chemicals were of reagent
quality and used without further purification. MnSO4·5H2O (99.5%), Na2SO4 (99.0%), tetrabutylammonium
chloride (TBA+Cl–, 99.5%), NH4NO3 (99.0%), NH3 (28.0% in water), KMnO4 (99.3%), NaOH (97.0%), H2PtCl6·6H2O (98.5%), and H2SO4 (95.0%) were obtained
from Wako Pure Chemicals. AgNO3 (99.8%) was purchased from
Ishizu Chemical, Ltd. All solutions were prepared with doubly distilled
water and then deoxygenated by purging with purified nitrogen gas
for more than 20 min immediately before each experiment.