Preparation and characterization of CeOx@MnOx core–shell structure catalyst for catalytic oxidation of NO

The Structures and Compositions Design of the Hollow Micro-Nano-Structured Metal Oxides for Environmental Catalysis

In recent decades, with the rapid development of the inorganic synthesis and the increasing discharge of pollutants in the process of industrialization, hollow-structured metal oxides (HSMOs) have taken on a striking role in the field of environmental catalysis. This is all due to their unique structural characteristics compared to solid nanoparticles, such as high loading capacity, superior pore permeability, high specific surface area, abundant inner void space, and low density. Although the HSMOs with different morphologies have been reviewed and prospected in the aspect of synthesis strategies and potential applications, there has been no systematic review focusing on the structures and compositions design of HSMOs in the field of environmental catalysis so far. Therefore, this review will mainly focus on the component dependence and controllable structure of HSMOs in the catalytic elimination of different environmental pollutants, including the automobile and stationary source emissions, volatile organic compounds, greenhouse gases, ozone-depleting substances, and other potential pollutants. Moreover, we comprehensively reviewed the applications of the catalysts with hollow structure that are mainly composed of metal oxides such as CeO2, MnOx, CuOx, Co3O4, ZrO2, ZnO, Al3O4, In2O3, NiO, and Fe3O4 in automobile and stationary source emission control, volatile organic compounds emission control, and the conversion of greenhouse gases and ozone-depleting substances. The structure-activity relationship is also briefly discussed. Finally, further challenges and development trends of HSMO catalysts in environmental catalysis are also prospected.

Low-Temperature Selective Catalytic Reduction of NO x with NH3 over Mn-Ce Composites Synthesized by Polymer-Assisted Deposition

The Mn _x Ce _y binary catalysts with a three-dimensional network structure were successfully prepared via a polymer-assisted deposition method using ethylenediaminetetraacetic acid and polyethyleneimine as complexing agents. The developed pore structure could facilitate the gas diffusion and accelerate the catalytic reaction for NH₃ selective catalytic reduction (SCR). Moreover, the addition of Ce is beneficial for the exposure of active sites on the catalyst surface and increases the adsorption of the NH₃ and NO species. Therefore, the Mn₁Ce₁ catalyst exhibits the best catalytic activity for NO _x removal with a conversion rate of 97% at 180 °C, superior water resistance, and favorable stability. The SCR reaction over the Mn₁Ce₁ catalyst takes place through the E-R pathway, which is confirmed by the in situ diffuse reflectance Fourier transform analysis. This work explores a new strategy to fabricate multimetal catalysts and optimize the structure of catalysts.

Recent advances in selective catalytic oxidation of nitric oxide (NO-SCO) in emissions with excess oxygen: a review on catalysts and mechanisms

Nitric oxides (NO_x, which mainly include more than 90% NO) are one of the major air pollutants leading to a series of environmental problems, such as acid rain, haze, photochemical smog, etc. The selective catalytic oxidation of NO to NO₂ (NO-SCO) is regarded as a key process for the development of selective catalytic reduction of NO_x by ammonia (via fast selective catalytic reduction reaction) and also the simultaneous removal of multipollutant (pre-oxidation and post-absorption). Until now, scholars have developed various types of NO-SCO catalysts, dividing the main groups into noble metals (Pt, Pd, Ru, etc.), metal oxides (Mn-, Co-, Cr-, Ce-based, etc.), perovskite-type oxides (LaMnO₃, LaCoO₃, LaCeCoO₃, etc.), carbon materials (activated carbon, carbon fiber, carbon nanotube, graphene, etc.), and zeolites (ion-exchanged ZSM-5, CHA, SAPO, MCM-41, etc.) in this review. This paper summarizes the recent progress of the above typical catalysts and mostly analyzes the catalytic performance for NO oxidation in terms of the H₂O and/or SO₂ resistances and also the influencing factors, and their reaction mechanisms are described in detail. Finally, this review points out the key problems and possible solutions of the current researches and presents the application prospects and future development directions of NO-SCO technology using the above typical catalysts.

Promotion effect of urchin-like MnO x @PrO x hollow core-shell structure catalysts for the low-temperature selective catalytic reduction of NO with NH3

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃. The structural properties of the catalyst were characterized by FE-SEM, TEM, XRD, BET, XPS, H₂-TPR and NH₃-TPD analyses, and the performance of the low-temperature NH₃-SCR was also tested. The results show that the catalyst with a molar ratio of Pr/Mn = 0.3 exhibited the highest NO conversion at nearly 99% at 120 °C and NO conversion greater than 90% over the temperature range of 100–240 °C. Also, the MnO_x@PrO_x catalyst presented desirable SO₂ and H₂O resistance in 100 ppm SO₂ and 10 vol% H₂O at the space velocity of 40 000 h⁻¹ and a testing time of 3 h test at 160 °C. The excellent low-temperature catalytic activity of the catalyst could ultimately be attributed to high concentrations of Mn⁴⁺ and adsorbed oxygen species on the catalyst surface, suitable Lewis acidic surface properties, and good reducing ability. Additionally, the enhanced SO₂ and H₂O resistance of the catalyst was primarily ascribed to its unique core–shell structure which prevented the MnO_x core from being sulfated.

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃.

The Influence Of NO/O2 On The NOx Storage Properties Over A Pt-Ba-Ce/γ-Al2O3 Catalyst

With the purpose of studying the influence of NO/O2 on the NOx storage activity, a Pt-Ba-Ce/γ-Al2O3 catalyst was synthesized by an acid-aided sol-gel method. The physical and chemical properties of the catalyst were characterized by X-ray diffraction (XRD) and Transmission Electron Microscope (TEM) methods. The results showed that the composition of the catalyst was well-crystallized and the crystalline size of CeO2 (111) was about 5.7 nm. The mechanism of NO and NO2 storage and NOx temperature programmed desorption (NO-TPD) experiments were investigated to evaluate the NOx storage capacity of the catalyst. Pt-Ba-Ce/γ-Al2O3 catalyst presented the supreme NOx storage performance at 350℃, and the maximum value reached to 668.8 μmol / gcat. Compared with O2-free condition, NO oxidation to NO2 by O2 had a beneficial effect on the storage performance of NOx. NO-TPD test results showed that the NOx species stored on the catalyst surface still kept relatively stable even below 350℃.

Catalytic Oxidation of Dimethyl Disulfide over Bimetallic Cu–Au and Pt–Au Catalysts Supported on γ-Al2O3, CeO2, and CeO2–Al2O3

Dimethyl disulfide (DMDS, CH3SSCH3) is an odorous and harmful air pollutant (volatile organic compound (VOC)) causing nuisance in urban areas. The abatement of DMDS emissions from industrial sources can be realized through catalytic oxidation. However, the development of active and selective catalysts having good resistance toward sulfur poisoning is required. This paper describes an investigation related to improving the performance of Pt and Cu catalysts through the addition of Au to monometallic “parent” catalysts via surface redox reactions. The catalysts were characterized using ICP-OES, N2 physisorption, XRD, XPS, HR-TEM, H2-TPR, NH3-TPD, CO2-TPD, and temperature-programmed 18O2 isotopic exchange. The performance of the catalysts was evaluated in DMDS total oxidation. In addition, the stability of a Pt–Au/Ce–Al catalyst was investigated through 40 h time onstream. Cu–Au catalysts were observed to be more active than corresponding Pt–Au catalysts based on DMDS light-off experiments. However, the reaction led to a higher amount of oxygen-containing byproduct formation, and thus the Pt–Au catalysts were more selective. H2-TPR showed that the higher redox capacity of the Cu-containing catalysts may have been the reason for better DMDS conversion and lower selectivity. The lower amount of reactive oxygen on the surface of Pt-containing catalysts was beneficial for total oxidation. The improved selectivity of ceria-containing catalysts after the Au addition may have resulted from the lowered amount of reactive oxygen as well. The Au addition improved the activity of Al2O3-supported Cu and Pt. The Au addition also had a positive effect on SO2 production in a higher temperature region. A stability test of 40 h showed that the Pt–Au/Ce–Al catalyst, while otherwise promising, was not stable enough, and further development is still needed.

Effects of Ag0-modification and Fe3+-doping on the structural, optical and photocatalytic properties of TiO2

The reasons for the photocatalytic activity of 1% Ag–TiO₂ > pure TiO₂ > 1% Ag/1% FeTiO₂ > 1% Fe–TiO₂ are investigated systematically.

VOX supported on TiO2–Ce0.9Zr0.1O2 core–shell structure catalyst for NH3-SCR of NO

In this experiment, a TiO₂–Ce_0.9Zr_0.1O₂ support with core–shell structure was successfully prepared by a precipitation method and VO_X/TiO₂–Ce_0.9Zr_0.1O₂ catalyst was prepared by an impregnation method, and the catalyst was used to catalyze the NH₃-SCR of NO. Based on the results of HRTEM, XRD, BET, H₂-TPR, NH₃-TPD, XPS, Py-IR, it was speculated that due to the interaction between TiO₂ and Ce_0.9Zr_0.1O₂, more oxygen vacancies and Ce³⁺ are generated, which are beneficial to the existence of low-valence V by electron transfer between high valence state V and Ce³⁺and increase the acidic sites on the catalyst surface. The catalytic activity (>97%) of the VO_X/TiO₂–Ce_0.9Zr_0.1O₂ catalyst is superior to the current commercial catalyst (V₂O₅–WO₃/TiO₂) and has a higher N₂ selectivity (>97.5%) at 40 000 h⁻¹ GHSV and 250–400 °C.

VO_X/TiO₂–Ce_0.9Zr_0.1O₂ catalyst exhibits high activity and selectivity in a wide temperature window.

The synthesis of CuyMnzAl1−zOx mixed oxide as a low-temperature NH3-SCR catalyst with enhanced catalytic performance

A new type of low-temperature selective catalytic reduction (SCR) catalyst, Cu_yMn_zAl_1−zO_x, derived from layered double hydroxides is presented in this contribution.

Preparation of a monolith MnOx–CeO2/La–Al2O3 catalyst and its properties for catalytic oxidation of toluene

The catalyst DP-MnCe prepared by the deposition–precipitation method has the best catalytic activity for toluene oxidation.

Amorphous saturated cerium–tungsten–titanium oxide nanofiber catalysts for NOx selective catalytic reaction

A nano-fibrous, amorphous supersaturated CeO₂/W–TiO₂ SCR catalyst endowed with well-connected and open porosity, high reactivity, and tunable chemistry is herein proposed.

Natural manganese ore catalyst for low-temperature selective catalytic reduction of NO with NH3 in coke-oven flue gas

Different types of manganese ore raw materials were prepared for use as catalysts, and the effects of different manganese ore raw materials and calcination temperature on the NO conversion were analyzed. The catalysts were characterized by XRF, XRD, BET, XPS, H₂-TPR, NH₃-TPD, and SEM techniques. The results showed that the NO conversion of calcined manganese ore with a Mn:Fe:Al:Si ratio of 1.51:1.26:0.34:1 at 450 °C reached 80% at 120 °C and 98% at 180~240 °C. The suitable proportions and better dispersibility of active ingredients, larger BET surface area, good reductibility, a lot of acid sites, contents of Mn⁴⁺ and Fe³⁺, and surface-adsorbed oxygen played important roles in improving the NO conversion.

The enhanced resistance to P species of an Mn–Ti catalyst for selective catalytic reduction of NOx with NH3 by the modification with Mo

The modification of Mn–Ti catalyst by Mo could enhance its resistance to P species.

Influence of Ca doping and calcination temperature on selective catalytic oxidation of NO over Mn–Ca–Ox–(CO3)ycatalysts

Selective catalytic oxidation (SCO) is an unconventional technology for denitration.

Investigation of the active sites for NO oxidation reactions over MnOx–CeO2 catalysts

Mn⁴⁺/Mn³⁺ is the active site for NO oxidation reactions, according to the TOF calculated with respect to the initial reducibility measured by H₂-TPR quantification.

The enhanced performance of a CeSiOx support on a Mn/CeSiOx catalyst for selective catalytic reduction of NOx with NH3

A series of Mn/CeSiO_x catalysts were prepared by the wet impregnation method and used for selective catalytic reduction of NO with NH₃.

Fabrication and formation mechanism of Ce2O3–CeO2–CuO–MnO2/CNTs catalysts and application in low-temperature NO reduction with NH3

Ce₂O₃–CeO₂–CuO–MnO₂/CNTs catalysts were synthesized via a redox strategy, and presented 58–85% NO conversion at 80–180 °C.