Catalytic asymmetric cyanations of prochiral unsaturated compounds affording the corresponding nitrile products in high enantiomeric excess (≥90% in general) are summarized in this review. The ...nucleophilic cyanide addition onto aldehydes, ketones, and imines is promoted by chiral metal complexes and organocatalysts. Recent progress in asymmetric conjugate cyanation of α,β-unsaturated carbonyl compounds is also discussed. The asymmetric cyanation of unactivated alkenes is catalyzed by chiral transition-metal complexes. Current topics of intramolecular carbocyanation and aminocyanation in addition to the traditional hydrocyanation are reviewed.
Methanation of CO 2 on bulk Co-Fe catalysts Ischenko, Olena V.; Dyachenko, Alla G.; Saldan, Ivan ...
International journal of hydrogen energy,
2021, Letnik:
46, Številka:
76
Journal Article
Recenzirano
The efficiency of CO 2 methanation was estimated through gas chromatography in the presence of Co–Fe catalysts. Scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron ...spectroscopy, and Mössbauer spectroscopy were applied for ex-situ analysis of the catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy experiments were performed to identify gaseous species adsorbed at the catalyst surface. Based on the experimental results, surface reaction model of CO 2 methanation on Co–Fe catalysts was proposed to specify active ensemble of metallic atoms at the catalyst surface, orientation of adsorbed CO 2 molecule on the ensemble and detailed reaction mechanism of CO 2 →CH 4 conversion. The reaction step when OH group in the FeOOH complex recombined with the H atom adsorbed at the active ensemble to form H 2 O molecule was considered as the rate-limiting step.
Methanation of CO2 on bulk Co–Fe catalysts Ischenko, Olena V.; Dyachenko, Alla G.; Saldan, Ivan ...
International journal of hydrogen energy,
11/2021, Letnik:
46, Številka:
76
Journal Article
Recenzirano
The efficiency of CO2 methanation was estimated through gas chromatography in the presence of Co–Fe catalysts. Scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron ...spectroscopy, and Mössbauer spectroscopy were applied for ex-situ analysis of the catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy experiments were performed to identify gaseous species adsorbed at the catalyst surface. Based on the experimental results, surface reaction model of CO2 methanation on Co–Fe catalysts was proposed to specify active ensemble of metallic atoms at the catalyst surface, orientation of adsorbed CO2 molecule on the ensemble and detailed reaction mechanism of CO2→CH4 conversion. The reaction step when OH group in the FeOOH complex recombined with the H atom adsorbed at the active ensemble to form H2O molecule was considered as the rate-limiting step.
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•The efficiency of CO2 methanation was estimated in the presence of Co–Fe catalysts.•Dissociation of CO2 molecules into discrete C and O atoms was confirmed.•Fast production of CH4 and slow H2O formation was considered in Sabatier reaction.•OH group from the FeOOH complex recombine in H2O molecule using H atom at the surface.
Fenton catalysts composed of Fe3O4/CuO nanoparticles were synthesized using co-precipitation method for the purpose of decolorization of wastewater. A high dye removal efficiency of 94 % was achieved ...with 20 wt% Fe3O4/CuO composition at ambient conditions and a reaction time of 90 min. Catalytic activity analysis reveal that nanoparticles with 20 wt% Fe3O4/CuO composition outperform those with 10 wt% and 30 wt% nanoparticles toward removal of methylene blue (MB) dye due to their higher surface area and smaller nano-structural size. The synthesized catalysts were characterized using XRD, EDS, FE-SEM and BET techniques. The results confirm the formation of nano-sized particles with distinct pore size and surface area. The sample with 20 wt% Fe3O4/CuO showed higher surface area in comparison with other synthesized catalysts (e.g. hybrid 10 and 30 wt% Fe3O4/CuO, single Fe3O4 and CuO oxides). Additionally, the effects of the catalyst amount, solution pH, dosage of hydrogen peroxide (H2O2), and reaction time on the catalytic performance of 20 wt% Fe3O4/CuO were evaluated. The optimum conditions to achieve the highest color removal efficiency (i.e. 94 %) were obtained at pH = 7, 50 mL reaction solution containing 10 mg/L methylene blue, 16 mmol/L of hydrogen peroxide, 0.08 g of catalyst and the reaction time of 90 min. The synthesized catalyst shows great structural stability and reusability with a dye removal efficiency higher than 90 % after five successive reaction runs. The experimental results further highlight the efficient use of hybrid metal oxides as catalysts for the removal of organic compounds from industrial wastewaters.
Decolorization of wastewater by Fenton oxidation process was conducted using Fe3O4/CuO and MnO2-Fe3O4/CuO hybrid catalysts prepared via co-precipitation and impregnation methods, respectively. The ...chemical structure, surface area and porosity of hybrid nanoparticles were characterized using XRD, EDS, FE-SEM and BET techniques. The experimental results reveal that the introduction of MnO2 to Fe3O4/CuO catalyst (i.e. MnO2-Fe3O4/CuO) enhances the rate of decolorization reaction due to the larger catalyst surface area and narrower pore size distribution, in comparison with Fe3O4/CuO, as well as the direct oxidation of methylene blue via Mn4+ and Mn3+. In particular, MnO2-Fe3O4/CuO revealed higher methylene blue removal efficiency than Fe3O4/CuO during 45 min reaction time (90% vs. 79%). Reaction analyses show that both catalysts follow a second-order reaction kinetic where MnO2-Fe3O4/CuO exhibit a higher decolorization rate constant (118 vs. 57 L.mg−1min−1) toward removal of methylene blue (MB) dye. Additionally, the effect of reaction conditions including pH, dosage of hydrogen peroxide, and concentrations of catalyst and methylene blue on the catalytic performance of MnO2-Fe3O4/CuO were investigated. The synthesized hybrid nanostructured catalyst, MnO2-Fe3O4/CuO, revealed excellent color removal efficiency with high reusability after six runs (removal efficiency > 90%).
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•The introduction of Mn to Fe3O4/CuO catalyst (i.e. MnO2-Fe3O4/CuO) enhances the rate of decolorization reaction.•The degradation of methylene blue via hybrid catalysts follows a second-order reaction kinetic.•The reaction parameters such as H2O2 dosage, catalyst and dye concentrations influence the color removal efficiency.•MnO2-Fe3O4/CuO revealed excellent color removal efficiency (> 90%) with high reusability after six runs.
•An automotive thermoelectric generator (ATEG) is built and tested experimentally.•ATEG has been tested under four steady-state points and a WLTC driving cycle.•An electric heater is coupled to an ...ATEG to warm up the exhaust gases.•The system could reduce up to 97% the pollutant emissions of cold starts.
Low exhaust gas temperatures of internal combustion engines, resulting from cold-starts, low loads or low temperature ambient conditions, can compromise the performance of the automotive aftertreatment system. In this situation, the engine is running far from its design point, with a very low performance of the catalytic converter since it has not reached its optimal temperature. This effect can result in more than 20% of the total emissions for nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) during a driving cycle.
This study presents a new approach for vehicular thermoelectric generators to reduce NOx, HC and CO emissions and solve the cold-start issue. Authors propose the use of a thermoelectric generator coupled to an exhaust gas heater. This system, that operates autonomously, is designed to transform the waste heat into electric heat that can quickly raise the exhaust gas temperature to the optimal level.
Results show that an improvement up to 94% of the mean catalyst’s efficiency can be achieved. In addition, the system could reduce up to 94%, 91%, and 97% the pollutant emissions of NOx, HC and CO, respectively, in a WLTC driving cycle with a threshold exhaust gas temperature of 350 °C.
Nitrogen oxides (NOx) are highly reactive air pollutants that cause various environmental and health concerns. Selective catalytic reduction (SCR) has emerged as a vital tool for mitigating NOx ...emissions. This comprehensive review explores the advancements in SCR technologies and their critical role in NOx reduction by investigating various reducing agents. The performance of ammonia/urea, hydrocarbons, hydrogen, and carbon monoxide as reductants was investigated in various SCR systems. Ammonia, which is renowned for its efficiency in SCR, was examined for its ability to reduce NOx emissions. Urea-based SCR is a safe and low-toxicity alternative, particularly for mobile applications. Hydrocarbon-based SCR systems are versatile, enabling lower operating temperatures and improved efficiency. Hydrogen-based SCR technology is an environment-friendly option for NOx reduction, particularly in low-temperature scenarios. Carbon monoxide-SCR optimizes reduction processes, particularly in industrial sectors like steel and coking where CO abundance is beneficial for NO removal, addressing critical emission reduction needs. In-depth information on these reducing agents was examined, along with information on the catalyst components, mechanisms, and important factors that affect the NOx conversion efficiency. This study also highlights the need for research to address several challenges, future directions, and innovations in SCR technologies, emphasizing the importance of reducing environmental impacts and enhancing industrial processes.
Developing supported catalyst and elucidating the catalyst activation mechanism are of significant importance for large scale synthesis of single-walled carbon nanotubes (SWNTs). In the work, we ...designed an alumina-supported iron (Fe–Al2O3) catalyst for catalytic growth of carbon nanotubes by CO chemical vapor deposition (CVD). In the temperature range of 800–1000 °C, the prepared powder catalyst only affords the growth of carbon fibers or multi-walled carbon nanotubes. In contrast, when placing the powder catalyst on flat SiO2 substrates, such as SiO2/Si and quartz, SWNTs were observed to grow at the catalyst-substrate interface, highlighting the contributions of the SiO2 substrate in SWNT synthesis. Systematic characterizations on both the catalysts and carbon nanotubes revealed that the SiO2 substrate promotes the reduction of iron oxide in the adjacent Fe–Al2O3 catalyst, facilitating the generation of small Fe particles at the catalyst-substrate interface, which act as the active phase for the subsequent growth of SWNTs. This work opens a new avenue for growing SWNTs from supported catalyst and sheds light on enhancing SWNT growth efficiency by tuning catalyst reduction behaviors.
Pressing initially inactive powder Fe–Al2O3 catalyst onto a flat SiO2 substrate promotes the reduction of iron oxide at the catalyst-substrate interface, facilitating the generation of small Fe particles and the low temperature growth of SWNTs with a narrow diameter distribution. Display omitted
•The system presented in this study can reduce NOx emissions in a Diesel-powered Euro VI Heavy Duty truck working at low engine regimes by up to 80%.•The system is composed by an electric gas heater ...(EGH) coupled to an automotive thermoelectric generator (ATEG).•Simulations show that the EGH-ATEG system can work energetically independent of the vehicle, so there is no extra consumption of fuel.•This system can improve SCR efficiency up to 55% during low engine regimes.
Selective Catalytic Reduction systems (SCR) are very efficient on reducing NOx. However, they only perform properly when exhaust gas temperature is higher than 180 °C. This means that for low engine regimes combined with cold engine temperatures, SCR systems remains inactive.
This study presents a new approach to minimize the amount of NOx emitted by diesel engines of Heavy-Duty Vehicles during low engine regimes and low exhaust gases temperature conditions. We propose the addition of an Automotive Thermoelectric Generator (ATEG) coupled to an electric Exhaust Gas Heater (EGH) to make the SCR system inject the urea solution at low engine regimes. This EGH-ATEG system, which can be retrofitted in any existing vehicle, is designed to be energetically closed, so there is no extra consumption of fuel. Experimental results show that NOx emissions can be reduced up to 80% when an EGH is added to a standard diesel-powered Euro VI Heavy Duty truck configuration. Apart from that, the use of an ATEG installed downstream of the aftertreatment system can produce the energy required by the EGH, which means that the EGH-ATEG system can work energetically autonomous and independent from the vehicle’s electrical system. This system can improve SCR efficiency up to 55% during low engine regimes.
•Unsteady-state operation enhances catalytic processes.•Promoting methane oxidation by transient operation.•Utilisation of temporary states with high catalytic activity.
Total oxidation of methane ...over model monolith catalysts with platinum supported on alumina, alumina–ceria and ceria has been studied under unsteady-state operation of the feed gas stoichiometry. The general activity for methane oxidation follows the order Pt/alumina<Pt/alumina–ceria<Pt/ceria. Thanks to high catalytic activity at the gas composition switches, increased cycling frequency between oxygen excess and oxygen free conditions increases the average methane conversion significantly from 11% to 58% for Pt/alumina and from 25% to 87% for Pt/alumina–ceria. The corresponding stationary methane conversion is 10% and 19%, respectively. The underlying reason for the enhanced catalytic activity is likely twofold namely that periods with detrimentally high coverage of either oxygen or carbon are shortened and that the transients induce a highly active (chemical) state of the catalyst, thus, facilitating high average conversion of methane.
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