Phosphoric acid adsorbed on silica-ceria matrix obtained by sol-gel method: Studies of local structure, texture and acid property

SiO2/CeO2 mixed oxide (designated as SC) with variable ceria contents, SC1 = 5.6, SC2 = 8.0 and SO = 13.0 wt%, prepared by the sol-gel processing method, were obtained as amorphous solids possessing specific surface areas of SC1 = 463, SC2 = 474 and SC3 = 460 m(2)g(-1). Phosphate ions were immobilized onto the surface of these solids through Ce-O-P bonding by immersing SC into a phosphoric acid solution. Solids with the following P contents were obtained (in atom %): SCP1 = 0.60, SCP2 = 0.71 and SCP3 = 1.63. The binding energy peak P 2p(3/2) at ca. 134.0 eV, observed by XPS, and the P-31 MAS NMR single peak, observed at ca. -10 ppm, revealed that H2PO4- is the species present on the surface of the matrices. Using pyridine as a molecular probe, only Bronsted acid sites could be detected. The amount of these acid sites, determined by ammonia gas adsorption on the surface, reached the following values for each phosphate-treated solid: SCP1 = 0.37, SCP2 = 0.43 and SCP3 = 0.51 mmol g(-1).