Fischer-Tropsch reaction over alumina-supported cobalt catalyst : activation using H2 and CO

Abstract

Abstract: The catalytic activity for Fischer-Tropsch (FT) reaction over cobalt-based catalysts mainly depends on two parameters, namely the reducibility of cobalt precursors and cobalt dispersion. Therefore, a perfect catalyst would comprise of an optimal combination of these two parameters. Cobalt precursor’s reduction is usually performed in the presence of H2 and is usually limited by metal-support interactions which, in some cases, lead to the formation of metal-support compounds that are not reducible under a practical reduction temperature range. The water vapour that is formed during cobalt-based reduction by H2 has been reported to promote the formation of these metal-support compounds in some cases. An investigation on a reduction process that does not produce water would potentially offer opportunities for better cobalt-based catalyst reduction. Therefore, the aim of this project was to investigate the effect of activating Co/Al2O3 FT catalyst using H2 or CO on the catalyst structure and performance for FT reactions. The catalyst was prepared by impregnation of the support (Al2O3) with a cobalt nitrate (Co(NO3)2•6H2O) solution and calcined in air at 500°C for 10 hours to decompose and transform the cobalt nitrate to cobalt oxide. XRD analysis was performed to determine the structure of the catalyst prepared. BET analysis was performed to determine the surface area and porosity of the catalyst. Temperature programmed reduction (TPR) was performed on calcined Co/Al2O3 catalyst using a H2 and CO containing gas mixture respectively to study the reduction behaviour of the catalyst. The catalyst morphology was studied using scanning electron microscopy (SEM) analysis. The catalyst was tested for FT reaction in a fixed bed reactor and the outlet gas products were analysed using a Dani master gas chromatograph (GC) equipped with thermal conductivity detector (TCD) and flame ionisation detector (FID). It was found that CO activates the Co/Al2O3 catalyst at a lower temperature than H2 and is accompanied by carbon deposition on the catalyst surface. The main forms of cobalt species in catalyst samples reduced by CO or H2 at 300 oC were CoO. Co0 and CoO were the major cobalt phases for the catalyst samples respectively reduced by CO and H2 at 350 oC. The highest catalytic activity for FT reaction with the highest rate of C5+ hydrocarbons formation were measured on CO-activated catalyst samples. The deposited carbon on COreduced samples is believed to be a precursor for possible cobalt carbide formation during FT reaction that led to high methane selectivity.M.Tech. (Chemical Engineering