为的重要性。