Factors affecting quantum yields for chlorine formation in the solar photolysis of acidic chloride solutions containing hexachloroiridate (IV)

Abstract

We have examined quantum yields for chlorine production from solar photolysis of acidic chloride solutions containing the hexachloroiridate (IV) complex, IrCl 6 2−. Previous investigators found good quantum yields in the middle UV (Φ(Cl 2) = 0.128 at 254 nm), but the yield was found to decrease in the near UV, falling to zero at wavelengths above 488 nm. Under solar photolysis, we find nearly zero yields in closed or static systems. However, when the chlorine produced is swept out with an inert gas, Φ(Cl 2) under solar photolysis is approximately 0.0018, for a solution containing 0.001 M lrCl 6 2− in 6 M HCl solution. Improvements in the quantum yield result from increasing the IrCl 6 2− concentration from 0.001 M to 0.1 M or higher and from increasing the Cl − concentration from 1 M to 5 M; solutions were acidic in all cases. Using a solution of 0.025 M IrCl 6 2−, 0.5 M HCl, and 5 M NaCl, Φ(Cl 2) = 0.0070 was found. Further improvement in the yield is achieved if the hexachloroiridate (III) produced is simultaneously oxidized back to hexachloroiridate (IV) while photolysis is in progress. Under the best conditions of simultaneous photolysis and electrolysis, a chlorine quantum yield of 0.080 is found for solar light between UV cutoff (ca. 320 nm) and the maximum actinic wavelength of the IrCl 6 2−/Cl − system, at 488 nm.