Stefan Wehner

Abstraction of D adsorbed on Pt(111) surfaces with gaseous H atoms

The impact of gaseous H atoms at D covered Pt(111) surfaces at 85 K leads to the formation of gaseous HD and D2 products. The kinetics of formation of these products was measured simultaneously with H exposure for different initial D coverages. The HD and D2 rates as a function of H fluence from the reaction start exhibit common characteristics; a rate step, a rate maximum, and a subsequent exponential rate decay. The HD and D2 rate steps were observed not to scale linear with the D coverage and to increase if on D covered surfaces H was coadsorbed prior to reaction. Of the observed phenomena, only the exponential decay of the HD rate is in line with expectations if an Eley–Rideal mechanism acts in the present reaction. D2 formation, the HD rate step as a function of D coverage, and the presence of a H coadsorbate contradict the Eley–Rideal picture. The results suggest that the reactions towards HD and D2 proceed via hot atom type mechanisms.

The role of sticking and reaction probabilities in hot-atom mediated abstraction reactions of D on metal surfaces by gaseous H atoms

Recent experiments on the abstraction of D adsorbed on metal surfaces with gaseous hydrogen atoms revealed a kinetics of HD formation which is not compatible with the operation of Eley–Rideal (ER) mechanisms. Furthermore, homonuclear products were observed during abstraction, which are not expected through an ER reaction scheme. It was therefore suggested that hot-atom (HA) mechanisms are more appropriate to explain the measured kinetics and products. Random walk calculations of the abstraction kinetics are presented based on a model which exclusively relies on elementary reaction steps which are HA mediated processes. Within this model, the ratio of two variables, the probabilities for hot-atom sticking at empty sites ps and hot-atom reaction with adsorbed species pr, was found to control the kinetics of HD and D2 formation. The essential features of measured kinetic data at Ni(100), Pt(111), and Cu(111) surfaces were reproduced by simple and reasonable assumptions on ps/pr.

Interaction of gaseous D atoms with CH3I adsorbed on Pt(111), H/Pt(111), and C/Pt(111) surfaces: From hot-atom to Eley-Rideal phenomenology

The interaction of gaseous D atoms with methyl iodide molecules adsorbed on Pt(111), hydrogen saturated Pt(111), and graphite monolayer covered Pt(111) surfaces was studied. Direct product rate measurements were employed to determine the reaction kinetics. On all substrates, incoming D atoms abstract the methyl group from adsorbed CH3I via gaseous CH3D formation. In the monolayer regime of CH3I/Pt(111) pure hot-atom phenomenology was observed in the rates. With multilayers as targets, the fluence dependence of the rates get Eley–Rideal-type. With a coadsorbed H monolayer present, the CH3D rates at a CH3I monolayer on Pt(111) are affected by the suppression of hot-atom sticking. Accordingly, the rate curves exhibit similar features as expected for Eley–Rideal phenomenology. However, CH4 as a product and simultaneous abstraction of adsorbed H via gaseous HD and H2 formation clearly demonstrate that hot-atom reactions occur. With CH3I adsorbed on a graphite monolayer on Pt(111), the abstraction kinetics of m...

Kinetic phase transitions in the reaction CO+O→CO2 on Ir(111) surfaces

The oxidation of CO on Ir(111) surfaces was investigated under UHV conditions in the temperature range 360 K to 700 K by CO2 rate measurements utilizing mass spectroscopy. Steady-state CO2 rates were measured at constant total CO+O2 gas flux and variable gas composition (YCO=Y, YO2=1−Y) using mass flow controllers which allowed changes in the CO/O2 gas composition down to 0.1%. Between 360 K and 450 K the CO2 rates initially increase proportional to Y (T<400 K) or to Y1.5 (420 K<T<450 K) and exhibit a sudden drop to a negligible value at a temperature-dependent critical value Y*. The rate drop indicates a kinetic phase transition induced by CO poisoning of the surface. This behavior is similar to the features described by the ZGB and more recently developed lattice gas (LG) models of the CO+O reaction on surfaces. However, in contrast to the ZGB model but in accordance with LG models and experimental results on other platinum metal surfaces, no oxygen poisoning was observed at small Y, i.e., the surface w...

Interaction of gaseous D atoms with alkyl halides adsorbed on Pt(111), H/Pt(111), and C/Pt(111) surfaces: Hot-atom and Eley–Rideal reactions. I. Methyl bromide

The kinetics of reactions of D atoms with isopropyl iodide adsorbed on Pt(111), H covered Pt(111), and monolayer C covered Pt(111) surfaces were studied using direct product detection. Propane, C3H7D, was observed as gaseous product formed through propyl abstraction from the halide. At low temperatures, below the propane desorption temperature, and submonolayer coverages the reaction products remain adsorbed on the surface. At higher temperatures (coverages) they desorb. With C/Pt(111) as substrate, the reaction kinetics are compatible with the operation of an Eley–Rideal mechanism. On Pt(111) and H/Pt(111) substrates the abstraction kinetics of propyl from the halide is controlled by hot-atom mechanisms. Coadsorbed H leads to simultaneous abstraction of H from the surface towards HD and H2, as well as to a gaseous C3H8 product. Propyl abstraction cross-sections are in the range 0.5 to 1.5 A2, in line with direct processes. The results confirm the role of a metallic or nonmetallic substrate on the mechani...

Reaction hysteresis of the CO + O --> CO2 reaction on palladium(111).

Rate measurements of the reaction CO + O --> CO(2) on palladium(111) single crystal surfaces have been performed by means of mass spectroscopy under ultrahigh vacuum conditions. The total flux Phi of the impinging reactants CO and O(2) was held constant at 1 ML s(-1), whereas its CO fraction Y was varied between 0 (pure O(2)) and 1 (pure CO). The measurements have been performed for surface temperatures between 370 and 510 K and with a wide range of sampling times, evaluating the system parameter range for bistable behavior. Long-time measurements lasting several days proved the bistable behavior to result from two stable states rather than from slow processes not visible on usual experimental time scales. Pulselike modulations of the feed gas composition revealed the mechanisms confining the experimentally observed bistable range: the high CO fraction border of the bistability is given by the equistability condition of both states, whereas the other border is found to be associated with a saddle-node bifurcation in the corresponding system of reaction diffusion equations.

STOCHASTIC MODEL CALCULATION FOR THE CARBON MONOXIDE OXIDATION ON IRIDIUM(111) SURFACES

We study the effect of external noise on the catalytic oxidation of CO on an Iridium(111) single crystal under ultrahigh vacuum conditions. This reaction can be considered as a model of catalysis used in industry. In the absence of noise the reaction exhibits one or two stable stationary states, depending on control parameters such as temperature and partial pressures. When noise is added, for instance, by randomly varying the quality of the influx mixture, the system exhibits stochastic reaction rate and switching. In this work, we present two approaches: one for the monostable regime, and another for the bistable situation that relies on a white noise approximation. Both approaches rest on the assumption that spatial patterns of coverage on the Iridium plate can be neglected on a first approximation. Using mathematical models, it is possible to reconstruct stationary probability distribution functions that match experimental observations and provide support for the existence of a thermodynamic potential.

Time-dependent growth of crystalline Au0-nanoparticles in cyanobacteria as self-reproducing bioreactors: 2. Anabaena cylindrica

Summary Microbial biosynthesis of metal nanoparticles as needed in catalysis has shown its theoretical ability as an extremely environmentally friendly production method in the last few years, even though the separation of the nanoparticles is challenging. Biosynthesis, summing up biosorption and bioreduction of diluted metal ions to zero valent metals, is especially ecofriendly, when the bioreactor itself is harmless and needs no further harmful reagents. The cyanobacterium Anabaena cylindrica (SAG 1403.2) is able to form crystalline Au0-nanoparticles from Au3+ ions and does not release toxic anatoxin-a. X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and laser-induced breakdown spectroscopy (LIBS) are applied to monitor the time-dependent development of gold nanoparticles for up to 40 hours. Some vegetative cells (VC) are filled with nanoparticles within minutes, while the extracellular polymeric substances (EPS) of vegetative cells and the heterocyst polysaccharide layer (HEP) are the regions, where the first nanoparticles are detected on most other cells. The uptake of gold starts immediately after incubation and within four hours the average size remains constant around 10 nm. Analyzing the TEM images with an image processing program reveals a wide distribution for the diameter of the nanoparticles at all times and in all regions of the cyanobacteria. Finally, the nanoparticle concentration in vegetative cells of Anabaena cylindrica is about 50% higher than in heterocysts (HC). These nanoparticles are found to be located along the thylakoid membranes.

CO oxidation on Ir(111) surfaces under large non-gaussian noise.

In this article we consider the CO oxidation on Ir(111) surfaces under large external noise with large autocorrelation imposed on the composition of the feed gas, both in experiments and in theory. We report new experimental results that show how the fluctuations force the reaction rate to jump between two well defined states. The statistics of the reaction rate depend on those of the external noise, and neither of them have a gaussian distribution, and thus they cannot be modeled by white or colored noise. A continuous-time discrete-state Markov process is proposed as a suitable model for the observed phenomena. The model captures the main features of the observed fluctuations and can be modified to accommodate other surface reactions and other systems under non-gaussian external noise.

THE CO-OXIDATION REACTION ON Ir(111) SURFACES: BISTABILITY, NOISE AND SPATIO-TEMPORAL PATTERNS IN EXPERIMENT AND MODELING

This review summarizes recent studies on the catalytic CO oxidation on Iridium(111) surfaces. This was investigated experimentally under ultrahigh vacuum (UHV) conditions using mass spectroscopy to detect gaseous products and photoelectron emission microscopy (PEEM) to visualize surface species. The underlying reaction–diffusion system based on the Langmuir–Hinshelwood mechanism was analyzed numerically. The existence of bistability for this surface reaction was shown in experiment. For the first time the effect of noise on a bistable surface reaction was examined. In a surface science experiment the effects on product formation and the development of spatio-temporal patterns on the surface were explored. Steady state CO2 rates were measured under constant flux of the CO + O mixture as a function of sample temperature (360 K < T < 700 K) and gas composition, characterized by the molar fraction of CO in the feed gas (0 ≤ Y ≤ 1). The reaction reveals bistability in a limited region of Y and T. A rate hysteresis with two steady state rates was observed for cycling Y up and down, one of high reactivity (upper rate, oxygen covered surface) and one of low reactivity (lower rate, CO covered surface). The position of the hysteresis loop shifts to higher Y values and decreases in width with increasing temperature. For small CO content in the feed gas the CO2 rate is proportional to Y3/2. At 500 K extremely slow Y cycling measurements (about 100 hours per direction) were done and showed that bistability still exists and no slowly changing transients were observed. The requirements for the speed with which experiments can be executed without producing experimental artifacts were explored. Since over-sampling alters the measured hysteresis loop, a maximum rate for changing the gas composition in Y cycling experiments was determined. The influence of noise on the reaction rates and the formation of spatio-temporal patterns on the surface was explored by superposing noise of Gaussian white type on Y and on T. Noisy Y (deviation Δ Y) represents multiplicative and additive noise, noisy T (deviation Δ T) multiplicative noise only. Noisy T enters the reaction via the rate-determining step, the observed CO2 rates become noisy for low temperatures (around 420 K) when the surface is dominantly oxygen covered (CO + O reaction step is rate-limiting) and for higher temperatures (around 500 K) when the surface is dominantly CO covered (CO desorption step is rate-limiting). The effect of noisy Y was examined for a sample temperature of 500 K and is dependent on the selected average gas composition. In the regions with one steady state CO2 rate (outside the hysteresis) the recorded rates were noisy. The probability distribution of the rates is Gaussian shaped for the upper rate (below hysteresis) and asymmetric for the lower rate (above hysteresis). For large noise strength bursts, short-time excursions to and above the upper rate, were observed. Inside the hysteresis small noise made the steady state rates noisy, but above a Y-dependent Δ Y transients from the locally stable to the globally stable rate branch were observed. These transients take up to several ten thousand seconds and become faster with increasing noise. For larger Y noise strength bursts and switching between both steady state rates were detected. Photoelectron emission microscopy (PEEM) was done to visualize spatio-temporal adsorbate patterns on the surface as expected from the observations in the CO2 rate measurements. CO- and oxygen-covered regions on the Ir(111) surface are visible in PEEM images as gray and black areas as a consequence of their work function contrast. Islands of the adsorbate, corresponding to the globally stable branch, are formed in a background of the other one. The long transient times are the result of the extremely slow domain wall motion of these islands (around 0.05 μm s-1). In the hysteresis region CO oxidation on Iridium(111) surfaces is dominated by domain formation and wall motion for small to moderate noise strength. The island density increases with noise, but the wall velocity is independent of applied Δ Y. For larger noise amplitudes, fast switching between oxygen- and CO-dominated surfaces is observed as well as nucleation and growth of the minority phase in the majority phase. In the numerically analyzed reaction–diffusion system all parameters were taken from the experiment. Modeling the reaction–diffusion system shows qualitative up to quantitative agreement with the experimental observations. The length scale for the modeling grid is determined from wall velocity seen in the experiments.