Comments on the paper by S. Westerberg, C. Wang and G.A. Somorjai,
entitled �Heat of adsorption of CO on Pt(1 1 1) obtained by sum frequency
generation vibrational spectroscopy�a new technique to measure adsorption
isotherms� Surf. Sci. 582 (2005) 137
Surf. Sci., (2005)DOI: 10.1016/j.susc.2005.07.033
Abstract
It has been claimed S. Westerberg, C. Wang and G.A. Somorjai, Surf.
Sci. 582 (2005) 137 that the CO coverage on Pt(1 1 1) is proportional
to the intensity of the band of linear CO observed by sum frequency
generation vibrational spectroscopy, this assertion being based on
an assumed agreement between a literature value of the adsorption
heat of CO on Pt(1 1 1) and that calculated by applying the Langmuir
model to the adsorption isotherms obtained by assuming the proportionality
mentioned above. We show here, in the first place, that Langmuir�s
equation was wrongly applied by Westerberg et al., the value of the
adsorption heat obtained with a correct application of Langmuir�s
model to the assumed adsorption isotherms being 73.5 kJ mol-1 instead
of that of 113 kJ mol-1. In the second place, and far more importantly,
we emphasize that Langmuir�s model, based on a constancy of the adsorption
heat, cannot be applied to the adsorption of CO on Pt, a system for
which the adsorption heat decreases markedly with increasing coverage.
Therefore, the above value of 73.5 kJ mol-1 for the adsorption heat
of CO on Pt(1 1 1), although obtained through a correct application
of Langmuir�s equation, is invalid. Finally, application of the thermodynamic,
and therefore model-independent, Clausius�Clapeyron equation to the
adsorption isotherms given by Westerberg et al. yields adsorption
heats that increase markedly with increasing coverage, which conclusively
proves that the assumed adsorption isotherms are essentially wrong,
and therefore that the claimed proportionality between the CO coverage
on Pt(1 1 1) and the intensity of the sum-frequency-generation band
of linear CO does not obtain.