LOCAL INDEPENDENT OSCILLATION CENTERS PRESENT ON AN IRIDIUM FIELD EMITTER DURING THE NO-H-2 REACTION

The NO-H-2 reaction over Ir field emitters has been studied using fiel d emission microscopy (FEM) for NO partial pressures in the 10(-7)-10( -6) Torr range, with NO:H-2 ratios up to 1:40. Sustained isothermal os cillations were observed in the form of periodic changes in the emissi on intensity from several planes present on the tip. An increase in em ission, which was initiated on the Ir(510) plane, expanded to the (100 ) plane and then to surrounding stepped planes. In the following perio d, the emission from these bright planes decreased slowly, until the c ycle began again. Interestingly, the three (100) areas of a [111]-orie nted tip oscillated independently of one another. The dynamic behaviou r of the NO-H-2 reaction over a large single crystal of Ir(100) under similar conditions was also studied. Mass spectrometry revealed a hyst eresis in product formation during a heat-cool temperature cycle, and damped oscillations in N-2 and H2O production. A model for the observe d non-linear behaviour on Ir will be proposed. (C) 1998 Elsevier Scien ce B.V. All rights reserved.