The catalytic formation of water from adsorbed hydrogen and oxygen atoms on
Pt(111) was studied with scanning tunneling microscopy and high resolution
electron energy loss spectroscopy. The known complexity of this reaction i
s explained by the strongly temperature dependent lifetime of the product H
2O molecules on the surface. Below the desorption temperature water reacts
with unreacted O adatoms to OHad, leading to an autocatalytic process; at h
igher temperatures sequential addition of H adatoms to O-ad with normal kin
etics takes place.