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The
understanding and quantification of the CO adsorption modes
and strength on ultradispersed platinum catalysts supported on γ-Al2O3 is of prominent importance for analytic and
catalytic purposes. We report a multiscale experimental (AEIR, CO-TPD)
and theoretical approach to provide vibrational properties, adsorption
enthalpies, and desorption behaviors. First principles calculations
on Pt13(CO)
m
/γ-Al2O3 and Pt(111) surface models (using various exchange-correlation
functionals) provide a complementary view to experimental approches.
Adsorption enthalpies computed with the RPBE functional appear to
be the most compatible with the AEIR results. The occupation of top
sites by CO dominates the behavior of supported Pt clusters. CO coverage
reaches higher values in comparison to Pt(111) for similar operating
conditions, and considerable cluster reconstruction is observed at
high coverage. First principles calculations also confirm the IR assignment
related to the various adsorption modes on top and bridge sites and
demonstrate a particle size effect, lowering the frequency of linear
adsorption at top sites with respect to extended Pt(111) surfaces.
Finally, first principles-based microkinetic modeling of CO-TPD experiments
shows that the adsorption strengths predicted on the small-size cluster
by DFT are compatible with the experimental values. We discuss possible
reasons for the experimental desorption pattern to be much broader
than the computed pattern.
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