Photocatalytic Cleavage Of Water Research Articles

Cd-ZnGeON solid solution: the effect of local electronic environment on the photocatalytic water cleavage ability

The efficiency of hydrogen generation through photocatalytic water cleavage was elucidated for calcinated Cd-ZnGeON host systems. The electronic structures of the Cd-ZnGeON solid solution were explored via synchronized X-ray diffraction, X-ray absorption spectroscopy at the K-edge for elements O, N, Zn and Ge, X-ray photoelectron spectroscopy, diffuse reflection spectroscopy, and high-resolution transmission electron microscopy, etc., which were used as sensitive analytical probes. A good correlation was observed between the ability of Cd-ZnGeON for photocatalytic water splitting and the local atomic arrangement in its lattice. Based on advanced structural information about the solid solution, a concise lattice-model was proposed for hydrogen generation through water splitting. The electron-transfer route for hydrogen generation via photocatalytic water cleavage was also traced. The results reveal that the wurtzite Cd-ZnGeON calcinated at 600 °C generates a large amount of hydrogen (3647.25 μmol H2 h−1 g−1).

Sustainable hydrogen production by the application of ambient temperature photocatalysis

Hydrogen production by water splitting is a holy grail of science, being one route to a non-fossil fuel and a potential source of clean energy. It is essential to use a non-polluting energy source to drive the reaction and one way is to utilise sunlight energy, combined with a way to overcome the kinetic limitations of the reaction. The production of hydrogen by the photocatalytic cleavage of water can overcome these energetic hurdles. This area of science is briefly reviewed here, focussing on two routes, namely i) the use of sacrificial agents such as alcohols to act as oxygen scavengers and liberate hydrogen only and ii) by direct water splitting to produce both hydrogen and oxygen. The factors which are important in determining the characteristics of effective photocatalytic water splitting systems are elucidated.

Heterogeneous photocatalytic cleavage of water

The photocatalytic cleavage of water has become of increasing interest in recent years for the manufacture of hydrogen, a clean and renewable energy carrier. The production of H2 using heterogeneous photocatalysts is reviewed in this feature article. Particulate or electrode heterogeneous catalysts under light irradiation can successively reduce H+ into H2 and oxidize water to produce O2, achieving cleavage of water (H2O → H2 + 1/2O2).

Physicochemical properties and photocatalytic hydrogen evolution of TiO2 films prepared by sol–gel processes

Anatase TiO2 films were obtained on glass substrates using a sol–gel method using titanium isopropoxide as a precursor. The thickness of the film was about 140 nm for one coating, and the thickness is controlled by the number of coating cycles. The spectra of UV-VIS absorption indicated that the absorption edge of the TiO2 films is ca. 385 nm, corresponding to the band gap energy of 3.20 eV. We obtained TiO2 films having a high activity for the hydrogen evolution from photocatalytic water cleavage. By loading with 0.3 wt% Pt rate of hydrogen production increases. No influence of film thickness and calcination temperature on the photocatalytic property is observed.

Intercalation of titanium oxide in layered H2Ti4O9 and H4Nb6O17 and photocatalytic water cleavage with H2Ti4O9/(TiO2, Pt) and H4Nb6O17/(TiO2, Pt) nanocomposites

Titanium oxide and platinum have been intercalated into H 2 Ti 4 O 9 and H 4 Nb 6 O 17 by the successive reactions of H 2 Ti 4 O 9 and H 4 Nb 6 O 17 with [Pt(NH 3 ) 4 ]Cl 2 , n-C 3 H 7 NH 2 and acidic TiO 2 colloid solutions followed by UV light irradiation. The thicknesses of titanium oxide and platinum were less than 0.7 nm. The band gap energy of the titanium oxide incorporated was less than 3.4 eV. The rate constants for the charge injection from excited TiO 2 into the conduction band of H 2 Ti 4 O 9 and H 4 Nb 6 O 17 were determined to be 0.11×10 9 and 0.12×10 9 s -1 , respectively, by measuring the TiO 2 emission lifetimes in TiO 2 incorporatednanocomposites. H 2 Ti 4 O 9 /(TiO 2 , Pt) and H 4 Nb 6 O 17 /(TiO 2 ,Pt) nanocomposites (intercalated with both TiO 2 and Pt) werecapable of water cleavage into hydrogen and oxygen without a hole scavenger, at 0.088–0.104 mmol h -1 , following irradiation from a 450 W Hg lamp, although no noticeable gas was evolved in the presence of H 2 Ti 4 O 9 /TiO 2 , H 4 Nb 6 O 17 /TiO 2 , H 2 Ti 4 O 9 /Pt, H 4 Nb 6 O 17 /Pt and unsupported TiO 2 /Pt. The hydrogen and oxygen evolution was linear with time and the niobate system was fairly efficient.

Definition of the Intrinsic Rate of Photocatalytic Cleavage of Water over Pt-RuO 2/TiO 2 Catalysts

The photocatalytic cleavage of water over Pt-RuO 2/TiO 2 catalysts under near UV illumination is investigated in a semicontinuous flow photoreactor. The effects of operational variables on the intrinsic rate of H 2 production, expressed per unit mass of catalyst per unit "transparency" of the reaction slurry, are investigated. It is shown that the intrinsic rate is a linear function of the intensity of incident illumination, an exponential function of the reaction temperature, and an exponential function of the steady-state pH value of the slurry. An equation describing the intrinsic rate of H 2 production over Pt-RuO 2/TiO 2 slurries is developed and values of the kinetic parameters are defined.

Photogeneration of hydrogen using visible light with undoped/doped α-Fe 2O 3 in the presence of methyl viologen

The visible light-induced hydrogen production by water splitting reactions was studied by using undoped or Rh(III) Cu(II) -doped, with or without RuO 2 loaded α-Fe 2O 3 at different pH values in the presence of methyl viologen (MV 2+) as an electron relay. Loading with Rh(III) and Cu(II) ions enhances the hydrogen production efficiency of the photocatalyst. Rh(III) is more efficient than Cu(II) in increasing the photocatalytic activity of α-Fe 2O 3 This increased activity is explained on the basis of the role of the metal ions loaded in photogenerated electron-hole separation. Simultaneous loading of RuO 2 and Rh(III) Cu(II) ions further enhances the efficiency. The dark recombination reaction of the evolved hydrogen and oxygen gases has also been investigated and α-Fe 2O 3 is found to be a good catalyst for both formation (on illumination) and recombination (in the dark) of the two gases. A reaction mechanism involving all these observations has been proposed to explain the photocatalytic cleavage of water.

1,4-Bis(1,2,6-triphenyl-4-pyridyl)benzene as a novel hydrophobic electron relay for dihydrogen evolution in photocatalytic systems based on lipid vesicles

Abstract 1,4-Bis(1,2,6-triphenyl-4-pyridyl)benzene (“benzoviologen”) is an efficient lipophilic reversible electron relay, which can be used to build structurally organized molecular systems based on lipid vesicles for photocatalytic water cleavage. Benzoviologen has an advantage over the widely used conventional lipophilic viologens, since its reduced forms possess more negative reduction potentials. This promotes photostimulated transmembrane electron transfer and catalytic water reduction at neutral or even slightly basic pH. The kinetic properties of benzoviologen and its capacity to produce one- and two-electron-reduced forms were studied. The ability to couple vectorial transmembrane electron phototransfer with dihydrogen evolution from water over a dispersed Pd catalyst was proven experimentally.

Influence of catalyst parameters and operational variables on the photocatalytic cleavage of water

The photocatalytic cleavage of water over Pt-RuO 2/TiO 2 catalysts under illumination in the near UV (250–400 nm) is investigated in a batch and in a semicontinuous flow photoreactor cell. The effects of operational variables and parameters related to chemical and physical characteristics of the catalyst on the rate of hydrogen production are investigated. It is shown that the rate of the process is insensitive to the details of surface structure of Pt; the rate exhibits a weak maximum at a Pt loading of 0.5 wt% and is independent of Pt loading when this exceeds 1 wt%. The rate of H 2 production is strongly dependent on the pH of the slurry, increasing significantly at pH levels higher than 10. The rate was also observed to be a linear function of the intensity of illumination in the near UV.

Photocatalytic water cleavage using a mixture of ruthenium tris-bipyridyl chloride and zinc phthalocyanine as photosensitizers

Photoinduced electron transfer and hydrogen evolution from water were investigated, using each of the two photocatalysts ruthenium tris-bipyridyl complex [Ru(bip) 2+ 3], zinc phthalocyanine and their mixture, with methyl viologen (MV 2+) as an electron acceptor, ethylene diamine tetraacetic acid as electron donor and colloidal platinum in polyvinyl alcohol (Pt-PVA) as hydrogen redox catalyst, in cetyl trimethyl ammonium bromide (CTAB) (10 −2 M) at pH 6.5. The efficiency of methyl viologen reduction by zinc phthalocyanine increased on addition of [Ru(bip) 2+ 3] complex, using visible light (350–700 nm) from a xenon arc lamp. Optimum concentrations of colloidal platinum in the presence of each of the two metal complexes and their mixtures were also studied. Evolution of hydrogen took place using the two photosensitizers in absence of methyl viologen.

Structural aspects in the photooxidation and photoreduction of water in clay mineral suspensions

The photosensitized cleavage of water is hampered by fast recombination of usually charged intermediates. Charged surfaces carrying the appropriate catalyst for both the oxidation and reduction of the substrate might be used to compete with this recombination. In this study negatively charged clay minerals and positively charged Al(OH)x gels were examined as catalyst supports capable of exerting such electrostatic interactions. Eu3+ cations were used as quenchers of the excited state of the photosensitizer and vectors of the absorbed energy. It is shown that electrostatic effects do play a role in such systems, and that the tridimensional structure of the catalyst supports must be taken into account when designing a heterogeneous photocatalytic water cleavage system.