Schematic drawing of the capillary tip used for product gas sampling showing (a) the capillary geometry and (b) the principle of simulation. The yellow circle represents the capillary orifice, which is surrounded by the cylindrically shaped capillary wall (light gray area). The catalytically active surface area is subdivided into finite elements representing point sources for desorbing molecules. Desorbed molecules, which move between sample surface and capillary tip, interact with molecules in the gas phase (mainly ), as described by the MC simulations. This region between substrate and capillary plane is divided into finite radial elements, with characteristic properties of the local gas phase as determined by the CFD calculations (pressure, temperature, and velocity components). Molecular trajectories are followed until the capillary plane is reached. For comparison, an optical microscopy picture of the end of a capillary with comparable dimensions to that used for the present experiments is shown in the inset.
Spatial distribution of reactant (CO) and product partial pressures above a wide Pt stripe (length: ) at 3.7 mbar and 600 K determined by scanning mass spectrometry, showing the (local) consumption and production of reactant and product gases. (a) Line scans across the Pt structure at different capillary heights (upper panel); (b) two-dimensional plots representing a cross-sectional view of the spatial variation of the partial pressures of the components. The lower panel of (a) shows the variation of the SMS signal above the center of the Pt structure with increasing capillary height.
product partial pressure above a square Pt field (, left panels) and a Pt stripe (, right panels) at 0.37 (upper panels) and 3.7 mbar (lower panels). The line scans displayed in the center were taken at different vertical distances between capillary orifice and sample surface for capillary heights of (a) 4, 16, 26, 36, 73, 110, 355, 875, and ; (b) 3, 13, 24, 34, 79, 131, 291, 701, and ; (c) 4, 14, 35, 65, 108, 213, 353, 623, 873, and ; and (d) 4, 15, 37, 76, 126, 193, 298, 438, 708, and . The signal intensities of the line scans decrease in this order, except for the gray colored data points related to the lowest capillary height . The gray areas in the graphs represent the lateral extents of the Pt fields and the contour plots related to the respective sets of line scans visualize the two-dimensional product gas evolution above the fields.
Distribution of the (a) pressure , (b) radial velocity , (c) axial velocity , and (d) temperature T in the gap between sample surface and capillary tip at 3.7 mbar, computed by 2D CFD simulations. The diagram in the lower panel (e) displays the total pressure underneath the capillary tip at different relative capillary heights vs the lateral position relative to the center of the capillary orifice (marked area: capillary wall). The inset shows the evolution of the pressure at the capillary orifice with decreasing capillary height.
Simulated line scans above a (a) square Pt field and a (b) Pt stripe at 0.37 mbar, obtained by Monte Carlo simulations and considering the variation in reactant pressures derived from CFD calculations (open symbols, capillary heights of 2, 5, 20, 40, 80, 150, 350, 700, and ). For comparison, results from SMS measurements performed at capillary heights of 4, 26, 36, 73, 215, 355, and in (a) and 3, 24, 34, 79, 131, 701, and in (b) are included (solid symbols). (c) shows the evolution of the signal above the center of the Pt stripe (lateral displacement ). [Note that additional experimental data points have been used where for more clarity the line scans are not included in (b).] The red colored data points denote the corresponding results from the SMS measurements (Fig. 4); green dotted lines: simulation neglecting the variation in partial pressure determined in the CFD calculations [see inset of (c)].
Comparison of measured line scans above a square Pt microstructure at 3.7 mbar (solid symbols, capillary heights 14, 65, 108, 213, 353, 623, and ) and (a) the corresponding simulated line scans obtained by Monte Carlo simulations and by superposition of (b) the gas distribution from point sources, using capillary heights of 20, 40, 80, 150, 350, 700, and (open symbols). The inset shows the corresponding evolution of the signal intensity (simulation vs measurement) above the center of the Pt fields. Note that additional experimental data points were used which are not shown in the line scans.
Simulation of the signal intensity above the central position of a square Pt field vs the height of the capillary at different pressures and for varying reaction orders (, see legend). For comparison, a MC simulation using noncorrected parameters (no CFD based corrections) is included as well (black data set).
(a) Number of molecules hitting the orifice of a capillary positioned above the center of a square Pt field as a function of the radial position of their origin on the sample surface ( at the center of the capillary), both for the low pressure regime (upper panel) and the high pressure regime (lower panel). (b) Normalized volume integral of the radial distribution of the data presented in the lower panel of (a), representing the fraction of product molecules originating from the inside of a circular area under the capillary tip with the radius r as a function of the respective radius.
Article metrics loading...
Full text loading...