A Theoretical Study on Electronic Transition of CO Adsorption on Platinum Pt(111) Surface

It is now technically possible for Raman spectroscopy to investigate in detail the catalytic reaction on the transition metal surfaces. However, there are only few theoretical papers reported on the contribution of the electronic excited states to the spectroscopic properties. During the interaction of the visible light with the transition metal surface, there exist a number of low-lying excited states due to the unfilled d orbital Nakai and Nakatsuji studied theoretically adsorbed CO on the Pt₂ cluster to simulate CO adsorbed on plartinum surfaces.¹ However it is known that the cluster with only two platinum atoms is insufficient to simulate CO adsorbed at surface. It is suggested in literature that a good simulated result generally needs to adopt a cluster with more than seven atoms. In this paper, we use a duster with 8 platinum atoms in the surface layer, which has been used by Kua and Goddard to mimic the oxidation of methanol on the Pt(111) surface.² Based on the interstitial electron model, they found that the cluster is the smallest and the best cluster possible to be used to mimic Pt(111) surface.     

Cluster calculations of the H2O/Pt(111) system

The electronic interaction between water and a Pt(111) surface as evaluated for different Pt_x(H₂O)_y clusters is discussed. Hartree–Fock–Slater (HFS ) one-electron ground state energies, ionization potentials, partial densities of states, and Mulliken occupation numbers are related to bonding shifts, as well as initial and final state screening for different orientations of the molecule. The formation of Pt? H₂O bonds are sensitive to the orientation since surface oriented H atoms bridge the spatial separation between O 2p and Pt 5d orbitals and thus increase the intermixing of metal and adsorbate orbitals. The dipole moment and the net charge of the H₂O molecule is also discussed. Finally, approximations of the metal–H₂O potential for use in statistical models of the liquid–metal interface are suggested.     

DFT study of interaction of O, O2, and OH with unreconstructed Pt(hkl) (h, k, l = 0, 1) surfaces—similarities, differences, and universalities

Adsorption of O, O₂, and OH on Pt(111), Pt(100), and Pt(110) surfaces was studied using periodic DFT calculations. It was found that generally adsorbate-surface interaction strengths increase with the decrease in surface packing density. On the Pt(111) surface the dissociation of O₂ molecule was not predicted, but it was predicted on Pt(100) and Pt(110) surfaces. While the strength of the adsorbate-substrate interaction decreases with the rise in surface coverage by O atoms, in the case of OH adsorption adsorbate layer gets stabilized at higher surface coverage through the hydrogen bonding. In spite of all the mentioned differences, single parameter of surface electronic structure was identified, being useful for the explanation of the adsorption trends at different adsorption sites for O and OH adsorption on Pt surfaces of various crystallographic orientations and also provided a deeper understanding of atomic oxygen adsorption as a function of surface coverage.     

Two-photon photoemission study of the coverage-dependent electronic structure of chemisorbed alkali atoms on a Ag(111) surface

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].     

Density functional calculations on CO attached to PtnRu(10−n) (n = 6–10) clusters

B3LYP and SCF‐Xα calculations have been performed on Pt_nRu_(10−n)CO (n = 6–10) clusters. The work aims to simulate the adsorption of CO on the (111) surface of platinum metal and to examine the electronic effects that arise when some Pt atoms are replaced with Ru. Adsorption energies and Pt C and C O stretching frequencies have been calculated for each cluster. Ru does affect the electronic structure of the clusters, the calculated adsorption energies, and frequencies, the Pt C frequency more than the C O. The donation‐backbonding mechanism that accompanies the shift in CO stretching frequency that occurs when CO adsorbs on platinum does not explain the differences in frequency shift observed in CO on various Pt/Ru surfaces. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 589–598, 2000     

Electronic and atomic structure of platinum-cobalt nanocatalysts

X-ray diffraction in combination with X-ray emission and EXAFS spectroscopy were used to study the electronic and atomic structure of metal nanoparticles stabilized on a carbon support in novel Pt _x Co/C catalysts of different composition with the molar ratio Pt:Co (x) of 1 to 3. Cobalt atoms in nanoparticles, which average size was 2–4 nm, were shown to form chemical bonds both with platinum atoms and carbon atoms of the support material.     

The nature of the interactions between Pt4 cluster and the adsorbates *H, *OH, and H2O

The nature of the interactions between the platinum cluster Pt4 and the adsorbates (*)H, (*)OH, and H2O, as well as the influence of these adsorbates on the electronic structure of the Pt4 cluster, was investigated by density functional theory (B3LYP, B3PW91, and BP86) together with the effective core potential MWB for the platinum atoms, and 6-311++G(d,p) and aug-cc-pVTZ basis set for the H and O atoms. Identification of the optimal spin multiplicity state and the preferential adsorption sites were also evaluated. Adsorption changes the cluster geometry significantly, but the relaxation effects on the adsorption energy are negligible. The adsorbates bind preferentially atop of the cluster, where high bonding energies were observed for the radical species. Adsorption is followed by a charge transfer from the Pt4 cluster toward radical adsorbates, but this charge transfer occurs in a reversed way when the adsorbate is H2O. In contrast with water, adsorption of the radicals (*)H and (*)OH on platinum causes a remarkable re-distribution of the spin density, characterized by a spin density sharing between the (*)H and (*)OH radicals and the cluster. The covalent character of the cluster-adsorbate interactions, determined by electron density topological analysis, reveals that the Pt4-H interaction is completely covalent, Pt4-OH is partially covalent, and Pt4-H2O is almost noncovalent.     

基于第一性原理研究铱(111)和(100)表面的原子吸附（英文）

本文利用第一性原理密度泛函理论研究了九种不同的吸附原子在铱(Ir)的(111)和(100)表面上的吸附性质.探讨了Ir表面的功能化,因此吸附能、稳定的结构、态密度和磁矩,这将为进一步研究其在催化和其他表面应用中的可能展现的功能提供重要信息.研究表明,三/四重空位点是Ir(111)/(100)表面最有利的吸附位点.通过对大范围的覆盖率(从0.04到1个单层)的研究,表明吸附原子的吸附能具有很强的覆盖率依赖性.吸附能随着覆盖率的增加而增加,这意味着吸附物之间存在排斥相互作用.吸附原子和衬底电子态之间的强杂化会影响吸附性质,同时吸附原子的磁矩被抑制.通过Bader电荷分析,揭示了吸附原子和衬底之间的大量电荷转移.与(111)表面的结合相比,(100)表面吸附原子的结合更强.     

Noncovalent Axial I⋅⋅⋅Pt⋅⋅⋅I Interactions in Platinum(II) Complexes Strengthen in the Excited State

Coordination compounds of platinum(II) participate in various noncovalent axial interactions involving metal center. Weakly bound axial ligands can be electrophilic or nucleophilic; however, interactions with nucleophiles are compromised by electron density clashing. Consequently, simultaneous axial interaction of platinum(II) with two nucleophilic ligands is almost unprecedented. Herein, we report structural and computational study of a platinum(II) complex possessing such intramolecular noncovalent I⋅⋅⋅Pt⋅⋅⋅I interactions. Structural analysis indicates that the two iodine atoms approach the platinum(II) center in a “side-on” fashion and act as nucleophilic ligands. According to computational studies, the interactions are dispersive, weak and anti-cooperative in the ground electronic state, but strengthen substantially and become partially covalent and cooperative in the lowest excited state. Strengthening of I⋅⋅⋅Pt⋅⋅⋅I contacts in the excited state is also predicted for the sole previously reported complex with analogous axial interactions.     

Interface-controlled synthesis of CeO2(111) and CeO2(100) and their structural transition on Pt(111)

Ceria-based catalytic materials are known for their crystal-face-dependent catalytic properties. To obtain a molecular-level understanding of their surface chemistry, controlled synthesis of ceria with well-defined surface structures is required. We have thus studied the growth of CeO_x nanostructures (NSs) and thin films on Pt(111). The strong metal-oxide interaction has often been invoked to explain catalytic processes over the Pt/CeO_x catalysts. However, the Pt-CeO_x interaction has not been understood at the atomic level. We show here that the interfacial interaction between Pt and ceria could indeed affect the surface structures of ceria, which could subsequently determine their catalytic chemistry. While ceria on Pt(111) typically exposes the CeO₂(111) surface, we found that the structures of ceria layers with a thickness of three layers or less are highly dynamic and dependent on the annealing temperatures, owing to the electronic interaction between Pt and CeO_x. A two-step kinetically limited growth procedure was used to prepare the ceria film that fully covers the Pt(111) substrate. For a ceria film of ~3–4 monolayer (ML) thickness on Pt(111), annealing in ultrahigh vacuum (UHV) at 1000 K results in a surface of CeO₂ (100), stabilized by a c-Ce₂O₃(100) buffer layer. Further oxidation at 900 K transforms the surface of the CeO₂(100) thin film into a hexagonal CeO₂(111) surface.     

Synthesis and spectral studies on platinum complexes with mono- and bidentate N-donor organic ligands

Platinum(II) complexes of types PtLX₂, PtL₂X₂, PtLX″ and the Pt(IV) complexes PtLXY (where L = mono- or bidentate organic ligand containing nitrogen donor atoms; X = Cl or Br; X′ = oxalate or malonate and Y = Br) have been synthesized and characterized from their elemental analysis, IR and X-ray photoelectron spectral data. The Pt 4f_7/2 binding energies indicate that 1,8-naphthalene-diamine ligand is a better donor of electron density to the metal than other ligands studied here. The Cl 2p_3/2 binding energies in the square planar Pt(II) complexes are observed in the range 198.8 ± 0.8 eV. The ν (PtCl) vibrations (ca 335 and 320 cm^?1) corresponding to two cis-Cl ligands were observed in the IR spectra.The extent of the interaction between cis-dichloro-bis-(theophylline)platinum(II) with calf thymus DNA has beenstudied. The UV difference spectra resulting from aquated Pt^II(theoph)₂-DNA interaction exhibit bands at 282 and 292 nm attributable to the change in the electron distribution of the base moieties induced by binding with platinum and due to the loss of base stacking. Melting profiles for the DNA samples treated with Pt-complex showed decrease in the melting temperature. Binding of the guanine residues of the DNA, involving probably (N7)-0(6) positions to the metal is implied.     

Theoretical evidence of PtSn alloy efficiency for CO oxidation

The efficiency of PtSn alloy surfaces toward CO oxidation is demonstrated from first-principles theory. Oxidation kinetics based on atomistic density-functional theory calculations shows that the Pt3Sn surface alloy exhibits a promising catalytic activity for fuel cells. At room temperature, the corresponding rate outstrips the activity of Pt(111) by several orders of magnitude. According to the oxidation pathways, the activation barriers are actually lower on Pt3Sn(111) and Pt3Sn/Pt(111) surfaces than on Pt(111). A generalization of Hammer's model is proposed to elucidate the key role of tin on the lowering of the barriers. Among the energy contributions, a correlation is evidenced between the decrease of the barrier and the strengthening of the attractive interaction energy between CO and O moieties. The presence of tin modifies also the symmetry of the transition states which are composed of a CO adsorbate on a Pt near-top position and an atomic O adsorption on an asymmetric mixed PtSn bridge site. Along the reaction pathways, a CO2 chemisorbed surface intermediate is obtained on all the surfaces. These results are supported by a thorough vibrational analysis including the coupling with the surface phonons which reveals the existence of a stretching frequency between the metal substrate and the CO2 molecule.     

Pt/Cu(001)-p(2×2)-O表面吸附结构的总能计算

采用密度泛函理论(DFT)研究了氧吸附后Pt/Cu(001)表面合金的原子结构和表面性质. 计算结果表明, 在Pt/Cu(001)-p(2×2)-O表面最稳定结构中, 衬底表面原子层不发生再构, 氧原子吸附于4重对称的Pt原子谷位, 每个氧原子吸附能约为2.303 eV. 吸附结构的Cu—O和Pt—O键键长分别为0.202和0.298 nm, 氧原子的吸附高度ZCu—O约为0.092 nm. 吸附前后Pt/Cu(001)-1ML(monolayer)表面合金的表面功函数分别为4.678和5.355 eV. 吸附表面氧原子和衬底的结合主要来自氧原子2p轨道和衬底金属原子d轨道的杂化作用, 氧原子吸附形成的表面电子态主要位于费米能级以下约-2.7 eV 处.     

Sulfur the archetypal catalyst poison? The sulfur-induced promotion of the bonding of unsaturated hydrocarbons on Cu(111)

We have shown using a combination of temperature-programmed desorption and UV photoelectron spectroscopy that the presence of preadsorbed atomic sulfur promotes the bonding of cyclic unsaturated hydrocarbons (benzene and cyclohexene) to Cu(111). This promoting behavior of sulfur can be rationalized in terms of the ability of adsorbed sulfur to influence the balance between charge donation from the adsorbate to metal, and back-donation from the metal to adsorbate. The effects of sulfur on Cu(111) are dramatically different from those observed in previous studies on Pt(111), which found that it caused a downward shift in the desorption temperature of adsorbed benzene, through purely steric effects.     

Tellurium adatoms as an in-situ surface probe of (111) two-dimensional domains at platinum surfaces

Irreversibly adsorbed tellurium has been studied as a probe to quantify ordered domains in platinum electrodes. The surface redox process of adsorbed tellurium on the Pt(111) electrode and Pt(111) stepped surfaces takes place around 0.85 V in a well-defined peak. The behavior of this redox process on the Pt(111) vicinal surfaces indicates that the tellurium atoms involved in the redox process are only those deposited on the (111) terrace sites. Moreover, the corresponding charge density is proportional to the number of sites on (111) ordered domains (terraces) that are, at least, three atoms wide. Hence, this charge density can be used to measure the number of (111) terrace sites on any given platinum sample. Structural information about tellurium adsorption is obtained from atomic-resolution STM images for the Pt(111) and Pt(10, 10, 9) electrodes. A rectangular structure (2 x radical 3) and a compact hexagonal structure (11 x 8) were identified. However, the redox peak for adsorbed tellurium on (100) domains at 1.03 V overlaps with peaks arising from steps and (110) sites. Therefore, it cannot be used without problems for the determination of (100) sites on a platinum sample. On the (100) terraces, the surface structure of the adsorbed tellurium is c(2 x 2), as revealed by STM. Finally, tellurium irreversible adsorption has been used to estimate the number of (111) ordered domains terrace sites on different polycrystalline platinum samples, and the results are compared to those obtained with bismuth irreversible adsorption.     

Growth of a Pt film on non-reduced ceria: a density functional theory study

The growth of platinum on non-reduced CeO(2) (111) surface is studied by means of calculations based on the density functional theory. Particles of increasing size are formed on the oxide surface by incorporating the platinum atoms one by one until multilayer films are obtained. The main conclusion is that platinum atoms tend to maximize the number of metallic bonds and to approach the situation of the bulk, hence preferring films to particles, particles to isolated atoms, and a three-dimensional growth to a two-dimensional one. The supported particles and the films exhibit a contraction of the Pt-Pt distances, with respect to those of the Pt bulk, in order to match the ceria lattice. The density of states projected on the film surface platinum atoms shows important differences in shape and energy (lower d-band center) compared to the Pt(111) reference surface, which could be the major reason for the observed changes in catalytic reactivity when deposited particles are compared with single crystal surfaces.     

Electronic properties and charge transfer phenomena in Pt nanoparticles on γ-Al(2)O(3): size, shape, support, and adsorbate effects

This study presents a systematic detailed experimental and theoretical investigation of the electronic properties of size-controlled free and γ-Al(2)O(3)-supported Pt nanoparticles (NPs) and their evolution with decreasing NP size and adsorbate (H(2)) coverage. A combination of in situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculations revealed changes in the electronic characteristics of the NPs due to size, shape, NP-adsorbate (H(2)) and NP-support interactions. A correlation between the NP size, number of surface atoms and coordination of such atoms, and the maximum hydrogen coverage stabilized at a given temperature is established, with H/Pt ratios exceeding the 1?:?1 ratio previously reported for bulk Pt surfaces.     

Stability of atomic oxygen chemisorption on Pt‐alloy surfaces

A density functional theory calculation is used to investigate the atomic oxygen (O) stability over platinum (Pt) and Pt‐based alloy surfaces. Here, the stability is connected with the preferential adsorption sites for O chemisorptions and the adsorption energy. Thus, the interaction mechanism between atomic O and metal surfaces is studied by using charge transfer analysis. In this present paper, atomic structure and binding energy of oxygen adsorption on the Pt(111) are in a very good agreement with experiment and previous density functional theory calculations. Furthermore, we obtained that the addition of ruthenium (Ru) and molybdenum (Mo) on the pure Pt surface enhances the adsorption energy. Our charge transfer analysis shows that the largest charge transfer contributing to the metal‐O bonding formation is observed in the case of O/PtRuMo surface followed by O/PtRu surface. This is in consistency with metal d‐orbital characteristic, where Mo has much more empty d‐orbital than Ru in correspondence to accept electrons from atomic oxygen. Copyright © 2016 John Wiley & Sons, Ltd.     

CO在Pt低指数面上吸附行为的理论研究

用密度泛函理论(DFT)中的广义梯度近似(GGA)方法对CO-Pt低指数面吸附体系进行了结构优化, 并对吸附体系的吸附热、C—O键和C—Pt键的键长、布居数分析、电子态密度进行了研究. 计算结果表明, 在0.25 ML(monolayer)的覆盖率下, CO最容易在Pt(100)晶面的桥位、Pt(110)晶面的短桥位、Pt(111)晶面的hcp三重位吸附, 吸附热分别达到了2.11、2.37、1.96 eV; CO在吸附成键过程中伴有电子在CO分子和Pt之间的转移. 吸附后, C—O键被削弱, 键长变长, 金属内部的作用亦被削弱, 其表层Pt 原子的布居数明显降低; 态密度分析表明, CO在吸附过程中, 其4σ、1π、5σ、2π轨道均参与成键.