(689c) Gas-Phase Coupling of Reactive Surfaces by Oscillating Reactant Clouds

The collective, global behavior of a catalytic system depends on the effective communication of local reactivity variations to distant points in the system. One particularly efficient mode of communication occurs via partial pressure fluctuations in the gas-phase above the reactive surface. This gas-phase coupling mode is widely considered to be most effective under vacuum conditions, where the mean free path between molecular collisions is relatively large. However, this hypothesis has gone untested due to a number of experimental challenges associated with systematically creating and characterizing spatially resolved catalytic activity.

In this work, we take advantage of a spatially distributed system of spatially isolated chemical oscillators to investigate the details of gas-phase communication during CO oxidation and NO reduction using NH₃ on polycrystalline Pt. Characterization of local gas-phase variations using scanning mass spectrometry, in parallel with in situ surface imaging, provided a novel description of the surface/gas-phase interaction which differed from the conventional assumption of a gradient free, molecular flow environment near the surface even under vacuum conditions. This analysis further allowed for the determination of an effective gas-phase coupling length, which, based on the identification of oscillating reactant clouds above the surface, was calculated to be on the order of 500 µm for a total system pressure of 10^-3 Torr. This coupling length was found to be in agreement with surface imaging results which qualitatively showed coupling between individual oscillators.