Hydrogen Photoevolution Research Articles

Enhancing catalytic activity of TiO2 nanoparticles through acid treatment in Eosin-Y sensitized photohydrogen evolution reaction system

Light-driven water splitting has gained increasing attention as an eco-friendly method for hydrogen production. There is a pressing need to enhance the performance of catalysts for the commercial viability of this reaction. Many methods have been proposed to improve catalyst performance; however, an economical and straightforward approach remains a priority. This paper presents an uncomplicated technique called acid treatment, which augments the catalytic performance of nanoparticles. The method promotes a change in the catalytic reactivity by causing a deficit in electron density of Ti and O on the surface of TiO2 nanoparticles without altering their size, morphology, or crystal structure. In the Eosin Y sensitized photocatalytic hydrogen production system, nitric acid treated TiO2 (16.95 μmol/g) exhibited 1.5 times the hydrogen production compared to bare TiO2 (11.15 μmol/g).

Evidence for Electron Transfer from the Bidirectional Hydrogenase to the Photosynthetic Complex I (NDH-1) in the Cyanobacterium Synechocystis sp. PCC 6803.

The cyanobacterial bidirectional [NiFe]-hydrogenase is a pentameric enzyme. Apart from the small and large hydrogenase subunits (HoxYH) it contains a diaphorase module (HoxEFU) that interacts with NAD(P)+ and ferredoxin. HoxEFU shows strong similarity to the outermost subunits (NuoEFG) of canonical respiratory complexes I. Photosynthetic complex I (NDH-1) lacks these three subunits. This led to the idea that HoxEFU might interact with NDH-1 instead. HoxEFUYH utilizes excited electrons from PSI for photohydrogen production and it catalyzes the reverse reaction and feeds electrons into the photosynthetic electron transport. We analyzed hydrogenase activity, photohydrogen evolution and hydrogen uptake, the respiration and photosynthetic electron transport of ΔhoxEFUYH, and a knock-out strain with dysfunctional NDH-1 (ΔndhD1/ΔndhD2) of the cyanobacterium Synechocystis sp. PCC 6803. Photohydrogen production was prolonged in ΔndhD1/ΔndhD2 due to diminished hydrogen uptake. Electrons from hydrogen oxidation must follow a different route into the photosynthetic electron transport in this mutant compared to wild type cells. Furthermore, respiration was reduced in ΔhoxEFUYH and the ΔndhD1/ΔndhD2 localization of the hydrogenase to the membrane was impaired. These data indicate that electron transfer from the hydrogenase to the NDH-1 complex is either direct, by the binding of the hydrogenase to the complex, or indirect, via an additional mediator.

Elaboration and studies of physical and photo-electrochemical properties of La2NiO4 and its use with SnO2 in photo-evolution of hydrogen under visible light irradiation

La2NiO4 elaborated by nitrate route gives a positive result when used as a catalyst for the hydrogen photo-evolution. The prepared oxide crystallizes in the tetragonal K2NiF4 structure I4/mmm. The BET method was used to determine the specific surface area (4.91 m2/g); such value could suggest an interesting catalytic activity. Optical, electrical and photo-electrochemical properties have been studied to elaborate the energy diagram, a preamble for the photocatalysis. The band gap (Eg) has been evaluated at 1.51 eV and the transition is directly allowed. La2NiO4 is p-type semiconductor assigned to O2− intercalation in the two-dimensional crystal lattice; activation energy of 0.13 eV in the heating direction from the Arrhenius type law. The flat band potential Efb (−0.45 VSCE) and the density of the holes NA (1.13 × 1021 cm-3) were computed from the capacitance measurement. The conduction band illustrated in the energetic diagram for La2NiO4 (−1.77 VSCE) offers the possibility of H2 generation upon visible light. The best performance occurs at pH 13 in the presence of S2O3-3 as a hole scavenger. The H2 generation of the hetero-system (La2NiO4/SnO2) is significantly improved compared to La2NiO4. The higher photoactivity is ascribed to the electrons injection from the conduction band (La2NiO4-CB = −1.77 V/SCE), acting as generator and electrons pump, to (SnO2-CB = 0.35 V/SCE) inducing a better water reduction. The H2 evolution rate reaches 15.1 μmol g−1 min−1.

Dinuclear 2,4-di(pyridin-2-yl)-pyrimidine based ruthenium photosensitizers for hydrogen photo-evolution under red light.

In this study, we report two dinuclear Ru(II) complexes C1 and C2 and compare them to their mononuclear analogues Ref1 and Ref2. The dinuclear species exhibit a much stronger absorption, longer excited-state lifetimes and higher luminescence quantum yields than the mononuclear complexes. In addition, C1 and C2 are easier to reduce. An estimation of the driving forces for the electron transfer processes relevant to photocatalytic hydrogen evolution suggests that C1 and Ref2 possess similar activity as photosensitizer (PS). Yet, the improved photophysical properties of C1 make it a more promising candidate for hydrogen evolution. In hydrogen evolution experiments, C1 indeed exhibits increased activity as PS, however, the catalytic system loses its activity after only a few hours. C2 is less active than the mononuclear complexes despite its superior photophysical properties. This observation is attributed to a lack of driving force for the electron transfer towards the catalyst. Further studies of the dinuclear complex C1 show that it is indeed the PS, which decomposes under the catalytic conditions, presumably due to the electron transfer towards the catalyst being the rate-limiting step.

Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light

The photocatalytic reduction of water to form hydrogen gas (H2) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2-yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue- and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

Heteroleptic ruthenium bis-terpyridine complexes bearing a 4-(dimethylamino)phenyl donor and free coordination sites for hydrogen photo-evolution.

Motivated by the recent report of a heteroleptic ruthenium bis-terpyridine complex [Ru(toltpy)(bipytpy)](PF6)2 (toltpy: 4'-(tolyl)-2,2':6',2''-terpyridine; bipytpy: 4'-(4-bromophenyl)-4,4''':4'',4''''-di-pyridinyl-2,2':6',2''-terpyridine) capable of driving the photo-evolution of hydrogen with a constant rate of activity for 12 days [M. Rupp et al., Inorg. Chem., 2019, 58, 9127-9134], we investigated the impact of an internal electron donor on the photoactivity of three new ruthenium bis-terpyridine photosensitizers. We used 4'-(N,N-dimethylaminophenyl)-4,4''-di-tert-butyl-2,2':6',2''-terpyridine (Dtpy) as the donor ligand. These complexes also bear peripheral coordination sites at various positions on the terpyridine backbone, allowing for photosensitizer-catalyst interactions. Their performances in photocatalysis under blue and green light (450 and 525 nm, respectively) were measured, in the presence of triethanolamine, using a cobaloxime catalyst [Co(dmgH)2(H2O)2](BF4)2. Complexes C1[Ru(Dtpy)(pytpy)](PF6)2 (pytpy: 4'-(pyridin-4-yl)-2,2':6',2''-terpyridine) and C3[Ru(Dtpy)(bipytpy)](PF6)2 appeared to have a photostability similar to that of [Ru(toltpy)(bipytpy)](PF6)2 but with lower activity rates, while C2[Ru(Dtpy)(pz2bpy)](PF6)2 (pz2bpy: 2,6-di(pyrazin-2-yl)-4,4'-bipyridine) showed a better peak activity, however, followed by a progressive decay. After 24 hours, complexes C1, C2 and C3 had reached TONs of 18, 35 and 52 under blue light and 14, 20 and 47 under green light, respectively, and were still found to be active. Their photophysical and electronic properties are discussed to rationalize the photocatalytic trends.

On the photochemistry of Fe2(edt)(CO)4(PMe3)2, a [FeFe]‐hydrogenase model: A DFT/TDDFT investigation

AbstractThe photochemistry of the [FeFe] hydrogenases model [Fe2(edt)(CO)4(PMe3)2] (edt = 1‐2 ethane‐dithiolate, (1) is investigated at Time‐Dependent Density Functional Theory (TDDFT) level, focusing on the effect of the phosphine ligands on the early stages of the photodynamic of the system compared to that of the all CO model Fe2(pdt)(CO)6 (2) (pdt = 1‐3 propanedithiolate). We observed a role of the Fe→S charge transfer for the lower energy singlet transitions, unveiling a photoisomerization pathway between the lowest energy forms while the higher energy excitations are likely involved in the CO dissociation pathways. TDDFT shows that the average Fe‐CO bond elongation in 1 is shorter than that observed in 2, providing the electronic structure rationale on the observation that diiron dithiolates are more photo‐stable with respect to the CO photolysis than the all CO model. This is relevant for catalyst photo‐stability and is an advantageous and thus desirable feature for practical applications of photo‐hydrogen evolution.

Ultrasound-assisted synthesis of PPyCuS@GOPt nanocomposite and investigation of its electrocatalytic behavior towards photo-hydrogen evolution

Capability of reducing the overpotential and also increasing the current density of hydrogen evolution reaction, HER, through exposure to light can be aid in the development of effective HER driven by sunlight. With this in mind, herein, the hybrid electrode based on polypyrrole –CuS composite, PPyCuS, is fabricated with the aim to form an electrode photo-electrocatalyst for HER. Therefore, an in situ sonoelectrochemical method was employed to synthesis of hybrid nanocomposite film on copper substrate. The photoelectrochemical performance of PPyCuS@GOPt hetrostructure was investigated in acidic solution under visible light irradiation by linear sweep voltammetry, electrochemical impedance spectroscopy, EIS, and Tafel slopes evaluation. Actually, we could prepare the PPyCuS@GOPt nanocomposite film on Cu2O nano particles exhibited a much smaller onset overpotential with high current density for photo-HER. It was proven that GO, and Pt can act as effective dual co-catalysts to enhance the photocatalytic H2 production activity of PPyCuS. The higher photoactivity is attributed to the fact that the photoinduced interfacial charge transfer in the heteronanostructures follow by further transfer in GO sheets. This work is expected to provide a new concept for development of highly efficient polymer-sulfide compounds photoelectrocatalysts for HER under simulated sunlight.

Biochemical characterization of the hydrogen photoevolution in cyanobacterium Oscillatoria chalybea

The filamentous non-heterocystous cyanobacterium Oscillatoria chalybea showed molecular hydrogen photoevolution under different conditions. The major part of determined hydrogen evolution would be derived from a hydrogenase system distinct from the nitrogenase. To improve the hydrogen production, different buffers, temperatures, pH values and some inorganic cofactors were studied. The results revealed that the best buffer was Tricine-KCl, the optimum temperature was 25 °C, the optimum pH value was within the range 7–7.5 and the optimum concentration of nickel chloride was 50 μM that improved also the growth of O. chalybea. The influence of inorganic cofactors showed that the addition of 10−5 MnCl2 increases the photoevolution of molecular hydrogen.

Organophosphonate bridged anatase mesocrystals: low temperature crystallization, thermal growth and hydrogen photo-evolution.

The sol-gel co-condensation of organo-phosphonates to titanium alkoxides enables access to novel organic-inorganic hybrids based on phosphonate-bridged titanium dioxide. In this contribution, we bring new perspectives to the long established sol-gel mineralization of titanium alkoxide species, by harnessing the virtues of the well-designed phosphonate-terminated phosphorus dendrimers as reactive amphiphilic nanoreactor, confined medium and cross-linked template to generate discrete crystalline anatase nanoparticles at low temperature (T = 60 °C). An accurate investigation on several parameters (dendrimer generation, dendrimer-to-titanium alkoxide ratio, precursor reactivity, temperature, solvent nature, salt effect) allows a correlation between the network condensation, the opening porous framework and the crystalline phase formation. The evolution of the dendrimer skeleton upon heat treatment has been deeply monitored by means of (31)P NMR, XPS and Raman spectroscopy. Increasing the heteroatom content within a titania network provides the driving force for enhancing their photocatalytic water splitting ability for hydrogen production.

Hydrogen photo-evolution by Rhodopseudomonas palustris 6A using pre-treated olive mill wastewater and a synthetic medium containing sugars

Increasing costs of petroleum, associated with the escalating problems of global climate change, require always greater efforts in order to produce an energy carrier as bioH2. In this study, bioH2 production using photofermentative process was investigated. Two culture broths were used: (a) a synthetic medium rich in sugars (glucose and fructose) and (b) a pre-treated fresh olive-mill wastewater (OMWF) diluted with water (30%, v:v). The pre-treatment was carried out using two different vegetable matrices (dry-Azolla and granular active carbon) to decrease both the content of polyphenols and the dark colour of wastewater. Rhodopseudomonas palustris 6A isolated from soil spread with OMW was utilized for batch growth experiments, carried out indoors under continuous light (200μE/m2/s). When synthetic medium was used, the culture mixing was performed using either (i) a magnetic stir bar, and (ii) an impeller equipped with five turbines. The latter system made it possible to increase the bioH2 photo-evolution by 1.4 times. The specific hydrogen photo-evolution rate was 13.5mL/g(dw)/h in the broth containing diluted OMWF and 11.8mL/g(dw)/h in the synthetic medium containing sugars (glucose and fructose).

Photocatalytic hydrogen evolution over the hetero-junction CoAl2O4/ZnO

The photoevolution of hydrogen is demonstrated on the new hetero-junction CoAl2O4/ZnO. The nano sized spinel obtained from hydrotalcite calcination exhibits a high specific surface area (130 m2 g−1) and good photocatalytic activity. The optical transition of the spinel CoAl2O4 (1.56 eV), directly allowed, is due to Co2+: d–d transition. The capacitance measurements indicate a negative slope, characteristic of p type conduction with a flat band potential of −0.11 V SCE. The photoactivity is thermodynamically predicted from the energy band diagram. The potential of the conduction band (−1.51 V SCE) is more negative than the hydrogen level, leading to spontaneous hydrogen liberation. The spinel is activated by visible light and the electrons are transferred to water via ZnO. A reducing agent is required to prevent the photocorrosion, SO3 2− being particularly favorable and CoAl2O4 approaches 100 % stability. The hetero-junction consists of a mixture of powders (x%CoAl2O4/125 mg ZnO) exhibits self-sensitization for the hydrogen production in basic medium (pH~12.5) with an evolution rate of 2.04 × 10−2 cm3 h−1 (g catalyst)−1 under optimal conditions.

Preparation of a microalgal photoanode for hydrogen production by photo-bioelectrochemical water-splitting

In this study, a microalga Tetraselmis subcordiformis (synonym: Platymonas subcordiformis)-based photoanode was prepared by a novel method developed in our lab. The optimal photocurrent density of microalgae photoanode, 37 μA/cm2, was achieved under illumination of 145 μmol s−1 m−2 at anode potential of 0.5 V vs Ag|AgCl|sat. KCl, immobilized cell density of 2.08 × 106/cm2 and BQ concentration of 300 μmol/L. The results of measurements showed that oxygen evolution peak, hydrogen evolution peak and photocurrent response were all synchronous to light impulse in a three-electrode system. It revealed that there occurred a process of photo-bioelectrochemical water-splitting. Hydrogen can be produced by the method. The investigation for whole photo-bioelectrochemical process also indicated that the electrons for hydrogen evolution had two sources, microalgal metabolic process in dark condition and photosynthetic water oxidation. The photo-hydrogen evolution was twice more than hydrogen evolution in dark condition.

Photoelectrochemical properties of doped polyaniline: Application to hydrogen photoproduction

The semi conducting properties of doped polyaniline (emeraldine-salt, PANI) elaborated by chemical route are investigated by the photo-electrochemical technique. The band gap is found to be 1.48 eV and the transition is directly allowed. The electrical conduction obeys to an exponential law with activation energy of 0.13 eV. p-type conductivity is evidenced from the cathodic photocurrent. The energy band diagram clearly shows the spontaneous hydrogen photo evolution. The potential of the conduction band of PANI (−0.93 VSCE) determined from the capacitance measurements is suitably positioned with respect to H2O/H2 level (−0.66 VSCE). Therefore, the photocatalytic properties of this material has been evaluated according to the hydrogen generation. The best performance is achieved at pH ∼7 with a liberation rate of 0.113 mL h−1 (mg catalyst)−1 and a quantum efficiency of 0.18% under visible light (29 mW cm−2). An increase of 56% is obtained on the hetero-system PANI/TiO2.

Genetic Analysis of the Hox Hydrogenase in the Cyanobacterium Synechocystis sp. PCC 6803 Reveals Subunit Roles in Association, Assembly, Maturation, and Function

Hydrogenases are metalloenzymes that catalyze 2H(+) + 2e(-) ↔ H(2). A multisubunit, bidirectional [NiFe]-hydrogenase has been identified and characterized in a number of bacteria, including cyanobacteria, where it is hypothesized to function as an electron valve, balancing reductant in the cell. In cyanobacteria, this Hox hydrogenase consists of five proteins in two functional moieties: a hydrogenase moiety (HoxYH) with homology to heterodimeric [NiFe]-hydrogenases and a diaphorase moiety (HoxEFU) with homology to NuoEFG of respiratory Complex I, linking NAD(P)H ↔ NAD(P)(+) as a source/sink for electrons. Here, we present an extensive study of Hox hydrogenase in the cyanobacterium Synechocystis sp. PCC 6803. We identify the presence of HoxEFUYH, HoxFUYH, HoxEFU, HoxFU, and HoxYH subcomplexes as well as association of the immature, unprocessed large subunit (HoxH) with other Hox subunits and unidentified factors, providing a basis for understanding Hox maturation and assembly. The analysis of mutants containing individual and combined hox gene deletions in a common parental strain reveals apparent alterations in subunit abundance and highlights an essential role for HoxF and HoxU in complex/subcomplex association. In addition, analysis of individual and combined hox mutant phenotypes in a single strain background provides a clear view of the function of each subunit in hydrogenase activity and presents evidence that its physiological function is more complicated than previously reported, with no outward defects apparent in growth or photosynthesis under various growth conditions.

Fresh olive mill waste deprived of polyphenols as feedstock for hydrogen photo-production by means of Rhodopseudomonas palustris 42OL

In the present study, fresh olive mill waste (OMWF) was processed using an innovative pre-treatment based on the adsorption capacity of two natural matrices. The liquid fraction coming from the pre-treatment is an inexpensive feedstock for hydrogen photo-production by means of purple non-sulphur photosynthetic bacteria (PNSB). The culture broth, which contained OMWF (30%) and distilled water (70%) was tested for hydrogen photo-production using Rhodopseudomonas palustris 42OL. The photo-fermentative process was studied under batch growth conditions, at three different irradiances (18.5, 37 and 74 W/m2). The lowest cumulative amount of hydrogen (439 ml-H2/L-broth) was attained at the irradiance of 18.5 W/m2, while the highest one (1030 ml-H2/L-broth) was obtained at the irradiance of 74 W/m2. The resulting average hydrogen evolution rates (HPRavg), which were based on the culture volume and were attained at the three different irradiances (i.e. 18.5, 37 and 74 W/m2), were 1.40, 3.17 and 5.28 ml-H2/L-broth/h respectively. The culture age at which the hydrogen photo-evolution ceased showed an inverse proportionality as compared to the irradiance: i.e. the higher the irradiance, the lower the culture age. On the contrary, the COD removal efficiency (CODRE) increased depending on the irradiance. The maximal light conversion efficiency (1.07%) was attained at the irradiance of 37 W/m2, and decreased at both lower and higher irradiances.

Visible light induced hydrogen on the novel hetero-system CuFe 2O 4/TiO 2

The hydrogen photo evolution has been successfully achieved over the new hetero-system CuFe 2O 4/TiO 2 and the activity is dependent on the synthesis method. The spinel CuFe 2O 4 is p-type semi conductor with activation energy of 0.21 eV. It has been prepared via a variety of techniques and the best performance is achieved on the catalyst elaborated by sol gel. The onset potential of the photocurrent (−0.17 V SCE ) is slightly cathodic with respect to the flat band potential (0 V SCE ) implying a small existence of surface states within the gap region. The photo induced minority carriers (electrons) react with H 2O molecules via TiO 2 interfacial mechanism. The intimate contact between CuFe 2O 4 and TiO 2 particles is necessary for H 2 liberation. The generated photovoltage is used to catalyze the reaction ( H 2 O + S 2 O 3 2 - + 2 OH - → 2 H 2 + 2 SO 3 2 - , Δ G R ∘ = - 39.31 kJ mol - 1 ) and the oxide approaches 100% stability to photo driven corrosion. The rate of H 2 evolution is found to be 15 μmol g −1 min −1 with a polychromatic power efficiency of 1.3%. Over time, a pronounced deceleration occurred. The competitive reduction of sulfite is therefore though to be the reason of the regression of the photoactivity.

Hydrogen production over CuIn3Se5/WO3 hetero-junction

Abstract The hydrogen photo-evolution was successfully achieved over CuIn3Se5/WO3 hetero-junction. CuIn3Se5 powder has been synthesized by fusion technique. The compound crystallizes in the chalcopyrite structure and exhibits n-type conductivity. The optical gap (1.19 eV) was determined from the reflectance diffuse spectrum and the flat band potential (-0.22 V) from the Mott Schottky plot. The small forbidden band and the negative flat band potential make it promising for the solar energy conversion. In aqueous solutions, the material is stabilized by hole consumption involving X2- species (= S2- and S2O3 2-). H2 formation would become thermodynamically easy in alkaline media. The best performance was obtained in S2O3 2- solution (10-2 M, pH∼7) with an evolution rate of 0.54 mL g-1 min-1. The photo activity is ascribed to electrons transfer from the sensitizer CuIn3Se5-Conduction Band (CB), acting as electrons pump, to WO3-CB (-0.4 V VS SCE) resulting in the enhanced water reduction.

Hydrogen photo-evolution upon S deprivation stepwise: an illustration of microalgal photosynthetic and metabolic flexibility and a step stone for future biotechnological methods of renewable H2 production

The metabolic flexibility of some photosynthetic microalgae enables them to survive periods of anaerobiosis in the light by developing a particular photofermentative metabolism. The latter entails compounds of the photosynthetic electron transfer chain and an oxygen-sensitive hydrogenase in order to reoxidize reducing equivalents and to generate ATP for maintaining basal metabolic function. This pathway results in the photo-evolution of hydrogen gas by the algae. A decade ago, Melis and coworkers managed to reproduce such a condition in a laboratory context by depletion of sulfur in the algal culture media, making the photo-evolution by the algae sustainable for several days (Melis et al. in Plant Physiol 122:127-136, 2000). This observation boosted research in algal H(2) evolution. A feature, which due to its transient nature was long time considered as a curiosity of algal photosynthesis suddenly became a phenomenon with biotechnological potential. Although the Melis procedure has not been developed into a biotechnological process of renewable H(2) generation so far, it has been a useful tool for studying microalgal metabolic and photosynthetic flexibility and a possible step stone for future H(2) production procedures. Ten years later most of the critical steps and limitations of H(2) production by this protocol have been studied from different angles particularly with the model organism Chlamydomonas reinhardtii, by introducing various changes in culture conditions and making use of mutants issued from different screens or by reverse genomic approaches. A synthesis of these observations with the most important conclusions driven from recent studies will be presented in this review.

Photo-catalytic hydrogen production over Fe 2O 3 based catalysts

The hydrogen photo-evolution was successfully achieved in aqueous (Fe 1− x Cr x ) 2O 3 suspensions (0 ≤ x ≤ 1). The solid solution has been prepared by incipient wetness impregnation and characterized by X-ray diffraction, BET, transport properties and photo-electrochemistry. The oxides crystallize in the corundum structure, they exhibit n-type conductivity with activation energy of ∼0.1 eV and the conduction occurs via adiabatic polaron hops. The characterization of the band edges has been studied by the Mott Schottky plots. The onset potential of the photo-current is ∼0.2 V cathodic with respect to the flat band potential, implying a small existence of surface states within the gap region. The absorption of visible light promotes electrons into (Fe 1− x Cr x ) 2O 3-CB with a potential (∼−0.5 V SCE) sufficient to reduce water into hydrogen. As expected, the quantum yield increases with decreasing the electro affinity through the substitution of iron by the more electropositive chromium which increases the band bending at the interface and favours the charge separation. The generated photo-voltage was sufficient to promote simultaneously H 2O reduction and SO 3 2− oxidation in the energetically downhill reaction (H 2O + SO 3 2− → H 2 + SO 4 2−, ΔG = −17.68 kJ mol −1). The best activity occurs over Fe 1.2Cr 0.8O 3 in SO 3 2− (0.1 M) solution with H 2 liberation rate of 21.7 μmol g −1 min −1 and a quantum yield 0.06% under polychromatic light. Over time, a pronounced deceleration occurs, due to the competitive reduction of the end product S 2O 6 2−.