Enhanced photocatalytic hydrogen production from aqueous sulfide/sulfite solution by ZnO0.6S0.4 with simultaneous dye degradation under visible-light irradiation

Treasuring industrial sulfur by-products: A review on add-value to reductive sulfide and sulfite for contaminant removal and hydrogen production

Reductive sulfur-containing by-products (S-BPs) released from industrial process mainly exist in the simple form of sulfide and sulfite. In this study, recent advances to remove and make full use of reductive S-BPs to achieve efficient contaminant removal and hydrogen production are critically reviewed. Sulfide, serves as both reductant and nucleophile, can form intermediates with the catalyst surface functional group through chemical interaction, efficiently promoting the catalytic reduction process to remove contaminants. Sulfite assisted catalytic process could be classified to the advanced reduction processes (ARPs) and advanced oxidation processes (AOPs), mainly depending on the presence of dissolved oxygen (DO) in the solution. During ARPs, sulfite could generate reductive active species including hydrated electron (e_aq^-), hydrogen radical (H·), and sulfite radical (SO₃^•-) under the irradiation of UV light, leading to the efficient reduction removal of a variety of contaminants. During AOPs, sulfite could first produce SO₃^•- under the action of the catalyst or energy, initiating a series of reactions to produce oxysulfur radicals. Various contaminants could be effectively removed under the role of these oxidizing active species. Sulfides and sulfites could also be removed along with promoting hydrogen production via photocatalytic and electrocatalytic processes. Besides, the present limitations and the prospects for future practical applications of the process with these S-BPs are proposed. Overall, this review gives a comprehensive summary and aims to provide new insights and thoughts in promoting contaminant removal and hydrogen production through taking full advantage of reductive S-BPs.

Photocatalytic hydrogen evolution by degradation of organic pollutants over quantum dots doped nitrogen carbide

Semiconductor photocatalysts are of great importance for addressing current environmental and energy crises. In this study, we developed a simple exfoliation-sonication route to fabricate nitrogen carbide quantum dots (CNQDs) doped nitrogen carbide nanosheet (CNS) composite photocatalysts which were employed to produce hydrogen and degrade organic pollutants (methyl orange, acridine orange, aniline, and phenol) synchronously under visible light irradiation. The presence of acridine orange and aniline enhanced the hydrogen evolution efficiency from 8.8 mmol g^-1 h^-1 to 32.1 and 11.7 mmol g^-1 h^-1, respectively. On the contrary, methyl orange and phenol with the same concentration inhibited hydrogen evolution. Based on the proton chain and energy band analyses, the synchronous mechanism of photocatalytic hydrogen evolution and organic pollutant degradation on CNQDs/CNS was also proposed. On one side, the oxygen-containing functional groups on the surface of CNQDs and the surrounded water molecules constructed proton chains, increasing the combination probability between protons and photo-generated electrons. On the other side, the heterojunction of CNQDs/CNS induced the separation of photo-generated electron-hole pairs. The photo-generated electrons migrate to CNQDs, on which the protons were transformed into hydrogen molecules, while the holes migrated to CNS where the organic pollutants were oxidized synchronously.

Influence of QD photosensitizers in the photocatalytic production of hydrogen with biomimetic [FeFe]-hydrogenase. Comparative performance of CdSe and CdTe

Photocatalytic systems comprising a hydrogenase-type catalyst and CdX (X = S, Se, Te) chalcogenide quantum dot (QD) photosensitizers show extraordinary hydrogen production rates under visible light excitation. What remains unknown is the mechanism of energy conversion in these systems. Here, we have explored this question by comparing the performance of two QD sensitizers, CdSe and CdTe, in photocatalytic systems featuring aqueous suspensions of a [Fe₂ (μ-1,2-benzenedithiolate) CO₆] catalyst and an ascorbic acid sacrificial agent. Overall, the hydrogen production yield for CdSe-sensitized reactions QDs was found to be 13 times greater than that of CdTe counterparts. According to emission quenching experiments, an enhanced performance of CdSe sensitizers reflected a greater rate of electron transfer from the ascorbic acid (k_Asc). The observed difference in the QD-ascorbic acid charge transfer rates between the two QD materials was consistent with respective driving forces for these systems.

Bioremediation of Crude Glycerol by a Sustainable Organic-Microbe Hybrid System

Klebsiella pneumoniae with crude glycerol-utilizing and hydrogen (H₂)-producing abilities was successfully isolated from return activated sludge from Shatin Sewage Treatment Works. The H₂ production strategy used in this study was optimized with crude glycerol concentrations, and 1,020 μmol of H₂ was generated in 3 h. An organic–microbe hybrid system was constructed with metal-free hydrothermal carbonation carbon (HTCC) microspheres to enhance the H₂ production under visible light (VL) irradiation. Under optimized VL intensity and HTCC concentration, an elevation of 35.3% in H₂ production can be obtained. Electron scavenger study revealed that the photogenerated electrons (e^–) from HTCC contributed to the additional H₂ production. The variation in intercellular intermediates, enzymatic activity, and reducing equivalents also suggested that the photogenerated e^– interacted with K. pneumoniae cells to direct the metabolic flux toward H₂ production. This study demonstrated the feasibility of using an organic–microbe hybrid system as a waste-to-energy technology.

TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

The growing world energy consumption, with reliance on conventional energy sources and the associated environmental pollution, are considered the most serious threats faced by mankind. Heterogeneous photocatalysis has become one of the most frequently investigated technologies, due to its dual functionality, i.e., environmental remediation and converting solar energy into chemical energy, especially molecular hydrogen. H2 burns cleanly and has the highest gravimetric gross calorific value among all fuels. However, the use of a suitable electron donor, in what so-called “photocatalytic reforming”, is required to achieve acceptable efficiency. This oxidation half-reaction can be exploited to oxidize the dissolved organic pollutants, thus, simultaneously improving the water quality. Such pollutants would replace other potentially costly electron donors, achieving the dual-functionality purpose. Since the aromatic compounds are widely spread in the environment, they are considered attractive targets to apply this technology. In this review, different aspects are highlighted, including the employing of different polymorphs of pristine titanium dioxide as photocatalysts in the photocatalytic processes, also improving the photocatalytic activity of TiO2 by loading different types of metal co-catalysts, especially platinum nanoparticles, and comparing the effect of various loading methods of such metal co-catalysts. Finally, the photocatalytic reforming of aromatic compounds employing TiO2-based semiconductors is presented.

Hydrogen production from natural organic matter via cascading oxic-anoxic photocatalytic processes: An energy recovering water purification technology

Photocatalysis provides a "green" strategy to produce the clean energy of H₂. However, the realization of efficient H₂ production is usually accomplished by the consumption of electron donors, which are costly energy carriers themselves. Here, we attempted to utilize the naturally abundant humic acid (HA), a representative natural organic matter (NOM), as the source of electron donor in a cascading oxic-anoxic photocatalytic system. Results showed that degradation of HA and remarkable H₂ yield (1660.9 μmol g^-1 h^-1 at optimal condition) were obtained successively, whereas the anoxic photocatalytic treatment of pristine HA did not improve H₂ yield but substantially eliminated the H₂ production and HA degradation efficiency. These phenomena suggested the preoxidation process played a vital role in counteracting the detrimental effect of HA on photocatalytic H₂ production. Electrochemical measurement indicated that the preoxidized HA harbored more redox-active moieties than the untreated HA and thus leading to a higher photo-induced charge carrier separation efficiency. A variety of advanced spectroscopic analyses revealed that the photocatalytic oxic pre-treatment resulted in breakdown of chemically inert, electron mediating and chromophoric aromatic macrostructure of HA to form smaller sized oxygenated organic intermediates. These intermediates were more nucleophilic than the pristine HA and acted as sacrificial reagent in the subsequent anoxic process for boosting H₂ production. This study showcases an energy recovering water remediation process and paves the way for the design of novel photocatalytic technologies for environmental application.

Tailoring aromatic ring-terminated edges of g-C3N4 nanosheets for efficient photocatalytic hydrogen evolution with simultaneous antibiotic removal

A promising “one stone three birds” route to enhance photocatalytic H₂ production with simultaneous organic pollutant removal activity.

Relationship between the intermediate phases of the sputtered Zn(O,S) buffer layer and the conduction band offset in Cd-free Cu(In,Ga)Se2 solar cells

Intermediate phases are formed in Zn(O,S) thin films with different oxygen fluxes, affecting the device performance.

Photocatalytic Removal of Methyl Orange Azo Dye with Simultaneous Hydrogen Production Using Ru-modified ZnO Photocatalyst

The aim of this work is to demonstrate the effectiveness of the photocatalytic process in the Methyl Orange azo dye degradation and simultaneous H2 production by using ZnO doped with ruthenium. Ru-modified ZnO photocatalysts were prepared by precipitation method and were characterized by different techniques (XRF, Raman, XRD, N2 adsorption at −196 °C, and UV–vis DRS). The experiments were carried out in a pyrex cylindrical reactor equipped with a nitrogen distributor device and irradiated by four UV lamps with the main wavelength emission at 365 nm. Different Ru amounts (from 0.10 to 0.50 mol%) were tested in order to establish the optimal amount of the metal to be used for the ZnO doping. The photocatalytic activity was evaluated both in terms of Methyl Orange removal and hydrogen production. The experimental results showed that the best activity, both in terms of H2 production and Methyl Orange degradation, was obtained with the Ru-modified ZnO photocatalyst at 0.25 mol% Ru loading. In particular, after four hours of UV irradiation time, the discoloration and mineralization degree were equal to 83% and 78%, with a simultaneous hydrogen production of 1216 µmol L−1. This result demonstrates the ability of the photocatalytic process to valorize a dye present in wastewater, managing to obtain a hydrogen production comparable with the data present in the literature today in the presence of other sacrificial substances.

X-Shaped α-FeOOH with Enhanced Charge Separation for Visible-Light-Driven Photocatalytic Overall Water Splitting

Photocatalytic overall water splitting (POWS) is a promising route for converting solar energy into green and sustainable energy. Herein, we report a facile hydrothermal approach for the fabrication of x-shaped α-FeOOH photocatalysts containing high-index facets for POWS. The x-shaped α-FeOOH photocatalysts exhibited enhanced visible-light-driven POWS activities in comparison with that of FeOOH without x-structures, with a maximum H₂ and O₂ evolution rate of 9.2 and 4.7 μmol h^-1  g^-1 , respectively. The morphology and particle size of the α-FeOOH could be controlled by adjusting the NH₄ F concentration in the precursors. The photodeposition of Pt and RuO₂ on the x-shaped α-FeOOH revealed the specially separated reduction and oxidation centers on the surface of α-FeOOH, with the oxidation-active sites selectively located on the edges of the α-FeOOH x-structures. Electrochemical experiments further affirmed the enhanced charge separation in the x-shaped α-FeOOH. The smaller particle size and unique x-shape of the α-FeOOH photocatalyst were shown to enhance the POWS performance owing to the large specific surface area, high proportion of exposed high-index facets, high electron-transfer efficiency and effective separation of the photogenerated electron-hole pairs. The current study revealed that the x-shaped α-FeOOH products could serve as cost-effective and stable photocatalysts for POWS.