ADSORPTION, DESORPTION, AND INTERPARTICLE MOTION OF HYDROGEN ON SILICA-SUPPORTED RUTHENIUM - A STUDY BY IN-SITU NUCLEAR-MAGNETIC-RESONANCE

H-1 NMR line shapes of hydrogen adsorbed on silica-supported ruthenium at pressures of 10(-6)-10 Torr were studied by using selective excitation via DANTE sequences. A transition from inhomogeneous to homogeneous line broadening was observed at hydrogen coverage of similar to 0.5. The spectra were simulated by using generalized Bloch equations that included N-site exchange processes. The homogeneous line shape originates from increased hydrogen mobility, whereas proton-proton dipolar couplings are negligibly small. A rate parameter k obtained from this model quantifies the average mobility of hydrogen in the exchange process. This parameter increases by more than three orders of magnitude when the hydrogen coverage changes from 0.4 to 0.8. The simulations of line shapes obtained at variable temperatures showed that k exhibits Arrhenius behavior with an activation energy of 52 (+/-5) kJ/mol and preexponential factor k(0)=4X10(10) is implied that the motion of hydrogen must involve desorption, interparticle diffusion, and readsorption.