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

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 catalysis 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.