| Spatio-Temporal Forcing Of A Non-Linear Surface Reaction | ||
| Danny Bilbao, Chemical Engineering, University of Delaware, Newark, DE 19716, Christopher Snively, Chemical Engineering, University of Delaware, Department of Chemical Engineering, Newark, DE 19702 and Jochen Lauterbach, Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716
The oxidation of CO on Pt(100) exhibits complex, non-linear behavior, such as reaction rate oscillations and spatiotemporal adsorbate patterning. The current focus of our work is to investigate the dynamics of this system when it is subjected to external forcing. Using a novel gas microdosing technique to locally introduce reactants onto the catalytic surface and EMSI (Ellipsomicroscopy for Surface Imaging) to image adsorbate activity, unique behavior has been observed for systems at pressures on the order of 1x10-4 Torr. The versatility of the local dosing setup allows for a continuous or periodic perturbation to be applied to a single, or multiple, small regions of the surface. Using one particular doser configuration, four CO islands having a center to center distance of 350 μm and a diameter of 200 μm were placed on an O covered surface. Each island was maintained by a continuous supply of CO from the doser, and frequently exchanged CO waves with the other islands. The CO waves, which were only observed inside the region bounded by the four CO islands, were most likely promoted by an increased CO concentration in the gas phase due to overlap in the gas streams from the doser. Locally dosing various gasses (e.g. CO, O2, NH3, etc.) onto the catalyst surface was found to modify the surface in such a way as to lead to the formation of a ring shaped pattern due to aggregation of CO at the boundary of the dosed area. Further investigation of this behavior concluded that the appearance of the ring depends on several factors, including the surface temperature at the time of dosing, with a CO island forming instead of the ring at temperatures below 125 °C, as well as the time delay between dosing and the introduction of global CO into the gas phase. Global CO addition was delayed up to 45 minutes after dosing, increasing the initiation time for the ring by a factor of three. Finally, O2 pulses from the microdoser were used to drive small areas of the surface from a monostable, CO covered state into a state of self-sustained oscillation. The O2 pulses triggered the formation of a spontaneous O front, which traveled across the area of the surface previously affected by the doser. Under global reaction conditions of pO2 = 4.0x10-4 Torr, with pCO/pO2 = 0.025, and T = 237 °C, the O wave was quickly followed by a CO wave, returning the surface to the CO poisoned state. This cycle repeated approximately every 45 seconds without further external perturbation.
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