Confinement of subnanometric PdZn at a defect enriched ZnO/ZIF-8 interface for efficient and selective CO2 hydrogenation to methanol

Abstract

Methanol synthesis from CO₂ hydrogenation is an important reaction for future fuel and chemical production as the so-called “methanol economy” in the 21st century. Pd–ZnO catalysts are known to be active for this reaction, but the fabrication of ultrasmall PdZn alloy nanoparticles (NPs) on an oxygen defective ZnO surface as selective and efficient catalysts remains a challenge. Here, we report an epitaxial growth of a thin zeolitic imidazolate framework-8 (ZIF-8) overcoat on ZnO nanorods as a support to confine the synthesis of subnanometric Pd NPs at a ZnO/ZIF-8 interface for CO₂ hydrogenation. The thickness of the ZIF-8 overcoat can be tuned by the etching time and the etching process enables the creation of oxygen defects on the ZnO surface. It is also found that increasing the ZIF-8 thickness disfavors the formation of the PdZn alloy upon H₂ reduction due to the spatial isolation effect. In CO₂ hydrogenation, the optimized Pd-Z@Z8-1 catalyst shows the best performance, with a methanol selectivity of 66–78% and a methanol yield of 12.1–19.8 g g_Pd⁻¹ h⁻¹ at 523–563 K and 4.5 MPa, which are ranked among the top values over Pd based catalysts reported in the literature under comparable conditions. Interestingly, we obtain a volcanic relationship between the methanol yield and ZIF-8 thickness. It thus appears to require a proper balance to optimize the activity between decreasing PdZn alloy sites and increasing ZnO defects with the increase in ZIF-8 thickness. From this study, we propose a synergetic catalysis mechanism between the PdZn alloy and surface defect enriched ZnO. It is believed that this work provides a novel “killing two birds with one stone” strategy to fabricate noble metal–metal oxide/MOF hybrid materials for catalytic applications.