Electron energy loss spectroscopy of the decomposition of formic acid on Ru(001)

Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO₂ and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H₂,H₂O, CO and CO₂ as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.     

NEXAFS study of HCOO/Ag(110): Evidence for dynamic bending

The structure of the surface formate on Ag(110) at 300 K was studied by near-edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. Interpretation of the NEXAFS spectra of the formate intermediate with a localized bond picture is inconsistent with the findings of previous vibrational studies. Rather, the resonances must be assigned to transitions to delocalized molecular orbitals, whereupon the results agree with the bidentate configuration deduced from the vibrational experiments. A bonding geometry is determined with respect to the surface normal in which the O-O direction lies parallel to the plane of the surface; no azimuthal ordering was detected. The molecular plane appears to be inclined at an angle of 30 ± 15°. This apparent tilt may be due to a dynamical motion of the species about the surface normal in which the molecular plane oscillates and rotates about the surface normal. These results indicate different bonding geometries for formate on Ag(110) and Cu(110).     

Ru(0001)表面氮分子和钡原子的相互作用

本文采用密度泛函理论方法研究了Ru(0001)表面氮分子和钡原子的相互作用。计算结果表明,钡原子的作用弱化了氮分子键。氮分子键长从Ru(001)-N2表面的0.113 nm伸长至Ru(001)-N2/Ba表面的0.120 nm;分子的拉伸振动频率从2221 cm-1减小到1746 cm-1;氮分子得到的电荷数从清洁表面的0.3 e增加到1.1 e。电荷从钡原子6s轨道向钌原子4d轨道转移,转移电荷增强了氮分子 空轨道和钌原子4d轨道间的杂化作用,导致 分子轨道和 杂化轨道发生极化。轨道极化使分子电偶极矩增加了约-0.136 eÅ。金属钡在Ru(0001)表面氮分子活化过程中具备电子型助催剂的特征。     

Oxidation of a potassium monolayer on Ru(001) studied with photoemission,NEXAFS and vibrational EELS

The interaction of oxygen with a monolayer of potassium on a Ru(001) surface has been investigated with photoemission and NEXAFS. O 1s core-level data exhibit for low exposures of oxygen a single peak at 531.8 eV. This indicates together with the earlier observation of KO bond formation by EELS that a single K_xO_y species with equivalent oxygen atoms is formed. NEXAFS data indicate a partially filled O π* orbital and a strongly shifted σ-resonance. Photoemission, vibrational and Auger data suggest an ionic species which is close to potassium Superoxide with the OO bond oriented parallel to the surface.     

The short range of the electronic promoter effect of potassium

Photoemission of Adsorbed Xenon atoms (PAX) as a local work function probe is used to investigate the range of the electronic promoter effect of potassium submonolayers on a Ru(001) surface. Three Xe states on these bimetallic K/Ru surfaces are clearly distinguishable by their 5p photoemission and are associated with Xe probe atoms at basically unmodified Ru sites, at “mixed” K.Ru sites next to K ions, and on top of potassium, respectively. From the relative intensities of these three states as well as from their 5p electron binding energies as a function of potassium coverage it is concluded that the radius of the “sphere” of modified charge density around one K ion is ～ 6 Å.     

Structure transformation of Ir clusters on Ir surfaces

A small Ir cluster can assume either a one-dimensional linear-chain structure or a two-dimensional island-like structure. We present a study of the energetics of the 1 D to 2 D structure transformation of three-atom Ir clusters on the Ir(111) and (001) surfaces. On the (111) plane, the temperature dependence of the ratio of the probabilities of observing a three-atom cluster in the 1 D and 2 D structures exhibits a simple linear Arrhenius behavior. The 2 D island structure is found to be more stable with the cluster binding energy lower by 0.098±0.004 eV. On the (001) plane, the 1 D chain structure is more stable with the cluster binding energy lower by 0.335±0.015 eV. From these energies, the relative pair interaction at three different bond lengths can be derived. The relative pair potential is found to be non-monotonic in distance dependence. We explain the (1×5) reconstruction of the Ir(001) surface as being caused by the large difference in the pair binding energy of the first and second nearest-neighbor bonds. In addition, we find a significant deviation from the simple linear Arrhenius behavior at low temperatures for the three-atom Ir cluster on the Ir (001) plane, indicating that the entropy factor is temperature dependent.     

On the perfection of cleavage planes of potassium bichromate single crystals

The relative perfection of the 001, 100 and 100 cleavage planes of potassium bichromate single crystals is analysed by calculations of the surface energy of main crystallographic planes. The surface energy was determined for the three cleavage planes as well as for planes corresponding to other theoretically possible orientations. It was found that: (1) among different planes the calculated values of the surface energy of the 001, 100 and 010 cleavage planes of potassium bichromate are the lowest, (2) among the three cleavage planes the 001 perfect cleavage plane has the lowest surface energy, (3) the relative perfection of different cleavage planes can be explained by using the coefficient δ proportional to the ratio of the surface energy of the hkl cleavage plane to the energy of dislocations emerging on the plane, and (4) the lower the values of the coefficient δ for a cleavage plane, the better is the cleavage perfection.     

N_2在Co掺杂Ru(001)表面吸附的DFT研究

采用密度泛函理论与周期性平板模型相结合的方法,对N_2在Ru(001)表面top、fcc、hcp、bridge四个吸附位和Ru-Co(001)表面Ru-top、Co-top、Ru(Ru)Ru-bridge、Co(Co)Co-bridge、Ru(Co)Co-bridge、Ru(Ru)Co-bridge、Ru_2Co-hcp、RuCo_2-hcp、Ru_2Co-fcc、RuCo_2-fcc十个吸附位的14种吸附模型进行了构型优化、能量计算,得到了N_2较有利的吸附位;并对清洁表面进行能带分析,对最佳吸附位进行总态密度分析.结果表明:掺杂Co后,Ru催化剂的能带变宽,催化活性增强;N_2在Ru(001)表面的最稳定吸附位top的吸附能是-88.94 kJ·mol~(-1),在Ru-Co(001)表面的最稳定吸附位Ru-top的吸附能是-95.71 kJ·mol~(-1),而且N_2与金属表面成键,属于化学吸附.     

Benzene and ethylene chemisorbed on transition metals; The measurement of energy level shifts which accompany chemisorption

Ultraviolet photoemission spectra for ethylene chemisorbed on Ag(110) and benzene on Ag(110) and Ru(001) indicate adsorption with minimal molecular distortions. A survey of the photoemission data for ethylene and benzene similarly adsorbed on transition metals indicate that the relaxation shift in the binding energy of molecular Orbitals not contributing to the chemisorption bond is essentially independent of the metal when obtained using the value of the work function when the surface is saturated with adsorbate. This observation is in agreement with a theoretical analysis where final state image charge screening is the dominant relaxation mechanism. These observations support the choice of the work function of the surface saturated with adsorbate as the approximation of the adsorption site potential.     

Cl2 adsorption on MgO(001) surface supported sodium monolayers: a density functional theory study

The adsorption of Cl₂ Na monolayers supported on the MgO(001) surface has been studied by the density functional method using cluster models embedded in a large array of point charges (PCs). The value of PCs was determined by charge self-consistent technique. The results indicate that Na-promoted MgO(001) surface is an efficient catalyst toward Cl₂ adsorptive decomposition. Besides, it was found that the role of the MgO(001) surface is not passive, which is different from CO adsorption on MgO(001) surface supported Na metal monolayers. The analysis of band and projected density of states indicates that the electron transfer from the surface Mg 3s valence orbital and Na 3s valence orbital to the anti-bonding σ^∗ orbital of Cl₂ is the source of the Cl₂ bond weakening. This is also different from the CO adsorption on MgO(001) surface supported Na metal monolayers, where only the electrons from the Na valence orbital are transferred to the anti-bonding π^∗ orbital of adsorbed CO. Our study suggests that the essence of catalysis is different for CO and Cl₂ adsorption on Na metal monolayers supported an MgO(001) surface.     

BaxOy小团簇修饰Ru(0001)表面的结构稳定性和氮分子吸附性质

为了说明钡助剂的存在形式, 本文采用第一性原理方法研究了Ba_xO_y小团簇修饰Ru(0001)表面的结构稳定性和氮分子吸附性质. 基于总能的热力学分析发现, 在实验条件下(500 K, P_H2O/P_H2<10^-3), Ba₂O团簇比BaO₂, BaO, Ba和O等团簇(原子)更加稳定. 这证实含有金属性钡原子的团簇也是氧化钡助剂可能的工作状态. 表面电荷差分密度说明Ba₂O团簇的氧和钡原子与衬底的作用不同. 不过Ba₂O团簇氧和钡原子附近的氮分子吸附行为相似, Ba₂O团簇增强了氮分子和衬底的相互作用. Ba₂O团簇氧和钡原子附近的氮分子吸附能分别为0.78 和0.88 eV, 均大于清洁表面的0.67 eV. 氮分子间距和氮分子的拉伸振动频率都表明Ba₂O团簇在一定程度上活化了吸附氮分子. Ba₂O团簇氧和钡原子附近的N–N键长分别为0.117和0.116 nm, 大于清洁表面的0.114 nm. 氧和钡原子附近氮分子的拉伸振动频率分别为 1888 和1985 cm^-1, 小于清洁表面的2193 cm^-1. 电荷差分密度的计算结果说明, 削弱作用主要来自于Ba₂O团簇中钡离子和氮分子间的静电作用. 两者间的静电作用增加了氮分子π 反键轨道的占据数, 促进了氮分子极化, 从而削弱氮分子键.     

Hydrogen vibrations at transition metal surfaces

Chemisorbed hydrogen has been studied on a variety of simple and composite transition metal surfaces using local density functional calculations. The goal of these studies is to elucidate trends and general principles in bond energies, atomic geometries, vibrational frequencies, and dynamic effective charges. The theoretical approach, based on the linear augmented plane wave method, will be reviewed. Results will be discussed for H in 2-, 3-, and 4-fold sites on W(001), Ru(0001), Cu(111), Pt(111), Rh(001), and the metal overlayer systems Cu-Ru(0001) and Ni-W(001). In most cases, all the harmonic vibrational modes were studied. Some anharmonic effects were examined. While good agreement has been obtained with much of the available experimental data, some outstanding disagreements will be discussed. Our results will be compared with the nearest-neighbor harmonic potential model and effective medium approach.     

A DFT study of H adsorption on Pt(111) and Pt-Ru(111) surfaces

In this work a comparative analysis between different Pt-Ru(111) surface models and pure Pt(111) surface is presented. Some aspects of the electronic structure of the surfaces and hydrogen adsorption are analysed based on density functional theory calculations. The hydrogen adsorption energy is significantly reduced when Ru is present on the surface. The substitution of Pt atoms by Ru atoms reinforce the Pt-H bond while the metal-metal bond is strongly modified, making the system less stable.     

MoSi_2(001)表面的氧吸附行为

运用第一原理密度泛函理论方法,首先计算了MoSi_2各清洁表面的表面能,(001)Si-|-Si断面具有较低的表面能,是MoSi_2最可能的解理面;通过生成能及键布居分析研究了单氧原子、双氧原子及氧分子在(001)Si-|-Si断面的吸附行为,发现单氧原子在空位处吸附最稳定,此时O极易与Si结合,得到的Si-O-Si键长及键角与SiO_2的非常接近,表明低浓度下O极易与表面的Si结合生成SiO_2;双氧原子发生空位+顶位吸附时O原子除与Si有强作用外,可与Mo有一定相互作用;氧分子以平行的方式接近空位最有利于吸附,此时氧分子最易分解为氧原子,发生氧原子在空位的吸附.     

利用原子集团多重散射理论决定HCOO-Cu(110)的结构

利用原子集团多重散射理论计算甲酸在Cu(110)表面催化分解中间产物HCOO(formate)的O原子K边X射线吸收精细结构谱,证实formate吸附在Cu(110)表面长方元胞的短桥位,求得C—O键长等于1.26±0.01?,Cu—O键长为1.975±0.02?,O—C—O键角处于130°—134°范围内。上述结果与光电子衍射谱的分析结论一致。 关键词：     

Electron stimulated desorption ion angular distributions of H2O and NH3 on Ru(001)

The recent experiments of Madey et al. on the Electron Stimulated Desorption Ion Angular Distributions (ESDIAD) of H₂O and NH₃ on a Ru(001) surface are discussed using the reflection approximation for Franck-Condon ionization. In this approximation, ESDIAD data mirrors, the adsorbate wave function and therefore equilibrium bond angles as well as vibrational information can be extracted from it. It is also shown that image potential distortions of the desorbed ion trajectory from chemical bond directions are easily calculated and could be quite large although considerable uncertainty exists.     

Interaction between CO and NH3 coadsorbed on Ru(001): its effects on the ordering in mixed adlayers and the ammonia dissociation

The coadsorption of CO and ammonia on Ru(001) has been investigated by low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The main focus has been on the interaction between different admolecules on the surface and its important role in surface reaction. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed depending on the CO precoverage. One of the (2 × 2) structures (-phase) contains one CO and two ammonia molecules per (2 × 2) unit cell and the other (β-phase) contains two CO and one ammonia. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia (¹⁵NH₃ or ND₃). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased initially with increasing CO precoverage, passed a minimum at θ_CO = 0.25, increased with a further increase of CO coverage, and eventually reached a saturation value above θ_CO = 0.5. The dissociation of ammonia in the β−(2 × 2) structure was found to be enhanced by a factor of 4–6 as compared with the dissociation in the −(2 × 2) structure. The HREEL spectra indicated that the C_3v molecular axis of ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the β−(2 × 2) phase with a high CO partial coverage. This observation suggests that the tilting of ammonia due to the interaction with CO facilitates electron donation from Ru 4d to LUMO of ammonia, leading to the N-H bond dissociation. The microscopic model for the CO-NH₃ interaction on metal surfaces is presented.     

tetracene 分子在Ru(1010)表面的吸附结构及其电子态研究

利用紫外光电子能谱(UPS)、角分辨紫外光电子能谱(ARUPS)和扫描隧道显微镜(STM)等方法研究了tetracene分子在Ru(1010)表面上吸附的电子态，吸附位置和吸附取向.UPS实验显示，与tetracene分子有关的光电子谱峰在费米能级以下2.1, 3.5, 4.8, 6.0, 7.1和9.2 eV处；ARUPS 结果表明，tetracene分子的分子平面基本平行于衬底表面；从STM图像中可以看到tetracene分子的长轴沿［0001］和［1210］两个晶向.基于密度泛函理论的从头算计算证实了上述结论.当分子长轴沿［0001］晶向时，分子中心位置在衬底表面的“短桥位”上，当分子长轴沿［1210］晶向时，分子中心位置在衬底表面的“四原子中心空位”上. 关键词： tetracene分子 Ru(1010)表面 吸附结构 吸附电子态     

三氧化钨表面氢吸附机理的第一性原理研究

采用基于密度泛函理论的第一性原理平面波超软赝势方法,在广义梯度近似下,研究了立方WO_3,WO_3(001)表面结构及其氢吸附机理.计算结果表明立方晶体WO_3理论带隙宽度为0.587 eV.WO_3(001)表面有WO终止(001)表面和O终止(001)表面两种结构,表面结构优化后W—O键长和W—O—W键角改变,从而实现表面弛豫;WO终止(001)表面和O终止(001)表面分别呈现n型半导体特征和p型半导体特征.分别计算了H原子吸附在WO终止(001)表面和O终止(001)表面的H—O_(2c)—H,H—O_(2c)…H—O_(2c),H—O_(1c)—H和H—O_(1c)…H—O_(1c)四种吸附构型,其中H—O_(1c)—H吸附构型的吸附能最小,H—O键最短,H失去电子数最多,分别为-3.684 eV,0.0968 nm和0.55e,此吸附构型最稳定.分析其吸附前后的态密度,带隙从吸附前的0.624 eV增加到1.004 eV,价带宽度基本不变.H的1s轨道电子与O的2p,2s轨道电子相互作用,在-8和-20 eV附近各形成了一个较强的孤立电子峰,两个H原子分别与一个O_(1c)原子形成化学键,最终吸附反应生成了一个H_2O分子,同时产生了一个表面氧空位.     

The orientation of formate and carbonate on ZnO(101&#x0304;0)

Polarisation dependent CK-edge NEXAFS spectra of carbonate and formate species on ZnO(101&#x0304;0) indicate that both species are aligned by the substrate. The carbonate species, formed by adsorption of CO₂, is oriented with its molecular plane close to the [0001] azimuth. This suggests a bond geometry in which one O atom of CO²⁻₃ is in the substrate, and a second interacts with a surface cation, consistent with the results of previous cluster calculations. In contrast, the formate species, formed by adsorption of formic acid, is not azimuthally ordered, although the molecular rotation axis is aligned close to the surface normal, consistent with bidentate bonding to a single cation.