Non-linear processes on Pt, Ph, Pd, Ir and Ru surfaces during the NO-hydrogen reactions

The present paper reviews our research in the field of NO + H-2 reactions o ver Pt, Ph, Pd, Ir and Ru surfaces. Emphasis will be placed on the ability of these platinum group metals to produce oscillations in both reaction rat e and selectivity, in addition to other non-linear processes such as hyster esis phenomena. In the case of Ph, non-linearity is related to periodic tra nsitions between N- and O-rich surfaces, with O destabilising the N-adlayer and causing an acceleration in the N-2 production. However, in the case of Pt, the important step involves the creation of vacant sites required for NO dissociation, whereby products leaving the surface facilitate an autocat alytic rise in the concentration of such vacancies and, hence, reaction rat e. Ir positions itself between the former two, because oscillations have be en observed in two different regimes. Both a lower-temperature Pt-like and higher-temperature Rh-Like behaviours have been observed on different Ir su rfaces. New results gained from field emission microscopy (FEM) and field i on microscopy (FIM) measurements are presented to consolidate the lower-tem perature Pt-like behaviour, and confirm the dualistic nature of Ir. No osci llations in rate have been observed over Pd(111) or Ru(0001). The Ru surfac e exhibits selectivity to N-2 of almost 100% even in large excess of hydrog en. (C) 2000 Elsevier Science B.V. All rights reserved.