REAL-TIME OBSERVATION OF THE NUCLEATION AND PROPAGATION OF REACTION FRONTS ON SURFACES USING PHOTOEMISSION ELECTRON-MICROSCOPY

The catalytic oxidation of CO on Pt{100} has been studied in situ using photoemission electron microscopy (PEEM). The difference in work function between CO-covered regions of the surface and O-covered regions provides image contrast in PEEM using a deuterium lamp. The CO and O2 partial pressures were adjusted to give a CO coverage greater than 0.5 monolayer at a crystal temperature below 500 K corresponding to the low rate branch of the oxidation reaction in which CO inhibits the adsorption of oxygen. The transition to the high rate branch (characterized by low CO coverage) was then initiated by decreasing the CO partial pressure. This allows oxygen adsorption at various types of defect sites, forming holes in the CO film. The holes subsequently enlarge as reaction fronts propagate away from the defects at velocities from 0.5 to over 40 μm s-1 depending on temperature, pressure and directions of steps on the surface. The return to the low rate branch, induced by increasing the CO pressure again, does not give rise to propagating reaction fronts but rather to random adsorption of CO throughout the surface. © 1990 Elsevier Science Publishers B.V. (North-Holland).