Novel aspects of the physical chemistry of Co/SiO 2 Fischer–Tropsch catalyst preparations : The chemistry of cobalt silicate formation during catalyst preparation or hydrogenation

Abstract

Co/SiO 2 catalysts were prepared by both impregnation and precipitation techniques and analyzed by IR, TPR, XRD, and BET methods at intermediate stages of their preparation to define the stage and mechanism of cobalt silicate formation. To prove that reducing conditions can lead to cobalt silicate, one part of a catalyst prepared by impregnation of silica with cobalt nitrate was calcined in air at 350 °C, and another part in H 2–N 2 atmosphere at the same temperature. Infrared spectra revealed the presence of Co–O–Si absorption in the sample exposed to reducing atmosphere. Cobalt silicate formation is postulated to be a reaction between migrating silicic acid and hydrated cobalt hydroxide generated in the reduction process. Precipitation of cobalt nitrate solution at 80–90 °C with potassium hydroxide solution in the presence of Davison 952 silica resulted in amorphous cobalt silicate formation. Precipitation with sodium carbonate solution gave no IR-detectable quantities of cobalt silicate in the presence of Davison 952 silica, but cobalt silicate was detected when Cab-O-Sil HS-5 silica was used. Precipitations of Co and Mg nitrates (∼2:1 Co/Mg ratio) with sodium carbonate gave essentially quantitative yield of amorphous cobalt silicates whether the precipitation occurred in the presence of Davison 952 silica, or the silica was added after the precipitation. Cobalt silicate formation during precipitation either by KOH or Na 2CO 3 solutions is a heretofore unrecognized facile reaction of hydrated cobalt hydroxide with silicic acid (dissolved silica). The mechanism which can explain Co(OH) 2(H 2O) x formation during Na 2CO 3 precipitations is an acid–base reaction between hydrated Co ions and carbonate ions. This reaction also yields carbonic acid, which competes with silicic acid for Co(OH) 2(H 2O) x in secondary reactions.