Abstract
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 θ of deuterium is varied from 0.001 to 0.33, the diffusion activation energy Ediff falls from 12 to 7 kcal/mol, and the preexponential factor D0 falls from 3×104 to 0.5 cm2/s. Concentration profiles 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), Ediff for deuterium is nearly constant at 4 kcal/mol for 0.02<θ<0.33, while D0 remains near 7×10−4 cm2/s.
Published Version
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