Crystal face specificity in ammonia synthesis on tungsten carbide

Abstract

Decomposition of NH3 is an important reaction in the cleaning of syngas obtained from the gasification of biomass as well as for the production of hydrogen for fuel cells from easily condensed NH3. To the best of our knowledge, this paper reports for the first time a detailed study of NH3 decomposition on tungsten carbide (WC). Results for a commercially available Fe ammonia synthesis catalyst (Amomax-10) are also reported for comparison.The WC catalyst was characterized by BET, XRD, SEM, EDX and temperature programmed reaction (TPRx). The catalytic behavior of WC strongly depended on pretreatment conditions. The highest activity was obtained with WC samples pretreated in an 80/20 mixture of H2–CO. Complete decomposition of NH3 was observed at 550 °C for 4000 ppm of NH3 at a space velocity of 1,884,000 h−1. At lower temperatures, the activity of the WC catalyst reached steady-state after an induction period that decreased in time with increasing temperature. Reconstruction of the surface during pretreatment and during decomposition of NH3 is suggested to be responsible for the behavior of the catalyst observed during TPRx and time-on-stream (TOS) isothermal reaction. The commercial Fe NH3 synthesis catalyst, although active for NH3 decomposition, showed rapid partial deactivation following an induction period with a steady-state conversion of only 35% at 650 °C and the space velocity used. Thus, WC appears to be an excellent catalyst for use in ammonia decomposition.