Coverage dependent promoter action: K coadsorption and reactions with CO and CO2 on Pd{1 1 0}

The adsorption of CO and CO₂ on K-predosed Pd{1 1 0} at room temperature has been examined via reflection–absorption infrared spectroscopy (RAIRS). CO₂ adsorbs on 0.37 ML K-predosed Pd{1 1 0} with high sticking probability and a reactive chemisorbed intermediate, CO₂⁻, is detected in RAIRS at room temperature. Reaction of this species ultimately yields carbonate. The same high K precoverage induces dissociation of CO at low CO exposure. Carbonate is detected at higher CO exposure and is probably produced via stepwise oxidation of molecularly adsorbed CO. In contrast at low K precoverage (0.11 ML), CO remains intact but the C–O bond is considerably weakened with respect to CO chemisorbed on clean Pd{1 1 0}. These findings illustrate a dual promoter mechanism of K in the adsorption and reaction of CO or CO₂ at high K coverage. The alkali metal induces dissociation of these molecules and directly participates in the formation of a surface compound, K₂CO₃.     

Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型，用第一性原理计算方法，计算并分析了Pd在CeO₂(111)面上不同覆盖度时的吸附能，价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明：1)在单层吸附时，Pd的最佳吸附位置是O的顶位偏向Ce的桥位；在1/4单层吸附时，Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时，吸附原子Pd之间的作用较强；1/4单 关键词： 三元催化剂 Pd 2')" href="#">CeO₂ 吸附 密度泛函理论     

N2H4在NiFe(111)合金表面吸附稳定性和电子结构的第一性原理研究

采用基于色散校正的密度泛函理论进行了第一性原理研究, 详细分析了肼(N₂H₄)在Ni₈Fe₈/Ni(111)合金表面稳定吸附构型的吸附稳定性和电子结构及成键性质. 通过比较发现, 肼分子以桥接方式吸附在表面的两个Fe原子上是最稳定的吸附构型, 其吸附能为-1.578 eV/N₂H₄. 同时发现, 肼分子在这一表面上吸附稳定性的趋势为: 桥位比顶位吸附更有利, 且在Fe原子上比在Ni原子上的吸附作用更强. 进一步分析了不同吸附位点上稳定吸附构型的电子结构、电荷密度转移以及电子局域化情况. 结果发现: 相同吸附位点的电子态密度图基本一致, 并且N原子的p轨道和与之相互作用的表面原子的d轨道之间存在态密度上的重叠; 吸附后电荷密度则主要从肼分子转移到表面原子之上; 在电子局域化函数切面图中也发现吸附后电子被局域到肼分子的N原子和相邻的表面原子之间. 这些电子结构的表征都充分说明肼分子与表面原子之间通过电荷转移形成了强烈的配位共价作用.     

Defect induced surface chemistry: A comparison of the adsorption and thermal decomposition of C2H4 on Rh{111} and Rh{331}

The adsorption and thermal decomposition of C₂H₂ on Rh{111} is compared to the atomically stepped Rh{331} surface over a temperature range of 300 to 800 K. Using X-ray photoelectron spectroscopy (XPS) we find that the C 1s spectra as a function of C₂H₄ exposure exhibit a shift in binding energy (E_b) from 283.5 eV at 1 L C₂H₄ exposure on both surfaces to 283.8 eV on Rh{33 and to 284.1 eV on Rh{111} at saturation coverage (4 L). Careful analysis of the C 1s E_b value and full width at half maximum as a function of surface temperature after a 10 L exposure of C₂H₄ at 300 K reveals that a species consistent with a C₂H adsorbate composition is formed between 400 and 450 K on Rh{111}. This species is also observed on Rh{331} although at the lower temperature of 375 K. Computer peak deconvolution of the C 1s spectra between 500 and 700 K suggests that a CH_ads or C_ads surface fragment is formed and increases in concentration at the expense of the C₂H species as the surface temperature increases. Above 750 K a graphite overlayer is formed on both surfaces. This overlayer, however, exhibits a low degree of carbon π-character bonding on Rh{331}. The adsorption and decomposition mechanisms suggest that the 300 K C₂H₄ adsorbate on Rh{331} is ethylidyne and that the stepped surface is more thermally reactive than the flat Rh{111} surface.     

Study of ultrathin Fe films on Pd{111)}, Ag{111} and A1{111}

The growth of epitaxial Fe films of 1 to 10 layer equivalents (LE) on Pd{111}, Al{111} and Ag{111} has been studied with quantitative low-energy electron diffraction and Auger electron spectroscopy. On Pd{111}, both at room temperature and at 200° C, the growth starts with pseudomorphic layers which may involve intermixing of Fe and Pd. At both temperatures, when the coverage reaches 6 LE, the Fe films develop into large bcc {110} domains related to the substrate by the Kurdjumov-Sachs orientation. On Ag{111} Fe grows initially in a very similar manner to Fe on Ag{001}, i.e., by way of small islands of bcc Fe. These islands then grow into bcc Fe{110} domains in the Nishiyama-Wassennan orientation. On Al{111} Fe behaves differently, despite the fact that the lattice mismatch for Fe on A1{111} is nearly identical to that for Fe on Ag{111}: for coverages of less than 1 LE the surface region becomes completely disordered and no LEED pattern is visible.     

乙烯和乙炔基在Ni(110)表面上吸附结构的研究

采用平面波赝势方法，利用基于从头计算的软件包，对乙烯和乙炔基在Ni(110)表面上吸附的问题进行了计算. 在低覆盖度时，孤立的乙烯分子的吸附能比密集时高，乙烯分子的C-C 轴大致沿衬底的Ni原子链方向(即沿［110］晶向)，C-C轴与衬底Ni(110)表面有12°的倾斜角，乙烯分子的C—C键的键长为 0.147nm. 乙烯分子中接近顶位的C原子与衬底中距离最近的Ni原子为0.199nm. 在高覆盖度时，乙烯分子在Ni(110)表面上形成c(2×4)再构，每个表面二维元胞中有两个乙烯分子，每个乙烯分子的吸附位置与低覆盖度时相似，而C—C键长比低覆盖度时要短. 乙炔基是乙烯在Ni(110)表面上分解的产物. 关于乙炔基的计算结果表明：乙炔基的两个C原子的间距为0.131nm，比乙烯分子中C原子的间距更短. 与乙烯分子相比，乙炔基的吸附位置更靠近顶位. H原子与吸附在顶位上的C原子相连接，C—H键也大致沿衬底的Ni原子链方向，与Ni表面呈45°的倾斜角. 关键词： 乙烯和乙炔基 平面波赝势方法 吸附几何结构     

A full dynamical leed analysis of the NI(111) P(2 × 2)-C2H2 structure

The p(2 × 2) structure formed upon adsorption of acetylene on the Ni(111) surface, for 0.5 L exposure at 250 K, is studied by a full dynamical LEED analysis based on the comparison of only five I7ndash;V profiles at normal incidence. Among the several models tested, the most probable one is that with the adsorbed molecule parallel to the surface plane in a μ-bridging bonding site. The NiC distance is equal to 2.1 Å.     

The local adsorption site for sulphur on Ni {111} in the low coverage lattice gas

Angle-resolved sulphur L_2.3VV Auger electron spectra have been taken from sulphur adsorbed on Ni {111} at a range of coverages both below and above those corresponding to the ordered (2 × 2) structure. These data indicate that the local adsorption site in the low coverage lattice gas is the same single three-fold hollow site adopted in the ordered overlayer. This contrasts with the low coverage occupation of both three-fold hollow sites for the system I/Ag<{111}.     

CO2在金属有机骨架材料有机链上的吸附机理研究

金属有机骨架是一种新型的温室气体CO₂吸附材料。本文采用密度泛函理论（DFT）方法研究CO₂在MOF-5有机链上的6种不同吸附位置及其3种不同构型下的吸附行为。考虑GGA水平下三种不同泛函对计算结果的影响,对比发现GGA/PW91泛函能够较好地计算CO₂分子与有机链原子之间的弱范德华力;发现CO₂分子与苯环边相交以及与苯环上的碳原子平行是两种吸附能最大的构型;利用Br原子替代苯环上的H原子可以显著增强对CO₂分子的吸附能力,为设计具有较高CO₂吸附能力的MOFs材料提供理论依据。     

The structures of sulphur on Pd(111) studied by X-ray standing wavefield absorption and surface EXAFS

The surface structures of R30°-S and R19.1°-S on Pd(111) have been investigated by normal incidence X-ray standing wave (NIXSW) absorption and surface extended X-ray absorption fine structure (SEXAFS). NIXSW measurements show that the most likely site of S adsorption in the R30° phase is the threefold “fcc” hollow. The location of the S atoms at the “fcc” hollow site is consistent with S adsorption on the neighbouring fcc (111) transition metal surfaces. SEXAFS analysis revealed a S–Pd nearest neighbour bond distance of 2.28±0.04 Å. The results for the R19.1° phase suggest that the structure involves a mixed S–Pd overlayer, with the S–Pd vertical layer spacing equal to the Pd bulk 111 spacing.     

Promoter action of alkali in the activation of CO2 on Pd(111): A HREELS case study

We study the adsorption and reaction of CO₂ as a function of temperature between 100 and 700 K in the presence of Na on a Pd(111) surface using high resolution electron energy loss spectroscopy. While CO₂ does not react with a clean Pd(111) surface, we find various reaction channels on the Na precovered Pd(111) surface depending on the Na coverage. At intermediate coverage a bent CO₂⁸⁻ species with characteristic vibrational bands can be unambiguously identified. This species is stable up to 200 K, and dissociates into CO and oxygen similar to its behaviour on other surfaces, and as reported in a previous photoemission study [Wambach et al., Surface Sci. 209 (1989) 159]. In case the surface has been oxygen contaminated before Na and CO₂ exposure surface carbonates can be observed.     

The adsorption and reaction of CO and O2 on PdAu alloy wires

Temperature programmed desorption (TPD) of CO and O₂ on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction X_B^pd ? 0.83. The heat of adsorption, sticking coefficient and maximum coverage of O₂ were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + O₂ reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + O₂ on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pd_9±1 ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pd₉ ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.     

A structural study of a C3H3 species coadsorbed with CO on Pd(1 1 1)

The combination of chemical-state-specific C 1s scanned-energy mode photoelectron diffraction (PhD) and O K-edge near-edge X-ray absorption fine structure (NEXAFS) has been used to determine the local adsorption geometry of the coadsorbed C₃H₃ and CO species formed on Pd(1 1 1) by dissociation of molecular furan. CO is found to adopt the same geometry as in the Pd(1 1 1)c(4 × 2)-CO phase, occupying the two inequivalent three-fold coordinated hollow sites with the C–O axis perpendicular to the surface. C₃H₃ is found to lie with its molecular plane almost parallel to the surface, most probably with the two ‘outer’ C atoms in equivalent off-atop sites, although the PhD analysis formally fails to distinguish between two distinct local adsorption sites.     

The adsorption of oxygen on Pd1/W(110)

The behavior of oxygen on Pd₁/W(110) has been investigated from 25 to 200 K by thermal desorption, UPS, XPS, and work function measurements. At 25 K only dioxygen species are present. A weakly bound O₂layer, containing O₂/Pd = 0.31 or 4.4 × 10¹⁴ O₂ molecules/cm² is desorbed at 35 K, leaving a coverage of O₂/Pd = 0.35 or 5 × 10¹⁴ O₂ molecules/cm². Heating to 200 K results in desorption of molecular O₂ as well as conversion to O, with O/Pd = 0.3. The molecular states, except the very weakly bound one, have high dipole moments with electron transfer to O₂, and thus correspond to Superoxide and peroxide species. These have UPS spectra quite different from physisorbed O₂. At 90 K adsorption is still mostly molecular with a sticking coefficient s near unity. At 200 K, adsorption is atomic with an initial s₀ = 0.8. This must be contrasted with Cu₁/W(110) where s₀ is unity even at 300 K. The difference can be explained by the much better size match of Pd and W, than Cu and W which makes it easier for Cu to take up momentum of impinging O₂ molecules.

The behavior of oxygen on Pd₁/W(110) is very similar to that on bulk Pd(111), suggesting that for oxygen the former surface resembles bulk Pd. This is not so for CO adsorption which is much weaker on Pd₁/W(110) than on bulk Pd. The reasons for this difference are not presently understood.     

A theoretical analysis of adsorption and dissociation of CH3OH on the stoichiometric SnO2(110) surface

A theoretical analysis based on the Hartree–Fock pseudopotential method and a density-functional theory calculation using a hybrid combination of general gradient approximation with pseudopotential procedure has been carried out to study the adsorption and dissociation of methanol on the stoichiometric SnO₂(110) surface. The dependence of the results upon model system and computing method is discussed. An optimization procedure of adsorbate and substrate atom positions on a six-layer slab model has been selected to characterize the corresponding geometric parameters, adsorption energy and charge-transfer processes related with the molecularly adsorbed CH₃OH and dissociative channels to yield methoxy or methyl fragments. In the high-coverage limit (θ=1), we find that dissociation of the methanol molecule via the heterolytic cleavage of the C---O bond is favoured. At lower coverage (θ=1/2), this channel and the molecularly adsorbed methanol present similar adsorption energies.     

Benzene and thiophene formation from acetylene on sulfided Pd(111) studied by LITD/FTMS

We report the first laser-induced thermal desorption (LITD) studies of acetylene on Pd(111). LITD coupled with Fourier transform mass spectrometry (FTMS) probes the surface molecular composition. Our results show simultaneous formation of thiophene and benzene at 120 K after a 6 L dose of acetylene at 80 K on a Pd(111) surface with 0.06 ML of sulfur. Simultaneous formation implies that formation of the C₄H₄ intermediate is the slow step in the formation of both cyclic products. The relative amounts of thiophene and benzene observed with LITD/FTMS are comparable, while thermal desorption spectroscopy (TDS) yields integrated thiophene signals that are < 2% as intense as for benzene. This indicates that thiophene primarily decomposes upon heating. Low coverage (0.5 L) results confirm reports that the presence of sulfur enhances benzene production.     

O2 interaction with Pt{100}-hex-R0.7°: scattering, sticking and saturating

The interaction of oxygen with the Pt{100}-hex-R0.7° surface has been studied using supersonic molecular beams at incident translational energies from 0.06 to 0.9 eV and surface temperatures from 300 to 600 K. Scattering measurements show the existence of both intrinsic and extrinsic precursor states, and the trapping probability into these states is high at low incident energies. However, sticking probability measurements on the clean surface indicate that O₂ dissociative adsorption on Pt{100}-hex-R0.7° is a direct activated process, in contrast to that on Pt{100}-(1 × 1) or Pt{111}. Strong temperature enhancement of the initial sticking probability has been observed and accounted for partly by a dynamical barrier model. The sticking probability varies strongly with oxygen coverage, which is explained through computer simulations of island formation. The formation of small islands is demonstrated by TEAS measurements. Thermal desorption measurements show that, at high incident energies above 0.5 eV, new states are populated and higher coverages, up to a full monolayer, are reached.     

The adsorbate-induced removal of the Pt{100} surface reconstruction Part I: NO

The molecular adsorption of NO on both the reconstructed (hex) and unreconstructed (1 × 1) surfaces of Pt{100} has been studied using a combination of infrared reflection-absorption spectroscopy (IRAS) and low energy electron diffraction (LEED) at temperatures between 90 and 300 K. On the (1 × 1) surface at 300 K adsorbed NO gives rise to an N-O stretching band at initially 1596 cm⁻¹ shifting to 1641 cm⁻¹ at a coverage of θ = 0.5. The LEED pattern at this coverage is interpreted in terms of a c(4 × 2) structure in which all the molecules occupy a single type of adsorption site between the on-top and bridge positions. At temperatures below 300 K, a higher coverage disordered phase is observed, giving rise to an N-O stretching band at 1680 cm⁻¹ associated with an on-top NO species. On the (hex) phase surface above 210 K, NO adsorption gives rise to bands characteristic of adsorption on the (1 × 1) phase indicating that the reconstruction is immediately lifted. Below 200 K initial adsorption actually occurs directly on the (hex) phase, resulting in a band at 1680 cm⁻¹, which is assigned to on-top NO. This band increases in intensity until, at a critical coverage dependent on temperature, the (hex) → (1 × 1) surface phase transition is induced. This is indicated by the disappearance of the band at 1680 cm⁻¹ and its replacement by bands characteristic of adsorption on islands of the (1 × 1) structure.     

Imaging benzene on nickel and copper {110} surfaces with low temperature STM:: the adsorption site

Single benzene molecules have been imaged on the {110} surfaces of both copper and nickel with an Eigler-type low temperature scanning tunnelling microscope. Conditions were chosen such that the molecule and the lattice atoms could be resolved at the same time within one image frame. On Ni{110} benzene was found to adsorb in the hollow site between the close-packed rows. For the more weakly bound molecule on Cu{110} the imaging conditions had to be changed since, for the tunnelling conditions under which the lattice atoms are resolved, a strong interaction between the tip and the benzene takes place. This results in the molecule being “dragged” along the surface. By changing the tunnelling conditions within one image frame, however, it is possible via extrapolation of the substrate lattice to show that the molecule adsorbs in the long-bridge site. The shapes of the images of individual molecules are discussed.     

Au(111)表面甲基联二苯丙硫醇盐单层膜的原子结构

利用第一性原理研究了甲基联二苯丙硫醇盐(BP3S)单体、虚拟Au表面BP3S的分子链和单层膜及BP3S/Au(111)吸附系统的原子结构.计算表明BP3S单体呈对称结构,两苯环夹角为35°±10°.首先BP3S单体在虚拟Au(111)表面自组装成稳定的单一分子链.然后在虚拟Au(111)表面,分子链错位排列自组装成两种稳定的单层膜.在虚拟Au(111)-(3~(1/2)×7~(1/2))和Au(111)-(3~(1/2)×13~(1/2))表面,分子链与虚拟表面夹角分别为60°和30°.最后把两种稳定的单层膜吸附在Au(111)表面的四个吸附位,计算表明只有桥位和顶位稳定,且桥位的吸附能比顶位的吸附能低.比较吸附前后BP3S单层膜的结构变化,可知其变化不大,这说明吸附系统的结构参数主要取决于单层膜内的相互作用,衬底对其的影响不大.