Adsorption on transition aluminas from in situ capacitance measurements Monica Caldararu a,* , Georgeta Postole a , Mariana Carata a , Cristian Hornoiu a , Niculae I. Ionescu a , Tatiana Ioujakova b , Akos Redey b a I.G. Murgulescu Institute of Physical Chemistry of the Romanian Academy, Spl. Independentei 202, 77208 Bucharest, Romania b University of Veszprem, Department of Engineering Technology and Environmental, Egyetem u. 10, H-8201 Veszprem, POB 158, Hungary Received 22 October 2002; received in revised form 22 October 2002; accepted 17 December 2002 Abstract The interaction of water or propylene dipoles with surface of g-Al 2 O 3 and of a boehmite type alumina between room temperature and 400 8C was evaluated from evolution of the apparent capacity/dielectric constant during thermal cycling and under flushing with dry or humid gases (argon, oxygen) and with a mixture containing propylene and air (i.e. in conditions which mimic the catalytic experiment). Data allowed to stipulate that the extent of weak adsorption was higher on g-Al 2 O 3 , in relation with the higher amount of surface OH groups, as supported by IR data. # 2003 Elsevier Science B.V. All rights reserved. Keywords: 82.65.Jv; 77.22.-d; 82.80.-ch Keywords: Alumina; Capacity; Dielectric constant; Dehydration; Water adsorption; Propylene adsorption 1. Introduction Surface properties of transition aluminas were intensely studied, as these compounds are used as catalysts and as supports in many important catalytic processes. Adsorption on alumina is mainly controlled by its surface topography (i.e. by the local distribution of the surface OH groups, of coordinatively unsatu- rated Al 3þ cations and of oxygen ions), which in turn depends strongly on the category of the pretreatment used [1–7]. Most of the studies on hydration/dehydra- tion of aluminas have been performed under special pretreatment conditions (i.e. by evacuation at high temperature) which, besides having the potential of facilitating incipient transition to other phase, provide information which could be much different of the surface behavior in operating conditions of catalysis (i.e. at atmospheric pressure, under gas flow). Recently [8] we presented data based on in situ electrical conductivity measurements proving that between room temperature (RT) and 400 8C the sur- face conductivity of g-Al 2 O 3 is dominated by protonic conduction. Up to about 200 8C, proton movement occurs by ‘‘vehicle’’ mechanism (facilitated by the presence of molecular water species); above this temperature, as most of the physically adsorbed water species were removed, the mobile protons of acidic OH groups move most probably by hopping between Applied Surface Science 207 (2003) 318–326 * Corresponding author. Tel.: þ40-213-121147; fax: þ40-213-121147. E-mail address: mcaldararu@chimfiz.icf.ro (M. Caldararu). 0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(02)01510-6