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Ammonia is a central vector in sustainable global growth, but the usage of fossil feedstocks and centralized Haber-Bosch synthesis conditions causes >1.4% of global anthropogenic CO2 emissions. While nitrogenase enzymes convert atmospheric N2 to ammonia at ambient conditions, even the most active manmade inorganic catalysts fail due to low activity and parasitic hydrogen evolution at low temperatures. Here, we show that the [RuH6] catalytic center in ternary ruthenium complex hydrides (Li4RuH6) activates N2 preferentially and avoids hydrogen over-saturation at low temperatures and near ambient pressure by delicately balancing H2 chemisorption and N2 activation. The active [RuH6] catalytic center is capable of achieving high yield at low temperatures via a shift in the rate-determining reaction intermediates and transition states, where the reaction orders in hydrogen and ammonia change dramatically. Temperature-dependent atomic-scale understanding of this unique mechanism is obtained with synchronized experimental and density functional theory investigations.

International audience; Ru/gamma-Al2O3 catalysts were prepared using three different methods: wet impregnation, colloidal method and microemulsion. Ru-supported nanoparticles with different average sizes and distribution of sizes were obtained. The catalysts were tested in ammonia synthesis under mild reaction conditions, namely low temperature (100 degrees C) and low pressure (4 bar), and characterized by N-2 adsorption, XRD, XPS, TEM and TPR techniques. The results indicate that a good catalytic performance can be achieved by Ru supported nanoparticles fulfilling two requirements: (i) a relatively high average size (despite the usual assertion that only small particles are required) and (ii) a broad distribution of sizes that ensures the presence of both small particles, containing highly active sites, and large nanoparticles, which are shown to promote the reaction on small particles. This promotion results from a cooperative effect between small and large nanoparticles in good contact, which also allows keeping a highly reduced surface of ruthenium. It is proposed that, under mild reaction conditions, large Ru nanoparticles promote the ammonia synthesis reaction by allowing a more effective activation and transfer of hydrogen atoms, able to hydrogenate strongly adsorbed nitrogen atoms, and thus to release active sites for the activation of N2. (c) 2013 Elsevier B.V. All rights reserved.

The paper examined the catalysts of low-temperature ammonia synthesis Ru-Cs/Sibunit in which Sibunit, mesoporous carbon carrying agents, vary considerably in specific surface area and pore volume, but the pore sizes are similar. According to Raman spectroscopy, the Sibunit structures are same. Applied ruthenium dispersiveness, determined by CO chemisorption, are also close (∼ 13 - 16%) for all the examined catalysts. The difference in the activity of the samples is due to the different direct contact probability of the particles of ruthenium and modifier (CsO x ). This probability decreases with increasing specific surface area ( S BEТ Sibunit) that are supposed to cause the decrease of catalyst activity on high-superficial Sibunit in the ammonia synthesis reaction.