Ulrich Stimming

Visualization of Surfactants on Nanostructured Palladium Clusters by a Combination of STM and High-Resolution TEM

Scanning tunneling microscopy (STM) and high-resolution transmission electron microscopy (TEM) have been used to determine the dimensions of a series of palladium clusters stabilized by tetraalkylammonium salts. Electrochemically prepared colloids were used in which the average diameter of the inner metal core was varied between 2 and 4 nanometers, and the size of the ammonium ions was adjusted in the series +N(n-C4H9)4 < +N(n-C8H17)4 < +N(n-C18H37)4. The difference between the mean diameter determined by STM and that measured by TEM allows the determination of the thickness of the protective surfactant layer. On the basis of these studies, a model of the geometric properties of ammonium-stabilized palladium clusters has been proposed. Suggestions for the mechanism of the STM imaging process are also made.

Ethanol oxidation on PtRu electrodes studied by differential electrochemical mass spectrometry

Abstract The oxidation of ethanol in aqueous HClO 4 was studied on co-electrodeposited PtRu electrodes in a temperature range of 5–40°C by means of differential electrochemical mass spectrometry (DEMS). The formation of carbon dioxide and acetaldehyde were monitored by ion currents at appropriate masses. The presence of Ru promotes the surface reaction between the organic chemisorbate and the oxidant, whereas it may inhibit ethanol adsorption on the electrode. Optimum composition of the electrode for total oxidation was found to vary from Pt 0.67 Ru 0.33 at 5°C to Pt 0.85 Ru 0.15 at higher temperatures.

CO adsorption and oxidation on Pt and PtRu alloys: dependence on substrate composition

Adsorption and electrooxidation of CO on polycrystalline Pt and PtRu alloys (Pt0.5Ru0.5, Pt0.7Ru0.3) has been studied by means of in situ infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS). Within the resolution limits of this study (ΔR/R ≈ 10−3) only one adsorbed CO(a) species at ν ≈ 2070–2075 cm−1 (Pt) and 2050–2060 cm−1 (PtRu) was detected. No significant difference in CO frequency between Pt0.5Ru0.5 and Pt0.7Ru0.3 was found. The oxidation of CO adsorbed in the hydrogen region on PtRu shows distinct differences when compared with a polycrystalline Pt electrode. Whereas on Pt the oxidation of CO proceeds in two stages, on PtRu cyclic voltammetry shows only one oxidation peak at a less positive potential compared to Pt. The lowest peak potential for CO adsorbate oxidation was observed on Pt0.5Ru0.5 at about 380 mV vs. rhe. The results for CO oxidation obtained on Pt and Pt0.5Ru0.5 deposited onto a porous Au substrate by on line mass spectrometry were in accordance with the ir spectroscopic data. It was shown that Ru atoms at the surface of a PtRu bimetallic electrode increase the CO tolerance of the catalyst. The promoter effect of Ru is explained with the bifunctional mechanism for CO oxidation where oxygen containing species are bounded preferentially to ruthenium surface atoms.

Anion adsorption from sulfuric acid solutions on Pt(111) single crystal electrodes

Abstract Anion adsorption from sulfuric acid solutions on a Pt(111) electrode was investigated with in-situ STM and cyclic voltammetry. An ordered 2 1 1 2 adlattice is observed in the potential range between 0.5 and 0.7 V. This adlattice is attributed to specifically adsorbed anions and interpreted as a coadsorbate of sulfate anions and water. Similarities as well as differences to anion adsorption on Rh(111) and Au(111) are discussed.

CO adsorption and oxidation on a Pt(111) electrode modified by ruthenium deposition: an IR spectroscopic study

Abstract CO adsorption and electro-oxidation on a Pt(111) electrode modified by Ru deposition was studied by IR spectroscopy. The IR spectra in the region of the CO stretching vibration exhibit distinct bands which can be assigned to CO adsorbed on the Pt substrate (CO|Pt) and on Ru islands (CO|Ru). This is in contrast with IR studies of Pt  Ru alloy electrodes where only a single vibrational band of adsorbed CO is observed. The integrated band intensities of the CO|Ru and of CO|Pt bands on the Ru-modified Pt(111) electrode decrease simultaneously at the onset potential of CO oxidation. The results are interpreted within the framework of a bifunctional mechanism of CO electro-oxidation, postulating active Pt sites in close proximity to the Ru islands and a high mobility of the CO|Pt adsorbate layer. The differences in the IR spectra between the Ru|Pt(111) electrode and the Pt  Ru alloys are explained by disparate surface structures (Ru islands versus homogeneous distribution of Pt and Ru atoms).

Platinum electrodeposition on graphite: electrochemical study and STM imaging

The electrodeposition of Pt on a thermally oxidized HOPG surface was performed by single potential perturbation in dilute chloroplatinic acid solutions. The quantitative analysis of the current versus time transient responses, on the time scale of seconds, indicated a low saturation density of nuclei of 2×106 cm−2. The characterization by STM revealed a heterogeneous distribution of deposited Pt on the substrate surface. Most of the deposits were composed of agglomerates of spherical nanoparticles. Local particle densities exceeding 1010 cm−2 were observed, which is several orders of magnitude higher than the saturation coverage of nuclei evaluated from the analysis of the transient at t>0.3 s. From the electrochemical and the structural investigations, it appears that Pt electrodeposition on graphite involves several faradaic steps on the time scale. Firstly, at t 0.3 s, it is very likely that the more rapid growth of a small number (2×106 cm−2) of Pt particles gives a transient response, which is adequately described by the model of Scharifker and Hills for a diffusion controlled growth. As a result, it seems that cluster diffusion contributes significantly to the structural evolution of platinum electrodeposits on graphite.

Reaction of Hydrogen/Water Mixtures on Nickel‐Zirconia Cermet Electrodes: II. AC Polarization Characteristics

The reaction of hydrogen/water mixtures on nickel-zirconia cermet electrodes has been investigated as a function of potential and temperature at three different partial pressures of the reactants. Apparent kinetic parameters, namely, reaction orders, activation enthalpies, and pre-exponential factors, were determined as a function of potential in low-temperature (725--845 C) and high-temperature (845--950 C) regions for both the hydrogen oxidation and the hydrogen evolution reactions. In the low-temperature region, the charge-transfer coefficient, {alpha}{sub a}, calculated according to absolute rate theory was 0.7, independent of temperature. The apparent reaction order of the hydrogen oxidation reaction is in the region of 0.5 at 725 C, from which the authors conclude that at low temperatures, charge transfer involving atomically adsorbed hydrogen species determines the rate of this reaction. The results for the hydrogen oxidation reaction at temperatures above 845 C as well as for the hydrogen evolution reaction are not consistent with a charge-transfer-controlled electrode reaction. Adsorption and chemical reaction between adsorbed species are considered to be a dominant for the hydrogen oxidation reaction at temperatures above 845 C and the hydrogen evolution reaction over the entire temperature range studied.

Reaction of CO/CO2 gas mixtures on Ni–YSZ cermet electrodes

The reaction of carbon monoxide/carbon dioxide mixtures on Ni–YSZ cermet electrodes was investigated as a function of the electrode potential and the partial pressures of the reactants at 1273 K. Time-dependent reaction rates are observed for the CO oxidation reaction for oxygen activities corresponding to open circuit potentials in the range from −750 to −1010 mV. The electrode changes between a passive state and several active states for the CO/CO2 reaction. Periodic changes of the reaction rate for the CO oxidation are observed every 30 and 80 s. The impedance spectra recorded at the rest potential and the overpotential dependence of the CO oxidation rate indicate a change in the number of active sites in the reaction zone. In the active state, the CO oxidation reaction is more than one order of magnitude slower than the hydrogen oxidation reaction on these Ni–YSZ cermet electrodes. These results indicate clear differences in the kinetics of the CO and H2 oxidation reaction.

Quasielastic neutron scattering study of proton diffusion in SrCe0.95Yb0.05H0.02O2.985

Abstract By means of quasielastic neutron scattering (QENS) we have studied proton diffusion in SrCe 0.95 Yb 0.05 H 0.02 O 2.985 on microscopic scales in space and time. The diffusion process consists of a sequence of free diffusion and trapping/escape events. The resulting diffusion coefficient agrees with the diffusion coefficient determined by impedance spectroscopy measurements which were performed on the same sample batch.

INVESTIGATION OF PT PARTICLES ON GOLD SUBSTRATES BY IR SPECTROSCOPY PARTICLE STRUCTURE AND CATALYTIC ACTIVITY

Abstract The CO monolayer adsorption and electro-oxidation is investigated on model electrodes consisting of small Pt clusters (2–8 nm size) supported on polycrystalline gold in 0.1 M HClO 4 . The study is performed by in situ infrared spectroscopy and cyclic voltammetry. The electrodes were prepared from an aqueous Pt colloid and polycrystalline gold. The size and size distribution of the clusters is determined by TEM images. The cyclic voltammogram of the CO monolayer oxidation on 3 nm Pt particles exhibits three oxidation peaks located at 100 to 500 mV RHE higher potentials compared to a polycrystalline Pt electrode. A pronounced influence of the Pt particle coverage on the stretching vibration of linearly bonded CO is also found: while at low Pt coverages the band is located at 2013 cm −1 an upwards shift to 2060 cm −1 is observed when increasing the particle coverage. A particle size effect on the vibrational frequency can also be established by using electrodes prepared from an aged Pt colloid. TEM analysis of this colloid showed particles sizes of 2.2 nm (primary size) and 8.5 nm (secondary size) and aggregates of the larger particles. At low Pt particle coverage, two bands can be distinguished: a band at 2013 cm −1 is attributed to CO adsorbed on the small primary particles while a second band at 2046 cm −1 is assigned to linearly bonded CO adsorbed on the larger particles and on the aggregates. The importance of the lateral dipole field interactions between the adsorbed vibrating molecules is discussed and also the dependence of the vibrational frequency on the coordination of the adsorbate Pt site. Both interactions are too weak to account for the pronounced vibrational shifts in comparison with CO bonded on polycrystalline Pt and therefore the results are tentatively explained by particle-substrate interaction.