Acidic character of metal-loaded amorphous and crystalline silica-aluminasdetermined by XPS and adsorption calorimetry

The XPS technique in combination with microcalorimetry was used to picture the acid character of metal ion loaded zeolite and silica-alumina samples. Co, Cu, and Ni ions were loaded on ZSM-5 and SiO2-Al2O3 (about 1 mmol/g) by three different procedures: ion exchange, impregnation, and solid-state io n-exchange. The samples prepared by ion exchange of the two matrixes presen ted high values of the 2 p(3/2) XPS bands of the metal ions, indicating the occurrence of charge transfer from the ions to the support matrix. This pe rmitted the stabilization of the metal phases as isolated ions rather than as oxide clusters. Microcalorimetric experiments of ammonia adsorption were performed in order to determine the number, strength (i.e., adsorption ent halpy), and strength distribution of the acid sites on the samples and on t he relevant matrixes. The N 1s XPS lines of ammonia adsorbed on the surface s were decomposed into two component peaks, assigned to Bronsted (average v alue of BE, 402.2 eV) and Lewis (average value of BE, 400.4 eV) acid sites. The relative intensities of the two peak components were measured for the quantitative determination of Bronsted and Lewis acid site concentrations. Coupling the information from adsorption calorimetry and XPS spectroscopy o f N Is adsorbed lines, the absolute numbers of Bronsted and Lewis acid site s on each sample were determined. The two support matrixes were protonic ac ids (65 and 60% of Bronsted acid sites for ZSM-5 and SiO2-Al2O3, respective ly), and the acidity of ZSM-5 was greater than that of SiO2-Al2O3 consideri ng both the number of total acid sites and the acid strength. The presence of metal ions deposited on the two matrixes deeply changed the respective p roportions of Bronsted and Lewis sites. A huge increase in the Lewis acid s ite population of the ZSM-5-based samples (70, 85, and 90% of Lewis sites f or the samples containing Co, Cu, and Ni, respectively) and of the SiO2-Al2 O3-based samples prepared by ion exchange (55, 60, and 70% of Lewis sites f or the samples containing Co, Cu, and Ni, respectively) was observed.