Investigations of adsorption on Pt and Rh by laser-induced desorption

The adsorption, desorption, equilibrium, and diffusion behavior of NO, CO, and H₂ on Pt( 111) and Rh( 111) have been investigated using laser-induced thermal desorption. A pulsed infrared laser beam is focused onto a single-crystal surface, causing very rapid heating (~10¹⁰ K/s) and nearly instantaneous desorption of surface species which are detected with a quadrupole mass spectrometer. Isosteric heats of adsorption of NO and CO are found to decrease as the coverage increases from 26 ±2 to 12 kcal/mol and from 32 ±2 to 16 kcal/mol, respectively. By combining the equilibrium and adsorption data, preexponential factors for desorption are found to decrease by a factor of 10⁷ near saturation. Surface diffusion of deuterium and hydrogen on Pt( 111) varies strongly with coverage. As the initial surface coverage of deuterium is varied from 0.001 to 0.33, the diffusion activation energy E_diff, falls from 12 to 7 kcal/mol, and the preexponential factor Do falls from 3 × 10⁴ to 0.5 cm²/s. Concentration protiles measured directly by a variable aperture technique confirm that deuterium diffusion slows at low coverage. Hydrogen diffuses about twice as fast as deuterium. On Rh ( 111), E_diff for deuterium is nearly constant at 4 kcal/mol for 0.02 < 0 < 0.33, while Do remains near 7 × 10^-4 cm²/s.