吡啶氮或吡咯氮:氮掺杂碳催化剂在电催化还原CO_(2)中活性中心研究

随着化石燃料的过度燃烧以及污染的持续加剧,诸如海水酸化和气候变暖等问题逐渐显现,因此人们开始日益关注大气中CO_(2)的浓度.近些年,由于碳基材料具有成本低、化学和机械稳定性良好、电导率高和有效抑制析氢反应等优点而被广泛用于CO_(2)还原反应(CO_(2)RR).杂原子(B, P和N)掺杂可以有效地改变碳基材料电子结构并提高CO_(2)RR的选择性和活性.其中,由于氮原子具有独特的优势而被作为碳基材料的掺杂元素.在无金属氮掺杂碳材料中,吡啶氮被广泛地认为是催化活性中心,而吡咯氮则被认为不具催化活性,该观点仍存在争议.本文首先通过密度泛函理论计算证明了吡咯氮的本征活性足以有效催化CO_(2)RR,但由于与相邻吡啶氮位点的相互作用,吡咯氮的活性被显著抑制.此外,进一步通过实验制备了不含吡啶氮的无金属氮掺杂碳球(NCS)作为CO_(2)RR电催化剂.NCS中的吡咯氮与CO_(2)RR的性能呈直接正相关,表明其可以成为活性中心.性能最佳的NCS可在CO_(2)RR中产生范围较宽的合成气(CO/H_(2)(0.09-12)), CO选择性最高可达71%.在最优NCS中加入少量吡啶氮显著降低了CO_(2)RR的活性,进一步验证了相邻吡啶氮位点对吡咯氮活性的抑制作用.综上,本工作揭示了各种氮物种之间的交互作用,以及吡咯氮可以作CO_(2)RR活性位点的潜力;进一步提高了我们对电催化作用机制的理解,有助于高效电催化剂的设计和合成.

An Experimental Study on Fuel Combustion Under External Irradiance

In order to investigate the burning characteristics of a 0#diesel and 3#jet fuel,a small-scale experimental bench mainly composed of a cone calorimeter was arranged.The heat release and burning rates were investigated changing the external irradiance in order to clarify the triadic relationship among these quantities.The effective heat of combustion of 0#diesel and 3#jet fuel were 38.89 MJ/kg and 43.15 MJ/kg,respectively,with the corresponding combustion efficiencies being 96.78%and 99.60%(the effective peak heat of combustion being 1.665 times the mean value for both types of fuel).According to our experimental results,the heat release rate and burning rate of 0#diesel and 3#jet fuel both grow linearly with an increase in the external irradiance.Moreover,the heat release rate and burning rate of 3#jet fuel are greater than the equivalent values for the 0#diesel.The average smoke production rate of 0#diesel grows with an increase in the external irradiance,while for the 3#jet fuel it remains approximately the same.The specific extinction area of these two kinds of fuel shows a subtle decrease with the increase of external irradiance.The time to ignition of 3#jet fuel is smaller than 0#diesel for all the considered external irradiances,which indicates that 3#jet fuel is easier to ignite than the 0#diesel.

Large Eddy Simulation of Gasoline-Air Mixture Explosion in Long Duct with Branch Structure

Gas explosion is a process involving complex hydrodynamics and chemical reactions.In order to investigate the interaction between the flame behavior and the dynamic overpressure resulting from the explosion of a premixed gasoline-air mixture in a confined space,a large eddy simulation(LES)strategy coupled with sub-grid combustion model has been implemented.The considered confined space consists of a long duct and four branches symmetrically distributed on both sides of the long duct.Comparisons between the simulated and experimental results have been considered with regard to the flame structure,flame speed and overpressure characteristics.It is shown that the explosion process can qualitatively be reproduced by the numerical simulation.Due to the branch structure,vortices are generated near the joint of the branch and long duct.Vortices rotate in opposite directions in the different branches.When the flame propagates into the branch,the flame front is influenced by the flow field structure and becomes more and more distorted.The overpressure displays a similar behavior in the two branches which have a different distance from the ignition point.It is finally shown that the overpressure change law can directly be put in relation with the shape of flame front.

Electrocatalytic Hydrogenation and Oxidation in Aqueous Conditions

Water molecule contains one oxygen and two hydrogen atoms,making it a potential oxygen and hydrogen source.Electrocatalytic organic reduction and oxidation using water as oxygen and/or hydrogen donors provide an environmentally friendly and sustainable strategy to replace traditional chemical-driven stoichiometric reactions that use sacrificial reagents.Furthermore,the development of electrochemical synthesis provides a potential application for low tension photoelectricity,which is not cost-effective during boosted voltage and application.In the last decade,electrocatalytic redox reactions of organic molecules in aqueous media had shown progress owing to the development of electrode materials and water-splitting technology.This paper highlights several electrocatalytic systems and corresponding mechanisms for both hydrogenation and oxidative transformation of representative compounds.The activation process of protons and water on the working electrode surface has received special focus.Furthermore,paired electrolysis using water as the oxygen and hydrogen source has been demonstrated.This paired system combines hydrogenation and oxidation half-reactions in one cell using water as the hydrogen and oxygen source,resulting in high atomic and electron utilization rates.

Promotion of the oxygen evolution performance of Ni–Fe layered hydroxides via the introduction of a proton-transfer mediator anion

Developing efficient catalysts with high durability and activity for the oxygen evolution reaction(OER)is imperative for sustainable energy conversion technologies,including hydrogen generation and CO_(2) reduction,as well as other electrochemical energy storage systems.To this end,a comprehensive understanding of the mechanism for the water oxidation reaction is vital.Herein,a surfactant,nonafluoro-1-butanesulfonate(FBS),was introduced into Ni-Fe layered double hydroxide(Ni Fe-FBS/CFP)via electrochemical deposition on the surface of a carbon fiber paper(CFP)substrate.The as-prepared Ni Fe-FBS/CFP electrode exhibited excellent catalytic activities for OER compared to the Ni-Fe layered double hydroxide based electrode(Ni Fe-LDH/CFP),an excellent stability of 15 h,and an ultralow Tafel slope of 25.8 m V dec-1.Furthermore,by combining the results of p H-dependent kinetics investigations,chemical probing,proton inventory studies,and isotopic and atom-protontransfer measurements,it was observed that a proton-transfer process controls the reaction rates of both the Ni Fe-LDH and Ni Fe-FBS catalysts,and the residual sulfonate groups serve as proton transfer mediator to accelerate the proton transfer rate.