Abstract
Calorimetric heats of adsorption and sticking probabilities are reported
for NO and CO on both the reconstructed hex and the unreconstructed
(1 X 1) surfaces of Pt100 by single crystal adsorption calorimetry
(SCAC), at room temperature. The hex surface reverts to the (1 X
1 ) structure during adsorption of both gases, as previously reported.
The initial heat of adsorption on the (1 X 1) surface is 215 kJ/mol
for CO and 200 kJ/mol for NO. Adsorbate-adsorbate interactions determine
not only the dependence of the heat of adsorption on coverage but
also the formation of different ordered structures. A model is suggested
to explain the observed dependence of the differential heat on coverage
and the LEED patterns, and a Monte Carlo simulation is performed
to derive the corresponding differential heat, thus allowing estimates
to be made of the magnitude of adsorbate-adsorbate interactions.
For CO adsorption, the critical contribution is the pairwise interaction
energy epsilon(d) between molecules in nnn sites while for NO triplet
formation is suggested with significant repulsive interaction between
molecules in the same tripler (epsilon(t)) and an even stronger repulsion
between triplet pairs (epsilon(tt)). NO-NO repulsive interactions
(epsilon(t) = 20 kJ/mol, epsilon(tt) = 80 kJ/mol) are considerably
stronger than GO-CO interactions (epsilon(d) = 5 kT/mol); thus, at
half monolayer coverage CO gives rise to a c(2 X 2) pattern while
NO gives a c(2 X 4) pattern. Moreover, with CO the coverage can be
increased to 0.75 ML, with the formation of compressed structures,
while for NO the saturation coverage is just 0.5 ML. The differential
heat on the hex surface is also discussed showing the possible role
of adsorption at defect sites in the energetics of the system. The
surface energy difference between the clean (1 X 1) and hex surfaces
is obtained as 20 kJ(mol Pt-s)(-1) by comparing the integral heats
of adsorption of CO on both surfaces at theta = 0.5, when the final
states of the two systems are identical.
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