WPA was prepared according to the literature [30] and recrystallized
prior to use. Salts AgxH3xPW12O40 (x = 1.0, 2.0, 2.5) were
obtained by a standard procedure where stoichiometric quantities
of AgNO3 were added to a methanol–water mixture (1:1) of 0.1 M
WPA. An aqueous solution of AgNO3 (0.04 M) was added drop wise
at a rate of 1 mL/min under vigorous stirring for 6 h in the dark, at
room temperature.