Showing papers by "Jens K. Nørskov published in 2023"

Continuous-flow electrosynthesis of ammonia by nitrogen reduction and hydrogen oxidation

Abstract: Ammonia is a critical component in fertilizers, pharmaceuticals, and fine chemicals and is an ideal, carbon-free fuel. Recently, lithium-mediated nitrogen reduction has proven to be a promising route for electrochemical ammonia synthesis at ambient conditions. In this work, we report a continuous-flow electrolyzer equipped with 25–square centimeter–effective area gas diffusion electrodes wherein nitrogen reduction is coupled with hydrogen oxidation. We show that the classical catalyst platinum is not stable for hydrogen oxidation in the organic electrolyte, but a platinum-gold alloy lowers the anode potential and avoids the decremental decomposition of the organic electrolyte. At optimal operating conditions, we achieve, at 1 bar, a faradaic efficiency for ammonia production of up to 61 ± 1% and an energy efficiency of 13 ± 1% at a current density of −6 milliamperes per square centimeter. Description Protons from H2 for ammonia synthesis Electrochemical synthesis of ammonia from nitrogen (N2) and hydrogen (H2) could advantageously decentralize the current mass production of fertilizer. One promising method being explored involves lithium ion cathodic reduction in an organic solvent electrolyte, followed by reaction of the lithium with N2. However, conventional H2 oxidation catalysts for the complementary anodic process are unstable in these conditions. Fu et al. report that a gold–platinum alloy can robustly catalyze this oxidation and thus steadily produce the protons for ammonia under continuous flow conditions. —JSY A gold–platinum alloy catalyst proved stable for hydrogen oxidation to couple with nitrogen reduction in an ethereal solvent.

Spin Effects in Chemisorption and Catalysis

Abstract: In this work, we show using density functional theory calculations that controlling the spin state of the surface of magnetic metals has a substantial effect on their chemical properties. For a range of adsorbates, the adsorption energy is shown to be stronger on non-spin polarized surfaces than on spin polarized ground state surfaces. This is true for Fe, Co, and Ni surfaces, and the result is the same for three commonly used exchange–correlation functionals. We further discuss the origin of the effect in terms of the surface electronic structure and show that a simple model based on the d-band model of adsorption can explain the effect. Finally, we discuss how spin effects may be used to control surface reactivity and provide guidance on how to alter the surface spin state, e.g., adding a metal promotor.