Determination of the surface coverage of oxidic supports by oxidic and non-oxidic supported phases using potentiometric titration and electrophoretic mobility data. A study of Fe2O3/Al2O3 supported catalysts

Abstract

The point of zero charge (p.z.c.) and the isoelectric point (i.e.p.) of a series of Fe₂O₃/Al₂O₃ catalysts of varying iron contents, used as a model system, have been determined using potentiometric titrations and microelectrophoretic mobility measurements, respectively. Simple equations allowing the calculation of the percentage surface coverage (p.s.c.) by determing p.z.c. or i.e.p. values have been derived. Although the p.z.c. was found to differ from the i.e.p., both techniques provided similar, although not identical, p.s.c. values. These values correlated very well with X-ray photoelectron spectroscopy results. This fact suggests that both potentiometric titrations and microelectrophoretic mobility measurements can be used to estimate the variation in p.s.c. of a supported-metal oxide catalyst caused by the variation of a preparative parameter. The main advantage of these two methods is the low cost involved. The present work suggests that the combination of the above-mentioned electrochemical techniques could prove extremely useful in investigating the formation of small amounts of non-oxidic supported phases hardly detectable by X-ray spectroscopy. Finally, it has been concluded that three iron-supported phases are formed on the surface of alumina: (i) small amounts of a non-oxidic supported phase (e.g. adsorbed Fe³⁺ or iron aluminate), (ii) loosely bound α-Fe₂O₃ and (iii) strongly associated α-Fe₂O₃.