Bimetallic Palladium-based Hydrogenation Catalysts: Preparation, Characterization and Performance

Abstract

Bimetallic nanoparticles, composed of two different metals, hold great promise for improving the catalytic production of chemicals and fuels compared to using monometallic catalysts. Synergistic effects between the two metals can enhance catalytic activity, selectivity, and stability. However, achieving uniform nanoparticles in terms of size, composition, and spatial distribution remains a major challenge, especially when using industrially relevant preparation methods such as impregnation. We investigated impregnation-based synthesis of Pd-Ni nanoparticles supported on silica (SBA-15) as a model catalyst. We systematically studied the evolution of palladium and nickel species throughout impregnation, drying, calcination, and reduction. Citrate-based precursors were found to prevent the severe precursor redistribution typically seen with conventional nitrate precursors, thus yielding more uniform Pd-Ni nanoparticles. The type of Pd precursor strongly influenced the extent of Pd-Ni mixing. In CO₂ hydrogenation, increasing Pd-Ni nanoscale intimacy led to a lower activity and higher CO selectivity. A second strategy was employing ethylenediamine (EDA) as a chelating agent to enhance precursor adsorption during impregnation. This method proved even more effective, producing highly uniform Pd-Ni nanoparticles with controlled size, distribution, and composition. Extending the EDA-based approach to Pd-Cu and Pd-Zn systems demonstrated its general applicability. These catalysts showed excellent performance in the selective hydrogenation of 2-methyl-3-butyn-2-ol, an intermediate in vitamin synthesis, even surpassing the industrial catalyst in selectivity towards the desired alkenol product. Finally, we use advanced transmission electron microscopy techniques, including a novel workflow combining STEM-EDX tomography and colocation analysis for quantifying nanoscale elemental distributions. These findings highlight the critical role of careful nanoparticle preparation and characterization in unlocking the full potential of bimetallic catalysts.