Adsorption and desorption of NO, CO and H₂ on Pt(111): Laser-induced thermal desorption studies

The adsorption, desorption, and equilibrium behavior of NO. CO, and H₂ have been investigated using laser-induced thermal desorption (LITD). A pulsed infrared laser beam is focused onto a Pt(111) surface, causing very rapid heating ( ≈ 10¹⁰ K/s) and nearly instantaneous desorption of surface species. The desorbed molecules produce a pressure rise in the vacuum system that is proportional to the coverage and is detected with a quadrupole mass spectrometer. For surface temperatures between 160 and 280 K, S₀ for D₂ increases by ≈ 50% and for H₂ by ≈ 70%. Low-coverage sticking coefficients S₀ for both NO and CO are found to be independent of surface temperature in the range 160 to 430 K and indicate adsorption through a precursor state. Isosteric heats of adsorption of NO and CO decline as the coverage increases from initial values of 26 ± 2 and 32 ± 2 kcal/mol to 12 ± 2 and 16 ± 2 kcal/mol, respectively. By combining the equilibrium and adsorption data, pre-exponential factors for desorption have been determined as a function of coverage. Laser-induced surface damage does not appear to be significant in these results. However, repeated desorption without high temperature annealing causes a drop in S₀ for NO and CO by 30% and a rise in S₀ for D₂ by a factor of five. Measurements made with LITD agree well with available literature values obtained by other methods. However, LITD has several important advantages, especially the capability for the repeated measurement of surface coverages of a wide variety of adsorbates at high repetition rates in the presence of background pressures up to 10^-4 Torr.