Fischer–Tropsch synthesis over early transition metal carbides and nitrides: CO activation and chain growth

• Fischer–Tropsch synthesis over Mo, W, V, and Nb carbides and nitrides is performed.
• Rates decrease as follows: Mo2C ∼ W2C ∼ VN ∼ NbN > Mo2N ∼ W2N ≫ VC ∼ NbC.
• Carbides and nitrides of the same parent metal exhibit very different TOFs for FTS.
• Mo2N exhibits a larger activation barrier for CO dissociation than Mo2C.
• Molecular COads facilitated coupling over Mo2C, supporting the oxygenate mechanism.

A series of transition metal carbide and nitride catalysts (Mo2C, Mo2N, W2C, W2N, VC, VN, NbC, and NbN) were prepared and evaluated for Fischer–Tropsch synthesis. The activity trend was Mo2C ∼ W2C ∼ VN ∼ NbN > Mo2N ∼ W2N ≫ VC ∼ NbC, with carbides and nitrides of the same parent metal exhibiting significantly different turnover frequencies (TOF). For example, the Mo2C catalyst exhibited a TOF of 0.36 s−1 at 300 °C whereas the TOF for the Mo2N catalyst was 0.04 s−1. The carbides and nitrides favored light hydrocarbons (C1–C4) exhibiting α values between 0.31 and 0.43 at 290 °C, and were active for the water–gas shift reaction. Results from temperature programmed desorption and reaction indicated that both the Mo2N and Mo2C catalysts are capable of direct (without H2 assistance) CO dissociation; however, the activation barrier is much higher over Mo2N than Mo2C. The results also indicate that C–C coupling over the Mo2C surface was facilitated by molecularly adsorbed CO (without H2 pre-adsorption), suggesting the primary pathway to C2+ hydrocarbons was through the oxygenate mechanism.

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