Improving the Stability of Cobalt Fischer−Tropsch Catalysts by Boron Promotion

Abstract

Supported Co catalysts exhibit favorable activity and selectivity for Fischer−Tropsch synthesis (FTS) but deactivate slowly. To explore deactivation by carbon deposition, the stability of various forms of deposited carbon was evaluated using density functional theory (DFT). A surface carbide and graphene islands were calculated to be thermodynamically stable. Two forms of deposited carbon are also distinguished experimentally after 200 h of FTS. On the basis of this mechanistic insight, boron was proposed as a promoter to enhance the stability of Co catalysts. DFT calculations indicate that boron and carbon display similar binding preferences, and boron could selectively block the deposition of resilient carbon deposits. To evaluate the theoretical predictions, supported 20 wt % Co catalysts were promoted with 0.5 wt % boron and tested under realistic FTS conditions. Boron promotion was found to reduce the deactivation rate 6-fold, without affecting selectivity and activity.

The Optimally Performing Fischer–Tropsch Catalyst

“…With respect to the important question of selectivity of the FT reaction, according to one school of thought, small particles are not reactive, as strongly adsorbed CO inhibits chain growth;2, 3 according to others, step‐edge sites are required that are not stable on small particles 4–6. This explains the observation that selectivity toward the production of methane strongly increases and the rate of CO consumption decreases for smaller transition‐metal nanoparticles 5.…”

The Optimally Performing Fischer–Tropsch Catalyst

“…With respect to the important question of selectivity of the FT reaction, according to one school of thought, small particles are not reactive, as strongly adsorbed CO inhibits chain growth; [2,3] according to others, step-edge sites are required that are not stable on small particles. [4][5][6] This explains the observation that selectivity toward the produc-tion of methane strongly increases and the rate of CO consumption decreases for smaller transition-metal nanoparticles.…”

Structural Modification of Ni/γ‐Al2O3 with Boron for Enhanced Carbon Resistance during CO Methanation

“…Therefore, carbon is retained on the surface and can be easily removed. The modification of Ni and Co catalysts with boron has been proposed for applications in reforming reactions and the Fischer–Tropsch process, but the current understanding of boron's actual role relies mostly on theoretical calculations. Herein, we use boron as a catalyst promoter to increase the stability and carbon resistance of Ni for low‐temperature CO methanation with C 2 H 4 present in the feed.…”