A thiomolybdate cluster for visible-light-driven hydrogen evolution: comparison of homogeneous and heterogeneous approaches.

This study investigates the hydrogen evolution reaction (HER) efficiency of two photosystems incorporating an all-inorganic molecular thiomolybdate [Mo3S13]2- cluster as a HER catalyst. First, we delve into the performance of a homogeneous [Mo3S13]2-/[Ru(bpy)3]2+ (Mo3/Ru) dyad which demonstrates high turnover frequencies (TOFs) and apparent quantum yields (AQYs) at 445 nm approaching the level of 0.5%, yet its performance is marked by pronounced deactivation. In contrast, a heterogeneous approach involves anchoring [Mo3S13]2- onto graphitic carbon nitride (GCN) nanosheets through weak electrostatic association with its triazine/heptazine scaffold. [Mo3S13]2-/GCN (Mo3/GCN) displays effective H2 generation under visible light, with TOF metrics on par with those of its homogeneous analog. Although substantial leaching of [Mo3S13]2- species from the Mo3/GCN surface occurs, the remaining {Mo3}-based centers demonstrate impressive stability, leading to enduring HER performance, starkly distinguishing it from the homogeneous Mo3/Ru photosystem. Photoluminescence (PL) quenching experiments confirm that the performance of Mo3/GCN is not limited by the quality of the inorganic interface, but could be optimized by using higher surface area supports or a higher concentration of [Mo3S13]2- sites. Our findings showcase complexities underlying the evaluation and comparison of photosystems comprising well-defined catalytic centers and pave the way for developing analogous surface-supported (photo)catalysts with broad use in energy applications.

Synthesis of Vitamin B3 through a Heterogeneous Photocatalytic Approach Using Metal-Free Carbon Nitride-Based Catalysts.

Vitamin B3 (nicotinic acid, VB3) was synthesized through the photocatalytic oxidation of 3-pyridinemethanol (3PM) under visible-light-emitting diode (LED) irradiation using metal-free graphitic carbon nitride (GCN) - based materials. A bulk (GCN) material was prepared by a simple thermal treatment using dicyandiamide as the precursor. A post-thermal treatment under static air and nitrogen flow was employed to obtain the GCN-T and GCN-T-N materials, respectively. The conditions adopted during the post-treatment revealed differences in the resulting materials' morphological, electronic, and optical properties. The post-treated photocatalysts revealed an enhanced efficiency in the oxidation of 3PM into VB3, with the GCN-T-N photocatalyst being the best-performing material. The defective surface, reduced crystallinity, and superior photoabsorption of GCN-T-N account for this material's improved performance in the production of VB3. Nevertheless, the presence of nitrogen vacancies in the carbon nitride structure and, consequently, the creation of mid-gap states also accounts to its highly oxidative ability. The immobilization of GCN-T-N in sodium alginate hydrogel was revealed as a promising strategy to produce VB3, avoiding the need for the photocatalyst separation step. Concerning the mechanism of synthesis of VB3 through the photocatalytic oxidation of 3PM, it was possible to identify the presence of 3-pyridinecarboxaldehyde (3PC) as the intermediary product.

Light-driven oxygen evolution from water oxidation with immobilised TiO2 engineered for high performance.

Calcination treatments in the range of 500-900 °C of TiO2 synthesised by the sol-gel resulted in materials with variable physicochemical (i.e., optical, specific surface area, crystallite size and crystalline phase) and morphological properties. The photocatalytic performance of the prepared materials was evaluated in the oxygen evolution reaction (OER) following UV-LED irradiation of aqueous solutions containing iron ions as sacrificial electron acceptors. The highest activity for water oxidation was obtained with the photocatalyst thermally treated at 700 °C (TiO2-700). Photocatalysts with larger anatase to rutile ratio of the crystalline phases and higher surface density of oxygen vacancies (defects) displayed the best performance in OER. The oxygen defects at the photocatalyst surface have proven to be responsible for the enhanced photoactivity, acting as important active adsorption sites for water oxidation. Seeking technological application, water oxidation was accomplished by immobilising the photocatalyst with the highest OER rate measured under the established batch conditions (TiO2-700). Experiments operating under continuous mode revealed a remarkable efficiency for oxygen production, exceeding 12% of the apparent quantum efficiency (AQE) at 384 nm (UV-LED system) compared to the batch operation mode.

Aging assessment of microplastics (LDPE, PET and uPVC) under urban environment stressors.

The changes in the chemical structure, surface morphology and crystallinity are reported for three different polymers (LDPE, PET and uPVC) in microplastic form, after being artificially exposed to different aging agents that can affect microplastics in urban environments: ozone, UV-C, and solar radiation. In parallel to the laboratory experiments, the microplastics were exposed to real weathering conditions for three-months in a building rooftop located in the city of Porto (Portugal). By analysing the (virgin and aged) microplastic samples periodically through ATR-FTIR spectroscopy and estimating the Carbonyl Index, it was possible to sketch the aging degree evolution through time and identify the most aggressive agents for each polymer regarding the changes in their chemical structure. SEM and XRD measurements allowed to complement the ATR-FTIR results, giving a more complete picture of the effects of each treatment on each polymer and suggesting that ATR-FTIR measurements are not sufficient to correctly evaluate the aging degree of microplastics. The changes observed in the microplastic particles studied support the theory that microplastics in the environment undergo aging and change their characteristics through time, potentially affecting their behavior and intensifying their impacts.

Aqueous solution photocatalytic synthesis of p-anisaldehyde by using graphite-like carbon nitride photocatalysts obtained via the hard-templating route.

Graphite-like carbon nitride (GCN)-based materials were developed via the hard-templating route, using dicyandiamide as the GCN precursor and silica templates. That resulted in urchin-like GCN (GCN-UL), 3D ordered macroporous GCN (GCN-OM) and mesoporous GCN (GCN-MP). The introduction of silica templates during GCN synthesis produced physical defects on its surface, as confirmed by SEM analysis, increasing their specific surface area. A high amount of nitrogen vacancies is present in modified catalysts (revealed by XPS measurements), which can be related to an increase in the reactive sites available to catalyse redox reactions. The textural and morphological modifications induced in GCN an enhanced light absorption capacity and reduced electron/hole recombination rate, contributing to its improved photocatalytic performance. In the photocatalytic conversion of p-anisyl alcohol to p-anisaldehyde in deoxygenated aqueous solutions under UV-LED irradiation, the GCN-UL was the best photocatalyst reaching 60% yield at 64% conversion for p-anisaldehyde production after 240 min of reaction. Under oxygenated conditions (air), the process efficiency was increased to 79% yield at 92% conversion only after 90 min reaction. The GCN-based photocatalyst kept its performance when using visible-LED radiation under air atmosphere. Trapping of photogenerated holes and radicals by selective scavengers showed that under deoxygenated conditions, holes played the primary role in the p-anisaldehyde synthesis. Under oxygenated conditions, the process is governed by the effect of reactive oxygen species, namely superoxide radicals, with a significant contribution from holes.

Efficient removal of parabens from real water matrices by a metal-free carbon nitride photocatalyst.

Metal-free graphite-like carbon nitride (GCN-500) was obtained by thermal post-treatment of bulk polymeric carbon nitride at 500 °C. The catalyst was thoroughly characterized by morphological, optical and textural analysis techniques. The efficiency of GCN-500 was evaluated under visible (λexc = 417 nm) LED excitation for the photocatalytic degradation of methyl-, ethyl- and propyl-paraben in different water matrices either isolated or in a mixture of the three compounds. The GCN-500 proved to be more efficient than the benchmark TiO2 P25, with complete conversion of the individual parabens within 20 min of irradiation, contrasting with 120 min needed for total degradation using TiO2. Experiments in the presence of selected scavengers confirmed the high importance of superoxide radicals in the photocatalytic oxidation of parabens using GCN-500. The effect of the nature of the aqueous matrix in the kinetics of the photocatalytic process was assessed using ultrapure, tap and river waters spiked with a mixture of the three parabens. Although still very efficient, the complexity of the real water samples turned the degradation process slower due to the presence of other components such as ions and dissolved organic matter. GCN-500 proved to be stable in a continuous-flow system using GCN-500 coated glass rings (GCN-500-GR) to remove MP, EP and PP from real water matrices.