Au_7团簇吸附O_2和CO及其对CO的催化反应机理研究

本文采用密度泛函理论,研究了Au_7团簇催化CO的氧化反应机理.研究发现,二维平面结构的Au_7团簇更容易吸附CO和O_2分子. Au_7团簇吸附一个O_2分子的吸附能为0.64 eV,但在吸附多个O_2分子时,平均吸附能有了明显的下降,表明Au_7团簇进行多吸附O_2分子的可能性不大. Au_7团簇吸附一个CO分子的吸附能为1.26 eV,且在吸附多个CO分子时,平均吸附能虽有减少,但减小的幅度不大,说明Au_7团簇有可能吸附多个CO分子.此外,在Au_7团簇催化CO的氧化反应过程中,整个反应克服的最高势垒仅为0.34 eV,说明Au_7团簇有望成为良好的CO氧化催化剂.     

Fe、Co、Ni混合团簇磁性的密度泛函研究

基于13原子二十面体结构,采用密度泛函方法系统计算研究了Fe、Co及Ni单质及二元混合团簇的磁性.发现有限温度下团簇磁性随结构畸变的敏感性随Fe、Co、Ni顺序逐渐减弱,同时发现二十面体结构Fe_(13)及Co_(13)均具有不同磁矩的近简并低能态.对FeNi及CoNi混合团簇、其磁矩随组分的变化不存在反常现象,但对于FeCo混合团簇、其磁矩随组分的演化行为存在个别反常现象.我们认为:这种反常现象能够对FeCo非晶合金中的实验观测结果提供一种可能的理论解释.     

Wn (n= 1–6)团簇吸附CO的密度泛函研究

采用密度泛函理论(density functional theory, DFT) 在B3LYP/LANL2DZ基组水平上对钨团簇吸附CO分子进行了系统研究. 结果表明, W_nCO团簇的基态结构是在W_n团簇中性或阴离子基态结构的基础上吸附CO生长而成; CO的吸附以端位吸附为主,桥位吸附为辅; CO分子在W_n团簇表面发生的是非解离性吸附. 与优化的CO键长(0.116 nm)相比,吸附后C-O键长变长(0.120–0.123 nm), 表明吸附后C-O键被削弱, CO分子被活化了.稳定性分析表明,在所研究的团簇中, W₃CO和W₅CO团簇的稳定性较强;自然键轨道(NBO)分析表明, W原子与CO分子相互作用的本质是CO分子内的杂化轨道与W原子6s, 5d, 6p和6d轨道相互作用的结果. 关键词： nCO (n= 1–6)团簇')" href="#">W_nCO (n= 1–6)团簇 基态构型 稳定性 电子性质     

铁铜双核羰基簇合物结构的理论研究

采用密度泛函理论BPW91方法在6-311+G(d)基组水平上, 对FeCu团簇吸附CO过程中可能的几何结构和电子态进行了系统研究. 计算结果表明: FeCu双原子团簇饱和吸附CO分子数是7, 配位过程均为放热反应, 吸附能最大的结构是FeCu(CO)1. 金属原子满足18电子规则,对CO的吸附位置起主要决定作用, 即CO配位由于遵循18电子规律, 即Fe原子上形成Fe(CO)4Cu 之后CO与Cu原子发生配位形成Fe(CO)4Cu(CO)3. 各配合物中 Fe电荷密度最大的结构是Fe(CO)4Cu(CO)1, Cu电荷密度最大的结构Fe(CO)4Cu(CO)3.     

Yn(n=2-8)团簇吸附CO2的密度泛函理论研究

运用密度泛函理论, 研究了CO2在Yn (n=2-8) 团簇表面的吸附结构和电子性质。结果表明：CO2吸附于 Yn (n=2-8)团簇表面时,线型的分子结构畸变为三角形构型。YnCO2表现出了较大的吸附能 (大于3eV)。吸附使体系的能隙表现出了奇偶振荡效应。吸附后,C-O键伸长,C原子由电子施体变为受体,O原子所带电子数也显著增加。在所有尺寸中,Y4CO2 、Y6CO2稳定性最好。     

Yn(n=2-8)团簇吸附CO2的密度泛函理论研究

运用密度泛函理论, 研究了CO2在Yn (n=2-8) 团簇表面的吸附结构和电子性质。结果表明：CO2吸附于 Yn (n=2-8)团簇表面时,线型的分子结构畸变为三角形构型。YnCO2表现出了较大的吸附能 (大于3eV)。吸附使体系的能隙表现出了奇偶振荡效应。吸附后,C-O键伸长,C原子由电子施体变为受体,O原子所带电子数也显著增加。在所有尺寸中,Y4CO2 、Y6CO2稳定性最好。     

密度泛函理论研究CO与Nin（n=1—6）团簇的相互作用

采用密度泛函理论对CO吸附在镍团簇表面进行了系统研究.结果表明,Ni_nCO团簇的最低能量结构是在Ni_n团簇最低能量结构的基础上吸附CO生长而成,CO的吸附没有改变Ni_n团簇的结构;CO分子在Ni_n团簇表面发生的是非解离性吸附,与优化的CO键长（0.1138?nm）相比,吸附后C—O键长变长（0.1180—0.1214?nm）,表明吸附后C—O键被削弱,CO分子被活化.自然键轨道分析表明,CO分子只与最近邻的Ni原子发生相互作用;CO分子与Ni原子相互作用的本质是CO分子内的杂化轨道与Ni原子3d, 4s, 4p轨道相互作用的结果. 关键词： nCO团簇')" href="#">Ni_nCO团簇 n团簇')" href="#">Ni_n团簇 平衡结构 电子性质     

CO2在中性及带负电Be（n=4, 7, 10, 12, m=0, -1）团簇表面的吸附与解离

运用密度泛函理论,对中性及带负电的BeCO2 (n=4, 7, 10, 12, m=0, -1) 团簇进行了构型优化,稳定性和电子性质分析。结果表明：CO2吸附于Ben和Ben-1团簇表面时,C-O键长均有不同程度的伸长。其中Be4CO2-1,Be12CO2-1中CO2分子的一个C-O键自然断裂(伸长量达到了134%和156%),为典型的解离性吸附。Be团簇表现出了较好的吸附CO2分子的能力,特别是带负电以后,吸附能明显增大(约为3.16eV--5.965eV)。电子性质分析表明,带负电升高了相应团簇的前线轨道能级,使轨道杂化发生在费米能级处附近,从而增强了CO2分子与相应团簇的成键能力。     

Y_n(n=2-8)团簇吸附CO_2的密度泛函理论研究

运用密度泛函理论,研究了CO_2在Y_n(n=2-8)团簇表面的吸附结构和电子性质.结果表明:CO2吸附于Y_n(n=2-8)团簇表面时,线型的分子结构畸变为三角形构型.Y_nCO_2表现出了较大的吸附能(大于3eV).吸附使体系的能隙表现出了奇偶振荡效应.吸附后,C-O键伸长,C原子由电子施体变为受体,O原子所带电子数也显著增加.在所有尺寸中,Y_4CO_2、Y_6CO_2稳定性最好.     

Sc修饰Si@Al_(12)团簇吸附CO的第一性原理研究

本文采用密度泛函理论的广义梯度近似,研究了Sc原子修饰的Si@Al_(12)团簇与CO分子之间的相互作用.结果显示:Sc原子倾向于以穴位的形式吸附于Si@Al_(12)团簇表面;Sc周围最多可以吸附7个完整CO分子,CO的平均吸附能处于0.990~1.602 eV之间;Sc Si@Al_(12)·7CO团簇中CO质量分数可达33.07%,有望作为CO气体过滤材料.     

A theoretical study of CO adsorption on FeCo(100) and the effect of alloying

FeCo catalysts are modeled for optimizing the Fischer–Tropsch synthesis process since they can be tuned to enhance CO activity and resist poisoning. The electronic properties associated with CO adsorption are studied using plane-wave density functional theory (DFT). The range of computed adsorption energies from this work falls between the CO adsorption energies on pure Fe and Co surfaces. It was found that CO prefers to adsorb on the top site of the Co surface of FeCo alloys, whereas CO has stronger adsorption on pure Fe rather than pure Co surface. The trend in adsorption energy is top-Co > hollow-Fe > top-Fe > hollow-Co > bridge-Co > bridge-Fe. This change in preferable metal for adsorption (i.e., from Fe in a pure system to Co in the FeCo alloy surface in the current investigation) is due to the shift in the d-band center of the alloyed material. It implies that alloying Fe with Co changes the properties of the pure metal and ultimately affects the CO adsorption energy; however, the mechanism of adsorption remains similar and can be explained using the Nilsson–Pettersson model. Additional CO configurations consisting of hollow-site adsorption with a tilted geometry, was also investigated. The corresponding adsorption energy was found to be slightly higher than the adsorption energy when CO is adsorbed on the top-Co site.     

Structural evolution of FexCo1−x/NM (NM=Ag, Cu, Al2O3) discontinuous multilayers by XAS

We discuss the specific features of the K-edge X-ray absorption spectroscopy (XAS) at the Fe and Co K-edges in Fe_xCo_1−x/NM discontinuous multilayers with NM=Ag, Cu, and Al₂O₃. Thermal treatments lead to the breakup of the layers forming a heterogeneous alloy with magnetic clusters of the FeCo embedded in a nonmagnetic (NM) matrix. Stabilization of the BCC phase of the FeCo alloy is directly evidenced by XAS. Differences among Fe and Co K-edge signatures indicate a strong dependence on the chemical nature of the absorbing atom for annealed samples. The amplitude of the extended X-ray absorption fine structure signal higher at the Co K-edge than at the Fe one reflect a smaller structural disorder in the Co environment. This behavior depends neither on the composition of the alloy nor on the NM material. In the case of Al₂O₃ a higher disorder around Fe atoms is observed also in the as-deposited sample. All these results indicate that the Fe segregates at the interface of the magnetic clusters/NM matrix after thermal annealing.     

Phase characterization in the Fe−Co−V and Fe−Co−Nb systems

Two distinct phases of the Fe?Co?Nb and three of the Fe?Co?V systems have been studied by means of X-ray diffraction and Mössbauer spectroscopy. The dependence of the lattice parameter of the alpha phase on the nominal solute content in equiatomic FeCo together with the alterations of the Mössbauer spectra have shown a very limited solubility of niobium in alpha FeCo. There are indications that the solubility of vanadium in alpha FeCo increases with increasing nominal content of solute in the alloy. A mechanism involving the withdrawal of cobalt from the alpha matrix to form a phase rich in Co and V (gamma) is proposed to explain such a varying solubility. The vanadium content of the sigma phase in equiatomic FeCo alloys with 22 wt% V is proposed to be less than 50 at%.     

Spin polarization and charge transfer of Co nanoclusters coated with CO molecules

We have investigated magnetic properties of Co clusters coated with CO molecules by first-principles density functional calculations. Total magnetic moment of the system decreases with the increase of CO molecule concentration regardless of adsorption sites for CO molecules. Spin polarization slightly increases as CO molecules are adsorbed on the most stable sites of Co cluster, which is caused by the increased local spin polarization of p-electrons due to charge transfer from Co clusters to CO molecules.     

FeCo二元合金纳米线阵列的磁化反转

利用交流电化学沉积方法在氧化铝模板中制备了一维结构的Fe_xCo_1-x(0 ≤ x ≤ 0.51)二元合金纳米线阵列.X射线衍射结果显示,单质Co纳米线为(100)择优取向的hcp结构,FeCo合金纳米线则呈现(110)择优取向的bcc结构,而且衍射峰随纳米线中Fe含量的增加向低角度偏移.室温磁性测量结果显示, FeCo合金纳米线具有较好的磁特性.与Co纳米线相比,Fe的引入改善了Co纳米线的磁性能,使其呈现出较大的矫顽力和较高的矩形比.采用一致转动模型和对称扇形机理的球链模型分别计算了FeCo合金纳米线的矫顽力, 发现其磁化反转机理与对称扇形机理的球链模型相符合.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Effect of phosphorus and copper additions on the structure of Pt and Pt–Cu nanoparticles in a radiation-induced reduction method

A facile chemical process has been developed for the preparation of magnetic FeCo nanoparticles. The FeCo nanoparticles were mono-dispersed, obtained by the safe and ecofriendly method, possessed saturation magnetization up to 187 emu/g, and demonstrated excellent chemical stability. In this work, we have studied how to control Fe/Co ratio by variation of precursor ratio, and how to vary particle size from 9.3 to 12.3 nm by surfactant amount used. The cytotoxicity of as-synthesized nanoparticles was investigated after coating with the poly(methyl methacrylate-co-butyl acrylate) by the emulsion process and the results demonstrated high biocompatibility. Similarly, the same synthesis method was used with the single precursor FeO(OH) or Co₃O₄. The results showed that this method can also fabricate 10 nm mono-dispersed spherical Fe₃O₄ particles and self-assembly Co nanoneedles.     

Structural and magnetic characterization of soft-magnetic FeCo alloy nanoparticles

Soft-magnetic FeCo alloy nanoparticles with diameters less than 100 nm are prepared by ball milling. X-ray photoemission spectroscopy (XPS) and X-ray magnetic circular dichroism (XMCD) are used to characterize these particles. While the XPS spectrum from the as-prepared sample clearly shows Co photoemission peaks, no sign of Fe is observed in the same spectrum. However, Fe photoemission peaks appear after 1 h of Ar ion sputtering. A quantitative analysis of the XPS spectra shows an increase of Fe concentration versus sputtering time until the Fe:Co ratio of the bulk alloy is reached. In addition, the narrow scan Fe and Co 2p XPS spectra show that Co is more oxidized than Fe. All these measurements indicate that the nanoparticles have a Co shell and an Fe-rich core. They further demonstrate the usefulness of XPS combined with depth-profiling via sputtering to obtain element- and chemically-sensitive structural information on nanoparticles. XMCD as an element-specific magnetic analysis tool further reveals that Fe and Co are ferromagnetically coupled in these particles. The information obtained is useful for establishing a structure–property relation for the studied material that is expected to have applications as a soft magnetic material at high temperatures.