Article,
Effects of K, O, and H adatoms on the adsorption kinetics of CO on Pt(111)
Surf. Sci., 273 (3): 273 - 284 (1992)
DOI: DOI: 10.1016/0039-6028(92)90065-E
Abstract
The sticking coefficient, S, of CO on clean and K-, O- and H-modified
Pt(111) surfaces has been measured directly using a simple kinetic
uptake method, as a function of CO and modifier coverage and surface
temperature. A simple method is also introduced to determine the
coverage-dependent desorption energy from these measurements without
the need to know the pre-exponential factor or desorption order for
those systems where S is only a function of coverage or only weakly
dependent on temperature. The initial sticking coefficient of CO
on clean Pt(111) is 0.9 and is independent of substrate temperature.
The major influence of adsorbed K on the initial sticking coefficient
of CO on Pt(111) at 320 K is site blocking, and the magnitude of
this effect correlates well with the size of the adsorbed K ion or
atom. The shape of S as a function of CO coverage for low K coverages
in the ionic regime is similar to that of CO on clean Pt(111). A
dramatic change occurs for K coverages near or greater than 0.24
ML, where S passes through a maximum at about of the saturation CO
coverage and coincident CO and K desorption is observed. This behavior
of S, which has not been seen before, can be fit quite adequately
by a simple model that incorporates the reduced site blocking effects
of K due to the CO-induced transition of K from a metallic neutral
to an ionic state and the accompanying size change of K atoms. These
results are best described by a coverage-dependent alkali-CO interaction.
In additional studies, we find that adsorption of 0.25 ML O on Pt(111)
has no effect on S for low CO coverages at 100 K, and only the saturation
CO coverage is reduced from 0.59 to 0.31 ML. In contrast, adsorbed
H reduces not only the CO saturation coverage but also S even at
low CO coverages.
