Synthesis and characterization of discrete nickel phosphide nanoparticles: Effect of surface ligation chemistry on catalytic hydrodesulfurization of thiophene

Discrete, unsupported nanoparticles of Ni2P have been prepared by using a solution-phase method with bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)(2)] as the nickel source and trioctylphosphine (TOP) as the phosphorus source in the presence of the coordinating solvent trioctylphosphine oxide (TOPO). Ni2P nanoparticles prepared at 345 degrees C have an average crystallite size of to 10.2 +/- 0.7 nm and are capped with TOP and/or TOPO coordinating agents. The surface of the Ni2P nanoparticles can be modified by washing with CHCl3 or by exchanging TOP/TOPO groups with mercaptoundecanoic acid (MUA). The surface areas of these nanoparticles are on the order of 30-70 m(2) g(-1). As-prepared and MUA-capped nanoparticles undergo a phase transformation at 370 degrees C under reducing conditions, but CHCl3-washed Ni2P nanoparticles retain the Ni2P structure. CHCl3-washed and MUA-capped nanoparticles exhibit higher HDS catalytic activity than as-prepared nanoparticles or unsupported Ni2P prepared by temperature-programmed reduction of a phosphate precursor. The surface modifications have a clear effect on the catalytic activity as well as the thermal stability of Ni2P nanoparticles under reducing conditions.