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.
Kinetic oscillations during the catalytic CO oxidation on Pd(110): The role of subsurface oxygen
Abstract At high oxygen pressures ( P O 2 #62; 1 × 10 −3 Torr) and at fairly low temperature ( T = 400 K) the catalytic CO oxidation on Pd(110) exhibits kinetic oscillations which have been followed by means of measurements of the reaction rate and of the work function change. Under these conditions the kinetics of CO 2 formation develop pronounced hysteresis effects which differ significantly from those observed with Pt surfaces. The unusual features in the CO 2 hysteresis could be related to the formation of subsurface oxygen which has been demonstrated to take place under the same conditions. As subsurface oxygen decreases the catalytic activity of the surface, the characteristic hysteresis effects could be traced back to a slow filling and depleting of the subsurface oxygen reservoir under conditions of varying adsorbate coverages. A mechanism for the kinetic oscillations on Pd(110) is suggested in which the slow formation/removal of subsurface oxygen modulates the catalytic activity of the surface. The proposed mechanism is supported experimentally by the coincidence of the p O 2 , T conditions for kinetic oscillations with the conditions under which the characteristic features in the CO 2 hysteresis due to the formation of subsurface oxygen can be observed. The operation of a mechanism which is based on structural changes, e.g. a surface phase transition or reversible facetting, can be excluded on the basis of the property of the clean Pd(110) surface which does not reconstruct, as well as from the absence of kinetic oscillations in the low pressure regime ( p O 2 −3 Torr).
Robust solutions of inverse problems in electromagnetic non-destructive evaluation
Non-destructive testing (NDT) is the application of physical measurement technology based on energy interaction with the material and its nonconformities. The material's response is sensed by transducers and sensors which?in most cases?scan the component and document the results in inspection images. However, NDT measures a physically defined quantity or even an intrinsic property. The difference between non-destructive evaluation (NDE) and NDT is in the interpretation of the inspection data. NDE has to discuss the inspection results in terms of quality elements and characteristics which are relevant to describe the fitness of the material for use. In the case of macroscopic defects these are the kind of defect (cracklike, globular) and its size and orientation to the main stress directions; in the case of material property determination the parameters are mainly mechanical properties. Therefore, in NDE one has to solve inverse problems. The solution of inverse problems based on mathematical procedures such as integral equations is a strong developing discipline and most of the articles prepared for this special issue of the journal have the objective of discussing the latest state of the art in that field. However, practical NDE needs robust and quick solutions which are to be applied mainly online. Therefore, we present here inversion procedures based on multiple linear regression algorithms applied to inspection data. We describe the calibration procedure to fit the free parameters of the model functions and give examples of practical applications in industry.