Selective photocatalytic reduction–recovery of palladium using polyoxometallates

Using the polyoxometallate anions (POM) PW12O403−, SiW12O404−, H2W12O406− or P2Mo18O626− as photocatalysts and propan-2-ol as sacrificial electron donor, Pd2+ can be reduced and recovered in metallic form. The method leads to very low final palladium concentrations ( The process is efficient in both one-pot and two-pot system. However, reduced tungstates of the formula XW12O40n−, X = P, Si or H2 that react rapidly with oxygen are preferably photochemically produced in situ in a one pot system, where propan-2-ol/POM/Pd2+ species co-exist. On the other hand, the 2-equivalent reduced P2Mo18O626− is preferably used in a two-pot system, pre-formed upon photolysis of a propan-2-ol/POM system and in turn serves as an easily kept electron storage (since it is inert against reoxidation by oxygen) amenable to reduce Pd2+ ions at will. Some novel results are marked concerning the environmental photocatalysis with POM: i) Upon photolysis of a one-pot system where propan-2-ol/POM/Pd2+ species co-exist and under the same conditions, the rate of photocatalytic reduction of palladium follows the photoxidizing ability of POM (PW12O403−>SiW12O404−>H2W12O406−). That is, POM catalysts that are more efficient in photo-oxidative degradation of organic compounds are also more effective in reductive recovery of palladium. ii) Selectivity in recovery of palladium among mixture of non-noble metal ions (Cu2+ and Ni2+), can be achieved even when they are in more than 10-fold excess.