Narrow Results

The determination of the physical and chemical properties of small nanoparticles plays a fundamental role in homogeneous and heterogeneous catalysis since a large number of the surface atoms of these nanoparticles can be exposed to chemical reactions, as well as in chemisorption, solid state physics, laser physics, crystal growth, epitaxy and surface science in general as most recent experimental and theoretical investigations have been disclosed. An empirical many-body Potential Energy Function (PEF) incorporating two-plus three-body atomistic potential derived by fitting experimental data pertaining to bulk Iridium has been applied to study the structural stability and energetics of Iridium nanoparticles of Ir_n ($n=3$–13). A constant temperature Molecular Dynamics (MD) technique is employed in the simulations. It is found that the energetically most stable structures of Iridium nanoparticles are in three-dimensional distorted compact forms close to symmetric geometries with the MD technique. The theoretical predictions are compared to the available theoretical and experimental literature data for binding energies, bond lengths and the most stable nanocluster geometry.

In this paper, the lattice cohesive curve of iridium is investigated through first-principles calculations. The double-exponential function to fit the curve is presented. The inversion pair potential curve is generated through Chen’s inversion method. The accurate pair potential function is obtained through fitting by the new double-exponential function. The phonon spectra are calculated using the inversion potential data, the embedded atom method (EAM) potential theory and first-principles method, respectively, to verify the reliability of the inversion potential. The method combining Boltzmann statistics equation with accuracy fitting of lattice cohesive energy curve is proposed to calculate the thermal expansion coefficient. In addition, the bulk modulus and Grüneisen constant in the room temperature are calculated. The results are in good agreement with experiment results, which imply that the inversion potential is effective and accurate.

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 Y^3/2. 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.

Recently [M. Kunat et al., Surf. Sci.474, 114 (2001)] a quenching of adsorbate-assisted adsorption ("autocatalytic adsorption") by defects has been observed for the system CO/Cu(110). A conceptually similar effect for CO adsorption on O–Ir(110) had additionally been reported before [U. Burghaus et al., Surf. Sci.384, L869 (1997)]. Presented is a simple Monte Carlo simulation (MCS) scheme which includes basically one free fit parameter and can reproduce both effects consistently.

The adsorption of N₂O on Ir(110) was investigated with high-resolution resonant photoemission at 135 K. The results obtained show evidence of molecular adsorption of N₂O, along with some dissociation. It is found that the nitrogen photoemission spectra measured at the terminal and central nitrogen energy positions of the N₂O/Ir(110) system are equivalent to those of N₂O in the gas phase. In contrast, the oxygen spectrum shows little resemblance to the gas phase oxygen spectrum of N₂O. In the nitrogen resonant photoemission spectra one can only discover resonant behavior with constant binding energy peaks. In contrast, the oxygen resonant photoemission spectra shows dominantly Auger behavior with peaks at constant kinetic energies. Both observations reveal that the oxygen is bonded to the Ir surface. A systematic study as a function of coverage and temperature and a comparison with other surfaces differing in structure and composition is needed to understand the variations in the adsorption behavior of N₂O on metal surfaces.

The interaction between a solid inert metal Ir and an active liquid metal Ga during mechanical activation in a high-energy planetary mill is studied by X-ray diffraction and scanning electron microscopy with high-resolution energy dispersive X-ray microanalysis. The effect of mechanical activation on the formation of Ga_xIr_y intermetallic compounds and Ga_xIr_y/Ir composites and their solubility in acids was investigated. The subsequent extraction of Ga from intermetallic compounds and composites in the mixture of concentrated acids $(\mathrm{\text{HCl}}+{\mathrm{\text{HNO}}}_3)$ makes it possible to produce nanoscale Ir.