31 research outputs found

    Synergistic effect between ceria and tungsten oxide on WO3–CeO2–TiO2 catalysts for NH3-SCR reaction

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    AbstractWO3–CeO2–TiO2 catalysts for NO (nitrogen monoxide) reduction by ammonia were prepared by a sol–gel method. The catalysts were characterized by BET, XRD, Raman, NH3/NO adsorption and H2-TPR to investigate the relationships among the catalyst composition, structure, redox property, acidity and deNOx activity. WO3–CeO2–TiO2 catalysts show a high activity in a broad temperature range of 200–480°C. The low-temperature activity of catalysts is sensitive to the catalyst composition especially under low-O2-content atmospheres. It may be related to the synergistic effect between CeOx and WOx in the catalysts. On one hand, the interaction between ceria and tungsten oxide promotes the activation of gaseous oxygen to compensate the lattice oxygen consumed in NH3-SCR (selective catalytic reduction) reaction at low temperatures. Meanwhile, the Brønsted acid sites mainly arise from tungsten oxides, Lewis acid sites mainly arise from ceria. Both of the Brønsted and Lewis acid sites facilitate the adsorption of NH3 on catalysts and improve the stability of the adsorbed ammonia species, which are beneficial to the NH3-SCR reaction

    A rechargeable molecular solar thermal system below 0 °C

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    An optimal temperature is crucial for a broad range of applications, from chemical transformations, electronics, and human comfort, to energy production and our whole planet. Photochemical molecular thermal energy storage systems coupled with phase change behavior (MOST-PCMs) offer unique opportunities to capture energy and regulate temperature. Here, we demonstrate how a series of visible-light-responsive azopyrazoles couple MOST and PCMs to provide energy capture and release below 0 °C. The system is charged by blue light at -1 °C, and discharges energy in the form of heat under green light irradiation. High energy density (0.25 MJ kg-1) is realized through co-harvesting visible-light energy and thermal energy from the environment through phase transitions. Coatings on glass with photo-controlled transparency are prepared as a demonstration of thermal regulation. The temperature difference between the coatings and the ice cold surroundings is up to 22.7 °C during the discharging process. This study illustrates molecular design principles that pave the way for MOST-PCMs that can store natural sunlight energy and ambient heat over a wide temperature range.This work was supported by the National Key Research and Development Program of China (2017YFA0207500), National Natural Science Foundation of China (22022507 and 51973111), Beijing National Laboratory for Molecular Sciences (BNLMS202004), China Postdoctoral Science Foundation (2020M681279) and European Research Council (ERC) through CoG 101002131 “PHOTHERM”.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

    Low-temperature SCR activity and SO2 deactivation mechanism of Ce-modified V2O5–WO3/TiO2 catalyst

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    The promotion effect of ceria modification on the low-temperature activity of V2O5-WO3/TiO2 catalyst was evaluated for the selective catalytic reduction of NO with NH3 (NH3-SCR). The catalytic activity of 1 wt% V2O5-WO3/TiO2 was significantly enhanced by the addition of 8 wt% ceria, which exhibited a NOx conversion above 80% in a broad temperature range 190–450 °C. This performance was comparable with 3 wt%V2O5-WO3/TiO2, indicating that the addition of ceria contributed to reducing the usage of toxic vanadia in developing low-temperature SCR catalysts. Moreover, V1CeWTi exhibited approximately 10% decrease in NOx conversion in the presence of 60 ppm SO2. The characterization results indicated that active components of V, W and Ce were well dispersed on TiO2 support. The synergetic interaction between Ce and V species by forming V–O–Ce bridges enhanced the reducibility of VCeWTi catalyst and thus improved the low-temperature activity. The sulfur poisoning mechanism was also presented on a basis of the designed TPDC (temperature-programmed decomposition) and TPSR (temperature-programmed surface reaction) experiments. The deposition of (NH4)2SO4 on V1CeWTi catalyst was much smaller compared with that on V1Ti. On the other hand, the oxidation of SO2 to SO3 was significantly promoted on the CeO2-modified catalyst, accompanied by the formation of cerium sulfates. Therefore, the deactivation of this catalyst was mainly attributed to the vanishing of the V–Ce interaction and the sulfation of active ceria

    Destructive and Protective Effects of NH3 on the Low-Temperature Hydrothermal Stability of SAPO-34 and Cu-SAPO-34

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    The influences of gaseous, weakly adsorbed, and strongly adsorbed NH3 on the low-temperature (<100 degrees C) hydrothermal stability of SAPO-34 and Cu-SAPO-34 were investigated. NH3 temperature-programmed desorption (NH3-TPD), H-1 magic angle spinning nuclear magnetic resonance (MAS NMR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were adopted to characterize the adsorption states of NH3 and H2O in SAPO-34, and the destruction of the SAPO-34 framework was revealed by direct and cross-polarization Si-29, Al-27, and P-31 MAS NMR. Gaseous NH3 coadsorbed with H2O inside SAPO-34 micropores and induced the hydrolysis of framework P-O-Al and Si-O(H)-Al bonds. Weakly adsorbed NH3 was released during aging and played a similar negative role to gaseous NH3. When being combined with hydrolyzed Al species from the framework, active Cu ions transformed to inactive CuAl2O4-like species, leading to deactivation in low-temperature SCR of Cu-SAPO-34. Strongly adsorbed NH4+ via 200 degrees C preadsorption protected the framework integrity of SAPO-34 and the SCR activity of Cu-SAPO-34

    A strategy to construct (reduced graphene oxide, γ-Fe2O3)/C3N4 step-scheme photocatalyst for visible-light water splitting

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    The interface architecture plays important role in the charge transfer and separation of S-scheme photocatalysis. Herein, we propose a strategy to synthesize (reduced graphene oxide, γ-Fe2O3)/C3N4 S-scheme heterojunctions by thermal treatment of MIL-101(Fe) and melamine. (rGO, γ-Fe2O3)/C3N4 presents a high oxygen evolution rate (OER) of 3.85 mmol·g−1·h−1 under visible irradiation, and overall water splitting activity with the hydrogen evolution (HER) and OER rates of 23.3 and 12 μmol·g−1·h−1, respectively. The band alignments by different Fermi levels of C3N4 and (rGO, γ-Fe2O3) result in internal electric field, which significantly enhances the separation efficiency of photogenerated electrons and holes

    Destructive and Protective Effects of NH3 on the Low-Temperature Hydrothermal Stability of SAPO-34 and Cu-SAPO-34

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    The influences of gaseous, weakly adsorbed, and strongly adsorbed NH3 on the low-temperature (<100 degrees C) hydrothermal stability of SAPO-34 and Cu-SAPO-34 were investigated. NH3 temperature-programmed desorption (NH3-TPD), H-1 magic angle spinning nuclear magnetic resonance (MAS NMR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were adopted to characterize the adsorption states of NH3 and H2O in SAPO-34, and the destruction of the SAPO-34 framework was revealed by direct and cross-polarization Si-29, Al-27, and P-31 MAS NMR. Gaseous NH3 coadsorbed with H2O inside SAPO-34 micropores and induced the hydrolysis of framework P-O-Al and Si-O(H)-Al bonds. Weakly adsorbed NH3 was released during aging and played a similar negative role to gaseous NH3. When being combined with hydrolyzed Al species from the framework, active Cu ions transformed to inactive CuAl2O4-like species, leading to deactivation in low-temperature SCR of Cu-SAPO-34. Strongly adsorbed NH4+ via 200 degrees C preadsorption protected the framework integrity of SAPO-34 and the SCR activity of Cu-SAPO-34

    Nitration Poisoning of Silver on Al2O3 for the Catalytic Oxidation of Soot in NOR-Containing Atmospheres

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    Silver-based catalysts are promising candidates for soot oxidation because of their good catalytic performances and low costs compared with platinum catalysts. NO2 acts as a strong agent for the oxidation of soot over Pt/Al2O3. However, an abnormal inhibition effect of NOx was observed over the Ag/Al2O3 catalyst during the processes of soot temperature programmed oxidation and transient reaction analysis. The formation of silver nitrates was confirmed by temperature-programmed decomposition of NOx, ultraviolet-visible spectroscopy, and temperature-programmed reduction with H2. The intrinsic reactivity of silver nitrate was found to be much lower than that of metallic silver in '' loose '' and '' tight '' contacts between soot and catalyst. Moreover, the formed silver nitrate was relatively thermally stable, with the decomposition completed at around 450 degrees C. At higher temperatures, the catalyst recovered and the nitrate poisoning effect no longer worked

    Nitration Poisoning of Silver on Al2O3 for the Catalytic Oxidation of Soot in NOR-Containing Atmospheres

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    Silver-based catalysts are promising candidates for soot oxidation because of their good catalytic performances and low costs compared with platinum catalysts. NO2 acts as a strong agent for the oxidation of soot over Pt/Al2O3. However, an abnormal inhibition effect of NOx was observed over the Ag/Al2O3 catalyst during the processes of soot temperature programmed oxidation and transient reaction analysis. The formation of silver nitrates was confirmed by temperature-programmed decomposition of NOx, ultraviolet-visible spectroscopy, and temperature-programmed reduction with H2. The intrinsic reactivity of silver nitrate was found to be much lower than that of metallic silver in '' loose '' and '' tight '' contacts between soot and catalyst. Moreover, the formed silver nitrate was relatively thermally stable, with the decomposition completed at around 450 degrees C. At higher temperatures, the catalyst recovered and the nitrate poisoning effect no longer worked

    Combining Cu-SSZ-13 with TiO2: promotion of urea decomposition and influence on SCR

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    Urea is applied as an ammonia source for SCR in diesel vehicles, but incomplete urea decomposition may lead to insufficiency of NH3 and deposition of solid by-products. To improve the urea decomposition activity of Cu-SSZ-13 SCR catalysts, Cu-SSZ-13 was combined with TiO2 nanoparticles by a simple impregnation method. TiO2/Cu-SSZ-13 composites exhibited lowered decomposition temperatures of urea and related solid by-products. Thus, the composited catalysts showed higher NOx conversion and a lower amount of urea-related deposits than unmodified Cu-SSZ-13 in a simulated urea deposition scenario. The superiority of the urea decomposition activity of the composited catalysts was maintained upon 800 degrees C hydrothermal aging due to stabilized anatase over the catalysts according to XRD and Raman spectroscopy. H-2-TPR, UV-vis, and STEM revealed the formation of Cu-Ti-O species due to Cu-Ti interactions, which was adverse to SCR activity, but no extra hydrothermal aging effect was observed on the composited catalysts compared with unmodified Cu-SSZ-13. This work showed the potential to improve the low-temperature urea-SCR performance of Cu-SSZ-13 via TiO2 modification

    Protection Effect of Ammonia on CeNbTi NH<sub>3</sub>-SCR Catalyst from SO<sub>2</sub> Poisoning

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    CeNbTi catalyst was poisoned in different sulfur poisoning atmospheres at 300 °C for 6 h and then was evaluated for selective catalytic reduction (SCR) of NOx with NH3. The catalyst deactivation upon SO2 exposure was effectively inhibited in the presence of NH3. Temperature-programmed decomposition (TPD) analyses were applied to identify deposit species on the poisoned catalysts by comparison with several groups of reference samples. Diffuses reflectance infrared Fourier transform spectroscopy (DRIFTS) over CeNbTi catalysts with different poisoning pretreatments and gas purging sequences were designed to investigate the roles of NH3 in the removal of surface sulfites and sulfates. More ammonium sulfates including ammonium bisulfate and ammonium cerium sulfate were generated instead of inert cerium sulfate in these conditions. The mechanisms about the formation and transformation of surface deposits upon sulfur poisoning w/wo NH3 were explored, which provided a basis for developing Ce-based mixed oxides as SCR catalysts for stationary sources
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