Surface changes of alumina induced by phosphoric acid impregnation

Abstract

A series of phosphorus-modified γ-Al 2O 3 catalysts, prepared by impregnation of alumina with various loadings of phosphoric acid, were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrophoretic migration measurements. The texture of the alumina grains was altered to a great extent by phosphoric acid treatments in aqueous media. Two well differentiated processes occurred: (i) partial solubilization of the Al 3+ cations, which precipitate as AlPO 4 during drying of the impregnate, and (ii) attack of the grain boundaries giving rise to crumbling of big aggregates into smaller ones which then spread on the remaining alumina substrate. The XRD technique was unable to detect crystalline AlPO 4 structures up to a 12 wt.-% load of P 2O 5. At contents below 2.5 wt.-% P 2O 5, energy-dispersive microanalysis data revealed that phosphorus was homogeneously distributed on the alumina surface, but at higher P 2O 5 contents inhomogeneity gradually increased. Zeta potential measurements also indicated that phosphorus was homogeneously dispersed, very probably as a monolayer, up to about 2.5 wt.-% P 2O 5, while poorly dispersed phosphate species appeared at higher P 2O 5 loadings.