Electronic state and catalytic activity of SiO₂ doped γ–Al₂O₃

Abstract

Silica-alumina catalysts have been applied for a long time in cracking and reforming reactions. Lewis type surface sites are generally accepted to serve as active centers. In this respect, surface properties of these materials are of main concern. In the present work S O z doped y A 1 2 0 3 with less than a monolayer coverage were prepared by the reaction of SiC14 (8) with -pA120j at 820-1120 K. Catalytic activity of the samples was tested by a model reaction of n-hexane cracking. Surface composition and electronic state were evaluated by XPS investigations performed on a KRATOS XSAM 800 instrument. Catalytic activity (conversion) varies remarkably with increasing Si content showing a sharp maximum a t low Si coverage. The observed shift of A1 2p lines with increasing Si content is interpreted as an enhancement of the acceptor or acidic character of the surface A1 atoms. The Auger parameter for Si(dSi), (composed by addition of Si 2p BE and Si (KLL) KE values) passes also through a maximum at the same Si-content, as does the conversion curve. A close correlation of the Auger parameter and the catalytic activity is established for samples with (Si/A1)57%. This leads to a conclusion, that the rise in extra-atomic relaxation energy may serve as an additional indication of enhanced catalytic activity.

XPS characterization of cobalt impregnated SiO2 and γ‐Al2O3

“…The observed shift of the Al 2p peak towards higher binding energies when cobalt is incorporated (Table 3) could be interpreted as an enhancement of the acceptor or acidic character of the surface Al atoms 47 . The formation of Co–O bonds in the surface could lessen the electron density around the Al atoms.…”

“…This could be the result of an increase in the silicon charge once the support is impregnated with cobalt. For example, Si–O–Co structures can be formed, so that the Si atoms might be in a relatively electron‐rich environment, 47 which would explain the lower BE of the peak in these solids (Figure 5A).…”

“…This could be the result of an increase in the silicon charge once the support is impregnated with cobalt. For example, Si-O-Co structures can be formed, so that the Si atoms might be in a relatively electron-rich environment, 47 which would explain the lower BE of the peak in these solids (Figure 5A). ever, the two energy levels resulting from the spin-orbit coupling are very close, 49 and the signal can be fitted with a single peak, similarly to the case of Si 2p spectrum.…”

“…The observed shift of the Al 2p peak towards higher binding energies when cobalt is incorporated (Table 3) could be interpreted as an enhancement of the acceptor or acidic character of the surface Al atoms. 47 The formation of Co-O bonds in the surface could lessen the electron density around the Al atoms. This behavior has been reported previously when thin layers of silica were built on the alumina surface 47 (silicon and cobalt have fairly similar electronegativities).…”

“…47 The formation of Co-O bonds in the surface could lessen the electron density around the Al atoms. This behavior has been reported previously when thin layers of silica were built on the alumina surface 47 (silicon and cobalt have fairly similar electronegativities). This result may suggest that the impregnation of alumina with cobalt would generate stronger acidic sites that could be important in catalytic performance in hydrogenation reactions involved in the hydrocarbon cracking.…”