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.
Structural effects on water formation from coadsorbed H + O on Rh(100)
The effect of adsorbate coverage, adsorption sequence and temperature on the structure, composition and reactivity of coadsorbed layers, produced by dissociative adsorption of O2 and H2 at 200 K on a Rh(100) surface, has been studied by combined TPD, XPS and LEED measurements. The emphasis is on the impact of the structure and composition of the mixed O + H layers on the synthesis of hydroxyl and water as a result of the O + H surface reaction. The difference in the O 1s binding energies of adsorbed O (529.9 eV) and OH species (530.8 eV) was used as a fingerprint to monitor the formation of the OH species. The H2O TPD spectra show substantial variations of the desorption temperature range and the amount of water evolved with coadsorbate coverage and structure: from 270 to 350 K and from 0 to 0.08 ML, respectively. It has been found that dense O + H adlayers, where the O coverage is in the range 0.25-0.4 ML, favor the formation of stable OH species. The maximum amount of stable hydroxyl OH species (∼ 0.16 ML) can be produced by heating of these dense adlayers to ∼ 260 K. This results in reordering of the adspecies to form a new O + OH − (2 × 6) structure, where hydroxyls react readily to evolve 0.08 ML of water in a sharp desorption peak at ∼ 280 K. The effect of the adlayer density and restructuring on the production of OH and H2O is discussed.
Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber
Abstract Two measurement campaigns were carried out at ENERGI E2's Asnaes Power plant, unit 5. The unit has a capacity of 620 MW e and is equipped with a wet flue gas desulphurisation (FGD) plant employing a counter-current spray absorber with five spray levels. In the first campaign, the power plant was firing Orimulsion ® with 2.85 wt% S resulting in a flue gas concentration of SO 2 exceeding 2000 ppmv. In the second campaign, the fuel applied was a low-S blended coal and the SO 2 concentration in the raw gas was around 400 ppmv. A novel probe for in situ sampling of gas phase concentrations in wet FGD spray absorbers was developed and applied for measuring axial profiles of the SO 2 gas phase concentrations in the absorber. The expected decrease in SO 2 concentrations along the height of the absorber was found in the spray section (from height 26.5 to 36.2 m) whereas the SO 2 concentration above the holding tank and below the gas inlet was quite low probably due to long local residence times in the region. Horizontal variations, due to somewhat different flow conditions near the column wall were investigated and the SO 2 concentrations were found to be higher near the wall. Measurements at different gross loads showed that the SO 2 gas phase concentration at a given position inside the absorber was roughly linearly related to the L/G ratio in the measuring interval. Turning off one of the lower spray levels, while burning coal with low S content, did not lower the overall removal efficiency of the absorber. However, the SO 2 gas phase concentration inside the lower part of the absorber was increased by a factor of 2–3. Measurements of slurry pH at different positions showed a decrease of approximately 0.5 units from the upper to the lower part of the absorber. The full-scale measurements provide a detailed set of experimental data for validation of mathematical models of a wet FGD spray absorber.