Metal particles coverage evidenced by 1H NMR spectroscopy in high surface Rh/CeO2 catalysts

H-2 reduction at increasing temperatures of high surface Rh/CeO2 catalyst (SBET approximate to 55 m(2)/g) has been followed with H-1 NMR spectroscopy. Metal-support interaction produces an important decrease in intensity of the shifted NMR signal ascribed to hydrogen adsorbed on metal particles, disappearing when the sample is reduced at 773 K. Oxidation at 473 K of reduced catalysts restores the position of this line, but intensity displays lower values than initial ones, suggesting that some metal coverage has been produced. Consecutive 773 K reduction/473 K oxidation treatments showed that metal coverage increases with the number of cycles. To analyze the origin of this phenomenon, the influence of temperature, time, and type of reduction has been studied. From results reported in this work, it has been concluded that diffusion of reduced species from the support is the main factor responsible for the metal coverage produced. Elimination of the metal coverage requires the oxidation at 673 K of the catalyst.