Semiconductor Nanowire-Supported Palladium Nanocatalysts

This work presents the innovative design, fabrication, and performance evaluation of aerosol palladium (Pd)-based catalysts supported by semiconductor gallium phosphide (GaP) nanowires (NWs), targeting their application in hydrogenation reactions, which are crucial in the industrial production of fine chemicals and pharmaceuticals.<br/><br/>Pd nanoparticles (NPs) are generated via spark ablation (Messing et al. 2010), a clean and versatile technique enabling the production of high-purity aerosols with controlled density, composition, morphology, and size. The support material plays a key role in maintaining the stability and catalytic performance of NPs, as well as facilitating their separation for reuse and minimizing NP loss. The use of semiconductors as supports has been investigated to enhance the catalytic activity of the well-established Pd catalyst (Hao et al. 2018; Liu et al. 2022).<br/><br/>GaP NWs, fabricated by metal-organic vapor phase epitaxy (MOVPE), offer a distinct advantage due to their high aspect ratio (length/diameter), providing a larger surface area to expose the Pd NPs, thereby increasing the number of active sites. The biocompatibility of GaP NWs (Franzén et al. 2021) further broadens their applications, making them appealing for potential synergy effects.<br/><br/>The fabrication of these nanocatalysts involves the generation of 10 nm Pd NPs with a particle density of 1000 #/μm². These Pd NPs are subsequently deposited along the tapered side walls of standing 〈111〉 GaP NWs supports by means of an electrostatic precipitator, as part of the spark discharge generator setup, ensuring a good NP-NW bond. The fabrication of the GaP NW support at 540 °C and 100 mbar, controlling the V/III ratio (phosphine to trimethylgallium), allows for precise tuning of the morphology and crystal structure (hexagonal wurtzite and/or cubic zinc blende with stacking faults).<br/><br/>Electron microscopy confirmed the preservation of the morphology of both Pd NPs and GaP NWs after nanocatalyst fabrication, in addition to their well-defined crystallinity according to their fabrication parameters. Wettability analysis revealed a correlation between nanocatalyst morphology, crystal structure, and surface energy, with surfaces ranging from hydrophilic to hydrophobic.<br/><br/>Catalytic performance was assessed through the partial hydrogenation of phenylacetylene to styrene, a valuable commodity in the polymer industry. These nanocatalysts present a promising alternative to the commercial Lindlar catalyst used in the hydrogenation of alkynes, consisting of Pd particles poisoned with lead and quinoline.<br/><br/>The results suggest catalytic performance predominantly linked to the surface energy of the NW support, which is dependent on its morphology and crystal structure. This study provides a controllable method for nanocatalyst fabrication, offering an advantage over typical wet chemical synthesis. The diverse behaviors of Pd-based nanocatalysts underscore the critical role of NW supports, contributing to a deeper understanding of key parameters that influence nanocatalyst functionality.<br/><br/>This work was supported by the LMK foundation for interdisciplinary research, Swedish Foundation for Strategic Research (Grant No. FFL18-0282) and the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 945378. Part of the work was performed at Lund Nanolab, part of Myfab.<br/><br/>References<br/>Franzén, S. M., Tasić, M., Poulie, C. B., Magnusson, M. H., Strand, D., & Messing, M. E. 2021, Scientific Reports, 11, 9276<br/>Hao, C.-H., et al. 2018, ACS Applied Materials & Interfaces, 10, 23029<br/>Liu, B., Xu, T., Li, C., & Bai, J. 2022, Molecular Catalysis, 528, 112452<br/>Messing, M. E., et al. 2010, The Journal of Physical Chemistry C, 114, 9257