Macroscopic and mesoscopic characterization of a bistable reaction system:CO oxidation on Pt(111) surface

The catalytic oxidation of CO by oxygen on a platinum (111) single-crystal surface in a gas-flow reactor follows the Langmuir-Hinshelwood reaction mec hanism. It exhibits two macroscopic stable steady states (low reactivity: C O-covered surface; high reactivity: O-covered surface), as determined by ma ss spectrometry. Unlike other Pt and Pd surface orientations no temporal an d spatiotemporal oscillations are formed. Accordingly, CO+O/Pt(111) can be considered as one of the least complicated heterogeneous reaction systems. We measured both the macroscopic and mesoscopic reaction behavior by mass s pectrometry and photoelectron emission microscopy (PEEM), respectively, and explored especially the region of the phase transition between low and hig h reactivity. We followed the rate-dependent width of an observed hysteresi s in the reactivity and the kinetics of nucleation and growth of individual oxygen and CO islands using the PEEM technique. We were able to adjust con ditions of the external control parameters which totally inhibited the moti on of the reaction/diffusion front. By systematic variation of these condit ions we could pinpoint a whole region of external control parameters in whi ch the reaction/diffusion front does not move. Parallel model calculations suggest that the front is actually pinned by surface defects. In summary, o ur experiments and simulation reveal the existence of an "experimental'' bi stable region inside the "computed'' bistable region of the reactivity diag ram (S-shaped curve) leading to a novel dollar ($)-shaped curve. (C) 1999 A merican Institute of Physics. [S0021-9606(99)70122-4].