INFLUENCE OF METALLIC SILVER AND OF PLATINUM-SILVER BIMETALLIC DEPOSITS ON THE PHOTOCATALYTIC ACTIVITY OF TITANIA (ANATASE AND RUTILE) IN ORGANIC AND AQUEOUS-MEDIA

The influence of deposited silver upon the photocatalytic activity of titania in the rutile and anatase allotropic forms has been studied in three different reactions. The common feature found for these three r eactions was an electron transfer from illuminated TiO2 to silver part icles. In the first reaction (platinum photodeposition), it has been s hown that the initial presence of metallic silver orientates the local ization of subsequent Pt photodeposits with, in particular, a 100% sel ectivity to Pt deposition on top of silver particles or agglomerates p reviously deposited on anatase. In the second reaction (2-propanol oxi dation), Ag deposit was found beneficial for the activity of rutile an d detrimental for that of anatase. For rutile which is less active, si lver helps for the electron-hole pair dissociation. By contrast, for a natase, the negatively charged Ag particles preferentially attract pho toholes and become recombination centers, thus decreasing the photocat alytic activity. For the third reaction (dehydrogenation of 2-propanol ), Ag/TiO2 catalysts were found very poorly active, as expected for a group I-B metal. However, in the additional presence of platinum depos its, Pt/Ag/TiO2 catalysts are as active as their Pt/TiO2 homologues, t hus confirming that Pt is deposited on top of silver with good electro nic contacts between both metals. The three photocatalytic reactions h ave common electronic processes based on the photoelectron generation on titania and the subsequent electron transfer to the metal(s). (C) 1 998 Elsevier Science S.A.