Photoinduced Hydrogen Production Research Articles

The hydrogenase from the hyperthermophilic archaeon Pyrococcus furiosus: From basic research to possible future applications

The tetrameric hydrogenase from the hyperthermophilic archaeon Pyrococcus furiosus has been genetically characterized and tested as catalyst in a light-mediated in vitro hydrogen production system under different experimental conditions. The genetic analysis of the hydrogenase points out a possible bifunctional nature of the enzyme in which the β and γ subunits might be responsible for elemental sulfur reduction, whereas the δ and α subunits could be associated to the hydrogenase activity. Experiments on photoinduced hydrogen production performed with the P. furiosus enzyme coupled to the semiconductor titanium dioxide in the presence of methylviologen as an electron carrier show that the system evolved H 2 with a rate as high as 0.5 μmoles/min/mg of TiO 2. Interestingly, H 2 is produced even in the absence of MV. Since the experimental data reported here indicate that the inactivation of the electron carrier is the limiting point of the system, the P. furiosus hydrogenase can be considered a promising catalyst for this kind of in vitro system.

Photoinduced hydrogen production from an aqueous solution of ethylene glycol over ultrafine gold supported on TiO 2

Photocatalytic H 2 production from aqueous solutions of ethylene glycol was studied using a catalyst consisting of gold supported on TiO 2. The catalysts were prepared by the deposition-precipitation method, using TiO 2 powder and an aqueous solution of chloroauric acid. Transmission electron microscopy showed that Au was deposited as highly dispersed particles with mean diameters of 2 to 5 nm, depending on the method of pretreatment of the precursor. The effects of precursor calcination temperature, duration of irradiation, gold loading, concentration of ethylene glycol, initial pH and temperature on H 2 production were investigated. The gas phase products of the reaction were mainly H 2 and CO 2. The precursor calcined in air at 673 K proved to be the most active photocatalyst. The yield of H 2 exhibited a weak maximum at a gold loading of about 2 wt.%. The rate of H 2 production was strongly dependent on the initial pH of the suspension; it increased with pH up to neutral pH and decreased thereafter. Increasing the concentration of ethylene glycol and the temperature had a positive effect on the H 2 yield. The activation energy of H 2 production, as given by an Arrhenius plot over the temperature range 284–340 K, was 15 kJ mol −1.

Photoinduced hydrogen production in dilute solutions and organized assemblies

A new class of covalently linked cyanine dye-electron relay compounds have been investigated. As well as 2-[1-ethyl-1,2-dihydro-chinolinyliden-(2)-methyl]-1-ethyl-chinolinium chloride (1), 1-[3-[2-[(1-ethyl-2(1H)-chinolinyliden)methyl]chinolinium-1-yl]propyl]-1′-methyl-4,4′-bi-pyridinium trichloride (2) and 1-[3-[2-[(1-ethyl-2(1H)-chinolinyliden)methyl]chinolinium-1-yl]pentyl]-1′-methyl-4,4′-bipyridinium triiodide (3) were able to form huge molecular associates with a well-defined three-dimensional structure in dilute, aqueous solutions (critical gel concentration ccrit > 2 × 10−3mol 1−1). Photochemical properties of compounds (1)-(3) were different in gels and in solutions (c < ccrit) without organized dye associates. Owing to antenna effects, organized assemblies provide an intense light harvesting effect. Covalently linked viologen groups quench the excited dye intramolecularly with a higher efficiency. Furthermore, the increased quenching efficiency was found to be dependent on the degree of organization. In dilute aqueous solutions (c < ccrit, λEX > 280 nm), compounds (1)-(3) could act as photosensitizers for light-induced hydrogen production (via intermolecular charge separation) with quantum yields of φ12H2 = 2%. In organized assemblies (c >ccrit), intramolecular charge separation [in compounds (2) and (3)] could be observed and hydrogen production experiments were performed successfully.

Immobilization of hydrogenase in nylon gel containing electron mediator and application to an artificial photosystem

Hydrogenase of Desulfovibrio vulgaris was immobilized in nylon gel containing an electron mediator. With the immobilization, the stability for storage and repeated use increased and the optimal pH was shifted to the acidic side. Photoinduced hydrogen production was observed in an artificial photosystem consisting of the hydrogenase gel, metal porphyrin, and mercaptethanol.

Photoreduction of viologens and hydrogen evolution with hydrogenase

Ethyl-, propyl-, and n-butyl-viologens are found to be electron carries for photoinduced hydrogen production and also can be cofactors of hydrogenase. As these viologens are photoexcited by visible light irradiation or sunlight, simpler systems without another photosensitizer can be developed for the photochemical utilization of solar energy.