The NO-CO reaction kinetics has been studied on Pd/MgO (1 0 0) model cataly
sts at low pressure by a pulsed molecular beam technique. The model catalys
ts have been prepared by epitaxial growth of metal clusters in UHV on cleav
ed MgO single crystals. Three samples with mean clusters sizes of 2.8, 6.9
and 15.6 nm have been studied. The reaction products are mainly N-2 and CO2
. The steady-state reaction rate has been measured between 150 and 400 degr
eesC at low pressures (10(-9)-10(-6) Torr). The rate limiting step for the
reaction is the dissociation of NO at low temperature and the adsorption of
CO at high temperature. The reaction probability of NO has been accurately
determined, taking into account the reverse spillover of NO from the MgO s
upport towards the Pd particles. It is shown that in these conditions the t
urnover number is not an appropriate parameter to determine the intrinsic e
ffect of particle size. The intrinsic activity depends not only on particle
size but also on particle shape. The medium sized particles exhibiting mai
nly (1 1 1) facets are found to be more active. The largest particles, whic
h exhibit principally (1 0 0) facets, are less active. (C) 2001 Elsevier Sc
ience B.V. All rights reserved.