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The presented study was focused on the simple, eco-friendly synthesis of composite hydrogels of crosslinked carboxymethyl cellulose (CMC)/alginate (SA) with encapsulated g-C3N4 nanoparticles. The structural, textural, morphological, optical, and mechanical properties were determined using different methods. The encapsulation of g-C3N4 into CMC/SA copolymer resulted in the formation of composite hydrogels with a coherent structure, enhanced porosity, excellent photostability, and good adhesion. The ability of composite hydrogels to eliminate structurally different dyes with the same or opposite charge properties (cationic Methylene Blue and anionic Orange G and Remazol Brilliant Blue R) in both single- and binary-dye systems was examined through adsorption and photocatalytic reactions. The interactions between the dyes and g-C3N4 and the negatively charged CMC/SA copolymers had a notable influence on both the adsorption capacity and photodegradation efficiency of the prepared composites. Scavenger studies and leaching tests were conducted to gain insights into the primary reactive species and to assess the stability and long-term performance of the g-C3N4/CMC/SA beads. The commendable photocatalytic activity and excellent recyclability, coupled with the elimination of costly catalyst separation requirements, render the g-C3N4/CMC/SA composite hydrogels cost-effective and environmentally friendly materials, and strongly support their selection for tackling environmental pollution issues.

The interfacial charge transfer (ICT) complex formation is a simple procedure to bring optical absorption of widebandgap oxide materials in the visible spectral range, crucial for enhancing their use in photo-driven reactions. The optical absorption of the prepared ICT complexes between ZnO and five different colorless benzene derivatives is red-shifted compared to pristine ZnO nanopowder. The density functional theory (DFT) calculations provided realistic energy level alignment in hybrid systems. Also, the DFT-calculated infrared spectra support the binding structures derived based on experimental measurements of free and adsorbed ligands onto ZnO surfaces. The photocatalytic performance of prepared hybrids was evaluated using photocatalytic hydrogen generation in the water-splitting reaction. The ZnO nanopowders modified with catechol and caffeic acid have over 50% higher hydrogen production rate than pristine ZnO, displaying steady hydrogen production under long-run working conditions.

Photocatalytic processes have been recognized as promising and sustainable methods for pollutant degradation and energy conversion. Due to the simplicity of the process, capacity for using renewable and pollution-free solar energy, mild reaction conditions, reasonable reaction time, complete pollutant degradation, no harmful byproducts, and cost-effectiveness, photocatalytic degradation processes are more promising than conventional physical methods. Due to its favorable properties, TiO2 semiconductor has been used as an ideal candidate for photocatalytic reactions. However, large band gap, fast e-/h+ recombination, and low light harvesting efficiency are the main obstacles that restrict its practical application. Using g-C3N4 semiconductor in combination with TiO2 is an excellent strategy to enhance the photocatalytic activity under visible light irradiation. In this study, g-C3N4/TiO2photocatalyst was successfully synthesized and used in single and simultaneous photodegradation of cationic Methylene Blue and anionic Orange G dyes. To examine the heterojunction between g-C3N4 and TiO2 and the mechanism of the transport pathway of photogenerated e-/h+, the photoluminescence spectra and radical scavenger study were applied. The present research offers a novel approach in reaction mechanism analysis for degradation of dye pollutants in single/mixed solutions and suggests a strategy for determination of heterojunction type in g-C3N4/TiO2 systems.

ZnFe photocatalysts have been increasingly investigated for water remediation due to the high demands in this field, such as activity, toxicity, cost, and stability. The presented study was focused on the simple, safe, non-toxic, and eco-friendly synthesis and characterization of ZnFe-mixed metal oxides in correlation with their functional properties. Photocatalytic performance of these materials was evaluated by rhodamine B photodegradation under simulated solar light irradiation. The synthesized mixed oxides contained hexagonal wurtzite ZnO as the predominant phase, whereas, after thermal treatment, the formation of the spinel-structured ZnFe2O4 phase was observed. The photocatalysts with the additional spinel phase and thermally treated at 300°C and 500°C exhibited superior photocatalytic activity probably due to the highest amount of the ZnFe2O4 spinel phase, favourable mesoporous structure, and an optimal energy band gap of ∼2.30 eV that initiated higher light-harvesting efficiency. The rhodamine B photodegradation followed zero-order kinetics, indicating complete coverage of active sites by the pollutant substrate. Additionally, photocatalysts showed the highest efficiency at the natural pH (6.8), being in accordance with green synthesis principles. Simple, green route assembling synthesis method, high photodegradation efficiency, and good reusability make these ZnFe-mixed oxides great candidates for potential application in practical wastewater treatments.

In this work hydrogen production activity of a series of the nickel-loaded, nitrogen-doped titanate-based photocatalysts was tested by water splitting reaction in isopropanol-water mixture. The utilization of ammonium ions during catalyst synthesis and subsequent calcination led to successful formation of nitrogen-doped nanotubular titanates with lower sodium content, favored titanate-to-anatase transformation and enhanced textural, morphological and interface properties. The photocatalytic tests revealed that hydrogen production activity of nitrogen-doped titanates is ten times higher compared to nitrogen-free titanates. The presence of highly photoactive anatase phase, improved anatase crystallinity and lower amount of sodium ions are found to be decisive factors for increased hydrogen production activity. The optimal nickel loading was determined and the variation in photocatalytic activity with reduction temperature was explained. Stability tests gave quantitative analysis of the effects of reaction products accumulation and photocorrosion on the catalyst deactivation.

Platinum doped titanate catalysts were synthesized via alkaline hydrothermal method, followed by incipient wetness impregnation method and temperature programmed reduction (TPR). The effects of sacrificial agents on the photocatalytic activity of platinum doped titanate catalysts were studied in water splitting/alcohol photoreforming process under simulated solar light irradiation. Mechanism of photocatalytic reactions were elucidated by additional experiments performed using temperature-programmed desorption/mass spectrometry (TPD/MS). The applied alcohols (methanol, ethanol and 1-propanol) have shown enhanced hydrogen production rate in water splitting/photoreforming. Decreasing hydrogen production rate were observed in order methanol>ethanol>1-propanol (with achieved 11.6, 12.5, and 5.9 mmol g-1 h-1, respectively) due to the decrease in carbon atoms in alcohol carbon chain.

Waste-based CaO was synthesized from chicken eggshell, which calcined form (900 ºC, 5 ºC/min, 4 h) was hydrated (80 ºC, S:L=1:5, 24 h) and rehydrated/calcined (600 ºC, 5 ºC/min, 4 h). The obtained catalyst was characterized (Hg-porosimetry and TPD-CO2) and tested in biodiesel synthesis from off-grade sunflower oil by transesterification to analyse the influence of water addition on reaction intensification. Water content was varied in the range from 1 wt% to 9 wt% by weight of oil. It is shown that water addition positively affects the catalytic activity leading to a reduction in induction time and to faster achievement of the reaction equilibrium (fatty acid methyl esters content > 95%), which for a water content higher than 1 wt% is achieved in 90 min. On the other hand, the equilibrium for the reaction without water addition was achieved considerably later (after 150 min).

Lignite coal fly ash (FA) from a domestic thermal power plant was converted into a pure cancrinite zeolitic material (ZMFA) using a novel, custom-made, rotating autoclave reactor system by a short-term alkali activation process. The obtained ZMFA was used as catalyst support of calcium oxide as an active component derived from waste chicken eggshells (ES). The ZMFA supported calcium oxide catalyst (xCaO/ZMFA) was synthesized by means of the ultrasound-assisted method. The influence of different concentrations of dispersed calcium oxide (x = 5–20 wt%) over ZMFA and thermal treatment at different temperatures (450–600 °C) were studied. The structural and morphological characterization showed that the original cancrinite structure was preserved. The basicity and textural properties indicated the presence of strong active sites in a well-defined pore network suitable for the reactions of bulky organic compounds such as triacylglycerols (TAGs). The highest activity (96.5% of fatty acid methyl esters) in the methanolysis of sunflower oil was achieved with the 20CaO/ZMFA catalyst under reaction conditions: temperature of 60 °C, methanol/oil molar ratio of 12:1, catalyst concentration of 4 wt%, and reaction time of 2 h. It was found that the optimal calcination temperature of the catalyst precursor was 550 °C. At calcination temperatures above 550 °C, the melting of the glassy phase became more intense whereby the molten phase partially reacted with calcium oxide forming the catalytically inactive calcium silicate compounds (wollastonite, larnite, etc.). The rate constants of the two tested kinetic models were correlated with the concentrations of active calcium oxide. The MRPD of both models was low indicating their reliability.

Understanding the degradation behavior of azo dyes in photocatalytic wastewater treatment is of fundamental and practical importance for their application in textile-processing and other coloration industries. In this study, quantum chemistry, as density functional theory, was used to elucidate different degradation pathways of azo pyridone dyes in a hydroxyl radical (HO•) initiated photocatalytic system. A series of substituted azo pyridone dyes were synthesized by changing the substituent group in the para position of the benzene moiety, ranging from strong electron-donating to strong electron-withdrawing groups. The effect of dye molecular structure on the photocatalytic degradation reaction mechanism was analyzed and quantification of substituent effects on the thermodynamic and kinetics parameters was performed. Potential energy surface analysis revealed the most susceptible reaction site for the HO• attack. The calculated reaction barriers are found to be strongly affected by the nature of substituent group with a good correlation using Hammett σp constants and experimentally determined reaction rates. The stability of pre-reaction complexes and transition state complexes were analyzed applying the distortion-interaction model. The increased stability of the transition state complexes with the distancing from the substituent group has been established.