Confinement of MnOx@Fe2O3 core-shell catalyst with titania nanotubes: Enhanced N2 selectivity and SO2 tolerance in NH3- SCR process

Changes of Nitrate Activity and Byproduct Distribution Characteristics for Synergistic NOx and Dioxin Abatement over V2O5/AC Catalyst

The simultaneous removal of NOx and dioxins has been considered an economical and effective technology of controlling multipollutant flue gas in the context of "carbon peaking and carbon neutrality". However, this technology has not yet been implemented in practical situations, because the interactive relationship between the selective catalytic reduction (SCR) reaction and dioxin catalytic oxidation lacks a deep understanding, especially on a carbon-based catalyst. In this research, the influence of NO and NH3 on the oxidation characteristics and byproducts distribution of dibenzofuran (DBF) was studied on V2O5/AC catalyst. Results indicated that NH3 has a stronger inhibition effect for DBF catalytic oxidation than NO due to obvious competitive adsorption between NH3 and DBF on the V2O5/AC catalyst. In addition, although both NO and NH3 inhibit the complete degradation of DBF, their effects on the byproduct distribution are not consistent. NO primarily affects the level of oxygen-containing byproducts, while NH3 primarily affects the level of alkane byproducts. Furthermore, the SCR reaction activity demonstrated a reduction when DBF was present. The occupation of V2O5 sites by DBF and its oxidizing intermediates has hindered the production of monodentate nitrate and the reactivity of bridged nitrate, resulting in a decrease in SCR activity via the L-H mechanism. This work aims to provide theoretical guidance for simultaneous removal of NOx and dioxins in industrial fumes.

Low-temperature CeCoMnOx spinel-type catalysts prepared by oxalate co-precipitation for selective catalytic reduction of NO using NH3: A structure-activity relationship study

CeCoMnOx spinel-type catalysts for the selective catalytic reduction of NO using NH3 (NH3-SCR) are usually prepared by alkaline co-precipitation. In this paper, a series of CeCoMnOx spinel-type catalysts with different calcination temperatures were prepared by acidic oxalate co-precipitation. The physicochemical structures and NH3-SCR activities of the CeCoMnOx spinel-type catalysts prepared by oxalate co-precipitation and conventional ammonia co-precipitation were systematically compared. The results show that the CeCoMnOx spinel-type catalysts prepared by the oxalate precipitation method (CeCoMnOx-C) have larger specific surface area, more mesopores and surface active sites, stronger redox properties and adsorption activation properties than those prepared by the traditional ammonia co-precipitation method at 400 °C (CeCoMnOx-N-400), and thus CeCoMnOx-C have better low-temperature NH3-SCR performance. At the same calcination temperature of 400 °C, the NO conversion of CeCoMnOx-C-400 exceeds 89 % and approaches 100 % within the reaction temperature of 100-125 °C, which is 14.8 %-2.5 % higher than that of CeCoMnOx-N-400 at 100-125 °C. In addition, the enhanced redox and acid cycle matching mechanisms on the CeCoMnOx-C surface, as well as the enhanced monoadsorption Eley-Rideal (E-R) and double adsorption Langmuir-Hinshelwood (L-H) reaction mechanisms, are also derived from XPS and in situ DRIFTS characterization.

Galvanic Replacement Reaction: Enabling the Creation of Active Catalytic Structures

The galvanic replacement reaction (GRR) is recognized as a redox process where one metal undergoes oxidation by the ions of another metal possessing a higher reduction potential. This reaction takes place at the interface between a substrate and a solution containing metal ions. Utilizing metal or metal oxide as sacrificial templates enables the synthesis of metallic nanoparticles, oxide-metal composites, and mixed oxides through GRR. Growing evidence showed that GRR has a direct impact on surface structures and properties. This has generated significant interest in catalysis and opened up new horizons for the application of GRR in energy and chemical transformations. This review provides a comprehensive overview of the synthetic strategies utilizing GRR for the creation of catalytically active structures. It discusses the formation of alloys, intermetallic compounds, single atom alloys, metal-oxide composites, and mixed metal oxides with diverse nanostructures. Additionally, GRR serves as a postsynthesis method to modulate metal-oxide interfaces through the replacement of oxide domains. The review also outlines potential future directions in this field.

Recent advances in core-shell structured catalysts for low-temperature NH3-SCR of NOx

Ammonia selective catalytic reduction (NH₃-SCR) of nitrogen oxides is an effective and well-established technology for NO_x removal, but current commercial denitrification catalysts based on V₂O₅-WO₃/TiO₂ have some obvious disadvantages, including narrow operating temperature windows, toxicity, poor hydrothermal stability, and unsatisfied SO₂/H₂O tolerance. To overcome these drawbacks, it is imperative to investigate new types of highly efficient catalysts. In order to design catalysts with outstanding selectivity, activity, and anti-poisoning ability, core-shell structured materials have been widely applied in the NH₃-SCR reaction, which exhibits numerous advantages including the large surface area, the strong synergy interaction of core-shell materials, the confinement effect, and the shielding effect from the shell layer to protect the core. This review summarizes recent developments of core-shell structured catalysts for NH₃-SCR, including basic classification, synthesis methods, and a detailed description of the performance and mechanisms of each type of catalyst. It is hoped that the review will stimulate future developments in NH₃-SCR technology, leading to novel catalyst designs with improved denitrification performance.

Excellent operating temperature window and H2O/SO2 resistances of Fe-Ce catalyst modified by different sulfation strategies for NH3-SCR reaction

Expecting to gain an excellent operating temperature window and superior catalytic activity of the catalyst in SCR reaction, the Fe-Ce bimetallic oxide catalyst was firstly prepared and sulfated with two different sulfation strategies by H₂SO₄. It is interestingly found that both the two sulfation strategies can significantly broaden the operating temperature window of the catalyst. In particular, the SFC and FCS both exhibit superior resistance to H₂O + SO₂, and the NO_x conversion of the SFC even displays no changes in the coexistence of H₂O and SO₂. The characterization results show that different sulfation strategies can generate amorphous sulfate species rather than bulk sulfate species. Furthermore, more surface-adsorbed oxygen as well as higher contents of Ce³⁺ and Fe³⁺ can be obtained on the sulfated catalysts, especially for the SFC catalyst. Meanwhile, different sulfation strategies will progressively enhance the redox ability and amounts of strong acid sites, which will contribute to broadening the operating temperature window for the NH₃-SCR reaction. Additionally, different sulfation methods do not change the reaction pathway of catalysts. However, the adsorption of ad-NH₃ species and reactivity of ad-NO_x species are significantly changed. These lead to the reaction pathway shifts to E-R direct over the SFC and the promotion of E-R and L-H mechanisms over the FCS catalyst.

Design an efficient photoelectrochemical aptasensor for PCB72 based on CdTe@CdS core@shell quantum dots-decorated TiO2 nanotubes

In this work, an efficient and novel photoelectrochemical (PEC) aptasensor for 2,3',5,5'-tetrachlorobiphenyl (PCB72) was constructed based on CdTe@CdS core@shell quantum dots (CdTe@CdS QDs)-decorated TiO₂ nanotubes (TiO₂ NTs). CdTe@CdS QDs were prepared by the combination of CdTe and CdS with a proper lattice mismatch. Due to their large band offsets, core@shell QDs can reduce undesirable carrier recombination, significantly improving their charge separation efficiency. Then the synthesized CdTe@CdS QDs were modified on TiO₂ NTs (CdTe@CdS QDs/TiO₂ NTs) through electrostatic adsorption method. The as-prepared composites exhibit a wide visible light absorption range, good PEC activity and high photoelectric conversion efficiency. Also, the PEC aptasensor prepared via the immobilization of anti-PCB72 aptamer on the composites exhibits outstanding analytical performance with high sensitivity and specificity for PCB72 under visible-light irradiation, achieving a detection limit as low as 0.03 ng/L. It was also applied to detect PCB72 in four different real environmental samples with satisfactory results.

Phosphotungstic Acid-Modified MnOx for Selective Catalytic Reduction of NOx with NH3

H3PW12O40-modified MnOx catalysts (denoted as Mn-HPW) were used for NOx elimination with co-fed NH3. The optimal Mn-HPW0.02 catalyst exhibited over 90% NOx conversion at 90–270 °C. The incorporation of HPW increased the amount of Lewis acid sites of the catalyst for adsorbing NH3, and accelerated the reaction between the adsorbed NH3 species and gas-phase NOx, thus, increasing the low-temperature catalytic activity. The oxidation ability of the Mn catalyst was decreased due to the addition of HPW, thus, mitigating the overoxidation of the adsorbed NH3 species and improving the de-NOx activity and N2 selectivity in the high-temperature region. DRIFT results revealed that the NH3 species on Lewis and Brønsted acid sites, bridged nitrate, and bidentate nitrate were important species/intermediates for the reaction. NH3-SCR over the Mn and Mn-HPW0.02 catalysts obeyed the Eley–Rideal and Langmuir–Hinshelwood mechanisms, simultaneously, at 120 °C.

In situ deposition of 0D CeO2 quantum dots on Fe2O3-containing solid waste NH3-SCR catalyst: Enhancing redox and NH3 adsorption ability

As NO_x has been turning into a crucial environmental problem, NH₃-SCR technology with relatively simple device, reliable operation and low secondary pollution, has become a widely used commercial and mature de-nitration technology. However, some weaknesses restricted the further application of commercialized V₂O₅-WO₃/TiO₂ NH₃-SCR catalysts, while Fe₂O₃-based catalysts have received much attention due to their high thermal stability, passable N₂ selectivity and low cost. In this study, Fe₂O₃-containing solid waste derived from Zn extraction process of electric arc furnace dust was exploited as the base material for catalyst preparing. Owing to the complementary and synergistic effect of CeO₂ and Fe₂O₃, 0D CeO₂ quantum dots (CeQDs) with fully-exposed active sites, large specific surface area, and rapid charge transfer have been introduced and deposited onto Fe₂O₃-containing solid waste nanorods. The in-situ deposition of CeQDs led to the admirable enhancement in NH₃-SCR catalytic activity, N₂ selectivity and SO₂ tolerance of the extremely low-cost Fe₂O₃ catalyst. Comprehensive characterizations and DFT calculations describing the adsorption of O₂ and NH₃ were applied to analyze the catalyst structure and further investigate the detailed relationship between structural properties and activity as well as reaction mechanism. This work provides new insights for the high-value utilization of iron-containing solid waste and a practical reference for boosting the performance of NH₃-SCR catalysts by introducing quantum dots.

Tailor the crystal planes of MIL-101(Fe) derivatives to enhance the activity of SCR reaction at medium and low temperature

Mainly exposed crystal facets and controllable morphology play a key role in the final performance of the preparation of specific nanomaterials. In the present study, a metal-organic framework pyrolysis method without adding solvent modifiers was developed. By adding CO in the calcination atmosphere to change atmosphere ratio, Fe₃O₄ nanostructures are exposed with different crystal planes and evaluated their performance in NH₃-SCR reaction. This study proves that SCR catalytic activity of Fe₃O₄ nanocrystals is dependent on morphology and crystal facet. Compared with materials exposed (100), catalysts with more (111) show stronger deNO_x performance. The preferential exposure of Fe₃O₄ (111) crystal facets increases the concentration of adsorbed oxygen on the catalyst, showing higher surface acidity, and enhances the interaction among NO, O₂ and catalyst, which is conducive to SCR reaction. This is supported by DFT calculations. The results present a great application prospect in preparing nanomaterials with specific crystal structures to effectively treat pollutants.