The Effect of Water on the Activity and Selectivity for γ-Alumina Supported Cobalt Fischer–Tropsch Catalysts with Different Pore Sizes

Abstract

The effect of water on the activity and selectivity for a series of c-Al 2 O 3 supported cobalt Fischer-Tropsch catalysts has been studied in an isothermal fixed-bed reactor at T=483 K, P=20 bar, and H 2 /CO=2.1. The catalysts were produced applying incipient wetness impregnation and consisted of 20 wt.% cobalt and 0.5 wt.% rhenium deposited on c-Al 2 O 3 supports with different pore characteristics. For the narrow-pore catalysts, addition of water corresponding to an inlet partial pressure ratio of P H2 O/P H2 =0.4 reduced the reaction rates. In contrast, for a catalyst with larger pores the same water pressure increased the reaction rates. For all catalysts, water amounts equal to P H2 O/P H2 =0.7 at the reactor inlet suppressed the reaction rates and led to permanent deactivation. The addition of water increased the C 5+ selectivity and decreased the CH 4 selectivity for all catalysts. The pore characteristics seem to determine the effect of water on the rates.

Promotion Effects of Platinum and Ruthenium on Carbon Nanotube Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

“…Water is one of the major products of FT synthesis. The decrease in the FT reaction rate is usually attributed to catalyst deactivation due to sintering of the cobalt, and segregation of the noble meal promoter, oxidation of the cobalt surface or small cobalt clusters [39][40][41][42]. The introduction of noble metal could increase the extent of reduction by dissociating hydrogen which then spills over to reduce the cobalt oxide to cobalt metal.…”

Evidence for Diffusion-Controlled Hydrocarbon Selectivities in the Fischer–Tropsch Synthesis Over Cobalt Supported on Ordered Mesoporous Silica

“…2 that all catalysts show a two-step reduction for cobalt oxides: a first peak (peak I) at 442-589 K, ascribed to reduction of Co 3 O 4 to CoO, is followed by a second peak (peak II) at around 622-728 K (less marked for the CoSBA-15 catalyst) corresponding to the reduction of CoO to Co [13,14]. CoSBA-15 shows a more complex profile with a broad H 2 -uptake at temperatures between 600 and 1,200 K, suggesting the presence of surface cobalt species with different degrees of interaction with the support [13,14]. It has also been reported that in the 1-D porous structure (with narrow pores), the slow removal of water formed upon reduction may affect the appearance of the TPR, and even promote the formation of barely reducible cobalt silicates [15,16].…”

“…It can be seen from Fig. 2 that all catalysts show a two-step reduction for cobalt oxides: a first peak (peak I) at 442-589 K, ascribed to reduction of Co 3 O 4 to CoO, is followed by a second peak (peak II) at around 622-728 K (less marked for the CoSBA-15 catalyst) corresponding to the reduction of CoO to Co [13,14]. CoSBA-15 shows a more complex profile with a broad H 2 -uptake at temperatures between 600 and 1,200 K, suggesting the presence of surface cobalt species with different degrees of interaction with the support [13,14].…”

Thermodynamics of oxidation of an alumina-supported cobalt catalyst by water in F-T synthesis

“…Water contributed to the sintering and thus caused the deactivation of the catalyst. Still, neither cobalt loss nor the surface compound formation between cobalt and zirconia was detected [13].…”

“…In turn, the proposed monomer dependencies in the F-T synthesis mechanism explain the lower methane selectivity and higher molecular weight products observed at increased water concentrations. The deactivation of coprecipitated Co-ZrO 2 catalyst during F-T synthesis was investigated in a fixed-bed reactor under dry and water addition conditions [13]. It was found that the activity of the catalyst decreased after reaction and the catalyst deactivated more with water addition during reaction.…”