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 photoemisson electron microscopy (PEEM) as spatially resolving method. The reactive removal of the (8×8)-O structure proceeds via a hole-eating mechanism. The shape and velocity of the front depends on the parameter p(H2), temperature and the exposure time to oxygen. At very high O2 exposures (≈106 L) triangular reaction fronts are seen oriented parallel to close-packed rows of the (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.