Role of P-containing species in phosphated CeO2 in the deterioration of its oxygen storage and release properties

The chemical nature of P-containing species incorporated on the surface and within the subsurface regions of CeO2 solid and their effects on the deterioration of the oxygen storage and release properties of phosphated ceria have been investigated. The samples were characterised by X-ray diffraction, X-ray photoelectron, Raman, and 31P nuclear magnetic resonance spectroscopies; Fourier transform infrared pyridine chemisorption, N2 physical adsorption isotherms, oxygen storage capacity, and 18O2 temperature-programmed isotopic exchange measurements. In the samples with P/Ce ratios <0.03, isolated orthophosphate species are present on the surface and in the subsurface regions of CeO2 solid. In these samples, deterioration of the oxygen storage and release properties correlates very well with the amount of P incorporated as isolated orthophosphates, because oxygen diffusion within the subsurface region of CeO2 is increasingly inhibited. These results show that the presence of CePO4 (monazite) is not essential for the deterioration of oxygen storage and release properties in CeO2. In the samples with P/Ce > 0.03, crystals of monazite are formed. Theoretical estimations indicate that about 5.5 P atoms/nm2 are required to fully cover the CeO2 surface. This is equivalent to a P/Ce ratio of 0.02 in the ceria solid studied in the present work (BET area = 12.8 m2/g). The experimental P/Ce ratio needed for complete surface saturation is higher (close to 0.03) very likely because part of P is incorporated into the subsurface region of ceria. Monazite also deteriorates the oxygen storage and release properties of ceria because it is a very stable Ce(III) phase. Once CePO4 crystals are formed, further incorporation of P does not result in a deeper deterioration of the oxygen storage and release properties of ceria. Larger monazite crystals are then formed on the surface at the expense of isolated orthophosphate species, leaving part of the ceria surface uncovered and thus available for surface and bulk oxygen exchange.