P. Maruthamuthu

Dye sensitized hydrogen evolution from water

Abstract Photochemically stable catalysts for continuous production of dihydrogen from water in the presence of visible light are yet to be developed. Most of the proposed systems suffer from loss of activity with use, at varying extent. In the present investigation, the photoactivity of Pt/TiO 2 system in the visible region is improved by the addition of the sensitizer ([Ru(dcbpy) 2 (dpq)] 2+ ) [where dcbpy=4,4′-dicarboxy 2,2′-bipyridine and dpq=2,3-bis-(2′-pyridyl)-quinoxaline] leading to efficient water reduction. This system is relatively inexpensive, reproducible, extremely stable and efficient in conversion of light into dihydrogen in aqueous solution. In order to obtain maximum information about the performance of TiO 2 –Pt catalysts, we varied the components and conditions of the system for water reduction. The dependence of the dihydrogen evolution rate on the amount of catalysts, concentration of the sensitizer and percentage of platinum on TiO 2 has been studied. In addition to the above, a comparative study on the photocatalytic activities of Pt/TiO 2 and Pt/ZnO in the presence of the above sensitizer for the production of hydrogen was also made.

Photocatalytic hydrogen production by dye-sensitized Pt/SnO2 AND Pt/SnO2/RuO2 in aqueous methyl viologen solution

Abstract Hydrogen photoproduction by a rarely studied, wide band gap semiconductor, SnO2, in powder form loaded with Pt and RuO2 and sensitized by Ru(bpy)32+ and organic dyes or with low band gap semiconductor, CdS, was investigated. Acriflavin, Eosin Blue, Rhodamine B, Rose Bengal and Fluorescein were used as photosensitizers in the presence of methyl viologen and with or without a sacrificial agent, EDTA. The effects of variation in concentrations of Ru(bpy)32+ and MV2+ were studied. The maximum rate of hydrogen production was observed at [Ru(bpy)32+] = 3.75 × 10−5 mol dm−3 and [MV2+] = 2.5 × 10−5 mol dm−3. The efficiency of the sensitizers in hydrogen production was found to decrease in the order: Eosin Blue > Rose Bengal > Ru(bpy)32+ > rhodamine B = acriflavin > fluorescein.

Hydrogen evolution from water with visible radiation in presence of Cu(II)/WO3 and electron relay

Cu(II)-doped WO3 particles in presence of methylviologen and visible radiation are found to evolve hydrogen from water more efficiently. Methylviologen has been used as an electron relay for the transfer of conduction band electron of the semiconductor to the hydrogen ion present in the aqueous solution. Increase in the molar percentage of Cu(II) ions is found to increase the hydrogen evolution rate.

Hydrogen production with visible light using metal loaded-WO3 and MV2+ in aqueous medium

Hydrogen production with visible light using Pt, Pd and Rh loaded-WO3 particles suspended in aqueous methyl viologen solution has been achieved with high efficiency in the absence of any sacrificial agent.

Hydrogen generation using Cu(II)/WO3 and oxalic acid by visible light

Abstract Radiations of wavelength ⩾ 410 nm (∼99% intensity at 436 nm) were found to effect the photo-Kolbe reaction in oxalic acid in the presence of Cu(II) doped WO3 particles, simultaneously generating CO2 and H2. The conduction band electrons of the semiconductors are, possibly, involved in the reduction of H+ to H2 in the presence of Cu(II) as dopant. The diffuse reflection measurements showed that Cu(II) doped WO3 enhanced the absorption of the semiconductor in the visible region of the solar radiation. The rate of disappearance of oxalic acid followed Langmuir kinetics. Experiments conducted under direct solar radiation also proved that the Cu(II)/WO3 catalyst is efficient in the decarboxylation of oxalic acid.

Photogeneration of hydrogen using visible light with undoped/doped α-Fe2O3 in the presence of methyl viologen

Abstract The visible light-induced hydrogen production by water splitting reactions was studied by using undoped or Rh(III) Cu(II) -doped, with or without RuO2 loaded α-Fe2O3 at different pH values in the presence of methyl viologen (MV2+) 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-Fe2O3 This increased activity is explained on the basis of the role of the metal ions loaded in photogenerated electron-hole separation. Simultaneous loading of RuO2 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-Fe2O3 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.

Photocatalytic hydrogen production with semiconductor particulate systems: An effort to enhance the efficiency

Abstract Hydrogen production in visible light (λ = 437 nm) with the following semiconductor particulate systems, Ag(I)/WO3, Fe(III)/WO3 and Cr(III)/WO3 and mixed semiconductors, WO3 -CdS and WO3 -Pt/CdS, has been investigated in the presence of an electron relay, methylviologen, MV2+. The efficiency of WO3-CdS mixed photocatalyst is found to be less than that of CdS alone. However, Pt loaded CdS powders mixed with WO3 powders (WO3-Pt/CdS) exhibited a notable enhancement in the photocatalytic activity. The effect of sintering temperature on the efficiency of WO3 showed that the sample sintered at 800°C was with higher efficiency than that sintered at 300°C. Similarly, loading of WO3 with Ag(I), Fe(III) and Cr(III) by high temperature (800°C) sintering enhanced the quantum yields when compared to those sintered at 300°C. Simultaneous use of the electron relay (MV2+) and a hole scavenger (oxalic acid) for water splitting reaction in the presence of Ag(I)/WO3 is found to be less efficient compared to that observed in the presence of MV2+ or oxalic acid alone, which might be due to the interaction of CO2 molecules formed, with methylviologen radical cations.

Factors influencing the photocatalytic efficiency of WO3 particles

Abstract WO3 particles were doped with the transition metal ions TiIII, VIV, CrIII, MnII, FeIII, CoII, NiII, CuII, ZnII, RuIII and V2O5 to increase the visible light absorption. The photocatalytic efficiencies of these doped samples were investigated by carrying out the decomposition of peroxomonosulphate (PMS, HSO5−) over the doped and undoped WO3 particles. PMS is capable of reacting with both the photogenerated holes and electrons. The effect of initial concentrations of PMS, amount of catalyst, pH, dopants and percentage doping on the photocatalytic efficiency of WO3 are investigated and are discussed.

Photobiocatalysis: hydrogen evolution using a semiconductor coupled with photosynthetic bacteria

Abstract Photobiocatalytic production of hydrogen in the presence of Bi2O3 semiconductor, methyl viologen (MV2+) as an electron mediator and three different bacteria (Rhodopseudomonas capsulata, Rhodospirillum rubrum and Escherichia coli) as hydrogen evolution enzyme catalysts has been carried out in different environments. Addition of intact bacterial cells is found to increase the hydrogen production efficiency. It has been suggested that the nitrogenase enzymes of the bacterial cells catalyze the hydrogen evolution process rather than the hydrogenase enzymes of the bacteria. The presence of carbohydrates (fructose, dextrose and starch) and organic acids (oxalic acid, EDTA and ascorbic acid) as electron donors with the above system further enhances the hydrogen production efficiency due to the reaction of photogenerated valence band holes of the semiconductor with these substrates, thereby preventing the e−-h+ recombination. The effects of loading the semiconductor with RuO2 and Rh2O3 and addition of divalent metal ions, Ca2+, Mg2+ and Mn2+ to the system towards hydrogen production efficiency were also studied and discussed.

Syntheses of mixed ligands complexes of Ru(II) with 4,4′-dicarboxy-2,2′-bipyridine and substituted pteridinedione and the use of these complexes in electrochemical photovoltaic cells

The synthesis, spectral and photoelectrochemical studies of mixed ligand complexes of [Ru(dcbpy)2(LL)]Cl2, where LL=2,4-(1,3-N,N′-dimethyl)pteridinedione (DMP), 6,7-dimethyl-2,4-(1,3-N,N′-dimethyl)pteridinedione (MDMP), 6,7-diphenyl-2,4-(1,3-N,N′-dimethyl)pteridinedione (PhDMP), dibenzo[h,j]-(1,3-N,N′-dimethyl)isoalloxazine (BIAlo), 6,7-bis(pyrid-2-yl)-2,4-(1,3-N,N′-dimethyl) pteridinedione (PyDMP) were carried out. These complexes were attached to sol–gel processed TiO2 electrodes and the photocells fabricated were illuminated with polychromatic radiation in the presence of I2/I3− as redox electrolyte. The incident photon to current conversion efficiency determined was found to be ∼20–48%.