A study of the growth and CO electrooxidation behaviour of PtRh alloys on Pt{1 0 0} single crystals

The formation of quasi-crystalline PtRh two dimensional films supported on Pt{1 0 0} is described. For the first time, the voltammetry of PtRh{1 0 0} single crystal alloys covering the whole range of alloy composition is reported. Synthesis follows a similar procedure to that described previously for the formation of PtPd alloys supported on Pt{h k l} but with some important provisos concerning the final annealing step. CO electrooxidation was used as a probe reaction and for certain PtRh{1 0 0} surface alloys, unusually high electrocatalytic activity was observed relative to monometallic Pt and Rh electrodes. The flame annealing of a “PtRhPt sandwich” precursor structure was found to be the best method of forming the PtRh alloy surfaces. For PtRh films annealed under nitrogen, significant phase separation was observed in agreement with previous surface science studies of PtRh adlayers on Pt{1 0 0} annealed in the absence of oxygen. In addition, an excess of Rh in the “sandwich structure” from which the alloy was formed tended to preclude good alloy formation. It is suggested that the protocols for thin film formation described may prove useful in many other important electrocatalytic systems.     

General trends of the carbon penetration in Si(0 0 1) surfaces: influences of relevant parameters

Due to its importance in Si-based devices, carbon incorporation in a silicon matrix has become an object of intensive research. However, the size difference between carbon and silicon makes this incorporation difficult, and only small amounts of carbon (a few percent) can be introduced without giving rise to SiC precipitation. Experimental and theoretical studies combined together have led to important progress in the general understanding of surface–subsurface carbon incorporation in the clean and hydrogenated Si(0 0 1) reconstructed surfaces. These results emphasize the role of the surface reconstruction and the carbon–carbon interactions. However, the Si(0 0 1) surface often presents defects such as dimer vacancies or ad-dimers. By modifying the local stress, these defects can play an important role in carbon incorporation. We review the results of recent studies and discuss the role of different relevant parameters such as local stress, carbon–defect interactions, carbon concentration, position and orientation of the defects, defect–defect distance, defect concentration… Finally, we show how, acting on the surface defects, one might improve carbon penetration in Si(0 0 1) and allow a better control of the carbon position in the subsurface layers.     

Preferential oxidation of carbon monoxide adsorbed on Pd submonolayer films deposited on Pt(1 0 0)

The electrochemical behavior of palladium modified Pt(1 0 0) in perchloric acid solution has been studied by means of cyclic voltammetry, CO bulk oxidation and in-situ Fourier transform infrared (FTIR) spectroscopy. FTIR spectra on Pd submonolayers reveal that at low potentials (about 0.4 V) Pd-bounded CO is oxidized preferentially, whereas at the same time the Pt–CO oxidation rate is rather slow.     

The S1 ← S0 0–0 transition energies of polychlorinated dibenzofurans (PCDFs) revisited: CIS(D) and MP2 calculations with correction for correlation energies

This paper reexamines the structures and energies of dibenzofuran and twenty PCDFs in S₁–S₃ states. It was demonstrated that, although the CIS method gives a false relative ordering of excited states, the false ordering can be remedied by the CIS(D) method. Moreover the CIS geometries of typical PCDF molecules reasonably agree with their SAC-CI geometries. It was found that molecules chlorinated at the 1- and 9-positions are twisted in the S₂ state but are planar in other states, except for 1,4,6,9-TeCDF and fully chlorinated dibenzofuran (OCDF). The twisted structure of 1,4,6,9-TeCDF occurs in the S₃ state, but the structure of OCDF is twisted in every state. We partitioned the molecule into the parent structure and four chlorine groups and measured the twist energy with reference to the ground state. Then, the S_i ← S₀ 0–0 transition energies (i = 1, 2) calculated using the CIS(D) and MP2 methods could be expressed as a multiple linear equation with components and twist energy. It was further confirmed that if the multiple linear equation is corrected for residual correlation energies of the parent structure, it can predict the S₁ ← S₀ 0–0 transition energies with high precision.     

Electro-oxidation of dimethyl ether on platinum nanocubes with preferential {1 0 0} surfaces

Platinum nanocubes with preferential {1 0 0} surfaces are synthesized by colloidal method and characterized by TEM image and hydrogen adsorption/desorption process. The mean diameter of Pt nanocubes is ca. 10.5 nm. The hydrogen adsorption/desorption wave shows clearly the <1 0 0> preferential orientation. It is proved that dimethyl ether oxidation on Pt nanocubes exhibits similar features with bulk Pt single crystal electrode. The catalytic activity of Pt nanocubes for DME electro-oxidation is nearly 3× higher than that on commercial available Pt black catalyst. After potential step to 1.2 V, the surfaces of Pt nanocubes are reconstructed and the activity for DME electro-oxidation decreases.     

Preparation and electrocatalytic activity of Rh adlayers on Pt(1 0 0) electrodes: reduction of nitrous oxide

Rhodium adlayers (submonolayer range) have been prepared on Pt(1 0 0) electrodes by electrodeposition from acidic solutions containing an excess of chloride. These Rh/Pt(1 0 0) electrodes give a well-defined voltammetric signal in the hydrogen adsorption region, which gives evidence of a high level of order in the Rh adlayer and allow a reliable estimation of the coverage. The voltammetric behavior of the Rh/Pt(1 0 0) electrodes points to an epitaxial growth with formation of rhodium islands. The well-ordered bimetallic surfaces freshly prepared were tested as electrocatalysts for nitrous oxide reduction and the responses were compared with those of the bulk Pt(1 0 0) and Rh(1 0 0) electrodes. The voltammogram for the bimetallic surface showed well separated N₂O reduction signals for Rh and Pt surface zones. An exceptionally high electrocatalytic activity for the Rh adlayer was found for low coverages. This behavior is explained on the basis of a high activity of the rhodium adatoms in the periphery of the islands.     

New understanding of the nature of OH adsorption on Pt(1 1 1) electrodes

The adsorption of hydroxyl on Pt(1 1 1) single crystal electrodes from aqueous acidic solutions is carefully reinvestigated. The effect of small additions (10⁻⁸–10⁻⁵ M) of chloride and bisulphate anions on the OH adsorption region in perchloric acid solution has been studied. Two regions can be differentiated in the voltammetric profile, that behave differently after the addition of the foreign anion. The initial broad adsorption process is unaffected until the highest concentration is attained. However, the sharper peak at higher potentials is affected even at the lower anion concentration. Since mass transport limitations allow to discard the anion adsorption as the main process giving this peak, we propose that the two processes are due to the dissociative adsorption of two different kinds of water, that are affected by the anion in a different way. From this idea, a new model, based on the Frumkin adsorption isotherm, is proposed, which gives an excellent fit of the experimental results.     

Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces and its role in the HOR

Hydrogen adsorption isotherms, evaluated by combination of cyclic voltammetry and chronoamperometry, are reported on Pt(1 1 1) and Pt(1 0 0) surfaces in 0.1 M HClO₄. We found that at E > 0.05 V Pt(1 1 1) and Pt(1 0 0) are only partially covered by the adsorbed hydrogen (H_ad). On both surfaces, a full monolayer of the adsorbed hydrogen is completed at −0.1 V, i.e. the adsorption of atomic hydrogen is observed in the hydrogen evolution potential region. We also found, that the activity of the hydrogen oxidation reaction is mirrored by the shape of the hydrogen adsorption isotherms, implying that H_ad is in fact a spectator in the HOR.     

Electroreduction of anions on chemically etched and electrochemically polished Bi(1 1 1) electrode

Electroreduction kinetics of to anions at chemically etched (CHE) and electrochemically polished (EP) Bi(1 1 1) electrodes has been studied using rotating disc electrode method. The surface nanostructure of CHE Bi(1 1 1) and EP Bi(1 1 1) electrodes has been studied by in situ STM and the very different values of root mean squared roughness (Rms) have been obtained (1000 times higher for CHE Bi(1 1 1) (Rms  143 nm) than for EP Bi(1 1 1) (Rms  0.145 nm)). The influence of the nanoroughness of CHE Bi(1 1 1) on the current density, heterogeneous reaction rate constant and corrected Tafel plots (cTp) has been demonstrated. For CHE Bi(1 1 1) the more pronounced inhibition of electroreduction reaction at moderate negative surface charge density has been observed in comparison with EP Bi(1 1 1), caused by the differences in surface charge density and also in diffuse layer ψ₀ potential drop values at crystallographically different homogeneous regions (planes) exposed at the surface of the macroheterogeneous polycrystalline CHE Bi(1 1 1) surface. The very low apparent transfer coefficient α_app obtained indicates the nearly activationless charge transfer mechanism for electroreduction at the CHE Bi(1 1 1) electrode similarly to EP Bi(1 1 1). However, α_app only very weakly depends on Rms for the Bi electrodes at high negative surface charge densities where the values of ψ₀ potential are nearly equal for different planes at fixed electrode potential. At very high negative surface charge densities the cationic catalysis through the adsorbed ion pairs is possible.     

Nickel-underpotential deposition on Pt(1 1 0) in sulphate-containing media

Nickel-underpotential deposition (UPD) on platinum electrodes has been studied in sulphuric media using cyclic voltammetry. The process seems structure-related: among the surfaces we did study, only (2 × 1)-reconstructed Pt(1 1 0) surface yields nickel-UPD. At low pH values, nickel-UPD peaks overlap with the hydrogen adsorption/desorption region. At higher pH (2–3) the overlapping is less severe: nickel coverage increases and can be estimated. Nickel-UPD on platinum (1 1 0) is a slow and irreversible process. The co-adsorption of sulphate or hydrogen-sulphate anions is probable. Nickel-UPD on Pt(1 1 0) occurs even in the presence of CO. Nickel submonolayer on Pt(1 1 0) exhibit remarkable activity towards CO-oxidation.     

A spectroscopic proof of a surface equilibrium between on top and bridge bonded CO at Pt(1 1 0) in acid solution

According to most of works in the literature, adsorbed carbon monoxide at Pt(1 1 0) electrodes in acid media presents only linear bonded (CO_L) so-called, atop geometry. In the present work, the formation of bridge bonded carbon monoxide (CO_B) is shown via in situ infrared FT spectra, measured on a Pt(1 1 0) electrode covered with 25% CO, in HClO₄ solutions. For the first time, the inter conversion between atop and bridge bonded CO at potentials in the hydrogen adsorption region is reported in acid solution. Band intensity and band center frequency indicate dipole–dipole coupling effects in spite of the low CO total coverage.     

A possible approach towards spin-polarized transport through single molecule magnets: Mn12 on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0)

The possibility to use the Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) system as a substrate for future spin-polarized transport measurements on Mn₁₂ single molecule magnets has been investigated by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy at room temperature. In particular, the stability of the iron layer during a wet chemical preparation of Mn₁₂ monolayers was studied. The results demonstrate that Mn₁₂ can be deposited on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) while preserving the metallic nature of the ferromagnetic iron layer which is required as a possible source of spin-polarized electrons in future studies.     

Nitrate reduction on Pt(1 1 1) surfaces modified by Bi adatoms

This work presents the effect of Bi modification of Pt(1 1 1) electrodes on the electroreduction of nitrate anions by using voltammetric and FTIR experiments. On Pt(1 1 1) nitrate consumption occurs at potentials lower than 0.35 V, but with Pt(1 1 1)/Bi this process is shifted to significantly higher potentials (0.6–0.7 V). In the latter surface N₂O was observed as the main product in solution. Different forms of adsorbed NO were detected on the adatom covered surfaces as well as on clean Pt(1 1 1).     

CI Study of CO adsorption on MgO(1 0 0)

Using CI embedding method, we have studied the adsorption of CO on MgO(1 0 0). The MgO(1 0 0) substrate is described by a Mg₉O₉ (3 × 3 × 2) core cluster, embedded in ionic (Mg²⁺/O²⁻) core potentials. The adsorption energy is calculated to be 0.11 eV at the CI level with a blue shift of 19 cm⁻¹ for CO stretching on MgO(1 0 0). The dispersion accounts only 35% of the total binding energy of CO on MgO(1 0 0). The CO/MgO(1 0 0) interaction is weak and mainly of the van der Waals type with only slight chemical bonding characters.     

Electrochemical deposition of PbTe onto n-Si(1 0 0) wafers

Electrochemical deposition of PbTe from 50 mM Pb(NO₃)₂ + 1 mM TeO₂ + 0.1 M HNO₃ solution onto n-Si(1 0 0) wafers was studied using cyclic voltammetry (CV), chronoamperometry, ex situ SEM, XRD and EDX. Electrochemical behavior of n-Si(1 0 0) electrode in electrolytes 50 mM Pb(NO₃)₂ + 0.1 M HNO₃ and 1 mM TeO₂ + 0.1 M HNO₃ was also studied. No underpotential deposition (UPD) of Pb and Te onto n-Si was observed in the investigated systems indicating weak Pb–Si and Te–Si interactions. Deposition of Pb and Te on n-Si occurred with overvoltage via 3D island growth. Electrosynthesis of PbTe (NaCl-like structure, a = 0.650 nm) takes place due to codeposition of Pb and Te at potentials E > E_{Pb²⁺/Pb⁰} (lead UPD onto tellurium). Cathodic deposition of PbTe onto n-Si(1 0 0) is irreversible – there is no anodic current in the CV curve. Oxidation of PbTe on n-Si is observed only under illumination, when photoelectrons and photoholes are generated in silicon substrate.     

A bonding study of cyclopentene (c-C5H8) adsorption on Ni(1 1 1) surface

The adsorption of cyclopentene (c-C₅H₈) on Ni(1 1 1) was studied using DFT and semiempirical calculations. Preferred site and geometry calculations were carried out considering a Ni(1 1 1) surface and a unit cell of 64-atoms. The tetrahedral threefold hollow position was identified as the most favorable site, with a surface-molecule minimum distance of 1.83 Å. A bending structure is adopted when the molecule is adsorbed where the carbon atoms of the double bond are closer to the surface forming an angle of 160° among non-equivalents carbon atoms. The metal surface was represented by a two-dimensional slab with an overlayer of c-C₅H₈/Ni of 1/9 ratio. We also computed the density of states (DOS) and the crystal orbital overlap populations (COOP) corresponding to CC, CNi, CH, and NiNi bonds. We found that both NiNi bonds interacting with the ring, and the CC bond are weakened after adsorption, this last bond is linked significantly to the surface. The hydrogen atoms belonging to the saturated carbon atoms also participate in the adsorbate–surface bonding. The main interactions include the 4s, 3p_z and 5d_z² bands of nickel and 2p_z bands of the carbon atoms of the double bond.     

Experimental (solid + liquid) phase equilibria of (alkan-1-ol + benzonitrile), (amine + benzonitrile) binary mixtures,and (decan-1-ol + decylamine + benzonitrile) ternary mixtures

(Solid + liquid) phase diagrams, SLE have been determined for (octan-1-ol, or nonan-1-ol, or decan-1-ol, or undecan-1-ol + benzonitrile) and for (hexylamine, or octylamine, or decylamine, or 1,3-diaminopropane + benzonitrile) using a cryometric dynamic method at atmospheric pressure. Simple eutectic systems with complete immiscibility in the solid phase and complete miscibility on the liquid phase have been observed. The solubility decreases with an increase of the number of carbon atoms in the alkan-1-ol, or amine chain. The temperature of the eutectic points increases and shifts to lower alkan-1-ol, or amine mole fractions as the alkyl chain length of the alkan-1-ol, or amine increases. The higher intermolecular interaction was observed for the (alkan-1-ol + benzonitrile) systems.