The catalytic decomposition of ammonia

Abstract

Chemical vapour impregnation was investigated as a novel preparation method for Ru/Al2O3 catalysts for the production of hydrogen through ammonia decomposition. These catalysts were shown to be more active than those produced by impregnation. TEM imaging showed that this is because more particles were within the optimal 3-5 nm range. Investigation by XPS also revealed that less Cl-, a known inhibitor, is present in the CVI prepared catalyst. CoMoN catalysts have previously been shown to exhibit a synergistic effect with activity higher than that of Ru. The effect of pH on the preparation of these catalysts was investigated. Catalytic testing did not present this synergy and further investigation by XRD revealed this to be due to incomplete nitridation of the CoMoO4 precursors. Fe-Pt, Fe-Pd and Fe-Ni/Al2O3 catalysts were prepared by CVI to investigate the robustness of periodic table interpolation as a catalyst design method. Although all catalysts were predicted to show enhanced activity only Fe-Pt demonstrated a large enhancement, with Fe-Pd showing limited synergy and Fe-Ni showing none. STEM investigation showed that small, alloyed Fe-Pt particles were prepared by CVI that rearranged under reaction conditions but were catalytically stable. XRD suggested that the enhancement observed in Fe-Pd catalysts was due to particle size effects and the same was demonstrated for Fe-Ni using N2O titration. The activity of Fe catalysts was shown to be enhanced significantly upon the addition of a Cs promotor. The optimal Cs loading was shown to be between 0.5 and 1 mol eq. with further increase in Cs leading to a decrease in activity. XPS and TPR studies suggest that the enhancement is due to an electronic interaction between the two metals. XRD and BET surface area investigations show the decrease in promotion at higher Cs loadings is due to an amorphous layer of CsOH forming over the support; blocking active sites and causing a decrease in catalyst surface area