Selective Dissolution of A-Site Cations in ABO3Perovskites: A New Path to High-Performance Catalysts

Insights into in situ surface reconstruction in cobalt perovskite oxides for enhanced catalytic activity

An depth understanding of the fundamental interactions between surface termination and catalytic activity is crucial to prompt the properties of functional perovskite materials. The elastic energy due to size mismatch and electrostatic attraction of the charged Sr dopant by positively charged oxygen vacancies induced inert A-site surface enrichment rearrangement for perovskites. Lower temperatures could reduce A-site enrichment, but it is difficult to form perovskite crystals. La0.8Sr0.2CoO3-δ (LSCO) as a model perovskite oxide was modified with additive urea to reduce the crystallization temperature, and suppress Sr segregation. The LSCO catalysts with 600 °C annealing temperature (LSCO-600) exhibited a 19.4-fold reaction reactivity of toluene oxidation than that with 800 °C annealing temperature (LSCO-800). Combined surface-sensitive and depth-resolved techniques for surface and sub-surface analysis, surface Sr enrichment was effectively suppressed due to decreased oxygen vacancy concentration and smaller electrostatic driving force. DFT calculations and in-situ DRIFTs spectra well revealed that tuning the surface composition/termination affected the intrinsic reactivity. The catalyst surface with lower Sr enrichment could easily adsorb toluene, cleave, and decompose benzene rings, thus contributing to toluene degradation to CO2. This work demonstrates a green and efficient way to control surface composition and termination at the atomic scale for higher catalytic activity.

Mullite-like SmMn2O5-Derived Composite Oxide-Supported Ni-Based Catalysts for Hydrogen Production by Auto-Thermal Reforming of Acetic Acid

The x%Ni/Sm2O3-MnO (x = 0, 10, 15, 20) catalysts derived from SmMn2O5 mullite were prepared by solution combustion and impregnation method; auto-thermal reforming (ATR) of acetic acid (HAc) for hydrogen production was used to explore the metal-support effect induced by Ni loadings on the catalytic reforming activity and product distribution. The 15%Ni/Sm2O3-MnO catalyst exhibited optimal catalytic performance, which can be due to the appropriate Ni loading inducing a strong metal-support interaction to form a stable Ni/Sm2O3-MnO active center, while side reactions, such as methanation and ketonization, were well suppressed. According to characterizations, Sm2O3-MnO mixed oxides derived from SmMn2O5 mullite were formed with oxygen vacancies; nevertheless, loading of Ni metal further promoted the formation of oxygen vacancies, thus enhancing adsorption and activation of oxygen-containing intermediate species and resulting in higher reactivity with HAc conversion near 100% and hydrogen yield at 2.62 mol-H2/mol-HAc.

Interface engineering-induced perovskite/spinel LaCoO3/Co3O4 heterostructured nanocomposites for efficient peroxymonosulfate activation to degrade levofloxacin

Conventional perovskite oxides (ABO₃) tend to suffer from their inactive surfaces and limited active sites that reduce their catalytic activity and stability, while interface engineering is a facile modulating technique to boost the catalyst's inherent activity by constructing heterogeneous interfaces. In this study, perovskite/spinel LaCoO₃/Co₃O₄ nanocomposites with heterogeneous interfaces were synthesized via sol-gel and in-situ gradient etching methods to activate peroxymonosulfate (PMS) for degrading levofloxacin (LEV). LaCoO₃ on the surface was etched into spinel Co₃O₄, and LaCoO₃/Co₃O₄ nanocomposites with two crystal structures of perovskite and spinel were successfully formed. The surface-modified LaCoO₃/Co₃O₄ exhibited superior catalytic performance with a reaction rate constant more than 2 times that of the original LaCoO₃, as well as excellent pH adaptability (3-11) and reusability (more than 6 recyclings) for LEV degradation. Besides, multiple characterization techniques were carried out to find that LaCoO₃/Co₃O₄ possessed a larger specific surface area and richer oxygen vacancies after surface modification, which provided more active sites and accelerated mass transfer rate. The mechanism of reactive oxygen species involved in the reaction system was proposed that LaCoO₃/Co₃O₄ not only reacted with PMS directly to produce SO₄^•- and ^•OH but also its surface hydroxyl group helped to form the [≡Co(Ⅲ)OOSO₃]⁺ reactive complex with PMS to produce O₂^•- and ¹O₂. In addition, electrochemical experiments demonstrated that the surface electronic structure of LaCoO₃/Co₃O₄ was effectively regulated, exhibiting a faster electron transfer rate and facilitating the redox process. By detecting and identifying degradation intermediates, three degradation pathways for LEV were proposed. Our work provided profound insights into the design of efficient and long-lasting catalysts for advanced oxidation processes.

Recent Status and Developments of Vacancies Modulation in the ABO3 Perovskites for Catalytic Applications

Perovskite oxides (ABO₃ ) have attracted comprehensive interest for wide range of functional applications (especially for chemical catalysis) due to their high design flexibility, controllable vacancies sites creation, abundant chemical properties, and stable crystal structure. Herein, the previous research and potential development of ABO₃ through adjusting the vacancy at different sites (A-site, B-site, and O-site) to enhance catalytic performance are systematically analyzed and generalized. Briefly, the ABO₃ with different vacancies sites prepared by multifarious direct and indirect methods, accompanied with the improved physical-chemical properties, endow them with distinct and intensified development of catalysis application. In addition, the impressive optimization proved by the vacancies sites adjustment over the ABO₃ is studied to continuously facilitate the advance in some common catalysis reactions, further expanding to other optimized functional applications. At last, the constructive suggestions for fine regulation and analysis of vacancies sites over ABO₃ are also put forward.

Acid Etching-Induced In Situ Growth of λ-MnO2 over CoMn Spinel for Low-Temperature Volatile Organic Compound Oxidation

Surface lattice oxygen is crucial to the degradation of volatile organic compounds (VOCs) over transition metal oxides according to the Mars-van Krevelen mechanism. Herein, λ-MnO₂ in situ grown on the surface of CoMn spinel was prepared by acid etching of corresponding spinel catalysts (CoMn-Hx-Ty) for VOC oxidation. Experimental and relevant theoretical exploration revealed that acid etching on the CoMn spinel surface could decrease the electron cloud density around the O atom and weaken the adjacent Mn-O bond due to the fracture of the surface Co-O bond, facilitating electron transfer and subsequently the activation of surface lattice oxygen. The obtained CoMn-H1-T1 exhibited an excellent catalytic performance with a 90% acetone conversion at 149 °C, which is 42 °C lower than that of CoMn spinel. Furthermore, the partially maintained spinel structure led to better stability than pure λ-MnO₂. In situ diffuse reflectance infrared Fourier transform spectroscopy confirmed a possible degradation pathway where adsorptive acetone converted into formate and acetate species and into CO₂, in which the consumption of acetate was identified as the rate-limiting step. This strategy can improve the catalytic performance of metal oxides by activating surface lattice oxygen, to broaden their application in VOC oxidation.

Recent Advances in Perovskite Catalysts for Efficient Overall Water Splitting

Hydrogen is considered a promising clean energy vector with the features of high energy capacity and zero-carbon emission. Water splitting is an environment-friendly and effective route for producing high-purity hydrogen, which contains two important half-cell reactions, namely, the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). At the heart of water splitting is high-performance electrocatalysts that efficiently improve the rate and selectivity of key chemical reactions. Recently, perovskite oxides have emerged as promising candidates for efficient water splitting electrocatalysts owing to their low cost, high electrochemical stability, and compositional and structural flexibility allowing for the achievement of high intrinsic electrocatalytic activity. In this review, we summarize the present research progress in the design, development, and application of perovskite oxides for electrocatalytic water splitting. The emphasis is on the innovative synthesis strategies and a deeper understanding of structure–activity relationships through a combination of systematic characterization and theoretical research. Finally, the main challenges and prospects for the further development of more efficient electrocatalysts based on perovskite oxides are proposed. It is expected to give guidance for the development of novel non-noble metal catalysts in electrochemical water splitting.

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

Hierarchical Regulation of LaMnO3 Dual-Pathway Strategy for Excellent Room-Temperature Organocatalytic Oxidation Performance

The performance-enhancing strategy of a single pathway for perovskite has been widely studied. In this work, the dual-pathway strategy of A-site Ce substitution and nitric acid selective dissolution was proposed. The catalytic oxidation performance of LaMnO₃ exhibits the characteristic of hierarchical regulation, that is, a steplike improvement, which avoids the limitation of performance improvement of the single pathway. The B-site Mn with catalytic activity was in situ reconstituted on the surface to build a Mn-rich surface. The obtained sdLa_0.7Ce_0.3MnO₃ has the advantages of good oxygen mobility, high Mn⁴⁺/Mn³⁺ molar ratio, and large specific surface area, and this material showed excellent catalytic oxidation performance for organics, which can realize colorimetric chemical oxygen demand detection at room temperature. Here, Ce substitution improved the oxidation capacity by improving the oxygen mobility and the ratio of Mn⁴⁺/Mn³⁺, and further nitric acid treatment not only accelerated the in situ reconstruction of B-site Mn but also increased the specific surface area.

In Situ Modulation of A-Site Vacancies in LaMnO3.15 Perovskite for Surface Lattice Oxygen Activation and Boosted Redox Reactions

Modulation of A-site defects is crucial to the redox reactions on ABO₃ perovskites for both clean air application and electrochemical energy storage. Herein we report a scalable one-pot strategy for in situ regulation of La vacancies (V_La ) in LaMnO_3.15 by simply introducing urea in the traditional citrate process, and further reveal the fundamental relationship between V_La creation and surface lattice oxygen (O_latt ) activation. The underlying mechanism is shortened Mn-O bonds, decreased orbital ordering, promoted MnO₆ bending vibration and weakened Jahn-Teller distortion, ultimately realizing enhanced Mn-3d and O-2p orbital hybridization. The LaMnO_3.15 with optimized V_La exhibits order of magnitude increase in toluene oxidation and ca. 0.05 V versus RHE (reversible hydrogen electrode) increase of half-wave potential in oxygen reduction reaction (ORR). The reported strategy can benefit the development of novel defect-meditated perovskites in both heterocatalysis and electrocatalysis.

Catalytic Oxidation of VOCs over SmMnO3 Perovskites: Catalyst Synthesis, Change Mechanism of Active Species, and Degradation Path of Toluene

Highly active samarium manganese perovskite oxides were successfully prepared by employing self-molten-polymerization, coprecipitation, sol-gel, and impregnation methods. The physicochemical properties of perovskite oxides were investigated by XRD, N₂ adsorption-desorption, XPS, and H₂-TPR. Their catalytic performances were compared via the catalytic oxidation of toluene. The perovskite prepared by self-molten-polymerization possessed the highest catalytic capacity, which can be ascribed to its higher oxygen adspecies concentration (O_latt/O_ads = 0.53), higher surface Mn⁴⁺/Mn³⁺ ratio (Mn⁴⁺/Mn³⁺ = 0.95), and best low-temperature reducibility (H₂ consumption = 0.27; below 350 °C). The most active catalyst also exhibited good cycling and long-term stability for toluene oxidation. After a multistep cycle reaction and a long-term reaction of 42 h, the toluene conversion maintained above 99.9% at 270 °C. Mechanistic study hinted that lattice oxygen was involved in toluene oxidation. The oxidation reaction was dependent on the synergism of lattice oxygen, adsorbed oxygen, and oxygen vacancies. The degradation pathway of toluene, researched by diffuse reflectance infrared Fourier transform spectroscopy and online mass spectrometry technologies, demonstrated that a series of organic byproducts existed at a relatively low temperature. This work provides an efficient and practical method for selecting highly active catalysts and for exploring the catalytic mechanism for the removal of atmospheric environmental pollution.

Architecture of Biomimetic Water Oxidation Catalyst with Mn4CaO5 Clusterlike Structure Unit

Mn₄CaO₅ cluster in green plant is considered as the ideal structure for water oxidation catalysis. However, this structure is difficult to be constructed in heterogeneous catalyst because of its distorted spatial structure and unique electronic state. Herein, we report the synthesis of two-dimensional biomimetic Ca-Mn-O catalyst with Mn₄CaO₅ clusterlike structure through ultrasonic-assisted reduction treatment toward Ca-birnessite. The synergistic effect between ultrasonic and reduction successfully reduced the Mn oxidation state in Ca-birnessite without breaking the structure of MnO₂ monolayers, forming a regular two-dimensional structure with Mn₄CaO₅ cubanelike structure unit for the first time. The biomimetic catalyst shows a superior water oxidation activity (turnover frequency = 3.43 s^-1), which is the best in manganese-based heterogeneous catalyst to date. This work provides a new strategy for the precise synthesis of specific structure and exhibits a great prospect of biomimic in heterogeneous catalyst.

Heterogeneous activation of peroxymonosulfate by LaCo1-xCuxO3 perovskites for degradation of organic pollutants

Recently cobalt-based heterogeneous catalysts have been widely investigated for peroxymonosulfate (PMS) activation in sulfate radical-based advanced oxidation processes. However, the improvement of the catalytic performance for PMS activation remains to be a challenge. As the limiting step, the rapid transformation of Co^II/Co^III redox pairs is crucial for PMS activation. Perovskites attract increasing attention due to their controllable oxidation state of B-site metal and formation of oxygen vacancies, which accelerates the cycle of redox pairs. LaCo_1-xM_xO₃ (M = Cu, Fe and Mn) perovskites as heterogeneous catalysts of PMS were synthesized for the degradation of phenol. The results showed that LaCo_0.4Cu_0.6O₃ exhibited the highest catalytic activity. The pseudo first-order kinetic constant of phenol degradation on LaCo_0.4Cu_0.6O₃ is 0.302 min^-1, being about 5 times as high as Co²⁺ with same molar concentration of cobalt in LaCo_0.4Cu_0.6O₃. XPS analysis confirmed that substitution of copper could promote the cycle of Co^II/Co^III, thus enhance the catalytic efficiency for PMS activation. The facilitated cycle of Co^II/Co^III played a crucial role in the generation of sulfate radicals, hydroxyl radicals and singlet oxygen. And sulfate radical was the primary radical responsible for pollutants degradation. The results provide insights into constructing novel perovskite catalysts for the removal of organic pollutants in water.

Hydrothermal shape controllable synthesis of La0.5Sr0.5MnO3 crystals and facet effect on electron transfer of oxygen reduction

Controllable growth of perovskite structure oxide crystals with well-defined facets is challenging, especially in the mixed-valence state manganites with both rare-earth and alkaline-earth cations in their A-site.

Polyoxometalates covalently combined with graphitic carbon nitride for photocatalytic hydrogen peroxide production

The polyoxometalate (POM) cluster [SiW₁₁O₃₉]⁸⁻ (SiW₁₁) with photoreductive ability has been successfully covalently combined with graphitic carbon nitride (g-C₃N₄) through the organic linker strategy.

Activation of surface lattice oxygen in single-atom Pt/CeO2 for low-temperature CO oxidation

To improve fuel efficiency, advanced combustion engines are being designed to minimize the amount of heat wasted in the exhaust. Hence, future generations of catalysts must perform at temperatures that are 100°C lower than current exhaust-treatment catalysts. Achieving low-temperature activity, while surviving the harsh conditions encountered at high engine loads, remains a formidable challenge. In this study, we demonstrate how atomically dispersed ionic platinum (Pt2+) on ceria (CeO2), which is already thermally stable, can be activated via steam treatment (at 750°C) to simultaneously achieve the goals of low-temperature carbon monoxide (CO) oxidation activity while providing outstanding hydrothermal stability. A new type of active site is created on CeO2 in the vicinity of Pt2+, which provides the improved reactivity. These active sites are stable up to 800°C in oxidizing environments.

Surface Tuning of La0.5Sr0.5CoO3 Perovskite Catalysts by Acetic Acid for NOx Storage and Reduction

Selective dissolution of perovskite A site (A of ABO3 structure) was performed on the La1 - xSrxCoO3 catalysts for the NOx storage and reduction (NSR) reaction. The surface area of the catalysts were enhanced using dilute HNO3 impregnation to dissolve Sr. Inactive SrCO3 was removed effectively within 6 h, and the catalyst preserved the perovskite framework after 24 h of treatment. The tuned catalysts exhibited higher NSR performance (both NOx storage and NO-to-NO2 oxidation) under lean-burn and fuel-rich cycles at 250 °C. Large amounts of NOx adsorption were due to the increase of nitrate/nitrite species bonding to the A site and the growth of newly formed monodentate nitrate species. Nitrate species were stored stably on the partial exposed Sr(2+) cations. These exposed Sr(2+) cations played an important role on the NOx reduction by C3H6. High NO-to-NO2 oxidation ability was due to the generation of oxygen defects and Co(2+)-Co(3+) redox couples, which resulted from B-site exsolution induced by A-site dissolution. Hence, our method is facile to modify the surface structures of perovskite catalysts and provides a new strategy to obtain highly active catalysts for the NSR reaction.