The effects of adsorbate coverage on catalytic surface reactions are not well understood. Here, we contrast the rates of ${\mathrm{O}}_2$ and ${\mathrm{NO}}_2$ dissociations, two competing reactions in NO oxidation catalysis, versus oxygen coverage at a Pt(111) surface. In situ x-ray photoelectron spectroscopy experiments show that the ${\mathrm{NO}}_2$ dissociation rate is less sensitive to O coverage than is ${\mathrm{O}}_2$. Density-functional theory simulations reveal an ${\mathrm{NO}}_2$ reaction pathway that is more adaptable to an increasingly crowded surface than is ${\mathrm{O}}_2$ dissociation. While the rates are comparable at low coverage, ${\mathrm{NO}}_2$ dissociation is many orders of magnitude faster at O coverages typical of NO oxidation catalysis.

• Received 2 June 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.076101