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 excita tion via DANTE sequences. A transition from inhomogeneous to homogeneo us 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 origin ates from increased hydrogen mobility, whereas proton-proton dipolar c ouplings are negligibly small. A rate parameter k obtained from this m odel quantifies the average mobility of hydrogen in the exchange proce ss. This parameter increases by more than three orders of magnitude wh en the hydrogen coverage changes from 0.4 to 0.8. The simulations of l ine shapes obtained at variable temperatures showed that k exhibits Ar rhenius behavior with an activation energy of 52 (+/-5) kJ/mol and pre exponential factor k(0)=4X10(10) is implied that the motion of hydroge n must involve desorption, interparticle diffusion, and readsorption.