Non-thermal plasma-assisted NOx storage and reduction on a LaMn0.9Fe0.1O3 perovskite catalyst

Mitigation approaches and techniques for combustion power plants flue gas emissions: A comprehensive review

Population growth and urbanization are driving energy demand. Despite the development of renewable energy technologies, most of this demand is still met by fossil fuels. Flue gases are the main air pollutants from combustion power plants. These pollutants include particulate matter (PM), sulfur oxides (SOx), nitrogen oxides (NOx), and carbon oxides (COx). The release of these pollutants has adverse effects on human health and the environment, including serious damage to the human respiratory system, acid rain, climate change, and global warming. In this review, a wide range of conventional and new technologies that have the potential to be used in the combustion power plant sector to manage and reduce flue gas pollutants have been examined. Nowadays, conventional approaches to emissions control and management, which focus primarily on post-combustion techniques, face several challenges despite their widespread use and commendable effectiveness. Therefore, studies that have proposed alternative approaches to achieve improved and more efficient methods are reviewed. The results show that new advances such as novel PM collectors, attaining an efficiency of nearly 100 % for submicron particles, microwave systems, boasting an efficiency of nearly 90 % for NO and over 95 % for SO2, electrochemical systems achieving above 90 % efficiency for NOx reduction, non-thermal plasma processes demonstrating an efficiency close to 90 % for NOx, microalgae-based methods with efficiency ranging from 80 % to 99 % for CO2, and wet scrubbing, exhibit considerable potential in addressing the shortcomings of conventional systems. Furthermore, the integration of hybrid methods, particularly in regions prioritizing environmental concerns over economic considerations, holds promise for enhanced control and removal of flue gas pollutants with superior efficiency.

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.

Constructing oxygen vacancy-rich MXene @Ce-MOF composites for enhanced energy storage and conversion

Oxygen vacancies can regulate the coordination structure and electronic states of atoms, thus promoting the formation of surface-active sites and increasing the conductivity of the electrode material. This work presents a design for MXene@Ce-MOF composites with abundant oxygen vacancies. The hydroxyl groups on the surface of monolayer MXene attract cerium ions, which create surface defects in Ce-MOF and further promote the formation of oxygen vacancies. This results in a significant improvement in energy storage capacity, as well as performance in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The MXene@Ce-MOF composite exhibits a specific capacity of 496 F g^-1, which is 1.8 times higher than that of pure Ce-MOF and 3.5 times higher than MXene alone. At a current density of 10 mA cm^-2, the overpotential for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is as low as 270 and 220 mV, respectively, and the composite exhibits excellent cycling stability. Oxygen vacancy-based MOF composites play a crucial role in electrocatalysis and energy conversion.

Morphology Effects on Structure-Activity Relationship of Pd/Y-ZrO2 Catalysts for Methane Oxidation

Pd/Y-ZrO2 catalysts were prepared by Y-ZrO2 with different morphologies (flower-like, spherical, reticulated, and bulk-specific morphology), which were prepared by hydrothermal synthesis. Activity evaluation and characterization results show that the morphology influences the microstructures of Y-ZrO2 and the chemical states of active Pd species, thus affecting the activity of methane oxidation. Bulk Pd/Y-ZrO2 exhibits the best CH4 oxidation activity and thermal stability due to the block shape exposed (101) surface, and the single tetragonal phase structure maintained after high-temperature aging. The relatively large-sized Pd particles and Pd0 jointly promote the catalytic oxidation of CH4.

Mesoporous LaCoO3 perovskite oxide with high catalytic performance for NOx storage and reduction

A mesoporous LaCoO₃ perovskite oxide (LaCoO₃-Meso) with three-dimensionally ordered helical interwoven structure was synthesized by a nano-casting method using KIT-6 as the hard template. The obtained LaCoO₃-Meso with high surface area was tested for its catalytic performance in the NO_x storage and reduction (NSR) reaction and compared with a sample synthesized by the conventional sol-gel method. The LaCoO₃-Meso showed a significant advantage for NO_x storage, with a NO_x storage capacity 2 times higher than the regular sample. LaCoO₃-Meso also exhibited improved NSR catalytic performance in the 150-450 °C temperature range, especially within 350-400 °C, where the NO_x conversion was raised for 40%. The results of X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements suggested the presence of a high concentration of oxygen defects on the LaCoO₃-Meso surface. Further results provided by temperature programmed reduction and temperature programmed desorption indicated that the oxygen defects not only increase the amount of trapped NO_x, but also improve the low-temperature redox performance of the catalyst. The lower stability of NO_x species adsorbed on oxygen defects promotes the NO_x release step in the NSR reaction and benefits the regeneration of storage sites.

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.

In-plasma-catalysis for NO x degradation by Ti3+ self-doped TiO2-x /γ-Al2O3 catalyst and nonthermal plasma

In an attempt to realize the efficient treatment of NO_x, a mixed catalyst of Ti³⁺ self-doped TiO_2−x and γ-Al₂O₃ was constructed by reducing commercial TiO₂. The degradation effect on NO_x was evaluated by introducing the mixed catalyst into a coaxial dual-dielectric barrier reactor. It was found that the synthesized TiO_2−x could achieve considerable degradation effects (84.84%, SIE = 401.27 J L⁻¹) in a plasma catalytic system under oxygen-rich conditions, which were better than those of TiO₂ (73.99%) or a single plasma degradation process (26.00%). The presence of Ti³⁺ and oxygen vacancies in TiO_2−x resulted in a relatively narrow band gap, which contributed to catalyzing deeply the oxidation of NO_x to NO₂⁻ and NO₃⁻ during the plasma-induced “pseudo-photocatalysis” process. Meanwhile, the TiO_2−x showed an improved discharge current and promoted discharge efficiency, explaining its significant activation effect in the reaction. Reduced TiO_2−x could achieve an impressive degradation effect in a long-time plasma-catalysis process, and still maintained its intrinsic crystal structure and morphology. This work provides a facile synthesis procedure for preparing Ti³⁺ self-doped TiO_2−x with practical and scalable production potential; moreover, the novel combination with plasma also provides new insights into the low-temperature degradation of NO_x.

TiO_2−x has a smaller forbidden band width, more abundant Ti³⁺ and oxygen vacancies, so as to obtain a better and more stable degradation effect of NO_x in plasma-catalysis process.

Degradation of gas-phase o-xylene via combined non-thermal plasma and Fe doped LaMnO3 catalysts: Byproduct control

A series of Fe doped LaMnO₃ catalysts were prepared to control the production of byproducts such as O₃, N₂O, and CO, during the degradation of volatile organic compounds with a non-thermal plasma. Eliminating these potentially toxic byproducts will make non-thermal plasma technologies applicable for a wider range of commercial applications. The modified LaMnO₃ catalysts are combined in NTP-catalysis reactor with optimal configuration. Experimental results show that doping Fe on LaMnO₃ catalysts can not only enhance the oxidation of o-xylene, but also lower the emission levels of byproducts. LaMn_0.9Fe_0.1O₃ catalyst shows the best catalytic activity among the formulations tested herein. In addition to the strong mineralization of 88.1 %, the catalyst has the highest performance for o-xylene conversion (91.3 %), O₃ inhibition efficiency (84.9 %), and N₂O inhibition efficiency (61.2 %) due to the strong concentration of active oxygen species on the surface of the catalyst. Moreover, the high reducibility of Fe³⁺ demonstrated with H₂-TPR (hydrogen temperature-programed reduction) further enhances the removal of O₃ by oxygen species exchange between Mn³⁺/Mn⁴⁺ and Fe²⁺/Fe³⁺.

Perovskite-based catalysts for the control of nitrogen oxide emissions from diesel engines

Nitrogen oxides removal over a wide range of perovskite-based catalysts together with their property-activity relationships.

Plasma Oxidation of H2S over Non-stoichiometric LaxMnO3 Perovskite Catalysts in a Dielectric Barrier Discharge Reactor

In this work, plasma-catalytic removal of H2S over LaxMnO3 (x = 0.90, 0.95, 1, 1.05 and 1.10) has been studied in a coaxial dielectric barrier discharge (DBD) reactor. The non-stoichiometric effect of the LaxMnO3 catalysts on the removal of H2S and sulfur balance in the plasma-catalytic process has been investigated as a function of specific energy density (SED). The integration of the plasma with the LaxMnO3 catalysts significantly enhanced the reaction performance compared to the process using plasma alone. The highest H2S removal of 96.4% and sulfur balance of 90.5% were achieved over the La0.90MnO3 catalyst, while the major products included SO2 and SO3. The missing sulfur could be ascribed to the sulfur deposited on the catalyst surfaces. The non-stoichiometric LaxMnO3 catalyst exhibited larger specific surface areas and smaller crystallite sizes compared to the LaMnO3 catalyst. The non-stoichiometric effect changed their redox properties as the decreased La/Mn ratio favored the transformation of Mn3+ to Mn4+, which contributed to the generation of oxygen vacancies on the catalyst surfaces. The XPS and H2-TPR results confirmed that the Mn-rich catalysts showed the higher relative concentration of surface adsorbed oxygen (Oads) and lower reduction temperature compared to LaMnO3 catalyst. The reaction performance of the plasma-catalytic oxidation of H2S is closely related to the relative concentration of Oads formed on the catalyst surfaces and the reducibility of the catalysts.

Non-thermal-plasma-activated de-NOx catalysis

The combination of non-thermal plasma (NTP) with catalyst systems as an alternative technology to remove NO_x emissions in the exhaust of lean-burn stationary and mobile sources is reviewed. Several factors, such as low exhaust gas temperatures (below 300°C), low selectivity to N₂ and the presence of impurities, make current thermally activated technologies inefficient. Various hybrid plasma-catalyst systems have been examined and shown to have a synergistic effect on de-NO_x efficiency when compared with NTP or catalyst-alone systems. The NTP is believed to form oxygenated species, such as aldehydes and nitrogen-containing organic species, and to convert NO to NO₂, which improves the reduction efficiency of N₂ during hydrocarbon-selective catalytic reduction reactions. The NTP has been used as a pretreatment to convert NO to its higher oxidation states such as NO₂ to improve NO_x reduction efficiency in the subsequent processes, e.g. NH₃-selective catalytic reduction. It has been applied to the lean phase of the NO_x storage to improve the adsorption capacity of the catalyst by conversion of NO to NO₂ Alternatively, a catalyst with high adsorption capacity is chosen and the NTP is applied to the rich phase to improve the reduction activity of the catalyst at low temperature.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

An ultrafast approach for the syntheses of defective nanosized lanthanide perovskites for catalytic toluene oxidation

In this article, Sr²⁺ and/or Fe³⁺-doped LaMnO₃ perovskites were rapidly synthesized using supercritical water (sc-H₂O) in a continuous hydrothermal flow reactor.

Non-Thermal Plasma Activation of Gold-Based Catalysts for Low-Temperature Water-Gas Shift Catalysis

Non-thermal plasma activation has been used to enable low-temperature water-gas shift over a Au/CeZrO4 catalyst. The activity obtained was comparable with that attained by heating the catalyst to 180 °C providing an opportunity for the hydrogen production to be obtained under conditions where the thermodynamic limitations are minimal. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), structural changes associated with the gold nanoparticles in the catalyst have been observed which are not found under thermal activation indicating a weakening of the Au-CO bond and a change in the mechanism of deactivation.

Low-temperature H2-plasma-assisted NOx storage and reduction over a combined Pt/Ba/Al and LaMnFe catalyst

With the assistance of H₂ plasma in rich phase, higher NO_x conversions could be obtained over the PBA+LMF sample over a wide temperature range (200–350 °C).

Combining nonthermal plasma with perovskite-like catalyst for NOx storage and reduction

A new NOx storage and reduction (NSR) system is developed for NOx removal by combining perovskite-like catalyst with nonthermal plasma technology. In this hybrid system, catalyst is mainly used for oxidizing NO to NO2 and storing them, while nonthermal plasma is applied as a desorption-reduction step for converting NOx into N2. An innovative catalyst with a high NOx storage capacity and good reduction performance is developed by successive impregnation. The catalysts prepared with various metal oxides were investigated for NOx storage capacity (NSC) and NOx conversion. Characterization of the catalysts prepared reveals that addition of cobalt (Co) and potassium (K) considerably increases the performance for NSC. Results also show that SrKMn0.8Co0.2O4 supported on BaO/Al2O3 has good NSC (209 μmol/gcatalyst) for the gas stream containing 500 ppm NO and 5 % O2 with N2 as carrier gas. For plasma reduction process, NOx conversion achieved with SrKMn0.8Co0.2O4/BaO/Al2O3 reaches 81 % with the applied voltage of 12 kV and frequency of 6 kHz in the absence of reducing agents. The results indicate that performance of plasma reduction process (81 %) is better than that of thermal reduction (64 %). Additionally, mixed gases including 1 % CO, 1 % H2 and 1 % CH4, and 2 % H2O(g) are simultaneously introduced into the system to investigate the effect on NSR with plasma system and results indicate that performance of NSR with plasma can be enhanced. Overall, the hybrid system is promising to be applied for removing NOx from gas streams. Graphical abstract ᅟ.