The increase of carbon dioxide in the atmosphere has generated dramatic weather perturbations and change of the global average temperature of the terrestrial atmosphere and the oceans. The CO₂ conversion in harmless species to the environment or that can be useful as fuels, appears as a feasible alternative to decrease this gas concentration⁽¹⁾. The electrochemical or photo-electrochemical transformation of CO₂ are promising processes. In this work the electrochemical reduction of CO₂ is studied in aqueous solution, using cationic metallo-tetraruthenated porphyrins (MTRP, where M= Co, Ni,Mn, Zn). The macrocycle consists in a tetrapyridylporphyrin coordinated with four Ru (II) complexes in the periphery of the macrocycle. These kinds of compounds have been chemically modified and as a result several modifications on electrodic surfaces have been carried out. The modified electrodes are based in conducting polymers and electrostatically assembled layers. The latter have been achieved by the incorporation of polyoxometallates and anionic porphyrins or phthalocyanines. The modified electrodes have been characterized by Raman, uv-visible and IR-ATR spectroscopies; AFM microscopy and conventional electrochemical techniques. The reduction of CO₂ was verified at aprox. -0.7 V vs Ag/AgCl. After potential controlled electrolysis in presence of CO₂ a wide distribution of reaction products was determined by GC, GC-MS and HPLC-ESI-MS analytical methods. Main products are formic acid, formaldehyde and methanol with traces of CO and H₂. The selectivity is controlled by the metal center in the cavity of the porphyrin and also by the roughness of the modified electrode where a subtle interplay between coordination properties and adsorption of initial CO₂ derivatives are the key steps but remain unclear. When electrolysis was carried out under white light irradiation, distribution of products changed but turn over frequency increases.  Several models of reaction intermediaries have been detected by flash photolysis and IR spectroelectrochemistry in suitable solvents.  Some of them are carbonyl complexes of the electrochemically reduced macrocycles, but the presence of oxalate species has also been detected.  In consequence the probable mechanisms may involve a primary coordination of CO₂molecule in a reduced metal center of the macrocycle followed by a series of electrochemical steps that, depending on the morphology of the electrode (microenvironment, proton availability) , can originates reduction products with different number of transferred electrons.  

⁽¹⁾ M. Alvarez-Guerra, J. Albo, E. Alvarez-Guerra, A. Irabien. Energy Environ. Sci. 2015, 8, 2574

Acknowledgements: Financial support from  FONDECYT  #1141199 and RC 130006, granted by the Fondo de Innovación para la Competitividad del Ministerio de Economía, Fomento y Turismo, Chile, are acknowledged. J.H. , D.Q. and C.D thank CONICYT  scholarships .