Pt on graphene monolayers supported on a Ni(111) substrate: relativistic density-functional calculations

The structural, energetic, and magnetic properties of Pt atoms and dimers adsorbed on a Ni-supported graphene layer have been investigated using density-functional calculations, including the influence of dispersion forces and of spin-orbit coupling. Dispersion forces are found to be essential to stabilize a chemisorbed graphene layer on the Ni(111) surface. The presence of the Ni-substrate leads not only to a stronger interaction of Pt atoms and dimers with graphene but also to a locally increased binding between graphene and the substrate and a complex reconstruction of the adlayer. The stronger binding of the dimer also stabilizes a flat adsorption geometry in contrast to the upright geometry on a free-standing graphene layer. These effects are further enhanced by dispersion corrections. Isolated Pt adatoms and flat dimers are found to be non-magnetic, while an upright Pt dimer has strongly anisotropic spin and orbital moments. For the clean C/Ni(111) system, we calculate an in-plane magnetic anisotropy, which is also conserved in the presence of isolated Pt adatoms. Surprisingly, upright Pt-dimers induce a re-orientation of the easy magnetic axis to a direction perpendicular to the surface, in analogy to Pt(2) on a free-standing graphene layer and to the axial anisotropy of a gas-phase Pt(2) dimer.     

Geometric and magnetic properties of Pt clusters supported on graphene: relativistic density-functional calculations

The geometric and magnetic structures of small Pt(n) clusters (n = 1 - 5) supported on a graphene layer have been investigated using ab initio density functional calculations including spin-orbit coupling. Pt-Pt interactions were found to be much stronger than the Pt-C interactions promoting the binding to the support. As a consequence, the equilibrium structure of the gas-phase clusters is preserved if they are deposited on graphene. However, the clusters bind to graphene only via at most two Pt-C bonds: A Pt(2) dumbbell prefers an upright position, the larger clusters are bound to graphene only via one edge of the planar cluster (Pt(3) and Pt(5)) or via two terminal Pt atoms of a bent Pt(4) rhombus. Evidently, the strong buckling of the graphene layer induced by the Pt-C bonds prevents the formation of a larger number of cluster-support bonds. As the local spin and orbital magnetic moments are quenched on the Pt atoms forming Pt-C bonds, the magnetic structure of the supported clusters is much more inhomogeneous as in the gas-phase. This leads to noncollinear magnetic structures and a strongly reduced magnetic anisotropy energy.     

Force-constant calculations for Ni(PF3)4, Pd(PF3)4 and Pt(PF3)4

Force constants for a SVFF approximation have been obtained for the compounds Ni(PF₃)₄, Pd(PF₃)₄ and Pt(PF₃)₄ using the FG matrix method and vibrational assignments (A₁, E and F₂ symmetry species) on the basis of regular tetrahedral symmetry, point group T_d. Six primary force constants involving bond stretching and bond angle changes and five interaction force constants were used to simultaneously fit the observed wavenumbers. The PF stretching force constant differs by only ± 3% for all three compounds and for the free ligand, PF₃. The metal—phosphorus stretching force constant, however, increases significantly in the order Ni < Pd < Pt, and is 65% larger for Pt(PF₃)₄ than for Ni(PF₃)₄. These changes are discussed in terms of the metal—phosphorus—fluorine bonding in M(PF₃)₄ compounds.     

Pt（111）和Pt3Ni（111）表面上CO催化氧化反应的密度泛函理论研究

采用密度泛函理论,对Pt(111)和Pt3Ni(111)表面上CO和O的单独吸附、共吸附以及CO的氧化反应进行了系统的研究. 结果表明, Pt3Ni(111)表面上CO的吸附弱于Pt(111)表面, O的吸附明显强于Pt(111)表面. 两个表面表现出相似的CO催化氧化活性. 表面Ni的存在不但稳定了O的吸附,同时也降低了过渡态O的能量.     

Equilibrium properties of Ag clusters on Pt(111)

Molecular dynamics simulations (MD) and Embedded Atom Method (EAM) potentials are employed to investigate the properties of small Ag clusters deposited on the (111) surface of Pt. The computation of the dissociation energies reveals the most stable structures up to 19 atoms, while MD simulations allow to extract the dynamical diffusion barrier for adatom, dimer and trimer. We obtain dissociation energies in the range of 0.3 eV for the trimer and 0.45 eV for the heptamer and a migration energy as low as 0.05 eV for the adatom.     

State-resolved reactivity of CH4(2nu3) on Pt(111) and Ni(111): effects of barrier height and transition state location

Quantum state-resolved sticking coefficients on Pt(111) and Ni(111) surfaces have been measured for CH4 excited to the first overtone of the antisymmetric C-H stretch (2nu3) at well-defined kinetic energies in the range of 10-90 kJ/mol. The ground-state reactivity of CH4 is approximately 3 orders of magnitude lower on Ni(111) than on Pt(111) for kinetic energies in the range of 10-64 kJ/mol, reflecting a difference in barrier height of 28+/-6 kJ/mol. 2nu3 excitation of CH4 increases its reactivity by more than 4 orders of magnitude on Ni(111), whereas on Pt(111) the reactivity increase is lower by 2 orders of magnitude. We discuss the observed differences in the state-resolved reactivity for the ground state and 2nu3 excited state of methane in terms of a difference in barrier height and transition state location for the dissociation reaction on the two metal surfaces.     

Hydrogen chemisorption on Pt/Ni (111) systems

Extensive self-consistent real-space recursion-method calculations were performed for the Pt overlayer or the Pt₇ cluster on the Ni(111) surface and for hydrogen chemisorption on these systems. Correlations between the surface-atom local density of electronic-state properties before chemisorption, surface reactivity, and the initial-state contribution to the metal core-level shifts, respectively, is documented and discussed. The experimentally observed catalytic properties of Pt Ni alloy surfaces are also briefly considered. © 1996 John Wiley & Sons, Inc.     

The adsorption and co-adsorption of oxygen and carbon monoxide on Pt3Ni(111): a vibrational study

High-resolution electron energy loss spectroscopy has been used to investigate the adsorption and co-adsorption of oxygen and CO on the Pt(3)Ni(111) surface. For the sake of comparison, similar measurements have also been performed on the Pt(111) surface. We find that CO adsorbs at the same manner on both surfaces. By contrast, significant differences between the two surfaces exist concerning the adsorption of O and the co-adsorption of O with CO.     

Preoxidation of CO on Os-modified Pt(111): a comparison with Ru-modified Pt(111)

The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 × 2)-α to (√19 × √19) via the transient structures of (2 × 2)-β and (1 × 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 × 2)-β and (1 × 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (√19 × √19), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.     

Electrodeposition of Au monolayer on Pt(111) mediated by self-assembled monolayers

In-situ scanning tunneling microscopy (STM), cyclic voltammetry (CV), and infrared reflection-adsorption spectroscopy (IRRAS) have been used to examine the electrodeposition of gold onto Pt(111) electrodes modified with benzenethiol (BT) and benzene-1,2-dithiol (BDT) in 0.1 M HClO4 containing 10 microM HAuCl4. Both BT and BDT were attached to Pt(111) via one sulfur headgroup. STM and IRRAS results indicated that the other SH group of BDT was pendant in the electrolyte. Both BT and BDT formed (2 x 2) structures at the coverage of 0.25, and they were transformed into (square root(3) x square root(3))R30 degrees as the coverage was raised to 0.33. These two organic surface modifiers resulted in 3D and 2D gold islands at BT- and BDT-coated Pt(111) electrodes, respectively. The pendant SH group of BDT could interact specifically with gold adspecies to immobilize gold adatoms on the Pt(111) substrate, which yields a 2D growth of gold deposition. Molecular resolution STM revealed an ordered array of (6 x 2 square root(13)) after a full monolayer of gold was plated on the BDT/Pt(111) electrode. Since BDT was strongly adsorbed on Pt(111), gold adatoms only occupied free sites between BDT admolecules on Pt(111). This is supported by a stripping voltammetric analysis, which reveals no reductive desorption of BDT admolecules at a gold-deposited BDT/Pt(111) electrode. It seems that the BDT adlayer acted as the template for gold deposit on Pt(111). In contrast, a BT adlayer yielded 3D gold deposit on Pt(111). This study demonstrates unambiguously that organic surface modifiers could contribute greatly to the electrodeposition of metal adatoms.     

氧原子在具有Pt皮肤的Pt3Ni(111)表面的吸附和扩散

用基于密度泛函理论的第一性原理方法研究了氧原子在具有Pt皮肤的Pt3Ni(111)[记为Pt-skin-Pt3Ni(111)]表面的吸附和扩散特性. 重点研究了氧原子在Pt-skin-Pt3Ni(111)表面的扩散问题, 这对理解Pt-skin-Pt3Ni(111)催化剂的高催化活性有重要意义. 结果表明: 氧原子容易吸附在fcc位; 催化剂Pt3Ni中的Ni原子对催化剂的电子结构有很大影响, 从而改变了其对氧原子的吸附. 用推拉弹性带(NEB)方法搜索氧原子的扩散势垒, 并解释了Pt-skin-Pt3Ni(111)催化剂的高催化活性.     

Sub-surface nucleation of graphene precursors near a Ni(111) step-edge

Graphene nucleation on Ni(111) has been modeled using QM/MD simulations. We demonstrate that graphene precursor nucleation can occur underneath the catalyst surface. In addition, a Ni(111) step-edge is not a static structure, as is often assumed; it is instead highly malleable, being deformed and subsequently healed during graphene nucleation.     

Molecular dynamics simulations of surface oxide-water interactions on Pt(111) and Pt/PtCo/Pt3Co(111)

Classical molecular dynamics simulations of the interactions of water with oxidized Pt(111) and Pt/PtCo/Pt(3)Co(111) surfaces are performed by modeling water with the CF1 central force model that allows molecular dissociation and therefore the presence of other intermediates of the oxygen reduction reaction different from atomic oxygen. It is found that the water-surface oxide interactions do not affect the overall structure of the catalyst represented by an extended periodic slab. However, such interactions are affected by changes in the electrochemical potential which are simulated by higher values of the surface and atomic oxygen charges at increased oxygen coverage. Thus, electrochemical potential as well as the presence of protons and anions products of acid dissociation define the identity and the amount of oxygen reduction reaction intermediates such as OH or H(3)O. We observe agglomerations of water molecules over regions of the surface and the presence of OH and H(3)O in their vicinity. Our simulation model is able to qualitatively reproduce features of the degradation of the catalyst surface after oxidation and reduction cycles.     

Syntheses and structural analyses of variable-stoichiometric Au-Pt-Ni carbonyl/phosphine clusters, Pt3(Pt(1-x)Ni(x))(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 and Pt2(Pt(2-y)Ni(y))(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2, with ligation-induced site-specific Pt/Ni substitutional disorder within butterfly-based Pt3(Pt(1-x)Ni(x))Au2 and Pt2(Pt(2-y)Ni(y))Au2 core-geometries

In ongoing attempts of directed synthesis of high-nuclearity Au-Pt carbonyl/phosphine clusters with [Ni6(CO)12]2- used as reducing agent and CO source, we have isolated and characterized two new closely related variable-stoichiometric trimetallic clusters, Pt3(Pt(1-x)Ni(x))(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1) and Pt2(Pt(2-y)Ni(y))(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2). Their M4Au2 cores may be envisioned as substitutional disordered butterfly-based M4Au2 frameworks (M = Pt/Ni) formed by connections of the two basal M(B) atoms with both (Au-Au)-linked Au(PPh3) moieties. Based upon low-temperature CCD X-ray diffraction studies of eight crystals obtained from different samples, ligation-induced site-specific Pt/Ni substitutional disorder (involving formal insertion of Ni in place of Pt) in a given crystal was found to occur only at the one OC-attached basal M(B) site in 1 or at both OC-attached basal M(B) sites in 2 corresponding to a crystal composite of the Pt3(Pt(1-x)Ni(x))Au2 core in 1 or of the Pt2(Pt(2-y)Ni(y))Au2 core in 2; the Ph3P-attached M(B) site (M(B) = Pt) in 1 and two wingtip M(w) sites (M(w) = Pt) in 1 and 2 were not substitutionally disordered. The resulting variable stoichiometry of the M4Au2 core in 1 may be viewed as a crystal composite of two superimposed individual stereoisomers, Pt4(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1a) and Pt3Ni(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1b), in the averaged unit cell of a given crystal. Likewise, 2 represents the crystal-averaged composite of three individual stereoisomers, Pt4(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2a), Pt3Ni(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2b), and Pt2Ni2(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2c). Formal Ni substitution for Pt at only the basal M(B) site(s) in the four crystal composites each of 1 and 2 was found to vary widely from 17% to 79% Ni in 1 and from 21% to 95% Ni in 2. Nevertheless, reasonably close Pt/Ni occupancy factors were found within each of the four pairs of composite crystals selected from samples obtained from duplicate syntheses. Both 1 and 2 may be formally derived from the electronically equivalent classic butterfly Pt4(mu2-CO)5(PPh3)4 cluster by replacement of its bridging mu2-CO ligand spanning the basal M(B)-M(B) edge with two one-electron donating (Au-Au)-linked AuPPh3 moieties along with the substitution of a terminal CO in place of one or both M(B)-attached PPh3 ligands in 1 and 2, respectively; site-specific Pt/Ni substitutional disorder occurs only at the CO-attached M(B) sites. The variable-stoichiometric 1 and 2 re also electronically equivalent and geometrically related to the crystal-ordered butterfly-based Pt4(mu2-CO)4(PR3)4(mu3-HgX)2 clusters (R3 = Ph3, MePh2; X = CF3, Br, I).     

NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.     

Macroscopic single‐domain graphene sheet on Ni(111)

We observed in situ growth of a single graphene sheet on Ni(111) by low‐energy electron microscopy. The sheet was grown epitaxially beyond the steps on the substrate. The crystalline shapes of graphene islands were clearly seen; the straight edges of the island are crossed at either 60 or 120°, and the linear edges shifted perpendicular to the edge keeping the equilibrium shape. Graphene islands were united to form a single sheet without any grain boundaries and any wrinkles. The Ni substrate of several centimeters in size was covered with a single‐domain graphene sheet. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental and theoretical study of reactivity trends for methanol on Co/Pt(111) and Ni/Pt(111) bimetallic surfaces

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on NiPt(111) and CoPt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various NiPt(111) and CoPt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni-Pt-Pt(111), as compared to the subsurface monolayer Pt-Ni-Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on NiPt(111) and CoPt(111) bimetallic surfaces. On Ni-Pt-Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300 K; this species reacted at approximately 300 K to form CO and H2. On Co-Pt-Pt(111), methoxy was stable up to approximately 350 K and decomposed to form CO and H2. Overall, trends in methanol reactivity on NiPt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.     

A unique vibrational signature of rotated water monolayers on Pt(111): predicted and observed

Six H-bonds in the periodic di-interstitial structure that accounts for scanning tunneling microscope images of "√37" and "√39" wetting layers on Pt(111) are some 0.2 A? shorter than H-bonds are in ice Ih. According to a broadly obeyed correlation, this density functional theory result implies a stringent test of the di-interstitial motif, namely the presence of an OH-stretch band red-shifted from that of ice Ih by more than 1000 cm(-1). Infrared absorption spectra satisfy the test, in showing a feature centered at about 1965 cm(-1), which grows in as deposited water orders.     

Molecule-substrate interaction channels of metal-phthalocyanines on graphene on Ni(111) surface

Molecule-substrate interaction channels of metal-phthalocyanines (MPcs, including NiPc, CuPc, ZnPc, FePc, and CoPc) on graphene on Ni(111) were investigated by employing high-resolution electron energy loss spectroscopy (HREELS). Except the expected IR-active modes, some Raman-active modes were also observed in all of MPcs, which are considered in this study. From the origination of the Raman-active features, it was deduced that MPcs are coupled with the substrate mainly through their central metal atom. The Raman-active modes appear as symmetric peaks in the HREELS in the case of MPcs with Ni, Cu, and Zn, whereas they are asymmetric and appear as a Fano line shape in the case of MPcs with Fe and Co. This spectroscopic difference indicates that the molecule-substrate coupling is completely different in the two cases mentioned above. The molecule-substrate interaction strength is considerably weak and comparable with the π-π interaction between molecules in the case of MPcs with Ni, Cu, and Zn, whereas it is much stronger in the case of MPcs with Fe and Co. From the HREELS observations, it can be suggested that the whole molecule can be effectively decoupled from the underneath Ni(111) by inserting a single layer of graphene between them in the case of MPcs with Ni, Cu, and Zn, whereas only benzene rings can be completely decoupled in the case of MPcs with Fe and Co.     

Electron attachment to Ni(PF(3))(4) and Pt(PF(3))(4)

An experimental study has been made of thermal electron attachment to the transition-metal trifluorophosphine complexes Ni(PF(3))(4) and Pt(PF(3))(4) using a flowing-afterglow Langmuir-probe apparatus. Both complexes are efficient at electron attachment, although the rate constants are somewhat less than collisional. The rate constant for electron attachment to Ni(PF(3))(4) is 1.9 x 10(-7) cm(3) s(-1) at room temperature, about a factor of 2 less than collisional. The activation energy is 39+/-5 meV for the attachment reaction. The rate constant for electron attachment to Pt(PF(3))(4) is 5.4 x 10(-8) cm(3) s(-1) at room temperature, and the activation energy is 84+/-8 meV. For both complexes, a PF(3) ligand is lost on electron attachment, and only the M(PF(3))(3)(-) ion is observed in the negative-ion mass spectrum. Density functional calculations were carried out on Ni(PF(3))(4) and various fragments in order to describe the thermochemistry of the attachment reaction.