Local fluctuations during catalytic CO oxidation on Pt : a field emission study

Field electron microscopy (FEM) is employed to monitor local brightness fluctuations that occur during catalytic CO oxidation on a Pt field emitter tip and that are caused by local variations in the adsorbate coverages. The reaction is studied in the bistability range at T = 310 K with fixed p(O2) = 4.0 x 10(-4) Torr and p(CO) = 3.0 x 10(-7) Torr. The measurements are conducted on the apex plane of a [100]-oriented Pt tip. Local time series are obtained by digitization of the video-recorded FEM image (40 ms time resolution) and integration of the intensity in windows of 100 x 100 Angstrom and 20 x 100 Angstrom size. From the measured time series, the autocorrelation and cross-correlation functions were determined. Fluctuations on the reactive (oxygen covered) and on the inactive (CO covered) branch of the reaction exhibit quits substantial differences. On the reactive branch, spatial correlations extend over much longer distances (several hundred angstroms) than on the unreactive branch, and time correlations decay much more slowly. These differences are discussed on the basis of the Langmuir-Hinshelwood mechanism for catalytic CO oxidation. The fluctuations on the unreactive branch apparently originate from CO diffusion alone. (C) 1998 Elsevier Science B.V. All rights reserved.