ELECTROCATALYTIC REDUCTION OF CARBON-DIOXIDE BY NICKEL(II) COMPLEXES OF CYCLAM AND C-ALKYLATED AND N-ALKYLATED CYCLAMS

The electrocatalytic reduction of CO2 on mercury in water with nickel(II) complexes of cyclam and C- and N-alkylated cyclams was studied in terms of the activity, the stability and selectivity. First, the mechanism of the electrocatalysis with Ni(II)-cyclam was studied in detail by cyclic voltammetry, polarography and electrocapillarity of aqueous solutions with and without the catalyst under N2, CO and CO2. It is concluded from these results that i) the active catalytic species was the adsorbed Ni(I)-cyclam on mercury and ii) the electrocatalytic reduction of CO2 proceeded not by the surface EC catalytic mechanism, but by the electrocaltalytic mechanism. Namely, the adsorbed Ni(I)-cyclam did not behave as a surface bound mediator, but acted as a surface catalytic site and thus the rate of the electrocatalysis was potential dependent. Secondly, the effect of alkylation of cyclam as the ligand of nickel(II) complexes on the activity and the stability was studied by CV. In terms of the reduction potential and the stability, the complex of N-monomethylated cyclam was the best catalyst among the catalysts studied, although the highest catalytic peak current was observed with Ni(II)-cyclam. Finally, the selectivity was studied with DEMS and the selective formation of CO and H2 at low and high negative potentials, respectively, was found for all the complexes studied.