The nonlinear behavior in the NO−H2 reaction over Ir(110) was studied with X-ray photoelectron spectroscopy
(XPS). At a NO pressure of 7.7 × 10-8 mbar, three different H2/NO mixtures were examined in the temperature
range from 300 to 750 K, in addition to a mixture with a 10 times higher NO pressure. During the heating−cooling cycle in the reaction mixture, three different N species could be observedmolecular NO, atomic N,
and NH
x
as well as two O species which could be assigned to molecular NO and atomic O. Simultaneously,
the formation rate of N2, H2O, and NH3 was followed. It appeared that under low H2 pressure conditions no
NH3 is formed, and as a result, no NH
x
could be detected on the surface. The results show that the unreactive
surface is covered with atomic O and molecular NO, whereas the active surface is either covered with atomic
N (and NH
x
) or adsorbate free. The presence of atomic O can suppress the NO dissociation up to 600 K, and
it has a strongly destabilizing effect on atomic N. Furthermore, the dissociative adsorption of hydrogen can
be blocked by a critical N coverage or by the combination of molecular NO and atomic O. On the basis of
these results, a model is proposed for the oscillations in activity and selectivity.