From catalysis by metals to bifunctional photocatalysis

Catalysis by metals, was transposed to "Bifunctional Photocatalysis", based on noble metals deposited on titania. Both, participation (i) of the photo-active oxidic support and (ii) of the deposited metal have been successfully analyzed, delimited and associated. For both reactions involving hydrogen (Cyclopentane-Deuterium Isotope Exchange (CDIE) and alcohol dehydrogenation), the respective roles of electrons and holes were clearly put in evidence. The photogenerated electrons were found to be spontaneously transferred to the metal nano-particles because of the alignment of the Fermi levels of both solid phases. They neutralize protons H+ (or deuterons D+) into atoms before their recombination and evolution of dihydrogen (or HD). Simultaneously, photogenerated holes h(+) neutralize anionic species in oxidation reactions responsible for the activation of organic molecules. These two anionic species were OD- deuteroxyl surface groups in CDIE and alcoholate anions RO- in alcohol dehydrogenation, respectively. Both reactions exhibited a stoichiometric threshold which was exceeded several hundreds of times, clearly defining the real catalytic character of both reactions. The titania-deposited noble metals appeared as auxiliaries or co-catalysts working independently of UV-irradiation for making the reaction run catalytically. Eventually, the concepts of bifunctional photocatalysis were applied to account for interphasic photocatalytic processes such as hydrogen transfer, electron transfer, photosensitization in the visible via CdS addition.