Intermetallic GaPd₂ Nanoparticles on SiO₂ for Low-Pressure CO₂ Hydrogenation to Methanol: Catalytic Performance and In Situ Characterization

A nanodispersed intermetallic GaPd₂/SiO₂ catalyst is prepared by simple impregnation of industrially relevant high-surface-area SiO₂ with Pd and Ga nitrates, followed by drying, calcination, and reduction in hydrogen. The catalyst is tested for CO₂ hydrogenation to methanol at ambient pressure, revealing that the intrinsic activity of the GaPd₂/SiO₂ is higher than that of the conventional Cu/ZnO/Al₂O₃, while the production of the undesired CO is lower. A combination of complementary in situ and ex situ techniques are used to investigate the GaPd₂/SiO₂ catalyst. In situ X-ray diffraction and in situ extended X-ray absorption fine structure spectroscopy show that the GaPd₂ intermetallic phase is formed upon activation of the catalyst via reduction and remains stable during CO₂ hydrogenation. Identical location-transmission electron microscopy images acquired ex situ (i.e., micrographs of exactly the same catalyst area recorded at the different steps of activation and reaction procedure) show that nanoparticle size and dispersion are defined upon calcination with no significant changes observed after reduction and methanol synthesis. Similar conclusions can be drawn from electron diffraction patterns and images acquired using environmental TEM (ETEM), indicating that ETEM results are representative for the catalyst treated at ambient pressure. The chemical composition and the crystalline structure of the nanoparticles are identified by scanning TEM energy dispersive X-ray spectroscopy, selected area electron diffraction, and atomically resolved TEM images.