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Electrocatalysis of greenhouse gases to fuels or chemical feedstocks on well-characterized materials

Final Report Summary - GHGELCAT (Electrocatalysis of greenhouse gases to fuels or chemical feedstocks on well-characterized materials)

The fellow studied electrochemical reactions involving nitrogen- or carbon-containing species on well-defined surfaces. Such reactions are highly important for applications related to (i) the energy conversion, storage and utilization, namely transformations of electrical energy to chemical energy by making or breaking chemical bonds (ii) the environmental protection, namely cuts in greenhouse gas emissions,
water remediation, removal of toxic compounds from waste streams etc (iii) and to the chemical industry, namely production of useful chemicals or feedstock molecules by electrochemical means (electrosynthesis). The main scientific objective of this project was to develop a better fundamental understanding of the underlying processes during the electrochemical conversions within the nitrogen or carbon cycle. In addition to the scientific objective, the second central focus of the work program was to provide training on complementary skills and
facilitate the career development of the research fellow.

The main achievements during this project are summarized in the following:

* The fellow was trained on single-crystal preparation techniques, on performing electrochemical experiments using single-crystal electrodes, and on utilizing complementary methods such as vibrational spectroscopy, online electrochemical mass spectrometry, online ion chromatography.
* The fellow improved his project-management skills, presentation, communication and networking capabilities and finally reached an independent position after the end of this project.


The main conclusion in this project is that the selectivity and even the mechanism of electrochemical reactions involving nitrogen- or carbon- containing species can be strongly dependent on the surface structure:

* Pt(100) is unique in carrying out the ammonia oxidation and the nitrite reduction to nitrogen gas. Instead, Pt(111) is inactive for ammonia oxidation. The mechanism of the ammonia oxidation reaction on Pt(100) involves a deprotonation step preceding the electron transfer, before the N-N forming step, a feature which is not captured by the existing mechanisms. *NO formation takes place on surface defects in parallel to ammonia oxidation. The key step for N-N coupling involves the dimerization of *NH or *NH2 species; the formation of *N is highly unfavorable on Pt(100).

* The nitric oxide reduction yields the same final product (ammonium) on Pt(111) and Pt(100), but this is done via two different mechanisms. At low coverages, *NO reduction proceeds via *NHO on Pt(100) and *NOH on Pt(111). At high coverages, *NOH is the first hydrogenation product on Pt(100), while on Pt(111) both *NHO and *NOH can be formed. On Pt(100), the coverage-dependent reaction pathways, surface availability and surface atom occupancy is reflected to a transition from first- to second- order kinetics as the *NO coverage decreases. The above have a very important implication in electrocatalysis: A reaction mechanism obtained for a certain surface and coverage cannot be directly extrapolated to structurally different electrodes, where the surface abundance and stability of intermediates and spectators are dissimilar.

* The nitric acid reduction in acidic solutions forms HNO2, which is in equilbrium with *NO which adsorbs on Pt(111), Cu(111) or Cu(100). However, Pt(100) can form *NOH which is oxidized to *NO at more positive potentials. In alkaline solutions, platinum is not active for nitrate reduction. Cu(111) reduces nitrate to nitrite, while the main product on Cu(100) is hydroxylamine.