While the oxidation of CO has long been considered to be structure insensit
ive, nano-assembled model catalysts consisting of size-selected metal clust
ers (Au-n, Pt-n, Pd-n, and Rh-n, n = 1-20), supported on thin MgO(1 0 0) fi
lms, reveal distinct size-effects. When adding a single Pt atom to Pt-14,Pt
- platinum clusters increase their reactivity by a factor of three. Rh-20 s
hows the highest reactivity of all the investigated clusters, oxidizing abo
ut 13 CO molecules per cluster at 350 K. While F-centers on MgO films trans
form Aug from an inert to an active catalyst, the reactivity of Pds is not
suppressed when deposited on defect-poor films. These different catalytic p
roperties are rationalized within simple frontier orbital models, whereas f
or AUg the reaction mechanism for the low temperature oxidation of CO is el
ucidated within first-principle calculations. (C) 2000 Elsevier Science B.V
. All rights reserved.