A. Auroux et al., Acidic character of metal-loaded amorphous and crystalline silica-aluminasdetermined by XPS and adsorption calorimetry, J PHYS CH B, 103(34), 1999, pp. 7195-7205
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.