Search Results (16)

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, MnO_x, CuO_x, 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. Full article

Structure–performance relationships in functional catalysts allow for controlling their performance in a wide range of reaction conditions. Here, the structural and compositional peculiarities in CTAB-templated CeO₂-ZrO₂-MnO_x catalysts prepared by co-precipitation of precursors and their catalytic behavior in CO oxidation and soot combustion are discussed. A complex of physical–chemical methods (low-temperature N₂ sorption, XRD, TPR-H₂, Raman, HR TEM, XPS) is used to elucidate the features of the formation of interphase boundaries, joint phases, and defects in multicomponent oxide systems. The addition of Mn and/or Zr dopant to ceria is shown to improve its performance in both reactions. Binary Ce-Mn catalysts demonstrate enhanced performance closely followed by the ternary oxide catalysts, which is due the formation of several types of active sites, namely, highly dispersed MnO_x species, oxide–oxide interfaces, and oxygen vacancies that can act individually and/or synergistically. Full article

Dry reforming of methane with ratio CH₄/CO₂ = 1 is studied using supported Ni catalysts on SBA-15 modified by CeMnO_x mixed oxides with different Ce/Mn ratios (0.25, 1 and 9). The obtained samples are characterized by wide-angle XRD, SAXS, N₂ sorption, TPR-H₂, TEM, UV–vis and Raman spectroscopies. The SBA-15 modification with CeMnO_x decreases the sizes of NiO nanoparticles and enhances the NiO–support interaction. When Ce/Mn = 9, the NiO forms small particles on the surface of large CeO₂ particles and/or interacts with CeO₂, forming mixed phases. The best catalytic performance (at 650 °C, CH₄ and CO₂ conversions are 51 and 69%, respectively) is achieved over the Ni/CeMnO_x/SBA-15 (9:1) catalyst. The peculiar CeMnO_x composition (Ce/Mn = 9) also improves the catalyst stability: In a 24 h stability test, the CH₄ conversion decreases by 18 rel.% as compared to a 30 rel.% decrease for unmodified catalyst. The enhanced catalytic stability of Ni/CeMnO_x/SBA-15 (9:1) is attributed to the high concentration of reactive peroxo (O⁻) and superoxo (O₂⁻) species that significantly lower the amount of coke in comparison with Ni-SBA-15 unmodified catalyst (weight loss of 2.7% vs. 42.2%). Ni-SBA-15 modified with equimolar Ce/Mn ratio or Mn excess is less performing. Ni/CeMnO_x/SBA-15 (1:4) with the highest content of manganese shows the minimum conversions of reagents in the entire temperature range (X(CO₂) = 4–36%, X(CH₄) = 8–58%). This finding is possibly attributed to the presence of manganese oxide, which decorates the Ni particles due to its redistribution at the preparation stage. Full article

Ce-MnO_x composite oxide catalysts with different proportions were prepared using the coprecipitation method, and the CO-removal ability of the catalysts with the tested temperature range of 60–140 °C was investigated systematically. The effect of Ce and Mn ratios on the catalytic oxidation performance of CO was investigated using X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), H₂ temperature programmed reduction (H₂-TPR), CO-temperature programmed desorption (CO-TPD), and in situ infrared spectra. The experimental results reveal that under the same test conditions, the CO conversion rate of pure Mn₃O₄ reaches 95.4% at 170 °C. Additionally, at 140 °C, the Ce-MnO_x series composite oxide catalyst converts CO at a rate of over 96%, outperforming single-phase Mn₃O₄ in terms of catalytic performance. With the decrement in Ce content, the performance of Ce-MnO_x series composite oxide catalysts first increase and then decrease. The Ce MnO_x catalyst behaves best when Ce:Mn = 1:1, with a CO conversion rate of 99.96% at 140 °C and 91.98% at 100 °C. Full article

Novel yttrium-doped CeO₂, MnO_x, and CeMnO_x composites are investigated as catalysts for low-temperature NH₃-SCR. The study involves the preparation of unmodified oxide supports using a citrate method followed by modification with Y (2 wt.%) using two approaches, including the one-pot citrate method and incipient wetness impregnation of undoped oxides. The NH₃-SCR reaction is studied in a fixed-bed quartz reactor to test the ability of the prepared catalysts in NO reduction. The gas reaction mixture consists of 800 ppm NO, 800 ppm NH₃, 10 vol.% O₂, and He as a balance gas at a WHSV of 25,000 mL g⁻¹ h⁻¹. The results indicate that undoped CeMnO_x mixed oxide exhibits significantly higher deNO_x performance compared with undoped and Y-doped MnO_x and CeO₂ catalysts. Indeed, yttrium presence in CeMnO_x promotes the competitive NH₃-SCO reaction, reducing the amount of NH₃ available for NO reduction and lowering the catalyst activity. Furthermore, the physical-chemical properties of the prepared catalysts are studied using nitrogen adsorption/desorption, XRD, Raman spectroscopy, temperature-programmed reduction with hydrogen, and temperature-programmed desorption of ammonia. This study presents a promising approach to enhancing the performance of NH₃-SCR catalysts at low temperatures that can have significant implications for reducing NO emissions. Full article

Pt nanoparticles and a CeMnO_x composite were loaded on the surface of the natural diatomite material to generate the Pt/CeMnO_x/diatomite using the redox precipitation and impregnation methods. The physicochemical properties of the catalysts were characterized by means of various techniques. The catalytic properties and resistance to H₂O and SO₂ of the catalysts were measured for the oxidation of typical volatile organic compounds (i.e., toluene and ethyl acetate). Among all of the as-prepared samples, Pt/CeMnO_x/diatomite exhibited the highest catalytic activity: the temperatures (T_90%) at a toluene or ethyl acetate conversion of 90% were 230 and 210 °C at a space velocity (SV) of 20,000 mL g⁻¹ h⁻¹, respectively, and the turnover frequency (TOF_Pt) at 220 °C was 1.04 μmol/(g_cat s) for ethyl acetate oxidation and 1.56 μmol/(g_cat s) for toluene oxidation. In particular, this sample showed a superior catalytic activity for ethyl acetate oxidation at low temperatures, with its T_50% being 185 °C at SV = 20,000 mL g⁻¹ h⁻¹. In addition, the Pt/CeMnO_x/diatomite sample possessed good sulfur dioxide resistance during the toluene oxidation process. In the presence of SO₂, some of the SO₂ molecules were adsorbed on diatomite, which protected the active sites from being poisoned by SO₂ to a certain extent. The pathways of ethyl acetate and toluene oxidation over Pt/CeMnO_x/diatomite or Pt/CeMnO_x were as follows: The C–C and C–O bonds in ethyl acetate are first broken to form the CH₃CH₂O* and CH₃CO* species or toluene is first oxidized to benzaldehyde and benzoic acid, and all of these intermediates are then converted to CO₂ and H₂O. This work can provide a strategy to develop efficient catalysts with high catalytic activity, durability, low cost, and easy availability under actual working conditions. Full article

In the present study CeO₂, MnO₂ and CeMnO_x mixed oxide (with molar ratio Ce/Mn = 1) were prepared by sol-gel method using citric acid as a chelating agent and calcined at 500 °C. The silver catalysts (1 wt.% Ag) over the obtained supports were synthesized by the incipient wetness impregnation method with [Ag(NH₃)₂]NO₃ aqueous solution. The selective catalytic reduction of NO by C₃H₆ was investigated in a fixed-bed quartz reactor using a reaction mixture composed of 1000 ppm NO, 3600 ppm C₃H₆, 10 vol.% O₂, 2.9 vol.% H₂ and He as a balance gas, at WHSV of 25,000 mL g⁻¹ h⁻¹.The physical-chemical properties of the as-prepared catalysts were studied by several characterization techniques, such as X-ray fluorescence analysis, nitrogen adsorption/desorption, X-ray analysis, Raman spectroscopy, transmission electron microscopy with analysis of the surface composition by X-ray energy dispersive spectroscopy and X-ray photo-electron spectroscopy. Silver oxidation state and its distribution on the catalysts surface as well as the support microstructure are the main factors determining the low temperature activity in NO selective catalytic reduction. The most active Ag/CeMnO_x catalyst (NO conversion at 300 °C is 44% and N₂ selectivity is ~90%) is characterized by the presence of the fluorite-type phase with high dispersion and distortion. The characteristic “patchwork” domain microstructure of the mixed oxide along with the presence of dispersed Ag⁺/Ag_n^δ+ species improve the low-temperature catalyst of NO reduction by C₃H₆ performance compared to Ag/CeO₂ and Ag/MnO_x systems. Full article

Chlorobenzene (CB) is a volatile and harmful organic molecule that may result in deformities, cancer, etc. Catalytic oxidization of CB may be a way to manage it. The development of nonprecious catalysts with high catalytic activity is the key but is still a challenge. In this work, a series of Ce-Mn-O_x/TiO₂ modified by citric acid monohydrate were developed and exhibited a composite pore structure. This pore structure leads to a large specific surface area, highly exposed activity sites, and excellent catalytic activity. The as-prepared 10C-CM/T exhibited nearly 100% efficiency for CB oxidization in the temperature range of 300–350 °C. The in situ DRIFT measurements demonstrated that the main intermediates at 250 °C are maleate and phenolic acid, whereas when the temperature is 350 °C, the main intermediates are carbonate, bidentate carbonate, and maleate. Full article

Recently, manganese oxides (MnO_x)/cerium(IV) oxide (CeO₂) composites have attracted widespread attention for the selective catalytic reduction (SCR) of nitrogen oxides (NO_x) with ammonia (NH₃), which exhibit outstanding catalytic performance owing to unique features, such as a large oxygen storage capacity, excellent low-temperature activity, and strong mechanical strength. The intimate contact between the components can effectively accelerate the charge transfer to enhance the electron–hole separation efficiency. Nevertheless, MnO_x/CeO₂ still reveals some deficiencies in the practical application process because of poor thermal stability, and a low reduction efficiency. Constructing MnO_x/CeO₂ with other semiconductors is the most effective strategy to further improve catalytic performance. In this article, we discuss progress in the field of MnO_x/CeO₂-based ternary composites with an emphasis on the SCR of NO_x by NH₃. Recent progress in their fabrication and application, including suitable examples from the relevant literature, are analyzed and summarized. In addition, the interaction mechanisms between MnO_x/CeO₂ catalysts and NO_x pollutants are comprehensively dissected. Finally, the review provides basic insights into prospects and challenges for the advancement of MnO_x/CeO₂-based ternary catalysts. Full article

A series of CeO₂–MnO_X catalysts with different Ce contents was prepared using Mn–BTC MOF as a sacrificial template for toluene oxidation. Interestingly, the performance of CeO₂–MnO_X increased rapidly only when the Ce content lower than 5%. The 1%-, 3%- and 10%-Ce-content samples exhibited the T₉₀ value of 325 °C, 291 °C and 277 °C, respectively. XRD shows that the catalyst phase changes significantly before (Mn₃O₄ only) and after (Mn₂O₃, Mn₃O₄ and CeO₂) 3% Ce loading. All other results indicated that the Ce–Mn interface properties of different Ce content composite oxides was quite distinguishable in terms of removal and energy efficiency. XRD and XPS results further showed that there a Ce monolayer dispersion threshold existed on the interface of MnO_X (3.2 wt%, confirmed by XPS), which caused the difference in performance increment. The dispersed Ce could be divided into a monolayer dispersion state (1–3%) and a crystalline phase state (>3%), according to the existence form, which corresponded to the significant and minor enhancements of toluene conversion rate. Importantly, the Ce in monolayer dispersion state obviously improved the redox properties of catalysts interface, while the Ce in crystal state not. The interfaces with monolayer dispersion Ce result in more abundant metal ion states, oxygen vacancies, better electron transfer performance and catalytic activity. Full article

Novel Mn–Ce–Ti–O composite aerogels with large mesopore size were prepared via a one-pot sol–gel method by using propylene oxide as a network gel inducer and ethyl acetoacetate as a complexing agent. The effect of calcination temperature (400, 500, 600, and 700 °C) on the NH₃–selective catalytic reduction (SCR) performance of the obtained Mn–Ce–Ti–O composite aerogels was investigated. The results show that the Mn–Ce–Ti–O catalyst calcined at 600 °C exhibits the highest NH₃–SCR activity and lowest apparent activation energy due to its most abundant Lewis acid sites and best reducibility. The NO conversion of the MCTO-600 catalyst maintains 100% at 200 °C in the presence of 100 ppm SO₂, showing the superior resistance to SO₂ poisoning as compared with the MnO_x–CeO₂–TiO₂ catalysts reported the literature. This should be mainly attributed to its large mesopore sizes with an average pore size of 32 nm and abundant Lewis acid sites. The former fact facilitates the decomposition of NH₄HSO₄, and the latter fact reduces vapor pressure of NH₃. The NH₃–SCR process on the MCTO-600 catalyst follows both the Eley–Rideal (E–R) mechanism and the Langmuir–Hinshelwood (L–H) mechanism. Full article

NO oxidation was conducted over MnO_x-CeO₂ catalysts, which were synthesized by the co-precipitation method. The calcination temperature and third metal doping were the main considerations. MnCe catalysts calcined at 350 °C and 450 °C attained the highest NO conversion efficiency, compared to 550 °C. XRD results suggested that the higher the calcination temperature, the higher the crystallization degree, which led to a negative effect on catalytic activity. Subsequently, Sn, Fe, Co, Cr, and Cu were separately doped into MnCe composites, but no improvement was observed for these trimetallic catalysts in NO conversion. Nevertheless, MnCeSn, MnCeFe, and MnCeCo still exhibited a desirable NO conversion efficiency, so they were tested under SO₂ addition together with MnCe catalyst. Among them, MnCeFe exhibited the highest NO conversion after whole poisoning testing. XPS results indicated that Fe could protect Mn and Ce metal oxides from being reduced during SO₂ poisoning process. Furthermore, in in-situ DRIFTS measurement, part of nitrate species maintained undestroyed on the MnCeFe catalyst surface after SO₂ poisoning. These characteristics reinforced that Fe dropping would achieve better performance under SO₂ atmosphere. Full article

The catalytic activities of CeO₂-MnO_x composite oxides synthesized through oxalate method were researched. The results exhibited that the catalytic properties of CeO₂-MnO_x composite oxides were higher than pure CeO₂ or MnO_x. When the Ce_at/Mn_at ratio was 3:7, the catalytic activity reached the best. In addition, the activities of CeO₂-MnO_x synthesized through different routes over benzene oxidation were also comparative researched. The result indicated that the catalytic property of sample prepared by oxalate method was better than others, which maybe closely related with their meso-structures. Meanwhile, the effects of synergistic interaction and oxygen species in the samples on the catalytic ability can’t be ignored. Full article

A series of Mn−Fe−Ce−O_x/γ-Al₂O₃ nanocatalysts were synthesized with different Mn/Fe ratios for the catalytic oxidation of NO into NO₂ and the catalytic elimination of NO_x via fast selective catalytic reduction (SCR) reaction. The effects of Mn/Fe ratio on the physicochemical properties of the samples were analyzed by means of various techniques including N₂ adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (TPR), NH₃-temperature-programmed desorption (TPD) and NO-TPD, meanwhile, their catalytic performance was also evaluated and compared. Multiple characterizations revealed that the catalytic performance was highly dependent on the phase composition. The Mn15Fe15−Ce/Al sample with the Mn/Fe molar ratio of 1.0 presented the optimal structure characteristic among all tested samples, with the largest surface area, increased active components distributions, the reduced crystallinity and diminished particle sizes. In the meantime, the ratios of Mn⁴⁺/Mnⁿ⁺, Fe²⁺/Feⁿ⁺ and Ce³⁺/Ceⁿ⁺ in Mn15Fe15−Ce/Al samples were improved, which could enhance the redox capacity and increase the quantity of chemisorbed oxygen and oxygen vacancy, thus facilitating NO oxidation into NO₂ and eventually promoting the fast SCR reaction. In accord with the structure results, the Mn15Fe15−Ce/Al sample exhibited the highest NO oxidation rate of 64.2% at 350 °C and the broadest temperature window of 75–350 °C with the NO_x conversion >90%. Based on the structure–activity relationship discussion, the catalytic mechanism over the Mn−Fe−Ce ternary components supported by γ-Al₂O₃ were proposed. Overall, it was believed that the optimization of Mn/Fe ratio in Mn−Fe−Ce/Al nanocatalyst was an extremely effective method to improve the structure–activity relationships for NO pre-oxidation and the fast SCR reaction. Full article

Highly dispersed Mn–Ce binary metal oxides supported on carbon nanofibers (MnO_x–CeO₂/CNFs(OX)) were prepared for Hg⁰ removal from coal-fired flue gas. The loading value of the well-dispersed MnO_x–CeO₂ was much higher than those of many other reported supports, indicating that more active sites were loaded on the carbon nanofibers. In the present study, 30 wt % metal oxides (15 wt % MnO_x and 15 wt % CeO₂) were successfully deposited on the carbon nanofibers, and the sorbent yielded the highest Hg⁰ removal efficiency (>90%) within 120–220 °C under a N₂/O₂ atmosphere. An increase in the amount of highly dispersed metal oxides provided abundant active species for efficient Hg⁰ removal, such as active oxygen species and Mn⁴⁺ cations. Meanwhile, the carbon nanofiber framework provides the pathway for charge transfer during the heterogeneous Hg⁰ capture reaction processes. Under a N₂+6%O₂ atmosphere, a majority of Hg⁰ was removed via chemisorption reactions. The effects of flue gas composition were also investigated. O₂ replenished the active oxygen species on the surface and thus greatly promoted the Hg⁰ removal efficiency. SO₂ had an inhibitory effect on Hg⁰ removal, but NO facilitated Hg⁰ capture performance. Overall, carbon nanofibers seems to be a good candidate for the support and MnO_x–CeO₂/CNFs(OX) may be promising for Hg⁰ removal from coal-fired flue gas. Full article