On the origin of the low-temperature ( ≈ 150 K) H 2 desorption from Pt(111)

Abstract

Chemisorption of hydrogen on Pt(111) has been reinvestigated using thermal-energy atom scattering (TEAS). A nearly defect-free ( ⩽ 0.1% step atoms) Pt(111) surface was used. In the absence of coadsorbed species, desorption from a saturated H monolayer occurs only at surface temperatures T s>180 K. The state frequently observed to desorb around T s = 150 K is atypical of clean H 2/Pt(111). TEAS measurements imply that this state is related to the incorporation of hydrogen in ice layers and thermal desorption data further support this interpretation. Hydrogen monolayer saturation on the clean, nearly defect-free Pt surface at T s = 80 K requires exposures in excess of 3×10 4 L, two orders of magnitude larger than observed previously.