Rationalizing the Onset and Extent of β-PdZn Formation as a Function of Pd:Zn Ratio and Its Effect on Methanol Yield for CO2 Hydrogenation Studied under In Situ and Operando Conditions

Abstract

Direct hydrogenation of CO2 to methanol offers a route to low-carbon liquid fuels, and Pd/ZnO/TiO2 catalysts containing PdZn alloy nanoparticles have emerged as promising alternatives to conventional Cu-based systems. However, the structural evolution of PdZn phases during activation and reaction remains poorly understood. In this work, combined in situ and operando synchrotron X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are used to clarify the formation, stability, and catalytic relevance of β-PdZn as a function of the Pd:Zn ratio. The results show that β-PdZn formation is strongly dependent on Zn content, with the onset temperature decreasing from ∼400 °C for Pd1Zn1/TiO2 to ∼330 °C for Zn-rich formulations. While alloy crystallite sizes remain relatively small (3–7 nm), increasing Zn in combination with using less TiO2 leads to substantial growth of ZnO crystallites, reducing surface area. Operando XAS/XRD confirms that β-PdZn remains structurally stable under reaction conditions, attributed to the solvent-free synthesis method that prevents particle aggregation. Catalytic testing reveals that optimal methanol productivity correlates with moderate β-PdZn contents (4–6 wt %) and alloy nanoparticle sizes near 7 nm. Using lower Zn levels leaves residual metallic Pd, increasing methane formation, whereas excessive Zn diminishes the alloy–ZnO interface, essential for CO2 hydrogenation. These insights provide a structural basis for tailoring PdZn catalysts for efficient CO2-to-methanol conversion.