Electronic Structure Of Platinum Research Articles

Dimensional‐Transformation of Ternary‐Alloy through the Manipulation of Reduction Kinetics

AbstractAlthough significant progress is achieved for the synthesis of noble metal‐based nanocrystals (NCs), the construction of nanostructures with switchable dimensionality is not reported yet. Herein, a facile and general approach for the synthesis of multimetallic (PtCuPb and PtAgPb) NCs with dimensional‐switching properties by manipulating the reduction rate involved in the growth of 2D nanosheets (2D‐NS) and 1D nanowires (1D‐NW) is presented. At a slow reduction rate, PtCuPb and PtAgPb NCs grow to form 2D‐NS, in contrast, at a fast reduction rate, dimensionality alteration occurs to form 1D‐NW. This 2D‐NS to 1D‐NW transition generates compressive surface strain in PtCuPb‐NW with unique intermetallic core, active surface composition and exposes facet to boost oxygen reduction reaction (ORR). In PtCuPb‐NW, compressive strain shifting the d‐band electronic structure of platinum and weakening chemisorption of oxygenated species are shown. In detail, PtCuPb‐NW/C exhibits superior specific and mass activities (4.91 mA cm−2 and 2.65 A mgPt−1) for ORR that are ≈19 and ≈15 times higher than the commercial Pt/C. The observations made and chemical principles unveiled in this effort indicate that a general approach exists for dimensional‐switching and show a new class of Pt‐based electrocatalysts with enhanced performance for fuel cells.

Facile synthesis and enhanced catalytic activity of electrochemically dealloyed platinum–nickel nanoparticles towards formic acid electro-oxidation

Abstract To obtain the electrocatalyst with an improved electrocatalytic performance towards formic acid electro-oxidation (FAEO), a simple impregnation method is used to prepare Pt3Ni nanoparticles loaded on carbon black, assisted with electrochemically dealloying process. The X-ray powder diffraction (XRD) results as well as transmission electron microscopy (TEM) analysis of as-synthesized electrocatalyst demonstrates that the reduction temperature has a great influence on the FAEO activity of the dealloyed Pt3Ni nanoparticles. X-ray photoelectron spectroscopy (XPS) analyses confirm the variation in the electronic structure of platinum by incorporation of nickel atoms which reduces chemisorption of toxic carbon monoxide and promotes the dehydrogenation pathway of FAEO. The size of the dealloyed Pt3Ni nanoparticles remains within the range of about 2.7 nm. All electrochemical results illustrate that the performance of the as-obtained electrocatalyst towards the FAEO is significantly enhanced. Moreover, the carbon black content, incorporation of Ni atoms, and reduction temperature conditions have been proven to be the key factors for modification of the crystal structure and morphology which leads to enhanced catalytic performance.

Electronic and Geometric Structures of Supported Platinum, Gold, and Platinum−Gold Catalysts

X-ray absorption spectroscopy (XAS) on the Pt and Au L3 edges showed that the structure of platinum and gold nanoclusters differs from that of the bulk metals. The interatomic distances are contracted and the s, p, and d orbitals rehybridize, which results in less filled 5d valence bands. The XAS spectra of the monometallic clusters were used to interpret the effect of alloying platinum and gold. Preparation of SiO2- and TiO2-supported PtAu catalysts from a Pt2Au4(C⋮CBut)8 precursor resulted in well-mixed bimetallic clusters. Gold was preferentially located on the surface of the small clusters. X-ray absorption near-edge spectroscopy (XANES) showed that the electronic structure of platinum and gold differed from that in the monometallic clusters. The Fermi levels of platinum and gold shifted in opposite directions: the position of the Pt L3 edge shifted to higher energy and the Au L3 edge shifted to lower energy. Furthermore, the white-line intensity increased for gold and decreased for platinum. These c...

Electronic structure of platinum complex/Zn-porphyrinato assembled macrosystems, related precursors and model molecules, as probed by X-ray absorption spectroscopy (NEXAFS): theory and experiment

Macromolecular arrays containing Zn-diethynylporphyrin coordinated to platinum tethers were studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the N K-edge. Porphin (Pf), 2,8,12,18-tetraethyl-5,15-diethynyl-3,7,13,17-tetramethylporphyrin (PfNH) and 2,8,12,18-tetraethyl-5,15-diethynyl-3,7,13,17-tetramethylporphyrinatozinc(II) (Zn-Pf2), have been taken as experimental models. The resonances assignment is supported by static-exchange (STEX) calculations for the porphin (Pf) and Zn-porphin (Zn-Pf1) molecules. By calculation, four π* resonances occur for porphin (Pf) and two for Zn-Pf1. Experimentally we detect four resonances for porphin and PfNH, and two for the Zn-complexed ones. They can be assigned to core excitations to a couple of energy split π* orbitals for each not equivalent N atom. Our calculations justify the observed low π* intensity as due to an extended delocalization of the π* orbitals. The NEXAFS investigations on platinum complex/Zn-porphyrinato assembled macrosystems (i.e. Pt-ZnPf1, Pt-ZnPf2 and Pt-ZnPf3) show the influence of the Pt tethers bound to the porphyrin through the ethynyl moiety on the electronic structure of the tetrapyrrolic cycle.

Quantum Chemical Studies on Molecular and Electronic Structure of Platinum and Tin Adducts with Guanine

The electronic structure of model platinum-guanine and tin-guanine adducts has been studied both at semiempirical level and by using ab initio methods at the correlation level. The possible binding energies have been evaluated and most probable strustures were determined. In all cases the binding enegies of the gaseous tin chelates are rather large. The stabilizing role of the hydrogen bonds has been pointed out in tinguanine chelates as well as comparison with the relevant platinum complexes is discussed.

The Effect of Ruthenium on the Binding of CO, H2, and H2O on Pt(110)

Ruthenium is known to improve the CO tolerance of platinum based fuel cell catalysts, but the mechanism is unclear. Some investigators believe that the main role of ruthenium is to dissociate water, and thereby provide a faster route for the oxidation of CO. Other investigators believe that ruthenium changes the electronic structure of platinum, in a way that lowers the binding energy of the CO and thereby promotes easy reaction. In this study, we have used TPD to measure the magnitude of the two effects in UHV. When we deposit 0.25 monolayers of ruthenium on Pt(110), we find that the adsorption properties of the surface change substantially. The surface only adsorbs about half as much CO and H2. The sticking probability of water is also reduced. The β2 hydrogen peak disappears, whereas the α1 CO peak is attenuated by a factor of 2. The binding energy of the α1 CO decreases from 31 to 29 kcal/mol when ruthenium is added to the surface, whereas the binding energy of the α2 CO decreases from 25 to 23 kcal/m...

H2 and H2/CO oxidation mechanism on Pt/C, Ru/C and Pt–Ru/C electrocatalysts

The oxidation kinetics of H2 and H2 + 100 ppm CO were investigated on Pt, Ru and Pt–Ru electrocatalysts supported on a high-surface area carbon powder. The atomic ratios of Pt to Ru were 3, 1 and 0.33. XRD, TEM, EDS and XPS were used to characterize the electrocatalysts. When alloyed with ruthenium, a decrease in mean particle size and a modification of the platinum electronic structure were identified. Impedance measurements in H2SO4, at open circuit potential, indicated different mechanisms for hydrogen oxidation on Pt/C (Tafel–Volmer path) and Pt–Ru/C (Heyrowsky–Volmer path). These mechanisms also occur in the presence of CO. Best performances, both in H2 and H2 + CO, were achieved by the catalyst with the ratio Pt/Ru = 1. This is due to a compromise between the number of free sites and the presence of adsorbed water on the catalyst. For CO tolerance, an intrinsic mechanism not involving CO electroxidation was proposed. This mechanism derives from changes in the electronic structure of platinum when alloyed with ruthenium.

X-ray Absorption Edges in Platinum-Based Alloys. 2. Influence of Ordering and of the Nature of the Second Metal

Changes in the electronic structure of platinum−3d transition single-crystal alloys (Pt1-xMx with M = Fe, Co, and Ni) are studied as a function of composition, order and nature of the 3d metal. These changes are detected by using synchrotron radiation to analyze the absorption edges. Both the Pt LII and LIII edges and the 3d metal K edge exhibit correlated changes, which are most sensitive to the minority element in the alloy. The modifications on the Pt LII and LIII edges are interpreted as resulting from a variation of the number of holes in the 5d band, corresponding to an electron transfer to platinum from the 3d metal. The corresponding changes observed on the 3d metal K edge point to a population rearrangement between the probed levels originating from the atomic 3d, 4s, and 4p levels. An effect of ordering has also been observed in the Pt−Co alloys the magnitude of which is greater at the Co K-edge for the lowest concentration of this element. Finally, we compare our results to available calculations and experimental results obtained by other techniques.

Electronic structure of platinum at ultrahigh pressure

Abstract Platinum is studied, theoretically, under very high compression. The calculated equation of state is found to agree well with the recent experimental data. At V/V0 = 0.4, where V0 is the experimental equilibrium volume, we find a transition from the face centered cubic structure (fcc), found at ambient pressure, to the body centered cubic structure (bcc). The calculated transition pressure is 26 Mbar. The stabilization of the bcc structure is explained by the eigen value sum.

Electronic structure of platinum(II) antitumor complexes and their interactions with nucleic acid bases. Part II

Using the semiempirical all-valence method (GRINDOL) together with the virtual-charge method to represent the solvent effect, the solvation (hydration) energies of some platinum (II) complexes were calculated. Based on these results and on the results of calculations of formation energies in the gas phase, the formation of the platinum dimers and some possible interactions of Pt(II) antitumor complexes with guanine, thioguanine and the phosphate group are discussed. It is concluded that: (i) the solvent more strongly stabilizes the Pt(II) 06-N7 chelate than it does the intrastrand N7-N7 crosslink, but the latter is enthalpy favoured; (ii) binding of Pt(II) to the phosphate group is less likely than binding to guanine in N7 region; (iii) platinum dimers, if formed, react with guanine and will disappear from solution under normal conditions; and (iv) the solvent effect does not change the main conclusions obtained for the gas phase in cases of the interaction of cis-Pt(II) with thioguanine and trans-Pt(II) with guanine.

Electronic Structure of Platinum in Silicon

The negatively charged state of substitutional platinum in silicon is observable by electron paramagnetic resonance (EPR). The g-tensor of the EPR spectrum (labelled Si-Pt[I]) reveals orthorhombic-I symmetry of the centre. The principal g-values, which are g[110] = 1.3867, g[11̄0] = 1.4266 and g[001] = 2.0789, respectively, deviate significantly from the pure spin value g = 2.0023, indicating substantial contributions from orbital momentum. The g-tensor data were analysed on a model of one electron, with spin S = 1/2, in an orbital triplet state, L = 1. Spin-orbit coupling and crystal field interactions of cubic, tetragonal or orthorhombic symmetry were included in the model. The theoretical analysis can account in a satisfactory manner for the experimentally observed values. The electronic structure of Pt- is concluded to be the 5d96s6p configuration. This is consistent with predominant bonding of platinum with two silicon neighbours and dihedral distortion. The results are incompatible with alternative models, such as the vacancy model or a 5d-version of the Ludwig-Woodbury model. The orbital g-factor is reduced by about 30% by covalency.

The Electronic Structure of Platinum, Palladium and Nickel in Silicon

The Electronic Structure of Platinum, Palladium and Nickel in Silicon

ChemInform Abstract: Electronic Structure of Platinum(II) Antitumor Complexes and Their Interactions with Nucleic Acid Bases. Part 1.

AbstractElectronic structure and intermolecular interactions of the model antitumor Pt(II) complexes with guanine and thioguanine are calculated using the semi‐empirical all‐valence method (GRINDOL).

Electronic structure of platinum(II) antitumor complexes and their interactions with nucleic acid bases. I

Using the semi-empirical all-valence method (GRINDOL) (recently modified and extended to transition series elements), electronic structure and intermolecular interactions of the model antitumor Pt(II) compounds with guanine and thioguanine have been calculated. Several possible models of antitumor action of platinum compounds are discussed. It is concluded that cis-Pt(II) complexes with guanine form stable intrastrand N7N7 cross-links (but chelation to the O6 atom is also possible). The trans-isomers of platinum(II) exclusively form interstrand cross-links, but the cis-Pt(II) complexes with thioguanine form almost entirely the N7S five-membered chelates.

Electronic Structure of Platinum and Palladium Alloys

The electronic structure of some dilute alloys of Pd and Pt is studied in relation with the band structure of the pure metals (PdRh, Ru… PtIr, Re…). We qualitatively discuss the magnetic susceptibility and specific heat measurements taking account of the band-structure effects (deviation from the rigid-band model) and of the exchange-enhancement effects. We point out that NMR measurements allow the determination of the local susceptibility on the impurity site and of its exchange enhancement factor as shown by new experimental results on Pt-Ir alloys.