Ammonia is a precursor for synthesis of many important chemicals including pharmaceutical products and fertilizers; it is also a promising chemical for efficient hydrogen capture and release. Traditionally, ammonia is produced via Haber-Bosch process, which requires high temperature (400-600°C) and pressure (20-40 MPa), leading to high energy consumption and safety concerns. Search for an efficient and stable catalyst enabling ammonia synthesis under mild conditions is a major topic in catalysis research. Recent experimental studies demonstrated that flat Ru nanoparticles on calcium amide [Ca(NH2)2] enable continuous synthesis of NH3 at temperatures as low as ~200 °C and ambient pressure. In order to extend these results to a broader class of catalysts, we aim to understand the atomistic origin of this low-temperature activity and to reveal effects specific to the supporting surface and the size and shape of Ru nanoparticles. In this work, we investigate thermodynamic stability of Ca(NH2)2 surfaces in different crystallographic orientations and interactions of small Ru clusters and flat nanoparticles with the amide support using ab initio simulations. It is shown that (NH2)- anions reorient and/or donate protons to facilitate the formation of N-Ru bonds and, thus, anchor the nanoparticles. Formation of a precursor layer by Ru adsorption at hollow sites is also found to promote a two-dimensional growth of Ru nanoparticles. We found that extra electrons induced by the NH2 vacancies are transferred to the nearest to them Ru atoms. We propose that these effects enhance reduction of dinitrogen on the nanoparticles and hence render them catalytically active.

Structure and activation of Ru catalyst on Ca(NH2)2: Effect of hydrogen and electron transfer

254th National Meeting (2017)

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