Volume 226, Issue 9-10 - Memorial Prof. Kolb

In Memoriam Dieter M. Kolb (1942–2011)

The contribution deals with the thermodynamics of charge carriers in crystals, in particular with the distinction between building and structure elements. In this context the use of the basic mechanistic processes (addition, elimination, substitution) usually applied for describing reactions with organic molecules proves helpful. It is shown that as far as thermodynamics is concerned, the use of building elements is to be preferred. The only advantage of structure elements is that they automatically include site exhaustibility effects, which have to be included by using Fermi-Dirac statistics in the case of building elements. A rigorous proof is given for the applicability of the structure element approach for the interaction free case. If interactions have to be considered and multinary crystals are addressed, the structure element approach becomes artificial. In the case of kinetic considerations structure elements naturally appear in the rate equation. If the rates approach zero the description has to match the building element description. How interactions behave as a function of the deviation from equilibrium is a matter of specific considerations.

The great majority of models that have been developed to predict localized corrosion damage are empirical in nature. As such, they lack the ability to effectively predict damage outside of their immediate realm of calibration and generally can only “predict” what is already known. Furthermore, empirical models generally cannot predict new phenomena or relationships. It is shown that it is possible to describe propagation of corrosion damage deterministically, i.e. to describe how the systems evolves from the present state to the future state on the basis of natural laws [conservation of charge, mass-energy, and mass-charge equivalence (Faraday's law), etc. ], subject to constraint by the natural laws. It is clear that deterministic models have much more predictive power than do empirical models. However, in the general case, it is possible to describe the development of localized corrosion damage in terms of the propagation of an ensemble of corrosion events, rather than as individual cavities. Accordingly, the prediction can be made in terms of statistical terms, for example, probability that the deepest pit will exceed the critical dimension that defines failure ( e.g. , thickness of a pipe wall). In doing so, the statistical parameters (mean depth of the deepest pit with its standard deviation, etc. ) can be calculated deterministically. In order to perform such calculations, we must possess deterministic models for every stage of cavity development (pit nucleation, propagation and repassivation, transition pit into crack, crack propagation, and so forth). Some of these models are outlined in the current review. The deterministic theory outlined here has been applied for the predation of localized corrosion damage in important, practical systems, including pitting in oil field components, in low pressure steam turbine blades and discs, and in condensing heat exchangers, to name but a few of the current and past applications, some of which are reviewed in this paper.

La 0.6 Sr 0.4 CoO 3- δ (LSC) thin film electrodes of about 80 nm thickness were prepared via a sol-gel route on yttria-stabilized zirconia (YSZ) and gadolinium doped ceria (GDC) solid electrolytes. Impedance measurements on microelectrodes fabricated from these films revealed very low polarization resistances for electrochemical oxygen exchange at intermediate temperatures (500–600 ºC). Prerequisite for the very fast oxygen reduction kinetics was a limitation of the preparation (annealing) temperature to 600 ºC. Already after ca. 15 minutes annealing of the sol-gel prepared layer at this temperature, electrodes were electrochemically highly active despite low crystallinity (almost XRD amorphous). LSC electrodes on GDC showed much higher polarization resistances. However, this originated from an additional interfacial resistance rather than from kinetically slow LSC surfaces.

Nitrate reduction on well-oriented platinum surfaces modified with Bi adatoms has been studied. The quantification of the electrocatalytic enhancement of the reaction rate due to the presence of Bi at different coverages was made on Pt(111) and the vicinal surfaces Pt(554) and Pt(332). These contain 9 and 5 atoms-width (111) terraces, respectively, separated by (110) monoatomic steps. The study was then extended to preferentially {111}Pt oriented nanoparticles. In all cases, Bi catalyzes nitrate reduction at high potentials, but the catalytic current suddenly drops when Bi is reduced. The analysis of the variation of catalytic activity with Bi coverage reveals the participation of a third body effect, meaning that Bi impedes the NO formation on the surface that acts as a poison for the nitrate reduction. The poisoning effect was also quantified by measuring the stripping of adsorbed NO spontaneously formed by contacting, with nitrate solutions, electrodes with different Bi coverage. The results of both single crystals and preferentially oriented nanoparticles agree with the supposed third body effect. The comparison of Pt nanoparticles with the stepped surfaces, Pt(554) and Pt(332), suggests that the main differences are related to the size of the terraces and not to the existence of defects/steps that do not seem to play any significant contribution to the catalysis.

The hydrogen evolution reaction (HER) is of fundamental importance in electrochemistry, and despite the large number of theoretical and experimental investigations, it is still far from being completely understood. In the present work, the HER is studied on a Pt(111) surface in a wide range of temperatures down to 120 K. The overall reaction and the first reaction step, the Volmer reaction, were evaluated separately using potentiostatic pulses in order to obtain a detailed understanding of the different reaction steps. At room temperature, the Volmer reaction exhibits a Tafel slope of 120 mV/dec, which is the expected value for a single electron transfer. For the overall reaction a change in Tafel slope at - η ≈ 50 mV is found. While the Tafel slope is ≈ 30 mV/dec for small overpotentials, it is 120 mV/dec for larger overpotentials. For both the overall reaction and the Volmer reaction, a strong dependence of the apparent transfer coefficient on temperature is found. This temperature dependence cannot be explained using classical electron transfer theory.

Impedance spectra of Pt(100) in aqueous 0.5 M H 2 SO 4 and 1 M HCl solutions have been measured in a potential range which includes the hydrogen adsorption/desorption peak and the double layer region. In the latter potential region the positive surface charge and the large dc capacitance imply that the double layer consist mostly of adsorbed anions; the capacitance spectra are markedly different for the two systems, revealing that adsorption/desorption is much faster for sulphate than for chloride.

A study of the current-potential and capacity curves of Au(100) electrode prepared by flame method provide evidence that not only anions or organic molecules but also cations (protonated molecules of dopamine) can lift the reconstruction. Our investigation indicate also that coumarin hinders the adsorption of these cations and thus delays lifting of the reconstruction. It was found that the deprotonated form of dopamine also lifts the reconstruction but at much more negative potentials.

The underpotential deposition (UPD) of Copper on Au(111) in nonaqueous acetonitrile/perchlorate electrolyte has been investigated. The deposition mechanism is thereby strongly influenced by coadsorbed acetonitrile molecules and the stability of the solvation shell around the Cu(I) ions. The UPD mechanism obeys a two step mechanism even in presence of only none specifically adsorbing perchlorate ions. Thereby the first sub monolayer of copper is stabilized by coadsorbed acetonitrile molecules. However already 5% vol water content changes the structure within the double layer and consequently degrades the copper sub monolayer. At about 20% vol water content the Cu(I) ion/solvent complex is still stable. The reduction of Cu(I) ions from this complex is much faster compared to the reduction from the respective Cu(II) water complex.

Lead underpotential deposition on Au(111) surface modified with submonolayer of Pt is studied using cyclic voltammetry and in situ scanning tunneling microscopy methods. The two-dimensional Pt submonolayers (nanoclusters) on Au(111) were obtained by spontaneous Pt deposition on Au(111) from × 10 3 M {PtCl 6 } 2− + 0.1 M HClO 4 solution. The in situ scanning tunneling microscopy data were analyzed using statistical image processing algorithm which enabled quantification of the morphology change on Pt-modified Au(111) surface as a function of applied underpotential. The results suggest that Pb underpotential deposition starts on Au steps and other surface defects, similar to Pb underpotential deposition on Au(111). The further process proceeds by Pb monolayer nucleation and growth on Au terraces into a complete layer. In parallel, the Pb monolayer starts to nucleate on top of the Pt nanoclusters. The final stage of the Pb underpotential deposition is formation of the compact Pb nanoclusters/layer on top of the pre-existing Pt nanoclusters. The scanning tunneling microscopy data suggests that morphology of underpotentially deposited Pb monolayer on Pt-modified Au(111) is similar to the starting surface in terms of the areal density of nanoclusters, their size and shape. The morphological changes of the Pt modified Au(111) surface during Pb underpotential deposition are correlated with cyclic voltammetry results.

By employing high resolution in-situ STM, the adsorption of alkylimidazolium-based cations of EMI + , PMI + , BMI + and OMI + on Au(111) and Au(100) surfaces are investigated systematically. The cation adsorption on both Au(111) and Au(100) are composed of double rows arising from counter-facing imidazolium-based cation pairs. On Au(100), the double rows associated with the four cations show micelle-like appearance along the two √ 2 directions of the Au(100) surface lattice units. The width of the double rows varies depending on the side chain length of the cations, but is constrained by the periodicity along the √ 2 directions. Anions of BF 4 - , PF 6 - , CF 3 SO 3 - and Tf 2 N - do not influence the micelle-like adsorption structure. On Au(111), the double rows are formed only when the terraces are etched to several atoms wide. Most likely, the underneath Au surface experiences restructuring to accommodate the double row structure, and the worm-like orientation of the double rows is the consequence of strain release. Both the micelle-like and worm-like adsorption structures would be lifted upon cathodic potential excursions when the surfaces are driven to undergo ordinary Au(100)-hex and Au(111)-( √ 3 × 22 ) reconstructions. These results reveal that the ordered micelle-like structure on Au(100) and the irregular worm-like structure on Au(111) are of the same nature.

The immersion method was used to measure the potential of zero free charge ( E pzfc ) of a bare Au(111) electrode and the electrode modified by self-assembled monolayers (SAMs) of 1-thio- β -D-glucose ( β -Tg) and 2,3-di- O -phytanyl- sn -glycerol-1-tetraethylene glycol-D,L- α -lipoic acid ester (DPTL). The measurements were performed in three electrolyte solutions: 0.1 M NaH 2 PO 4 , 0.1 MKClO 4 and 0.1 M NaF. The E pzc of the Au(111) electrode has different values in these electrolyte solutions due to the specific adsorption of phosphate and fluoride anions on the Au(111) surface. In contrast, when the gold surface is covered by a SAM of β -Tg, the adsorption of anions is suppressed and similar values of E pzfc were measured in the three electrolytes. Additional chronocoulometric experiments were performed to determine the potentials of the zero total charge ( E pztc ) for the thiol covered electrode. Significant differences between numerical values of potentials of the zero free charge E pzfc and potentials of the zero total charge E pztc were observed.

The adsorption behavior of tert -pentanol (TPT) on Ag(100) and Ag(110) single-crystal faces from aqueous 0.05 M KClO 4 was investigated on the basis of chronocoulometric and differential capacity measurements. The adsorption data on Ag(100) were analyzed thermodynamically and the standard Gibbs energy of adsorption, Δ G 0 ads , was determined as a function of the electrode potential and of the charge density, σ M , on the metal. The adsorption parameters for TPT adsorption on Ag(100) were compared with those on Ag(111), Hg, liquid Ga and Bi(001), available in the literature. The surface dipole potential of the TPT film at maximum coverage on the above metal surfaces was estimated by a novel approach that requires no a priori assumptions as to the predominance of either water desorption or TPT adsorption on the value of the shift, E N , of the potential of zero charge from zero to maximum coverage. Instead, the E N values for the different metals were estimated a posteriori, under the assumption that the water molecules that are still present at maximum TPT coverage maintain the same orientation as on the bare metal surface at the same charge density σ M . The lack of a correlation between the Δ G 0 ads values on the different metal surfaces and their hydrophilicity indicates that TBT adsorption depends primarily on its particular conformation and on the metal surface structure.

The recent availability of tetrahedral palladium (Pd TH ) nanocrystals with cleaned surfaces allowed us to evaluate their facet-specific electrochemical properties as a new support of platinum monolayer (Pt ML ) catalysts. The Pd–Pt ML core-shell electrocatalyst was examined by combining structural analyses and Density Functional Theory (DFT) with electrochemical techniques. The surfaces of the Pd TH core are composed of (111) facets wherein the Pd atoms are highly coordinated and have low surface energy. Our results revealed that in comparison with sphere Pd (Pd SP )-supported Pt ML or pure Pt, the Pd TH -supported Pt ML features more surface contraction and a downshift of d -band relative to the Fermi level. These geometric- and electronic-effects determine the higher activity of Pt ML /Pd TH /C for the oxygen reduction reaction (ORR) compared to that of Pt ML /Pd SP /C. This shape-property interdependence illuminated new approaches to basic- and applied- research on Pt-based ORR electrocatalysts of significant importance to the widespread use of fuel cells.

Motivated by our previous studies on metallic substrates, in the present work we addressed the functionalization and the subsequent metallization of a hydrogen-terminated n -Si(111) electrode. DFT provides atomistic insights on the grafting mechanism of 4-pyridinecarboxaldehyde (C 6 H 5 NO) what encouraged electrochemical investigations, i. e. cyclic voltammetry and in-situ STM, combined with XPS measurements which together provide evidence for a successful transfer of the so far obtained knowledge from metal single crystal to semiconductor surfaces.

Redox-driven ion transfer processes accompanying p-doping and undoping were investigated for poly(3,4-ethylenedioxythiophene) (PEDOT) films. The PEDOT films were potentiostatically grown from acetonitrile solutions of monomer onto the Au electrodes of 10 MHz AT-cut quartz crystal resonators. The films were then sequentially exposed to 0.1 M LiClO 4 /CH 3 CN solution and to two deep eutectic solvent (DES) media, Ethaline 200 (choline chloride and ethylene glycol in 1:2 molar ratio) and Propaline 200 (choline chloride and propylene glycol in 1:2 molar ratio). In each case, the electron and ion transfer processes accompanying PEDOT redox switching were investigated using the EQCM. The maintenance of film electroneutrality was achieved in quite different ways in the three media. In contrast to anion transfer domination in 0.1 M LiClO 4 /CH 3 CN solution, cation transfer (in the opposite direction) was dominant in Ethaline 200. In Propaline 200, a two stage mixed mechanism was observed, with cation transfer domination in the early stages of p-doping (later stages of undoping) and anion transfer domination in the later stages of p-doping (earlier stages of undoping). The roles of thermodynamic and kinetic factors in these competitive situations were explored by observing the effect of experimental timescale (potential scan rate) in voltammetric experiments.

The search for effective additives is important for the development of new metal deposition processes. The hard/soft concept is a possible way for the selection of suitable substances. Hardness or softness are connected with molecular properties like electronegativity or the chemical potential of electrons. The density functional theory (DFT) provides methods for calculating the respective values. Corresponding function of the side of the electrode is the Fermi energy, the upper level of occupied electron energy levels. Hardness and softness of the electrode depend on the electrode potential. This dependence will be discussed.