Zusammenfassung
The kinetics of the oxidation of carbon monoxide on Pt(111) surfaces
was studied isothermally by using an effusive directional molecular
beam in an arrangement based on a variation of the dynamic method
originally devised by King and Wells. Three temperature regimes were
identified for this reaction on surfaces precovered with atomic oxygen.
Below 300 K no reaction is observed, and the presence of preadsorbed
atomic oxygen on the surface does not significantly affect the initial
sticking coefficient of CO but only reduces its saturation coverage
by less than half, which it does by preferentially blocking the bridge
sites. Above 400 K, on the other hand, the desorption of CO2 from
oxygen-covered surfaces is controlled by the impinging frequency
of the incoming CO. The most interesting temperature range is that
between 300 and 400 K, where the rate of surface recombination of
CO with oxygen competes with that of CO adsorption; under those conditions
the overall dynamic behavior is fairly complex, and not all the surface
oxygen is reactive. Furthermore, the reaction rates in this regime
not only depend on the coverages of the reactants, but also on how
the surface is prepared. Two kinetically distinct types of oxygen
atoms develop during the course of reaction in spite of the fact
that they all sit on identical sites at the start of the kinetic
runs, suggesting that the reactivity of chemisorbed CO depends on
the local oxygen coverage of neighboring sites. We propose that such
local arrangements modify the adsorption energy for atomic oxygen,
and that this in turn changes the activation energy for the oxidation
reaction. Previous reported molecular beam experiments were also
extended to cover a wider range of surface coverages in order to
better determine the dependence of the rate constant for the surface
oxidation step on the coverages of CO and oxygen. It was found that
while the presence of oxygen on the surface helps the production
of CO2, increasing CO coverages augment the activation barrier for
this reaction, an observation that is in direct contrast with previous
reports. Finally, the adsorption sites for CO during the surface
CO+O recombinatory reaction were characterized by reflection�absorption
infrared spectroscopy. The data reported here is analyzed and discussed
in terms of possible kinetic models.
Nutzer