氧官能团MXenes用于电催化合成氨的理论筛选

电催化合成氨技术以绿色可再生的电能为驱动力,通过在室温条件下改变外加电压来克服速控步能垒,被认为是一种取代哈伯工艺的潜在选择.然而,该技术存在法拉第效率较低、氨气产率不高等问题.因此,设计高效的电合成氨催化剂是目前亟待解决的关键问题.氧官能团的二维过渡金属碳化物和氮化物(MXenes)由于具有独特的几何结构、高导电性和表面易调变等特点,在全水解、碳转化、氧还原或固氮等电催化过程中应用十分广泛.其中,表面氧官能团不饱和覆盖的MXenes材料的电催化合成氨性能较好,这是因为适量的氧空位能够调节活性中心的电荷分布,从而优化关键中间体的结合强度;同时,氧空位的存在为反应提供了足够的活性位点.然而,氧官能团MXenes家族庞大,种类众多,如何从中筛选出合适的合成氨电催化剂尚且缺乏理论指导.本文设计了一系列氧官能团的二维过渡金属碳化物和氮化物(MXenes)作为合成氨电催化剂,并通过密度泛函理论从稳定性、选择性和活性角度出发提出了一套较完善的筛选流程.以纯MXenes表面覆盖17/18氧官能团所需的极限电位来判断氧官能团MXenes的稳定性.通过对催化剂上氮气分子和氢原子的吸附行为进行比较来证明其选择性.通过比较潜在热力学决速步的吉布斯自由能(包括G_(N2-NNH),G_(NH-NH_(2))和G_(NH_(2)-NH3))快速获得反应所需的极限电位,可以快速筛选出最佳催化剂.在此基础上,通过吉布斯自由能计算,完整分析了Nb_(3)C_(2)O_(x)上电催化合成氨的反应机理并利用催化性能来验证筛选的准确性.计算结果表明,Nb_(3)C_(2)O_(x)催化剂上的热力学决速步为*NH_(2)加氢,吉布斯自由能仅上升了0.45 eV,与筛选结果一致.此外,结合套索算法,回归了氧官能团MXenes电催化合成氨性能Gx的表达式,探寻了催化活性来源.同时,由吸附能组成的描述符被证明是描述热力学决速步吉布斯自由能的重要变量.

单原子负载型MXene用于电催化合成氨过程的理论计算（英文）

合成氨在地球氮循环中扮演着重要角色.工业上传统的合成氨方法采用高温高压的反应条件,对反应设备要求高,并且导致了巨大的能耗.因此,以电力为驱动的电催化合成过程作为一种新型的合成氨方法引起了广泛关注.选择和设计合适的催化剂以降低所需的过电势是该过程的一个重要研究课题.常用的电催化剂包括金属基、金属氧化物、聚合物及其他复合性催化剂.其中,单原子催化剂因其极高的原子利用率而广受关注,但必须选择合适的基底使其成为兼具高催化活性和高稳定性的催化剂.二维过渡金属碳/氮化物(MXene)作为一种新型二维材料,拥有和石墨烯类似的电导性质,并与金属有良好的相互作用,是一种富有希望的载体.本文采用密度泛函理论研究了氮气在一系列MXene负载的过渡金属单原子催化剂上的吸附和活化,通过吉布斯自由能计算研究了电催化合成氨的反应路径,给出了相应的过电势.同时,通过研究可能的决速步骤的吉布斯自由能,分析了吉布斯自由能和过电势之间的关系.计算结果表明,在所有的MXene负载的过渡金属单原子上,氮气更倾向于一端吸附.根据吉布斯自由能的定义,负值显示这些催化剂具有良好的氮气活化性能,特别是铁基催化剂(–0.75 eV),这就不难理解工业上广泛应用铁基催化剂.而负载不同的过渡金属对电催化合成氨的过电势具有一定影响.通过吉布斯自由能计算发现,该系列金属的过电势在0.68–2.33 eV, Mo/Ti3C2O2需要的外加电压最少.这对实验上催化剂的选择具有一定的指导意义.同时,我们发现电催化合成氨过程有两个可能的决速步骤:氮气加氢生成NNH和NH2生成氨气.通过比较这两个步骤的吉布斯自由能可快速得到催化剂的过电势.因此,我们可以得出结论,该系列MXene负载的过渡金属单原子催化剂能够有效地改变反应路径,免出现传统反应中氮氮键断裂的巨大能垒,从而有效降低了反应的过电势.这为实验上选择合适的催化剂提供了理论依据.并且,这种通过直接比较决速步骤的吉布斯自由能得到过电势的方法对电催化合成氨以及其他类似反应的催化剂筛选和理性设计具有指导意义.

Building highly active hybrid double-atom sites in C_(2)N for enhanced electrocatalytic hydrogen peroxide synthesis

Two-electron(2 e^(-))oxygen reduction reaction(ORR)shows great promise for on-site electrochemical synthesis of hydrogen peroxide(H_(2)O_(2)).However,it is still a great challenge to design efficient electrocatalysts for H_(2)O_(2)synthesis.To address this issue,the logical design of the active site by controlling the geometric and electronic structures is urgently desired.Therefore,using density functional theory(DFT)computations,two kinds of hybrid double-atom supported on C_(2)N nanosheet(RuCu@C_(2)N and PdCu@C_(2)N)are screened out and their H_(2)O_(2)performances are predicted.PdCu@C_(2)N exhibits higher activity for H_(2)O_(2)synthesis with a lower overpotential of 0.12 V than RuCu@C_(2)N(0.59 V),Ru_(3)Cu(110)facet(0.60 V),and PdCu(110)facet(0.54 V).In aqueous phase,the adsorbed O_(2)is further stabilized with bulk H_(2)0 and the thermodynamic rate-determining step of 2 e^(-) ORR change.The activation barrier on PdCu@C_(2)N is 0.43 eV lower than the one on RuCu@C_(2)N with 0.68 eV.PdCu@C_(2)N is near the top of 2 e^(-) ORR volcano plot,and exhibits high selectivity of H_(2)O_(2.)This work provides guidelines for designing highly effective hybrid double-atom electrocatalysts(HDACs)for H_(2)O_(2)synthesis.

A first-principles study of reaction mechanism over carbon decorated oxygen-deficient TiO_(2) supported Pd catalyst in direct synthesis of H_(2)O_(2)

The choice of support is one of the most significant components in the direct synthesis of H_(2)O_(2).Aiming to improvement of activity and selectivity of H_(2)O_(2) on Pd/TiO_(2) surface,we systematically investigated the important elementary steps on Pd/TiO_(2)-Vo@C,Pd/TiO_(2)-Vo,Pd/TiO_(2)-2 Vo,Pd/TiO_(2),and Pd/C using the first-principles calculations.The Bader charge analysis and charge density difference of O_(2) adsorption elucidate the relationship between the electronic distribution and chemisorption energy.The effective barrier analysis further enables to quantitatively estimate the reactivity of H_(2)O_(2) and H2O.We demonstrate unambiguously that the selectivity of H2O formation is boosted as the oxygen vacancy concentration raised.Moreover,the introduction of C into a TiO_(2) with appropriate oxygen vacancies can slightly reduce the effective barrier for H_(2)O_(2) formation and increase the effective barrier for H2O formation leading to a higher activity and selectivity of H_(2)O_(2) formation.Our finding suggests that carbon-doped oxygen vacancy TiO_(2) supported Pd is potential alternative catalyst compared with the Pd/TiO_(2).

Pyrolysis of vulcanized styrene-butadiene rubber via ReaxFF molecular dynamics simulation

Styrene-butadiene rubber(SBR)is widely used in tires in the automotive segment and vulcanization using sulfur is a common process to enhance its mechanical properties.However,the addition of sulfur as the cross-linking agent usually results in impurities in pyrolysis products during rubber recycling,and thus the desulfurization during tire pyrolysis attracts much attention.In this work,the pyrolysis of vulcanized SBR is studied in detail with the help of Reax FF molecular dynamics simulation.A series of crosslinked SBR models were built with different sulfur contents and densities.The following Reax FF MD simulations were performed to show products distributions at different pyrolysis conditions.The simulation results show that sulfur products distribution is mainly controlled by sulfur contents and temperatures.The reaction mechanism is proposed based on the analysis of sulfur products conversion pathway,where most sulfur atoms are bonded with hydrocarbon radicals and the rest transfer to H_(2)S.High sulfur contents tend to the formation of elemental sulfur intermediate,and temperature increase facilitates the release of H_(2)S.

Trace water triggers high-efficiency photocatalytic hydrogen peroxide production

Photocatalytic production of hydrogen peroxide(H_(2)O_(2))has attracted much attentions as a promising method for sustainable solar fuel.Here,we demonstrate that trace water can drastically boost highefficiency photocatalytic production of H_(2)O_(2) with a record-high concentration of 113 mmol L^(-1) using alkali-assisted C_(3)N_(4) as photocatalyst in water/alcohol mixture solvents.By electron paramagnetic resonance(EPR)measurement,the radical species generated during the photocatalytic process of H_(2)O_(2) are identified.We propose alcohol is used to provide and stabilize-OOH radicals through hydrogen bond,while trace water could trigger photocatalytic production of H_(2)O_(2) via providing and transferring indispensable free protons to completely consume OOH radicals,which breaks the reaction balance of-OOH radical generation from alcohol.Thus-OOH radicals could be supplied by alcohol continuously to serve as a reservoir for high-efficiency production of H_(2)O_(2).These results pave the way towards photocatalytic method on semiconductor catalysts as an outstanding approach for production of hydrogen peroxide.

Curvature effects on electric-double-layer capacitance

Understanding the microscopic structure and thermodynamic properties of electrode/electrolyte interfaces is central to the rational design of electric-double-layer capacitors(EDLCs).Whereas practical applications often entail electrodes with complicated pore structures,theoretical studies are mostly restricted to EDLCs of simple geometry such as planar or slit pores ignoring the curvature effects of the electrode surface.Significant gaps exist regarding the EDLC performance and the interfacial structure.Herein the classical density functional theory(CDFT)is used to study the capacitance and interfacial behavior of spherical electric double layers within a coarse-grained model.The capacitive performance is associated with electrode curvature,surface potential,and electrolyte concentration and can be correlated with a regression-tree(RT)model.The combination of CDFT with machine-learning methods provides a promising quantitative framework useful for the computational screening of porous electrodes and novel electrolytes.

Remote Tracking Gas Molecular via the Standalone-Like Nanosensor-Based Tele-Monitoring System

Remote tracking the variation of air quality in an effective way will be highly helpful to decrease the health risk of human short-and long-term exposures to air pollution.However,high power consumption and poor sensing performance remain the concerned issues,thereby limiting the scale-up in deploying air quality tracking networks.Herein,we report a standalone-like smart device that can remotely track the variation of air pollutants in a power-saving way.Brevity,the created smart device demonstrated satisfactory selectivity(against six kinds of representative exhaust gases or air pollutants),desirable response magnitude(164–100 ppm),and acceptable response/recovery rate(52.0/50.5 s),as well as linear response relationship to NO2.After aging for 2 weeks,the created device exhibited relatively stable sensing performance more than 3 months.Moreover,a photoluminescence-enhanced light fidelity(Li-Fi)telecommunication technique is proposed and the Li-Fi communication distance is significantly extended.Conclusively,our reported standalone-like smart device would sever as a powerful sensing platform to construct high-performance and low-power consumption air quality wireless sensor networks and to prevent air pollutant-induced diseases via a more effective and low-cost approach.

A generalized formula for two-dimensional diffusion of CO in graphene nanoslits with different Pt loadings

Catalytic performance of supported metal catalysts not only depends on the reactivity of metal,but also the adsorption and diffusion properties of gas molecules which are usually affected by many factors,such as temperature,pressure,properties of metal clusters and substrates,etc.To explore the impact of each of these macroscopic factors,we simulated the movement of CO molecules confined in graphene nanoslits with or without supported Pt nanoparticles.The results of molecular dynamics simulations show that the diffusion of gas molecules is accelerated with high temperature,low pressure or low surface-atom number of supported metals.Notably,the supported metal nanoparticles greatly affect the gas diffusion due to the adsorption of gas molecules.Furthermore,to bridge a quantitative relationship between microscopic simulation and macroscopic properties,a generalized formula is derived from the simulation data to calculate the diffusion coefficient.This work helps to advise the diffusion modulation of gas molecules via structural design of catalysts and regulation of reaction conditions.

The Structural and Chemical Reactivity of Lattice Oxygens on β-PbO_(2 ) EOP Electrocatalysts

The oxygen evolution reaction(OER)and electrochemical ozone production(EOP)attracted considerable attention due to their wide applications in electrocatalysis,but the detailed reaction mechanism of product formation as well as the voltage effect on O_(2)/O_(3)formation still remains unclear.In this work,density functional theory calculations were used to systematically investigate the possible reaction mechanisms of OER and EOP on the PbO_(2)(110)surface,with the possible reaction network involving surface lattice oxygen atoms(LOM)proposed.The results show that the LOM-2 reaction pathway involving two surface lattice oxygen atoms(Olatt)and one oxygen atom from H_(2)O was the most thermodynamically reactive.Different potential determining step(PDS)was obtained depending on the multiple reaction pathway,and the results show that the facile diffusion of Olattwould proceed the LOM pathway and promote the formation of surface oxygen vacancies(O_(vac1)/O_(vac2)).Furthermore,O_(vac1)/O_(vac2)formation on the surface would trigger further reactions of H_(2)O adsorption and splitting,which refilled the oxygen vacancy and ensured the considerable stability of the PbO_(2)(110)surface.Multiple H_(2)O dissociation pathways were proposed on PbO_(2)(110)with oxygen vacancy sites:the acid-base interaction mechanism and the vacancy fulfilling mechanism.

Mechanical behavior and wrinkling patterns of phaseseparated binary polymer blend film

The wrinkling of phase-separated binary polymer blend film was studied through combining the Monte Carlo(MC)simulation for morphologies with the lattice spring model(LSM)for mechanical properties.The information of morphology and structure obtained by use of MC simulation is input to the LSM composed of a three-dimensional network of springs,which allows us to determine the wrinkling and the mechanical properties of polymer blend film,such as strain,stress,and Young’s modulus.The simulated results show that the wrinkling of phase-separated binary polymer blend film is related not only to the structure of morphology,but also to the disparity in elastic moduli between polymers of blend.Our simulation results provide fundamental insight into the relationship between morphology,wrinkling,and mechanical properties for phase-separated polymer blend films and can yield guidelines for formulating blends with the desired mechanical behavior.The wrinkling results also reveal that the stretching of the phase-separated film can form the micro-template,which has a wide application prospect.