INTERACTION OF HYDROGEN AND OXYGEN WITH NIOBIA-SUPPORTED AND NIOBIA-PROMOTED RHODIUM CATALYSTS

Niobia-supported and niobia-promoted rhodium catalysts, which exhibit significant SMSI behavior, have been characterized by temperature-programmed reduction, oxidation and desorption (TPR, TPO and TPD, respectively) of hydrogen. Hydrogen desorption from the rhodium metal decreased with increasing catalyst reduction temperature for the niobia-supported and niobia-promoted rhodium catalysts (SMSI), while no great change was observed for the unpromoted Rh/SiO2 catalyst (non-SMSI). For the Rh/Nb2O5 catalyst, no spillover of hydrogen occurred even at high temperature (773 K). In contrast, the Rh/SiO2 catalyst exhibited hydrogen spillover, and the amount was increased substantially by the deposition of niobia promoter. The SMSI states of these catalysts were studied by temperature-programmed oxidation. For the Rh/Nb2O5 catalysts, those partially reduced niobia species covering rhodium particles were oxidized around 460 K, and the partially reduced niobia support was oxidized around 600 K. For the Nb2O5-promoted Rh/SiO2 catalysts, the amount of reduced niobia species on rhodium particles was estimated by oxygen consumption measurement: the ratio of reduced niobia species with surface rhodium atoms (Rh(s)) is about 1.5 in terms of NbO2/Rh(s) for the state of the strong Rh-Nb2O5 interaction.