Oxygen Coverage Dependence of NO Oxidation on Pt(111)

The interaction of NO with adsorbed atomic oxygen on Pt(111) was studied with temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and low-energy electron diffraction (LEED). Atomic oxygen adlayers with 0.25 and 0.75 ML coverages were prepared on a Pt(111) single crystal by dissociative chemisorption of O-2 at 300 K and NO2 at 400 K, respectively. These two oxygen precovered surfaces were used to study the oxygen coverage dependence of NO oxidation at different sample temperatures. The well-ordered p(2 x 2)-O layer, corresponding to Theta(o) = 0.25 ML, does not react with NO to form NO2 in the temperature range 350-500 K, in contrast to CO oxidation, which takes place readily at a sample temperature as low as 300 K. At Theta(o) = 0.75 ML the NO oxidation reaction is facile, and the formation of NO2 is observed even at 150 K. However, the NO oxidation reaction completely stops as the atomic oxygen coverage drops below 0.28 ML, because all the weakly bound oxygen atoms available only at higher O coverages have been consumed. The remaining oxygen atoms are bound too strongly to the Pt(111) surface and, therefore, unable to participate in NO oxidation in the 150-500 K temperature range.