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

Hydrogen species in both SiO2 and Rh/SiO2 catalysts pretreated in different atmospheres (H2, O2, helium or air) at different temperatures (773 or 973 K) were investigated by means of 1H MAS NMR. In SiO2 and O2-pretreated catalysts, a series of downfield signals at -7.0, 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) and SiOH under the screening of oxygen vacancies in SiO2 lattice, respectively. Besides the above signals, both upfield signal at -110 and downfield signals at 3.0 and 0.0 were also detected in H2-pretreated catalyst, respectively. The upfield signal at -110 originated from the dissociative adsorption of H2 over rhodium and was found to consist of both the contributions of reversible and irreversible hydrogen. There also probably existed another dissociatively adsorbed hydrogen over rhodium, which was known to be p hydrogen and in a unique form of "delocalized hydrogen". It wa