Ammonia Decomposition on Tungsten-Based Catalysts in the Absence and Presence of Syngas

Synthesis gas produced from biomass gasification can serve as a starting point for producing electricity, low-to-medium energy fuels, and even hydrogen for fuel cells. The major barrier in commercialization of biomass gasification is the presence of impurities such as NH 3, tars, and H 2 S in the gas products that are detrimental to downstream processes. This paper reports the results of a study of NH 3 decomposition to N 2 and H 2 on tungsten-based catalysts, tungsten carbide (WC) and tungstated zirconia (WZ), for a gasification gas cleanup strategy that involves removal of tars first followed by NH 3 decomposition. The effects of the presence of H 2 and CO on the behavior of these catalysts are also reported. At the NH 3 decomposition reaction conditions used in the present study (1 atm, 465−650 °C, 4000 ppm), both WC and WZ showed an induction period. The main reason for this induction period is hypothesized to be a restructuring of the catalyst surface by NH 3 so that the surface is more favorable for reaction. Both WC and WZ displayed superior activity compared to a commercial Fe-based NH 3 synthesis catalyst (Amomax-10). At 600 °C and in the presence of syngas, no conversion was observed on WC while ca. 20% and 10% conversions were observed on WZ and Amomax-10, respectively, at steady state for the reaction conditions used. This corresponds to intrinsic rates of 1.2 μmol/g cat./s and 0.53 μmol/g cat./s (0.021 μmol/m 2 cat./s and 3.1 μmol/m 2 cat./s), respectively, for WZ and Amomax-10.