Switching on photocatalytic NO oxidation and proton reduction of NH2-MIL-125(Ti) by convenient linker defect engineering

Environmental catalytic city: New engine for air pollution control

Air pollution is a major challenge to the improvement of urban environmental quality. The control of air pollution still faces severe challenges, especially in developing countries, such as ozone pollution control. Ozone is a typical secondary air pollutant, and its formation chemistry from its precursors (NOx and volatile organic compounds) is highly nonlinear, which caused the emission reduction of its precursors is not always effective and therefore new assisted approaches to control of ozone pollution are needed. Photocatalysis and ambient catalysis technology are expected to be applied in open atmosphere as a new booster to the direct purification of air pollutants in emission sources. In this perspective, we summarize the current knowledge about the photocatalysis and ambient catalysis technology for the removal of air pollutants under natural photothermal conditions. Based on these technologies, we propose the concept of "Environmental Catalytic City", which refers to the spontaneous purification of low concentration urban air pollutants in the atmosphere by catalytic materials coating on the artificial surfaces, such as building surfaces in the city. In this way, the urban city with self-purification function can remove air pollution without additional energy consumption. The further improvement, development, and application of the "Environmental Catalytic City" is also discussed.

Highly efficient photocatalytic removal of NO and synchronous inhibition of NO2via heterojunction formed by ZnAl-LDH and MXene-Ti3C2-derived TiO2@C

The key challenge in oxidizing NO using photocatalysis is controlling the selectivity of products to avoid the generation of toxic byproducts like NO2. Here, we propose regulating the generation of reactive oxygen species by constructing Type-II heterojunctions to facilitate the deep oxidation of NO to nitrates. Experimental characterization and Density functional theory (DFT) simulations demonstrate that the outstanding photocatalytic activity of heterojunction materials stems from their superior charge separation efficiency and stronger adsorption capacity for NO and O2 molecules, promoting the formation of reactive oxygen species. These results indicated that the best-performing sample, ZATC15, demonstrated an impressive NO removal efficiency of 65.43 %. However, the selectivity rate of NO2 was only 4.78 %, much lower compared to the NO2 selectivity rates of pure ZnAl-LDH (48.17 %) and TiO2@C (72.46 %). The intermediate and final products, the generation pathways of active free radicals (h+ and •O2-) and the mechanism behind the profound oxidation of NO were elucidated based on in-situ Fourier Transform Infrared Spectroscopy (in-situ FT-IR), Electron spin resonance (ESR), and capture experiment. This investigation will offer valuable insights for modifying LDH in order to effectively remove ppb-level NO through photocatalysis.

NH2-MIL-125(Ti) and its functional nanomaterials - a versatile platform in the photocatalytic arena

Titanium (Ti)-based MOFs are promising materials known for their porosity, stability, diverse valence states, and a lower conduction band (CB) than Zr-MOFs. These features support stable ligand-to-metal charge transfer (LMCT) transitions under photoirradiation, enhancing photocatalytic performance. However, Ti-MOF structures remain a challenge owing to the highly volatile and hydrophilic nature of ionic Ti precursors. The discovery of MIL-125 marked a breakthrough in Ti-cluster coordination chemistry. Combining it with NH2 chromophores to form NH2-MIL-125 enhanced its structural design and extended its activity into the visible light region. This review delves into the high-performance photocatalytic properties of NH2-MIL-125, focusing on its applications in H2O2 and H2 production, CO2 and N2 reduction, drug and dye degradation, photocatalytic sensors, and organic transformation reactions. The discussion considers the influence of the Ti precursor, coordination environment, synthesis process, and charge transfer mechanisms. Numerous strategic methods have been discussed to improve the performance of NH2-MIL-125 by incorporating linker modification, metal node modification, encapsulation of active species, and post-modification for enhancing light absorption ability, promoting charge separation, and improving photocatalytic efficiency. Moreover, future perspectives include methods to investigate how the efficiency of NH2-MIL-125-based materials can be planned in promoting research by highlighting their versatility and potential impacts in the area of photocatalysis.

Metal-Organic Frameworks for Dual Photo-Oxidation and Capture of Arsenic from Water

Despite rapid technological progress, heavy metal water pollution, and particularly arsenic contamination, remains a significant global challenge. In addition, the stabilization of trivalent arsenic as neutral arsenite (AsIII) species hinders its removal by conventional sorbents. While adsorption of anionic arsenate (AsV) species is in principle more feasible, there are only few adsorbents capable of adsorbing both forms of arsenic. In this work, we explore the potential of two well-known families of Metal-Organic Frameworks (MOFs), UiO-66 and MIL-125, to simultaneously adsorb and photo-oxidize arsenic species from water. Our results demonstrate that the formation of AsV ions upon light irradiation promotes the subsequent adsorption of AsIII species. Thus, we propose the combined utilization of photocatalysis and adsorption with Metal-Organic Framework photocatalysts for water remediation purposes.

Regulating the Electronic Band Structure of the Ti-Based Metal-Organic Framework toward Boosting Light-Driven Hydrogen Evolution

The photocatalytic H2 evolution rate on the isomorphic nanosheet-based Ti metal organic-frameworks (MOFs) is regulated through changing the length of aromatic carboxylate ligands. For the series of Ti-MOFs, when increasing the length of organic linkers, the band gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) narrow based on density functional theory (DFT) calculation, accompanied by a degree of increase of organic ligand involvement in the LUMO. When increasing the linker length, both the intensities of photoluminescence (PL) and electron paramagnetic resonance (EPR) signals related to Ti3+ gradually decrease, which are opposite to their photocatalytic performance, where the longer the linkers, the higher the hydrogen evolution rate. It is suggested that the bound photoelectrons by Ti3+ compete with the transfer of photoelectrons for H2 evolution. When increasing the length of the organic linker, more photoelectrons could be generated, in addition to electron transfer overwhelming electrons bound by Ti3+. Both of them engender the super photocatalytic hydrogen evolution. This work highlights a specific way of regulating the electronic structure of Ti-based photocatalysts toward promoting the utilization efficiency of photoelectrons, which will shed light on the design of efficient photocatalysts for the generation of solar fuels.

Fabrication of core-shell nanostructure via novel ligand-defect reassembly strategy for efficient photocatalytic hydrogen evolution and NO removal

The core-shell structure often exhibits unique properties, resulting in superior physical and chemical performance distinct from individual component in the field of photocatalysis. However, traditional prepared methods such as template synthesis and layer-by-layer self-assembly are relatively complex. Therefore, it is necessary to explore an efficient and expedient approach. Here, we have proposed a convenient method to gradually destroy the terephthalic acid (BDC) of MIL-125 from the outer to inner layers through hydrothermal stirring, followed by reassembling with photosensitive 2-amino-terephthalic acid (BDC-NH2) into the exposed Ti-oxo clusters left by the BDC to create photocatalysts featuring a core-shell configuration. The special core-shell sample with the analogous mixture of MIL-125 and MIL-125-NH2 function as a high-performance dual-functional photocatalyst for hydrogen generation and NO elimination. The optimal core-shell material (M-125-45-N) exhibits an outstanding photocatalytic hydrogen production rate of 3.74 mmol·g-1·h-1 and an excellent photocatalytic NO removal rate of 70.15 %. The apparent quantum yield (AQY) value and solar-to-hydrogen energy conversion efficiency (STH) at specific wavelengths are also investigated. The Density functional theory (DFT) calculation, In-situ Fourier transform infrared (In-situ FT-IR) and Electron spin resonance (ESR) have suggested that the enhanced photocatalytic activity of optimal core-shell material arised from its stronger adsorption capacity towards reactants, promoting the production of reactive oxygen species (ROS) conducive to photocatalytic reactions. This study represents the first investigation of a dual functional core-shell MOFs formed via ligand-defect reassembly, showcasing the excellent efficacy in photocatalytic hydrogen evolution and NO removal, which contributes to the feasible development of novel dual-functional photocatalysts with core-shell structures.

High-efficiency removal of microcystis aeruginosa using Z-scheme AgBr/NH2-MIL-125(Ti) photocatalyst with superior visible-light absorption: Performance insights and mechanisms

Algal blooms have become a widespread concern for drinking water production, threatening ecosystems and human health. Photocatalysis, a promising advanced oxidation process (AOP) technology for wastewater treatment, is considered a potential measure for in situ remediation of algal blooms. However, conventional photocatalysts often suffer from limited visible-light response and rapid recombination of photogenerated electron-hole pairs. In this study, we prepared a Z-scheme AgBr/NH2-MIL-125(Ti) composite with excellent visible light absorption performance using co-precipitation to efficiently inactivate Microcystis aeruginosa. The degradation efficiency of AgBr/NH2-MIL-125(Ti) for chlorophyll a was 98.7 % after 180 min of visible light irradiation, significantly surpassing the degradation rate efficiency of AgBr and NH2-MIL-125(Ti) by factors of 3.20 and 36.75, respectively. Moreover, the removal rate was maintained at 91.1 % even after five times of repeated use. The experimental results indicated that superoxide radicals (•O2-) were the dominant reactive oxygen species involved. The photocatalytic reaction altered the morphology and surface charge of algal cells, inhibited their metabolism, and disrupted their photosynthetic and antioxidant systems. In conclusion, this study presents a promising material for the application of photocatalytic technology in algal bloom remediation.

Boosted visible-light-induced photo-Fenton degradation of organic pollutants over a novel direct Z-scheme NH2-MIL-125(Ti)@FeOCl heterojunction catalyst

Improving the charge separation, charge transfer, and effective utilization is crucial in a photocatalysis system. Herein, we prepared a novel direct Z-scheme NH2-MIL-125(Ti)@FeOCl (Ti-MOF@FeOCl) composite photocatalyst through a simple method. The prepared composite catalyst was utilized in the photo-Fenton degradation of Rhodamine B (RhB) and ciprofloxacin (CIP). The Ti-MOF@FeOCl (10FeTi-MOF) catalyst exhibited the highest catalytic performance and degraded 99.1 and 66% of RhB and CIP, respectively. However, the pure NH2-MIL-125(Ti) (Ti-MOF) and FeOCl catalysts achieved only 50 and 92% of RhB and 50 and 37% of CIP, respectively. The higher catalytic activities of the Ti-MOF@FeOCl composite catalyst could be due to the electronic structure improvements, photoinduced charge separations, and charge transfer abilities in the catalyst system. The composite catalysts have also enhanced adsorption and visible light-responsive properties, allowing for efficient degradation. Furthermore, the electron paramagnetic resonance (EPR) signals, the reactive species trapping experiments, and Mott-Schottky (M - S) measurements revealed that the photogenerated superoxide radical (•O2-), hydroxyl radical (•OH), and holes (h+) played a vital role in the degradation process. The results also demonstrated that the Ti-MOF@FeOCl heterojunction composite catalysts could be a promising photo-Fenton catalyst system for the environmental remediation. Environmental implications The discharging of toxic contaminants such as organic dyes, antibiotics, and other emerging pollutants to the environment deteriorates the ecosystem. Specifically, the residues of organic pollutants recognized as a threat to ecosystem and a cause for carcinogenic effects. Among them, ciprofloxacin is one of antibiotics which has biological resistance, and metabolize partially in the human or animal bodies. It is also difficult to degrade ciprofloxacin completely with traditional treatment methods. Similarly, organic dyes are also toxic and a cause for carcinogenic effects. Therefore, effective degradation of organic pollutants such as RhB and ciprofloxacin with appropriate method is crucial.

Balanced Mass Transfer and Active Sites Density in Hierarchical Porous Catalytic Metal-Organic Framework for Enhancing Redox Reaction in Lithium-Sulfur Batteries

Developing highly efficient catalysts, characterized by controllable pore architecture and effective utilization of active sites, is paramount in addressing the shuttle effect and sluggish redox kinetics of lithium polysulfides (LiPSs) in lithium-sulfur batteries (LSBs), which, however, remains a formidable challenge. In this study, a hierarchical porous catalytic metal-organic framework (HPC-MOF) with both appropriate porosity and abundant exposed catalytic sites is achieved through time-controlled precise pore engineering. It is revealed that the evolution of the porous structure and catalytic site density is time-dependent during the etching processes. The moderately etched HPC-MOF-M attains heterogeneous pores at various scales, where large apertures ensure fast mass transfer and micropores inherit high-density catalytic sites, enhancing utilization and catalytic kinetics at internal catalytic sites. Capitalizing on these advantages, LSB incorporating the HPC-MOF-M interlayer demonstrates a 164.6% improvement in discharge capability and an 83.3% lower decay rate over long-term cycling at 1.0C. Even under high sulfur loading of 7.1 mg cm-2 and lean electrolyte conditions, the LSB exhibits stable cycling for over 100 cycles. This work highlights the significance of balancing the relationship between mass transfer and catalytic sites through precise chemical regulation of the porous structure in catalytic MOFs, which are anticipated to inspire the development of advanced catalysts for LSBs.

Modulating the Activity and SO2 Resistance of α-Fe2O3 Catalysts for NH3-SCR of NOx via Crystal Facet Engineering

The development of Fe-based catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx) has garnered significant attention due to their exceptional SO2 resistance. However, the influence of different sulfur-containing species (e.g., ferric sulfates and ammonium sulfates) on the NH3-SCR activity of Fe-based catalysts as well as its dependence on exposed crystal facets of Fe2O3 has not been revealed. This work disclosed that nanorod-like α-Fe2O3 (Fe2O3-NR) predominantly exposing (110) facet performed better than nanosheet-like α-Fe2O3 (Fe2O3-NS) predominantly exposing (001) facet in NH3-SCR reaction, due to the advantages of Fe2O3-NR in redox properties and surface acidity. Furthermore, the results of the SO2/H2O resistance test at a critical temperature of 250 °C, catalytic performance evaluations on Fe2O3-NR and Fe2O3-NS sulfated by SO2 + O2 or deposited with NH4HSO4 (ABS), and systematic characterization revealed that the reactivity of ammonium sulfates on Fe2O3 catalysts to NO(+O2) contributed to their improved catalytic performance, while ferric sulfates showed enhancing and inhibiting effects on NH3-SCR activity on Fe2O3-NR and Fe2O3-NS, respectively; despite this, Fe2O3-NR showed higher affinity for SO2 + O2. This work set a milestone in understanding the NH3-SCR reaction on Fe2O3 catalysts in the presence of SO2 from the aspect of crystal facet engineering.

Copper-decorated strategy based on defect-rich NH2-MIL-125(Ti) boosts efficient photocatalytic degradation of methyl mercaptan under sunlight

Photocatalysis has received significant attention as a technology that can solve environmental problems. Metal-organic frameworks are currently being used as novel photocatalysts but are still limited by the rapid recombination of photogenerated carriers, low photogenerated electron migration efficiency and poor solar light utilization rate. In this work, a novel photocatalyst was successfully constructed by introducing Cu species into thermal activated mixed-ligand NH2-MIL-125 (Ti) via defect engineering strategy. The constructed defect structure not only provided 3D-interconnected gas transfer channels, but also offered suitable space to accommodate introduced Cu species. For the most effective photocatalyst 0.2Cu/80%NH2-MIL-125 (300 °C) with optimized Cu content, the photocatalytic degradation rate of CH3SH achieved 4.65 times higher than that of pristine NH2-MIL-125 under visible light (λ > 420 nm). At the same time, it showed great degradation efficiency under natural sunlight, 100 ppm CH3SH was completely removed within 25 min in full solar light illumination. The improved catalytic efficiency is mainly due to the synergistic effect of the integrated Schottky junction and rich-defective NH2-MIL-125, which improved the bandgap and band position, and thus facilitated the separation and transfer of the photo-generated carriers. This work provided a facile way to integrate Schottky junctions and rich-defective MOFs with high stability. Due to its excellent degradation performance under sunlight, it also offered a prospective strategy for rational design of high-efficiency catalysts applied in environmental technologies.

Ag Nanoparticle and Ti-MOF Cooperativity for Efficient Inactivation of E. coli in Water

Because of the limitations of traditional chlorine-based bactericidal water treatment, such as the formation of disinfection byproducts (DBPs) and resistance to chlorine, novel approaches and materials are required for effective disinfection of water. This study focuses on the development of a new sterilization material, Ag/NH2-MIL-125(Ti), which was designed to effectively inactivate Escherichia coli in water. The effectiveness of the as-designed material stems from the synergistic interactions between Ag nanoparticles (NPs) and photoactive metal-organic frameworks (MOFs). In this complex material, the MOFs play a critical role in dispersing and isolating the Ag NPs, thus preventing undesirable aggregation during bacterial inactivation. Simultaneously, Ag NPs enhance the photocatalytic performance of the MOFs. Sterilization experiments demonstrate the remarkable rapid E. coli inactivation performance of Ag/NH2-MIL-125(Ti) under illuminated and nonilluminated conditions. Within 25 min of visible light exposure, the as-prepared material achieves a >7-log E. coli reduction. In addition, Ag/NH2-MIL-125(Ti) efficiently decomposes acetic acid, which is the main DBP precursor, under visible light irradiation. Mechanistic investigations revealed that •O2- and h+ were the primary active substances responsible for the inactivation of E. coli and the decomposition of acetic acid, respectively.

Research landscape and hotspots of selective catalytic reduction (SCR) for NOx removal: insights from a comprehensive bibliometric analysis

Selective catalytic reduction (SCR) has been one of the most efficient and widely used technologies to remove nitrogen oxides (NO_x). SCR research has developed rapidly in recent years, which can be reflected by the dramatic increase of related academic publications. Herein, based on the 10,627 documents from 2001 to 2020 in Web of Science, the global research landscape and hotspots in SCR are investigated based on a comprehensive bibliometric analysis. The results show that SCR research has developed positively; the annul number of articles increase sharply from 246 in 2001 to 1092 in 2020. People's Republic of China and Chinese Academy of Sciences are the most productive country and institution, respectively. The global collaboration is extensive and frequent, while People's Republic of China and USA have the most frequent research cooperation. Applied Catalysis B-Environmental is the leading publication source with 711 records. Five major research areas on SCR are identified and elaborated, including catalyst, reductant, deactivation, mechanism, and others. Zeolite is the most widely studied SCR catalyst, while copper, silver, platinum, and iron are the most popular metal elements in catalyst. Ammonia (NH₃) is dominated among various SCR reductants, while hydrocarbon reductant has gained more attention. Sulfur dioxide (SO₂) and vapor are the two most concerned factors leading to catalyst deactivation, and catalyst regeneration is also an important research topic. Density functional theory (DFT), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and kinetics are the most widely used methods to conduct mechanism study. The studies on "low temperature," "atomic-scale insight," "elemental mercury," "situ DIRFTS investigation," "arsenic poisoning," "SPOA-34," "Cu-CHA catalyst," "TiO₂ catalyst," and "Ce catalyst" have been the hotspots in recent years.

Effectively enhanced photocatalytic performance of layered perovskite Bi2NdO4Cl by coupling piezotronic effect

The coupling between piezoelectricity and photoexcitation is an attractive method for improving the photocatalytic efficiency of semiconductors. Herein, a novel layered perovskite photocatalyst Bi₂NdO₄Cl (BNOC) has been successfully prepared via solid-state reaction. PFM results confirm that BNOC has piezoelectricity, and its piezo-photocatalytic degradation performance was evaluated for the first time using tetracycline hydrochloride (TH) as a pollutant model. The results show that the piezo-photocatalytic degradation rate constant is about 1.5 times higher than the sum of the individual photo- and piezo-catalytic components. This synergistic enhancement can be attributed to the band tilting-induced piezoelectric polarization charges and formation of a piezoelectric field, which accelerates the photoinduced charge carrier separation and effectively enhanced the photocatalytic performance. This work may facilitate the development of novel piezoelectric photocatalytic materials that are highly sensitive to the mechanical energy of discrete fluids, and offer ideas for piezo-photocatalysis in environmental applications.

Synergistic effect of cyano defects and CaCO3 in graphitic carbon nitride nanosheets for efficient visible-light-driven photocatalytic NO removal

Photo-oxidation with semiconductor photocatalysts provides a sustainable and green solution for NO_x elimination. Nevertheless, the utilization of traditional photocatalysts in efficient and safe photocatalytic NO_x removal is still a challenge due to the slow charge kinetic process and insufficient optical absorption. In this paper, we report a novel porous g-C₃N₄ nanosheet photocatalyst modified with cyano defects and CaCO₃ (xCa-CN). The best performing sample (0.5Ca-CN) exhibits an enhanced photo-oxidation NO removal rate (51.18 %) under visible light irradiation, largely surpassing the value of pristine g-C₃N₄ nanosheets (34.05 %). Such an enhancement is mainly derived from an extended visible-light response, improved electron excitation and transfer, which are associated with the synergy of cyano defects and CaCO₃, as evidenced by a series of spectroscopic analyses. More importantly, in-situ DRIFTS and density functional theory (DFT) results suggest that the introduction of cyano defects and CaCO₃ enables control over NO adsorption and activation processes, making it possible to implement a preference pathway (NO → NO⁺ → NO₃¯) and reduce the emission of toxic intermediate NO₂. This work demonstrates the potential of integrating defect engineering and insulator modification to design highly efficient g-C₃N₄-based photocatalysts for air purification.

Dual-Ligand Ti-MOFs with Push-Pull Effect for Photocatalytic H2 Production

Enhancing the photogenerated electrons transfer efficiency is crucial for photocatalytic reactions. Herein, a dual-ligand-induced push-pull effect was manipulated to intensify the transfer of photogenerated electrons between organic ligands and metal clusters using NH₂-MIL-125(Ti), a kind of Ti-based metal-organic framework (MOF), as the model system. The dual-ligand MOF, NH₂/Cl-MIL-125, was designed and synthesized based on the Hammett constant (σ_m), in which -NH₂ (σ_m = -0.16) and -Cl (σ_m = 0.37) were selected as the electron-pushing group and the electron-pulling group, respectively. Meanwhile, -CH₃ (σ_m = -0.07, electron-pushing) and -H (σ_m = 0, neither electron-pushing nor electron-pulling) were selected as the reference groups to prepare NH₂/CH₃-MIL-125 and NH₂/H-MIL-125, respectively, to validate the electron push-pull effect. NH₂/Cl-MIL-125 (5.32 mmol g^-1 h^-1) exhibits a higher photocatalytic H₂ evolution activity than single-ligand NH₂-MIL-125 (1.93 mmol g^-1 h^-1), NH₂/CH₃-MIL-125 (4.45 mmol g^-1 h^-1), and NH₂/H-MIL-125 (4.73 mmol g^-1 h^-1) under full-spectrum irradiation. The result can be attributed to the electron push-pull effect between -NH₂ and -Cl, which boosts the electron transfer along the ligand-metal-ligand direction. Our dual-ligand-induced push-pull strategy for enhancing the electron transfer may offer some novel insights into the rational design and synthesis of photocatalysts for many other reactions.

Multi-channel charge transfer of hierarchical TiO2 nanosheets encapsulated MIL-125(Ti) hollow nanodisks sensitized by ZnSe for efficient CO2 photoreduction

Metal-organic frameworks-based hybrids with desirable components, structures, and properties have been proven to be promising functional materials for photocatalysis and energy conversion applications. Herein, we proposed and prepared ZnSe sensitized hierarchical TiO₂ nanosheets encapsulated MIL-125(Ti) hollow nanodisks with sandwich-like structure (MIL-125(Ti)@TiO₂\ZnSe HNDs) through a successive solvothermal and selenylation reaction route using the as-prepared MIL-125(Ti) nanodisks as precursor. In the ternary MIL-125(Ti)@TiO₂\ZnSe HNDs hybrid, TiO₂ nanosheets were transformed from MIL-125(Ti) and in situ grown on both sides of the MIL-125(Ti) shell, forming sandwich-like hollow nanodisks, and the ratio of MIL-125(Ti)/TiO₂ can be tuned by changing the solvothermal time. The ternary hybrids possess the advantages of enhanced incident light utilization and abundant accessible active sites originating from bimodal pore-size distribution and hollow sandwich-like heterostructure, which can effectively promote CO₂ photoreduction reaction. Especially, the formed multi-channel charge transfer routes in the ternary heterojunctions contribute to the charge transfer/separation and extend the lifespan of charge-separated state, thus boosting CO₂ photoreduction performance. The CO (513.1 μmol g^-1h^-1) and CH₄ (45.1 μmol g^-1h^-1) evolution rates over the optimized ternary hybrid were greatly enhanced compared with the single-component and binary hybrid photocatalysts.