Non-linear behaviour in the NO–H2 reactions over single crystals and field emitters of some Pt-group metals

Abstract

Field emission microscopy studies of the NO–H₂ reactions on Pt, Rh and Ir field emitters in the 10^–7 to 10^–4 mbar total pressure range exhibit non-linear behaviour. In the cases of Ir and Rh field emitters, sustained isothermal oscillations in the form of periodic changes in the emission current from various planes present on the tips were observed. In the case of Ir, a bright wave initiated at the Ir(510) surface and moved to the (100) and surrounding planes. In the case of Rh, a bright wave initiated at the Rh(533) surface moved around the (111) plane essentially parallel to the steps around that plane. Subsequently, in both cases, the emission from these bright planes decreased slowly, until the cycle began again. The dynamic behaviour of the NO–H₂ reactions over a number of selected large single-crystal surfaces of Pt, Rh and Ir, as studied by mass spectrometry and Auger electron spectroscopy, showed a rich variety of non-linear behaviour. The reaction over Pt(100) and various (111)-terraced Rh planes with (100) steps showed sustained isothermal oscillations in product formation when the temperature was held constant on the cooling branch of a heat–cool heating cycle. In both cases, N₂ and NH₃ were formed, but for Rh, formation of these products occurred out of phase with each other, whereas for Pt, it occurred in phase. The Ir(100) surface showed a hysteresis in N₂ production. Models for the observed non-linear behaviour and the differences in behaviour between Pt, Rh and Ir will be discussed.