NO oxidation activity of Ag-doped perovskite catalysts

Understanding the effects of A-site Ag-doping on LaCoO3 perovskite for NO oxidation: Structural and magnetic properties

The partial substitution of A-site in perovskites is a major strategy to enhance the catalytic oxidation activity. This study explores the use of silver (Ag) to partially replace the lanthanum (La) ion at the A-site in LaCoO3 perovskite, investigating the role of Ag in the ABO3 perovskite structure, elucidating the nitric oxide (NO) oxidation mechanism over La1-xAgxCoO3 (x = 0.1-0.5) perovskites. La0.7Ag0.3CoO3 with an Ag-doping amount of 0.3, exhibited the highest NO oxidation activity of 88.5% at 275 °C. Characterization results indicated that Ag substitution enhanced the perovskite, maintaining its original phase structure, existing in the form of a mixture of Ag0 and Ag+ in the La1-xAgxCoO3 (x = 0.1-0.5) perovskites. Notably, Ag substitution improved the specific surface area, reduction performance, Co3+, and surface adsorption oxygen content. Additionally, the study investigated the relationship between magnetism and NO oxidation from a magnetism perspective. Ag-doping strengthened the magnetism of La-Ag perovskite, resulting in stronger adsorption of paramagnetic NO. This study elucidated the NO oxidation mechanism over La-Ag perovskite, considering structural and magnetic properties, providing valuable insights for the subsequent development and industrial application of high oxidation ability perovskite catalysts.

Role of La-based perovskite catalysts in environmental pollution remediation

Since the advent of the industrial revolution, there has been a constant need of efficient catalysts for abatement of industrial toxic pollutants. This phenomenon necessitated the development of eco-friendly, stable, and economically feasible catalytic materials like lanthanum-based perovskite-type oxides (PTOs) having well-defined crystal structure, excellent thermal, and structural stability, exceptional ionic conductivity, redox behavior, and high tunability. In this review, applicability of La-based PTOs in remediation of pollutants, including CO, NO x and VOCs was addressed. A framework for rationalizing reaction mechanism, substitution effect, preparation methods, support, and catalyst shape has been discussed. Furthermore, reactant conversion efficiencies of best PTOs have been compared with noble-metal catalysts for each application. The catalytic properties of the perovskites including electronic and structural properties have been extensively presented. We highlight that a robust understanding of electronic structure of PTOs will help develop perovskite catalysts for other environmental applications involving oxidation or redox reactions.

Perovskite-based electrocatalysts for oxygen evolution reaction in alkaline media: A mini review

Water electrolysis is one of the attractive technologies for producing clean and sustainable hydrogen fuels with high purity. Among the various kinds of water electrolysis systems, anion exchange membrane water electrolysis has received much attention by combining the advantages of alkaline water electrolysis and proton exchange membrane water electrolysis. However, the sluggish kinetics of the oxygen evolution reaction, which is based on multiple and complex reaction mechanisms, is regarded as a major obstacle for the development of high-efficiency water electrolysis. Therefore, the development of high-performance oxygen evolution reaction electrocatalysts is a prerequisite for the commercialization and wide application of water electrolysis systems. This mini review highlights the current progress of representative oxygen evolution reaction electrocatalysts that are based on a perovskite structure in alkaline media. We first summarize the research status of various kinds of perovskite-based oxygen evolution reaction electrocatalysts, reaction mechanisms and activity descriptors. Finally, the challenges facing the development of perovskite-based oxygen evolution reaction electrocatalysts and a perspective on their future are discussed.

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.

The role of synthesis vis-à-vis the oxygen vacancies of Co3O4 in the oxygen evolution reaction

The combustion synthesized Co3O4 due to high oxygen vacancies exhibited a significant oxygen evolution reaction as has been probed by electrocatalytic experiments and DFT calculations.

NO oxidation performance and kinetics analysis of BaMO3 (M=Mn, Co) perovskite catalysts

Perovskite is an efficient and emerging catalyst for NO oxidation. In this study, BaMnO₃ and BaCoO₃ perovskite catalysts were synthesized by the sol-gel method, and their catalytic oxidation performances of NO were studied. The catalytic performances indicated that BaMnO₃ and BaCoO₃ perovskites had the highest NO oxidation activities with the NO conversions of 78.2% at 350 °C and 84.3% at 310 °C, respectively. The high activities of BaMnO₃ and BaCoO₃ perovskite catalysts were related to the abundant surface adsorption oxygen (O_A = 76.21% and 78.57%, respectively) and the high concentration of Mn⁴⁺ (Mn⁴⁺/Mn = 66.95%) and Co³⁺ (Co³⁺/Co = 63.8%). Moreover, the results of FT-IR and kinetics revealed that NO and O₂ adsorbed on the surface of samples and combined with the B-O band to form bidentate nitrate and bridging nitrate, which eventually was converted into NO₂. The kinetics analysis revealed that the NO oxidation reaction followed the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. In addition, the activation energies were 36.453 kJ/mol for BaMnO₃ and 30.081 kJ/mol for BaCoO₃, implying that BaMnO₃ and BaCoO₃ provide low-cost and efficient catalysts, which can be comparable to Pt noble metal catalysts.

Promotional removal of oxygenated VOC over manganese-based multi oxides from spent lithium-ions manganate batteries: Modification with Fe, Bi and Ce dopants

In this work, cathode materials of spent lithium-ions manganate batteries are recovered as the precursor of manganese-based oxides catalysts and furthermore, different amount of Fe, Bi, Ce are introduced to modify their properties. A series of MnOx(MS)-X Fe, MnOx(MS)-X Bi and MnOx(MS)-X Ce samples with crystal phase of Mn₅O₈ are synthesized using combustion method and then the catalytic behavior and physicochemical properties of prepared catalysts are investigated. Compared to binary MnOx-5% Fe, MnOx-15% Bi and MnOx-10% Ce samples, multi MnOx(MS)-5% Fe, MnOx(MS)-15 Bi and MnOx(MS)-10% Ce catalysts display enhanced catalytic performance significantly in the removal of oxygenated VOC, which could be attributed to larger specific surface area, higher concentration of surface active oxygen species and Mn⁴⁺ ions and better reducibility at low temperature. In-situ DRIFTS results imply that main oxygen-containing functional groups such as carbonyl (-C=O), carboxyl (-COO), hydroxyl (-OH) can be observed during VOC oxidation and by comparison, it can be found that gas-phase O₂ plays a crucial role in facilitating the further oxidation of by-products into CO₂. In addition, TD/GC-MS results point out that the main by-products are formaldehyde; 2-propanol, 1-methoxy-; ethanol, 2-methoxy-, acetate; 2-ethoxyethyl acetate; acetic acid during VOC oxidation.

Low-temperature NO oxidation using lattice oxygen in Fe-site substituted SrFeO3-δ

Improvement of the low-temperature activity for NO oxidation catalysts is a crucial issue to improve the NO_x storage performance in automotive catalysts. We have recently reported that the lattice oxygen species in SrFeO_3-δ (SFO) are reactive in the oxidation of NO to NO₂ at low temperatures. The oxidation of NO using lattice oxygen species is a powerful means to oxidize NO in such kinetically restricted temperature regions. This paper shows that Fe-site substitution of SFO with Mn or Co improves the properties of lattice oxygen such as the temperature and amount of oxygen release/storage, resulting in the enhancement of the activity for NO oxidation in a low-temperature range. In particular, NO oxidation on SrFe_0.8Mn_0.2O_3-δ is found to proceed even at extremely low temperatures <423 K. From oxygen release/storage profiles obtained by temperature-programmed reactions, Co doping into SFO increases the amount of released oxygen owing to the reducibility of the Co species and promotes the phase transformation to the brownmillerite phase. On the other hand, Mn doping does not increase the oxygen release amount and suppresses the phase transformation. However, it significantly decreases the oxygen migration barrier of SFO. Substitution with Mn renders the structure of SFO more robust and maintains the perovskite structure after the release of oxygen. Thus, the oxygen release properties are strongly dependent on the crystal structure change before and after oxygen release from the perovskite structure, which has a significant effect on NO oxidation and the NO_x storage performance.

Insight into structure defects and catalytic mechanism for NO oxidation over Ce0.6Mn0.4Ox solid solutions catalysts: Effect of manganese precursors

The effects of Mn precursors on structure defects and NO catalytic mechanism over Ce_0·6Mn_0.4O_x catalysts were fully investigated. The Ce_0·6Mn_0.4O_x-Ac catalyst, synthesized by using MnAc₂ as a Mn precursor, showed the best catalytic activity for NO conversion (86.9%) at 250 °C under high space velocity (40,000 mL g^-1 h^-1). Detailed structure-activity relationship reveals that the abundant oxygen vacancies and the highly migratory oxygen species formed on Ce_0·6Mn_0.4O_x are the crucial factors that leading to the better NO oxidation activity than that of the other Ce_0·6Mn_0.4O_x-Y (YNO₃, SO₄, Cl) catalysts. In situ DRIFTS technique confirms that the differences in formation mode and desorption ability of N-based (nitrates, nitrites, and dimer nitroso) intermediate species are the vital factors for NO high-efficiency catalytic oxidation. The highly reactive surface intermediate species, like monodentate nitrates, were observed particularly on Ce_0·6Mn_0.4O_x-Ac catalyst, so that the NO oxidation performance on Ce_0·6Mn_0.4O_x-Ac catalyst was more active comparing with other Ce_0·6Mn_0.4O_x-Y catalysts. This study can broaden the horizons for understanding NO catalytic oxidation mechanism on serial Ce_0·6Mn_0.4O_x catalysts and serve as a reference guide in design of structure defects for functional materials by modulating precursor species.

Perovskite-Based Catalysts as Efficient, Durable, and Economical NOx Storage and Reduction Systems

Diesel engines operate under net oxidizing environment favoring lower fuel consumption and CO2 emissions than stoichiometric gasoline engines. However, NOx reduction and soot removal is still a technological challenge under such oxygen-rich conditions. Currently, NOx storage and reduction (NSR), also known as lean NOx trap (LNT), selective catalytic reduction (SCR), and hybrid NSR–SCR technologies are considered the most efficient control after treatment systems to remove NOx emission in diesel engines. However, NSR formulation requires high platinum group metals (PGMs) loads to achieve high NOx removal efficiency. This requisite increases the cost and reduces the hydrothermal stability of the catalyst. Recently, perovskites-type oxides (ABO3) have gained special attention as an efficient, economical, and thermally more stable alternative to PGM-based formulations in heterogeneous catalysis. Herein, this paper overviews the potential of perovskite-based formulations to reduce NOx from diesel engine exhaust gases throughout single-NSR and combined NSR–SCR technologies. In detail, the effect of the synthesis method and chemical composition over NO-to-NO2 conversion, NOx storage capacity, and NOx reduction efficiency is addressed. Furthermore, the NOx removal efficiency of optimal developed formulations is compared with respect to the current NSR model catalyst (1–1.5 wt % Pt–10–15 wt % BaO/Al2O3) in the absence and presence of SO2 and H2O in the feed stream, as occurs in the real automotive application. Main conclusions are finally summarized and future challenges highlighted.

Taming NO oxidation efficiency by γ-MnO2 morphology regulation

Nitric oxide (NO) emitted from the combustion of fossil fuels has drawn global concern, and the oxidation of NO contributes greatly to the DeNO_x process.

Enhanced Bifunctional Catalytic Activity of Manganese Oxide/Perovskite Hierarchical Core-Shell Materials by Adjusting the Interface for Metal-Air Batteries

LaMnO₃ perovskite is one of the most promising catalysts for oxygen reduction reaction (ORR) in metal-air batteries and can be compared to Pt/C. However, the low catalytic activity toward oxygen evolution reaction (OER) limits its practical application in rechargeable metal-air batteries. In this work, the MnO₂/La_0.7Sr_0.3MnO₃ hierarchical core-shell composite materials with a special interface structure have been designed via the selective dissolution method. The core of La_0.7Sr_0.3MnO₃ particles is wrapped by the porous and loose MnO₂ nanoparticles. The as-prepared MnO₂/La_0.7Sr_0.3MnO₃ materials have excellent catalytic activity toward ORR/OER and are used as bifunctional oxygen electrocatalysts for metal-air batteries. Based on results of transmission electron microscopy, X-ray photoelectron spectroscopy, valence-band spectroscopy, and O₂ temperature-programmed desorption analysis, we conclude that the bifunctional catalytic activity of the MnO₂/La_0.7Sr_0.3MnO₃ materials can be effectively promoted due to the specific interface structure between the La_1-xSr_xMnO₃ core and the MnO₂ shell. This can be attributed to three aspects: (a) the electronic conductivity, which is beneficial for providing the faster charge-transfer paths and kinetics at the oxide/solution interface than that of the MnO₂ sample; (b) the enhancement of oxygen adsorption capacity due to surface defects (oxygen vacancies) and chemical adsorption, which is helpful to improve the reaction kinetics during the process of oxygen catalysis; and (c) the tuning of oxygen adsorption ability via the moderate Mn-O bond strength, which may be conducive to getting for obtain an enhanced Mn-O bond strength on the surfaces for ORR and a weakened Mn-O bond in the lattice for OER.

Facile fabrication of Ce-decorated composition-tunable Ce@ZnCo2O4 core-shell microspheres for enhanced catalytic propane combustion

The design of heterogeneous catalysts of high efficiency for complete oxidation of volatile organic compounds (VOCs) is a challenge. In the present study, propane is adopted as a VOC representative, and core-shell structured ZnCo2O4@CeO2 catalysts with Ce decoration were synthesized and tested for propane combustion. Through SEM, STEM, and EDX analyses, the structure of the ZnCo2O4@CeO2 catalysts was characterized. The results of activity evaluation demonstrate that the presence of Ce can significantly promote catalytic performance, and the most suitable Ce content has been verified. Furthermore, the optimized ZnCo2O4@CeO2 catalyst exhibits excellent thermal stability and strong resistance toward water. The superior catalytic performance over the optimized ZnCo2O4@CeO2 catalyst is attributed to the high concentration of surface lattice oxygen (O2-) and the presence of strong interactions between Ce and Co.

Synergistic Effect of Cu/CeO2 and Pt-BaO/CeO2 Catalysts for a Low-Temperature Lean NO x Trap

A lean NO _x trap (LNT) catalyst has been widely used for removing NO _x exhaust from lean-burn engines. However, the operation range of LNT has been limited because of the poor activity of LNT catalysts at low temperatures (≤300 °C), especially in urban driving conditions. To increase NO _x removal efficiency during lean-rich cycle operation, a Cu/CeO₂ (CC) catalyst was added to a Pt-BaO/CeO₂ (PBC) catalyst. In comparison to PBC- or CC-only catalysts, the physical mixture of PBC and CC catalysts (PBC + CC) exhibited a significant synergy for both NO _x storage and reduction efficiencies. In particular, low-temperature activity below 200 °C was greatly enhanced. A Pt-BaO-Cu/CeO₂ (PBCC) catalyst, which was synthesized by depositing Pt and Cu together on a ceria support, showed poorer NO _x removal efficiency. The origin of the synergistic effect over PBC + CC was investigated. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NO _x storage on PBC. Under rich conditions, H₂ was generated in situ on the CC by a water-gas shift reaction then accelerated the reduction of NO _x, which had been stored on PBC, with a higher selectivity to N₂. This simple modification in the catalyst can provide an important clue to enhance low-temperature activity of the commercial LNT system.

In situ fabrication of highly active γ-MnO2/SmMnO3 catalyst for deep catalytic oxidation of gaseous benzene, ethylbenzene, toluene, and o-xylene

γ-MnO₂, SmMnO₃, and γ-MnO₂/SmMnO₃ catalysts were prepared by facile methods, wherein the SmMnO₃ (SMO) perovskite was synthesized through one-step calcination and the γ-MnO₂/SmMnO₃ was formed by an in situ growth of γ-MnO₂ on the surface of SMO. These materials ware characterized by XRD, SEM-mapping, N₂-adsorption, XPS and H₂-TPR to investigate their textural properties. Compared with that of SMO and γ-MnO₂, the γ-MnO₂/SMO shows better performance for catalytic oxidation of aromatic VOCs in wet air (10 vol.%), which may be attributed to its higher surface molar ratio of lattice oxygen to adsorbed oxygen (O_latt/O_ads) and better low-temperature reducibility. Besides, for γ-MnO₂/SMO catalyst, a successive oxidation route and the inner principle of BETX (benzene, ethylbenzene, toluene, and o-xylene) oxidation were also revealed via various tests and a comprehension of dynamics investigation. Meanwhile, the experiments under simulated realistic exhaust conditions displayed that the γ-MnO₂/SmMnO₃ is also a good catalyst with high stability for aromatic VOCs oxidation, and fulfilled endurability to high humidity (20 vol.%).

Defective Mn xZr1- xO2 Solid Solution for the Catalytic Oxidation of Toluene: Insights into the Oxygen Vacancy Contribution

Oxygen vacancy is conducive to molecular oxygen adsorption and activation, and it is necessary to estimate its contribution on catalysts, especially the doped system for volatile organic compound (VOC) oxidation. Herein, a series of doped Mn _xZr_{1- x}O₂ catalysts with oxygen vacancy were prepared by partially substituting Zr⁴⁺ in a zirconia with low-valent manganese (Mn²⁺). Compared with the corresponding mechanically mixed samples (MB-x) without oxygen vacancy, Mn _xZr_{1- x}O₂ catalysts exhibited better toluene conversion and specific reaction rate, where the differential values were calculated to estimate the contribution of oxygen vacancy on catalytic performance. The increase in oxygen vacancy concentrations in Mn _xZr_{1- x}O₂ catalysts can boost the differential values, implying the enhancement of oxygen vacancy contribution. Density functional theory (DFT) calculations further confirmed the contribution of oxygen vacancy, and molecular oxygen is strongly absorbed and activated on a defective Mn-doped c-ZrO₂ (111) surface with oxygen vacancy rather than a perfect m-ZrO₂ (-111) surface or a perfect Mn-doped c-ZrO₂ (111) surface, thus resulting in the significant improvement in catalytic activity for toluene oxidation. In situ DRIFTS spectra revealed that the oxygen vacancy can alter the toluene degradation pathway and accelerate the intermediates to convert into CO₂ and H₂O, thus leading to a low activation energy and high specific reaction rate.

Selective hydrogenation of CO2 over a Ce promoted Cu-based catalyst confined by SBA-15

Highly efficient generation of methanol and CO relying on the synergistic effect of Cu, ZnO, and CeO_x dispersed in SBA-15.

Fe-Doped α-MnO2 nanorods for the catalytic removal of NOx and chlorobenzene: the relationship between lattice distortion and catalytic redox properties

Controllably tuning redox performance is one of the key targets in catalysis.

Superior low-temperature NO catalytic performance of PrMn2O5 over SmMn2O5 mullite-type catalysts

PrMn₂O₅ is demonstrated as a superior catalyst compared to SmMn₂O₅ for low temperature NO oxidation, both experimentally and theoretically.

Interactional effect of cerium and manganese on NO catalytic oxidation

To preferably catalyze the oxidation of NO to NO₂ in diesel after-treatment system, a series of CeO₂-MnO _x composite oxides was supported on silica-alumina material by the co-impregnation method. The maximum conversion of NO of the catalyst with a Ce/Mn weight ratio of 5:5 was improved by around 40%, compared to the supported manganese-only or cerium-only sample. And its maximum reaction rate was 0.056 μmol g^-1 s^-1 at 250 °C at the gas hourly space velocity of 30,000 h^-1. The experimental results suggested that Ce-Mn solid solution was formed, which could modulate the valence state of cerium and manganese and exhibit great redox properties. Moreover, the strong interaction between ceria and manganese resulted in the largest desorption amount of strong chemical oxygen and oxygen vacancies, leading to the maximum O _α area ratio of 62.26% from the O 1s result. These effective oxygen species could be continually transferred to the surface, leading to the best NO catalytic activity of 5Ce5Mn/SA catalyst. Graphical abstract.

Synthesis of Pt/K2CO3/MgAlOx-reduced graphene oxide hybrids as promising NOx storage-reduction catalysts with superior catalytic performance

Pt/K₂CO₃/MgAlO_x–reduced graphene oxide (Pt/K/MgAlO_x–rGO) hybrids were synthesized, characterized and tested as a promising NO_x storage and reduction (NSR) catalyst. Mg–Al layered double hydroxides (LDHs) were grown on rGO via in situ hydrothermal crystallization. The structure and morphology of samples were thoroughly characterized using various techniques. Isothermal NO_x adsorption tests indicated that MgAlO_x–rGO hybrid exhibited better NO_x trapping performance than MgAlO_x, from 0.44 to 0.61 mmol · g⁻¹, which can be attributed to the enhanced particle dispersion and stabilization. In addition, a series of MgAlO_x–rGO loaded with 2 wt% Pt and different loadings (5, 10, 15, and 20 wt%) of K₂CO₃ (denoted as Pt/K/MgAlO_x–rGO) were obtained by sequential impregnation. The influence of 5% H₂O on the NO_x storage capacity of MgAlO_x–rGO loaded with 2 wt% Pt and 10% K₂CO₃ (2Pt/10 K/MgAlO_x–rGO) catalyst was also evaluated. In all, the 2Pt/10 K/MgAlO_x–rGO catalyst not only exhibited high thermal stability and NO_x storage capacity of 1.12 mmol · g⁻¹, but also possessed excellent H₂O resistance and lean–rich cycling performance, with an overall 78.4% of NO_x removal. This work provided a new scheme for the preparation of highly dispersed MgAlO_x–rGO hybrid based NSR catalysts.

Catalytic oxidation of nitric oxide (NO) over different catalysts: an overview

Nitrogen oxides (mainly NO) are one of the major air pollutants that lead to a number of environmental problems such as photochemical smog, acid rain and haze.

La1−xAgxMnO3 electrocatalyst with high catalytic activity for oxygen reduction reaction in aluminium air batteries

Ag doping is one of the best methods for improving the catalytic activity of LaMnO₃ perovskites, and the mass specific activity of LAM-30 (La_0.7Ag_0.3MnO₃) can reach 48.0 mA mg⁻¹ which is about 32 times that of LAM-0 (LaMnO₃).

Promotional effect of the TiO2 (001) facet in the selective catalytic reduction of NO with NH3: in situ DRIFTS and DFT studies

NO on anatase-TiO₂ (001) was mainly in the form of NO₂ which could trigger the subsequent ‘fast SCR’ reaction.

Performance optimization of a MnO2/carbon nanotube substrate for efficient catalytic oxidation of low-concentration NO at room temperature

Performance optimization of a MnO₂ NS/CNT series of catalysts could be realized via doping methods or acid treatment.

A highly efficient CeWOx catalyst for the selective catalytic reduction of NOx with NH3

Characterization was used to investigate the main reasons for the highly efficient NO_x abatement by the CeWO_x catalyst.