Manganese-based catalysts are used in ammonia synthesis

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Source: China Chemical Industry News “Activating” the catalytic behavior of transition metals before use According to Sinochem New Network, a research team from the Research Group on Composite Hydride Materials Chemistry at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has made new progress in the field of ammonia synthesis using manganese-based catalysts. This research outcome may provide an effective catalyst design strategy for “activating” the synthetic ammonia catalytic behavior of pre-transition metals. 　　The synthesis of ammonia on transition metals is an important research topic in the field of heterogeneous catalysis. Ruthenium (Ru) and iron (Fe) exhibit excellent catalytic performance for ammonia synthesis due to their moderate nitrogen (N) adsorption energy, and are thus used in the industrial ammonia synthesis process. Pre-transition metals such as vanadium (V), chromium (Cr), and manganese (Mn) exhibit strong adsorption affinity for N species; thus, they readily form stable nitride phases in the reaction atmosphere of ammonia synthesis. This hinders the subsequent hydrogenation step, resulting in poor ammonia synthesis activity. Consequently, these metals have not attracted much attention from researchers over the years. 　　To address this issue, building on previous work, the research team selected Mn metal as a representative and systematically studied the effect of alkali (alkaline earth) metal hydrides on the ammonia synthesis catalytic activity of Mn metal. Experimental results show that the addition of alkali (earth) metal hydrides can increase the catalytic activity of Mn for ammonia synthesis by 1 to 3 orders of magnitude. Furthermore, the research results also showed that the order of the promoting effect of alkali (alkaline earth) metal hydrides on Mn differs significantly from that of conventional alkali (alkaline earth) metal oxide electron donors. 　　Thermodynamic analysis and characterization results reveal that under the conditions of ammonia synthesis, the phase transformation between alkali (earth) metal hydrides and imine compounds, along with their interactions with manganese nitride, are the fundamental reasons for their catalytic effect. Through in-depth studies on the structure-activity relationship of the Mn-LiH catalytic system, researchers have found that the active phase of the catalyst and its kinetic behavior (apparent activation energy, rate-determining steps, etc.) depend strongly on reaction conditions such as temperature and space velocity; this is another characteristic that distinguishes it from conventional ammonia synthesis catalysts.