Copper modified-TiO2 catalysts for hydrogen generation through photoreforming of organics. A short review

An intense scientific activity was recorded during the last several years in the field of preparation, characterization and use of copper-based TiO2 photocatalysts for hydrogen generation through photocatalytic reforming of organics. Different copper species were used dissolved in aqueous solution or incorporated on the TiO2 surface as single co-catalyst or in the presence of a second catalyst (e.g., graphene, carbon fibers) to (1) effectively separate the electron-hole pairs, thus reducing the occurrence of the recombination reaction, and (2) extend the light absorption to the visible range of the solar spectrum. Many organic species (e.g., methanol, glycerol, formic acid) were proposed as sacrificial agents for hydrogen generation, although the prevailing idea is that of using organic compounds currently found in industrial wastewaters. The pH value was recognized as a fundamental variable in photocatalytic H2 generation via copper modified-TiO2 catalysts. A positive effect to promote hydrogen generation was associated to an increase in pH until moderate alkaline values. On the other hand, a release in the solution of cupric ions and a consequent decrease in photocatalytic activity were observed when decreasing pH. A relevant lack of information was recorded about the efficiencies of hydrogen generation which were reported only in few papers. Therefore, this critical literature review has been performed with the aim of providing a complete background to select the most efficient approaches and eventually promote new competitive systems for hydrogen generation via photoreforming for industrial applications.