Local independent oscillation centres present on an iridium field emitter during the NO-H2 reaction

The NO-H-2 reaction over Ir field emitters has been studied using field 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 oscillations were observed in the form of periodic changes in the emission intensity from several planes present on the tip. An increase in emission, which was initiated on the Ir(510) plane, expanded to the (100) plane and then to surrounding stepped planes. In the following period, the emission from these bright planes decreased slowly, until the cycle began again. Interestingly, the three (100) areas of a [111]-oriented tip oscillated independently of one another. The dynamic behaviour of the NO-H-2 reaction over a large single crystal of Ir(100) under similar conditions was also studied. Mass spectrometry revealed a hysteresis in product formation during a heat-cool temperature cycle, and damped oscillations in N-2 and H2O production. A model for the observed non-linear behaviour on Ir will be proposed. (C) 1998 Elsevier Science B.V. All rights reserved.