Work

Resolving the Methanol-Promoting Alloy–Oxide Ensemble in Pd–Zn/C Catalysts for CO2 Hydrogenation

Year: 2026

Type: preprint

Abstract: Pd–Zn bimetallic catalysts are a promising alternative to traditional Cu-based systems for the hydrogenation of CO2 to methanol. Yet a complete understanding of their structure-activity relationships has been hindered by methodological limitations, such as high-temperature synthesis and the use of bulk characterization techniques. Here, we use an inert graphitic carbon support to prepare a Pd–Zn/C catalyst library with systematically varied Zn:Pd ratio and pre-treatment temperature. In situ Pd and Zn K-edge X-ray absorption spectroscopy (XAS), coupled with multivariate curve resolution, resolves the coexisting Pd–Zn and Zn-derived oxide components, including Pd-rich FCC α-PdZny, ordered tetragonal Zn-rich β1-PdZnz, crystalline ZnO, and disordered ZnOx. The components identified by XAS provide a reference database for the detailed chemical speciation across the catalyst library. In situ XAS showed that catalysts pre-treated in H2 at 250-450 °C do not restructure during catalysis, and therefore it is sufficient to analyze their structure before catalysis to uncover structure-activity correlations. Combined with kinetic measurements and statistical correlation analysis, this framework identifies the structural motifs associated with methanol and CO formation. Methanol productivity is linked to the coexistence of ordered tetragonal β1-PdZnz (z > 1) and disordered ZnOx, whereas CO forms across a broader range of Pd-containing bimetallic environments. In situ restructuring experiments for the catalysts pre-treated at mild conditions (50 °C) further show that the hydride-forming FCC α-PdZny phase acts as a precursor that can evolve toward the more methanol-selective β1-phase to form an active interface under catalytic conditions in the presence of Zn oxide. These results resolve the methanol-promoting alloy–oxide ensemble in Pd–Zn catalysts and show that selective methanol synthesis depends on stabilizing the appropriate local oxidic environment at the alloy boundary, rather than simply maximizing Zn loading or alloy fraction.

Source: ChemRxiv

DOI: 10.26434/chemrxiv.15010759/v1