Investigation of Dynamic Material Changes During the Preparation of ZnPd Nanoparticles Supported on ZnO and their Catalytic Application in Methanol Steam Reforming onthe Atomic Level

Given its high energy density and sustainability, hydrogen is regarded as a crucial energy carrier in the pursuit of a carbon-free energy economy. In the search for a storage medium of hydrogen, methanol is emerging as a promising chemical storage. The recovery of hydrogen is achieved through the process of methanol steam reforming, whereby methanol and water are transformed into hydrogen and carbon dioxide. The intermetallic ZnPd nanoparticle supported on ZnO has been demonstrated to function as an excellent catalyst for the reforming, due to its high CO₂ selectivity and activity. However, the favourable catalytic performance is first established during catalysis. The reason for the enhancement of the catalytic properties appears to be a dynamic structural evolution of the catalyst, as evidenced by the formation of ZnO patches. The nanostructural dynamics and the underlying cause of the improved catalytic properties can be elucidated by analysing the synthesis and catalysis of the catalytic system in situ. The comprehensive work studies the preparation, structural evolution, and catalytic application of ZnPd nanoparticles supported on ZnO during methanol steam reforming (MSR), using in situ scanning transmission electron microscopy (STEM). The preparation stages, including calcination of supported palladium nitrate and reduction of palladium oxide, were analysed. In situ calcination revealed that palladium nitrate transforms into palladium oxide at ~170 °C, leading to nanoparticle growth, with a stable size window between 200-400 °C. At temperatures above 460 °C, PdO decomposes into elemental palladium, triggering particle mobility, agglomeration, and ZnO nanorod formation. Further heating above 660 °C under high vacuum caused ZnO faceting and decomposition, which was facilitated by the evaporation of elemental zinc and oxygen. In situ reduction of supported PdO resulted in the formation of intermetallic ZnPd via two distinct formation mechanisms: hydrogen spillover-induced ZnO migration and encapsulation of Pd nanoparticles, followed by ZnPd nucleation and core-shell structure formation. The findings align with those of ex situ experiments and existing literature, confirming that the electron beam enhances the reaction but does not activate it. In situ STEM experiments in open and closed cell configurations demonstrated that methanol acts as a strong reducing agent for ZnO, while hydrogen and water steam stabilise the system. Under MSR conditions, ZnPd nanoparticles are subject to compositional and morphological changes, including Zn enrichment and nanoparticle faceting. The formation of ZnO patches, which was observed for the first time in situ, was found to preferentially occur on ZnPd facets and Znenriched areas. This formation requires a precise balance between hydrogen, water, and methanol and thus is sensitively dependent on the chemical potential. The research provides novel insights into the dynamic behaviour of ZnPd/ZnO catalysts under operational conditions, advancing the methodology for studying catalysts in steam environments and contributing to the broader understanding of catalytic systems.