Modified UiO-66 as photocatalysts for boosting the carbon-neutral energy cycle and solving environmental remediation issues

Metal-Organic Frameworks as Photocatalysts for Solar-Driven Overall Water Splitting

Metal-organic frameworks (MOFs) have been frequently used as photocatalysts for the hydrogen evolution reaction (HER) using sacrificial agents with UV-vis or visible light irradiation. The aim of the present review is to summarize the use of MOFs as solar-driven photocatalysts targeting to overcome the current efficiency limitations in overall water splitting (OWS). Initially, the fundamentals of the photocatalytic OWS under solar irradiation are presented. Then, the different strategies that can be implemented on MOFs to adapt them for solar photocatalysis for OWS are discussed in detail. Later, the most active MOFs reported until now for the solar-driven HER and/or oxygen evolution reaction (OER) are critically commented. These studies are taken as precedents for the discussion of the existing studies on the use of MOFs as photocatalysts for the OWS under visible or sunlight irradiation. The requirements to be met to use MOFs at large scale for the solar-driven OWS are also discussed. The last section of this review provides a summary of the current state of the field and comments on future prospects that could bring MOFs closer to commercial application.

Capacitive Desalination and Disinfection of Water Using UiO-66 Metal-Organic Framework/Bamboo Carbon with Chitosan

The zirconium-based metal-organic framework (MOF) (UiO-66)/bamboo carbon (BC) composite with chitosan was prepared using hydrothermal and impregnation methods and used for capacitive desalination (CDI) and disinfection of water. The results showed that these composites had fast ion exchange and charge transfer properties. During the CDI process, these composites' electrodes exhibited good cycle stability, electrosorption capacity (4.25 mg/g) and excellent bactericidal effect. These carbon-based composites electrodes' bactericidal rate for Escherichia coli could reach 99.99% within 20 minutes; therefore, they had good performance and were a good choice for high-performance deionization applications.

A novel, Z-scheme ZnO@AC@FeO photocatalyst, suitable for the intensification of photo-mediated peroxymonosulfate activation: Performance, reactivity and bisphenol A degradation pathways

In this study, the intensification of a UVC-based PMS activation treatment is performed by a novel photocatalyst. Using ZnO nanoparticles coupled with activated carbon (AC), impregnated by ferroferric oxides (FO, magnetite), as an effective Z-scheme photocatalyst (ZACFO), the effective Bisphenol A (BP-A) removal was attained. Several techniques were applied for the characterization of the as-prepared catalyst and proved the successful preparation of ZACFO. The photocatalytic activity of pristine ZnO was significantly improved after its combination with ACFO. It was found that the fabrication of ZACFO heterostructures could inhibit the charge carriers recombination and also accelerate the charge separation of photo-induced e^-/h⁺ pairs. Under this UVC-based photocatalysis-mediated PMS activation system, ZACFO showed an excellent potential as compared to the single constituent catalysts. The complete degradation of 20 mg/L concentration of BP-A was attained in just 20 min with excellent reaction rate constant of 27.3 × 10^-2 min^-1. Besides, over 60% of TOC was eliminated by the integrated ZACFO/PMS/UV system within 60 min of reaction. The minor inhibition by most matrix components, the high recycling capability with minor metals' leaching and the effectiveness in complex matrices, constitute this composite method an efficient and promising process for treating real wastewater samples. Finally, based on the photo-produced reactive intermediates and by-products identified, the Z-scheme photocatalytic mechanism and the plausible pathway of BP-A degradation were proposed comprehensively. The presence and role of radical and non-radical pathways in the decontamination process of BP-A over ZACFO/PMS/UV system was confirmed.

Photocatalytic Hydrogen Production from Glycerol Aqueous Solutions as Sustainable Feedstocks Using Zr-Based UiO-66 Materials under Simulated Sunlight Irradiation

There is an increasing interest in developing cost-effective technologies to produce hydrogen from sustainable resources. Herein we show a comprehensive study on the use of metal-organic frameworks (MOFs) as heterogeneous photocatalysts for H₂ generation from photoreforming of glycerol aqueous solutions under simulated sunlight irradiation. The list of materials employed in this study include some of the benchmark Zr-MOFs such as UiO-66(Zr)-X (X: H, NO₂, NH₂) as well as MIL-125(Ti)-NH₂ as the reference Ti-MOF. Among these solids, UiO-66(Zr)-NH₂ exhibits the highest photocatalytic H₂ production, and this observation is attributed to its adequate energy level. The photocatalytic activity of UiO-66(Zr)-NH₂ can be increased by deposition of small Pt NPs as the reference noble metal co-catalyst within the MOF network. This photocatalyst is effectively used for H₂ generation at least for 70 h without loss of activity. The crystallinity of MOF and Pt particle size were maintained as revealed by powder X-ray diffraction and transmission electron microscopy measurements, respectively. Evidence in support of the occurrence of photoinduced charge separation with Pt@UiO-66(Zr)-NH₂ is provided from transient absorption and photoluminescence spectroscopies together with photocurrent measurements. This study exemplifies the possibility of using MOFs as photocatalysts for the solar-driven H₂ generation using sustainable feedstocks.

Altered zirconium dioxide based photocatalyst for enhancement of organic pollutants degradation: A review

Heterogeneous advanced oxidation processes are a promising approach for cost-efficient removal of pollutants using semiconductors. Zirconium dioxide (ZrO₂) is an auspicious material for photocatalytic activity owning to its suitable bandgap, stability, and low cost. However, ZrO₂ suffers from fast recombination rate, and poor light harvesting ability. Nonetheless, extra modification has also shown improvements and therefore is worth investigating. The endeavour of this paper initially discusses the fundamentals with respect to reactive species, classification, and synthesis methods for ZrO₂. Furthermore, with particular consideration to stability and reusability, several additional modification approaches for ZrO₂-based photocatalysts such as doping and noble metals loading. Furthermore, the formation of heterojunctions has also been shown to boost photocatalytic activity while inhibiting charge carrier recombination. Finally, photocatalyst separation via magnetic-based photocatalysts are elucidated. As a result, ZrO₂-based photocatalysts are regarded as a promising emerging technology that warrants further development and research.

Carbon and N conservation during composting: A review

Composting, as a conventional solid waste treatment method, plays an essential role in carbon and nitrogen conservation, thereby reducing the loss of nutrients and energy. However, some carbon- and nitrogen-containing gases are inevitably released during the process of composting due to the different operating conditions, resulting in carbon and nitrogen losses. To overcome this obstacle, many researchers have been trying to optimize the adjustment parameters and add some amendments (i.e., pHysical amendments, chemical amendments and microbial amendments) to reduce the losses and enhance carbon and nitrogen conservation. However, investigation regarding mechanisms for the conservation of carbon and nitrogen are limited. Therefore, this review summarizes the studies on physical amendments, chemical amendments and microbial amendments and proposes underlying mechanisms for the enhancement of carbon and nitrogen conservation: adsorption or conversion, and also evaluates their contribution to the mitigation of the greenhouse effect, providing a theoretical basis for subsequent composting-related researchers to better improve carbon and nitrogen conservation measures. This paper also suggests that: assessing the contribution of composting as a process to global greenhouse gas mitigation requires a complete life cycle evaluation of composting. The current lack of compost clinker impact on carbon and nitrogen sequestration capacity of the application site needs to be explored by more research workers.

Recent Advances in Vehicle Exhaust Treatment with Photocatalytic Technology

Vehicle exhaust has been acknowledged as an essential factor affecting human health due to the extensive use of cars. Its main components include volatile organic compounds (VOCs) and nitrogen oxides (NOx), which can cause acute irritation and chronic diseases, and significant research on the treatment of vehicle exhaust has received increasing attention in recent decades. Recently, photocatalytic technology has been considered a practical approach for eliminating vehicle emissions. This review highlights the crucial role of photocatalytic technology in eliminating vehicle emissions using semiconductor catalysts. A particular emphasis has been placed on various photocatalytic materials, such as TiO2-based materials, Bi-based materials, and Metal–Organic Frameworks (MOFs), and their recent advances in the performance of VOC and NOx photodegradation. In addition, the applications of photocatalytic technology for the elimination of vehicle exhaust are presented (including photocatalysts combined with pavement surfaces, making photocatalysts into architectural coatings and photoreactors), which will offer a promising strategy for photocatalytic technology to remove vehicle exhaust.

The application of transition metal-modified biochar in sulfate radical based advanced oxidation processes

Sulfate radical (SO₄^•-) based advanced oxidation processes (SR-AOPs) is a very important chemical oxidation technology for the degradation of recalcitrant organic pollutants in water and has been well developed. Recently, transition metals or their oxides-modified biochar has been widely used as the catalyst to catalyze peroxymonosulfate (PMS) and peroxydisulfate (PS) in SR-AOPs due to their outstanding properties (e.g., large surface area, high stability, abound catalytic sites, and diversity of material design, etc.). These composite materials not only combine the respective beneficial characteristics of biochar and transition metals (or their oxides) but also often present synergistic effects between the components. In this review, we present the synthesis of different types of transition metal (or metal oxides)/biochar-based catalysts and their application in SR-AOPs. The catalytic mechanism, including the generation process of free radicals and other reaction pathways on the surface of the catalyst were also carefully discussed. Particular attention has been paid to the synergistic effects between the components that result in enhanced catalytic performance. At the end of this review, the future development prospects of this technology are proposed.

Recent Progress in the Development of MOF-Based Photocatalysts for the Photoreduction of Cr(VI)

There has been a direct correlation between the rate of industrial development and the spread of pollution on Earth, particularly in the last century. The organic and inorganic pollutants generated from industrial activities have created serious risks to human life and the environment. The concept of sustainability has emerged to tackle the environmental issues in developing chemical-based industries. However, pollutants have continued to be discharged to water resources, and finding appropriate techniques for the removal and remedy of wastewater is in high demand. Chromium is one of the high-risk heavy metals in industrial wastewaters that should be removed via physical adsorption and/or transformed into less hazardous chemicals. Photocatalysis as a sustainable process has received considerable attention as it utilizes sunlight irradiation to remedy Cr^(VI) via a cost-effective process. Numerous photocatalytic systems have been developed up to now, but metal-organic frameworks (MOFs) have gained growing attention because of their unique versatilities and facile structural modulations. A variety of MOF-based photocatalysts have been widely employed for the photoreduction of Cr^(VI). Here, we review the recent progress in the design of MOF photocatalysts and summarize their performance in photoreduction reactions.