Precise Control of Cu Nanoparticle Size and Catalytic Activity through Pore Templating in Zr Metal-Organic Frameworks

Composite materials composed of nanoparticles trapped within metal-organic frameworks (MOFs) combine the broad functionality of nanotechnology with the structural regularity of crystalline scaffolds. Still, leveraging the tunability of MOF pore sizes to control nanoparticle diameter and spatial arrangement in these composites remains a great challenge. Here we present two Zr-based MOFs, NU-901 and NU-907, with distinct pore diameters that serve as templates for the controlled growth of Cu nanoparticles (CuNPs) of different sizes (∼1.5 nm and ∼0.9 nm, respectively). In situ synchrotron X-ray scattering and diffraction experiments, along with pair distribution function and difference envelope density analyses, provide crucial insight into the size and location of these CuNPs in the pores of each MOF. These composites (denoted as CuNPs@NU-901 and CuNPs@NU-907) are shown to be competent catalysts for the selective hydrogenation of acetylene to ethylene, with a clear structure-property relationship indicating that larger CuNPs exhibit higher activity than smaller particles. This counterintuitive trend is further explored using density functional theory calculations of transition state energies to understand the role of CuNP structure on catalytic functionality. The calculations show that the activation energy for semihydrogenation is higher for a Cu cluster of finite size than for a Cu surface. This work demonstrates the utility of templated nanoparticle growth within MOF pores as a general strategy to achieve precise control over the composite structure and functionality.