Ammonia

About: Ammonia is a research topic. Over the lifetime, 16217 publications have been published within this topic receiving 271940 citations. The topic is also known as: NH3 & azane.

Splicing the active phases of copper/cobalt-based catalysts achieves high-rate tandem electroreduction of nitrate to ammonia

Abstract: Electrocatalytic recycling of waste nitrate (NO3-) to valuable ammonia (NH3) at ambient conditions is a green and appealing alternative to the Haber-Bosch process. However, the reaction requires multi-step electron and proton transfer, making it a grand challenge to drive high-rate NH3 synthesis in an energy-efficient way. Herein, we present a design concept of tandem catalysts, which involves coupling intermediate phases of different transition metals, existing at low applied overpotentials, as cooperative active sites that enable cascade NO3--to-NH3 conversion, in turn avoiding the generally encountered scaling relations. We implement the concept by electrochemical transformation of Cu-Co binary sulfides into potential-dependent core-shell Cu/CuOx and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in-situ Raman spectra reveal that the inner Cu/CuOx phases preferentially catalyze NO3- reduction to NO2-, which is rapidly reduced to NH3 at the nearby Co/CoO shell. This unique tandem catalyst system leads to a NO3--to-NH3 Faradaic efficiency of 93.3 ± 2.1% in a wide range of NO3- concentrations at pH 13, a high NH3 yield rate of 1.17 mmol cm-2 h-1 in 0.1 M NO3- at -0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.

Modified palladium metal‐oxide‐semiconductor structures with increased ammonia gas sensitivity

Abstract: It is known that palladium metal‐oxide‐semiconductor (Pd‐MOS) structures are sensitive detectors for hydrogen gas. We show that the evaporation of a thin film of catalytically active metals on top of the structure can increase the sensitivity towards ammonia considerably. It was found that the thin metal must be in contact with the oxide to cause the increased sensitivity. The largest increase was observed with the transition metals Ir and Pt. The ammonia sensitivity could be enhanced about 60 times compared to that of an unmodified structure

Control of ureogenesis

Abstract: Control of urea synthesis was studied in rat hepatocytes incubated with physiological mixtures of amino acids in which arginine was replaced by equimolar amounts of ornithine. The following observations were made. Intramitochondrial carbamoyl phosphate was always below 0.1 mM. Only when ornithine was absent and when, in addition, the concentration of amino acids was higher than four times their plasma concentration, intramitochondrial carbamoyl phosphate rose up to about 3 mM; under these conditions ammonia accumulated in the medium. The relationship between ornithine-cycle flux and the concentration of the cycle intermediates at varying amino acid concentration indicated that under near-physiological conditions the ornithine-cycle enzymes are far from being saturated with their subsidiaries. Moderate concentrations of norvaline had no effect on the rate of urea synthesis unless the cells were severely depleted of ornithine. Activation of carbamoyl-phosphate synthetase (ammonia) by addition of N-carbamoylglutamate only slightly stimulated urea production at all amino acid concentrations. However, in the presence of the activator the curve relating ornithine-cycle flux to the steady-state ammonia concentration was shifted to lower concentrations of ammonia. The intramitochondrial concentration of carbamoyl phosphate in rat liver in vivo was below 0.1 mM. This value is far below the concentration required for substantial inhibition of carbamoyl-phosphate synthetase. It is concluded that in vivo the function of activity changes in carbamoyl-phosphate synthetase, via the well-documented alterations in the intramitochondrial concentration of N-acetylglutamate, is to buffer the intrahepatic ammonia concentration rather than to affect urea production per se. At constant concentration of ammonia the rate of urea production is entirely controlled by the activity of carbamoyl-phosphate synthetase.