Synthesis, Characterization, and Catalytic Hydrogenation Activity of Highly Dispersed Fe, Rh, and Rh/Fe Powders

Abstract

The preparation of submicrometer-sized metal particles by chemical reduction of homogeneous solutions of metal salts in the absence of surfactants has been investigated. Reduction of FeCl3 with LiBEt3H in THF resulted in the formation of solutions containing crystalline, monodispersed Fe particles with average sizes ∼200 and 2-4 nm, depending on the reaction conditions. The reduction of [(1,5-COD)RhCl]2 (where 1,5-COD = 1,5 cyclooctadiene) with LiBEt3H in THF resulted in formation of a black solution containing 2-4 nm sized Rh particles, as shown by TEM and SAXS, together with small amounts of [(l,5COD)RhH]4. Stepwise reduction of first FeCl3 with LiBEt3H in THF followed by addition of [(1,5COD)RhCl]2 resulted in formation of Rh/Fe particles. The particles were characterized by SEM, TEM, and X-ray diffraction experiments. The catalytic activity of these highly dispersed metals toward pyrene hydrogenation has been investigated. The 2-4 nm and agglomerated Rh particles showed an activity of 31.8 mg product/g catalyst/s and 4.8 mg product/g catalyst/s, respectively, while at 100°C, the Fe particles were inactive. The 2-4 nm Rh/Fe particles have shown pyrene conversion of 1.1 mg product/g catalyst/s, while larger ∼150-nm-sized Rh/Fe particles exhibited lower reactivity, 0.076 g product/g catalyst/s. These activities were compared to those of hydrous titanate (HTO) supported Pd catalysts which have been used in studies of direct coal liquefaction. The results show that the activities of the highly dispersed metal colloids are comparable to those of supported metal catlaysts on a per-gram-of-metal basis.