Electrical conductivity of g-Al 2 O 3 at atmospheric pressure under dehydrating/hydrating conditions M. Caldararu * , G. Postole, C. Hornoiu, V. Bratan, M. Dragan, N.I. Ionescu Instituteof Physical Chemistry ``I.G. Murgulescu'' of the Romanian Academy, Spl. Independentei 202, 77208 Bucharest 12, Romania Received 3 April 2001; accepted 28 June 2001 Abstract Changes of electrical conductance and capacity of g-Al 2 O 3 werestudiedinsituduringdehydration/rehydrationingas¯ow. The measurements were performed at atmospheric pressure by following a standard protocol of experiments, and were coupled with simultaneous monitoring of the composition of the inlet/ef¯uent gas. The electrical properties are dominated by the protonic conduction. It was shown that at low temperature this occurs by vehicle mechanism, being facilitated by the presence of water molecule adsorbed on g-Al 2 O 3 surface; the decrease of electrical conductance on heating is obviously connected with dehydration. At higher temperature t > 200  C conduction is dominated by proton hopping between OH groups Grotthuss mechanism). It is suggested that in current operating conditions in catalysis, g-Al 2 O 3 is only partially dehydrated/dehydroxylated, the bulkactingasasourceofwater/OHgroupsforthesurface;thisprocesscontrolstheacidityofaluminaandalumina-supported catalysts. # 2001 Elsevier Science B.V. All rights reserved. Keywords: AC electrical conductivity; g-Al 2 O 3 ; Protonic conduction; Water adsorption 1. Introduction Aluminum oxide exists in several crystallographic modi®cations, of which g-Al 2 O 3 is the most widely usedasacatalystandassupportforcatalysts[1,2].It was shown that the catalytic properties of g-Al 2 O 3 depend very much upon the extent to which it has been dried [3±5]. Extended studies [2,6±9] based mainly on IR data and structural considerations) focused on the nature and stability of surface OH groups in conditions of catalysis, since the catalytic activityof g-Al 2 O 3 aremainlyattributedtoLewisacid sites created on the surface by removal of hydroxyl groups on heating above 4008C). However, g-Al 2 O 3 easily adsorbs water even at room temperature and wateradsorptionwouldchangepartoftheLewisacid sites in acid sites of Brùnsted type. The thermal stability of physically and chemically adsorbed water onaluminaisstillnotclear.Informationaboutsurface behavior in hydrating/dehydrating environments is clearlyneededforunderstandingcatalysisonalumina and alumina-supported catalysts. Besides the crucial role in controlling surface acidity, the incorporated water could generate parasite reactions or contribute tothespillovereffects[10]and,asthemostimportant atmospheric contaminant, sometimes it competes with reactants for adsorption on the same adsorption sites [11]. Severalstudiesonaluminaweredevotedtotheelec- tricalproperties.Thereporteddataaresomewhatcon- tradictory, because a wide range of alumina samples Applied Surface Science 181 2001) 255±264 * Corresponding author. Tel./fax: 40-1-312-1147. E-mail address: mcaldararu@chimfiz.icf.ro M. Caldararu). 0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0169-433201)00393-2