A first-principles study of methanol decomposition on Pt(111)

A periodic, self-consistent, Density Functional Theory study of methanol decomposition on Pt(111) is presented. The thermochemistry and activation energy barriers for all the elementary steps, starting with O[bond]H scission and proceeding via sequential hydrogen abstraction from the resulting methoxy intermediate, are presented here. The minimum energy path is represented by a one-dimensional potential energy surface connecting methanol with its final decomposition products, CO and hydrogen gas. It is found that the rate-limiting step for this decomposition pathway is the abstraction of hydroxyl hydrogen from methanol. CO is clearly identified as a strong thermodynamic sink in the reaction pathway while the methoxy, formaldehyde, and formyl intermediates are found to have low barriers to decomposition, leading to very short lifetimes for these intermediates. Stable intermediates and transition states are found to obey gas-phase coordination and bond order rules on the Pt(111) surface.     

A first-principles study of K adsorption on Pb(111)

Ab initio total-energy density functional theory calculations with supercell models have been employed to investigate the R30 degrees and (2 x 2) structures of K on the Pb(111) surface. Four "on-surface" sites and a substitutional site were considered. The calculations showed that the substitutional site is more stable than all the on-surface sites, due to its low vacancy formation energy. The calculated R30 degrees geometry agrees well with the LEED results. The density-of-states analysis indicates that the K atom loses part of its loosely bound valence s electron. From the electron density distributions, it was found that the lowering of the work function after the substitutional adsorption can be attributed to the dipole moment, associated with the positively polarized adsorbate atom that is characterized by charge depletion from the K vacuum sides and charge accumulation in the region between K and Pb atoms. Our results indicate that the bonding of K with the Pb(111) surface has a mixed ionic and metallic bond character.     

STM Images of Individual Porphyrin Molecules on Cu(100) and Cu(111) Surfaces

Simply sublime! Samples of monomeric and dimeric zinc porphyrins were sublimed onto a Cu surface under ultrahigh vacuum conditions. Images obtained by scanning tunneling microscopy at room and low temperature (98 K) show features attributed to individual porphyrin molecules with excellent resolution. In the case of the (relatively large) linear dimer shown, two distinct conformations were detected on a surface with low coverage area. R=CH₂CH₂COOCH₃.     

CO和H2分子在Cu(111)面的吸附和溶剂化效应

采用广义梯度近似(GGA)密度泛函理论(DFT)的PW91方法结合周期性模型, 在DNP基组下, 利用Dmol3模块研究了CO和H2在真空和液体石蜡环境下在Cu(111)表面上不同位置的吸附. 计算结果表明, 溶剂化效应对H2和CO的吸附结构参数和吸附能的影响非常显著. 在液体石蜡环境下, H2平行吸附在Cu(111)表面是解离吸附, 而CO 和H2在两种环境下的垂直吸附都是非解离吸附. 相比真空环境吸附, 在液体石蜡环境中, Cu(111)吸附CO时, 溶剂化效应能够提高CO吸附的稳定性, 同时有利于CO的活化. 在真空中, H2只能以垂直方式或接近垂直方式吸附在Cu(111)表面. 当Cu(111)顶位垂直吸附H2, 相比真空环境吸附, 溶剂化效应能够提高H2吸附的稳定性, 但对H2的活化没有明显影响. Cu(111)表面的桥位或三重穴位(hcp和fcc)垂直吸附H2时, 溶剂化效应能明显提高H2的活化程度, 但降低H2的吸附稳定性; 在液体石蜡中, 当H2平行Cu(111)表面吸附时, 溶剂化效应使H—H键断裂, 一个H原子吸附在fcc位, 另一个吸附在hcp位.     

Water Formation for the Metalation of Porphyrin Molecules on Oxidized Cu(111)

Herein the formation of water molecules in the intermediate step of the redox reaction of porphyrins self‐metalation on O/Cu(111) is demonstrated. Photoemission measurements show that the temperature on which porphyrins pick‐up a substrate metal atom on O/Cu(111) is reduced by about 185±15 K with respect to the pure Cu(111). DFT calculations clearly indicate that the formation of a water molecule is less expensive than the formation of H₂ on the O/Cu(111) substrate and, in some cases, it can be also exothermic.     

Structure and Conformation of Individual Molecules upon Adsorption of a Mixture of Benzoporphyrins on Ag(111), Cu(111), and Cu(110) Surfaces

The front cover artwork is provided by the groups of Prof. Dr. Hans-Peter Steinrück and Prof. Dr. Norbert Jux at the Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg. The image shows a mixture of six 2H-tetrakis-(3, 5-di-tert-butyl-phenyl)(x)benzoporphyrins (2H-diTTBP(x)BPs, x = 0, 1, 2-cis, 2-trans, 3, or 4) molecules forming a porous square structure on Ag(111) as observed in scanning tunneling microscopy (STM) at room temperature. Read the full text of the Research Article at 10.1002/cphc.202300355 .     

An Efficient Synthesis of Optically Pure (R)- and (S)-2-(aminomethyl)alanine ((R)- and (S)-ama) and (R)- and (S)-2-(aminomethyl)leucine ((R)- and (S)-aml)

An efficient synthesis of enantiomerically pure (R)- and (S)-2-(aminomethyl)alanine ((R)- and (S)-Ama) 1a and (R)- and (S)-2-(aminomethyl)leucine ((R)- and (S)-Aml) 1b is described (Schemes 1 and 2). Resolution of the racemic amino acids was achieved using L -phenylalanine cyclohexylamide ( 2 ) as chiral auxiliary. The free amino acids 1a, b were converted to the N^α-Boc,N^γ-Z-protected derivatives 11a, b (Scheme 3) ready for incorporation into peptides. Based on the three crystal structures of the diastereoisomeric peptides 8a, 8b , and 9b , the absolute configurations in both series were determined. β-Turn type-I geometries were observed for structures 8b and 9b , whereas 8a crystallized in an extended backbone conformation.     

由(S)-和( R )-天冬氨酸合成( R )-和(S)-(2-苄氧乙基)环氧乙烷的改良方法

苄氧乙基环氧乙烷;不对称合成;由(S)-和( R )-天冬氨酸合成( R )-和(S)-(2-苄氧乙基)环氧乙烷的改良方法     

The chemisorption of hydrogen on Cu(111): A dynamical study

Ab initio band-structure calculations within a density functional formalism were performed to compute the binding energy curves of atomic hydrogen with the high-symmetry adsorption sites of the (111) surface of copper. For a two-layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.25, 3.12, and 3.24 eV, for on-top, bridge, and threefold sites, so that the chemisorption of H₂ on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on-top sites. Starting from these results, an LEPS potential for the interaction of H₂ with the Cu(111) surface was built. In this model potential, the most favored approaches correspond to a H₂ molecule parallel to the Cu surface, and for them, the activation barrier is located at the corner between the entrance and the exit channels of the reaction, and its lowest value is 0.6 eV. The LEPS potential was used in quasi-classical trajectories calculations to simulate the adsorption of a beam of H₂ molecules on Cu(111). The results show that (a) when H₂ is in the ground vibrational state the dissociative adsorption probability P_a increases from 0 to .90 along a roughly sigmoidal curve by increasing the collision kinetic energy from 0.4 to 1.3 eV, and (b) the vibrational energy can be as effective as the translational one in promoting dissociative chemisorption, in agreement with the experimental results. © 1994 John Wiley & Sons, Inc.     

Electron-induced isomerisation of dichlorobenzene on Cu(111) and Ag(111)

The constitutional isomerisation of single dichlorobenzene molecules adsorbed on the surfaces of Ag(111) and Cu(111) between their meta- and para-isomers is induced and investigated by means of a low temperature scanning tunneling microscope. On both substrates similar isomerisation thresholds are found indicating that the excitation mechanism of this reaction does not depend significantly on the underlying substrate. The isomerisation threshold voltage of (170 +/- 7) meV most likely corresponds to excitation of a C-C stretch mode whose gas-phase energies we calculated ab initio to lie at 174 and 172 meV for meta- and para-isomers respectively. Though the reaction is found to be localized on the submolecular scale, it depends heavily on the second substituent both in terms of excitation energy and reaction outcome.     

Scanning tunneling microscope imaging of (CH3S)2 on Cu(111)

Scanning tunneling microscope (STM) images of isolated molecules of dimethyl disulfide, (CH(3)S)(2), adsorbed on the Cu(111) surface were successfully obtained at a sample temperature of 4.7 K. A (CH(3)S)(2) molecule appears as an elliptic protrusion in the STM images. From density functional theory calculation, it was suggested that the bright part in the protrusion corresponds to the molecular orbital which is widely spread around H atoms in each CH(3) group in the (CH(3)S)(2) molecule. The STM images revealed that the molecules have a total of six equivalent adsorption orientations on Cu(111), which are given by the combination of three equivalent adsorption sites and two conformational isomers for each adsorption site.     

Methane dehydrogenation on Rh@Cu(111): a first-principles study of a model catalyst

The issue of tuning the relative height of the first two dehydrogenation barriers of methane (CH(4) --> CH(3) + H and CH(3) --> CH(2) + H) is addressed using density-functional theory. It is shown that the combination of a very active reaction center-such as Rh-with a more inert substrate-such as Cu(111)-may hinder the second dehydrogenation step with respect to the first, thus resulting in the reverse of the natural order of the two barriers' heights.     

First-principles study on the adhesive and electronic property of c-BN(111)/Cu(111) interface

The interface properties of c-BN/Cu composite play an important role in its application. In this work, we employed first-principles calculation to investigate the bonding properties and electronic characteristics of the c-BN(111)/Cu(111) interface. The adhesion properties, partial density of states (PDOS), charge density, and charge density difference of different interfaces were analyzed. The results show that the interface of B-termination “OT” stacking mode is the most stable one. The density of states at the c-BN(111)/Cu(111) interface is similar to that of c-BN bulk phase, indicating that the electronic states of the c-BN layer are not affected by the Cu atoms. The PDOS diagram shows that the 2p orbital of B atoms and the 2p orbital of N atoms are hybridized in the c-BN layer. Besides, 2p orbital of B(N) atoms and 3d orbital of Cu atoms are hybridized in the interface. The covalent bonds and ionic bonds in the interface of N-termination and B-termination OT stacking mode structures are stronger than that of “SL” and “TL” stacking mode. So, the OT stacking mode has larger adhesive energy. Furthermore, Cu and c-BN can form a good coherent interface, which can be used to prepare c-BN/Cu composites and functional materials with excellent mechanical properties.     

CH3O decomposition on PdZn(111), Pd(111), and Cu(111). A theoretical study

Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH(3)O) have been carried out, namely, dehydrogenation to formaldehyde and C-O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd-Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd-Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C-H scission of CH(3)O on various substrates under study was demonstrated to proceed easier than C-O bond breaking. The energy barrier for the dehydrogenation of CH(3)O on PdZn(111) (113 kJ mol(-)(1)) and Cu(111) (112 kJ mol(-)(1)) is about 4 times as high as that on Pd(111), due to the fact that CH(3)O interacts more weakly with Pd than with PdZn and Cu surfaces. Calculated results showed that the decomposition of methoxide to formaldehyde is thermodynamically favored on Pd(111), but it is an endothermic process on PdZn(111) and Cu(111) surfaces.     

氧化锌有序结构在Au(111)和Cu(111)上的生长

利用NO₂或O₂作为氧化剂,研究了氧化锌在Au(111)和Cu(111)上的生长和结构。NO₂表现了更好的氧化性能,有利于有序氧化锌纳米结构或薄膜的生长。在Au(111)和Cu(111)这两个表面上,化学计量比氧化锌都形成非极性的平面化ZnO(0001)的表面结构。在Au(111)上,NO₂气氛下室温沉积锌倾向于形成双层氧化锌纳米结构;而在更高的沉积温度下,在NO₂气氛中沉积锌则可同时观测到单层和双层氧化锌纳米结构。O₂作为氧化剂时可导致形成亚化学计量比的ZnO_x结构。由于铜和锌之间的强相互作用会促进锌的体相扩散,并且铜表面可以被氧化形成表面氧化物,整层氧化锌在Cu(111)上的生长相当困难。我们通过使用NO₂作为氧化剂解决了这个问题,生长出了覆盖Cu(111)表面的满层有序氧化锌薄膜。这些有序氧化锌薄膜表面显示出莫尔条纹,表明存在一个ZnO和Cu(111)之间的莫尔超晶格。实验上观察到的超晶格结构与最近理论计算提出的Cu(111)上的氧化锌薄膜结构相符,具有最小应力。我们的研究表明,氧化锌薄膜的表界面结构可能会随氧化程度或氧化剂的不同而变化,而Cu(111)的表面氧化也可能影响氧化锌的生长。当Cu(111)表面被预氧化成铜表面氧化物时,ZnO_x的生长模式会发生变化,锌原子会受到铜氧化物晶格的限域形成单位点锌。我们的研究表明了氧化锌的生长需要抑制锌向金属基底的扩散,并阻止亚化学计量比ZnO_x的形成。因此,使用原子氧源有利于在Au(111)和Cu(111)表面上生长有序氧化锌薄膜。     

Plasma-assisted oxidation of Cu(100) and Cu(111)

Oxidized copper surfaces have attracted significant attention in recent years due to their unique catalytic properties, including their enhanced hydrocarbon selectivity during the electrochemical reduction of CO₂. Although oxygen plasma has been used to create highly active copper oxide electrodes for CO₂RR, how such treatment alters the copper surface is still poorly understood. Here, we study the oxidation of Cu(100) and Cu(111) surfaces by sequential exposure to a low-pressure oxygen plasma at room temperature. We used scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and low energy electron diffraction (LEED) for the comprehensive characterization of the resulting oxide films. O₂-plasma exposure initially induces the growth of 3-dimensional oxide islands surrounded by an O-covered Cu surface. With ongoing plasma exposure, the islands coalesce and form a closed oxide film. Utilizing spectroscopy, we traced the evolution of metallic Cu, Cu₂O and CuO species upon oxygen plasma exposure and found a dependence of the surface structure and chemical state on the substrate''s orientation. On Cu(100) the oxide islands grow with a lower rate than on the (111) surface. Furthermore, while on Cu(100) only Cu₂O is formed during the initial growth phase, both Cu₂O and CuO species are simultaneously generated on Cu(111). Finally, prolonged oxygen plasma exposure results in a sandwiched film structure with CuO at the surface and Cu₂O at the interface to the metallic support. A stable CuO(111) surface orientation is identified in both cases, aligned to the Cu(111) support, but with two coexisting rotational domains on Cu(100). These findings illustrate the possibility of tailoring the oxidation state, structure and morphology of metallic surfaces for a wide range of applications through oxygen plasma treatments.

A low-pressure oxygen plasma oxidized Cu(100) and Cu(111) surfaces at room temperature. The time-dependent evolution of surface structure and chemical composition is reported in detail for a range of exposure times up to 30 min.     

Structure and bonding of alkanethiols on Cu(111) and Cu(100)

The local structure of the sulfur atom of methanethiolate and ethanethiolate on the Cu(111) and Cu(100) surfaces was investigated from first principles employing the periodic supercell approach in the framework of density functional theory. On the 111 surface, we investigated the (square root 3 x square root 3)R30 degrees and (2 x 2) structures, whereas on the 100 surface, we investigated the p(2 x 2) and c(2 x 2) structures. The landscape of the potential energy surface on each metal surface presents distinctive features that explain the local adsorption structure of thiolates found experimentally. On the Cu(111) surface, the energy difference between the hollow and bridge sites is only 3 kcal/mol, and consequently, adsorption sites ranging from the hollow to the bridge site were observed for increasing surface coverages. On the Cu(100) surface, there is a large energy difference of 12 kcal/mol between the hollow and bridge sites, and therefore, only the 4-fold coordination was observed. The high stabilization of thiolates on the hollow site of Cu(100) may be the driving force for the pseudosquare reconstruction observed experimentally on Cu(111). Density of states analysis and density difference plots were employed to characterize the bonding on different surface sites. Upon interaction with the metal d bands, the pi* orbital of methanethiolate splits into several peaks. The two most prominent peaks are located on either edge of the metal d band. They correspond to bonding and antibonding S-Cu interactions. In the case of ethanethiolate, all the back-bonds are affected by the surface bonding, leading to alternating regions of depletion and accumulation of charge in the successive bonds.     

Embedded atom method study of Cu deposition on Ag(111)

A new model is applied to the study of Cu deposition on Ag(111), It links elements from the embedded atom model and Monte Carlo simulations. In agreement with the experiment, the present results favor overpotential upon underpotential deposition. A diffusion coefficient of 1.93 × 10⁻⁶ cm² s⁻¹ is estimated for the system. The predicted structure of the adsorbed monolayer is more compact than the electrochemical observation, a fact which may be due to some simplifications of the model like neglecting the solvent and anion effects or the nature of the potentials employed.