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

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 similar to W2C similar to VN similar to NbN > MO2N similar to W2N >> VC similar to 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 degrees C whereas the TOF for the MO2N catalyst was 0.04 s(-1). The carbides and nitrides favored light hydrocarbons (C-1-C-4) exhibiting alpha values between 0.31 and 0.43 at 290 degrees 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 Hy 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 Hy pre-adsorption), suggesting the primary pathway to C2+ hydrocarbons was through the oxygenate mechanism. (C) 2015 Elsevier Inc. All rights reserved.