Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
The development of catalysts for the Hydrogen Evolution Reaction (HER) on large scales require to use abundent and cheap materials. Research efforts have been directed in the last years to the preparation and study of transition metal-based inorganic catalysts for the HER. Cobalt-based systems have thrived, as molecules, nanoparticles or amorphous materials. The cobaloxime class of compounds have been thoroughly studied in the recent years, and it has recently been shown that these molecules undergo a reductive damage of their ligands under electrocatalytic conditions. This phenomenon leads to the formation of nanoparticles, which are active for the HER reaction.
We have used Co K-edge X-ray absorption spectroscopy to study the formation of these nanoparticules in situ, as well as their structure under electrocatalytic conditions. We show that, in acetonitrile, the particles formed are about 100 nm in size and made of metallic cobalt with a very low level of order. Sub-domains no larger than 1 nm (about 20 atoms) can be evidenced by EXAFS analysis. Under aqueous conditions, the particles are essentially converted into an oxide, which is turned back into a metal as the cathodic potential is decreased. An oxide layer remains at the surface of the particles, which thickness depends on the potential applied to the system. This layer does not, however, completely disappear at high potentials. Although the activity scales with the oxide layer, it is not directly related to the metallic surface available, suggesting a more complex mechanism. These findings further support mechanistic proposals concerning the reduction of protons in to hydrogen at metlalic surfaces and have implications for the design of both electro and photocatalysts ofr solar-to-fuel conversion reactions.
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