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 NH3, tars, and H2S in the gas products that are detrimental to downstream processes. This paper reports the results of a study of NH3 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 NH3 decomposition. The effects of the presence of H2 and CO on the behavior of these catalysts are also reported. At the NH3 decomposition reaction conditions used in the present study (1 atm, 465-650 degrees 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 NH3 so that the surface is more favorable for reaction. Both WC and WZ displayed superior activity compared to a commercial Fe-based NH3 synthesis catalyst (Amomax-10). At 600 degrees 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 mu mol/g cat./s and 0.53 mu mol/g cat./s (0.021 mu mol/m(2) cat./s and 3.1 mu mol/m(2) cat./s), respectively, for WZ and Amomax-10.