ELECTROREDUCTION OF CARBON-DIOXIDE BY METAL PHTHALOCYANINES

Experimental results on the electroreduction of CO2 on gas diffusion electrodes modified by M-Pc, reported in a previous paper [N. Furuya and K. Matsui, J. electroanal. Chem. 271, 181 (1989)], are interpreted qualitatively by means of Taube's theory based on the Huckel model [R. Taube, Z. Chem. 6, 8 (1966); R. Taube, Pure appl. Chem. 38, 429 (1974)]. The reason why some of the M-Pc produce CO or CH4 while others yield HCOOH is explained by the electron configurations in the metallic atoms. The reduction of CO2 to CO is attributed to the strongly electron-donating HOMO of [M-Pc]n- arising from the electron-accepting LUMO of neutral M-Pc which spreads over the nitrogen atoms surrounding M. The final production of CO occurs when the created CO is promptly desorbed from the metallic atom M having such a bare d-shell with the doubly occupied 2a1g (d(z2)) orbital. If, on the contrary, the CO molecule is bound to the metallic atom by a sigma-bonding, the reduction may proceed up to CH4 production. In metals having outermost s or p electrons, the electron occupation of the spread LUMO by ionization will not take place and such a strong reduction as to produce CO may not be possible. Then the electron transfer from M-Pc to CO2 will result in the production of [CO2]- followed by HCOOH creation instead of CO. The anion [CO2]- bents as angle OC-O = 134-degrees, and the C- will probably form a sigma-bond with d(z2) or p(z) of M, making the so-called carbon-dioxide complex, until further reduction to HCOOH.