Effect of Promoter Deposition Order on Platinum-, Ruthenium-, and Rhenium-Promoted Cobalt Fischer-Tropsch Catalysts

Abstract

The effects of noble metal (NM) promoter (Pt, Re, or Ru) deposition order were investigated in commercially-representative Co Fischer-Tropsch catalysts (nominally 25 wt% Co on a La stabilized alumina support). The effects of deposition sequence (co-deposition vs. sequential deposition) were studied in terms of NM retention and distribution, Co crystallite size, Co reducibility, and catalyst activity and selectivity. Ru retention was <48% with more severe losses for co-deposition (85% loss). Sequentially deposited Co/Ru showed poor Ru distribution, unlike co-deposited Co/Ru. Excellent retention and uniform distribution were observed for both Pt and Re promoted catalysts. Cobalt crystallite sizes were smaller for all three co-deposited catalysts (~4.2 nm for all NMs) when compared to any of the sequentially deposited catalysts. Although sequentially deposited catalysts required higher reduction temperatures (TPR), they had greater extents of reduction (EORs) relative to co-deposited catalysts. CO depletion rates and turnover frequencies (TOFs) for co-deposited Co/Pt and Co/Re were 2-3 times higher than their sequentially deposited counterparts. Methane selectivities were lower for co-deposited Co/Pt and Co/Re. It is interesting to note that the smaller Co crystallite sizes (co-deposition) resulted in more active catalysts than those with larger crystallites (sequentially deposition), which is counter to previous work on unpromoted Co catalysts. The co-deposited Co/Pt catalyst was found to be statistically more active than the unpromoted Co catalyst, while the corresponding sequentially deposited catalyst was less active. Overall, co-deposited Co/Pt and Co/Re catalysts showed superior catalytic performance by all standards of judgment when compared to their sequentially deposited counterparts.