Three-dimensionally ordered macroporous perovskite materials for environmental applications

Recent progress in defect-engineered metal oxides for photocatalytic environmental remediation

Rapid industrial advancement over the last few decades has led to an alarming increase in pollution levels in the ecosystem. Among the primary pollutants, harmful organic dyes and pharmaceutical drugs are directly released by industries into the water bodies which serves as a major cause of environmental deterioration. This warns of a severe need to find some sustainable strategies to overcome these increasing levels of water pollution and eliminate the pollutants before being exposed to the environment. Photocatalysis is a well-established strategy in the field of pollutant degradation and various metal oxides have been proven to exhibit excellent physicochemical properties which makes them a potential candidate for environmental remediation. Further, with the aim of rapid industrialization of photocatalytic pollutant degradation technology, constant efforts have been made to increase the photocatalytic activity of various metal oxides. One such strategy is the introduction of defects into the lattice of the parent catalyst through doping or vacancy which plays a major role in enhancing the catalytic activity and achieving excellent degradation rates. This review provides a comprehensive analysis of defects and their role in altering the photocatalytic activity of the material. Various defect-rich metal oxides like binary oxides, perovskite oxides, and spinel oxides have been summarized for their application in pollutant degradation. Finally, a summary of existing research, followed by the existing challenges along with the potential countermeasures has been provided to pave a path for the future studies and industrialization of this promising field.

Unlocking Efficiency: Minimizing Energy Loss in Electrocatalysts for Water Splitting

Catalysts play a crucial role in water electrolysis by reducing the energy barriers for hydrogen and oxygen evolution reactions (HER and OER). Research aims to enhance the intrinsic activities of potential catalysts through material selection, microstructure design, and various engineering techniques. However, the energy consumption of catalysts has often been overlooked due to the intricate interplay among catalyst microstructure, dimensionality, catalyst-electrolyte-gas dynamics, surface chemistry, electron transport within electrodes, and electron transfer among electrode components. Efficient catalyst development for high-current-density applications is essential to meet the increasing demand for green hydrogen. This involves transforming catalysts with high intrinsic activities into electrodes capable of sustaining high current densities. This review focuses on current improvement strategies of mass exchange, charge transfer, and reducing electrode resistance to decrease energy consumption. It aims to bridge the gap between laboratory-developed, highly efficient catalysts and industrial applications regarding catalyst structural design, surface chemistry, and catalyst-electrode interplay, outlining the development roadmap of hierarchically structured electrode-based water electrolysis for minimizing energy loss in electrocatalysts for water splitting.

Perovskites as oxygen storage materials for chemical looping partial oxidation and reforming of methane

The traditional partial oxidation, dry reforming and steam reforming of methane technologies are separated into two reactors for execution by chemical looping technology, which can avoid the defects exposed in the traditional process (avoiding carbon accumulation, reducing costs, etc.). The key to chemical looping technology is to find suitable oxygen carriers (OCs), which can store and release oxygen to form a closed loop in the chemical looping. The purpose of this review is to summarize the current status of perovskite oxides for partial oxidation and reforming of methane in chemical looping, describe the structure, oxygen capacity, oxygen migration rate and common synthesis methods of perovskites in chemical looping. In addition, the effects of impregnation loading, ion doping, and structural morphology on the catalytic conversion of CH4 by perovskite OCs and the reaction mechanism on the OCs are also discussed.

Effect of acid/alkali treatment on the structure and catalytic performance of 3DOM CeCo0.7Mn0.3O3 catalyst

In this work, Ce was used as the A-site element and three-dimensional ordered macroporous (3DOM) materials as the template to obtain 3DOM CeCo0.7Mn0.3O3 catalyst via excessive impregnation method. The catalyst was subjected to acid/alkali treatment with dilute nitric acid and sodium hydroxide solutions. The results revealed that the catalysts subjected to acid/alkali treatment exhibited better structural and catalytic activity characteristics than the bulk catalyst. Specifically, the specific surface area of the catalyst treated with acid increased from 34.86 to 60.67 m2·g-1, and the relative contents of Oads and Mn4+ species increased. Moreover, the T90% further decreased to 174 °C. As for the catalyst treated with alkali, it exhibited a rougher surface and a wider pore size distribution, producing more lattice defects which were favorable for reaction progress. The T90% was 183 °C, indicating that acid/alkali treatment both had a positive effect on the catalytic oxidation of toluene.

Recent Progress of Three-dimensionally Ordered Macroporous (3DOM) Materials in Photocatalytic Applications: A Review

In recent years, three-dimensionally ordered macroporous (3DOM) materials have attracted tremendous interest in the field of photocatalysis due to the periodic spatial structure and unique physicochemical properties of 3DOM catalysts. In this review, the fundamentals and principles of 3DOM photocatalysts are briefly introduced, including the overview of 3DOM materials, the photocatalytic principles based on 3DOM materials, and the advantages of 3DOM materials in photocatalysis. The preparation methods of 3DOM materials are also presented. The structure and properties of 3DOM materials and their effects on photocatalytic performance are briefly summarized. More importantly, 3DOM materials, as a supported catalyst, are extensively employed to combine with various common materials, including metal nanoparticles, metal oxides, metal sulfides, and carbon materials, to enhance photocatalytic performance. Finally, the prospects and challenges for the development of 3DOM materials in the field of photocatalysis are presented.

Heterogeneous Activation of Persulfate by LaMO3 (M=Co, Fe, Cu, Mn, Ni) Perovskite Catalysts for the Degradation of Organic Compounds

Sulfate radical-based advanced oxidation processes (SR-AOPs) are lately applied for the degradation of various pollutants through the formation of reactive oxidant species (ROS) from activation of oxidants, such as persulfate (PS) or peroxymonosulfate (PMS). In this study, LaMO3 (M=Co, Fe, Cu, Mn, Ni) perovskite catalysts were synthesized, characterized by several techniques, and tested for the activation of persulfate towards the degradation of phenolic pollutants. The effect of substitution of position B of La-based perovskites as well as calcination temperature was studied. Overall, the results showed that the decisive role in the catalytic activity was the presence of structures that enhance the transfer of electrons between perovskite and oxidant. LaNiO3 followed by LaCoO3 were found as the most active catalysts. Finally, the stability of the catalysts was studied, showing that B-metal leaching is significant for both catalysts, with LaCoO3 being the most stable one.

Cadmium Sulfide 3D Photonic Crystal with Hierarchically Ordered Macropores for Highly Efficient Photocatalytic Hydrogen Generation

Cadmium sulfide is a potential candidate for photocatalytic water splitting. However, CdS nanoparticles have a high recombination rate of photoinduced carriers induced by aggregation. Therefore, decreasing the recombination rate and increasing the migration rate of photogenerated carriers are essential to drive the development and application of CdS in hydrogen production. In this study, we design CdS with a three-dimensional ordered macroporous (3DOM) structure using polymethylmethacrylate as a template. It not only retains the excellent visible light response of CdS but also improves the easy recombination of photogenerated carriers in CdS nanoparticles by taking advantage of the unique ability of mass transfer, charge separation, and migration in the 3DOM structure. Meanwhile, the highly ordered periodic structure of 3DOM CdS can produce a slow photon effect of photonic crystals to obtain more photoinduced carriers. In particular, we found that a suitable stop-band position is beneficial to maximize the utilization of the slow photon effect. The photocatalytic hydrogen evolution rate of Pt-CdS is considerably improved after constructing the 3DOM structure. This study provides a new design strategy of ordered macroporous sulfide catalysts to achieve high photocatalytic activity.

Efficient hydrogen generation of vector Z-scheme CaTiO3/Cu/TiO2 photocatalyst assisted by cocatalyst Cu nanoparticles

Herein, the CaTiO₃/Cu/TiO₂ all-solid-state Z-scheme heterojunction is successfully designed via Cu nanoparticles situating at the interface between CaTiO₃ and TiO₂ with a new synthesis route. Interestingly, TiO₂ nanosheets are generated in-situ on the surface of CaTiO₃ in the second step hydrothermal reaction. The lifetimes of photoexcited carriers, photoluminescence emission spectra and transient photocurrent response tests have confirmed that the efficient Z-scheme charge transmission path of the CaTiO₃/Cu/TiO₂ is beneficial to facilitate the separation of photogenerated carriers and reduce their recombination efficiency. As expected, the hydrogen generation rate of CaTiO₃/Cu/TiO₂ is increased to 23.550 mmol g^-1h^-1 with the appropriate amount of copper loading, which is about 981 times and 93 times higher than that of pristine CaTiO₃ (0.024 mmol g^-1h^-1) and CaTiO₃/TiO₂ (0.253 mmol g^-1h^-1), respectively. Furthermore, the CaTiO₃/Cu/TiO₂ sample shows good stability in cycle experiments. Particularly, experimental results show that the non-noble metal Cu nanoparticles can be an effective electron mediator. And these merits strongly demonstrate that the CaTiO₃/Cu/TiO₂ composites have potential application in photocatalytic field. This study can provide fundamental guidance for designing rationally efficient non-noble metal vector Z-scheme system photocatalysts with outstanding photocatalytic H₂ generation performance.

Exsolution of Iron Oxide on LaFeO3 Perovskite: A Robust Heterostructured Support for Constructing Self-Adjustable Pt-Based Room-Temperature CO Oxidation Catalysts

Constructing highly active and stable surface sites for O₂ activation is essential to lower the barrier of Pt-based catalysts for CO oxidation. Although a few active Pt-metal oxide interfaces have been reported, questions about the stability of these sites under the long-term storage and operation remain unresolved. Here, based on developing a robust FeO_x/LaFeO₃ heterostructure as a support, we constructed stable Pt-support interfaces to achieve highly active CO oxidation at room temperature. Even after it is kept in the air for more than 6 months, the catalyst (without pretreatment) still maintains the high activity like a fresh one, which is superior to metal hydroxide-Pt interfaces, and meets the requirements of long-term storage for emergency use. In situ characterizations and systematic reaction results showed that CO oxidation occurs through an alternative mechanism, which is triggered by intrinsic reactants and self-adjusted to a more active interface in the reaction process. Theoretical calculations and ⁵⁷Fe Mössbauer spectra revealed that abundant cation vacancies significantly increase the activity of surface oxygen species and should be responsible for this unique process. This work demonstrates an alternative concept to fabricate robust and highly active Pt-based catalysts for catalytic oxidation.

Improving the Catalytic CO2 Reduction on Cs2AgBiBr6 by Halide Defect Engineering: A DFT Study

Pb-free double halide perovskites have drawn immense attention in the potential photocatalytic application, due to the regulatable bandgap energy and nontoxicity. Herein, we first present a study for CO2 conversion on Pb-free halide perovskite Cs2AgBiBr6 under state-of-the-art first-principles calculation with dispersion correction. Compared with the previous CsPbBr3, the cell parameter of Cs2AgBiBr6 underwent only a small decrease of 3.69%. By investigating the adsorption of CO, CO2, NO, NO2, and catalytic reduction of CO2, we found Cs2AgBiBr6 exhibits modest adsorption ability and unsatisfied potential determining step energy of 2.68 eV in catalysis. We adopted defect engineering (Cl doping, I doping and Br-vacancy) to regulate the adsorption and CO2 reduction behavior. It is found that CO2 molecule can be chemically and preferably adsorbed on Br-vacancy doped Cs2AgBiBr6 with a negative adsorption energy of -1.16 eV. Studying the CO2 reduction paths on pure and defect modified Cs2AgBiBr6, Br-vacancy is proved to play a critical role in decreasing the potential determining step energy to 1.25 eV. Finally, we probe into the electronic properties and demonstrate Br-vacancy will not obviously promote the process of catalysis deactivation, as there is no formation of deep-level electronic states acting as carrier recombination center. Our findings reveal the process of gas adsorption and CO2 reduction on novel Pb-free Cs2AgBiBr6, and propose a potential strategy to improve the efficiency of catalytic CO2 conversion towards practical implementation.

Surface acid etching for efficient anchoring of potassium on 3DOM La0.8Sr0.2MnO3 catalyst: An integration strategy for boosting soot and NOx simultaneous elimination

The emission of soot and NOx is one of the most severe environmental issues, and the key factor is the development of catalysts in after-treatment systems. In this study, an innovative non-noble metal catalyst, named HKLSM, was fabricated by etching 3DOM La_0.8Sr_0.2MnO₃ with citric acid and synchronously anchoring potassium salt, for soot and NOx simultaneous removal. The citric acid could not only slightly erode the 3DOM skeleton, thereby beneficial to the dispersion of potassium, but also react with high-valence state Mn to generate abundant coordination unsaturated Mn³⁺ sites, which could produce more active oxygen species. Moreover, HKLSM showed a higher NOx adsorption capability than the samples that were not subjected to acid etching. This adsorbed NOx could be stored as NO₃^- species, which could facilitate soot combustion. Among all the as-prepared catalysts, HKLSM demonstrated a competitive soot combustion activity with a T₅₀ value of 368 °C, a TOF value of 3.24 × 10^-4 s^-1, a reaction rate of 1.87 × 10^-7 molg^-1s^-1, a total NOx to N₂ yield of 42.0% and favorable reusability and water-resistance. This integration strategy can rationalize an alternative protocol to soot and NOx simultaneous elimination or even other catalysis systems.

Removal of gaseous pollutants by using 3DOM-based catalysts: A review

Catalytic oxidation is a promising technique to control the emission of gaseous pollutants. Three-dimensionally ordered macroporous (3DOM)-based catalysts have aroused widespread attention because of their high porosity, large surface area and pore volume, superb ability of mass transfer. Therefore, they have been widely used in gaseous pollutants control field, such as soot and methane catalytic combustion, VOCs catalytic oxidation, photocatalytic CO₂ reduction and so on. In this review, the recent studies about the preparation and applications of 3DOM catalysts are summarized. At the same time, the advantages and mechanism of the 3DOM catalysts used in gaseous pollutants control are introduced in depth. Finally, the perspective and future direction of 3DOM-based catalysts for gaseous pollutants control are proposed.

Current State of Porous Carbon for Wastewater Treatment

Porous materials constitute an attractive research field due to their high specific surfaces; high chemical stabilities; abundant pores; special electrical, optical, thermal, and mechanical properties; and their often higher reactivities. These materials are currently generating a great deal of enthusiasm, and they have been used in large and diverse applications, such as those relating to sensors and biosensors, catalysis and biocatalysis, separation and purification techniques, acoustic and electrical insulation, transport gas or charged species, drug delivery, and electrochemistry. Porous carbons are an important class of porous materials that have grown rapidly in recent years. They have the advantages of a tunable pore structure, good physical and chemical stability, a variable specific surface, and the possibility of easy functionalization. This gives them new properties and allows them to improve their performance for a given application. This review paper intends to understand how porous carbons involve the removal of pollutants from water, e.g., heavy metal ions, dyes, and organic or inorganic molecules. First, a general overview description of the different precursors and the manufacturing methods of porous carbons is illustrated. The second part is devoted to reporting some applications such using porous carbon materials as an adsorbent. It appears that the use of porous materials at different scales for these applications is very promising for wastewater treatment industries.