Chemical waves and adsorbate-induced segregation on a Pt(100) surface microstructured with a thin Rh/Pt film

Reaction fronts formed during the O2 + H-2 and NO + H-2 reactions on a 70 A ngstrom thick microstructured Rh film on a Pt(100) substrate have been inve stigated using photoemission electron microscopy (PEEM) and scanning photoe lectron microscopy (SPEM). The reactions were carried out in the 10(-7) mba r range at T = 670 and 700 K. The photoelectron spectra revealed that the t hin Rh film was alloyed with Pt due to vertical intermixing with the underl ying Pt(100) substrate. The surface Rh/Pt ratio was found to vary from simi lar to 4 for an oxygen-free film to 4.8 in oxygen ambient. The O 1s spectra indicated the presence of oxide species along with chemisorbed oxygen on t he alloyed film. The chemical maps revealed an enhanced tendency for oxide formation at the boundaries of the Rh/Pt film leading, under reducing condi tions, to residual oxide and Ph enrichment expanding laterally to similar t o 10 mu m inside the film. This composition inhomogeneity can explain the p eculiar ring patterns near Pt/Rh interfaces that were observed by PEEM unde r the reaction conditions. (C) 1999 Elsevier Science B.V. All rights reserv ed.