Carbide and Nitride Based Catalysts for Synthesis Gas Conversion.

Abstract

The production of fuels and chemicals from syngas (H2 and CO) plays a critical role in our economy and will play an even greater role in the future as we transition from petroleum-derived products to biomass-derived products. This transition will require new catalysts that exhibit high activities, selectivities and durabilities for syngas conversion reactions. Research described in this dissertation investigated the catalytic properties of early transition metal carbide and nitride based materials for two industrially relevant syngas conversion reactions: water gas shift (WGS) and Fischer-Tropsch Synthesis (FTS). In addition to kinetic measurements, the materials were characterized using bulk and surface techniques to develop structure-function relationships.

For WGS, the effects of sulfur on the catalytic performance and structures of Mo2C and Pt/Mo2C catalysts were investigated. In the presence of 5 ppm H2S, Mo2C deactivated by 90% from its initial activity and was only partially regenerable. The deactivation was caused by the formation of MoS2 on the catalyst surface. These domains are known to be slightly active for WGS. Oxygen deposited on the Mo2C surface under reaction conditions may have facilitated the formation of MoS2, suggesting that minimizing the amount of surface oxygen could lead to improved sulfur tolerance. Pt/Mo2C was irreversibly poisoned by H2S, primarily due to the formation of PtS.

For FTS, the rates were a function of the metal and the interstitial atom, suggesting that it may be possible to tune the activity of the catalyst. The intrinsic rate trend for the carbide and nitride materials was as follows: Mo2C ~ W2C ~ VN ~ NbN > Mo2N, W2N >> VC, NbC. The materials were capable of direct CO dissociation, a key step in the production of hydrocarbons. Mo2N appeared to catalyze the FTS reaction via the carbide mechanism while Mo2C catalyzed either the oxygenate or CO-insertion mechanism. Regarding selectivity, the materials favored light hydrocarbons and CO2. The latter may have been a consequence of the high WGS activities for these catalysts. Based on these results, the early transition metal carbides and nitrides may be promising catalysts for the production of short chain hydrocarbons or olefins from biomass-derived syngas.