Structural and catalytic insights into Pd-UiO-67 frameworks for CO2 hydrogenation to methanol

Abstract

This study investigates the catalytic performance of palladium nanoparticles supported on UiO-67, a zirconium-based metal–organic framework (MOF), for CO₂ hydrogenation to methanol, emphasising the influence of the size and location of Pd particles in relation to the MOF matrix. Depending on the synthesis conditions, Pd particles were either supported on the outer surface of the MOF, forming larger nanoparticles (∼11–18 nm), or embedded within the MOF pores as smaller particles (∼1 nm), with their size constrained by the host framework. Advanced characterisation techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM), coupled with catalytic testing, revealed that Pd clusters embedded within the MOF exhibited higher CO₂ conversion and methanol selectivity. This superior performance is attributed not only to the increased surface area-to-volume ratio of the smaller Pd clusters, but also to the enlarged metal–MOF interface, which promotes favourable electronic interactions and enhances the accessibility of active sites. Notably, the confined Pd clusters suppressed methane formation, producing CO as the sole by-product. Despite local distortions at elevated temperatures, the UiO-67 framework maintained its structural integrity under reaction conditions, highlighting its thermal and chemical robustness. These findings deepen the understanding of structure–activity relationships in MOF-based catalysts and underscore the critical role of precise control over metal dispersion and metal-support interfaces in optimising catalytic efficiency and selectivity for CO₂ hydrogenation.