TRIANGULAR-SHAPED REACTION FRONTS IN A CATALYTIC SURFACE-REACTION

The reaction between oxygen adsorbed on a Rh(111) surface and hydrogen has been investigated in titration experiments at low pressure (p<10( -5) mbar) using photoemission electron microscopy (PEEM) as spatially resolving method. The reactive removal of the (8 X 8)-O structure proc eeds via a hole-eating mechanism. The shape and velocity of the front depends on the parameter p(H-2), temperature and the exposure time to oxygen. At very high O-2 exposures (approximate to 10(6) L) triangular reaction fronts are seen oriented parallel to close-packed rows of th e (111) surface. The anisotropic front propagation on the isotropic Rh (111) surface is attributed to elastic stress at the interface between surface phases with different periodicities. (C) 1998 Elsevier Scienc e B.V.