1-D Reversible Phase of Chemisorbed N₂ on Stepped Pt Surfaces

The behavior of the N 2 phase adsorbed exclusively on the steps of the Pt(335) and Pt(779) surfaces was studied at 88 K under N 2 gas flux by reflection−absorption infrared spectroscopy and molecular beam adsorption experiments. Under equilibrium N 2 pressure, the saturated 1-D chemisorbed layer is able to be additionally reversibly populated (by 2−20%). Under these conditions, fast isotopic exchange between 14 N 2 and 15 N 2 occurs. No chemisorption is detected on the terraces. The reversibly chemisorbed N 2 species is spectrally indistinguishable from the irreversibly preadsorbed N 2 (N−N frequency 2236−2230 cm -1 ) and therefore must be chemisorbed on vacant Pt step sites. The infrared feature undergoes a frequency decrease with increasing N 2 coverage, which is a result of two opposite effects: a vibrational coupling shift (upward) and a dominant static shift (downward). The proposed kinetic model involving terrace- and step-bound precursors explains the experimentally observed Langmuirian adsorption isotherm for the reversible N 2 phase. This is the first reported reversible precursor-mediated 1-D chemisorption, which obeys the same “pseudo-Langmuirian behavior” as is known for its 2-D analogues.