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 (<0.2 ppm) while the soluble POM catalyst is self-separated from the pure precipitated metal that is obtained, retaining its catalytic function. The process is efficient in both one-pot and two-pot system. However, reduced tungstates of the formula XW12O40n-, X = P, Si or H-2 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. (C) 2004 Elsevier B.V. All rights reserved.