The direct CO2 electrochemical reduction on model platinum single crystal electrodes Pt(hkl) is studied in [C2mim(+)][NTf2(-)], a suitable room temperature ionic liquid (RTIL) medium due to its moderate viscosity, high CO2 solubility and conductivity. Single crystal electrodes represent the most convenient type of surface structured electrodes for studying the impact of RTIL ion adsorption on relevant electrocatalytic reactions, such as surface sensitive electrochemical CO2 reduction. We propose here based on cyclic voltammetry and in situ electrolysis measurements, for the first time, the formation of a stable adduct [C2mimH-CO2(-)] by a radical-radical coupling after the simultaneous reduction of CO2 and [C2mim(+)]. It means between the CO2 radical anion and the radical formed from the reduction of the cation [C2mim(+)] before forming the corresponding electrogenerated carbene. This is confirmed by the voltammetric study of a model imidazolium-2-carboxylate compound formed following the carbene pathway. The formation of that stable adduct [C2mimH-CO2(-)] blocks CO2 reduction after a single electron transfer and inhibits CO2 and imidazolium dimerization reactions. However, the electrochemical reduction of CO2 under those conditions provokes the electrochemical cathodic degradation of the imidazolium based RTIL. This important limitation in CO2 recycling by direct electrochemical reduction is overcome by adding a strong acid, [H(+)][NTf2(-)], into solution. Then, protons become preferentially adsorbed on the electrode surface by displacing the imidazolium cations and inhibiting their electrochemical reduction. This fact allows the surface sensitive electro-synthesis of HCOOH from CO2 reduction in [C2mim(+)][NTf2(-)], with Pt(110) being the most active electrode studied.
ABSTRACT:Electrooxidation of CO at the Pt(hkl)-electrolyte interface in two different room temperature ionic liquids (RTILs) is probed to be surface sensitive. Provided data reveal a specific surface structure, (110) sites, which selectively activate CO oxidation in RTILs. This new knowledge is crucial for designing the next generation of Pt nanosized electrocatalysts for the CO oxidation reaction by increasing that type of site on the catalyst surface.A great deal of attention has been devoted to the use of room temperature ionic liquids (RTILs) especially as substitutes for volatile organic solvents used in organic synthesis at industrial scale, 1 but also for the synthesis of inorganic and organometallic compounds.2 Air and moisture-stable RTILs have been regarded as environmentally friendly and alternative solvents thanks to its unique physicochemical features.3-4 For these reasons, RTILs particularly concentrate a great interest in the field of energy storage.5 Carbon monoxide is a common C1 building block in synthesis, extensively used for many different homogenous and heterogeneous catalyzed reactions, in both liquid and gas phase. One of the most relevant reactions conducted via gas phase heterogeneous catalysis is CO hydrogenation following Fischer-Tropsch reaction to form long-chain hydrocarbons. Among the liquid phase catalyzed reactions, electrochemical CO oxidation corresponds to one of the most technologically relevant reactions, 6 since adsorbed CO acts as a poisoning intermediate in many catalytic processes. For this reason, this reaction represents the rate determining step for the total electrochemical combustion of the liquid fuels used in direct fuel cells, such a direct formic acid (DFAFC), methanol (DMFC) or ethanol (DEFC) fuel cells.
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