Mechanistic modeling of cobalt based catalyst sintering in a fixed bed reactor under different conditions of fischer-tropsch synthesis

Abstract

A three-step sintering mechanism is proposed for Co-based catalysts under Fischer-Tropsch reaction conditions. This mechanism includes an intermediate formation of oxide layer on cobalt metal nanoparticles in the presence of water. The partially reversibly oxidized surface accelerates sintering by both reducing the surface energy and enhancing the diffusion rates of cobalt particles. The proposed mechanism is then employed for a fixed-bed unsteady state reactor. The effect of particle growth on the catalytic activity was analyzed within a diverse range of operating conditions (syngas ratio = 1.5-4, water co-feed ratio = 0-6, inert co-feed ratio = 0-6). It is found that, at the same gas space velocity, sintering proceeds faster at higher H 2/CO ratios. At the same initial conversion, a low H 2/CO syngas ratio increases sintering severity, i.e., catalyst deactivation due to the crystallite growth, as it brings about higher relative water partial pressure. Dilution of syngas with different amounts of inert gas does not affect the cobalt sintering rate. Cobalt sintering proceeds more rapidly if water is co-fed during the reaction. © 2012 American Chemical Society.