Hydrogen production through photoreforming of oxygenated organic substrates over Cu/TiO2 catalysts

Hydrogen is the ideal candidate to fulfill the growing energy demand in a sustainable manner because of its high energy content and no emission of greenhouse gases from its combustion. Currently most of hydrogen generation techniques involve the employment of fossil fuels, with consequent production of toxic greenhouse gases. The possibility to produce hydrogen by means of photocatalytic processes using the solar radiation as energy source fits in perfectly with the switch to a more sustainable energy production. The solar photocatalytic hydrogen generation can be achieved by reforming organic substances contained in civil or industrial wastewaters. This could allow to combine water decontamination with production of an energy carrier starting from a renewable source, the solar radiation. Hydrogen production through photoreforming of organic species using copper-modified TiO2 photocatalysts is attracting a considerable attention during last years. It is reported that the doping of TiO2 with copper species helps enhance to separate the electron-hole pairs, thus reducing the occurrence of the recombination reaction, and extend the light absorption to the visible range of the solar spectrum. The choice of copper is supported by its low-cost and abundance in Earth’s crust. In particular, the use of catalysts prepared by in situ photodeposition processes, with nanometric size, could represent a straightforward promising strategy to improve the process efficiency. In this study, the production of hydrogen by photocatalytic reforming of oxygenated organic species was investigated using metal copper-modified TiO2 nanoparticles, prepared “in situ” by reduction of cupric ions. The behavior of different alcohols and organic acids to undergo photoreforming with hydrogen production was investigated and compared. A characterization of the catalysts recovered at the end of the runs revealed the formation of zero-valent copper nanoparticles on the catalysts surface. The effect of adopting different crystallographic phases of TiO2 was also assessed. In particular, three TiO2 commercial samples of different crystalline phases (mixed-phase P25, pure anatase and pure rutile) were employed to prepare Cu-doped TiO2 materials by in situ copper photo-deposition. The resulting samples were extensively characterized by several complementary techniques and tested as photocatalysts for hydrogen production through photoreforming of alcohols. Correlations between hydrogen production rates and physical-chemical properties (structural, compositional and optical properties) of the samples are discussed. The analyses highlighted the major roles played by physical sizes and surface properties of TiO2 particles in determining the morphology, the dispersion of zero-valent copper nanoparticles on TiO2 surface and, ultimately, the photocatalytic performances. A modeling investigation was performed through the development of a simplified kinetic model taking into account the mass balance equations for the main reactive species involved in the photocatalytic system. The kinetic model was tested to predict hydrogen generation rates for experimental runs carried out at different initial concentrations of sacrificial agent (methanol and glycerol) and at varying photocatalyst load. The modeling investigation allowed to estimate for the first time the equilibrium adsorption constants and the kinetic constant for the hole-capture by sacrificial agents, as well as the quantum yield and the rate constant of electron-hole recombination for the copper modified-TiO2 nano-photocatalyst. The simultaneous presence in the aqueous matrix of an inorganic ion, that is chloride, was also investigated when formic acid was adopted as sacrificial agent. The effect on hydrogen generation rate of the initial concentrations of formic acid, chloride and cupric ion, and pH values was evaluated. These experimental outcomes were rationalized within a consistent reaction mechanism able to predict the system behavior under different operating conditions. Therefore, this critical literature review has been performed with the aim of providing a complete and reliable approach to promote new competitive processes able to use waste organic streams for hydrogen generation through photacatalytic system based on solar energy.