Bistability and formation of low work function areas in the O-2+H-2 reaction on a Rh(111) surface

The catalytic O-2 + H-2 reaction on Rh(111) has been investigated in the 10 (-6)-10(-5) mbar range using photoelectron emission microscopy (PEEM) as sp atially resolving method. Depending on the pretreatment of the sample in wh ich the sample was exposed to pO(2) = 2 X 10(-4) mbar at T = 770 K for vary ing times (t(OX)) different dynamic behavior was found. For t(OX) = 12 h we found simple bistable behavior with reaction fronts initiating transitions between an unreactive high oxygen coverage state and a reactive almost bar e surface. For t(OX) = 36 h low work function (WF) areas developed in the a rea where two reaction fronts collided. For very long oxygen pretreatments with t(OX) = 48 h the formation of secondary fronts traveling backwards fro m the area where two primary fronts collided were seen. The properties of t he low WF areas which have been tentatively assigned to subsurface oxygen w ere studied in adsorption and titration experiments. (C) 2000 American Inst itute of Physics. [S0021-9606(00)70635-3].