Surface Study of Nickel Phosphide and Cobalt Sulfide Nanoparticles for Heterogeneous Catalysis

Abstract

Compound metal nanoparticles have a large surface area-to-volume ratio, a high concentration of low coordination sites, and surface vacancies that give rise to properties favorable for heterogeneous catalytic processes. Nanoparticles are incorporated onto exterior supports, as they are unable to withstand harsh reaction conditions. The use of supports decreases the nanoparticles’ surface area, which hampers their catalytic activity. The integration of a new ligand binding mode, known as "crystal-bound," will remove the need of exterior supports because the ligands will sit in higher coordination number sites, ultimately increasing their stability and robustness. If the ligands are crystal-bound, more metal sites will be exposed, which is expected to increase the catalytic activity of the metal compound nanoparticles. Nickel phosphide and cobalt sulfide nanoparticles are used as model systems, as they have been used as heterogeneous catalysts for chemical syntheses and electrochemical processes. Three different tertiary phosphine and primary aryl thiol ligands, as well as reaction temperature, were used to target crystal-bound ligands for Ni_xP_y and Co_xS_y nanoparticles. The ligand binding mode was characterized via proton nuclear magnetic resonance and Fourier transform infrared spectroscopy. The results of Ni_xP_y experiments suggest that the ligands were not crystal-bound, but did lead to the discovery of a new mechanism of formation. As for Co_xS_y, optimization of synthesis parameters are still needed in order to obtain better control of morphology and structure to facilitate the characterization of Co_xS_y surfaces.