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 H2/CO ratios. At the same initial conversion, a low H2/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.

The role of external acid sites of ZSM-5 in deactivation of hybrid CuZnAl/ZSM-5 catalyst for direct dimethyl ether synthesis from syngas

“…Copper sintering seems to proceed according to particle migration and coalescence mechanism. Similar mechanism of nanoparticle sintering facilitated by the presence of water was previously observed for supported nickel and cobalt catalysts [42,43]. In addition to water, hydroxyl groups on the zeolite outer surface can obviously contribute to higher copper mobility in hybrid catalysts.…”

Ostwald ripening on a planar Co/SiO2 catalyst exposed to model Fischer–Tropsch synthesis conditions

“…Therefore, a particle migration and coalescence mechanism, as previously suggested by Sadeqzadeh et al [40] and Tsakoumis et al [41], is not observed on the model catalyst under the reaction conditions used. On supported catalysts, the mechanism of loss of metal surface area is typically derived from changes in the shape of the particle size distribution [40,41], while in the present study, changes are observed at the single-particle level (using TEM).…”

Heterogeneous Catalyst Deactivation and Regeneration: A Review

“…Primarily, X-ray absorption near edge (XANES) analysis shows simultaneous increasing extent of reduction and increasing Co-Co coordination, due both to removal of oxygen and increases in particle size. Second, water is believed to cause chemical-assisted sintering [80,367,[372][373][374], especially at high partial pressures that occur at CO conversions above about 65% [223], although the exact mechanisms are debated. Minor surface oxidation [373,374] and surface wetting [375] have been proposed, although Saib et al have shown that cobalt oxidation is not an important deactivation route [79] in catalysts with Co particles >~8 nm, which are typical in commercial FTS catalysts.…”