Fixed Energy X-ray Absorption Voltammetry and Extended X-ray Absorption fine Structure of Ag nanoparticle electrodes

•In-situ and operando X-ray Absorption spectroelectrochemistry•Direct observation of adsorption of CHCl3 on Ag nanoparticles•Ag/AgX/X− transitions: Ag states of charge in inorganic halide solutions•Direct observation of Cl− adsorption–desorption on Ag nanoparticles

FEXRAV (Fixed Energy X-ray Absorption Voltammetry) has been recently introduced as a powerful tool in electrochemistry. It consists in measuring the X-ray absorption coefficient at a fixed X-ray energy while the electrochemical potential of the material under investigation is varied at will in a conventional three-electrode electrochemical cell. In the case of Ag electrodes, the usefulness of FEXRAV might appear limited: first of all, Ag cannot assume an extended variety of oxidation state values as other transition metals. In addition, only the Ag–K edge is accessible in the hard X-ray region that is needed for operando experiments. In turn, the final states are of p character, and therefore any charge state variation bound to the d orbitals (as happens in the electrochemistry of Ag) is difficult to observe in the XANES (X-ray Absorption Near Edge Spectroscopy) region. However, Ag has plentiful applications in electrochemistry, in particular for what concerns the reduction of halogenated organic compounds. In this work we evidence the capabilities of FEXRAV in the case of Ag electrodes under working conditions in the presence of organic and inorganic halides. More specifically and for the first time, the adsorption of aqueous trichloromethane at a potential preceding the electron transfer reaction is observed. This paper therefore represents a significant advance for FEXRAV, and a fundamental contribution to the elucidation of the exceptional electrocatalytic activity of silver toward hydrodehalogenation reactions.

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