Enhanced ammonia oxidation on BDD induced by inhibition of oxygen evolution reaction

Electrochemical oxidation of ammonia (NH3 and NH4+) on boron-doped diamond (BDD) electrode was studied using differential electrochemical mass-spectrometry (DEMS) and chronoamperometry. Electro-oxidation of ammonia induces inhibition of the oxygen evolution reaction (OER) due to adsorption of the ammonia oxidation products on the BDD surface. The inhibition of the OER enhances ammonia electro-oxidation, which becomes the main reaction. The amino radicals, formed during ammonia oxidation, trigger a reaction chain in which molecular oxygen dissolved in solution is involved in the ammonia electro-oxidation. Nitrogen, nitrous oxide, and nitrogen dioxide were detected as the ammonia oxidation products, with nitrogen being the main gaseous product of the oxidation. (C) 2010 Elsevier B.V. All rights reserved.

Electrochemical behavior of ammonia at Ni/Ni(OH)2 electrode

Christos Comninellis, Agnieszka Kapalka

The electrochemical oxidation of ammonia was investigated on a Ni/Ni(OH)(2) electrode prepared by potential cycling of a Ni electrode in 1 M NaClO4. It was found that oxidation of ammonia is strongly pH dependent and proceeds mainly at pH values above 7. This indicates that NH3 rather than NH4+ is oxidized on nickel electrodes. Oxidation of ammonia occurs in the potential region of Ni(II)/Ni(III) redox activity resulting in formation of a clear peak. Ni/Ni(OH)(2) is not deactivated during ammonia oxidation even at high ammonia concentrations. A considerable fraction of the ammonia was oxidized to nitrate (11%), while the rest were gaseous nitrogen compounds. It is postulated that nitrogen was formed via a mechanism involving direct electron transfer from ammonia to the anode whereas the formation of nitrate involved oxygen transfer from water to an ammonia molecule. (C) 2009 Elsevier B.V. All rights reserved.

Elsevier2010

Ammonia oxidation to nitrogen mediated by electrogenerated active chlorine on Ti/PtOx-IrO2

Christos Comninellis, Agnieszka Kapalka, Nina-Luisa Michels

The electrochemical oxidation of ammonia (NH3 and/or NH4+) in the presence of chloride was investigated on a Ti/PtOx-IrO2 electrode. It was shown that ammonia is effectively removed from solution via electrogenerated active chlorine. Based on mass balances, nitrogen is postulated to be the main product of ammonia electrolysis. In the bulk, the concentration of chloramines was low. This could be explained by the fact that the oxidation of ammonia takes place close to the electrode surface where an excess of chlorine relative to ammonia is ensured during the process. This results in the oxidation of ammonia to N-2 and in a local pH decrease. As a result, chloramines were decomposed in the proximity of the electrode prior to diffusing into the bulk. (C) 2010 Elsevier B.V. All rights reserved.

Elsevier2010

Transition metal clusters on h-BN/Rh(111)

Hamed Achour

The main aim of this dissertation is to study the catalytic aspects of very small transition metal clusters supported on h-BN/Rh(111). The catalytic reactions of interest were ammonia synthesis and CO oxidation on mass selected and soft landed iron and platinum clusters, respectively. A special focus was devoted to the investigation of the stability of these very small clusters on h-BN before and after the reaction. It was found that Pt7 clusters soft landed at room temperature with an energy of 1.2 eV per atom and annealed to 700 K exhibit Smoluchowski ripening. Above this temperature, these clusters undergo partial intercalation between the h-BN monolayer and the Rh(111). The intercalation initiates at 900 K and becomes more pronounced when the clusters are annealed under gas reaction conditions. A high catalytic activity was observed when the Pt catalyst remains supported on h-BN. In this case, the reaction starts at 480 K and follows Langmuir-Hinshelwood mechanism. However, the catalytic activity strongly reduces when the Pt clusters undergo partial intercalation. Nevertheless, in this case the reaction starts at only 380 K, revealing a reduction by 100 K in the Pt poisoning as a result of substrate effect and charge redistribution. In a second trail, the cluster-h-BN interaction was studied through h-BN irradiation with Pt clusters, at room temperature, within an energy window of 30-416 eV/atom. The results show that even irradiation at 30 eV/atom can lead to entire non thermal intercalation of the clusters. The energetic Pt clusters were found to be site selective as they settle only under the h-BN wires, in contrast to soft landed Pt7 clusters that were found to settle at the side edge of the h-BN depressions. After being exposed to irradiated Pt clusters, at an energy above 100 eV/atom, the h-BN layer starts to display visual cavity-type defects. These defects, induced by collision, disappeared after annealing to 600 K under gas reaction leaving behind an h-BN with no visual defects. CO temperature desorption spectroscopy (TDS) indicates that the intercalated Pt is inactive towards CO oxidation due to h-BN screening. Subsequently, we were able to produce a catalyst system made of soft-landed Pt/h-BN/intercalated Pt/Rh(111) using a combination between soft and energetic Pt deposition with which a reduction by 100 K in CO poisoning was obtained. Finally, ammonia synthesis was conducted on soft landed Fe clusters, supported on h-BN/Rh(111) and deposited at 100 K under Ultra High Vacuum (UHV). Scanning tunneling microscopy (STM) displays two imaging states of the as deposited clusters; a ring state surrounding h-BN depression which disappears above 300 K and a dot like structure. The Fe clusters are found to grow by Ostwald ripening after annealing to 600 K and above this temperature, they endure partial intercalation. It was found that, in the presence of iron, the nitrogen reaction gas experienced an exchange with the nitrogen species forming the h-BN layer. TPR (Temperature Programmed Reduction) of N

_2

with hydrogen revealed that nitrogen reduction occurs at 620 K under high vacuum following the Haber-Bosch method. The detection of reaction intermediates NH and NH

_2

together with desorbed atomic nitrogen, during ammonia synthesis, confirms that NH

_3

formation involves the stepwise hydrogenation of adsorbed nitrogen.

EPFL2017