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

Toward synergetic reduction of pollutant and greenhouse gas emissions from vehicles: a catalysis perspective

It is a great challenge for vehicles to satisfy the increasingly stringent emission regulations for pollutants and greenhouse gases. Throughout the history of the development of vehicle emission control technology, catalysts have always been in the core position of vehicle aftertreatment. Aiming to address the significant demand for synergistic control of pollutants and greenhouse gases from vehicles, this review provides a panoramic view of emission control technologies and key aftertreatment catalysts for vehicles using fossil fuels (gasoline, diesel, and natural gas) and carbon-neutral fuels (hydrogen, ammonia, and green alcohols). Special attention will be given to the research advancements in catalysts, including three-way catalysts (TWCs), NOx selective catalytic reduction (SCR) catalysts, NOx storage-reduction (NSR) catalysts, diesel oxidation catalysts (DOCs), soot oxidation catalysts, ammonia slip catalysts (ASCs), methane oxidation catalysts (MOCs), N2O abatement catalysts (DeN2O), passive NOx adsorbers (PNAs), and cold start catalysts (CSCs). The main challenges for industrial applications of these catalysts, such as insufficient low-temperature activity, product selectivity, hydrothermal stability, and poisoning resistance, will be examined. In addition, the future development of synergistic control of vehicle pollutants and greenhouse gases will be discussed from a catalysis perspective.

Efficient photocatalytic NO removal with inhibited NO2 formation and catalyst loss over sponge-supported and functionalized g-C3N4

Though photocatalytic purification of NO has been widely studied, how to avoid secondary pollution during gas-solid reaction is still a challenge, especially in inhibiting the formation of toxic intermediates (NO2) and avoiding the blow away of powdery photocatalyst. Herein, we proposed a one-step solvothermal method to prepare melamine sponge (MS) supported and functionalized g-C3N4 (CN), which simultaneously realizes the inhibition of NO2 formation and catalyst loss. Sodium hydroxide, which plays a dual role, has been introduced during the preparation of supported photocatalyst. Specifically, sodium atom, as the modifier of performance, could facilitate the randomly distributed charge of pristine CN to be converged, which accelerates the adsorption/activation of reactants for efficient and deep NO oxidation. Hydroxyl group, as the binder between CN and MS, induces the interaction by forming hydrogen bonds, which contributes to the firm immobilization of powdery photocatalyst. The supported sample exhibits outstanding NO removal rate (58.90%) and extremely low NO2 generation rate (1.41%), and the mass loss rate of photocatalyst before and after reaction is less than 1%. The promotion mechanism of performance also has been elaborated. This work takes environmental risks as a prerequisite to propose a feasible strategy for perfecting the practical application of photocatalytic technology.

Electrification-Enhanced Low-Temperature NOx Storage-Reduction on Pt and K Co-Supported Antimony-Doped Tin Oxides

NOx storage-reduction (NSR), a promising approach for removing NOx pollutants from diesel vehicles, remains elusive to cope with the increasingly lower exhaust temperatures (especially below 250 °C). Here, we develop a conceptual electrified NSR strategy, where electricity with a low input power (0.5-4 W) is applied to conductive Pt and K co-supported antimony-doped tin oxides (Pt-K/ATO), with C3H6 as a reductant. The ignition temperature for 10% NOx conversion is nearly 100 °C lower than that of the traditional thermal counterpart. Furthermore, reducing the power in the fuel-lean period relative to that in the fuel-rich period increases the maximum energy efficiency by 23%. Electrically driven release of lattice oxygen is revealed to play vital roles in multiple steps in NSR, including NO adsorption, desorption, and reduction, for improved NSR activity. This work provides an electrification strategy for developing high-activity NSR catalysis utilizing electricity onboard hybrid vehicles.

General Route to Colloidally Stable, Low-Dispersity Manganese-Based Ternary Spinel Oxide Nanocrystals

While certain ternary spinel oxides have been well-explored with colloidal nanochemistry, notably the ferrite spinel family, ternary manganese (Mn)-based spinel oxides have not been tamed. A key composition is cobalt (Co)-Mn oxide (CMO) spinel, CoxMn3-xO4, that, despite exemplary performance in multiple electrochemical applications, has few reports in the colloidal literature. Of these reports, most show aggregated and polydisperse products. Here, we describe a synthetic method for small, colloidally stable CMO spinel nanocrystals with tunable composition and low dispersity. By reacting 2+ metal-acetylacetonate (M(acac)2) precursors in an amine solvent under an oxidizing environment, we developed a pathway that avoids the highly reducing conditions of typical colloidal synthesis reactions; these reducing conditions typically push the system toward a monoxide impurity phase. Through surface chemistry studies, we identify organic byproducts and their formation mechanism, enabling us to engineer the surface and obtain colloidally stable nanocrystals with low organic loading. We report a CMO/carbon composite with low organic contents that performs the oxygen reduction reaction (ORR) with a half-wave potential (E1/2) of 0.87 V vs RHE in 1.0 M potassium hydroxide at 1600 rpm, rivaling previous reports for the highest activity of this material in ORR electrocatalysis. We extend the general applicability of this procedure to other Mn-based spinel nanocrystals such as Zn-Mn-O, Fe-Mn-O, Ni-Mn-O, and Cu-Mn-O. Finally, we show the scalability of this method by producing inorganic nanocrystals at the gram scale.

Fe-Doped Metal Complex (LaCo0.9Fe0.1O3) and g-C3N4 Formed a Nanoheterojunction for the Photocatalytic Decomposition of Water

Photocatalytic water decomposition provides an environmentally friendly method of hydrogen production similar to "photosynthesis", while current research aims to develop affordable yet efficient photocatalysts. Oxygen vacancy is one of the most significant defects in metal oxide semiconductors, including perovskite, which substantially influences the semiconductor material's efficiency. To enhance the oxygen vacancy in the perovskite, we worked on doping Fe. A perovskite oxide nanostructure of LaCo_xFe_1-xO₃ (x = 0.2, 0.4, 0.6, 0.8, and 0.9) was prepared by the sol-gel method, and a series of LaCo_xFe_1-xO₃ (x = 0.2, 0.4, 0.6, 0.8, and 0.9)/g-C₃N₄ nanoheterojunction photocatalysts were synthesized using mechanical mixing and solvothermal methods for LaCo_xFe_1-xO₃ (x = 0.2, 0.4, 0.6, 0.8, and 0.9). Fe was successfully doped into the perovskite (LaCoO₃), and the formation of an oxygen vacancy was verified by various detection methods. In our photocatalytic water decomposition experiments, we observed that LaCo_0.9Fe_0.1O₃ demonstrated a significant increase in its maximum hydrogen release rate, reaching 5249.21 μmol h^-1 g^-1, which was remarkably 17.60 times higher than that of LaCoO₃-undoped Fe. Similarly, we also explored the photocatalytic activity of the nanoheterojunction complex LaCo_0.9Fe_0.1O₃/g-C₃N₄, and it exhibited pronounced performance with an average hydrogen production of 7472.67 μmol h^-1 g^-1, which was 25.05 times that of LaCoO₃. We confirmed that the oxygen vacancy plays a crucial role in photocatalysis.

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.

Engineering surface segregation of perovskite oxide through wet exsolution for CO catalytic oxidation

Cation segregation occurring near the surface or interfaces of solid catalysts plays an important role in catalytic reactions. Unfortunately, the native surface of perovskite oxides is dominated by passivated A-site segregation, which severely hampers the catalytic activity and durability of the system. To address this issue, herein, we present a wet exsolution method to reconstruct surface segregation in perovskite cobalt oxide. Under reduction etching treatment of glycol solution, inert surface Sr segregation was transformed into active Co₃O₄ segregation. By varying the reaction time, we achieved differing coverage of the active Co₃O₄ segregation on the La_0.5Sr_0.5CoO_3-δ (LSCO) perovskite oxide surface. This study reveals that CO oxidation activity exhibits a volcano-shaped dependence on the coverage of Co₃O₄ segregation at the surface of a perovskite cobalt oxide. Furthermore, we find that a suitable coverage of Co₃O₄ segregation can dramatically improve the catalytic activity of the perovskite catalyst by enhancing interface interactions. Co K-edge, Co L-edge, and O K-edge X-ray absorption spectra confirm that the synergistic effect optimizes the covalence of the metal-oxygen bond at the surface and interface. This work not only contributes to the design and development of perovskite-type catalysts, but also provides important insight into the relationship between surface segregation and catalytic activity.

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.