Coupled theoretical and experimental analysis of surface coverage effects in Pt-catalyzed NO and O2 reaction to NO2 on Pt(111)

Batch reactor results and analysis are reported for the reaction of NO with O-2 to form NO2 over a Pt(1 1 1) single crystal at atmospheric pressure. The apparent activation energy and NO, O-2, and NO2 reaction orders are found to be 80 kJ mol(-1), 1.3, 1, and -2 and are comparable to previous studies on supported Pt catalysts which take inhibition by the product NO2 into account. The absolute rates on a per Pt atom basis are the highest yet reported 0.34 +/- 0.02 s(-1), at 300 degrees C, 73 ppm, NO, 27 ppm, NO2 and 5% O-2. Auger electron spectroscopy and X-ray photoelectron spectroscopy are used to show that the surface chemisorbed oxygen coverage under reaction conditions is 0.76 +/- 0.06 ML, consistent with a coverage controlled by NO2 dissociation. DFF calculations are used to compare the stability of possible surface intermediates on a clean Pt(1 1 1) surface with those on ap(root 3 x root 3)-2O (2/3 ML) ordering surface. In contrast to the clean surface, O-2 adsorption and dissociation are endothermic at 2/3 ML oxygen, but a peroxynitrite intermediate OONO* is slightly stable and may provide an alternative, associative pathway to NO2 formation that is consistent with the observed first order reaction kinetics in O-2. (C) 2008 Elsevier B.V. All rights reserved.