H-1 MAS NMR characterization of hydrogen over silica-supported rhodium catalyst

Hydrogen species in both SiO2 and Rh/SiO2 catalysts pretreated in different atmospheres (H-2. O-2, helium or air) at different temperatures (773 or 97 3 K) were investigated by means of H-1 MAS NMR. In SiO2 and O-2-pretreated catalysts, a series of downfield signals at similar to7.0, 3.8-4.0, 2.0 and 1.5-1.0 were detected. The first two signals can be attributed to strongly adsorbed and physisorbed water and the others to terminal silanol (SiOH) a nd SIGH under the screening of oxygen vacancies in SiO2 lattice, respective ly. Besides the above signals, both upfield signal at similar to -110 and d ownfield signals at 3.0 and 0.0 were also detected in H-2-pretreated cataly st, respectively. The upfield signal at similar to -110 originated from the dissociative adsorption of H-2 over rhodium and was found to consist of bo th the contributions of reversible and irreversible hydrogen. There also pr obably existed another dissociatively adsorbed hydrogen over rhodium, which was known to be beta hydrogen and in a unique form of "delocalized hydroge n". It was presumed that the beta hydrogen had an upfield shift of ca. -20- -50, though its H-1 NMR signals, which, having been masked by the spinning sidebands of Si-OH, failed to be directly detected out. The downfield sign al at 3.0 was assigned to spillover hydrogen weakly bound by the bridge oxy gen of SiO2. Another downfield signal at 0.0 was assigned to hydrogen held in the oxygen vacancies of SiO2 (Si-H species), suffering from the screenin g of trapped electrons. Both the spillover hydrogen and the Si-H resulted f rom the migration of the reversible hydrogen and the beta hydrogen from rho dium to SiO2 in the close vicinity. It was proved that the above migration of hydrogen was preferred to occur at higher temperature than at lower temp erature.