N₂ Chemisorption on Stepped Pt Surfaces. Control by 2-D and 1-D Precursor Behavior

Nitrogen adsorption uptake at 88 K and temperature-programmed desorption (TPD) experiments were done on two stepped platinum surfaces, Pt(335) and Pt(779). N 2 adsorption occurs only on the step sites, and the saturation coverages measured on these surfaces were (1.51 ± 0.09) × 10 14 and (1.01 ± 0.16) × 10 14 cm -2, respectively, indicating a higher fractional population of widely separated step sites compared to closely spaced step sites. The adsorption kinetics revealed that the adsorption is mediated by terrace and step precursors. The N 2 initial sticking probability is 0.91 ± 0.06 (average of six measurements) on the Pt(335) surface and 0.68 ± 0.07 (average of five measurements) on the Pt(779) surface. The binding energy of N 2 at step sites, determined from experimental and simulated TPD curves, is −0.50 eV at zero coverage and decreases monotonically to −0.44 eV at saturation coverage on both surfaces. The equilibrium thermodynamics of N 2 adsorbed on either of the two stepped Pt surfaces are the same. The role of terrace width on the initial sticking probability is discussed for adsorption on stepped surfaces.