REDUCTION OF CU2+(AQ) BY CO2- - 1ST STEPS AND THE FORMATION OF COLLOIDAL COPPER

Aqueous solutions of Cu(ClO4)2 and HCOONa were pulse-radiolyzed and the changes in optical absorption and conductivity recorded. CO2- reduces Cu2+ to yield Cu+ (k = 2 x 10(9) M-1 s-1). Cu+ reacts with CO2- to yield CuCO2 (absorption at 480 nm (pH = 7.3); k = several 10(10) M-1 s-1). CuCO2 exists in two forms depending on the pH of the solution (385 nm at lower pH's). CuCO2 reacts with Cu2+ to form 2Cu+ + CO2 (k = 5 x 10(8) M-1 s-1) and with Cu+ to form Cu2+ (350 nm; k approximately 10(8) M-1 s-1). Cu2+ and Cu+ are the precursors to the formation of colloidal copper particles. Three distinct time periods during the reduction of Cu2+ to copper metal can be distinguished: (1) formation and disappearance of the first intermediates, i.e., CuCO2 and Cu2+, within about 100-mu-s; (2) formation of nuclei within milliseconds to seconds; and (3) rather sudden formation of larger aggregates within seconds. At the beginning of this sudden formation, the particles absorb in the UV; they are probably oligomers of nonmetallic character. In the later stages of formation, larger metallic particles absorbing in the UV and visible are present.