第一性原理研究荷电状态对Pd_(13)团簇催化分解NO性能的影响

文章运用了第一性原理计算方法研究一氧化氮在中性和带电荷背景的Pd_(13)团簇上的吸附和分解.研究结果表明:NO在中性和带电荷的Pd_(13)团簇上均倾向于吸附在Pd_(13)团簇的空位.带负电的Pd_(13)团簇比中性、带正电的Pd_(13)团簇对于NO的催化分解更有利,体现在带负电的Pd_(13)团簇上有更强的吸附能和更低的反应能垒.     

First-principles calculations of ammonia decomposition on Ni(110) surface

First-principles calculations based on density functional theory (DFT) have been performed to study the adsorption and decomposition of NH₃ on Ni(110). The adsorption sites, the adsorption energies, the transition states and the activation energies of the stepwise dehydrogenation of NH₃ and the associative desorption of N are determined, and the zero point energy correction is included, which makes it possible to compute the rate constants of the elementary steps in NH₃ decomposition. Combined DFT calculations and kinetic analysis at 350 K indicate that the associative desorption of N has a reaction rate lower than NH_x dehydrogenation and is therefore the rate determining step. The distinctly different rate constants over Ni(110), Ni(111) and Ni(211) imply that ammonia decomposition over Ni-based catalyst is a structure-sensitive reaction.     

Role of vacancy and metal doping on combustive oxidation of Zr/ZrO2 core-shell particles

We studied self-propagated combustion synthesis of transition-metal-doped tetragonal ZrO₂ (t-ZrO₂) with first principles-based one-dimensional diffusion reaction model. The optimal reaction condition for the combustion process was investigated by calculating energetic stability and surface reactivity of oxygen vacancy defects on (101) surface termination of t-ZrO₂ using first-principles density functional methods. In the first-principles model, the surface was doped with 14 different metal impurities in the 4th and 5th row of the periodic table to examine the role of transition-metal doping on the combustion process. Results indicate that there are clear trends in the defect stability and reactivity depending upon the type of metal impurity and their relative location with respect to the oxygen vacancy. Surface density of states and charge density information also show that there is a trade-off between the vacancy stability and chemical activity of the surface defect states. Based on the thermodynamic information obtained from first principles, we analyze the combustion process of a Zr metal particle by using a one-dimensional diffusion-reaction model. The competition between the vacancy-assisted chemisorption and the vacancy diffusion results in an optimal point for rate of combustion reaction with respect to the vacancy stability. From this, we suggest a plausible screening strategy for metal-doping which can be applied at different temperatures and pressures, as well as with different particle sizes. Our analysis indicates that first-principles calculation provides key information that can be subsequently used for an optimization of the reaction rate for a self-sustained combustion process. An explicit inclusion of rates of defect and ionic transport will be introduced into our model in future work.     

Density functional study of the decomposition pathways of SiH? and GeH? at the Si(100) and Ge(100) surfaces

By means of first-principles calculations we studied the decomposition pathways of SiH? on Ge(100) and of GeH? on Si(100), of interest for the growth of crystalline SiGe alloys and Si/Ge heterostructures by plasma-enhanced chemical vapor deposition. We also investigated H desorption via reaction of two adsorbed SiH?/GeH? species (β? reaction) or via Eley-Rideal abstraction of surface H atoms from the impinging SiH? and GeH? species. The calculated activation energies for the different processes suggest that the rate-limiting step for the growth of Si/Ge systems is still the β? reaction of two SiH? as in the growth of crystalline Si.     

First-principles calculations of K-shell x-ray absorption spectra for warm dense ammonia

The x-ray absorption spectroscopy is a powerful tool for the detection of thermodynamic conditions and atomic structures on warm dense matter. Here, we perform first-principles molecular dynamics and x-ray absorption spectrum calculations for warm dense ammonia, which is one of the major constituents of Uranus and Neptune. The nitrogen K-shell x-ray absorption spectrum(XAS) is determined along the Hugoniot curve, and it is found that the XAS is a good indicator of the prevailing thermodynamic conditions. The atomic structures at these conditions are ascertained. Results indicate that the ammonia could dissociate to NH_x(x = 0, 1, or 2) fragments and form nitrogen clusters, and the ratios of these products change with varying conditions. The contributions to the XAS from these products show quite different characteristics,inducing the significant change of XAS along the Hugoniot curve. Further model simulations imply that the distribution of the peak position of atomic XAS is the dominant factor affecting the total XAS.     

The UBI-QEP method: Mechanistic and kinetic studies of heterogeneous catalytic reactions

Applications of the unity bond index-quadratic exponential potential (UBI-QEP) method in mechanistic and kinetic studies of complex heterogeneous catalytic reactions are discussed. It is shown how UBI-QEP energetics helps to answer specific questions regarding the mechanisms of various reactions on metal surfaces. Examples of the following reactions are considered: elementary reactions of hydrogen transfer (from one carbon atom connected to the metal surface to another such atom in a different surface species), H-X bond breaking with the assistance of coadsorbed oxygen, methanol synthesis, Fischer-Tropsch synthesis, ammonia synthesis and decomposition, and partial methanol oxidation to formaldehyde. Then, examples of kinetic simulations using UBI-QEP energetics are considered for the following processes: ethane hydrogenolysis, watergas shift reaction, selective hydrogenation of acetylene in ethylene-rich mixtures, oxidative conversions of hydrogen and methane on Pt and Rh surfaces, ammonia decomposition, C₁–C₂ product formation in Fischer-Tropsch synthesis over cobalt, and steam reforming of methane. It is shown how the use of UBI-QEP and Monte Carlo methods makes it possible to calculate reaction parameters that depend on temperature in the equilibrium and kinetic regimes. To increase the accuracy of simulations, we added nonenergetic parameters affecting energetics at nonzero coverages: a spatial constraint on the distance between adsorbed atoms and the distance of hot atom traveling upon oxygen dissociation on metal surfaces. The UBI-QEP/Monte Carlo simulation method is illustrated by the study of molecular oxygen adsorption on single crystalline nickel surfaces. In most UBI-QEP-based mechanistic and kinetic studies of heterogeneous catalytic reactions, good agreement is observed with experimental observations, in many cases on a quantitative level. Taking into account diversity of reactions to which the method has been applied the agreement with experiments supports the efficiency of the method in solving mechanistic and kinetic problems.     

液态硝基甲烷热分解行为及压力效应的第一性原理研究

采用基于密度泛函理论的第一性原理分子动力学方法对液态硝基甲烷的热分解行为进行了模拟,结合各产物布居数随时间的演化,讨论了热分解初期可能发生的3种反应,即分子内/分子间的质子迁移反应和C—N键的断裂.在长时间（30 ps）的模拟过程中,H₂O是主要产物.研究了液态硝基甲烷在不同密度（压力）条件下热分解的动力学行为.发现不同密度（压力）条件下液态硝基甲烷热分解呈现明显不同的变化趋势,并给出了解释. 关键词： 硝基甲烷 分子动力学 热分解 压力效应     

Mg-TM-H (TM=Sc,Ti, V,Y, Zr,Nb)晶体形成能力和电子性能

基于密度泛函理论的第一性原理的Vienna Ab initio Simulation Package (VASP)软件系统研究了Mg₇TMH₁₆ (TM=Sc, Ti, V, Y, Zr, Nb)的晶体结构和电子性能. 分析了结合能和形成焓,计算得到的结合能显示Mg₇TiH₁₆和Mg₇NbH₁₆的稳定性最强,形成焓的变化表明Mg₇TMH₁₆具有比MgH₂更低的分解温度,电子态密度显示Mg₇TMH₁₆表现出明显的金属特性. 关键词： 第一原理 7TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb)')" href="#">Mg₇TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb) 结合能 形成焓     

The decomposition of ammonia on an oxygen-precovered Ni(110) surface studied by scanning tunneling microscopy

The decomposition of ammonia on a Ni(110) surface with preadsorbed oxygen has been investigated in ultra-high vacuum at room temperature using scanning tunneling microscopy (STM). We propose a reaction model in which the high reactivity observed at low O coverage is ascribed to a direct interaction between the NH₃ molecules and the terminating atoms of the short, mobile -Ni-O- added rows which are observed on the surface under these conditions. This model is consistent with the observation that the surface becomes inert at high O coverage. We believe that the present reaction model can also explain results from some other experiments in which preadsorbed oxygen has been found to act as a promoter for dissociation of H-containing species, such as for NH₃ on Cu(110) and H₂O on Ni(110).     

Modeling ammonia oxidation over a Pt (533) surface

We present a new reaction model for ammonia oxidation on a Pt (533) surface and perform numerical simulations using mean field equations. Kinetic parameters were taken from experiments and Density Functional Theory (DFT) calculations. The model is based on an oxygen-activated ammonia decomposition and includes NHx (x = 0, 1, 2) intermediates. Reaction rates and coverages obtained from calculations show semiquantitative agreement with values from kinetic and in-situ XPS measurements up to 0.1 mbar pressures. Pathways for ammonia oxidation were analyzed by varying kinetic parameters in the model, which provides new insights into the relative importance of different reaction steps.     

V2CN2负载过渡金属双原子电催化固氮的第一性原理研究

电催化剂固氮能够在温和条件下催化氮气还原制氨.本工作基于第一性原理计算,系统地探究了分散在二维V₂CN₂上过渡金属二聚体(Fe, Mo, Ru)形成的双原子催化剂用于电催化固氮的可行性.双原子的协同作用使氮气得到较好的活化.吉布斯自由能计算表明,在V₂CN₂负载双铁原子(Fe₂@V₂CN₂)体系进行的催化反应过电位最低,仅为0.25 eV.进一步的分子动力学计算表明Fe₂@V₂CN₂具有较好的结构稳定性,并且该体系能很好的抑制HER反应的发生.通过与Mo₂@V₂CN₂,Ru₂@V₂CN₂体系的比较可知,氮气分子的吸附构型以及HER反应的竞争对催化剂的选择影响很大.我们的计算能够为双原子催化剂的设计提供更多的依据.     

Predicting experimental yields as an index to rank synthesis routes II: application to the Curtius rearrangement

Synthesis yields of organic reactions are one of the most important factors in ranking synthesis routes created by synthesis route design systems such as Transform‐Oriented Synthesis Planning and Knowledge base‐Oriented Synthesis Planning. If it is possible to predict the yields of synthesis reactions before starting experiments, one can easily determine an order of synthesis routes for experimental works. In the present study, the reaction profiles of the Curtius rearrangement with different substituents were calculated to generate an equation predicting experimental yields of this reaction. Reactions followed by the formation of isocyanates were also analyzed to consider the relationship between reaction times and experimental yields. A partial least squares (PLS) regression was used to correlate the experimental yields with the calculated activation energies, E_a(calc), together with experimental conditions such as dielectric constants of solvents, reaction times, and reaction temperatures as explanatory variables. Although the PLS regression using all the data gave very poor results, we succeeded in making a model equation with R² = 0.887 using a modified data set. However, there is a conflict between the predictability and the interpretability on the reaction time. This discrepancy mainly comes from unnecessarily long reaction times in the experiments for azides with calculated E_a values of less than 33 kcal mol^–1. To construct a good model equation for the experimental yields of the Curtius reaction, we have to use data sets obtained from within 90 min of the reaction for the PLS regression. Copyright © 2011 John Wiley & Sons, Ltd.     

Excited-state forces within a first-principles Green's function formalism

We present a new first-principles formalism for calculating forces for optically excited electronic states using the interacting Green's function approach with the GW Bethe-Salpeter-equation method. This advance allows for efficient computation of gradients of the excited-state Born-Oppenheimer energy, allowing for the study of relaxation, molecular dynamics, and photoluminescence of excited states. The approach is tested on photoexcited carbon dioxide and ammonia molecules, and the calculations accurately describe the excitation energies and photoinduced structural deformations.     

The two-dimensional Peierls-Nabarro model for interfacial misfit dislocation networks of cubic lattice

A model is developed to investigate the two-dimensional interfacial misfit dislocation networks that follows the original Peierls-Nabarro idea. Structure and energies of heterophase interfaces are considered for the cubic lattice. To examine the energy contribution of misfit dislocations, where interactions between two dislocation arrays are concerned, a generalized stacking fault energy is proposed. Combined with first-principles calculations, we apply this model to a practical metal-ceramic example: the Ag/MgO(100) interface. An important correction to the adhesive energy is proposed in addition to its dislocation structure being confirmed.     

First-principles-based kinetic Monte Carlo simulation of nitric oxide decomposition over Pt and Rh surfaces under lean-burn conditions

First-principles-based kinetic Monte Carlo simulation was used to track the elementary surface transformations involved in the catalytic decomposition of NO over Pt(100) and Rh(100) surfaces under lean-burn operating conditions. Density functional theory (DFT) calculations were carried out to establish the structure and energetics for all reactants, intermediates and products over Pt(100) and Rh(100). Lateral interactions which arise from neighbouring adsorbates were calculated by examining changes in the binding energies as a function of coverage and different coadsorbed configurations. These data were fitted to a bond order conservation (BOC) model which is subsequently used to establish the effects of coverage within the simulation. The intrinsic activation barriers for all the elementary reaction steps in the proposed mechanism of NO reduction over Pt(100) were calculated by using DFT. These values are corrected for coverage effects by using the parametrized BOC model internally within the simulation. This enables a site-explicit kinetic Monte Carlo simulation that can follow the kinetics of NO decomposition over Pt(100) and Rh(100) in the presence of excess oxygen. The simulations are used here to model various experimental protocols including temperature programmed desorption as well as batch catalytic kinetics. The simulation results for the temperature programmed desorption and decomposition of NO over Pt(100) and Rh(100) under vacuum condition were found to be in very good agreement with experimental results. NO decomposition is strongly tied to the temporal number of sites that remain vacant. Experimental results show that Pt is active in the catalytic reaction of NO into N₂ and NO₂ under lean-burn conditions. The simulated reaction orders for NO and O₂ were found to be +0.9 and ?0.4 at 723?K, respectively. The simulation also indicates that there is no activity over Rh(100) since the surface becomes poisoned by oxygen.     

Bond-counting rule for carbon and its application to the roughness of diamond (001)

Despite that carbon is tetravalent identical to silicon, first-principles calculations reveal that stable step structures on diamond (001) are entirely different from those on silicon. Moreover, pristine Si(001) is flat; pristine diamond (001) could be rough due to negative step formation energies. A generic bond-counting rule is established, which should apply to most carbon structures where sp2 and sp3 hybrids coexist: e.g., it provides a qualitative account of the step energy order without detailed calculation. Our findings agree with experimental observations.     

Parabolic model of the concerted molecular decomposition of chloroalkanes

The intersecting parabolas model (IPM) is used to analyze the measured kinetic parameters for the concerted molecular decomposition of chloroalkanes RCl to olefin and HCl. According to this model, the configuration of the transition state is formed by three atoms: C…H…Cl. The activation energy E and the rate constant k for 12 previously unstudied reactions of concerted molecular decomposition of RCl are calculated based on the enthalpy of reaction by using the IPM algorithms. The factors that influence the activation energy E for RCl decomposition are established: the enthalpy of reaction, energy of stabilization of radical R^?, presence of a π bond adjacent to the reaction center, and dipole–dipole interaction for the decomposition of polychloroalkanes. The values of E and k for reverse reactions of addition of HCl to olefins are evaluated. The energy spectrum of partial activation energies for the concerted molecular decomposition of RCl is constructed.     

Chemisorption and reaction of NH3 on Ni(111)

The chemisorption of ammonia on Ni(111) has been investigated using LEED, thermal desorption, and angle-resolved photoemission. For exposures at 200 K, thermal desorption shows a coverage-dependent binding energy associated with dipole-dipole interactions. A (2 × 2) LEED pattern occurs at 2–4 L exposure. Time dependence of the LEED pattern and changes in the thermal desorption induced by the LEED beam indicate that the (2 × 2) pattern is due to a stable intermediate decomposition species. Using synchrotron radiation photoemission all three valence orbitals of ammonia have been observed for the first time. The energies of the ammonia-induced features in the photoemission (?22.0, ?11.0 and ?6.7 eV below the Fermi energy) and the observed symmatries positively identify the absorbed species as molecular ammonia. Additional structure observed in the photoemission spectra after electron bombardment is associated with the stable adsorbed intermediate.     

Diffraction effects in low-energy electron emission

The origin of diffraction peaks in the energy distribution of intensity of low-energy (< 1000 eV) electron emission from crystals is discussed from the standpoint of the dynamical theory of diffraction. The emitted electrons are considered to originate at relatively incoherent point sources in the crystal. The two-beam approximation of dynamical theory is used. The theory accounts for the chief regularities of diffraction peaks: temperature-dependence of peak intensities like that for low-energy electron diffraction (LEED) peaks, correlation of peak energies with X-ray absorption fine structure, and correlation of peak energies with the energies of normal-incidence LEED peaks in specular reflection. It is shown that the conditions for diffraction peaks coincide with the conditions for emergence of Kikuchi lines. It is predicted that for energies just above those of diffraction peaks, such emergences should be observable in the angular distribution of emission as intensity minima for emission along low-index crystal axes. Theory of Kikuchi band profiles is developed in an Appendix.     

The reaction kinetic description of processes in an alkaline metal solution in liquid ammonia

In this work the scheme of the decomposition of alkaline metals solutions in liquid ammonia is proposed. On the basis of this scheme and by the means of fundamental laws of reaction kinetics (Guldberg-Waage's law and the principle of superposition for partial reaction velocities) a system of differential equations describing the course of decomposition of the mentioned solutions is set up and solved. By means of the found solution and on the basis of experimental data the rate constant characterizing the dissociation of liquid ammonia is estimated.