In situ dynamic study of hydrogen oxidation on rhodium.
The reaction of hydrogen/oxygen gas mixtures with rhodium single crystals was studied using video-FIM (Field Ion Microscopy) at temperatures between 350 and 550 K and up to 2 x 10(-2) Pa total pressure. Imaging at 500 K in a hydrogen rich gas mixture (H2:O2 = 9) revealed considerable morphological changes of the (0 0 1)-oriented field emitter tip, i.e. the growth of low-index at the expense of high-index planes and strong crystal coarsening. Decreasing the hydrogen partial pressure led to chemical and structural changes of the Rh sample. Starting on the [1 1 0] planes a surface oxide formed, which spread anisotropically across the surface until it finally covered the whole visible surface area. The transformation was reversible upon increasing the hydrogen pressure back to its initial value. However, a hysteresis behavior was observed, i.e. a larger hydrogen partial pressure was found to be necessary to re-establish the initial patterns of a reactive Oad/Had layer. By varying the temperature from 400 to 500 K a phase diagram was established for the Oad/Had system. Increasing the electric field proved to shift the phase diagram towards higher H2 pressures. At 550K self-sustained kinetic oscillations with a cycle time of approximately 40s could be observed.
The role of fluctuations in bistability and oscillations during the H2 + O2 reaction on nanosized rhodium crystals.
A combined experimental and theoretical study is presented of fluctuations observed by field ion microscopy in the catalytic reaction of water production on a rhodium tip. A stochastic approach is developed to provide a comprehensive understanding of the different phenomena observed in the experiment, including burst noise manifesting itself in a bistability regime, noisy oscillations, and nanopatterns with a cross-like oxidized zone separating the surface into four quadrants centered on the {111} facets. The study is based on a stochastic model numerically simulating the processes of adsorption, desorption, reaction, and transport. The surface diffusion of hydrogen is described as a percolation process dominated by large clusters corresponding to the four quadrants. The model reproduces the observed phenomena in the ranges of temperature, pressures, and electric field of the experiment.
Design and performance of a novel field electron/field ion microscope combination based on image processing
Abstract The construction of a novel field emission/field ion microscope (FEM/FIM) combination based on image processing techniques is described. The field electron/field ion image formed on a fluorescent screen is converted into a computer image. From these images more detailed information can be obtained about processes taking place upon adsorption of gases or during reactions. The most reactive areas on a tip can be more easily determined and colouring of the different areas makes the differences in work function of these tip areas more pronounced. The working of this new microscope is demonstrated using the reaction between oxygen and a tungsten field emitter tip. The results obtained with this experimental set-up are compared with literature data on this well-studied system for the sake of calibration. It is shown that the reproducibility of work function measurements is about 0.03 eV.