Atom-resolved investigation of surface reactions: Ammonia and oxygen on Cu(110) at 300 and 400 K

Atomic oxygen adsorbed on Cu(110) forms isolated, added -Cu-O-rows at low cpverages and p(2 x 1)-O islands at high coverages. The reactivity of both types of species for reactions with ammonia has been investigated at 300 and 400 K by in-situ imaging with an atomic-resolution scanning tunnelling microscope (STM). We find that at 300 K oxygen in both forms is reactive with ammonia. At low coverages all oxygen adatoms are consumed readily, while at high precoverages reaction proceeds from the boundaries of oxygen islands. The reaction can initiate from both the end and the side of the -Cu-O-rows. During the exposure of an ammonia and oxygen mixture the reactivity of the oxygen species present in the steady state is estimated to be comparable to that of oxygen preadsorbed at low coverages. At 400 K, reactions can be initiated inside the p(2 x 1)-O islands. The N adatoms produced frequently block the ends of -Cu-O-rows inhibiting further reactions and creating stable, two-dimensional (2D) mixed N-O structures on the surface. Step defects on the surface are found to have a strong effect on the reactivity of oxygen adatoms. Reactivity is high at the top and bottom of a [1(1) over bar0$] step and at the bottom of a [001] step, whereas it is very low at the top of a [001] step.