Thermal barrier energetic significance

DFT calculations [17,331,332] of 02/Pt(l 11) find the two different molecular precursors defined in the discussions above and observed by STM [153,326]. However, DFT calculations of 02 dissociation on surfaces are problematical and two different standard approximations for the exchange correlation functional (PW91 and RPBE) get somewhat different results. The DFT calculations are in only fair agreement with experiments. Most troubling is that all DFT barriers to dissociation from both molecular precursors are significant, 0.6-0.9 eV relative to the 02+ Pt(lll) asymptote. This is qualitatively inconsistent with experiment, which shows that thermal dissociation is energetically favored over desorption. 

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