In whatever photocatalytic process you MUST have a photosensitizer. If you use a dye as a photosensitizer, what is a substrate? What is a role of HPA in overall process? If you want to use HPA as a photosensitizer, why do you need a dye? As an example, take H4SiW12O40 at pH < 4, add some ethanol and expose the solution in quartz reactor to sunlight. The solution becomes blue due to a formation of a heteropolyblue, HBA, which has a reduced W-atom.