The photochemical splitting of water into H2 and O2 has fascinated photochemists since the OPEC oil embargo in 1973, yet progress has been slow largely due to the challenge of oxidizing water, an energetically uphill reaction. Much of this research has utilized solar light, semiconductor catalysis and “sacrificial electron donors” (SED); that is, molecules that facilitate H2 evolution at the expense of the degradation of valuable chemicals, an approach has been criticized as it consumes valuable chemicals. Photochemical H2 generation and water treatment are usually considered as orthogonal processes, that is, photocatalytic processes are usually designed with either one of the two outcomes in mind. Instead, we propose them as parallel or concurrent processes that in the future may provide simultaneous solutions to both H2 generation and water decontamination. Whereas alcohols have been the preferred choice for SED, many molecules that can be easily oxidized can fulfill this role, including contaminants that are present in most rivers. In our own research with decorated TiO2 we find that waters from regional river sources generate much more hydrogen than pure water. Preliminary results also show that the bacterial content in water is linked to the amount of H2 generated; indeed, bacterial growth is inhibited under these conditions. We propose that parallel technologies that couple H2 generation and water quality catalysis should be the preferred strategy and that strict water splitting may not be the most practical, valuable or efficient route to H2 generation.
Some of the materials used for hydrogen generation also have interesting properties in the context of synthetic organic chemistry. The use of heterogeneous catalysis has key advantages compared to its homogeneous counterpart, such as easy catalyst separation and reusability. However, one of the main challenges is to ensure good performance after the first catalytic cycles. Active catalytic species can be inactivated during the catalytic process leading to reduced catalytic efficiency, and with that, the loss of the advantages of heterogeneous catalysis. Here we present an unconventional approach in order to extend the catalyst lifetime based on the crop rotation system used in agriculture. The catalyst (Pd@TiO2) is used in alternating different catalytic reactions, which reactivate the catalyst surface, thus extending the reusability of the material, and preserving its selectivity and efficiency. As a proof of concept, different organic reactions were selected and catalyzed by the same catalytic material during target molecule rotation.
